FTS fixed-bed reactor based on Nano-structured Iron catalyst ...
FTS fixed-bed reactor based on Nano-structured Iron catalyst ... FTS fixed-bed reactor based on Nano-structured Iron catalyst ...
Petroleum & CoalISSN 1337-7027Available online at www.vurup.sk/petroleum-coalPetroleum & Coal 55 (2) 82-92, 2013MODELING OF FISCHER–TROPSCH SYNTHESIS PACKED BED REACTOR FORPRODUCING LIQUID FUELS FROM NATURAL GASMohammad IraniGas Division, Research Institute of Petroleum Industry (RIPI), Tehran, Iran.Received January 2, 2013, Accepted March 30, 2013AbstractA tubular pilot scale
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Petroleum & CoalISSN 1337-7027Available <strong>on</strong>line at www.vurup.sk/petroleum-coalPetroleum & Coal 55 (2) 82-92, 2013MODELING OF FISCHER–TROPSCH SYNTHESIS PACKED BED REACTOR FORPRODUCING LIQUID FUELS FROM NATURAL GASMohammad IraniGas Divisi<strong>on</strong>, Research Institute of Petroleum Industry (RIPI), Tehran, Iran.Received January 2, 2013, Accepted March 30, 2013AbstractA tubular pilot scale <str<strong>on</strong>g>fixed</str<strong>on</strong>g>-<str<strong>on</strong>g>bed</str<strong>on</strong>g> <str<strong>on</strong>g>reactor</str<strong>on</strong>g> for the Fischer–Tropsch Synthesis (<str<strong>on</strong>g>FTS</str<strong>on</strong>g>) was simulated to numericallydem<strong>on</strong>strate the c<strong>on</strong>versi<strong>on</strong> of synthesis gas (CO+H 2 ) to higher hydrocarb<strong>on</strong>s over an ir<strong>on</strong> <str<strong>on</strong>g>based</str<strong>on</strong>g> <strong>catalyst</strong>.The <str<strong>on</strong>g>reactor</str<strong>on</strong>g> was a 3.1 cm diameter and 2.75 m length steel tube. The feed temperature were between563- 583 K. Saturated water was employed to c<strong>on</strong>trol the peak temperature within the <strong>catalyst</strong> <str<strong>on</strong>g>bed</str<strong>on</strong>g>,which is critical for the stability of ir<strong>on</strong> <str<strong>on</strong>g>based</str<strong>on</strong>g> <strong>catalyst</strong>. In this research, computati<strong>on</strong>al fluid dynamics(CFD) was used to model the interacting phenomena such as n<strong>on</strong>-ideality of mixture, reacti<strong>on</strong> andhydrodynamics in a <str<strong>on</strong>g>fixed</str<strong>on</strong>g>-<str<strong>on</strong>g>bed</str<strong>on</strong>g> <str<strong>on</strong>g>reactor</str<strong>on</strong>g> for the <str<strong>on</strong>g>FTS</str<strong>on</strong>g>. Synthesis gas c<strong>on</strong>versi<strong>on</strong>, product selectivity, andtemperature were measured as a functi<strong>on</strong> of <str<strong>on</strong>g>reactor</str<strong>on</strong>g> length. <str<strong>on</strong>g>FTS</str<strong>on</strong>g> can be c<strong>on</strong>ducted with very lowtemperature range between 563 and 583K within a wide range of CO c<strong>on</strong>versi<strong>on</strong> using saturated wateras coolant. By using saturated water, <str<strong>on</strong>g>reactor</str<strong>on</strong>g> exhibited good isothermal c<strong>on</strong>diti<strong>on</strong> under most simulatedc<strong>on</strong>diti<strong>on</strong>s. Very good agreement between pilot experimental data and the model was achieved. Resultsshow that increase in feed temperature have a significant effect <strong>on</strong> the yield of products.Keywords: Numerical modeling; Fischer–Tropsch synthesis; packed <str<strong>on</strong>g>reactor</str<strong>on</strong>g>; CFD methods; saturated water.1.Introducti<strong>on</strong>The Fischer–Tropsch synthesis (<str<strong>on</strong>g>FTS</str<strong>on</strong>g>) is a key step in the c<strong>on</strong>versi<strong>on</strong> of natural gas and coalor biomass into liquid fuels (synfuels) through the so-called gas-to-liquids (GTL) and coal/biomass-to-liquids (CTL/BTL) processes. Am<strong>on</strong>g them, GTL is particularly attractive from thepoint of view of operating costs, carb<strong>on</strong> and thermal efficiencies as well as envir<strong>on</strong>mentalimpact [1- 3] . The <str<strong>on</strong>g>FTS</str<strong>on</strong>g> is the catalytic c<strong>on</strong>versi<strong>on</strong> of synthesis gas (CO+H 2 ) into a multi-comp<strong>on</strong>entmixture of hydrocarb<strong>on</strong>s and water. The commercial <str<strong>on</strong>g>FTS</str<strong>on</strong>g> plants have utilized either Fe or Co<strong>catalyst</strong>s using <str<strong>on</strong>g>FTS</str<strong>on</strong>g> to produce liquid fuels (gasoline and diesel) [4, 5] .The <str<strong>on</strong>g>FTS</str<strong>on</strong>g> can