T H E MECHANISM OF DEMINERALIZATION OF AQUEOUS SODIUM CHLORIDE SOLUTIONS BY FLOW, UNDER PRESSURE, THROUGH POROUS MEMBRANES S. S O U R I R A J A N Division of dfifilied Chemistry, National Research Council, Ottawa, Canada
On the basis of Gibbs adsorption equation, a mechanism has been proposed for the demineralization of aqueous sodium chloride solutions using porous membranes. Data are presented to illustrate the relative effectiveness of the above technique for the separation of several inorganic and organic substances in aqueous solutions. In principle, the above separation mechanism is applicable to both inorganic and organic solutes in aqueous or nonaqueous solutions involving the negative adsorption of solutes a t interfaces. HE SEPARATION of sodium chloride and other inorganic Tsubstances in aqueous solution by simply passing the solution under pressure through cellulose acetate and other membranes has been succesrifully accomplished by Reid and coworkers (72, 7 3 ) . According to them, the transfer of water and ions through cellulo'se acetate membranes is governed by two different mechanisms (72). Those ions and molecules that can associate with the membrane through hydrogen bonding actually combine with the membrane and are transported through it by alignmen>:-type diffusion; those which cannot enter into hydrogen bonding with the membrane are transferred by hole-type diffusion. Ticknor also proposed that permeation of cellophane membranes by liquids involves both a viscous and a diffusive fiow mechanism (74). Loeb and Sourirajac: developed techniques for making porous cellulose acetate membranes capable of demineralizing aqueous sodium chloride solutions a t rates of industrial significance ( 7 5 ) . From a brine solution containing 5.25% sodium chloride, potable water containing less than 500 p.p.m. of salt was obtained in a >;inglepass a t the rate of 5 to 11 gallons per sq. ft. of film area per day, under a n operating pressure of 1500 to 2000 p.s.i.g. ( 9 ) . This paper postulates a possible mechanism of the above process and points out the feasibility of developing a widely applicable general process for the separation of substances in solution.
where u = adsorption of the salt in moles per sq. cm. of surface u =
surface tension of the solution
a = activity of the salt R = gas constant
T = absolute temperature A = area of the surface of the solution a = activity coefficient of the salt in solution m = molality of the solution (moles per 1000 grams of solvent) Figure 1 gives the thickness of the surface film of pure water o n aqueous solutions of different concentrations of sodium chloride calculated by Harkins and McLaughlin (5)and Goard ( 4 ) . McBain and Dubois (70) estimated that the thickness of the fresh water surface film was about 4 A. in a solution containing 110 grams of sodium chloride per liter. Langmuir ( 7 ) estimated that the thickness of pure water film on potassium
I
I
1
1
0 - - . 4
DATA
I
I
DATA OF H A R K I N S MCLAUGHLIN OF
AND
GOARD
Basic Concept
Let us consider a dilute aqueous solution of a n inorganic salt-such as sodium chloride-kept open to the atmosphere. The topmost layer of such a solution is fresh water due to the negative adsorption of the dissolved salt a t the air-solution interface. From the Gibbs adsorption equation written in the form
the thickness, t, of the fresh water layer a t the air-solution interface can be derived to be ( 5 )
!-
q , , 5 0 I
O
8
4
WEIGHT
Tc
NaCl
IN
I
,
,
12
16
20
AQUEOUS
SALT
, I 24
SOLUTION
Figure 1. Thickness of the surface film of pure water on aqueous solutions of sodium chloride VOL. 2
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THE
B U L K OF SOLUTION DEMINERALIZED
WATER
AT
THE
INTE RFACF
1
_ _ _ - - - - - - - - - - - - - - - - -- - (IH,O
INTERFACE
FILM
H,O
H20
POROUS FILM SURFACE OF APPROPRIATE CHEMICAL NATURE
H20
H20 H20a
H20
P O R W S FIL SURFACE OF APPROPRIATE CHEMICAL NATURE
PORE OF
-1;
chloride solutions varied from 3.3 to 4.24 A. All of the above data tend to point out that the thickness of the pure water film a t the air-solution interface of inorganic electrolytes, is indeed very small, probably of the order of a monomolecular layer. However, the Gibbs adsorption equation shows that the magnitude of the depth of the negatively adsorbed surface layer depends on the variation of the interfacial tension with the activity of the salt in solution. This means that the thickness of the fresh water layer on an aqueous solution of a n inorganic salt at the solution-contacting medium interface depends on the nature of the solution as well as that of the surface in contact with the solution. This was confirmed by the observations of McLewis ( 7 7 ) who studied the adsorption of nonelectrolytes, electrolytes, and several organic compounds at the oil-water and mercury-water interfaces, and found that the interfacial adsorption of the solute was 3 to 70 times higher than that calculated for the air-solution interface. If now the aqueous sodium chloride solution is brought into contact with a solid membrane of appropriate chemical nature such that a n interface is created giving rise to the formation of multimolecular layers of deionized water at the interface. then such a system offers a potential means of separating fresh water from the bulk of the solution. The above possibility was first suggested by Yuster (76). The practical separation of the fresh water at the interface from the bulk of the solution can be accomplished by simply letting the fresh water flow out through pores of appropriate size suitably created in the membrane. using the pressure necessary to effect the continuous flow of the interfacial water through the capillary pore.
