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Composite Catalytic-Permselective Membranes: A Strategy for Enhancing Selectivity and Permeation Rates via Reaction and Diffusion Benjamin A. Wilhite* Artie McFerrin Department of Chemical Engineering, Texas A&M University, College Station, Texas 77843-3122, United States ABSTRACT: This manuscript presents a new strategy for enhancing permselective films via the addition of a porous, catalytically active layer in a composite catalytic-permselective membrane design. A general mathematical analysis of reaction and diffusion within the catalytic layer is presented in order to establish design criteria. Numerical simulations are presented for the case of the water-gas-shift reaction supporting hydrogen purification, to demonstrate the advantages of the composite catalytic-permselective design concept. For the case of a water-gas-shift catalytic coating placed atop a hydrogen-permselective dense palladium film, an 84% reduction in carbon monoxide (CO) exposure is predicted, along with a mild (8%) increase in the overall hydrogen permeation rate; this reduction in carbon monoxide contamination of the palladium surface represents a significant improvement in palladium film utilization. For the case of the same catalytic coating placed atop a carbon dioxide (CO2)-permselective polymer film, CO2CO permselectivities are increased by two orders of magnitude.
’ INTRODUCTION Membrane technology has emerged as a powerful alternative to cryogenic distillation or adsorption systems for several gas purification applications, with the primary advantages of reduced system footprint, energy, and capital costs.13 Gas separation membranes have garnered special attention in energy applications, primarily in the removal of carbon dioxide (CO2) and other contaminants from natural gas4,5 and in the purification of hydrogen from reformate mixtures.68 The advent of hydrogendriven fuel cell power systems over the past two decades9,10 and the continued growth of hydrotreatment processes in the petrochemical industry11 has driven substantial research into the development of novel inorganic7,12 and polymeric13,14 permselective membranes capable of breakthroughs in hydrogen permeation rates and permselectivities for extracting high-purity hydrogen from reformate mixtures. Hydrocarbon fuels may be converted to hydrogen-rich mixtures through exothermic partial oxidation (see reaction R1) and/or endothermic steam reforming (see reaction R2). Additional gains in hydrogen yields are achieved through the watergas-shift reaction (see reaction R3), with the additional advantage of reducing the undesired carbon monoxide (CO) content. n ðR1Þ O2 f n CO þ ðn þ 1ÞH2 Cn H2nþ2 þ 2 Cn H2nþ2 þ n H2 O f n CO þ ð2n þ 1ÞH2
ðR2Þ
CO þ H2 O T CO2 þ H2
ðR3Þ
There exists a valuable synergy between the reforming reactions described by reactions R1R3 and hydrogen purification, in that undesired permeate species (e.g., carbon monoxide (CO), hydrocarbon fuels) are catalytically converted to the desired permeate species for membrane purification. This synergy has r 2011 American Chemical Society
been explored using catalytic packed-bed membrane reactors for combining fuel reforming with hydrogen-permselective membranes,1517 such that the permselective removal of hydrogen reaction product enhances the reforming rates and yields via LeChatelier’s Principle.18,19 The same synergy can be utilized by coupling the water-gas-shift with either hydrogen-permselective inorganic films2022 or carbon dioxide (CO2)-permselective polymeric films23,24 to obtain high-purity hydrogen from reformate mixtures of CO and hydrogen (H2). In both membrane reactor design strategies, the catalytic conversion of undesired permeates (reactants) to desired permeate (products), and their subsequent permselective removal, occur in parallel, which often exposes the limitations of the permselective film. Dense palladium films remain a leading technology for hydrogen purification, because of the infinite theoretical permselectivities and an established knowledge base developed over the past 50 years.12,25,26 Typical film permeabilities of ∼1 108 mol m1 s1 Pa0.5 or ∼1 1012 mol m1 s1 Pa1 have been reported in the literature over a range of film thicknesses (∼2100 μm) and compositions.7,12,27 In practice, such films are often limited by the competitive adsorption of CO and other species, which substantially inhibit permeation rates.16,2729 In addition, irreversible corrosion by hydrocarbons30 and sulfur31,32 becomes an increasing concern as permselective film thickness is reduced. Research to date aimed at reducing inhibition and/or corrosion has focused upon the development of novel alloy compositions33,34 and alternative permselective materials. Polymeric membranes have emerged as a promising alternative for hydrogen purification via the selective removal of CO2.13,14,24 These systems promise a dramatic reduction in Received: January 20, 2011 Accepted: August 2, 2011 Revised: July 21, 2011 Published: August 02, 2011 10185
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Figure 1. Illustration of composite catalytic-permselective membrane design concept. Inset shows a schematic of the well-mixed permeate and retentate model employed in the present study.
