MEMBRANE SE PARAT/ON PR 0CESSES N. N , 11 he mythical Maxwell Demon is a modern reality.
TWe have, today, a number of polymeric membranes capable of selecting individual molecules from a single-
phase homogeneous mixture of chemicals. Membranes and membrane processes have been proposed and developed for separating : -salt from seawater -helium from natural gas -water from organic alcohol, ketone, and acid-water mixtures -hydrocarbons (toluene from naphtha and Ce fractions from isobutane-butylene) --spent acids from metallurgical liquors -caustic soda from viscose - C 0 2 - 0 2 , K 2 - 0 2 ,and NH3-N2-H2 mixtures -aromatic-saturated and unsaturated hydrocarbon mixtures -catalytically cracked gasolines All of these, except the caustic and acid recovery processes, have been proposed and developcd within the last decade and are in various pilot plant stages. Whether or not large scale industrial applications follow will depend to a major extent on whether or not one can endow the modern Maxwell Demon with the necessary longevity, ruggedness, and low cost. A membrane is a thin barrier separating two fluids. The barrier prevents all hydrodynamic flow so that transport through the membrane is by diffusion. The property of the membrane describing the rate of transport is its permeability. A membrane is semipermeable if, under identical conditions, it transports different molecular species at different rates. This review is limited to those permeation processes where the driving force for transport is pressure (hydrodynamic or osmotic). We have arbitrarily excluded electrodialysis and related processes where membrane selectivity is achieved by virtue of fixed electrical charges. Porous barrier diffusion is also excluded since it is, basically, a hydrodynamic not a diffusion process. 18
I N D U S T R I A L A N D E N G I N E E R I N G CHEMISTRY
R. €3. LONG
E. 1. HENLEY
The bibliography on gaseous diffusion covers January 1960 to June 1364, that concerned with liquid permeation the period from 1961 to June 1964, and that concerned with liquid dialysis covers only recent work.
GAS DIFFUSION PROCESSES Interest in gas permeation of polymers is not restricted to those concerned with separation processes. The penetration of plastics by gases and vapors, for example, is important to the food industry, where packaging materials are often selected for their ability either to permit or prohibit the transmission of gases. Whatever the application. the characterization of the diffusion process through the membrane i s necessary, and this is usually done b) measuring the rate coefficients of the transport processes and associated parameters. The usual rate coefficient is the diffusivity D , The most common method of measuring the diffusivity is the time-lag method which permits the simultaneous determination of the solubility, S. The permeability, P, of the membrane is given by the equation
P
=
DS
Diffusion, in some cases, ma)- be too slow for time-lag measurement. Michaels, Vieth, and Bixler ( A 4 4 suggested a sorption technique in which the rate of pressure drop due to sorption of polymer samples in a constant temperature cell is measured and related to diffusion coefficient through a mathematical expression derived by Crank ( A 12). Diffusion rates in polymers can also be measured bp an optical technique which consists of observing the movement of a colored, diffusing boundary, as described by Barker ( A 2 ) . This method can be used to conveniently determine the diffusivity of the membrane under stress. A semimicro unit was designed b) Park (A50). It measures the volume of permeating gas by means of a liquid slug moving inside the diffusion tube. This device. like the one developed earlier by Brubaker and Kammermeyer ( A 10, A 7 I ) , is said to be insensitive to leaks and does not require a large sample size.
Separation of fluid mixtures by selective permeation is about to
pass from pilot to commercial scale
An interesting method for measuring permeability of water and moisture waa recently reported by Sivadjian and Corral (A57),and by Sivadjian and Ribeiro (A58). This technique is simple and requires a minimum of measuring instruments. It involves the determination of the degree of discoloration of a hygrophotographic plate endosed in the plastic film whose permeation constant is to be measured. For high pressure studies, Barrer’s time-lag method is not suitable, because the back pressure of a membrane is restricted initially to be zero and the forward pressure is then limited by a small pressure difference across the film, since too large a pressure difference will either deform or break the film. A method of measuring a change in volume under conditions of constant temperature and pressure, which permits relatively rapid determination of permeation rate at atmospheric pressures or above, was described by Li and Henley (A36).
I
A
E h d of Temperature
The temperature dependence of the permeation constant is usually given by an Arrhenius relationship. This is true for permanent gases. For organic vapors such as methyl bromide and isobutene, Sobolev, Meyer, Stannett, and Szwarc (A56) found that the diffusivitytemperature relationship at varioKs pressures less than one atmosphere departed tremendously from the Arrhenius relationship. Only at sufficiently low pressures, e.g., 100 mm., were plots of log D us. 1 / T linear. It has been generally assumed that the activation energies for permeation and diffusion are independent of the temperature. Kumins and Roteman (A37) have reported that for small gas molecules, this assumption remains valid at and above the glass transition temperatures. However, for carbon dioxide and water vapor, the transport processes are affected appreciably by the glass transition temperature. Mears (A47) studied, specifically, the influence of glass transition temperature on gas solubility S in polyvinyl acetate, and found that log S varies linearly with 1 / T only above the glass transition temperature. The plot of log S us. 1/Tis convex to the 1 / T axis and exhibits a minimum.
L.
uc L
I
VOL 57
NO. 3
MARCH 1965
19
H h d s of Pressum and Concentration The permeation of permanent gases through plastic films has been found to be independent of pressure. However, the permeability of organic gases and vapors shows a complicated dependence on pressure and concentration by virtue of the strong interaction between solute and membrane. Early work on the pressure effect, largely carried out at pressures equal to or smaller than one atmosphere, demonstrated the swelling or plasticizing effects of organic vapors on plastic films. Exponential equations relating the solubility constant and the integrated diffusivity to vapor activity were proposed by Rodgers, Stannett, and Szwarc (A53). These relationships, however, usually hold only at low temperatures, according to Mears (A42) and to Fujita, Kishimoto, and Matsu-
A 160
Figure. 1. P m e a f i o n consfants, IhroughpolyefhyImJilm of fhickmss z, for nitrous oxide (black circles, I = 0.W in.), nihm oxide (whifc circles, z = 0.01 in.), nitrous oxide (black spumes, z = 0.002 in.), efhylenc (whife hionglcs, z = 0.094 in.), carbon dioxide (whife squares, z=0.004 in.), dichiorod~uoromefhane(crosm, r=0.004in.)
