Hydrogen Recovery from a H2−H2O−HBr Mixture Utilizing Silica

Department of Materials Physics and Chemistry, Graduate School of Engineering, Kyushu University, .... silane (TEOS, ShinEtsu Chemical), was vaporized...
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Ind. Eng. Chem. Res. 1998, 37, 2502-2508

Hydrogen Recovery from a H2-H2O-HBr Mixture Utilizing Silica-Based Membranes at Elevated Temperatures. 1. Preparation of H2O- and H2-Selective Membranes Bong-Kuk Sea,† Eddy Soewito,† Midori Watanabe,‡ Katsuki Kusakabe,† Shigeharu Morooka,*,† and Sung Soo Kim§ Department of Materials Physics and Chemistry, Graduate School of Engineering, Kyushu University, Fukuoka, 812-8581 Japan, and Department of Environmental Science and Technology, Jisan Junior College, Pusan, 609-323 Korea

Amorphous silica-based membranes were prepared for use in recovering hydrogen at elevated temperatures from a H2-H2O-HBr mixture produced by a thermochemical water decomposition process (the UT-3 process). The silica-based membranes, which were formed on porous tubes by chemical vapor deposition of tetraethoxysilane at 650 °C with a forced cross-flow through the porous wall, exhibited a H2 or H2O selectivity, which was related to the preparation and posttreatment conditions. The microporous silica membrane formed on a macroporous R-alumina tube and heat-treated at 900 °C in an inert atmosphere showed a H2 permeance of (3-4) × 10-7 mol m-2 s-1 Pa-1 and a H2/H2O selectivity of 7-15 at permeation temperatures of 200-400 °C. The microporous silica membrane which was formed on a mesoporous γ-alumina layer coated on the R-alumina tube and then calcined in air showed a H2O permeance of the order of 10-6 mol m-2 s-1 Pa-1 and a H2O/H2 selectivity of 12-25 at 200-400 °C. This membrane rejected HBr at a H2O/HBr selectivity of 300-1000. 1. Introduction The thermochemical decomposition of water has the potential to meet the demand for hydrogen in the future, when process heat can be provided from solar energy collectors or high-temperature gas reactors. Although a number of thermochemical processes have been considered, most have been abandoned. The UT-3 process proposed by Yoshida and his collaborators (Kameyama and Yoshida, 1978; Aochi et al., 1989; Sakurai et al., 1996) represents a rare example of a continuously operated process. The reactions utilized in the UT-3 process are as described below:

CaBr2(s) + H2O(g) f CaO(s) + 2HBr(g)

(1)

CaO(s) + Br2(g) f CaBr2(s) + 1/2O2(g)

(2)

Fe3O4(s) + 8HBr(g) f 3FeBr2(s) + 4H2O(g) + Br2(g) (3) 3FeBr2(s) + 4H2O(g) f Fe3O4(s) + 6HBr(g) + H2(g) (4) All chemical intermediates are recycled internally, and water is the sole raw material. This scheme has a specific feature in which only gaseous reactants flow through four fixed-bed reactors, which are connected in series. The gaseous flows to and from the Ca reactors are interchanged when CaBr2 and CaO are consumed * To whom correspondence should be addressed. Fax: 8192-651-5606. E-mail: [email protected]. † Kyushu University. ‡ Kyushu University, Center for Advanced Instrumental Analysis. § Jisan Junior College.

Figure 1. A scheme describing the UT-3 process.

via reactions 1 and 2, respectively. The flows to and from the Fe reactors are similarly interchanged. As shown in Figure 1, hydrogen is separated from the H2H2O-HBr mixture at point A and oxygen is recovered from the O2-H2O mixture at point B. Steam is used as the reactant for hydrolysis in reactors 1 and 4, as well as the heat carrier for the entire system. Reactions 1 and 4 are endothermic, while reactions 2 and 3 are exothermic. Thus, the reaction Gibbs energies for reactions 2 and 3 are negative at the temperatures employed, and those of reactions 1 and 4 are positive. As a result, the hydrolysis of CaBr2 and FeBr2, especially that of the former, is controlled by chemical equilibria. All reactors used in the process are plug flow reactors, in which no backmixing of reactants occurs. Steam moves forward through reactor 1 and reacts with CaBr2 to produce CaO and HBr. The hydrolysis ceases at an axial position where the HBr concentration becomes higher than the value allowed by the chemical equilibrium, and the mixture of H2O and HBr then flows downstream through the reactor without further reaction. Thus, it is necessary to increase the flow rate of steam in reactors 1 and 4 to enhance the hydrolysis. Since excessive steam prevents reactions 2 and 3,

