I
ROBERT C. BINNING,' ROBERT J. LEE,? JOSEPH F. JENNINGS, and EUGENE C. MARTIN American Oil Co., Texas City, Tex.
Separation of Liquid Mixtures
by Permeation
Liquid-phase permeation through thin plastic films has commercial potential for separating azeotropes and other organic mixtures
1
I- H.XS been hno~vn since 1831 that some gases permeate nonporous plastic films fasrer than others ( g ) , but only recently has i t been suggested that this principle be applied to the commercial separation of gaseous mixtures (2>7 I? 72). T h e manner in \vhich vapors are adsorbed and desorbed by plastic films has been studied 1. 7. SI. and several patents have been issued on the separation of b!permeation vaporous mixtures through nonporous plastic films. Howe\.er! no kno\\m commercial application has been made of such a process. Dialysis-type processes. in \\-hich molecules move from one liquid phase to another through soiiic type of mcmbrane, have been used for the purification of proteins and other biological materials for many years (.?). This report describes a liquid phase permeation system in which rhe liquid charge misrure is maintained in contact with rhin plastic films and the permearing product is removed from rhe opposite side of the film as a vapor. This liquid permearion process is physically different from gas permeation. Significant differences in rates also exist between liquid and vapor permeation. .A mechanism is proposed 10 explain rhe observed behavior of liquid permeation. The rapid pFrinearion rates attainable b!- liquid phase operation, along with escellent separation selectibities, indicate good commercial potential for this process. .A major advantage is unique selectivity in many separations, such as those involvinq azeorropic mixtures. Process design and rconomic cvaluation of one such application have already proved the attractiveness of chis process. hlany applications \vi11 be feasible in separatinq mixtures of organic chemicals and of hydrocarbons Ivhich cannot be separated b!- conventional means.
1
Present address, hfonsanto Chemical
Co., Dayton, Ohio.
* Present address, Standard (Indiana), it'hitinq, Ind.
Oil
Co.
Although no commercial installation has been made, -American Oil Co. has given Ionics, Inc., Cambridge, Mass.; exclusive rights to the commercial development of this process. Several commercial installations are presently being investigated.
CHARGE UIXTuFIE ILIOLl'D PHASE'
\
,FILM
L l O U ' 3 PHASE-
Process Description
A variety of polymeric plastic films can be used in the liquid permeation process to separate mixtures of organic compounds. Choice of film depends on the chemical nature of the mixture being separated and the stability of the polymer at the required operating temperature. O n e of the components of the charge mixture must be sufficiently soluble in the film polymer so that permeation \vi11 occur a i a n appreciable rare \vithout sofiening the film io the extent that it becomes weakened and ruptures. .Also, the film must be very thin-preferably about 0.001 inch thick, or less-to obtain high permeation rates. Thin filnis can be used because selectivity is independent of film thickness in the range of practical film thicknesses. T h e films are nonporous; they do not contain discrete holes or pores, and they do nor function by a molecular sieving action nor by Graham-type diffusion through holes. Instead, permeation requires solubility of material in the polymer comprising the film and movement of
Diagram o f liquid permeation process shows unit divided into two compartments b y the film. The more p e r meable molecules are shown in white
the material through the polymer struccure by a n activated diffusion process. A diagram of the liquid permeation process is shown above. The permeation unit is divided into t5vo compartments by the film. The charge mixture is introduced into the left compartment and maintained in the liquid phase a t the temperature of operation. T h e operating pressure is atmospheric or tvhatever pressure may be required to keep the charge in the liquid phase. T h e prrmeating material is removed rapidly from the right compartment, which is usuallp
Size, Shape, and Solubility of Molecules Affect Their Permeability
b b
Lower molecular weight molecules in a homologous series permeate faster
Molecules o f smaller cross section with the same molecular weight and chemical nature permeate faster
b Shape and size factors may predominate for materials with small differences in chemical nature, but b Molecules with large differences in chemical nature are not as affected b y shape and size factors in the range of normally liquid materials
VOL. 53, NO. 1
JANUARY 1961
45
Laboratory film holder provides 22 square inches of film surface on five faces of the cube. Films are supported by porous stainless steel elements, visible on the open face
kept a t reduced pressure, thereby maintaining a high concentration gradient across the film and preventing attainment of equilibrium. T h e more permeable molecules (sho\vn in white) preferentially permeate through the film and are enriched in the permeate. In this example, typical of many actual separations by permeation. the permeate product contains 80% of the white molecules, starting with a charge stock containing a 50 to 50 mixture of white and black. T h e process can be operated in either a coptinuous or batchwise manner, and the amount of material taken through the film can be varied from a small percentage u p to 50% or more. T h e product passing through the film is referred to as the permeate. while the nonpermeating residue or
remnant is called the nonpermeant. T h e composition of the nonpermeant leabing the unit will depend, of course, on ho\\ much permeate was taken through the membrane.
