treatment did not degrade any of the polymer solutions to the viscosity or V - T slope levels of the base oil used. Polymethacrylates could be degraded further than polyisobutylenes when polymers of equal thickening power were compared. Literature Cited
(1) American Society for Testing Materials, Philadelphia, Pa., “ASTM Standards cm Petroleum Products and Lubricants,” Vol. 1, pp. 183-9, December 1962. (2) American Society for Testing Materials, Philadelphia, Pa., “ASTM Standards on Petroleum Products,” Part 17, pp. 93845, January 1965. (3) American Society for Testing Materials, Philadelphia, Pa., “Supplementary Preprint to the 1961 Report of Committee D-2 on Petroleum Products and Lubricants,” Appendix VII, pp. 61-6, 1961. ( 4 ) Billmeyer, F., “Textbook of Polymer Chemistry,” pp. 128-33, Interscience. Xew York. 1957. ( 5 ) Foster, T.; Mueller, E., SOC.Testing Materials, Spec. Tech. Pub. 382,14-32 (May 1965). (6) Gironda, A,, Essing, E., Kubin, B., “Sonic Shear Method for Determination of SLiear Breakdown in Hydraulic Fluids and Lubricatinc Oils.” T2’right Air Development Center, Dayton, Ohio. TVADC Tech. ReDt. 55-62 (March 19551. (7) Haiton, R., ‘61ntroduction to Hydraulic Fluids,” p. 97, Reinhold, New York, 1962. (8) Jellinek, H., TVhite, G., J . Polymer Scz. 6, 745-66 (1951). ( 9 ) Lawson, N., Am. SOC.Testing- Materials, Spec. Tech. Bull. 182 (September 1955). 110) LeMar. R. L.. “Evaluation of Sonic Shear Stabilitv Test Methods. ’ Literature Report,” Rock Island Arsenal,’ Rock Island, Ill., Tech. Rept. 61-3178 (August 29, 1961).
__
(11) LeMar, R. L., Bootzin, D., Am. SOC.Testing Materials, Spec. Tech. Pub. 382,70-83 (May 1965). (12) Nejak, R., Dzuna, E., Society of Automotive Engineers International Congress, Jan. 9-13, 1961. (13) Neunherz, P., Symposium on Polymers in Lubricating Oils, Division of Petroleum Chemistry, ACS, Vol. 7, No. 4-B, September 1962. (14) Prudhomme, R., Grabar, P., J . Chim. Phys. 46, 667-70 (1949). (15) Saini, G., Ostacoli, G., Ric. Sci.26,514-22 (1956). (16) Selby, T., Am. SOC.Testing Materials, Spec. Tech. Pub. 382, 58-69 (May 1965). (17) Stringer, H., Rohm and Haas Co., Philadelphia, Pa,, private communication, 1964. (18) Thomas, J., J . Phys. Chem. 63,1729 (1959). (19) Trop, D., “Shear Stability of Hydraulic Fluids by Sonic Shear,” Wright Air Development Center, Dayton, Ohio, WADD Rept. 60-467 (July 1960). (20) U. S. Military Specification, “MIL-H-5606B Hydraulic Fluid, Petroleum Base, Aircraft, Missile and Ordnance,” June 26, 1963. (21) U. S. Military Specification, “MIL-H-I3866A(ORD) Hydraulic Fluid, Petroleum Base, Artillery Recoil, Special,” Oct. 2, 1956. (22) Van Horne, W., Division of Petroleum Chemistry, American Chemical Society, Preprints 1, No. 4, pp. 26-31 (1956). (23) Vick, G., Goodson, R., Am. SOC.Testing Materials, Spec. Tech. Pub. 382 (Mav 1965’1. (24) Zuidema, H:, “Performance of Lubricating Oils,” p. 33, Reinhold, New York, 1952. RECEIVED for review December 9, 1965 ACCEPTED December 12, 1966
~
D ESA LI NATION M EM BRAN ES FROM ORGANIC CASTING SOLUTIQNS SEROP M A N J I K I A N
Universal Water Corp., Del M a r , Calif. Optimization of a cellulose acetate desalination membrane was undertaken in an experimental investigation to find techniques for simplifying membrane fabrication and improve performance. This was achieved by minimizing fabrication variables and b y reducing the number of casting solution components. A major portion of this study deals with the use of organic compounds as additives and/or solvents in the casting solution. Valrious organic compounds to b e used as additives were examined. The most promising, formamide, was optimized as a casting solution component in the ternary system, cellulose acetate-acetoneformamide. The use of formamide as an additive simplified the preparation of membranes and eliminated the need for casting temperature restrictions, thus increasing the feasibility of economic exploitation of the reverse osmosis desalination process. Lastly, binary systems were developed where a single organic compound was utilized as both solvent and additive.
in the demineralization of saline waters has increased during the past decade. Of particular interest has been the reverse osmosis process, where a semipermeable membrane is used to desalinize saline waters with pressure as the driving force. The term “osmosis” is used to describe spontaneous flow of water into a solution, or from a dilute to a more concentrated solution when separated from each other by a semipermeable membrane. To obtain fresh water from saline water, the flow must be reversed-Le., from the solution into a fresh \vater stream; hence, the term “reverse osmosis.”
