Dehydrogenation of Methylcyclohexane To Produce High-Purity

Oct 1, 2010 - These results indicate that the residence time was sufficient to reach ...... A.; Satoh , K.; Nanba , T. Hydrogen recovery from cyclohex...
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Ind. Eng. Chem. Res. 2010, 49, 11287–11293

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Dehydrogenation of Methylcyclohexane To Produce High-Purity Hydrogen Using Membrane Reactors with Amorphous Silica Membranes Kazunori Oda,† Kazuki Akamatsu,*,†,| Takashi Sugawara,† Ryuji Kikuchi,† Atsushi Segawa,‡ and Shin-ichi Nakao§ Department of Chemical System Engineering, The UniVersity of Tokyo, 7-3-1 Hongo, Bunkyo-ku, Tokyo 113-8656, Japan, Hydrogen & New Energy Research Laboratory, Nippon Oil Corporation, 8, Chidori-cho, Naka-ku, Yokohama, 231-0815, Japan, Department of EnVironmental and Energy Chemistry, Faculty of Engineering, Kogakuin UniVersity, 2665-1 Nakano-machi, Hachioji-shi, Tokyo 192-0015, Japan

We developed a membrane reactor that can produce high-purity hydrogen in one step from methylcyclohexane. This membrane reactor combined a hydrogen-selective amorphous silica membrane prepared with dimethoxydiphenylsilane and oxygen and employing counter-diffusion chemical vapor deposition, and Pt/Al2O3 catalyst. The silica membrane showed excellent hydrogen permeance at 573 K of the order of 10-6 mol m-2 s-1 Pa-1 and high hydrogen/sulfur hexafluoride permselectivity of around 104. The membrane reactor exhibited equilibrium shifts as expected under reaction temperatures ranging from 473 to 553 K and reaction pressures ranging from 0.1 to 0.25 MPa, and these performances were successfully predicted using a simulation model, which was also developed in this study. Finally, we demonstrated that hydrogen with purity as high as 99.95% was produced from methylcyclohexane in the membrane reactor without using carrier gas or sweep gas. 1. Introduction Hydrogen has received increasing attention as a promising clean secondary energy source. However, there are many difficulties in storing and transporting hydrogen because hydrogen is a gas at standard temperature and pressure. Many studies have been conducted on materials and methods for hydrogen storage and transportation; nevertheless, further examination is required to establish such techniques. Among materials, organic chemical hydrides, such as cyclohexane, methylcyclohexane, and decaline, are promising candidates for hydrogen storage materials.1 Organic chemical hydrides could be used as hydrogen carriers through hydrogenation reactions, and hydrogen could be obtained from organic chemical hydrides via dehydrogenation reactions. Moreover, the dehydrogenated organic chemical hydrides could be used again as hydrogen carriers after hydrogenation. Organic chemical hydrides have large hydrogen storage capacities, and they are easy to handle, just like the handling of gasoline, because they are liquids under normal conditions. To obtain pure hydrogen from organic chemical hydrides and recycle the hydrides as hydrogen carriers, it is necessary to separate hydrogen from organic gas formed in the dehydrogenation of organic chemical hydrides. There have been many reports on hydrogen production from organic chemical hydrides;1-12 however, the separation of hydrogen from other gases remains an unsolved problem. In addition, from the viewpoint of energy efficiency, it should be noted that it is preferable to obtain high-purity hydrogen from organic chemical hydrides in one step. This study proposes the application of membrane reactors to the dehydrogenation of organic chemical hydrides to obtain hydrogen of higher purity. The dehydrogenation reactions of * To whom correspondence should be addressed. Tel.: +81-42-6284584. Fax: +81-42-628-4542. E-mail: [email protected]. † The University of Tokyo. ‡ Nippon Oil Corp. § Kogakuin University. | Present address: Department of Environmental and Energy Chemistry, Faculty of Engineering, Kogakuin University, 2665-1 Nakanomachi, Hachioji-shi, Tokyo 192-0015, Japan.

