Ind. Eng. Chem. Res. 2006, 45, 7997-8001
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KINETICS, CATALYSIS, AND REACTION ENGINEERING Methanol Steam Reforming to Hydrogen in a Carbon Membrane Reactor System Xiaoyong Zhang, Haoquan Hu,* Yudong Zhu, and Shengwei Zhu State Key Laboratory of Fine Chemicals, Institute of Coal Chemical Engineering, Dalian UniVersity of Technology, 129 Street, Dalian 116012, P.R. China
In this study, the methanol steam reforming reaction was carried out in a carbon membrane reactor (CMR) for hydrogen production. The behavior in the CMR was compared to that in a fixed-bed reactor (FBR) at the same experimental conditions. The parameters temperature, carrier gas flow rate, and feed ratio were investigated to better understand the separation effect of hydrogen in the CMR on the methanol conversion and product selectivity. The results showed that the CMR gives a higher methanol conversion than the FBR at all investigated operating conditions; the overall yields of hydrogen in the CMR and FBR are identical, but a CO-free hydrogen stream can be produced with the CMR, which could be directly used in a proton-exchangemembrane fuel cell. 1. Introduction Hydrogen production has been a matter of great importance in the past decade, but it is now becoming a key process as a result of spectacular advances in fuel cell technology. In particular, its economical production from abundant sources with reduced purification costs is one of the goals toward the commercialization of fuel-cell-powered systems.1,2 The use of methanol as an onboard hydrogen source is advantageous when considering distribution infrastructures, safety aspects, and vehicle driving ranges. Methanol has some advantages as a fuel and raw material for several chemical production processes. For example, it is more easily transported than methane or other gas fuels, it has a high energy density, and it does not require desulfurization. Furthermore, methanol reacts at moderate temperatures (473673 K)3 in reactions such as partial oxidation (473-493 K), steam reforming (473-573 K), and decomposition (up to 673 K).4 In particular, the steam reforming of methanol is an endothermic reaction and is considered as an important method for hydrogen generation in terms of feasibility for various types of on-site energy systems. According to the literature,4-6 the chemical reactions considered in methanol steam reforming are as follows
CH3OH + H2O T CO2 + 3H2
∆H298 K ) +49.5 kJ/mol (1)
CO + H2O T CO2 + H2
∆H298 K ) -41.2 kJ/mol (2)
CH3OH T CO + 2H2
∆H298 K ) +90.7 kJ/mol (3)
where reactions 1 and 3 are both reversible and endothermic and proceed with a volume increase, suggesting that high methanol conversion could be obtained at high temperature and low pressure. Exothermic reaction 2 is the so-called water-gas shift reaction, which proceeds simultaneously with methanol steam reforming and without a volume change. When carried * To whom correspondence should be addressed. E-mail: hhu@ chem.dlut.edu.cn. Tel. and Fax: (86)-0411-88993966.
out in a traditional reactor these reactions produce a hydrogencontaining mixture, so the resulting hydrogen requires purification before it can be fed to a proton-exchange-membrane fuel cell (PEMFC). Separation is mainly directed toward removing CO, which poisons the anodic catalyst of the fuel cell.7 For this reason, recent studies on the methanol steam reforming reaction have considered the application of a membrane reactor that is highly selective toward hydrogen production; such an approach might allow for the replacement of the traditional reformer and its associated gas-cleaning unit. In this scenario, the potential benefit of a hydrogen-selective membrane reactor for the recovery of pure hydrogen from the methanol steam reforming reaction appears. Pd,8 Pd/V/Pd, Pd75Ag25, Pd60Cu40,9 and Pd-supported10-12 membrane reactors have been studied in the pressure range of 0.1-2.5 MPa and at temperatures between 533 and 593 K. However, it is often pointed out that palladium membranes have some problems in terms of cost and permeability for the industrial application of palladium membrane reactors. Such a situation emphasizes that other membrane materials should be explored and developed as alternatives to palladium. From this point of view, one can expect that carbon membranes, gaining great interest recently, represent one of the candidates because of their high hydrogen permselectivity resulting from a molecular-sieving effect.13-15 Research on methanol steam reforming in fixed-bed reactor (FBR) has mainly focused on catalyst optimization to reduce the CO concentration in the gaseous mixture exiting the reactor. There are no reports dealing with carbon membrane reactors (CMRs) applied to the methanol steam reforming reaction. Therefore, in this work, the objective was to study the behavior of a CMR in terms of methanol conversion and hydrogen and CO production in comparison with an FBR operated at the same experimental conditions. 2. Experimental Section 2.1. Preparation of Carbon Membrane. The carbon membrane used in the CMR was prepared by spraying an N,Ndimethyl formamide (DMF) solution containing phenol form-
10.1021/ie060414m CCC: $33.50 © 2006 American Chemical Society Published on Web 10/26/2006
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Figure 1. Schematic view of the carbon membrane reactor.
