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Reactor Configuration and Concentration Polarization in Methane Steam Reforming by a Membrane Reactor with a Highly Hydrogen-Permeable Membrane Nobuhiko Mori,* Toshiyuki Nakamura, Ken-ichi Noda, Osamu Sakai, Akira Takahashi, Naoyuki Ogawa, and Hitoshi Sakai NGK Insulators Ltd., Mizuho, Nagoya, 467-8530, Japan
Yuji Iwamoto Japan Fine Ceramics Center, Atsuta, Nagoya, 456-8587, Japan
Tadashi Hattori Department of Applied Chemistry, Aichi Institute of Technology, Toyota, 470-0392, Japan
To examine the effect of so-called “concentration polarization” on the performance of a membrane reactor with a highly hydrogen-permeable membrane, methane steam reforming was conducted, using a Pd/Ag membrane with a thickness of a few micrometers. First, the relation between the methane conversion and the hydrogen recovery was experimentally examined, and the relation was compared with that predicted by a rather simple simulation that assumes the instant achievement of equilibrium. When the hydrogen recovery was smallest, the experimental results agreed well with the simulation results. With increasing reaction pressure, the experimental methane conversion became lower than the simulated conversion. These results suggest that the reaction is limited by reduced hydrogen removal, because of the concentration polarization. The influence of concentration polarization was confirmed by the comparison of the experimental results of hydrogen permeation from a mixture of H2 and N2 with the simulation results based on the plug-flow model. It then was experimentally attempted to reduce the concentration polarization by changing the configuration of the reactor. The methane conversion was successfully improved using reactors that had narrower inner diameters and baffle plates, probably because of the reduction in concentration polarization. It was concluded that the reactor configuration was quite essential to make the best use of a membrane reactor with a highly permeable membrane. 1. Introduction Steam reforming of methane is one of the major candidates of hydrogen production for the polymer electrolyte fuel cell (PEFC), because the reaction is widely used to produce synthesis gas for various chemicals, such as ammonia and methanol. In the steam reforming of methane, hydrogen is supposedly generated by two successive reverse reactions:
(Reforming reaction): CH4 + H2O ) CO + 3H2 (-∆H0298 ) -206 kJ/mol) (CO shift reaction): CO + H2O ) CO2 + H2 (-∆H0298 ) +41 kJ/mol) The overall reaction is endothermic and favored at high temperatures, because of the limitation by equilibrium; in addition, the reaction requires a large amount of steam to achieve high methane conversion and avoid deactivation of the catalyst, because of the coking. Typically, the reaction is performed at temperatures up to 1123 K and using the reactant with a steamto-methane (S/C) ratio of 2-4.1 Moreover, producing hydrogen with a CO impurity of 2) (8b)
the composition in the next unit was calculated in the same way as that previously described. The hydrogen flux from this unit then was calculated. This procedure of calculation was
Figure 6. Comparisons in (a) methane conversion and (b) hydrogen recovery between experimental and simulation results at various reaction pressures. Solid line represents equilibrium conversion without hydrogen removal. The structure of the membrane reactor is that depicted in Figure 1a, the catalyst size is 3 mm, and the flow rate of reactant gas is (0, 9) 2000 cm3/min or (], [) 8000 cm3/min. Open symbols represent experimental results, and closed symbols represent simulation results. (Pressure on the permeation side is 0.1 atm.)
repeated for 35 units, and the methane conversion (X(n ) 35)), the overall hydrogen recovery (z(n ) 35)), and total amount of hydrogen permeated through the membrane (TH(n ) 35)) were determined from the gas composition at the outlet of the last unit. It was confirmed that the calculated results are dependent only negligibly on the number of units at N > 35. One might suspect that the reaction would not achieve equilibrium in each unit. This assumption might be suspicious, especially in the first unit, because, after the second unit, the amount of reaction required to attain equilibrium in the next unit is not so large. Note that the equilibrium was attained even in the runs that used only one-tenth of the catalyst around the membrane used under the standard conditions, as shown in Figure 4. The assumption in the first unit can be also rationalized by the fact that enough amount of catalyst was packed before the membrane so that the reaction could achieve equilibrium. Figure 6a, which compares the experimental methane conversion with the simulated conversion, as a function of reaction pressure, indicates that the experimental conversion was always less than the simulation value. The experimental conversion was similar to the simulation value only at the lowest pressure and at the higher flow rate of reactant gas, where hydrogen recovery rate was the smallest, as shown in Figure 6b. With increasing reaction pressure, both the experimental and simulation values of hydrogen recovery increased, although the increase in simulation value was much larger. Thus, the difference in hydrogen recovery between the experiment and the simulation also increased as the reaction pressure increased, or, in other words, with increasing hydrogen recovery. The difference in methane conversion also increased with increasing reaction pressure, or with increasing hydrogen recovery, as shown in Figures 6a and 6b. This result agreed well with the published results about the effect of concentration polarization on gas separation and the membrane reactor. He et al.,18 Takaba and Nakao,19 and Mourgues and Sanchez20 theoretically predicted that, when the permeability became higher, the gas permeation through the membrane in the gas separation could be greatly reduced by the concentration polarization. Itoh and Haraya24 reported that, in the dehydrogenation of cyclohexane in a membrane reactor, the deviation of the experimental conversion from the simulation value became larger, because of concentration polarization, as the feed rate increased and with the permeate pressure decreased (i.e., with increasing hydrogen permeation). These agreements strongly suggest that the difference between the experiment and
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Figure 7. Hydrogen recovery from a 50% H2:50% N2 mixture gas. (The structure of the membrane reactor is that depicted in Figure 1a, and the catalyst size is 3 mm. The pressure on the feed side is 5 atm, and the pressure on the permeation side is 0.1 atm.)