organic chemical hydrides are endothermic and equilibrium reactions; therefore, using membrane reactors, conversions in dehydrogenation reactions of organic hydrides exceed equilibrium conversions as the produced hydrogen is removed from the reaction side. Furthermore, high-purity hydrogen is obtained from the permeation side at the same time. In this way, the dehydrogenation reaction and separation are carried out in one step in membrane reactors, and, therefore, the use of membrane reactors could make the process more efficient and solve the unsolved problems. In fact, palladium membranes,13-19 zeolite membranes,20 porous Vycor membrane,21 and γ-alumina membrane22 have been applied to the dehydrogenation of organic chemical hydrides in a membrane reactor. The availability of membrane reactors was also demonstrated for other equilibrium reactions such as the dehydrogenation of hydrogen sulfide23 and steam-reforming of methane.24 However, in general, zeolite membranes and γ-alumina membrane have low selectivity of hydrogen-hydride mixtures, and the porous Vycor membrane has low hydrogen permeance. Palladium membranes have both high hydrogen permeance and high selectivity of hydrogen-hydride mixtures; however, palladium is an expensive rare metal, and its total reserves are insufficient for such use. From the viewpoint of practical use, economical materials that have sufficient total reserves are desirable for membranes. Therefore, silica is a major candidate material for membranes. Silica membranes have both high hydrogen permeance and high selectivity of hydrogen.25-28 Recently, we carried out the dehydrogenation of cyclohexane, as one of the organic chemical hydrides, using silica-derived membrane reactors,29 and produced high-purity hydrogen of 99.9%.30 However, for some applications, hydrogen of higher purity is needed. Moreover, from a transporting viewpoint, hydrogen carriers in a liquid state at ordinary temperature and pressure are preferable. The melting points of cyclohexane and benzene are around 280 K; therefore, they are frozen in cold regions. On the other hand, the methylcyclohexane-toluene system has several merits. First, the selectivity of hydrogenmethylcyclohexane mixtures through silica membranes would be higher than that of hydrogen-cyclohexane mixtures because

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methylcyclohexane is larger than cyclohexane. Second, methylcyclohexane and the reaction product toluene are liquid at standard temperature and pressure, and they have sufficiently low melting points not to freeze even in cold regions. Third, the equilibrium conversion of methylcyclohexane is higher than that of cyclohexane at a certain temperature; therefore, hydrogen could be produced from methylcyclohexane at lower temperature. Fourth, the carcinogenic material benzene, which was detected in small amounts on the permeate side when using a membrane reactor for the dehydrogenation of cyclohexane in our previous study, is not produced in the dehydrogenation of methylcyclohexane. In this study, we develop membrane reactors to produce hydrogen from methylcyclohexane using silica membranes. The silica membranes were prepared employing the counter-diffusion chemical vapor deposition (CVD) method. To investigate the merits of the membrane reactors, the dehydrogenation of methylcyclohexane was examined with carrier gas and sweep gas at a reaction temperature ranging from 473 to 553 K and reaction pressure ranging from 0.1 to 0.25 MPa. Moreover, we demonstrate the production of hydrogen with purity higher than 99.9% from methylcyclohexane in a membrane reactor without using carrier gas or sweep gas. 2. Experimental Section 2.1. Membrane Preparation and Permeation Measurements. A porous R-alumina tube (diameter of 6.3 mm, length of 330 mm) with 70 nm pores was kindly supplied by Noritake Co. Ltd., Japan, and used as the substrate. Effective membrane area was a center of the substrate (50 mm), and other parts were glazed with a sealant. A γ-alumina layer was coated on the effective membrane area of a substrate to reduce the pore size, according to the report of Yoshino et al.31 The effective area of the substrate was dipped in boehmite sol for 5 s, dried in air, and then calcined at 873 K for 3 h. This coating was repeated twice. The pore size of the γ-alumina layer was found to be around 4 nm through permporometry measurements. An amorphous silica layer was deposited on the γ-alumina layer of the substrate by counter-diffusion CVD. Dimethoxydiphenylsilane (DMDPS) was purchased from Shin-Etsu Chemical Co. Ltd., Japan, and used as the silica precursor. Substrates coated with γ-alumina layers were placed in the membrane fabrication module with a furnace attachment, and DMDPS was supplied through the exterior of the substrate from a bubbler with 200 mL min-1 N2 as a carrier gas. The bubbler temperature was maintained at 433 K. O2 was supplied to the interior of the substrate at a rate of 200 mL min-1. The substrate temperature was measured by a thermocouple and controlled at 873 K by a temperature regulator. The CVD time was 1 h. Permeation measurements were conducted at 373, 473, and 573 K using single-component hydrogen, nitrogen, and sulfur hexafluoride. The permeance of hydrogen and nitrogen gas was measured using a bubble flow meter (Horiba Co. Japan), and that of sulfur hexafluoride was determined employing the pressure difference method. 2.2. Membrane Reactor to Dehydrogenate Methylcyclohexane. A schematic diagram of the membrane reactor apparatus is shown in Figure 1. Pt/Al2O3 particles (1 wt %) were used as catalysts. Aqueous platinum solution was prepared using hydrogen hexachloroplatinate hexahydrate, and alumina particles (reference catalyst ALO-6, kindly supplied by the Catalysis Society of Japan) with a size of 500-710 µm were impregnated with solution heating at 353 K in a water bath for 3 h. The particles were then dried at 588 K and calcined at 773 K for

