CO-Free Hydrogen Production by Membrane Reactor Equipped with

NGK Insulators Ltd., Mizuho, Nagoya, 467-8530, Japan, Japan Fine Ceramics Center, Atsuta, Nagoya,. 456-8587, Japan, and Nagoya Industrial Science ...
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Ind. Eng. Chem. Res. 2008, 47, 1421-1426

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CO-Free Hydrogen Production by Membrane Reactor Equipped with CO Methanator Nobuhiko Mori,*,† Toshiyuki Nakamura,† Osamu Sakai,† Yuji Iwamoto,‡ and Tadashi Hattori§ NGK Insulators Ltd., Mizuho, Nagoya, 467-8530, Japan, Japan Fine Ceramics Center, Atsuta, Nagoya, 456-8587, Japan, and Nagoya Industrial Science Research Institute, Chikusa, Nagoya 464-0819, Japan

A novel membrane reactor system equipped with a CO methanator was proposed for the efficient production of CO-free hydrogen by a simple and compact reactor unit without any additional purification of hydrogen. It is expected that a trace amount of CO in the permeate can be removed through the methanation at the sacrifice of a minimum amount of hydrogen. The effectiveness of this system was experimentally examined for the steam reforming of methane in the membrane reactor using a thin Pd/Ag membrane with a few defects and a Ru/Al2O3 catalyst for methanation. It was demonstrated that a highly concentrated and practically COfree hydrogen could be successfully obtained with attaining the high methane conversion beyond the limit imposed by equilibrium, and the amount of hydrogen consumed by the CO and CO2 methanations was much lowered; that is, it was less than ca. 1% of hydrogen permeated through the membrane. Furthermore, the catalytic performance for methanation at relatively high temperature was investigated using a packed bed type reactor with a simulative permeate, because the loss of hydrogen due to the methanations can be more reduced by a catalyst with the ability for CO selective methanation in the presence of CO2. The experimental result that the selectivity toward CO methanation strongly depended on the metal catalyst and the support suggested the possibility of further improvement of the proposed system. 1. Introduction Effective and economical production of high-purity hydrogen is quite essential for the widely practical use of polymer electrolyte fuel cells (PEFCs). Conventionally, hydrogen is produced on a large scale by the steam reforming of hydrocarbons such as natural gas or naphtha oil. The steam reforming of methane (SRM) includes the two successive reactions:

reforming reaction: CH4 + H2O ) CO + 3H2 -∆H°298 ) -206 kJ/mol (1) CO shift reaction: CO + H2O ) CO2 + H2 -∆H°298 ) 41 kJ/mol (2) The overall reaction is endothermic and favored at high temperatures, because of the limitation by equilibrium, and thus, typically, the reaction needs high temperatures up to 1123 K. In addition, a large amount of steam is necessary for achieving high methane conversion and avoiding the deactivation of the catalyst, because of the coking. Consequently, hydrogen production via the SRM has been conventionally operated with a highly energy-consuming process. Moreover, the concentration of a CO impurity in the fuel gas for the PEFC must be below 10 ppm due to the prevention of the anodic catalyst from the deactivation by the CO adsorption, resulting in a complicated and costly whole process for hydrogen production. The membrane reactor (MR) is one of the most promising processes for highly effective hydrogen production.1,2 In the MR, hydrogen production by the SRM and hydrogen separation from the reformed gas by the membrane are simultaneously carried out in a single unit. The preferential extraction of hydrogen from * To whom correspondence should be addressed. Tel.: +81 52 872 7984. Fax: +81 52 872 7537. E-mail: [email protected]. † NGK Insulators Ltd. ‡ Japan Fine Ceramics Center. § Nagoya Industrial Science Research Institute.