Permeation Involves Three Steps: ,Solution of liquid in the film surface in contact with the liquid charge mixture ,Migration through the body of the film Vaporization of the permeating material at the downstream interface where permeate product is immediately swept away
ch/cN
TO
T h e general mechanism of the liquid permeation process is similar to gas permeation, but there are important differences in the state of the permeable membrane for liquid permeation and in the rate of transport of material betlveen the liquid charge phase and the membrane. I n gas permeation. polymer film characteristics are not appreciably changed by the permeating gas because it has a very low degree of solubility in the film; hence. it is possible to calculate permeate composition from permeation rates of the pure q s e s and composition of the charge mixtures (6. 70. 72). it'hen a steady state of flow is established, the amount of gas which permeates through unit area of the polymer film in unit time satisfies one form of Fick's first law of diffusion ( 7 . 3) :
where p l and p:! are the pressures of the gas on the two sides of the film, L is film thickness, D is the diffusion coefficient, 5' is the solubility coefficient, and P is the permeability constant Ivhich is equal to
DS. \\'hen dealing with the permeation of organic vapors (7), or in the present liquid permeation process, it is not possible to calculate the composition of the permeate from a knowledge of charge composition and the rates of permeation of pure components. 'The swollen membrane (i.e., the membrane containing the dissolved permeating components) has a substantially different permeability than the original membrane; likeivise, the membrane condition when permeating individual compounds differs from that when permeating a mixture of dissimilar compounds. T h e mor? fundamental form of Fick's first law must be used to describe the liquid permeation process in the steady state:
in which q is the amount of liquid which permeates unit area of film in unit time, and C? - C1 is the concentration differential between the two sides of the film in some consistent units which express the number of molecules per unit volume. Equipment
With this laboratory assembly, pressure on both sides of the membrane can be varied independently over a wide range A. Film holder 6. Cover flange C . Permeation vessel D . Heating jacket E . Reflux condenser
46
INDUSTRIAL AND ENGINEERING CHEMISTRY
F. Exit pipe G. Condenser H. Manometer 1. Thermowell
T h e heart of the laboratory equipment is the film holder which provides the means for mounting the thin plastic films. T h e film holder shown above is a hollow cube provided with circular openings on five of the faces for mounting the membranes. T h e sixth face is fitted with a pipe connection for removing the permeate product. After mounting the membranes, the film holder assembly is tested for leaks
LIQUID PER ME.ATI O N Table 1. Charge Pressure Does Not Affect Rate or Selectivity in Liquid Permeation
CHARGE SIDE
PERMEATE SIDE
LIQUID PHASE
VAPOR PHASE
Charge composition: 50-50 n-heptane-isooctane Operating temperature: 100' C.
