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KINETICS, CATALYSIS, AND REACTION ENGINEERING Efficient Hydrogen Production via Methanol Steam Reforming by Preventing Back-permeation of Hydrogen in a Palladium Membrane Reactor N. Itoh,*,† Y. Kaneko,‡ and A. Igarashi‡ National Institute of Advanced Industrial Science and Technology, AIST Central 5, Tsukuba 305-8565, Japan, and Kogakuin University, Hachioji 192-0015, Japan
It was found that hydrogen recovery in a palladium membrane reactor is depressed by the backpermeation of hydrogen from the perm-side to the reforming side near the inlet of the reactor, where the partial pressure of hydrogen on the reforming side is lower than that of the permside because the reaction has just started. To diminish this phenomenon, three methods, (1) use of a sweep gas, (2) premixing hydrogen into the feed, and (3) extension of the catalyst bed, were examined by simulation as well as experimentally. As a result, it was shown that the first method, if one uses steam as the sweep gas, is useful in a fuel cell system requiring humidified hydrogen. The second one is not as effective because the hydrogen amount recovered is less than that premixed. The third one is the best for obtaining pure hydrogen. Introduction Hydrogen is of great interest as a fuel source for polymer-electrolyte fuel cells (PEFCs), being produced economically by steam reforming of methanol or hydrocarbons such as natural gas, gasoline, diesel fuel, and the like.1-3 In the future, solar and wind energy and hydropower will also be used for hydrogen production.4 For generating hydrogen from hydrocarbons, however, water-gas shift reactors of significant size are required because their higher processing temperatures produce unacceptably large CO contents. In this sense, methanol seems to have an advantage because of higher conversion even at low processing temperatures ranging from 250 to 350 °C. Because the gas mixture produced via methanol steam reforming includes 103 to 104 ppm of CO,5 an apparatus in addition to the reformer is needed to remove the remaining CO, where CO must be at least less than several tens of parts per million in order to maintain the PEFC performance. To solve this technical issue, a palladium membrane reactor, which produces highly pure hydrogen as well as attains a high conversion, would be useful because it is possible to design a compact type of reactor combining a reformer and a CO remover.6-10 To obtain pure hydrogen from the permeation side, because the operation modes of the reactor using sweep gas can never be used, another mode, in which the reforming side of the membrane reactor must be kept at a pressure higher than that of the permeation side, should be used.11 In this mode, however, because the * Corresponding author. Telephone and Fax: +81-298-614733. E-mail:
[email protected]. † National Institute of Advanced Industrial Science and Technology. ‡ Kogakuin University.
Figure 1. Back-permeation phenomenon of hydrogen occurring in a membrane reactor using palladium when no sweep gas is used.
permeation side is filled entirely with pure hydrogen, “back-permeation” of hydrogen as illustrated in Figure 1 necessarily takes place near the reactor entrance, where the partial pressure on the reform side is so low that the permeation from the permeation side to the reforming side proceeds. Such back-permeation becomes a disadvantage in terms of hydrogen recovery as well as the kinetic progress of the reaction. Therefore, some ways of improving the hydrogen recovery (hydrogen output from the permeation side) are required and are proposed to realize pure hydrogen production and the supply systems for the PEFCs and so forth. In this study, to depress the back-permeation as much as possible, three methods, as illustrated in Figure 2, were considered and examined analytically and experimentally. First, the use of an inert sweep gas would make the partial pressure of hydrogen at the inlet on the permeation side zero and therefore prevent the backpermeation, although the permeated hydrogen is diluted with inert gas. Second, premixing hydrogen into the feed
10.1021/ie020349q CCC: $22.00 © 2002 American Chemical Society Published on Web 08/24/2002
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pre-reforming is examined; this corresponds to 9.4 g of additional pellets. A schematic of the experimental setup is presented in Figure 4. The feed of a methanolwater mixture (1:1.2 in mole) is sent to the membrane reactor with a high-pressure syringe pump (Harvard, Ltd.), and the product including H2, CO2, CO, CH3OH, and H2O is analyzed with a gas chromatograph, where it is assumed that CO is formed as a secondary product by the reverse water-gas shift reaction.12 The conversion of methanol is determined by considering the following reactions.