Figure 3. Relationship between the pure-gas permeation rate of the carbon membrane and the molecular size of the gas.
Figure 2. Flowsheet of the experiment setup.
aldehyde novolac resin (PFNR) and poly(ethylene glycol) (PEG) onto a PFNR-based green membrane support. The method for preparing the green membrane support has been described elsewhere.16,17 PFNR/PEG was dissolved in DMF to form a mixed matrix coating solution. A small quantity of mixed matrix coating solution was sprayed onto a finely polished surface of a support tube by means of spray coating technique and was then carbonized in a tubular furnace under an inert gas stream from room temperature to 1073 K at a heating rate of 0.5 K/min. 2.2. Preparation of Cu/ZnO/Al2O3 Composite Catalysts. The catalyst for the methanol steam reforming reaction was synthesized by a coprecipitation method.18,19 The hydroxyl carbonate precursor was prepared by precipitation from metal nitrate solutions with sodium carbonate at pH 7. The concentration of the nitrate solutions was adjusted to a nominal catalyst composition with a Cu/ZnO/Al2O3 molar ratio of 70:15:15 for the Cu/ZnO/Al2O3 catalyst. The resulting precipitate was aged in the mother liquor under continuous agitation for 2 h. The aged precipitate was filtered and washed with deionized water. Drying was performed overnight at 393 K, followed by calcination in air at a flow rate of 40 mL/min from room temperature to 593 K at a constant heating rate of 2 K/min, with the final temperature held for 3 h. The calcined precursor was pressed and sieved, yielding a grain fraction of 250-350 µm for use. 2.3. Fixed-Bed Reactor and Carbon Membrane Reactor. The FBR consisted of a stainless steel tube with an inner diameter of 6 mm, and the CMR consisted of a tubular carbon membrane with an inner diameter of about 6 mm. The carbon membrane tube was a pinhole-free carbon composite membrane layer having a thickness of 20-30 µm and was sealed inside a stainless steel tube with a length of 30 cm and an inner diameter of 2 cm, as shown in Figure 1. In the FBR and CMR, catalyst pellets were packed in the reaction zone, and a quartz fiber was placed into lumen on both ends of the reaction tube. Figure 2 shows the flowsheet of the experimental setup. The temperature in the reactor was controlled by an external tubular furnace with a temperature programming controller. N2 as a carrier gas was supplied at a constant flow rate through a mass flow controller. When necessary, the pressure inside the reactor was kept higher than atmospheric pressure with a pressure regulator.