the reaction zone through the membrane shifts the thermodynamic equilibrium toward the product side, resulting in the enhancement of conversion. Consequently, at moderate temperature, 773-873 K, the MR enables us to achieve high methane conversion beyond the limitation imposed by the equilibrium.3-5 Moreover, the MR equipped with a membrane exhibiting high H2 selectivity can produce CO-free hydrogen without complicated H2 purification units, such as a CO catalytic converter and pressure swing adsorption (PSA). Thus, the MR is expected to be a more energy-effective and compact hydrogen production system, compared to the conventional system. A hydrogen permselective membrane is the key component for the MR. A Pd-based membrane is viewed as one of the promising membranes for hydrogen separation, because of high stability at elevated temperature and extremely high hydrogen permselectivity based on the solution-diffusion transport mechanism, despite the expensiveness of Pd. Conventionally, a thin Pd-based layer contributing the separation of hydrogen is deposited on the ceramic3 or metallic porous support,4,5 for decreasing the thickness of the Pd-based layer in order to both increase hydrogen flux and decrease the cost of Pd. However, a thinner Pd-based layer inevitably results in greater difficulty in depositing the layer on the porous support without the defect, because of the roughness of the surface of the porous support. Thus, considerable efforts for fabricating Pd-based membrane free from defects have been reported so far. Recently, thin Pdbased membranes owning both high H2 permeability and moderately high H2 selectivity have been successfully obtained.6-10 Gade et al. synthesized a Pd membrane composed of a Pd layer with a thickness of 3.8 µm and a ZrO2-coated stainless steel porous support, exhibiting a hydrogen permeability of 0.57 mol m-2 s-1 and a hydrogen selectivity (H2/N2) of more than 850.9 Remarkably, the membrane showed a highly stable performance for over 250 h at 673 K. Mardilovich et al. fabricated a Pdbased membrane by depositing a Pd-based layer with a thickness of 7.9 µm on a Pd/Ag-coated stainless steel support.10 The values

10.1021/ie070925o CCC: $40.75 © 2008 American Chemical Society Published on Web 02/12/2008

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of the H2 permeability and the ideal selectivity of H2/He of the membrane were 68.5 m3 m-2 h-1 atm0.5 and 149, respectively. Moreover, studies for the long-term stability of the Pd-based membrane under the steam reforming conditions have been reported. Lin and Rei reported the MR for the steam reforming of methanol at 623 K, using a Pd membrane with a thickness of about 20 µm.11 The membrane exhibited a stable hydrogen flux for nearly 900 h. According to the overview by Ma,12 Matzakos could obtain hydrogen with a purity of greater than 99% from the Pd membrane in the MR for natural gas steam reforming at 773 K for 6000 h. The Japan Gas Association developed the membrane reformer for steam reforming of natural gas using membrane modules with a Pd alloy film of less than 20 µm thickness.13 The constant hydrogen production rate of 28 Nm3/h with its purity of over 99.999% was obtained for cumulative operation time of 450 h with total start-up and shutdown cycles of 27. On the other hand, Ma pointed out many technical challenges, such as making even thinner membranes for the practical use of the Pd-based membrane and the MR.12 However, in the case of thinner membrane, the defect is more likely to form and, consequently, the concentration of CO impurity in the permeate would be higher. Therefore, developing the technique for the removal of CO becomes a newly addressed matter in hydrogen production systems using thin membranes. For the removal of CO impurity in the fuel gas for PEFCs, CO preferential oxidation has been widely extended. However, the reaction needs the production of pure O2 for not declining the purity of H2 or the fine control of the O2 feed rate for suppressing the consumption of H2. Another alternative is CO methanation.14-16 The removal of CO by its methanation has been extensively operated in the production of H2 for NH3 synthesis. Recently, a few attempts have been made to apply the methanation for removing CO in the fuel gas for PEFCs. The removal of CO by its methanation has some advantages in terms of no external supplies for reactant gas and heat,14-16 possibly enabling the CO reduction by a small and simple additional reactor. On the other hand, it is well-known that methanation has a disadvantage for the loss of H2 by the intended reaction of CO with H2 (CO methanation) and the undesired side reaction of coexisting CO2 with H2 (CO2 methanation) as expressed in eqs 3 and 4:

CO + 3H2 ) CH4 + H2O CO2 + 4H2 ) CH4 + 2H2O

-∆H°298 ) 206 kJ/mol (3) -∆H°298 ) 165 kJ/mol (4)

