the use of nanoporous carbon membranes for high temperature ...

THE USE OF NANOPOROUS CARBON MEMBRANES FORHIGH TEMPERATURE CARBON DIOXIDE SEPARATIONCLARE JULIA ANDERSONSUBMITTED IN TOTAL FULFILMENT FOR THE REQUIREMENTS OFTHE DEGREE OF DOCTOR OF PHILOSOPHYMAY, 2009DEPARTMENT OF CHEMICAL AND BIOMOLECULAR ENGINEERINGTHE UNIVERSITY OF MELBOURNE

DEDICATIONThis thesis is dedicated to my two grandmothers, Julia (deceased) and Vonda and theirunfaltering belief in the importance of education.ii

ABSTRACTTechnology designed to capture and store carbon dioxide (CO 2 ) will play a significant role inthe near-term reduction of CO 2 emissions and is considered necessary to slow globalwarming.Nanoporous carbon (NPC) membranes, made from the pyrolysis of a polymer, may be thenew generation of gas separation membranes suitable for CO 2 capture. The research workpresented in this thesis has assessed the suitability of NPC membranes for CO 2 capture fromnatural gas (methane, CH 4 ) and air-blown synthesis gas purification (nitrogen, N 2 ) at highoperating temperatures and in the presence of condensable and non-condensable impurities.Three different types of NPC membranes, namely supported, modified-supported andunsupported, have been manufactured. The elimination of cracking within the NPCmembranes, which is a significant concern for the commercialisation of these membranes,was most successful for supported NPC membranes made from polyfurfuryl alcohol (PFA).Characterisation results of the produced NPC membranes showed a pore size range suitablefor gas separation via molecular sieving (3 - 5Å). The characterisation results also indicated asignificant increase in porosity with pyrolysis temperature. The membrane performanceresults supported these findings with a significant increase in permeance observed withincreasing pyrolysis temperature.Mixed gas performance measurements revealed an increase in the CO 2 /CH 4 selectivity overthe pure gas CO 2 /CH 4 permselectivity due to the competitive adsorption of the more polarCO 2 .Mixed gas performance measurements also showed an increase in CH 4 permeance as theoperating temperature was increased from 35 to 200 o C, which can be related to an increase inthe rate of diffusion. However, the selectivity decreased with increasing operatingtemperature due to the smaller changes in the CO 2 permeance. These smaller changes in CO 2permeance are related to the stronger adsorption of this gas to the carbon surface at loweroperating temperatures as confirmed by gas adsorption experiments and molecular modellingsimulation.The exposure of the NPC membranes to condensable and non-condensable impuritiestypically found in natural gas and synthesis gas saw a small reduction in permeance and aminimal reduction in selectivity, particularly at elevated operating temperatures.iii

Overall, NPC membranes have good potential as a technology suitable for CO 2 capture asthey traditionally have a better performance than their polymeric counterparts and can beoperated at higher temperatures with only a slight impact on performance in the presence ofimpurities. However, the challenges of cracking and aging are considerable and have not asyet been fully resolved.The very recent emergence of new membrane technologies that combine the performance of aNPC membrane with the durability of a polymeric membrane are certainly now attractingmore attention and may well reveal themselves as the best of both worlds.iv

PREFACESome of the findings of this thesis have been published elsewhere as follows.Journal PublicationsC. J. Anderson. S. J Pas, G. Arora, S. E. Kentish, A. J. Hill, S. I. Sandler and G. W. Stevens,“Effect of Pyrolysis Temperature and Operating Temperature on the Performance ofNanoporous Carbon Membranes,” Journal of Membrane Science 322 (2008) 19-27.Peer-Reviewed Conference PublicationsC. J Anderson. G. Arora, S. E. Kentish, S. I. Sandler and G. W. Stevens, “Nanoporous CarbonMembranes for CO 2 Capture,” in Chemeca 2007 – 35th Australasian Chemical EngineeringConference. 2007. Melbourne, Australia.Conference ProceedingsC. J Anderson. G. Arora, S. E. Kentish, S. I. Sandler and G. W. Stevens, “Nanoporous CarbonMembranes for CO 2 Capture,” in North American Membrane Society Annual Meeting. 2007.Orlando, United States of America.C. J Anderson, “Bridging the Gap from Fossil Fuels to Sustainable Energy Using CarbonDioxide Sequestration,” in Youth Symposium of the 19th World Energy Congress. 2004.Sydney, Australia.v

ACKNOWLEDGEMENTSThe research work for this PhD thesis was carried out during the very busy time in which mytwo daughters, Estelle and Elise, were born. I would never have been able to complete suchan undertaking as a PhD without the support and understanding of several people whom Iwould like to acknowledge here.Firstly, to my primary supervisor A/Prof. Sandra Kentish (the University of Melbourne) forher dedication and ability to provide the most comprehensive support despite her numerouscommitments to other students and to the Department of Chemical and BiomolecularEngineering.Secondly to my other supervisors, Prof. Geoff Stevens (University of Melbourne) and Prof.Stan Sandler (University of Delaware) for setting aside time in their busy schedules and forproviding access to invaluable resources.Thanks to Barry Hooper and the Cooperative Research Centre for Greenhouse GasTechnologies (CO2CRC) for the funding and also enabling me the opportunity to becomeinvolved in assisting on such important matters as reducing greenhouse gas emissions. Iwould also like to acknowledge the University of Melbourne, the Particulate FluidsProcessing Centre (PFPC) and the Department of Chemical and Biomolecular Engineering forfinancial assistance related to visiting the University of Delaware in the USA.To Wendy Tan, the CO2CRC research assistant, for all your hard work in the lab especially inrunning experiments and handling chemicals whilst I was pregnant and on maternity leave.Your diligence and tenacity (especially in the face of cracked membranes and brokenequipment) is exceptional.Thank you also to Drs Steven Pas and Anita Hill (CSIRO) for providing the PALS analysispresented in this thesis. It has been a pleasure working with you both. Additionally, thank youto Roger Curtin (University of Melbourne) for the SEM analysis and Dr Peng Ling (RMIT)for the HRTEM analysis presented in this thesis.To Drs Gaurav Arora and Jianwen Jiang (formerly of the University of Delaware) forallowing access to their molecular simulation models and for their patience and support withhelping to get the models up and running in Australia. Additionally, thanks to Dr DaltonHarvey (University of Melbourne) for providing access to better computing power and forteaching me the basics of the Unix Language.vi

To the Department of Chemical and Biomolecular Engineering, it has been lovely to be backin the building after my eight year break in industry following undergraduate studies. It is avery welcoming and supportive work place. Additionally, thank you to Kevin Smeaton andthe workshop team for making the seemingly impossible possible.To my parents, Kate and Kevin, thank you both for my upbringing. Being a parent myselfnow, I realise how much work goes in often without acknowledgement. So thank you. I feelproud of the values you have installed along with the unconditional support for all of my lifechoices.Great thanks also to our next door neighbour, Nancy Vella, for providing a loving home forEstelle, which enabled me some time to complete this thesis.Finally I would like express my gratitude to my husband, Darren Millard. Darren, you are themost amazing person I know and the love of my life. Your untiring support of both my studiesand in the home with our children is beyond words.vii

DECLARATIONThis is to certify that:The thesis comprises only my original work.Due acknowledgement has been made in the text to all other material used.The thesis is less than 100,000 words in length, exclusive of tables, maps, bibliographies.Clare AndersonMay 2009viii

TABLE OF CONTENTSDEDICATION ........................................................................................................................ IIABSTRACT ...........................................................................................................................IIIPREFACE ................................................................................................................................VJOURNAL PUBLICATIONS ....................................................................................................... VPEER-REVIEWED CONFERENCE PUBLICATIONS..................................................................... VCONFERENCE PROCEEDINGS.................................................................................................. VACKNOWLEDGEMENTS .................................................................................................. VIDECLARATION ................................................................................................................ VIIITABLE OF CONTENTS ...................................................................................................... IXLIST OF FIGURES..............................................................................................................XVLIST OF TABLES.............................................................................................................XXIINOMENCLATURE ........................................................................................................ XXVINON-ENGLISH CHARACTERS .......................................................................................... XXVIISUBSCRIPTS.................................................................................................................... XXVIII1 INTRODUCTION.............................................................................................................11.1 GLOBAL WARMING......................................................................................................11.2 CO 2 CAPTURE AND SEQUESTRATION...........................................................................21.3 CAPTURE TECHNOLOGIES ............................................................................................32 LITERATURE REVIEW.................................................................................................72.1 INTRODUCTION.............................................................................................................72.2 GAS SEPARATION MEMBRANES...................................................................................72.2.1 Performance Indicators........................................................................................72.2.1.1 Permeance and Selectivity ................................................................................72.2.1.2 Diffusivity and Heat of Adsorption...................................................................92.2.2 Performance Measurement.................................................................................102.2.2.1 Permeance and Selectivity ..............................................................................102.2.2.2 Diffusivity .......................................................................................................122.2.2.3 Heat of Adsorption..........................................................................................132.2.3 Materials and Transport Mechanisms................................................................152.2.3.1 Microporous Membranes ................................................................................15ix

2.2.3.2 Nanoporous Membranes .................................................................................172.2.3.3 Nonporous Membranes ...................................................................................182.2.3.4 Membranes of Mixed Materials......................................................................192.2.3.5 Summary .........................................................................................................192.3 MANUFACTURE OF NPC MEMBRANES ......................................................................202.3.1 Supported NPC Membranes ...............................................................................212.3.1.1 Polymer ...........................................................................................................212.3.1.2 Porous Support................................................................................................242.3.1.3 Modified Porous Support ................................................................................242.3.1.4 Deposition Method..........................................................................................252.3.1.5 Pyrolysis..........................................................................................................252.3.1.6 Number of Coats .............................................................................................292.3.2 Unsupported NPC Membranes...........................................................................292.3.2.1 Polymer ...........................................................................................................292.3.2.2 Pyrolysis Temperature ....................................................................................302.3.3 Characterisation.................................................................................................312.3.3.1 Membrane Thickness ......................................................................................312.3.3.2 Surface Structure.............................................................................................312.3.3.3 Pore Size Distribution .....................................................................................312.4 PERFORMANCE OF NPC MEMBRANES .......................................................................342.4.1 Supported NPC Membranes ...............................................................................342.4.2 Unsupported NPC Membranes...........................................................................352.5 GAS TRANSPORT MECHANISMS OF NPC MEMBRANES .............................................362.5.1 Experimental Observations ................................................................................362.5.2 Molecular Modelling ..........................................................................................372.5.2.1 Monte Carlo Simulation..................................................................................372.5.2.2 Molecular Dynamics Simulation.....................................................................392.5.2.3 Published Simulation Studies of NPC.............................................................402.6 HIGH TEMPERATURE OPERATION OF NPC MEMBRANES ..........................................432.6.1 Arrhenius Relationships......................................................................................432.7 EFFECT OF IMPURITIES ON THE PERFORMANCE NPC MEMBRANES ..........................472.7.1 Water ..................................................................................................................472.7.2 Air .......................................................................................................................492.7.3 Condensable Impurities......................................................................................512.7.4 Non-Condensable Impurities ..............................................................................532.8 SCOPE OF THIS RESEARCH WORK ..............................................................................55x

3 EXPERIMENTAL MATERIALS, APPARATUS AND METHODS........................573.1 INTRODUCTION...........................................................................................................573.2 MEMBRANE MATERIALS, MANUFACTURING EQUIPMENT AND TECHNIQUES ...........573.2.1 Materials.............................................................................................................573.2.2 Manufacturing Equipment and Techniques........................................................593.2.2.1 Supported NPC Membranes............................................................................593.2.2.2 Modified Supported NPC Membranes............................................................643.2.2.3 Unsupported NPC Membranes .......................................................................663.3 MEMBRANE CHARACTERISATION TECHNIQUES ........................................................673.3.1 Surface Structure ................................................................................................673.3.2 Pore Size.............................................................................................................683.3.2.1 PALS...............................................................................................................683.3.2.2 WAXD ............................................................................................................693.4 MEASUREMENT OF MEMBRANE PERMEANCE AND SELECTIVITY..............................703.4.1 Pure Gas .............................................................................................................703.4.2 Mixed Gas...........................................................................................................723.4.3 At Elevated Temperatures...................................................................................753.4.4 Exposure to Impurities........................................................................................763.4.4.1 Condensable Impurities...................................................................................763.4.4.2 Non-Condensable Impurities ..........................................................................793.5 MEASUREMENT OF MEMBRANE ADSORPTION...........................................................823.5.1 Theoretical Adsorption .......................................................................................823.5.1.1 Model Development........................................................................................823.5.1.2 Model Description...........................................................................................823.5.1.3 Fitting the Adsorption Isotherms ....................................................................833.5.1.4 Calculating the Adsorption Separation Factor ................................................833.5.2 Experimental Adsorption ....................................................................................843.5.2.1 Buoyancy Correction ......................................................................................853.5.2.2 Fitting the Adsorption Isotherms ....................................................................874 MANUFACTURE AND HIGH TEMPERATURE OPERATION OF NPCMEMBRANES........................................................................................................................894.1 INTRODUCTION...........................................................................................................894.2 SUPPORTED NPC MEMBRANES..................................................................................894.2.1 Manufacture........................................................................................................894.2.2 Characterisation.................................................................................................914.2.2.1 Surface Structure.............................................................................................91xi

4.2.2.2 Pore Size .........................................................................................................934.2.3 Pure Gas Performance .......................................................................................974.2.4 Mixed Gas Performance ...................................................................................1024.2.4.1 Effect of Feed Pressure .................................................................................1034.2.5 High Temperature Performance.......................................................................1054.2.5.1 Effect on Permeance .....................................................................................1054.2.5.2 Effect on Selectivity......................................................................................1074.2.5.3 Effect of Thermal Cycling ............................................................................1094.2.6 Concluding Remarks on Supported NPC Membranes......................................1104.3 MODIFIED-SUPPORTED NPC MEMBRANES..............................................................1124.3.1 Manufacture and Characterisation ..................................................................1124.3.2 Membrane Performance ...................................................................................1144.3.3 Concluding Remarks on Modified-Supported NPC Membranes ......................1165 UNDERSTANDING THE HIGH TEMPERATURE PERFORMANCE OF NPCMEMBRANES THROUGH GAS ADSORPTION MOLECULAR SIMULATION ANDEXPERIMENTS...................................................................................................................1175.1 INTRODUCTION.........................................................................................................1175.2 MOLECULAR SIMULATION .......................................................................................1175.2.1 Pure Gases........................................................................................................1175.2.1.1 Heats of Adsorption ......................................................................................1195.2.2 Mixed Gases......................................................................................................1215.2.2.1 Carbon Dioxide / Methane............................................................................1215.2.2.2 Carbon Dioxide / Nitrogen............................................................................1235.3 GAS ADSORPTION EXPERIMENTS.............................................................................1255.3.1 Pure Gases........................................................................................................1255.3.1.1 Carbon Dioxide.............................................................................................1255.3.1.2 Nitrogen ........................................................................................................1285.3.1.3 Heats of Adsorption ......................................................................................1305.3.1.4 Relating the CO 2 Heat of Adsorption to the CO 2 Permeance .......................1315.4 CONCLUDING REMARKS ON UNDERSTANDING THE HIGH TEMPERATUREPERFORMANCE OF NPC MEMBRANES THROUGH GAS ADSORPTION MOLECULARSIMULATION AND EXPERIMENTS ........................................................................................1326 PERFORMANCE OF NPC MEMBRANES IN THE PRESENCE OFCONDENSABLE AND NON-CONDENSABLE IMPURITIES .....................................1336.1 INTRODUCTION.........................................................................................................133xii

6.2 CONDENSABLE IMPURITIES......................................................................................1336.2.1 Water ................................................................................................................1346.2.2 Hexane ..............................................................................................................1376.2.3 Toluene .............................................................................................................1406.2.4 Comparison of Condensable Impurities ...........................................................1436.3 NON-CONDENSABLE IMPURITIES.............................................................................1446.3.1 Hydrogen Sulphide ...........................................................................................1446.3.2 Carbon Monoxide .............................................................................................1466.3.3 Comparison of Non-Condensable Impurities ...................................................1496.4 CONCLUDING REMARKS ON THE PERFORMANCE OF NPC MEMBRANES IN THEPRESENCE OF IMPURITIES ...................................................................................................1507 CONCLUSIONS AND RECOMMENDATIONS ......................................................1537.1 CONCLUSIONS ..........................................................................................................1537.2 RECOMMENDATIONS FOR FURTHER WORK .............................................................1568 REFERENCES..............................................................................................................159APPENDICES ......................................................................................................................175APPENDIX A – RELEVANT BACKGROUND INFORMATION ................................177A.1 GLOBAL WARMING..................................................................................................177A.2 INTERNATIONAL RECOGNITION OF CLIMATE CHANGE............................................178A.3 AUSTRALIAN RECOGNITION OF CLIMATE CHANGE.................................................179A.4 REDUCING CO 2 EMISSIONS FROM AUSTRALIA’S ENERGY PRODUCTION ................180A.5 OPPORTUNITIES FOR CO 2 SEQUESTRATION IN AUSTRALIA .....................................181A.6 CHALLENGES FOR CO 2 SEQUESTRATION IN AUSTRALIA.........................................182A.7 CO 2 CAPTURE TECHNOLOGIES ................................................................................183A.7.1 Absorption.........................................................................................................183A.7.2 Adsorption.........................................................................................................183A.7.3 Cryogenics ........................................................................................................184A.7.4 Gas Cryogenics.................................................................................................184A.7.5 Gas Separation Membranes .............................................................................184APPENDIX B – STANDARD OPERATING PROCEDURES ........................................187B.1 STANDARD OPERATING PROCEDURE FOR THE CPVV (MIXED GAS) APPARATUS....187B.2 STANDARD OPERATING PROCEDURE FOR THE CPVV (MIXED GAS) APPARATUS INTHE PRESENCE OF CONDENSABLE IMPURITIES ....................................................................199xiii

B.3 STANDARD OPERATING PROCEDURE FOR THE CPVV (MIXED GAS) APPARATUS INTHE PRESENCE OF NON-CONDENSABLE IMPURITIES............................................................211B.4 RISK ASSESSMENT FOR THE OPERATION OF THE CPVV (MIXED GAS) APPARATUS INTHE PRESENCE OF NON-CONDENSABLE IMPURITIES............................................................223APPENDIX C – EXPERIMENTAL METHODS..............................................................231C.1 PURE GAS PERMEANCE MEASUREMENT..................................................................231C.1.1 Error Analysis...................................................................................................231C.1.2 Sample of Calculated Results ...........................................................................234C.2 MIXED GAS PERMEANCE MEASUREMENT ...............................................................236C.2.1 Error Analysis...................................................................................................236C.2.2 Sample of Calculated Results ...........................................................................238C.3 ADSORPTION MEASUREMENT ..................................................................................239C.3.1 Error Analysis...................................................................................................239C.3.2 Sample of Calculated Results ...........................................................................241C.4 MOLECULAR MODELLING........................................................................................242C.4.1 Sample Input File..............................................................................................242C.4.2 Sample Output File...........................................................................................243C.4.3 Sample Isotherm Data ......................................................................................244APPENDIX D – UNSUPPORTED NPC MEMBRANES .................................................245D.1 MANUFACTURE AND CHARACTERISATION ..............................................................245D.1.1 Matrimid Polyimide..........................................................................................245D.1.2 Kapton Polyimide .............................................................................................247D.2 PERFORMANCE .........................................................................................................248D.2.1 Matrimid Polyimide..........................................................................................248D.2.2 Kapton Polyimide .............................................................................................249D.3 CONCLUDING REMARKS ON UNSUPPORTED NPC MEMBRANES .............................250xiv

LIST OF FIGURESFigure 1-1 : CO 2 capture and sequestration process. Diagram provided by the CooperativeResearch Centre for Greenhouse Gas Technologies (CO2CRC) [5].......................2Figure 1-2 : Separation of CO 2 from (a) natural gas and (b) air-blown synthesis gas .............4Figure 2-1 : Membrane separation unit.....................................................................................7Figure 2-2 : Equipment used for permeance or permeability measurement; (a) constantpressure variable volume apparatus (CPVV) for higher flow membranes and (b)constant volume variable pressure (CVVP) for lower flow membranes ................10Figure 2-3 : An extension of the constant pressure variable volume apparatus (CPVV) usedfor permeance and selectivity measurement using a mixed gas feed. ....................11Figure 2-4 : Calculation of the diffusion coefficient using the time-lag method, once thegradient is constant and steady state flow through the membrane has beenreached, an extrapolation of the steady state flow line back to the x-axis where thegas flow is 0 reveals the value of the time lag (θ). .................................................12Figure 2-5 : Summary of transport mechanisms indicating as the size of the membrane poresdecrease, transport changes from bulk flow to molecular sieving. ........................19Figure 2-6 : Structure of a typical fullerene (the Buckminster Fullerene C60) discovered in1985 by Kroto et al [56].........................................................................................20Figure 2-7 : Structure of a hexagonal sheet, which when randomly bonded to other sheetsforms the curved nanoporous carbon structure. Figure adapted from [61]. .........21Figure 2-8 : Typical furnace temperature profile for the pyrolysis of a polymer to producenanoporous carbon.................................................................................................26Figure 2-9 : Performance of unsupported NPC membranes [17] with respect to the RobesonUpper Bound [38] for polymeric membranes, which shows that the carbonisationof a Matrimid polymeric membrane increases the permeability and selectivity suchthat it breaks the upper bound................................................................................35Figure 2-10 : The process that is used by molecular simulation to create flux and separationfactors for nanoporous carbon. The two simulation methods used are (1) GrandCanonical Monte Carlo (GCMC) simulation for determining the adsorptionproperties and (2) molecular dynamic (MD) simulation for determining the selfdiffusivities.Combining the results from both methods produces a predicted fluxand separation factor. ............................................................................................40Figure 2-11 : Structure of C168 Schwarzite used to represent nanoporous carbon [106]......40Figure 3-1 : Chemical structure of Matrimid polyimide (Vantico, Switzerland) .....................59Figure 3-2 : Chemical structure of Kapton polyimide (Dupont, USA) ....................................59xv

Figure 3-3 : (a) Photograph and (b) schematic diagram of the spin coating apparatus. Theporous support is held onto a spinning chuck using a vacuum pump and a sealpurge gas. The PFA solution is sprayed onto the spinning porous support using apneumatic spray nozzle...........................................................................................60Figure 3-4 : (a) Photograph and (b) schematic diagram of the quartz tube furnace with acontinuous purge of ultra high purity argon. .........................................................61Figure 3-5 : Quartz tube furnace typical temperature profile with time during pyrolysis ofsupported NPC membranes....................................................................................62Figure 3-6 : The impact of oxidising the first coat of carbon of membranes manufactured at550°C. The increase in permeance is 1 to 2 orders of magnitude. This result issimilar to that achieved from the addition of silica particles [28] [27].................63Figure 3-7 : Apparatus for creating the colloidal solution of silica nanoparticles in which asolution containing ethanol and ammonia was mixed with a solution containingethanol and TEOS and reacted at a constant temperature.....................................65Figure 3-8 : Millipore filtration cell used for filtering diluted silica nanoparticle colloidsolution through the porous stainless steel support. ..............................................66Figure 3-9 : Quartz tube furnace typical temperature profile with time during pyrolysis ofunsupported NPC membranes................................................................................67Figure 3-10 : (a) Photograph and (b) process flow diagram of the constant volume, variablepressure (CVVP) apparatus for measuring pure gas permeance andpermselectivity........................................................................................................70Figure 3-11 : Photograph of the CVVP membrane cell showing one inlet connection for thefeed gas and one outlet connection for the permeate gas.......................................71Figure 3-12 : Process flow diagram of the constant pressure, variable volume apparatus(CPVV) for measuring mixed gas permeance and selectivity. ...............................72Figure 3-13 : Photograph of the CPVV membrane cell showing two inlet connections for thefeed and sweep gases and two outlet connections for the retentate and permeatestreams....................................................................................................................73Figure 3-14 : Photograph of the vessel used to saturate the feed gas of the constant pressurevariable volume (CPVV) apparatus for measuring mixed gas permeance andpermselectivity in the presence of condensable impurities.....................................76Figure 3-15 : Process flow diagrams of (a) the constant pressure, variable volume (CPVV)apparatus for measuring mixed gas permeance and selectivity in the presence ofcondensable impurities (b) vessel for contacting feed gas with condensableimpurities................................................................................................................77xvi

Figure 3-16 : Process flow diagram of the constant pressure, variable volume apparatus(CPVV) for measuring mixed gas permeance and selectivity in the presence ofnon-condensable impurities....................................................................................79Figure 3-17 : Photograph of the gas extraction system installed around the gas bottlecontaining H 2 S or CO impurities............................................................................81Figure 3-18 : Structure of C 168 Schwarzite from the side (left) and rotated at 20°C (right)....83Figure 3-19 : (a) Photograph and (b) schematic diagram of the microbalance used formeasuring gas adsorption isotherms at elevated temperatures. ............................84Figure 4-1 : The creation of a NPC membrane within 3 coats of the polymeric precursor.....89Figure 4-2 : The cumulative carbon density per coat for NPC membranes manufactured atpyrolysis temperatures between 400 and 600°C. Also depicted in this figure is thecumulative carbon density of an NPC membrane made by Rajagopalan and Foleyat 600°C [82] with a final O 2 /N 2 permselectivity of 5............................................91Figure 4-3 : (a) SEM pictures showing (clockwise from top left) the uncoated stainless steelsupport, the carbon membrane after three coats with no defects, macroscopiccracking in the carbon surface and microscopic cracking in the carbon surface,(b) HRTEM pictures shown (from left) nanoporous carbon and enlarged photo toshow pore size detail. .............................................................................................92Figure 4-4 : Pore size distribution of NPC produced at 600°C obtained from the HRTEMpicture using the image processing program Image J. The distribution range isfrom 2.7 to 5.2 Å centred around 4.0 Å. .................................................................93Figure 4-5 : Characterisation of nanoporous carbon material manufactured at pyrolysistemperatures from 400 to 600°C using positron annihilation lifetime spectroscopy(PALS). The primary x-axis (τ 2 ) is the lifetime of position annihilating in the pores,which indicates a decrease in pore size with increasing pyrolysis temperature. Thesecondary y-axis (I 2 ) is the number of positrons annihilating in the pores, whichindicates an increase in the number of pores with increasing pyrolysistemperature.............................................................................................................94Figure 4-6 : Characterisation of nanoporous carbon material manufactured at pyrolysistemperatures of 400, 600 and 800°C using wide angle x-ray diffraction (WAXD).The primary x-axis is the diffraction angle whilst number of counts or intensity isgiven on the primary y-axis. The values of 2θ are converted to d-spacing usingBragg’s Law. ..........................................................................................................95Figure 4-7 : The effect of pyrolysis temperature on permeance showing a trend of increasingpermeance with increasing pyrolysis temperature.................................................98Figure 4-8 : The effect of pyrolysis temperature on permselectivity showing no clear trendbetween selectivity and pyrolysis temperature. ......................................................99xvii

Figure 4-9 : Change in permeance and selectivity with number of coats for NPC membrane Dmade at 400°C. Knudsen diffusion dominates until a selective membrane is formedwith the third coat. However, the final 4 th coat results in cracking of the membranelayer and a subsequent decrease in selectivity and increase in permeance.........100Figure 4-10 : Change in (a) CO 2 /CH 4 permselectivity and (b) CO 2 / N 2 permselectivity withamount of carbon deposited for selected membranes manufactured at pyrolysistemperatures of 400 and 550 °C. The decrease in selectivity after a maximumvalue has been reached represents the point at which cracking occurs. For themembranes manufactured at 550°C, this critical point is at ~ 6 mg/cm 2 , whereasfor the membranes manufactured at 400°C, this critical point is at ~ 8 mg/cm 2 . 101Figure 4-11 : Change in CO 2 permeance with increasing feed pressure. Increasing the feedpressure reduces the permeance...........................................................................104Figure 4-12 : Change in CO 2 /N 2 selectivity with increasing feed pressure. Increasing the feedpressure has the slightly increasing the selectivity...............................................104Figure 4-13 : Effect of operating temperature on mixed gas permeance. The permeanceincreases with increasing operating temperature due to the increased diffusion.The membrane manufactured at 550°C maintains a higher permeance across theoperating temperature range................................................................................105Figure 4-14 : Arrhenius plot of permeance and operating temperature. The gradient of thefitted lines indicate the activation energies of permeance. The fitted lines aresteeper for CH 4 resulting in higher activation energies as presented in Table 4-10...............................................................................................................................106Figure 4-15 : Effect of operating temperature on mixed gas selectivity for NPC membranesmanufactured at pyrolysis temperatures of 400 and 550°C. Increasing theoperating temperature has the effect of decreasing the selectivity due to thedisproportional increase in the CH 4 permeance. This is due the significantreduction in the CO 2 adsorption at the higher temperatures. ..............................108Figure 4-16 : Extrapolated effect of operating temperature on mixed gas selectivity for NPCmembranes manufactured at pyrolysis temperatures of 400 and 550°C.Extrapolating the CO 2 /CH 4 selectivity relationship with operating temperatureusing an exponential fit shows that the selectivity will approach the Knudsen valueof 0.6 at an operating temperature between 400 and 500°C................................108Figure 4-17 : Effect of thermal cycling on an NPC membrane. After eleven regenerationcycles the CO 2 and CH 4 permeance increased dramatically whilst the selectivitydropped to 1, indicating the presence of significant cracks. ................................109Figure 4-18 : Silica nanoparticles, with a size less than 300 nm produced from the reaction ofethanol, ammonia and TEOS at a temperature of 50 °C for 45 mins...................112xviii

Figure 4-19 : Effect of the addition of silica nanoparticles on reducing the permeance of thestainless steel support suggesting some reduction in pore size and porosity.......113Figure 4-20 : Effect of the addition of carbon on permeance to supports filled with silicananoparticles. The membrane layer begins to form around 1.5 – 2 mg/cm 2 ofcarbon but cracks at 3.5 mg/cm 2 . .........................................................................115Figure 4-21 : Effect of the addition of carbon on selectivity to supports filled with silicananoparticles. The selectivity results agree with the permeance results, whichsuggest cracking of the formed membrane layer occurs around 3.5 mg/cm 2 .......115Figure 5-1 : Simulated adsorption isotherms of pure CO 2 onto nanoporous carbon.Increasing the temperature decreases the amount of CO 2 adsorbed. ..................118Figure 5-2 : Simulated adsorption isotherms of pure CH 4 onto nanoporous carbon.Increasing the temperature decreases the amount of CH 4 adsorbed. ..................118Figure 5-3 : Simulated adsorption isotherms of pure N 2 onto nanoporous carbon. Increasingthe temperature decreases the amount of N 2 adsorbed. .......................................119Figure 5-4 : Straight line plot of 1/T versus the simulated Ln(b) for pure gases fordetermining the heat of adsorption (∆H ads ). .........................................................120Figure 5-5 : Adsorption isotherms of mixed CO 2 /CH 4 onto nanoporous carbon. Increasing thetemperature decreases the amount of CO 2 and CH 4 adsorbed.............................121Figure 5-6 : CO 2 /CH 4 mixed gas adsorption separation factor as a function of pressure forsystem temperatures of 35 and 200°C..................................................................122Figure 5-7 : Adsorption isotherms of mixed CO 2 /N 2 onto nanoporous carbon. Increasing thetemperature decreases the amount of CO 2 and N 2 adsorbed. ..............................123Figure 5-8 : CO 2 /N 2 mixed gas adsorption separation factor as a function of pressure forsystem temperatures of 35 and 200°C..................................................................124Figure 5-9 : Experimental adsorption isotherms of pure CO 2 onto nanoporous carbon.......125Figure 5-10 : Experimental and simulated adsorption isotherms of pure CO 2 onto NPC.....126Figure 5-11 : Experimental adsorption isotherms of pure N 2 onto nanoporous carbon. ......128Figure 5-12 : Experimental and simulated adsorption isotherms of pure N 2 onto nanoporouscarbon. The large discrepancy at 35°C is attributed to not achieving equilibriumin the adsorption experiments...............................................................................129Figure 5-13 : Straight line plot of 1/T versus the simulated Ln(b) for pure gases fordetermining the heat of adsorption (∆H ads ). .........................................................130Figure 6-1 : Normalised effect of water concentration on the reduction in CO 2 permeance of aNPC membrane manufactured at 550°C as a function of time. ...........................134Figure 6-2 : Normalised effect of water concentration on the CO 2 /CH 4 selectivity of a NPCmembrane manufactured at 550°C as a function of time. ....................................135xix

Figure 6-3 : Normalised effect of water concentration on the permeance of a NPC membranemanufactured at 550°C. The effect appears to be more pronounced on CO 2permeance and at lower operating temperatures.................................................136Figure 6-4 : Normalised effect of water concentration on the selectivity of a NPC membranemanufactured at 550°C. The effect is minimal. ....................................................136Figure 6-5 : Normalised effect of hexane concentration on the reduction in CO 2 permeance ofa NPC membrane manufactured at 550°C as a function of time. ........................137Figure 6-6 : Normalised effect of hexane concentration on the CO 2 /CH 4 permeance of a NPCmembrane manufactured at 550°C as a function of time. ....................................138Figure 6-7: Normalised effect of hexane concentration on the permeance of a NPC membranemanufactured at 550°C. The effect appears to be more pronounced on the CO 2permeance and at lower operating temperatures.................................................139Figure 6-8 : Normalised effect of hexane concentration on the CO 2 /CH 4 selectivity of a NPCmembrane manufactured at 550°C. The impact is minimal. ................................139Figure 6-9 : Normalised effect of toluene concentration on the reduction in CO 2 permeance ofa NPC membrane manufactured at 550°C as a function of time. ........................140Figure 6-10 : Normalised effect of toluene concentration on the CO 2 /CH 4 permeance of aNPC membrane manufactured at 550°C as a function of time. ...........................141Figure 6-11 : Normalised effect of toluene concentration on the permeance of a NPCmembrane manufactured at 550°C. The effect is more pronounced on the CO 2permeance and at lower operating temperatures.................................................142Figure 6-12 : Normalised effect of toluene concentration on the selectivity of a NPCmembrane manufactured at 550°C. The effect is minimal. ..................................142Figure 6-13 : Normalised effect of H 2 S on the reduction in CO 2 permeance of an NPCmembrane manufactured at 550°C. The effect is minimal. ..................................145Figure 6-14 : Normalised effect of H 2 S on the reduction in CO 2 /N 2 selectivity of an NPCmembrane manufactured at 550°C. The effect is minimal. ..................................145Figure 6-15 : Normalised effect of CO on the reduction in CO 2 permeance of an NPCmembrane manufactured at 550°C. The effect is greatest at the lowest pressureand temperature....................................................................................................147Figure 6-16 : Normalised effect of CO on the reduction in CO 2 /N 2 selectivity of an NPCmembrane manufactured at 550°C. The effect is minimal. ..................................147Figure A-1 : Correlation between the Earth’s temperature and atmospheric carbon dioxide(data sourced from [163])....................................................................................177Figure A-2 : Predicted greenhouse gas emissions from by sector for 1990 and the predicted2008 to 2012 period (data sourced from [166])...................................................180xx

Figure A-3 : Australia’s large stationary sources of carbon dioxide by industry (data courtesyof the CRC for Greenhouse Gas Technologies) ...................................................182Figure C-1 : Permeate pressure rise for pure gases CO 2 , N 2 and CH 4 through MembraneNPC550B. The respective permeances were calculated from the steady stategradients...............................................................................................................234Figure C-2 : Input file used for the molecular simulation of the adsorption of a CO 2 :CH 4mixture (10:90 vol%) at a system temperature of 300 K and pressure of 100 kPa...............................................................................................................................242Figure C-3 : Output file produced from the molecular simulation of the adsorption of aCO 2 :CH 4 mixture (10:90 vol%) at a system temperature of 300 K and pressure of100 kPa.................................................................................................................243Figure D-1 : Weight loss of unsupported NPC membranes made from Matrimid polyimidewith increasing pyrolysis temperature. The transition from polymer to carbon iswithin 400 and 600°C...........................................................................................246Figure D-2 : SEM photographs of Matrimid (a) after pyrolysis at 420°C and (b) afterpyrolysis at 430°C. ...............................................................................................247Figure D-3 : Weight loss of unsupported NPC membranes made from Kapton polyimide withincreasing temperature. The transition from to carbon is between 500 and 700°C...............................................................................................................................247Figure D-4 : SEM photograph of Kapton after pyrolysis at 450°C. The surface was coatedwith a gold layer to enable SEM imaging suggesting the absence of carbon and thepresence of polymer..............................................................................................248xxi

LIST OF TABLESTable 1-1 : Operating conditions of CO 2 separation from natural gas and synthesis gas.........5Table 2-1 : Kinetic diameter, molecular weight and Knudsen selectivities of gas molecules ofinterest. ...................................................................................................................16Table 2-2 : H 2 /N 2 permselectivity for NPC membranes made from PFA and polyimide. TheNPC membranes made from PFA have a higher selectivity but lower permeancethan those made from polyimide.............................................................................23Table 2-3 : Permeance and permselectivity of NPC membranes made from PFA at differentpyrolysis temperatures [75]. Increasing the temperature of pyrolysis has asignificant effect upon increasing the permeance. However, the permselectivitygoes through a maximum at 450 °C. ......................................................................28Table 2-4 : PALS data for NPC membranes manufactured from the pyrolysis of the polyimideP84 and Matrimid [96]. The lifetime of ortho-positronium annihilating in thepores (τ 2 ) indicates the size of the pores. The number or intensity of orthopositroniumannihilating in the pores (I 2 ) indicates the number of pores or theporosity...................................................................................................................32Table 2-5 : WAXD data for NPC membranes manufactured from the pyrolysis of thepolyimide Matrimid and P84 [17, 96, 97] along with powder x-ray diffraction datafor bulk nanoporous carbon manufactured from PFA [71]. ..................................33Table 2-6 : Permeance results of various gas molecules for membranes made from PFA [19].The increase in permeance with decreasing molecular size is a clear indicator of amolecular sieving mechanism.................................................................................36Table 2-7 : Activation energy of permeance of various gas molecules for NPC membranesmade from polyamic acid. The activation energy for CO 2 is 0 indicating that theCO 2 permeance will remain constant with increasing temperature.......................43Table 2-8 : Activation energies of diffusion and heats of adsorption of various gas moleculesfor NPC membranes made from PFA and Cellulose..............................................44Table 2-9 : Activation energy of permeance of various gas molecules for NPC membranesmade from PFA and Cellulose obtained by the addition of the diffusion andactivation energies presented in Table 2-8. Interestingly, for CO 2 the activationenergy of permeance is negative because adsorption is such a large contributor tothe permeance.........................................................................................................45Table 3-1 : Materials used to manufacture supported NPC membranes. ................................57Table 3-2 : Extra materials used to manufacture modified supported NPC membranes.........58Table 3-3 : Materials used to manufacture unsupported NPC membranes. ............................58xxii

Table 3-4 : The composition of the two solutions prepared for the manufacture of the silicananoparticles used for filling the porous support prior to coating with PFA. .......65Table 3-5 : Mixed gas testing conditions..................................................................................75Table 3-6 : The concentration of condensable impurities in the feed gas based on theoperating temperature of the saturating vessel containing the liquid impurity. ....78Table 3-7 : Details of the adsorbents, temperature and pressures used for obtaining simulatedadsorption isotherms of nanoporous carbon using GCMC simulation..................82Table 3-8 : Details of the adsorbents, temperatures and pressures used for obtainingexperimental adsorption isotherms of nanoporous carbon....................................85Table 3-9 : Equipment mass and densities used to calculate buoyed volume. The equipmentmasses and densities were provided by the supplier (VTI). The density ofnanoporous carbon was assumed to be the standard value of 1.6 g/cm 3 [79].......87Table 4-1 : Volumes of polymer solution (40 wt% PFA in acetone) added to each coat viaspin-coating at 1000 rpm and the resulting carbon mass added after pyrolysis....89Table 4-2 : Manufacture results of the membranes made from the spin-coating of PFA ontostainless steel supports and then pyrolysised between 400 and 600°C..................90Table 4-3 : Basic Statistics of the pore size distribution of NPC produced at 600°C obtainedfrom the HRTEM picture using the image processing program Image J...............93Table 4-4 : PALS data for NPC membranes made from Matrimid and P84 [96] and PFA.Values of τ 2 are similar, which indicates a comparable pore size range...............96Table 4-5 : WAXD data for NPC membranes made from Matrimid and P84 [17, 96, 97] andPFA. The values of the d-spacing indicate that pore size decreases with increasingtemperature.............................................................................................................96Table 4-6 : Pure gas performance results for the final coats of membranes made between 400and 600°C...............................................................................................................97Table 4-7 : Pure gas performance results for each coat for membranes at 400 and 550°C..100Table 4-8 : Pure gas vs. mixed gas results for membranes manufactured at 400 and 550°C.An increased CO 2 /CH 4 selectivity is seen with the mixed gas testing. .................103Table 4-9 : Activation Energy (E p ) of CO 2 and CH 4 Permeance (kJ/mol) for NPC membranesmade at 400 and 550°C. Activation energies were calculated from the permeancedata over the range of operating temperatures using Equation 2-38. Data fromother authors is included for comparison.............................................................106Table 4-10 : Effect of varying reaction temperature and time on the size of the produced silicananoparticles. The fourth sample with a reaction temperature of 50°C and time of45 mins gave monodisperse silica nanoparticles with an average size less than 300nm. ........................................................................................................................112xxiii

Table 4-11 : Effect of the addition of silica nanoparticles on the permeance and selectivity ofthe porous stainless steel support. The error in the permeance is ± 1.38% and theerror in the permselectivity is 1.95%....................................................................113Table 4-12 : Addition of carbon to supports modified with silica nanoparticles. The error inthe permeance is ± 1.38% and the error in the permselectivity is 1.95%. ...........115Table 5-1 : Values of the Langmuir constants (b and V m ) for the simulated adsorptionisotherms of pure CO 2 , CH 4 and N 2 on NPC with increasing temperatures. .......119Table 5-2 : Simulated values of the heat of adsorption (∆H ads ) of pure gases on nanoporouscarbon compared with experimental values reported in the literature. ...............120Table 5-3 : Experimental values of the Langmuir constants (b and V m ) for the adsorptionisotherms of pure CO 2 on nanoporous carbon.....................................................125Table 5-4 : Buoyed volume for CO 2 adsorption onto NPC at 35°C and 200°C calculated fromthe methods of (1) helium adsorption and (2) equipment masses. .......................127Table 5-5 : Experimental values of the Langmuir constants (b and V m ) for the adsorptionisotherms of pure N 2 on nanoporous carbon........................................................128Table 5-6 : Values of the experimental and simulated heats of adsorption of pure gases onnanoporous carbon compared with values reported in the literature..................131Table 5-7 : A comparison of the experimental CO 2 heat of adsorption (∆H ads ) with theactivation energy of permeance (E p ) for a NPC membrane manufactured at 550°C(refer Section 4.2.5) to calculate the activation energy of diffusion (E d ) viaEquation 2-38. ......................................................................................................131Table 6-1 : Normalised stabilised reduction in CO 2 permeance with impurity presence. .....143Table 6-2 : Normalised stabilised reduction in CO 2 /CH 4 selectivity with impurity presence...............................................................................................................................143Table 6-3 : Normalised stabilised reduction in CO 2 permeance as a function of impurity....149Table 6-4 : Normalised stabilised reduction in CO 2 /N 2 selectivity as a function of impurity...............................................................................................................................149Table C-1 : Error Analysis Relationships [175] ....................................................................231Table C-2 : Error analysis of the calculation of the pure gas CO 2 CH4 permselectivity fromduplicate measurements of Membrane NPC400C taken 6 days apart. ................232Table C-3 : Error analysis of the calculation of the pure gas CO 2 permeance for MembraneNPC550B. The error analysis is independent of the absolute values of thevariables as the error is a percentage of the absolute value with the exception ofthe values of the membrane area. As the membrane area reading is the same forthe CH 4 permeance, the error analysis for the CH 4 permeance gives the sameresult.....................................................................................................................233xxiv

