Conference topics: cr11 TIChE International Conference 11 Two-dimensional mathematical modeling of oxidative coupling of methane in a membrane reactor Salamah Manundawee 1, Suttichai Assabumrungrat 1, Wisitsree Wiyaratn 2* 1 Center of Excellence in Catalysis and Catalytic Reaction Engineering, Department of Chemical Engineering, Faculty of Engineering, Chulalongkorn University, Bangkok 3, Thailand 2 Department of Production Technology and Education, Faculty of Industrial Education and Technology, King Mongkut's University of Technology Thonburi, Bangkok 1, Thailand * Email: wisitsree.wiy@kmutt.ac.th Abstract The oxidative coupling of methane (OCM) in membrane reactors (MR) has been studied by two-dimensional numerical simulations under non-isothermal conditions using COMSOL Multiphysics Program. The performances of several membrane types; i.e. membranox (alumina membrane), a dense Ba.5 Sr. 5CO.8 Fe.2 O 3-δ (BSCFO) and La.4 Sr.6 Ga.4 Fe.6 O 3-δ (LSGFO) membrane were studied over Na-W-Mn/SiO 2 catalyst packed in membrane reactors. The effects of operating condition such as air flow rate and methane flow rate were investigated. The simulation results showed that Ba.5 Sr.5 Co.8 Fe.2 O 3-δ (BSCFO) membrane reactor showed the best performances among all the membrane types. The highest CH 4 conversion and C 2 selectivity was 36.3 % and 7.59 %, respectively under 993 K, air flow rate.28m 3 /s CH 4 flow rate.5 m 3 /s. Moreover, increasing of methane feed rate resulted in lower CH 4 conversion but increased C 2 selectivity. The effect of air feed rate showed the contrary results. Keyword: OCM, membrane reactor, dense membrane, two-dimensional, simulation 1
Conference topics: cr11 TIChE International Conference 11 1. Introduction Natural gas consists mainly of methane and a raw material for a number of synthetic products. The one of many processes for natural gas utilization was oxidative coupling of methane (OCM); it can directly convert methane to precious C 2 hydrocarbons. The OCM reaction has been reported since 1982 by the pioneering work of Keller and Bhasin [1]. However, many researchers have been still interested on OCM because improvement to obtain higher conversion of methane and high C 2 hydrocarbons selectivity in the production process is opened. Reactor configuration was found to play an important role in producing high yield and selectivity of ethylene production in OCM reaction. Various types of reactors such as fixed bed reactor, fluidized-bed reactor, moving bed chromatographic reactor, and membrane reactor have been reported [2-4]. Applications of membrane reactor combining the separation and reaction in one unit to control oxygen concentration along the reactor were promising for enhancing OCM performance. Inorganic membrane for OCM process can be divided in 2 classes of porous and dense membranes. Porous membranes exhibit high permeability but relatively low selectivity. On the other hand, dense membranes show much better selectivity but lower permeability [5]. Moreover, the design of the membrane reactor using two-dimensional mathematical model considering both axial and radial directions was more realistic and provides better prediction accuracy. Therefore in this study, two-dimensional mathematical modeling of OCM to C 2 hydrocarbons in a membrane reactor operated under non-isothermal conditions was investigated using COMSOL Multiphysics Program. The model was useful for prediction of OCM performance under different three types of membrane i.e. Membranox (alumina membrane), a dense Ba.5Sr.5Co.8Fe.2O3 -δ (BSCFO) and La.4 Sr.6 Ga.4 Fe.6 O 3-δ (LSGFO) membrane and Effect of operating condition such as methane flow rate and air flow rate were investigated. 2. Simulation 2.1 Kinetic model The reaction rate expression was proposed by Danespayeh et al. [6] for NaWMn/SiO 2. The stiochiometric equations and reaction rates of the proposed models were described in literatures [6] 2.2 Reactor model The two-dimensional pseudo-homogeneous model was developed for simulation of OCM reaction in this work to study the reactor behavior. The mass balance for each of the eight components (C i = CH 4, O 2, C 2 H 4, C 2 H 6, CO, CO 2, H 2, and H 2 O) can be written as (1) The rates of generation for each component were calculated from the stoichiometry of the reaction The energy balance for the reactor can be written as (2) The momentum balance for reactor describes flow in porous media. The equation extends Darcy s law combination with the continuity equation (3) 2.3 Geometry and boundary condition The reactor geometry was 2D cylindrical (shell and tube side) and the flow enters the computational domain at a known velocity, composition and temperature. At the outlet, it was assumed that the convective part of the mass and heat transport vector was dominating. At the tube wall, a mass transfer O 2 flux was defined. The oxygen permeation flux is given by following formula: For LSGFO membrane [7], For BSCFO membrane [8], (4) ln (5) For Membranox membrane [9],,, (6). (7) 2.4 Numerical implementation The finite element method was proposed to formulate the PDE problem. The developed finite element model the reactor was solved with quadratic finite element basis functions, using a commercial finite element simulation environment COMSOL. The software runs the finite element analysis together with meshing which was a partition of the geometry model into small units of simple shapes and error control using a variety of numerical solvers. Three application modes were needed; heat transfer by conductive and convective was used to model. Mass transfer by convective and diffusion was used to show the 2