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however, the flow rate of steam in reactors 2 and 3 needs to be restricted within a value which is required as the heat carrier. Thus, steam is recycled through reactors 1 and 4, after H2 and HBr are separated at the exit of reactor 4 (point A). The concentration of oxygen at the outlet of reactor 2 is higher than that at the outlet of reactor 4, since the flow rate of steam through reactors 2 and 3 is less than that through reactors 1 and 4. Thus, the separation of oxygen from steam at point B, using an oxygen ion conducting membrane, is easier than that of hydrogen from steam and hydrogen bromide vapor at point A. For this reason, the separation of hydrogen at point A is the focus of this study. Ceramic membranes are preferable for this purpose. Polymeric and palladium membranes cannot be used because of the elevated temperature and the presence of corrosive HBr, respectively. A number of studies on ceramic membranes have been reported (Scott, 1995; Burggraaf and Cot, 1996; Krishna and Wesselingh, 1997). However, the majority of porous ceramic membranes used for gas separation are H2O-selective. Silicalite membranes (Sano et al., 1994, 1997) separated alcohols and carboxylic acids from water by a pervaporation mechanism at ambient temperature, but they could not be used in the separation of gases including steam. A membrane with a high zirconia content permeated hydrogen preferentially to steam, but the H2/ H2O selectivity was not reproducible (Ohya et al., 1994b). Only a silica membrane formed by the crossflow chemical vapor deposition (CVD) possessed H2/H2O and H2/HBr selectivities of 10 and 100, respectively (Morooka et al., 1996), although the H2 permeance of the membrane was less than 5 × 10-8 mol m-2 s-1 Pa-1. This study describes the preparation of highly H2- or H2O-permeable silica-based membranes on a porous R-alumina tube or a γ-alumina-coated porous R-alumina tube by chemical vapor deposition using tetraethoxysilane (TEOS). The membranes were examined with reference to hydrogen recovery for a mixture of H2H2O-HBr.

temperature of the reactor was increased from 30 to 650 °C at a rate of 15 °C min-1 and then maintained at that temperature for the duration of the experiment. When the macropores of the support were plugged with silica, the pressure inside the tube was reduced to 10-6000 Pa (referred to as Pfe). The value of Pfe was affected by the balance between rates of permeation and evacuation and was dependent on the reactor system and reactants used. However, Pfe served as a useful and practical indicator of the completeness of pore plugging. Details of this preparation have been reported previously (Yan et al., 1994; Morooka et al., 1995, 1996; Sea et al., 1996, 1997). In the case of the production of H2-selective membranes, silica membranes formed on the support tube were heat-treated in oxygen-free argon at 900 °C for 1 h, which removed surface hydroxy groups. In the case of production of H2O-selective membranes, silica membranes were formed on the γ-alumina-coated R-alumina tube and were calcined in air at 900 °C for 1 h, resulting in the expected formation of silica-alumina. Membrane morphology was examined with a field emission scanning electron microscope (SEM, Hitachi S-900). Gas Permeation Test. Gas permeation experiments were performed at 30-600 °C using single-component H2, N2, and H2O. Argon was used as the sweep gas on the permeate side, and ambient pressure was maintained on both sides of the membrane. The partial pressure of the feed gas on the permeate side was maintained at less than 2000 Pa by dilution with the sweep gas. Gas permeance tests were also performed for a mixture of H2-H2O-HBr in a molar ratio of 0.49: 0.50:0.01 at 30-400 °C unless otherwise stipulated. Tubings and connectors were fabricated from poly(tetrafluoroethylene) and were heated to 110 °C with ribbon heaters. Concentrations of H2, N2, and H2O were analyzed using a TCD-GC (Shimadzu, GC-8A). HBr in the permeate side was absorbed in water and determined with an inductively coupled plasma mass spectrometer (Yokogawa, ICP-MS 2000). Permeance to i component, ki, is defined as follows:

2. Experimental Section Membrane Preparation. A porous R-alumina tube (2.5-mm o.d. and 1.9-mm i.d.), having 110-180 nm pores, was used as the support. The permeating portion of the membrane was 10-15 mm in length, and the remaining portions were glazed with a SiO2-BaO-CaO sealant at 1100 °C. The γ-alumina was formed from a boehmite sol, prepared using procedures described by Yoldas (1975). The concentration of the boehmite sol was 0.6 mol L-1. The lower end of the support tube was plugged, and the outer surface of the unglazed portion was dipped in the boehmite sol for 2 min. After dipping, the membrane was air-dried overnight. It was then heated to 750 °C at a rate of 1 °C min-1. This dippingdrying-firing sequence was repeated for a total of three processings. The average pore size of the γ-alumina support tube was determined with a BET unit (Shimadzu, Micromeritics ASAP 2000) and determined to be 6-9 nm. The support tube was coaxially fixed in a quartz tube of 9.8-mm i.d. and 150-mm length and placed in an electric tubular furnace. The silicon source, tetraethoxysilane (TEOS, ShinEtsu Chemical), was vaporized at 40 °C and introduced into the reactor via a nitrogen carrier. The gas was continuously evacuated from the outside end of the tube with a rotary vacuum pump. The

ki )

(mole of i component permeated per unit time) (membrane area)(partial pressure difference) (5)

Selectivity of the i component to the j component was calculated from the ratio of ki to kj. 3. Experimental Results Morphology of Membranes. Figure 2 shows the fractured sections of silica membranes formed on the R-alumina tube with Pfe ) 500 Pa and on the γ-aluminacoated R-alumina tube with Pfe ) 20 Pa. The silicamodified layer was extended to depths of 500 and 100 nm for Figure 2a,b, respectively. No pinholes were detected of either of the membranes. Silica was actually deposited in the macropores of the R-alumina support or the mesopores of the γ-alumina layer coated on the R-alumina support (Yan et al., 1994). However, the silica top layer thickness was not greatly affected by the final evacuation pressure for the same support tube. The pore-plugging mechanism during CVD with a cross-flow has been reported previously (Morooka et al., 1995, 1996). H2-Selective Membranes. Silica, which contains no surface hydroxy groups, is essentially hydrophobic (Sano

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Figure 2. Fractured surfaces of membranes formed on (a) an R-alumina tube and (b) a γ-alumina-coated R-alumina tube.

Figure 3. Effect of final evacuation pressure on H2 permeance and H2/N2 selectivity for membranes formed on the R-alumina tube. Single-component gases, permeation temperature: (4, 2) 200 °C; (O, b) 400 °C; (3, 1) 600 °C.

et al., 1994, 1997) and suitable as a material for H2selective membranes. Figure 3 shows permeances to single-component H2 and N2, as well as H2/N2 selectivity, for the silica membranes formed on the R-alumina tube with Pfe ) 10-2000 Pa. The membranes prepared with Pfe ) 200-2000 Pa showed H2 and N2 permeances, which were nearly independent of the permeation temperature of 30-600 °C. The H2/N2 selectivity of the membranes was 4-5, which was close to the value estimated from the Knudsen diffusion mechanism, 3.7. The H2 permeance was higher than 10-7 mol m-2 s-1 Pa-1. In addition, the silica membranes prepared with Pfe ) 10-50 Pa permeated hydrogen by an activated diffusion mechanism, and the H2 permeance increased with increasing permeation temperature. The permeance to N2 was unaffected by permeation temperature. The H2/N2 selectivity was higher than 100 at permeation temperatures of 400-600 °C. The membranes prepared with Pfe ) 10 and 20 Pa did not permeate nitrogen within the limit of detection used in this study. Figure 4 shows permeances to H2, H2O, and HBr from the H2-H2O-HBr mixture for the membranes formed on the R-alumina tube with Pfe ) 500, 1000, and 2000 Pa. The heat treatment at 900 °C in the inert atmosphere improved the H2/H2O selectivity of the silica membrane by removing surface hydroxy groups. The heat treatment decreased the H2 permeance by sintering micropores in the silica structure, but the extent of this was not substantial. The concentration of HBr was