CH3OH + H2O f CO2 + 3H2 CO2 + H2 f CO + H2O
Figure 2. Three methods envisaged for depressing the backpermeation of hydrogen.
would reduce the difference in hydrogen partial pressure between the reforming and permeation sides, which could be kept positive by maintaining the initial partial pressure of hydrogen on the reform side the same as or larger than the pressure on the permeation side. A similar effect should be attained by extension of the catalyst bed prior to the membrane because the feed entering the membrane-installed zone has already been pre-reformed and contains some hydrogen Experimental Section Figure 3 shows a cross-sectional view of the shell-andtube type of membrane reactor used. The membrane tube made of palladium alloy (Pd91Ru7In2), a one-endclosed type, 200 mm long, 7.1 mm in diameter, and 0.2 mm thick, was prepared, where the metal composition was selected from the standpoint of high resistance to hydrogen brittleness at lower temperatures. The CuZn-Al catalyst used was composed of CuO (38 mol %), ZnO (41 mol %), and Al2O3 (21 mol %) (MDC-3, supplied by Toyo CCI, Japan). Cylindrical catalyst pellets (93.5 g, 3.5 mm o.d. × 2.5 mm) were packed around the membrane tube. The catalyst length was extended by 20 mm prior to the membrane tube when the effect of
The reforming reaction is carried out in the range 2-7 atm at 200 °C, while the permeation side is kept at atmospheric pressure. Since the reactor was placed in an air circulation oven, the temperature along the reactor was almost uniform, excluding near the inlet, in which several degrees drop in temperature compared with the initial one was observed. The hydrogen output from the permeation side is measured with a soap-film flow meter. When the sweep gas effect is tested, argon gas is introduced by opening the stop valve (SW) as indicated in Figure 4, which is closed in the other two cases. Results and Discussion 1. Use of Sweep Gas. It is evident that once a sweep gas is used, pure hydrogen can no longer be obtained from the permeation side. However, considering that, in the PEFC system, hydrogen must be humidified and then supplied to maintain its proton conductivity, the idea of using water vapor (steam) as the sweep gas is envisaged. This means that using a sweep gas is advantageous in terms of allowing no back-permeation and providing a direct supply of hydrogen saturated with water vapor. Here, the sweep gas effect is quantitatively evaluated by means of computer simulation, in which a set of simultaneous ordinary differential equations derived for an ideal flow reactor13 was numerically solved. The following semiempirical expression for the methanol steam reforming is used, which has been derived by modifying the equation postulated
Figure 3. Cross-sectional view of the shell-and-tube type of membrane reactor used.
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Figure 4. Schematic of the experimental setup. Table 1. Kinetic Parameters Used in Calculations rate constant, k equilibrium constant, Kp adsorption constant of methanol, KM adsorption constant of hydrogen, KH permeability of hydrogen
4.0 mol/(m3 bar s) 12740 bar2 0.9 bar-1 0.75 bar-1 2.52 × 10-9 mol/(m s Pa0.5)
by Idem and Balchshi14 so as to fit the data obtained over 450-500 K and 2-5 bar.
rM ) -
kM(pMpW - pCp3H/KP) (1 + kMpM + KHpH)4pH0.5
(M, CH3OH; W, H2O; C, CO2; H, H2) where rM (mol/(m3 s)) is the disappearing rate of methanol, k (mol/(m3 bar s)) is the rate constant, pi (bar) is the partial pressure of component i, Kp (bar2) is the equilibrium constant, and kad (bar-1) is the adsorption constant. In Table 1, the parameters used are listed. Three cases are compared in Figure 5, that is, reaction only, reaction and separation without a sweep gas, and reaction and separation with a sweep gas. In the case where only the reaction proceeds, the conversion decreases with increasing reforming pressure. On the other hand, in the two cases where the combined reaction and H2 separation are carried out, one can find a maximum in the methanol conversion around 3 atm of reforming pressure, whereas there is no maximum in the reaction without H2 separation. The increase in the low-pressure range is due to a depression of the back-permeation, which is caused by an increase in the partial pressure of hydrogen on the reforming side. The decrease after the maximum is due to a decrease in the reaction rate followed by an increase in back-permeation. Furthermore, it can be seen that, by using even a small amount of sweep gas, higher conversions are obtained in comparison with the case of “reaction only”.