TIChE International Conference 11 concentration and transport of eight species of interest. Brinkman equation was applied to describe fluid flow. 3. Result and discussion 3.1 Characterization ofdifferent membrane types The characteristics of membrane reactor using three different membranes were investigated. Na-W-Mn/SiO 2 OCM catalyst was packed in membrane reactor. The condition was considered at air flow rate of.28 m 3 /s, CH 4 flow rate of.55 m 3 /s. and temperature at 993 K. Figure 1 shows the concentration profiles of oxygen concentration at different MR a. Ba.5 Sr.5 Co.8 Fe.2 O 3-δ (BSCFO) MR, b. La.4 Sr.6 Ga.4 Fe.6 O 3-δ (LSGFO) MR and c. Membranox MR. It can be seen that the oxygen feed in the shell side of the reactor permeates through the membrane along the length of the reactor. The membrane acts as a distributor. Oxygen was transported through membrane in form of the lattice oxygen (O - and O - 2 ). The nature of the distribution in each position in the reactor was not uniform in radial direction. In contrast to the assumptions in the onedimensional model report in literature [3, 9] they assume uniform permeation of oxygen along reactor length. There was maximum radial oxygen concentration distribution in the middle of the reactor. Flux of oxygen in BSCFO MR was higher than those of LSGFO and Membranox MR, respectively. The BSCFO membrane exhibits higher oxygen flux and higher oxygen vacancy concentration than LSGFO due to substitution of La 3+ by the lower valence state of Ba 2+ and Sr 2+. The higher oxygen vacancy concentration in the BSCF membrane could lead to a higher C 2 formation rate []. Figures 2 and 3 show concentration profiles of methane (reactant) and ethylene in three MRs, a. BSCFO MR, b. LSGFO MR and c. Membranox MR. Methane was fed into tube side of reactor. Methane concentration decreases from the inlet due to reaction. In case of Membranox MR, methane (reactant) and ethylene (product) were lost from the tube side to the shell side as shown in Figures 2c and 3c. Also, the other species (C 2 H 6, CO, CO 2, H 2, and H 2 O) can permeate to the shell side in the same manner. Permeation of methane results in low methane conversion. Although permeation of products to the shell side could prevent the oxidation of the products but they would be present in the shell side stream at low concentration and become different to recover them. In case of dense membranes, i.e. BSCFO and LSGFO MR, no loss of products to the shell side was observed as only oxygen species could permeate through the dense membranes. The specific characteristics of the dense membrane prevent other species to the trans-membrane side. c. c. Fig.2. CH 4 concentration profiles of three membrane reactors. Fig.1. O 2 concentration profiles of three membrane reactors a. LSGFO MR, b. BSCFO MR and c. Membranox MR Paper Code-3
TIChE International Conference 11 c. c. Fig.3. C 2H 4 concentration profiles of three membrane reactors. Figure 4 shows temperature profiles in the three membrane reactors, a. BSCFO MR, b. LSGFO MR and c. Membranox MR. The increase of temperature was gradually increased at the center of the reactor caused by the reaction with oxygen permeable into this area of the reactor, which was observed in Figure 5.18. It shows that the reaction site was everywhere in the reactor and heat was accumulated in the bed of catalyst. Heat can transfer from the reactor to the shell side via the tube wall and therefore the temperature change near the wall becomes not significant. The temperature profiles for the different MRs were similar but there were slight differences in a rising of temperature: BSCFO > LSGFO > Membranox. Fig.4. Temperature profiles of three membrane reactors. Table 1 shows summary of OCM performances between three membrane types in membrane reactor at air flow rate of.28 m 3 /s, methane flow rate of.55 m 3 /s and temperature of 993 K. The C 2 yield of BSCFO membrane was 25.43 %, the highest compared to 24.81 % for LSGFO and 22.41 % for Membranox membrane. Performance Table1. Summary OCM performance Membrane types Membranox BSCFO LSGFO CH 4 conversion, % 28.39 36.3 33.37 C 2 selectivity, % 78.93 7.59 74.34 C 2 yield, % 22.41 25.43 24.81 3.2 Effect of operating condition From the previous section, we know that BSCFO membrane provides the best performance for MR and therefore it was selected in this present study. 3.2.1 Effect of methane feed rate The effect of methane feed flow rate in a range.138.688 m 3 /s at temperature 993 K and air flow rate of.28 m 3 /s on CH 4 conversion and C 2 selectivity were presented in Figure 5 CH 4 conversion was decreased, whereas, C 2 selectivity was increased with the increase of methane feed flow rate. Increasing methane feed flow rate results in lower contact time, and hence, CH 4 conversion was decreased and more favorable for the C 2 hydrocarbon production because of shortening contact time between C 2 products with the oxygen, which limits the formation of carbon oxides. Paper Code-4