mot0 (A79). They also noted that the apparent activation energy for diffusion decreased continuously with increasing temperature. In addition, Kishimoto and Matsumoto (A%), as well as Mears, observed that the exponential concentration dependence of diffusivity applies only in the region of high concentration. This finding indicates that it is a dangerous practice to evaluate the diffusivity at zero penetrant concentration by extrapolating the plot of log D us. C. The linear dependence of diffusivity on concentration sometimes observed at low vapor activity was recently discussed by Nishijima and Ostet (A49). It is of interest to note that, in a recent paper, Stannett and Yasuda (A59) demonstrate that permeability of organic vapors increases with increasing vapor pressure up 20
INDUSTRIAL A N D E N G I N E E R I N G CHEMISTRY
to saturation; the permeation of liquid penetrant that follows has the same permeability constant as that of the corresponding saturated vapor. Investigation of the phenomenon of gas permeation at elevated pressures was recently made by Li and Henley (A36), Figure 1. Henry’s and Fick’s laws were obeyed by C 0 2 at 11 atmospheres. A modified diffusion law was proposed to describe the pressure dependent permeability of other gases. Since permeability is defined as the product of the solubility and diffusion constants, increase of permeability with increase of pressure may mean that both solubility and diffusion constants are functions of pressure. Solubility constants of methane and ethane were recently measured at various pressures by Eastburn (A16), Lundberg, Wik, and Huyett (A38), and Kittelberger (A28) and were shown to depend strongly on pressure. Effect of the Notum of Penetrant
Permeation of molecules through a polymer film depends significantly on the physical properties of the penetrant. The activation energy for diffusion, Ed, was found to be an increasing function of the penetrant molecular diameter by Mears (A43) and Barrer (A3). Recently Kumins and Roteman (A37) discussed the exponential dependence of Ed on the molecular diameter. Correlations for the activation energies of diffusion and solution with the penetrant molecular diameter and the Lennard-Jones force constants were presented by Michaels and Bixler (A45, A46). Their correlations are based on the model of completely amorphous polyethylene. If crystallites are present in the polymer, the effect of the size, shape, and size distribution of crystallites on the activation energy of permeation, if any, has to be considered. Solubility constants were demonstrated by Hsieh (A27) to vary linearly with the Lennard-Jones force potential. He measured the solubility and permeability of ethyl cellulose and nitrocellulose to different gases and vapors. Diffusion coefficients, calculated from the solubility and permeability data, were found to be influenced by the shape factor of the penetrant molecules and to decrease linearly with the increase of molecular weight of penetrant. Permeation of organic vapors and gases has recently been shown to be greatly affected by the gas solubility in a polymer, which, in turn, is dictated by the mutual compatibility of the penetrant and the polymer and is related to the ease of condensation of the penetrant. Except for small and simple molecules, this effect of gas solubility usually overshadows the influence of molecular diameter. In general, an organic vapor is more easily condensed than organic gas, and since condensation is a function of pressure, the permeation of organic vapor is more pressure sensitive. Plasticizing ERecI
The swelling and plasticizing mechanisms have not been well understood and are a subject of increasing interest. Water vapor was observed to hsvr strong plasti-
cizing effect. The equilibrium sorption isotherms of hydrophilic films such as ethyl cellulose were determined by Yasuda and Stannett (A77) to be a function of vapor pressure. The diffusion coefficients were found to be concentration independent. Their work on the effect of emulsifier in polymer provides additional information about the diffusion process. The permeation constants were found to remain unchanged despite considerable increase of the water vapor sorption due to the presence of the hydrophilic emulsifiers. I t would appear that the emulsifiers in polymers act as clustering centers for the water vapor sorption, but that the clusters do not participate in the diffusion process. The diffusivities of nylon and cellulose, both hydrophilic films, were reported, however, to increase with increasing concentration by Stannett and coworkers (A60). Recent work of Thornton, Stannett, and Szwarc (A65) has furnished further evidence of the clustering of water molecules. It was found that the permeation rate of water through Mylar, a hydrophilic polymer, is greater than that of water vapor. This is interpreted as due to the clustering effect when saturation is approached. The plasticizing effect was also demonstrated by Stannett and Yasuda (A5Q) on acetone and acetonitrile vapors permeating through vulcanized rubber. Acetone, whose molar volume is much greater than that of acetonitrile, has lower permeability at relatively low vapor pressure. At higher vapor pressures, the plasticizing effect of the bulkier acetone molecule increases the diffusion rate, leading to a permeability of acetone surpassing that of acetonitrile. The sorption of water vapor at high relative humidity is normally described by solution theory. Kwai recently discussed his work from this aspect (A34) and pointed out the significance of polymer elastic contribution to the sorption isotherms. Clustering of penetrant molecules in polymer is now generally regarded as the main cause for the nonideal sorption and diffusion behavior of water vapor. A “clustering function” was derived by Zimm and Lundberg (A73) to express the clustering tendency of the penetrant molecules. Direct experimental evidences for the existence of penetrant clusters in polymer were recently provided by Barrer and Barrie ( A 4 ) , who measured the light scattering by ethyl cellulose films at high relative humidities, by Veith (A66) from the studies of the dielectric property of polystyrene film, and by Mayne (A40)from observing globules of water in polymeric films through microscopes. It is interesting to compare the permeation constants of carbon dioxide and nitrous oxide from the viewpoint of plasticizing effect. Although both of these gases have the same molecular weight and similar molecular forms, the work by Li and Henley (A36) showed that the nitrous oxide permeability varied with pressure, while carbon dioxide maintained a constant permeability throughout the pressure range investigated (17.7 p.s.i.a. to 162.5 p.s.i.a.). Moreover, nitrous oxide gave a much higher permeability than carbon dioxide at the same pressures. It seems that, because nitrous oxide is polar while carbon
dioxide is nonpolar, it may be easier for nitrous oxide to be adsorbed on the polymer molecules and to exert the plasticizing effect. Also, adsorption depends on pressure, therefore the permeability of nitrous oxide is pressure dependent and greater than that of COz. Effect of Polymer Structure
Since polymers are composed of distinct crystalline and amorphous phases, and crystalline polymer acts toward gases as an impermeable dispersed phase, the model of amorphous polymer structure recently proposed by Di Benedetto ( A15) has special significance in interpreting diffusion data. There is much experimental evidence that the commonly held concept-that macromolecules in amorphous polymer are random coils like a ‘6bowlful of spaghetti”-is far from correct. It is suggested that the important units in an amorphous polymer are bundles of molecular chains which actually possess some short range order. Gaseous diffusion data from various literature sources are said to be successfully analyzed based on this model. Nevertheless, as Berger and Smith ( R 6 ) pointed out, there are still a number of problems with the “bundle” theory yet to be answered. Another model for predicting diffusion coefficient was proposed earlier by Brandt ( A Q ) ,wherein the activation energy for diffusion is assumed to be the energy required for symmetrical separation of two polymer chain segments. The predicted activation energies were shown to be 30 to 75y0of the experimental values. Gas permeation through crystalline polymer is known to be negligible. Its rate was shown by Bent ( A 5 ) to be proportional to the volume fraction of amorphous phase P = Pa where Pa is the permeability for amorphous polymer and X, is the volume fraction of amorphous phase. Lasoski and Cobbs (A35) and Alter ( A I ) have suggested that both gas solubility and diffusion are linear functions of the volume fraction of amorphous material. Consequently, permeability is proportional to the square of the volume fraction of amorphous material. The former have also shown that the rate of permeation increases with increase in the number of short chain branches. The latter has correlated permeability with polymer density by the following equation : P M X lo-’’ (1 - p p ) n where A4 is a constant characteristic of each gas, p p is the density of the polyethylene film. Considering the geometry of the crystalline population as well as the amorphous phase fraction, Michaels and Bixler (A45, A46) have recently proposed correlations for solubility and permeation : I