Figure 5. Simulation results showing the effect of using a sweep gas of 2.0 cm3(STP)/min on the methanol conversion.
A comparison between the hydrogen recovery with and without a sweep gas when the reforming pressure is changed at a fixed flow rate of sweep gas (4.7 cm3 (STP)/min) is shown in Figure 6. The hydrogen recovery, defined as the amount of hydrogen obtained as permeate, is found to be larger when using a sweep gas. As the reforming pressure becomes small, the ratio of the recovery with to that without the sweep gas becomes large. Next, experimental results with varying flow rate of the sweep gas are shown in Figure 7. Clearly, increasing the sweep gas is found to lead to a rise in
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Figure 8. Conversion and H2 recovery changes with increasing amount of premixed hydrogen (no sweep used).
Figure 6. Improvement in hydrogen recovery using a sweep gas of 4.7 cm3(STP)/min, where a maximum possible rate of hydrogen produced is 33.2 cm3(STP)/min.
Figure 7. Experimental results when the flow rate of sweep gas is varied.
Figure 9. Simulated profiles of hydrogen partial pressures on both the reforming and the permeation sides when the catalyst bed is extended by 20% (no sweep used).
conversion as well as hydrogen recovery, which are always higher than those without the sweep gas. Thus, it is verified experimentally that use of a sweep gas is effective in terms of both enhancing the reforming reaction and increasing the amount of hydrogen permeate. 2. Premixing of Hydrogen into the Feed. Figure 8 shows both the conversion and the H2 recovery changes with increasing amount of premixed hydrogen. Compared with the case where no hydrogen is premixed, a significant increase in both conversion and H2 recovery is observed. In such a case, the availability of this idea should be judged by whether the amount of hydrogen recovered exceeds that premixed. The linear dotted line in Figure 8 represents the sum of the recovered hydrogen with no premix (1.9 cm3(STP)/min) and the premixed hydrogen. If the hydrogen recovery is over this line, it can be said that the premix is effective. Unfortunately, the experimentally obtained H2 recovery is located at a lower position than the line. This is mainly because the premix of hydrogen has caused a decrease in the reforming rate rather than a depression of the back-permeation. 3. Extension of Catalyst Bed. First, a computer simulation was done.11 The profiles of hydrogen partial
pressures on both the reforming and the permeation sides of the reactor are shown in Figure 9. Before entering the catalyst bed with the membrane tube, the reforming proceeds to a certain extent in the extended bed and the hydrogen partial pressure at the entrance (at L ) 0) is higher than that on the permeation side, so that the back-permeation disappears and only a desirable forward permeation occurs. It is expected that preventing the back-permeation can avoid a drop in conversion as shown in Figure 10. When there is no extended bed, the conversion (break line) decreases with an increase in the reforming pressure. Such a decrease is not clearly seen in the case of placing a catalyst bed prior to the membrane tube, and high conversions (solid line) are maintained over the pressure range considered. It can be found that there is an optimum reaction pressure (around 5 atm) giving the highest conversion. Experimental results, although there is some scatter, are also shown in Figure 10. Figure 11 shows the experimental results that are compared with the simulation. Agreement between them suggests that simulation, even with a simple model, is a very useful tool for the reactor design. Next, the reactor performances between with and without the extended bed at a fixed space velocity (feed rate/catalyst weight) were also compared in Figure 11, where the feed
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permeation of hydrogen from the perm side to the reforming side. As methods of diminishing this undesirable phenomenon, (1) the use of a sweep gas, (2) premixing hydrogen into the feed, and (3) extension of the catalyst bed are proposed, analyzed by computer simulation, and experimentally confirmed. As a result, it was clarified that the first and the third methods are useful for preventing the disadvantage due to the backpermeation. The second one is not as effective because the hydrogen amount recovered is less than that premixed. The third one is considered to be the best from the viewpoint of obtaining pure hydrogen. Literature Cited
Figure 10. Simulated result comparing the methanol conversion with and without a catalyst bed extension of 2 cm (no sweep used).