Thus, the consumption of H2 inevitably increases, if coexisting CO2 besides CO is methanated. Moreover, the CO2 methanation is highly exothermic, and thus, the methanation of a large amount of CO2 possibly leads to the increase in the reaction temperature by a self-catalytic reaction, resulting in difficulty in controlling the reaction temperature.14 Therefore, the selective methanation toward CO is preferential, because of both suppressing the consumption of H2 and avoiding the risk of uncontrolled temperature in the reactor. Thus, complex methods for preferential CO methanation in the presence of CO2, such as multistage CO methanation14 and temperature-staged methanation,15 have been proposed so far. In this study, a novel hydrogen production system composed of the MR for the SRM and the CO methanator was proposed for producing hydrogen-rich gas with CO < 10 ppm through a

Figure 1. Schematic drawing of concept of proposed membrane reactor system.

thin Pd-based membrane even with a few defects.17 The concept of the proposed system is indicated in Figure 1: a supplemental catalyst for CO methanation was installed in the permeate of the membrane in order to eliminate CO impurity permeated through the defect of the membrane. It is expected that the problems mentioned above, such as the consumption of H2 and the increase in the temperature of the reactor by the CO2 methanation, can be solved, because the membrane should cut off the majorities of CO and CO2 in the reformed gas. In this article, the production of highly concentrated and practically CO-free hydrogen by the proposed system was experimentally investigated using the thin Pd-based membrane with a few defects. Furthermore, in order to examine the possibility of further improvement of the system, some additional runs of CO and CO2 methanations were conducted by using a simulated permeate and a conventional packed bed type reactor. 2. Experimental Section 2.1. Performance of Proposed System. A dense Pd/Ag tubular membrane was prepared by electroless and electroplating techniques on the outer surface of an in-house porous ceramic support. The support was of asymmetric structure and was composed of surface and intermediate layers and substrate. Their pore sizes were about 0.1, 1, and 10 µm, respectively. The outer diameter and length of the membrane were about 10 mm and 40 mm, respectively. Three pieces of the membrane with a Pd/ Ag layer of ca. 2.5 µm thickness were used in the series of the experiments. A Pd-based membrane was installed in a reactor with an inner diameter of 25 mm. Commercial 2% Ru/Al2O3 catalyst with a particle size of 1 mm was packed before and around the membrane.18 The amounts of the catalyst packed before and around the membrane were ca. 5 and 10 g, respectively. In the experiment for the MR with the methanator, 3 g of catalyst for the methanation was packed in the shell side of the membrane. The catalyst for the methanation was identical to that for the SRM. Before the SRM by the MR, pure hydrogen was fed and the permeability of the membrane was determined by measuring the flow rate of the hydrogen permeated through the membrane. The SRM by the MR was conducted with reactant gas composed of methane and steam. Steam was supplied by evaporating water at 463 K. The steam-to-methane ratio in the reactant gas was 3, and the total flow rate was 400 cm3 min-1. The reaction

Ind. Eng. Chem. Res., Vol. 47, No. 5, 2008 1423 Table 1. Compositions of Permeate and Retentate in SRM by MRa run no. membrane H2 permeability/cm3 cm-2 min-1 atm-1/2 methanation catalyst CH4 conversion/% H2 concn/% dry base total C-compds/ppm dry base impurity concn/ppm dry base (fraction/%) CH4 CO CO2 total C-compds/% dry base C-compd concn/% dry base (fraction/%) CH4 CO CO2

1

2

3

M1 200 absent 81.3

M2 220 present 83.6

M3 220 present 84.6

Permeate 99.668 3316.6

99.992 85.1

99.811 1892.5

746.8 (22.5) 194.3 (5.9) 2375.5 (71.6)

82.7 (97.2) 0.2 (0.2) 2.2 (2.6)

1876.7 (99.2) 4.1 (0.2) 11.7 (0.6)

Retentate 67.7

68.1

71.6

12.7 (18.8) 5.2 (7.7) 49.8 (73.6)

11.2 (16.4) 3.8 (5.6) 53.1 (78.0)