varied over the range 0.05-5 mol L-1, where the H2 permeance of the membranes was determined. The results show that HBr concentration had no effect on the permeation properties. Figure 5 shows H2, H2O, and HBr permeances from the H2-H2O-HBr mixture for the membranes prepared on the R-alumina tube with Pfe ) 20, 50, and 100 Pa. In this case, no heat treatment was applied. The membrane prepared with Pfe ) 20 Pa permeated H2 at a permeance of 3 × 10-9 mol m-2 s-1 Pa-1 at 400 °C and permeated no water within a detection limit of 10-10 mol m-2 s-1 Pa-1. Figure 6 shows the relationship between H2 permeance and H2/H2O selectivity at 400 °C, using the H2H2O-HBr mixture as the feed, for the H2-selective membranes formed on the R-alumina tube. The permeation properties of the membranes produced in the present study can be divided into the following three regions. Region I: H2 permeance is of the order of 10-7 mol m-2 s-1 Pa-1, and H2/H2O selectivity is 3-6. Molecules which are inert to the pore walls permeate through the silica membrane by the Knudsen diffusion mechanism. Hydrogen interacts to some extent with the pore walls, and the H2/H2O selectivity then exceeds the value of the Knudsen diffusion mechanism. Heat treatment in an inert atmosphere increases the surface diffusion of hydrogen molecules in the pores as indicated by the open keys in Figure 6. Region II: The pore sizes are close to the sizes of H2 and H2O molecules, and these molecules may permeate by a single-file diffusion mechanism. The H2/H2O selectivity is then less than the Knudsen diffusion value. Region III: The H2 permeance is low, but the H2/H2O selectivity is high. As shown in Figure 3, the H2/N2 selectivity for singlecomponent systems is approximately 100. Thus, the membrane appears to be capable of recognizing the sizes of hydrogen and water. H2O-Selective Membranes. Figure 7 shows permeances to single-component H2, H2O, and N2 for the membranes formed on the γ-alumina layer. The H2 and N2 permeances increased with increasing Pfe, and these gases were transported by the Knudsen diffusion mechanism, except for the membrane formed with Pfe ) 100 Pa. However, the permeance to H2O differed from that to H2 and N2 and decreased with increasing Pfe. For the membrane formed with Pfe ) 2000 Pa, the H2/H2O selectivity was approximately 3, which is consistent with the Knudsen diffusion mechanism. The membranes prepared with Pfe ) 100-1000 Pa was H2O-selective, and the membrane formed with Pfe ) 100 Pa showed an especially high H2O/H2 selectivity of approximately 100 at permeation temperatures below 200 °C. This suggests that H2O is transported by the surface diffusion mechanism and that the permeation rate increases with decreasing pore size (i.e., increasing surface area). Figure 8 shows H2, H2O, and HBr permeances for the H2-H2O-HBr mixed system. No differences between single- and mixed-component permeances were observed for the membrane prepared with Pfe ) 2000 Pa. For the membranes formed with Pfe ) 100 and 500 Pa, however, the H2O permeance was decreased, while the H2 and HBr permeances were not greatly altered, by the feed of the mixture. The membrane prepared with Pfe ) 100 Pa was H2O-selective. The effect of Pfe on the H2/H2O selectivity of the silica membranes formed on the mesoporous γ-alumina layer for the mixed feed

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Figure 4. Permeances for membranes formed on the R-alumina tube. Mixed feed: (a) without heat treatment; (b) after heat treatment. (4, 2) H2O; (O, b) H2; (0, 9) HBr.