be viewedas a polymerizati<strong>on</strong> reacti<strong>on</strong> where chain growth is the basic feature. Various types of <str<strong>on</strong>g>reactor</str<strong>on</strong>g>s(including fluidized <str<strong>on</strong>g>bed</str<strong>on</strong>g>, multi-tubular <str<strong>on</strong>g>fixed</str<strong>on</strong>g>-<str<strong>on</strong>g>bed</str<strong>on</strong>g> and slurry) have been c<strong>on</strong>sidered during the<str<strong>on</strong>g>FTS</str<strong>on</strong>g> process development. The <str<strong>on</strong>g>fixed</str<strong>on</strong>g>-<str<strong>on</strong>g>bed</str<strong>on</strong>g> FT process, being <strong>on</strong>e of the most competing <str<strong>on</strong>g>reactor</str<strong>on</strong>g>technologies, occupies a special positi<strong>on</strong> in <str<strong>on</strong>g>FTS</str<strong>on</strong>g> industrial processes [6-9] . Due to the high demand<strong>on</strong> gasoline in the world and its higher price relative to that of diesel, producti<strong>on</strong> of gasolinefrom the FT process, becomes more favorable. The octane number of FT gasoline is lower thanthat of the gasoline obtained from crude oil processing, since the FT gasoline mainly c<strong>on</strong>sistsof n-paraffin. To promote the yield and quality of the gasoline from <str<strong>on</strong>g>FTS</str<strong>on</strong>g>, bi-functi<strong>on</strong>al <strong>catalyst</strong>shave received extensive attenti<strong>on</strong> in the recent years [10] .In this research, the modified bi-functi<strong>on</strong>al Fe-HZSM5 <strong>catalyst</strong> has been used. Such a processremoves the need for cumbersome upgrading units for GTL plants [11] . To achieve optimumc<strong>on</strong>diti<strong>on</strong> for the best performance of process, the <str<strong>on</strong>g>reactor</str<strong>on</strong>g> should be simulated exactly byc<strong>on</strong>sidering n<strong>on</strong>-idealities. Clearly, due to the complexity of <str<strong>on</strong>g>FTS</str<strong>on</strong>g> reacti<strong>on</strong> system, variety ofproducts (C 1 -C 20 ) and operating c<strong>on</strong>diti<strong>on</strong> of this process, a suitable <str<strong>on</strong>g>reactor</str<strong>on</strong>g> model with c<strong>on</strong>sideringthermodynamic n<strong>on</strong>-idealities ,from which the selectivity and heat transfer informati<strong>on</strong>can be determined in a qualitative fashi<strong>on</strong>, has not been presented yet [12,13] .Bub et al. [14] and Jess et al. [15] developed a two-dimensi<strong>on</strong>al, pseudo-homogeneous modelthat was used to predict product distributi<strong>on</strong> and performed the c<strong>on</strong>ceptual design of a <str<strong>on</strong>g>fixed</str<strong>on</strong>g>-<str<strong>on</strong>g>bed</str<strong>on</strong>g>
eactor for c<strong>on</strong>verting nitrogen rich syngas. Atwood and Bennet [16] have proposed a <strong>on</strong>edimensi<strong>on</strong>al,homogeneous plug flow model. Wang et al. [17] devised a comprehensive <strong>on</strong>edimensi<strong>on</strong>alheterogeneous <str<strong>on</strong>g>reactor</str<strong>on</strong>g> model to simulate the performance of <str<strong>on</strong>g>fixed</str<strong>on</strong>g>-<str<strong>on</strong>g>bed</str<strong>on</strong>g> FT<str<strong>on</strong>g>reactor</str<strong>on</strong>g>s for hydrocarb<strong>on</strong> producti<strong>on</strong>. Liu et al. [18] have studied the steady state and dynamicbehavior of the FT <str<strong>on</strong>g>fixed</str<strong>on</strong>g>-<str<strong>on</strong>g>bed</str<strong>on</strong>g> <str<strong>on</strong>g>reactor</str<strong>on</strong>g> using a two-dimensi<strong>on</strong>al heterogeneous model. Also,Marvast et al. [19] have simulated the behavior of the FT <str<strong>on</strong>g>fixed</str<strong>on</strong>g>-<str<strong>on</strong>g>bed</str<strong>on</strong>g> <str<strong>on</strong>g>reactor</str<strong>on</strong>g> using a twodimensi<strong>on</strong>alheterogeneous model at steady state c<strong>on</strong>diti<strong>on</strong>.In the present work, we report a computati<strong>on</strong>al fluid dynamic (CFD) study to investigateperformance of a bi-functi<strong>on</strong>al Fe-HZSM-5 <strong>catalyst</strong> in a pilot scale tubular <str<strong>on</strong>g>fixed</str<strong>on</strong>g>-<str<strong>on</strong>g>bed</str<strong>on</strong>g> <str<strong>on</strong>g>reactor</str<strong>on</strong>g>for <str<strong>on</strong>g>FTS</str<strong>on</strong>g>, coupling the exothermic <str<strong>on</strong>g>FTS</str<strong>on</strong>g> with saturated water for heat removal. The CFD is apowerful tool for simulating complex reacting flows [20-24] . The aim of our study was to analyzethe c<strong>on</strong>diti<strong>on</strong>s under which the synthesis reacti<strong>on</strong> could be c<strong>on</strong>ducted at a reas<strong>on</strong>ably highCO c<strong>on</strong>versi<strong>on</strong> close to the isothermal c<strong>on</strong>diti<strong>on</strong> at temperatures between 563 and 583 K.The simulati<strong>on</strong> results have been validated with pilot experimental data.2. Process descripti<strong>on</strong>The <str<strong>on</strong>g>FTS</str<strong>on</strong>g> has been investigated in a pilot scale of a <str<strong>on</strong>g>fixed</str<strong>on</strong>g> <str<strong>on</strong>g>bed</str<strong>on</strong>g> <str<strong>on</strong>g>reactor</str<strong>on</strong>g> which was packed withbi-functi<strong>on</strong>al Fe-HZSM5 <strong>catalyst</strong> (metal part: 100 Fe/5.4 Cu/7K2O/21SiO2, acidic part:SiO2/Al 2 O 3 =14). There was a jacket around the <str<strong>on</strong>g>reactor</str<strong>on</strong>g> which boiling water flowing throughit to remove the heat of reacti<strong>on</strong>. The temperature of boiling water could be c<strong>on</strong>trolled measuringpressure of the system. The pilot has been designed and c<strong>on</strong>structed by the Research Instituteof Petroleum Industry, Nati<strong>on</strong>al Iranian Oil Company (RIPI-NIOC) in 2008 [12] . The characteristicsof the pilot plant are shown in Table 1. The object is to find the profiles of temperature andc<strong>on</strong>centrati<strong>on</strong> of various species al<strong>on</strong>g the <str<strong>on</strong>g>fixed</str<strong>on</strong>g>-<str<strong>on</strong>g>bed</str<strong>on</strong>g> <str<strong>on</strong>g>reactor</str<strong>on</strong>g> length at different c<strong>on</strong>diti<strong>on</strong>sand recognize the optimum c<strong>on</strong>diti<strong>on</strong>.Table1 <str<strong>on</strong>g>FTS</str<strong>on</strong>g> pilot plant characteristicsTube Dimensi<strong>on</strong>(mm) 31×3×2.75Number of tubes 1Molar Ratio of H2/Co 1Feed Temperature(K) 563,573 and 583Catalyst sizes (mm) 1.5×4Catalyst density(kg/m 3 ) 1290Bulk density(kg/m 3 ) 730Reactor pressure(bar) 17Cooling temperature (K) Saturated water (563,573 and 583)GHSV(Nl/hr) 3Bed Voidage 0.383. Mathematical Model3.1. Governing equati<strong>on</strong>sIn order to model the problem, five sets of equati<strong>on</strong>s should be solved; c<strong>on</strong>tinuity equati<strong>on</strong>,momentum balance, energy and species transport equati<strong>on</strong>s. The mass c<strong>on</strong>servati<strong>on</strong>,momentum, and total enthalpy equati<strong>on</strong>s, may be expressed as follows, respectively:.( v ) 0(1) .(v v) P.i Tvv g Sn (3).(v( H.( vCi– Di P)) .( C) Riii1h j ) .(q) SiM. Irani/Petroleum & Coal 55(2) 82-92, 2013 83R(2)(4)
22S DijvjC j1jij1 21 v v jSR jhMjjR j0 (5)Where, represents mixture density, v is velocity vector, and H, h i are total enthalpyand enthalpy of species, respectively and C i stands for c<strong>on</strong>centrati<strong>on</strong> of chemical species. Pis the static pressure, S is the model-dependent source term from porous-media and S R isthe heat of reacti<strong>on</strong> . The <strong>catalyst</strong> z<strong>on</strong>e c<strong>on</strong>sidered as a porous media that modeled by theadditi<strong>on</strong> of a momentum source term to the standard momentum equati<strong>on</strong>. The source termis composed of two parts: a viscous loss term (Darcy, the first term <strong>on</strong> the right-hand sideof Equati<strong>on</strong> 5) and an inertial loss term (the sec<strong>on</strong>d term <strong>on</strong> the right-hand side of Equati<strong>on</strong>5). v is the magnitude of the velocity and D and C are prescri<str<strong>on</strong>g>bed</str<strong>on</strong>g> matrices. The momentumsink c<strong>on</strong>tributes to the pressure gradient in the porous cells, causing a pressure drop that isproporti<strong>on</strong>al to the fluid velocity in the cells [25] .Due to complexity of the <str<strong>on</strong>g>FTS</str<strong>on</strong>g> reacti<strong>on</strong> system, variety of products (C 1 - C 20 ) and operatingc<strong>on</strong>diti<strong>on</strong> of the process, n<strong>on</strong>-ideal thermodynamic behavior of system should be c<strong>on</strong>sideredusing Eqs 6 to 8 [12,13] . Hereup<strong>on</strong>, a suitable EOS (equati<strong>on</strong> of state) should be used forcalculati<strong>on</strong> of mixture density and species fugacity (f i ). For this study Peng-Rabins<strong>on</strong> equati<strong>on</strong>was chosen as a suitable EOS which its parameters has been listed in table 2.Table 2 Parameters of Peng-Rabins<strong>on</strong> EOSσ ε Ω Ψ1 21 2(Tr;) 1 Z bPbeta RTf P y 20.37464 1.542260.26992 1 TZ Z1 Z q 0.07779 0.45724Pr, Tr12r2(Tr) , q Tbiln i ( Z 1) ln( Z beta ) I qibmaq bRT aqiq xibi[i] 2. - a (6)b1 Z 1 betaI ln() Z beta1i2ii2M. Irani/Petroleum & Coal 55(2) 82-92, 2013 84r3.2 Computati<strong>on</strong>al domain and boundary c<strong>on</strong>diti<strong>on</strong>sThe schematic of the setup is given in figure1, A two- dimensi<strong>on</strong>al system (2.75 m x 31mm)which was fed with synthesis gas (a mixture of carb<strong>on</strong> m<strong>on</strong>oxide and hydrogen). Furthermore,the <str<strong>on</strong>g>reactor</str<strong>on</strong>g> was divided in two secti<strong>on</strong>s:1) top z<strong>on</strong>e fluid, 2) reacti<strong>on</strong> (<strong>catalyst</strong>) z<strong>on</strong>e. Packed
M. Irani/Petroleum & Coal 55(2) 82-92, 2013 85<str<strong>on</strong>g>bed</str<strong>on</strong>g> was c<strong>on</strong>sidered as a porous media due to the large value of N (tube-to-<strong>catalyst</strong> diameterratio) [26] .Figure.1 Schematic of <str<strong>on</strong>g>fixed</str<strong>on</strong>g>-<str<strong>on</strong>g>bed</str<strong>on</strong>g> <str<strong>on</strong>g>reactor</str<strong>on</strong>g> and its boundary c<strong>on</strong>diti<strong>on</strong>sThe mass-flow-inlet boundary c<strong>on</strong>diti<strong>on</strong> was used for gas flow to the modeled <str<strong>on</strong>g>reactor</str<strong>on</strong>g>.The pressure-outlet boundary c<strong>on</strong>diti<strong>on</strong> was specified at the bottom of the <str<strong>on</strong>g>reactor</str<strong>on</strong>g>. The <str<strong>on</strong>g>FTS</str<strong>on</strong>g>reacti<strong>on</strong> is exothermic and if the heat of reacti<strong>on</strong> isn’t removed, temperature run away willbe taken place. In this study saturated water was used for heat removal, so heat absor<str<strong>on</strong>g>bed</str<strong>on</strong>g>by water generates steam and <str<strong>on</strong>g>reactor</str<strong>on</strong>g> was assumed to be isothermal. The source term (S)for boundary cells (walls in reacti<strong>on</strong> z<strong>on</strong>e) should be included as follows:S H ( T) H ( T)(7)Which H (T )of reacti<strong>on</strong>) and (T ) is enthalpy at temperature T (the increased temperature due to releasing heatH is enthalpy at temperature T (the desired temperature for reacti<strong>on</strong>).3.3. Products and rate equati<strong>on</strong>sWhen rate equati<strong>on</strong>s include in the CFD simulati<strong>on</strong>, chemical kinetic calculati<strong>on</strong>s are verycomputer time c<strong>on</strong>suming, especially for a complex reacti<strong>on</strong> scheme like <str<strong>on</strong>g>FTS</str<strong>on</strong>g> [17] . In somesimulati<strong>on</strong> studies the complexity of the <str<strong>on</strong>g>FTS</str<strong>on</strong>g> chemistry has been reduced by using a set ofglobal <strong>on</strong>e-step reacti<strong>on</strong> for selected mechanism [19] . The Fischer–Tropsch reactants andproducts which c<strong>on</strong>sidered in this study c<strong>on</strong>sist of: CO, H 2 , CO, CO 2 , H 2 O, CH4, C 2 H 4 , C 2 H 6 ,C 3 H 8 , n-C 4 H 10, i- C 4 H 10 and C 5 +. The following reacti<strong>on</strong>s listed in Table 3 are leading Fischer–Tropsch reacti<strong>on</strong>s [27] :Table 3 List of <str<strong>on</strong>g>FTS</str<strong>on</strong>g> reacti<strong>on</strong>sNum.Reacti<strong>on</strong> stoichiometry1 CO 3H2CH4 H2O2 2CO 4H2 C2H4 2H2O3 2 5HCH H OCO2 2 6224 3CO 7H2 C3H8 3H2O5 4CO 9H2 n C4H10 4H2O6 4CO 9H2 iC4H10 4H2O7 8.9618.05HCH ( C ) 8.H O8CO HCO2 8.96 12.36 59622OCO2 H2The general form of reacti<strong>on</strong> rate equati<strong>on</strong> is like Eq. (8) and for <str<strong>on</strong>g>FTS</str<strong>on</strong>g> its kinetic parameters [27]are given in table 4:Rmoli hr.grcat E fi kiexp RT f2h2co(8)