11.9 (16.6) 3.1 (4.3) 56.6 (79.1)

a Reaction conditions for SRM: temperature, 823 K; pressure in reaction side, 3 atm; pressure in permeation side, 0.1 atm; flow rate of reactant, 400 cm3 min-1.

temperature for the SRM was controlled by the thermocouple attached to the reactor wall and kept at about 823 K. The temperature in the shell side of the membrane was measured by the thermocouple. The pressures of the reaction side and permeation side were 3 and 0.1 atm, respectively. The compositions of retentate and permeate were analyzed by thermal conductivity detector (TCD) and flame ionization detector (FID) type gas chromatographies, respectively. The flow rates of the retentate and permeate were measured by gas flow meters. 2.2. Methanation of CO and CO2 by Conventional Packed Bed Reactor. In addition to 2 wt % Ru/Al2O3 used in the abovementioned experiment, two commercial palletized catalysts, 0.5 wt % Rh/Al2O3 and 2 wt % Ru/ZrO2, were used after being crushed and sieved (0.5-1 mm). Prior to the reaction experiment, the catalysts were reduced by 10% H2 with Ar at 673 K for 1 h. A catalyst test was conducted using a conventional tubular reactor with an inner diameter of ca. 10 mm at atmospheric pressure. The reactant mixture was a simulated permeate containing 98.3% H2, 4000 ppm CH4, 1300 ppm CO, and 12 000 ppm CO2 in dry base. It also contained 2% H2O in wet base. The composition of the outlet gas was analyzed by the FIDtype gas chromatography. Reaction was carried out under a space velocity of ca. 10 000 or 30 000 h-1. The space velocity was defined as the ratio of the flow rate of the reaction mixture including steam at room temperature to the volume of the catalyst bed. 3. Results and Discussion 3.1. Performance of Proposed System. Three runs were conducted using the membrane reactor unit shown in Figure 1 to examine the performance of the proposed system. The first run was conducted without methanation catalyst for comparison. Methane conversion was about 81.3% at the reaction pressure of 3 atm and was higher than the thermodynamic equilibrium conversion without hydrogen permeation of 40.8%. Although the retentate contained 67.7% C-compounds, such as CH4, CO, and CO2, the permeate contained only 0.33% impurities and the H2 concentration attained 99.67%, as indicated in Table 1. This result that the ratio of hydrogen concentration to total C-compound concentration in the permeate was ca. 300 suggests that the membrane used in this study exhibited high selectivity toward hydrogen, despite the high hydrogen permeability.

However, gases other than hydrogen, such as CH4, CO, and CO2, were detected, and thus, there seemed to be a few defects in the membrane. The fractions of CH4, CO, and CO2 in the impurities in the permeate roughly agreed with that in the retentate, which suggests that the permeation through the defect is nonselective or, in other words, the fractions in the permeate are mainly determined by those in the retentate. In the second and third runs, the methanation catalyst was packed in the permeate side of the membrane reactor as shown in Figure 1. The temperature at the permeation side of the membrane, where the catalyst for the methanation was packed, was about 773 K. Methane conversions in the MR with M2 and M3 were 83.6 and 84.6%, respectively, and thus, there was no remarkable difference in the methane conversion between them. The conversions of methane in the second and third runs were slightly higher than that in the first run, which agreed well with the slight difference in the permeability of H2. The conversions were higher in the second and third runs where the permeabilities were higher than that in the first run. On the other hand, the concentrations of H2 in the permeate reached 99.992 and 99.811%, respectively, and the total concentrations of C-compounds in the permeate were lower in these runs, especially in the second run, which indicates that the membranes M2 and M3 had less defect than M1. The hydrogen selectivities (H2/C-compounds) of M2 and M3 correspond to 11 750 and 527, respectively. The difference between the runs with and without methanation catalyst was most remarkably observed in the concentration of CO in the permeate: CO concentration that was 194.3 ppm in the first run was much reduced to 0.2 and 4.1 ppm in the second and the third runs, respectively. These results obviously meet the requirement of CO concentration less than 10 ppm. CO2 concentration was also much reduced: it was 2.2 and 11.7 ppm, respectively. Since there is no profound difference in the fractions of CH4, CO, and CO2 in the retentate between the runs with and without methanation catalyst, much lowered fractions of CO and CO2 in the permeate can be attributed to the conversions of CO and CO2 into CH4 as expected. This was confirmed by a separate run using a conventional packed bed type reactor with the simulative permeate of the membrane owning a few defects. The composition of the reactive gas including H2O was determined by the calculation on the assumption that the composition of the retentate was the