C'liarge I're-ure. P.S.I.G. 15 115
Permeation Rate for Permeate 1-31il Filiu, Composition, Gal. /(Hr.) (Sq. Ft.) Tol. % n-Heptane X lo3 75 75
I
I
t +
140 140
I 1
I
by evacuating the inside to 10 m m . of mercury. \Vhen the membranes are properly mounted, the rate of air leakage \vi11 be less than 4 m m . of mercury pressure rise per hour. T h e complete laboratory assembly is shoivn schematically. T h e film holder. -1. is installed in the cover flange, B. of the permeation vessel? C: by means of a packing gland. T h e permeation vessel is provided \\.ith a heating jacket, D,and a reflux condenser! E. T h e exit pipe, F, from the inside of the film holder: is provided with the necessary fittings so that the permeate may be conducted to the condenser. G . A manometer, H . is connected to the p-rmeate exit line. A thermo\vell. I. is attached to the cover flange so rhat the temperat.u-e of the liquid can be measured. T h e dimensions of the film holder and permeation vessel are such that the liquid charge covers all of the films throughout a permeation run. In cases where runs \yere made a t pressures higher than atmospheric in the permeation vessel. the reflux condenser lvas replaced \Vith a pressure gage, and a bottom-entering agitator was employed to keep the liquid phase in rapid motion. In all cases: agitation of the liquid contents \vas proirided either by vigorous reflux or by stirring to assure that fresh charge material was continually brought in contact \\.it11 the film surfaces. \2:ith the assembly described. the pressure on both sides of the membrane can be varied indepTndent1)- over a \vide rang?. All of the data reported were obtained under steady-state operating conditions and are based on several samples of the permeate and nonpermeani. taken over a period of .5 to 7 hours of continuous operation. T h e amount of charge material used \vas large compared to the amount of permeate removed. so that the composition of the liquid contacting the films changed relatively slowly. I n some experiments? the liquid composition \vas maintained approximately constant by frequent addition of the charge components in a ratio equal to that removed in the permeate.
Polymer film under permeation conditions is assumed to consist of solution phase zone and vapor phase zone. Selectivity in separating mixtures occurs a t the two interfaces of the former
superatmospheric pressure on the charge has no effect on either permeability or selectivity, in ranges of pressure which are considered practical (Table I). In this experiment, all variables were maintained constant in the two runs except the charge pressure, which was varied by using a nitrogen pressure above the liquid charge mixture in contact with one side of the film. Changing charge pressure from 1 to 8 atm. had no detectable effect upon either rate or selectivity. These results were based on observations for 6 to 7 hours of steady-state operation in rach case. Operation at intermediate pressures, observed for shorter times, \vas found to be the same.
T h e rates of permeation are expressed, in most cases, as gallons or pounds per hour per 1000 square feet of film surface to aid the reader in visualizing more readily the size of equipment necessary to carry out separations on a commercial scalc. Metric units are also used. Factors Affecting Permeation
T h e efficiency of the permeation process is dependent upon both rate and selectivity. Relatively high rates of throughput, as well as good selectivity, must be achieved to develop an economical process. Operating Variables. CHARGE PREYSL-RE. In liquid phase permeation,
Table II.
Permeation Rate Remains Constant over a W i d e Range of Pressure Differentials across the Film Charge: pure n-heptane Charge pressure: 760 mm. Hg Temperature: 99' C.
Pressure on Permeate Side of Film, Mni. Hg.
Pressure Differential across Film ( P ? - P,j, 11ni. Hg
20 40 50 100 200 300 400 500
740 720 710 660 560 460 360 260
Permeation Rate. Gal./(Hr.j (Sq. Ft.) X I O 3 1-mil 0.55-1nil film film
...
Air i n Perm eate (0.55 3Iil Filinj, Mole % ~~
...
236 233
2.4 2.4
134 119 125 121 125 116
230 230 236 226 230
2.5 2.9 2.5 2.3 2.7
...
VOL. 53, NO. 1
...
JANUARY 1961
47
I
1
r
I
r:,
i
300
10
I
1
29
28
+.,,x
1 73' Figure 1. perature
i 80'
I
27
26
103
1
90'
I
IOO~C.