x,
s
=
saxa
P = (xaDaKa)/d where S, and D, are the solubility constant and diffusivity, respectively, in a hypothetical, completely amorphous polymer, 7 is the tortuosity factor, and $ is the chain immobilization factor. Stretching membranes was found by Lasoski and Cobbs (A35), Brandt ( A Q ) ,Walters (A69), and Roberts VOL. 5 7
NO. 3 M A R C H 1 9 6 5
21
(A51) to induce temporary crystallization and, therefore, change the volume fraction of amorphous polymers. Brandt also discovered that the change in void content on cold drawing was associated with changes of the opposite sign in the activation energy of diffusion, resulting in weaker temperature dependence of diffusion. The structural change in polyethylene terephthalate after stretching and also the effectsof strain relaxation on crystalline spacing and orientation were specifically studied by Heffelfinger and Schmidt (AZO). The permeabilities of strained polyethylene and polypropylene films were measured most recently by Yasuda, Stannett, Peterlin, and Frisch ( A 7 2 ) . They found that, for small molecules, permeability increases with increasing stress, possibly due to an increase in free volume of the membrane. This is in contrast to the results of Rosen ( A M ) with polystyrene, who found the selectivity of polyethylene films is the same when strained as when unstrained. It is interesting to note that uniaxial orientation and solvent conditioning were recently used by Bixler and Michaels (A7) to change the permeation characteristics of polymer films. The effect of microvoids on permeation was a subject of recent attention. Thompson and Woods (A6.3) demonstrated that the abnormally low density of polyethylene terephthalate is due to the presence of “needleshaped voids.” In like manner, Rybnikar (1155) showed an analogous behavior in poly-6-caproamide, Schulz and Gerrens (A62) furnish experimental evidence of an internal capillary system in polystyrene below the glass transition temperature. A theoretical analysis of diffusion together with mass transfer through micropores was offered by Frisch (A17). The presence of microvoids in polymers may lower the apparent energy of activation and the rate of overall temperature variation, since fine pores permit mass transfer by Knudson flow, which is dependent upon the square root of temperature rather than upon the Arrhenius exponential expression. The presence of microvoids in either the amorphous or crystalline phases of polymer may also account for the discrepancy of literature data on permeation, as suggested by Klute and Franklin (A29). The additional mechanism of permeation due to slip flow of gaseous molecules through microvoids has been investigated by Veith (A67). I t is interesting to note that, despite the considerable evidence for the existence of voids, hydrophobic films such as polyethylene do not provide centers for initiating clustering of water vapor through hydrophobic film. The diffusion and solubility of HnO vapor, therefore, have been observed to be independent of Concentration. Whether or not water vapor permeation can be affected by the presence of nonreinforcing solid in the polymer was investigated by Kumins and Roteman (A31) and also by these two authors with Rolle (A33). They found that the glass transition temperature was changed due to the polymer-solid interaction and, also, that the solid interfered in the segmental mobility of the polymer chain. ‘The shift of glass transition temperature, in turn, affected diffusivity. 22
INDUSTRIAL AND ENGINEERING C H E M I S T R Y
Anomalous concentration gradients for vapor diffusing through glassy polymer were reported by Long and RichInan (A37). They also discovered that the polymer surface attains its equilibrium concentration very slowly. A time-dependent surface concentration was, therefore, taken into account in solving the Fick’s diffusion equation for anomalous absorption curves. It is not uncommon that polymer inhomogeneities exist owing to the gradient of internal composition or local stress. The dependence of non-Fickian diffusion on such inhomogeneities was discussed recently ( A 78). Study of the effect of cross-linking on the permeability of methyl bromide through polyethylene (A47, A60) has shown that the diffusion coefficients decrease with increasing degree of cross-linking, while the solubility is relatively unaffected. The swelling of highly cross.. linked epoxy polymer film by organic vapors was studied by Kwai (A34). The molecular weight per cross-linked unit was obtained by applying the equation of rubber elasticity to sorption isotherms as well as by chemical considerations. Permeability of Graft Polymers
Study of graft polymer permeability is of interest since it shows the effect of branching on permeation. Meyers, Rodgers, Stannett, and Szwarc (A47) found that the permeability of nitrogen through 60’3, vinylpyridine graft polyethylene is only one tenth of its original value through polyethylene, whereas 30 to 40% styrene or acrylonitrile grafted to polyethylene resulted in a reduction by one third of the permeability. Effect of lrradiatio n
The effect of irradiation on gas permeation has been reported in several papers (A8, A74, A47, A48, A60). The irradiation was usually by Co60in air with maximurn dose of lo8 roentgens. The major conclusion of these investigations was that irradiation induced cross-linking, which brought about a decrease of diffusion constant. The solubility constant, on the contrary, increased markedly, possibly due to the effect of chemical composition changes. Effect of Film Thickness
The effect of film thickness on sorption of organic vapors in amorphous polymers slightly above their transition temperature was recently investigated by Kishimoto and Matsuinoto ( A 2 7 ) . It was found that sorption was not controlled by a purely Fickian diffusion mechanism-. for thin films the sorption curve was almost linear up to two thirds of the total vapor uptake. With increasing film thickness two-stage sorption appeared at low equilibrium concentration. The work by Li and Henley ( A 3 6 ) has shown that thickness of polyethylene film, ranging from 1 to 10 mils, had no influence on the permeation constant at various pressures up to eleven atmospheres. It should be noted, however that if film is in a swollen condition, indicating the existence of a liquid solution of the transmitting vapor and the polymer film, increasing thickness might lead to l o ~ permer ability values due to possible effect of thickness on concentration gradients within the film (AZ2, A23).
Permeation of Water Vapor
Recent publications which provide permeation data of water vapor are the papers by Klute (A30), Starkweather (A67), and Marie (A39). Special mention should be given to Klute and Franklin's survey of the literature of water vapor permeation through polyethylene (A29). It is an excellent source of the references published before 1958. Also, two monographs on permeation were published in 1962; one furnishes a table of summarized literature data (A64), and the other devotes specifically a chapter discussing water vapor permeating through both hydrophilic and hydrophobic polymeric films (A60).
process, and Ionics Inc. began commercializauon studies. The resulting publications through 1961 were critically reviewed with particular emphasis on engineering economics by C. Y. Choo in his chapter on membrane C ~ Petroleum permeation (BS) appearing in A ~ U Q W in Chmistry and Refining. Measunmeni of Permedion Constann
Liquid permeation is carried out in a cell divided into two compartments by a plastic film (Figure 2). The
Gas Separation
Recent studies in this field have been extended from binary mixtures to multicomponent mixtures and from single stage processes to multistage cascades. Theoretically, as stated by Walters (A68), a multicell system with recompression between the stages could give any desired degree of separation of multicomponent mixtures. Walters has also found out that separation in-. creases rapidly as temperature increases. For instance, in a gaseous mixture of eight components, the separation ratio of n-pentane to methane increases from 4 to 15 when the temperature is raised from 73' to 120' F. Since the permeation constant for a gas through a membrane is the product of the solubility of the gas in the film times the diffusivity, the key to high permeation rates, as well as membrane selectivity, lies in maximizing the solubility. Kammermeyer (A24) found that silicone rubber has a preferential selectivity of 7 to 1 for carbon dioxide over oxygen. The selectivity of carbon dioxide over helium is as much 11 to 1 at room temperature. Of current interest is a Lindedeveloped process for recovering helium from natural gas by diffusion through fluorocarbon membranes ( A 7 3 ) ; see also I&EC, Feb. '65, page 49). Other processes, such as the N r 0 2and NIHI-NH8 separations, are discussed by Tuwiner et al. ( A 6 4 ) . Both NASA and the Air Force are currently sponsoring the development of prototype membrane in spacecraft separation units to separate COSfrom 0% cabins (A68).