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Figure 2. Concentrations of CO and CO2 after methanation by 2 wt % Ru/Al2O3. Weight of catalyst: 1.98 g. SV: 10 000 h-1 (wet basis). Composition of reactive gas (dry base): 98.3% H2, 4000 ppm CH4, 1300 ppm CO, and 12 000 ppm CO2. Dashed line indicates temperature of methanation in runs 2 and 3.

equilibrium state with hydrogen removal18 and the fractions of CH4, CO, CO2, and H2O in the permeate were identical to those in the retentate. The space velocity in this run was about 10 000 h-1 and was higher than that in the second and third runs for the MR, which was about 4500 h-1. As can be seen in Figure 2, the concentration of CO could be reduced to less than 10 ppm above 423 K. The reduction of CO2 was also observed at temperatures of more than 573 K, and this result agreed with the simultaneous reduction of CO and CO2 at the methanation temperature of ca. 773 K in the second and third runs. Thus, it was clearly demonstrated that CO permeated through the defect of the membrane could be successfully reduced to less than 10 ppm by the proposed MR system. The amount of hydrogen consumption for the removal of CO could also be much reduced by the present method. For example, if only CO in the permeate of run 1 was reduced to 10 ppm by methanation, it would consume ca. 550 ppm hydrogen that corresponded to only 0.055% of hydrogen permeated through the membrane. Even if both of CO and CO2 were completely methanated, the amount of consumed hydrogen would correspond to only 1% of hydrogen permeated through the membrane. Thus, it can be concluded that there was no significant consumption of H2 by the methanation in the MR. This is in marked contrast to the conventional packed bed reactor system equipped with the methanator. In the reformed gas after the CO shift reactor in the conventional system for hydrogen production,14 the concentration of CO2 was 24% and so high that almost all of the hydrogen was consumed by the CO2 methanation, and therefore, the highly selective CO methanation was essential. In the case of the completely selective CO methanation that minimizes the amount of consumed hydrogen, the ratio of the amount of methanated hydrogen to all of hydrogen produced in the conventional system, i.e., the loss of hydrogen, could be decreased to ca. 2.1% with the decrease in the concentration of H2 from 78.5 to 78.1%. However, this loss of hydrogen was still larger compared to that required for the methanations of CO and CO2 in the permeate in the run 1, i.e., ca. 1%. As mentioned above, it was proved that the membrane reactor equipped with the methanator could effectively reduce CO concentration to less than 10 ppm with only a small loss of hydrogen. It should be noted, however, that such a fine result could be achieved by using a highly selective membrane. The use of a less selective membrane would result in a rather large loss of hydrogen, because the loss of hydrogen should increase with the increase in the concentrations of CO and CO2 or, in other words, with the decrease in the concentration of hydrogen

Figure 3. Relation between concentration of hydrogen before methanation and loss of hydrogen. Fraction of gases, CH4:CO:CO2 ) 747:194:2376. For the conventional system, see the text.