materials costs (as compared to palladium alloys) with competitive permeabilities. Several polyvinyl-based CO2-permselective membranes have been reported with permeabilities of ∼(0.11) 1012 mol m1 s1 Pa1 and CO2:H2 permselectivities ranging from 30:1 to 200:1.5,8 Advances in polymer thin-film deposition methods35,36 and manufacturing5,37 provide reduction in the active film thicknesses to the nanometer scale, thus allowing order-of-magnitude improvements in permeation rates, compared to palladium-based films for hydrogen separation. Current research in polymeric membranes has focused upon the modification of existing polymers to enhance selectivities13,14 to compete with palladium-based membranes. Reaction engineering provides a promising alternative to materials design for addressing corrosion/inhibition and permselectivity challenges in hydrogen purification, by employing a composite catalytic-permselective membrane design (see Figure 1). In this design, the membrane is assembled from a permselective thin film, combined with a porous catalytic film, such that the catalytic conversion of undesired permeates to desired permeates and/or inerts directly supports the permselective removal of the desired permeate. In this manner, the catalytic film can significantly improve performance of an underlying permselective film via: (1) catalytic conversion of contaminants or inhibitors, thus providing a corrosion-protection layer, and/or (2) catalytic production of additional desired permeate in the immediate vicinity of the permselective film, thus providing gains in observed permeation rates, and/or (3) catalytic modification of the ratio of desired to undesired permeate concentrations at the permselective film surface, thus providing significant gains to observed permselectivities. Wilhite and colleagues experimentally demonstrated the composite catalytic-permselective membrane design approach for addressing the issue of palladium thin-film corrosion by methanol in a miniaturized methanol membrane reformer.30 By creating a composite membrane structure comprised of a 200-nm-thick palladium permselective film protected by a ∼500-μm catalyst layer (8:1 LaNi0.95Co0.05O3:Al2O3), corrosion was effectively prevented without introducing any additional diffusional limitations from the catalyst layer. A similar composite catalyticpermselective membrane design was reported by Nomura and co-workers,38 who applied a Ni-based methane steam reforming catalyst over a hydrogen-permselective silica film; although hydrogen production rates comparable to those of a packed-bed membrane reactor were observed, no comparisons of membrane
permeability or permselectivity toward hydrogen were presented. Although these initial investigations have illustrated both the promise and potential tradeoffs in utilizing the composite catalytic-permselective membrane design, there remains a need for fundamental modeling analysis to provide a rigorous basis for implementing this new design approach for gas separations membranes. This manuscript presents a detailed analysis of the composite catalytic-permselective membrane strategy with the aim of establishing design criteria for its successful use in enhancing the performance of permselective thin films. In the present study, design analysis is focused upon the instantaneous, or localized, enhancement in membrane permselectivity and permeation rates. In addition to a generalized dimensionless analysis, an application-specific design study is presented for the case of the water-gas-shift reaction directly supporting the performance of (i) high-temperature, hydrogen-permselective palladium film and (ii) low-temperature, carbon dioxidepermselective polymeric thin film. These results provide valuable insight for applying the composite catalytic-permselective membrane design approach to hydrogen purification membranes.
’ MODEL DEVELOPMENT A generalized model is developed for the composite catalyticpermselective membrane, assuming that a single elementary chemical reaction (reaction R4) occurs within the catalytic layer, which converts undesired permeate species (A, B) to desired (C) and undesired permeate (D) products, i.e., k
νA A þ νB B S νC C þ νD D K
ðR4Þ
such that the rate expression is written as r ¼k
jν j jν j pA A pB B
jν j jν j
p Cp D C D K
! ð1Þ
Reaction heat is assumed to be negligible, such that the present analysis may focus upon a fundamental understanding of how the overall membrane performance is manipulated solely through mass transport and reaction within the catalytic layer. For the case of the mildly exothermic water-gas° = 42 000 J mol1) and assuming an shift reaction (ΔHrxn effective thermal conductivity for a porous alumina-based catalyst washcoating of 3 W m 1 s1, Prater temperatures are 10186
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Table 1. Comparison of Reformate Compositions for Methanol and Diesel Hydrocarbon Fuelsa
a
yCO,in
yH2,in
yH2O,in
yCO2,in
θCO,in
θH2,in
θH2O,in
θCO2,in
(θH2θCO2/θH2O)in
methanol SR
0.01
0.65
0.18
0.16
1
65
18
16
55
methanol ATR
0.01
0.41
0.10
0.15
1
41
10
15
65
diesel ATR
0.09
0.28
0.15
0.03
1
3.3
1.8
0.3
0.6
ATR = autothermal reforming; SR = steam reforming.