Table 4 <str<strong>on</strong>g>FTS</str<strong>on</strong>g> parameters of reacti<strong>on</strong> ratesK 1 (mol s -1 kg -1 cat /kPa) E 1 (J mol -1 )1 2.044 e-2 156932 6.255 e-5 20.3843 3.423 e-4 1.56074 5.972 e-6 164.065 6.482 e-6 86.9346 6.482 e-6 81.7537 3.168 e-5 738.788 3.168 e-5 5532.93.4. Material propertiesThe specific heat of each species was defined as piecewise-polynomial functi<strong>on</strong> of temperature.Besides to c<strong>on</strong>sider variati<strong>on</strong> of density with temperature and compositi<strong>on</strong>, compressibilityfactor (Z) was calculated using Peng-Rabins<strong>on</strong> EOS and also the density of gas mixture wasdefined as follow:P M Z RTWhere R is the universal gas c<strong>on</strong>stant, M G is the molecular weight of gas mixture and P isthe operating pressure (taken to be 17 bar). For other thermal properties of the mixture suchas molecular viscosity, thermal c<strong>on</strong>ductivity and diffusivity coefficient, the relati<strong>on</strong>s in appendixwere used [28].The steps implemented in the soluti<strong>on</strong> procedure are as follows:1. Calculati<strong>on</strong> of the gas-phase transport properties.2. Calculati<strong>on</strong> of the density of gas mixture using Peng-Rabins<strong>on</strong> EOS.3. Calculati<strong>on</strong> of the thermodynamic properties such as fugacity coefficients andcompressibility factors using special subroutines.4. Determinati<strong>on</strong> of the rate of c<strong>on</strong>sumpti<strong>on</strong> of reactants and producti<strong>on</strong> of products per unitvolume.5. Determinati<strong>on</strong> of the source terms for all the c<strong>on</strong>servati<strong>on</strong> equati<strong>on</strong>s.6. Solving the c<strong>on</strong>servati<strong>on</strong> equati<strong>on</strong>s.The finite volume method has been used to discrete the partial differential equati<strong>on</strong>s ofthe model, using the SIMPLE method for pressure–velocity coupling.4. RESULTS AND DISCUSSIONM. Irani/Petroleum & Coal 55(2) 82-92, 2013 86The geometry and grid was created in Gambit (Figure 2), the pre-processor of Fluent. Thegeometry c<strong>on</strong>sists of a rectangle, 2.75m in length and 31mm in width, with uniform <strong>structured</strong>grids to simulate fluid flow. A parametric study was d<strong>on</strong>e by varying the gas inlet temperature.In all cases, <str<strong>on</strong>g>reactor</str<strong>on</strong>g> pressure and H 2 /CO ratio are kept c<strong>on</strong>stant at 17 bar and 1, respectively.Based <strong>on</strong> the variati<strong>on</strong> of gas inlet temperatures, three set of simulati<strong>on</strong>s were performed.(9)Figure 2 Mesh c<strong>on</strong>figurati<strong>on</strong> of computati<strong>on</strong>al domainFigure 3 presents the c<strong>on</strong>tour of velocity in the <str<strong>on</strong>g>reactor</str<strong>on</strong>g>. As can be seen, in the first z<strong>on</strong>eof the <str<strong>on</strong>g>reactor</str<strong>on</strong>g> the velocity has the maximum value and in <strong>catalyst</strong> regi<strong>on</strong>, it is decreased dueto the pressure drop in the reacti<strong>on</strong> z<strong>on</strong>e.Figure 4 shows the mass fracti<strong>on</strong> profiles of product species al<strong>on</strong>g the <str<strong>on</strong>g>reactor</str<strong>on</strong>g> length forthe case with the inlet gas temperature of 573 K. As shown in figure 4 the mass fracti<strong>on</strong> ofproduct species is c<strong>on</strong>tinuously increasing from the inlet to the outlet and there is a jump in
Mass Fracti<strong>on</strong>M. Irani/Petroleum & Coal 55(2) 82-92, 2013 87the end z<strong>on</strong>e of the <str<strong>on</strong>g>reactor</str<strong>on</strong>g> due to the decrease in mass fracti<strong>on</strong> of CO. (c<strong>on</strong>sidering reacti<strong>on</strong>rates, denominator is fugacity of CO).Figure.3 C<strong>on</strong>tour of velocity (operating c<strong>on</strong>diti<strong>on</strong>: T=573, P=17 bar)0.140.120.10.08methaneetilenpropaanEthaneButhaneC5+CO0.060.040.0200 0.25 0.5 0.75 1 1.25 1.5 1.75 2 2.25 2.5 2.75Reactor length(m)Figure. 4. Mass fracti<strong>on</strong> of species al<strong>on</strong>g the <str<strong>on</strong>g>reactor</str<strong>on</strong>g> length at T=573As depicted in figure 5 in the case with temperature of 563 the C 5+ selectivity was lessthan the case with temperature of 573. When the temperature was increased to 583K, C 5+selectivity was declined because of activati<strong>on</strong> of water-gas-shift reacti<strong>on</strong> at this temperatureand increasing of CO 2 producti<strong>on</strong>.Based <strong>on</strong> the producti<strong>on</strong> of heavier products in reacti<strong>on</strong> z<strong>on</strong>e, density increases al<strong>on</strong>g the<str<strong>on</strong>g>reactor</str<strong>on</strong>g> length (Figure 7).