VAfM fW
LIQUID M S E
1
1
I
NOWflRMvlNT
&ATE
Figure 2. Diagram of liquid permeation process, M a c pmwabls moledes are open circles
LIQUID PERMEATION PROCESS Binning, Lee, Jennings, and Martin (B4, 8.5) were the first to report that permeating mixtures from the liquid phase on one side of the film to the vapor phase on the other side has good commercial separation potential. They called this process liquid permeation, and pointed out that liquid permeation rates can be up to 1000 times gas permeation rates. Binning and James (86)gave cost data showing that commercial drying of isopropanol is cheaper via liquid permeation than by azeotropic distillation with hexane. After these first disclosures by Binning and various coworkers, several other laboratories became interested in the potential of liquid permeation as a separation
/
SURFACE LAYER
I
MMWNE THICKNESS
Figuru 3. Thc sequence of steps in the pmneation process upstream compartment contains the liquid feed, and the downstream compartment contains vapors which have permeated through the film. The downstream compartment is maintained at a lower pressure than the liquid compartment to ensure that only vapors and no liquid are present. The film itself is nonporous and, to function properly, must contain no pinholes. VOL 57
NO. 3
MARCH 1965
23
Mechanism of Permeation
The permeation process is believed to involve the following steps, Figure 3 ( B 4 ) -solution of permeating molecules at the film surface in contact with the liquid -diffusion of these molecules through the film -evaporation of these molecules from the downstream surface of the film The steady state permeation process may then be described by a form of Fick's law, dc q = -D(1) dL where q is the permeation rate, D is the diffusivity, and d c / d L is the concentration gradient. Unfortunately, the value of the diffusivity, D , depends very strongly on the concentration of solvents in the plastic film. Many expressions have been proposed to relate D to the solubility of solvent in the film and to a diffusivity, Do,obtained at zero concentration of solvents. The equation quite widely accepted is
D = Do . eac (2) where D o and a are constants at a given temperature and the constant a essentially describes the plasticizing action of solvent on the film. Substituting Equation 2 in Equation 1 gives the steady state permeation rate fgr a single component (3)
in terms of the concentration of permeating material in the plastic film at the upstream (GI) and downstream (62) sides, the diffusivity Do,the plasticizing constant a, and the film thickness L. Measured Parameters
The values of Do, C I , c2, and q were all determined experimentally for n-heptane, methylcyclohexane, and toluene in polypropylene by Long (BTZ),and values of a were computed from Equation 3 , With concentration given in grams per cc. of swollen polymer, the values of a varied with temperature from 20 at 53' C. for methylcyclohexane to 49 at 0' C. The values for n-heptane were 33 at 53' C and 87 at 0' C, with toluene falling in between. The values of a were also very sensitive to crystallinity of the swollen film, increasing as crystallinity increases. The values of a are of the same order of magnitude as those obtained for C4 and Cb hydrocarbon vapors in vinyl chloride by Kokes and Long (B71), for methyl iodide in polyvinyl acetate by Richman and for benzene and n-hexane in polyethylene Long (B75), by McCall (B13) and for styrene in polyethylene by Chandler and Henley ( B 7 ) . Effect of Solvent Concentration
There is a very strong dependence of diffusion rate on solvent concentration in the polymer film, and there appears to be no basic difference between the values obtained for liquid permeation and for vapor diffusion provided the concentrations in the film are known. 24
INDUSTRIAL A N D ENGINEERING CHEMISTRY
A recent paper by Stannett and Yasuda (B79)on permeation of benzene and cyclohexane through polyethylene and acetone and acetonitrile through rubber tends to confirm this, since they found no difference in permeation rates for liquid versus vapor when using film equilibrated with the permeating solvent. Thus, when vapor and liquid permeation rates are different, the diffusion step must not be rate limiting. On the basis of his measurements, Long (B12) calculated the solvent concentration gradients through the film and found a very steep gradient near the downstream side of the film (Figure 4 ) . This indicates that essentially all the permeation resistance is near the downstream side of the film. However, while tending to support the solution phase-vapor phase two-zone permeation model proposed by Binning (B4, B 5 ) , this gradient curve indicates that the concentration varies smoothly through the film and the distinction of dissolved solvent as liquid or vapor is unrealistic. 'The meaningful factor is diffusivity as it is influenced by solvent concentration. The calculated concentration profiles of Long are very similar to those experimentally determined by Richman and Long (B75)for vapor diffusion of methyl iodide into polyvinyl acetate. Effect of Pressure and Downstream Concentration
Long (B12) also showed that the independence of diffusion rate from downstream pressure is only apparent. Calculation of the effect of downstream concentration of solvent in the film shows no appreciable loss of permeation rate until c2 is about 70 to SOYGof c1 (Figure 5). To get such concentrations of c2, the downstream pressure must be close to saturation pressure of the vapor. No experiments have been run in this region so far. The calculation further shows a very sharp drop in permeation rate as cp approaches C I . Long's overall conclusion was that liquid permeation is merely a special case of ordinary diffusion, with its apparently unusual properties being due to the extreme concentration dependence of diffusion in plastics. Effect of Polymer Structure
One of the difficulties in arriving at a good theoretical model of liquid permeation is the variability in polymer structure with previous thermal history, pre\,ious solvent history, and with degree of crystallinity from batch to batch. Furthermore, in actual permeation operation, the polymer often gradually changes its characteristic, but at relatively slow rates (B2, B9, B l 2 ) . Michaels, Baddour, Bixler, and Choo (B74) have shown that pretreating polyethylene by swelling it with p-xylene and then annealing it in the swollen state increases the permeation rate for xylenes through the film and increases the selectivity of the film for p-xylene with respect to 0- and m-xylenes. They noted also that the permeability and
E . J . Henley is il. I . D . Professor o f Chemical Engineering at the University of Brazil, on leave of absence from the Stevens Institute. 4'. A'. Li and R . B . Long are Chemical Engineers with the Process Research Division of the E m Research and Engineering Company.
