Ind. Eng. Chem. Prod, Res. Dev. 1983, 22, 313-319
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Organic Liquid Mixtures Separation by Permselective Polymer Membranes. 1. Selection and Characteristics of Dense Isotropic Membranes Employed in the Pervaporation Process Israel Cabasso The Polymer Research Institute, State Universw of New York, College of Environmental Science and Forestty, Syracuse, New York 13210
Separation of binary llquld mixtures by various types of isotropic, dense membranes is discussed. A pervaporation process was employed, and flux rates and separation factors were obtained for group modes (binary mixtures such as benzene/cyclohexane, methanoVhexane, methanol/styrene) and for liquid mixtures composed of components having similar chemical natures (cyclohexene/cyclohexane and styrene/ethylbenzene). High separations were obtained by matching the chemlcal affinity of tailored membranes to that of one of the permeating components. The separation characteristlcs of three types of membranes-homogeneous polymer alloys, highly cross-linked quaternary PPO, and Nafion-are presented; rationales for membrane selection employing solubility parameter diagrams are discussed.
an immiscible phase which dissolves (i.e., extracts) one component of the azeotropic mixture. The extractant is subsequently stripped from the extracting phase via distillation or other appropriate techniques. The membrane separation process (Figure 1) differs from the extraction process in that extraction and stripping occur concurrently, thus providing the driving force for steady-state mass transport; in such a process a thin amorphous polymer film is brought into contact with the liquid mixture (feed) and the desired component dissolves and permeates the film (membrane) by diffusion mechanism along a concentration gradient. The permeate is stripped from the membrane on the side opposite to the feed mixture-the “downstream” side (phase 111, Figure 1). The process prevails by maintaining a low chemical potential for the permeant at phase 111. The practical methods that are employed in such processes (phase I to phase IV) have a large impact on the separation efficiency (e.g., separation factor, permeation rate, energy consumption). Most of the reports that deal with membrane separation of organic liquid mixtures focus on the “pervaporation” process, which maintains phase 111as a gas phase at a low permeate vapor pressure. The permeating component evolves at the downstream in a gaseous state and is condensed and collected, as a liquid or solid, in phase IV. A schematic for a hollow fiber membrane pervaporation unit (Cabasso and Leon, 1975) that was employed in the present study is shown in Figure 2. Aptel et al. (1976) described a “thermopervaporation”where condensation takes place on cold walls surrounding the membrane downstream. An alternative procedure has been demonstrated by Cabasso et al. (1974~)in which the permeant is stripped by circulating a nonpermeable acceptor liquid at the downstream membrane interface (phase 111). In most cases where diffusion transport prevails and no viscous flow is allowed, steady-state permeation rates through the membranes can be described by an integrated version of Fick’s first law
Introduction The separation of organic liquid mixtures by synthetic membrane technology was suggested more than two decades ago by Binning et al. (1958). In the same year, Binning and James (1958) evaluated the economics of drying 2-propanol via a selective synthetic membrane, claiming that the membrane separation process was more feasible than a conventional azeotropic distillation with hexane. Elaborate analyses of such processes were given later by Choo (1962). Since then, a number of studies dealing with the separating of organic liquids via pervaporation processes were reported. Nevertheless, this technology has not kept pace with other membrane separation processes such as reverse osmosis, electrodialysis, and hemodialysis, which have experienced major progress in the past two decades and have materialized in industry as commercial operating units. The reasons, among others, are the poor separations obtained with commercial films, e.g., polyethylene ( H u g and Lin, 1968) that served as membrane components, and the lack of proven feasibility of the conventional pervaporation process. The purpose of the present series of papers is to describe the adaptation of novel membranes and methods of membrane selection for the separation of azeotropics and related liquid mixtures. Various advanced membranes possessing unique chemical specifications, configurations, and morphologies, such as hollow fiber, tubular, anistropic (asymmetric),and composite membranes, were used in the study. This paper addresses the question of membrane selection and some observation was made of the relationship between permeation rate, separation, and chemical nature of the components in the membrane and liquid mixture. The separations reported in this paper were carried out employing dense isotropic membranes via a pervaporation process. Subsequent papers will describe the performance and problems associated with high-flux anistropic and composite membranes and the effect of various alternative separation processes on membrane performance. Identification of the Principles and Components in the Membrane Separation System. In principle, liquid mixture separation by membrane techniques is similar to well-established extractive distillation or liquid-liquid extraction processes. In the extraction process for example, an azeotropic liquid mixture is brought into contact with 01 96-4321 ~133/1222-0313~01.50/0
where J is the permeation rate (cm3/cm2s), 1 is the membrane thickness, C1and C2 are concentrations of the permeant in the feed and product interfaces of the membrane, 0
1983 American Chemical Society
Ind. Eng. Chem. Prod. Res. Dev., Vol. 22, No. 2, 1983
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Table I. Polymers and Membranes Employeda polyphosphonates
PPN-I PPN-I1 PPN-IIIC PPOPhd QPPOe CA Nafion
poly( phosphate)
poly [styrene( diethy1)phosphonatel copolymer [vinylidene chloride-styrene( diethy1)phosphonatel poly [bromophenylene oxide( dimethy1)phosphonatel poly [phenylene oxide hydroquinone( dimethy1)phosphatel quaternary poly(pheny1ene oxide) cellulose acetate (Eastman 394-45) copolymer (polysulfonyl fluoride vinyl ether-polytetrafluoroethylene) (Du Pont)
a PPN-111, PPOPh, and QPPO are derivatives of poly( 2,6-dimethyl-1,4-phenylene oxide). Cabasso et al. (1974d). e Cabasso (1980). Cabasso and Tran (1979).