Figure 1. Schematic diagram of the membrane reactor apparatus.

4 h. The catalysts were loaded into the tubular membrane and reduced in hydrogen at 673 K. As a feed material, methylcyclohexane was sent to a vaporizer and carried to the membrane. The conversion of methylcyclohexane was determined from the concentration ratios of unreacted methylcyclohexane, and the conversion produced toluene and benzene at the outlet gas of the feed side, the concentrations of which were measured by a gas chromatograph (GC-14B, Shimadzu Corp., Japan). First, the dehydrogenation of methylcyclohexane in a packedbed reactor was examined to determine the reaction rate constant of this reaction. According to Ali et al.,32 the reaction rate can be described as follows:

{

rd ) k1FPMCH 1 -

PTOLPH3 KeqPMCH

}

(1)

The flow rate of the methylcyclohexane feed was 1.31 × 10-6 to 6.53 × 10-6 mol s-1, and that of the nitrogen carrier gas was 1.49 × 10-5 to 7.44 × 10-5 mol s-1. Experimental temperatures ranged from 473 to 553 K, and the experimental pressure was 0.1 MPa. Second, the dehydrogenation of methylcyclohexane in a membrane reactor using carrier gas and sweep gas was carried out. N2 gas was used as the carrier gas and sweep gas. To investigate the effect of hydrogen extraction, a blind tube and the DMDPS-derived membrane were used. The flow rate of the methylcyclohexane feed was 1.31 × 10-6 mol s-1, that of the carrier gas was 3.72 × 10-6 mol s-1, and that of the sweep gas was 15 mL min-1. To investigate the performances of the reactors, various temperature conditions ranging from 473 to 553 K and pressure conditions ranging from 0.1 to 0.25 MPa were used. The production of hydrogen from methylcyclohexane in the membrane reactor without using carrier gas or sweep gas was also examined under the following conditions. The DMDPSderived membrane was used. The methylcyclohexane was supplied at 1.31 × 10-5 mol s-1, the experimental temperature was 573 K, and the experimental pressure was 0.3 MPa. The purity of hydrogen obtained from the permeation side was determined from the concentrations of the contamination gases (methylcyclohexane, toluene, benzene, and methane) on the permeation side, which were measured by the gas chromatograph. The cycle of startup-operation (6 h)-shutdown was repeated three times in a reaction test.

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Table 1. H2, N2, and SF6 Permeances through DMDPS-Derived Membranes at 473 and 573 Ka

membrane 1 membrane 2

473 K 573 K 473 K 573 K

H2 permeance [mol m-2 s-1 Pa-1]

N2 permeance [mol m-2 s-1 Pa-1]

SF6 permeance [mol m-2 s-1 Pa-1]

6.7 × 10-7 6.9 × 10-7 1.3 × 10-6 1.3 × 10-6

8.0 × 10-9 1.0 × 10-8 4.6 × 10-8 4.6 × 10-8

9.2 × 10-11 1.0 × 10-10 3.5 × 10-11 3.5 × 10-11

a Dehydrogenation reaction of methylcyclohexane was carried out in membrane reactors using these membranes. Membrane 1 was used in a membrane reactor for the dehydrogenation of methylcyclohexane using carrier gas and sweep gas. Membrane 2 was used in a membrane reactor for hydrogen production from methylcyclohexane without using carrier gas or sweep gas.