TIChE International Conference 11 CH 4 conversion., % 45 35 25 15 5 Fig.5. The effect of CH 4 feed rate on CH 4 conversion and C 2 selectivity.2.4.6.8 CH 4 flow rate, m 3 /s conv, % sel, % 9 8 7 6 C 2 selectivity, % side by permeating through the membrane. On the other hand, in case of dense membranes i.e. BSCFO and LSGFO, no reactant and product loss was present due to the high selectivity. The specific characteristics of the dense membrane prevent other species to the transmembrane side. The results showed that C 2 yield and CH 4 conversion of BSCFO higher than LSGFO and Membranox, respectively in contrast of C 2 selectivity. Operating conditions, such as methane flow rate and air flow rate have influences on performance of OCM reaction operated in BSCFO membrane reactor. Increasing of methane feed flow rate results in lower CH 4 conversion but increased C 2 selectivity. The effect of air flow rate showed the contrary results. To achieve the best of OCM performance, it requires further optimal operating condition and dimension of reactor. 3.2.1 Effect of air feed rate The effect of air feed flow rate in range.4.52 m 3 / s for temperature of 993 K and methane flow rate of.55 m 3 /s on CH 4 conversion and C 2 selectivity were presented in Figure 6 CH 4 conversion was increased, whereas, C 2 selectivity was decreased with the increase of air feed flow rate. When increasing air feed rate, the flux of oxygen permeation increases due to higher driving force of oxygen across the membrane. As a result, CH 4 conversion increases. C 2 selectivity was decreased because higher amount of oxygen could react with the methyl radical and C 2 product to carbon oxides. CH 4 conversion, % 6.2.7.12 air flow rate, m 3 /s Fig.6. Effect of air flow rate on CH 4 conversion and C 2 selectivity 3. Conclusions conv, % sel, % The MR model was applied to examine its performance of three MRs and study effect of operating condition in OCM. Characteristics of three membranes including BSCFO, LSGFO and membranox membranes were investigated. Flux of oxygen in BSCFO MR was higher than those of LSGFO and Membranox MR, respectively. In case of Membranox MR, reactant and products were lost from the reaction side to the shell 9 8 7 6 C 2 selectivity, % Acknowledgements The authors gratefully acknowledge the Thailand Research Fund, Higher Education Commission and King Mongkut's University of Technology Thonburi for financial support. References [1] G.E. Keller, M.M. Bhasin, Synthesis of ethylene via oxidative coupling of methane determination of active catalysts. Journal catalyst 73 (1982): 9-19. [2] Al-Zahrani, S.M., The effects of kinetics, hydrodynamics and feed conditions on methane coupling using fluidized bed reactor. Catalysis Today 64 (1): 217 225. [3] Kiatkittipong, W., Tagawa, T., Goto, S., Assabumrungrat, S., Silpasup, K. and Praserthdam, P., Comparative study of oxidative coupling of methane modeling in various types of reactor. Chemical Engineering Journal 115 (5): 63 71. [4] Prodip, K., Yan, Z. and Ajay, K.R., Multi-objective optimization of simulated countercurrent moving bed chromatographic reactor for oxidative coupling of methane. Chemical Engineering Science 64 (9): 4137 4149. [5] Louis, O., Stephane, H., Claude, M. and Andre C.V.V., Oxidative coupling of methane using catalyst modified dense perovskite membrane reactors. Catalysis Today 142 (9): 34 41. [6] Daneshpayeh, M., Abbasali, K., Navid, M., Yadolah, M., Rahmate, S. and Alireza, T., Kinetic modeling of oxidative coupling of methane over Mn/Na 2WO 4/SiO 2 catalyst. Fuel Processing Technology 9 (9): 3 4. [7] Shaula, A.L., Yaremchenko, A.A., Kharton, V.V. Logvinovich, D.I., Naumovich, E.N., Kovalevsky, A.V., Frade, J.R. and Marques, F.M.B., A Oxygen permeability of LaGaO 3-based ceramic membranes. Journal of Membrane Science 221 (3): 69 77. [8] Wang, H., You, C. and Yung, W., Oxygen permeation study in a tubular Ba.5Sr.5Co.8Fe.2O 3-δ oxygen permeable membrane. Journal of Membrane Science 2 (2): 259 271 [9] Kao, Y.K., Lei, L., and Lin, Y. S., A comparative simulation study on oxidative coupling of methane in fixed-bed and membrane reactors. Industrial & Engineering Chemistry Research 36 (1997): 3583-3593. [] Wang, H., You, C. and Yung, W., Oxidative coupling of methane in Ba.5Sr.5Co.8Fe.2O 3-δ (BSCF) tubular membrane reactors. Catalysis Today 4 (5): 16 167. Paper Code-5