50-50 volume mixture of n-heptane and iso-octane
Thus, pressures of this magnitude have no significant influence on the concentration of the componmts in the liquid charge system, nor upon the solubility of the liquid charge components in the polymer film. Hence. the concentration gradient across the film is unaltered, and the rate of permeation remains unchanged. I n gas permeation processes, the rate of permeation is affected by the charge pressure as this affects the concentration gradient across rhe film. PERMEATEPRESSUREA N D PRESSURE DIFFERESTIAL.I n liquid permeation, the conditions on the downstream side of the film are maintained such that the permeate product can be rapidly removed as a vapor. So long as this condition is satisfied. the pressure on the permeate side of the film has very little influence on the rate of permeation unless the pressure is so high as to limit the removal step (Table 11). Data \vere obtained by varying the pressure on the permeate side of the film, Tvhile maintaining the n-heptanr charge in the liquid phase a t atmospheric pressure and 99' C. Permeation rate remained practically constant between 20 and 500 m m . of mercury permeate pressure, equivalent to pressure differentials ranging from 260 to 740 m m . of mercury, shoning that the concentration gradient (C? - Cl)> is not affected appreciably by changes in pressure differential of this order of magnitude. Actually. simple calculations show that molecular concentration in the liquid charge phase is so great compared to the INDUSTRIAL AND ENGINEERING CHEMISTRY
I
I
Permeation rate increases linearly with tem-
Charge stock:
48
1
I
1
I
,
I
10 RECIPROCAL THICKNESS
05
I
I
I
I
I
I 5
H1I.S
Figure 2. Permeation rate i s inversely proportional to film thickness; selectivity i s independent of film thickness Charge stock:
50-50 volume mixture of n-heptane and iso-octane
Concentration in the vapor permeate phase, that the concentration gradient is scarcely affected by these changes in permeate pressure. I n the experiment Ivith 0.55-miI films, analytical measurements shoived that the measured pressure in the permeate zone was essentially due only to n-heptane vapors. 'This was done by inserting a 8-inch copper tube. through the permeate take-off line, into the center of the interior of the film holder. This 8-inch sample tube \vas connected directly to a mass spectrometer, so that samples of the permeate immediately adjacent to rhe films could be dralvn into the mass spectrometer for determination of mole percentage air and mole percentage n-heptane. Duplicate analyses Jverr run during each period of operation a t the different permeate pressures. .Air content averaged 2.5 mole yc (Table 11). 'These measurements show conclusively that the partial pressure of n-heptane in the permeate zone is 97 to 985; of the measured pressure throughout the pressure range investigated. and hence the permeate pressure is a valid measure of the pressure differential across the film. In vieiv of this, it is concluded that in liquid permeation the rate of permeation is independent of the pressure differential across the film. as long as the permeate is removed as a vapor. TEMPERATURE. In general, an increase in temperature causes an increase in the rate of permeation and a decrease in selectivity (Table 111). I n ail experiments in this series, the charge in contact .
I
Lvith the films \vas i n the liquid phase) and the pressure on the permeate side of the films was maintained at 35 mm. of mercury to remove thc~ permeate as a vapor a t each temperature used. T h e variations of permration rate \cere plotted (Fiqure 1 i on a logarithmic scale as a function of reciprocal temperature for the four film thicknrsses. .As expected, permcatioii is a temperature-
Table 111. Higher Temperatures Increase the Permeation Rate and Moderately Reduce Selectivity
'
Charge composition: 50-50 vol. n-heptaneiso-octane Charge: liquid phase, atmospheric pressure Permeate zane pressure: 35 mm. Hg
I'ertiieate
Filii1
(Coin- I'errrieatioii l~ositioii, Rate, Permeati~rii 1-dcc (;al./ ( H r . I
Thickness, Mils
Temp.. C.