AUTHORS
selectivity changes were highly dependent on degree of swelling, treatment temperature, polymer crystallinity, and treating compound. They explained their data on the basis of changes in the crystalline texture of the polymer and predicted that by proper tailoring of the struc'ture of the polymeric membrane it could be made much more effective. On the other hand, Long (872)found
Figure 4. Calculated concentration gradknt far n-hcptanc through polypropyleneflm
100
P
cr E
560
8
e e
;40
20
that structural changes made by treating polypropylene with solvents can be obtained either by preannealing in solvent or simply by operating under permeating conditions for long periods of time; Le., days or weeks at relatively high temperatures of 50' to 80° C. Furthermore, the nature of the solvent used during the solvent annealing was not important. Thus, the importance and effectiveness of tailoring will vary among polymers. Baddour, Michaels, Bixler, de Filippi, and Barrie (B7) studied further the transport of liquids in structurally modified polyethylene. They showed that vapor phase solvent treating of fresh polyethylene film at 80" C. caused annealing in the film. In general, solvent-annealed polyethylene fJm showed greater equilibrium sorption of solvents per unit volume of amorphous polymer as annealing temperature or solvent concentration were increased. This results in correspondingly higher transport rates. These results as well as the effects of solvent and thermal pretreating on permeation rates are discussed in terms of a structural model of the polymer. This model considers the changes in crystallinity of the polymer due to temperature and solvent effects in terms of osmotic pressure gradients, crystalline melt-out, and recrystallization. Prior to obtaining steady-state permeation, the xylene and acetylene dichloride permeation rates through polyethylene pass through a maximum. The initial increase with time is attributed to fragmentation of crystallites due to osmotic pressure buildup. The following decline in flux to the steady state value is attributed to crystallization and/or stress relaxation after rearrangement of chain segments in the swollen state. The maxima usually occur the first hour with steady state achieved in four to five hours. Eisenman and Berger (BS) and Eisenman (B70)note that increases in film crystallinity almost invariably decrease permeation rates and enhance selectivity. They show a marked effect of downstream pressure on both selectivity and rate. This result is in direct opposition to other investigators (85,B72),and may be due to the generally high levels of downstream pressure they used. Frequently, they operated close to the saturation pressure of the permeate and, thus, their results may have been influenced by downstream evaporation effects. Eisenman also reaffirmedthe fact established by Schrodt, Sweeny, and Rose (B78)that it is still impossible to predict selectivities in mixtures from the permeation rates of individual compounds. This is not too unexpected because of the large number of solvent-solvent and solvent-polymer interactions normally encountered. This situation will probably not be remedied until solubilities of mixtures versus composition of the dissolved mixture are known and concentration profiles for the individual components of mixtures in the polymer can be determined. Prediction of Permeation Rater
0
20
40
69
DOWNSNIM CONCENTRATION OF nC, II
Figure 5. Effectof downstream concentration on pmniation of nhepbme through polypropylencfilm
Salame (B76)and Salame and Pinsky (877) have proposed a system for predicting liquid permeation rates for solvents in polyethylene and polypropylene. This system is based on analyis of the extensive data'reVOL 57
NO, 3
MARCH 1965
25
ported by Bent and Pinsky (B3) on weight loss of solvents from plastic bottles held at various tempera tures. This technique makes use of additive “Permachor” values which are obtained for various chemical groups, lengths of carbon chains, types of unsaturation, and degree of branching in the permeating molecule. These values are obtained by plotting the log of the permeation rate for normal hydrocarbons versus the number of carbon atoms in the molecule to get a straight line master curve. Then, other classes of compounds are plotted in the same way and usually give a straight line roughly parallel to the master curve. The curves are then superimposed and the correction in carbon number needed to superimpose the curves is known as the Permachor. In this way a series of correction values or Permachors is built up for various molecular groupings and can be used to predict permeation rates for new compounds. The method is remarkably powerful as a prediction tool, but it does fall down in certain cases. One difficulty is that there is a single temperature correction for all molecules, whereas actual systems differ in the effect of temperature on permeation rate. Further, effects of temperature or symmetry on polar group interactions are not included. However, this technique is still the most powerful prediction method available toda)- for pure solvents in hydrocarbon polymers. The permeation mechanism is still not clearly established. Schrodt e t al. (B78) suggested that hydrogen bonding between the polymer film and solvent played an important role; Binning et al. (B5)proposed that selectivity occurs at the interface between a “vapor phase zone” and a “liquid phase zone” in the film. Michaels et al. (B74) suggests a type of sieving action between polymer crystallites as controlling selectivity, and Long (B1Z) suggests that classical diffusion and the concentration gradients established by the individual components in the mixtures should define permeation. Before any of the suggested mechanisms can be accepted, more experimental work is necessary.
LIQUID DIALYSIS AND REVERSE OSMOSIS PROCESSES Dialysis as a unit operation considerably antedates gas and liquid permeations. Membrane dialysis was used by Graham in 1861 for the practical purpose of separating colloids from crystalloids. The first large industrial dialyzers, for recovery of caustic from rayon steep liquor, were installed in the United States and abroad in the 1930’s. Industrial dialysis units for recovery of spent acid from metallurgical liquors have been widely used in the metallurgical industry since 1958. Reverse osmosis, on the other hand, is a very recent process; the construction of the first pilot plant, a desalination unit, has only recently been completed ( C 3 ) . In general, the only reverse osmosis processes proposed to date are those where aqueous salt or organic solutions are pumped through membranes which selectively pass water. The process is often called ultrafiltration. 26
INDUSTRIAL A N D E N G I N E E R I N G CHEMISTRY
Mechanisms of Dialysis and Reverse Osmosis
Mass transfer between two liquid phases separated by a membrane involves diffusion through the liquid phases as well as the membrane. The overall dialysis coefficient, U,,, is defined by a linear conductance equation of the type
W U O AAC (4) where W = weight transferred, A = area, and AC = concentration driving force. The LTo, in turn, is related to the pseudo-film coefficients of the membrane, Urn,and the liquid, U,, in a manner analogous to heat transfer. 1
7
__ -
Ll,
7
7
urn E; +
(5)
Equation 5 can be applied with some confidence to dilute solutions, but for concentrated solutions where there is both a flow of solvent into the dialysis compartrnent as well as a flow of ions into the diffusates compartment, the equation must be modified. In general, however, the overall dialysis coefficient is considered obtainable from a knowledge of the diffusion coefficient, particle diameter of the solute, and the average pore diameter and membrane thickness. There is considerably more controversy regarding reverse osmosis. Reid and Bretton (C13) who were among the first to study desalination of salt water by reverse osmosis through cellulose acetate membranes, postulated that the transfer of HzO and ions through cellulose acetate is governed by two mechanisms : movement of those ions and molecules which can combine with the membrane by alignment diffusion; movement of ions and molecules which do not align, by hole-type diffusion. Merten (C9) and to a large extent Ticknor (C76) consider the membrane as a pure diffusion barrier through which the flow is diffusive or viscous, depending on the membrane-solvent interaction. Merten, in particular, has examined the hydrodynamics of th.e process and, on the basis of a viscous, Poiseuille flow, has derived the equation q+ K ( P -- T ) q+ = water transmission rate K = membrane transmission coefficient P = operating pressure T = osmotic pressure of the feed solution
(6 1
A somewhat contrasting viewpoint is held by Sourirajan (C75)who analyses the reverse osmosis process in terms of interface phenomena. A s a first step, he calculates the depth of the fresh water layer on the surface of the salt solution. This layer exists since salts raise the surface tension of water and thus, according to the Gibbs equation, tend to move away from the interface. Hence, reasons Sourirajan, there is always a fresh water layer on top of the membrane of about 4 A . thickness, and the water can be transmitted through the membrane by pore diffusion, the optimum pore size being about 2 t , where t = thickness of pure H20 layer above the membrane. In some instances, at least, osmosis appears to take
place through evaporation of the water at one membrane surface, passage through the membrane as a vapor, followed by condensation at the opposite membrane surface (C5). In other cases, solubility appears to be a controlling factor (C77). Meosunment of Membrane Permeation Constants
Apparatus for measuring dialysis membrane coefficients is markedly similar to that used for obtaining gas and liquid coefficients, with the exception that, as a rule, one tries to adjust the hydrodynamics so that UL+ m , In a dialysis apparatus, this is done by inserting variable speed stirrers in each compartment and increasing the speed until U, reaches a maximum (C7, C78). Continuous dialyzers resembling filter presses are also used (C7, C70, C74,C79). Van Soyce (C77) has described the commercially available Graver Hi-Step unit in detail. I t is interesting to note that liquid film resistances for dialysis have been measured and found to be functions of the acid and base concentrations (CZ). Reverse osmosis cells generally do not contain stirring devices because of the high cell pressures. The experiments are usually carried out at relatively high permeate flow rates, which are obtained either by circulating the feed, or by having a very small feed chamber. Figure 6, which is due to Merten (C9) shows the effect of liquid boundary layer thickness on the flow rate of water through a membrane. The ordinate is upstream salt concentration; the lines represent calculated values.
