Ind. Eng. Chem. Res. 2001, 40, 5391-5397
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Permeability, Selectivity, and Testing of Hydrogen Diffusion Membranes Suitable for Use in Steam Reforming K. Jarosch and H. I. de Lasa* Chemical Reactor Engineering Centre (CREC), Faculty of Engineering Science, University of Western Ontario, London, N6A 5B9 Ontario, Canada
Three composite hydrogen-permeable membranes were produced by the electroless deposition of palladium on macroporous Inconel. The permeability and selectivity for hydrogen to argon was assessed over the range 750-850 °C using a 156-µm-thick membrane. The permeability coefficient was 1.874 × 10-6 mol/(m s kPa0.5), with an activation energy of 22.6 kJ/mol. Membranes produced by the electroless technique exhibited hydrogen/argon molar selectivities in the range (336-1187):1. Two membranes were used to assess the effect of hydrogen permeation on the conversion of methane over a 20 wt % Ni/R-alumina catalyst in a CREC riser simulator used to model a fast fluidized bed. Hydrogen permeation from the reactor produced conversions in excess of those attainable at equilibrium and favorably modified the composition of the synthesis gas. 1. Introduction Steam reforming is often the subject of studies in the use of hydrogen-permeable membranes in multifunctional reactors because of its importance as an industrial reaction, the presence of a strong equilibrium limitation, and the presence of hydrogen in the product. With the exception of Adris et al,1,2 most work has been focused on the production of tubular composite membranes that are then employed in studies using fixed beds of catalyst (packed in the annular space) at low temperature and pressure.3,4 The authors of the present work have proposed a novel conceptual process for the steam reforming of methane in a multifunctional reactor, called catforming, that combines a circulating fluidizedbed reactor with high-temperature hydrogen-permeable membranes.5-7 In the catformer reactor concept (Figure 1), the reactant gas meets with catalyst, and the resulting gassolid suspension enters the primary downflow section of the catformer. As the suspension flows down the reactor tube, the reforming and water-gas shift reactions take place, and hydrogen is continuously removed from the mixture via diffusion through a membrane. Hydrogen removal shifts the conversion of methane to values above those that can be attained at equilibrium and also favorably affects the selectivity. In Figure 1, hydrogen is shown as being removed from the permeate side of the membrane using a sweep stream (e.g., steam). Increased functionality can be achieved by integrating a hydrogenation reaction as a hydrogen sink on the permeate side, as suggested by Armor.8 After exiting the catformer, the synthesis gas product is separated from the catalyst and sent downstream for further processing or direct use in the synthesis of alcohols. During steam reforming, coke can be formed either by kinetic processes when the reactant gas composition is far from its equilibrium value or by equilibrium processes when graphitic carbon is predicted at equilibrium.9 In the catformer, coke formation is expected via kinetic processes, as the reforming reaction is under kinetic rather than heat transfer control, and therefore, some or all of the catalyst leaving the downflow section will need to be regenerated. After regeneration, hot
Figure 1. Schematic flow diagram of the catformer concept.
catalyst is recirculated to the downflow section, thus relieving a portion of the heat duty required in the primary section of the catformer. To assess the viability of the catformer concept, a research project was conceived and executed, the scope of which covered the development of and comparison of the performance of various catalytic materials,10,11 as well as the development and comparison of membranes suitable for the catforming process. To this end, four palladium/Inconel composite membranes were manufactured, three by the electroless deposition of palladium and one via thermal spraying. The comparisons were made under conditions of temperature (750-850 °C) and pressure (1.6-2.8 MPa) closely approximating those found in industrial use. The objective of this paper is to present the characterization results of the composite palladium/Inconel membranes. This includes both the permeability to and selectivity for hydrogen, as well as a comparison between the expected effects (based on model prediction) on the conversion and selectivity and those measured experimentally. 2. Experimental Section 2.1. The Riser Simulator. The riser simulator (Figure 2) is a bench-scale reactor used to simulate the
10.1021/ie0011425 CCC: $20.00 © 2001 American Chemical Society Published on Web 08/30/2001
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Figure 2. Cross-sectional view of a riser simulator unit modified for use in the catforming process.