Table C-4 : Summary of calculated pure gas permeances and permselectivities of MembraneNPC550B based upon the steady state gradients from the permeate pressure risecurves. The system constants were permeate volume (V p ) = 525 cm 3 , temperature(T) = 306 K and membrane area = 9.62 cm 2 . ......................................................235Table C-5 : Error analysis of the calculation of the mixed gas CO 2 /CH 4 permselectivity fromduplicate measurements of Membrane NPC550B................................................236Table C-6 : Error analysis of the calculation of the CO 2 permeance for Membrane NPC550B.The error analysis is independent of the absolute values of the variables as theerror is a percentage of the absolute value with the exception of the values of thepermeate flow and membrane area. As the permeate flow and membrane areareadings are the same for the CH 4 permeance, the error analysis for the CH 4permeance gives the same result. .........................................................................237Table C-7 : Sample of the calculated mixed gas performance of Membrane NPC550B. Thesystem constants were temperature = 35 °, feed gas composition = 10:90(CO 2 :CH 4 vol%), feed pressure (p f ) = 1200 kPag, retentate pressure = 1150 kPag,retentate flow = 100 ml/min, sweep gas flow = 20 ml/min, permeate pressure = 0kPag and membrane area (A) = 9.62 cm 2 . ...........................................................238Table C-8 : Error analysis of the calculation of the adsorbed volume of CO 2 onto nanoporouscarbon 303 Torr (40 kPa) at 35°C. ......................................................................240Table C-9 : Sample of the calculated results for the adsorption of CO 2 on nanoporous carbonat 35°C..................................................................................................................241Table C-10 : Data produced for the 300 K isotherm from the molecular simulation of theadsorption of CO 2 :CH 4 (10:90 vol%) on nanoporous carbon represented by C 168Schwartize.............................................................................................................244Table D-1 : Performance results of raw Matrimid and partially pyrolysised NPC membranesmade from Matrimid at 420 and 430°C. The results suggest significant cracking inthe partially pyrolysised NPC membranes. ..........................................................249Table D-2 : Performance results of raw Matrimid and partially pyrolysised NPC membranesmade from Kapton at 420°C. The results suggest some improvement inperformance with partial pyrolysis.......................................................................250xxv

NOMENCLATUREA Membrane Surface Area (m 2 )bCDdLangmuir ConstantConcentrationDiffusion Coefficient (m/s)D Spacing (nm)d m Diameter of the Membrane (m) in Equation 2-23d p Diameter of the Particle (m) in Equation 2-17dEEEf∆HJDimensionality of the System in MD SimulationActivation Energy (kJ/mol)Energy of the System in GCMC SimulationError in Variable A in Appendix CFugacity (Pa)Heat of Adsorption (kJ/mol)Flux (sm 3 /m 2 .s)k BBoltzmann ConstantMWNMolecular WeightNumber of Molecules in GCMC and MD Simulationn Integer in Equation 2-28PPermeability (sm 3 .m/m 2 .s.Pa) or (mol/m 2 .s.Pa)P’ Permeance (mol/m.s.Pa)pPressure (Pa)xxvi

pp∆pPartial Pressure (Pa)Pressure Drop (Pa)p 0 Driving-Force Pressure (Pa) in Equation 2-27QRVolumetric flow rate (m 3 /s)Ideal Gas Constant = 8.314 (J/mol.K)r Separation Distance in Equation 2-32SStTuVSeparation FactorSolubility Coefficient (mol/m 3 .Pa)Time (s)Temperature (K)Interaction PotentialVelocity (m/s)V Volume (m 3 )V m Molar Volume at Standard Temp and Pressure (mol/m 3 ) in Equation 2-6V m Langmuir Constant in Section 2.2.2.3x∆xyConcentration of a Component on the Feed Side (mol fraction)Membrane Thickness (m)Concentration of a Component on the Permeate Side (mol fraction)Non-English Charactersα A/BSelectivityεδWell DepthEffective Membrane Thickness (m)xxvii

λ Specific Radiation Wavelength (nm) in Equation 2-28λ Coefficient of Laminar or Turbulent Flow in Equation 2-23λ De Broglie Wavelength in Equation 2-31ρ Density (kg/m 3 )π Mathematical Constant (~3.14159)φFugacity Coefficientθ Angle of Diffraction (°C) in Equation 2-28θ Time Lag (s) in Equation 2-11ФστPartition Function in GCMC SimulationSite Collision DiameterTime Axis Intercept of the Integral Molar Flux (s)µ Chemical Potential in GCMC SimulationSubscriptsAadsBComponent AAdsorptionComponent BC Convective Flow in Equations 2-25fIMPKMpSFeed SideImpurityKnudsenMolecular SievingPermeate SideSurface Diffusionxxviii

SDSSSolution DiffusionSteady State0 Initial or at Zero Concentrationxxix

xxx

1 INTRODUCTION1.1 Global WarmingThe energy demands of our modern society are primarily met by burning fossil fuels, whichproduce large quantities of the greenhouse gas carbon dioxide (CO 2 ). CO 2 is termed agreenhouse gas because it absorbs infrared radiation and thereby reduces the energy flow outof the atmosphere, which in turn leads to an increase in the earth’s temperatures. Thisphenomenon is known as “Global Warming”.Definitive changes in the Earth’s global climate, such as increases in the average air andocean temperatures, increased melting of snow and ice leading to rising sea levels, havealready been observed. Additionally, the global emissions of greenhouse gases, in particularCO 2 , are projected to grow over the coming decades leading to even greater changes in theEarth’s climate [1].Locally, Southern Australia is experiencing the worst drought on record [2]. Whilst there havebeen other impacts of climate change such as coral bleaching of the Great Barrier Reef [3], itis the relentlessness, severity and ultimately personal impact of the drought which has led tosome shift in the attitude towards climate change of the Australian People.In the 2007 federal election, the Australian Labor Party led by Kevin Rudd was elected for thefirst time in eleven years. The more progressive environmental policies, including signing theKyoto Protocol, are said to have been a contributor to Rudd’s landslide win [4].However, the cost impact of reducing Australia’s greenhouse gas emissions is a major barrierfor further significant support from the Australian people. The distress caused by recentincreases in petrol prices due to the increasing oil prices unrelated to climate change are anindication of attitudes toward paying more for energy. This is particularly true for theproportion of the population living away from the major centres where energy pricessignificantly influence the transportation costs of essential goods.Australia has also enjoyed inexpensive electricity due to plentiful black and brown coalreserves. In fact, the presence of cheap energy and other substantial mineral reserves are thebiggest contributors to Australia’s economy. This was the principal argument for the lack ofaction on climate change by the previous Australian government.1

1.2 CO 2 Capture and SequestrationOne promising process for significantly reducing greenhouse gas emissions in the shorterterm with potential for lower cost is CO 2 capture and sequestration. Whilst the energy orelectricity is still produced from fossil fuels, the CO 2 produced as part of the process iscaptured and stored underground preventing release into the atmosphere and furthercontribution to global warming.The Australian Cooperative Research Centre for Greenhouse Gas Technologies (CO2CRC)was established in 2003 and is a collaboration of research participants, government andindustry partners working towards commercialisation of CO 2 capture and sequestration inAustralia.The CO 2 capture and sequestration process as shown in Figure 1-1 involves (i) capturing CO 2from the source, (ii) pressurising CO 2 so that it forms a dense phase, (iii) transportation of theCO 2 to a storage site (via pipeline) and finally (iv) storing the CO 2 underground in an existinggeological formation such as an oil and gas or saline reservoir.Figure 1-1 : CO 2 capture and sequestration process. Diagram provided by theCooperative Research Centre for Greenhouse Gas Technologies (CO2CRC) [5].Looking at CO 2 capture in more detail, there are three major points within stationary energyproduction where CO 2 is produced and then emitted to the atmosphere. The first is in theproduction of natural gas (primarily methane, CH 4 ) from an underground reservoir. In thiscase, the CO 2 coexists with methane in the reservoir. The CO 2 must be removed from thenatural gas before it can be used in a commercial or domestic application. In Australia, this is2

usually done via an onshore gas processing plant. The second emission source comes from theproduction of synthesis gas (hydrogen, H 2 ) using fossil fuels such as natural gas, oil and coal.CO 2 capture from this emission source is often referred to as pre-combustion capture. Thefinal emission source is the flue gas from electricity power stations that are from directcombustion of fossil fuels, such as natural gas, oil or coal. CO 2 capture from this emissionsource is often referred to as post-combustion capture.The present cost of CO 2 sequestration is estimated by the International Energy Agency (IEA)to be between $40 and $60 US per tonne of CO 2 emissions avoided [6, 7] of which capturecosts contribute approximately 90 %. Therefore, significant improvements to the capturetechnology remain a focus for major reduction in the cost of CO 2 sequestration process.1.3 Capture TechnologiesTechnologies that exist to separate CO 2 from other gases include gas absorption, gasadsorption, cryogenics, gas hydrates and gas separation membranes. The focus of currentresearch is now on improving these technologies with new materials to create more efficientand hence less expensive processes.Nanoporous carbon is one such new material that has attracted a lot of interest in recent years.Made from the pyrolysis (heating) of a polymer at high temperatures, nanoporous carbon is astructure related to the fullerene group of carbon structures (the Buckyball, nanotubes etc.),which contains nanosized pores.Initial testing of nanoporous carbon (NPC) membranes revealed superior performance overthe traditional polymeric membranes. High performance NPC membranes have beenmanufactured from the pyrolysis of cellulose [8-13], polyvinylindene chloride (PVDC) [14],phenolic resin [15], polyimide [16, 17], polyamic acid [18] and polyfurfuryl alcohol (PFA)[19]. The CO 2 /CH 4 separation factors of these NPC membranes range from 10 to as high as200 whilst retaining a good permeance or flow through the membrane.The high values of performance published sparked a flurry of research work on NPCmembranes. The majority of the research work was focused on the transport mechanism [8,15, 20-26] and further development of high performing NPC membrane [27, 28]. While initialstudies utilised an NPC layer supported on a porous support, thinner unsupported NPCmembranes were also produced from thin polymer sheets to increase the flow. However,difficulties with brittleness and cracking made working with such NPC membranes verydifficult [29]. In fact, cracking still remains an impediment to the further development ofsupported and unsupported membranes alike [30].3

Following on from this, there have been several studies related to the effect of oxygen, waterand condensable impurities (such as heavier hydrocarbons) on the performance of NPCmembranes. Oxygen has been shown to react with the carbon creating hydrophilic carbonyl(C=O) functional groups which attract water from the atmosphere resulting in a severereduction in membrane performance [31-33]. The presence of condensable hydrocarbons wasalso shown to result in a severe reduction in membrane performance [34, 35].The objective of this research is to follow-on from the aforementioned ground breaking workand specifically look at the performance of NPC membranes in the application of CO 2capture. The two emission sources examined for the application of NPC membranes are (1)separation of CO 2 from natural gas (primarily methane CH 4 ) at high temperature and in thepresence of the impurities – water, hexane, toluene and hydrogen sulphide (H 2 S) and (2) theseparation of CO 2 from air-blown synthesis gas production (nitrogen, N 2 after initialhydrogen, H 2 separation) for electricity or hydrogen production at high temperature and in thepresence of the impurities – water, H 2 S and carbon monoxide (CO).CO 2 capture from power station flue gases using NPC membranes is not considered as part ofthis research work given the high concentrations of both oxygen and water.Details of the operating conditions of natural gas and synthesis gas separation used toevaluate the suitability of NPC membranes for these two applications are given in Figure 1-2and Table 1-1. The data was provided by the Cooperative Research Centre for GreenhouseGas Technologies (CO2CRC) [36].(a)(b)CO 2 /CH 4Feed> 2000 kPag> 100 °CH 2 0, H 2 SFocus of thisresearch workCooler, Knockout Potand FilterCH 4CO 2H 2 /CO 2 /N 2Feed> 2000 kPag250 °CH 2 0, CO, COSCooler, KnockoutPot and FilterFocus of thisresearch workN 2N 2 , CO 2 CO 2H 2 , H 2 OFigure 1-2 : Separation of CO 2 from (a) natural gas and (b) air-blown synthesis gas4

Table 1-1 : Operating conditions of CO 2 separation from natural gas and synthesis gasNatural GasSynthesis GasPressure kPag >2000 2800 – 5700Temperature °C >100 >250Composition Mol %H 2 0 55 – 56CO 0 2.5 – 2.8CO 2 10 37 – 40N 2 0 0.68 – 3.10O 2 0 0CH 4 90 0.02 – 0H 2 S + COS ppm levels 0.18 – 0.22 (no COS)H 2 O Saturated 0.19 – 0.31Hexane Saturated 0Toluene Saturated 0The structure of this thesis presents the findings of the research work beginning with themanufacture and high temperature operation of NPC membranes (Chapter 4). Themechanisms behind the transport of gases through NPC membranes at elevated temperaturesare then further studied using the results of gas adsorption experiments and molecularmodelling (Chapter 5). Finally, the impact of condensable and non-condensable impuritiesrelevant to the specific application of CO 2 capture from the two emission sources beingreviewed is assessed (Chapter 6).For further details on global warming, the CO 2 sequestration process and capture technologiesplease refer to Appendix A.5

2 LITERATURE REVIEW2.1 IntroductionThe review of the published knowledge related to this research work begins with a generaldiscussion on gas separation membranes, which includes typical materials of manufacture,performance measurement and gas transport mechanisms. Following on from this, themanufacturing methods, performance and gas transport mechanisms relevant to nanoporouscarbon (NPC) membranes is discussed.Relating to the specific application of this research work, the performance of NPCmembranes at high temperatures and in the presence of particular impurities is then presented.2.2 Gas Separation MembranesA membrane separates one component from another on the basis of size, shape or chemicalaffinity. A feed stream of mixed components enters a membrane unit where it is separatedinto a retentate and permeate stream as depicted in Figure 2-1 below. The retentate stream istypically the purified product stream and the permeate stream contains the waste component.FeedRetentateMembranePermeateFigure 2-1 : Membrane separation unit2.2.1 Performance Indicators2.2.1.1 Permeance and SelectivityThe performance of membranes is generally described by the flux (J) or permeability (P),which is equivalent to the throughput of the membrane, and the selectivity, which isequivalent to the separation efficiency of the membrane.Membrane flux is a measure of the flow through the membrane per unit area (mol/(m 2 /s)). Asper Fick’s first law, for a component to flow through the membrane a driving force is7

equired. Gas flow or flux through a membrane of a certain thickness (∆x) is driven by thefugacity differential across the membrane (∆f) and is described by the following equation.JP= ∆fEquation 2-1∆xFor ideal gases, the fugacity differential is simply equal to the pressure differential (∆p). Fornon-ideal gases, the fugacity differential is related to the pressure differential via the fugacitycoefficient (φ) as shown in Equation 2-2.∆ f = φ − φEquation 2-21p12p2For the range of pressures and gas components studied in this research works the fugacitycoefficients are all greater than 0.95 and usually close to 1.0. Therefore, it has been assumedthat fugacity can be approximated by pressure. All of the following equations and resultspresented in this thesis will use pressure.The permeability (P) is more commonly used to describe the performance of a membranethan flux. This is because the permeability of a homogenous stable membrane material isconstant regardless of the pressure differential or membrane thickness and hence it is easier tocompare membranes made from different materials. The permeability is usually given in theunits “Barrer”, which is equivalent to 10 -10 scm 3 .cm/(cm 2 .s.cmHg) or 3.347 x 10 -13mol.m/(m 2 .s.Pa) in S.I. Units.The term permeance (P’) is more commonly used in the literature for supported nanoporouscarbon (NPC) membranes. The permeance is equal to the permeability multiplied by themembrane thickness and is given the S.I. units mol/(m 2 .s.Pa). As stated in the ASTM standardtest method for determining gas permeability characteristics [37], permeability is onlymeaningful for homogenous films where the membrane thickness can be accuratelydetermined. Supported NPC membranes are not homogenous due to the supporting porouslayer and as such the term permeance is generally used for this research work.Selectivity is a measure of the ability of the membrane to preferentially permeate onecomponent over the other. An indication of selectivity can be obtained from the ratio of purecomponent permeances for a gas pair (A/B) as shown in Equation 2-3. This term is referred toas the ideal selectivity or permselectivity (α A/B ).P'AαA / B=Equation 2-3P'B8

For a binary mixture, the ideal selectivity can be calculated from the component (A/B) molfractions in the permeate stream (y) and in the feed stream (x) along with the feed (p f ) andpermeate (p p ) stream pressures as per Equation 2-4 below.⎡ pp⎤⎢xB− yB⎥y ⎢⎣pAf ⎥α =⎦A / BEquation 2-4yB⎡ pp⎤⎢xA− yA) ⎥⎢⎣pf ⎥⎦Another performance measurement indicator used in mixed gas experiments is the separationfactor (S A/B ). The separation factor is simply the ratio of components (A/B) in the permeatestream (y) and feed stream (x) as shown in Equation 2-5. However, the value of the separationfactor is specifically related to the feed composition and pressure drop used in the experiment.S =yAyBA / BEquation 2-5xAxBThe permeances and selectivities quoted are usually at 35 °C (308 K) unless otherwise stated.The relationship between the permeability (for membranes with a known thickness) andselectivity, is often described by Robeson’s theory [38] where the selectivity of the membranedecreases with increasing permeability.2.2.1.2 Diffusivity and Heat of AdsorptionOther indicators of membrane performance often used are the diffusivity and heat ofadsorption. These indicators are used to further understand the transport mechanisms of gasesthrough membranes as discussed later in Section 2.2.3.The diffusion coefficient or diffusivity (D) indicates the speed of the gas molecules travellingthrough the membrane.The heat of adsorption (∆H ads ) is the energy released when a gas molecule adsorbed onto thesurface of the membrane and thus indicates the affinity of a particular gas to the membranesurface.9

2.2.2 Performance Measurement2.2.2.1 Permeance and SelectivityThere are several techniques that can be used to measure permeance and selectivity. Thesimplest method is to subject the membrane to a pressure differential, measure the flowthrough the membrane and calculate the permeance (or permeability) from Equation 2-1. Thismethod works well for membranes that have a high permeance and as such the flow caneasily be measured using conventional flow metering techniques such as a rotameter or abubble flow meter. The equipment used for this method is referred to as a constant pressurevariable volume (CPVV) apparatus and is shown in Figure 2-2 (a).(a)Flow Meter(b)P fP pP fP pCylinder ofKnown VolumeMembraneMembraneGas CylinderGas CylinderFigure 2-2 : Equipment used for permeance or permeability measurement; (a) constantpressure variable volume apparatus (CPVV) for higher flow membranes and (b)constant volume variable pressure (CVVP) for lower flow membranesHowever, most membranes prepared on a laboratory scale when subjected to a reasonablepressure difference have a flow that is too low to be measured conventionally. In this case, itis more accurate to calculate the flow and therefore permeance by measuring the rise inpressure on the permeate side of the membrane within a known volume. This equipment usedfor this method is referred to as a constant volume, variable pressure (CVVP) apparatus and isshown in Figure 2-2 (b).Feed gas is introduced on top of the membrane at pressure (p f ). The rise in pressure on thepermeate (p p ) side of the membrane within the known volume (V p ) is recorded against timeand used to calculate permeance using the following equation [39].10

ppVpP ' ∆∆x= P =∆xVm∆t(p − p ) ARTEquation 2-6fpPlotting ∆p p against time (t) gives a line with a slope equal to the permeability (P) or gaspermeance (P’) if the membrane thickness (∆x) is removed from the equation. The term ARTrefers to the membrane surface area (A), the universal gas constant (R) and the experimentaltemperature (T), whilst the term V m refers to the molar volume at standard temperature andpressure.The two methods mentioned above are generally used for measuring the permeance of a puregas and from which permselectivity can be calculated using Equation 2-3.To measure the separation performance of a mixed gas stream, an extended version ofconstant pressure variable volume apparatus is used as shown in Figure 2-3.RetentateFlow MeterP rP fMembranePermeateFlow MeterGasChromatographMixed Gas Cylinder(Known Composition)P pFigure 2-3 : An extension of the constant pressure variable volume apparatus (CPVV)used for permeance and selectivity measurement using a mixed gas feed.In the mixed gas stream, the permeance of a component (A for example) is calculated fromthe flux (J) and the component pressure drop (∆p A ) by rearranging Equation 2-1 forpermeance as shown in Equation 2-7.PJA'A=Equation 2-7∆pAThe component flux (J A ) is calculated from the permeate molar flow rate (Q p ), permeatecomponent mol fraction (y) and membrane surface area (A) as presented in Equation 2-8.11

QpyAJA= Equation 2-8AThe change in component partial pressure is calculated from the feed (x) and permeate (y)component mol fraction along with the pressure of the feed (p f ) and permeate streams (p p ) aspresented in Equation 2-9.∆ p = x p − y pEquation 2-9AAfApSubstituting Equations 2-8 and 2-9 into Equation 2-7 yields Equation 2-10 which gives thefinal equation for calculating the component permeance using a mixed gas feed.P'AJ QpyAA= =Equation 2-10∆pA(x p − y p )AAfAp2.2.2.2 DiffusivityThe diffusivity of pure gases through membranes can be calculated using the time-lag method[40]. A plot of the gas flow (Q) through the membrane versus time (t) reveals an initialtransient permeation followed by steady state permeation. Extending the linear section of theplot back to the intersection of the x-axis gives the value of the time-lag (θ) as shown inFigure 2-4.Gas Flow (Q)Transient PermeationSteady-State PermeationθTime (t)Figure 2-4 : Calculation of the diffusion coefficient using the time-lag method, once thegradient is constant and steady state flow through the membrane has been reached, anextrapolation of the steady state flow line back to the x-axis where the gas flow is 0reveals the value of the time lag (θ).12

The time lag relates to the time it takes for the first gas molecules to travel through themembrane and is thus related to the diffusivity. The diffusion coefficient (D) (m/s) can becalculated from the time-lag and the membrane thickness (∆x) as shown in Equation 2-11.This method produces a value of the diffusivity that is applicable to ideal gases. CO 2 is a nonidealgas with a diffusivity that is concentration dependent. Wang et al [41, 42] have studiedthe permeation of CO 2 in carbon molecular sieves and have developed an extended version ofthe time-lag method that allows for the concentration dependency.D = ∆xEquation 2-11 Adapted from Reference [40]6θ2.2.2.3 Heat of AdsorptionThe heat of adsorption (∆H ads ) of a gas onto the surface of a membrane is calculated fromadsorption isotherms at various temperatures obtained using a gravimetric micro-balance [12].The adsorption behaviour of gases on nanoporous carbon generally falls outside the simplestmodel for adsorption, Henry’s Law, especially for the more adsorptive gases like CO 2 [43].Type 1 adsorption [44], where pores are extremely small, is more common for materials likenanoporous carbon and can be fitted well using the Langmuir Equation.The Langmuir model assumes that adsorption energy is constant and independent of surfacecoverage and that the maximum adsorption (V m ) occurs when the surface is covered by amonolayer of adsorbate. The amount of gas adsorbed (V) at a particular pressure (p) isdescribed by the Langmuir Equation as shown in Equation 2-12 [42].VVmbP= Equation 2-121 + bPRearranging the Langmuir Equation to a linear form as given in Equation 2-13 below givesthe value of b and V m from a straight line plot of p/V versus p.pVp= 1 +Equation 2-13V bmV mThe Langmuir rate constant, b, determined at various temperatures can then be used tocalculate the heat of adsorption (∆H ads ) from the Arrhenius-type relationship shown inEquation 2-14 below [43].13

⎛ ∆Hads ⎞b = bo exp ⎜ ⎟Equation 2-14⎝ RT ⎠2.2.2.3.1 Variations on the Langmuir ModelThe deviation from ideal Langmuir behaviour of the adsorption of a single component gas athigh pressures can often be more accurately modelled using a double-layer Langmuir model[45] as shown in Equation 2-15 below.VVm1b1PVm2b2P= +Equation 2-151 + b P 1+b P12For a mixed gas with components A and B, the fractional adsorption (θ) of one component(A) onto the substrate surface from bulk phase can be described using the dual site orextended Langmuir model [46, 47] as described in Equation 2-16.bPθ A AA= 1+ bAPA+ bBPEquation 2-16BHowever, the extended Langmuir isotherm is most accurate when the bulk concentrations ofthe two components are similar [46], which is not the case for this modelling work.Another approach is to use the ideal adsorbed solution theory (IAST), which predicts mixtureadsorption behaviour from the shape of the pure gas adsorption isotherms [48]. Chen andSholl [49] have examined the accuracy of the IAST method for the adsorption of CO 2 /CH 4 oncarbon nanotubes using transition matrix Monte Carlo simulations. The authors have foundthe IAST method based upon pure gas isotherms fitted with the dual-site Langmuir model tobe accurate over a large pressure range.Ritter and Yang [50] have examined the accuracy of several adsorption models for thecompetitive adsorption of CO 2 /CH 4 onto activated carbon and have found IAST to provide abetter fit to the experimental data than the extended Langmuir isotherm.14

2.2.3 Materials and Transport MechanismsThe sizes of the pores within a membrane dictate the gas transport mechanism and theresulting separation performance.2.2.3.1 Microporous MembranesMicroporous membranes are generally manufactured from materials such as silica ceramicsand have pore sizes > 3 nm (30 Å) [40]. The transport of gases in microporous membranesoccurs via Knudsen diffusion [8, 23] or in the case of very large pores via convective (bulk)diffusionKnudsen diffusion occurs when the membrane pores are smaller than the mean free path ofthe gas molecules [40]. In this case, the gas molecules collide more frequently with the porewalls than with each other. The Knudsen diffusion coefficient (D K ) of a gas (A) is inverselyproportional to the square-root of the molecular weight (MW) as shown in Equation 2-17below, where d p is the pore radius of the substrate. Therefore, gases with a low molecularweight have a greater ability to diffuse through the membrane than higher molecular weightgases and hence separation occurs.DK ( A)d 8RT= Equation 2-17p3 πMW(A)Fick’s law describes the relationship between the flux (J) and the concentration (C) drivingforce of a gas (A) as shown in Equation 2-18. If it is assumed that ideal gas behaviour applies,Equation 2-18 is integrated to Equation 2-19, where ∆p is the pressure drop, A is themembrane area and ∆x is the thickness.∂C(A)J( A)= −D(A)Equation 2-18∂xD(A)∆p(A)J( A)=Equation 2-19RT∆xSubstituting Equation 2-17 into 2-19, gives the following expression for the flux (J K ) of a gas(A) through substrate via the Knudsen diffusion mechanism.JK ( A)d ∆p8RT= Equation 2-20p ( A)3RT∆xπMW(A)15

Using Equation 2-1, to relate the flux to the permeance (P’ K ), then gives the followingexpression for the permeance of a gas (A) through substrate via the Knudsen diffusionmechanism.P'K ( A)dp 8RT= Equation 2-213RT∆xπMW( A)The Knudsen selectivity for a pair of gases (A and B) is simply described by the ratio of theirrespective Knudsen diffusion coefficients, which reduces to the inverse square-root ratio ofthe molecular weights as shown in Equation 2-22. The Knudsen selectivities for the gasmolecules of interest in relation to CO2 are presented in Table 2-1.DK( A)MW(B)αK ( A / B)= =Equation 2-22D MWK ( B)( A)Table 2-1 : Kinetic diameter, molecular weight and Knudsen selectivities of gasmolecules of interest.GasKinetic Diameter Molecular Weight Knudsen Selectivity(nm)(g/mol) In Relation to CO 2CO 2 0.330 44 1.00O 2 0.346 32 0.85H 2 S 0.360 34 0.88N 2 0.364 28 0.80CO 0.376 28 0.80CH 4 0.380 16 0.60When the pore radius increases further bulk / convective flow will dominate. Under theseconditions, the gas molecules will collide more frequently with each other and no separationwill occur.Convective flow through a macroporous medium is described by Darcy’s Law, which is theproportional relationship between the velocity (v), density (ρ) and pressure drop of the gas(∆p) as shown in Equation 2-23 [51].Equation 2-23 is also known as the Darcy-Weisbach equation, where λ is the coefficient oflaminar or turbulent flow and d m is the membrane diameter.16

2x ρ(A)v(A)∆ p(A)= λ ∆Equation 2-23d 2mRelating the velocity of the component to the volumetric flow (Q) using Equation 2-24,substituting back into Equation 2-19 and rearranging for flow gives Equation 2-25.2⎛ dm( ) ( )2 ⎟ ⎞QA= vAπ ⎜Equation 2-24⎝ ⎠QC ( A)⎛ d 2∆pd= π ⎜Equation 2-25⎝2m ⎞( A)m⎟2 ⎠ ∆xλρ( A)Convective flow selectivity (α C(A/B) ) for ideal gas behaviour is equal to unity.2.2.3.2 Nanoporous MembranesNanoporous membranes are generally made from carbon and zeolites [40]. These nanoporousmembranes are usually asymmetric, i.e. they have a selective membrane layer on top of asupport. Porous membranes can be made without a support but they generally have littlemechanical strength due to brittleness.Nanoporous carbon (NPC) membranes are porous membranes made from the pyrolysis(heating) of a polymer with or without a porous support.The major advantage of membranes like NPC membranes is an ability to operate at hightemperatures. However, unlike polymeric counterparts which are already used commerciallyfor natural gas separation, NPC membranes are still only at a research level. There are only acouple of companies (namely, Carbon Membranes Ltd and UBE Industries) producing NPCmembranes for specific applications [23].In nanoporous membranes, with pore sizes < 30 Å, other flow mechanisms dominate overKnudsen diffusion and bulk flow.If the pore sizes are relatively large (between 10 and 30 Å), gas separation can occur via thepartial condensation of one component of the gas mixture. The condensed componentproceeds to diffuse through the membrane as a liquid and separation takes place. Carbonnanotubes have a diameter generally in the range of 1 to 2 nm (10 to 20 Å). Andrews et al[52] have demonstrated through a packed bed arrangement of carbon nanotubes that CO 217

selectively condenses in the pores of the nanotubes over N 2 and therefore produces theseparation of the two gases.As the pore sizes decrease below around 10 Å, as is the case for NPC membranes, amechanism of surface diffusion becomes important [20]. More polar gas molecules areadsorbed onto the surface of the membrane and move through via surface diffusion.Surface diffusion is the mechanism proposed by Rao and Sircar for gas flow through theirSSF (Surface Selective Flow) membrane manufactured from the pyrolysis of polyvinylindenechloride (PVDC). In the case of SSF membranes, the pore sizes are slightly larger than thegas molecules and as such the larger and more polar molecules are adsorbed onto the porewalls and penetrate through the membrane [20]. Surface diffusion follows a transportmechanism activated by temperature, which is described in detail in Section 2.6.Gas separation via molecular sieving occurs when the pore sizes of the membrane approachthe size of gas molecules (< 5 Å). Gas molecules smaller than the membrane pores will passthrough whilst the larger molecules will be retained on the high pressure side of themembrane. Like surface adsorption, transport via molecular sieving also follows atemperature activated transport mechanism, which is also further described in Section 2.6.2.2.3.3 Nonporous MembranesNonporous membranes used for gas separation are generally made from polymers and can besupported (asymmetric spiral wound sheets) or unsupported (integrally skinned hollowfibres). Membranes made from Palladium used in hydrogen separation are another example ofnonporous membranes. These membranes are similar to NPC membranes in that theycomprise a selective membrane layer upon a porous support [53].For mechanical strength, polymeric membranes are usually operated at temperatures belowthe glass transition temperature, where the structure of the polymer changes is glassy ratherthan rubbery. Additionally, glassy membranes have a high free volume and are size selectivewhereas rubbery membranes select on the basis of solubility.Common polymers used in gas separation include cellulose acetate and polyimide. Inparticular, polyimide is a popular choice for gas separation (particularly CO 2 from natural gas- CH 4 ) because it has a high glass transition temperature (300 °C).Gas transport through a nonporous membrane, such as a polymeric membrane, is described bya solution diffusion mechanism where the permeability (P SD ) is the product of the diffusivity(D SD ) and the solubility (S SD ) as shown in Equation 2-26.18

P = D SEquation 2-26SD( A)SD(A)SD(A)Diffusivity is a kinetic term, which gives an indication of how fast a gas molecule travelsthrough a membrane. Solubility is a thermodynamic term that represents the amount of gasmolecules adsorbed by the membrane. This selectivity of two gases (α A/B ) is then simply theratio of the permeances or permeability.Equation 2-26 is also sometimes used to simply model the transport of gases through NPCmembranes as being a combination of diffusion and adsorption representing the molecularsieving and surface diffusion mechanisms.2.2.3.4 Membranes of Mixed MaterialsOne example of membranes comprising of different materials are mixed matrix membranes.Mixed matrix membranes are generally polymeric membranes impregnated with molecularsieving materials, such as nanoporous carbon or zeolites. Mixed matrix membranes arepromising materials as they have selectivity from the molecular sieving domains [54, 55].2.2.3.5 SummaryA diagrammatic summary of the transport mechanisms discussed is presented in Figure 2-5below. The transport mechanisms are grouped according to the membrane pore size. Typicalmaterials used to manufacture membranes of various pore sizes are also presented.TypicalMaterialsPolymericMembranesNanoporous CarbonMembranesCarbonNanotubesSilicaCeramicsMacroporousMaterialsTransportMechanismMolecularSievingCapillaryCondensationBulkDiffusionCO 2N 2 or CH 4SolutionDiffusionSurfaceDiffusionKnudsenDiffusionNone 0 5 10 30 100 1000Pore Size (Å)Figure 2-5 : Summary of transport mechanisms indicating as the size of the membranepores decrease, transport changes from bulk flow to molecular sieving.19

2.3 Manufacture of NPC MembranesThe three known crystalline forms of carbon are as follows; (i) graphite, (ii) diamond and (iii)fullerenes. The nanoporous carbon structure is most closely related to the fullerene group ofcarbon. In each form of carbon the atoms are arranged differently leading to alteredproperties. Graphite, for example, is one of the softest known structures whilst diamond is thehardest. The carbon atoms in graphite have a sp 2 bonding arrangement whereas the carbonatoms in diamond have a sp 3 bonding arrangement.Fullerenes are closed ball-like structures with the carbon in a pentagon or hexagon ringformation and were discovered in 1985 by Kroto et al [56]. The bonding arrangement inFullerenes is predominately sp 2 bonding with some sp 3 bonding [57].Figure 2-6 : Structure of a typical fullerene (the Buckminster Fullerene C60) discoveredin 1985 by Kroto et al [56].A ribbon-like structure consisting of carbon sheets was originally proposed as the structure ofNPC by Jenkins and Kawamura in 1976 [58]. However, in relation to work on carbogenicmolecular sieves (CMS) manufactured from NPC, Foley et al [59] presented new theoriesrelating the microstructure of NPC to fullerenes and proposed “mixed ring structures as theultimate building blocks”. This proposal comes from the postulation that defects in the formsof rings must be present to enable the high curvature that explains the narrow pore sizedistribution of 4 – 5 Å. Building on these findings, Acharya et al [60] have obtained threedimensionalstructures for nanoporous carbon using simulation on the basis of known keyproperties of NPC, such as complete sp 2 carbon hybridisation. The model developed simulatesthe real-life pyrolysis process by beginning with basic building blocks to randomly formulatebonds between hexagonal sheets (refer to Figure 2-7), which lead to the curvature. Acharya etal [60] have concluded that the ribbon-like structure originally proposed by Jenkins andKawamura [58] does not represent the curved structures obtained from the pyrolysis ofpolymers and is more applicable to glassy carbons.20

Figure 2-7 : Structure of a hexagonal sheet, which when randomly bonded to othersheets forms the curved nanoporous carbon structure. Figure adapted from [61].2.3.1 Supported NPC MembranesThe manufacture of a supported NPC membrane is undertaken in a series of steps. The initialstep requires the selection of an appropriate polymer precursor and porous support. In the nextstep, the polymer is deposited onto the porous support using a selected technique. Theresultant polymer-coated porous support is then pyrolysised (heated) under an inertatmosphere and a nanoporous carbon layer is formed on the porous support. Finally, thepolymer deposition and pyrolysis steps are repeated a certain number of times in order toincrease the amount of NPC deposited.2.3.1.1 PolymerA variety of polymers can be used to manufacture NPC membranes on condition that they arethermosetting and hence undergo cross-linking at high temperatures and so prevent theformation of graphite layers, which decrease the microporosity of the material [62]. Polymersthat can be used include cellulose, PVDC (polyvinylindene chloride), phenolic resin,polyimide and PFA (polyfurfuryl alcohol).It is difficult to directly compare the permeances and selectivities obtained from differentpolymer precursors as deposition techniques, pyrolysis conditions and the gas moleculesemployed are often not the same.From a general analysis of the literature data, NPC membranes made from PVDC displayextremely high permeances that are several orders of magnitude higher than those made frompolyimide. Similarly, NPC membranes made from the pyrolysis of phenolic resin or PFAexhibit permeances an order of magnitude lower again than those made from polyimide.Selectivities are a little harder to compare because the gas molecules employed for measuringpermeances differ due to different targeted commercial applications. The published results as21

shown in Table 2-2 at the end of this section suggest that membranes with lower permeances(such as those made from PFA) display higher selectivities as is consistent with Robeson’stheory discussed earlier in Section 2.2.2. Some detail of the results presented in the literaturefor each polymer is presented below.2.3.1.1.1 CelluloseHollow-fibre NPC membranes made from pyrolysis cellulose were patented in 1999 [13] withcommercialisation starting soon after by a company named Carbon Membranes Ltd, Thiscompany closed in 2001 [12]. These NPC membranes with a CO 2 /CH 4 selectivity around 3 at25°C [8] have been used for extensive research into the transport mechanism behind theseparation of gas by NPC membranes [8-13].2.3.1.1.2 Polyvinylindene Chloride (PVDC)Air Products and Chemicals have commercialised a membrane for the separation of hydrogencalled the Surface Selective Flow (SSF TM ) membrane, which is made from the pyrolysis ofPVDC on tubular supports [14, 20, 63-66]. In an initial paper, Rao and Sircar reported H 2 /N 2and CH 4 /H 2 selectivities of 1.7 and 5, respectively with extremely high correspondingpermeances [20]. In later work, Anand et al reported that the SSF membrane can achieve ahigh pressure (retentate side) H 2 recovery of 50 % from effluent gas containing methane,ethane, ethylene, propane, propylene and carbon dioxide [63].2.3.1.1.3 Phenolic ResinFuertes [21, 67] has also produced a selective flow NPC membrane from the pyrolysis ofphenolic resin. Oxidative treatment following pyrolysis was used to increase the pore size to 5– 7 Å. The increased pore size allows larger and more polar molecules such as hydrocarbonsto selectively adsorb and diffuse through the membrane. Selectivities as high as 87 arereported for CO 2 /CH 4 [67].2.3.1.1.4 PolyimidePolyimide is already established as a material for use in non-porous polymeric gas separationmembranes due to the high selectivity for CO 2 and the stability for operating temperatures upto 300 °C. Pyrolysis of polyimide for the manufacture of NPC has also been researched.Kusakabe et al [68] report CO 2 /N 2 selectivities for NPC membranes manufactured frompolyimide as high as 51. Hayashi et al [69] have manufactured NPC membranes from thepyrolysis of polyimide and have decreased the pore size even further by the chemical vapourdeposition (CVD) of propylene at 650 °C. CVD was shown to increase the CO 2 /N 2 selectivityat 35 °C from 47 to 73.22

2.3.1.1.5 Polyfurfuryl Alcohol (PFA)PFA has also been widely reported in the literature for the manufacture of NPC. PFA is afuran polymer and is derived from wood and other biomass sources. PFA has very highcarbon yield of 50 % on pyrolysis, which makes it an attractive material for the manufactureof NPC. Bird and Trimm completed the early work using PFA for gas separation membranesbut did not have any success with making a membrane without significant cracks [70].Following this, there was further research into the use of PFA to manufacture NPC as amolecular sieve material [62, 71, 72].The research then developed to the current situation where PFA is usually coated onto aporous support to make an NPC membrane (often referred to as the “Carbon Molecular SieveMembrane”) [19, 25, 43, 73-82]. Such membranes are reported with O 2 /N 2 permselectivitiesranging from 2 to 60. Most of the research published has measured permeances of O 2 and N 2through NPC membranes at room temperature with the proposed application being airseparation. There is little data published on CO 2 and CH 4 separation for NPC Membranesmade from PFA.As discussed earlier, it is difficult to directly compare permeance and selectivities of NPCmembranes made from different polymers. However, the comparison of H 2 /N 2permselectivity between NPC membranes made from PFA [75] and those made frompolyimide [68] is presented in Table 2-2 below. The pyrolysis temperature in both cases was600 °C.Table 2-2 : H 2 /N 2 permselectivity for NPC membranes made from PFA and polyimide.The NPC membranes made from PFA have a higher selectivity but lower permeancethan those made from polyimide.PolymerH 2 Permeance N 2 PermeanceH 2 /N 2(mol/m 2 /s/Pax10 10 ) (mol/m 2 /s/Pax10 10 ) PermselectivityPFA [75] 14 0.5 30Polyimide [68] 300 40 8Subsequent discussions on deposition method and pyrolysis of supported NPC membraneswill focus on literature that has used PFA. PFA has been selected for this research as theappropriate polymer precursor for the manufacture of supported NPC membranes on the basisof the high selectivity and yield of carbon produced.23