the simulation in the present study is also due to the effect of concentration. Hara et al.23 reported, on the other hand, that, in methanol decomposition, the CO hindrance as well as concentration polarization has a significant effect on the performance of the membrane reactor. In the present case, however, the effect of CO hindrance can be regarded as negligible, because the reaction temperature (823 K) was much higher than the criterion temperature for CO hindrance. Hara et al.23 reported that CO hindrance caused the decline in hydrogen permeation at inner diameter of 25 mm > inner diameter of 40 mm. This agreement strongly supports the aforementioned conclusion that the methane conversion in the membrane reactor was improved by enhancing hydrogen permeation, as a result of reducing the concentration polarization. The concentration polarization was ascribed to two factors: the radial mixing diffusion of hydrogen in the packed bed of catalyst particles,22-24 and the diffusion of hydrogen in the boundary layer near the surface of membrane.18-21 It is known
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Figure 9. Effect of membrane reactor structure and catalyst size on hydrogen recovery using a 50% H2:50% N2 mixture gas. Dotted line represents the simulated hydrogen recovery. (The membrane reactor structures and catalyst sizes are as follows: (O) Figure 1a, catalyst size ) 3 mm; (]) Figure 1b, catalyst size ) 1 mm; (∆) Figure 1c, catalyst size ) 1 mm; and (0) Figure 1d, catalyst size ) 1 mm. The flow rate of feed gas is 5000 cm3/min, and the pressure on the permeation side is 0.1 atm.)
Figure 10. Predicted composition of gases on the reaction side with hydrogen removal: (O) methane, (∆) carbon monoxide, (]) carbon dioxide, (0) hydrogen, and (×) water. (The pressure on the reaction side is 5 atm.)
that the effects of both factors decrease as the linear flow velocity increase, which is in good agreement with the experimental results shown in Figures 8 and 9. On the other hand, the catalyst particle size has an effect only on the mixing: smaller particles result in better mixing. Figure 8b shows the experimental results obtained using catalyst particles with a diameter of 1 mm, with the other reaction conditions remaining unchanged from those of the runs shown in Figure 8a, where catalyst particles with the size of 3 mm in size were used. As can be observed from the figures, the particle size did not make an appreciable difference in the methane conversion. This suggests that the concentration polarization is caused by the slow diffusion of hydrogen in the boundary layer near the surface of the membrane. On the other hand, the maximum value of the Reynolds number, Rep calculated according to the reference (Rep ) Favusdp/µav),22 was ∼10, which was less than the critical value for the transition between laminar and turbulent flow (i.e., 50). This indicates the possibility that the turbulent flow was not sufficiently achieved in this experiment, and that there might be a concentration gradient of hydrogen in the catalyst bed. Although the quantification of both effects requires further study, Figures 8a and 8b seem to suggest that the concentration gradient in the boundary layer had a greater effect than that in the catalyst bed. 3.4. Performance of Membrane Reactor. Figure 10 shows the gas composition in the reaction side calculated via thermodynamic calculation. The partial pressure of hydrogen changed from 28% to 15%, when methane conversion was increased from 34% to 80% with hydrogen permeation through the membrane. This implies that, to make the best use of a membrane reactor, hydrogen must be effectively removed from a gas mixture that contains only 15%-28% hydrogen. One
Figure 11. Dependence of (a) methane conversion and (b) hydrogen recovery on pressure on the reaction side. Solid line represents equilibrium conversion without hydrogen removal. (The structure of the membrane reactor is that depicted in Figure 1e, the catalyst size is 1 mm, and the flow rate of reactant gas varies: (O) 500, (∆) 1000, and (0) 2000 cm3/min. The pressure on the permeation side is 0.1 atm.)