in the permeate before methanation. This problem can be serious, when both CO and CO2 are unselectively methanated. Figure 3 shows the losses of hydrogen in the present system under completely selective and unselective methanation conditions as a function of hydrogen concentration before methanation on the assumption of a fixed CH4/CO/CO2 ratio in the permeate before methanation. When CO can be selectively methanated, the loss of hydrogen in the present MR system is much lower than that in the conventional system (2.1%) shown by the horizontal dotted line in Figure 3. On the other hand, when both CO and CO2 are completely methanated, the loss of hydrogen reaches ca. 2.1% at the hydrogen concentration of 99.3%. That is, when the hydrogen concentration is less than 99.3%, the proposed MR system, equipped with a completely unselective methanator, would lose the advantage toward the conventional packed bed reactor system equipped with the complete CO selective methanator in terms of the loss of hydrogen. Thus, the membrane, which can produce enriched hydrogen with a concentration of more than 99.3% as of the state before the methanation, might be one of the criterions for the membrane adapted to the proposed system. 3.2. Catalyst Test for Selective CO Methanation at High Temperature. As shown in Figure 3, the proposed MR system can be further improved, if CO could be selectively methanated in the presence of CO2. In the above-mentioned case, for example, the hydrogen consumption would be reduced from ca. 1% to only 0.055% of produced hydrogen, if the selective methanation of CO could be achieved. As shown in Figure 2, CO was preferentially reduced only in the specific temperature range on the catalyst used in the above-mentioned runs: Only CO was reduced to less than 10 ppm in the temperature range between 423 and 473 K, though the simultaneous reductions of CO and CO2 were observed at temperatures higher than 573 K. This result agreed well with the published papers devoted to the application of selective CO methanation for the removal of CO in the fuel gas for PEFCs.14-16 However, the CO selective methanation by lowering the temperature possibly spoils the compactness of the overall MR system for the SRM, because of the large temperature difference between the MR unit for 773-873 K and the methanator for 423-473 K. Therefore, in order to examine the possibility of developing the catalyst for selective CO methanation at higher temperatures, some catalyst tests were conducted using the packed bed type reactor with a simulative permeate. Figure 4 shows the effect of reaction temperature on the changes in the concentrations of CO and CO2 by methanation using Ru/Al2O3, Rh/Al2O3, and Ru/ZrO2, respectively. All of the experiments were carried out under the space velocity (SV) of 30 000 h-1. In the case of the Ru/Al2O3 catalyst, selective CO methanation was attained at

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Figure 4. Concentrations of CO and CO2 after methanation by (a) 2 wt % Ru/Al2O3, (b) 0.5 wt % Rh/Al2O3, and (c) 2 wt % Ru/ZrO2 catalysts. Weight of catalyst: (a) 0.68 g, (b) 1.05 g, and (c) 0.97 g. SV: 30 000 h-1 (wet basis). Composition of reactive gas (dry base): 98.3% H2, 4000 ppm CH4, 1300 ppm CO, and 12 000 ppm CO2. Dashed lines indicate the temperature of methanation in runs 2 and 3.

473 K, but not at 573 K, in agreement with the results shown in Figure 2. In the case of the Rh/Al2O3 catalyst, the CO concentration was reduced to less than 10 ppm without the remarkable CO2 reduction at around 573 K, which indicates that the Rh/Al2O3 catalyst had the ability of selective CO methanation in the high-temperature region compared to the Ru/ Al2O3 catalyst. The Ru/ZrO2 catalyst also had the ability for selective CO methanation in a relatively higher temperature range, i.e., 473-573 K. Thus, it was indicated that the combination of metal and support had a large effect on the temperature region for selective CO methanation, suggesting the logical possibility of CO selective methanation in the hightemperature region by designing the catalyst. The experimental results in this study indicated the difficulty in the CO selective methanation at 773 K. However, in the SRM by the MR, hydrogen removal by the membrane was the ratedetermining step18 and, thus, using a more highly hydrogenpermeable membrane for the MR supposedly enabled the lowering of the reaction temperature to less than 773 K, leading to the possibility of selectively removing CO even with a catalyst used in this study, such as Ru/ZrO2. It was also claimed that in the steam reforming of methanol,19 where the reaction was conventionally operated at less than 573 K, CO selective methanation could be possible by using the Ru/ZrO2 catalyst. Finally, it can be concluded that a highly effective system will be realized through the developments of the catalyst for selective hydrogenation of CO at high temperature and of the highly permeable membrane. 4. Conclusions In this study, we proposed a novel hydrogen production system in which a supplemental catalyst for CO methanation was installed in the permeate of a membrane reactor for the purpose of eliminating a trace of CO impurity permeated through the defect of the membrane. Its performance was experimentally examined to show that highly concentrated hydrogen, higher than 99.8%, with a CO impurity of less than 10 ppm could be successfully obtained through the Pd/Ag membrane with a few defects in the MR for the steam reforming of methane and with attaining a methane conversion of 81-85%. This result strongly demonstrated that the proposed system could produce highly enriched hydrogen, meeting the requirement for use in PEFCs without spoiling the characteristics of the membrane reactor, namely compactness and simplification. Thus, it was concluded that the proposed MR system could be an attractive system for an energy-effective and compact hydrogen production by using a membrane with the ability for producing highly concentrated hydrogen of, for example, 99.3%, and preferably a catalyst with

