1124
Ind. Eng. Chem. Res. 1996, 35, 1124-1132
Pervaporation Using Adsorbent-Filled Membranes Wenchang Ji† and Subhas K. Sikdar*,‡ CeraMem Corporation, 12 Clematis Avenue, Waltham, Massachusetts 02154, and Water and Hazardous Waste Treatment Research Division, NRMRL, U.S. EPA, 26 West Martin Luther King Drive, Cincinnati, Ohio 45268
Membranes containing selective fillers, such as zeolites and activated carbon, can improve the separation by pervaporation. Applications of adsorbent-filled membranes in pervaporation have been demonstrated by a number of studies. These applications include removal of organic compounds from water, dehydration of azeotropic mixtures, and organic/organic separation. Three different types of adsorbents have been evaluated: zeolites, activated carbon, and carbon molecular sieves. Experimental results show the most promising filled membrane is the silicalitefilled poly(dimethylsiloxane) (PDMS) membrane which can be effectively used in the removal of organic compounds from water. The transport of species through adsorbent-filled membranes occurs by a sorption-diffusion mechanism. Sorption of species in the filled rubbery membranes can be described by a dual sorption model. Modeling of the transport process through filled membranes involves mass-transfer resistances in the different phases of the membranes: in the polymer phase, in the adsorbent phase, and in a polymer-adsorbent interface. Several models based on resistance-in-series mechanisms that were developed to describe the experimental results were reviewed. 1. Introduction Pervaporation has become an important membrane separation process. Applications of pervaporation include (a) removal of organic compounds from water, (b) dehydration of an organic/water azeotropic mixture, and (c) separation of organic/organic mixtures. Membranes used in the pervaporation process are dense polymeric membranes. Mass transport through membranes in pervaporation follows a sorption-diffusion mechanism: sorption of liquid into the membrane at the feed side, transport through the membrane, and desorption into the vapor phase at the permeate side of the membrane. It has been shown that preferential sorption of a permeating organic species determines its overall selectivity to a large extent for rubbery membranes used for organic removal. But preferential diffusion through membranes usually results in preferential permeation for glassy polymeric membranes (Lee et al., 1989). Owing to the development of poly(vinyl alcohol) (PVA)-poly(acrylonitrile) (PAN) composite membranes with extremely high selectivity and permeation flux for water permeation, pervaporation has been successfully commercialized by GFT (Gesellschaft fu¨r Trenntechnik, Hamburg, Germany) for dehydration of an organic/water azeotropic mixture. More than 90 industrial solvent dehydration units using the pervaporation process were installed worldwide (Rautenbach et al., 1992). However, removal of organic compounds, particularly alcohols, from aqueous solutions by pervaporation is still in the pilot plant stage, mostly because of the unavailability of organophilic membranes with high selectivity. PDMS membranes are currently used in most of these applications. However, selectivity for a PDMS membrane is low, for example, less than 10 for ethanol over water for an ethanol/water mixture. As a result, permeate concentration with a PDMS membrane is low, no more than 50% in the case of pervaporation of dilute ethanol aqueous solutions. * Author to whom correspondence is addressed. Telephone: (513) 569-7528. Fax: (513) 569-7787. † CeraMem Corp. ‡ NRMRL.