2.4. Methanol Steam Reforming. The methanol steam reforming reaction was carried out in both the FBR and CMR with 1.05 g of Cu/ZnO/Al2O3 catalyst. The experimental conditions were as follows: reaction temperature, between 473 and 523 K; flow rate of inert carrier (N2), between 30 and 300 mL/min; flow rate of water and methanol, 0.02-0.03 mL/min, corresponding to a weight hourly space velocity (WHSV) of the reactants of 1.0 h-1 with H2O/CH3OH molar ratios of 1.0 and 1.5; reaction pressure, 0.2 MPa. For the CMR, the permeating pressure was held at 0.1 MPa, and Ar was used as the sweep gas with a flow rate of 30 mL/min. Before the reaction, the catalyst was heated at a constant rate of 1 K/min to 453 K in a N2 stream under atmospheric pressure, and then a mixture of N2 with 10% H2 was applied as the pretreatment gas at that temperature for 5 h. Afterward, the reactor was heated at a constant rate of 1 K/min to the reaction temperature and held at that temperature for 30 min. The overall reaction time was 5 h for both the CMR and FBR. During the experiments, H2O and CH3OH were mixed and vaporized at 423 K and then carried by an inert carrier gas (N2) to the reactor. The outlet stream of the reactor was completely condensed to remove the unreacted H2O and CH3OH, and then the flow rate of the dry gaseous stream was measured with a soap-film flow meter; The composition of the gas was determined using a gas chromatograph (GC 7890T) equipped with two packed columns (GDX602 and molecular sieve 5A) and a thermal conductivity detector (TCD). For the CMR, which has two outlet streams (permeation and retentate), two TCD detectors were simultaneously used for measuring the gas composition. 3. Results and Discussion 3.1. Gas Permeation of Carbon Membrane. Pure-gas permeation rates of the carbon membrane used in the CMR are shown in Figure 3. It can be seen that the permeation rate of H2 is much higher than those of the other gases at 298 K. The ideal H2/CO separation factor is 118. The permeabilities of all gases show that the permeation rates through the membrane are correlated with the kinetic diameters, rather than the molecular weights, of the gas molecules and decrease with increasing gas molecular size. The effect of temperature on gas permeation rate is illustrated in Figure 4. It can be seen that the hydrogen permeance through the membrane hardly varies with temperature, whereas the permeation rates of CO2 and CO increase with temperature. This will result in a decrease of the ideal H2/CO separation factor, but it still remains between 33 and 54 at actual reaction temperatures. These results imply that the prepared carbon membrane exhibits a good
Ind. Eng. Chem. Res., Vol. 45, No. 24, 2006 7999 Table 1. Conversion and Yields of Reaction in the CMR and FBR at Different Temperaturesa 473 K CH3OH conversion (%) H2 yield (%) CO2 yield (%) CO yield (%)
498 K
523 K
FBR
CMR
FBR
CMR
FBR
CMR
57.57 75.31 24.55 0.14
64.35 75.75 24.16 0.09
89.21 75.42 24.10 0.48
93.27 73.97 25.62 0.41
96.20 75.49 23.33 1.18
99.87 75.77 23.16 1.07
a Data are the average values calculated over 4 h of reaction; WHSV ) 1.0 h-1, H2O/CH3OH molar ratio ) 1.5, pressure ) 0.2 MPa.
Figure 4. Effect of temperature on the gas permeation rate through the carbon membrane.
performance for use as a membrane reactor in the methanol steam reforming reaction to generate a product with high-purity H2. 3.2. Definitions. The following formulas were used to describe the performance of methanol steam reforming in the FBR and CMR12
CH3OH conversion (XCH3OH, %) )
COtotal + CO2,total × 100 CH3OHin (4)
H2 yield (YH2, %) )
H2,total × 100 (5) H2,total + COtotal + CO2,total
CO yield (YCO, %) )
COtotal × 100 (6) H2,total + COtotal + CO2,total
CO2 yield (YCO2, %) )
CO2,total × 100 H2,total + COtotal + CO2,total (7)
where each species indicates the molar flow rate of the species in the feed or product stream and the subscript “total” indicates the total outlet flow rate of each species. In particular, for the FBR, only one outlet stream is present for each species, whereas for the CMR, two outlet streams (permeate and retentate) are considered for each species. The selectivity is defined only for the CMR and can be considered as the hydrogen selectivity throughout the shell side compared to the total amount of permeation gas. The selectivities for the three gas species are defined by the following expressions
H2 selectivity (SH2, %) )
H2,out × 100 H2,out + COout + CO2,out (8)
CO2 selectivity (SCO2, %) )
CO selectivity (SCO, %) )
CO2,out × 100 H2,out + COout + CO2,out (9)
COout × 100 H2,out + COout + CO2,out (10)
where the subscript “out” indicates the shell-side flow rate of the carbon membrane. 3.3. Conversion and Yield. Table 1reports the average values of the CH3OH conversion and product (H2, CO, and CO2) yields achieved during 4 h of reaction time both in the FBR and CMR.