behavior of riser/downer reactors operating in the fast fluidization regime.12 A sample of catalyst in a basket is inserted into the body of the reactor beneath an impeller. When the impeller is spun, reactant gas is drawn through the basked, causing the sample of catalyst to become fully fluidized. Simulation of the presence of a membrane in the walls of the catformer was achieved by inserting between the upper and lower shells of the reactor a membrane mounted in a flange. Hydrogen generated in the upper shell of the reactor diffuses through the membrane and enters the permeate chamber, from which it is removed by a neutral sweep gas. In the absence of a catalyst, the reactor was used as a permeation cell. 2.2. Membrane Preparation. Membranes were prepared from circular tokens, 2.54 cm in diameter, cut from a sheet of porous Inconel with a nominal pore size of 0.5 µm that was supplied by Mott Metallurgical Corporation. The tokens were then welded into a flange for mounting in the riser simulator. After welding, the surface of the token and the token/weld joint were polished with 0.25-µm diamond paste. The flange assembly was then immersed in an ultrasound bath for 1 h, after which surface contamination was removed by drawing distilled water through the token, followed by benzene and acetone. 2.2.1. Membranes Prepared by Electroless Deposition. Deposition of the palladium film was accomplished by first activating the surface of the token with aqueous solutions of stannous chloride and tetraamminepalladium(II) chloride.7 The surface of the token was well wetted with an aliquot of the stannous chloride solution, and vacuum was used to draw this sample into the pores of the token. Sufficient solution was left standing on the surface that the entire surface was submerged. Ten minutes were allowed to elapse before the solution was rinsed through the token. The token was then washed by drawing distilled water through the surface under vacuum. The treatment was then repeated with the palladium solution. This cycle was repeated 10-12 times until the surface had lost its mirror finish and became a uniform field gray. A Plexiglas cylinder (with an inner diameter just encompassing the diameter of the weld) was then fixed to the plate using silicon
cement, and a 25-mL aliquot of electroless palladium plating solution was applied.7 After each aliquot was depleted of palladium, the membrane was rinsed with distilled water, and a fresh aliquot was applied. When the deposition had proceeded to the point at which the plating solution would not flow through the membrane under the influence of gravity (3-4 aliquots), vacuum was used to draw the plating solution into the pores. Sealing of the surface pores was tested by fixing the plate to the cell and applying pressure to the unplated side. Preparation of a membrane was considered complete when a pressure differential in excess of 30 psig was required for helium bubbles to form on the surface of the palladium. Note that holes smaller than this can be sealed by annealing the membrane at temperatures above 500 °C in hydrogen atmospheres.5 2.2.2. Membrane Prepared by Thermal Spraying. Thermal spraying is a technique for the production of dense, adherent, crack-free films by the high-speed impact of small molten metal particles on the surface of a substrate. For this study, one membrane was produced using a variant of thermal spraying called combustion spraying. In combustion spraying, molten particles are produced by introducing powered metal into an oxygen-fed flame. The molten particles are carried to the target at speeds in the range of 340-350 m s-1 using a neutral carrier. Thermal spraying was found to produce a uniform film of excellent adherence over complex and difficult geometries such as weld joints. However, judging from the membrane used in these trials, thermal spraying did not produce a film that was sufficiently gastight for the resultant composite membrane to be of use as a permeation barrier for hydrogen. 2.3. Hydrogen Permeation and Selectivity Measurement. The permeability and selectivity for hydrogen to argon of membrane MIV was assessed at three temperatures, 750, 800, and 850 °C at each of two hydrogen partial pressures, 324 and 552 kPag. The assessment was performed by first placing the membrane flange in the catformer, sealing the reactor, and allowing a neutral carrier gas (30/70 nitrogen/helium mixture) to flow through the upper and lower shells. The reactor was then heated to the first target temperature. Once the apparatus reached the target temperature, the flow of carrier gas to the upper shell was replaced with a flow of hydrogen permeation gas (5 mLSTP/s, 90/10 hydrogen/argon). The neutral carrier continued to flow through the lower shell at a rate of 13 mLSTP/s. The total transmembrane pressure differential was maintained at 20 kPa, with the higher pressure in the upper shell. One hour was allowed to elapse, after which time samples were taken every 20 min and analyzed until the system was judged to be in equilibrium. Analysis was done using a Perkin-Elmer Sigma 115 GC fitted with a thermal conductivity detector and 1.8 m of Porapak-Q column in series with 4.6 m of Carbonex 1000 column. As the sweep gas flow rate, hydrogen partial pressures, membrane area available for permeation, and hydrogen molar fraction in the sweep gas are known, the hydrogen permeation at steady state for each condition can be determined. 2.4. Testing under Reaction Conditions. Specific details pertaining to the operation of the riser simulator as a catformer have been reported previously.6,7 The catalyst used in the reaction trials, 20 wt % Ni on
Ind. Eng. Chem. Res., Vol. 40, No. 23, 2001 5393 Table 1. Summary of Membrane Service membrane
preparation technique
MI MII MIII MIV MB
electroless thermal spray electroless electroless electrolessa
a
thickness Tmax TOS (µm) (°C) (h) 78 175 156 244
750 850 750 850 550
9 8 32 36
service/ observations reaction residual porosity delamination permeation/reaction permeation
Prepared as described in ref 5.