0.8
70 80 90 100 70 80 90 100 70 80 90 100 70 80 90 100
1 .o
1.4
1.9
ri-
Heptane
(Sq.Ft.) X 103
79 78 76 75 77 77 75 75 76 77 75
78 144 205 58 80 112 156 33 50 69
75
93
76 76 77 75
22 33 47 66
105
LIQUID P E R M E A T I O N activated process. T h e rate is approsimatel>-double. Lvith a 20' C. increase in temperature for all four film thicknesses. T h e apparent energy of activation is about 8.5 kcal. per mole for this system and is independent of film thickness. F u r THICKNESS. A study of permeability as a function of film thickness shoived that the rate is inversely proportional to film thickness and that selectivity is independent of film thickness in the range of thickness considered practical for commercial use (Table 111: Figure 2). A linear relationship bet\veen rate and the reciprocal of film thickness \\-as established a t four temperatures. Selectivity. as measured hy the krolume percentage of +heptane in the permeate, \vas essentially the same a t all four film thicknesses (Table 111). Similar results \ v e x obtained \vith other mixtures and films. Thus, i t is practical to operate \vith very thin films and still retain selectivity and rapid permeation rates. Successful runs on the separation of organic mistures have been made ivith films as thin as 0.3 mil. Film Polymer Properties. T h e chemical nature of the pol>-merLvliich composes the film and the presence of plasticizers and solvents influence the permearion rate and extent of separation obtained. .\nother important characteristic of films is their stahilit?., both thermal and chemical. in the presence of the charge mixture under operating conditions. .Uthough the nature of the films is not discussed in this report. certain thin p o l ~ m e rfilms are much more stable and selective under permearion conditions. T h e authors have carried out permeation separations for hundreds of hours of continuous operation ivith films 0.5 to 1.0 mil thick lvithout any detectable change of film stability and 1vithou.t film failure. Effect of Structure and Solubility of Permeating Molecules. T h e properties of molecules jvhich cause differences in rate of permeation arlz size: shape, and chemical nature. Some information on these factors has been obtained by invrsrigaiing permeation rates of a number of pure hydrocarbons. I n a homologous series of normal paraffins. permeation rate through the same film under the same conditions decreases with an increase in the nuniher of carbon atoms [Table I\-). T o observe the effect of molecular shape, relative permeation rates of hexane isomers \yere studied. There was a wide variation in permeation rate of these isomers \vhich correlates with the shape facror (Table I\'). T h e unbranched n-hexane permeated more than three times faster than the singly branched methylpentanes and about a hundred times faster than the doubly
Table IV. Olefins Permeate Faster than Paraffins and Unbranched Hydrocarbons Faster than Branched Isomers Liquid charge: 5 2 " C., atmospheric pressure Film thickness: 0.0046 cm. Permeate zone pressure: 3 6 0 mm. H g
Table V.
Reversal of Selectivity
Benzene-methyl alcohol mixtures can b e separ a t e d b y selectively permeating either component Liquid charge: 60' C., atmospheric pressure Film thickness: 1 mil Permeate zane pressure: 40 mm. H g
I'erineation
Periiiea-
LIeter)
tion Rate, Gd.1
Rate. 111. ' (Hr.) (Sq.
(Per 0.001-
Cin. Fil~ii
Thlcknes?
Hydrorar bon ,:-Hexane n-Heptane is-Octane ii-Nonane
479 197 145 31
2-Methylpentane 3-Methylpentane 2,2-Dimethylbutane
145 115
A
B
69.5M
105
30.5B 10M
104
5 1437 532
I-Hexene 2-Heptene
branched 2.2-dimethylbutane \vhich has r h e g m structure. T h e effect of chemical nature is quite pronounced. even among hydrocarbons. as illusrrated by observations of the relative permeability of hydrocarbon pairs \vhich do not differ greatly in size and shape. For example. the relative permeability of olefins compared to paraffins or the same numher of carbon atoms and structure clearly sho\i,s this effecr (Table 11'). T h e presence of a double bond in I-hesene produces a sufficient difference in chemical nature so that it permeates about three times faster than n-hexane. and 2-heptene permeates about three times fasttr than n-heptane. I n the data presented above. differences in permeation rate Lvere compared for molecules \vhich differ onl! in size, shape. or chemical nature. The molecular shape factor will predominate \\.hen the difference in chemical nature is not large. However: \vhen the dif-
Table VI.
32M 68B 32M
ference in chrmical naturr or solubility character is very large. size and shape factors may have \ r r j . little influence on permeability. I n such instances. tht, striking phcof selectivity-can nomenon-reversal be observed b>- changinq the type of film (Table 1.). There is considerable difference in the size and shape of benzene and meth!-l alcohol. hut there is also a large difference in chemical naturr. hiethyl alcohol is more sOluble in Film .A than is benzene, and methyl alcohol is selectively permeated through Film A . I n the second esample. henzene is more soluble in Film B than methyl alcohol, and bcnzene is selectivt-l>- permeated. This type of phenomenon can probably be correlated Lt1-ith the Flory-Huggins 3 parameter for the interaction of solvents \\it11 polymers. Differences in Vapor Phase and Liquid Phase Permeation. T h e authors have observed substantially hiSher rates of permearion \\-hen the charge is maintained in the liquid phase. i n contrast Lvith the vapor phase method of operation. Experiments \vere conducted in ivhich tivo permeation unirs were
Higher Permeation Rates Were Obtained When Liquid Charge Was in Contact with Films, Compared with Vapor Phase Operation Charge: 59.5' C., atmospheric pressure Permeate zone pressure: 35 mm. H g MeOH-Charge composilion: 39.1 wt. Film thickness: 1.0 mil Film area: 1 4 2 sq. cm. ( 2 2 sq. in.)