It can be seen that the CH3OH conversion increases with increasing temperature for both reactors. The increase of the CH3OH conversion with temperature can be explained by considering that reaction 1 and the reverse water-gas shift (rWGS) reaction (reaction -2) are endothermic, and hence, increasing the temperature will favor a move toward the products. The CH3OH conversion increases with increasing temperature and is higher in the CMR than in the FBR. For example, at 473 K, the CH3OH conversion (average value) is about 57.6% in the FBR versus about 64.4% in the CMR, whereas at 523 K, the CH3OH conversion (average value) is about 96.2% in the FBR versus about 99.9% in the CMR. The better performance of the CMR, due to the effect of hydrogen permeation, can be explained by considering the changes in the equilibrium compositions. The removal of hydrogen through permeation from the product stream makes the overall reaction shift toward the products. In particular, the CO yield increases with increasing temperature in both reactors; for example, it is 0.14% at 473 K and 1.18% at 523 K in the FBR, compared to 0.09% at 473 K and 1.07% at 523 K in CMR. The increasing CO yield with temperature can be explained by considering the rWGS reaction (reaction -2) and methanol decomposition reaction 3 for their endothermic characteristics, i.e., high temperature is beneficial for producing CO, whereas the lower CO yield in the CMR than in the FBR can be explained from the effect of hydrogen on the equilibrium of the chemical reaction. CO formation occurs mainly through the steam reforming reaction (reaction 1) to form CO2 followed by the rWGS reaction (reaction -2) at temperatures between 473 and 523 K over Cu-based catalysts;20,21 however, the direct decomposition of methanol (reaction 3), followed by the water-gas shift (reaction 2), also occurs to a small extent. The only difference between the two reactors is the presence of the carbon membrane that separates H2 from the reaction products. This changes the equilibrium of chemical reaction 1 and makes the reaction shift toward the products (CO2 and H2), while also restraining the rWGS reaction (reaction -2) to form CO. Therefore, as hydrogen is being removed from the system, reaction 2 moves toward the products, thus reducing the CO concentration. Figures 5 and 6 present the effects of the H2O/CH3OH feed ratio on the CH3OH conversion at different temperatures in the CMR and FBR, respectively. Considering reaction 1, increasing the H2O/CH3OH feed ratio promotes the reaction to move toward the products. As shown in the figures, when the feed ratio increases from 1 to 1.5, the CH3OH conversion increases from 59.7% to 64.4% in the CMR and from 48.2% to 57.6% in the FBR at 473 K. 3.4. Gas Selectivity. By using the CMR, it is possible to recover most of the hydrogen produced in the shell-side stream. Table 2 reports the H2 selectivity in the shell side of the CMR. With increasing temperature, the H2 selectivity exhibits a slightly decrease, whereas the CO2 selectivity exhibits a slightly increasing trend because of the increasing permeation
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Figure 5. Effect of H2O/CH3OH feed ratio on CH3OH conversion at different temperatures in the CMR. Conditions: Plumen, 0.2 MPa; Pshell, 0.1 MPa; carrier gas (N2) flow rate, 120 mL/min.