~
c CRITICAL
SIZE
Figure 2. Schematic representation of the mechanism of demineralization of aqueous sodium chloride solutions b y flow under pressure through a porous membrane
52:;
ON THE AT THE
Figure 3. The critical size of the pores on the area of the film a t the interface for the mechanism of demineralization shown in Figure 2
also work if the pores were smaller than 2t. although at a reduced capacity. The magnitudes of the pore radius, contact angle, and surface tension of water contribute to the pressure needed to establish the initial flow of the interfacial \vater through the pore. I t is necessary to maintain the pore size equal to 2t or less only on the area of the film at the interface; the connecting pores at the interior of the bulk of the film material. away from the film-solution interface, could be bigger ; in fact, this is desirable from the practical point of view because then the total resistance to flow will be less, and. consequently, the operating pressure needed to maintain the continuous f l o ~ c of the interfacial water at the desired rate will also be less. Thus the mechanism of separating fresh water from aqueous salt solutions postulated above involves a system of four parameters: the nature of the solution, the nature of the film surface in contact with the solution, the critical diameter of the pores in the film? and the optimum pressure necessary to effect the continuous flow of the interfacial liquid through the pores at the desired rate. The first two parameters determine the nature of the interface o n which the third depends; and the fourth depends on the size, number, and distribution of the pores on the surface as well as the physical structure of the main bulk of the porous material.
Table I. Effects of Variation of Pore Size and Operating Pressure on Demineralization of an Aqueous Sodium Chloride Solution Data of
Mechanism
Schematically, the process of demineralization of a n aqueous sodium chloride solution by flow under pressure through a porous membrane may be conceived to operate as shown in Figure 2. The maximum pore size in a successful film material which can effect the separation of a continuous stream of fresh water from brine solution by the above technique, may be considered as the critical pore size for this particular system. If t is the thickness of the fresh water layer a t the film-solution interface, then the critical pore diameter for the film material is easily seen to be equal to 2t as shown in Figure 3. The system could 52
l&EC FUNDAMENTALS
PORE DIAMETER AREA OF THE FILM INTERFACE
Loeb (8)
Product ion Rate of Operating Demineralization Solution, Pressure, Obtained, Gal.j(Day) % P.S. I.G.a % (Sq. F t . ) 15 0 700 0 1.06 1500 36.4 8.3 61.2 0.55 13.3 1500 18.8 1500 80.0 0.09 18.8 2000 80.0 0.15 18.8 4000 77.0 0.25 ... Negligible 25.0 5000 a Increase of operating pressure probably resulted also in considerable plastic deformation of the ,film material, thereby afecting the pore si:e and pore sire distribution at thejlm-solution interface. Shrinkage of F i l m Diameter,
Table II. Demineralization of Aqueous Sodium Chloride Solutions by Flow under Pressure through a Preshrunk Schleicher and Schuell Cellulose Acetate Film Operating pressure:
Concn of Salt in Feed Solution, lt’t 7 0 iVaCI
1500 p.s.i.g.