2.3.1.2 Porous SupportThe porous support must contain pore sizes larger than the pore sizes of the resultant NPCmembrane. Mesoporous materials such as graphite, alumina or porous stainless steel can beused. With the porous support providing no contribution to gas separation, the choice of aporous support is governed by mechanical strength, ease of manufacture and cost. Porousstainless steel supports are advantageous because of mechanical strength and reduced costgiven they can be welded to ensure a gas tight fitting suitable for high temperature operation.The Foley research group [73, 74, 79, 80, 82] use 2 micron porous stainless steel supports inthe form of disks with a diameter of 46 mm and thickness of 1 mm. The same research group[43, 75, 77, 78, 83, 84] has also used tubular stainless steel supports to manufacture NPCmembranes. The tubular support is used for creating membrane modules.The porous supports are often cleaned to remove contaminants such as oxides and greasesprior to coating. Acharya and Foley [74] cleaned the porous supports with chloroform in asonification bath for 15 mins followed by air drying, before depositing the polymericprecursor solution.2.3.1.3 Modified Porous SupportDuring the pyrolysis process, the polymer enters the pores of the support creating a membranethat is thick and heavy with carbon, which can lead to cracking of the NPC membrane surface[27, 28].One method for producing ultra-thin crack-free membrane surfaces involves filling the poresof the porous support with nanoparticles prior to adding the membrane layer. Thenanoparticles act to prevent the polymer entering the pores of the support during pyrolysisand ensure that carbon is only formed as a thin layer on the surface of the support [27, 28].With a support pore size around 2 micron, Rajagopalan et al [28] have used silicananoparticles ranging from 40 to 500 nm in size to fill the support. In this case, themodification of the support has provided a two-order of magnitude increase in gas permeance,whilst maintaining selectivity. Merritt et al [27] have used Silica nanoparticles and carbonblack particles to modify a porous stainless steel support resulting in a two-order ofmagnitude increase in gas permeance.Adding a γ-alumina layer to a porous support prior to forming a nanoporous carbonmembrane is another method for reducing the carbon membrane thickness and reduces thechance of cracking. Sea and Lee [85] have altered tubular α-alumina supports by adding a24

coating of γ-alumina. The presence of the γ-alumina layer on top of the support reduces theoverall support pore size from 1 micron to between 5 to 7 nm and thus prevents the polymerentering the support during pyrolysis. This technique of adding a γ-alumina layer has beenused by other researchers for reducing the pore size of a support [86-88].2.3.1.4 Deposition MethodThe first step in depositing the polymer on the porous support is the preparation of apolymeric precursor solution. Polymeric precursor solutions containing 25 to 60 wt% PFA inacetone have been reported in the literature [75, 78, 83] depending on the viscosity requiredfor the chosen deposition method.Deposition methods include brush-coating, spray-coating or ultrasonic deposition. For eachmethod, the porous support is rotated at a constant speed to allow uniform coverage of theprecursor. For the disk-shaped porous supports, the rotation is achieved using a spin coater.Brush-coating is the simplest method of deposition using a paint brush to apply the polymericprecursor (60 wt% PFA) to the porous support. Brush-coating has been used by Acharya et al[73] to produce NPC membranes with an O 2 /N 2 selectivity of 3.In subsequent work, Acharya and Foley [74, 76] have sprayed the polymeric precursor (50wt% PFA) to the support using an external mix airbrush (Badger Model 250). The NPCmembranes produced using this spray-coating method demonstrated O 2 /N 2 selectivitiesranging from 3 to 9.Shiflett and Foley [83] argue that ultrasonic deposition of the polymeric precursor (30 wt%PFA) provides a greater degree of control and therefore results in better reproducibly. Usingan automated ultrasonic spray system designed and built by Sono-Tek Corporation (USA),Shiflett and Foley [75] have manufactured 7 membranes under the same pyrolysis conditions(450 °C and 2 hours). The O 2 /N 2 selectivity produced ranged from 2 to 6 with permeances allwithin the same order of magnitude. In his patent for the NPC Membrane using ultrasonicdeposition, Shiflett [77] has quoted O 2 /N 2 selectivities from 2 to as high as 60. It is suggestedthat higher selectivities are observed using ultrasonic deposition due to the thinner and moreuniform coats obtained, which have less cracks and inconsistencies.2.3.1.5 PyrolysisThe generic method for the pyrolysis referenced in the literature [75, 78, 83] is as follows; thepolymer on the porous support is heated under an inert atmosphere increasing at a set rate to apredetermined temperature (termed the pyrolysis or soak temperature) where it is held at that25

temperature for a predetermined time period (termed the pyrolysis or soak time). The newlyformed NPC membrane is then allowed to cool to room temperature under the inertatmosphere. An example of the furnace temperature profile with time is given in Figure 2-8.Furnace Temperature (°C)800Oven HeatingOven Off (Natural Cooling)700Soak Temperature & Time600500400300200100Solvent Evaporation00 100 200 300 400 500 600 700 800 900 1000Time (min)Figure 2-8 : Typical furnace temperature profile for the pyrolysis of a polymer toproduce nanoporous carbon.Pyrolysing PFA under inert conditions liberates H 2 O, CO 2 , CO, CH 4 and H 2 gases, leaving ananoporous carbon structure according to the following reactions.600°C PFA→ H2O + FA + CO2 + CO + CH4 + H2+ Cwhere PFA = Polyfurfuryl Alcohol FA = Furfuryl AlcoholO CH 2O CH 2OH O CH 2OH26

Pyrolysis of PFA is generally performed under a He or Ar atmosphere. Shiflett [89] obtainedthe highest O 2 /N 2 selectivity factors when using such an inert atmosphere. Geiszler and Koros[90] have demonstrated whilst making NPC membranes from the pyrolysis of polyimide, thatresidual trace levels of O 2 in the inert atmosphere (> 3 ppm) will affect the structure of thenanoporous carbon formed.The final pyrolysis temperature has a significant impact on the resulting performance of theNPC membrane. The evolution of the nanoporous carbon structure with increasing pyrolysistemperature has been studied for carbon molecular sieves (CMS) derived from PFA [59, 62,71]. Foley et al [59] explain that as the pyrolysis temperature is increased from 300°C, thePFA precursor begins to produce the nanoporous carbon. At temperatures above 500°C, thehydroxyl groups disappear and when the pyrolysis temperature is increased further beyond600°C, dehydrogenation occurs and finally at temperatures above 1200°C where all of thehydrogen has been removed, a rearrangement of the carbon atoms occurs that ultimatelyproduces a graphite-like structure.Nuclear magnetic resonance (NMR) spectra of CMS samples prepared at 400, 500 and 600°Cby Mariwala and Foley [71] have showed the presence of oxygen atoms, as are present in theoriginal PFA polymer, at pyrolysis temperatures up to 500°C whilst at 600°C elementalanalysis reveals the presence of only carbon with less than 0.5% hydrogen. Powder X-raydiffraction on samples prepared at higher pyrolysis temperatures have revealed a shift in thed-spacing from between 3.9 and 5.2Å at 500°C to between 3.7 and 3.9Å at 800°C and then0.36 Å at 1000°C.The narrowing of the pore size distribution with increasing pyrolysis temperature is said to becaused by the growth of the aromatic microdomains which also cause a subsequent increasein porosity. However, the removal of oxygen and carbon-hydrogen bonds with increasingpyrolysis temperature leads to a fragile structure and the collapse of the micropores [71].In a more recent study on bulk nanoporous carbon, Burket et al [61] used a variety ofcharacterisation techniques to further understand the carbonisation of PFA in the range from300°C up to 600°C. It was concluded that between 300 and 400°C, carbon and polymercarbon chains co-exist resulting in a mesoporous structure. As the temperature is increasedbeyond 400 to 600°C, with the major weight loss at 500 °C, carbonisation completes and themesopores collapse leaving a microporous structure.Looking specifically at membranes, Shiflett and Foley [75] have produced three NPCmembranes from PFA at pyrolysis temperatures of 300, 450 and 600°C using the method of27

ultrasonic deposition. The results are presented in Table 2-3. Increasing the pyrolysistemperature from 300 to 600°C increased the oxygen and nitrogen permeance by an order ofmagnitude, which the authors attribute to the increase in porosity.Table 2-3 : Permeance and permselectivity of NPC membranes made from PFA atdifferent pyrolysis temperatures [75]. Increasing the temperature of pyrolysis has asignificant effect upon increasing the permeance. However, the permselectivity goesthrough a maximum at 450 °C.Pyrolysis Temp (°C)O 2 Permeance(mol/m 2 /s/Pax10 12 )@ 293 KN 2 Permeance(mol/m 2 /s/Pax10 12 )@ 293 KO 2 /N 2 Permselectivity300 0.11 0.01 13450 0.56 0.02 30600 1.27 0.46 3The permselectivity results are perhaps explained by the change in proportions of smaller tolarger pore sizes as the pyrolysis temperature is increased. Burns et al [91] explain theoccurrence of a selectivity maximum for carbon molecular sieving materials as being causedby the subtle changes in the distribution of selective elements with pyrolysis temperature.Interestingly, the majority of NPC membranes that have been manufactured from PFA are atpyrolysis temperatures at and below 600°C [73, 75, 78, 83, 84]. Whilst a completenanoporous carbon structure is produced at a pyrolysis temperature of 600°C with acontinuous reduction in pore size and increase in porosity achieved at temperatures greaterthan 600°C, the apparent fragility of the structure beyond 600°C makes it difficult to workwith in a membrane form.In his PhD Thesis, Shiflett [89] discusses the effect of the heating ramp rate. Heating ramprates above 10 °C/min can lead to the formation of bubbles on the surface due to insufficienttime for the evolving gases to escape. A value of 5 °C/min is commonly used.Increasing the pyrolysis time is generally reported to have a similar impact on the structure ofthe nanoporous carbon as increasing the pyrolysis temperature. Rajagopalan and Foley [82]have demonstrated that the pyrolysis time causes significant changes in the morphology of themembranes. As the pyrolysis time is increased, the size of globular domains is increased andthe structure becomes disordered, reducing the selectivity. A pyrolysis time of 2 hours alongwith a pyrolysis temperature of 450 °C for NPC membrane manufactured from PFA is mostcommonly reported in the literature [24, 73, 75, 77, 78, 83, 84].28

2.3.1.6 Number of CoatsThe amount of nanoporous carbon deposited onto the stainless steel support can be increasedwithout producing cracks by applying additional thin coats of polymer. The pyrolysis processis repeated for each additional polymer coat applied.Using a spin-coating deposition method, Rajagopalan and Foley [82] have reviewed therelationship between the number of coats applied, weight gain (carbon deposition) andpermeability. For the first two coats applied, the nanoporous carbon fills the pores of theporous support. By the third coat, a NPC membrane layer is formed. They have also shownthat the permeability increases with increasing carbon deposition whilst the O 2 /N 2permselectivity reaches a maximum of 5 after 8 coats with the weight gain having reached 6mg/cm 2 . The reduction in selectivity is thought to be due to cracking in the initial layer of themembrane due to a long exposure to high temperatures.2.3.2 Unsupported NPC MembranesWhilst unsupported NPC membranes lack the mechanical strength of supported NPCmembranes, they often display a higher permeance due to having a thinner membrane layer.The manufacturing process for unsupported NPC membranes is much simpler than supportedNPC membranes. The polymer is cast as a thin film or extruded as a hollow fibre; the solventis then evaporated through drying the polymer over several days and then finally pyrolysed toproduce an unsupported carbon membrane.2.3.2.1 PolymerA polymer can be used to make an unsupported carbon membrane provided it (a) undergoescross-linking at high temperatures to prevent the formation of graphite layers as discussed inSection 2.3.1.1 and (b) can be produced as a flat sheet or hollow fibre.By far the most commonly used polymer in the production of unsupported carbon membranesis polyimide as it is a well established polymer used for gas separation membranes. Kapton(DuPont, USA) and Matrimid (Ciba Specialty Chemicals, Japan) are commonly used as theyare commercially available. NPC membranes made from Kapton are generally morepermeable and less selective than those made from Matrimid [16, 17]. More recently, Liu et al[92] produced highly selective and permeable unsupported NPC membranes from the novellow cost polymer poly(phthalazinone ether sulfone ketone).29

2.3.2.2 Pyrolysis TemperatureThe pyrolysis temperature required to produce carbon is dependent on the original polymer.Barsema et al [29] carbonised Matrimid at temperatures between 300 and 525°C and foundthat carbonisation did not begin to occur until 425°C. Similarly, Suda and Haraya [93]carbonised Kapton at temperatures between 500 and 1000°C and found that carbonisation didnot begin until 527°C. As the temperature was increased up to 1000°C, the permeabilitydecreased whilst the CO 2 /N 2 permselectivity increased.Suda and Haraya [93] also reported a increase in permeability upon decreasing the heatingrate from 1.33 °C/min to 13.3 °C/min. As discussed in Section 2.3.1.5, this same result hasbeen reported for supported membranes.In a recent study, Su and Lua [94], have examined the effect of pyrolysis atmosphere on thecarbonisation of Kapton at 600 and 800°C. It was concluded that the permeance of all gaseswas lowest under vacuum, whilst O 2 /N 2 permselectivity was highest under vacuum andCO 2 /CH 4 permselectivity was highest under Argon.Characterisation of the nanoporous carbon produced revealed the lowest d-spacing (anindication of pore size) under the Argon atmosphere, which concurs with the highpermselectivity observed. Likewise, the BET surface area was lowest for the samples madeunder vacuum, which arguably explains the low permeance observed for the membranesmade under vacuum. Whilst this study was undertaken with unsupported NPC membranes,the findings apply to the structure of the carbon produced and as such are also relevant tosupported NPC membranes.Additionally, a clear trend of decreasing permeance and increasing permselectivity withincreasing the pyrolysis temperature from 600 to 800°C was also reported [94].In the manufacture of NPC membranes from poly(phthalazinone ether sulfone ketone) [92], itis argued that the thermostabilisation of the membrane in air between 400 and 500°C prior topyrolysis under argon between 650 and 850°C allows formation of oxygen bridges betweenthe aromatic molecules that prevent rearrangement during carbonisation and hence result in agreater porosity.30

2.3.3 Characterisation2.3.3.1 Membrane ThicknessMembrane thickness is usually calculated by the mass of carbon deposited on the poroussupport and the known standard density of nanoporous carbon (1.6 g/cm 3 ). Strano and Foley[79] have also calculated an effective membrane thickness (δ) using the steady state flux (J ss )and driving force pressure (p 0 ) of Helium (a non-adsorbing gas) from the diffusion time-lag(τ) as shown in Equation 2-27, which they have derived from Equation 2-11. The term R isthe ideal gas constant and T is the experimental temperature.6J ssτRTδ = Equation 2-27p02.3.3.2 Surface StructureVarious microscopy techniques are used to visually examine the surface structure of NPCmembranes. Foley et al. [59] have used High Resolution Tunnelling Electron Microscopy(HRTEM) and Energy Dispersive X-Ray Spectrometry (EDX) to capture and describe thechanging structure of nanoporous carbon as it is pyrolysised from 400 to 800°C as wasdescribed previously.Shiflett et al. [84] have used Atomic Force Microscopy (AFM), EDX and HRTEM to obtainfurther information on the membrane surface. AFM was used to obtain information on poresizes; however, the resolution of AFM was not powerful enough to detect the poredimensions. Some metallic impurities on the surface of the membrane were detected by EDX.2.3.3.3 Pore Size DistributionVarious porosity techniques are reported in the literature for characterising nanoporouscarbon. Shiflett et al [84] have used methyl chloride porosity to confirm the presence ofnanopores and to determine the size distribution from the Horvath-Kawazoe (HK) and Kelvinmodels. Adsorption of methyl chloride was measured using a microbalance from C. I.Electronics (USA). The results from the methyl chloride porosity showed an average poresize of 4.5 – 5 Å. Shiflett [89] has compared these results with digitally enhanced HRTEMimages that showed slit shaped pores within the range of 3 to 6 Å.In their paper dedicated to the use of methyl chloride adsorption for calculating pore sizes,Mariwala and Foley [95] conclude that methyl chloride provides advantages over thetraditional nitrogen adsorption technique due to higher rates of diffusion. Similarly, given thevery slow diffusion rate of N 2 , Steel and Koros [17] have used CO 2 adsorption and a31

Micromeritics ASAP 2010 apparatus to obtain a pore size distribution centred around 5 Å forNPC membranes manufactured from the pyrolysis of polyimide.A different technique, positron annihilation lifetime spectroscopy (PALS), has recently beenused by Tin et al [96] to characterise unsupported carbon membranes made from the pyrolysisof the polyimides Matrimid and P84 at 800°C. PALS characterises samples by bombardingthe sample with positrons and then analysing subsequent lifetime and intensity of producedortho-positronium. The lifetime of ortho-positronium annihilating in the pores (τ 2 ) indicatesthe size of the pores whist the number or intensity of ortho-positronium annihilating in thepores (I 2 ) indicates the number of pores or the porosity. The values of τ 2 and I 2 determined forNPC membranes made by Tin et al [96] are presented in Table 2-4.Table 2-4 : PALS data for NPC membranes manufactured from the pyrolysis of thepolyimide P84 and Matrimid [96]. The lifetime of ortho-positronium annihilating in thepores (τ 2 ) indicates the size of the pores. The number or intensity of ortho-positroniumannihilating in the pores (I 2 ) indicates the number of pores or the porosityPolymer Precursor Pyrolysis Temperature (°C) τ 2 (ps) I 2 (%)P84 800 378 70Matrimid 800 381 68In the same work, Tin et al [96] have also used wide angle x-ray diffraction (WAXD) todetermine the d-spacing of the NPC membrane. The d-spacing, which can be interpreted asthe distance between carbon molecules or the pore size, is determined from WAXD byexposing the sample to a specific radiation wavelength (λ), measuring the angle of diffraction(θ) and relating this to the d-spacing (d) using Bragg’s Law as shown in Equation 2-28. Theterm n is an integer related to the order of reflection.n λ = 2d sinθEquation 2-28In a related study, Tin et al [97] use WAXD to specifically review the effect of pyrolysistemperature on the morphology of NPC membranes made from P84. This is a similar study tothat done by Steel and Koros [17] who also used WAXD to characterise NPC membranesmade from Matrimid at different pyrolysis temperatures. The values of the d-spacingdetermined for NPC membranes made from the different polyimide [17, 97] are presented inTable 2-5. With a pore size around 4 Å, these results are in line with those determined usingadsorption as discussed above. For the highest pyrolysis temperature used to make NPCmembranes from P84 and Matrimid (800 °C) there is a peak located greater than 40° 2θ or 2.1Å, which the authors of both papers attribute to the formation of a more graphitic structure.32

Looking specifically at bulk nanoporous carbon manufactured from PFA, Powder x-raydiffraction was used by Mariwala and Foley [71] to obtain d-spacing values at increasingpyrolysis temperature. These results are also included in Table 2-5 for comparison.Table 2-5 : WAXD data for NPC membranes manufactured from the pyrolysis of thepolyimide Matrimid and P84 [17, 96, 97] along with powder x-ray diffraction data forbulk nanoporous carbon manufactured from PFA [71].Polymeric Precursor Pyrolysis Temperature (°C) d (Å)Polyimide – P84 ([97]) 550 4.2Polyimide – P84 ([96, 97]) 800 3.5Polyimide – Matrimid ([17]) 550 3.8Polyimide – Matrimid ([96]) 800 3.7PFA [71] 500 3.9 – 5.2PFA [71] 800 3.7 – 3.9PFA [71] 1000 3.633

2.4 Performance of NPC Membranes2.4.1 Supported NPC MembranesThe permeance of NPC membranes reported in the literature range from less than 1 x 10 -10 tomore than 1000 x 10 -10 mol/(m 2 .s.Pa) depending on the flowing gas molecule, polymer usedand conditions of manufacture. Similarly, there is also a large range of reported selectivitiesfrom as low as 1 (no separation achieved) to over 200.Looking specifically at the gas molecule pair CO 2 /CH 4 , separation factors between 80 and 90have been reported for NPC membranes made from the pyrolysis of the polymer PFA byWang et al [19]. Sedigh et al [25] have reported a separation factor of 33 for a NPCmembrane made from the pyrolysis of PFA on an alumina tube. Centeno et al [15] have alsoreported a high separation of 87 for NPC membranes made from the pyrolysis of phenolicresin. Ogawa and Nakano [18] have made hollow fibre NPC membranes from the pyrolysis ofpolyamic acid and report a very high CO 2 /CH 4 separation factor of 181.Similarly, for the gas molecule pair of CO 2 /N 2 , Wang et al [19] reported separation factors of79 and 40 for a NPC membranes made from the pyrolysis of the polymer polyfurfuryl alcohol(PFA). This compares favourably against a zeolite membrane tested by Bernal et al [98] witha CO 2 /N 2 permselectivity of 13.7. Hayashi et al [99] have demonstrated that supported NPCmembranes manufactured from the pyrolysis of polyimide have much greater CO 2permeabilities and CO 2 /N 2 permselectivities than polymer membranes made from the sameprecursor. The sorptivity and diffusivity through the nanoporous carbon membranes wererespectively 2 to 3 and 300 to 1000 times larger than with the polyimide polymer membranes.As discussed earlier, the separation factor from mixed gases provides a more realisticindicator of membrane performance than permselectivity from pure gases. Variousresearchers [18, 25] have obtained permeance measures for CO 2 /CH 4 mixtures throughsupported NPC membranes and conclude that mixture selectivities are higher than idealselectivities due to the selective adsorption of CO 2 on the pore walls, which hinders thetransport of CH 4 .Whilst the focus of this research is on the CO 2 /CH 4 and CO 2 /N 2 gas pairs, it should be notedthat also in the area of CO 2 separation and capture very promising selectivities have beenreported for the H 2 /CO 2 gas pair where CO 2 is retained on the retentate side [14, 20, 22, 25,29, 63, 64, 100-103] for molecular sieving membranes. However, for the SSF membraneswhere the pore size is larger and surface diffusion dominates the H 2 is actually retained on theretentate side [63].34

There is also research work in synthesis gas production focused on H 2 separation via catalyticmembranes where the catalyst is impregnated into the polymer prior to pyrolysis [79].2.4.2 Unsupported NPC MembranesThe performance of unsupported NPC membranes is generally reported as permeability as thethickness of the membrane is known. This makes it possible to directly compare theperformance of unsupported NPC membranes with polymeric membranes.As discussed earlier in Section 2.2.2, Robeson’s theory relates to the relationship betweenpermeability and selectivity. Based upon the published data on the performance of polymericmembranes, Robeson [38] developed relationships for the theoretical performance limits ofpolymeric membranes for particular gas pairs. The performance of unsupported NPCmembranes made from the pyrolysis of polyimide [17] in relation to the Robeson upperbound for the CO 2 /CH 4 gas pair for polymeric membranes is presented in Figure 2-9.1000Target PerformanceHigh Selectivity and PermeabilitySelectivity (CO 2 /CH 4 )10010Robeson Upper BoundRaw Polyimide Polymer MembranePolyimide NPC Membrane11 10 100 1000 10000CO 2 Permeability (Barrer)Figure 2-9 : Performance of unsupported NPC membranes [17] with respect to theRobeson Upper Bound [38] for polymeric membranes, which shows that thecarbonisation of a Matrimid polymeric membrane increases the permeability andselectivity such that it breaks the upper bound.As can be seen from Figure 2-9, the pyrolysis of Matrimid to form a NPC membrane [17]increases both the permeability and selectivity compared with the original Matrimidmembrane resulting in a shift to above the Robeson upper bound.35

2.5 Gas Transport Mechanisms of NPC MembranesAn ideal NPC membrane will have a pore size distribution that is very narrow and centred at asize just larger than that of the desired penetrant thus facilitating gas separation via molecularsieving. In this study, the desired penetrant is CO 2 , which has a kinetic diameter of 3.3 Å andas such a pore size distribution centred around 4-5 Å would result in reasonable molecularsieving ability for this molecule. However, in a real manufactured NPC membrane there areoften some pores present within the 5 – 10 Å range [84], which leads to the surface diffusiontransport mechanism also contributing the performance of the NPC membrane.2.5.1 Experimental ObservationsThe research work of Fuertes [21] provides a good explanation of the transition betweenmolecular sieving and surface diffusion. Fuertes has oxidised NPC membranes produced fromthe pyrolysis of phenolic resin to increase the pore size of the nanoporous carbon from 4 to 5– 7 Å. Comparing the permeances of gases through oxidised and non-oxidised membranes, itwas noted that in the non-oxidised membranes the permeance of a gas increases withdecreasing molecular size, which suggests a molecular sieving mechanism. However, in theoxidised membranes with the larger pore size, permeance was no longer correlated withmolecular size as the larger more polar molecules like hydrocarbons displayed higherpermeance.A molecular sieving trend has been observed by Wang et al [19] for a single NPC membranemade from the vapour deposition on PFA (with a narrow pore size range of 4- 5 Å) onalumina tubes. As shown in Table 2-6, the permeance decreases with increasing kineticdiameter.Table 2-6 : Permeance results of various gas molecules for membranes made from PFA[19]. The increase in permeance with decreasing molecular size is a clear indicator of amolecular sieving mechanism.Permeance (mol/m 2 /s/Pax10 10 ) @ 293 KH 2 (0.289 nm) CO 2 (0.33 nm) O 2 (0.346 nm) N 2 (0.364 nm) CH 4 (0.38 nm)225.00 58.20 7.75 0.73 0.63The permeance of low-adsorbing gases (such as N 2 or CH 4 ) will be constant for a molecularsieving membrane over a range of driving force pressure [15, 24]. However, for gases thatadsorb well (such as CO 2 ), the permeance will be pressure-dependent [104] with highestpermeances observed at low pressures.36

If, however, there is insufficient coverage of carbon or cracking occurs, the gas moleculeswill proceed directly through the porous support via bulk flow or Knudsen diffusion. Todetermine the contributions of each mechanism to the overall performance requires a detailedunderstanding of the pore distribution along with the number of defects. Several researchers[12, 105] have developed a parallel resistance transport models to account for the presence ofdefect pores in NPC membranes.2.5.2 Molecular ModellingMolecular modelling or simulation can be used to predict the adsorption and diffusion ofgases through a substrate (e.g. nanoporous carbon). Molecular simulation is a very usefulcomputational tool for predicting performance and for understanding the transportmechanisms behind the performance observed experimentally.There are two methods of molecular simulation used for predicting the adsorption anddiffusion of gases through nanoporous carbon. The first is Monte Carlo simulation which isused for simulating equilibrium properties such as adsorption isotherms [106-109]. Thesecond is molecular dynamic simulation which is used for simulating the movement ordiffusion of molecules through the substrate. Equilibrium properties calculated from usingMonte Carlo simulation are also used as an input to the molecular dynamic simulation [110,111].2.5.2.1 Monte Carlo SimulationThe Monte Carlo (MC) method is a stochastic (nondeterministic) process that uses repeatedrandom sampling to compute the results.Monte Carlo (MC) simulations, which like molecular dynamic simulations employ computersto solve the algorithms, use the MC method to calculate the equilibrium properties of asystem. This is achieved by sampling the phase space of an ensemble of state points andcomputing the averages of mechanical properties over the ensemble [112].Adsorption properties and therefore adsorption isotherms can be calculated using an ensembleof constant chemical potential (µ), volume (V) and temperature (T) known as the GrandCanonical (GC) ensemble.Each system within the GC ensemble is in equilibrium with the environment and as such theenergy (E) and number of particles (N) of each system is allowed to fluctuate at constantµVT. The partition function (Ф), which describes the statistical properties of a system in37

equilibrium, for the GC ensemble is represented in Equation 2-29, where k B is the Boltzmannconstant and f is the fugacity related directly to the chemical potential via equation 2-30.iEi−kBTNiΦ ( f , V , T ) = ∑ f eEquation 2-29µ = k BT ln fEquation 2-30For each GCMC simulation at constant bulk pressure and temperature, the number ofparticles adsorbed from the bulk phase in the equilibrium state is determined from byperforming trial moves of displacement, rotation, insertion and deletion. Each move isselected at random and has an associated probability function [45].The adsorption isotherms are created by completing several such Grand Canonical MonteCarlo (GCMC) simulations at varying values of chemical potential to calculate the associatednumber of particles adsorbed from the bulk phase. The bulk phase pressure is related to thechemical potential via Equation 2-31 as shown below, where λ is the de Broglie wavelengthand φ is the fugacity coefficient of the fluid in the bulk phase [112].1 3 1µ = ln λ + ln( pφ)Equation 2-31k Tk TBBLikewise, adsorption isotherms at different temperatures can be created by completinganother family of simulations to determine the number of adsorbed particles at varying bulkpressure (chemical potential).Inputs into the GCMC simulation include the surface structure of the substrate (e.g.nanoporous carbon), the initial position of the gas molecules and potential models to describethe interactions within the gas molecules and between the gas molecules and the substrate.The Lennard Jones potential model is used to provide the interaction potential (u) between thegas molecules and is described by Equation 2-32, where r ij is the separation distance betweensites i and j, ε i,j is the well depth and σ i,j is the site collision diameter [112].126⎡⎛σ⎤i,j⎞ ⎛ σi,j⎞u ⎢⎜⎟ − ⎜ ⎟ ⎥i, j( ri, j) = 4ε Equation 2-32⎢⎥⎣⎝ri, r ⎠ ⎝ ri, r ⎠ ⎦The Steele potentials are used in the simulation to provide the interaction potential betweenthe gas and carbon molecules and are based upon adsorption on planar graphite sheets [113].38

Ab-initio potentials can be used to provide the interaction potential between gas-carbonmolecules, which incorporate the presence of the non-hexagonal rings of nanoporous carbon[113].2.5.2.2 Molecular Dynamics SimulationMolecular dynamic (MD) simulation uses numerical methods to calculate the properties of asystem over a period of time. Numerically, MD simulation uses an algorithm to integrateNewton’s equations of motion from initial positions and velocities to create moleculartrajectories, which can then be analysed to obtain equilibrium and time-dependent properties[112].For the study of gas adsorption and diffusion through porous substrates, MD simulations arecarried out in the Grand Canonical ensemble of constant number of particles, volume andtemperature (NVT). Newton’s equations of motions are integrated using the Verlet velocityalgorithm. The Nose-Hoover thermostat is used to maintain a constant temperature[45].The sum of kinetic and potential energy functions of the system is known as the Hamiltonian(H). The Hamiltonian of a given system is constant and this value can be used to check theaccuracy of MD calculations.To obtain the diffusivity of a gas particle through the porous substrate, the Einstein relation isused. The Einstein relation, as shown in Equation 2-33, relates the time (t) limit of the meansquared displacement of particles (N) at position r to the component diffusivity (D). Thedimensionality of the system is denoted d [45].1NN2lim ∑[ ri( t)− ri(0)] = 2dDtt→∞i=1Equation 2-33The potential energy models employed to describe the interactions within molecules andbetween the molecule and the substrate are the same as discussed for GCMC simulation in theprevious section.The combination of the adsorption results from GCMC simulation and component diffusivityresults from MD simulation can be to calculate the predicted flux and ideal selectivity at agiven temperature and pressure [45]. A simplified summary of the GCMC and MDSimulation processes is shown in Figure 2-10.39

Substrate structure / coordinatesInitial positions and velocities ofgas moleculesGrand CanonicalMonte CarloSimulationAdsorption propertiesPotential model for describinginteractions between gas molecules(e.g. Lennard Jones, Steele or Ab-Initio)• Stochastic / Random• System of constant µVT• Equilibrium statedetermined from trial movesEquilibrium propertiesFlux &IdealSelectivitySubstrate structure / coordinatesInitial positions and velocities ofgas moleculesPotential model for describinginteractions between gas molecules(e.g. Lennard Jones, Steel or Ab-Initio)Molecular DynamicSimulation• Numerical• System of constant NVT• Solving Newton’s Equationsof Motion (Nose-HooverThermostat) using the VerletAlgorithm for obtainingtrajectories• Einstein relation for obtainself-diffusivitySelf-diffusivitiesPhenomenologicalcoefficients for mixturesFigure 2-10 : The process that is used by molecular simulation to create flux andseparation factors for nanoporous carbon. The two simulation methods used are (1)Grand Canonical Monte Carlo (GCMC) simulation for determining the adsorptionproperties and (2) molecular dynamic (MD) simulation for determining the selfdiffusivities.Combining the results from both methods produces a predicted flux andseparation factor.2.5.2.3 Published Simulation Studies of NPCNanoporous carbon is commonly modelled using the coordinates of C 168 Schwarzite(presented in Figure 2-11), which is a very similar curved-surface carbon compound.Figure 2-11 : Structure of C168 Schwarzite used to represent nanoporous carbon [106].40

Klauda et al [113] have specifically reviewed the effect of the curved carbon surface on gasadsorption using molecular modelling of planar graphite, curved C 60 Fullerene and curvedC 168 Schwarzite. The interaction potential between nitrogen and the C 60 and C 168 surfaceswere found to be different than those for the planar graphite, resulting in higher adsorptionenergies with the curved surfaces. However, the curved surface only had a small effect on theinteraction potential between oxygen and the C 60 and C 168 surfaces. The authors suggest thatthis difference in adsorption energies on the curved carbon surface may explain the highO 2 /N 2 selectivity observed experimentally in NPC membranes by Shiflett and Foley [75, 83].Using grand canonical Monte Carlo (GCMC) simulation, Jiang et al [106] have modelled theadsorption of oxygen (O 2 ) and nitrogen (N 2 ) in C 168 Schwarzite. At low pressure theadsorption of N 2 is favoured but as the pressure is increased the adsorption of O 2 dominatesdue to its smaller size. The simulation isotherms are fitted well to the two-site Langmuirmodel. In a later publication, Jiang and Sandler [107] have examined the mixed gascompetitive adsorption of O 2 and N 2 using both GCMC and Gibbs ensemble Monte Carlosimulation. At pressures above 400 kPa and at 27 °C, the O 2 /N 2 selectivity increases above 1to 1.2 at 10,000 kPa. However, increasing the temperature to 127 °C then reduces theselectivity back to 1.Arora and Sandler [110, 111] argue that the adsorption O 2 /N 2 selectivity of the curved C 168Schwarzite surface predicted by molecular modelling can not solely explain the experimentalresults observed by Shiflett and Foley [75, 83] and it is necessary to understand the diffusionof these gases through nanoporous carbon. In the initial publication, the authors [110]conclude that using the ab initio potential for describing the interaction between the gasmolecules and the carbon surface better explains the high selectivity seen experimentally. Tomodel the pore size of nanoporous carbon, the authors [111] subsequently performedsimulations using carbon nanotubes with constrictions that represented the pore size ofnanoporous carbon, which allow molecular sieving. If the correct constriction is present, O 2will pass through the nanoporous carbon much more readily than N 2 resulting in a muchgreater overall O 2 /N 2 selectivity.With the purpose of reviewing the suitability of nanoporous carbons for separating CO 2 fromN 2 in a typical power station flue gas, Jiang and Sandler [108] have used a combination ofquantum mechanics (to obtain the ab initio potential) and GCMC (to predict the adsorption).The authors also explain that the ab initio potential provides a more accurate adsorbateadsorbentinteraction potential for nanoporous carbon, with an adsorption selectivity ofCO 2 /N 2 around 160 being observed for a binary mixture (21:79 mol& CO 2 :N 2 ) at ~ 100 kPa.This work has only considered the effect of adsorption for separating CO 2 and N 2 . If using41

NPC membranes, the diffusion effect on selectivity must also be considered. Based on theobservations of Arora and Sandler [111] for O 2 /N 2 selectivity, it can be suggested that theselectivity based on the diffusion of CO 2 and N 2 provided that correct pore size is present willbe larger due to the even greater difference in molecule size. As such it is expected that theCO 2 /N 2 selectivity of NPC membranes would be much larger than that for O 2 /N 2 because ofthe greater difference in both the adsorption and molecule size.Molecular simulations of carbon nanotubes have been undertaken by various researchers[109, 114-122] and considering that nanoporous carbon and nanotubes have a similarstructure, these simulation results are also of interest. Arora et al [118] have used GCMC andmolecular dynamic simulations to analyse the adsorptive and diffusive behaviour of N 2 insingle wall carbon nanotubes of varying diameter. It was found that as the diameter of thenanotube was decreased from 15.7 to 8.6 Å, the adsorption of N 2 was increased. Similarly, theself-diffusivity of N 2 increased with decreasing nanotube diameter due to the localisation ofthe N 2 molecules in the core of the nanotube rather than in bilayers as is seen with nanotubeswith higher diameters. However, when considering the adsorption of a N 2 /O 2 (air) mixture oncarbon nanotubes, it was discovered through simulation [109, 117] that O 2 adsorptiondisplaces N 2 with increasing pressure because of O 2 being a smaller molecule and being ableto reach the smaller cavities.In a recent study, Skoulidas et al [121] have used molecular modelling to understand theadsorption and diffusion of CO 2 and N 2 in carbon nanotubes at room temperature as afunction of nanotube diameter. Whilst the authors have not given separation factors in theirpublication, they have observed that the diffusion mechanism is not Knudsen and shouldresult in a large CO 2 /N 2 selectivity.In summary, molecular simulations of nanoporous carbons reveal theoretically highadsorptive selectivity due to the curved structure of the carbon compared with planar graphitealong with high theoretical diffusive selectivity provided the appropriate sized porerestrictions are present to allow separation via molecular sieving. These results are thus verypromising for considering nanoporous carbons as a new material for gas separation.42

2.6 High Temperature Operation of NPC MembranesGiven the high temperatures at which NPC membranes are manufactured, they are ideal foroperation above ambient temperature and indeed up to the original pyrolysis temperature.High temperature operation is advantageous for CO 2 separation from natural gas or synthesisgas production as it eliminates the need for pre-cooling, which is required when usingpolymeric membranes. Therefore, understanding how high temperature operation affects theperformance of NPC membranes is pertinent.2.6.1 Arrhenius RelationshipsThe relationship between permeance (P’) and operating temperature (T) of an NPC membranecan be described by an Arrhenius-type relationship as given in Equation 2-34 [22].⎡− ( Ep) ⎤P' = P'0exp⎢⎥Equation 2-34⎣ RT ⎦Rearranging Equation 2-34 to a linear form results in Equation 2-35 shown below.− Ep 1Ln( P')= + Ln(P'0)Equation 2-35R TPlotting Ln (P’) against 1/T reveals a straight line from which the permeance activationenergy (E p ) can be calculated from the gradient. Examples of activation energies calculatedfor NPC Membranes made from polyamic acid [26] are presented in Table 2.7. Relating thisback to the experimental data, as the operating temperature increases, the permeance ofmethane will increase at the faster rate than N 2 , O 2 or CO 2 as it has the highest value of E p .Table 2-7 : Activation energy of permeance of various gas molecules for NPCmembranes made from polyamic acid. The activation energy for CO 2 is 0 indicating thatthe CO 2 permeance will remain constant with increasing temperature.Activation Energy of Permeance (E p ) (kJ/mol)NPC MembraneMade fromCO 2 (0.33 nm) O 2 (0.346 nm) N 2 (0.364 nm) CH 4 (0.38 nm)Polyamic Acid [26] 0 3.1 9.8 14.5Looking now in more detail at the effect of temperature on the transport mechanisms, thepermeance of a gas (A) via surface diffusion (P’ S ) or molecular sieving (P’ M ) are bothactivated by temperatures as presented below in Equations 2-36 and 2-37 [8, 20, 23]. The43

diffusion coefficient of surface diffusion is represented by D 0S and the diffusion coefficient ofmolecular sieving is represented by D 0M .D0S( A)⎡−( ES( A)− ∆Hads(A)) ⎤P's( A)= exp⎢⎥Equation 2-36RT∆x⎣ RT ⎦P'MD0M( A)⎡−( ED(A)− ∆Hads(A)) ⎤( A)= exp⎢⎥Equation 2-37RT∆x⎣ RT ⎦In the case of surface diffusion, whether the permeance increases or decreases withtemperature depends on the relative values of the activation energy for diffusion (E S(A) ) andadsorption (∆H ads ). If ∆H ads > E S(A) , then the permeance will decrease with increasingtemperature as is expected for strongly adsorbing gases. If ∆H ads < E S(A) then diffusion isdominant and the permeance will increase with increasing temperature.In the case of molecular sieving, it is assumed that adsorption is a very small contributor andas such E D(A) >>∆H ads resulting in an increase in permeance with increasing temperature.To simplify the situation when the relative contributions of molecular sieving and surfacediffusion are unknown, Suda and Haraya [22] developed a relationship for the permeanceactivation energy using the activation energy of diffusion (E d ) to approximate molecularsieving and the heat of adsorption (∆H ads ) to approximate surface diffusion (Equation 2-38).Ep= E − ∆HEquation 2-38dadsPublished values for the activation energies of diffusion (E d ) and heats of adsorption (∆H ads )for NPC membranes made from PFA and Cellulose are presented in Table 2-8.Table 2-8 : Activation energies of diffusion and heats of adsorption of various gasmolecules for NPC membranes made from PFA and Cellulose.Activation Energy of Diffusion (E d ) (kJ/mol)NPC MembraneMade fromCO 2 (0.33 nm) O 2 (0.346 nm) N 2 (0.364 nm) CH 4 (0.38 nm)PFA [43] 9.2 17.3 24.7 Data N/ACellulose [12] 23.4 21.6 26.5 Data N/AHeat of Adsorption (∆H ads ) (kJ/mol)NPC MembraneMade fromCO 2 (0.33 nm) O 2 (0.346 nm) N 2 (0.364 nm) CH 4 (0.38 nm)PFA [43] 10.6 9.3 10.2 Data N/ACellulose [12] 29.6 20.8 10.5 Data N/A44

The activation energies for diffusion are positive implying that diffusion increases withincreasing temperature, whereas the activation energies for adsorption are >0 implying thatadsorption increases with decreasing temperature as per Equation 2-38. For both the NPCmembranes made from PFA and Cellulose, the activation energy of diffusion increases withincreasing molecular size suggesting molecular sieving. Additionally, the heat of adsorption isgreatest for the most polar molecule, CO 2 , suggesting surface diffusion also occurs.The expected activation for permeance can then be obtained from adding the two activationenergies of diffusion and adsorption as shown in Table 2-9. Like the data presented earlier inTable 2-7, the permeance activation energy for CO 2 is negative, which indicates that the valueof the CO 2 permeance will decrease with increasing temperature. This is due to the increase indiffusion being smaller than the decrease in adsorption with increasing temperature.Table 2-9 : Activation energy of permeance of various gas molecules for NPCmembranes made from PFA and Cellulose obtained by the addition of the diffusion andactivation energies presented in Table 2-8. Interestingly, for CO 2 the activation energyof permeance is negative because adsorption is such a large contributor to thepermeance.Activation Energy of Permeance (E p = E d + ∆H ads ) (kJ/mol)NPC MembraneMade fromCO 2 (0.33 nm) O 2 (0.346 nm) N 2 (0.364 nm) CH 4 (0.38 nm)PFA [43] -1.4 8 14.5 Data N/ACellulose [12] -6.2 0.8 16.0 Data N/AStrano and Foley [24] argue that at low operating temperatures adsorption dominates while athigh temperatures gas diffusion dominates. It is suggested that as the temperature increases,gas diffusion will approach Knudsen diffusion and very little separation will occur. Gilronand Soffer [8] also suggest that at higher operating temperatures the permeability andselectivity observed is caused by Knudsen diffusion despite the average pore size of the NPCmembranes being within the range for molecular sieving (less than 5 Å). Knudsen diffusiondoes not follow an Arrhenius relationship rather the permeance is related to operatingtemperature through a square root relationship as presented earlier in Equation 2-17. In thiscase, permeance increases as temperature increases.In another experimental and modelling study, Sedigh et al [25] have examined the effect ofoperating temperature on tubular NPC membranes made from PFA using mixed gas feeds.For a binary feed mixture of CO 2 /CH 4 , the selectivity decreases from 33 at 22°C to 5 at140°C. This reduction in selectivity is concluded to be due to the increase in CH 4 diffusion45

and therefore CH 4 permeance and the simultaneous decrease in CO 2 permeance due to theloss of CO 2 adsorption. Molecular simulations of the adsorption of a binary gas mixture inslit-like carbon nanopores have shown that CO 2 is adsorbed more strongly than CH 4 onto thepore wall and that adsorption decreases with increasing temperature.Fuertes and Centeno have looked specifically at the transport of gases through NPCmembranes produced from the pyrolysis of phenolic resin [15] and polyamic acid [26].Performance measurements were taken for both membranes at temperatures up to 150 °C andit was found that the permeance of CH 4 increases with temperature suggesting activated gasdiffusion. However, the permeance of CO 2 does not change with temperature. It is againsuggested that this is because of the adsorption of CO 2 occurring in the micropores. As thetemperature increases, the decrease in the adsorption of CO 2 is compensated by the increaseof gas diffusion of CO 2 resulting in no net change in the permeance.In a related study, Fuertes [67] has reviewed the separation of a non-adsorbable component(N 2 ) from a strongly adsorbable component (n-Butane) using NPC membranes also madefrom the pyrolysis of phenolic resin. For the n-Butane/N 2 separation, increasing thetemperature from 25 to 150°C reduces the selectivity from 65 to 5 because of the highcontribution of adsorption to the permeance at the lower temperatures of the more adsorbablecomponent. In the same paper, Fuertes also reviews the effect of operating temperature on theseparation of the two non-adsorbable components, N 2 /CH 4 , and shows that for the sametemperature range, the selectivity reduces from 3 to 2. It is concluded that aside frommolecular sieving by size, the relative adsorption of components onto NPC membranes is asignificant contributor to selectivity at low operating temperature whilst not at high operatingtemperatures.46