Figure 4. Hydrogen permeance of membrane MIV as a function of the difference between the square roots of the hydrogen partial pressures on the retentate and permeate sides of the membrane.
Figure 3. SEM image of a cross section (obtained by fracturing) of membrane MIV prepared by the electroless deposition technique and used for both a reaction trial at 800 °C and a permeation study (750-850 °C). Membrane MIV was ductile, and the fracturing process caused the membrane to delaminate. The thickness of the palladium layer was estimated to be 156 µm.
R-alumina, was developed specifically for the catforming process and characterized by Tarek El Solh.10,11 Two membranes, MI and MIV, were used to assess the effect of the membrane on the steam reforming of methane. Using membrane MI, steam reforming was performed at a temperature of 750 °C and a total pressure of 1692 kPag using an average molar steamto-methane ratio (SMR) of (1.3 ( 0.2):1. The pressure of the carrier gas in the permeate chamber (30% nitrogen/70% helium mixture) was set to 152 kPag, and the flow was set to 2.21 mLSTP/s. Using membrane MIV, steam reforming was performed at a temperature of 800 °C, a total pressure of 2068 kPag, and an average molar steam-to-methane ratio (SMR) of (2.1 ( 0.2):1. The pressure of the carrier gas was set to 303 kPag, and the flow was set to 1.84 mLSTP/s. For both membranes, the catalyst-to-oil (C/O) ratio was 6 gCH4/gcat of 20 wt % Ni R-alumina catalyst, a total of 0.395 g of catalyst being used. Reaction trials were performed in a randomized order at each of three contact times: 30, 60, and 90 s. The GC described above was used to analyze the product gas for hydrogen, carbon monoxide, carbon dioxide, and methane. The methane conversion could then be calculated as the number of moles of carbon monoxide and carbon dioxide detected in the GC divided by the total number of moles of all carbon species detected in the GC. For each membrane, repeat trials were carried out at the 60-s contact time. Five trials were carried out using membrane MI and four using membrane MIV. 3. Results and Discussion Four membranes were produced for use in the course of the study, three by the electroless deposition tech-
nique described above (MI, MIII, and MIV) and one by the thermal spray technique (MII). An additional membrane, MB, was produced in a previous study using a technique that involved seeding the surface with nickel and copper prior to deposition of the palladium film using both electrolytic and electroless methods.5 A summary of the service history of each membrane can be found in Table 1. 3.1. Palladium Film Thickness. To determine the palladium film thickness, membranes MI and MIV (Figure 3) were cut free from the flange and fractured. This had the effect of producing a sharp break that exposed a cross section of the membrane showing a portion of the porous support, the support palladium interface, and the palladium film. As membrane MIII experienced complete delamination, the film thickness was determined by weighing the film. 3.2. Permeation Study Results. When the diffusion of hydrogen through the bulk of the palladium film is the limiting step, the hydrogen flux through the membrane can be represented by14
J H2 )
( )
Qo -ED (pH2,10.5 - pH2,20.5) exp z RT
(1)
If this is true, then for a given temperature, a plot of the hydrogen permeation versus the difference in the square roots of the hydrogen partial pressures on the two sides of the membrane should yield a straight line (Figure 4). As this was the case, the parameters Qo and ED were estimated using the nonlinear least-squares regression routine, CURVEFIT.M, available in the Optimization Toolbox (version 2) of MATLAB release 11. As the model form (eq 1) is highly nonlinear in the parameters Qo and ED, the data were centered on the mean temperature.6,7 For membrane MIV, the hydrogen-permeability coefficient, Qo, was estimated to be 1.874 × 10-6 mol/(m s kPa0.5) with a 95% confidence interval (linearized) of (0.052 × 10-7. The value of the activation energy, ED, was estimated to be 22.6 kJ/mol with a 95% confidence interval (linearized) of (4.9 kJ/mol. The correlation between Qo and ED was estimated to be -0.04662. For