determined to be 0.5) to achieve substantial increases (by a factor of >200) in the overall membrane CO2:CO permselectivity. Figure 4b illustrates the importance of identifying molecular-sieve type materials capable of achieving high relative hydrogen permeances (i.e., order-of-magnitude reduction in catalyst CO2:H2 permselectivity) in order to minimize reduction in the overall CO2:H2 permselectivity. In all of the above analyses, a constant fluid composition is assumed; e.g., changes in bulk fluid composition, in response to membrane separations, are neglected. This has enabled a direct analysis of instantaneous or localized enhancement of permselective film performance via composite catalytic-permselective membrane designs, and the establishment of general design criteria. It is important to recognize that enhancement/reduction in film performance will vary over the axial length of the membrane system as the design criteria (K/θ) changes. For cases wherein membrane permeation rates are substantially greater than reaction rates, undesired permeate/coreactant concentrations may be expected to increase along the flow length of the membrane, because of removal of the desired permeate, creating a greater need for the composite catalytic-permselective membrane design. In cases where reaction rates exceed permeation rates, the depletion of both desired and undesired reactants may reduce the need for enhancements via the composite catalytic-permselective membrane design. Therefore, care must also be taken at the systems level when designing membrane systems that employ composite catalytic-permselective membranes to ensure that desirable conditions are maintained along the entire axial length of the system. This will be explored in detail in a subsequent manuscript.
’ CONCLUSIONS The present analysis demonstrates that composite catalytic-permselective membrane designs promise substantial gains in corrosion prevention, permeation rates, and permselectivities for membrane separations. At the heart of this design strategy is the use of chemical reactions to modify permeate partial pressures at the surface of the permselective film; thus, the catalytic reaction must be capable of (i) destroying undesired inhibitors/corrosives for corrosion prevention, and/or (ii) converting undesired permeate to
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desired permeate for enhancing permeation rates and permselectivities. Analysis of the composite catalytic-permselective membrane design for hydrogen purification from diesel reformates illustrates the advantages of this new approach to membrane design. For the specific case of an aluminasupported water-gas-shift catalytic film placed atop a hydrogen-permselective dense palladium film, application of the above design rules dictate that a catalytic coating 470 106 m thick is required to achieve an 84% reduction in carbon monoxide (CO) exposure, along with a mild (8%) increase in the overall hydrogen permeation rate; thus, the addition of a catalytic thin film prevents corrosion and/or competitive inhibition of the palladium film by CO, which is a leading limitation to current palladium membrane systems. Application of the same design criteria to the case of an identical catalytic film placed atop a carbon dioxide (CO 2 )-permselective polymer film dictates a coating thickness of 6.3 10 3 m, which corresponds to a predicted enhancement of CO 2 CO permselectivity enhancement by more than two orders of magnitude.