Figure 6. Boundary layn, limitedpow ratesfor sea wafer
Effect of Osmotic PMSSUN
The osmotic pressure which is a colligative property, plays a key role in both dialysis and reverse osmosis processes, since a molal salt solution exerts a pressure of 381 p.8.i. This presents a problem in dialysis units insofar as it tends to drive water into the dialyzate compartment. In many respects, the osmotic pressures limit the recoveries which can be obtained in reverse osmosis process such as desalination, since it is questionable whether pressure much over a few thousand p.s.i. can easily and economically be maintained.
6.8 64
6.0 5.6
7
7
-i 5.2
z d
4.8
? .
Membrane Diffusion Coefficients
In dialysis, the coefficients become functions of salt concentrations, and ion ratios and membrane-solution interactions (CI, C78). In reverse osmosis processes, the membrane diffusion coefficients are generally evaluated with pure water (u = 0 in Equation 6). The best membranes prepared to date have K values in the range of 0.1 - 0.02 lb./sq. ft.-br.-atm., depending on the total pressure (C4, C8, C9, C75). The dependence of flux on total pressure (compaction) for a typical cellulose acetate membrane is shown in Figure 7. Effect of Membrone slruclure
To be suitable for dialysis applications, a membrane must have a high permeability and suitable pore size, and it must have mechanical stability. The membranes originally employed for caustic recovery were of parchment or cellulose. With the introduction of the acid-
tl
4.4 L
4.0
3.6 3.2 2.8 2.4 40
80 100 120 140 OPEMTING PRESSURE lAlM.1
60
16n
Figure. 7. Membrane constnnt (1s a fum6ion of Opnaring pressure for pure watnpow rhough ccllulosc accfate
VOL. 5 7
NO. 3
M A R C H '1965
27
TABLE I.
Wet Thickness: Mils
j Membrane Parchment, 45 lb. Cellophane, 600 PT D N T nitrocellulose Nalfilm D-3OC -~ a
1
4.6 3.5 4.1 4.2
1
1
w,,= ( ~ ~ . / m i n . ) / c mfor .2
I
'
SOME PROPERTIES OF DIALYSIS MEMBRANES
Water Transport Osmotic, W o a X loa
0 75 0 68 0 84 1.54
Hydrostatic, Whb X 70a
I
'
Membrane Dialysts Coeficient,
1 32 0 42 1 14 2 74
iVa GI
0 64
i:i
2 12 c
I
1 92 3 34 258 5 40
~
,1
Approximate Membrane Life
i
HCl
y ' :4H 2 37 I 1 9 7 2 23
W h = (cc./min.)/cm.2for one f o o t head.
7M NaCl.
I
(cm./min.) X 7 0 2
One month One week Two years
Na$lm 0-30 is a synthetic uinylplastic membrane made by
National Aluminate Gorp. TABLE I I . MEMBRANE SELECTIVITY TOWARD AQUEOUS S O D I U M CHLORIDE
1
Membrane Tested
% Semipermeabilitya 26 None None None None 6 No flow None None 96-97.4
Polyvinyl alcohol Amberplex A-I Amberplex C-1 Ethyl cellulose Nylon Cellophane Rubber hydrochloride Polystyrene Saran Cellulose acetate
TABLE I l l .
stable vinyl films in 1958, acid recovery processes became commercially practical. Table I lists some of the properties of commercially available membranes (CIO). Membranes suitable for reverse osmosis processes have even more stringent and complex requirements than those used in dialysis processes. Reid and coworkers ( C I Z , C13) appear to be the first to have discovered that cellulose acetate membranes have the relatively unique property of rejecting salt, while passing water at a small but significant rate. Loeb (C8) in 1960, found that by casting the membrane from perchlorate solutions, the permeation constants could be increased by several orders of magnitude, with no attending loss in selectivity. Table I1 lists some typical membrane permeabilities.
SEPARATION OF SOME INORGANIC AND ORGANIC SUBSTANCES I N AQUEOUS SOLUTION USING PRESHRUNK SCHLEICHER AND SCHUEL CELLULOSE ACETATE MEMBRANES ( C j 5 )
Concentration of Feed Solution Film No.
Solute
Molality
B B
Potassium chloride Sodium chloride Lithium chloride
0 5 0 5
B B
Sodium bromide Sodium nitrate
B 9 9 9
Sodium iodide Ethyl alcohol n-Propyl alcohol Isopropyl alcohol
15 15
n-Butyl alcohol Isobutyl alcohol
B
28
1 1 1
Ethyl alcohol Acetone Acetic acid
5
Glycerol
8 8 8
Sorbitol Pentaerythritol Sodium formate
~
j
0 5 0 5
0 5
0 25 0 25
Feed Rate, Cc./Min. 15 15
1500 1500
76 5 82 5
4 08
15
1500 1500
78 0 69 0
2 92
15 30 30 30
1 82
30
1 14 1 43 1 48
30 30 30
Weight Per Cent
30
I 1
0 25 0 07 025
INDUSTRIAL AND ENGINEERING C H E M I S T R Y
,
I
Operating Pressure, P.S.I.G.