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Table 2. Comparison of Permeation Parameters source
Qo (mol/(m s kPa0.5))
ED (kJ/mol)
range(°C)
support type
thickness(µm)
Katsuta12
10-5
20.5 8.88 14.45 15.7 15.7 ( 5.2 22.6 ( 4.9
470-950 450-600 450-600 400-500 250-550 750-850
foil R-alumina R-alumina SS316L Inconel Inconel
940 11.4 17 20 244 156
Collins13,a Collins13,b Shu4 MB5,c MIV7,c a
1.105 × 8.903 × 10-7 2.71 × 10-6 2.018 × 10-5 (4.948 ( 0.71) × 10-6 (1.874 ( 0.052) × 10-6
Exponent on H2 pressure ) 0.58. b Exponent on H2 pressure ) 0.57. c With 95% confidence interval.
Figure 5. Selectivity of membrane MIV for hydrogen over argon.
membrane MB, Qo was estimated to be 4.948 × 10-6 mol/(m s kPa0.5) with a 95% confidence interval of (0.71 × 10-7. The value of the activation energy, ED, was estimated to be 15.7 kJ/mol with a 95% confidence interval of (5.2 kJ/mol. The correlation between Qo and ED was estimated to be -0.5563. A comparison between the values estimated for the parameters of eq 1 for membranes MIV, MB, and a selection of those reported in the literature can be found in Table 2. As can be seen in Table 2, although there is significant variation in the values reported for the permeation parameters, it is evident that the hydrogen permeabilities of the palladium/Inconel composite membranes produced for this study fall within the range of those reported for other composite combinations. 3.3. Selectivity. In theory, a palladium film membrane free of flaws should exhibit an infinite selectivity for hydrogen over any other species. In practice, most thin films contain some degree of residual porosity. Depending on the environment to which the membrane is exposed, cracks and pinholes can develop in the film as a reult of phase change in the palladium/hydrogen system.15 For these reasons, the selectivity is often found to have a finite value. For membrane MIV, the selectivity was found to be high, to rise with temperature (Figure 5), and to decrease with increasing differential hydrogen partial pressure. The latter two observations are a result of the combination of bulk hydrogen diffusion through the palladium and Knudsen diffusion of hydrogen and argon through the pores. For a given differential hydrogen pressure, the rate of hydrogen diffusion through the bulk of the palladium increases with temperature, whereas the rate of Knudsen diffusion decreases. For a given temperature, the selectivity falls with increasing differential hydrogen partial pressure because hydrogen
diffusion through bulk palladium is proportional (eq 1) to the difference in the square root of the hydrogen partial pressures on the two sides of the membrane whereas Knudsen diffusion through the pores is directly proportional to the partial pressure difference. The selectivity values reported here, ranging from 336:1 to 1187:1, are comparable to those observed by other workers. Li et al.16 found that the selectivity for hydrogen over nitrogen for a composite palladium/ stainless steel (316L) membrane produced using electroless deposition ranged from 400 to 1600 over the temperature range 325-475 °C. Nam et al.17 reported hydrogen-to-nitrogen selectivities between 500 and 4700 over the temperature range 350-500 °C for composite palladium/stainless steel membranes produced by vacuum electrodeposition. 3.4. Influence on the Steam Reforming of Methane. 3.4.1. Modeling the Catformer. Simulation of the catformer membrane trials was based on the following two independent equilibrium reactions describing the steam reforming of methane and the water-gas shift reactions respectively
CH4 + H2O 79 8 CO + 3H2 K
(2)
8 CO2 + H2 CO + H2O 79 K
(3)
1
2
Under the conditions used, the steam reforming reaction (eq 2) was found to be under kinetic control, whereas the water-gas shift reaction was found to proceed such that it was at or near equilibrium with the reforming reaction.6,7 The extent of the methane reforming reaction, X, was represented in terms of the rate of consumption of methane per gram of catalyst, w, as
dX ) -rCH4w dt
(4)
From the results of model discrimination carried out over the course of a larger study,7 the rate of reforming, rCH4, was found to follow a one-adsorbate, nondissociative sorption of the Langmuir-Hinshelwood (HougenWatson) type as represented in eq 5.