I
NTEREST
Background Work
From the first cornlpendium on “Existing and Potential Separation Processes” (U. S. Department of the Interior,
1952) until now, the search has gone on for a practical semipermeable membrane. Reid and Breton (Breton, 1957), a t the University of Florida, were the first to investigate the potential of osmotic membranes in desalination. Their major contribution was the discovery of a suitable membrane material, cellulose acetate. Original work by Dobry (1936) on the miscibility of cellulose acetate in aqueous magnesium perchlorate prompted Loeb and Sourirajan (1960), a t the University of California, to modify the preparation of membranes by incorporating aqueous magnesium perchlorate in a n acetonic cellulose acetate casting solution. Resulting membranes had the selectivity of Reid and Breton membranes with the added advantage of high permeabilities. VOL. 6
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MARCH 1967
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Exhaustive experiments a t the University of California have increased the number of inorganic electrolytes that could Serve as substitutes for magnesium perchlorate. Loeb (1961) investigated the influence of various aqueous inorganic salts on membrane performance when added to casting solutions. H e concluded that an acceptable electrolytic additive should have a favorable anionic shape, charge, and size. I n assigning the anion the primary role in improving the performance of cellulose acetate membranes, Loeb differs sharply with the hypothesis of Kesting (1963), who attributes the effect of the electrolyte to the cation and stresses the necessity of a highly hydrated cation for the swelling of cellulose acetate. Neither case seems to have conclusive experimental or theoretical backing. I n the fabrication of cellulose acetate membranes using magnesium perchlorate, the following casting and curing variables are controlled. All equipment and the casting solution are held between -11' and -8' C. T h e membrane is cast on plate glass with 0.010-inch side runners by use of a doctor blade. Two and one half minutes are allowed for acetone to evaporate a t -10' C . The film on the plate glass is immersed in icz water for 1 hour. The membrane is removed from the glass and heated in a water bath for 5 minutes. T h e membrane used in a 6-month life test for brackish water desalination was reported by Loeb and Manjikian (1965) as Solution 11.
Cellulose acetate Acetone \Yater Magnesium perchlorate Hydrochloric acid
Weight % 23.15 69.50 5.46 1.65 0.33
The first fundamental change in membrane casting solution composition was made in 1965, when formamide-modified membranes were first disclosed (Manjikian et al., 1965). Unanswered questions as to the role of casting solution components in determining the properties of semipermeable membranes continue to pose a challenge. I t was the quest for a simplified system, in both the casting solution composition and the fabrication procedure, that led to the following experimental investigation. Casting Solution Constituents
Membrane Material. CELLULOSEESTER TYPE. T h e basic membrane material used in reverse osmosis membranes for the past 5 years has been cellulose acetate, specifically Eastman E-398-3. I n the optimization of membranes for application, the two major considerations are the performance (flux and per cent salt rejection) and the mechanical properties of the membrane. Both are largely determined by the composition of the casting solution. Experimentally, it was found that cellulose acetate type E398-3 was best suited for testing new additives because of its good miscibility characteristics. However, after the casting solution had been standardized, other types of cellulose acetate powders were tested in order to improve the mechanical properties of the membrane while retaining its high desalinizing performance. The effect of the following variables on membrane performance and strength was studied : cellulose acetate chain 24
I & E C P R O D U C T RESEARCH A N D D E V E L O P M E N T
~~
Table I. Effect of Cellulose Acetale Chain Length Variation at Constant Acetyl Content on Membrane Performance
Curing Temp.,
c.
Desalinized Wafpr Salt Flux, content, gal./sq. f t . day $.p.m.
~
75.5
30.2 29.7 28.5 33.7 31.6 32.6
79.0 79.0 76.5
306 340 290 560 510 442
Cellulose Acetate Type E-398-3 E-398-6 E-398-10 E-394-30 E-394-45 E-394-60
Table II. Membrane Performance as a Function of Cellulose Acetate Molecular Weight Distribution (CA similar to type E-398-3) Desalinized Water Curing Flux, Salt content, Mol. Wt. Zdentijcation TzmP., gal./sq.'jt. C. day p.p.m. Dtstrt bution AVO.
72 74.5 75.5 75.4 77.8
32.4 40.5 38 . o 27 .O 24.3 27.0 24.3 21.6 21.6
374 476 340 238 136 204 119 i 02 102
Narrow Normal \Vide Narrow Normal i+'ide
Narrow Normal \Vide
CAE-2196-1 CAE-21 96-2 CAE-2196-3 CAE-2196-1 C.4E-2196-2 CAE-21 96-3 CAE-2196-1 CAE-2196-2 C.4E-2196-3
length as determined by viscosity, per cent cellulose acetate acetyl content, and cellulose acetate molecular weight distribution. Membranes were prepared from a solution of 25y0 cellulose acetate dissolved in a 3 to 2 mixture of acetone and formamide by weigh:, and cast a t ambient temperatures. Testing conditions were 600-p.s.i.g. operating pressure, with 0.5% NaCl feed brine. I n general, these were the standard conditions under which membranes were tested in this study. Test results are given in Tables I and I1 and Figure 1. Cellulose acetate chain length variation (viscosity) and molecular weight distribution do not markedly affect membrane performance characteristics. However, adequate performance could be achieved at lower curing temperatures when longer chain polymers were used. Water flux through the membrane was expected to be directly proportional to the hydroxyl content of cellulose acetate. This is not supported by membrane performance data as given in Figure 1 in the acetylation range investigated. As membrane desalinizing ability approached loo%, the effect of chain length on curing temperature was reduced to a minimum. Membranes prepared from E-398-10 were physically superior to those made from E-398-3. E-398-10 also gives membranes that are plastically deformable. I n general, flat membranes were fabricated with type E-398-3 cellulose acetate and tubular membranes with type E-398-10. Other cellulosic derivations such as cellulose triacetate, cellulose acetate-butyrate, and cellulose propionate were tested as membrane material. Successful films were prepared but none approached the performance of cellulose acetate. RECOVERY OF CELLULOSE ACETATE.Membranes prepared from cellulose acetate type E-398-3 and tested under synthetic brines for a period of time were used for recovery tests. These membranes were washed clear and oven-dried below 90' C. A standard casting solution was prepared using the dried films in place of cellulose acetate powder. Membranes were fabricated and tested with the following results:
Table 111.
