DRYING CELLULOSE ACETATE REVERSE OSMOSIS MEMBRANES K E N N E T H
D .
VOS'
A N D
F .
0 . BURRIS',
JR.
G u l f General Atomic I n c . , S a n Diego, Calif. 921 12
Cellulose acetate reverse osmosis membranes, after being soaked in a surface active agent, were dried at either ambient or elevated temperatures ( 1 10OC.). Upon being rewet, the membranes showed no loss in desalination or physical properties as compared with the undried control membrane. All types of surface active agents (nonionic, anionic, cationic, and amphoteric) function well. Because the dry membranes are somewhat brittle, glycerol and ethlyene glycol have been used as plasticizers for the dry membrane.
THEmembranes
currently used in reverse osmosis for desalination are made of cellulose acetate, using the technique of either Loeb and Sourirajan (1963) or Manjikian et al. (1965). These membranes are cast from a mixed solvent and are sometimes called modified membranes. Electron micrographs have shown that these modified membranes consist of two parts, a thin rejecting layer and a larger porous region (Riley et al., 1964, 1966). The rejecting layer performs the actual separation; the porous region, which makes up the bulk of the membrane, is simply a backing for the rejecting layer that offers a low resistance to the flow of water through the membrane. Modified membranes, because they contain a large number of pores, contain large quantities of water, 60 t o 70 weight 5 (Lonsdale et al., 1965). If this water is allowed to evaporate under ambient conditions, the membranes suffer an irreversible loss in desalination properties. In the electron microscopy referred to above, special techniques were used to remove the water from the membrane. In one case the water was replaced with carbon tetrachloride by liquid extraction (Riley et al., 1964). This method, while fairly successful, is very slow and requires considerable time and effort to handle large quantities of membrane. The second method was freeze-drying (Riley et al., 1966), which can be done successfully, although some care is needed to follow this method properly. This paper describes a method whereby the water in the membrane is allowed to evaporate under ambient or elevated temperatures after the membrane has been soaked in a surface active agent. These membranes, after being rewet, show no loss in desalination characteristics in comparison with undried control membranes. Experimental
Modified membranes were prepared by a procedure similar to that used by Sourirajan and Govindan (1965). The membranes were heat-treated by immersion in water a t 85°C. for 30 minutes. The reverse osmosis experiments ' Present address, Research Department, S. C. Johnson and Son, Inc., Racine, Wis. 53402 'Deceased.
84
I & E C PRODUCT RESEARCH A N D DEVELOPMENT
were made in a desalination cell a t brine flow rates known to minimize boundary layers (Merten, 1963). The brine was 1 i 0.3 weight c > NaC1; the salt content of the brine and the product was measured either by the Mohr titration method for C1- or by conductivity using standardized conductance cells. The operating pressure of the reverse osmosis cells was 800 30 p s i . , and the temperature was 25. k 5.C. The desalination properties of the membrane are expressed as the per cent salt rejection and the membrane constant. The salt rejection is the ratio of the difference between the salt content of the brine and the product divided by the salt content of the brine. The membrane constant is the water flux per unit membrane area per unit time divided by the net pressure, where the net pressure is the applied pressure difference minus the osmotic pressure difference across the membrane (Lonsdale et al., 1965). Membranes to be dried with surface active agents were soaked in an aqueous solution for 15 minutes and then hung up to dry in ambient air, unless otherwise noted. The concentration is given in volume per cent. The ambient air was about 23" C. and 60% relative humidity. After drying for a t least 1 hour, the membranes were soaked in distilled water and then run in reverse osmosis. Control samples of the same membrane, which were not dried, were also run in reverse osmosis. I n the tabulated data, the results for the control membranes follow each set of data for which the control applies. The uncertainties listed in the tabulated data are the standard deviations of several measurements, usually four but occasionally two. I n some cases, only one measurement was made and no uncertainty is listed.