rCH4 )
-k′KApCH4 (1 + KApCH4)
[
1-
pCOpH23 pCH4pH2OK1
]
(5)
As the water-gas shift reaction was found to be in equilibrium with the reforming reaction, the extent could be calculated from the equilibrium relationship represented in eq 6, where K2 and Y are, respectively, the equilibrium constant and extent of the water-gas shift reaction and Z is the extent of hydrogen permeation.
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[Y][3X + Y - Z]
K2 )
(6)
[X - Y][NH2Oo - X - Y]
Equation 6 can be transformed algebraically such that the extent of the water-gas shift reaction can be found as one of the roots of a quadratic equation. The extent of hydrogen permeation, Z, was found by modeling the lower shell (permeate chamber) as if it were a CSTR.18 As the flux of hydrogen, JH2, can be calculated using eq 1 and the membrane area available for hydrogen permeation, A, is known, the extent of hydrogen permeation can be found by integrating the expression
( )
-ED QoA dZ ) JH2A ) exp (pH2,us0.5 - pH2,ls0.5) (7) dt z RT The hydrogen partial pressure in the lower shell, pH2,ls, was calculated using the total pressure in the lower shell, Pls; the molar flow of sweep gas into the lower shell, Fsweep; and the molar flow of hydrogen permeating into the lower shell from the upper shell.
( ) (
pH2,ls ) Pls
FH2,ls
Ftotal,ls
) Pls
J H 2A
Fsweep + JH2A
)
(8)
In addition to the extents of the steam reforming and water-gas shift reactions, another indicator, Qr, was calculated using eq 9
Qr )
FH2 - FCO2 FCO + FCO2
(9)
where the molar ratio Qr (“Q ratio”) is an indicator used to judge the suitability of a synthesis gas for direct use in the production of alcohols. In general, steam reforming of methane produces synthesis gas that is hydrogenrich, the value of Qr at equilibrium being 3.0. From stoichiometry, the Q ratio is 2.0 for the production of methanol, although in practice the value usually lies between 1.8 and 2.2.19 3.4.2. Reaction Run Results. Two often-cited justifications for the inclusion of hydrogen-permeable membranes in reactors are the potential for the production of large quantities of pure hydrogen and the attainment of conversions greatly in excess of those that can be attained at equilibrium without a membrane.15 Uemiya et al.20 and Shu et al.4 report such results using benchscale tubular membrane reactors and suggest that the primary result of membrane reforming is to induce a large change in the conversion with respect to that at equilibrium. Both performed the steam reforming of methane in a tubular reactor whose wall included a supported palladium membrane (20 µm). Both used similar conditions: a steam-to-methane ratio of 3.0 and temperatures in the range 350-500 °C, with neither reactor being governed by kinetics. Results indicate that the inclusion of the membrane could double4 or triple20 the conversion of methane with respect to the value that can be obtained at equilibrium without the membrane. The flaw with this argument is that, although the thickness of the membrane does influence the amount of hydrogen that can permeate, equally important is the membrane surface-to-reactor volume ratio. In each case, this ratio was high (400020 and 421 m-1 4) when compared to those available if the tubular reactors were
Figure 6. Experimental and simulated methane conversions at three contact times for reforming performed with membranes MI (left) and MIV (right). For each case, the equilibrium conversion at infinite contact time without the presence of a membrane is shown for comparison. The equilibrium conversion is presented for each case as the experimental conditions varied slightly from run to run.