possibility is to increase the reaction pressure. However, as shown in Figure 2a, the methane conversion decreased as the reaction pressure increased. This implies that the effect of pressure on the equilibrium conversion exceeds the effect on the hydrogen permeation, despite a highly permeable Pd/Ag membrane with a thickness of a few micrometers. In Figure 11a, on the other hand, the methane conversion in the membrane reactor increased with the reaction pressure, indicating that the improved configuration of the membrane reactor with a highly permeable membrane can overcome not only the thermodynamic limitation in the methane conversion but also the decreasing effect of reaction pressure on the equilibrium conversion by minimizing the effect of concentration polarization. As can be observed from Figures 2a and 11a, the methane conversion was improved, using the baffle plates, especially at lower flow rates and higher pressures. The reactor configurations proposed in this study were those with narrowed inner diameters and an installation of baffle plates, as shown in Figures 1c and 1d. In the former, the inner diameter of the reactor may have a direct effect on reducing the concentration polarization in the catalyst bed. In the present case, however, because the flow rate of reacting gas was kept constant, the narrowed inner diameter was followed by the large linear flow velocity of reacting gas. This can enhance the mixing in the catalyst bed and the diffusion of hydrogen through the boundary layer near the membrane surface, resulting in a smaller effect of concentration polarization. Thus, the concentration polarization can be reduced by increasing the linear flow velocity, even when the inner diameter of reactor is kept unchanged. Because the contact time decreases as the linear velocity increased, it is necessary to use a highly active catalyst to make the best use of the membrane reactor. In the case of baffle plates, which are more effective than narrowing the inner diameter, the key is creating a configuration of the baffles plates that accelerates the linear velocity of reacting gas, both in the catalyst bed to enhance the mixing and near the surface of the membrane, to enhance the hydrogen diffusion through the boundary layer. 4. Conclusions It was experimentally confirmed that the concentration polarization had a large effect on the methane conversion and the hydrogen recovery in the steam reforming using a membrane reactor with a highly permeable Pd/Ag membrane with a thickness of a few micrometers. Two reactor configurations were determined to be effective for reducing the concentration polarization, resulting in the improvement of the hydrogen
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recovery and the methane conversion: one involves a reactor with a narrower inner diameter, resulting in a faster linear flow velocity of reacting gas, and the other one, which seemed more effective, is the installation of baffle plates in the reactor. It is concluded that optimizing the reactor configuration was quite essential for making the most use of the membrane performance, especially for a highly hydrogen-permeable membrane. Nomenclature dp ) diameter of catalyst [m] FM ) flow rate of methane in reactant [cm3/min] H ) hydrogen flux [cm3/min] K ) permeability of membrane [cm3/cm2 min atm1/2] K1 ) equilibrium constant of steam reforming of methane [atm2] K2 ) equilibrium constant of CO sift conversion N ) number of minute units P ) pressure [atm] Rep ) Reynolds number S ) surface area of membrane [cm2] TH ) total amount of hydrogen permeated through membrane [cm3/min] us ) superficial velocity of gas on reaction side [m/s] X ) conversion of methane Y ) yield of carbon dioxide z ) hydrogen recovery µav ) average viscosity [kg m-1 s-1] Fav ) average density [kg/m3] Subscripts n ) nth minute unit P ) permeation side R ) reaction side total ) total Acknowledgment This work was performed as a part of the R&D project for High Efficiency Hydrogen Production/Separation System Using Ceramic Membrane, funded by New Energy and Industrial Technology Development Organization (NEDO). Literature Cited (1) Rostrup-Nielsen, J. R.; Sehested, J.; Nøerskov, J. K. Hydrogen and synthesis gas by steam- and CO2 reforming. AdV. Catal. 2002, 47, 65139. (2) Uemiya, S.; Sato, N.; Ando, H.; Matsuda, T.; Kikuchi, E. Steam reforming of methane in a hydrogen-permeable membrane reactor. Appl. Catal. 1991, 67, 223-230. (3) Shu, J.; Grandjean, B. P. A.; Kaliaguine, S. Methane steam reforming in asymmetric Pd- and Pd-Ag/porous SS membrane reactors. Appl. Catal., A 1994, 119, 305-325. (4) Shu, J.; Grandjean, B. P. A.; Kaliaguine, S. Aymmetric Pd-Ag/ stainless catalytic membranes for methane steam reforming. Catal. Today 1995, 25, 327-332. (5) Oertel, M.; Schmitz, J.; Weirich, W.; Jendryssek-Neumann, D.; Schulten, R. Steam reforming of natural gas with integrated hydrogen separation for hydrogen production. Chem. Eng. Technol. 1987, 10, 248255.
