(BAT) Reference Document for the Production of Chlor-alkali ...

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Chapter 6 6.2 Oxygen-depolarised cathodes in modified membrane cells Description This technique consists in replacing the common metal cathodes in membrane cells with oxygen-depolarised cathodes that reduce oxygen to produce hydroxide instead of converting water to hydrogen and hydroxide. Technical description The utilisation of oxygen-depolarised cathodes (ODC) in the chlor-alkali electrolysis is an integration of an alkaline fuel cell cathode process into the membrane electrolysis cell: the cathode half cell reduces oxygen instead of producing hydrogen. This lowers the cell voltage by about 1 volt and means a substantial energy saving, as predicted from the theory and taking into account the typical electrode overpotentials. Figure 6.1 shows the expected potential differences for the two cathode types. This lowers the cell voltage by about 1 V at current densities of industrial relevance (e.g. 4 kA/m 2 ) and thus the electrical energy consumption by about 30 %. A higher reduction of the cell voltage of 1.23 V would be expected from the difference in standard electrode potentials. However, the overpotential of oxygen reduction shows a stronger increase with current density than that of hydrogen formation [ 49, Euro Chlor 2010 ], [ 52, Moussallem et al. 2008 ]. Figure 6.1:The influence of oxygen-depolarised cathodes on the electrode potentials, NaCl [Gestermann, 1998] {The figure was deleted.} The cathode reaction in membrane cells with ODC is [ 52, Moussallem et al. 2008 ]: The overall reaction is: O2 + 2 H2O + 4 e - V 4 OH - 4 NaCl + O2 + 2 H2O V 4 NaOH + 2 Cl2 The ODC requires the use of pure oxygen (92 – 99 %), but research is ongoing on the use of plain or oxygen-enriched air instead of pure oxygen [ 3, Euro Chlor 2011 ], [ 49, Euro Chlor 2010 ]. The ODC is a gas diffusion electrode which separates caustic from the oxygen side, from where the gas diffuses into the porous electrode structure to the catalyst centres and reacts with the cathodic water to produce OH - ions. All other functions are the same as in the normal membrane electrolysis. The ODC has to meet strict requirements for successful operation [ 52, Moussallem et al. 2008 ]: chemical stability in concentrated sodium hydroxide solution at temperatures between 80 and 90 °C; high mechanical stability in technical electrolysers with areas of several m 2 ; high electrical conductivity and low thickness; high surface area and activity of electrocatalyst; suitable hydrophobic/hydrophilic pore structure for easy access of gases and liquids without passing through of gas and flooding by liquid electrolyte even at different pressure differences between gas and liquid; high long-term stability; affordable costs. WORKING DRAFT IN PROGRESS These strict requirements explain why it took about forty years from the beginning of research on ODCs in the early seventies to the construction of an industrial installation in 2010. During the 1990s two large programmes were started by the Japanese Ministry of Trade and Industry in 290 December 2011 TB/EIPPCB/CAK_Draft_1
Chapter 6 cooperation with the Japanese Soda Industry Association as well as by Bayer in cooperation with Uhde and DeNora in Germany. In 2011, limited information is available on developments in Japan, but at least two demonstration units have been undergoing tests since 2007 [ 49, Euro Chlor 2010 ]. In Germany, the Federal Ministry of Education and Research funded a programme from 2006 – 2010 to render the technique economically viable by reducing the electrolysis voltage to 2.0 V and by reducing ODC production costs [ 51, Bulan 2007 ], [ 53, Bulan et al. 2009 ]. As a result, a membrane cell plant using the ODC technique operated by Bayer and UHDENORA/Uhde with a chlorine capacity of 20 kt/yr was put into operation in summer 2011 [ 49, Euro Chlor 2010 ]. The use of ODCs for the electrolysis of hydrogen chloride is, however, simpler and Bayer has operated two installations in Brunsbüttel (Germany) with a chlorine capacity of 20 kt/yr since 2003 and in Caojing (China) with a chlorine capacity of 215 kt/yr since 2008 [ 115, Jörissen et al. 2011 ]. Oxygen-depolarised cathodes for chlor-alkali electrolysis can be divided into gas-liquid permeable ODCs used in two-compartment, zero-gap cells and gas-liquid permeable ODCs used in three-compartment, finite-gap cells. In zero-gap cells, the ODC is in direct contact with the membrane. Gaseous oxygen and liquid electrolyte flow countercurrently through the electrode. This design has two potential advantages. Firstly, the problem of height-dependent pressure difference is solved (see below) and secondly, the ohmic losses in the catholyte gap are minimised. On the other hand, oxygen transport may be hindered because the produced sodium hydroxide is removed against the direction of oxygen flow. In addition, the sodium ions do not carry enough water through the membrane, therefore the caustic concentration has to be adjusted by the addition of water to the oxygen feed in order not to exceed the membrane stability limits [ 52, Moussallem et al. 2008 ]. The only existing plant of industrial scale is based on three-compartment, finite-gap cells (Figure 6.1). WORKING DRAFT IN PROGRESS Source: [ 49, Euro Chlor 2010 ] Figure 6.1: Schematic view of a three-compartment, finite-gap membrane cell with an oxygendepolarised cathode (Bayer/Uhde design) TB/EIPPCB/CAK_Draft_1 December 2011 291
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EUROPEAN COMMISSION JOINT RESEARCH
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PREFACE 1. Status of this document
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Reference Document on Best Availabl
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3.4.7 Emissions of noise ..........