’ APPENDIX A: ANALYTICAL SOLUTION FOR FIRSTORDER REVERSIBLE REACTION, NEGLIGIBLE DIFFUSION LIMITATIONS UPON PERMEATION For the case of an elementary, first-order reversible reaction (i.e., νB = νD = 0), the mass balances for species A and C can be cast as a system of coupled linear differential equations: " # " # uA d2 u A ¼ ϕ2 M ðA-1aÞ uC ds2 uC The assumption of negligible diffusion resistance upon permeation (i.e., ζ f 0) yields boundary conditions of the form " # " # " # 0 0 uA d uA ¼ at s ¼ 0 ðA-1bÞ 0 ζ 3 uC ds uC and "
uA uC
where
# ¼ ðI 3 PÞ 2 1
6 M ¼6 4 1 Rcat C=A
at s ¼ 1
3 1 K 7 1 7 5 and KRcat C=A
ðA-1cÞ
"
P ¼
1 θC
# ðA-1dÞ
Recognizing that the solution may be cast as a series of i functions defined by the eigenvalues and eigenvectors of matrix M, i.e., ~uj ¼
∑i ai ðλi ; sÞ 3 j~i
ðA-2Þ
then eq A-1a can be written in terms of a position function, ai(s) d2 ai ðλi ; sÞ ¼ ϕ2 λi 2 ai ðλi ; sÞ ds2
ðA-3aÞ
where M I 3 λ2 ¼ 0 10191
ðA-3bÞ
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which has the general solution ai ðsÞ ¼ Ci sinh λi ϕs þ Di cosh λi ϕs
ðA-4Þ
Corresponding eigenvalues and eigenvectors are obtained from eq A-3b: λ1 2 ¼ 0
and
λ2 2 ¼ 1 þ
1 KRcat C=A
which correspond to eigenvectors, 3 2 1 6 KRcat 7 6 C=A 7 j1 ¼ 6 1 7 and j2 ¼ 5 4 Rcat C=A
3 1 6 7 K 6 7 4 1 5 cat KRC=A
ðA-5aÞ
2
ðA-5bÞ
such that uA ¼
D1 C2 D2 sinh λ2 ϕs þ cosh λ2 ϕs cat þ KRC=A K K
ðA-6aÞ
and uC ¼
D1 C2 D2 sinh λ2 ϕs cat cosh λ2 ϕs cat cat RC=A RC=A K RC=A K ðA-6bÞ
Application of the boundary conditions described in eq A-1b yields a value for C2: C2 ¼ 0
ðA-7aÞ
Application of the boundary conditions described in eq A-1c allows solution for B1 and B2: 1 þ θC Rcat C=A 1 þ KRcat C=A and
"
KRcat C=A
KRcat C=A ¼ D1
ðA-7bÞ
# K θC 1 ¼ D2 cat 1 þ KRC=A cosh λ2 ϕ
such that the final solution is uA ¼
1 þ 1 þ
θC Rcat C=A KRcat C=A
þ
2
6 6 KRcat C=A 4
ðA-7cÞ
3 θC 7cosh λ2 ϕs K 7 5 þ KRcat C=A cosh λ2 ϕ 1
1
ðA-8aÞ 2 uC ¼
1 þ 1 þ
θC Rcat C=A KRcat C=A
6 K K6 4
3 θC 7cosh λ2 ϕs K 7 5 þ KRcat C=A cosh λ2 ϕ 1
1
’ ACKNOWLEDGMENT Support of this work was provided by the National Science Foundation (CBET-CBS, through Award No. 0730820). Additional support of this effort was provided by the DuPont de Nemours Corporation, through a Young Faculty Award. ’ NOTATION Di = effective diffusivity of the ith species in catalytic film (m2 s1) k = forward rate coefficient for supporting reaction in catalytic film (m3 s1 mol1) K = equilibrium coefficient for supporting reaction in catalytic film L = catalytic film thickness (m) pi = partial pressure of the ith species in catalytic film (m) Pi = permeability of the ith species in permselective film (mol m1 s1) 3 ri = molar rate of reaction for the ith species (mol/(m s1)) n = order dependency of gas flux upon partial pressure for permselective film R = ideal gas constant (Pa m3 mol1 K1) s = dimensionless depth of catalytic film; s = z/L t = thickness of permselective film (m) T = membrane temperature (K) ui = dimensionless concentration of reactant, undesired permeate species; ui = pi/pA,f z = catalytic film depth (m) Symbol
Rcat C/i = permselectivity toward the desired permeate (C) over the ith undesired species in catalyst film; Rcat C/i = DC/Di perm RC/i = permselectivity toward the desired permeate (C) over the ith undesired species in permselective film; Rperm C/i = PC/Pi ϕ = Thiele modulus for catalyst film; ϕ = (pA,fkL2/DA)1/2 ηperm = membrane enhancement factor for transport of the C desired permeate ηsel C/i = membrane enhancement factor for permselectivity toward the desired permeate νi = stoichiometry coefficient for the ith species θi = dimensionless fluid-phase concentration of the ith species; θi = pi,f/pA,f ζ = ratio of permselective to catalytic film permeance; ζ = (PC/t)(RTL/DC) Subscript
A = reactant and undesired permeate B = coreactant and undesired permeate C = desired reaction product and desired permeate D = undesired reaction product and undesired permeate f = evaluated within the feed (retentate) bulk fluid p = evaluated within the permeate (sweep) bulk fluid
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’ AUTHOR INFORMATION Corresponding Author
*Tel.: (979) 845 0406. Fax: (979) 845 6446. E-mail: Benjamin.
[email protected].
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