0888-5885/96/2635-1124$12.00/0
Based on the sorption-diffusion model, membrane performance can be enhanced by improving either the selective sorption or selective diffusion or, if possible, both. Therefore, incorporating microporous adsorbents with high sorption selectivity uniformly into a polymeric matrix could enhance the overall selectivity of filled membranes. Microporous adsorbents of this kind are zeolite, carbon molecular sieve, and activated carbon. According to te Hennepe et al. (1987), an appropriate zeolite used for the preparation of filled pervaporation membranes should have the following properties: (1) selectivity toward the same molecules as the pure polymeric membrane has; (2) desorption under pervaporation conditions. When adsorbent-filled membranes are used, the pervaporation process combines the advantages of the high sorption capacity of microporous hydrophobic adsorbents with the continuous operation of membrane separation processes. Advantages of using polymeric membranes containing selective adsorbents have been amply demonstrated. Both selectivity and flux were improved by adding hydrophobic zeolites into silicone rubber membranes. Hydrophobic zeolites used in studies are silicalite (te Hennepe et al., 1987; Jia et al., 1992) and ZSM-5 zeolite (Ping et al., 1992; Cen and Lichtenthaler 1992; BartelsCaspers et al., 1992; Chen et al., 1993). A hydrophilic zeolite was used to facilitate water transport and to increase the selectivity of water over ethanol in breaking ethanol-water azeotrope (Goldman et al., 1989; Gao et al., 1993). Higher O2 permeability and O2/N2 selectivities were obtained for silicalite-filled membranes than for those without filler (Jia et al., 1991). Duval et al. (1993) studied the effect of addition of various kinds of zeolites on the gas separation properties of polymeric membranes. They found that zeolites such as silicate1, 13X, and KY improved the separation of CO2/CH4 mixtures by rubber polymers. Other adsorbents such as activated carbon and carbon molecular sieve materials were also evaluated (Duval et al., 1993, 1994; Ji et al., 1995). Some pervaporation performance data using adsorbent-filled membranes are given in Table 1. The objective of this paper is to review the works performed in pervaporation using adsorbent-filled membranes. Applications of filled membranes in the removal © 1996 American Chemical Society
Ind. Eng. Chem. Res., Vol. 35, No. 4, 1996 1125 Table 1. Performance of Filled Membranes in Pervaporation filler conc
t (µm)
mixtures
feed conc (wt %)
T (°C)
0 vol % 31.0 vol %
100 100
0 wt % 70 wt % silicalite/synt. 67 wt % 77 wt % PDMS silicalite/synt. 0 wt % 67 wt % 77 wt % 77 wt % PDMS silicalite/synt. 0 wt % 30 wt % 60 wt % 0 wt % 40 wt % 60 wt % 0 wt % 30 wt % 60 wt % 0 wt % 30 wt % 60 wt % EPDA 0 vol % 48.6 vol % CPb 52.0 vol % SXb 28.0 vol % W20b 31.0 vol % NY20b EVA 0 vol % 45.0 vol % CPb b 24.0 vol % 4S 0 vol % NBR 45 SXb 44.0 vol % 55.0 vol % PVC NaA 0 wt % 59.0 wt % modified PVC NaA 0 wt % 43.3 wt % 59.0 wt % 0 wt % PEBA ACb 5 wt % 10 wt % 5 wt % CAc
100 100 100 100 3 4 12 20 150 150 150 150 150 150 150 150 150 150 150 150 100 100 100 100 100 100 100 100 100 100 100 73 260 50 98 115
toluene/EtOH toluene/EtOH 1-octene-3-ol/water EtOH/water EtOH/water EtOH/water EtOH/water EtOH/water EtOH/water EtOH/water EtOH/water MtOH/H2O MtOH/H2O MtOH/H2O EtOH/water EtOH/water EtOH/water 2-PtOH/water 2-PtOH/water 2-PtOH/water 1-PtOH/water 1-PtOH/water 1-PtOH/water toluene/EtOH toluene/EtOH toluene/EtOH toluene/EtOH toluene/EtOH toluene/EtOH toluene/EtOH toluene/EtOH toluene/EtOH toluene/EtOH toluene/EtOH ethanol/water ethanol/water ethanol/water ethanol/water ethanol/water TCAd/water TCA/water TCA/water TCA/water
10 10 100 ppm 7.0 7.0 7.0 7.0 7.0 7.0 6.5 5.1 5.3 5.3 5.3 5.3 5.3 5.3 5.3 5.3 5.3 5.3 5.3 5.3 10 10 10 10 10 10 10 10 10 10 10 azeotrope azeotrope azeotrope azeotrope azeotrope 40-300 ppm 40-300 ppm 40-300 ppm 40-300 ppm
30 30 30 22 22 22 22 22 22 22 22 22.5 22.5 22.5 22.5 22.5 22.5 22.5 22.5 22.5 22.5 22.5 22.5 30 30 30 30 30 30 30 30 30 30 30 25 25 25 25 25 30 30 30 30
polymer
adsorbent
PDMS
W20b
PDMS PDMS
silicalite silicalite/UOP
Pa (Torr)
0.04