Performance of Membranes Prepared from Ethylcellulose at Room Temperature Casting (Evaporation time, 20 minutes) Desalinized Water Casting Solution Curing Salt Temp., Gal./sq. content, Composit ion, yo Dioxane EC TEP C. f t . day $.p.m.
-t/ o
i
I
I
e/0
f
/*-
69'C
/-\
e/-*-\ /
-
I
i -
37
72'C
0.
75oc
0-
1
I
38
39
PERCENT ACETYL CONTENT
Figure 1. Membrane performance as a function of cellulose acetate acetyl content at various curing temperatur'es
Membrane From recovered cellulose acetate From fresh cellulose acetate (control)
Flux, Gal./ Sq. Ft. Day
Salt Rejection,
24
95
21
95
70
This indicates the po4bilities that cellulose acetate does not change chemically during and after membrane fabrication, and that it could be recovered for successful re-utilization. Previous work with ethylcellulose (Loeb, ETHYLCELLULOSE. 1961) as a replacemeni for cellulose acetate did not produce promising results. These membranes were prepared from casting solutions using acetone as the solvent and aqueous magnesium perchlorate as the modifying agent. T h e failure of ethylcellulose as a membrane material in the above casting solution was attributed i o the absence of carbonyl groups in the polymer. T h e recent success of organic compounds as additivesolvents in the preparai ion of semipermeable membranes has prompted the re-investigation of ethylcellulose as a membrane material. Since a number of investigations in this field have considered the carbonyl groups as responsible for the semipermeability of cellulose acetate, it was hoped that comparison of membranes fabricated from ethylcellulose and cellulose acetate would be a means of evaluating the importance of the carbonyl group in the desalination phenomenon and clarifying the structural make-up necessary in a successful membrane material.
Solubility was found to be a determining factor in the preparation of casting solutions from ethylcellulose. Ethylcellulose of 49.3% ethoxyl content was readily soluble, but membranes prepared from it did not have acceptable desalinizing properties. With ethylcellulose of 50% ethoxyl content, the maximum solubility in dioxane was about 12%. This amount was sufficient for the casting of films. T h e additive-solvent used in the ternary casting solution was triethyl phosphate. Membranes prepared from this ethylcellulose had good salt-rejection properties (Table 111). These membranes were not optimized because of their relatively low flux characteristics. The Solvent. A number of organic solvents could be successfully used in the preparation of casting solutions of cellulose acetate. I t is assumed that the role of the solvent is merely to provide cellulose acetate and the modifying agent, the proper matrix from which films could be cast. This assumption is clarified below. Some general properties looked for in a solvent were its ability as a solvent, its miscibility with water and with the modifying agent, its lack of chemical reactivity with other components of the casting solution, and its ability to leach out of the film a t a faster rate than the modifying agent. T h e following compounds have been utilized as solvents : acetic acid, acetone, dimethyl formamide, dimethyl acetoacetamide, dimethylsulfoxide, dioxane, methyl pyrrolidone, tetrahydrofuran, and triethyl phosphate. Water. Of immediate interest is the role of water in the preparation of semipermeable membranes. Water comes into the picture a t two stages: as a component of the casting solution and when the cast film is immersed in water during fabrication. Water (Loeb, 1964) has been assigned a major role in the "proper" structurization of cellulose acetate membranes prepared from casting solutions containing aqueous electrolytes. Specifically, the "modified structure" of water was assumed to be essential for the preparation of a successful semipermeable membrane. This modification, breaking the residual ice structure of water, was thought to be brought about by certain inorganic electrolytes used in the casting solution. Thus, the physical features necessary for the fabrication of osmotic membranes were to be established by the inclusion of water and the proper electrolyte in a solution of cellulose acetate in acetone. T o test this hypothesis, a substitute for water was sought in the form of a n organic liquid with a high dielectric constant and miscible with acetone. A casting solution was prepared using a formamide solution of magnesium perchlorate with cellulose acetate and acetone. Membranes prepared from this solution gave acceptable results. This led to the testing of formamide as a replacement for both water and the inorganic salt in the casting solution. Very successful membranes were fabricated from the ternary system cellulose acetate, acetone, and formamide. The addiVOL. 6
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MARCH 1967
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tion of water to this system did not appreciably change the performance of the membrane. I t could be surmised from the above that water itself is not a fundamental component of the casting solution, but could be considered a vehicle for the introduction of inorganic electrolytes in a n acetonic solution of cellulose acetate. T h e second use of water in the membrane fabrication process is a t the stage where the cast film is immersed in water. The role of water a t this point is rather specific and important. Here; via diffusion and imbibition, water displaces the components other than cellulose acetate in the cast film and becomes a permanent structural component of the finished membrane. Formation or gelation of the membrane is finalized a t this stage. There are indications that the rate of structurization and structurization itself are regulated primarily by the components in the casting solution, the casting technique, the formation of an ultrathin skin on the surface of the film, and the temperature of the immersion water. Since the final membrane is a gel of cellulose acetate and water, the amount and distribution of water in the membrane become important. A very fine and uniform dispersion of water throughout the finished membrane is desirable. Membranes prepared from casting solutions containing water were far less homogeneous than membranes made from mixtures containing no water. Additives (Conditioning Agents). An additive could be characterized as the flux-inducing component of a casting solution for the preparation of semipermeable cellulose acetate membranes. T o date, investigations have been concerned mainly with aqueous solutions of inorganic electrolytes as additives (Kesting, 1963; Loeb, 1964). Since magnesium perchlorate was one of the first to be used in this capacity, it is not surprising that later efforts were directed toward the investigation of salts similar to magnesium perchlorate in their physical and chemical properties. In