2.7 Effect of Impurities on the Performance NPC MembranesFor the commercial applications of NPC membranes being considered as part of this researchwork additional minor components are also present in the feed stream.In the case of CO 2 removal from natural gas, saturated amounts of water, heavier andaromatic hydrocarbons along with trace amounts of hydrogen sulphide (H 2 S) are present.In the case of CO 2 removal from air-blown synthesis gas for hydrogen or electricityproduction, trace amounts of H 2 S and water are present along with significant quantities ofcarbon monoxide (CO). Refer to the process details presented earlier in Table 1-1 Section 1.3.Assessing the robustness of NPC membranes in the presence of these impurities is critical inestablishing the suitability of NPC membranes for the aforementioned commercialapplications. For the purpose of reviewing the available literature, the “impurities” have beengrouped as (a) water, (b) condensable impurities (heavier and aromatic hydrocarbons) and (c)non-condensable impurities (H 2 S and CO).An additional discussion on the published effects of exposing the NPC membranes to air isalso presented after the discussion on water. Whilst no air or oxygen is present in applicationsinvestigated in this research, the effect of air exposure on the performance of NPCmembranes is relevant to the storage of the membranes.2.7.1 WaterWater is in equilibrium with natural gas when produced from a reservoir. Similarly, waterwill be present in synthesis gas production because of the water gas shift reaction.Due to the problems if operating equipment (such as polymeric membranes, compressors andtransmission pipelines) with a humid feed, gas streams are dehydrated using establishedtechnologies such as glycol dehydration or a solid desiccant [123]. If the water tolerance ofthe equipment (such as membranes) can be increased by using nanoporous carbon instead ofpolyimide, this will decrease the amount of pre-dehydration required and therefore the cost.To specifically address the impact of water vapour on the performance of NPC membranes,Jones and Koros [31] have completed a comprehensive study using humidified feeds. Hollowfibre polyimide membranes made at 500 to 550°C were tested using a mixed oxygen andnitrogen feed at relative humidities of 23, 44 and 85 %. The flux through the membrane wasdecreased dramatically by 50, 60 then 70 % with increases in the relatively humidity.Interestingly, the impact on the O 2 /N 2 selectivity was negligible at relative humidities of 2347

and 44 % but decreased by 20 % in the case of 85 % humidity. The authors suggest that theinitial phase of water sorption is rapid upon the surface and is then followed by a then slowerpore filling. Gawrys et al [124] also concluded that at low relative humidities water is weaklyadsorbed to the polar sites on carbon molecular sieves whilst at higher relative humiditiesmicropore filling will prevail resulting in a greater uptake of water.Both Groszek [125] and Avraham et al [126] have investigated the impact of gas streamcomposition on the adsorption of water onto carbon molecular sieves. From gas adsorptionexperiments, Groszek [125] concludes that water adsorption is much higher from ahumidified methane stream compared with a humidified nitrogen stream due to the formationof methane-hydrates which occur in the micropores. Avraham et al [126] have studiedoxygen/nitrogen adsorption in the presence of water. At low relative humidities, theadsorption kinetics for oxygen and nitrogen are faster than water. As the humidity isincreased, water fills the pores displacing first the oxygen and then the nitrogen leading to asignificant reduction in the adsorption of these two gases.In a more recent study upon exposure to a 32.5 % relative humidity feed, Lagorsse et al [127]report a 50 % reduction in the permeance of NPC membranes at 29°C. To further understandthis result, the authors have obtained fractional uptake curves for water, N 2 and CO 2 on themembrane at 29 °C. The uptake curves show that whilst the carbon has a slower uptake ofwater than CO 2 but not N 2 , the total amount of water absorbed is greater than CO 2 and N 2 .Interestingly, it is concluded that effect of humidity exposure should be viewed as a stronglyadsorbing component diffusing slowly through the carbon rather than a pore-blockingmechanism as such. This result carries on from an earlier publication where Lagorsse et al[128] conducted a detailed study into the nature of the adsorption isotherms of water in NPCmembranes. In this study, the authors developed a model which concludes that water adsorbsinitially on the hydrophilic oxygen functional groups, which allows a cluster of watermolecules to grow via hydrogen bonding. When the number of water molecules in the clusterreaches a critical size and has enough dispersion energy to be released it is adsorbed directlyonto the carbon surface.Jones and Koros [31] suggest that a reduction in water adsorption onto a NPC membranes canbe achieved by changing the feed flow configuration of the membrane such that the feed isfirstly exposed to the more porous support. By doing this a hydrophobic guard layer isproduced in the porous support resulting in a less dramatic effect on membrane performance.In a follow-up publication, Jones and Koros [129] have further improved the stability of NPCmembranes in the presence of water by coating the finished membrane with a very thin layer48

of unique polymeric materials which are hydrophobic but do not impact upon the permeanceof other molecules. The unique polymeric materials were DuPont’s Teflon products AF1600and AF2400 (perfluro-2,2,-dimethyl-1,3-dioxole and tetrafluoroethylene, respectively).2.7.2 AirIn the late 1960s and early 1970s, Mattson et al published a series of papers exploring thesurface chemistry of carbon materials in the presence of air at room temperature [130-132].The authors use an infrared spectrophotometer to detect the presence of functional groups onactivated carbons that have been exposed to air and conclude that carbonyl (C=O) functionalgroups are present on the surface. The hydrophilic nature of these carbonyl groups providessites for water adsorption, which blocks the pores of the carbon as discussed previously inSection 2.7.1. More recently, Petit and Bahaddi [133] demonstrated that the uptake of water isgreater on oxidised activated carbon than on non-oxidised carbon confirming the importanceof oxygen in creating a hydrophilic surface.At higher temperatures (600°C), carbon will rapidly oxidise to carbon dioxide or carbonmonoxide resulting in complete disintegration of the carbon [134]. However, as discussedearlier in Section 2.2.3.2, carefully controlled oxidation can be used to open the pores ofnanoporous carbon membranes. Hayashi et al [135] have oxidised NPC membranes madefrom polyimide at 700 °C in an oxygen-nitrogen stream at 300 °C and found that the gaspermeance was increased for a range of gas molecules due to the widening of the pore sizedistribution. In the same paper, Hayashi et al [135] have also tested the stability of CMSmembranes when exposed to air at 100 °C over a period of 30 days. Over the period of 30days, the membrane lost a total of ~ 17 % of the initial weight due to the loss of carbon fromthe formation of the carbonyl groups. The permeance of the exposed membrane decreaseddramatically with time due to the adsorption of water but was largely recovered by a post-heattreatment using nitrogen at 600 °C for 4 hours.To specifically review the impact of the storage environment upon the aging of themembrane, Menendez and Fuertes [32] have tested the performance of NPC membranes madefrom phenolic resin at 700°C stored under air, nitrogen and propylene over a period of time.Upon storage under air, the permeance of various gases through the membrane drops by 60 %over a period of one week. With storage under nitrogen, the drop in permeance is limited toless than 20 % after the same period of time, whereas under propylene storage, the permeanceis actually increased by up 100 %. The authors suggest that the increase in permeance of thesamples stored in propylene is possibly related to the expansion of the pores due tohydrocarbon exposure. The effect on permselectivity is the reverse of permeance. In the case49

of air and nitrogen storage, permselectivity is slightly increased whilst for propylene storagepermselectivity is decreased by ~ 50 % for the CO 2 /CH 4 and CO 2 /N 2 pair over a period of oneweek.Furthermore, Menendez and Fuertes [32] have also studied the impact of humidity on agingby storing the NPC membranes under normal laboratory air, 100 % humidity air and dry air.The permeance of the NPC membranes stored under lab air and dry air decreased in a similarmanner to around 10 % of the original permeance after 3 months. Interesting, for themembranes stored in 100 % humid air, the permeance was only decreased to 80 % of theoriginal performance. The reasoning for a less dramatic decrease in the performance for themembranes stored under humidity is the formation of a protective surface layer of watermolecules which prevents deeper oxygen attack. Regeneration of membranes at 120°C undervacuum for 3 hours resulted in a partially recovery of the original performance.In the most recent study, Lagorsse et al [127] conducted a detailed experimental investigationinto the effect of air exposure on NPC membranes. The membranes were initially exposed toair for several days, then pure nitrogen for 3 months, air for another 40 days, propylene forseveral days and then finally pure oxygen for another 38 days. The rate of reduction inpermeance was greatest during the first exposure to oxygen and air. The final exposure tooxygen resulted in a much smaller reduction in permeance due to an apparent stabilisation ofthe carbon surface. The authors believe that the reaction of oxygen with carbon forms oxygensurface groups, which are subsequently gasified to CO 2 and CO. This explains the 4 % weightreduction of the NPC membrane when exposed to oxygen and air for 30 days.Exposure to nitrogen and propylene resulted in very low reductions in permeance and in factthe CO 2 permeance increased upon exposure to propylene [127].This increase in CO 2permeance observed after exposure to propylene concurs with the findings of Menendez andFuertes [32] suggesting that propylene could be used as a regeneration agent. However, theimpact of selectivity is unclear. Menendez and Fuertes [32] observed a significant decrease inthe CO 2 /N 2 selectivity whilst Lagorsse et al [127] observed a slight increase in the CO 2 /N 2selectivity.Interestingly, Lagorsse et al [127] employ a rather extreme regeneration technique to restorethe membrane performance after exposure to oxygen or air. The technique involves heattreatingthe membrane at 620°C in a reducing atmosphere (H 2 ) to remove any oxygen surfacegroups. The amount of oxygen present in the NPC membrane was reduced from 4.2 to 3.3 %.The fractional uptakes of N 2 and CO 2 were also improved after the heat treatment.50

2.7.3 Condensable ImpuritiesCondensable impurities (heavier and aromatic hydrocarbons) are an omnipresent part of thenatural gas processing system. Even reservoirs which are largely comprised of methane willhave some heavier and aromatic hydrocarbons (usually termed “gas condensate”). Themajority of the gas condensate will drop-out in one or a series of upstream pressure separationvessels but there will invariably be some small carry-over into the gas stream. As such, gasprocessing facilities using the traditional polymeric membranes are equipped with expensivepre-treatment equipment to remove these condensable impurities.With NPC membranes being touted as more robust than polymeric membranes in thepresence of impurities, there is potential for a reduction in the pre-treatment required andtherefore cost. At the research level, hexane (C 6 H 14 ) or heptane (C 7 H 16 ) is commonly used tosimulate the impact of heavier hydrocarbons whilst toluene (C 7 H 8 ) is used to simulate theimpact of aromatic hydrocarbons.Tanihara et al [103] present an interesting comparison between the effect of toluene exposureon polymeric polyimide membranes compared with NPC membranes also made frompolyimide. Using a feed gas comprising H 2 /CH 4 (50:50 vol%), the authors demonstrate thatwith increasing Toluene concentration the permeance and selectivity fall significantly for thepolymeric membranes but not for the NPC membranes. It is suggested that the significanteffect on the polymeric membranes is due to toluene adsorbing in the Langmuir domainsalong with irreversible compaction of the membrane cause by plasticisation. The resistance ofthe NPC membranes to plasticisation is considered to prevent the adverse effect onperformance in the presence of toluene. Regeneration of the membranes using dry feed gas at50 °C resulted in a 50 % recovery for the polymeric membranes and a complete recovery forthe NPC membranes.Conversely, Jones and Koros [34] found a dramatic decrease in O 2 /N 2 separation performanceof NPC membranes made from polyimide when exposed to trace amounts of hexane andtoluene. It was also observed that with exposure to trace amounts of even higherhydrocarbons (up to C 16 ), the performance of the membranes drops by up to 99%. On theother hand, exposing the NPC membranes to lighter hydrocarbons such as propane andpropylene resulted in a much smaller drop in performance when compared with hexane,toluene and the higher hydrocarbons. A novel regeneration technique using pure propylene at1000 kPag for 2 to 3 hours was developed resulting in complete recovery of the membraneperformance after hydrocarbon exposure.51

In a later work, Vu et al [35] have reviewed the effect of condensable impurities on the mixedgas CO 2 /CH 4 separation performance of NPC membranes made from polyimide. In thepresence of either heptane or toluene, there is a considerable initial drop in the permeancewhich then reaches equilibrium after approximately 1 hour. Interestingly, there is no changein the CO 2 /CH 4 selectivity presumably because the CO 2 and CH 4 permeances have reduced ina similar manner. In the instance of toluene exposure, increasing the concentration from 70.4to 295 ppm resulted in a similar % reduction in performance. In the instance of hexaneexposure, increasing the concentration from 100 to 302 ppm actually resulted in a reducedeffect on the membrane performance. No explanation is given for this result. Completerecovery of the permeance and selectivity is obtained from regeneration at 90°C in flowingnitrogen at 300 kPag for at least 12 hours suggesting that the impurities are only physicallyadsorbed to the nanoporous carbon.In a similar paper, Vu et al [136] have exposed mixed matrix membranes comprising carbonmolecular sieve particles within a polyimide matrix (Matrimid) to a low concentration (70ppm) of toluene in a CO 2 /CH 4 mixed feed gas. A membrane made from pure Matrimid wasalso exposed to 70 ppm of toluene for direct comparison. Over a period of 60 hours, there isno change in the CO 2 permeability for the mixed matrix membrane and only a slight decreasein the CO 2 permeability for the pure polymer membrane. Selectivity is fairly stable in bothcases. The authors postulate that the toluene may only occupy the larger non-selective poresof the mixed matrix membrane and as such the effect on the performance is minimal. Thismay well be true for very small concentrations of toluene, as used in this work.Zeolite membranes, like NPC membranes, have pores sizes < 1 nm and generally follow amolecular sieving mechanism. Li et al [137] have exposed SAPO-34 zeolite membranes toethylene (C 2 H 4 ), propane (C 3 H 8 ) and butane (C 4 H 10 ) to review the impact on performance ofcondensable impurities in a CO 2 /CH 4 mixed gas feed. With a 1 mol% hydrocarbon impurityin the feed, the CO 2 permeance and CO 2 /CH 4 selectivity were reduced with the greatestreduction in performance being from exposure to the heaviest hydrocarbon (butane in thiscase). This is explained by the increasing heat of adsorption of hydrocarbons on zeolite withincreasing carbon number. Whilst increasing the partial pressure or concentration of thehydrocarbon in the feed also increases the reduction in performance, the performance is stableif the partial pressure of the feed gases is increased in the same proportion. Regeneration ofthe zeolite membranes was achieved by exposing the membrane to feed gas containing noimpurities or by calcination.In summary, condensable impurities do seem to impact upon the performance of NPCmembranes by blocking the pores and reducing permeance and selectivity. This effect is more52

pronounced as the concentration or molecular weight of the condensable impurity increases.However, it appears that adsorption of the impurity onto the surface is physical and as suchthe NPC membrane can be regenerated using fairly benign techniques.2.7.4 Non-Condensable ImpuritiesNon-condensable impurities are impurities that remain in a gaseous form upon contact withthe NPC membrane and may impact on the performance of the membrane by physicaladsorption and pore blocking or by chemically reacting with the carbon.In the case of CO 2 removal from natural gas, hydrogen sulphide (H 2 S) is present due tosulphide producing bacteria present in the reservoir. In the case of CO 2 removal fromsynthesis gas, H 2 S is present again from the sulphur producing bacteria present in the originalhydrocarbon source (such as coal). Another impurity present in synthesis gas, which is knownfor poisoning catalyst materials due to its reactivity, is carbon monoxide (CO) produced bythe water-gas shift reaction used to make the synthesis gas (H 2 ).It is generally necessary to remove H 2 S from natural gas to a very low level to meet the salesgas specification. This is done using a chemical scavenger or molecular sieve if theconcentration of H 2 S is low. If the concentration of H 2 S in the natural gas is high then largerequipment for removing H 2 S must be employed. The most commonly used de-sulphurisationprocess is the Claus Process, which oxidises H 2 S to produce elemental sulphur (S) [123]. Theequipment required for the Claus process is extensive due to the high temperatures andcatalysts necessary to obtain a good yield.The traditional polymeric membranes are susceptible to plasticisation upon prolongedexposure to H 2 S [138]. It has also been shown that H 2 S the reduces the performance of Pd/Cumembranes, which are traditionally used in synthesis gas purification, due to the formation ofsurface sulphides that block H 2 adsorption sites [139]. If NPC membranes could be shown tobe resistant to H 2 S whilst also removing some H 2 S into the CO 2 stream for sequestration,savings could be made in terms of the H 2 S removal systems required.Whilst it sometimes necessary to remove the CO present in synthesis gas as it can poisoncatalysts, it is often desired to retain the CO in the synthesis gas if it is being used to makesynthetic fuels (Fisher-Tropsh Process) as shown in the reaction below [140].~ 400 °C (2n+1)H 2 + nCO → C n H (2n+2) + nH 2 OStudies of polymeric membranes, in particular polyimide, show that CO is retained on theretentate side as it has a larger kinetic diameter and lower solubility than CO 2 [138].53

Whilst there is little known specifically of the effect of H 2 S and CO on the performance ofNPC membranes, there is considerable literature available on the adsorption of these gases onactivated carbon. This is a good beginning for understanding how these non-condensableimpurities may affect the performance of NPC membranes.Ritter and Yang [50] have obtained adsorption isotherms of CO and H 2 S along with H 2 , CO 2and CH 4 on activated carbon at room temperature. For single gas experiments, the volume ofgas adsorbed is as follows; H 2 S>CO 2 >CH 4 >CO>H 2 , with all gases following the Langmuirmodel of adsorption. For the dual gas experiments, CO 2 adsorbed more than CO, H 2 Sadsorbed more than CO 2 and CO 2 adsorbed more than CH 4 . An extended Langmuir Equationincorporating interaction parameters was used to model the adsorption of mixed gas systems.In a later article, Yan et al [141] have studied the mechanism of the adsorption of H 2 S ontoactivated carbon. Using kinetic adsorption and characterisation data, it was determined thatH 2 S both chemically and physically adsorbs onto the carbon surface. The presence ofpotassium hydroxide (KOH) in the activated carbon reacts with the H 2 S to produce potassiumhydrosulphide (KHS) or potassium sulphide (K 2 S).Additionally and more importantly when relating Yan et al’s [141] results to NPCmembranes, it was found that H 2 S reacts with adsorbed oxygen upon the activated carbon toform elemental sulphur, which in the presence of water can form sulphuric acid. In fact, theauthors go on to comment that activated carbons can deteriorate through the formation ofsulphuric acid.Activated carbons impregnated with metal salts are often used for the removal of CO. Tamonet al [142] have impregnated activated carbon with palladium chloride (PdCl 2 ) and copperchloride (CuCl) and found that this increases the adsorption of CO onto the activated carbonsurface by over an order of magnitude. Taking a different approach and using a metal oxide,Mohamad et al [143] have impregnated activated carbon with tin oxide (SnO 2 ) and found anincrease in the adsorption capacity due to CO reacting with the SnO 2 to produce CO 2 .Assuming that NPC membranes will exhibit the same adsorptive behaviour as activatedcarbon, it is anticipated that based on the adsorptive component of permeance H 2 S shouldtravel through the membrane with the CO 2 , whilst the CO should be retained on the retentate.Additionally, if we consider the kinetic diameters of the gas molecules (Table 2-1) in relationto a molecular sieving mechanism, H 2 S should travel through with the CO 2 , whilst CO maybe retained on the retentate side.54

2.8 Scope of this Research WorkBased upon a promising lab-scale performance, there has been significant research into theproduction of nanoporous carbon membranes over the last decade.Methods to produce NPC membranes from the pyrolysis of various polymers are welldocumented although the problems with cracking and aging of the membrane surface duringpyrolysis are still to be resolved.The published performance of NPC membranes is frequently measured at ideal conditions,i.e. with a pure gas and at room temperature. The performance of NPC membranes in acommercial setting, i.e. with a mixed gas feed, at elevated temperatures and in the presence ofimpurities, is not as well understood.The focus of this research work is to develop a better understanding of the performance ofNPC membranes in commercial applications and as such assess the suitability for theseparation of CO 2 from natural gas and synthesis gas (pre-combustion capture). Specifically,the aims of the research work are therefore to obtain a better understanding of1. Avoiding cracking in NPC membrane manufacture (Chapter 4),2. Mixed feed gas performance of NPC membranes (Chapter 4),3. High temperature operation of NPC membranes (Chapter 4),4. The transport mechanisms of gases through NPC membranes at high operatingtemperatures (Chapter 5),5. The impact of impurities present in natural gas and synthesis gas on the performanceof NPC membranes (Chapter 6).55

3 EXPERIMENTAL MATERIALS, APPARATUS AND METHODS3.1 IntroductionThe reagents, materials and methods used to manufacture supported, modified supported andunsupported NPC membranes are presented in this chapter. The techniques employed tophysically characterise the NPC membranes are then described.Following on from this, the experimental equipment used to measure the permeance andselectivity of the manufactured membranes is outlined. Finally, the equipment and methodsused for the measurement of gas adsorption onto the NPC membranes both experimentallyand theoretically via molecular simulation are presented.3.2 Membrane Materials, Manufacturing Equipment and Techniques3.2.1 MaterialsLists of the materials used to manufacture supported, modified supported and unsupportedNPC membranes are presented in Tables 3.1, 3.2 and 3.3, respectively. Please note that thematerials used to make modified supported NPC membranes presented in Table 3.2 are inaddition to those presented in Table 3-1.Table 3-1 : Materials used to manufacture supported NPC membranes.Material Use Supplier PurityPolymer, which uponPolyfurfuryl Alcohol100 %pyrolysis becomes Polysciences, USA(PFA)(Note 1)nanoporous carbonSolvent to makeAcetonepolymer precursorsolutionMerck, Germany 99.8 %Porous Stainless SteelDisks (44 mm Diameter,1 mm thick)ChloroformNitrogenSupportCleaning agent forsupport disksSpray nozzle andspin-coater purge gasMott MetallurgicalCompany, USAMerck, Germany 99.8 %BOC, AustraliaArgon Pyrolysis purge gas BOC, AustraliaHigh Purity99.99%Ultra High Purity99.999%(Note 2)57

Note 1 – Whilst the purity of the PFA from the supplier was documented as 100%, themolecular weight varied. The PFA received prior to August 2007 had a high molecularweight, which gave a good conversion to nanoporous carbon upon pyrolysis. All of theperformance data presented in Chapter 4 and in the presence of condensable impurities weremanufactured from this lot of PFA. Unfortunately, manufacturing difficulties experienced bythe supplier led to cessation of supply and then a replacement lower molecular weightpolymer in January 2008. Numerous attempts were made to manufacture NPC membranesfrom this polymer but were unfortunately unsuccessful as the polymer could not be spincoatedadequately and also did provide a good conversion to nanoporous carbon. In July2008, the supplier provided a higher molecular weight polymer, similar to the originalpolymer used. NPC membranes capable of gas separation were made out of this polymer.These membranes were used for testing with non-condensable impurities. The quality of thePFA has a significant impact on the ability to manufacture NPC membranes as was alsodiscovered by Sedigh et al [25].Note 2 – It was necessary to use ultra high purity Argon to limit the oxygen content duringpyrolysis. Using Argon with a lower purity (e.g. high purity 99.99%) resulted in very poorconversion of the polymer to nanoporous carbon.Table 3-2 : Extra materials used to manufacture modified supported NPC membranes.Material Use Supplier PurityTetraethylorthosilicate Stoeber synthesis of(TEOS)silica particlesFluka 99.5 %Ethanol Stoeber synthesis Chem-Supply 99.5 %Ammonia Stoeber synthesis Merck 25 %Table 3-3 : Materials used to manufacture unsupported NPC membranes.Material Use Supplier PurityMatrimid Polyimide Polymer, which uponVantico,Pellets (structure shown pyrolysis becomes100 %Switzerlandin Figure 3-1) nanoporous carbonDichloromethaneKapton PolyimideFlatsheet 100 HN and200 HN (structureshown in Figure 3-2)Solvent to makeMatrimid precursorsolutionPolymer, which uponpyrolysis becomesnanoporous carbonAjax Finechem,Australia99.8 %Dupont, USA 100 %Ultra High Purity Argon Pyrolysis purge gas BOC, Australia 99.999%58

OOONCCCCCNOOCH 3CH 3H 3 CnFigure 3-1 : Chemical structure of Matrimid polyimide (Vantico, Switzerland)OOCCNCCNOOOnFigure 3-2 : Chemical structure of Kapton polyimide (Dupont, USA)3.2.2 Manufacturing Equipment and Techniques3.2.2.1 Supported NPC MembranesThe polyfurfuryl alcohol (PFA) polymer was diluted in acetone to reduce the viscosity.Different concentrations of PFA in acetone were trialled but ultimately a solution with 40wt% PFA in acetone gave a solution viscosity similar to water and was thus ideal for coating.Porous stainless steel disks with a pore size of 0.2 micron were used for support. Prior tocoating, the disks were sonicated in chloroform for 15 mins to remove any grease, washedtwice with distilled water and then dried in the oven at 120 °C overnight.The PFA solution was coated onto the porous support using a spin coater (LaurellTechnologies, USA) and a pneumatic spray nozzle (Spraying Systems Co. Pty Ltd, Australia).The set up of the coating equipment is presented in Figure 3-3. The porous support wasplaced centrally onto the spinning chuck and secured by a vacuum line (V4) and seal purgegas (V3). Using an inbuilt controller, the support was then accelerated to a constant spinningspeed of 1000 rpm.The PFA solution was pipetted into the measuring cylinder. The solution was then atomisedand sprayed onto the spinning porous support by simultaneously opening valves V1 and V2.The amount of PFA solution sprayed onto the support was estimated using the graduations onthe measuring cylinder before and after coating. For the initial coat of the PFA solution, 3-459

ml was sprayed and for the subsequent coats (after carbonisation), 1 -2 ml of solution wassprayed.The pressure of the atomising gas was controlled at a pressure of less than 40 kPag to ensure asteady and gradual flow of the PFA solution through the nozzle. The distance between thespray nozzle and the spinning porous support was optimised at 6 cm as this distanceminimised the overall loss of PFA solution whilst still giving an even coating.(a)(b)PFA SolutionMeasuringCylinderSpray NozzleAtomising GasPGV1V2N 2PGSeal Purge GasV3Porous SupportSpinning ChuckV4Vacuum PumpN 2Figure 3-3 : (a) Photograph and (b) schematic diagram of the spin coating apparatus.The porous support is held onto a spinning chuck using a vacuum pump and a sealpurge gas. The PFA solution is sprayed onto the spinning porous support using apneumatic spray nozzle.60

Immediately after coating, the support and polymer were pyrolysed in a quartz tube furnace(Ceramic Engineering, Australia) (refer to Figure 3-4) at a temperature of between 400 and600°C for 2 hours. The temperature was ramped to the final pyrolysis temperature at 5°C/minwith a pause at 100°C for 60 mins to allow evaporation of the acetone solvent. A typicaltemperature profile of the furnace with time is shown in Figure 3-5.An ultra high purity argon flow of 200 ml/min was maintained at all times until the furnacehad returned to room temperature. The argon flow rate of 200 ml/min was chosen based uponexperience of other researchers presented in the literature. It is generally agreed that thehigher the inert gas rate, the more permeable the membrane due to faster sweeping of theliberated gases upon pyrolysis [90]. Additionally, argon was chosen in this case but otherinert gases such as helium can be used provided the oxygen content is limited to less than 3ppm [90, 144].(a)(b)Quartz TubeInsulated FurnaceFlow MeterExhaustPGCoated PorousSupportSupport HolderPurge GasArFigure 3-4 : (a) Photograph and (b) schematic diagram of the quartz tube furnace with acontinuous purge of ultra high purity argon.61

Furnace Temperature (°C)800Oven HeatingOven Off (Natural Cooling)7002 hours @ final pyrolysis temperature6005004003002001001 hour @ 100°C to remove solvent00 100 200 300 400 500 600 700 800 900 1000Time (min)Figure 3-5 : Quartz tube furnace typical temperature profile with time during pyrolysisof supported NPC membranes.The coating and pyrolysis process was repeated up to four times as was necessary to form acomplete membrane layer. The presence of a complete membrane layer was establishedvisually from coverage of continuous shiny black surface on the porous support and viaperformance testing to determine the permeance and selectivity.The membranes were weighed before and after coating as well as before and after pyrolysis.Additionally, the membranes were stored in a dessicator filled with ultra high purity argon tolimit the chemisorption of oxygen and physisorption of water, which reduce the performanceof the membrane as referred to in Section 2.7.2.3.2.2.1.1 Repeatability in Supported NPC Membrane ManufactureObtaining a smooth coating of the polymer solution was paramount. Any bubbles in thesolution or an uneven coating led to defeats in the carbon surface. Bubbles or an unevencoating occurred if the spray nozzle became partially blocked and did not spray evenly.Additionally, the change in the purity and molecular weight of the PFA by the polymersupplier (in January 2008) greatly affected the viscosity, the dilution required for coating andalso the ability of the solution to wet the stainless steel support as discussed earlier in Note 1to Table 3-1.Additionally, to obtain a crack-free carbon surface during pyrolysis, it was necessary toensure that the quartz tube furnace was completely flat to prevent any pooling of polymer onone side of the support and also that the furnace was free of loose particles. It was alsonecessary to ensure that all equipment related to weighing the carbon membranes before andafter pyrolysis was free of any loose particles.62

3.2.2.1.2 Manufacturing Method Development Enabling Support Pore OpeningDuring the initial stages of the research work, it was noticed that if the support was exposedto air during the first and second coats, the permeance was significantly larger.The following trial was then undertaken. Two NPC membranes were manufactured at 550°C.For the first membrane, the second and third coats of carbon were applied directly after thefirst coat and thus minimising air contact. The second membrane was manufactured byallowing the first coat to sit in atmospheric air for approximately one week before the secondand third coats were added.The permeance of the membranes was then measured. The results presented in Figure 3-6,show an increase in the permeance of the oxidated-membrane by 1 to 2 orders of magnitude.It should also be noted that the oxidation of the first coat did not compromise the selectivityas similar selectivities were observed for the both membranes.The results are presented in terms of the N 2 permeance for comparison with the publishedresults of Rajagopalan et al [28] and Merritt et al [27] who added silica nanoparticles to theporous stainless steel support. The addition of silica nanoparticles prevented nanoporouscarbon ingress into the support, which decreased the thickness of the membrane and henceincreased the permeance.1.00E-07N 2 Permeance (mol/m 2 .s.Pa)1.00E-081.00E-091.00E-101.00E-111.00E-120 NPC Membrane 1 NPC 2MembraneNPC 3Membranes4NPC Membranes 5Not Oxidised OxidisedNo Silica Particles With Silica Particles[Rajagapolan et al] [22, 21]Figure 3-6 : The impact of oxidising the first coat of carbon of membranesmanufactured at 550°C. The increase in permeance is 1 to 2 orders of magnitude. Thisresult is similar to that achieved from the addition of silica particles [28] [27].63

Interestingly, Hayashi et al [135] reported a widening of the pore size distribution withexposure to oxygen at 300°C prior to pyrolysis at a higher temperature. Whilst it has not beenpossible to confirm the mechanism of pore opening via pore size characterisation techniquesas part of this research work, it is suggested that oxygen adsorption from the air onto the firstcoat has increased the pore size of the NPC within the support during the pyrolysis of thesecond coat.As this technique was developed in the early stages of this research work, the resultspresented in this thesis for supported NPC membranes were obtained using membranes withan oxidised first coat.3.2.2.2 Modified Supported NPC MembranesThe alternative method of modifying the porous supports with silica particles to reduce theoverall thickness of the membrane proposed by Rajagopalan et al [28] and Merritt et al [27]was also trialled.The pore size of the supports was reduced by the addition of silica nanoparticles. The silicananoparticles were produced via the reaction of tetraethylorthosilicate (TEOS) with water inlow-molecular weight alcohols using an ammonia-based catalyst, which is also known as theStoeber synthesis [145].The reaction of TEOS (Si(OR) 4, R = C 2 H 4 ) with water in alcohol to produce silica (SiO 2 ) isdescribed by the two reactions presented below [146, 147].Si OR)+ H 0 → ( OR)SI(OH ) + ROH(4 23( RO ) Si(OH H O → SiO + ROH3) +22The ultimate size of the silica particles produced from the Stoeber synthesis depends upon (1)the concentration of TEOS, (2) the concentration of ammonia, (3) the concentration of water,(4) the reaction temperature and (5) the alcohol which is used as a solvent [146].Various authors have studied the effect of the above five variables and correlations exist forproducing silica nanoparticles of a particular size [145-148]. However, in this case, highlymonodisperse silica nanoparticles were made using a modification to the conventional methodproposed by Zhang et al [149]. By diluting the TEOS with alcohol prior to the reaction, theneed for prior distillation of the TEOS is eliminated which in turn dramatically decreases thecomplexity of the manufacture.64

Using the method of Zhang et al [149], two solutions as shown in Table 3-4 were prepared.Table 3-4 : The composition of the two solutions prepared for the manufacture of thesilica nanoparticles used for filling the porous support prior to coating with PFA.Solution Ethanol Ammonia TEOS1 46 ml 10 ml -2 4 ml - 1 mlThe two solutions were then mixed in a conical flask contained within a water bath controlledat a constant temperature. The reaction time and temperature of the water bath were varied toobtained silica particles within the desired size range (< 300 nm). The particle sizes of theproduced colloid solution were determined using Scanning Electron Microscopy (SEM) as isdescribed in Section 3.3.A schematic diagram of the equipment used for the manufacture of the silica nanoparticles ispresented in Figure 3-7.Produced SilicaNanoparticlesSolution of Ethanol,Ammonia and TEOSWater Bath atConstant TemperatureMagnetic StirrerHot PlateFigure 3-7 : Apparatus for creating the colloidal solution of silica nanoparticles in whicha solution containing ethanol and ammonia was mixed with a solution containingethanol and TEOS and reacted at a constant temperature.To impregnate the porous support with the silica nanoparticles, the colloidal solution wasfiltered through the support using a filtration cell (Millipore, USA) as shown in Figure 3-8.The porous support was clamped between the upper and lower parts of the filtration cell. Thecolloidal silica solution was then diluted with methanol (1 to 5.5 by volume) and poured ontop of the porous support. The vacuum line (V1) was then opened to allow a driving forceacross the porous support and allow the colloidal solution to flow through at a reasonable rate.65

The filtering process was repeated several times until the solution contained within the bottomof the filtration cell had become clear indicating the presence of few silica particles.Colloidal SilicaSolutionPorous StainlessSteel SupportClampV1Vacuum PumpSolution Containing VeryFew Silica ParticlesMillipore Filtration CellFigure 3-8 : Millipore filtration cell used for filtering diluted silica nanoparticle colloidsolution through the porous stainless steel support.The support containing the silica nanoparticles was then dried overnight at 100 °C in avacuum oven to remove the liquid. The filtering and drying processes were repeated up to 4times until a white coat was present on the surface of the support indicating saturation of thesupport with silica nanoparticles. The supports were weighed before and after the addition ofthe silica.3.2.2.3 Unsupported NPC MembranesTwo different types of polyimide (Matrimid and Kapton) were used to make unsupportedmembranes. Flat sheet polyimide membranes were cast from Matrimid pellets. The Matrimidwas dissolved in dichloromethane on a 2 wt% basis and then dispensed into glass rings in thefume hood. The membranes were be left overnight in the fume hood to allow evaporation ofthe solvent and then dried in the vacuum oven for 4 days at 150 °C. Kapton polyimide flatsheet (100 HN and 200 HN) was obtained directly from Dupont.Both the Matrimid membranes and the Kapton polyimide flat sheets were then cut to adiameter of 47 mm using a punch and placed in the quartz tube furnace between two graphiteblocks.Under an UHP argon flow of 200 ml/min, the polymeric membranes were carbonised at afinal pyrolysis temperature between 400 and 800°C for 2 hours. A typical temperature profileof the quartz tube oven is shown in Figure 3-9 below. The argon purge gas was maintained atall times until the furnace had returned to room temperature.66

800700Oven HeatingOven Off (Natural Cooling)Furnace Temperature (°C)6005004003002001002 hours @ final pyrolysis temperature00 100 200 300 400 500 600 700 800 900 1000Time (min)Figure 3-9 : Quartz tube furnace typical temperature profile with time during pyrolysisof unsupported NPC membranes.3.3 Membrane Characterisation TechniquesThe supported and unsupported membranes were all weighed before and after pyrolysis tocheck weight loss. Details of the surface structure and pore size distribution were obtainedusing the techniques described in the following sections.3.3.1 Surface StructureA Philips XL30 and a FEI Quanta FEG 200 scanning electron microscope (SEM) were usedto detect the presence of carbon and also to ensure the absence of micro-cracking. For thepartially carbonised unsupported membranes, which were not conductive, the samples werecoated in gold using a Dynavac mini sputter coater prior to using the SEM.High resolution transmission electron microscopy (HRTEM) was used to estimate the poresize and shape of NPC manufactured at 600°C. Images were recorded using a Jeol-2010exelectron microscope equipped with a Gatan parallel electron energy loss spectrometer(PEELS), STEM and Gatan Imaging Filter (GIF) located at the Royal Melbourne Institute ofTechnology (RMIT). The microscope was operated at 200keV and most images wererecorded at 600k magnification. The spherical aberration coefficient of the ultra-highresolution pole-pieces was CS = 1.00 mm and the effective image resolution was 0.2 nm.67

3.3.2 Pore SizeAn Image J analysis of the HRTEM image of nanoporous carbon was undertaken to estimatethe pore size distribution. Image J is a publicly available java-based image processingcomputer program developed by the American National Institutes of Health. The pore sizes of100 pores were measured individually using the measuring tool available in Image J and thencalibrated back to a relative size of 1 nm.3.3.2.1 PALSPositron Annihilation Lifetime Spectroscopy (PALS) was also used to indicate porosity andthe pore size distribution of the nanoporous carbon produced. In order to obtain enough NPCfor the PALS analysis, a crucible of PFA was placed in the quartz tube furnace whilst makingthe membranes.The PALS experiment measures the lifetimes of positrons by detecting start quanta of 1.28MeV associated with their birth, and stop quanta of 0.511 MeV associated with their death(annihilation radiation) [150]. When a positron is emitted from a radioactive isotope, such as22Na, it is injected into the sample under consideration with high energy (up to 544 keV).Once in the sample, the positrons are thermalised via inelastic collisions. Positrons canannihilate with electrons in the bulk of the material. Alternatively, as positrons are repelled bythe constituent nuclei of the material and are annihilated by electrons, positrons can belocalised in voids or regions of lower than average electron density. As a positron is of similarsize as an electron, it is able to access small voids.The lifetime of the positron is dependent on the overlap of the positron wavefunction with thewavefunctions of the electrons in the material. Therefore, as the probability of wavefunctionoverlap is decreased with increasing pore size, an increase in the average positron lifetime ismeasured [151]. In materials with high electron density, such as carbon membranes, twoseparate positron lifetimes are observed. The first lifetime (τ 1 ) is attributed to positrons beingannihilated in the bulk of the material (~150-280 ps). The second lifetime, τ 2 , is attributed topositrons being annihilated in defects (300-500 ps). The relative number of positrons beingannihilated in defects (I 2 ) compared to those annihilating in the bulk (I 1 ) gives information onthe number of defects in the material [150, 152, 153].Positron Annihilation Lifetime Spectroscopy (PALS) experiments were performed using anEG&G Ortec fast-fast coincidence system with fast plastic scintillators and a resolutionfunction of 260 ps FWHM (60Co source with the energy windows set to 22Na events).Approximately 500 mg of each sample was packed on either side of a 30 µCi 22NaCl foil68

(2.54 µm Ti) source. At least 5 spectra of one million integrated counts were collected witheach spectrum taking about 0.47 h to collect. Data analysis was performed using LT9 [154].The spectra were best fitted with two components. No evidence of source irradiation damagewas observed and no source correction was used.3.3.2.2 WAXDWide angle X-ray diffraction (WAXD) was also used to determine the pore size distribution.X-ray diffractograms were measured using a Bruker Advance, D8 diffractometer operating in2θ mode, using copper Kα radiation at 40 mA, 40 kV at room temperature in air. Theinstrument was equipped with a graphite diffracted beam monochromators. The powderedsample was mounted in a standard Bruker sample holder, giving minimal background scatter.Diffractograms were measured at 0.02° steps over the range 2θ = 5° to 65° with a step time of3 s. Bragg’s Law (Equation 2-28) was used to determine d-spacing from 2θ values.While not a direct indication of pore size, the d-spacing values give an indication of theaverage spacing between individual layers of the nanoporous carbon structure [155].69

3.4 Measurement of Membrane Permeance and SelectivityPure gas performance measurements were initially made in order to screen the membranes formixed gas testing and to determine the effect of pyrolysis temperature on performance. Mixedgas measurements were used to determine the effect of operating temperature and thepresence of impurities on the NPC performance.3.4.1 Pure GasThe permeance of pure CH 4 , N 2 and CO 2 (99.99%, BOC Gases, Australia) of each membranewas measured using a custom built constant volume, variable pressure (CVVP) apparatus[156] as shown in Figure 3-10 below(a)(b)ExhaustPGV2PTData LoggerV1V3Pure CH 4 , N 2 or CO 2OvenV4PSVV5PTV6Calibrated Volumewithin Water Bathat Constant TemperatureExhaustVacuumPumpFigure 3-10 : (a) Photograph and (b) process flow diagram of the constant volume,variable pressure (CVVP) apparatus for measuring pure gas permeance andpermselectivity70

The NPC membrane was placed in a purpose-built 44 mm membrane holder, which was fittedto the CVVP apparatus. The membrane unit (Figure 3-11) was housed within an oven(Scientific Equipment Manufacturers, Australia) to keep the temperature constant at 35°C.Similarly, a cylinder of calibrated volume (150 or 500 ml Swagelok, USA) was immersed in awater bath maintained at 27°C using a water bath circulator (Bioblock Scientific Polystat,France). Duthie et al [156] has shown that the errors associated with the temperature variationbetween the oven and the bath are small.Figure 3-11 : Photograph of the CVVP membrane cell showing one inlet connection forthe feed gas and one outlet connection for the permeate gasAs the first step, the vacuum connections (V4 and V5) were opened to allow the system to beevacuated. In order to draw a deep vacuum the system was often left to evacuate overnight.The vacuum connections were then sealed off and feed gas introduced on top of themembrane at a pressure (p f ). The pressure of the feed gas was measured using a 0 to 6985 kPapressure transducer (MKS Baratron, USA) and the pressure on the permeate side wasmeasured using a 0 to 1.33 kPa pressure transducer (MKS Baratron, USA). During thesubsequent pressure rise on the permeate side, both pressure measurements were recordedevery second using a PC installed with Labview 6.2 (National Instruments, USA). A pressuresafety valve (PSV) was installed directly upstream of the permeate pressure transducer toprevent overpressure from the feed gas in the instance of a membrane failure.For each gas, the rise in pressure within the known permeate volume (V p ) was then used tocalculate the permeance (P’) as discussed earlier in Section 2.2.2 and as shown in Equation 3-1. It should also be noted that prior to introducing a feed gas, the vacuum valves were closedand the pressure rise recorded in the absence of a feed gas in order to calculate the leak rate ofthe system which was then subtracted from the pressure rise readings of each gas.71

