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Effect of catalytic activity on methane steam reforming in hydrogen-permeable membrane reactor
Applied Catalysis A: General 286 (2005) 226–231 www.elsevier.com/locate/apcata
Effect of catalytic activity on methane steam reforming in hydrogen-permeable membrane reactor Jianhua Tong a, Yasuyuki Matsumura b,* b
a Research Institute of Innovative Technology for the Earth, Kizu-cho, Soraku-gun, Kyoto 619-0292, Japan National Institute of Advanced Industrial Science and Technology (AIST), Kansai Center, Ikeda, Osaka 563-8577, Japan
Received 12 July 2004; received in revised form 3 March 2005; accepted 9 March 2005 Available online 27 April 2005
Abstract Steam reforming of methane, mainly to hydrogen and carbon dioxide, proceeds over nickel catalysts at 800 K in an equilibrium-shift reactor with a thin palladium membrane 11 mm-thick supported on a stainless steel porous metal filter. The methane conversion greatly exceeds its equilibrium conversion due to the hydrogen separation with the membrane. The catalytic activity affects the conversion much more than in the cases using an equilibrium reactor. The conversion in the membrane reactor decreases with an increase in the space velocity of the reaction mixture, mainly because of a decrease in the hydrogen separation ratio (rate of hydrogen separation/total production rate of hydrogen). The rate of hydrogen separation depends basically on the hydrogen permeability of the membrane, but an active catalyst also increases the rate; that is, the hydrogen production rate depends on the catalytic activity. Thus, the higher rate causes the higher partial pressure of hydrogen in the reactor, while the hydrogen flux through the membrane depends on the pressure. # 2005 Elsevier B.V. All rights reserved. Keywords: Membrane reactor; Hydrogen separation; Palladium membrane; Methane steam reforming; Nickel catalyst
1. Introduction Industrial hydrogen production is based on steam reforming of methane, while hydrogen is mainly consumed in ammonia plants, methanol plants, and oil refineries. The process requires a high operation temperature, above ca. 1000 K, because of the equilibrium of the endothermic reactions: CH4 + H2O = 3H2 + CO (DG and DH at 1000 K are 28 and 225 kJ mol1, respectively) and CH4 + 2H2O = 4H2 + CO2 (30 and 191 kJ mol1, respectively) [1]. Such a high temperature causes some demerits, e.g., expensive tubular reformers made of high alloy nickel chromium steel with fire burners, irreversible carbon formation in the reactor, and large energy consumption. A lower reaction temperature is energetically and economically desirable, but the low-temperature reforming produces an insufficient conversion for the industrial process. For * Corresponding author. Tel.: +81 72 751 7821; fax: +81 72 751 9623. E-mail address: [email protected] (Y. Matsumura). 0926-860X/$ – see front matter # 2005 Elsevier B.V. All rights reserved. doi:10.1016/j.apcata.2005.03.013
example, when steam reforming of methane is carried out at 800 K at 2.0 MPa with H2O/CH4 molar ratio (S/C ratio) equal to three, the equilibrium conversion of methane is only 17%. In order to reduce the reaction temperature, researchers have proposed several membrane reactor [2–21]. In the reactor, hydrogen is usually separated from the reaction mixture using palladium-based membranes, and the separation of hydrogen significantly increases the equilibrium conversion of methane. Although a number of studies have been performed on membrane reactors for methane steam reforming, there are few detailed experimental reports [4,9,13]. Since separation of hydrogen is promoted by the pressure difference of hydrogen between the reaction sides and the permeation side of the reactor, the hydrogen pressure on the reaction side should be high. However, the hydrogen separation drops the pressure. Active catalysts promote the further methane conversion that recovers the hydrogen pressure to the new equilibrium produced by the hydrogen separation. Hence, catalytic activity is an important factor in
J. Tong, Y. Matsumura / Applied Catalysis A: General 286 (2005) 226–231
the membrane reaction, but no study has been made on this issue. In the present paper, we will show that the catalytic activity greatly affects the performance of the equilibriumshift reactor with a palladium membrane supported on porous sintered metal.