Figure 8. H2 and CO2 selectivities vs carrier gas (N2) flow rate in the CMR. Conditions: H2O/CH3OH ratio, 1.5; Plumen, 0.2 MPa; Pshell, 0.1 MPa; sweep gas (Ar) flow rate, 30 mL/min.
deposition on the catalyst surface increases and affects the activity of the catalyst. For values of the H2O/CH3OH ratio of less than 2 and temperatures above 473 K, carbon deposition on the catalyst is the main problem in methanol steam reforming over Cu-containing catalysts,23,24 which leads to decreases in the CH3OH conversion and H2 selectivity. On the other hand, increasing the flow rate, and thereby lowering the contact time with the catalyst, can lead to a decrease of the conversion, that is, an optimum carrier gas flow rate exists for methanol conversion. 4. Conclusions Figure 6. Effect of H2O/CH3OH feed ratio on CH3OH conversion at different temperatures in the FBR. Conditions: Plumen, 0.2 MPa; Pshell, 0.1 MPa; carrier gas (N2) flow rate, 120 mL/min. Table 2. Selectivity (%) of Gas in the Shell-side Stream of CMR at Different Temperaturea
H2 CO2
473 K
498 K
523 K
97.6 2.4
97.4 2.6
96.9 3.1
a Reaction conditions: WHSV, 1.0 h-1; H O/CH OH molar ratio, 1.5, 2 3 pressure ) 0.2 MPa.
Figure 7. CH3OH conversion vs carrier gas (N2) flow rate in the CMR. Conditions: H2O/CH3OH ratio, 1.5; Plumen, 0.2 MPa; Pshell, 0.1 MPa; sweep gas (Ar) flow rate, 30 mL/min.
of CO2 with temperature.22 In particular, the H2 selectivity is 97.4% at 498 K. This means that a gas product with a H2 composition of 97.4% and almost no CO can be obtained in the shell side. The CH3OH conversion and H2 selectivity in the CMR increase as the carrier gas flow rate increases, whereas the CO2 selectivity decreases, as shown in Figures 7 and 8. When the carrier gas flow rate is lower, the probability of carbon
The methanol steam reforming reaction has been studied in both an FBR and a CMR. The results confirm the potential of using a membrane reactor to increase the methanol conversion, H2 yield, and H2 selectivity over those obtained with an FBR operated at the same experimental conditions. By using a CMR, it is easy to separate H2 from all of the other gases (including CO) to obtain a CO-free H2 stream for direct utilization in a PEMFC. Moreover, in a CMR, the CO selectivity of the methanol steam reforming reaction is lower than that in an FBR. Improvements in both CH3OH conversion and H2 selectivity were observed upon changes in several reaction parameters, including the H2O/CH3OH feed ratio, reaction temperature, and carrier gas flow rate. The main results can be summarized as follows: (1) The CH3OH conversion increases with increasing H2O/ CH3OH feed ratio in both the FBR and CMR. (2) The CH3OH conversion increases with temperature in both the CMR and FBR. About 99.9% methanol conversion can be achieved at 523 K in the CMR, and the CH3OH conversion in the CMR is higher than that in the FBR in the temperature range investigated. (3) In principle, an increase in the carrier gas flow rate results in an increase in the CH3OH conversion up to a plateau. (4) With regard to permeation selectivity, the hydrogen selectivity is quite high (around 97%) in the CMR. The CO yield is lower in the CMR than in the FBR in the temperature range investigated. Literature Cited (1) Heinzel, A.; Vogel, B.; Hu¨bner, P. Reforming of natural gass Hydrogen generation for small scale stationary fuel cell systems. J. Power Sources 2002, 105, 202. (2) Vasileiadis, S.; Ziaka-Vasileiadou, Z. Efficient catalytic reactorsprocessors for fuel cells and synthesis applications. Sep. Purif. Technol. 2004, 34, 213.