P∆xV pp f− pp0t = P' t = lnEquation 3-1 [84]ART p − pfpThe calculated error in the pure gas permeance is +/- 1.4% and in the selectivity is +/- 2.0%.Details of the error analysis and sample calculations can be found in Appendix CC.1– Puregas permeance measurement.3.4.2 Mixed GasThe permeance of a mixed CO 2 :CH 4 or CO 2 :N 2 feed (10:90 vol%, BOC Gases Australia) wasmeasured using a custom-built constant pressure variable volume (CPVV) apparatus builtfrom ¼” stainless steel tubing and fittings (Swagelok USA) as shown in Figure 3-12 below.ExhaustPGFeedPGRetentateMF1 1BPC 1MFI 2ExhaustV1V2PGPGCH 4 /CO 2V3V4OvenTGV5V6ExhaustPGSweep GasPGTGPermeateMFC 1 MFI 3V7GC On-line SamplingLegendHeOn/Off ValveNeedle ValveMFCMFIMass Flow ControllerMass Flow IndicatorBPCBack Pressure ControllerFigure 3-12 : Process flow diagram of the constant pressure, variable volume apparatus(CPVV) for measuring mixed gas permeance and selectivity.Similar to the CVVP apparatus, the NPC membrane was placed in a custom-built 44 mmmembrane holder (Figure 3-13), which was fitted to the CPVV apparatus. The membrane unitwas housed within an oven (Scientific Equipment Manufacturers, Australia) to keep thetemperature of the membrane constant.72

Before introducing the mixed gas feed, the feed line was connected to a nitrogen cylinder(BOC Gases Australia) and the membrane regenerated overnight at 30 ml/min and 120°C toremove any water that had adsorbed onto the membrane from the atmosphere during handlingand installation.Figure 3-13 : Photograph of the CPVV membrane cell showing two inlet connections forthe feed and sweep gases and two outlet connections for the retentate and permeatestreams.The next morning, the N 2 was disconnected, valves V2, V3, V4, V5 and V6 were closed andthe mixed gas feed introduced. Valve V1 was always set to flow towards the feed mass flowindicator (MFI 1) (Aalborg, USA) unless the feed line needed depressuring as is the case inshutdown. The pressure of the mixed gas feed was increased at the regulator to the desiredpressure, usually between 1000 and 2000 kPag.If the flowrate reading given by MFI 1 was not 0 ml/min, the fittings were tightened wherepossible. Valves V2 and V4 were then opened to allow a flow through the retentate line. Thesetting of the retentate back pressure controller (BPC 1) (Aalborg USA) was adjustedaccordingly to give the desired retentate flow as shown by the retentate mass flow indicator(MFI 2) (Aalborg, USA), usually 100 ml/min.Helium sweep gas (BOC Gases Australia), was introduced by opening valve V5 and thensetting the desired flow through the sweep gas mass flow controller (MFC 1) (Aalborg, USA).The purpose of the inert sweep gas was to reduce the partial pressure of the carbon dioxide inthe permeate flow and hence increase the driving force. The sweep gas flow was optimised at20 ml/min. If the flow was much higher than this, it was difficult to obtain accurate readings73

of the permeate concentration from the gas chromatograph and if the flow was lower thanthis, there was inadequate flow to the gas chromatograph.Finally, the permeate flow was initiated by opening valve V6. Valve V7 was always settowards the gas chromatograph unless recording the permeate flowrate (MFI 3) (AalborgUSA) in which case V7 was switched toward the exhaust system. Doing this preventedexcessive backpressure on MFI 3, which in turn gave a more accurate reading.The system was allowed 2 hours to equilibrate before the first readings were taken. Duringthese initial 2 hours, the gas chromatograph (GC) (Varian CP-3800 USA) fitted with both athermal conductivity detector (TCD) and flame ionisation detector (FID) was brought on-line.The GC was calibrated once per month with CH 4 , N 2 and CO 2 .Once all flow rates were steady and 2 hours had elapsed, the first readings were taken. Thiswas done by opening the integral sampling valve on the GC to allow the permeate gas to flowthrough the GC columns. A further two permeate compositions were then sampled in quicksuccession using the GC to ensure consistency of readings. Valve V7 was then directed tovent and readings of the flowrate and pressure of each stream recorded. Further GC samplesalong with pressure and flowrate readings were then taken approximately every 30 to 60 minsfor up to 8 hours.As described in Section 2.2.2, the permeance of each component was calculated from the flux(F) and the partial pressure drop (∆p). The flux was calculated from the permeate flow rate(Q p ), permeate component mol fraction (y) and membrane surface area (A). Similarly, thechanges in partial pressure of each component were calculated from the feed (x) and permeate(y) component mol fractions and the pressure of the feed (p f ) and permeate streams (p p ) aspresented in Equation 3-2.P'AJ=∆pAA=A(xAQpfpyA− yApp)Equation 3-2The ideal CO 2 /CH 4 (α CO2/CH4 ) selectivity was calculated from the ratio of permeances asshown earlier in Section 2.2.1 as Equation 2.3. The CO 2 /CH 4 separation factor (S CO2/CH4 ) ofthe membrane was calculated as shown earlier in Section 2.2.1 as Equation 2.5.The experimental conditions for which the NPC membranes were tested using a mixed gasfeed are shown in Table 3-5. These conditions were chosen as the best representation of CO 2separation from natural gas within the limits of the experimental equipment.74

Table 3-5 : Mixed gas testing conditionsStream Property Controlled? ValueFeed Composition Yes 10:90 mol% CO 2 :CH 4Feed Flow Yes 100 ml/minFeed Temperature Yes 35°CFeed Pressure Yes 500 to 2000 kPagSweep Gas Composition Yes 100 mol% HeSweep Gas Flow Yes 20 ml/minSweep Gas Temperature Yes 35°CSweep Gas Pressure Yes ~ 50 kPagRetentate Composition No Determined by system performanceRetentate Flow No Determined by system performanceRetentate Temperature Yes 35°CRetentate Pressure Yes 450 to 1950 kPagPermeate Composition No Determined by system performancePermeate Flow No Determined by system performancePermeate Temperature Yes 35°CPermeate Pressure Yes 0 kPagA more detailed operating procedure is given in Appendix BB.1 – The standard operatingprocedure for the CPVV (Mixed Gas) apparatus.The calculated error in the mixed gas permeance is +/- 5.2% and in the selectivity is +/- 7.3%.Details of the error analysis and sample calculations can be found in Appendix CC.2 – Mixedgas permeance measurement.3.4.3 At Elevated TemperaturesThe separation performance of mixed gases at elevated temperatures was measured byincreasing the oven temperature, in which the membrane unit was housed, from 35°C tobetween 75°C and 200°C. After each temperature increase, the system was allowed 1 hour toequilibrate before GC samples and readings were taken.Temperatures greater than 200 °C were not exceeded due to the pressure-temperaturelimitations of the piping and fittings used. The temperature of the gas exiting the oven wasmeasured using a thermocouple to ensure that the gas flowing through the mass flow metershad cooled back to room temperature again because of the temperature-pressure limitations ofthe flow meters.75

3.4.4 Exposure to Impurities3.4.4.1 Condensable ImpuritiesThe permeance of a mixed CO 2 :CH 4 or CO 2 :N 2 feed (10:90 vol%, BOC Gases Australia) inthe presence of condensable impurities was measured using the CPVV apparatus withmodification as shown in Figures 3-14 and 3-15.The CPVV (mixed gas) apparatus was started using the method as described previously inSection 3.4.2 and a baseline performance reading established. Condensable impurities werethen introduced into the mixed gas feed by directing the feed gas through valve V2 tostainless steel vessel (Swagelok, USA) containing 50 ml of liquid impurity (refer to Figure 3-14). The vessel was filled with steel ballotini and was equipped with a sparger to allow thegas to become saturated with the liquid impurity. The saturated gas was then directed back tothe membrane cell through a liquid droplet filter (Swagelok, USA) via valve V7. The systemwas allowed 30 mins to allow the permeate gas under the presence of impurities to reach theGC. The permeate composition along with the stream flowrates and pressures were recordedas described in Section 3.4.2. Measurements were then taken every 30 mins until the values ofpermeance and selectivity had stabilised (usually less than 4 hours).Figure 3-14 : Photograph of the vessel used to saturate the feed gas of the constantpressure variable volume (CPVV) apparatus for measuring mixed gas permeance andpermselectivity in the presence of condensable impurities.76

(a)ExhaustTo Condensable ImpuritiesFrom Condensable ImpuritiesPGFeedPGRetentateV1MFI 1V2V7BPC 1V8MFI 2ExhaustPGPGCH 4 /CO 2V9V10OvenTGV11V12ExhaustPGSweep GasPGTGPermeateMFC 1 MFI 3V13GC On-line SamplingLegendHeOn/Off ValveNeedle ValveMFCMFIMass Flow ControllerMass Flow IndicatorBPCBack Pressure Controller------------------------------------------------------------------------------------------------------(b)PGFilling Line for Condensable ImpurityLab Exhaust SystemV3V4GC Sample PtV5From FeedFilterTo Membrane UnitCondensable ImpurityV6LegendOn/Off ValveNeedle ValveCheck ValveMFCMFIBPCMass Flow ControllerMass Flow IndicatorBack Pressure ControllerFigure 3-15 : Process flow diagrams of (a) the constant pressure, variable volume(CPVV) apparatus for measuring mixed gas permeance and selectivity in the presenceof condensable impurities (b) vessel for contacting feed gas with condensable impurities77

The impurities tested were water, hexane (99.8 %, Merck Germany) to represent the impact ofheavier hydrocarbons and toluene (99.8 %, Merck Germany) to represent the impact ofaromatic hydrocarbons. The concentration of impurity in the feed gas was controlled bysubmerging the vessel in a temperature controlled solution of glycol. The concentration ofimpurity (x IMP ) (mol %) was then calculated from the partial pressure of the impurity (pp IMP )at the vessel temperature and the feed gas pressure (p f ) using Equation 3-3. As discussedpreviously in Section 2.2.1 Partial pressures rather than fugacities have been used as thefugacity coefficients of the components at the vessel temperatures and pressure are greaterthan 0.95.ppIMPxIMP= Equation 3-3pfThe partial pressure of the impurity (pp IMP ) at the vessel temperature was determined from theHYSYS simulation package (Aspentech, USA) using the Peng-Robinson equation of state.The typical concentrations of impurities used are presented in Table 3-6.Table 3-6 : The concentration of condensable impurities in the feed gas based on theoperating temperature of the saturating vessel containing the liquid impurity.Component Vessel Temp (°C) Vapour Press (kPa) Impurity Concentration (ppm)Water 25 3.1 2390Water 10 1.2 918Water 5 0.9 650Hexane 25 20.2 15538Hexane 10 10.2 7844Hexane 5 7.8 6128Toluene 25 4.1 2549Toluene 10 1.8 1146Toluene 5 1.4 860Regeneration of the membrane after impurity exposure was completed using nitrogen at atemperature between 120 and 150 °C at a flow of 30 ml/min and pressure of 200 kPag formore than 12 hours.A detailed description of the operating procedure in the presence of condensable impurities isgiven in Appendix B2 – The standard operating procedure for the CPVV (mixed gas)apparatus in the presence of condensable impurities.78

3.4.4.2 Non-Condensable ImpuritiesThe permeance of a mixed CO 2 :CH 4 or CO 2 :N 2 feed (10:90 vol%, BOC Gases Australia) inthe presence of condensable impurities was measured using the CPVV apparatus withmodification as shown in Figure 3-16.To expose the membrane to H 2 S or CO, two speciality bottles were purchased from BOCAustralia. The first bottle contained 500 ppm of H 2 S in a CO 2 :N 2 mixture (10:90 vol%) whilstthe second contained 1000 ppm of CO in a CO 2 :N 2 mixture (10:90 vol%).The CPVV (mixed gas) apparatus was started using the method as described previously inSection 3.4.2 using a CO 2 :N 2 mixture (10:90 vol%, BOC Gases Australia) and a baselineperformance reading established. Valves V5, V6, V7 and V8 were then closed to keep themembrane at pressure. The CO 2 :N 2 mixture (10:90 vol%) cylinder was then closed and thefeed line up to valve V5 depressured by venting through V2. Valve V3 was then directedtoward valve V2. Locked valve V1 was opened and the pressure at the regulator on theimpurity cylinder (containing either H 2 S or CO) was slowly increased to the desired valve.The valves V4, V5 and V6 were then opened and BPC 1 adjusted so that a flow of 100ml/min was seen on MFI 2. Sweep gas was introduced through V7 and the permeate flowestablished by opening V8 and directing V9 to the GC.ExhaustGlove BoxPGV1 (Locked)V2V3FeedMFI 1PGPGBPC 1V4PGRetentateMFI 2ExhaustCO 2 /N 2 /H 2 SPGV5V6OvenExhaustTGN 2 or CO 2 /N 2V7V8ExhaustPGSweep GasPGTGPermeateMFC 1 MFI 3V9GC On-line SamplingLegendHeOn/Off ValveNeedle ValveMFCMFIMass Flow ControllerMass Flow IndicatorRelief ValveBPCBack Pressure ControllerFigure 3-16 : Process flow diagram of the constant pressure, variable volume apparatus(CPVV) for measuring mixed gas permeance and selectivity in the presence of noncondensableimpurities.79

The system was allowed 30 mins to allow the permeate gas under the presence of impuritiesto reach the GC. The permeate composition along with the stream flowrates and pressureswere recorded as also described in Section 3.4.2. Measurements were then taken every 30mins until the values of permeance and selectivity had stabilised (usually less than 4 hours).Regeneration of the membrane after impurity exposure was completed using nitrogen at atemperature between 120 and 150 °C at a flow of 30 ml/min and pressure of 200 kPag formore than 12 hours. The CO 2 :N 2 mixture (10:90 vol%) without the impurity was thenreconnected and the performance of the membrane tested to evaluate the effectiveness ofregeneration.The gases H 2 S and CO are both very toxic and can cause death quickly even when in very lowconcentrations, such as those present in the gas bottles used for the experiments. A detailedrisk assessment was undertaken before these gases were used in the lab. Five scenarios inwhich H 2 S or CO could leak into the lab were identified from the risk assessment. For eachscenario the volumetric rate of H 2 S or CO leaked into the lab was calculated using theuniversal gas equation for flow through an orifice [157]. Assuming ideal mixing, theconcentration of H 2 S or CO in the lab with time was calculated. From the knownconcentration effects of H 2 S and CO, the timing to an alarm, unconscious state and fatalitycould be determined.For each scenario, the risk of each scenario occurring was significantly reduced throughengineering and operator training. Engineering risk reduction measures included installing anextraction system around the gas bottle and from the oven (refer to Figure 3-17 below), a keylocksystem on the gas bottle, replacing all existing nylon seals and gaskets within the CVPPapparatus with Kalrez – a H 2 S resistant material and minimising fittings. Operators weretrained using a detailed operating procedure and a register of those trained was kept with theoperating procedure. The operating pressure of the rig did not exceed 1000 kPag to minimisethe volume leaked in the event of a leak. The operators were also trained to return the gasbottle containing either H 2 S or CO to the outdoor gas cage when not in use for more than 2consecutive days and advise all lab personnel if either gas was in use. A buddy system wasfollowed at all times.80

Figure 3-17 : Photograph of the gas extraction system installed around the gas bottlecontaining H 2 S or CO impurities.In the unlikely event of a leak occurring, emergency procedures were also developed and alllab personnel were trained to follow these procedures. These procedures included immediateevacuation of the lab upon sounding of one of two alarms for detecting H 2 S and CO alongwith availability and training of all lab personnel in the use of self-contained breathingapparatus’ (SCBA) for use when re-entering the lab. In addition to two of the lab personnelbeing trained in first aid, first aid was also available within minutes via the security system.A detailed description of the operating procedure in the presence of non-condensableimpurities is given in Appendix B.3 – The standard operating procedure for the CPVV (mixedgas) apparatus in the presence of non-condensable impurities.The details of the risk assessment, leakage calculations and safety precautions implementedare presented in Appendix B.4 – Risk assessment for the operation of the CPVV (mixed gas)apparatus in the presence of non-condensable impurities.81

3.5 Measurement of Membrane AdsorptionThe adsorption of gases onto NPC membranes was calculated theoretically using a molecularsimulation and measured experimentally using a microbalance.3.5.1 Theoretical AdsorptionMolecular modelling (GCMC simulation) was undertaken to obtain simulated adsorptionisotherms of pure CO 2 , N 2 and CH 4 along with the mixtures of CO 2 :N 2 (10:90 vol%) andCO 2 /CH 4 (10:90 vol%) on nanoporous carbon at various temperatures.3.5.1.1 Model DevelopmentThe molecular model used was developed by Dr Jianwen Jiang and Professor Stanley Sandlerat the University of Delaware, USA.3.5.1.2 Model DescriptionMonte Carlo simulations were carried out in the grand canonical ensemble of constantchemical potential, volume and temperature (µVT) to determine the number of molecules (N)adsorbed from the bulk phase onto the substrate.An adsorption isotherm was generated by running the model at the same temperature andcomposition at various pressure points. All pressures quoted in Chapter 5 are absolute (i.e.kPaa).A summary of the pressure and temperature conditions modelled using GCMC simulation ispresented in Table 3-7 below.Table 3-7 : Details of the adsorbents, temperature and pressures used for obtainingsimulated adsorption isotherms of nanoporous carbon using GCMC simulation.Condition 1 Condition 2 Pressure PointsAdsorbent Temperature (°C) (kPaa)CO 2 27 0.01N 2 35 0.10CH 4 75 1.00CO 2 /N 2 100 10.00CO 2 /CH 4 125 100.00150 1000.00175 10000.00200 100000.0082

Inputs to the model included the coordinates of nanoporous carbon (modelled as C 168Schwarzite as shown in Figure 3-18), the Lennard-Jones potential model to provide the gasgasmolecule interactions and the ab-initio potential model to provide the gas-carboninteractions.Figure 3-18 : Structure of C 168 Schwarzite from the side (left) and rotated at 20°C (right)The GCMC simulation model developed by Dr Jianwen Jiang was written in FORTRAN 90and was run on a high speed UNIX machine using a standard FORTRAN compiler. The errorin the statistical average of the number of molecules adsorbed was calculated using the blocktransformation technique [106, 112].Further details on the inputs to the model are presented in Appendix C.4.3.5.1.3 Fitting the Adsorption IsothermsThe simulated adsorption isotherms of the pure gases were fitted using the Langmuir modeland the heat of adsorption (∆H ads ) was calculated from the Langmuir constant, b, as describedpreviously in Section 2.2.2.3.3.5.1.4 Calculating the Adsorption Separation FactorThe adsorption separation factor of the mixed gases at various operating temperatures andpressures was calculated from the component (A and B) proportions in the bulk phase (x) andthe adsorbed phase (y) as shown in Equation 3-4.SyA/ yB= Equation 3-4(x / xads A / B)AB83

3.5.2 Experimental AdsorptionThe adsorption isotherms of pure gases on nanoporous carbon were measured using aGravimetric High Pressure (GHP-300) analyser fitted with a Cahn microbalance (VTICorporation, USA) as shown in Figure 3-19. The GHP-300 is a gravimetric analyser thatoperates at both high and low temperatures and pressures. Some measurements were conductedin house, but due to equipment failure other results were obtained on a comparable machine atthe VTI site in the USA.(a)(b)MicrobalancePTWater CooledFurnaceV8Reference SideV7MFMVPTVacuumPumpV9V6ConstantTemperatureWater BathSampleV5VentLegendV1 V2 V3 V4Pneumatic ValveVessel forOrganicVapourExposurePorts for Adsorbate GasesMFMVSolenoid ValveMass Flow Metering ValveFigure 3-19 : (a) Photograph and (b) schematic diagram of the microbalance used formeasuring gas adsorption isotherms at elevated temperatures.84

Due to time constraints it was only possible to complete pure gas isotherms at two temperaturesas presented in Table 3-8 below.Table 3-8 : Details of the adsorbents, temperatures and pressures used for obtainingexperimental adsorption isotherms of nanoporous carbon.Condition 1 Condition 2 Pressure Points Point TypeAdsorbent Temperature (°C) (kPaa +/- 5)CO 2 25 (N 2 only) 0 AdsorptionN 2 35 (CO 2 only) 41 Adsorption200 67 Adsorption131 Adsorption264 Adsorption520 Adsorption777 Adsorption968 Adsorption1283 Adsorption1522 Adsorption1280 Desorption1023 Desorption517 Desorption260 Desorption131 DesorptionTo obtain the adsorption isotherms for this research work, the GHP-300 was operated in staticmode. A sample of bulk nanoporous carbon that had been manufactured in a crucible at 550°Cfor a soak time of 2 hours under an ultra high purity argon purge at 200 ml/min was placed inthe balance. Due to equipment limitations, the maximum sample size allowed was ~100 mg.The sample was evacuated and then heated at 350 °C for 3 hours to dry the sample as much aspossible. The sample was then cooled to the experimental temperature and the adsorptionisotherm constructed by measuring the weight change with increasing pressure. Likewise, thedesorption isotherms was constructed by measuring the weight reduction with decreasingpressure.3.5.2.1 Buoyancy CorrectionDuring adsorption and desorption, the sample was buoyed by the gas in the sample chamber,which led to a reduction in the weight change measured. The weight changes measuredtherefore need to be corrected to remove the buoyancy effect.85

The buoyancy correction was made using either of the following two methods.3.5.2.1.1 Buoyed Volume from Helium AdsorptionFollowing an adsorption experiment, the sample chamber was fully evacuated and dried for 30mins at 150°C. The sample was then heated or cooled to the experimental temperature, whichwas the same as the preceding adsorption experiment. An adsorption isotherm using helium asthe adsorptive gas was then obtained. On the assumption that helium doesn’t adsorb to thenanoporous carbon, the weight changes observed were attributed to the buoyancy of the sample.The “helium” adsorption isotherms constructed show a linear decrease in the weight withincreasing pressure as the sample became more buoyant.The buoyed mass (per unit pressure P) was then calculated from the gradient of the straight linefit of the helium isotherm. The buoyed mass (m) was then converted to a buoyed volume (V)using the ideal gas law (Equation 3-5) at the temperature (T) of the helium adsorptionexperiment. The compressibility factor (z) allows for non-ideality of the gas and is particularlyimportant at higher temperatures and pressures. R is the ideal gas constant.mRTV = zEquation 3-5MWPThe buoyed volume was then used to correct the original adsorption isotherm by converting theweights measured at each pressure to volumes using Equation 3-5 with a compressibility factor(Z), adding the buoyed volume and converting back to a final mass again using Equation 3-5.3.5.2.1.2 Buoyed Volume from Equipment MassesThe buoyed volume can also be obtained from the difference in the weight of the microbalancesample and reference sides (as shown previously in Figure 3-19) provide the two sides are at thesame temperature. The weights of each component of the sample and reference side areconverted to volumes using standard densities as given in Table 3-9. The sum of the volumes onthe reference side is then subtracted from the sum of the volume on the sample side to give thebuoyed volume.The buoyed volume is then used to correct the original adsorption isotherm as discussedpreviously in Section 3.5.2.1.1.The calculated error in adsorption isotherms is +/- 8.35 %. Details of the error analysis andsample calculations can be found in Appendix C.3 – Adsorption measurement.86

Table 3-9 : Equipment mass and densities used to calculate buoyed volume. Theequipment masses and densities were provided by the supplier (VTI). The density ofnanoporous carbon was assumed to be the standard value of 1.6 g/cm 3 [79].Side Equipment MassDensity @35°C(g) (g/cm 3 )Reference SS 316 Fittings 0.6303 7.99Sample SS 316 Hang Downwire 0.1054 7.99Sample SS 316 Holder Bail 0.0119 7.99Sample Aluminium Sample Holder 0.4917 2.72Sample Nanoporous Carbon Varied with Experiment 1.60 [79]3.5.2.2 Fitting the Adsorption IsothermsThe experimental adsorption isotherms were fitted using the Langmuir model and the heats ofadsorption (∆H ads ) were calculated from the Langmuir constant, b, as described previously inSection 2.2.2.3.87

4 MANUFACTURE AND HIGH TEMPERATURE OPERATION OF NPCMEMBRANES4.1 IntroductionThe manufacture and characterisation of NPC membranes along with the pure gas and mixedgas performance results at high operating temperatures will be presented in this chapter. Bothsupported NPC membranes and modified-supported NPC membranes are considered. Theperformance results for unsupported membranes are presented in Appendix D.4.2 Supported NPC Membranes4.2.1 ManufactureSupported NPC membranes were manufactured from the pyrolysis of polyfurfuryl alcohol asdescribed in Section 3.2.2.1 at pyrolysis temperatures between 400 and 600°C. Between 2 and 4coats of the polymeric precursor were required to produce a selective membrane. During the 1stcoat, the polymer filled the pores of the support and carbon was not yet seen on the surface. Itwas upon the 2nd coat that carbon appeared on the surface. If the carbon upon the surface of thesupport had not formed a complete layer after the 2nd coat then a 3rd and sometimes 4th coatwas required. The creation of a membrane is depicted in Figure 4-1. The approximate volume ofpolymer solution added to each coat via spin-coating and the resulting carbon mass added afterpyrolysis is presented in Table 4-1.Figure 4-1 : The creation of a NPC membrane within 3 coats of the polymeric precursorTable 4-1 : Volumes of polymer solution (40 wt% PFA in acetone) added to each coat viaspin-coating at 1000 rpm and the resulting carbon mass added after pyrolysis.Coat Volume of Polymer Solution (ml) Carbon Mass Added After Pyrolysis (mg/cm 2 )1 4 – 9 2.5 – 6.82 2 – 3 1.5 – 2.53 1 0.84 0.5 – 1 0.489

All of the membranes reported in this chapter were manufactured using the higher molecularweight PFA received from Polysciences. The mass of carbon added for each membrane ispresented below in Table 4-2.Table 4-2 : Manufacture results of the membranes made from the spin-coating of PFAonto stainless steel supports and then pyrolysised between 400 and 600°C.MembranePyrolysisTemperature# ofCoatsCumulative Carbon Mass Added Per Coat(mg/cm 2 )°C 1 2 3 4 TOTALA 400 2 6.23 7.04 7.04B 400 2 6.51 7.58 7.58C 400 3 5.56 7.38 7.87 7.87D 400 4 4.69 6.71 7.57 8.77 8.77A 450 2 4.14 5.89 5.89B 450 2 4.88 6.15 6.15C 450 4 3.22 5.44 6.31 6.91 6.91A 500 2 3.64 5.29 5.29B 500 2 3.45 6.10 6.10C 500 3 4.81 5.81 6.13 6.13D 500 3 6.78 7.64 8.12 8.12A 550 2 2.99 5.18 5.18B 550 4 3.05 3.86 4.52 5.19 5.19C 550 3 2.89 4.46 5.22 5.22D 550 4 3.60 5.11 5.15 5.58 5.58E 550 2 4.09 6.07 6.07F 550 3 3.09 5.27 6.88 6.88A 600 2 2.84 4.62 4.62B 600 3 2.79 4.85 5.73 5.73C 600 4 3.11 4.42 5.31 6.28 6.28A graphical representation of the cumulative carbon mass achieved for each coat for themembranes made is presented in Figure 4-2. As is subsequently discussed in Section 4.2.3, aselective membrane was formed when the total carbon added was between 4 and 8 mg/cm 2 .Membranes made at higher pyrolysis temperatures were selective with a lower total amount ofcarbon added whilst membranes membrane made at the lower pyrolysis temperatures wereselective with a higher total amount of carbon added. The effect of pyrolysis temperature on thecarbon structure and resulting performance of the NPC membranes is further discussed in thefollowing sections.90

Rajagopalan and Foley @ 600 °C 400°C 450°C 500°C 550°C 600°C10.009.008.00Carbon Mass (mg/cm 2 )7.006.005.004.003.002.001.000.000 1 2 3 4Number of CoatsFigure 4-2 : The cumulative carbon density per coat for NPC membranes manufactured atpyrolysis temperatures between 400 and 600°C. Also depicted in this figure is thecumulative carbon density of an NPC membrane made by Rajagopalan and Foley at600°C [82] with a final O 2 /N 2 permselectivity of 5.4.2.2 Characterisation4.2.2.1 Surface StructureScanning electron microscopy (SEM) was useful as an initial characterisation tool forconfirming the presence of nanoporous carbon on the surface of the support and any cracking.SEM pictures of a nanoporous carbon surface with and without cracks are presented in Figure 4-3 (a).Likewise, high resolution transmission electron microscopy HRTEM was very useful in theearly stages of nanoporous carbon manufacture to identify that the correct pore size range toenable molecular sieving was being produced. Figure 4-3 (b) shows a HRTEM picture of theproduced nanoporous carbon clearly showing the presence of pores less than 1 nm.91

(a)Uncoated Stainless Steel Support3 Coats of NPC on SS SupportMicroscopic CrackingMacroscopic Cracking(b)Larger Detail of Nanoporous CarbonPore Size Detail of Nanoporous Carbon0 1 2 3 4 nmFigure 4-3 : (a) SEM pictures showing (clockwise from top left) the uncoated stainless steelsupport, the carbon membrane after three coats with no defects, macroscopic cracking inthe carbon surface and microscopic cracking in the carbon surface, (b) HRTEM picturesshown (from left) nanoporous carbon and enlarged photo to show pore size detail.92

4.2.2.2 Pore SizeAn estimate of the pore size distribution was obtained from measuring the size of 100 poreswithin a random selection of the HRTEM picture of NPC produced at 600°C using the imageprocessing program Image J. The estimated pore size distribution range is from 2.7 to 5.2 Åcentred on 4.0Å as shown in Figure 4-4. This result is consistent with the findings of Shiflett[89] who used digitally enhanced HRTEM images of NPC to calculate pores within the range of3 to 6 Å. The basis statistics of the pore size distribution are presented in Table 4-3.1410012# of PoresCummulative # of PoresMean = 4.0 AMedian = 4.0 A9080# of Pores108647060504030Cummulative # of Pores20210002.00 2.50 3.00 3.50 4.00 4.50 5.00 5.50 6.00Pore Size (A)Figure 4-4 : Pore size distribution of NPC produced at 600°C obtained from the HRTEMpicture using the image processing program Image J. The distribution range is from 2.7 to5.2 Å centred around 4.0 Å.Table 4-3 : Basic Statistics of the pore size distribution of NPC produced at 600°Cobtained from the HRTEM picture using the image processing program Image J.Statistic Unit Value# of Samples 100Minimum Å 2.70Maximum Å 5.20Range Å 2.50Mean Å 3.96Median Å 4.00Mode Å 3.60Standard Deviation Å 0.5693

Two other techniques, PALS and WAXD, were used to characterise the pores of nanoporouscarbon with increasing pyrolysis temperature.The PALS results shown in Figure 4-5 indicate an increase in the number of pores (increasingnumber of positrons annihilating in pores, I 2 ) and decrease in the average pore size (decreasinglifetime of positrons annihilating in pores, τ 2 ) with increasing pyrolysis temperature.Figure 4-5 : Characterisation of nanoporous carbon material manufactured at pyrolysistemperatures from 400 to 600°C using positron annihilation lifetime spectroscopy (PALS).The primary x-axis (τ 2 ) is the lifetime of position annihilating in the pores, which indicatesa decrease in pore size with increasing pyrolysis temperature. The secondary y-axis (I 2 ) isthe number of positrons annihilating in the pores, which indicates an increase in thenumber of pores with increasing pyrolysis temperature.Likewise, the WAXD results shown in Figure 4-6 tell a similar story. In general, the threesamples show high intensity between 2θ = 12° and 2θ = 27° and low intensity above 2θ = 35°.The lack of a single peak suggests a very wide distribution of d-spacings for the 400 and 600°Csamples, whereas at 800°C a more crystalline structure is evidenced by the peaks at 2θ = 23°and 40°. The intensity for the region 2θ = 12° to 35° is greatest for the sample pyrolysed at800°C and least for the sample pyrolysed at 400°C. This result suggests increasing poreconcentration with increased pyrolysis temperature, as concluded by the PALS results. For thesample pyrolysed at 400°C the peak in intensity occurs at 2θ = 18.8° or a d-spacing of 4.7 Å(marked in Figure 4-6). The sample pyrolysed at 600°C has a corresponding intensity peak at ad-spacing of 4.1Å while for the sample pyrolysed at 800°C this peak occurs at a d-spacing of94

3.9 Å. The d-spacing is thus decreasing with increasing pyrolysis temperature, in agreementwith the PALS results and HRTEM analysis.800700400°C 600°C 800°CCounts6005004004.7 A3.9 A4.1 A300200100012 17 22 27 32 37 42 47 52 57 622θ (°)Figure 4-6 : Characterisation of nanoporous carbon material manufactured at pyrolysistemperatures of 400, 600 and 800°C using wide angle x-ray diffraction (WAXD). Theprimary x-axis is the diffraction angle whilst number of counts or intensity is given on theprimary y-axis. The values of 2θ are converted to d-spacing using Bragg’s Law.PALS has been used previously by Tin et al [96] to characterise unsupported carbon membranesmade from the pyrolysis of the polyimides Matrimid and P84 at 800°C. Table 4-4 depicts thevalues of τ 2 and I 2 determined for these membranes in comparison with the membranes of thisresearch work. Whilst it is difficult to compare directly as different polymers and pyrolysisconditions have been used, the values of τ 2 are similar indicating that the pore sizes are withinthe range typical for nanoporous carbons. In the same work, the authors have also used WAXDto determine d-spacing of 3.68 and 3.51 Å, which is slightly lower than our pore size range asshown in Table 4-5 but again within the 3 – 6 Å range typical for nanoporous carbon made fromthe pyrolysis of PFA [84, 89].In a related study, Tin et al [97] use WAXD to specifically review the effect of pyrolysistemperature on the morphology of NPC membranes made from P84. This is a similar study tothat done by Steel and Koros [17] who also used WAXD to characterise NPC membranes madefrom Matrimid at different pyrolysis temperatures. Although the pyrolysis temperature range95

used for the polyimide NPC membranes is different to ours, the results show a decrease in the d-spacing or pore size with increasing pyrolysis temperature as shown in Table 4-5. Tin et al [97]also observe an increase in crystallinaty at 800°C, consistent with our results.For the highest pyrolysis temperature used to make NPC membranes from P84 and Matrimid(800°C) there is a peak located greater than 40 degrees-2-theta or 2.1 Å, which the authors ofboth papers attribute to a transition towards a more graphitic structure. Likewise, for oursamples made from PFA pyrolysed at 600°C and 800°C, there is a peak located greater than 40degrees-2-theta or 2.1 Å, which suggests a similar tendency towards a graphitic structure.The WAXD and PALS results are also consistent with those obtained by Burket et al [61] usingmethyl chloride adsorption. These workers find that pyrolysis of PFA at 300°C to 400°Cgenerates both mesopores (> 20 Å) and micropores (

Finally, it should be noted that both the WAXD and PALS analysis was conducted at ambienttemperature. In comparing these results to the permeation data at elevated temperatures below,it should be noted that the pore size may vary slightly with temperature. Furthermore, theexpansion of the metal substrate may also assist slightly in pore opening at higher temperatures.This means that the pore size of a membrane tested at elevated operating temperatures might besomewhat larger than the values given above.4.2.3 Pure Gas PerformanceA summary of the pure gas performance results for the membranes made between 400 and600°C is given below in Table 4-6. The pure gas performance results were used to assess theimpact of pyrolysis temperature upon the performance of the resulting NPC membranes.Table 4-6 : Pure gas performance results for the final coats of membranes made between400 and 600°C.Membrane Temp Coats CarbonPermeancemol/m 2 .s.Pa x10 -10Permselectivity°C mg/cm 2 CO 2 N 2 CH 4 CO 2 /N 2 CO 2 /CH 4A 400 2 7.04 0.28 0.05 N/A 5.5 N/AB 400 2 7.58 0.23 0.04 N/A 6.1 N/AC 400 3 7.87 2.69 0.21 0.18 12.6 14.8D 400 4 8.77 3.44 1.80 2.56 1.9 1.3A 450 2 5.89 0.24 0.10 N/A 2.4 N/AB 450 2 6.15 3.20 0.91 N/A 3.5 N/AC 450 4 6.91 0.07 0.01 0.01 9.7 12.0A 500 2 5.29 1.2 1.0 1.3 1.2 0.9B 500 2 6.10 7.3 1.9 2.2 3.9 3.3C 500 3 6.13 25.7 17.7 22.4 1.5 1.1D 500 3 8.12 61.4 53.5 74.0 1.1 0.8A 550 2 5.18 22.6 1.9 1.7 11.7 13.0B 550 4 5.19 45.6 4.1 N/A 11.2 N/AC 550 3 5.22 31.8 2.4 3.6 13.0 8.9D 550 4 5.58 48.9 2.4 N/A 20.6 N/AE 550 2 6.07 22.4 2.0 2.1 11.5 10.6F 550 3 6.88 57.3 16.2 19.2 3.5 3.0A 600 2 4.62 78.6 31.1 40.8 2.5 1.9B 600 3 5.73 334.3 287.2 444.1 1.2 0.8C 600 4 6.28 45.9 24.7 35.6 1.9 1.397

Figure 4-7 shows that the pure gas CO 2 permeance is consistently higher than that of N 2 andCH 4 . This trend indicates that Knudsen diffusion is not dominant, as in this case, the relativemolecular weights would imply a lower CO 2 permeance. Rather, the trend is indicative of eithermolecular sieving, as the kinetic diameter of CO 2 (3.3 Å) is substantially smaller than that of N 2(3.64 Å) and CH 4 (3.8 Å), and/or surface diffusion as CO 2 is more condensable than the othergases.For all gases, there is an increase in the pure gas permeance with increasing pyrolysistemperature. This permeance increase can be directly related to the increased in porosity withincreasing pyrolysis temperature observed in both the PALS and WAXD results as discussed inthe previous section, which would increase the rate of diffusion.1.00E-061.00E-07CO2 N2 CH4Expon. (CO2) Expon. (N2) Expon. (CH4)Permeance (mol/m 2 .s.Pa)1.00E-081.00E-091.00E-101.00E-111.00E-121.00E-13300 350 400 450 500 550 600 650Pyrolysis Temperature (°C)Figure 4-7 : The effect of pyrolysis temperature on permeance showing a trend ofincreasing permeance with increasing pyrolysis temperature.However, there was no observed correlation with increasing pyrolysis temperature andpermselectivity as shown in Figure 4-8. This result suggests that the change in pore size asobserved by PALS and WAXD is possibly too insignificant to have any impact on thepermselectivity.98

25.00CO2/N220.00CO2/CH4Permselectivity15.0010.005.000.00300 350 400 450 500 550 600 650Pyrolysis Temperature (°C)Figure 4-8 : The effect of pyrolysis temperature on permselectivity showing no clear trendbetween selectivity and pyrolysis temperature.As discussed in the following paragraph, the amount of carbon added to each membrane andtherefore the presence of microcracking appears to have a greater effect on the selectivity thanthe pyrolysis temperature.The change in permeance and permselectivity with increasing number of coats is presented for asingle NPC membrane in Figure 4-9. A selective membrane is not observed until the membranelayer is complete upon the second or third coat. However, as further coats are applied, crackingbegins to occur in the membrane layer. If the cracks are microscopic, Knudsen diffusion willdominate and the permselectivity will again drop below 1. If the cracks are significant,convective flow will dominate and permselectivity will be 1 as is the case for the exampleshown in Figure 4-9.The permselectivity results as a function of the amount of carbon deposited and the pyrolysistemperature for membranes made at 400 and 550°C including previous coats, where data wasavailable, are presented in Table 4-7 and Figure 4-10.These results reveal that for the lower manufacturing temperature, it is necessary to deposit alarger amount of carbon (~ 8 mg/cm 2 ) before a selective membrane is produced. Conversely, atthe higher manufacturing temperature, a smaller amount of carbon (~6 mg/cm 2 ) is required toproduce a selective membrane.99

1.00E+0010.00CO 2 Permeance (mol/m 2 .Pa.s)1.00E-011.00E-021.00E-031.00E-041.00E-051.00E-061.00E-071.00E-081.00E-09Carbon filling the supportand creating top layerKnudsen DiffusionCO 2 / N 2 Selectivity ~ 0.8Membrane LayerCreatedSelectivity > 1Cracking BeginsSelectivity Permeance 5.000.00-5.00-10.00-15.00CO 2 /N 2 Permselectivity1.00E-101.00E-11-20.000 1 2 3 4# of CoatsFigure 4-9 : Change in permeance and selectivity with number of coats for NPCmembrane D made at 400°C. Knudsen diffusion dominates until a selective membrane isformed with the third coat. However, the final 4 th coat results in cracking of the membranelayer and a subsequent decrease in selectivity and increase in permeance.Table 4-7 : Pure gas performance results for each coat for membranes at 400 and 550°C.Membrane Temp Coats CarbonPermeancemol/m 2 .s.Pa x10 -10Permselectivity°C mg/cm 2 CO 2 N 2 CH 4 CO 2 /N 2 CO 2 /CH 4A 400 2 7.04 0.28 0.05 N/A 5.5 N/AB 400 2 7.58 0.23 0.04 N/A 6.1 N/AC 400 2 7.38 3.2 0.91 1.1 3.5 2.9C 400 3 7.87 2.7 0.21 0.18 12.6 14.8D 400 2 6.71 2.2 2.0 N/A 1.1 N/AD 400 3 7.57 1.9 0.27 0.24 6.8 7.8D 400 4 8.77 3.4 1.8 2.6 1.9 1.3A 550 2 5.18 22.6 1.9 1.7 11.7 13.0B 550 4 5.19 45.6 4.1 N/A 11.2 N/AC 550 2 4.46 98.4 52.2 75.2 1.9 1.3C 550 3 5.22 31.8 2.4 3.6 13.0 8.9D 550 4 5.58 48.9 2.4 N/A 20.6 N/AE 550 2 6.07 22.4 2.0 2.1 11.5 10.6F 550 3 6.88 57.3 16.2 19.2 3.5 3.0100

(a)25.0550 °C 400 °C20.0CO 2 /CH 4 Permselectivity15.010.05.00.02.00 3.00 4.00 5.00 6.00 7.00 8.00 9.00 10.00Carbon Deposited (mg/cm 2 )(b)25.0550 °C 400 °C20.0CO 2 /N 2 Permselectivity15.010.05.00.02.00 3.00 4.00 5.00 6.00 7.00 8.00 9.00 10.00Carbon Deposited (mg/cm 2 )Figure 4-10 : Change in (a) CO 2 /CH 4 permselectivity and (b) CO 2 / N 2 permselectivity withamount of carbon deposited for selected membranes manufactured at pyrolysistemperatures of 400 and 550 °C. The decrease in selectivity after a maximum value hasbeen reached represents the point at which cracking occurs. For the membranesmanufactured at 550°C, this critical point is at ~ 6 mg/cm 2 , whereas for the membranesmanufactured at 400°C, this critical point is at ~ 8 mg/cm 2 .101