2. Experimental A palladium membrane was prepared by electroless plating on a porous stainless steel tube modified with cerium hydroxide, as described elsewhere [22]. The membrane thickness was 11 mm and the effective area of the membrane for hydrogen separation was 20.6 cm2. The hydrogen flux, J, through the membrane can be generally expressed as 0:5 J ¼ Q0 expðEA =RTÞð p0:5 1 p2 Þ=l, where p1 and p2 are the hydrogen pressures in the upstream and downstream sides, respectively, and l the thickness of the palladium layer [23]. The values, Q0 and EA, measured without a sweep gas were 2.1 mol mm m2 s1 kPa1/2 and 11.0 kJ mol1, respectively [22]. The membrane was almost defect-free, while the ratio of hydrogen flux to argon flux was ca. 2000 at 773 K when the upstream pressure was 0.20 MPa and that for the downstream side was atmospheric pressure. The membrane tube was installed in a reactor with a supported nickel catalyst (5.4 cm3). The structure of the reactor is shown in Fig. 1. When the reaction was carried out without the membrane, a dummy stainless steel tube without pores was installed in the reactor. Two alumina-supported nickel catalysts were supplied by Osaka Gas Co. One was a highly active commercial catalyst whose nickel content was
Fig. 1. Structure of membrane reactor.
227
47.2 wt.% (Ni–H), and the other was less active, containing 9.9 wt.% of nickel (Ni–L). The densities of Ni–H and Ni–L were 2.8 and 2.6 g cm3, respectively. In the reactor, the catalyst was preheated to 800 K in an argon stream, then reduced in a stream of 50 vol.% hydrogen diluted with argon (12 dm3 h1) at the same temperature. Steam reforming of methane was carried out at 800 K. A mixture of methane and steam with the S/C ratio of 3.0 was supplied quantitatively to the catalyst bed in up-flow. The reaction temperature was measured at the top of the catalyst bed. The effluent gas from the reactor (unpermeated gas) was dried with an ice trap and a silica-gel drier, then analyzed with an on-stream Shimadzu GC-8A gas chromatograph (activated carbon, 1 m; Ar carrier) equipped with a thermal conductivity detector. No significant coke formation was observed in the experiment. Nitrogen (1.29 mol h1 or 0.063 mol cm2 h1 unless otherwise mentioned) was concurrently led inside the membrane (permeation side) to sweep out the hydrogen that permeated. The composition of the permeated gas was also analyzed with the same gas chromatograph using a flow-switch. Methane conversion and selectivities were calculated on the basis of carbon numbers. The mass balance was within the error of 5%.
3. Results and discussion 3.1. Catalytic activity under equilibrium condition The activities of catalysts were evaluated at 0.10 MPa without a membrane. At low space velocities, methane conversions with Ni–H were discernibly larger than the equilibrium conversion, assuming the reactions of CH4 + 2H2O = 4H2 + CO2 and CH4 + H2O = 3H2 + CO at 800 K (Fig. 2). The conversion decrease with an increase in the space velocity, while the selectivity to carbon monoxide was always lower than the equilibrium value. The activity of Ni–L was appreciably lower than that of Ni–H by 6% when the space velocity was less than 10,000.
Fig. 2. Activity of nickel catalysts for steam reforming of methane in equilibrium reactor at 800 K.
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Fig. 3. Steam reforming of methane with membrane reactor at 800 K. Space velocity, 1120 h1.
Fig. 5. Steam reforming of methane with membrane reactor at 800 K. Space velocity, 3360 h1.
3.2. Effect of catalytic activity to membrane reaction Although the equilibrium conversion of methane decreases with the reaction pressure, the conversion increased in the membrane reactor at a low space velocity of 1120 h1 (Fig. 3). The conversion with Ni–H was significantly higher than that with Ni–L by 20% at 0.10 MPa, showing that the catalytic activity affects the performance of the membrane reactor more than that of the equilibrium reactor. Since the higher conversion results in the higher hydrogen pressure at the reaction side, the hydrogen flux through the membrane with Ni–H was also higher than that with Ni–L (Fig. 4). In the cases of the space velocity at 3360 h1, the methane conversion with Ni–H at 0.20 MPa was smaller than that at 0.10 MPa, but it increased with an increase in the reaction pressure above 0.20 MPa (Fig. 5). The selectivity to carbon monoxide decreased with an increase in the reaction pressure (see Figs. 3 and 5), showing that the water-gas shift reaction, CO + H2O ! CO2 + H2, is also promoted by the removal of hydrogen in the membrane reaction.