the ability for highly CO selective methanation in the temperature range of, e.g., 773-873 K. Acknowledgment This work was partially performed as a part of the research and development project for High Efficiency Hydrogen Production/Separation System using Ceramic Membrane funded by the New Energy and Industrial Technology Development Organization (NEDO). Literature Cited (1) 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. (2) Asaberg-Petersen, K.; Neilsen, C. S.; Jørgensen, S. L. Membrane reforming for hydrogen. Catal. Today 1998, 46, 193-201. (3) 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. (4) Shu, J.; Grandjean, B. P. A.; Kaliaguine, S. Methane steam reforming in asymmetric Pd- and Pd-Ag/porous SS membrane reactors. Appl. Catal., A: Gen. 1994, 119, 305-325. (5) 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. (6) 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. (7) Tong, J.; Shirai, R.; Kashima, Y.; Matsumura, Y. Preparation of a pinhole-free Pd-Ag membrane on a porous metal support for pure hydrogen separation. J. Membr. Sci. 2005, 260, 84-89. (8) Sun, G. B.; Hidajat, K.; Kawi, S. Ultra thin Pd membrane on R-Al2O3 hollow fiber by electroless plating: High permeance and selectivity. J. Membr. Sci. 2006, 284, 110-119. (9) Gade, S. K; Keeling, M. K.; Steele, D. K.; Way, J. D. Thoen, P. M. High flux Pd membranes deposited on stainless steel supports by electroless plating. Proc. Conf. Inorg. Membr. 2006, 9th, 65-68. (10) Mardilovich, I. P.; Engwall, E. E.; Ma, Y. H. Proc. Conf. Inorg. Membr. 2006, 9th, 92-95. (11) Lin, Y. M; Rei, M. H. Study on the hydrogen production from methanol steam reforming in supported palladium membrane reactor. Catal. Today 2001, 67, 77-84. (12) Ma, Y. H. Proc. Conf. Inorg. Membr. 2006, 9th, 28-37. (13) 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. 15th World Hydrogen Energy Conference; 2004; 28D-01. (14) Echigo, M.; Tabata, T. CO Removal from Reformed Gas by Catalytic Methanation for Polymer Electrolyte Fuel Cell Applications. J. Chem. Eng. Jpn. 2004, 37, 75-81. (15) Xu, G.; Chen, X.; Zhang, Z.-G. Temperaure-staged methanation: An alternative method to purify hydrogen-rich fuel gas for PEFC. Chem. Eng. J. 2006, 121, 97-107.

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(16) Choudhury, M. B. I.; Ahmed, S.; Shalabi, M. A.; Inui, T. Preferential methanation of CO in a syngas involving CO2 at lower temperature range. Appl. Catal., A 2006, 314, 47-53. (17) Soma, T.; Takahashi, T.; Isomura, M. JP Patent 3432892. (18) Mori, N.; Nakamura, T.; Noda, K.; Sakai, O.; Takahashi, A.; Ogawa, N.; Sakai, H.; Iwamoto, Y.; Hattori, T. Reactor Configuration and concentration polarization in methane steam reforming by a membrane reactor with a highly hydrogen-permeable membrane. Ind. Eng. Chem. Res. 2007, 46, 1952-1958.

(19) Itoh, N.; Kaneko, Y.; Igarashi, A. Efficient hydrogen production via methanol steam reforming by preventing back-permeation of hydrogen in a palladium membrane reactor. Ind. Eng. Chem. Res. 2002, 41, 47024706.

ReceiVed for reView July 6, 2007 ReVised manuscript receiVed November 28, 2007 Accepted December 5, 2007 IE070925O