Figure 6. Relationship between H2/H2O selectivity and H2 permeance for membranes formed on the R-alumina tube. Mixed feed at 400 °C. Figure 5. Permeances for membranes formed on the R-alumina tube. Mixed feed: without heat treatment. (4, 2) H2O; (O, b) H2; (0, 9) HBr.

system is similar to that for the single-component feed systems shown in Figure 7. The H2O selectivity of the silica membranes prepared on the γ-alumina-coated R-alumina support tube can be explained by the formation of acidic sites as a result of interaction between silica and alumina (Sano et al., 1997). To enhance the formation of silica-alumina, the silica membranes prepared on the γ-alumina layer with Pfe ) 100 and 2000 Pa were calcinated in air at 900 °C for 1 h. As indicated in Figure 8a, the calcination increased the H2O permeance by more than 1 order of magnitude to 2 × 10-6 mol m-2 s-1 Pa-1. The H2O/H2 selectivity of the membrane formed with Pfe ) 100 Pa was then increased to 25 at a permeation temperature of 400 °C. This membrane rejected HBr at a H2O/HBr selectivity of 300-1000, apparently because of the large size of hydrated Br- ions. The permeances of the membrane formed at 2000 Pa were not improved by calcination, since permeation through this membrane

was basically controlled by the Knudsen diffusion mechanism. Sea et al. (1996) also prepared silica membranes on the γ-alumina-coated R-alumina tube. A membrane prepared with Pfe ) 50 Pa was H2O-selective at permeation temperatures of 200-300 °C, but its H2O permeance was 10-7 mol m-2 s-1 Pa-1, which was 1 order of magnitude lower than that of the calcinated membrane shown in Figure 8a. Thus, both H2O permeance and H2O/H2 selectivity were greatly improved by optimizing the pore size and membrane thickness and by calcinating the as-formed membrane, compared to those reported in our previous study (Sea et al., 1996). Figure 9 indicates the relationship between H2/H2O selectivity and H2 permeance at 400 °C for the membranes formed in mesopores of the γ-alumina layer which was coated on the R-alumina tube. The H2/H2O selectivity depends on the preparation conditions of the membranes. The membrane formed with Pfe ) 2000 Pa possesses mesopores belonging to the Knudsen diffusion regime and is H2-selective. The membrane formed with Pfe ) 100 Pa possesses micropores where the surface diffusion mechanism prevails and is H2O-selective. When the final evacuation pressure is lower than 40 Pa, the membrane becomes H2-selective, consistent with

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Figure 7. Effect of final evacuation pressure on permeances for membranes formed on the γ-alumina layer. Single-component gases, without calcination: (4), H2O; (O) H2; (0) N2.

Figure 8. Effect of final evacuation pressure on permeances for membranes formed on the γ-alumina layer. Mixed feed: (4, 2) H2O; (O, b) H2; (0, 9) HBr. Open keys, without calcination; closed keys, after calcination.

our previous report (Sea et al., 1996). Membranes with pores in which molecules cannot pass one another are permselective to smaller molecules (i.e., H2 in this case). Thus, H2O selectivity is realized when pores possess abundant surface hydroxy groups and, at the same time, when the pores permit molecules to pass one another. To increase H2O selectivity, the formation of acidic silica-alumina was maximized by the following procedure: A silica membrane was formed by CVD with Pfe ) 6000 Pa on the mesoporous γ-alumina layer. The membrane was somewhat H2O-selective for the feed of the H2-H2O-HBr mixture, as shown by the open keys in Figure 10. It was then coated with the boehmite sol, dried at room temperature, and calcinated in air at 900 °C for 1 h. After the membrane was coated with boehmite, H2 permeance was greatly decreased, while H2O permeance was slightly decreased, as shown by the closed keys in Figure 10. The test was repeated using another membrane formed separately under the same conditions. The H2O/H2 selectivity was higher at lower permeation temperatures and was 10-400 (H2/H2O selectivity ) 0.0025-0.1) in the range of 120-200 °C. 4. Discussion Durability of H2-Selective Membranes. The thermal and chemical stability of a membrane is important in terms of practical applications. Thus, the stability of the prepared silica membranes was tested using the H2-H2O-HBr mixture at 400 °C. Figure 11 shows changes in permeances to H2 and H2O in the H2-H2O-

Figure 9. Relationship between H2/H2O selectivity and H2 permeance for membranes formed on the γ-alumina layer. Mixed feed at 400 °C. Open keys, without calcination; closed keys, after calcination.

