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Pervaporation of aqueous organic mixtures through Ge-ZSM-5 zeolite membranes
DESALINATION ELSEVIER
Desalination 147 (2002) 327-329 www.elsevier.com/locate/desal
Pervaporation of aqueous organic mixtures through Ge-ZSM-5 zeolite membranes Travis C. Bowen, Shiguang Li, Vu A. Tuan, John L. Falconer, Richard D. Noble* Department of Chemical Engineering, Universi O' of Colorado, Boulder, CO 80309-0424, USA Tel. +1 (303) 492-6100; Fax +1 (303) 492-4341; e-mail: [email protected]
Received l February 2002: accepted 7 March 2002
Abstract
Separanon factors for pervaporation of 5 wt% organic/water mixtures through Ge-ZSM-5 zeolite membranes are strongly correlated with the liquid-phase fugacities of the organic components in the feed. Competitive adsorption coverages probably depend on fugacity, This correlation allows qualitative prediction of zeolite membrane performance and is an important advancement since it is possible to group the separation factors by functionality of the organic component. Therefore, we now have a means to determine the approximate range of separation factors for various mixtures. Keywords: Zeolite membranes; Pervaporation; Driving force; Fugacity; Ge-ZSM-5 zeolite; Organic/water separations
Zeolites are crystalline structures made mostly of SiO 2 with uniform, molecular-sized pores. Zeolite membranes can separate mixtures by molecular sieving, selective adsorption, and/or diffusion differences. Hydrophobicity or hydrophilicity of the zeolite is important for selective adsorption in organic/water separations, lsomorphous sub*Corresponding author.
stitution of silicon by other elements in the zeolite framework has been shown to change the relative hydrophobicity/hydrophilicity of the membrane. Although only small changes in the unit cell parameters and lattice vibration frequencies were observed when Ge was substituted for Si in MFI zeolite powders, the adsorption properties could change with substitution due to the changes in the T-O-T bond angle and T-O bond length [1].
Presented at the International Congress on Membranes and Membrane Processes (ICOM), Toulouse, France, July 7-12, 2002.
0011-9164/02/$- See front matter © 2002 Elsevier Science B.V. All rights reserved Pll: S 0 0 1 1 - 9 1 6 4 ( 0 2 ) 0 0 5 6 0 - X
328
T.C. Bowen et al. / Desalination 147 (2002) 327-329
Understanding the transport mechanisms and driving force of pervaporation is necessary for predicting membrane performance and designing membranes for specific applications. This study investigates the effects of feed fugacity on competitive adsorption and driving force in pervaporation through a zeolite membrane. A Ge-ZSM-5 (Si/Ge = 25) zeolite membrane was synthesized by in-situ crystallization on the inner surface of porous stainless steel tubes. Characterization by XRD and SEM showed that the membrane had an approximately 30-gin thick layer of intergrown crystals with the ZSM-5 structure (~0.53 nm X R D pore size). The membrane had a n-C 4Hj0/i-C4Hj0 ideal selectivity of 2.1 at 473 K and a 0.2 g/m2.h 1,3,5-tri-isopropyl benzene (0.85 nm kinetic diameter) flux at 303 K, indicating that it had few non-zeolite pores larger than 0.85 nm. Pervaporation through this membrane was used to separate aqueous, binary feeds of 5 wt% organics, which were all smaller than the zeolite pore size and exhibited a three order-of-magnitude range of fugacities. Acetic acid, propionic acid, methanol, ethanol, 1-propanol, 2-propanol, 1butanol, acetone, MEK, methyl acetate, ethyl acetate, propanal, and diethyl ether were used. The organic component preferentially permeated through the membrane for all feeds at 303 to 333 K. Ethanol had an organic/water separation factor of 47, a 0.22 kg/m2.h total flux, and a 1.30 kPa fugacity at 303 K. The lowest separation factor was 2.7 for acetic acid, which had a fugacity of 0.12 kPa at 303 K. This mixture had a 0.075 kg/m2.h total flux. The highest separation factor was 1200 for diethyl ether, which had a 147 kPa fugacity at 333 K and its total flux was 1.23 kg/m2.h. Water fluxes for most of the mixtures were significantly lower than the pure water fluxes of 0.24-0.66 kg/m2-h for 303-333 K, indicating that the organics preferentially adsorbed on the zeolite and inhibited water adsorption and transport. Therefore, the separation occurred mainly by
competitive adsorption. The separation factors, however, are not highly correlated with the diffusivities or isosteric heats of adsorption. In addition to diffusivity and heat of adsorption, fugacity is also important in zeolite membrane pervaporation separations. Fig. 1 shows that the organic/water separation factors strongly depend on the liquid-phase fugacities of the organic components in the feed for temperatures of 303, 313,323, and 333 K. The organic fluxes exhibit a similar dependence on organic fugacity. The water fugacities for these mixtures were not affected significantly by the presence of the organic because the organic concentrations were low, and the fugacities were therefore similar to the pure water fugacity for all feeds. Analysis of literature data shows that fugacity correlates with selectivity for other high-quality zeolite membranes. The feeds include high and low concentrations of organics in water and organic-organic mixtures. In general, the flux of the preferentially-permeated molecule and the separation factor for a given membrane increase as the feed fugacity of the preferentially-permeated molecule increases.