The methane conversions at a space velocity of 3360 h1 were smaller than those at 1120 h1 (cf. Figs. 3 and 5), while the hydrogen pressures at the exit of reaction side were higher than the corresponding values at 1120 h1 (cf. Figs. 4 and 6). In Figs. 7 and 8, the methane conversions measured under different space velocities (1120–5600 h1 for Ni–H and 1120–3360 h1 for Ni–L) were plotted against the hydrogen separation ratios (the relative rate of hydrogen separation to the total production rate of hydrogen in the reaction). The higher space velocity resulted in the lower hydrogen separation ratio because the rate of hydrogen separation does not increase so much as the increase in the total production rate of hydrogen, which is related to the product of the space velocity times the methane conversion. The methane conversions produced with Ni–H at 0.10 MPa (open circles in Fig. 7) increased with an increase in the hydrogen separation ratio and were close to the equilibrium (solid curve), which is shifted by hydrogen separation; however, the conversions with Ni–L (solid circles) were clearly lower than the equilibrium, showing that the catalytic
Fig. 4. Permeation flux of hydrogen and partial pressure of hydrogen at reactor exit in steam reforming of methane with membrane reactor shown in Fig. 3.
Fig. 6. Permeation flux of hydrogen and partial pressure of hydrogen at reactor exit in steam reforming of methane with membrane reactor shown in Fig. 5.
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Fig. 7. Relationship between methane conversion and hydrogen separation ratio at 0.10 MPa.
Fig. 9. Effect of sweep gas on steam reforming of methane with membrane reactor at 800 K.
activity is insufficient to catch up with the equilibrium-shift. At 0.30 MPa, a similar tendency was observed (see Fig. 8). The partial pressure of the hydrogen decreases with an increase in the ratio due to the separation of hydrogen (see broken curves in Figs. 7 and 8).
than that measured without any sweep gas (Fig. 10, the broken line shows the flux without a sweep gas as described in Section 2 [22]). In the figure, p2 is the partial pressure of hydrogen at the exit of permeation side, while p1 was always 0.10 MPa. It is noteworthy that the palladium membrane employed was supported on porous stainless steel modified with cerium oxide particles. The result shows that diffusion of the sweep gas into the support is insufficient and that the actual hydrogen pressure on the surface of the palladium layer is significantly higher than that in the permeation gas. That is, an increase in the flux of the sweep gas does not effectively increase the hydrogen flux through the membrane; hence, the methane conversion is saturated at an excessive feed rate of the sweep gas. This shows that the reaction at a high pressure is more effective to increase the hydrogen flux than the employment of a large quantity of sweep gas, which is economically unfavorable (expensive) in the industrial process [24].
3.3. Effect of sweep gas to hydrogen separation rate Since the hydrogen flux through a palladium membrane depends on the pressure difference between the reaction side and the permeation side, a larger quantity of sweep gas that decreases the partial pressure of hydrogen on the permeation side results in the higher hydrogen separation rate and methane conversion. However, the effect was not as large as expected (Fig. 9), while no enhancement in the reaction took place without the sweep gas. To evaluate the performance of the membrane in presence of the sweep gas, we fed hydrogen to the membrane reactor without the catalyst by changing the flux of the sweep N2 gas. The hydrogen flux produced was considerably lower
Fig. 8. Relationship between methane conversion and hydrogen separation ratio at 0.30 MPa.
Fig. 10. Hydrogen flux through palladium membrane by feeding pure hydrogen in the presence of sweep gas. Broken line, the flux measured without sweep gas [22].