addition, the successful preparation of high flux membranes was attributed to the structural properties of these electrolytes. Thus, to clarify their effect, the properties of these electrolytes were investigated. Exhaustive experiments have increased the number of inorganic electrolytes that serve as successful additives. In general, they could not be classified in a single group or possess a common characteristic that could account for their success as additives. Attempts were made (Loeb, 1964) to show, on the basis of observed changes in water viscosity, that these electrolytes, by virtue of their anions, have a common property in their ability to ‘.break” the structure of water. Since this was the only common characteristic, it was concluded that the modification of the structure of water, in the casting mixture, was the primary role of the electrolytic additive. Aside from a number of exceptions to the above hypothesis, recent findings indicate that water in the casting solution is not necessary for the preparation of semipermeable membranes. Thus, the above role assigned to the additive becomes rather irrelevant. Other investigators (Kesting, 1963) have thought of the additive as a swelling agent for cellulose acetate. Again the need for an electrolyte has been emphasized; however, in this case the cation was assigned the active role. I t has been established that water added to an acetonic solution of cellulose acetate is not a successful agent in the absence of electrolytes. A nonelectrolyte very close to water, hydrogen peroxide, was tested as a replacement for magnesium perchlorate (Table IV). 26
l&EC PRODUCT RESEARCH A N D DEVELOPMENT
Table IV.
Performance of Membranes Prepared with Hydrogen Peroxide as the Additivea Desalinized Water Curing Flux, gal ./ Salt content, Temp., O C. sp .f t . day p.j.m, 78.5 20.3 600 80.5 18.4 150 a Casting solution composition, %: acetone 65.0, CA 22.0, HzO 70.4, HZOZ 2.6. Evaporation time ( 14‘ F.), 70 minutes.
The success of hydrogen peroxide indicates that an inorganic additive does not have to be an electrolyte. I n addition, the question as to the relative importance of the cation and the anion need not cloud the total picture. In a n all-organic casting system, such as cellulose acetateacetone-formamide, the number of variables is reduced to a level where each component can be investigated. As in the case of electrolytes, organic additives do not seem to possess a single common quality that would account for their success as conditioning agents. Some general requirements for a successful additive are miscibility with a cellulose acetatesolvent system, solubility in water, lack of chemical reactivity with other components of the system, and ability to leach out of the film after the solvent has been removed from the critical skin portion of the membrane. It seems reasonable to assume that the main function of the additive is to create the proper substructure of the membrane. If the fabrication technique has served to create a very thin layer (Riley et al., 1964) of a membrane having transport properties virtually identical with those of a homogeneous membrane described by Breton (19571, it could be said that the additive acts to create a substructure of ideal thickness and porosity. After the film is immersed in water, the solvent begins to diffuse out first; this induces the start of gelation. Since the additive is the last to leave, it keeps the polymer chains apart, particularly in the substructure, until gelation is completed. Thus, the final membrane becomes a gel of cellulose acetate and water with a very thin, relatively dense skin and a very porous substructure. I t is assumed that initially the solvent is removed at the skin and the bulk of the additive is leached out through the sublayer. I t may be concluded that the additive does not contribute to the demineralizing ability of cellulose acetate membranes and that its only function is to help reduce the resistance to water flow through the membrane by structurally modifying the film. The main feature of a n additive seems to be that of an unreactive, temporary filler which is subsequently removed for the creation of a desirable structure from the system. Organic Systems
The successful use of formamide as a replacement for aqueous inorganic electrolytes in the casting solution opened a new field of investigation in the study of osmotic membranes. Various organic compounds were tested as additive-solvents. (The term “additive-solvent” is used because most of these compounds are partial solvents for cellulose acetate.) These compounds were initially chosen for having t\vo necessary qualities : high solubility in water and miscibility with the primary solvent. I n general, these compounds were not optimized as additives. Only a cursory survey of organic compounds most likely to succeed was made. Of these, formamide \vas optimized as a casting solution component because of its promise for practical application.
Formamide. T h e unique properties of formamide (patent pending) as a n additive-solvent in the preparation of semipermeable membranes resulted in the over-all simplification of the membrane fabrication process. MEMBRANE FABRICATION PROCEDURE.Preparation of Casting Solution. Cellulose acetate was dissolved in a mixture of acetone and formamide. The functional ranges of each of the components are: Acetone Formamide Cellulose acetate
35-65 % 10-4070 20-3070
Casting of Film. Films were cast on plate glass with 0.010inch side runners by use of a doctor blade or in 0.402-inch'i.d. stainless steel tubes using a 0.384-inch 0.d. plum bob. Films were cast at ambient temperature. Evaporation Period and Immersion in Water. After a certain evaporation period, depcmding upon casting solution composition and temperature a t which the film is cast, the film is immersed in tap water (approximately 300 p.p.m.). T h e range of evaporation period is between 1/4 and 2l/2 minutes for flat membranes. This phase of the fabrication procedure was eliminated in the preparation of tubular membranes. The immersion period was 1 hour with water temperature a t 0 ' to 3' C. Membranes with acceptable performance could be prepared by immersing the film in water a t ambient temperature. However, these membranes are less homogeneous and mechanically poorer than those immersed in ice water. Curing of Film. Before use, membranes are cured in water for 5 minutes. The temperature range of the water bath could be between 23' and 90' C., depending upon performance requirements. Sheet membranes are cured prior to assembly in the test system. Tubular membranes need to be cured after unit assembly and under pressure to retain their tubular form and to conform to the shape of the supporting body. Membranes were tested a t an applied TESTING CO~DITIONS . pressure of 600 p.s.i.g. with a feed brine of 0.570NaCl in tap water.