scaled up to industrial size, approximately 79 m-1 for a 5.08-cm (2-in.) tube or 52 m-1 for a 7.62-cm (3-in.) tube. Consequently, bench-scale tubular reactors tend to overemphasize the role of the membrane. The membrane surface-to-volume ratio in the catformer was 13 m-1 for membrane MI and 10 m-1 for membrane MIV; thus, the results tend to underemphasize the membrane effect. Recognizing this fact, it was originally postulated that membranes of the thickness used in this study would have a small effect on the overall conversion relative to what could be obtained at equilibrium and that the primary effect of the membrane would be in the adjustment of the Q ratio. When the effect of the presence of a membrane on methane conversion is considered, in the case where the available membrane surface-to-volume ratio more closely approximates that available at the pilot scale or larger, one conclusion can be drawn. This is, the increase in conversion over what can be obtained at equilibrium will likely not be great both in absolute and relative terms. Modest relative increases in conversion, 5-20% in excess of equilibrium, are to be expected. When the effect of the presence of a membrane on the Q ratio is considered, it can be concluded that, even when the effect on conversion is moderate, the effect on the Q ratio can be large. What is found when experimental conversions from the catformer are compared to equilibrium values (Figure 6) is that, in all cases but one, the inclusion of the membrane did lead to supra-equilibrium conversion. The degree to which equilibrium was exceeded increased with increasing contact time. For membrane MI, after 30 s, the experimental conversion of methane was 4.7% in excess of the equilibrium value. After 60 s, the conversion was 8.9% in excess of equilibrium, and after 90 s, the conversion was 22.2% in excess of equilibrium. For membrane MIV, after 30 s, the experimental conversion did not exceed equilibrium, but after 60 s,
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the conversion was 16.7% in excess of equilibrium. After 90 s, the conversion was 18.4% in excess. Although the conversions achieved in the case of membrane MIV exceeded those achieved for membrane MI (except at 30 s), the conversion in excess of equilibrium was higher for membrane MI. This is related to the relative palladium thickness on each membrane, membrane MIV being twice as thick as MI. Consequently, although the temperature increases from 750 to 800 °C, increasing the permeability by a factor of 1.13, once the thickness is taken into account, the permeability of membrane MIV is lower than that of MI by a factor of 0.565. When the conversion of methane in excess of equilibrium is considered, it can be observed that the methane conversion rises with the contact time and with the absolute conversion of methane (as this also rises with time). This is different from the behavior reported by Adris et al.2 in which the conversion in excess of equilibrium falls with increasing absolute conversion, that is, the relative effect produced by the membrane decreases with increasing absolute conversion of methane. This discrepancy is a result of the difference in operational modes between the catformer and the reactor as proposed by Adris et al.2 Itoh et al.18 proposed classifications for palladium membrane reactors based on the flow regimes on the reaction and separation sides of the membrane and evaluated the relative behaviors of each model type. The flow pattern of the catformer, as envisioned on the pilot/ industrial scale, is classified as a countercurrent plugflow type. The reactant mixture will flow up or down the tube as a plug, while the sweep gas will flow countercurrently to the reactant as a plug. This mode ensures a high and constant differential hydrogen pressure (except near the inlet of the reactant side) across the membrane and, as Itoh et al.18 demonstrated, produces a greater effect than all other modes of operation. In this study, the bench-scale catformer operates as a batch reactor in the upper shell and as a CSTR in the lower shell. The mathematical equivalent of this configuration is a reactor of the cocurrent plug-flow type.18 In this mode of operation, the hydrogen partial pressure difference will begin small but will increase quickly as the reaction proceeds. If the contact time is long enough or the permeability is high enough, the differential pressure will later begin to fall. Simulation results indicated that the sweep gas flow rates used in the experiments would be greater than the hydrogen permeation by a factor of 10-100. At this level, the hydrogen permeation would contribute significantly to the flow leaving the lower shell, but the sweep flow into the lower shell would be sufficient to prevent the driving force from declining. As previously mentioned, hydrogen permeation that causes relatively small changes in conversion in excess of equilibrium can cause large changes in the Q ratio. When membrane MI was employed for 30 s, the conversion in excess of equilibrium was 4.7%, while the Q ratio had fallen to 2.06 (Figure 7). After 60 s, the conversion in excess of equilibrium was 8.9%, and the Q ratio had fallen to 1.32. After 90 s of contact time, the conversion in excess of equilibrium was 22.2%, and the Q ratio was 1.29. As the permeability is lower for membrane MIV, a smaller effect on the Q ratio is expected. Figure 7
Figure 7. Experimental and simulated Q ratios at three contact times for reforming performed with membranes MI (left) and MIV (right) corresponding to the conversions presented in Figure 6. The dashed line represents the ratio predicted by simulation if the membrane thickness is set to infinity, i.e., in the absence of a membrane.