Figure 11. Improvement in hydrogen recovery from the membrane reactor with the extended catalyst bed of 2 cm, where the feed rate of methanol is presented in the parentheses (no sweep used).
rate of methanol was increased to 22 µL/min, corresponding to a 10% increment of the catalyst weight. It is found that a higher hydrogen permeation flux in the extended-bed reactor could be obtained. It can be recognized that an improvement in the hydrogen recovery is possible by this reactor configuration. Conclusions It has been shown that hydrogen recovery in a palladium membrane reactor is depressed by the back-
(1) Donitz, W. Fuel Cell for Mobile Applications, Status, Requirements and Future Application Potential. Int. J. Hydrogen Energy 1998, 23, 611. (2) Brown, L. F. A Comparative Study of Fuels for On-board Hydrogen Production for Fuel-cell-powered Automobiles. Int. J. Hydrogen Energy 2001, 26, 381. (3) Ogden, J. M.; Steinbugler, M. M.; Kreutz, T. G. A Comparison of Hydrogen, Methanol and Gasoline as Fuels for Fuel Cell. J. Power Sources 1999, 79, 143-168. (4) Bockris, J. O’M. Hydrogen Economy in the Future. Int. J. Hydrogen Energy 1999. 24, 1. (5) Amphlett, J. C.; Creber, K. A. M.; Davis, J. M.; Mann, R. F.; Peppley, B. A.; Stokes, D. M. Hydrogen Production by Stem Reforming of Methanol for Polymer Electrolyte Fuel Cells. Int. J. Hydrogen Energy 1994, 19, 131. (6) Buxbaum, R. E. Membrane Reactor Advantages for Methanol Reforming and Similar Reactions. Sep. Sci. Technol. 1999, 34, 2113. (7) Lin, Y. M.; Lee, G.-L.; Rei, M. H. An Integrated Purification and Production of Hydrogen with a Palladium Membrane-catalytic Reactor. Catal. Today 1998, 44, 343. (8) Lin, Y.-M.; Rei, M.-H. Process Development for Generating High Purity Hydrogen by Using Supported Palladium Membrane Reactor as Steam Reformer. Int. J. Hydrogen Energy 1999, 25, 211. (9) Lin, Y. M.; Rei, M. H. Study on the Hydrogen Production from Methanol Steam Reforming in Supported Palladium Membrane Reactor. Catal. Today 2001, 67, 77. (10) Aasberg-Petersen, K.; Nielsen, C. S.; Jorgensen, S. L. Membrane Reforming for Hydrogen. Catal. Today 1998, 46, 193. (11) Itoh, N. Development of One-side Uniform Model for Palladium Membrane Reactors. J. Chem. Eng. Jpn. 1992, 25, 336. (12) Breen, J. P.; Meunier, F. C.; Ross, J. R. H. Mechanistic Aspects of the Steam Reforming of Methanol over a CuO/ZnO/ ZrO2/Al2O3 Catalyst. Chem. Commun. 1999, 22, 2247. (13) Itoh, N. Ideal Flow Models for Palladium Membrane Reactors. J. Chem. Eng. Jpn. 1990, 23, 420. (14) Idem, R. O.; Bakhshi, N. N. Kinetic Modeling of the Production of Hydrogen from the Methanol-steam Reforming Process over Mn-promoted Coprecipitated Cu-Al Catalyst. Chem. Eng. Sci. 1996, 51, 3697.
Received for review May 9, 2002 Revised manuscript received July 15, 2002 Accepted July 17, 2002 IE020349Q