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Fig. 11. Plots of hydrogen flux vs. hydrogen partial pressure at reactor exit in membrane reactor. Open circles, Ni–H at 1120 h1; solid circles, Ni–H at 3360 h1; open squares, Ni–L at 1120 h1; solid squares, Ni–L at 3360 h1; solid triangles, hydrogen flux by feeding hydrogen diluted with argon at 0.10 MPa in the presence of sweep gas.
3.4. Effect of catalytic activity to hydrogen flux through membrane To obtain the relationship between the hydrogen pressure in the catalyst side and the hydrogen flux, we measured the hydrogen flux by feeding hydrogen diluted with argon to the membrane reactor at a total pressure of 0.10 MPa in the presence of the sweep gas (solid curve in Fig. 11, the pressure was the mean of the values at the entrance and the exit). On the basis of the data in Fig. 4, we plotted the hydrogen flux versus hydrogen pressure at the exit of reaction side in the reaction with Ni–H at 1120 h1 (open circles) in Fig. 11. The plots were close to the solid curve, showing that the mean hydrogen pressure in the reaction side is close to that at the exit. However, the fluxes at 3360 h1 were appreciably lower than expected from the exit hydrogen pressures (open squares on the basis of Fig. 6). Hence, the mean hydrogen pressure is lower than that at the exit, suggesting insufficient hydrogen production in the middle part of the catalyst layer. In the cases of Ni–L, the flux values at 1120 h1 were lower than those expected (solid circles). This shows that the hydrogen pressure in the middle part of the reactor is significantly lower than the exit pressure even at the low space velocity, and that the hydrogen production does not catch up with the hydrogen removal in the middle part. Thus, the catalytic activity is important for the large hydrogen flux as well as for the performance of the membrane itself.
4. Conclusions The catalytic activity to steam reforming of methane greatly affects the performance of the membrane reactor.
Separation of hydrogen through the palladium membrane shifts the equilibrium to the larger conversion of methane, but the less active catalyst produces the lower conversion than the values expected from the shifted equilibrium, which depends on the hydrogen separation ratio (hydrogen separation rate/total rate of hydrogen production). The higher space velocity of the reactant results in the lower conversion in the membrane reactor, mainly because the hydrogen separation ratio decreases with an increase in the space velocity. Although the active catalyst produces the methane conversion close to the equilibrium expected from the hydrogen separation ratio at the reactor exit, the conversion in the middle part of the catalyst layer is insufficient to catch up the equilibrium-shift at a high space velocity. Since the hydrogen flux through the membrane relates to the partial pressure of hydrogen in the reaction side, the lower conversion in the middle part of the catalyst layer results in the lower hydrogen flux. Thus, catalytic activity also affects the hydrogen flux through the membrane.
Acknowledgement The financial support of the New Energy and Industrial Technology Development Organization is gratefully acknowledged.
References [1] J.R. Rostrup-Nielse, in: J.R. Anderson, M. Boudard (Eds.), Catalysis, Science and Technology, vol. 5, Springer-Verlag, New York, 1984, p. 3. [2] M. Oertel, J. Schmitz, W. Weirich, D. Jendryssek-Neumann, R. Schultem, Chem. Eng. Technol. 10 (1987) 248. [3] J. Schmitz, H. Gerke, CLB Chem. Labor Betr. 39 (1988) 75. [4] S. Uemiya, N. Sato, H. Ando, T. Matsuda, E. Kikuchi, Appl. Catal. 67 (1991) 223. [5] J. Shu, B.P.A. Grandjean, A. Van Neste, S. Kaliaguine, Can. J. Chem. Eng. 69 (1991) 1036. [6] A.M. Adris, S.S.E.H. Elnashaie, R. Hughes, Can. J. Chem. Eng. 69 (1991) 1061. [7] S.I. Prokopiev, Y.I. Aristov, V.N. Parmon, N. Giordano, Int. J. Hydrogen Energy 17 (1992) 275. [8] M. Chai, M. Machida, K. Eguchi, H. Arai, Appl. Catal. A 110 (1994) 239. [9] J. Shu, B.P.A. Grandjean, S. Kaliaguine, Appl. Catal. A 119 (1994) 305. [10] S.L. Jørgensen, P.E.H. Nielsen, P. Lehrmann, Catal. Today 25 (1995) 303. [11] J. Shu, B.P.A. Grandjean, S. Kaliaguine, Catal. Today 25 (1995) 327. [12] E. Kikuchi, Catal. Today 25 (1995) 333. [13] G. Barbieri, V. Violante, F.P. Di Maio, A. Criscuoli, E. Drioli, Ind. Eng. Chem. Res. 36 (1997) 3369. [14] J. Sogge, T. Strøm, Stud. Surf. Sci. Catal. 107 (1997) 561. [15] J.S. Oklany, K. Hou, R. Hughes, Appl. Catal. A 170 (1998) 13. [16] K. Aasberg-Petersen, C.S. Nielsen, S.L. Jørgensen, Catal. Today 46 (1998) 193. [17] J.-H. Kim, B.-S. Choi, J. Yi, J. Chem. Eng. Jpn. 32 (1999) 760.