Another interesting observation is the order of permeance for the three molecules, CO 2 , N 2 andCH 4 as additional carbon is added. Under Knudsen diffusion (based upon molecular weight), therate of permeance is CH 4 >N 2 > CO 2 and therefore 1> CO 2 /N 2 > CO 2 /CH 4 . This is the trend thatis observed when the amount of carbon added is too low and a membrane layer has not yetformed or too high and microscopic cracking has occurred.As the amount of carbon is increased, the trend becomes CO 2 > CH 4 > N 2 , suggesting amechanism based upon surface diffusion where CO 2 is the most adsorptive component and N 2 isthe least adsorptive component. In this case the selectivities are CO 2 /N 2 > CO 2 /CH 4 > 1 as isapparent for the membranes made at 550°C shown in Figure 4-10.When the amount of carbon is close to the critical point, the membrane layer is complete andwithout any larger pores or defects. In this case the rate of permeance is CO 2 >N 2 > CH 4 andhence CO 2 /CH 4 > CO 2 /N 2 >> 1 suggesting a mechanism that is based upon molecular sieving asis apparent for the membranes made at 400°C shown in Figure 4-10.4.2.4 Mixed Gas PerformanceMixed gas testing was performed for several of the higher performing supported NPCmembranes. The conditions for testing the membranes using a mixed gas feed and the methodsused for calculating the permeance and selectivity were as previously presented in Section 3.4.2.The differences in the results for the pure gas and mixed gas testing of two membranes, onemade at a pyrolysis temperature of 400°C (membrane C from Table 4-2) and one made at apyrolysis temperature of 550°C (membrane C from Table 4-2) are given in Table 4-8. Themixed gas testing of the membrane made at 400°C was completed immediately following thepure gas testing, whilst the testing of the membrane made at 550°C was completed usingregeneration that removes the majority of the adsorbed water as discussed in Section 3.4.2.The results as shown in Table 4-8, indicate that mixed gas testing results in a higher CO 2 /CH 4selectivity. Unfortunately, the results can not be compared directly because of the differing feedpartial pressures of CO 2 and CH 4 , which is limited by the operating pressure of the equipment.With CO 2 being a more adsorptive molecule, as is discussed in Chapter 5, it is suggested thatwith a mixed gas feed the adsorption of CO 2 molecules within the porous carbon actuallyinhibits the transport of CH 4 molecules resulting in a higher selectivity than with pure gas feedtesting.102

Table 4-8 : Pure gas vs. mixed gas results for membranes manufactured at 400 and 550°C.An increased CO 2 /CH 4 selectivity is seen with the mixed gas testing.PyrolysisTempFeedPressure∆P CO 2 ∆P CH 4°C kPag kPa kPaCO 2Permeancemol/m 2 .s.Pax 10 -10CH 4Permeancemol/m 2 .s.Pax 10 -10CO 2 /CH 4Selectivity400Pure Gas 850 950 950 3.2 ± 0.04 0.2 ± 0.00 16.0 ± 0.31Mixed Gas 1500 150 1350 6.3 ± 0.33 0.3 ± 0.0221.0 ± 1.53(↑ 31 %)550Pure Gas 400 500 500 31.8 ± 0.44 3.6 ± 0.05 8.8 ± 0.17Mixed Gas 1200 120 1080 22.7 ± 1.18 1.5 ± 0.0815.1 ± 1.10(↑ 70 %)4.2.4.1 Effect of Feed PressureTwo membranes made at 550°C (B and D from Table 4-2) were each tested using a CO 2 /N 2mixed gas at feed pressures of 500 and 1000 kPa and at feed temperatures of 35 and 100°C.There is a reduction in the CO 2 permeance with increasing pressure for both membranes asshown in Figure 4-11. The reduction in the permeance is more pronounced at the loweroperating temperature. The reduction in permeance with increasing pressure is attributed to adecrease in sorptivity as the molecular sieving transport mechanism is not dependent on feedpressure [84, 105]. Whilst there is an increase in the number of molecules adsorbed as thepressure is increased as shown in Chapter 5, the value of the sorptivity, which is normalised forpressure, decreases.Interestingly, the relative reduction in the N 2 permeance with increased feed pressure is slightlyhigher than that for the CO 2 permeance resulting in a slight increase in the CO 2 /N 2 selectivitywith increased feed pressure as shown in Figure 4-12. This is a curious result given that N 2molecules are much less adsorptive than CO 2 as discussed in Chapter 5. With the exception ofMembrane D at 35°C, a firm conclusion can not be made on the impact of increasing feedpressure on the CO 2 /N 2 selectivity due to the magnitude of the error margins.103

CO 2 Permeance (mol/m 2 .s.Pa)1.0E-089.0E-098.0E-097.0E-096.0E-095.0E-094.0E-093.0E-092.0E-09Membrane B 35°C Membrane B 100°CMembrane D 35°C Membrane D 100°C1.0E-09400 500 600 700 800 900 1000 1100 1200Feed Pressure (kPag)Figure 4-11 : Change in CO 2 permeance with increasing feed pressure. Increasing the feedpressure reduces the permeance.30.0Membrane B 35°C Membrane B 100°C25.0Membrane D 35°C Membrane D 100°CCO 2 /N 2 Selectivity20.015.010.05.00.0400 500 600 700 800 900 1000 1100 1200Feed Pressure (kPag)Figure 4-12 : Change in CO 2 /N 2 selectivity with increasing feed pressure. Increasing thefeed pressure has the slightly increasing the selectivity.104

4.2.5 High Temperature PerformanceThe mixed gas separation performance of membranes made at 400°C (membrane C in Table 4-2) and 550°C (membrane A in Table 4-2) was tested at operating temperatures up to 200°C.4.2.5.1 Effect on PermeanceIncreasing the operating temperature increases the energy of the gas particles and therefore thediffusion of the gas particles through the carbon membrane. However, as depicted in Figure 4-13, the membranes manufactured at higher temperatures, maintain a higher permeance over theoperating temperature range.The results show that permeance increases at a faster rate for CH 4 than CO 2 . It is thought thatwhilst the diffusion component of the permeance increases with temperature in a similar mannerfor both CH 4 and CO 2 , there is a significant reduction in the adsorption component of the CO 2permeance. Therefore, increasing the operating temperature has the effect of decreasing theselectivity due to the disproportional increase in the methane permeance.Mixed Gas Permeance (mol/m 2 .s.Pa)1.00E-07CH4 - 550 °C CH4 - 400 °CCO2 - 550 °C CO2 - 400 °C1.00E-081.00E-091.00E-101.00E-110 50 100 150 200 250Operating Temperature (°C)Figure 4-13 : Effect of operating temperature on mixed gas permeance. The permeanceincreases with increasing operating temperature due to the increased diffusion. Themembrane manufactured at 550°C maintains a higher permeance across the operatingtemperature range.105

An Arrhenius plot of the membrane permeance as a function of 1/T reveals a straight line (witha gradient equal to the activation energy) and is provided in Figure 4-14.Mixed Gas Permeance (mol/m 2 .s.Pa)1.00E-07CH4, 400°C PyrolysisCO2, 400°C PyrolysisCH4, 550°C PyrolysisCO2, 550°C PyrolysisExpon. (CH4, 400°C Pyrolysis) Expon. (CO2, 400°C Pyrolysis)1.00E-08Expon. (CH4, 550°C Pyrolysis) Expon. (CO2, 550°C Pyrolysis)1.00E-091.00E-101.00E-110.0020 0.0022 0.0024 0.0026 0.0028 0.0030 0.0032 0.00341/T (K -1 )Figure 4-14 : Arrhenius plot of permeance and operating temperature. The gradient of thefitted lines indicate the activation energies of permeance. The fitted lines are steeper forCH 4 resulting in higher activation energies as presented in Table 4-10.The accompanying values of the activation energy E p as calculated from Equation 2-38 arepresented in Table 4-9. These results are consistent with previously published work [15, 25, 26].Table 4-9 : Activation Energy (E p ) of CO 2 and CH 4 Permeance (kJ/mol) for NPCmembranes made at 400 and 550°C. Activation energies were calculated from thepermeance data over the range of operating temperatures using Equation 2-38. Data fromother authors is included for comparison.Polymer PrecursorPyrolysisTemperatureActivation Energy(E p ) CO 2Activation Energy(E p ) CH 4°C kJ/mol kJ/molPFA (this work) 400 3.9 ± 0.3 9.1 ± 1.3PFA (this work) 550 -0.8 ± 1.2 6.7 ± 0.2BPDA-pPDA polyimide [26] 550 0 14.5Phenolic Resin [15] 700 6.2 17.7106

The steep increase in permeance with temperature for CH 4 at both pyrolysis temperatures isreflected in positive activation energies (E p ) and indicates that molecular sieving is the dominantmechanism of transport for this species. As discussed in Section 4.2.2.2, the WAXD resultsindicated a distribution of pore sizes of less than 4.7 Å and 4.0 Å respectively for each pyrolysistemperature, which would suggest that a large number of the pores of both samples would becomparable in size to the kinetic diameter of methane (3.8 Å). If Knudsen diffusion and/orsurface diffusion were dominant then permeance would fall as operating temperature increased.While the two activation energies for CH 4 are comparable, the value of E p for CO 2 fallssignificantly as the pyrolysis temperature increases from 400 to 550°C. The narrowing of poresize predicted by both PALS and WAXD as discussed in Section 4.2.2.2 would be expected toincrease the activation energy for diffusion (E d ) for CO 2 , which has a kinetic diameter of 3.3 Å.The fall in the total activation energy (E p ) is thus unlikely to result from changes to the rate ofdiffusion. Rather, it would seem that the fall in total activation energy is related to an increase inthe heat of adsorption (∆H ads ).Burket et al [61] show that at pyrolysis temperatures up to 400°C, nanoporous carbon generatedfrom PFA retains residual polymer in polyaromatic domains as well as a number of oxygen andhydrogen moieties. These hetero-atoms and unreacted polymer are driven out above 400°C,leading to a pure carbon structure at 600°C. It would follow that the hetero-atoms and unreactedpolymer restrict the adsorption of CO 2 at the lower pyrolysis temperatures. Conversely, at550°C, the structure is almost entirely carbon, which would be expected to adsorb CO 2 strongly,leading to an enhanced ∆H ads and lower E p .4.2.5.2 Effect on SelectivityAs the permeance increases at a faster rate for CH 4 than CO 2 , increasing the operatingtemperature has the effect of decreasing the selectivity. The results as displayed in Figure 4.15show that the selectivity declines continuously as the operating temperature is increased. Thiscan be related to the relative importance of surface diffusion declining at these highertemperatures. The decline is slightly faster for the membrane prepared at 550°C, due to thehigher heat of adsorption. This means that the two curves converge at higher temperatures.Extrapolation suggests that this selectivity might fall to levels consistent with purely Knudsendiffusion (CO 2 /CH 4 selectivity = 0.6) at operating temperatures of around 400 to 500°C asshown in Figure 4-16. This is consistent with other researchers [8, 24] who also indicate thatKnudsen diffusion is the major contributor to the transport mechanism at high temperatures.107

98550 °C 400 °CCO 2 /CH 4 Selectivity765432100 50 100 150 200 250Operating Temperature (°C)Figure 4-15 : Effect of operating temperature on mixed gas selectivity for NPCmembranes manufactured at pyrolysis temperatures of 400 and 550°C. Increasing theoperating temperature has the effect of decreasing the selectivity due to thedisproportional increase in the CH 4 permeance. This is due the significant reduction in theCO 2 adsorption at the higher temperatures.CO 2 /CH 4 Selectivity9.008.007.006.005.004.003.002.001.00550 °C400 °CCO2/CH4 Knudsen SelectivityExpon. (550 °C)Expon. (400 °C)0.000 100 200 300 400 500 600Operating Temperature (°C)Figure 4-16 : Extrapolated effect of operating temperature on mixed gas selectivity forNPC membranes manufactured at pyrolysis temperatures of 400 and 550°C.Extrapolating the CO 2 /CH 4 selectivity relationship with operating temperature using anexponential fit shows that the selectivity will approach the Knudsen value of 0.6 at anoperating temperature between 400 and 500°C.108

4.2.5.3 Effect of Thermal CyclingThe impact on the performance of an NPC membrane from thermal cycling is presented inFigure 4-17. This NPC membrane was manufactured at 550°C and underwent 11 cycles ofregeneration for > 12 hours at 150°C.1.00E-05CH4 Permeance CO2 Permeance CO2/CH4 Selectivity20.0RRMembrane Exposed to Impurities15.0Permeance (mol/m 2 .s.Pa)1.00E-061.00E-071.00E-081.00E-09R R RR R R R RR10.05.00.0-5.0-10.0CO 2 /CH 4 Selectivity-15.01.00E-10-20.00 50 100 150 200 250 300 350 400 450 500Time (Hours)Figure 4-17 : Effect of thermal cycling on an NPC membrane. After eleven regenerationcycles the CO 2 and CH 4 permeance increased dramatically whilst the selectivity droppedto 1, indicating the presence of significant cracks.Following the 11 th regeneration, there was a dramatic increase in the permeance of both CH 4and CO 2 along with a drastic drop of the CO 2 /CH 4 . This significant drop in performancesuggests that a crack or several cracks had developed due to the continuous cycling of themembrane temperature from operation to regeneration.This result raises obvious concerns about the continual regeneration of NPC membranes in acommercial setting. Adsorbents (such as zeolite molecular sieve) are commonly known forhaving a limited lifetime due to the thermal cycling of the regeneration process [158]. Thisresults in change-out of the molecular sieve material after a particular time period (depending onthe adsorbent and the severity of regeneration process), which is a costly undertaking both dueto the new material required and the shutdown of production for the change-out if a sparesystem is not available.109

One of the supposed benefits of NPC membranes over polymeric membranes is their ability tobe regenerated resulting in lower maintenance and replacement costs. The effect of repeatedthermal cycling on NPC membranes is an area for further work.4.2.6 Concluding Remarks on Supported NPC MembranesIn summary, the results of the characterisation and pure gas performance testing conclude that ahigher permeance is achieved by the manufacture of the NPC membranes at a higher pyrolysistemperature (e.g. 550°C) as was also shown by Shiflett and Foley [75]. The PALS and WAXDcharacterisation results reveal that the porosity of the carbon increases with increasing pyrolysistemperature, which greatly influences the increase in permeance.There was no observed correlation between pyrolysis temperature and permselectivity. ThePALS and WAXD characterisation results suggest that the pore size change may be too small topresent any change in the permselectivity or that the presence of cracking has a more significantimpact.To produce a selective molecular sieving membrane with a minimal amount of cracking at550°C, it is necessary to limit the total carbon deposited to just under 6 mg/cm 2 due to the moreporous and therefore fragile nature of the carbon produced at this high temperature.Increasing the pyrolysis temperature beyond 550°C did not produce selective membranes,possibly due to the even more fragile nature of the carbon at 600°C.Testing the performance of an NPC membrane with a mixed gas feed provides a more accuraterepresentation of how the membranes would perform in a commercial setting than pure gasperformance testing. This is because mixed gas testing incorporates the interaction effectsbetween the different components in the feed gas.The results of pure and mixed gas testing for the same membranes presented in this chapterrevealed that the CO 2 /CH 4 mixed gas selectivity was greater than the CO 2 /CH 4 permselectivitydetermined from the pure gas testing. This result is probably explained by the competitiveadsorption effect of the more adsorbable component CO 2 “blocking” the way for CH 4permeance.The carbon dioxide/methane (CO 2 /CH 4 ) mixed gas performance measurements generally showan increase in permeance as the operating temperature is increased due to the dominance ofmolecular sieving. The exception is for CO 2 permeance of membranes prepared at a higher110

pyrolysis temperature (550°C) where adsorption processes are competitive with molecularsieving effects. This leads to a CO 2 permeance value that is relatively independent oftemperature. While this results in a membrane that is slightly more selective at 35°C, themembrane selectivities converge at higher operating temperatures. Indeed, extrapolationsuggests that the membrane selectivity would reduce to the Knudsen (CO 2 /CH 4 = 0.6) value atan operating temperature between 400 and 500°C.Whilst the selectivities of the membranes manufactured at different pyrolysis temperaturesconverge at higher operating temperatures, the permeance is still related the original pyrolysistemperature. This result implies that it is important to manufacture a membrane with highporosity/permeance rather than high adsorption/selectivity when considering high operatingtemperature applications such as CO 2 capture from natural gas.In conclusion, the results of mixed gas high temperature performance trials show that CO 2 /CH 4selectivity remains above 1 at operating temperature less than ~ 300°C. This is a promisingresult for high temperature applications. However, further work needs to be undertaken toevaluate the impact of thermal cycling on cracking of the NPC membrane surface.111

4.3 Modified-Supported NPC MembranesBy modifying the pore size of the support and thereby reducing the weight of carbon required tocreate the membrane layer, the chance of cracking is reduced as discussed in Section 2.3.1.3.Silica nanoparticles were produced, filtered into the stainless steel supports and then coated with2 to 3 layers of carbon to create the modified-supported NPC membranes.The manufacture and testing of the modified supported NPC membranes work was completedby Budi Suharto and Yik Koon (Evan) Ang as the final year research project for theirundergraduate engineering degree.4.3.1 Manufacture and CharacterisationThe two precursor solutions, as described earlier in Section 3.2.2.3 Table 3-4, required formanufacturing silica nanoparticles were reacted at various temperature and times until silicananoparticles with a size less than 300 nm were formed. The size of the silica nanoparticles wasdetected using scanning electron microscopy. A summary of the manufacturing results ispresented in Table 4-10. Typical SEM pictures of the nanoparticles are given in Figure 4-18.Table 4-10 : Effect of varying reaction temperature and time on the size of the producedsilica nanoparticles. The fourth sample with a reaction temperature of 50°C and time of 45mins gave monodisperse silica nanoparticles with an average size less than 300 nm.Method Reaction Temperature Reaction Time Particle Size°C mins nm1 21 120 500 +/- 202 39 30 310 +/- 203 39 60 295 +/- 204 (Selected for Filling) 50 45 275 +/- 20Figure 4-18 : Silica nanoparticles, with a size less than 300 nm produced from the reactionof ethanol, ammonia and TEOS at a temperature of 50 °C for 45 mins.112

The supports were then filled up to 4 times with the silica nanoparticles (< 300 nm) until silicawas clearly seen on the surface of the support after drying at 100°C overnight.To determine the effect of the silica nanoparticles on the stainless steel supports, pure gasperformance testing was undertaken and the results are shown in Table 4-11 and Figure 4-19.The addition of the silica nanoparticles decreases the permeance of the porous stainless steelsupport by almost an order of magnitude due to the reduction in the pore size and porosity.However, the selectivity does not alter significantly indicating that the reduction in the porositywas insufficient to cause a change in the transport mechanism and as such bulk flow was stilldominant.Table 4-11 : Effect of the addition of silica nanoparticles on the permeance and selectivityof the porous stainless steel support. The error in the permeance is ± 1.38% and the errorin the permselectivity is 1.95%.SupportPyrolysisTempSilica Permeance (mol/m 2 .s.Pa) x 10 -5 Permselectivity°C mg/cm 2 CO 2 N 2 CH 4 CO 2 /N 2 CO 2 /CH 4Blank 0 5.9 6.3 5.1 0.9 1.2A 550 0.0526 1.6 1.5 1.3 1.0 1.2B 550 0.3288 1.3 1.3 1.0 1.0 1.3C 550 0.5459 0.9 0.8 0.6 1.1 1.6D 550 2.4728 0.6 0.7 0.6 0.9 1.01.00E-04CO2 N2 CH4Permeance (mol/m 2 .s.Pa)1.00E-051.00E-060 0.5 1 1.5 2 2.5 3Mass of Silica Nanoparticles Added (mg/cm 2 )Figure 4-19 : Effect of the addition of silica nanoparticles on reducing the permeance ofthe stainless steel support suggesting some reduction in pore size and porosity.113

4.3.2 Membrane PerformanceAs expected for these modified supports, less carbon was required to create the membrane layerthan the un-modified supported membranes as the carbon was not filling the pores of thesupport. Likewise, less carbon was required for the onset of cracking. Two silica containingsupports (B and D) were both initially coated with 5 ml of PFA solution, which in turn gavegreater than 4 mg/cm 2 of carbon deposited onto the modified-support. Cracking was clearlyvisible on the surface of these membranes. A further two silica containing supports (A and C),were coated with 1 – 2 ml of PFA solution leading to less than 2 mg/cm 2 of carbon depositedonto the modified support. No cracking was seen on the surface of these two membranes and theperformance results showed Knudsen selectivity (CO 2 /N 2 = 0.8 and CO 2 /CH 4 = 0.6), whichindicated that the membrane layer was not complete. A second coat was then added to completethe membrane layer for these two supports.The performance results as each coat of carbon was added are presented in Table 4-12. Figures4-20 and 4-21 show the effect on permeance and selectivity, respectively, as the amount ofcarbon added to the modified support was increased. As the amount of carbon added wasincreased above 1.5 mg/cm 2 , the permeance dropped dramatically and the selectivity increasedabove 1, which suggested the formation of a selective layer. However, when the amount ofcarbon added was increased above 4 mg/cm 2 cracking appeared. Comparing this result to themembranes manufactured at 550°C without silica nanoparticles this is a reduction in the criticalcracking point from 6 mg/cm 2 to 4 mg/cm 2 .Interestingly, the membranes manufactured with silica nanoparticles in the support showed anorder of magnitude lower overall permeance than those manufactured without silicananoparticles which had a similar or higher selectivity. This is an interesting result given theamount of carbon added for the modified-supported membranes was lower, suggesting that thethickness of the membrane was lower and therefore the permeance should be higher. However,there was a slight but important difference in the manufacturing procedure of the two differenttypes of membranes. The membrane manufactured with the silica nanoparticles in the supportwere coated over 2 consecutive days whereas the membranes manufactured without the silicananoparticles were coated with a 5 day break between the first and the second coat. During the 5day break, the first coat was exposed to the atmosphere. As was discussed earlier in Section3.2.2.1.2, it is believed that exposing the first coat to oxygen opens the pores of carbon that hasfilled the pores of the support such that when the second coat is added, it is only this coat thatforms the membrane layer.114

Table 4-12 : Addition of carbon to supports modified with silica nanoparticles. The errorin the permeance is ± 1.38% and the error in the permselectivity is 1.95%.Membrane Coat Carbon Permeance (mol/m 2 .s.Pa) x 10 -10 Permselectivitymg/cm 2 CO 2 N 2 CH 4 CO 2 /N 2 CO 2 /CH 4A 1 1.6 908 1010 1370 0.9 0.7A 2 2.1 3.2 1.0 1.3 3.3 2.5B 1 4.2 444 395 504 1.1 0.9C 1 1.6 679 660 991 1.0 0.7C 2 3.2 6.8 2.6 2.8 2.7 2.5D 1 4.5 632 635 737 1.0 0.91.00E-041.00E-05CO2 N2 CH4Permeance (mol/m 2 .s.Pa)1.00E-061.00E-071.00E-081.00E-091.00E-101.00E-110 0.5 1 1.5 2 2.5 3 3.5 4 4.5 5Mass of Carbon Added (mg/cm 2 )Figure 4-20 : Effect of the addition of carbon on permeance to supports filled with silicananoparticles. The membrane layer begins to form around 1.5 – 2 mg/cm 2 of carbon butcracks at 3.5 mg/cm 2 .5.04.54.03.5CO2/N2CO2/CH4Selectivity3.02.52.01.51.00.50.00 0.5 1 1.5 2 2.5 3 3.5 4 4.5 5Mass of Carbon Added (mg/cm 2 )Figure 4-21 : Effect of the addition of carbon on selectivity to supports filled with silicananoparticles. The selectivity results agree with the permeance results, which suggestcracking of the formed membrane layer occurs around 3.5 mg/cm 2 .115

4.3.3 Concluding Remarks on Modified-Supported NPC MembranesThe addition of silica nanoparticles to the porous support prior to coating with PFA andpyrolysis reduced the amount of carbon required to produce a selective layer. The onset ofcracking occurred at a lower amount of carbon added making it more difficult to produceuncracked membranes.This was a disappointing outcome given the promising work published by Rajagopalan [28] etal and Merritt et al [27] who successfully demonstrated a factor of two increase in permeancealong with no sacrifice in selectivity upon the addition of silica nanoparticles.116

5 UNDERSTANDING THE HIGH TEMPERATURE PERFORMANCE OF NPCMEMBRANES THROUGH GAS ADSORPTION MOLECULAR SIMULATIONAND EXPERIMENTS5.1 IntroductionThe adsorption of gases onto nanoporous carbon has been studied using molecular simulationand mass uptake experiments to further understand the transport mechanism and performance ofNPC membranes. The results are presented in this chapter.5.2 Molecular SimulationThe adsorption of pure and mixed gases on nanoporous carbon (represented by C 168 Schwarzite)was simulated using a molecular simulation model based upon the grand canonical Monte Carlomethod as described in Section 3.5.1.5.2.1 Pure GasesThe adsorption isotherms of pure gases (CH 4 , N 2 and CO 2 ) from 27 to 200°C up to 10,000 kPawere simulated in order to calculate the individual heats of adsorption of the three gases onnanoporous carbon.The adsorption isotherms computed for the adsorption of CO 2 , CH 4 and N 2 onto NPC at selectedtemperatures are presented in Figures 5-1, 5-2 and 5-3, respectively. As the operatingtemperature of the system is increased, the loading decreases more prominently for CO 2 than forCH 4 and N 2 . The loading of CO 2 also begins at lower pressures than CH 4 and lower again thanN 2 .The adsorption isotherms were then fitted using the Langmuir model, as described in Section2.2.2.3. The Langmuir model fitted well for CH4 and N2 for all pressures, and for CO2 itprovided a good fit for the region where the pressure is greater than 0.01 kPa and less than 1000kPa. For CO 2 adsorption at pressures higher than 1000 kPa, it becomes necessary to use adouble Langmuir model, as it more accurately captures the deviation from the ideal Langmuirmodel at higher densities [45]. However, the partial pressure of CO 2 in the typical mixed gasfeeds studied in this research work is less than 1000 kPa and as such the single Langmuir fit forCO 2 up to pressures of 1000 kPa is deemed sufficient.117

The values of the Langmuir constants, b and V m for the adsorption of pure CO 2 , CH 4 and N 2 arepresented in Table 5-1. For each gas, the value of the Langmuir rate constant b decreases withincreasing temperature indicating the reduction of adsorption with temperature. Likewise theconstant V m , which indicates the maximum number of adsorbed particles decreases withincreasing temperature.The values of the Langmuir rate constant b are highest for CO 2 followed by CH 4 and then N 2 ,reasserting that the adsorptive strength of the pure gases upon nanoporous carbon is of the orderCO 2 >>CH 4 >N 2 .35°C 100°C 150°C 200°C8.00Langmuir Fit (35°C) Langmuir Fit (100°C) Langmuir Fit (150°C) Langmuir Fit (200°C)7.00Loading (mmol/gram NPC)6.005.004.003.002.001.000.000 1 10 100 1000 10000Pressure (kPa)Figure 5-1 : Simulated adsorption isotherms of pure CO 2 onto nanoporous carbon.Increasing the temperature decreases the amount of CO 2 adsorbed.35°C 100°C 150°C 200°C8.00Langmuir Fit (35°C) Langmuir Fit (100°C) Langmuir Fit (150°C) Langmuir Fit (200°C)7.00Loading (mmol/gram)6.005.004.003.002.001.000.000 1 10 100 1000 10000Pressure (kPa)Figure 5-2 : Simulated adsorption isotherms of pure CH 4 onto nanoporous carbon.Increasing the temperature decreases the amount of CH 4 adsorbed.118

35°C 100°C 150°C 200°C8.00Langmuir Fit (35°C) Langmuir Fit (100°C) Langmuir Fit (150°C) Langmuir Fit (200°C)7.00Loading (mmol/gram)6.005.004.003.002.001.000.000 1 10 100 1000 10000Pressure (kPa)Figure 5-3 : Simulated adsorption isotherms of pure N 2 onto nanoporous carbon.Increasing the temperature decreases the amount of N 2 adsorbed.Table 5-1 : Values of the Langmuir constants (b and V m ) for the simulated adsorptionisotherms of pure CO 2 , CH 4 and N 2 on NPC with increasing temperatures.Temperature K 300 308 348 373 398 423 448 478°C 27 35 75 100 125 150 175 200CO 2b bar -1 16.4 12.5 3.55 1.71 0.924 0.523 0.323 0.231V m mmol/gram NPC 7.02 6.91 6.56 6.33 6.05 5.75 5.49 5.08CH 4b bar -1 2.02 1.53 0.533 0.295 0.178 0.12 0.091 0.059V m mmol/gram NPC 7.93 7.90 7.83 7.74 7.61 7.32 6.99 6.64N 2b bar -1 0.315 0.252 0.110 0.075 0.057 0.040 0.031 0.026V m mmol/gram NPC 7.62 7.55 6.99 6.57 6.10 5.76 5.48 5.065.2.1.1 Heats of AdsorptionBy rearranging Equation 2-14 to provide a linear relationship, as shown as Equation 5-1, theheat of adsorption can be calculated from the gradient of the plot of 1/T versus Ln(b).∆HadsLn( b)= Ln(b0 ) +Equation 5-1RT119

The straight line plots of the values of b determined at various temperatures for the adsorptionof pure CO 2 , CH 4 and N 2 onto nanoporous carbon are presented in Figure 5-4.4.03.02.01.0Carbon Dioxide Nitrogen Methaney = 3.5576x - 9.022R 2 = 0.9994y = 2.8649x - 8.8724R 2 = 0.999ln(b)0.0-1.0y = 2.0464x - 8.029R 2 = 0.9977-2.0-3.0-4.0-5.01.0 1.5 2.0 2.5 3.0 3.51000/T (K -1 )Figure 5-4 : Straight line plot of 1/T versus the simulated Ln(b) for pure gases fordetermining the heat of adsorption (∆H ads ).The values of the resulting heats of adsorption (∆H ads ) are given in Table 5-2. Also presented inTable 5-2, are the experimental values of ∆H ads as reported in the literature for graphite [159],nanoporous carbon from PFA [43] and nanoporous carbon from cellulose [12].Table 5-2 : Simulated values of the heat of adsorption (∆H ads ) of pure gases on nanoporouscarbon compared with experimental values reported in the literature.Gas CO 2 CH 4 N 2∆H ads kJ/mol kJ/mol kJ/molThis Work (Simulation) 33.2 26.7 19.1Graphite [159] 16.8 Data N/A 8.9NPC from PFA [43] 10.6 Data N/A 10.2NPC from Cellulose [12] 29.6 Data N/A 10.5As expected, the heats of adsorption for the pure gases as calculated from the molecularmodelling are higher than those for graphite due to the curved nature of the Schwarzite surface[113]. However, the literature experimental values obtained for adsorption of CO 2 and N 2 onnanoporous carbon manufactured from PFA ([43]) and Cellulose [12] are also both lower thanpredicted here via simulation. This difference between simulated and experimental results isdiscussed further in Section 5.3.120

5.2.2 Mixed GasesThe adsorption behaviour of two mixtures was also modelled. The two mixtures were (a)CO 2 :CH 4 (10:90 vol%) representing CO 2 separation from natural gas and (b) CO 2 :N 2 (10:90vol%) representing CO 2 separation from synthesis gas.5.2.2.1 Carbon Dioxide / MethaneThe simulated adsorption isotherms of CO 2 and CH 4 from a CO 2 :CH 4 (10:90 vol%) mixed gasat 35 and 200°C are shown in Figure 5-5. As expected from the pure gas simulation results, theadsorption of both components decreases with increasing temperature.8.007.0035°C CO2 200°C CO235°C CH4 200°C CH46.00Loading (mmol/gram)5.004.003.002.001.000.000.01 0.1 1 10 100 1000 10000Pressure (kPa)Figure 5-5 : Adsorption isotherms of mixed CO 2 /CH 4 onto nanoporous carbon. Increasingthe temperature decreases the amount of CO 2 and CH 4 adsorbed.From the pure gas Langmuir isotherm fits shown in Figures 5-1 and 5-2, the saturation of CO 2(the point at which additional molecules are no longer adsorbed onto the NPC surface) at 35°Coccurs at around 100 to 1000 kPa and saturation of CH 4 occurs at a pressure >1000 kPa.Likewise, for the mixed bulk gas at 35°C, as shown in Figure 5-5, the NPC surface is saturatedwith CO 2 molecules at around 100 kPa whilst CH 4 adsorption continues until 10,000 kPa.At a system temperature of 200°C, adsorption is greatly reduced and as such much higherpressures are required for surface saturation to occur. From Figures 5-1 and 5-2, saturation ofCO 2 and CH 4 molecules onto the NPC surface at 200°C does not occur at pressures lower than121

10,000 kPa. Similarly, in the mixed gas scenario as shown in Figure 5-5, saturation has not beenachieved for either component for the pressure range shown.The impact on the mixed gas separation factor with increasing pressure for system temperaturesof 35 and 200°C is presented in Figure 5-6. In all cases, the simulated NPC is selective for CO 2with the CO 2 /CH 4 separation factor being greater than 1.9.008.00Feed Temperautre = 35°C Feed Temperature = 200°CCO 2 /CH 4 Separation Factor7.006.005.004.003.002.001.000.001 10 100 1000 10000 100000Pressure (kPa)Figure 5-6 : CO 2 /CH 4 mixed gas adsorption separation factor as a function of pressure forsystem temperatures of 35 and 200°C.At 35°C, the CO 2 /CH 4 separation factor reduces with increasing pressure because CO 2saturation has been reached at 100 kPa whereas CH 4 adsorption continues to occur. However, at200°C the CO 2 /CH 4 separation factor remains relatively constant for the pressure range studiedbecause saturation of neither component has occurred and thus the relative adsorption of CO 2and CH 4 from the bulk phase is the same.This result has surprising consequences for the application of CO 2 /CH 4 separation in that theCO 2 /CH 4 adsorptive selectivity is similar whether operating at a feed temperature of 35 or100°C and for feed pressures of 1000 kPa or greater.122

5.2.2.2 Carbon Dioxide / NitrogenThe simulated adsorption isotherms of CO 2 and N 2 from a CO 2 :N 2 (10:90 vol%) mixed gas at 35and 200°C are shown in Figure 5-7. As expected from the pure gas simulation results, theadsorption of both components decreases with increasing operating temperature.8735°C CO2 200°C CO235°C N2 200°C N26Loading (mmol/gram)5432200°C CO 2200°C N 2100 0 1 10 100 1000 10000Pressure (kPa)Figure 5-7 : Adsorption isotherms of mixed CO 2 /N 2 onto nanoporous carbon. Increasingthe temperature decreases the amount of CO 2 and N 2 adsorbed.From Figures 5-1 and 5-3, saturation of CO 2 at 35°C occurs at around 100 to 1000 kPa andsaturation of N 2 occurs at a pressure >> 10,000 kPa. Similarly, for the CO 2 /N 2 mixed gasadsorption at 35°C, as shown in Figure 5-7, the NPC surface is saturated with CO 2 molecules ataround 1000 kPa whilst N 2 adsorption continues beyond the pressure range shown.At a system temperature of 200°C, adsorption is greatly reduced and as such much higherpressures are required for surface saturation to occur. From Figures 5-1 and 5-3, saturation ofCO 2 and N 2 molecules onto the NPC surface at 200°C does not occur at pressures lower than10,000 kPa. Likewise, in the mixed gas scenario as shown in Figure 5-7, saturation has not beenachieved for either component for the pressure range shown.The impact on the CO 2 /N 2 mixed gas separation factor with increasing pressure for systemtemperatures of 35 and 200°C is presented in Figure 5-8. In all cases, the simulated NPC isselective for CO 2 with the CO 2 /N 2 separation factor being greater than 1.123

8070Feed Temperature = 35°C Feed Temperature = 200°CCO 2 /N 2 Separation Factor60504030201001 10 100 1000 10000 100000Pressure (kPa)Figure 5-8 : CO 2 /N 2 mixed gas adsorption separation factor as a function of pressure forsystem temperatures of 35 and 200°C.At both 35 and 200°C, the CO 2 /N 2 separation factor is relatively constant for the pressure rangebecause saturation of either component has not occurred and thus the relative adsorption of CO 2and N 2 from the bulk phase is the same.The absolute values of the CO 2 /N 2 separation factors are significantly higher than thecorresponding CO 2 /CH 4 separation factors. This result concurs with the larger difference in thepure gas CO 2 and N 2 heats of adsorptions (Section 5.2.1.1). Whilst there is a substantialdecrease in the CO 2 /N 2 separation factor when increasing the operating temperature from 35 to200 °C, it is still a promising result for the separation of CO 2 from N 2 at high temperatures.124

5.3 Gas Adsorption Experiments5.3.1 Pure Gases5.3.1.1 Carbon DioxideThe adsorption of pure CO 2 on nanoporous carbon measured experimentally at 35 and 200°C ispresented in Figure 5-9 below. The adsorption behaviour follows a Type 1 isotherm with nohysteresis as is expected for a material with nano-sized pores [44].CO 2 Uptake (mmol/gram NPC)6.05.04.03.02.01.0Adsorption at 35°C Desorption at 35°C Langmuir Fit at 35°CAdsorption at 200°C Desorption at 200°C Langmuir Fit at 200°C0.00 200 400 600 800 1000 1200 1400 1600 1800Pressure (kPa)Figure 5-9 : Experimental adsorption isotherms of pure CO 2 onto nanoporous carbon.The adsorption isotherms were then fitted using the Langmuir Equation, as described in Section2.2.2.3. The values of the Langmuir constants, b and V m are given in Table 5-3.Table 5-3 : Experimental values of the Langmuir constants (b and V m ) for the adsorptionisotherms of pure CO 2 on nanoporous carbon.Operating Temp K 308 478°C 35 200b bar -1 0.47 0.02V m mmol/gram NPC 3.6 5.6125

The values of b and V m are lower than those determined via pure gas simulation (as presentedearlier in Table 5-1). However, the value of b is only a little lower than the 1.5 bar -1 for CO 2adsorption at 30°C on NPC (from pyrolysis of cellulose) published by Lagorsse et al [12].A plot of the experimental results with the simulation results presented in Figure 5-10 highlightsthat the amount of adsorption seen experimentally is much lower than that predicted viasimulation.12Adsorption at 35°C ExperimentalAdsorption at 35°C SimulatedCO 2 Uptake (mmol/gram NPC)108642Adsorption at 200°C ExperimentalAdsorption at 200°C Simulated00 200 400 600 800 1000 1200 1400 1600 1800 2000Pressure (kPa)Figure 5-10 : Experimental and simulated adsorption isotherms of pure CO 2 onto NPC.There are a few possible explanations for the difference between the simulated and experimentalresults. The first is that the coordinates of the substrate used in the simulation was that ofSchwarzite (C 168 ) not nanoporous carbon. It is not possible to model nanoporous carbon directlybecause of the amorphous structure but the curved surface and sp 2 hybridized bonding ofSchwarzite is similar to that of nanoporous carbon [45]. However, Schwartize has a larger poresize than nanoporous carbon providing greater internal access to adsorbing gases and hence agreater total amount of adsorption at the various system pressures.The second possible explanation is that equilibrium was not reached in our adsorptionexperiments. At 35°C, the CO 2 adsorption experiment was run with a maximum equilibriumtime of 30 mins. Comparing this to work of Lagorsse et al [12] who determined an equilibriumtime of 30 min for CO 2 adsorption on NPC at 35°C and 200 kPa, it can be concluded thatequilibrium has been achieved at 35°C. The equilibrium time of 60 mins used in running theCO 2 adsorption experiments at 200°C is therefore more than sufficient.126

Another possible explanation for the difference in the results is an error within the buoyancycalculation. As discussed in Section 3.5.2, a correction is made to the adsorbed weights to allowfor the buoyed volume of the sample.The standard method for making this correction uses non-adsorbing Helium gas to calculate thebuoyed weight. Another method that can be used calculates the buoyed volume using themasses and densities of the sample and reference sides as is also discussed in Section 3.5.2.The buoyed volumes calculated using these two methods for the adsorption of CO 2 at 35 and200°C are presented below in Table 5-4. The NPC sample size used at 35°C was 50 mg and 107mg at 200°C.The error bands shown on the graphs in this chapter have been calculated to include this error inthe buoyed volume (detailed in Appendix C Section C.3.1) and are concluded to be small.Table 5-4 : Buoyed volume for CO 2 adsorption onto NPC at 35°C and 200°C calculatedfrom the methods of (1) helium adsorption and (2) equipment masses.Buoyed Volume ml Difference from He Method35°CHelium Adsorption Method 199 0Equipment Masses Method 185 - 7 %200°CHelium Adsorption Method 216 0Equipment Masses Method 221 + 2 %The final explanation is in the “freshness” of the sample. Oxygen chemisorption and waterphysisorption are well reported problems with nanoporous carbon, as discussed previously inSection 2.7.2.The drying of the sample at 350°C prior to adsorption testing would have removed most of thephysisorbed water however very high temperatures are necessary to remove chemisorbedoxygen. The occupation of adsorption sites with oxygen were not accounted for in the molecularsimulations.127

5.3.1.2 NitrogenThe adsorption of pure N 2 on nanoporous carbon measured experimentally at 35 and 200°C ispresented in Figure 5-12. The adsorption follows a Type 1 isotherm behaviour although there issome hysteresis upon desorption at 35°C. The hysteresis is most likely related to incompleteadsorption at 25°C particularly at the lower pressures.2.01.81.6Adsorption at 25°C Desorption at 25°C Langmuir Fit at 25°CAdsorption at 200°C Desorption at 200°C Langmuir Fit at 200°CN 2 Uptake (mmol/gram NPC)1.41.21.00.80.60.40.20.00 200 400 600 800 1000 1200 1400 1600 1800Pressure (kPa)Figure 5-11 : Experimental adsorption isotherms of pure N 2 onto nanoporous carbon.The adsorption isotherms were then fitted using the Langmuir Equation, as described in Section2.2.2.3. The values of the Langmuir constants, b and V m are given in Table 5-5.Table 5-5 : Experimental values of the Langmuir constants (b and V m ) for the adsorptionisotherms of pure N 2 on nanoporous carbon.Operating Temp K 298 478°C 25 200b bar -1 0.05 0.01V m mmol/gram NPC 3.2 4.7The value of b is lower in comparison with 0.19 bar -1 for N 2 adsorption at 30°C on NPC (frompyrolysis of cellulose) published by Lagorsse et al [12]. The values of b and V m are also lowerthan those determined via pure gas simulation (as presented earlier in Table 5-1).128

A plot of the experimental results with the simulation results presented in Figure 5-13 againhighlights that the amount of adsorption seen experimentally is significantly lower than thatpredicted via simulation, particularly at 25°C.N 2 Uptake (mmol/gram NPC)1098765432Adsorption at 25°C ExperimentalAdsorption at 25°C SimulatedAdsorption at 30°C Experimental Literature (Lagorsse et al)Adsorption at 200°C ExperimentalAdsorption at 200°C SimulatedAdsorption at 25°C Simulated10Adsorption at 200°C Simulated0 200 400 600 800 1000 1200 1400 1600 1800 2000Pressure (kPa)Figure 5-12 : Experimental and simulated adsorption isotherms of pure N 2 ontonanoporous carbon. The large discrepancy at 35°C is attributed to not achievingequilibrium in the adsorption experiments.As discussed in Section 3.5.2, the experimental data was collected using a maximumequilibrium time of 60 mins at both 25 and 200°C. This equilibrium time was confirmed to besufficient at 200°C but too short at 25°C. Lagorsse et al [12] state that it takes 14 hours for theadsorption of N 2 at 30°C and 40 kPa, confirming that the kinetics of N 2 adsorption on NPC arevery slow. This explains the lower b value obtained relative to the Lagorsse et al [12] results.Like the CO 2 adsorption data, the discrepancy between the experimental and simulated data at200°C is most likely due to the different substrate (C 168 Schwarzite) used in the simulation andthe reduction in available NPC sample adsorption sites due to exposure to air (oxygen).129