4 36 0 94 167
1
I
~
' I
1
1500 1500 1500 1500
Mole Per Cent Solute Remofied
66 36 45 66
0 0 7 7
1500 1500
35 8 67 2
1
750 7 50
26 9 24 3 20 5
750
87 4
1
750 750 750
1
30
30 30
1 I
1 1
92 5 79 7
Commercial Applications
Commercial dialysis methods for recovering acids and caustic are described in detail by Tuwiner (A64) and in reference (C14). Numerous other applications for recovering salts from organic and aqueous systems have been propased, However, large scale applications have not been publicized. A relatively virgin field for dialysis is the separation of organic mixtures. Rubber membranes have been used by Hill and Munsel to separate petroleum hydrocarbons from colloids (C6). Similar membranes have been used to separate hydrocarbons from sugars, amino acids, and fatty acids ( C I ) , as well as hydrocarbon isomers. Commercial applications await further improvements in membrane selectivity and permeability. The first reverse osmosis process to go on stream is the 1000 gallon-per-day desalination unit at Newport Beach, Calif,, financed by the Office of Saline Water and built by Aerojet-General. (C3). Reverse osmosis may also be the preferred way of recovering water from urine for space vehicles. Figure 9 shows a prototype reverse osmosis cell developed by Radiation Applications, Inc., for NASA (C4). Cellulose acetate membranes are used in this unit also. A number of other reverse osmosis applications are suggested by Table 111, taken from Sourirajan (C75). REFERENCES Gas Diffusion ( A l ) Alter, H., J. PolymerSci. 57, 925 (1962). (A2) Barker, R . E., Jr., Ibid., 58, 553 (1962). (A3) Barrer, R. M., J. Phys. Chem. 61, 178 (1957). (A4) Barrer, R. M., Barrie, J. A , , J. Polymer Sci. 28, 377 (1958). (A5) Bent, H. A,, Ibid., 24, 387 (1957). (A6) Berger, M., Smith, R. P., Private Communication. (A7) Bixler, H . J., Michaels, A. S., Paper presented to the 53rd National Meeting, A.I.Ch.E., 1964. (A8) Bixler, H . J., Michaels, A . J., Salame, M., J. Polymer Sci. A l , 895 (1963). (A9) Brandt, W. W., J. Phys. Chem. 63, 1060 (1959). (A10) Brubaker, D. W., Kammermeyer, K . A,, Anal. Chem. 25, 424 (1953). ( A l l ) Brubaker, D. W., Karnmermeyer, K. A., IND.ENO.CHEM.45,1148 (1953). (A12) Crank, J., “The Mathematics of Diffusion,” Oxford Univ. Press, London (1956). (A13) Chcm. Eng. Nrws 41, No. 17, 48 (April 29, 1963). See also IND. ENO. CHEM. 57,No. 2,49 (1965). (A14) Chmutov, K., Finkel, E., Zhur. Fir. Khim. 33, 93 (1959). (A15) Di Benedetto, D. J., J . Polymer Sci. A i , 3459 (1963). (A16) Eastburn, F., M.S. Thesis, Stevens Institute of Technology, 1963. (A17) Frisch, H . L., J . Phys. Chem. 60, 1177 (1956). (A18) Frisch, H . L., J . Polymer Sci. A2, 1115 (1964). (A19) Fujita, H., Kishimoto, A., Matsumoto, K., Trans. Foraday Sot. 5 6 , 424 (1960). (AZO) Heffelfinger, C. J., Schmidt, P. G., Paper presented to the 53rd National Meeting, A.I.Ch.E., 1964. (A21) Hsieh, P. Y., J . Appl. Polymer Si. 7, 1743 (1963). (A22) Joffe, J., Private Communication. (A23) Kammermeyer, K. A., Chem. Eng. Prog. Symp. Ser. 5 5 , 115 (1959). (A24) Kammermeyer, K . A., U. S. Patent 2,966,235 (December 26, 1960). (A25) Kawai, T., Ibid., 37, 181 (1959). (A26) Kishimoto, A., Matsumoto, K., J . Phys. Chem. 63,1529 (1959). (A27) Kishimoto, A,, Matsumoto, K., J.Polymer Sci. A2, 679 (1964). (A28) Kittelberger, K . F., “Ch. E. Research Report,” Stevens Inst. Tech., 1962. (A29) Klute, C. H., Franklin, P. J., J . Polymer Sci. 32, 161 (1958). (A30) Klute, C. H., J . Appl. Polymer Sci. 1, 3.40 (1959). (A31) Kumins, C. A,, Roteman, J., J . Pdymer Sci. 5 5 , 683 (1961). (A32) Ibid., 699 (1961). (A33) Kumins, C. A,, Roteman, J., Rolle, C., J . Appl. Polymer Sci. 1, 541 (1963). (A34) Kwai, T. K., J. Poiymer Sci. A i , 2977 (1963). (A35) Lasoski, S. W., Jr., Cobbs, W. H., Jr., J . Polymer Sci. 36, 21 (1959). (A36) Li, N. N., Henley, E. J., A.2.Ch.E. J . 10, 666 (1964). (A37) Long, F. A , , Richman, D., J . A m . Chem. Sac. 82, 513 (1960). (A38) Lundberg, J. L., Wilk, M. B., Huyett, M. J., J . Polymer Sa. 57, 275 (1962). (A39) Marie, K., Publ. Sci. Tech. M i n . Air (France) Notes Tech., No. 124 (1963). (A40) Mayne, J. E. O., J . Oil Color Chem. Assoc. 40, 183 (1957). (A41) Mears, P., Trans. Faraday Sot. 54, 40 (1958). (.442) Mears, P., J . Polymer Sci. 27, 391 (1958). (A43) Mears, P., Trans. Faraday Sot. 53, 101 (1957). (A44) Michaels, A . S., Veith, W. R., Bixler, H . J., J. Polymer Sci. B1, 19 (1963). (A45) Michaels, A . S., Bixler, H . J., J. Polymer Sci. 50, 393 (1961).