Table VI.
Effect of
Evaporation Period on Membranea Performance (Casting temperature 23" C.) Desalinized Water Evaporation Salt Period, Curing Flux, content, Min. Temp., C. gal./sq. ft. day #.p.m. 0.25 71.3 83.7 1870 0.5 71.6 47.3 646 1. 0 71 . O 25.6 578 731 1.5 71.0 33.8 2.0 71.6 25.i ll0i 2.5 71.5 22.7 1190 3.0 71.5 25.1 1700 a Casting solution composition: 45y0 acetone, 30y0 formamide, 257, cellulose acetate E-398-3.
The film is cast a t room temperature (23' C.) with a n evaporation period of minute, then immersed in ice Tvater for 1 hour. This membrane (standard) was used as a control for all later work. M e m b r a n e Characteristics. VARIABLES AFFECTING MEMBRANE PERFORMASCE. Certain variables in the fabrication and application of the membrane have a critical effect on performance. I n the fabrication process, the following factors should be considered : Casting solution composition. Temperature a t which film is cast. c. Evaporation period after casting. d. Temperature of water in which film is immersed. e . Curing temperature and procedure. f. Quality of water used in d and e. a.
6.
The above could be varied and easily controlled for experimental optimization of the membrane for a given application PRESSURE REGULATOR
PRESSUREGAUGE OSMOTIC CELL
A f l o ~ vdiagram of the slstem is given in Figure 2, and the cell in mhich flat membranes were pressurized is shown in Figure 3. Tables V, V I , and VI11 summarize the work done with membranes prepared from casting solutions consisting of acetone, formamide, and cellulose acetate. Figure 4 is a ternary diagram of the casting system. T h e shaded area represents combination of the three components from which successful membranes were prepared. T h e optimum composition was found to be Acetone
Formamide Cellulose acetate E-398-3
I I I
FILTER
'r
i i I I I I I 1
SOLUTION^ Figure 2.
Schematic of flow system
Table V. Performance of Membranes Cast from Ternary System Cellulose Acetate-Acetone-Formamide Casting Solution Composition, 5% Cellulose Casting Desalinized Water acetate Curing Euaporation Temp., Flux, Salt content, Acetone Formamide E-398-3 Temp., O C. Time, Min . O c. gal./sq. f t . day p.p.m, 4 6 68.5 10 25 10 2 72 10 65 77 . O 25 10 IO 17.0 221 20 55 2 7 . 0 6 25 7 9 . 5 10 476 25 50 25 2 1 . 6 8 1 . 5 10 187 30 45 25 1 3 . 8 8 5 . 3 10 35 153 40 1 5 . 1 25 65 . O 23 357 20 55 1 5 . 4 7 1 . 5 25 272 23 2 5 50 1 30.0 74.0 25 23 408 30 45 45 . O 72.0 25 23 600 30 '/z 45 30.0 76.0 25 23 250 30 1/2 45
VOL. 6
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MARCH 1967
27
OSMOTIC MEMBRANE SOLUTION PATH THROUGH C E L L
Figure 3.
Cross-sectional view of
desalination cell
OPTIMUM CAST N G SOLUTION COMPOSITION L I E S IN SHADED A R E A
ACETOYE RANGE 3 5 % 65 %
CELLULOSE ACETATE
Figure 4. Casting solution composition which good performance can b e obtained
FORMAMIDE
region
within
Some important variables in the application of membranes are : Applied pressure. Composition and concentration of brine. Circulation rate of brine over membrane surface. Brine characteristics : pH, turbidity, temperature, etc. Type and nature of membrane backing material. Time effects. Certain system variables are more critical than others and results obtained by using simulated brines as feed could be considered only as approximations for expected field conditions. EVAPORATION PERIOD us. MEMBRANE PERFORMANCE. Although casting solution composition is a primary factor in the preparation of semipermeable membranes, fabrication vari28
I & E C PRODUCT RESEARCH A N D DEVELOPMENT
ables could determine the range and effectiveness of membrane performance and control its optimization. Sensitivity of membrane performance to evaporation period is a function of casting temperature and nature of the conditioning agent used in the casting solution. For the standard formamide mixture cast a t ambient temperature this effect is reflected in Table VI. A I/Z- to 11/2-minute period is best for flat membranes. For members cast a t -10' C. the optimum evaporation period was 6 minutes. Tubular membrane casting controls are different from those of flat membranes. Here no evaporation period is necessary. Continuous casting and direct water immersion could be satisfactorily accomplished. Evaporation period is important in the proper structurization of the membrane. T h e formation and thickness of the socalled skin layer are radically affected by this fabrication step and its duration. Immediately after casting, the solvent (acetone) diffuses out into the atmosphere or water, creating a solvent-poor region on the surface of the film. This produces the selective portion of the membrane referred to as the skin. The rate of solvent removal decreases as the skin is formed and as the boundary layer adjacent to the film becomes enriched with acetone. At this stage the desired skin thickness is achieved and any further increase in thickness checked. Skin formation is terminated by controlling the temperature and nature of the environment adjacent to the film surface. This is partially determined by the properties of the conditioning agent utilized in the casting solution. Structurization of membrane substructure is completed during immersion in water. CURINGOF MEMBRANE.The main purpose of heating membranes a t elevated temperatures is to improve their demineralizing properties. Membranes with acceptable desalination properties have been prepared without heating. However, heating, in general, is a most effective control on the eventual performance of the membrane and for the preparation of membranes of near-perfect semipermeability. The characteristics induced in reverse osmosis membranes via heating d o not seem to be irreversible. Some of the beneficial effects of heating are lost during storage (in water) for a prolonged period. Only the desalinizing power of the membrane suffers with time. Flux either stays a t its original value or increases. T h e quality of water in which membranes are stored determines the rate and amount of this change. The temperature a t which membranes are cured is determined by the application requirements and the casting solution composition. Table VI1 shows the effect of curing temperature on performance of the standard membrane. I t could be postulated that the curing process has no net effect on membrane desalinizing characteristics other than that of minimizing the negative aspects of fabrication and improper casting solution formulation. As postulated previously, the additive serves to keep the polymer chains apart until gelation of membrane substructure is completed. This is in no way inducive of a n ordered and stable structure. Therefore, when pressure is applied on the membrane in use, certain sections of the substructure may collapse, resulting in a highly compacted substructure. This will increase resistance to water flow through the membrane and, if compaction is nonuniform, will damage the skin. To avoid the above, membranes are cured a t high temperatures where polymer inter-intra chain spaces are reduced, the system is stabilized (low energy state), and membrane substructure is consolidated. Deterioration in desalinizing ability and flux was lower when membranes were cured a t higher temperatures.