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[22] J. Tong, H. Xu, D. Wang, Y. Matsumura, J. Jpn. Petrol. Inst. 47 (2004) 64. [23] R. Dittmeyer, V. Ho¨ llein, K. Daub, J. Mol. Catal. A 173 (2001) 135. [24] Y. Matsumura, M. Uragami, T. Nakamori, S. Morimoto, J. Jpn. Petrol. Inst. 46 (2003) 166.
Effect of catalytic activity on methane steam reforming in hydrogen-permeable membrane reactor Jianhua Tong a, Yasuyuki Matsumura b,* b
a Research Institute of Innovative Technology for the Earth, Kizu-cho, Soraku-gun, Kyoto 619-0292, Japan National Institute of Advanced Industrial Science and Technology (AIST), Kansai Center, Ikeda, Osaka 563-8577, Japan
Received 12 July 2004; received in revised form 3 March 2005; accepted 9 March 2005 Available online 27 April 2005
Abstract Steam reforming of methane, mainly to hydrogen and carbon dioxide, proceeds over nickel catalysts at 800 K in an equilibrium-shift reactor with a thin palladium membrane 11 mm-thick supported on a stainless steel porous metal filter. The methane conversion greatly exceeds its equilibrium conversion due to the hydrogen separation with the membrane. The catalytic activity affects the conversion much more than in the cases using an equilibrium reactor. The conversion in the membrane reactor decreases with an increase in the space velocity of the reaction mixture, mainly because of a decrease in the hydrogen separation ratio (rate of hydrogen separation/total production rate of hydrogen). The rate of hydrogen separation depends basically on the hydrogen permeability of the membrane, but an active catalyst also increases the rate; that is, the hydrogen production rate depends on the catalytic activity. Thus, the higher rate causes the higher partial pressure of hydrogen in the reactor, while the hydrogen flux through the membrane depends on the pressure. # 2005 Elsevier B.V. All rights reserved. Keywords: Membrane reactor; Hydrogen separation; Palladium membrane; Methane steam reforming; Nickel catalyst
1. Introduction Industrial hydrogen production is based on steam reforming of methane, while hydrogen is mainly consumed in ammonia plants, methanol plants, and oil refineries. The process requires a high operation temperature, above ca. 1000 K, because of the equilibrium of the endothermic reactions: CH4 + H2O = 3H2 + CO (DG and DH at 1000 K are 28 and 225 kJ mol1, respectively) and CH4 + 2H2O = 4H2 + CO2 (30 and 191 kJ mol1, respectively) [1]. Such a high temperature causes some demerits, e.g., expensive tubular reformers made of high alloy nickel chromium steel with fire burners, irreversible carbon formation in the reactor, and large energy consumption. A lower reaction temperature is energetically and economically desirable, but the low-temperature reforming produces an insufficient conversion for the industrial process. For * Corresponding author. Tel.: +81 72 751 7821; fax: +81 72 751 9623. E-mail address: [email protected] (Y. Matsumura). 0926-860X/$ – see front matter # 2005 Elsevier B.V. All rights reserved. doi:10.1016/j.apcata.2005.03.013