(A46) Ibid., p. 413. (A47) Myers, A. W., Rodgers, C. E., Stannett, V., Szwarc, M., Modern Plasricr, 157 (May, 1957). (A481 Myers, A. W., Rodgers, C. E., Stannett, V., Szwarc, M., TAPPI 39, 734 (1956). (A49) Nishijima, Y., Osier, G., J. Chem. Phys. 27, 269 (1957). (A50) Park, W . R . R., Anal. Chem. 29, 1897 (1957). (A51) Roberts, R. W., Ph.D. Dissertation, State University of Iowa, 1962. (A52) Rodgers, C. E., Stannett, V., Szwarc, M., J . Polymer Sci. 45, 61 (1960). (A531 Rodgers, C. E., Stannett, V., Szwarc, M., J.Phys. Chem. 63, 1406 ( 1 9 5 9 ) . (A541 Rosen, B., J . Polymer Sci. 47, 19 (1960). (A55) Rybnikar, F., Ibid., 26, 104 (1957). (A56) Sobolev, I., Meyer, J. A., Stannett, V., Szwarc, M., IND.ENO. CHEM.499 441 (1957). (A571 Sivadjian, J., Corral, F., J . Appi. PolymerSci. 6, 561 (1962). (A58) Sivadjian, J., Ribeiro, D., Ibid. 8 , 1403 (1964). (A59) Stannett, V., Yasuda, H., Polymer Letters 1, 289 (1963). Szwarc, M., Bhargava, R . L., Meyer J. A. Meyer A. W (A601 Stannett, Rogers, E. E., Permeability of Plastic Films and Coated Pa;er to Gises an; Vapors,” TAPPI, ,1962. (A61) Starkweather, H . W., Jr., J.Appl. Polymer Sci. 2, 129 (1959). (A62) Schulz, G. V., Gerrens, H., 2. Phys. Chem. (Frankfurt) 7, 182 (1956). (A63) Thompson, A . B., Woods, D. W., Nature 176, 78 (1955). (A64) Tuwine:: S B. Miller L. P Brown, W. E., “Diffusion and Membrane Technoloav, ”. Rkiniold. Ne& York: 1962. (A65) Thornton, E. R., Stannett, V.,’Szwarc, M., J . Polymer Sci. 28, 465 (1958). (A66) Veith, H., Koll. 2. 152, 36 (1957). (A671 Veith, H., Sc.D. Thesis, MIT, 1961. (A6B) Walters, C. J., Petrol. ReJner 5 , 147 (1959). (A69) Walters, M. H., J . Polymer Sci. A l , 3091 (1963). (A701 Weinstock, J., Henley, E. J., Progress Report, USAF Contr. 47489. (A711 Yasuda, H., Stannett, V., J. Polymer Sci. 57, 907 (1962). (A72) Yasuda H. Stannett V. Peterlin, A., Frisch, H . L., paper presented to 53rd Meet& oiA.1.Ch.E.: 19i4. (A73) Zimm, B. H., Lundberg, J. L., J. Phys. Chem. 60, 425 (1956).
y.,
Liquid Permeation
(Bl). Baddour, R. F., Michaels, A. 5., Bixler, H . J., de Filippi, R. P Barrie, J. A. Division of Industrial and Engineering Chemistry, 143rd Meetyng, ACS, Lo; Angeles, March 1963. (B2) Barrer, R. M., Barrie, J. A., Slater, J., J. Polymer Sci. 27, 177 (1958). (B3) Bent, H . A., Pinsky, J., WADC Rep. 53-133, Vol. 2, 1955. (B4) Binning, R. C., Lee, R. J., Jennings, J. F., Martin, E. C., ACS Div. Petrol. Chem., Preprints 3, No. 1, 131 (1958). (B5) Binning, R. C., Lee, R. J., Jennings, J. F., Martin, E. C., IND.ENO.CHEM. 53.. 45 (1961). . . (B6) Binning, R . C., James, F. E.,Petroi. Rejner37,214 (1958). (B7) Chandler, H. W., Henley, E. J., A.I.Ch.E. J. 7, 295 (1961). (B8) Choo, C. Y., Advan. Petrol. Chem. Rejning 6, 73 (1962). (B9) Eisenman, J. L., Berger, C., Division of Colloid and Surface Chemistry, 141st Meeting ACS, Washington, March 1962. (B10) Eisenman, J. L. “Purification of Or anic Com ounds by Membrane Permeation,” Defense Documentation Center Wep. AD 488670, 1963. (B11) Kokes, R . J., Long, F. A,, J . A m . Chem. Sot. 75,6142 (1953). (B12) Long, R . B., Division of Petroleum Chemistry, 148th Meeting ACS, Chicago, September 1964. (B13) McCall, D. W., J . Polymer Sci. 26, 151 (1957). (B14) Michaels, A. S., Baddour, R. F., Bixler, H . J., Choo, C. Y., IND.ENG.CHEM. PROCESS DES.DEV. 1. 14 (19621. (B15) Richman, D., Long, F. A., J . Am. Chem. Sot. 82, 509 (1960). (Bl6) Salame, M., S.P.E. Trans., 153 (October, 1961). (B17) Salame, M., Pinsky, J., “Permeability Prediction,” Packaging Inst. Meeting, 1962. (B18) Schrodt, V. N., S w e e z R. F., Rose, A,, Division of Industrial and Engineering Chemistry, 144th eeting, ACS, Los Angeles, March 1963. (B19) Stannett, V., Yasuda, H., J.PolymerSct. B i , 289 (1963). I
.
,
Dialysis and Reverse Osmosis
(C1) Carr, C. W., “Physical Methods in Chemical Analysis,” 4, 1-43, Academic Press, New York, 1961. (CZ) Chamberlain, N. S., Vromen, B. H., Chem. Eng. 66, 117 (1959). (‘23) Chem. Eng. News 42, No. 29, 48 (July 20, 1964). (‘24) Everett, R., Meier, E., Odian, G., Henley, E. J., Katz, A., Hofstetter, “Water Recovery Studies,” Progress Rep. NAS 520, April 1954. (C5) Hassler, G. L., McCutchan, J. W., Advan. Chem. Ser. 27, ACS, Washington, 1960. (C6) Hill, M. W., Munsel, M. W., Division of Petroleum Chemistry, 138th Meeting ACS, New York, September 1960. (C7) Lane, J. A., Riggle, J. W., Chem. Eng. Prog. Symp. Ser., No. 24, 20 (1962). (C8) Loeb, S., Contribution No. 36, Water Resources Center, Univ. California, Los Angeles, 1961, (C9) Merten, U., IND.ENO.CHEM.FUNDAMENTALS 2, 229 (1963). ((210) Mindick, M., Oda, R., North Jersey Section, ACS, Oct. 27, 1958. ((211) Monet, G. E., Vermeulen, T., Chem. Eng. Prog. Symp. Ser., No. 24, 127 (1959). (C12) Reid, C. E., Spencer, H . G . , J . Appl. Polymer Sci. 4, 354 (1960) and J . Phyr Chcm. 64, 1587 (1960). (C13) Reid, C. E., Bretton, E. J., J . Appl. Poly. Sci. 1, 33 (1959). ((214) Riggle, J. W., in Chemical Engineers’ Handbook, 4th ed. p. 17, McGrawHill, New York, 1963. 2,51 (1963). (C15) Sourirajan, S., IND.END.CHEM.FUNDAMENTALS (Cl6) Ticknor, L. B., J . Phys. Chem. 62, 1483 (1958). (C17) Van Soyce, C . C., Chem. Eng. 66, 89 (1959). ((218) Vromen, B. H., IND.ENO.CHEM.54, No. 6, 20 (1962). (C19) Vromen, B. H., Chamberlain, N. S., 42nd Meeting, A.I.Ch.E., Atlanta, February 1960.
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MARCH 1965
29