5.3.1.3 Heats of AdsorptionThe straight line plots of the values of b determined experimentally at the two testedtemperatures for the adsorption of pure CO 2 and N 2 onto nanoporous carbon are presented inFigure 5-14. Also included for comparison is the pure gas CO 2 and N 2 adsorption data publishedby Lagorsse et al for NPC manufactured from cellulose [12] and the results of the molecularsimulation presented in Section 5.2.1.6.04.0CO2 (This Work Experimental) CO2 (This Work Simulation) CO2 (Lagorsse et al)Nitrogen (This Work Experimental) N2 (This Work Simulation) Nitrogen (Lagorsse et al)ln(b)2.00.0∆H = 33 kJ/mol∆H = 19 kJ/mol-2.0∆H = 27 kJ/mol∆H = 10 kJ/mol∆H = 19 kJ/mol∆H = 13 kJ/mol-4.0-6.01.5 2.0 2.5 3.0 3.5 4.01000/T (K -1 )Figure 5-13 : Straight line plot of 1/T versus the simulated Ln(b) for pure gases fordetermining the heat of adsorption (∆H ads ).As seen from the calculated heats of adsorption shown in Table 5-6, CO 2 has a greater heat ofadsorption than N 2 in all cases, which indicates that CO 2 is better adsorbed than N 2 on NPC.This result concurs with the simulated mixed gas results presented in Section 5.2.2.2 that show agood CO 2 /N 2 separation factor with a 10:90 vol % CO 2 :N 2 feed.The heats of adsorption for CO 2 and N 2 predicted via molecular simulation are higher than thatseen experimentally. This difference is attributed the use of different substrates and to thepresence of chemisorbed oxygen from the atmosphere assuming potential adsorption sites forCO 2 and N 2 on the experimental NPC samples.Interestingly, despite having greater values of the Langmuir constants and therefore greaterextents of adsorption, the experimental heat of adsorption values for CO 2 and N 2 on NPCreported by Lagorsse et al [12] are lower than the experimental values reported here. The reason130

for this difference is not clear. One possible explanation is a difference in the internal structureor the proportion of surface impurities as the two carbons are made from different polymers.Table 5-6 : Values of the experimental and simulated heats of adsorption of pure gases onnanoporous carbon compared with values reported in the literature.Gas CO 2 N 2∆H ads kJ/mol kJ/molThis Work (Experimental) 27.0 12.6This Work (Simulated) 32.6 19.0NPC from Cellulose [12] 18.6 10.4Graphite [159] 16.8 8.95.3.1.4 Relating the CO 2 Heat of Adsorption to the CO 2 PermeanceThe activation energy of permeance (E p ) for CO 2 was presented earlier in Section 4.2.5. UsingEquation 2-38 from Section 2.6.1, it is possible to back calculate the activation energy ofdiffusion (E d ) for CO 2 from the CO 2 heat of adsorption (∆H ads ) presented above in Table 5-6.As expected when the value of E p is close to 0, the results shown in Table 5-7 indicate that theCO 2 diffusion activation energy is similar to that of the heat of adsorption. At lower operatingtemperatures CO 2 adsorption will dominate and at higher operating temperatures diffusion withdominate. A similar conclusion was reached by Strano and Foley [43], the results of which arealso given in Table 5-7 for comparison.Table 5-7 : A comparison of the experimental CO 2 heat of adsorption (∆H ads ) with theactivation energy of permeance (E p ) for a NPC membrane manufactured at 550°C (referSection 4.2.5) to calculate the activation energy of diffusion (E d ) via Equation 2-38.GasPermeanceActivation Energy(E p )Heat ofAdsorption(∆H ads )DiffusionActivation Energy(E d )kJ/mol kJ/mol kJ/molCO 2 (This Work) -0.800 27.0 26.2CO 2 [43] -1.43 10.6 9.17131

5.4 Concluding Remarks on Understanding the High Temperature Performanceof NPC Membranes through Gas Adsorption Molecular Simulation andExperimentsThe simulated and experimental isotherms for the adsorption of pure gases on NPC revealed thevalue of the heats of adsorption are in order of CO 2 > CH 4 > N 2 . Mixed gas simulations of theadsorption of CO 2 :CH 4 (90:10 vol%) and CO 2 : N 2 (90:10 vol%) mixtures, attested that CO 2 isindeed more adsorptive than CH 4 and N 2 with adsorptive separation factors greater than 1. Thegreater difference between the values of heat of adsorption for the CO 2 : N 2 pair compared withthe CO 2 :CH 4 pair has resulted in a significantly larger separation factor for the CO 2 :N 2 pair.Comparing the experimental and simulation results for CO 2 and N 2 revealed a significantdifference in the amount of pure gas molecules adsorbed to the NPC surface. This difference isattributed to the different substrate (C 168 Schwarzite) used in the simulation and also to thepossibility of a smaller amount of adsorption sites available for gas adsorption on theexperimental samples due to oxygen adsorption from the air.The adsorption results indicate that nanoporous carbon shows good promise as a material for theseparation of CO 2 from N 2 at high temperatures as is the case for the synthesis gas purification.However, they also indicate that any exposure of the nanoporous carbon to air may reduce theadsorption capacity.132

6 PERFORMANCE OF NPC MEMBRANES IN THE PRESENCE OFCONDENSABLE AND NON-CONDENSABLE IMPURITIES6.1 IntroductionThis chapter details the results of experiments undertaken to assess the impact of the presence ofsmall quantities of condensable (water, hexane and toluene) and non-condensable (H 2 S and CO)on the performance of supported NPC membranes manufactured from the pyrolysis ofpolyfurfuryl alcohol at 550°C.6.2 Condensable ImpuritiesThe impact of the presence of condensable impurities on the performance of NPC membranesmust be understood as part of assessing the suitability for commercial applications.For the existing polymeric membrane technology, the presence of condensable impurities canhave a significant effect on reducing the performance [103, 138], which often makes itnecessary to remove the impurities from the feed gas (via expensive pretreatment) prior toexposure to the membrane.The supported NPC membranes manufactured from PFA were exposed to three differentimpurities, namely water, hexane (representing heavier hydrocarbons) and toluene (representingaromatic hydrocarbons). Water, heavier and aromatic hydrocarbons are present as condensableimpurities in the separation of CO 2 from natural gas (CH 4 ).However, only water is present as a condensable impurity in the separation of CO 2 fromsynthesis gas (N 2 ). Assuming that the interaction of water with the NPC membrane will besimilar with a CO 2 /N 2 feed as with a CO 2 /CH 4 feed, water impurity tests have been completedwith only a mixed gas CO 2 /CH 4 feed. The impact of non-condensable impurities with a mixedgas CO 2 /N 2 feed is discussed in Section 6.3.The concentrations of condensable impurities in natural gas depend upon the operatingconditions of the separators used to stabilise the natural gas as it is produced from the reservoir.The condensable impurities carried over into the natural gas overhead from the final stage ofseparation will be at saturated levels.In order to assess the impact of the condensable impurities on the performance of the NPCmembranes, the concentration of the condensable impurity in the feed was increased by133

increasing the temperature of the saturating vessel containing the impurity at a constant pressureof either 1500 kPag or 1200 kPag. Details of the temperatures and saturation concentrationsused were presented previously in Table 3-6.In this work, the NPC membranes were regenerated overnight under a nitrogen purge (50ml/min) at 200 kPag. Overnight (12 - 16 hours) was deemed sufficient based upon the resultspresented in the literature as discussed in Section 2.7.3.6.2.1 WaterThe temperature of the saturating vessel containing water at 1200 kPag, was decreased from25°C to 10°C and then to 5°C, resulting in a concentration drop from 2390 ppm (0.24 mol%) to918 ppm (0.09 mol%) and finally 650 ppm (0.07 mol%).Firstly, looking at the impact of the water on the performance of a NPC membrane over a periodof time, it is evident that there is an initial drop in the CO 2 permeance as the membrane isexposed to water impurity as shown in Figure 6-1. The extent of this reduction in permeance isgreater with the increased concentration of water. This trend is observed whether the data isplotted as a percentage of the original permeance (as done in Figure 6-1) or on an absolute scale.130%Water 2390 ppm, Membrane at 100 °C% of Original CO 2 Permeance120%110%100%90%Water 918 ppm, Membrane at 35 °CWater 918 ppm, Membrane at 100 °CWater 650 ppm, Membrane at 100 °C80%70%0 50 100 150 200 250 300Time (min)Figure 6-1 : Normalised effect of water concentration on the reduction in CO 2 permeanceof a NPC membrane manufactured at 550°C as a function of time.As time progresses the CO 2 permeance is somewhat recovered and stabilises after 100 mins.134

The reduction in the CH 4 permeance also drops initially although not to the same extent as withCO 2 . This results in an initial drop in the CO 2 /CH 4 selectivity as shown in Figure 6-2. However,the reduction in the CH 4 permeance once the system has stabilised is similar to that of CO 2resulting in no net change in the stabilised CO 2 /CH 4 selectivity as also shown in Figure 6-2.130%Water 2390 ppm, Membrane at 100 °C% of Original CO 2 /CH 4 Selectivity120%110%100%90%80%Water 918 ppm, Membrane at 35 °CWater 918 ppm, Membrane at 100 °CWater 650 ppm, Membrane at 100 °C70%0 50 100 150 200 250 300Time (min)Figure 6-2 : Normalised effect of water concentration on the CO 2 /CH 4 selectivity of a NPCmembrane manufactured at 550°C as a function of time.The stabilised permeance as a function of increasing water concentration is presented in Figure6-3. The reduction in permeance is very small even at the highest concentration of water. Theimpact appears to be even smaller at the higher operating temperature due to the loweradsorptive strength of water although the results at 35 and 100°C are within the same margin oferror so it is not possible to make a firm conclusion.These results are quite different to those of Jones and Koros [31], who found a significantreduction in the O 2 and N 2 flux of NPC membranes made from polyimide when exposed tohumidified feeds. In their case, humidified feeds of up to 85% relative humidity were used andflux reductions as high as 70% were observed. It is possible that the physisorbed water alreadypresent upon our membranes from the exposure to atmosphere during installation is lesseningthe impact when exposed to more water in the feed stream.The impact of increasing water concentration upon selectivity is very slight (+/- 10%). Theselectivity change is minimal both at 35 and 100°C as shown in Figure 6-4.135

130%120%CH4 Permeance @ 35 °C CO2 Permeance @ 35 °CCH4 Permeance @ 100 °C CO2 Permeance @ 100 °C% of Original Permeance110%100%90%80%70%0 500 1000 1500 2000 2500 3000Water Concentration (ppm)Figure 6-3 : Normalised effect of water concentration on the permeance of a NPCmembrane manufactured at 550°C. The effect appears to be more pronounced on CO 2permeance and at lower operating temperatures.120%115%CO2/CH4 Selectivity @ 35 °C CO2/CH4 Selectivity @ 100 °C% of Original CO 2 /CH 4 Selectivity110%105%100%95%90%85%80%75%70%0 500 1000 1500 2000 2500 3000Water Concentration (ppm)Figure 6-4 : Normalised effect of water concentration on the selectivity of a NPCmembrane manufactured at 550°C. The effect is minimal.136

6.2.2 HexaneThe temperature of the saturating vessel containing hexane at 1200 kPag, was decreased from25°C to 10°C and then to 5°C, resulting in a concentration drop from 15,538 ppm (1.55 mol%)to 7844 ppm (0.78 mol%) and finally 6128 ppm (0.61 mol%).Firstly, looking at the impact of the hexane on the performance of the NPC membrane over aperiod of time, it is evident that there is an initial drop in CO 2 performance as the membrane isexposed to hexane impurity as shown in Figure 6-5.120%110%Hexane 15538 ppm, Membrane at 35 °C Hexane 15538 ppm, Membrane at 100 °CHexane 7844 ppm, Membrane at 35 °C Hexane 7844 ppm, Membrane at 100 °CHexane 6128 ppm, Membrane at 35 °C Hexane 6128 ppm, Membrane at 100 °C% of Original CO 2 Permeance100%90%80%70%60%0 50 100 150 200 250Time (min)Figure 6-5 : Normalised effect of hexane concentration on the reduction in CO 2 permeanceof a NPC membrane manufactured at 550°C as a function of time.The extent of the reduction in performance is larger with the increased concentration of hexanebut not necessarily with the decreased operating temperature as expected. After 60 mins, thepermeance is somewhat recovered but continues to then gradually decrease particularly at theoperating temperature of 35°C. In the longer term, the data indicates (albeit within the margin oferror) that the impact on the reduction in permeance is less at the operating temperature of100°C than 35°C. This is probably due to the lower adsorptive strength of hexane at the higheroperating temperature.Interestingly, when the NPC membrane is exposed to hexane there is little change in the initialCH 4 permeance resulting in a sharp drop in the CO 2 /CH 4 selectivity as shown in Figure 6-6.However, the CH 4 permeance then declines in a similar manner to the CO 2 permeance resultingin no net change in the stabilised CO 2 /CH 4 selectivity as also shown in Figure 6-6.137

% of Original CO 2 /CH 4 Selectivity120%110%100%90%80%70%Hexane 15538 ppm, Membrane at 35 °C Hexane 15538 ppm, Membrane at 100 °CHexane 7844 ppm, Membrane at 35 °C Hexane 7844 ppm, Membrane at 100 °CHexane 6128 ppm, Membrane at 35 °C Hexane 6128 ppm, Membrane at 100 °C60%0 50 100 150 200 250 300 350Time (min)Figure 6-6 : Normalised effect of hexane concentration on the CO 2 /CH 4 permeance of aNPC membrane manufactured at 550°C as a function of time.The stabilised permeance as a function of hexane concentration is presented in Figure 6-7.There is a trend of reducing permeance with increasing concentration at 35°C although this isless obvious at 100°C. Despite the uncertainty in the permeance measurement, it can beconcluded that up to hexane concentrations of ~ 15,600 ppm (1.5 mol%) the reduction in theCO 2 and CH 4 permeance is limited less than 20 %.These results for the lowest concentration of hexane are similar those of Vu et al [35] who alsoinvestigated the impact of condensable impurities on the performance of NPC membranes forthe separation of CO 2 and CH 4 . Whilst Vu et al [35], used n-heptane to represent a straight chainhydrocarbon rather than n-hexane, their results showed a small decrease in permeance with lowconcentrations of the impurity (100 to 300 ppm). The effect of a larger concentration of theimpurity was not investigated in their publication.The impact of the presence of hexane on the CO 2 /CH 4 selectivity is relatively small as presentedin Figure 6-8. Whilst it was commented in the previous paragraphs that the reduction in the CO 2permeance was slightly greater, the difference is not large enough to dramatically affect theselectivity. Despite the uncertainly in the permeance measurement as depicted by the error band,it can still be concluded that up to hexane concentrations of ~ 15600 ppm (1.5 mol%), thereduction in the selectivity is limited to less than 10%.138

This result is similar to that of Vu et al [35] who observed no net change in the CO 2 /CH 4selectivity of an NPC membrane made from polyimide when exposed to heptane.130%120%CH4 Permeance @ 35 °C CO2 Permeance @ 35 °CCH4 Permeance @ 100 °C CO2 Permeance @ 100 °C% of Original Permeance110%100%90%80%70%60%0 2000 4000 6000 8000 10000 12000 14000 16000 18000Hexane Concentration (ppm)Figure 6-7: Normalised effect of hexane concentration on the permeance of a NPCmembrane manufactured at 550°C. The effect appears to be more pronounced on the CO 2permeance and at lower operating temperatures.130%CO2/CH4 Selectivity @ 35 °C CO2/CH4 Selectivity @ 100 °C% of Original CO 2 /CH 4 Selectivity120%110%100%90%80%70%60%0 2000 4000 6000 8000 10000 12000 14000 16000 18000Hexane Concentration (ppm)Figure 6-8 : Normalised effect of hexane concentration on the CO 2 /CH 4 selectivity of aNPC membrane manufactured at 550°C. The impact is minimal.139

6.2.3 TolueneThe temperature of the saturating vessel containing toluene at 1500 kPag, was decreased from25°C to 10°C and then to 5°C, resulting in a concentration drop from 2549 ppm (0.25 mol%) to1146 ppm (0.11 mol%) and finally 860 ppm (0.09 mol%).Firstly, looking at the impact of the toluene on the performance of the NPC membrane over aperiod of time, it is evident that there is also a definite initial drop in CO 2 permeance as themembrane is exposed to toluene impurity as shown in Figure 6-9.120%110%Toluene 2549 ppm, Membrane at 35 °C Toluene 2549 ppm, Membrane at 100 °CToluene 1146 ppm, Membrane at 35 °C Toluene 1146 ppm, Membrane at 100 °CToluene 860 ppm, Membrane at 35 °C Toluene 860 ppm, Membrane at 100 °C% of Original CO 2 Permeance100%90%80%70%60%50%40%0 50 100 150 200 250 300 350Time (min)Figure 6-9 : Normalised effect of toluene concentration on the reduction in CO 2permeance of a NPC membrane manufactured at 550°C as a function of time.After 150 mins, the permeance is somewhat recovered and stabilised. The impact on thereduction in permeance appears to less at the lower concentrations of toluene and at the higheroperating temperature of 100°C. This is due to the lower adsorptive strength of toluene at thehigher operating temperature.Like the results with hexane exposure, when the NPC membrane is exposed to toluene there islittle change in the initial CH 4 permeance resulting in a sharp drop in the CO 2 /CH 4 selectivity asshown in Figure 6-10. However, the CH 4 permeance then declines in a similar manner to theCO 2 permeance resulting in no net change in the stabilised CO 2 /CH 4 selectivity as also shown inFigure 6-10.140

% of Original CO 2 /CH 4 Selectivity140%130%120%110%100%90%80%70%60%50%Toluene 2549, Membrane at 35 °C Toluene 2549 ppm, Membrane at 100 °CToluene 1146 ppm, Membrane at 35 °C Toluene 1146 ppm, Membrane at 100 °CToluene 860 ppm, Membrane at 35 °C Toluene 860 ppm, Membrane @100 °C40%0 50 100 150 200 250 300 350Time (min)Figure 6-10 : Normalised effect of toluene concentration on the CO 2 /CH 4 permeance of aNPC membrane manufactured at 550°C as a function of time.The stabilised value permeance as a function of toluene concentration is presented in Figure 6-11. There is a more definitive trend of reducing permeance with increasing concentration,particularly at 35°C.The impact upon the CO 2 permeance is greater than the CH 4 permeance at both 35 and 100°Cand more so than was observed with hexane. The greater reduction in CO 2 permeance in thepresence of toluene rather than hexane is presumably due to the ability of toluene to assumemore available adsorption sites and reduce CO 2 adsorption.The impact of the presence of toluene on the CO 2 /CH 4 selectivity, as presented in Figure 6-12,although small appears to be more pronounced than both water and hexane. It can be concludedthat up to toluene concentrations of ~ 2500 ppm (0.25 mol%), the reduction in the selectivity islimited to less than 15%.This result is similar to that of Vu et al [35] who observed no net change in the CO 2 /CH 4selectivity of an NPC membrane made from polyimide when exposed to toluene. Using mixedmatrix membranes made from NPC and polyimide, Vu et al [136] also reported a minimalimpact on the performance in the presence of toluene although the toluene concentrations werevery small (70.4 ppm).141

However, Tanihara et al [103] who have used much higher concentrations of toluene (7500ppm) reported a minor change in the H 2 /CH 4 selectivity of an NPC membrane. This is incomparison to the significant change in the H 2 /CH 4 selectivity of un-pyrolysised polyimidemembrane when exposed to toluene [103].130%120%CH4 Permeance 35 °C CO2 Permeance 35 °CCH4 Permeance 100 °C CO2 Permeance 100 °C% of Original Permeance110%100%90%80%70%60%0 500 1000 1500 2000 2500Toluene Concentration (ppm)Figure 6-11 : Normalised effect of toluene concentration on the permeance of a NPCmembrane manufactured at 550°C. The effect is more pronounced on the CO 2 permeanceand at lower operating temperatures.130%120%CO2/CH4 Selectivity 35 °C CO2/CH4 Selecitivity 100 °C% of Original Selectivity110%100%90%80%70%60%0 500 1000 1500 2000 2500Toluene Concentration (ppm)Figure 6-12 : Normalised effect of toluene concentration on the selectivity of a NPCmembrane manufactured at 550°C. The effect is minimal.142

6.2.4 Comparison of Condensable ImpuritiesAs a summary, the reduction in the CO 2 permeance and CO 2 /CH 4 selectivity for eachcondensable impurity and concentration level tested are presented in Tables 6-1 and 6-2. Thepercentage errors on each data point are +/- 7 %.The reduction in NPC membrane performance is greatest upon exposure to toluene then hexaneand lowest upon exposure to water. Likewise, it appears that the impact of the impurities isgreatest at the lower operating temperature where impurity adsorption is highest.The data is of a similar magnitude as the error margin making it difficult to make absoluteconclusions. However, it can be concluded that the case with the worst reduction in performance(2549 ppm of toluene at 35°C) is still only a 30% reduction on the original CO 2 permeance anda 20% reduction on the original CO 2 /CH 4 selectivity.Table 6-1 : Normalised stabilised reduction in CO 2 permeance with impurity presence.Impurity Feed Temp. Impurity Conc. Impurity Conc. Impurity Conc.°C ppm ppm ppmWater 650 918 2390Water 100 0 % 2 % 1 %Hexane 4980 6375 12627Hexane 35 6 % 13 % 13 %Hexane 100 0 % 2 % 5 %Toluene 860 1147 2549Toluene 35 7 % 18 % 24 %Toluene 100 6 % 13 % 12 %Table 6-2 : Normalised stabilised reduction in CO 2 /CH 4 selectivity with impurity presence.Impurity Feed Temp. Impurity Conc. Impurity Conc. Impurity Conc.°C ppm ppm ppmWater 650 918 2390Water 100 0 % 0 % 0 %Hexane 4980 6375 12627Hexane 35 2 % 3 % 1 %Hexane 100 2 % 1 % 4 %Toluene 860 1147 2549Toluene 35 6 % 13 % 12 %Toluene 100 5 % 4 % 3 %143

6.3 Non-Condensable ImpuritiesThe impact of the presence of two non-condensable impurities, hydrogen sulphide (H 2 S) andcarbon monoxide (CO), on the performance of NPC membranes has been assessed.Hydrogen sulphide (H 2 S) is present as an impurity in CO 2 removal from natural gas and fromsynthesis gas. The concentration of H 2 S in natural gas is dependent on the amount of sulphideproducing bacteria present in the reservoir and can range from only a few ppm to a severalmol%. Likewise, the concentration of H 2 S in synthesis gas is dependent on the concentration ofsulphur in the original hydrocarbon feed stock.Carbon Monoxide (CO) is present in synthesis gas as a product from the water gas shiftreaction. The concentration of CO in synthesis gas can be as high as several mol percent.Both gases are extremely toxic and often poisonous to catalysts and materials. Therefore, it isoften necessary to remove or reduce these impurities via pre-treatment. If a membraneseparation material, such as nanoporous carbon, is not adversely affected by the presence of H 2 Sor CO, a significant reduction in pre-treatment equipment, footprint and cost is possible.6.3.1 Hydrogen SulphideTo reduce the risk of H 2 S exposure to lab personnel in the instance of a leak, the experimentswere conducted using a pre-purchased bottle of CO 2 /N 2 with a specified 500 ppm of H 2 S.Therefore, it was not possible to change the gas phase concentration. It is assumed that theinteraction of H 2 S with the NPC membrane will be similar with a CO 2 /CH 4 feed as with aCO 2 /N 2 feed.The impact of the H 2 S on the CO 2 permeance of an NPC membrane manufactured at 550°Cover time is illustrated in Figure 6-13. The data has been normalised to the original value ofpermeance for the each operating temperature and pressure for a clearer comparison.There is an initial decline in the CO 2 permeance which then stabilises around 100 mins. Thestabilised drop in the CO 2 permeance is within 20 % of the original value at feed operatingtemperatures of 35 and 100°C and feed pressures of 500 and 1000 kPag. The impact of H 2 S onthe N 2 permeance is very similar to that of CO 2 resulting in a small drop (

Whilst the reduction in permeance and selectivity of the membrane is minimal, the results dosuggest that H 2 S is adsorbing onto the nanoporous carbon and slightly inhibiting the transport ofCO 2 and N 2 . This selective adsorption of H 2 S is in agreement with the experimental adsorptionstudies of H 2 S on activated carbon by Ritter and Yang [50] who observed that H 2 S was moreadsorptive than CO 2 .120%115%35°C 1000 kPa 35°C 500 kPa% of Original CO 2 Permeance110%105%100%95%90%85%80%75%100°C 1000 kPa 100°C 500 kPa70%0 50 100 150 200 250 300 350 400Time (min)Figure 6-13 : Normalised effect of H 2 S on the reduction in CO 2 permeance of an NPCmembrane manufactured at 550°C. The effect is minimal.120%% of Original CO 2 /N 2 Selectivity115%110%105%100%95%90%85%80%75%35°C 1000 kPa 35°C 500 kPa100°C 1000 kPa 100°C 500 kPa70%0 50 100 150 200 250 300 350Time (min)Figure 6-14 : Normalised effect of H 2 S on the reduction in CO 2 /N 2 selectivity of an NPCmembrane manufactured at 550°C. The effect is minimal.145

The impact of feed temperature and pressure on the reduction in CO 2 permeance with increasingH 2 S partial pressure is not distinctive. It was expected that with increasing feed temperature, theimpact of H 2 S would be less as the adsorption of H 2 S should be lower at higher temperatures.The NPC membrane performance results show that the impact of H 2 S is fairly minimal. This isa very promising result in comparison with the significant impact H 2 S has upon reducing theperformance of Cu/Pd membranes [139] and plasticising polymeric membranes [138].However, further work is required to assess the long term stability of NPC membranes in thepresence of H 2 S and also at higher concentrations of H 2 S. Studies [141] have shown that H 2 Sadsorption on activated carbon in the presence of adsorbed oxygen and water has lead to theformation of sulphuric acid which degrades the carbon.As discussed earlier in Section 2.7.2, NPC membranes readily adsorb oxygen and water whenexposed to air. If NPC membranes are stored or exposed to air prior to installation and thenexposed to H 2 S during operation, rapid deterioration of the membranes could occur. Longerexperiments could not be run as part of this research work due to the associated health andsafety risks.Additionally, it will be necessary to determine the permeance of H 2 S through NPC membranesin order to determine whether H 2 S will concentrate in the retentate or the permeate stream. It isexpected that H 2 S will concentrate in the permeate stream as it is a smaller and more adsorptivemolecule than CO 2 . This could not be done as part of this research work as the concentrationswere too low to be measured by the Gas Chromatograph and increasing the H 2 S concentrationbeyond 500 ppm posed a significant health and safety risk.6.3.2 Carbon MonoxideHealth and safety risks associated with CO did not allow testing with CO levels as high as thosefound in synthesis gas (2.8 mol% refer Table 1-1 in Chapter 1). A risk assessment was approvedon the basis of a feed gas CO concentration of 1000 ppm.The impact of the presence CO on the CO 2 permeance of an NPC membrane manufactured at550°C over a period of time is illustrated in Figure 6-15. As with H 2 S, the data has beennormalised to the original value of permeance for the particular feed operating temperature andpressure in order to clearly identify the trends.146

There is an initial decline in the CO 2 permeance which then stabilises in around 50 to 100 mins.The stabilised drop in the CO 2 permeance is within 30 % of the original value at feed operatingtemperatures of 35 and 100°C and feed pressures of 500 and 1000 kPag. The greatest drop inpermeance is observed at feed conditions of 35°C and 500 kPag.120%115%35°C 1000 kPa 35°C 500 kPa% of Original CO 2 Permeance110%105%100%95%90%85%80%75%100°C 1000 kPa 100°C 500 kPa70%0 50 100 150 200 250 300 350Time (min)Figure 6-15 : Normalised effect of CO on the reduction in CO 2 permeance of an NPCmembrane manufactured at 550°C. The effect is greatest at the lowest pressure andtemperature.The impact of CO on the N 2 permeance over time is slightly lower than that of CO 2 resulting ina small drop (

The adsorption studies of H 2 S and CO on activated carbon by Ritter and Yang [50] observedthat CO is less adsorptive than CO 2 and much less than H 2 S suggesting that unlike H 2 S, CO willnot be competitively adsorbed over CO 2 . Although in this case, the impact on the permeance asshown in Figure 6-15 suggests that CO is adsorbing to the surface particularly at the lowertemperature and pressure, where adsorption is greater.However, the NPC membrane performance results show that impact of CO is fairly minimal.This is a promising result for the use of NPC membranes in CO 2 capture from synthesis gas.Further work is required to assess the long term stability of NPC membranes in the presence ofCO and at higher concentrations of CO. Longer experiments could not be run as part of thisresearch work due to the associated health and safety risks.Additionally, it will be necessary to determine the permeance of CO through NPC membranesin order to determine whether CO will concentrate in the retentate as is the case for polymericmembranes [138] or the permeate stream. It is expected that CO will concentrate in the retentatestream as it is a larger and less adsorptive molecule than CO 2 . This could not be done as part ofthis research work as the concentrations were too low to be measured and increasing the COconcentration beyond 1000 ppm posed a significant health and safety risk.148

6.3.3 Comparison of Non-Condensable ImpuritiesAs a summary, the stabilised reduction in the CO 2 permeance and CO 2 /N 4 selectivity for eachcondensable impurity and concentration level tested are presented in Tables 6-3 and 6-4. Thepercentage errors on each data point are +/- 7 %.The reduction in NPC membrane performance is slightly greater upon exposure to CO than H 2 S.However, this is most likely due to the higher feed concentration of CO (1000 ppm) comparedwith H 2 S (500 ppm).Table 6-3 : Normalised stabilised reduction in CO 2 permeance as a function of impurity.ImpurityOperatingTemperatureImpurityPartial PressureImpurityPartial Pressure°C kPaa kPaaHydrogen Sulphide 0.30 0.55Hydrogen Sulphide 35 7 % 5 %Hydrogen Sulphide 100 5 % 11 %Carbon Monoxide 0.60 1.10Carbon Monoxide 35 22 % 8 %Carbon Monoxide 100 12 % 10 %Table 6-4 : Normalised stabilised reduction in CO 2 /N 2 selectivity as a function of impurity.ImpurityOperatingTemperatureImpurityPartial PressureImpurityPartial Pressure°C kPaa kPaaHydrogen Sulphide 0.30 0.55Hydrogen Sulphide 35 2 % 4 %Hydrogen Sulphide 100 1 % 3 %Carbon Monoxide 0.60 1.10Carbon Monoxide 35 4 % - 1%Carbon Monoxide 100 2 % 2 %149

6.4 Concluding Remarks on the Performance of NPC Membranes in thePresence of ImpuritiesThe presence of condensable and non-condensable impurities on the performance of supportedNPC membranes made from the pyrolysis of polyfurfuryl alcohol at 550°C is relatively minor,particularly at elevated temperatures. This is a very promising result for the commercial use ofNPC membranes for the separation of CO 2 from CH 4 and N 2 .Upon exposure to condensable impurities, the CO 2 permeance of the NPC membranes isreduced by up to 24 % (+/- 7 %) with the greatest reduction being observed in the presence oftoluene, followed by hexane and then water. The reduction in the CH 4 permeance is not assignificant as that seen for CO 2 due to the lower dependence of the CH 4 permeance on surfaceadsorption. However, the reduction in the CO 2 /CH 4 selectivity is less than 13 % (+/- 7%).The performance reductions caused by water are significantly lower than those reported forpolymeric membranes [138]. However, the performance results are also significantly lower thanthose reported for NPC membranes by Jones and Koros [31]. This suggests that ourregeneration technique may not be removing the majority of the water adsorbed from theatmosphere during installation and hence limiting water adsorption from the feed stream.The performance reductions caused by the presence of hexane and toluene are similar to thoseobserved by Vu et al [35] even though higher impurity concentrations have been in this researchwork. Tanihara et al [103], who have used a higher concentration of toluene albeit in a H 2 /CH 4feed stream, have reported that the impact on performance of a NPC membrane to be minimalespecially in comparison to the impact on a polymeric membrane.In the case of non-condensable impurities, CO seems to have a greater effect on reducing theperformance of NPC membranes than H 2 S but this is most likely due to the higher concentrationof CO used. Upon exposure to these non-condensable impurities, the CO 2 permeance of theNPC membranes is reduced by up to 12 % (+/- 7 %). The impact on the N 2 permeance is similargiving a reduction in the CO 2 /N 2 permeance of less than 4 % (+/- 7%).In particular, the minor impact on NPC membrane performance by H 2 S is hopeful consideringthe performance of Cu/Pd membranes often used in synthesis gas purification are adverselyaffected by H 2 S [139].150

Whilst these results are promising for the use of NPC membranes in the separation of CO 2 fromnatural gas production and synthesis gas production, further work on the longer term impact ofexposure to impurities is required.151

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7 CONCLUSIONS AND RECOMMENDATIONS7.1 ConclusionsFor the last decade, nanoporous carbon has been touted as a superior material for the separationof gases. Manufactured from the pyrolysis of polymers at high temperature, nanoporous carbonsare structurally related to the fullerene group of carbon and contain nanosized sized poressuitable for the separation of gases based upon molecular size.Membranes can be manufactured from nanoporous carbon by coating a porous support with apolymeric precursor prior to pyrolysis (supported NPC membranes) or from the direct pyrolysisof a flat sheet or hollow fibre of polymer (unsupported NPC membranes). The ability to makemembranes from nanoporous carbon is advantageous due to reduced equipment size, footprintand potentially cost when compared with other separation processes such as gas absorption.The separation performance of NPC membranes is well documented as being above theRobeson line, which is considered the theoretical limit of performance for the traditionalpolymeric membranes. Additionally, the high temperatures used for the manufacture of NPCmembranes makes them ideal candidates for high temperature separation resulting in significantpotential for cost reduction by removing pre-cooling requirements.However, NPC membranes are not without drawbacks. The brittleness and cracking ofnanoporous carbon is a significant manufacturing challenge. Likewise, the significant reductionin performance seen when exposed to the atmosphere (termed “aging”) makes transportationand storage of the NPC membranes difficult along with the inability of these membranes tooperate in the presence of oxygen even when present in trace quantities.The objective of this research work was to put NPC membranes to the test and specificallyreview the performance of NPC membranes in the application of CO 2 capture from natural gas(methane) and from air-blow synthesis gas production (nitrogen). A critical component of theresearch work was to assess the performance under realistic conditions – high operatingtemperature and in the presence of condensable and non-condensable impurities.A substantial amount of time and effort was spent on developing a method to produce selectiveNPC membranes. Cracking proved to be the most significant obstacle of this research work andas such dedicated attempts to eliminate cracking was critical to obtaining working membranesthat could be used for fulfilling the objectives of the research work.153

Of the three types of NPC membranes produced, namely supported, modified-supported andunsupported, the most success in eliminating cracking and producing selective membranes, wasfound in the manufacture of supported NPC membranes from the pyrolysis of PFA.The critical mass of carbon deposited onto the support that marked the onset of cracking wasdetermined to be 8 mg/cm 2 and 6 mg/cm 2 for pyrolysis temperatures of 400 and 550°C,respectively. The characterisation results revealed a significant increase in the porosity withincreasing pyrolysis temperature, which explains the earlier onset of cracking at highermanufacturing temperatures due to the more porous and therefore delicate structure.The increase in permeance observed with increasing pyrolysis temperature is also explained bythe increase in porosity. However, the pore size change with increasing pyrolysis temperaturewas too small to impact upon the permselectivity. In fact the pore size range for the pyrolysistemperatures studied stayed within the published range of 4 – 5Å [61]. The permselectivity wasthus found to be more greatly effected by the formation of a complete membrane layer and theabsence of cracks than the pore size.Testing the performance of an NPC membrane with a mixed gas feed provides a more accuraterepresentation of how the membranes would perform in a commercial setting than thetraditional pure gas performance testing. The results of pure and mixed gas testing for the sameNPC membranes revealed that the CO 2 /CH 4 mixed gas selectivity was greater than the CO 2 /CH 4permselectivity determined from the pure gas testing. This result is explained by the competitiveadsorption effect of the more adsorbable component CO 2 “blocking” the way for CH 4permeance.The carbon dioxide/methane (CO 2 /CH 4 ) mixed gas performance measurements generally showan increase in permeance as the operating temperature is increased due to the dominance ofmolecular sieving. The exception is for the CO 2 permeance of membranes prepared at a higherpyrolysis temperature (550°C) where adsorption processes are competitive with molecularsieving effects. This leads to a CO 2 permeance value that is relatively independent oftemperature. Whilst this leads to a membrane that is slightly more selective at 35°C, themembrane selectivities converge at higher operating temperatures. Indeed, extrapolationsuggests that the membrane selectivity would reduce to the Knudsen (CO 2 /CH 4 = 0.6) value atan operating temperature between 400 and 500°C.Although the selectivities of the membranes manufactured at different pyrolysis temperaturesconverge at higher operating temperatures, the permeance is still related the original pyrolysis154

temperature. This result implies that it is important to manufacture a membrane with highporosity/permeance rather than high adsorption/selectivity when considering high operatingtemperature applications such as CO 2 capture from natural gas or synthesis gas.Molecular simulation of pure and mixed gas adsorption on nanoporous carbon (represented byC 168 Schwarzite) confirmed that CO 2 is a more strongly adsorbing molecule than CH 4 and muchmore so than N 2 . Whilst adsorption does decreases with increasing operating temperature theselectivity for CO 2 still remains at temperatures as high as 200°C.The greater difference between the values of heat of adsorption for the CO 2 / N 2 pair comparedwith the CO 2 /CH 4 pair gives a significantly larger separation factor for the CO 2 /N 2 pair. This isa particularly promising result for the separation of CO 2 from N 2 at high temperatures as is thecase for the air-blown synthesis gas purification.A significant difference between the simulated and experimental adsorption results wasobserved. This difference is attributed to the different substrate (C 168 Schwarzite) used in thesimulation and to a smaller amount of adsorption sites available for gas adsorption on theexperimental samples due to oxygen adsorption from the air.The presence of condensable and non-condensable impurities on the performance of NPCmembranes is small, particularly at elevated operating temperatures where the impurityadsorption is lower.The small reductions in the CH 4 and CO 2 permeance of a NPC membrane observed in thepresence of condensable impurities is of the order toluene > hexane > water. The impact of thecondensable impurities on the CH 4 and CO 2 permeances is very similar resulting in no netchange in the selectivity.Likewise, small reductions in the CH 4 and CO 2 permeance of a NPC membrane were observedin the presence of H 2 S and CO. The impact of the non-condensable impurities on the CH 4 andCO 2 permeances is very similar resulting in no significant change in the selectivity.In summary, provided crack-free NPC membranes can be easily manufactured and handled suchthat exposure to oxygen is prevented, NPC membranes do show some promise for the use inhigh temperature CO 2 capture processes such as separation from natural gas or synthesis gas.The supported NPC membranes tested as part of this research work maintain selectivity above 1for operating temperatures as high as 200°C. Additionally, the impact of condensable and noncondensableimpurities is relatively small particularly at higher operating temperatures.155

7.2 Recommendations for Further WorkThe rapid increase in publications on NPC membranes over the last decade has slowedsomewhat over the last few years and unfortunately no industrial success has been reported asyet. The significant problems with cracking and aging appear to have had a great impact uponfurthering this technology to commercialisation.The emerging membrane technologies that are presently attracting a lot of attention are thosethat endeavour to achieve the performance of a NPC membrane with the flexibility of apolymeric membrane, such as mixed matrix or partially pyrolysised membranes. Park et al [160]recently developed a novel technique utilising high manufacturing temperatures for producingpolymeric membranes containing highly tunned pore sizes for gas transport. These membranessurpass the Robeson upper bound whilst maintaining the robustness of a polymeric membrane.Whilst interest in NPC membranes appears to have been overtaken by the emerging compositemembrane technologies discussed above, the possibility of solving the problems with crackingand aging of NPC membranes still remains.The recent work of Rajgopalan et al [28] and Merritt et al [27] in regards to the addition of silicaparticles is promising for reducing the amount of carbon added. However, attempts to makeNPC membranes upon supports filled with silica nanoparticles as part of this research workwere unsuccessful again due to tendency for cracking. It is possible that the use of alternativesupports (such as porous ceramics or carbon), which have a similar thermal expansion tonanoporous carbon, could reduce cracking. However, obtaining high pressure feed/permeateseal using these supports at high temperatures is likely to be difficult and costly.The work of Jones and Koros [129] on applying a thin layer of unique polymeric materials toprotect the finished membrane from aging shows potential. However, further tests on thestability of this polymeric layer at high operating temperatures and in the presence of impuritieswould need to be undertaken.The impact of condensable and non-condensable impurities has been shown here to be fairlyminimal. However, further work is required to assess the long term impact of the impurities.Considering the health and safety risks associated in particular with H 2 S and CO, the mostappropriate way to further assess the impact of these impurities in the long term would be CO 2capture trials on existing processing plants where these impurities are ever present andcontrolled within the rigorous health and safety systems of an operating company.156

As part of the Victorian Government’s Energy Technology Innovation Strategy (ETIS), variouscapture technologies including membrane systems are being tested using actual synthesis gasproduced by HRL’s coal gasifier in Mulgrave (pre-combustion capture) and actual flue gasproduced by International Power at the Hazelwood Power Station in the Latrobe Valley (postcombustioncapture). Testing NPC membranes as part of the ETIS scheme, in particular for precombustioncapture, is recommended as the logical follow-on from this research work.157

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APPENDICES175

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APPENDIX A – RELEVANT BACKGROUND INFORMATIONA.1 Global WarmingGreenhouse gases absorb infrared radiation and therefore directly affect the energy flow throughthe Earth’s atmosphere. Increasing the quantity of greenhouse gases within the Earth’satmosphere leads to a build up in energy and hence an increase in temperature. Thisphenomenon is known as global warming.Altering the Earth’s climate in such a way is predicted to have devastating consequences, suchas increased ocean levels and changes to weather patterns which will destroy the delicatebalance for various ecological systems.Greenhouse gases are specified in the Kyoto Protocol as carbon dioxide (CO 2 ), methane (CH 4 ),nitrous oxide (N 2 O), hydrofluorocarbons (HFCs), perfluorocarbons (PFCs) and sulphurhexafluoride (SF 6 ) [161]. Activities within the global community are increasing theconcentrations of these greenhouse gases in the atmosphere. Carbon dioxide is produced fromthe burning of fossil fuels; methane and nitrous oxide are produced through land clearing andagriculture; and industrial processes produce hydrofluorocarbons, perfluorocarbons and sulphurhexafluoride [162].375Current Atmospheric Level of CO 2= 370 ppmvCarbon Dioxide Concentration (ppmv)350Methane Concentration (ppmv)Temperature over AntarcticaAtmospheric carbon dioxide concentrationAtmospheric methane concentrationTemperature relative to present climate (°C)Thousands of years before present (Ky BP)Figure A-1 : Correlation between the Earth’s temperature and atmospheric carbondioxide (data sourced from [163])177