HBr mixture for the silica membrane prepared with Pfe ) 1000 Pa with the subsequent heat treatment. The H2 and H2O permeance of the membrane formed with

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densation mechanism. When water is condensed in a pore whose wall is hydrophilic, the pressure exerted by the condensation, ∆Pc, is expressed by

∆Pc ) 4σ/dp

Figure 10. H2 and H2O permeances for membranes which were formed on the γ-alumina layer, coated with the boehmite sol and calcinated in air. Mixed feed. (4, 2) H2O permeance for membrane 1; (O, b) H2 permeance for membrane 1; (1) H2O permeance for membrane 2; (9) H2 permeance for membrane 2.

(6)

where σ is the surface tension and dp is the pore diameter. The surface tension of water decreases from 75.6 mN m-1 at 0 °C to 58.9 mN m-1 at 100 °C under atmospheric pressure, but values at higher temperatures and pressures are not available. If the surface tension of a H2O-HBr mixture is assumed to be of the order of 10 mN m-1, the pressure in a hydrophilic pore of, for instance, dp ) 4 nm is 10 MPa, which is equivalent to the pressure of saturated steam at approximately 310 °C. In pores of that size, if the pore wall is hydrophilic, water would be expected to exist as the liquid at temperatures below 310 °C. Thus, the occlusion of hydrogen may be realized when membranes with mesopores are used at temperatures lower than the limit of eq 6, as well as the critical temperature of water, 374 °C. The membranes shown in Figure 10 are for the case described above, although the reproducibility is not high. The presence of large pores substantially decreases the maximum temperature range where capillary condensation occurs. A membrane which possesses mesopores of a narrow size distribution may permit the capillary condensation to occur beyond the temperature of reactor 3, 220 °C. 5. Conclusions

Figure 11. Stability of membranes exposed to the H2-H2O-HBr mixture at 400 °C. Solid and broken lines, silica membrane, and SiC-based membrane formed on the R-alumina support and heattreated at 900 °C in an inert atmosphere, respectively.

Pfe ) 1000 Pa gradually decreased, although the H2/ H2O selectivity remained unchanged. Figure 11 also shows the durability of a SiC-based membrane, which was formed on the porous R-alumina tube by CVD using triisopropylsilane (Sea et al., 1998). The SiC deposition was continued at 750 °C until Pfe reached 1000 Pa, and the membrane was heat treated at 1000 °C for 1 h in a flow of deoxidized argon. The SiC-based membrane was more stable than the silica membrane in the H2-H2OHBr mixture at 400 °C. As shown in Figure 1, hydrogen should be separated at point A in the temperature range of 220-560 °C. Figure 4 shows that the permeances to H2 and H2O were not affected by permeation temperatures in the range of 200-400 °C. The H2-selective silica and SiC-based membranes may be resistant to steam for a longer period when they are used at the temperature of reactor 3, 220 °C. Capillary Condensation. Water was not condensed in the pores of the membranes shown in Figure 8, since the permeances were barely altered when the permeation temperature exceeded the critical temperature of water. However, Ohya et al. (1994a) reported that a porous zirconia-silica composite membrane permeated H2O and HBr and blocked H2 from a H2-H2O-HBr mixture at elevated temperatures by a capillary con-

Silica-based membranes were formed in a porous alumina tube by chemical vapor deposition using tetraethoxysilane at 650 °C. The membranes were modified so as to acquire H2 or H2O selectivity and were applied to hydrogen recovery at 220-560 °C from the H2-H2O-HBr mixture produced by the UT-3 process. The silica membranes which were formed on the porous R-alumina support tube were H2-selective. The H2 selectivity was improved by heat treatment in an inert atmosphere. However, the silica membranes which were formed on the γ-alumina-coated R-alumina support were dependent on formation conditions. The membranes formed with Pfe values lower than 20-40 Pa were H2-selective, suggesting that the membranes formed with low Pfe values contained micropores which permeated H2 and retarded H2O, based on molecular size. The membranes formed with Pfe ) 100-500 Pa were H2O-selective. Acknowledgment This work was supported by the Ministry of Education, Science, Sports and Culture, Japan (Grant-in-Aid for Scientific Research on Priority Area “Principle of Exergy Regeneration”) and by the New Energy and Industrial Technology Development Organization (NEDO). We express our gratitude to NOK Corp. for supplying the R-alumina support tubes. Literature Cited Aochi, A.; Tadokoro, T.; Yoshida, K.; Kameyama, H.; Nobue, M.; Yamaguchi, T. Economical and technical evaluation of UT-3 thermochemical hydrogen production process for an industrial scale plant. Int. J. Hydrogen Energy 1989, 14, 421-429.