SelectivityM. Irani/Petroleum & Coal 55(2) 82-92, 2013 882015T=563 K10T=573 KT=583 K500 0.5 1 1.5 2 2.5Reactor length (m)Figure. 5. Selectivity of species at different temperatures al<strong>on</strong>g the <str<strong>on</strong>g>reactor</str<strong>on</strong>g> length4.1. C<strong>on</strong>trol of the <str<strong>on</strong>g>FTS</str<strong>on</strong>g> temperatureAs menti<strong>on</strong>ed before, the aim of this study was to analyze the c<strong>on</strong>diti<strong>on</strong>s that the <str<strong>on</strong>g>FTS</str<strong>on</strong>g>could be c<strong>on</strong>ducted in a <str<strong>on</strong>g>fixed</str<strong>on</strong>g>-<str<strong>on</strong>g>bed</str<strong>on</strong>g> <str<strong>on</strong>g>reactor</str<strong>on</strong>g> at a reas<strong>on</strong>ably high CO c<strong>on</strong>versi<strong>on</strong> near isothermalc<strong>on</strong>diti<strong>on</strong> at any desired reacti<strong>on</strong> temperature between 563 and 583 K. To this end, a seriesof CFD simulati<strong>on</strong>s have been carried out in which the significant result of this simulati<strong>on</strong>study has been c<strong>on</strong>firmati<strong>on</strong> of the isothermicity exhibiti<strong>on</strong> of the <str<strong>on</strong>g>fixed</str<strong>on</strong>g>-<str<strong>on</strong>g>bed</str<strong>on</strong>g> <str<strong>on</strong>g>reactor</str<strong>on</strong>g>. As anexample, figure 6 shows the distributi<strong>on</strong> of temperatures al<strong>on</strong>g the <str<strong>on</strong>g>reactor</str<strong>on</strong>g> length at steadystate c<strong>on</strong>diti<strong>on</strong> for a simulati<strong>on</strong> c<strong>on</strong>ducted under c<strong>on</strong>diti<strong>on</strong>s of T=573 K, P=17 bar.Figure.6 C<strong>on</strong>tour of temperature (operating c<strong>on</strong>diti<strong>on</strong>: T=573, P=17 bar)As shown in this figure, temperature profiles of two cases were presented. In case 1 heatremoval was d<strong>on</strong>e with saturated water and in case 2 without heat removal. As can be seen,the maximum difference of temperature inside the <str<strong>on</strong>g>fixed</str<strong>on</strong>g>-<str<strong>on</strong>g>bed</str<strong>on</strong>g> <str<strong>on</strong>g>reactor</str<strong>on</strong>g> is lower than 14K, andthe mean temperature of the solid is about 575 K, close to the coolant. Interestingly, the tempe-
M. Irani/Petroleum & Coal 55(2) 82-92, 2013 89rature of the fluid inside the <strong>catalyst</strong> z<strong>on</strong>e, where the reacti<strong>on</strong> takes place, is almost c<strong>on</strong>stant atabout 577 K, that is well-suited. If there isn’t any heat removal in <str<strong>on</strong>g>reactor</str<strong>on</strong>g>, the temperatureat the fr<strong>on</strong>t of catalytic <str<strong>on</strong>g>bed</str<strong>on</strong>g> is increased to 685 K and the average temperature of the fluid incatalytic <str<strong>on</strong>g>bed</str<strong>on</strong>g> will remain <strong>on</strong> 650 K. This temperature is not suitable for <strong>catalyst</strong> performanceand <strong>catalyst</strong> degradati<strong>on</strong> will occurred. These results presage a very efficient heat exchangeperformance of the saturated water. It should be noted that the saturated water temperaturecan be c<strong>on</strong>trolled with pressure. The maximum change in temperature of the <str<strong>on</strong>g>FTS</str<strong>on</strong>g> stream insidethe <str<strong>on</strong>g>reactor</str<strong>on</strong>g> from the entry to the exit is about 10–15 K (Figure 7). Therefore, almost isothermal<str<strong>on</strong>g>FTS</str<strong>on</strong>g> can be achieved by setting the operating pressure at a value which the saturati<strong>on</strong> temperatureof liquid water is slightly below the temperature of the FT synthesis. The pressure ofboiling water system has to be reduced when the CO c<strong>on</strong>versi<strong>on</strong> increases, to have efficientheat-removal capacity.Figure.7 Profile of density (operating c<strong>on</strong>diti<strong>on</strong>: T=573, P=17 bar)4.2. Quantitative validati<strong>on</strong>Despite the fact that the results shown in figures 3 to 8 were used for qualitative validati<strong>on</strong>of the model and the soluti<strong>on</strong> procedure, in order to ensure the accuracy of the model, quantitativecomparis<strong>on</strong> of experimental data obtained from pilot and their corresp<strong>on</strong>ding simulated resultsis shown in table 5. Since it was limited to use the finite number of thermocouples, (point 1:beginning of catalytic <str<strong>on</strong>g>bed</str<strong>on</strong>g>; point 2: middle of catalytic <str<strong>on</strong>g>bed</str<strong>on</strong>g>; point 3: end of catalytic <str<strong>on</strong>g>bed</str<strong>on</strong>g>) <strong>on</strong>lythree experimental data of temperatures have been obtained and compared against theircorresp<strong>on</strong>ding simulated results. As this table shows, the errors at all times are less than fivepercent, since