Figure 2. Model of the membrane reactor in the simulation.

3. Simulation Model The membrane reactor was composed of a cylindrical stainless-steel tube and tubular hydrogen-selective membrane. The membrane was coaxially fixed inside the cylindrical tube. Figure 2 is a conceptual illustration of a membrane reactor. The inner side (reaction side) of the membrane was filled with catalyst particles. The outer side of the membrane was the permeation side. Feed gas and permeate gas flowed in the same direction. This simulation model was based on the following assumptions. (1) The reactor is isothermal. (2) The gas flows on both the reaction side and the permeation side are plug flows. (3) There is no radial distribution of pressure in the membrane reactor. (4) The diffusions in both the radial direction and the flow direction can be ignored. (5) The gas permeation through the membrane is proportional to the difference between the partial pressures on the reaction side and permeation side. (6) Dehydrogenation occurs only on the reaction side. The possibility that assumption (1) is not valid cannot be denied because the enthalpy change of this reaction is comparatively large, but in this study we accepted the assumption to develop the simple model based on the viewpoints of chemical engineering, without using the CFD technique. The mass balance of the reaction side and permeation side is described by the following equations: dNi,R ) VirdSR - 2πrQi(PRxi - PPyi) dl

(2)

dNi,P ) 2πrQi(PRxi - PPyi) dl

(3)

Equation 1 was used as the reaction rate equation, with the reaction rate constant decided employing the method mentioned above. 4. Results and Discussion 4.1. Permeance of a Single Gas through the SilicaDerived Membrane. The single-gas permeances of hydrogen, nitrogen, and sulfur hexafluoride are summarized in Table 1. In this study, two membranes were prepared. Membrane 1 was used for the tests of the temperature and pressure dependency, and membrane 2 was used for the continuous operation test described below. The permeation characteristics between membrane 1 and membrane 2 seemed to be slightly different from

Figure 3. Arrhenius plot of H2, N2, and SF6 permeances for the DMDPSderived membrane.

each other. However, hydrogen permeance through the DMDPSderived membrane would change so easily, especially in the range from 7 × 10-7 to 1 × 10-6 mol m-2 s-1 Pa-1. Now we think that the atmospheric condition before the permeation test is one of the reasons, and this would also bring the effect on the permeance of nitrogen or sulfer hexafluoride. In this study, we regarded that the two membranes showed similar performance. Figure 3 is an Arrhenius plot of single-gas permeance through membrane 2. The figure shows that the single-gas permeances of the three gases were virtually constant at temperatures ranging from 373 to 573 K. The permeance of hydrogen was around 10-6 mol m-2 s-1 Pa-1, that of nitrogen was around 5 × 10-8 mol m-2 s-1 Pa-1, and that of sulfur hexafluoride was less than 10-10 mol m-2 s-1 Pa-1. The selectivity of hydrogen/sulfur hexafluoride, which was regarded as an indicator of permeation characteristics of methylcyclohexane (kinetic diameter 0.60 nm33) or toluene (kinetic diameter 0.59 nm33), was around 104 in this temperature range. These performances had the same tendencies as those found in our previous study,27 in which we reported the performance of the DMDPS-derived membranes prepared employing the one-side diffusion CVD method. On the other hand, Table 1 and Figure 3 present results for the membrane derived with the same precursor but prepared employing the counter-diffusion CVD method. We can say that the DMDPS-derived membrane prepared employing the counter-diffusion CVD method, as well as that prepared employing the one-side diffusion CVD method, has excellent performance, probably because of the formation of the same membrane structures despite the different preparation method. 4.2. Dehydrogenation Reaction Rate Constant for Methylcyclohexane. Using the simulation model under the assumption that the permeances of all gases are zero and the experimental data of the reaction in the packed-bed reactor under various experimental conditions are as mentioned above, the

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Figure 4. Arrhenius plot of the dehydrogenation reaction rate constant for methylcyclohexane.