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Liquid-Phase Orgarti¢ Fuga¢ity (kPa)
Fig. 1. Organic flux for pervaporation of 5 wt% organic/ water mixtures through a Ge-ZSM-5 membrane at 303, 313,323, and 333 K.
T.C. Bowen et al. /Desalination 147 (2002) 327-329
The driving force for transport through zeolite membranes is the coverage gradient across the zeolite layer. For pure liquid feeds, the fugacity is usually sufficiently high that the adsorption sites are saturated and intracrystalline flow is not dependent on the feed fugacity [2]. However, in mixtures, molecules compete for adsorption sites and a higher fugacity probably correlates with a higher fractional coverage at the feed-membrane interface. Using the relationship of selectivity and fugacity, measurements of pervaporation separations for one mixture in a given zeolite membrane should
329
allow qualitative prediction of performance for other feed mixtures.
References [I] H. Kosslick, V.A. Tuan, R. Fricke, C. Peuker, W. Pilz and W. Storek, Synthesis and characterization of Ge-ZSM-5 zeolites,J. Phys. Chem., 97 (1993) 56785684. [2] M. Nomura,T. Yamaguchiand S.-I. Nakao,Transport phenomena through intercrystalline and intracrystalline pathways of silicalite zeolite membranes, J. Membr.Sci., 187 (2001) 203-212.
Desalination 147 (2002) 327-329 www.elsevier.com/locate/desal
Pervaporation of aqueous organic mixtures through Ge-ZSM-5 zeolite membranes Travis C. Bowen, Shiguang Li, Vu A. Tuan, John L. Falconer, Richard D. Noble* Department of Chemical Engineering, Universi O' of Colorado, Boulder, CO 80309-0424, USA Tel. +1 (303) 492-6100; Fax +1 (303) 492-4341; e-mail: [email protected]
Received l February 2002: accepted 7 March 2002
Abstract
Separanon factors for pervaporation of 5 wt% organic/water mixtures through Ge-ZSM-5 zeolite membranes are strongly correlated with the liquid-phase fugacities of the organic components in the feed. Competitive adsorption coverages probably depend on fugacity, This correlation allows qualitative prediction of zeolite membrane performance and is an important advancement since it is possible to group the separation factors by functionality of the organic component. Therefore, we now have a means to determine the approximate range of separation factors for various mixtures. Keywords: Zeolite membranes; Pervaporation; Driving force; Fugacity; Ge-ZSM-5 zeolite; Organic/water separations
Zeolites are crystalline structures made mostly of SiO 2 with uniform, molecular-sized pores. Zeolite membranes can separate mixtures by molecular sieving, selective adsorption, and/or diffusion differences. Hydrophobicity or hydrophilicity of the zeolite is important for selective adsorption in organic/water separations, lsomorphous sub*Corresponding author.
stitution of silicon by other elements in the zeolite framework has been shown to change the relative hydrophobicity/hydrophilicity of the membrane. Although only small changes in the unit cell parameters and lattice vibration frequencies were observed when Ge was substituted for Si in MFI zeolite powders, the adsorption properties could change with substitution due to the changes in the T-O-T bond angle and T-O bond length [1].
Presented at the International Congress on Membranes and Membrane Processes (ICOM), Toulouse, France, July 7-12, 2002.