Liquid I'hahe perrlieate coniposition.
Elaped Tiiiie, H r . 1 2 3 4 5
Average
TTt. 7c benzene
91.8 91.5 93.7 91.5 91.8 92.0
Permeation R a t e
'
Grains (llr,) (142 -q, cin.) 5; 47 47 47 47 48
-
Lh.yllr.) (.q. ft.) 0.76 0.68 0.68 0.68 0.68 0.70
60.9 wt.
'i
benzene
I.;L I'lin*e ~Jo~ perllleation ' < a t e vompositioii. Grsiii, ' Tvt. % (1ir.i (1-12 Lb,'(lir.) henzc:ie q.( Y I I . ) (xi.f t . ) ~~
l'eriiieate
89.3 90.6 90.6 90.6 90.6 90.3
VOL. 53, NO. 1
~
~
28 28 28 26 26 27
0.40 0.40 0.40 0.37 0.37 0.39
JANUARY 1961
49
operated under identical conditions except for the method of contacting the film with the charge mixture. T h e same film (1 mil thick) was used in each unit. I n the liquid phase operation, the film holder was immersed in a n azeotropic mixture of benzene and methyl alcohol. In the second unit, the film holder was positioned in the vapor phase above the azeotropic mixture which was refluxed a t a vigorous rate. I t is estimated that the reflux rate in the condenser \vas a hundred times the rate of permeation, so that saturated vapor of the azeotropic composition was contacting the films a t all times in the vapor phase unit. T h e permeate product was returned to the permeation vessel after each hour so there would be no large variations in charge composition during the experiments. T h e results from five hourly periods of observation of the two methods are shown in Table V I . T h e permeation rate ivith liquid contact was almost twofold greater than that obtained in the vapor phase operation. Even a t this higher permeation rate, separation selectivity was also slightly better when the films were in direct contact with the liquid charge. This is a typical result observed with several other charge systems and film compositions. For example: when charging a ivater-dioxane azeotrope a t 150' C. and 30 mm. of mercury permeate pressure. the average permeation rate over a 5-hour period was 0.32 pound/(hour) (square foot) for liquid phase contact; with vapor contact, the rate was only 0.12 pound (hour) (square foot). T h e lower rates observed for vapor phase operation appear to be due to a lower rate of transport of material into the film surface. Fresh vapors may not be supplied fast enough to maintain a continual high concentration of the more permeable component at the film surface. This may also explain the slightly lower selectivity observed in the vapor phasp operation. It is also possible that the film surface is in a more swollen and "open" condition when in direct contact with the liquid charge than with vapor phase contact. Mechanism
Although the mechanism of gas permeation has been well established. it does not adequately describe liquid permeation because there are important differences behveen these two processes. T h e usual rates of gas permeation are of the order of 1 pound per hour per 1000 square feet. whereas liquid permeation rates of over 1000 pounds per hour per 100 square feet are easily obtainable (Table 111). I n fact, liquid permeation rates as high as 7000 pounds per hour per 1000 square feet have been observed by the authors in other studies not reported here.
50
Another important difference is the condition of the film under permeation conditions. I n liquid permeation, the permeating liquid dissolves to an appreciable extent in the polymer film; hence, the permeable film under operating conditions is a s\\ollen "solution" of polymer and permeating organic compounds. This is entirely different from the condition of the "dry" film which exists in gas permeation. Consequently. the composition of the permeate in liquid permeation cannot be calculated with any certainty from the composition of the charge mixture and the known rates of permeation for the pure components. Further, liquid permeation differs from gas permeation in that the rate of permeation is independent of the pressure differential across the film under liquid permeation conditions because of the tremendous concentration gradient which inherently exists in a liquid-film-vapor system (Table 11). However. liquid and gas permeation both follow Fick's first law in that the steadystate rate is inversely proportional to film thickness.