Figure 5. Relationships between reaction temperature and methylcyclohexane conversion for the packed-bed reactor and the DMDPS-derived membrane reactor.

reaction rate constant k1 was determined for each condition. The Arrhenius plot of the reaction rate constant of this reaction is shown in Figure 4. All data taken from 473 to 553 K in Figure 4 fall on the same line; thus, it seems that the mechanism of reaction is the same in this temperature range, and we consider that the preexponential factor and activation energy can be decided uniquely. For this temperature range from 473 to 553 K, we determine an approximate expression as follows: ln k1 ) 11.4 -

13 500 T

(4)

R2 for the above relationship was over 0.99. From this result, the dehydrogenation reaction rate constant of methylcyclohexane was determined as follows.

(

k1 ) exp 11.4 -

112 000 RT

)

(5)

The activation energy determined from our experimental results coincided well with the activation energy proposed by Ali et al.;32 therefore, our reaction rate constant is valid. We used this reaction rate constant in the simulation, and the simulation results were compared to experimental results. 4.3. Dehydrogenation of Methylcyclohexane Using the Membrane Reactor at 0.1 MPa. Figure 5 shows the relationship between the reaction temperature and methylcyclohexane conversion for two reactors: the packed-bed reactor and the membrane reactor using membrane 1. The reaction pressure was fixed at 0.1 MPa. The methylcyclohexane conversion increased

Figure 6. Relationships between reaction pressure and methylcyclohexane conversion for the packed-bed reactor and the DMDPS-derived membrane reactor.

as the reaction temperature increased in both reactors, regardless of the extraction of the produced hydrogen. This is because the dehydrogenation of methylcyclohexane is endothermic and the equilibrium conversion of methylcyclohexane increases as the reaction temperature increases. In the case of the packed-bed reactor, the methylcyclohexane conversion is similar to the equilibrium conversion of methylcyclohexane. The hydrogen could not be extracted from the packed-bed reactor; therefore, the methylcyclohexane conversion could not exceed the equilibrium conversion of methylcyclohexane. These results indicate that the residence time was sufficient to reach reaction equilibrium. In the case of the membrane reactor using the DMDPSderived membrane, the methylcyclohexane conversion exceeds the equilibrium conversion under all temperature conditions and is larger than that of the packed-bed reactor. This is because the produced hydrogen was selectively extracted from the reaction side to the permeation side, and thus there was an equilibrium shift. It is desirable to produce hydrogen from the organic chemical hydride at lower temperature in some practical applications, and higher methylcyclohexane conversion would be obtained in an even lower temperature range using the membrane reactor. For example, we see from Figure 5 that the membrane reactor can be operated at a temperature that is around 20 K lower than that of the packed-bed reactor to achieve 80% of the methylcyclohexane conversion. In addition, in the case of the membrane reactor, methylcyclohexane conversion in the experiment is in agreement with that predicted by the simulation. In the simulation studies, the permeance of SF6, as an indicator of permeation characteristics of the organic chemical hydrides, was used for those of methylcyclohexane and toluene. Therefore, the equilibrium shift using the membrane reactor can be attributed to the high hydrogen permeance and selectivity through the membrane, and the increment in the methylcyclohexane conversion using the membrane reactor can be well predicted by the simulation model. The temperature dependency of the methylcyclohexane conversion using the membrane reactor has been investigated experimentally and through simulation in the case of the dehydrogenation of cyclohexane,29 and we investigated the dehydrogenation of methylcyclohexane in this study. 4.4. Dehydrogenation of Methylcyclohexane in the Membrane Reactor at Pressures Higher than 0.1 MPa. Figure 6 shows the relationships between reaction pressure and methylcyclohexane conversion for the packed-bed reactor and membrane reactor using membrane 1. The reaction temperature was fixed at 533 K.

