Numerical Simulations of Methane Aromatization with and without a Ceramic Hydrogen Separation Membrane

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1 Numerical Simulations o Methane Aromatization with and without a Ceramic ydrogen Searation Membrane Zuoan Li*, Christian ølseth, Selene ernandez Moreudo, and eidar augsrud University o Oslo, Deartment o Chemistry, FEMiO, Gaustadalleen, NO-9 Oslo, Norway Protia AS, Gaustadalleen, NO-9 Oslo, Norway University o Oslo, Deartment o Chemistry, InGAP, Sem Sælandsvei, NO-7 Oslo, Norway *Corresonding author: zuoan.li@smn.uio.no Abstract: Methane dehydroaromatization is romising or direct roduction o valuable industrial roducts such as benzene and hydrogen via the reaction C C 9. By removing hydrogen rom the roduct stream, the yield o benzene can be increased. In the resent investigation, reaction kinetics, mass transort and luid dynamics have been couled to simulate eects o various arameters such as low rate and temerature on the reaction yield. Moreover, eects o coking on the reaction is also exlored. eywords: methane aromatization, catalytic reactor, hydrogen searation membrane, coking eect. Introduction Non-oxidative reaction o methane dehydroaromatization over cation-modiied zeolites has been attracting growing attention due to a otential means or roducing high valuable roducts such as aromatics and hydrogen via the reaction C C 9 [], as schematically shown in Fig.. Many studies have been ocused on catalysts screening and characterization, and elementary thermodynamic stes o the reaction [, ]. owever, little attention has been aid to luid dynamics such as low rate and ressure gradient, which are imortant or an industrial alication. The highly endothermic nature o methane conversion to aromatics (e.g., kj/mol at 9 ), however, results in very low equilibrium conversions o methane and yields o benzene. One way to imrove the methane dehydroaromatization rocess is the elimination o the hydrogen ormed during the reaction, which inhibits the orward rates o the reaction. Thus, the use o a membrane reactor with continuous removal during the reaction could be ossible to change the thermodynamic limitations and increase the methane conversion. Ceramic roton conductors based on Ln -x WO - (Ln = rare earths) have shown high chemical stability in carbon-containing atmosheres [, 5] and can be used to transort the roduced hydrogen in the reactor via ambiolar transort o rotons and electrons. owever, couling o methane aromatization reaction and hydrogen removal oses signiicant design and otimization issues, which need to be solved numerically. In the resent contribution, COMSOL Multihysics. has been used to study the methane dehydroaromatization. First, a ixed catalyst bed reactor has been simulated in order to veriy the model used, and then a hydrogen searation membrane has been couled to study the eect o hydrogen removal. Moreover, the inluence o various arameters, such as sace velocity, temerature, and hydrogen transort roerties, on the reaction, have also been studied. Figure. Schematic illustration o a catalytic reactor or the methane aromatization.. Two-dimensional model aroaches

2 For a tubular reactor, an axisymmetric geometry can been used to reduce the comutation time. The catalyst reactor can be divided into two ree low regions and one orous catalyst bed region, where the methane aromatization takes lace (c. Fig. ). Detailed chemical reactions are as ollows.. eaction kinetics Using Mo/-ZSM5 as catalysts, two main indeendent chemical reactions are C () C 9 C C () Although eaction has oten been neglected, it is necessary to include it here to study otential eects o coking. The rate equations or these two reactions were otimized according to exerimental data rom 9 to 97, and described by the ollowing Langmuir- inshelwood exressions []: r r C k C k C C C C 9 C C C C C C C () () where i is the ugacity o comonent i; and is the equilibrium constant o reactions and, resectively, which can be calculated rom thermodynamic data [7]; k and k are the rate constants o eactions and, resectively; i is the adsortion equilibrium constant o comonent i. The ollowing kinetic arameters are used or the resent simulations. k ex T 9.5 (5) k.5 C 5.9 ex T ex.9.9 T ex T 5 ex C T. Mass transort () (7) (8) (9) Since the concentration o C and in the catalytic bed is high [,, ], the transort o concentrated secies i including Maxwell-Stean diusion and convection equations has to be alied iu n i ~ Dix () x i where denotes the density (kg/m ) o the luid which can be calculated by ideal gas law, u is the velocity o the luid (m/s), is the ressure (Pa), i is the mass raction o secies i, x is the molar raction o secies, and i is the i comonent o the multicomonent Fick diusivity (m /s), which is strongly deendent on the comosition and given by [8, 9] ~ D ~ D D ~ xd xd xd x x x D D D D D D xd x D D xd xd x x D D D D () ()