simulati<strong>on</strong> is <str<strong>on</strong>g>based</str<strong>on</strong>g> <strong>on</strong> the CFD approach and no empirical correlati<strong>on</strong> has beenused in the simulati<strong>on</strong>.Table 5 Comparis<strong>on</strong> between measured and predicted values for the pilot-scale <str<strong>on</strong>g>FTS</str<strong>on</strong>g>processesexperimental simulati<strong>on</strong> Error (%)X CO (%) 60.7 58.59 3.476112C5+ selectivity (g/g feed) 18 17.29 3.944444Temperature at point 1(K) 588 590 0.340140Temperature at point 2(K) 575 573 0.347826Temperature at point 3(K) 573 573 05. C<strong>on</strong>clusi<strong>on</strong>sThe detailed kinetics is em<str<strong>on</strong>g>bed</str<strong>on</strong>g>ded into the <str<strong>on</strong>g>reactor</str<strong>on</strong>g> model .The n<strong>on</strong>-ideal thermodynamicbehavior of mixture is correlated by using Peng-Rabins<strong>on</strong> EOS. The <str<strong>on</strong>g>reactor</str<strong>on</strong>g> model is tested
against the measured data from pilot-scale and satisfactory agreements are found betweenpredicti<strong>on</strong>s of model and experimental results. The numerical investigati<strong>on</strong> reveals that theproposed model can allow us to gain a quantitative insight into the complicated <str<strong>on</strong>g>fixed</str<strong>on</strong>g>-<str<strong>on</strong>g>bed</str<strong>on</strong>g><str<strong>on</strong>g>FTS</str<strong>on</strong>g> system. From the simulati<strong>on</strong> data presented in this paper, a <str<strong>on</strong>g>reactor</str<strong>on</strong>g> optimizati<strong>on</strong> strategycan be deduced; operating at the highest possible c<strong>on</strong>versi<strong>on</strong> to achieve the maximum C 5+yield while minimizing temperature run away. The simulated <str<strong>on</strong>g>reactor</str<strong>on</strong>g> exhibited good isothermicityunder most simulated c<strong>on</strong>diti<strong>on</strong>s by using saturated water as heat removing system.Increasing of feed temperature can increase syngas c<strong>on</strong>versi<strong>on</strong>; however it suppressesremarkably the overall yield of C 5+ products. Therefore the selecti<strong>on</strong> of feed temperature isimportant <strong>on</strong> the premier of keeping a satisfied syngas c<strong>on</strong>versi<strong>on</strong> for enhancement of theoverall yield of C 5+ products.NomenclatureM. Irani/Petroleum & Coal 55(2) 82-92, 2013 90P Pressure, bar T Temperature, Kg gravity accelerati<strong>on</strong>, m.s -2 f fugacity, barv velocity, m.s -1 k i Kinetic C<strong>on</strong>stant, mol.hr -1 .gr -1 .bar -1H Total enthalpy, kJ.kg -1 .s -1 E i Activati<strong>on</strong> Energy, kJ.kmol -1h Enthalpy of species, kJ.kg -1 .s -1 R Global Gas Factor, kJ.kmol -1 .K -1j Mass flux, kg.m -2 .s -1 M i Molecular Weight, kg.kmol -1q Heat flux, kJ.m -2 .s -1 V Volume, m 3S Momentum source term, x i mass fracti<strong>on</strong>D ij Diffusivity coefficient, D Total Diffusivity Coefficient, m 2 .s -1C i C<strong>on</strong>centrati<strong>on</strong>, kmol.m 3 k m Thermal C<strong>on</strong>ductivity, kJ.m -1 .K -1Z Compressibility FactorGreek LettersSubscriptρ Density, kg.m -3 i species numberμ Viscosity, kg.m -1 .s -1 j sec<strong>on</strong>d species numberφ i Fugasity coefficient m mixtureReferences[1] Bartholomew, C.H., Farrauto, R. J.: Fundamentals of Industrial Catalytic Processes,New Jersey: John Wiley & S<strong>on</strong>s Inc. – AIChE, Hoboken, 2006, 398.[2] Dry, M.E., Steynberg, P.: Studies in Surface Science and Catalysis, 2004, 152:406.[3] Davis, B.H.: Fischer−Tropsch Synthesis: Comparis<strong>on</strong> of Performances of Ir<strong>on</strong> andCobalt Catalysts, Ind. Eng. Chem. Res., 2007, 46 (26), pp 8938–8945.[4] Reidel, T., Schulz, H., Schaub, G., Jun, K., Hwang, J., Lee, K.: Topics in Catalysis,2003, 26:41.[5] De Deugd, R.M., Kapteijn, F., Moulijn, J. A.: Topics in Catalysis, 2003, 26:29.[6] Steynberg, A.P., Dry, M. E., Havis, B.H., Berman, B. B.: Studies in Surface Scienceand Catalysis, 2004, 152:64.[7] Dry, M.E.: Practical and Theoretical Aspects of the Catalytic Fischer-Tropsch Process,Applied Catalysis A: General, 1996, 138:319.[8] Sie, S.T. : Process development and scale up: IV Case history of the development ofa Fischer-Tropsch synthesis process, Rev. Chem. Eng. 1998, 14, 109–157.[9] Davis, B.H.: Topics in Catalysis, 2005, 32:143.[10] Nakhaei Pour, Zamani, Y., Tavasoli, A., Shahri, S.M. K., Taheri , S.A.: Study <strong>on</strong>products distributi<strong>on</strong> of ir<strong>on</strong> and ir<strong>on</strong>–zeolite <strong>catalyst</strong>s in Fischer–Tropsch synthesis ,Fuel, 2008, 87:2004.[11] Fischer–Tropsch pilot plant of Research Institute of Petroleum Industry and Nati<strong>on</strong>alIranian Oil Company (RIPI-NIOC), Tehran 18745-4163, Iran, 2004.[12] Irani, M., Bozorgmehry, R.B., Pishvaie, M. R., Tavasoli, A.: Investigating the effectsof mass transfer and mixture n<strong>on</strong>-ideality <strong>on</strong> multiphase flow hydrodynamics usingCFD methods, Iran. J. Chem. and Chem. Eng., 2009.