0011-9164/02/$- See front matter © 2002 Elsevier Science B.V. All rights reserved Pll: S 0 0 1 1 - 9 1 6 4 ( 0 2 ) 0 0 5 6 0 - X
328
T.C. Bowen et al. / Desalination 147 (2002) 327-329
Understanding the transport mechanisms and driving force of pervaporation is necessary for predicting membrane performance and designing membranes for specific applications. This study investigates the effects of feed fugacity on competitive adsorption and driving force in pervaporation through a zeolite membrane. A Ge-ZSM-5 (Si/Ge = 25) zeolite membrane was synthesized by in-situ crystallization on the inner surface of porous stainless steel tubes. Characterization by XRD and SEM showed that the membrane had an approximately 30-gin thick layer of intergrown crystals with the ZSM-5 structure (~0.53 nm X R D pore size). The membrane had a n-C 4Hj0/i-C4Hj0 ideal selectivity of 2.1 at 473 K and a 0.2 g/m2.h 1,3,5-tri-isopropyl benzene (0.85 nm kinetic diameter) flux at 303 K, indicating that it had few non-zeolite pores larger than 0.85 nm. Pervaporation through this membrane was used to separate aqueous, binary feeds of 5 wt% organics, which were all smaller than the zeolite pore size and exhibited a three order-of-magnitude range of fugacities. Acetic acid, propionic acid, methanol, ethanol, 1-propanol, 2-propanol, 1butanol, acetone, MEK, methyl acetate, ethyl acetate, propanal, and diethyl ether were used. The organic component preferentially permeated through the membrane for all feeds at 303 to 333 K. Ethanol had an organic/water separation factor of 47, a 0.22 kg/m2.h total flux, and a 1.30 kPa fugacity at 303 K. The lowest separation factor was 2.7 for acetic acid, which had a fugacity of 0.12 kPa at 303 K. This mixture had a 0.075 kg/m2.h total flux. The highest separation factor was 1200 for diethyl ether, which had a 147 kPa fugacity at 333 K and its total flux was 1.23 kg/m2.h. Water fluxes for most of the mixtures were significantly lower than the pure water fluxes of 0.24-0.66 kg/m2-h for 303-333 K, indicating that the organics preferentially adsorbed on the zeolite and inhibited water adsorption and transport. Therefore, the separation occurred mainly by
competitive adsorption. The separation factors, however, are not highly correlated with the diffusivities or isosteric heats of adsorption. In addition to diffusivity and heat of adsorption, fugacity is also important in zeolite membrane pervaporation separations. Fig. 1 shows that the organic/water separation factors strongly depend on the liquid-phase fugacities of the organic components in the feed for temperatures of 303, 313,323, and 333 K. The organic fluxes exhibit a similar dependence on organic fugacity. The water fugacities for these mixtures were not affected significantly by the presence of the organic because the organic concentrations were low, and the fugacities were therefore similar to the pure water fugacity for all feeds. Analysis of literature data shows that fugacity correlates with selectivity for other high-quality zeolite membranes. The feeds include high and low concentrations of organics in water and organic-organic mixtures. In general, the flux of the preferentially-permeated molecule and the separation factor for a given membrane increase as the feed fugacity of the preferentially-permeated molecule increases.
103
A
~ 102
oo o ° ~ "/ .~ 0 0
°o "~ 10 ~ / ~
,5. ether
¢~ carbox'ylic acids ~, aldehyde .
10-I
.
.
.
.
.
.
.
i
100
.
.
.
.
.
.
.
.
i
.
.
.
.
.
.
.
.
101
t
102
Liquid-Phase Orgarti¢ Fuga¢ity (kPa)
Fig. 1. Organic flux for pervaporation of 5 wt% organic/ water mixtures through a Ge-ZSM-5 membrane at 303, 313,323, and 333 K.
T.C. Bowen et al. /Desalination 147 (2002) 327-329
The driving force for transport through zeolite membranes is the coverage gradient across the zeolite layer. For pure liquid feeds, the fugacity is usually sufficiently high that the adsorption sites are saturated and intracrystalline flow is not dependent on the feed fugacity [2]. However, in mixtures, molecules compete for adsorption sites and a higher fugacity probably correlates with a higher fractional coverage at the feed-membrane interface. Using the relationship of selectivity and fugacity, measurements of pervaporation separations for one mixture in a given zeolite membrane should
329
allow qualitative prediction of performance for other feed mixtures.
References [I] H. Kosslick, V.A. Tuan, R. Fricke, C. Peuker, W. Pilz and W. Storek, Synthesis and characterization of Ge-ZSM-5 zeolites,J. Phys. Chem., 97 (1993) 56785684. [2] M. Nomura,T. Yamaguchiand S.-I. Nakao,Transport phenomena through intercrystalline and intracrystalline pathways of silicalite zeolite membranes, J. Membr.Sci., 187 (2001) 203-212.