example, when steam reforming of methane is carried out at 800 K at 2.0 MPa with H2O/CH4 molar ratio (S/C ratio) equal to three, the equilibrium conversion of methane is only 17%. In order to reduce the reaction temperature, researchers have proposed several membrane reactor [2–21]. In the reactor, hydrogen is usually separated from the reaction mixture using palladium-based membranes, and the separation of hydrogen significantly increases the equilibrium conversion of methane. Although a number of studies have been performed on membrane reactors for methane steam reforming, there are few detailed experimental reports [4,9,13]. Since separation of hydrogen is promoted by the pressure difference of hydrogen between the reaction sides and the permeation side of the reactor, the hydrogen pressure on the reaction side should be high. However, the hydrogen separation drops the pressure. Active catalysts promote the further methane conversion that recovers the hydrogen pressure to the new equilibrium produced by the hydrogen separation. Hence, catalytic activity is an important factor in
J. Tong, Y. Matsumura / Applied Catalysis A: General 286 (2005) 226–231
the membrane reaction, but no study has been made on this issue. In the present paper, we will show that the catalytic activity greatly affects the performance of the equilibriumshift reactor with a palladium membrane supported on porous sintered metal.
2. Experimental A palladium membrane was prepared by electroless plating on a porous stainless steel tube modified with cerium hydroxide, as described elsewhere [22]. The membrane thickness was 11 mm and the effective area of the membrane for hydrogen separation was 20.6 cm2. The hydrogen flux, J, through the membrane can be generally expressed as 0:5 J ¼ Q0 expðEA =RTÞð p0:5 1 p2 Þ=l, where p1 and p2 are the hydrogen pressures in the upstream and downstream sides, respectively, and l the thickness of the palladium layer [23]. The values, Q0 and EA, measured without a sweep gas were 2.1 mol mm m2 s1 kPa1/2 and 11.0 kJ mol1, respectively [22]. The membrane was almost defect-free, while the ratio of hydrogen flux to argon flux was ca. 2000 at 773 K when the upstream pressure was 0.20 MPa and that for the downstream side was atmospheric pressure. The membrane tube was installed in a reactor with a supported nickel catalyst (5.4 cm3). The structure of the reactor is shown in Fig. 1. When the reaction was carried out without the membrane, a dummy stainless steel tube without pores was installed in the reactor. Two alumina-supported nickel catalysts were supplied by Osaka Gas Co. One was a highly active commercial catalyst whose nickel content was
Fig. 1. Structure of membrane reactor.
227
47.2 wt.% (Ni–H), and the other was less active, containing 9.9 wt.% of nickel (Ni–L). The densities of Ni–H and Ni–L were 2.8 and 2.6 g cm3, respectively. In the reactor, the catalyst was preheated to 800 K in an argon stream, then reduced in a stream of 50 vol.% hydrogen diluted with argon (12 dm3 h1) at the same temperature. Steam reforming of methane was carried out at 800 K. A mixture of methane and steam with the S/C ratio of 3.0 was supplied quantitatively to the catalyst bed in up-flow. The reaction temperature was measured at the top of the catalyst bed. The effluent gas from the reactor (unpermeated gas) was dried with an ice trap and a silica-gel drier, then analyzed with an on-stream Shimadzu GC-8A gas chromatograph (activated carbon, 1 m; Ar carrier) equipped with a thermal conductivity detector. No significant coke formation was observed in the experiment. Nitrogen (1.29 mol h1 or 0.063 mol cm2 h1 unless otherwise mentioned) was concurrently led inside the membrane (permeation side) to sweep out the hydrogen that permeated. The composition of the permeated gas was also analyzed with the same gas chromatograph using a flow-switch. Methane conversion and selectivities were calculated on the basis of carbon numbers. The mass balance was within the error of 5%.