2508 Ind. Eng. Chem. Res., Vol. 37, No. 6, 1998 Burggraaf, A. J.; Cot, L. Membrane Science and Technology Series 4, Fundamentals of Inorganic Membrane Science and Technology; Elsevier: Amsterdam, 1996. Kameyama, H.; Yoshida, K. Br-Ca-Fe water decomposition cycles for hydrogen production. Proc. 2nd World Hydrogen Energy Conf. Zurich 1978, 829-850. Krishna, R.; Wesselingh, J. A. The Maxwell-Stefan approach to mass transfer. Chem. Eng. Sci. 1997, 52, 861-911. Morooka, S.; Yan, S.; Kusakabe, K.; Akiyama, Y. Formation of hydrogen-permselective SiO2 membrane in macropores of R-alumina support tube by thermal decomposition of TEOS. J. Membr. Sci. 1995, 101, 89-98. Morooka, S.; Kim, S.-S.; Yan, S.; Kusakabe, K.; Watanabe, M. Separation of hydrogen from an H2-H2O-HBr system with an SiO2 membrane formed in macropores of an R-alumina support tube. Int. J. Hydrogen Energy 1996, 21, 183-188. Ohya, H.; Nakajima, H.; Togami, N.; Aihara, M.; Negishi, Y. Separation of hydrogen from thermochemical process using zirconia-silica composite membrane. J. Membr. Sci. 1994a, 97, 91-98. Ohya, H.; Hisamatsu, T.; Sato, S.; Negishi, Y. Hydrogen purification of thermochemically decomposed gas using zirconia-silica composite membrane. Int. J. Hydrogen Energy 1994b, 19, 517521. Sakurai, M.; Bilgen, E.; Tsutsumi, A.; Yoshida, K. Adiabatic UT-3 thermochemical process for hydrogen production. Int. J. Hydrogen Energy 1996, 21, 865-870. Sano, T.; Yamaguchi, H.; Kiyozumi, Y.; Mizukami, F.; Haraya, K. Separation of ethanol/water mixture by silicalite membrane on pervaporation. J. Membr. Sci. 1994, 95, 221-228.

Sano, T.; Ejiri, S.; Yamada, K.; Kawakami, Y.; Yagishita, H. Separation of acetic acid-water mixtures by pervaporation through silicalite membranes. J. Membr. Sci. 1997, 123, 225233. Scott, K. Handbook of Industrial Membranes; Elsevier: Oxford, 1995. Sea, B.-K.; Watanabe, M.; Kusakabe, K.; Morooka, S.; Kim, S.-S. Formation of hydrogen permselective silica membrane for elevated temperature hydrogen recovery from a mixture containing steam. Gas Sep. Purif. 1996, 10, 187-195. Sea, B.-K.; Kusakabe, K.; Morooka, S. Pore size control and gas permeation kinetics of silica membranes by pyrolysis of phenylsubstituted ethoxysilanes with cross-flow through a porous support wall. J. Membr. Sci. 1997, 130, 41-52. Sea, B.-K.; Ando, K.; Kusakabe, K.; Morooka, S. Separation of hydrogen from steam using a SiC-based membrane formed by chemical vapor deposition of triisopropylsilane. J. Membr. Sci. 1998, in press. Yan, S.; Maeda, H.; Kusakabe, K.; Morooka, S. Hydrogen-permselective SiO2 membrane formed in pores of an alumina support tube by CVD with TEOS. Ind. Eng. Chem. Res. 1994, 33, 20962101. Yoldas, B. E. Alumina sol preparation from alkoxides. Ceram. Bull. 1975, 54, 289-290.

Received for review October 20, 1997 Revised manuscript received March 5, 1998 Accepted March 11, 1998 IE970730O