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4.3.6.3.3 Chemical reduction ......
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List of Tables Table 2.1: Main char
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List of Figures Figure 1.1: Share p
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SCOPE WORKING DRAFT IN PROGRESS Sco
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1 GENERAL INFORMATION 1.1 Industria
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Chlorine production in Mt/yr 12 11
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Chapter 1 Figure 1.4 shows the annu
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Share of total capacity in % 70 70%
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1.4 Chlor-alkali products and their
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Total consumption: 9 801 kt Miscell
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1.4.5 Consumption of hydrogen Chapt
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Chapter 1 and hazardous waste incin
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2 APPLIED PROCESSES AND TECHNIQUES
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Chapter 2 WORKING DRAFT IN PROGRESS
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Chapter 2 The main characteristics
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2.2 The mercury cell technique proc
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Chapter 2 Characteristics of the ca
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2.3 The diaphragm cell technique pr
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Source: [ 2, Le Chlore 2002 ] [USEP
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2.4 The membrane cell technique pro
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Chapter 2 (carcinogenic) [ 76, Regu
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Chapter 2 The membranes used in the
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Table 2.2: Typical configurations o
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2.5 Brine supply 2.5.1 Sources, qua
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Chapter 2 centrifuges before dispos
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Source: [ 29, Asahi Glass 1998 ] (p
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Impurity Source Upper limit of brin
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Chapter 2 No such dechlorination tr
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Chapter 2 The cooling water is gene
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Chapter 2 composition of the chlori
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2.6.11 Dealing with impurities 2.6.
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Chapter 2 amount of chlorine, and t
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2.6.12.2 Chemical reactions Chapter
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2.7 Caustic processing production,
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Chapter 2 2.8 Hydrogen processing p
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3 CURRENT PRESENT EMISSION AND CONS
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Chapter 3 Table 3.1: Overview of em
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3.3 Consumption levels of all cell
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3.3.3 Ancillary materials Ancillary
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Further materials and/or further us
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Chapter 3 current) and the efficien
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Chapter 3 distance means a higher f
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3.3.4.3.6 Production of caustic pot
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Chapter 3 hydrogen at low pressure
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28 kg of hydrogen is produced {Thes
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Chapter 3 depleted brine is recircu
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Chapter 3 Table 3.10: Emissions of
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Chapter 3 When measuring chlorine i
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Chapter 3 concentrations of organic
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Chapter 3 3.4.3 Emissions and waste
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Chapter 3 {Please TWG provide infor
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Chapter 3 in process and waste wate
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Chapter 3 produced per year has bee
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Chapter 3 of chlorine capacity. The
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Table 3.23: Mercury emissions from
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Mercury emissions in g Hg/t Cl capa
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Chapter 3 Table 3.24: Mercury emiss
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Chapter 3 KCl are higher than from
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Chapter 3 When concentrated solid c
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Chapter 3 Generally, storage of con
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Chapter 3 recycled brine systems si
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Process Maintenance To solid treatm
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3.5.9.5 Graphite and activated Acti
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Chapter 3 Table 3.31: Yearly waste
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Chapter 3 The fact that the differe
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Chapter 3 3.6 Emissions Emission an
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Reported asbestos concentrations ar
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Chapter 3 3.7 Emissions Emission an