3. Results and discussion 3.1. Catalytic activity under equilibrium condition The activities of catalysts were evaluated at 0.10 MPa without a membrane. At low space velocities, methane conversions with Ni–H were discernibly larger than the equilibrium conversion, assuming the reactions of CH4 + 2H2O = 4H2 + CO2 and CH4 + H2O = 3H2 + CO at 800 K (Fig. 2). The conversion decrease with an increase in the space velocity, while the selectivity to carbon monoxide was always lower than the equilibrium value. The activity of Ni–L was appreciably lower than that of Ni–H by 6% when the space velocity was less than 10,000.
Fig. 2. Activity of nickel catalysts for steam reforming of methane in equilibrium reactor at 800 K.
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Fig. 3. Steam reforming of methane with membrane reactor at 800 K. Space velocity, 1120 h1.
Fig. 5. Steam reforming of methane with membrane reactor at 800 K. Space velocity, 3360 h1.
3.2. Effect of catalytic activity to membrane reaction Although the equilibrium conversion of methane decreases with the reaction pressure, the conversion increased in the membrane reactor at a low space velocity of 1120 h1 (Fig. 3). The conversion with Ni–H was significantly higher than that with Ni–L by 20% at 0.10 MPa, showing that the catalytic activity affects the performance of the membrane reactor more than that of the equilibrium reactor. Since the higher conversion results in the higher hydrogen pressure at the reaction side, the hydrogen flux through the membrane with Ni–H was also higher than that with Ni–L (Fig. 4). In the cases of the space velocity at 3360 h1, the methane conversion with Ni–H at 0.20 MPa was smaller than that at 0.10 MPa, but it increased with an increase in the reaction pressure above 0.20 MPa (Fig. 5). The selectivity to carbon monoxide decreased with an increase in the reaction pressure (see Figs. 3 and 5), showing that the water-gas shift reaction, CO + H2O ! CO2 + H2, is also promoted by the removal of hydrogen in the membrane reaction.
The methane conversions at a space velocity of 3360 h1 were smaller than those at 1120 h1 (cf. Figs. 3 and 5), while the hydrogen pressures at the exit of reaction side were higher than the corresponding values at 1120 h1 (cf. Figs. 4 and 6). In Figs. 7 and 8, the methane conversions measured under different space velocities (1120–5600 h1 for Ni–H and 1120–3360 h1 for Ni–L) were plotted against the hydrogen separation ratios (the relative rate of hydrogen separation to the total production rate of hydrogen in the reaction). The higher space velocity resulted in the lower hydrogen separation ratio because the rate of hydrogen separation does not increase so much as the increase in the total production rate of hydrogen, which is related to the product of the space velocity times the methane conversion. The methane conversions produced with Ni–H at 0.10 MPa (open circles in Fig. 7) increased with an increase in the hydrogen separation ratio and were close to the equilibrium (solid curve), which is shifted by hydrogen separation; however, the conversions with Ni–L (solid circles) were clearly lower than the equilibrium, showing that the catalytic
Fig. 4. Permeation flux of hydrogen and partial pressure of hydrogen at reactor exit in steam reforming of methane with membrane reactor shown in Fig. 3.
Fig. 6. Permeation flux of hydrogen and partial pressure of hydrogen at reactor exit in steam reforming of methane with membrane reactor shown in Fig. 5.
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Fig. 7. Relationship between methane conversion and hydrogen separation ratio at 0.10 MPa.
Fig. 9. Effect of sweep gas on steam reforming of methane with membrane reactor at 800 K.
activity is insufficient to catch up with the equilibrium-shift. At 0.30 MPa, a similar tendency was observed (see Fig. 8). The partial pressure of the hydrogen decreases with an increase in the ratio due to the separation of hydrogen (see broken curves in Figs. 7 and 8).
than that measured without any sweep gas (Fig. 10, the broken line shows the flux without a sweep gas as described in Section 2 [22]). In the figure, p2 is the partial pressure of hydrogen at the exit of permeation side, while p1 was always 0.10 MPa. It is noteworthy that the palladium membrane employed was supported on porous stainless steel modified with cerium oxide particles. The result shows that diffusion of the sweep gas into the support is insufficient and that the actual hydrogen pressure on the surface of the palladium layer is significantly higher than that in the permeation gas. That is, an increase in the flux of the sweep gas does not effectively increase the hydrogen flux through the membrane; hence, the methane conversion is saturated at an excessive feed rate of the sweep gas. This shows that the reaction at a high pressure is more effective to increase the hydrogen flux than the employment of a large quantity of sweep gas, which is economically unfavorable (expensive) in the industrial process [24].