[13] Irani, M., Bozorgmehry, R.B., Pishvaie, M. R.,: Impact of Thermodynamic N<strong>on</strong>idealitiesand Mass Transfer <strong>on</strong> Multi-phase Hydrodynamics,Sci. Iranica, 2010.[14] Bub, G., Bearns, M., Bussemier, B., Frohning, C.: Chemical Engineering Science,2003, 58:867.[15] Jess, A., Popp, R., Hedden, K.: Fischer-Trospch synthesis with nitrogen- rich syngas:Fundamentals and <str<strong>on</strong>g>reactor</str<strong>on</strong>g> design aspects, Applied Catalysis A, 1999, 186:321.[16] Atwood, H.E., Bennet, C.O.: Ind. Eng. Chem. Proc. Des. Dev.1979, 18:163.[17] Wang, Y.N., Xu, Y.Y., Li, Y.W., Zhao, Y.L., Zhang, B.J.: Chem. Eng. Sci., 2003,58:867.[18] Liu, Q. S., Zhang, Z.X., Zhou, J. L.: J.Nat.Gas Chem. 1999, 8:137.[19] Marvast, M.A., Sohrabi, M., Zarrinpashneh, S., Baghmisheh ,Gh.: Chem. Eng. andTech., 2005, 1:28.[20] Amirshaghaghi, H., Zamaniyan, A., Ebrahimi, H., Zarkesh, M.: Numerical simulati<strong>on</strong>of methane partial oxidati<strong>on</strong> in the burner and combusti<strong>on</strong> chamber of autothermalreformer, Applied Mathematical Modelling,34(2010) 2312-2322.[21] Jazbec, M., Fletcher, D. F., Haynes, B. S.: Simulati<strong>on</strong> of the igniti<strong>on</strong> of lean methanemixtures using CFD modelling and a reduced chemistry mechanism, AppliedMathematical Modeling, 24, 8-9(2000) 689-696.[22] Pougatch, K., Salcudean, Gartshore,M. I.: A numerical model of the reactingmultiphase flow in a pulp digester, Applied Mathematical Modeling, 30 (2006) 209–230.[23] Shang, Z.: CFD of turbulent transport of particles behind a backward-facing stepusing a new model—k-e-Sp, Applied Mathematical Modeling, 29 (2005) 885–901.[24] Gerber, A.G., Dubay, R., Healy, A.: CFD-<str<strong>on</strong>g>based</str<strong>on</strong>g> predictive c<strong>on</strong>trol of melt temperaturein plastic injecti<strong>on</strong> molding, Applied Mathematical Modeling 30 (2006) 884–903.[25] Fluent, Incorporated: FLUENT 6 USER Manual, Leban<strong>on</strong> (NH): Fluent Inc., 2001.[26] Foumeni, E.A., Moallemi, H.A., Mcgreavy, C., Castro, A.A.: Canad. J. Chem. Eng.,1991, 69:1010.[27] Marvast, M.A.: Ph.D. Thesis, Amirkabir University of Technology: Tehran- Iran, 2005.[28] Poling, B.E., Prausnitz, J. M., O’C<strong>on</strong>nell, J.P.: The Properties of Gases & Liquids, 5 thed., New York: McGraw-Hill, 2000.[29] Mas<strong>on</strong>, E.A., Saxena, S.C.: Phys. Fluids, (1958) 1:361.Corresp<strong>on</strong>ding author, PhD of Chemical Engineering. postal address: Gas Divisi<strong>on</strong>, ResearchInstitute of Petroleum Industry (RIPI), West Blvd., Near Azadi Sports Complex, Tehran, Iran.Ph<strong>on</strong>e: +98 (21) 48252403, Fax: +98 (21) 44739716, E-mail address: Iranim@ripi.ir orMohammadirani@yahoo.comAppendixThermophysical properties in the gas phase were calculated using the following relati<strong>on</strong>s [28].Binary DiffusivityFuller’s method was used to determine diffusivity:AndDDM M/ M M1.75 1 2 1 2 0.04357 T(A.1)1 13 3 2P(V1V2)(1 xixjDi, m ) nj1ij(A.2)Where V i is found for each comp<strong>on</strong>ent by summing atomic diffusi<strong>on</strong> volumes given in [28] .Gas-Phase Mixture ViscosityM. Irani/Petroleum & Coal 55(2) 82-92, 2013 91Wilke’s method has been used to determine the gas-phase viscosity. It is expressed as
nmx (A.3)x WhereAndiji ni1j 10.5 M1i j M81i MjijjjMi 0.50.25 2 (A.4)ji (A.5)j Mi ijiMjGas-Phase Mixture Thermal C<strong>on</strong>ductivityThermal c<strong>on</strong>ductivity of the gas mixture was determined as follows. However, anadditi<strong>on</strong>al factor proposed by Mas<strong>on</strong> and Saxena [29] was used:kmWhereAndn ni1j 1x kixjiAij k Mj 1i k jM i Aij0.5 M 81i M j 1.065AM0.5 0.252(A.6)(A.7)j iji Aij(A.8)iMjSpecific heatCmPni1x CiiPM. Irani/Petroleum & Coal 55(2) 82-92, 2013 92(A.9)