confirms this result, as membrane MIV produced Q ratios of 2.90 after 30 s, 2.75 after 60 s, and 1.92 after 90 s. 4. Conclusions (a) Composite membranes produced in the course of this study via the electroless deposition of palladium on macroporous Inconel are as gastight as and have hydrogen permeabilities that are comparable to those of composite membranes produced on other supports. (b) The use of such membranes during the steam reforming of methane was found to produce methane conversions that were not only greater than those predicted for operation without the membrane but in excess of those that could be attained at equilibrium. (c) The membranes were also able to modify the composition of the synthesis gas produced such that it would be suitable for direct use in methanol synthesis. (d) The membrane surface-to-reactor volume ratio obtained in the bench-scale catformer was of the same order of magnitude as that obtained at the industrial scale. (e) The proposed operational mode of the catformer, countercurrent flow of the reactant and sweep gases, is a major advantage as it maximizes the effect of the membrane. Acknowledgment The authors acknowledge the Natural Science and Engineering Research Council of Canada for funding provided under Strategic Grant STR167310. The authors extend thanks to Volker Ho¨lein and Dr.-Ing. Roland Dittmeyer of DECHEMA for producing a membrane via the thermal spray technique. Nomenclature A ) area of membrane, m2
Ind. Eng. Chem. Res., Vol. 40, No. 23, 2001 5397 ED ) apparent activation energy for hydrogen permeation, J mol-1 F ) molar flow, mol s-1 JH2 ) hydrogen flux, mol m-2 s-1 KA ) adsorption constant for methane, kPa-1 K1 ) equilibrium constant for steam reforming, kPa2 K2 ) equilibrium constant for water-gas shift reaction k′ ) rate constant for the reforming reaction, mol gcat-1 kPa-1 s-1 P ) total pressure, kPa p ) partial pressure, kPa Qo ) hydrogen-permeability coefficient, mol m-2 s-1 kPa0.5 Qr ) Q ratio, [NH2 - NCO2]/[NCO + NCO2] R ) universal gas constant, J mol-1 or kPa cm3 mol-1 K-1 r ) rate of consumption, mol gcat-1 s-1 T ) temperature, K t ) time, s w ) mass of catalyst, g X ) extent of the methane reforming reaction, mol Y ) extent of the water-gas shift reaction, mol Z ) extent of hydrogen permeation, mol z ) palladium film thickness, m Subscripts/Superscripts CH4 ) methane H2O )water/steam H2 ) hydrogen CO ) carbon monoxide CO2 ) carbon dioxide sweep ) sweep gas us ) upper shell ls ) lower shell o ) initial 1,2 )retentate/permeate side of membrane
Literature Cited (1) Adris, S.; Elnashaie, S.; Hughes, R. A Fluidized Bed Membrane Reactor for Steam Reforming of Methane. Can. J. Chem. Eng. 1991, 69, 1061. (2) Adris, A.; Lim, C.; Grace, J. The Fluidized Bed Membrane Reactor System: A Pilot Scale Experimental Study. Chem. Eng. Sci. 1994, 49, 5833. (3) Uemiya, S.; Kude, Y.; Sugino, K.; Sato, N.; Matsuda, T.; Kikuchi, E. A Palladium/Porous-Glass Composite Membrane for Hydrogen Separation. Chem. Lett. 1988, 10, 1687.
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Received for review December 30, 2000 Revised manuscript received June 6, 2001 Accepted June 18, 2001 IE0011425