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In the case of the packed-bed reactor, the methylcyclohexane conversion decreased monotonically as the reaction pressure increased. The dehydrogenation reaction is an equilibrium reaction, and this reaction generates 3 mol of hydrogen and 1 mol of toluene from 1 mol of methylcyclohexane. Therefore, from a thermodynamic viewpoint, the methylcyclohexane conversion should decrease as the reaction pressure increases. In the case of the packed-bed reactor, the methylcyclohexane conversion was similar to the equilibrium conversion of methylcyclohexane. The hydrogen could not be extracted from the reaction side in the case of the packed-bed reactor; therefore, the methylcyclohexane conversion could not exceed the equilibrium conversion. These results indicate that the reaction time was sufficient to reach equilibrium. In the case of the membrane reactor using the DMDPSderived membrane, the methylcyclohexane conversion exceeded the equilibrium conversion. The conversion was virtually constant at pressures ranging from 0.1 to 0.25 MPa, and its value was as high as 0.99. From a thermodynamic viewpoint, the methylcyclohexane conversion decreases as the reaction pressure increases as mentioned above. However, the permeate flux of hydrogen is higher at higher pressures because of the larger driving force for permeation, which results in an equilibrium shift that increases methylcyclohexane conversion owing to the large effect of hydrogen extraction. That is to say, methylcyclohexane conversion should increase as the pressure increases from an extraction point of view. As a consequence, methylcyclohexane conversion is determined by a balance of these two effects. In the case of our experimental results, the increase in methylcyclohexane conversion due to extraction seems to be sufficient to cover the decrease due to the thermodynamic effect; therefore, the methylcyclohexane conversion was virtually constant at pressures ranging from 0.1 to 0.25 MPa. The methylcyclohexane conversion in the case of the membrane reactor was much higher than that in the case of the packedbed reactor, and the difference in methylcyclohexane conversion between the two reactors increased as the reaction pressure increased. This is because methylcyclohexane conversion is decreased by the thermodynamic effect in the case of the packedbed reactor, whereas the effect of hydrogen extraction covers the thermodynamic effect in the case of the membrane reactor. This reaction indicates that the application of the membrane reactor to the dehydrogenation of methylcyclohexane at pressures higher than 0.1 MPa is efficient. In the case of the membrane reactor, the experimental methylcyclohexane conversion is also in good agreement with the methylcyclohexane conversion in the simulation. Therefore, the pressure dependency of methylcyclohexane conversion can be predicted by simulation. The equilibrium shift when using the membrane reactor is attributed to the high hydrogen permeance and selectivity through the membrane, and thus the higher methylcyclohexane conversion when using the membrane reactor can be well predicted by the simulation model. The pressure dependency of the methylcyclohexane conversion in the dehydrogenation of cyclohexane using the membrane reactor has been investigated experimentally and in simulation,29 and we studied the effects on the dehydrogenation of methylcyclohexane in this study. 4.5. Continuous Hydrogen Production from Methylcyclohexane in the Membrane Reactor without Using Carrier Gas and Sweep Gas. From a practical viewpoint, the use of carrier gas and sweep gas is unfavorable because an additional process of separating hydrogen from the sweep gas is needed. When we can obtain pure hydrogen at