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{A list of techniques used on full
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3.8.3 PCDDs/PCDFs, PCBs, PCNs and P
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Chapter 3 The ‘dioxin’ pattern
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Chapter 4 4 TECHNIQUES TO CONSIDER
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Heading within the sections Cross-m
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4.1 Mercury cell plants 4.1.1 Overv
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Chapter 4 plants at comparable capa
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Chapter 4 Table 4.2: Data from the
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Chapter 4 environment can be expect
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4. Plant size Chapter 4 An increase
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Table 4.6: Comparison of reported c
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Chapter 4 Generally, conversion cos
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4.1.3 Decommissioning 4.1.3.1 Decom
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3. Emptying of the cells and handli
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Chapter 4 Table 4.7: Overview of co
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Table 4.8: Techniques for monitorin
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Chapter 4 Table 4.10: Decommissioni
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4.2 Diaphragm cell plants 4.2.1 Aba
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Source: [ 146, Arkema 2009 ] Figure
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Chapter 4 diaphragms [ 31, Euro Chl
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Driving force for implementation Th
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Chapter 4 4.2.3 Conversion of asbes
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Chapter 4 directly from the cells.
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4.3 All Diaphragm and membrane cell
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Chapter 4 non-standardised) will be
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Chapter 4 removal of conductive com
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Chapter 4 Cross-media effects Plant
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Environmental performance and opera
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Example plants Dow, Stade (Germany)
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Chapter 4 current density resulting
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Chapter 4 modifications to auxiliar
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Table 4.14: Cost calculation for th
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Chapter 4 membrane lifetimes range
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Chapter 4 Cross-media effects Some
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Chapter 4 The drawback of using fer
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Chapter 4 evaporation unit integrat
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Chapter 4 Environmental performance
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Chapter 4 Technical description The
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Chapter 4 Table 4.17: Monitoring te
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Chapter 4 Environmental performance
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Environmental performance and opera
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Chapter 4 Prevention of chlorine re
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Chapter 4 Good pipework design to m
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Chapter 4 was the solution at that
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differential pressure at inlet and
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Chapter 4 chlorine scrubber, immedi
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Chapter 4 Achieved environmental be
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{Please TWG provide more informatio
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environmental legislation; generati
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Chapter 4 Table 4.21: Examples of o
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Chapter 4 Driving force for impleme
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Page 285 and 286: General considerations In these BAT
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Page 295 and 296: 5.6 Monitoring of emissions Chapter
Page 297 and 298: [This BAT conclusion is based on in
Page 299 and 300: 5.9 Generation of waste Chapter 5 1
Page 301 and 302: 5.11 Site remediation Chapter 5 20.
Page 303 and 304: Chapter 5 inorganic chloramines, di
Page 305: 6 EMERGING TECHNIQUES 6.1 Introduct
Page 309 and 310: Chapter 6 In December 1998 a test w
Page 311 and 312: 6.3 Use of fuel cells in chlor-alka
Page 313 and 314: Chapter 6 hydrogen that is burnt to
Page 315 and 316: Chapter 7 7 CONCLUDING REMARKS AND
Page 317 and 318: REFERENCES References [1] Ullmann's
Page 319 and 320: References [63] Euro Chlor, Euro Ch
Page 321 and 322: References [112] Santorelli et al.,
Page 323 and 324: References [161] Germany, 'Verordnu
Page 325 and 326: [212] Florkiewicz, Long life diaphr
Page 327 and 328: References [259] Rappe et al., 'Lev
Page 329 and 330: GLOSSARY OF TERMS AND ABBREVIATIONS
Page 331 and 332: V. Units · VI. Chemical elements T
Page 333 and 334: VIII. Acronyms and definitions AC A
Page 335 and 336: EOX Extractable Organically-bound H
Page 337 and 338: Glossary Technique Ensemble of both
Page 339 and 340: ANNEX Annex {The CAK plant capaciti
Page 341 and 342: Euro Chlor No. Country code Company
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