(6) Barbieri, G.; Violante, V.; Maio, F. P. D.; Criscuoli, A.; Drioli, E. Methane steam reforming analysis in a palladium-based catalytic membrane reactor. Ind. Eng. Chem. Res. 1997, 36, 3369-3374. (7) Oklany, J. S.; Hou, K.; Hughes, R. A simulative comparison of dense and microporous membrane reactors for the steam reforming of methane. Appl. Catal., A 1998, 170, 13-22. (8) Lin, Y.-M.; Lee, G.-L.; Rei, M.-H. An integrated purification and production of hydrogen with a palladium membrane-catalytic reactor. Catal. Today 1998, 44, 343-349. (9) Madia, G. S.; Barbieri, G.; Drioli, E. Theoretical and experimental analysis of methane steam reforming in a membrane reactor. Can. J. Chem. Eng. 1999, 77, 698-706. (10) Kikuchi, E.; Nemoto, Y.; Kajiwara, M.; Uemiya, S.; Kojima, T. Steam reforming of methane in membrane reactors; comparison of electroless-plating and CVD membranes and catalyst packing modes. Catal. Today 2000, 56, 75-81. (11) Kikuchi, E. Membrane reactor application to hydrogen production. Catal. Today 2000, 56, 97-101. (12) Tong, J.; Matsumura, Y.; Suda, H.; Haraya, K. Thin and dense Pd/CeO2/MPSS composite membrane for hydrogen separation and steam reforming of methane. Sep. Purif. Technol. 2005, 46, 1-10. (13) Tong, J.; Matsumura, Y. Effect of catalytic activity on methane steam reforming in hydrogen-permeable membrane reactor. Appl. Catal., A 2005, 286, 226-231. (14) Kusakabe, K.; Mizoguchi, H.; Eda, T. Methane steam reforming in a zirconia membrane reactor. J. Chem. Eng. Jpn. 2006, 39, 444-447. (15) Yasuda, I.; Shirasaki, Y.; Tsuneki, T.; Asakura, T.; Kataoka, A.; Shinkai, H.; Yamaguchi, R. Development of membrane reformer for highlyefficient hydrogen production from natural gas. Presented at the 15th World Hydrogen Energy Conference, 2004, Paper No. 28D-01. (16) Haraya, K.; Hakuta, T.; Yoshitomi, H. A study of concentration polarization phenomenon on the surface of a gas separation membrane. Sep. Sci. Technol. 1987, 22, 1425-1438. (17) Ludtke, O.; Behling, R.-D.; Ohlrogge, K. Concentration polarization in gas separation. J. Membr. Sci. 1998, 146, 145-157. (18) He, G.; Mi, Y.; Yue, P. L.; Chen, G. Theoretical study on concentration polarization in gas separation membrane processes. J. Membr. Sci. 1999, 153, 243-258. (19) Takaba, H.; Nakao, S. Computational fluid dynamics study on concentration polarization in H2/CO separation membranes. J. Membr. Sci. 2005, 249, 83-88. (20) Mourgues, A.; Sanchez, J. Theoretical analysis of concentration polarization in membrane modules for gas separation with feed inside the holllow-fibers. J. Membr. Sci. 2005, 252, 133-144. (21) Zhang, J.; Liu, D.; He, M.; Xu, H.; Li, W. Experimental and simulation studies on concentration polarization in H2 enrichment by highly permeable and selective Pd membranes. J. Membr. Sci. 2006, 274, 83-91. (22) Itoh, N.; Xu, W.-C.; Haraya, K. Radial mixing diffusion of hydrogen in a packed-bed type of palladium membrane reactor. Ind. Eng. Chem. Res. 1994, 33, 197-202. (23) Hara, S.; Sakaki, K.; Itoh, N. Decline in hydrogen permeation due to concentration polarization and CO hindrance in a palladium membrane reactor. Ind. Eng. Chem. Res. 1999, 38, 4913-4918. (24) Itoh, N.; Haraya, K. A carbon membrane reactor. Catal. Today 2000, 56, 103-111. (25) Shirasaki, Y.; Tsuneki, T.; Seki, T.; Yasuda, I. Presented at the 35th Autumn Meeting of the Society of Chemical Engineers, Japan, 2002, Paper No. W120. (26) Amano, M.; Nishimura, C.; Komaki, M. Effects of high concentration CO and CO2 on hydrogen permeation through the palladium membrane. Mater. Trans., JIM 1990, 31, 404-408.
ReceiVed for reView July 28, 2006 ReVised manuscript receiVed January 9, 2007 Accepted January 18, 2007 IE060989J