,
1-
I I
Intermediate slate
I
mixture
* A f B
j
I I
I
Phase
I
Phase
II
I
, j
Phase
m
j
Phase
E
Figure 1. Liquid mixture separation scheme: phase I, feed mixture chamber (upstream); phase 11, membrane; phase 111, intermediate state (downstream) where permeants are held at lower chemical potential than phase I; phase IV, where permeants are brought to the pure liquid state.
' Cabasso e t al. (1974).
for which all the parameters can be obtained experimentally (Cabasso et al., 1974~).However, it should be emphasized that the relationships predicted by eq 2-4 cannot be taken for granted, especially when the polymer membrane is being highly cross-linked or is in its glassy state; i.e., at a temperature below its glass transition temperature. For such membranes any use of eq 3 will be only a conjecture unless it is used within an experimentally established range for which the relationship expressed in eq 2 was determined. A review of the literature shows that since the studies by Binning et al. (1961), there have been only a few systematic studies dealing with the relationship between the chemical nature of a membrane and its separation potential (e.g., Sweeny and Rose, 1965). This aspect will be dealt with extensively in the following sections. The conventional pervaporation processes, though not proven to have commercial feasibility yet, is especially appropriate for the task of assessing the capability of a membrane to separate organic liquid mixtures and hence was employed in this section of the study. Experimental Section Membranes and Materials. Three types of dense isotropic membranes were employed (Table I): (a) PPN/CA and PPOPh/CA (1/1by weight) which are alloys of polyphosphoryl esters and cellulose acetate (Cabasso and Tran, 1979); (b) QPPO, which is a highly cross-linked quaternary derivative of polyphenylene oxide (PPO); these polymers were cast, or spun, into flat-sheet or hollow fiber configurations; (c) flat-sheet membranes of Nafon 125 (E.I. du Pont de Nemours and Co., Wilmington, DE) and hollow fibers of Nafion 811. Nafon is an ion-exchange membrane consisting of sulfonic acid groups attached to a fluorohydrocarbon copolymer matrix (copolymers of polysulfonyl-fluoride vinyl ether and polytetrafluoroethylene, PTFE). The organic solvents used were of analytical grade. Various liquid mixtures were employed. Four binary systems (which are considered difficult to separate by conventional methods) were thoroughly investigated: (1)methanol/hexane, 26.9/73.1 (percent composition in azeotrope); (2) benzene/cyclohexane, 55/45 (percent composition in azeotrope); (2a) cyclohexene/cyclohexane; and (3) sytrene/ethylbenzene, close boiling components, heat-sensitive mixture. Apparatus. A typical pervaporation scheme is illustrated in Figure 2. Hollow fibers were epoxy-potted in a glass shell-tube, and flat-sheet membranes were mounted in a fluorinated polyethylene cell (Figure 3). The feed mixture was circulated through the hollow fiber bore, or over the flat sheet membrane, while the downstream side was evacuated to (0.1-25) X lo2 Pa or as specified. The permeate was collected in the vacuum trap condensers cooled by liquid air. The flow rate through the hollow fiber unit was adjusted so as to minimize concentration gradient along the fiber bore due to permeation of components through the wall; thus feed composition was determined at the entry and exit ports. Fiber dimensions of 400-800 pm in diameter and 20-30 cm in length were employed in
I == Circulating pump
I
reservoir
I
I
Hollow fiber cell
' uu h i i -
vacuum
Condensers
Figure 2. Scheme of pervaporation by hollow fiber permeator.
and D(c)is the diffusion coefficient, which for many solute-membrane systems is concentration dependent. For many of the systems reported in the literature, eq 1 can be solved satisfactorily if an empirical correlation can be found between D,) and C. This correlation has been shown in various studies (Park, 1950; McCall, 1957; Rogers et al., 1960; Frensdorf, 1964; Mears, 1965; Cabasso et al., 1974b) to obey the relationship D = Do exp(yC) (2) where Do is the diffusion coefficient of the permeate in the membrane a t zero concentration (C 0) and y is a constant characteristic of a given permeant-polymer system and reflects the interaction between the two. A useful expression can be obtained by combining eq 1 and 2
-
DO
J = -[exp(yCJ Yl
-
-
exp(yCJ1
(3)
If a permeation process produces a concentration gradient whereby C2 0, then eq 3 takes a simple form Do J = -[exp(yC, - 111 (4) 71
Ind. Eng. Chem. Prod. Res. Dev., Vol. 22, No. 2, 1983 315
Membrane
P e r m e o n t Vapor Outlet
Figure 3. Flat-sheet membrane pervaporation cell.