If it is assumed that the membrane is perfectly semipermeable and that salt in the product water is due to brine flow through imperfect sites, the product water could be divided into fresh water and brine. From experimental data it is possible to calculate the net fresh water flux across the membrane a t various pressures. For example, a t 600 p.s.i.g. the observed data were:
Effect of Curing Temperature on Membrane Performancea Desalinized Water Curing Temp., Flux, Salt content, gal./sq. f t . day p.p.m.
Table VII.
c.
3800 95 Unheated (23 a C.) 45 800 70 250 30 76 110 17 78 12 85 81 a Casting solution composition: 457. acetone, 30% formamide, 25.70 cellulose acetate E-398-3. Evaporatzon period, '/z minute. Operating pressure, 6OOp.s.i.g. Feea' salt content, 5000p.p.m.
Then, the net fresh water flow is given by 2.4
MEMBRANE CONSTANT. For the purpose of this test, a standard membrane (E-398-3 cellulose acetate) was prepared and cured a t 80' C. F'erformance data were taken a t various operating pressures (Table V I I I ) . Testing conditions were modified to minimize boundary layer effects. The membrane was prepressurized a t 1495 p.s.i.g. to eliminate compaction as a1 possible variable. Membrane performance a t the start arid completion of the experiment was:
Start Finish
Flow Rate, M l . / S q . Cm. Hr. 5.77 5.67
Salt Rejection, 96.5 96.2
2 . 4 ml./sq. cm. hr. 340 p.p.m. 4930 p.p.m.
Product flow rate Product salt content Brine salt content
-
(340 X 2.4) = 2.2 ml./sq. cm. hr. 4930
The membrane constant, L, as given in J = L ( A P where
- AT)
J
= product flux, ml./sq. cm. hr. AP = applied pressure, atm. AT = net osmotic pressure across membrane, atm.
was calculated from corrected values of product water flow rates. (The prepressurization of the methane and the use of corrected flow rates will result in lower value for the membrane constant.) As seen from Table VI11 and Figure 5, the membrane constant is independent of applied pressure, in agreement with results obtained a t General Atomic by Lonsdale et al. (1965). Of particular interest is the relationship between calculated brine flow and applied pressure. Product water salt content
yo
This indicates that the experimental procedure did not have a n adverse effect on membrane performance.
PRESSURE otrn
Figure
Applied Pressure, Atm .
a
5. Membrane constant I from flux and pressure relationships
Table VIII. Net , Osmotic Pressure, Atm .
Membrane Characteristicsa at Various Net Applied Pressures Net Desalinized Water Operating Pressure, Flux, Ml./Sq. Cm. Hr. Salt content, Atm . Obsd. Cor7 . p.p.m.
27.6 3.7 40.8 3.8 55.9 3.8 69.7 3.9 82 .o 3.9 Membrane constant, 0.06 ml./sq. em. hr.-atm.
24 37 52 66
78
1.7
2.5 3.4 4.2 5 .O
1.5 2.3 3.2 4.0 4.8
460
VOL. 6
Brine fiow, ml./sq. em. hr.
...