3 where D i is the Maxwell-Stean diusivity (m /s), and can be calculated with an emirical equation based on the kinetic gas theory []:.75 T D i k / / () v i v Mi M where k is a constant with the value. -8 Pa m /s, M i is the molar mass o secies i (kg/mol), and v i equals the molar diusion volume o secies i (m /mol). To calculate i in Eq., reaction kinetics in Section. must be couled and the ollowing relations can be obtained: C r r C r 9r r / () (5) () The boundary condition at the inlet is the weight raction o each secies: or methane and or other secies. At the outlet, the convective lux dominates the mass transort x n ~ n i Di x (7) This imlies that the concentration gradient in the direction erendicular to the outlet boundary is negligible, which is common or tubular reactors with a high degree o transort by convection in the direction o the main reactor axis. u u (9) where is the orosity o the catalyst bed, and is the ermeability o the catalyst orous media. For the sake o simlicity, the luid viscosity is oten assumed to be the same as that in ree low regions []. The orosity and ermeability are two key actors that govern the luid low in the orous region. Using the Carman-ozeny model [], the ermeability or a acked-bed with randomly distributed sherical articles can be exressed as d () 5( ) where d is the article size o the catalyst. At the inlet, the boundary condition is set to the velocity o the luid, which can be calculated through the low rate and cross sectional area o the reactor. At the outlet, the ressure is set to atm unless the calculation aims to study the eect o ressure.. ydrogen ermeation When a hydrogen searation membrane is alied to the catalyst reactor, the hydrogen roduced inside the reactor can be removed by uming, lushing or reaction with oxidizing gas as illustrated in Fig... Momentum transort Fluids in two ree low regions are described by the Navier-Stokes equation: u u u (8) where denotes the dynamic viscosity (Pa s) and is aroximated to that o methane due to being the main comonent in the luid, and the other terms have the same meanings as deined in Eq.. In the orous catalyst region, low can be described by the Brinkman equation as Figure. Schematic illustration o a catalytic membrane reactor.

4 ere, a dense ceramic mixed roton electron conductor based on Ln -x WO - (Ln = rare earths) is chosen as the hydrogen searation membrane due to its high stability, % hydrogen selectivity, and low cost as comared to Pd-based alloys [, 5, ]. Ambiolar transort o roton and electron gives a net lux o hydrogen []: Permeate - T σ t -dln e F L eactor () where L is the thickness o the membrane, σ is the roton conductivity, t - is the transerence e number o electrons, and the other terms have their usual meanings. To simliy, we have assumed that the material is either redominated by electronic ( t - ) or roton conduction ( e t ) and that the conductivity o both charge carriers is roortional to /. Thereby the above equation can be integrated yielding P / - Tσ () F L / where σ is the conductivity (either roton or electron) at standard hydrogen ressure ( atm). Due to high low or oxidizing condition at the ermeate side, the hydrogen artial ressure at the ermeate side is much lower than that in the catalytic reactor. Thus, Eq. can be urther simliied to / Tσ F L (), When the conductivity is indeendent o the integration o Eq. yields P ln - Tσ () F L ln In this case, the lux will be determined by the ratio o at both sides; the lower the the ermeate side, the higher the hydrogen lux. Since it is easier to achieve low at the ermeate side by lushing with high low or at reaction with oxidizing gas than increase the conductivity o the membrane, this may rovide us a way to select suitable membrane materials.. esults and discussion. Inluence o low rate, temerature and ressure To test the model arameters, the molar raction o and C at the outlet was irst calculated and comared to exerimental results at same temeratures and with a same sace velocity. As evident rom Fig. a and b the agreement is accetable, veriying the validity o the models suggested in this work. Molar raction o Molar raction o C (a) (b). Cal. this work Ex. Yao et al ml/g/h Cal. this work Ex. Yao et al. 7 ml/g/h T () T () Figure. Comarison o the ormed and C at the outlet between exeriments [] and simulations. Fig. demonstrates that the conversion rate o methane increases with increasing temerature and decreasing sace velocity. It is worth noting that the conversion rate increases signiicantly by reducing the sace velocity rom 5 to 7 ml/g/h, but that the dierence between 5 and ml/g/h is much smaller. This is instructive when choosing a suitable low rate or a seciic

5 Converstion o C (%) Conversion o C to C and (mol%) Conversion o C (%) industrial requirement. Again we see that the simulation reroduces the exerimental data nicely. As the ressure at the outlet increases, the conversion rate decreases signiicantly as shown in Fig. 5. This may be due to thermodynamic limitations o the reactions. 7 ml/g/h (cal.) 5 ml/g/h (cal.) 5 ml/g/h (Ex.) ml/g/h (cal.) T () Figure. Conversion rate o methane as a unction o temerature under various sace velocities. decomosition []. Since the concentration o charge carriers limiting hydrogen transort in Ln -x WO - is usually deendent on, Eq. rather than Eq. was selected or the simulation. By assuming that only one charge carrier limits the hydrogen lux, Fig. resents simulations o how the conductivity,, o a m thick membrane aects the conversion rate at 9 and 97. There is no signiicant increase in the conversion rate until the conductivity reaches value o - S/cm above which the eiciency o the membrane gradually increases. More methane is converted to benzene as hydrogen is extracted, but this eect is not so signiicant at 9 as comared to 97 due to kinetic limitations. Since the hydrogen lux is roortional to /L according to Eq., the obtained value o /L =.5 S/cm where a membrane starts to take eect, can be easily alied to membranes with dierent thicknesses. 8 7 T = 9 7 ml/g/h 8 (97 ) (9 ) C (97 ) C (9 ) 7 ml/g/h 5 8 _out (atm) (S/cm) Figure 5. Conversion rate o methane as a unction o ressure at the outlet.. Imact o hydrogen removal To study the eect o extracting hydrogen, a m Ln -x WO - based membrane has been included in the model. Although thinner membranes deinitely increase the hydrogen lux (see Eqs. -), micron-scale membranes on orous suorts are challenging or abrication and material stability may as well become an issue due to otential kinetic demixing or Figure. Conversion rate o C to C and vs. conductivity o the membrane with m thick. Although removing hydrogen increases the yield o benzene, coking can not be ignored (see eaction ). Fig. 7 reveals that the more hydrogen is removed, the more coke is roduced, in articular at high temerature. Coke can cause the catalysts to be oisoned via either active site suression or ore blocking [5], resulting in a decreasing conversion rate o methane as time goes, esecially or a catalytic membrane reactor []. The deactivation o catalysts may be weakened by diluting catalysts, and this will cause a change in the luid dynamics. To get a