Concn. of Salt in Demineralized Solution, Wt. % ,VaCl
Data of Loeb and Sourirajan ( 9 )
Demineralization Obtained,
%
Production Rate of Solution, Gal. /( Day) (sq Ft 1
area of the porous film at the interface, constitutes the indispensable twin requirement for the success of the separation process by the above mechanism. If the surface of the membrane has a preferential sorption for water and has pores of appropriate size, then salt separation will take place; otherwise, no significant separation can take place by the above mechanism. Thus the mechanism of the process is governed by a surface phenomenon, and the success of the practical development of the process is governed by this twin requirement. Effect of Pressure and Concentration
The effect of pressure on the parameter
Considering the process of demineralization of aqueous sodium chloride solutions by flow under pressure through porous membranes, only as a particular case of the general separation process outlined above. certain consequences follo\v. Film Surface and Pore Size
The film surface in contact \vith the brine solution must have a preferential sorption for ivater; otherwise, a continuous removal of fresh water from brine solution will be impossible by the above process. T o dlefine the criteria of preferential sorption of the film surface in specific chemical and physicochemical terms is desirable, and this should be possible Tvhen more experimental data become available. However, in a salt ivater demineralization system involving the use of a given porous medium in the manner shown in Figure 2: the degree of demineralization obtainable Lvould depend upon the nature of the interface. Early experiments indeed showed that while the use of plain cellophane film as the porous medium for the flojv of the brine solution did not result in any significant demineralization, the use of cellophane films initially surfacetreated Lvith different kinds of silicones did result in considerable demineralization; further, the degree of demineralization obtained depended on the exact nature of the prior surface treatment given to the cellophane film (77). But no prior surface treatment of any kind was necessary for the cellulose acetate and cellulose acetate butyrate films for their successful use in the demineralization of brine solutions (77). These results are not surprising. Cellulosic structure has a high sorption capacity for water due to its ability for hydrogen bonding. Cellulose esters are more repellent to sodium and chlorine ions than pure cellulose. Consequently, the surfaces of cellulose acetate and cellulose acetate butyrate membranes in contact with aqueous solutions of sodium chloride possess a preferential sorption for water. Silicone-treated cellophane membranes have apparently similar surface properties. The film must have pores of appropriate size to effect the continuous flow of the preferentially sorbed interfacial liquid by the above mechanism. If the pores are too big. then the solute from the bulk of the solution will leak through the pores and there could be no significant demineralization. When the pore size is successive1.y reduced, the degree of demineralization will be progressively increased with a corresponding reduction in the rate of production of demineralized water a t a given pressure. When there are no pores a t all in the film, then there could be no flow of the interfacial liquid through the film material by the above mechanism, whatever be the chemical nature of the film at the interface. Thus an appropriate chemical nature of the film surface in contact with the solution, as well as the existence of pores of appropriate size on the
Gibbs adsorption equation u ill naturally affect the thickness of the interfacial liquid and hence the degree of desalting obtainable in a given system. If this effect is negligible (of course, this need not be so in all cases) and if Poiseuille flow through pores is assumed. then an increase of operating pressure should increase only the rate of production of demineralized water and should have no effect on the degree of desalting obtainable provided the porous structure of the film remains unchanged. O n the other hand, a positive pressure coefficient of preferential sorption of the membrane surface \vi11 increase the extent of separation and add to the rate of production of demineralized water. Further, the application of pressure on the film can itself change the size. number, and distribution of the pores in the film, in which case the degree of desalting obtainable will also change appropriately. Thus the effect of pressure on the above separation process will vary depending upon the nature of the system and that of the porous structure of the film. In any case, the minimum pressure necessary to effect the continuous separation of fresh lvater from brine solution by flow through the porous film is the osmotic pressure of the
Table 111. Separation of Some Inorganic and Organic Substances in Aqueous Solution Using Preshrunk Schleicher and Schuell Cellulose Acetate Membranes
Film n.0.