340 255 221
0.2 0.2 0.2
187
0.2
NO. 1
MARCH 1967
29
is not a function of pressure. Thus the observed improvement in desalination a t higher pressures could be due to the diluting effect of increased fresh water production. Both the membrane constant and the calculated brine flow are critically affected by membrane curing temperature (Figure 6). Only membranes cured a t relatively high temperatures exhibit constant brine flow with respect to applied pressure. This constant is always greater than zero, indicating that there is a limit to membrane desalinizing ability below the 100% mark. WATERFLUXAS A FUXCTION OF AMOUNT OF ADDITIVE IN CASTING SOLUTION.An attempt was made to obtain a relationship between the amount of additive (formamide) in the casting solution and the corresponding flux through the membrane. To clarify and/or neutralize the role of the solvent, two types of casting solutions were prepared using the same components. I n one, the ratio of cellulose acetate to acetone was held constant while the amount of formamide was varied. I n the other, the amount of cellulose acetate was held constant while the ratio of formamide to acetone was changed. I n both cases the amount of cellulose acetate was above 20%. Since the quantity of formamide was increased a t the expense of the solvent, solubility limitations were to be expected. However, the breaking point in membrane performance occurred before solubility difficulties were encountered. The membranes were tested in the uncured conditionLe., the high temperature heating process was deleted to avoid the introduction of intermediate variables. Results of these experiments are given in Figures 7 and 8. From the plots of flux us. per cent formamide in the casting solution, the following numerical relationship was derived :
2.5
-
CURING TEMPERATURE:
62.0'C,L*O.O85 rnl/hr.cm~alrn c E ," 2.0 E
-
-
0 71 .4'C, L = 0.070 m l / hr. cmz a t m A B I . 4 ' C , L : 0 . 0 3 6 ml/hr.crn2alm
W
9
1.5
-
1.0
-
W J W
z
w cl
tz
-I
I
0
1
I
0
20
4
A
I 100
APPLIED PRESSURE, atm
Figure 6. Effect of applied pressure on calculated brine leakage at various curing temperatures
2 5C
'-"\CA: I '-"\CA: I
200
AC CONSTANT 5.11 AC CONSTANT 5 I1
'
I ;
~
150 CA' (AC+FI CONSTANT 1:3
U D
y = 0.074eo.262Z
-
I PO
60
40
-' F
rn
v)
1
where
L7
y = flux, gal./sq. ft. day x = per cent formamide The above relationship holds up to 30 to 35% formamide, beyond which the membrane breaks down under pressure. From Figures 7 and 8, it could be concluded that the optimum composition for the given system is 25% cellulose acetate, 30y0 formamide, and 45y0 acetone. Two additional observations could be made. First, the solvent (acetone) seems to have a neutral role, aside from providing the matrix from which cellulose acetate and formamide determine the structurization of the membrane. A plot of similar characteristics was obtained when acetic acid was substituted for acetone as a solvent. In contrast to acetone, membranes prepared from cellulose acetate solutions in acetic acid do have a certain flux without the use of a n additive. A correction for this flux was made to determine the net effect of formamide (Figure 9). Second, the increase in the amount of formamide in the casting solution corresponds to a n exponential decrease in resistance to water flux through the membrane, again due to modification of membrane substructure. Further work in this direction with various additives and solvents is necessary. ' L G ~ us.~ "BAD" ~ " SIDEOF MEMBRANES. A unique characteristic of the reverse osmosis membrane is the anisotropic nature of its two surfaces. I t has been reported (Loeb and Sourirajan, 1960), that the surface away from the glass plate during casting is the desalinizing (good) side, while the surface in contact with the glass plate is the nondesalinizing (bad) side. In short, the good side, when in contact with brine, desalinizes while the bad side does not. 30
l&EC PRODUCT RESEARCH A N D DEVELOPMENT
X
a
i
0
100
k-0074
1=02 6 2
b
50 .
% FORMAMIDE IN CASTING SOLUTION
Figure 7.
Effect of additive on flux
Ternary system CA-AC-F. minute dry time, unheated
Roam temperature casting,
%.
I t is believed that differences in properties of these two surfaces are brought about by the fabrication technique. I t would be possible to prepare membranes with desalinizing characteristics a t either surface. Here the evaporation period could be the determining factor. As the membrane is cast, a skin is formed a t both surfaces; however, the skin in contact with the glass plate has no chance to gel and redissolves. Tests were made with tubular membranes cast and immersed directly with evaporation time almost nil. I n some cases the properties of the two sides were found to be interchanged. Extruded films could have desalinizing skins a t both surfaces. However, the
c c
100
300
lo
x 3
Ft
I i
i
y: k e a ' k = 0.074
LL J
CORRECTED FOR F L U X DUE TO S O L V E N T
a = 0.262
t .I
c'
0 C!\:AC
-
CONSTANT 5:ll
S C I \ . ( A C C F l C O N S T A N T 1:3
bL 01
L---L.-
1
I
1
20
30
40
% FORMAMIDE IN C A S T I N G S O L U T I O N
Figure 8.
% FORMAMIDE I N CASTING SOLUTION
Figure 9.
Effec:t of additive on flux
Table IX.
Compound Dimethylformamide
Dimethylsulfoxide Urea
Glyoxal
Tetrahydrofurfuryl phosphate Triethyl phosphate Acetic acid Methylpyrrolidone (N-M-2-P)
Amount
of pore producing additive vs. flux
Ternary system CA-AC-F. time, unheated
Ternary system CA-AC-F. Roam temperature casting, %-minute d r y time, unheated
Room temperature casting, 6-minute d r y
Performance of Successful Organic Additives
Casting Soln. Composition, 7 0 Acetone 64.3 CA 14.3 DMF 21.4 Acetone 37.5 25 . O CA DMSO 37.5 Acetone 62.5 CA 21 .o Hz0 13.2 Urea 3.3 Acetone 65.2 CA 21.7 H2 0 9.2 Glyoxal 4.5 Acetone 60 c.4 20 THFP 20 Acetone 50 cA 25 TEP 25 Acetone 68.4 C.A 22.5 Acetic 9.1 Acetone 65 CA 22
Curing Temp., C . Unheated
Eva). Per., M i n .
3.5
Desalinized Water Salt Flux, content, gal./sg. f t . day $.p.m. 19.4 550
Casting Temp., C. Ambient 23
60
3.5
5.4
300
Ambient 23
62
3
5.0
750
Ambient
Unheated
6
9.0
935
-10
63.3
6
2.6
375
-10
70.3
17
19.0
375
-10
75.0
10
18.1
175
-10
79.0
2
20.0
205
-10
77.5
2
5.0
750
- 10
75.0
16
5.7
250
-10 ~
VOL. 6
NO. 1
MARCH 1967
31
Table X.