3.3. Effect of sweep gas to hydrogen separation rate Since the hydrogen flux through a palladium membrane depends on the pressure difference between the reaction side and the permeation side, a larger quantity of sweep gas that decreases the partial pressure of hydrogen on the permeation side results in the higher hydrogen separation rate and methane conversion. However, the effect was not as large as expected (Fig. 9), while no enhancement in the reaction took place without the sweep gas. To evaluate the performance of the membrane in presence of the sweep gas, we fed hydrogen to the membrane reactor without the catalyst by changing the flux of the sweep N2 gas. The hydrogen flux produced was considerably lower
Fig. 8. Relationship between methane conversion and hydrogen separation ratio at 0.30 MPa.
Fig. 10. Hydrogen flux through palladium membrane by feeding pure hydrogen in the presence of sweep gas. Broken line, the flux measured without sweep gas [22].
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Fig. 11. Plots of hydrogen flux vs. hydrogen partial pressure at reactor exit in membrane reactor. Open circles, Ni–H at 1120 h1; solid circles, Ni–H at 3360 h1; open squares, Ni–L at 1120 h1; solid squares, Ni–L at 3360 h1; solid triangles, hydrogen flux by feeding hydrogen diluted with argon at 0.10 MPa in the presence of sweep gas.
3.4. Effect of catalytic activity to hydrogen flux through membrane To obtain the relationship between the hydrogen pressure in the catalyst side and the hydrogen flux, we measured the hydrogen flux by feeding hydrogen diluted with argon to the membrane reactor at a total pressure of 0.10 MPa in the presence of the sweep gas (solid curve in Fig. 11, the pressure was the mean of the values at the entrance and the exit). On the basis of the data in Fig. 4, we plotted the hydrogen flux versus hydrogen pressure at the exit of reaction side in the reaction with Ni–H at 1120 h1 (open circles) in Fig. 11. The plots were close to the solid curve, showing that the mean hydrogen pressure in the reaction side is close to that at the exit. However, the fluxes at 3360 h1 were appreciably lower than expected from the exit hydrogen pressures (open squares on the basis of Fig. 6). Hence, the mean hydrogen pressure is lower than that at the exit, suggesting insufficient hydrogen production in the middle part of the catalyst layer. In the cases of Ni–L, the flux values at 1120 h1 were lower than those expected (solid circles). This shows that the hydrogen pressure in the middle part of the reactor is significantly lower than the exit pressure even at the low space velocity, and that the hydrogen production does not catch up with the hydrogen removal in the middle part. Thus, the catalytic activity is important for the large hydrogen flux as well as for the performance of the membrane itself.
4. Conclusions The catalytic activity to steam reforming of methane greatly affects the performance of the membrane reactor.
Separation of hydrogen through the palladium membrane shifts the equilibrium to the larger conversion of methane, but the less active catalyst produces the lower conversion than the values expected from the shifted equilibrium, which depends on the hydrogen separation ratio (hydrogen separation rate/total rate of hydrogen production). The higher space velocity of the reactant results in the lower conversion in the membrane reactor, mainly because the hydrogen separation ratio decreases with an increase in the space velocity. Although the active catalyst produces the methane conversion close to the equilibrium expected from the hydrogen separation ratio at the reactor exit, the conversion in the middle part of the catalyst layer is insufficient to catch up the equilibrium-shift at a high space velocity. Since the hydrogen flux through the membrane relates to the partial pressure of hydrogen in the reaction side, the lower conversion in the middle part of the catalyst layer results in the lower hydrogen flux. Thus, catalytic activity also affects the hydrogen flux through the membrane.
Acknowledgement The financial support of the New Energy and Industrial Technology Development Organization is gratefully acknowledged.
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