permeator units containing 1-50 fibers. Flow rates were controlled by a Model 12-41-303gear pump (Micro Pump Co.) with a Teflon head. Assessment of Membrane Performance. Membrane performance was measured in terms of permeate flux rate and separation factors. The separation factor an,,, is defined as
where XmF,XnF,Xmp,Xnpdenote the weight fractions of component n and component m in the feed solution (F) and the product (P). Solution compositions were analyzed on a Shimadzu (GC-4B) gas chromatograph equipped with 6-ft columns packed with Chromosorb 101 80/100 mesh for the alcohol/hydrocarbon binaries and 20 w t % DEGA polyester (with 2% phosphoric acid) on Chromosorb W for the aromatic solutions. Equilibrium sorption of organic liquid in the membranes was obtained by deducting the weight of the vacuum dried (for 3 weeks at 35 “C) membrane from the swollen equilibrated membrane (the membranes were equilibrated in the liquid for 2 weeks at 21 OC). Solubility parameter diagrams were delineated by methods described by Hansen and Beerbower (1971) and Klein and Smith (1972a,b). The solubility area of the cross-linked QPPO was determined by obtaining the noncross-linked polymer derivative of the same charge density. Consideration for Membrane Selection. The separation mechanism which determines membrane selectivity can be roughly divided into two categories: (a) those separations which are based on differences in mass and shape of the permeate, displaying differences in the selfdiffusion coefficient and spatial cross section, and (b) those separations which exploit differences in the chemical nature of the permeates, and which result in changes of the relative values of y and C. Qualitative or even quantitative (Greenlaw et al., 1977) predictions for separations of the first type are relatively simple; more complicated schemes are required to predict separations involving permeates which can interact with the membrane matrix. A practical aid would be a parameter that can help in classifying group interactions between solvents and polymers, thus providing a framework for the selection of polymer membranes (rather than picking them up at random). Naturally there is a strong tendency to revert to Hildebrand solubility parameter 6 (Carter and Jagannadhaswamy, 1964) which provides a useful frame for the characterization of the magnitude of the cohesive energy density and, hence, to the interaction in simple-mostly apolar-liquids. The solubility parameter approach when extended to polymer-solvent system (as proposed by Burrel (1957, 1975) has gained universal appeal in the coating industry; however, the difficulties encountered in
Figure 4. Solubility parameter diagram of QPPO membranes as plotted on two-dimensional grid, 6,-6h.
the determination of accurate 6 for polymers, especially for those which possess highly polar pendent groups, reduced the significance of 6, for most polymers, from a quantitative absolute value to a rather qualitative reference value. Numerous attempts are reported in the literature (see review by Hansen and Beerbower, 1971), dividing the Hildebrand solubility parameter 6 to its fractional components such as those which are responsible for the dispersive forces contribution ad, the polar (and induced polar interaction) 6, and the hydrogen bonding 6h [which can effectively further be divided to 6hA (acceptor) and 6hD (donor), e.g., Snyder (1978)l. Such division of b is most convenient for qualitative prediction of the nature of the interaction between solvents and a certain polymer system; it is especially useful if the polymers can be placed within a solubility parameter diagram (e.g., Shaw, 1974; Hansen, 1969) employing the fractional parameters as coordinates. The overall correlation among these values is given by the expression
+
= ad2 6; + 6h2 (6) The accuracy and significance of the above division leaves much to be desired. Nevertheless, the fact that the calculated values assigned to ad, 6,, and 6 h are sometimes only crude estimations of the relevant forces and interactions does not eliminate their usefulness [as long as the values that are employed were derived from the same formulation (Hansen, 1982)l. The solubility parameter values that were employed in the present paper are those reported by Hansen and Beerbower (1971). In this version the plane, or space, which is defied by the use of either two or three 6 coordinates serves as a locus for every organic compound. That is, each compound can be identified by either twoor three-dimensional coordinates given by its solubility parameter value, as shown for methanol and hexane in Figure 4. Similarly, every polymer can be defined by an area or volume in such a coordinate system, bounded by the coordinates for all those compounds which serve as solvents for this polymer; for example, the striped oval areas in Figure 4 represent two different QPPO membranes on the two-dimensional ”-& grid. Styrene is shown to be a solvent for QPPO(A), ethylbenzene, which is located outside the bounded area, is a nonsolvent; both are nonsolvents for QPPO(B) (which would, if not cross-linked, readily dissolve in methanol or DMF). Such a characterization of a polymer by its solubility area, or space, has been successfully employed in “tailoring” polymeric membranes for specific assignments (Cabasso et al., 1977; Cabasso and Tran, 1979; Klein and Smith, 62Hildebrmd
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Ind. Eng. Chem. Prod. Res. Dev., Vol. 22, No. 2, 1983
Table 11. Absorption of Methanol and Hexane in QPPO-B and CA/PPN-I11 Membranes (21 "C)
=7 4 )
absorption, wt %'
a
organic liquid
QPPO-B
methanol hexane methanollhexane (1/50 w/w)
35