Performance of Successful Solvent Additives
Compound Dimethylformamide
Casting Soh. Composition, 70 DMF 75 CA 25
Curin$ Temp., C . 93.0
Acetic acid
Acetic C-4
80 20
Unheated
Triethylp hosphate
TEP CA
80 20
93.0 94.0
formation of a dense skin on both surfaces could result in increased resistance to water flux through the membrane. Other Organic Additives. After the discovery of formamide as a successful additive, other organic compounds were tested in the same role. Table IX summarizes organic compounds that are acceptable as conditioning agents. T h e following compounds were unsuccessful as additives : acetonitrile, acrylic acid, diethanolamine, diethylamine, diethyl phthalate, isobutyl acetate, methylamine, oleic acid, phenol, propionic acid, pyrocatechol, tert-butyl acetoacetamide, and tetrahydrofuran.
Solvent- Additives
Certain organic compounds when used as solvents for cellulose acetate casting solution permit the preparation of semipermeable membranes bvithout the need of an additive. Of these, triethyl phosphate (TEP) is the most promising. T E P \vas initially tested as a n additive with good results. Table X lists successful binary systems and the performance of membranes cast from these solutions.
Conclusions
The ideal casting solution for the preparation of semipermeable membranes should comprise cellulose acetate and a solvent-additive-i.e., cellulose acetate dissolved in a single compound which would serve as both solvent and additive. T o date, three such systems have been discovered. but have not yet produced optimum membranes in terms of application. However, the value of a binary system lies in its simplicity. By minimizing the variables in the casting solution it provides ideal conditions for the study of cellulose acetate membranes. Since the final membrane is composed of cellulose acetate and water, an explanation of the selectivity of the membrane should be sought in the properties of cellulose acetate and the nature of water. I t has been demonstrated by Breton (1957) and others that cellulose acetate films are naturally selective. Therefore, it has to be concluded that the transitory role of the additive-solvent is to provide the matrix which permits the fabrication of reverse osmosis membranes with the desired structurization. Thus the additive-solvent is responsible for the high flux properties of the membranes and has no effect on the selectivity of cellulose acetate films. I t has been hypothesized that the curing process reduces the effects of the additive by consolidating and stabilizing the membrane structure. Membranes that do not require curing a t elevated temperatures have been prepared, indicating that by judicious control of the amount of additive used and the proper fabrication technique, this phase of the process could be eliminated, although it does not seem desirable to d o so. 32
l & E C P R O D U C T RESEARCH A N D D E V E L O P M E N T
Evap. Per., Min. 8
Desalinized M’ater Salt Flux, content, gal./sq. f t . day p.p.m. 10.8 140
10
1.5 1 .o
Castin8 Temp., a C. Ambient
6.2
650
Ambient
8.0 9.0
220 240
-10 Ambient
The ternary system, cellulose acetate-acetone-formamide, has produced the best membranes cast a t ambient temperatures, in both sheet form and tubular. In addition, this system permits the analysis of the role of each component independently of the others. Because of the improved mechanical properties of membranes cast from the above combination, the study of other membrane characteristics is made easier and the experimental results are reproducible and more reliable. The use of formamide as a n additive has simplified the fabrication of membranes and eliminated the need for casting temperature restrictions, thus increasing the feasibility of economic exploitation of cellulose acetate membranes for the demineralization of saline water.
Acknowledgment
Samples for tests of membrane materials were provided through the courtesy of William Gearhart, Eastman Chemical Products, Inc., Kingsport, Tenn.
literature Cited
Breton, E. J., Jr., “Water and Ion Flow Through Imperfect Osmotic Membranes,” U. S. Dept. Interior, Office of Saline LYater, Rept. 16 (1957). Dobry, A., Bull. SOC. Chim. France (5e Ser. T ) 111, 312-18 (1936). Kesting, R. E., “Role of the Membrane Salt in the Cellulose Acetate Desalination Membrane,” Symposium on Saline Water Conversion, Division of FYater and Waste Chemistry, 144th Meeting, ACS, Los Angeles, Calif., March 31 to -4pril 5, 1963. Loeb, S., “Appropriate Electrolytic Additives in a Casting Solution Used for the Production of High Performance Cellulose Acetate Membranes Used in Reverse Osmosis Desalination,” Ph.D. thesis, Department of Engineering, University of California, Los Angeles, May 1964. Loeb. S.. UCLA Deut. Engineering. Reut. 61-62 11961). Loebj S’., Manjikian, S., Ind. EEg. d e m . Procek Design Develop. 4. 207 11965). Lo;b, S.,‘Sourjrajan, S., “Sea Water Demineralization by Means of a Semipermeable Membrane,” UCLA Dept. Engineering, Rept. 60-60 (July 1960). Lonsdale, H. K., Merten, U., Riley, R. L., J . Appl. Polymer Sci. 9, 1341 (1965). Manjikian, S., Loeb, S., McCutchan, J., “Improvement in Fabrication Techniques for Reverse Osmosis Desalination Membranes,’’ Proceedings of First International Symposium on IVater Desalination, Washington, D. C., 1965. Riley, R., Gardner, J. O., Marten, U., Science 143, 801 (1964). U. S. Dept. Interior, “Demineralization of Saline Waters, Preliminary Discussion of a Research Program with an Outline and Description of Potential Processes and a Bibliography,” OTS, PB 161373 (October 1952). RECEIVED for review April 19, 1966 ACCEPTED January 6, 1967 Presented in part at First International Symposium on Water Desalination, October 1965.