B B B B B B B B B 9 9 9 15 15 15 15 1 1 1 1 5 5 3 8 8 8 8
Concn. of Feed Solution Weight Solute Molality yo Potassium chloride 0 . 5 3.59 Sodium chloride 0.5 2.84 Lithium chloride 0,5 2.07 Barium chloride 0,5 9.44 Strontium chloride 0 . 5 7.34 Sodium sulfate 0.5 6.63 Sodium bromide 0.5 4.89 Sodium nitrate 0.5 4.08 Sodium iodide 0.5 6.98 Ethyl alcohol 0.5 2.25 n-Propyl alcohol 0.5 2.92 2.92 Isopropyl alcohol 0,5 n-Butyl alcohol 0,25 1.82 Isobutyl alcohol 0.25 I .82 sec-Butyl alcohol 0.25 1.82 tert-Butyl alcohol 0.25 1.82 Acetaldehyde 0.25 1.09 Ethyl alcohol 0.25 1.14 Acrtone 0.25 1.43 Acetic acid 0.25 1.48 Glycerol 0.5 4.40 Ethylen? glycol 0.5 3.01 Sucrose 0.5 14.61 Dextrose 0.25 4.31 Sorbitol 0.25 4.36 Pentaerythritol 0.07 0.94 Sodium formate 0,25 1.67
VOL. 2
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OperatFerd ine :Mole Rate Pre;% Cc./ sure. Solute ,Win. P.S.I.G. Removed 15 1500 76.5 15 I500 82.5 15 1500 91 . O 15 1500 95.5 1 5 1500 9 8 . 5 1500 99.5 15 1500 78.0 15 1500 69.0 15 1500 15 66.0 1500 30 36.0 1500 45.7 30 30 1500 66.7 30 150C 35.8 1500 67.2 30 1500 64.2 30 1500 30 90.8 750 39.4 30 750 26.9 30 750 24.3 30 750 20.5 30 3c 750 87.4 750 60.0 30 750 30 99.1 750 30 97.1 750 30 96.8 92.5 750 30 ’50 30 79.7
FEBRUARY 1963
53
solution, a t which pressure the rate of production of fresh water is, of course, zero. Loeb’s experimental data ( 8 ) using the Schleicher and Schuell “ultrafine, superdense” cellulose acetate membranes for the demineralization of a n aqueous 3.5% sodium chloride solution, given in Table I, tend to confirm the effects of variation of pore size and operating pressure stated above on the efficiency of the separation process. Figure 1 shows that the thickness of the fresh water surface layer increases with a decrease in the concentration of sodium chloride in the solution, and, conversely, it decreases with an increase in the salt concentration. The magnitude of the changes in the thickness of the fresh water layer in any particular system depends on the nature of the interface. A particular form of the Schleicher and Schuell cellulose acetate film used with a feed solution containing 3.j5y0NaCl gave a concentration of 0.27% NaC1 in the demineralized water (amount salt removed = 92.4T0) with a rate of production of 1.96 gallons/(day) (sq. ft.) of film area under a n operating pressure of 1500 p.s.i.g. O n the basis of the separation mechanism postulated above, the effect of variation of concentration of the feed brine on the efficiency of the separation process may be expected to be as follows. If the NaCl concentration in the feed solution is more than 3.55/’0, then the extent of salt removed and the rate of production of demineralized water will both be less owing to the decrease in the thickness of the fresh water interface, as well as in the effective pressure applied-Le., the difference between the operating pressure and the osmotic pressure. The reverse situation should be true if the concentration of sodium chloride in the feed solution is less than 3.557,. The actual experimental data ( 9 ) , given in Table 11, do confirm these expectations. The above considerations also point out the importance of preventing any concentration buildup a t the vicinity of the film surface for the efficient operation of any practical separation process by the technique sholvn in Figure 2 ; a good circulation of the solution above the film surface will result not only in a higher degree of separation but also in a higher rate of production of desalted interfacial liquid than is otherwise possible. General Applicability of the Technique
In principle, the above separation mechanism is applicable to both inorganic and organic solutes in aqueous or nonaqueous solutions involving the preferential sorption of substances a t interfaces. The technique was tested recently for the separation of several inorganic and organic solutes in aqueous solutions using preshrunk Schleicher and Schuell cellulose acetate membranes. Data on these systems are given in Table I11 to illustrate the fact that the technique was successful in all the cases studied. More extensive data o n the above and other systems will be published separately. T h e actual extent of solute separation in any particular system can be varied widely depending upon the extent of film shrinkage and other operating conditions. T h e relative amounts of the different substances removed a t a constant molal concentration and other identical experimental conditions, are particularly important from the point of view of the over-all mechanism of the above separation process. Already a pattern has emerged. T h e ability of the given cellulose acetate membrane to separate the various inorganic ions in aqueous solution was found to be in the order: Sr 54