6 Formation rate o Coke (g/g_ca/s) better understanding o coking, time-deendent simulations is called or in the uture. 8.x -.x -.x -.x ml/g/h (S/cm) Figure 7. Coke ormation rate vs. conductivity o the membrane with m thick.. Conclusion eaction kinetics, mass transort and luid dynamics have been couled to simulate methane aromatization. The simulated results have revealed that higher conversion rates o methane can be achieved by decreasing sace velocity and ressure, and increasing temerature. A series o suitable arameters has been determined deending on the urose o the alication. To increase the yield o benzene by removing hydrogen in the reactor via a hydrogen searation membrane, the value o /L must be higher than.5 S/cm or a sace velocity o 7 ml/g/s and above 9. With the removal o hydrogen, the ormation rate o coke is becoming more signiicant and must be treated careully. 5. Acknowledgement Financial suorts rom the esearch Council o Norway (CN) under the roect Intensiied Conversion o Alkanes to Aromatics and Oleins Using Integrated Catalyst/Membrane eactor (ICO) (8), Nonoxidative Methane Activation by Co-conversion o Methane over Metal Containing Zeolite Catalysts (857), and inetics o igh-temerature Oxide Ion and Proton Conductors (INOXPO) (ENEGI 99/S) are grateully acknowledged.. eerences. Z.. Ismagilov, E.V. Matus, and L.T. Tsikoza, Direct converison o methane on Mo/ZSM-5 catalysts to roduce benzene and hydrogen: achievements and ersectives, Energy Environ. Sci.,, 5-5(8).. Y. Xu, and L. Lin, ecent advances in methane dehydro-aromatization over transition metal ion-modiied zeolite catalysts under nonoxidative conditions, J. Al. Catal., A, 88, 5-7 (999).. A.M. Dean, Detailed kinetic modelling o autocatalysis in methane yrolysis, J. Phys. Chem., 9, -9 (99)... augsrud, Deects and transort roerties in Ln WO (Ln=La, Nd, Gd, Er), Solid State Ionics, 78, (7). 5.. augsrud, and C. ølseth, Eects o rotons and accetor substitution on the electrical conductivity o La WO, J. Phys. Chem. Solids, 9, (8).. B. Yao, J. Chen, D. Liu and D. Fang, Intrinsic kinetics o methane aromatization under nonoxidative conditions over modiied Mo/ZSM-5 catalysts, Journal o Natural Gas Chemistry, 7, -8 (8) 7... Perry, and D.W. Green, Perry's Chemical Engineers andbook, 7 th ed., McGraw-ill Proessional, Bird, W. Stewart, and E. Lightoot, Transort Phenomena, nd ed., John Wiley & Sons,. 9. C.F. Curtiss and.b. Bird, Multicomonent Diusion, Ind. Eng. Chem. es., 8, 55-5 (999).. J.A. Wesselingh and. rishna, Mass Transer in Multicomonent Mixer, Delt University Press, ()...Y. Chein, L.C. Chen, Y.C. Chen and J.N. Chung, eat transer eects on the methanolsteam reorming with artially illed catalyst layers, International Journal o ydrogen Energy,, (9).. D.A. Nield, and A. Bean, Convection in orous media, Sringer Science-Business Media, New York ().. T. Norby, and. augsrud, Membranes or Energy Conversion, Wiley-VC Verlag Gmb & Co. GaA,.9- (8).. M. Martin, Materials in thermodynamic otential gradients, J. Chem. Thermodynamics, 5, 9-8 ().

7 5. A. De Lucas, P. Canizares, A. Duran, P. Sanchez and M.T. Garcia, Modiied W/ZSM-5 catalysts: structure and catalytic roerties, Alied Catalyst A: General, 5, 99-7 (997).. O. ival, B.P.A. Grandean, C. Guy, A. Sayari, and F. Larachi, Oxygen-ree methane aromatization in a catalytic membrane reactor, Ind. Eng. Chem. es.,, -9 ().