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Ind. Eng. Chem. Res. 2008, 47, 1416-1420
Dimethyl Ether Reforming in a Mesoporous γ-Alumina Membrane Reactor Combined with a Water Gas Shift Reaction Sang-Jun Park,†,‡ Dong-Wook Lee,† Chang-Yeol Yu,† Kwan-Young Lee,‡ and Kew-Ho Lee*,† National Research Laboratory for Functional Membranes, EnVironment and Energy, Research Center, Korea Research Institute of Chemical Technology, P.O. Box 107, Yuseong, Daejeon, 305-606, Korea, and Catalysis and Reaction Engineering Lab, Department of Chemical & Biological Engineering, Korea UniVersity, Seoul 136-701, Korea
Dimethyl ether (DME) steam reforming was performed in a γ-alumina/silica/stainless steel composite membrane reactor combined with a water gas shift (WGS) reaction to achieve three important aims simultaneously, such as DME conversion improvement, high hydrogen recovery, and CO elimination. The Knudsen membrane with high permeability was used to increase conversion improvement and hydrogen recovery. In one process of the DME steam reforming membrane reactor combined with the WGS reaction, the DME conversion was improved up to 35% in comparison with a conventional reactor, and hydrogen recovery was about 20%. The CO was not detected in the permeate side of the membrane reactor. The high CO removal efficiency was obtained from the WGS reaction with the Pt/TiO2 catalyst in the permeate side of the membrane reactor. Introduction Steam reforming of methanol and gasoline is being studied as a hydrogen source for fuel cells. Methanol steam reforming was easy to perform at low temperature. However, there is a problem in that the toxicity of methanol is high.1 The infrastructure for gasoline is well established, but gasoline steam reforming is difficult even at high temperature around 800 °C, and gasoline contains sulfur resulting in catalyst poisoning.2 In contrast, DME is a potential clean fuel and energy source of the next generation because it does not contain harmful materials and burns without producing NOx, smoke, or particulates. Moreover, because its ratio of hydrogen to carbon is high (CH3OCH3), the DME steam reforming has been attractive in recent years (eq 1). DME steam reforming seems to be a promising process as a hydrogen carrier for PEMFC.1-5
DME SR: (CH3)2O + 3H2O f 6H2 + 2CO2
∆H0 ) + 135 kJ/mol (1)
The PEMFC anode feed gas should contain less than 100 ppm CO and preferably less than 20 ppm because the anode catalyst is usually based on platinum, which is easily poisoned by even low concentrations of CO. To diminish CO concentration in the anode feed gas, several physical and chemical purification methods, such as, pressure swing adsorption, inorganic membranes, organic membranes, solvent absorption, water-gas shift reaction, and methanation and preferential oxidation, have been studied. In addition, new electrocatalyst materials with lower affinity for CO were investigated by several research groups.6-8 One of the effective solutions for CO removal is a membrane reactor system, from which reduction of the CO concentration can be achieved simultaneously with improvement in the DME conversion in one process. If the membrane reactor system is employed for DME reforming as a hydrogen carrier of PEMFC, * To whom correspondence should be addressed. Tel.: 82-42-8607240. Fax: 82-42-861-4151. E-mail:
[email protected]. † Korea Research Institute of Chemical Technology. ‡ Korea University.
there are three important aims that have to be achieved from the DME-reforming membrane reactor system. The first one is the improvement in the DME conversion, which is a basic objective in the membrane reactor field. The second one is high hydrogen recovery. From a viewpoint of its practical application in the PEMFC system, the high hydrogen recovery is considered to be the most important aim because the hydrogen recovery is directly associated with the capacity of the DME-reforming membrane reactor. The last one is CO removal for the prevention of CO poisoning on the platinum anode catalysts in the PEMFC. If dense and microporous membranes such as palladium and microporous silica membranes are used in the DME-reforming membrane reactor, CO can be efficiently eliminated. However, the dense and microporous membranes give considerably low hydrogen recovery. In contrast, utilization of mesoporous membranes leads to high hydrogen recovery combined with a significant falloff in CO removal efficiency, which can be overcome by additional CO cleanup processes such as the water-gas shift (WGS) reaction.9-13 The WGS reaction has attracted increasing interest recently because the WGS reaction not only reduces the amount of CO but also produces additional hydrogen (eq 2).
WGSR: CO + H2O(g) f H2 + CO2
∆H0 ) -41.2 kJ/mol (2)
In this study, we employed a DME-reforming membrane reactor combined with a WGS reaction to achieve three important aims simultaneously, such as the DME conversion improvement, high hydrogen recovery, and CO elimination. Experimental Section Sol Synthesis. Colloidal silica sol of 100 nm in particle size as a material for an intermediate layer were prepared to modify the pore structure of stainless steel supports. The colloidal silica sol was synthesized from base-catalyzed hydrolysis and the condensation reaction of tetraethyl orthosilicate (TEOS) purchased from Aldrich. A molar ratio of TEOS, water, ammonia, and ethanol was 1:53.6:0.64:40.1. The addition of the ammonia/ water mixture into the TEOS/ethanol mixture was carried out
10.1021/ie070910q CCC: $40.75 © 2008 American Chemical Society Published on Web 02/07/2008
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Figure 1. Schematic diagram of the DME steam reforming membrane reactor test apparatus.