> Ba > Li > Na > K
l&EC FUNDAMENTALS
and sulfate > chloride > bromide > nitrate > iodide. The above order is the same as the lyotropic series with respect to both cations and anions ( 3 ) ; the hydration of ions also follows the same order (7). With respect to the extent of separation of the organic substances in aqueous solution using cellulose acetate membranes, the following orders were established:
> EtOH iso-PrOH > n-PrOH iso-BuOH > n-BuOH tert-BuOH > sec-BuOH > n-BuOH n-PrOH
acetaldehyde > ethyl alcohol > acetone > acetic acid. The above orders appear significant in relation to the physicochemical properties of the solution-membrane system. Kammermeyer and Hagerbaumer ( 6 ) accomplished the separation of ethyl acetate-carbon tetrachloride, cyclohexaneethyl alcohol, benzene-methanol, and benzene-ethyl alcohol mixtures by pressure permeation through microporous glass having an average pore diameter of 40 A. In all cases, the permeated product was found to be richer in the more polar constituent of the mixture for which the surface of glass may be expected to possess a preferential sorption. Binning and Lee (2) reported the separation of a benzene-methanol mixture by permeation through polyethylene and cellulose triacetate membranes; the product permeated through the polyethylene membrane was richer in benzene, and that permeated through the cellulose triacetate membrane was richer in methanol. These results illustrate the applicability of the separation mechanism discussed above for the separation of substances in nonaqueous solutions. Thus it seems feasible to develop a general process for the separation of substances in solution based on the negative adsorption of the solute at liquid-solid interfaces. The process consists simply in letting the solution flow under pressure through a n appropriate porous medium; the liquid that flows out of the porous medium is enriched in the preferentially sorbed constituent. The industrial success of such a separation process depends upon developing: The porous medium of appropriate chemical nature, whose surface in contact with the solution gives the greatest thickness of the negatively adsorbed interface The practical technique of obtaining the porous medium with the best possible bulk structure, and, at the same time, containing the largest number of uniform pores of critical diameter on the area of the porous medium at the interface The optimum engineering system that makes the process economically feasible The possible fields of application of the above separation technique are obviously numerous. The general separation mechanism discussed above may contribute materially toward further advancements in the field of sea water demineralization and related processes; it may give rise to new technologies for the separation, concentration, and differential fractionation of a wide variety of inorganic and organic substances in aqueous or nonaqueous solutions; and, further, it may bring about a new point of view in the study of many natural physical and biological phenomena and in the development of new scientific techniques. Conclusion
The negative adsorption of solutes a t liquid-solid interfaces appears to offer a sound basis for the development of a practical technique for the separation of substances in solutions. While the parameters involved in the mechanism of this separation
(6) Kammermeyer, K., Hagerbaumer, D. H., A.Z.Ch.E.J. 1, 215 (1955). (7) Langmuir, I., J . Am. Chem. Soc. 39, 1848 (1917). (8) Loeb. S., “Characteristics of Porous Acetyl Cellulose Membranes for Pressure Desalinization of Dilute Sodium Chloride Solutions,” M.S. thesis, Department of Engineering, University of California, Los Angeles, 1959. (9) Loeb, S., Souiirajan, S., “Sea Water Demineralization by Means of a Semipermeable Membrane,” Department of Engineering, Vniversity of California, Los Angeles, Rept. No. 60-60,1961. (10) McBain, J. I V . , Dubois, R., J . Am. Chem. SOC.51, 3534 (1929). (11) McLrwis, C. \V. C., Phil. M a g . 15, 499 (1908); 17, 466 (1909) ; Z. Physik. Chem. 73, 129 (1910) ; Sci.Progr. (London) 11, 198 (1916). (12) Reid, C. E., Breton, E. J., J . ‘4Ppl. Polymer Sci. 1, 133 (1959). (13) Reid. C. E., Spencer, H. G., Zbid., 4, 354 (1960); J . Phys. Chem. 64, 1587 (1960). (14) Ticknor, L. B., Zbid.,62, 1483 (1958). (15) University of California, Office of Public Information, Press Release, “New water desalting process developed at UCLA,” August 23, 1960. (16) Yuster, S . T., personal communication, September 1956. (17) Yuster, S. T., Sourirajan: S., Bernstein, K., “Sea Water Demineralization by the Surface-Skimming Process,” Department of Engineering, University of California, Los Angeles, Rept. No. 58-26, 1958.