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atmospheric pressure from pressurized methylcyclohexane without using carrier or sweep gas, the hydrogen can be directly supplied at a hydrogen station and directly used as fuel in a fuel cell. Therefore, the dehydrogenation of methylcyclohexane was carried out in the membrane reactor without using carrier gas or sweep gas. Figure 7 shows the time courses of methylcyclohexane conversion, reaction pressure, hydrogen purity, the production rate of hydrogen, and hydrogen permeance for the DMDPSderived membrane during the dehydrogenation of methylcyclohexane. In this study, the membrane used was membrane 2. On the first day’s run, the conversion of methylcyclohexane and hydrogen purity were around 0.8 and as high as 99.95%, respectively, and these values were almost stable for 6 h. The methylcyclohexane conversion and hydrogen purity predicted by simulation were 0.86 and 99.98%, and these results are in good agreement with the experimental results. The purity of the hydrogen produced from the methylcyclohexane (99.95%) was higher than that in our previous study using cyclohexane,30 and this also means that the membrane maintained so high a selectivity as above 104 of hydrogen/ methylcyclohexane or hydrogen/toluene, according to the developed model. The conversion of methylcyclohexane and hydrogen purity on the second and third days were almost stable and almost the same as those on the first day. This result indicates that the DMDPS-derived membrane and Pt/ Al2O3 catalysts were stable during the experiment. The hydrogen permeance through the DMDPS-derived membrane was virtually the same before and after the experiment; this result indicates the stability of the membrane. In addition, no byproducts due to hydrocracking were observed during the periods of the operation in this study. Therefore, we can say that the purity of hydrogen produced from methylcyclohexane in the membrane reactor with a DMDPS-derived membrane operated without carrier gas or sweep gas is high enough for the hydrogen to be used as fuel in a fuel cell. 5. Conclusion We developed a membrane reactor that enables us to obtain hydrogen with higher purity from methylcyclohexane using a silica membrane. In addition, we developed a simulation model for predicting the performance of the membrane reactor, using the dehydrogenation reaction rate constant of methylcyclohexane determined from the experimental data of the reaction in a packed-bed reactor under various experimental conditions. The membrane reactor that consisted of the DMDPS-derived silica membrane and Pt/Al2O3 catalyst had excellent performance, as described below. We examined the dehydrogenation of methylcyclohexane with carrier gas and sweep gas under a reaction temperature ranging from 473 to 553 K and reaction pressure ranging from 0.1 to 0.25 MPa. An equilibrium shift was achieved under all conditions, and the methylcyclohexane conversion was in good agreement with the conversion predicted in simulation. The equilibrium shifts were due to the high hydrogen permeance and permselectivity of the DMDPSderived silica membrane. We demonstrated the production of hydrogen with purity of no less than 99.95% from methylcyclohexane in a membrane reactor without using carrier gas or sweep gas, and the methylcyclohexane conversion and hydrogen purity were in good agreement with those predicted in simulation. In addition,

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Figure 7. Course of the production of hydrogen from methylcyclohexane in the membrane reactor with the DMDPS-derived membrane. Changes in methylcyclohexane conversion, reaction pressure, hydrogen purity, production rate of hydrogen, and hydrogen permeance for the DMDPS-derived membrane during the dehydrogenation of methylcyclohexane are shown.

the performance of the membrane reactor was very stable during three cycles of startup-operation (6 h)-shutdown. Nomenclature k1 ) reaction rate [mol s-1 kg-cat-1 Pa-1] Keq ) equilibrium constant [Pa3] l ) reactor length [m] Ni ) R ) molar flow rate of component i on the reaction side [mol s-1] Ni ) P ) molar flow rate of component i on the permeation side [mol s-1] Pi ) partial pressure of component i on the reaction side [Pa] PR ) total pressure on the reaction side [Pa] PP ) total pressure on the permeation side [Pa] Qi ) permeance of component i [mol m-2 s-1 Pa-1] r ) outer radius of membrane [m] rd ) dehydrogenation rate of methylcyclohexane [mol s-1 m-3] R ) gas constant [m2 kg s-2 K-1 mol-1] SR ) sectional area of the reaction side [m2] xi ) mole fraction of component i on the reaction side [-] yi ) mole fraction of component i on the permeation side [-] Vi ) stoichiometric coefficient [-] F ) bulk density of catalyst [kg m-3]

Subscripts MCH ) methylcyclohexane TOL ) toluene H ) hydrogen i ) component i

Acknowledgment This research was supported by the Japan Petroleum Energy Center (JPEC) as a technological development project supported financially by the Ministry of Economy, Trade and Industry. We thank Noritake Corp., Tokyo, Japan, for kindly supplying the R-alumina substrate. The catalyst supports of Al2O3 were kindly supplied by the Catalysis Society of Japan. Literature Cited (1) Okada, Y.; Sasaki, E.; Watanabe, E.; Hyodo, S.; Nishijima, H. Development of dehydrogenation catalyst for hydrogen generation in organic chemical hydride method. Int. J. Hydrogen Energy 2006, 31, 1348–1356. (2) Ali, L. I.; Ali, A. G. A.; Fotouh, S. M. A.; Gheit, A. K. A. Dehydrogenation of cyclohexane on catalysts containing noble metals and their combinations with platinum on alumina support. Appl. Catal., A 1999, 177, 99–110.

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ReceiVed for reView June 3, 2010 ReVised manuscript receiVed August 19, 2010 Accepted September 15, 2010 IE101210X