Whilst providing the lowest global warming potential, carbon dioxide accounts for the majorityof greenhouse gases emitted to the atmosphere through human activity and as such greenhousegas emissions are generally quoted in tonnes of carbon dioxide equivalent. The correlationbetween the Earth’s temperature and the atmospheric concentrations of carbon dioxide andmethane is presented in Figure A-1.A.2 International Recognition of Climate ChangeThe United Nations and the World Meteorological Society created the Intergovernmental Panelon Climate Change (IPCC) in 1988 in recognition of the problem of global climate change. Therole of IPCC is to provide an objective source of accepted information on global climate change.The IPCC does not conduct research or monitor data, it collates and then presents peer-reviewedinformation on climate change from around the world such that a balanced viewpoint isobtained to enable policy making [164].The First Assessment Report published by the IPCC in 1990 presented a confirmation of theimpact of human activity on global climate change based on scientific data. Not surprisingly thisreport created concern within the international community and prompted the establishment ofthe Intergovernmental Negotiating Committee, which adopted the United Nations (UN)Framework Convention on Climate Change in 1992 [164]. The objective of the UN FrameworkConvention on Climate Change is to allow “stabilisation of greenhouse gas concentrations in theatmosphere at a level that would prevent dangerous anthropogenic interference with the climatesystem” [165].At the Earth Summit hosted in Rio de Janeiro in June 1992, the UN Framework Convention onClimate Change was open to signature by participating countries. Countries that signed theconvention were committed to monitoring greenhouse gas emissions, developing programs formitigating climate change and promoting public awareness of climate change. The frameworkprescribes that developed countries take the lead on addressing climate change [165].The Kyoto Protocol to the UN Framework Convention on Climate Change was established inKyoto, Japan in December 1997. The Second Assessment Report published by the IPCC in1996 contributed significantly to the negotiations of the Kyoto Protocol [164]. The primaryobjective of the Kyoto Protocol is to reduce the worldwide greenhouse gas emissions by 5 %below the 1990 levels during the commitment period of 2008 to 2012. Consequently, eachcountry that is a Party to the Kyoto Protocol has been assigned an emissions quota based on1990 levels for the commitment period [161]. As of the 16 th of February 2005, the Kyoto178

Protocol entered into force and as such 36 (Annex B) developed countries along with theEuropean Union are required to reduce emissions for the commitment period. Of the developingnations, including China and India, 137 countries have ratified the protocol but are only requiredto monitor and report emissions rather than reduce emissions. With Russia and Australia havingnow ratified the Kyoto Protocol, the only major country not to have joined is the United Statesof America.The most recent IPCC reports, the Third and Fourth Assessment Reports, were published by theIPCC in 2001 and 2007, respectively. Both reports provides stronger than ever evidence of theanthropological impact on climate change [1, 163].The global community is now looking towards what happens after 2012 when the commitmentperiod to the Kyoto Protocol ends. The United Nations Climate Change Conference held in Bali,Indonesia in December 2007 led to the formation of the “Bali Roadmap” for a futureinternational agreement on climate change, which is hoped to be completed by the end of 2009.A.3 Australian Recognition of Climate ChangeAustralia’s economy is dominated by the use of fossil fuels, especially coal, and it is a netexporter of energy. None of the energy generated by Australia is produced using nuclearresources. With the change of government in November 2007, Australia finally ratified theKyoto Protocol on the 3 rd of December 2007.Under the Kyoto Protocol, Australia was allocated the target of 108 % of 1990 emission levelsover the commitment period from 2008 to 2012 (Australia, like some other nations, has focusedon the year 2010 as a target date). Australia was one of only three Annex B countries allocatedan increased target over the 1990 emission levels [161]. This is attributed to Australia’s relianceon fossil fuels and it having a relatively high economic growth for a developed country.As shown in Figure A-2, the Australian Greenhouse Office has projected that Australia willapproach but not achieve this target as greenhouse gas emission levels are predicted to reach theKyoto Target (108 %) of the 1990 level (596 million tonnes of carbon dioxide equivalent) onaverage for the period from 2008 to 2012. The projected emission level of 108 % is on the basisthat greenhouse gas reduction measures will be introduced. Maintaining business as usual andwithout adopting any greenhouse gas reduction measures predicts an emission level of 124 %for the 2008 to 2012 period [166].179

Greenhouse gas measures adopted by the Australian Government include the implementation ofa range of polices and programs to reduce emissions, establishment of new forest plantationsand land use changes [166]. Under the Kyoto accounting rules, Australia is permitted to includeland use changes in calculating the emissions levels for the 2008 to 2012 period [161]. Land usechanges make a significant contribution to the reductions.CO2 Equivalent Emissions (Millions Tonnes/Year)7006005004003002001000-1001990 LevelsPredicted 2008 - 2012 Levels2874298893Predicted 596 mT/year (w ith reduction measures) = 108 % of 1990 Emissions Level18 1525Energy Agriculture Waste IndustrialProcessesPredicted 685 mT/year (business as usual) = 124 % of 1990 Emissions Level 685Kyoto Target 585 mT/year = 108 % of 1990 EmissionsLevel380-2113644554Forestry Sinks Land Use Change Total596Figure A-2 : Predicted greenhouse gas emissions from by sector for 1990 and the predicted2008 to 2012 period (data sourced from [166])A.4 Reducing CO 2 Emissions from Australia’s Energy ProductionIn 1990, energy production from stationary sources such as power stations accounted for 39 %of Australia’s total greenhouse gas emissions (primarily carbon dioxide, CO 2 ). Emission levelsfrom stationary sources is increasing and is predicted to account for 49 % of Australia’s totalgreenhouse gas emissions for the 2008 to 2012 period [166].Further reductions in greenhouse gas emissions from energy production can be achieved via (i)increasing energy efficiency and (ii) using less carbon intensive energy sources such asrenewable fuels or solar energy supplies [167].Increasing energy efficiency is achieved with both advances in technology and an increasedsocial awareness usually brought about by an increase in energy cost or with a threat to energyavailability. The Australian society does not have an inherent sense of urgency for increasingenergy efficiency as fossil fuels remain affordable and available.180

Developing an energy efficient society can be achieved through education and governmentpolicy. The introduction of a carbon tax is one example of how policy would promote energyefficiency through the higher cost of fossil fuels. A mix of measures, such as carbon taxes,levies, enforcing efficiency standards and providing incentives for market-available low-energyusage equipment, will eventually be required. The Australian Federal Government has not yetintroduced a carbon tax or a carbon trading market. However, the latter is being planned at theState and Territory level.Affordable and easily accessible renewable energy will not be available for several decadesunless there is a significant discovery in the short term. Renewable energy, with a capacity of16,000 GWh, presently only accounts for 10.5 % of Australia’s electricity generation with themajority sourced from hydro-electric power stations [168].The previous Australian Government introduced the Mandatory Renewable Energy Target(MRET) as method of increasing electricity production from renewable energy sources. TheMRET requires the production of an additional 9,500 GWh of renewable energy annually by2010 and will result in a reduction of 6.5 million tonnes of carbon dioxide emitted per annum[169]. While this reduction is modest it has allowed the emergence of a wind energy industry.Enhanced uptake of renewable energy will also occur as the cost is reduced with advances intechnology.Alternatively, geological storage (sequestration) of carbon dioxide has great potential for asocially acceptable solution that can be used to significantly reduce greenhouse gas emissions inthe interim while sustainable solutions are further advanced and implemented. Carbon dioxidesequestration is particularly applicable to Australia’s situation where the majority of electricityis generated from burning coal.A.5 Opportunities for CO 2 Sequestration in AustraliaCO 2 sequestration is best suited to large stationary emitters of carbon dioxide. Approximatelyhalf of Australia’s carbon dioxide emissions are from large stationary sources and could becaptured using carbon dioxide sequestration. A breakdown of Australia’s large stationarysources by industry is presented in Figure A-3.181

80.0%73.3%70.0%60.0%50.0%40.0%30.0%20.0%7.6% 6.0% 5.8% 4.5%10.0%2.8%0.0%PowerStationsSteel IndustryNon-FerrousMetalsPetroleumIndustryOtherIndustrialOil RefineriesFigure A-3 : Australia’s large stationary sources of carbon dioxide by industry (datacourtesy of the CRC for Greenhouse Gas Technologies)A.6 Challenges for CO 2 Sequestration in AustraliaThe major technical challenge for CO 2 sequestration is reducing the cost. Developing a positivepublic perception of the geological storage of carbon dioxide should also be recognised as animportant challenge for implementing carbon dioxide sequestration.The present cost of CO 2 sequestration is estimated by the IEA (International Energy Agency) tobe between $40 and $60 US per tonne of carbon dioxide emissions avoided [6, 7]. In the case ofpower generation, this cost translates to an increase of 1.5 to 3 US cents per kilowatt hour to theprice of electricity. Using the average 1998 electricity price for OECD countries, an increase of1.5 to 3 US cents per kilowatt hour represents a 11 to 43 % increase in the domestic electricityprice [7].From the data supplied by the IEA [7], it is estimated that capturing and compressing the carbondioxide from the source represents approximately 90 % of the sequestration cost. The cost oftransporting and storing the CO 2 is relatively small. Therefore, significant improvements to thecapture technology should remain a focus for major reduction in the cost of carbon dioxidesequestration process.182

A.7 CO 2 Capture TechnologiesSeparating CO 2 from other gases is an integral part of many existing processes, such as thepurification of natural gas before the liquefaction process. Existing gas separation technologyincludes gas absorption, gas adsorption, cryogenics, gas hydrates and gas separationmembranes.A.7.1AbsorptionIn the gas absorption (“scrubbing”) process, the gas is contacted with a liquid (solvent) and oneor more components of the gas are preferentially absorbed into liquid. The gas-rich liquid isthen sent to a regeneration process where the separated gas is released from the liquid.The gas absorption process is either a chemical or physical absorption process. In the chemicalabsorption process, the carbon dioxide is bonded to the solvent via a chemical reaction.Commercial chemical solvents are amine or carbonate based, such as monoethanolamine(MEA), diethanolamine (DEA) or hot potassium carbonate as used in the Benfield process. Inthe physical absorption process, the carbon dioxide is physically absorbed into the solventaccording to Henry’s law. Commercial physical solvents include Selexol (glycol) and Rectisol(methanol). The physical absorption process is favourable for higher pressure applications.All commercial carbon dioxide capture processes from power station flue gases currently usesolvent technology [170]. The advantage of gas absorption is that it is a mature technology andwell proven for the application. The disadvantage of gas absorption is the size and amount ofequipment required for the process. The application of chemical absorption (Fluor DanielEconamineSM FG Process) has been shown to separate and produce carbon dioxide at a cost ofUS$ 29.50 / tonne [171].A promising adaptation of the gas absorption process is the gas adsorption membrane contactordeveloped recently by Kvaerner Oil and Gas. The gas adsorption membrane contactor replacesthe traditional packed column resulting in a significant reduction in equipment size [172].A.7.2AdsorptionIn the gas adsorption process one or more of the components of the gas are preferentiallyabsorbed onto a solid desiccant. The solid desiccant is then regenerated via heat or pressureswing to remove the absorbed gases. Molecular sieves are a commonly used solid desiccant forthe removal of water or trace components of gases such as hydrogen sulphide.183

Solid desiccants for the bulk removal of carbon dioxide in flue gases are not presentlyconsidered for commercial use due to the limited adsorption capacity and low selectivity [173].A.7.3CryogenicsThe flue gases are cooled to cryogenic temperatures such that the carbon dioxide is condensedfrom the other gases. The recovery of carbon dioxide is dependent on the lowest achievabletemperature. Increasing the pressure will also increase the recovery.Separating methane from carbon dioxide is difficult and requires the use of cryogenicdistillation, such as the Ryan-Holmes process, which was developed in the 1980s for separatingacid gases (such as carbon dioxide) from light hydrocarbon gases [123].Cryogenic separation technology is best suited for flue gases that contain high levels of carbondioxide such as those produced from a pre-combustion or oxy-fuels process. The majordisadvantage of cryogenic separation technology is that it is inherently energy intensive, whichwill lead to a large operating cost.A.7.4Gas CryogenicsA hydrate is a crystalline structure that forms from water and a gas, such as carbon dioxide, andhas the appearance and characteristics of ice. If the flue gases are carefully cooled with a smallamount of water present, carbon dioxide hydrates can be selectively formed enabling separationfrom the other gases. The temperatures and pressures required are less extreme than cryogenicseparationA.7.5Gas Separation MembranesMembrane technology offers exciting promise for use in carbon dioxide capture and there issubstantial research activity in this area. Although membranes are not yet proven in theapplication of carbon dioxide capture from flue gases, they are currently used for the separationof carbon dioxide from natural gas in the oil and gas industry.Membranes have the advantage of being very compact in comparison with other capturetechnologies. If an improvement in separation efficiency can be achieved along with flexibilityto operate in higher temperatures, membrane technology could offer significant cost advantagesover other technologies.184

The membranes presently used in commercial carbon dioxide gas separation are polymer basednon-porous membranes such as spiral wound cellulose acetate membranes and polyimidehollow fibre membranes. These polymeric membranes are presently limited to operatingtemperatures below the glass transition temperature, where the structure of the polymer changesfrom a glassy to a rubbery state and the mechanical strength is drastically reduced. Polyimide,with a glass transition temperature of 300 °C, has one of the highest polymer glass transitiontemperatures [40]. The temperature limit therefore necessitates cooling of flue gases beforecarbon dioxide separation can take place.Porous membranes, from ceramic materials, with pore sizes less than 1 µm are presently beingexplored for use in gas separation as they offer the advantage of stability at higher operatingtemperatures [174]. Nanoporous carbon is an example of a material that can be used to makeporous membranes suitable for operation at high temperatures.185

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APPENDIX B – STANDARD OPERATING PROCEDURESB.1 Standard operating procedure for the CPVV (Mixed Gas) apparatus187

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B.2 Standard operating procedure for the CPVV (Mixed Gas) apparatus in thepresence of condensable impurities199

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B.3 Standard operating procedure for the CPVV (Mixed Gas) apparatus in thepresence of non-condensable impurities211

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B.4 Risk Assessment for the operation of the CPVV (Mixed Gas) apparatus inthe presence of non-condensable impurities223

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APPENDIX C – EXPERIMENTAL METHODSC.1 Pure Gas Permeance MeasurementC.1.1Error AnalysisThe permeance of a gas (A) using a pure gas feed was calculated via Equation 3-1 Section 3.4.1.The error analysis method used [175] uses the following relationships as shown in Table C-1 forcalculating the overall error from the individual errors. The variable Z depends on the variablesA and B. The respective errors are denoted as E Z , E A and E B .Table C-1 : Error Analysis Relationships [175]Relationship between Z, A and B Relationships Between the Errors of Z, A and B.Z = A +BZ = A − BZ = ABZ =ABZ = ln(A)⎛⎜⎝⎛⎜⎝22( EZ) = ( EA) + ( EB22( EZ) = ( EA) + ( EBEZZEZZ2⎞⎟⎠2⎞⎟⎠⎛= ⎜⎝⎛= ⎜⎝EEAAEAAZ =2⎞⎟⎠2⎞⎟⎠EAA))222⎛ EB⎞+ ⎜ ⎟⎝ B ⎠2⎛ EB⎞+ ⎜ ⎟⎝ B ⎠Using these relationships, the error analysis for the calculation of the permeance is as follows.⎛⎜⎝EP'P'2⎞⎟⎠⎛ E= ⎜⎝ VVpp⎞⎟⎠2⎛ EA+ ⎜⎝ A2⎞⎟⎠⎛ E+ ⎜⎝ TT2⎞⎟⎠⎛ Ep+ ⎜⎝ Pff⎞⎟⎠2⎛ Ep+ ⎜⎝ Ppp⎟ ⎞⎠2Equation C-1The error analysis for the CO 2 permeance from a typical pure gas experiment (MembraneNPC550B) is shown in Table C-3. Substituting the various values of E A /A into Equation C-1gives a 1.38% error in the CO 2 permeance. The error in the CH 4 permeance is also 1.38 % as therelative contributing errors are the same.The pure gas selectivity or permselectivity was calculated from the ratio of the pure gaspermeances as presented earlier as Equation 2-3 in Section 2.2.2. Following the error analysisrelationships presented in Table C-1, the error in the selectivity is described as shown inEquation C-2.231

2⎛ Eα⎞⎜ ⎟⎝ α ⎠⎛ E= ⎜⎝P'P'CO2CO2⎞⎟⎠2⎛ E+ ⎜⎝P'P'CH 4CH 4⎟ ⎞⎠2Equation C-2Substituting the errors in the CO 2 and CH 4 permeances (1.38%) gives a 1.95% error in thepermselectivity. Thus, for membrane NPC550B with an initial pure gas CO 2 /CH 4permselectivity of 8.9, the error band is +/- 0.2.The error method used here was checked using duplicate measurements from the samemembrane. The manufacturing variability between membranes is so large that comparingmeasurements between different membranes would not indicate the error in the pure gasperformance measurements.As duplicate measurements had not been made for membrane NPC550B, this analysis wascompleted using the 2 nd coat of membrane NPC400C. The CO 2 /CH 4 permselectivities calculatedfrom the two measurements taken 6 days apart are shown in Table C-2. The error is a largerthan that calculated via the addition of the errors in the instruments. However, the time betweenthe two measurements can not be ignored as attributing to the difference in the measurements assome exposure to the atmosphere has occurred.With this in mind, the errors calculated using the addition of the errors in the instruments hasbeen used for the error bands on the pure gas results presented in this thesis.Table C-2 : Error analysis of the calculation of the pure gas CO 2 CH4 permselectivityfrom duplicate measurements of Membrane NPC400C taken 6 days apart.Measurement Date CO 2 /CH 4 Permselectivity1 1 st Feb 2007 2.762 7 th Feb 2007 2.95Error 6.88%232

Table C-3 : Error analysis of the calculation of the pure gas CO 2 permeance for Membrane NPC550B. The error analysis is independent of theabsolute values of the variables as the error is a percentage of the absolute value with the exception of the values of the membrane area. As themembrane area reading is the same for the CH 4 permeance, the error analysis for the CH 4 permeance gives the same result.Variable (A) Value (A) Reading Error (E A ) Comments E A /A E A /A(%)Permeate Volume V p cm 3 525.00 5.00 +/- 5 cm 3 error in volume 0.0095 0.95Membrane Area A m 2 9.62 x 10 -4 7.86 x 10 -7 +/- 1 mm error indiameter0.0008 0.08Permeate Temperature T K 306.00 0.85% of ValueManufacturerspecification0.0085 0.85Feed Pressure p f kPa(a) 531.14 0.50% of ValueManufacturerspecification0.0050 0.50Permeate Pressure p p kPa(a) Increasing 0.13% of ValueManufacturerspecification0.0013 0.13233

C.1.2Sample of Calculated ResultsThe permeate pressure rise plots for the gases CO 2 , CH 4 and N 2 used to determine the respectivepure gas permeances from the steady state gradients as described in Section 3.4.1 is presented inFigure C-1.2018CO2 N2 CH41614CO 2 Steady State Gradient= 0.0636 Torr/secPressure (Torra)1210864CH 4 Steady State Gradient= 0.0070 Torr/sec20N 2 Steady State Gradient= 0.0051 Torr/sec0 100 200 300 400 500 600 700 800Time (secs)Figure C-1 : Permeate pressure rise for pure gases CO 2 , N 2 and CH 4 through MembraneNPC550B. The respective permeances were calculated from the steady state gradients.A summary of the calculated permeances and permselectivities based upon the steady stategradients as shown in Figure C-1 is given in Table C-4.234

Table C-4 : Summary of calculated pure gas permeances and permselectivities of Membrane NPC550B based upon the steady state gradientsfrom the permeate pressure rise curves. The system constants were permeate volume (V p ) = 525 cm 3 , temperature (T) = 306 K and membranearea = 9.62 cm 2 .GasUpstreamPressureUpstreamPressureRawSteady State Pressure GradientAdjusted (for Leak Rate)Steady State Pressure GradientPermeancePermselectivitywith respect to CO 2kPaa (Torr) (Torr/sec) (Torr/sec)mol/m 2 .s.Pax 10 -10Blank - - 0.0003 - - -CO 2 561.14 81.39 0.0636 0.0633 31.8 1.0N 2 553.60 80.29 0.0051 0.0048 2.5 13.0CH 4 528.53 76.66 0.0070 0.0067 3.6 8.9235

C.2 Mixed Gas Permeance MeasurementC.2.1Error AnalysisThe permeance of a gas (A) using a mixed gas feed was calculated using Equation 3-2 shown inSection 3.4.2. The error analysis method used [175] also employed the relationships as shown inTable C-1, resulting in Equation C-3 for calculation of the error analysis for the CO 2 permeance.⎛ Ep⎜⎝ P⎞⎟⎠2⎛ E= ⎜⎝ QQpp⎞⎟⎠2⎛ E+ ⎜⎝yyCO2CO2⎞⎟⎠2⎛ EA+ ⎜⎝ A2⎞⎟⎠⎛ E+ ⎜⎝xxCO2CO2⎞⎟⎠2⎛ E+ ⎜⎝ pp ff⎞⎟⎠2⎛ E+ ⎜⎝ pp pp⎞⎟⎠2Equation C-3The error analysis for the CO 2 permeance from a typical mixed gas experiment (MembraneNPC550B) is shown in Table C-6. Substituting the various values of E A /A into Equation C-3gives a 5.16 % error in the CO 2 permeance. The error in the CH 4 permeance is also 5.16 %.As the mixed gas selectivity was calculated from the ratio of the mixed gas permeances, thesame error analysis method as used for the permselectivity was employed. Substituting theerrors in the CO 2 and CH 4 permeances gives a 7.30% error in the mixed gas selectivity. Thus,for membrane NPC550B with an initial mixed gas selectivity of 15, the error band is +/- 1.1.This error method was checked using duplicate measurements from the same membrane. Thevariability between membranes is so large that comparing measurements between differentmembranes would not indicate the error in the pure gas performance measurements. Duplicatemeasurements of the CO 2 /CH 4 selectivity of NPC550B taken within 1 day of each other arepresented in Table C-5. However, the time between the two measurements can again not beignored as some exposure to hexane and a subsequent regeneration has occurred.With this in mind, the errors calculated using the addition of the errors in the instruments hasbeen used for the error bands on the pure gas results presented in this thesis.Table C-5 : Error analysis of the calculation of the mixed gas CO 2 /CH 4 permselectivityfrom duplicate measurements of Membrane NPC550B.Measurement Date CO 2 /CH 4 Selectivity1 15 th Jan 2008 15.42 16 th Jan 2008 13.9Error 9.74 %236

Table C-6 : Error analysis of the calculation of the CO 2 permeance for Membrane NPC550B. The error analysis is independent of the absolutevalues of the variables as the error is a percentage of the absolute value with the exception of the values of the permeate flow and membranearea. As the permeate flow and membrane area readings are the same for the CH 4 permeance, the error analysis for the CH 4 permeance givesthe same result.Variable (A) Value (A) Reading Error (E A ) Comments E A /A E A /A(%)Permeate Flow Q p ml/min 20.20 0.50 Reading fluctuation 0.0248 2.48Permeate CO 2ConcentrationMembraneAreaFeed CO 2Concentrationy CO2molfraction0.018 4% of ValueError in calibrationregression lineA m 2 9.62 x 10 -4 7.86 x 10 -7 1 mm error in diametermeasurementx CO2molfraction0.10 0.2% of ValueFeed Pressure p f kPa(a) 1301.33 1.5% of ValuePermeatePressurep p kPa(a) 101.33 1.5% of ValueManufacturerspecificationManufacturerspecificationManufacturerspecification0.0400 4.000.0008 0.080.0020 0.200.0150 1.500.0150 1.50237

C.2.2Sample of Calculated ResultsA sample of the calculated mixed gas permeances and selectivity for Membrane NPC550B calculated using Equation C-3 is presented in Table C-6.Table C-7 : Sample of the calculated mixed gas performance of Membrane NPC550B. The system constants were temperature = 35 °, feed gascomposition = 10:90 (CO 2 :CH 4 vol%), feed pressure (p f ) = 1200 kPag, retentate pressure = 1150 kPag, retentate flow = 100 ml/min, sweep gasflow = 20 ml/min, permeate pressure = 0 kPag and membrane area (A) = 9.62 cm 2 .Sample #Feed GasFlowPermeateFlowPermeate CompositionCH 4 / CO 2 / HeCH 4 Permeance CO 2 Permeance CO 2 /CH 4Selectivityml/min ml/min mol% mol/m 2 .Pa.s x 10 -10 mol/m 2 .Pa.s x 10 -10150108CHC1 100.2 20.2 1.09 / 1.82 / 97.09 1.46 22.5 15.4160108CHC1 100.2 20.2 1.00 / 1.52 / 97.47 1.34 18.6 13.9238

C.3 Adsorption MeasurementC.3.1Error AnalysisThere are two parts to the error in the adsorption measurement. Firstly, there is the accuracy ofthe balance and secondly there is the error in the buoyed mass correction.The accuracy of the balance is ± 0.1 microgram as specified by the equipment manufacturer.The buoyed mass correction was calculated from the buoyed volume using the ideal gas lawdescribed by Equation 3-5 in Section 3.5.2.1.The error analysis method used [175] also employed the relationships as shown in Table C-1,resulting in the error analysis for the calculation of the buoyed mass correction (Equation C-4)as follows. It is assumed that the errors in z, R and MW are negligible.⎛⎜⎝Emm2⎞⎟⎠⎛= ⎜⎝EVV2⎞⎟⎠⎛ E+ ⎜⎝ TT2⎞⎟⎠2⎛ EP⎞+ ⎜ ⎟⎝ P ⎠Equation C-4The error analysis for the adsorbed volume of CO 2 on nanoporous carbon at an examplepressure and temperature is shown in Table C-8. Substituting the various values of E A /A intoEquation C-4 gives an 8.35 % error in the buoyed mass. With the addition of the 0.0002 % errorin the balance accuracy gives and 8.3502 % error in the adsorption measurement.239

Table C-8 : Error analysis of the calculation of the adsorbed volume of CO 2 onto nanoporous carbon 303 Torr (40 kPa) at 35°C.Variable (A) Value (A) Reading Error (E A ) Comments E A /A E A /A(%)Measured Mass Micrograms 51366 0.1 Balance Accuracy 1.97 x 10 -6 0.0002Buoyed Volume V cm 3 20.20 0.50Maximum difference in twomethods for calculating thebuoyed volume as given in0.07 7.00Section 5.3.1.1, Table 5-7.Experimental Temperature T K 308 0.85 % of Value Manufacturer specification 0.0085 0.85Experimental Pressure P Torr 303 7.85E-07 Manufacturer specification 0.0050 0.50240

C.3.2Sample of Calculated ResultsA sample of the calculated results for the adsorption of CO 2 on nanoporous carbon at 35°C is shown in Table C-9.Table C-9 : Sample of the calculated results for the adsorption of CO 2 on nanoporous carbon at 35°C.Temperature Pressure Weight Buoyed Correction Total Uptake°C Torr kPa mg mg Weight Wt.% mmol/gram35.1 -0.2 0.0 49.7 0.0 49.7 0.0 0.034.6 303.3 40.4 51.4 0.1 51.5 3.7 0.834.6 493.5 65.8 51.9 0.2 52.1 4.9 1.134.6 1002.2 133.6 52.6 0.4 53.0 6.8 1.534.2 1971.1 262.8 53.1 0.8 54.0 8.8 2.034.3 3885.8 518.1 53.2 1.7 54.9 10.7 2.434.6 5826.2 776.8 52.8 2.6 55.4 11.9 2.734.7 7258.3 967.7 52.5 3.2 55.7 12.6 2.934.6 9615.0 1281.9 51.7 4.3 56.0 13.4 3.134.6 11514.7 1535.2 50.9 5.3 56.2 14.0 3.2241

C.4 Molecular ModellingFurther experimental details on the molecular simulation used to model the adsorption of pureand mixed gases on nanoporous carbon are presented in this section.C.4.1Sample Input FileAn example of an input file used by the FORTRAN compiler for the molecular modelling ispresented below in Figure C-2. Along with specifying the appropriate interaction andcoordinate files, the input file specifies the number of simulation steps (nstep), thetemperature (temp) and pressure (partb in kPa) of the system. In this example, the gas is amixed gas of 10 vol% CO 2 and 90 vol% CH 4 at a temperature of 300 K and system pressureof 100 kPa.topologyfile sequencefile forcefieldfile zeoforcefieldfile zeocoordfile'CO2CH4.top' 'CO2CH4.seq' 'ffCO2CH4' 'ffcarbon' 'C168.xyz'outputfile'1d2'iseed-1nstep nminit nsamp nmove iprint isave nofort nofort220000 1 10000 3000 200 20001 .true. .true.temp numcom LXY3D LXZ3D LYZ3D LCosAng LDL_POLY LUbind300. 2 .false. .false. .false. .false. .false. .false.partb in kPa(1..numcom) for GCMC0.1d2 0.9d2 ! (CO2,CH4)linit.true.LNVT LGrand.false. .true.Lexzeo Lzgrid Lnewtab Ltesttab LTube RTube.true. .true. .false. .true. .false. 0.d0pmswotch pmswap pmcb pmregr pmtra pmrot0.05 0.5 0.1 0.1 0.1 0.1stuk Imv Imvtot Iratp Igyra Isample0 200 100000 500 200 1lisotrop iratio iratv.false. 500 500boxlx1 boxly1 boxlz150. 50. 50.boxlx2 boxly2 boxlz250. 50. 50.rmtrax rmtray rmtraz rmzeo0.8d0 0.8d0 0.8d0 0.05d0rmrotx rmroty rmrotz rmvol0.05d0 0.05d0 0.05d0 0.1d0tatra tarot tazeo tavol0.5d0 0.5d0 0.5d0 0.5d0ntrmove rsmallmc150 0.4d0no of molecules10 10Figure C-2 : Input file used for the molecular simulation of the adsorption of a CO 2 :CH 4mixture (10:90 vol%) at a system temperature of 300 K and pressure of 100 kPa.242

C.4.2Sample Output FileAn example of an output file produced from the molecular modelling is presented below inFigure C-3. This is the output file related to the input file presented in Figure C-2 and as such,the gas is a mixed gas of 10 vol% CO 2 and 90 vol% CH 4 at a temperature of 300 K andsystem pressure of 100 kPa.RESULTS OF THE SIMULATION...RADIUS OF GYRATION COMP. 1 BOX 1 : 0.9897545E+00 +/- 0.6922732E-03END-TO-END DISTANCE COMP. 1 BOX 1 : 0.2320872E+01 +/- 0.2232917E-02RADIUS OF GYRATION COMP. 2 BOX 1 : 0.0000000E+00 +/- 0.0000000E+00END-TO-END DISTANCE COMP. 2 BOX 1 : 0.0000000E+00 +/- 0.0000000E+00AVERAGE TOTAL ENERGY BOX 1 : -0.1185288250E+07AVERAGE INTERMOLECULAR LJ ENERGY BOX 1 : -0.7076677914E+05AVERAGE LJ TAIL CORR. ENERGY BOX 1 : -0.1555018982E+04AVERAGE EXTERNAL ENERGY BOX 1 : -0.1154704794E+07AVERAGE TOTAL COULOMBIC ENERGY BOX 1 : -0.2518416855E+04AVERAGE INTRAMOLECULAR LJ ENERGY BOX 1 : 0.0000000000E+00AVERAGE BOND BENDING ENERGY BOX 1 : 0.4425675911E+05AVERAGE TORSIONAL ENERGY BOX 1 : 0.0000000000E+00AVERAGE STRETCHING ENERGY BOX 1 : 0.0000000000E+00AVERAGE VOLUME BOX 1 : 0.8288185600E+05AVERAGE NUMBER OF PARTICLES 1 BOX 1 : 0.1476132E+03 +/- 0.2168395E+01AVERAGE DENSITY OF PARTICLES 1 BOX 1 : 0.1781008E-02 +/- 0.2616248E-04AVERAGE NUMBER OF PARTICLES 2 BOX 1 : 0.2079234E+03 +/- 0.3075098E+01AVERAGE DENSITY OF PARTICLES 2 BOX 1 : 0.2508672E-02 +/- 0.3710218E-04AVERAGE LOADING OF PARTICLES IN [MMOL/GRAM ZEOLITE]VV1 COMPONENT 1VV1 COMPONENT 2AVERAGE LOADING OF PARTICLES IN [MOLECULES/UNIT CELL]VV2 COMPONENT 1VV2 COMPONENT 2************************** Ended Normally **************************Job CPU time:1.066 hours: 0.2288152E+01 +/- 0.3361227E-01: 0.3223019E+01 +/- 0.4766707E-01: 0.1845165E+02 +/- 0.2710493E+00: 0.2599043E+02 +/- 0.3843872E+00Figure C-3 : Output file produced from the molecular simulation of the adsorption of aCO 2 :CH 4 mixture (10:90 vol%) at a system temperature of 300 K and pressure of 100kPa.243

C.4.3Sample Isotherm DataThe data highlighted in yellow in Figure C-3 in the previous section along with the resultsproduced for other system pressures was then used to produce the adsorption isotherm, wherecomponent 1 is CO 2 and component 2 is CH 4 .The data for the 300 K isotherm of the mixed gas adsorption of CO 2 :CH 4 (10:90 vol%) ispresented in Table C-10 below.Table C-10 : Data produced for the 300 K isotherm from the molecular simulation of theadsorption of CO 2 :CH 4 (10:90 vol%) on nanoporous carbon represented by C 168Schwartize.System Pressure CO 2 Loading CH 4 Loading CO 2 /CH 4 Adsorptive Selectivitymmol/gram mmol/gram0.01 0.0021 0.0022 8.630.10 0.0208 0.0222 8.441.00 0.1913 0.2017 8.5410.00 1.1021 1.2149 8.16100.00 2.2882 3.2230 6.391000.00 2.2052 5.0822 3.9110000.00 2.0587 5.8848 3.15100000.00 2.4947 5.9408 3.78244

APPENDIX D – UNSUPPORTED NPC MEMBRANESThe advantage of unsupported NPC membranes over their supported counterparts is asignificant decrease in the membrane thickness leading to higher values permeance andtherefore flux.However, the significant disadvantage of unsupported NPC membranes is their intrinsic lackof mechanical strength, which makes handling and operation very difficult. The manufacture,characterisation and performance of unsupported NPC carbon membranes are discussed in thefollowing sections.The manufacture and testing of the unsupported NPC membranes work was completed byAmelia Russo and Timothy Carter as the final year research project for their undergraduateengineering degree.D.1 Manufacture and CharacterisationUnsupported carbon membranes were manufactured from as-cast Matrimid polyimide andpre-purchased Kapton polyimide.A significant amount of time was spent understanding the transition of the Matrimidpolyimide and Kapton polyimide from polymer to carbon. This was undertaken becauseinitial attempts to make carbon membranes from these polymers at the published hightemperatures that were required to produce fully carbonised membranes resulted inunworkable membranes due to brittleness. Thus, it was decided to focus on the transitionregime where a portion of polymer still exists resulting in a better mechanical strength.The same analysis of the transition region from polymer to carbon was not undertaken forPFA as these membranes were supported and did not require any contribution of the polymerto the mechanical strength. Therefore the pyrolysis temperature range used for the supportedmembranes was based solely on the literature data for the optimal manufacture of carbonfrom the polymer PFA as discussed earlier in Section 2.3.1.5.D.1.1Matrimid PolyimideThe weight loss of Matrimid with increasing temperature is given in Figure D-1. The weightbegins to drop at 400°C until a steady state is reached around 600°C, which is believed to bethe point at which the Matrimid has fully transformed into carbon with a total weight loss of245

40 %. This result is similar with that of Barsema et al [29] who reported a transition regionwithin the same temperature range as also shown in Figure D-1.10%Matrimid Matrimid [23]0%Weight Change-10%-20%-30%-40%-50%200 300 400 500 600 700 800Temperature (°C)Figure D-1 : Weight loss of unsupported NPC membranes made from Matrimidpolyimide with increasing pyrolysis temperature. The transition from polymer to carbonis within 400 and 600°C.NPC membranes that were made from Matrimid at pyrolysis temperature beyond 450°C wereextremely brittle and cracked upon testing. This result is similar to that of Barsema et al [29]who reported that at temperatures at and above 475°C, the membranes were very brittle anddifficult to handle.Characterisation using SEM of the surface of two selected membranes, made at pyrolysistemperatures of 420 and 430°C, respectively, are shown in Figure D-2 below. These twosamples were chosen for SEM analysis as the separation performance of these membraneswas known (as discussed in Section D.2).Interestingly, both samples had to be coated with a thin layer of gold before an image couldbe seen by the SEM. This means that the surface is not conductive and thus indicates that thesurface is mostly likely still polymer rather than carbon. However, several defects/cracks inthe surface were clearly seen even at this early stage in the pyrolysis process.246

(a)(b)Figure D-2 : SEM photographs of Matrimid (a) after pyrolysis at 420°C and (b) afterpyrolysis at 430°C.D.1.2Kapton PolyimideThe weight loss of Kapton polyimide with increasing pyrolysis temperature is presented inFigure D-3 below. The weight loss follows a similar pattern to that reported in the literaturefor this polymer [93]. Compared with Matrimid, carbonisation begins and finishes at muchhigher temperatures but with the same total weight loss of ~ 40 %.10%Kapton Batch 1 - 200HN Kapton Batch 2 - 100HN Kapton 100H [74]0%Weight Change-10%-20%-30%-40%-50%200 300 400 500 600 700 800 900 1000 1100Temperature (°C)Figure D-3 : Weight loss of unsupported NPC membranes made from Kapton polyimidewith increasing temperature. The transition from to carbon is between 500 and 700°C.247

Again, NPC membranes made from Kapton were extremely brittle and cracked upon testing.In fact, the only membrane (from the many that were made at temperatures up to 800°C) thatcould be tested was one made at 450°C.Characterisation using SEM of the surface of the membrane is shown in Figure D-4. Again,this sample had to be coated with a thin layer of gold before an image could be seen by theSEM, which means that the surface is not conductive and thus indicates that the surface ismostly likely still polymer rather than carbon. This result concurs with the minimal weightloss result at 450°C for Kapton.Figure D-4 : SEM photograph of Kapton after pyrolysis at 450°C. The surface wascoated with a gold layer to enable SEM imaging suggesting the absence of carbon andthe presence of polymer.D.2 PerformanceAs indicated in the previous section, testing of the NPC membranes was extremely difficultdue to the brittleness of the samples. Cracks would appear in the surface upon handling,inserting into the membrane cell for testing or upon testing itself. Despite trying differenttechniques, such as reducing the pyrolysis temperature and altering the supports used in thefurnace, the brittleness of the carbon produced could not be reduced. Eventually, the onlymembranes that could be tested were those that were partially pyrolysised.D.2.1Matrimid PolyimideA summary of the performance results for raw Matrimid and the partially pyrolysised NPCmembranes made from Matrimid at 420 and 430 °C is presented in Table D-1. Note that theperformance results are presented as permeability as the thicknesses of the membranes couldbe determined using a micrometer. Whilst there was a significant increase in permeability248

with partial pyrolysis, there was also a significant reduction in selectivity. Combining thisresult with the surface cracking seen by SEM it is evident that the cracking has producedKnudsen diffusion and and/or bulk flow, leading to the poor performance observed.As NPC membranes made from unsupported Matrimid polyimide could not be producedwithout cracking, it was not possible to compare the results with those of Barsema et al [29]who reported that the permeability increases exponentially with increasing temperature butwith a some sacrifice in permselectivity.Table D-1 : Performance results of raw Matrimid and partially pyrolysised NPCmembranes made from Matrimid at 420 and 430°C. The results suggest significantcracking in the partially pyrolysised NPC membranes.MembranePyrolysisTemperature Permeability (Barrer) Permselectivity°C CO 2 N 2 CH 4 CO 2 /N 2 CO 2 /CH 4Raw Matrimid N/A 6.49 0.58 0.49 11.26 13.19NPC420MA 420 198.61 287.95 278.85 0.69 0.86NPC430MB 430 14.64 19.07 14.21 0.77 0.90D.2.2Kapton PolyimideA summary of the performance results for raw Kapton and the partially pyrolysised NPCmembranes made from Kapton at 450 °C is presented in Table D-2 below. Again, theperformance results are given as permeability as the thickness of the membrane could bedetermined using a micrometer. In this case, cracking does not appear to have occurred as theselectivity has increased with the partial pyrolysis.The published results for Kapton polyimide [16, 22, 93, 94, 176-179] do not provideperformance results for partial pyrolysis for temperatures lower than 600°C. Su and Lua [94]report the CO 2 permeability of an NPC membrane made form the pyrolysis of Kapton at600°C to be 1820 barrer, which is significantly higher than the result shown in Table D-2 forraw Kapton and the partially carbonised membrane made at 450°C. Interesting, Su and Lua[94] also report a significant decrease in permeability and increase in permselectivity withincreasing the pyrolysis temperature from 600 to 800 and then to 1000°C. The authorsattribute this decrease in permeability to the narrowing pore size distribution with increasingpyrolysis temperature.It is therefore suggested that carbonisation of Kapton polyimide does provide an increase inthe permeability and selectivity over the original polymeric membrane. The permeability then249

decreases whilst the selectivity increases even further with increasing temperature above600°C.Despite the performance results obtained by Su and Lua [94], the problem with brittleness andhandling still remains. To obtain a workable unsupported NPC membrane, the optimummanufacturing temperature may still be within the transition region where there is an increasein the permeance even though the highest possible selectivity has not been obtained.Table D-2 : Performance results of raw Matrimid and partially pyrolysised NPCmembranes made from Kapton at 420°C. The results suggest some improvement inperformance with partial pyrolysis.MembranePyrolysisTemperature Permeability (Barrer) Permselectivity°C CO 2 N 2 CH 4 CO 2 /N 2 CO 2 /CH 4Raw Kapton N/A 1.32 0.39 0.32 3.38 4.13Kapton 420 2.51 0.61 0.46 4.11 5.46D.3 Concluding Remarks on Unsupported NPC MembranesLimited success was found in making crack-free unsupported NPC membranes from thepyrolysis of Matrimid or Kapton. This was a disappointing result as it was hoped that thepermeance or permeability of the NPC membranes could be greatly reduces by the thinnermembrane layer. Whilst the decrease in weight observed with increasing pyrolysistemperature was similar to that observed in the literature for both Matrimid and Kapton, theperformance of the membranes could not measured because of the brittleness of the carbon.Partial pyrolysis of Matrimid and Kapton was hoped to be a good compromise betweenobtaining a superior performance whilst maintaining the flexibility of the polymer. Some ofthe Matrimid membranes could be tested but cracking had still occurred even at such a lowpyrolysis temperature and as such the performance was poor. However, the partiallypyrolysised Kapton membrane did show some improvement in both selectivity andpermeability.Due to the difficulties in the manufacture of unsupported NPC membranes, the performanceof these membranes at higher temperature or in the presence of impurities has not beenexplored further. All of the results reported in the subsequent chapters were obtained usingsupported membranes manufactured from the pyrolysis of PFA without silica nanoparticles250