technique are clear, they have not yet been sufficiently defined to make it possible to predict in detail the most successful system for a given separation problem. This needs further research. A properly oriented program of experimental research o n the physics and chemistry of the interfaces, and the engineering development of process systems involving the parameters discussed above have every prospect of a wide field of scientific and industrial application. Acknowledgment
The author is grateful 1.0 I. E. Puddington and G. L. Osberg ior their valuable discussions and comments on this paper, and to Lou Pageau and A. 13. McIlhinney for their valuable assistance in the progress of these investigations. literature Cited (1) Ambard, L., Trautmann, S., “Ultrafiltration,” p. 16, Charles C
Thomas. SDrincfield. Ill.. 1960.
e.,
(2) Binning,’R. Lee, F:. J. (to The American Oil Co.), U. S. Patent 2,953,502 (Sept. 20, 1960). (3) Glasstone, S., “Text I3ook of Phvsical Chemistrv.” 2nd ed., ‘p. 1254, Van Nostrand, New York, 1946. (4) Goard, A. K.. J . Chem. SOC. 127. 2451 (1925) (5) Harkins, W. ’D., McLaughlin,”. MY, J . ‘Am. Chem. SOG. 47,
RECEIVED for review February 27, 1962 .ACCEPTED November 19. 1962
2083 (1925).
Issued as K.R.C. No. 6600.
EFFECT OF CONCENTRATION ON T H E GAS-LIQUID MASS TRANSFER COEFFICIENT R.
K. G I B B S ’ A N D
D. M . H I M M E L B L A U
Department of Chemical Engineering, The L’niuersity of Texas, Austin 12, Tex.
Using radioactive tracers, the influence of concentration on the interphase mass transfer coefficient in the COzwater system w~astested. All experimental data were collected with the gas and liquid phases in turbulent motion; these conditions were maintained consistently. Tracer COz was absorbed and desorbed, while the nontracer COZwas at equilibrium. The mass transfer coefficient defined by the model N = KA(C - C*) was found to be moderately concentration-dependent, and the character of the dependency was similar to that of the diffusion (coefficient for COZin water. The transfer coefficient was essentially the same (within for absorption and desorption at any given nontracer concentration.
THE
FACTORS that influence interphase mass transfer can be conveniently thought of as comprising two groups:
Hydrodynamic factors, such as the rate of mass flow, the geometry and size of equipment, the density, the viscosity, and the scale of turbulence. Concentration-dependent factors, such as the concentration itself, the equilibrium solubility of the solute, and the diffusion coefficient. Since in laminar systems the differential equations governing momentum and mass transfer can usually be written in a fairly simple fashion, and solved in many cases, such systems are easier to study quantitatively than turbulent systems. Present address, California Research Corp., Richmond, Calif.
f 5%)
I n the latter the time and spacial functional relationships for the velocity are usually unknown, and thus interphase mass transport cannot be predicted by solving the phenomenological equations of change. Ironically, the turbulent case is the one of most industrial importance. One approach toward a more basic understanding of turbulent transport which we have utilized is to investigate the effect of variables in systems in which the turbulent characteristics are consistent throughout the experiments. I t has to be admitted that the hydrodynamic influences o n mass transfer are probably of more consequence than the other factors, but assuming we could omit the former from consideration, what might be discovered then? I n particular, this investigation was directed toward a study of the influence of conVOL. 2
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