A Mechanism for Liquid Permeation Must Explain These Experimental Facts
)Rapid rate of liquid permeation compared with vapor permeation )Linear relationship between rate and film thickness )Selectivity independent of film thickness After considering the possible ways in which liquid permeation could occur to yield the observed results: one mechanism appears most plausible, as shown by the diagram (p, 47). For liquid permeation conditions, there exists a "solution phase." which comprises the ma,jor portion of the film, and a "vapor phase zone" Lvherein vaporization of the permeating material occurs. T h e latter zone occupies a relatively minor part of the film structure. There is a very rapid movement of liquid in the "solution phase'' and between the liquid charge phase and the '.solution phase." Some selectivity may occur in the movement across the interface bet\veen the liquid charge phase and the "solution phase." Probably most of the Selectivity occurs a t the interface between the "solution phase" and the "vapor phase." Thereafter, the material diffuses through the "vapor phase," and this diffusion is believed to be the rate-controlling step in the process. T h e so-called ..solution phase" may be visualized as a highly swollen state of the polymer in which there is a high concentration of permeating liquid in the polymer structure. T h e "vapor phase" may be considered a region in which permeating molecules are much more
INDUSTRIAL AND ENGINEERING CHEMISTRY
dispersed and polymer structure corresponds more nearly to that of the dry polymer film. Two completely different phases such as these can exist within the film because the opposite surfaces of the film are subjected to such different conditions. T h e existence of a "solution phase" as described here is demonstrated by the swollen condition of a plastic film immersed in a liquid which is almost a solvent for the film. I t is logical to assume that a "vapor phase zone" exists because vaporization must occur in a zone near the permeate surface of the film. There must be a very rapid movement of liquid in the "solution phase" and between the liquid charge phase and the "solution phase" to maintain selectivity under the steady-state of operation. T h e very rapid movement of liquid in the "solution phase" is supported by observations of sorption and desorption of organic vapors by polymer films (7). T h e diffusion coefficient varied about a thousand-fold for a film containing no solvent and the same film containing 0.12 gram of solvent per gram of polymer. Selectivity must occur at one or both of the interfaces between the liquid charge phase, the solution phase, and the vapor phase because selectivity is not a function of film thickness; therefore, the Selectivity function must occur a t distinct points or interfaces within the film. Acknowledgment
The authors acknowledge the contributions of James M. Stuckey in the initial stages of the development of this process. Literature Cited (1) Barrer. R . M.. "Diffusion
in and Through Solids," Cambridge Univ. Press. Cambridge. 1941. (2) Brubaker, D. W.. Kammermeyer, K., IND.EYG.CHEM.46, 733-72 (1954). (3) Crank, J.. Park, G. S., Trans. Faraday S i c . 47, 1072 (1951). (4) Doty. P. M., Aiken. W. H., Mark. H.. Ind. Eng. Chem. (Anal. E d . ) 46, 686
,-
(1 944). I
(5) Graham, T.. Trans. Roy. Soc. London 151, 183 (1861). (6) Huckins, H. E., Kammermeyrr. I(.. Chem. Ene. Pr9er. 49, 180-4, 295-8 (1953). (7) Kokes, R . J.. Long, F. J . .4m. Chem. Soc. 75, 6142 (1953). ( 8 ) Kokes, R. J.. Long, F. X.. Hoard. .T. L.. .I. Chem. Phys. 20, 1711 (1952). (9) Mitchell. J. V.. J . Roy. Inst. 2, 101: 307 (1831). (10) Othmer. D. F., Frohlich. G. .I.. IKD.ENG.CHEM.47, 1034 (1955). (11) IVeller: S . , Steiner. W . A., Cheni. Eng. Progr. 46, 585-90 (1950). (12) Weller. S.. Steiner. W. A., J . .4,b$. P/'Z?S. 21, 279-83 (1950). I
A\..
RECEIVED for review, May 5, 1958 RBSURMITTED May 12, 1960 XCCEPTED September 22, 1960 Division of Petroleum Chemistry, 133rd Meeting, ACS. San Francisco. Calif., April 1958.