Ind. Eng. Chem. Res., Vol. 45, No. 24, 2006 8001 (3) Sekizawa, K.; Yano, S.; Eguchi, K.; Arai, H. Selective removal of CO in methanol reformed gas over Cu-supported mixed metal oxides. Appl. Catal. A: Gen. 1998, 169, 291. (4) Fausto, G.; Luca, P.; Angelo, B. Hydrogen Recovery from Methanol Steam Reforming in a Dense Membrane Reactor: Simulation Study. Ind. Eng. Chem. Res. 2004, 43, 2420. (5) Peppley, B. A.; Amphlett, J. C.; Kearns, L. M.; Mann, R. F. Methanol steam reforming on Cu/ZnO/Al2O3. Part 1. The reaction network. Appl. Catal. A: Gen. 1999, 179, 21. (6) Peppley, B. A.; Amphlett, J. C.; Kearns, L. M.; Mann, R. F. Methanol steam reforming on Cu/ZnO/Al2O3 catalysts. Part 2. A comprehensive kinetic model. Appl. Catal. A: Gen. 1999, 179, 31. (7) Han, J.; Kim, I. S.; Choi, K. S. Purifier-integrated methanol reformer for fuel cell vehicles. J. Power Sources 2000, 86, 223. (8) Emonts, B.; Hansen, J. B.; Schmidt, H.; Grube, T.; Ho¨hlein, B.; Peters, R.; Tschauder, A. Fuel cell drive system with hydrogen generation in test. J. Power Sources 2000, 86, 228. (9) Wieland, S.; Melin, T.; Lamm, A. Membrane reactors for hydrogen production. Chem. Eng. Sci. 2002, 57, 1571. (10) 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. (11) 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 2000, 25, 211. (12) Angelo, B.; Fausto G.; Luca, P. A dense Pd/Ag membrane reactor for methanol steam reforming: Experimental study. Catal. Today 2005, 104, 244. (13) Itoh, N.; Haraya. K. A carbon membrane reactor. Catal. Today 2000, 56, 103. (14) Ismail, A. F.; David, L. I. B. A review on the latest development of carbon membranes for gas separation. J. Membr. Sci. 2001, 193, 1. (15) Koresh, J. E.; Soffer, A. Carbon molecular sieve membranes. General properties and the permeability of CH4/H2 mixture. Sep. Sci. Technol. 1987, 22, 973.
(16) Liang, C. H.; Sha, G. Y.; Guo, S. C. Carbon membrane for gas separation derived from coal tar pitch. Carbon 1999, 37, 1391. (17) Wei, W.; Hu, H. Q.; Qin, G. T.; You, L. B.; Chen, G. H. Pore structure control of phenol-formaldehyde based carbon microfiltration membranes. Carbon 2004, 42, 679. (18) Kurtz, M.; Strunk, J.; Hinrichsen, O.; Muhler, M.; Fink, K.; Meyer B.; Wo¨ll, C. Active sites on oxide surfaces: ZnO-catalyzed synthesis of methanol from CO and H2. Angew. Chem., Int. Ed. 2005, 44, 2790. (19) Shishido, T.; Yamamoto, Y.; Morioka, H.; Takaki, K.; Takehira, K. Active Cu/ZnO and Cu/ZnO/Al2O3 catalysts prepared by homogeneous precipitation method in steam reforming of methanol. Appl. Catal. A: Gen. 2004, 263, 249. (20) Purnama, H.; Ressler, T.; Jentoft, R. E.; Soerijanto, H.; Schlo¨gl, R. Schoma¨cke, R. CO formation/selectivity for steam reforming of methanol with a commercial CuO/ZnO/Al2O3 catalyst. Appl. Catal. A: Gen. 2004, 259, 83. (21) Lee, J. K.; Ko J. B.; Kim, D. H. Methanol steam reforming over Cu/ZnO/Al2O3 catalyst: Kinetics and effectiveness factor. Appl. Catal. A: Gen. 2004, 278, 25. (22) Teresa, A. C.; Antonio, B. F. Supported carbon molecular sieve membranes based on a phenolic resin. J. Membr. Sci. 1999, 160, 201. (23) Takahashi, T.; Inoue M.; Kai, T. Effect of metal composition on hydrogen selectivity in steam reforming of methanol over catalysts prepared from amorphous alloys. Appl. Catal. A: Gen. 2001, 218, 189. (24) Agarwal, V.; Patel, S.; Pant, K. K. H2 production by steam reforming of methanol over Cu/ZnO/Al2O3 catalysts: transient deactivation kinetics modeling. Appl. Catal. A: Gen. 2005, 279, 155.
ReceiVed for reView April 3, 2006 ReVised manuscript receiVed September 6, 2006 Accepted September 28, 2006 IE060414M