dropwise, followed by refluxing the mixture for 3 h with vigorous stirring, and then stable colloidal silica sol of 100 nm in particle diameter was obtained. Bohemite sol was synthesized according to the method suggested by Kusakabe et al.10 Preparation of Composite Membranes. Disks of 316L porous stainless steel purchased from Mott Metallurgical were used as a support of composite membranes. The porous stainless steel support with a thickness of 1 mm, surface area of 5 cm2, and average pore size of 0.5 µm was modified by the 100 nm sized silica xerogel to reduce the pore size and narrow the pore size distribution. The silica xerogel was infiltrated into macropores of the porous stainless steel support by a press under 10 MPa, followed by calcination at 650 °C for 2 h. The whole procedure of the first modification was repeated twice. To synthesize a mesoporous γ-alumina composite membrane, a mesoporous γ-alumina layer was coated on surface of the modified support with the boehmite sol by means of the soaking-rolling method.14 The back side of the support loaded on the O-ringsealed cell was evacuated by a rotary vacuum pump. The boehmite sol was poured onto the front side of the modified support, followed by maintaining the vacuum of the back side of the support for 3 min so that the boehmite sol could penetrate into the inner pores of the support. After the soaking process, the concentrated gel layer formed on the front side of the support was removed by rolling the membrane surface with a urethane rolling pin. After drying and calcination at 25 °C for 12 h and 650 °C for 2 h, the mesoporous γ-alumina membrane was successfully synthesized. We observed the surface morphology of the composite membrane by scanning electron microscope (SEM, JEOL JSM-840A). Gas Permeation Measurement. Permeation measurements for single gas were made with pure DME and hydrogen between 25 and 450 °C. The permeation area was 4.52 cm2. A single gas test was conducted by means of the pressure-drop method. A feed side of the membrane was pressurized by pure hydrogen or DME, whereas a permeate side of the membrane was under atmospheric pressure without sweeping gas. The transmembrane pressure was 0.042 MPa, and the flux of permeated gas was measured by a soap-film flow meter. The H2/DME permselectivity for the single gas permeation test is defined as a ratio of hydrogen permeance to DME permeance measured at the same transmembrane pressure and temperature. Membrane Reactor Tests. An apparatus for the DME steam reforming membrane reactor test is schematically shown in Figure 1. For the DME-reforming membrane reactor, 3 g of
Figure 2. Surface image of the composite membranes (a) purchased porous stainless steal disk, (b) modified substrate with 100 nm silica xerogel, and (c) γ-alumina/silica/stainless steel composite membrane.
Cu-Al2O3 catalysts purchased from NIKKI Chemical Co. was used in a whole experiment of the DME steam reforming. The DME steam reforming experiments were carried out in the range of reaction temperature from 250 to 500 °C. Water was fed to a side with a liquid-feed flow rate of 0.02 mL/min and evaporated in a preheating line. Subsequently, the water vapor was mixed with DME at 5 mL/min and an argon carrier gas at 100 mL/min. A flow rate of argon sweeping gas in the permeate side was 130 mL/min. The products and reactants in the retentate and permeate side were analyzed using a gas chromatographs (DS 6200, Donam Inc., Korea) equipped with Porapack T column and thermal conductivity detector. The hydrogen recovery is defined as a volumetric flow rate ratio of permeated hydrogen to total produced hydrogen. The DME conversion in
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Figure 3. Results of DME and H2 permeances and H2/DME selectivity of the composite membrane for the single gas test.
Figure 5. DME conversion and hydrogen recovery in the DRM and the DRMW reactor, (a) DRM reactor and (b) DRMW reactor.
For the confirmation of the DME conversion improvement via the membrane reactor system, DME steam reforming in a conventional reactor was also conducted by replacing the composite membranes in the reactor with a nonporous stainless steel disk. The mesoporous γ-alumina composite membrane was employed for the DME-reforming mesoporous membrane (DRM) reactor. In the case of the DME-reforming mesoporous membrane-water-gas shift (DRMW) reactor, we also used the mesoporous γ-alumina membrane and loaded 1.5 g of the Pt/ TiO2 catalyst into the permeate side of the membrane reactor to eliminate CO through the WGS reaction. The Pt/TiO2(KRICT-MT) catalyst for the WGS reaction was synthesized via the synthetic method reported in our previous publication.15,16 Results and Discussion
Figure 4. DME conversion and concentrations of products in the conventional reactor with different reaction temperatures, (a) DME conversion and (b) concentrations of products.
the membrane reactor was calculated from a definition described below.
DME conversion ) DMEfeed - DMEretentate - DMEpermeate × 100 DMEfeed
We have synthesized a γ-alumina/silica/stainless steel composite membrane for the DME-reforming membrane reactor. Figure 2 shows surface images of the composite membranes. As shown in part a of Figure 2, the stainless steel support has very rough surface and large pore size. We first modified the support using 100 nm silica xerogel to reduce the pore size and narrow the pore size distribution (part b of Figure 2). The second modification was carried out by coating a mesoporous γ-alumina layer on the first modified substrate. As appeared in part c of Figure 2, the crack-free γ-alumina layer was formed via the soaking-rolling method.
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Figure 6. Concentrations of products in the DRM and DRMW reactor, (a) retentate side of the DRM reactor, (b) permeate side of the DRM reactor, (c) retentate side of the DRMW reactor, and (d) permeate side of the DRMW reactor. Table 1. CO Concentrations with the Reactor Configurations Concentrations of CO configurations of reactor DRMW reactor DRM reactor conventional reactor a
side
250 °C
300 °C
350 °C
400 °C
450 °C
500 °C
retentate permeate retentate permeate