*
Results
If modified membranes are allowed to dry without taking special precautions, they suffer a nonrecoverable loss in desalination and physical properties. Tests were conducted in which membranes were allowed t o stand in ambient air, 25°C. and 7 0 5 relative humidity, for specified periods of time and then immersed in distilled water. The physical characteristics of the samples were
noted and the samples were run in reverse osmosis (Table I). After 5 minutes in air, the membranes showed irreversible physical changes (curling and wrinkling) that increased as the exposure increased. The desalination properties, on the other hand, improved slightly for a time and then degraded as the membrane dried further. I n further tests, membranes were dried after soaking in a surface active agent. Surface active agents may generally be divided into nonionic, anionic, cationic, and amphoteric. Examples of each class were used to dry modified membranes; membranes dried with each type retained their desalination and physical properties. Twelve nonionic surf,ace active agents were tested; eight gave dry membranes which, when rewet and tested, showed good desalination properties. Table I1 lists five examples. I n addition to the materials listed in Table 11, Elvanol 72-60, poly(vinyl alcohol) ( D u P o n t ) ; ethylene glycol (Union Carbide) ; Lutonal M40, poly(viny1 methyl ether) (BASF Colors and Chemicals, Inc.); Plurafac A-24 (Wyandotte Chemicals); and Zonyl A (Du Pont) all gave dry membranes which, when rewet and tested, showed undiminished transport properties. Membranes dried from solutions of Sterox AJ-100 (Monsanto) and Triton X-114 (Rohm and Haas Co.) gave dry membranes whose trans-
Table I. Reverse Osmosis Results for Membranes Exposbedto Ambient Air
c1-
Elapsed Drying Time, Min. 0 (control) 2 5 10 15 30 60
Rejection, C/c
97.6 98.2 98.7 98.0 98.1 98.0 95.4
= 0.l = 0.1 = 0:l = 0.4 = 0.l = 0.3 = 1.0
Membrane Const., I o - j G. Sq. Cm.Sec.- Atm. Physical Properties I
* * * *
1.46 0.02 1.27 0.02 1.22 0.02 1.24 =t0.20 1.33 0.02 1.20 + 0.05 0.66 0.5
*
... Unchanged Some curling 25% white, curling 50% white, curled White, badly curled White, badly curled
port properties were inferior to the controls. Four of the eight that worked adequately-ethylene glycol, glycerol, poly(viny1 alcohol) (PVA), and poly(viny1 methyl ether) (PVME)-are not usually considered surface active agents, although they do depress the surface tension of water. The surface tension of a 0.5 weight 5 solution of PVME is only 40 dynes per cm. as measured with a D u Nouy tensiometer (Harkins and Jordan, 1930). The surface tension of glycerol-water solutions is between 63 and 73 dynes per cm. (Washburn, 1928), and the surface tension of ethylene glycol-water solutions is reported by the manufacturer to be between 35 and 73 dynes per cm. Poly(viny1 alcohol) is also reported by the manufacturer to lower the surface tension of water. Eight different anionic, cationic, and amphoteric surface active agents were tested; the results for three are given in Table 111. The pH of the solutions containing the amphoteric surface active agents was adjusted with HC1 or NaOH to the p H of interest. Two additional anionic surface active agents were tested-Aerosol OT-B (American Cyanamid) and Tween 20 (Atlas Chemical Co.)-and both allowed the membrane to dry and retain its desalination properties. The two additional cationic surface active agents that were tested-Aerosol C-61 (American Cyanamid Co.) and Q-DOX 3280 (Cargill Chemical Co.)-performed adequately. The other amphoteric surface active agent tested, Deriphat 1SOC (General Mills, Chemical Division), did not give dry membranes with undimished properties; the water flux had degraded. A study was made of the concentrations of surface active agents that must be placed in the immersion water to obtain dry membranes which, when rewet and tested, would perform adequately. The immersion time was also examined (Table IV). For Tergitol15S7, the concentration can be as low as 0.5cc; however, for Triton X-100, a concentration of about 4 7 is needed. The immersion time can be short (1 minute is adequate). The time that the surface active agent has to diffuse into the pores of the
Table II. Results of Reverse Osmosis Tests of Membranes Dried with Nonionic Surface Agents
Concn, Sample
Chemical Composition
Glycerol"
Control FC-170' Control Sterox DJa Control Tergitol 15S7' Control Triton X-100' Control
5 20 30 40 60 100
Dodecyl phenoxy polyethoxyethanol (10 moles of ethylene oxide)
...
Polyoxyethylene (9 moles of ethylene oxide) substituted on C1. to Cli linear alkyl
...
Isooctyl phenoxy polyethoxyethanol (10 moles of ethylene oxide)
...
i
*
.Membrane Con,t, 10 G Sq Cm -Sec -Atm 1.28 = 0.05 1.28 zt 0.05 1.30 i 0.06 1.37 = 0.05 1.29 3 0.03 1.13 + 0.06 0.50 1.13 0.06 0.44
...
...
1
34
97.4
zt
...
...
98.2
* 0.8
4
28
95.3
3
...
...
95.0
* 0.8
1.13 i- 0.06
4
30
97.0
+ 0.3
1.16 = 0.05
...
...
97.1
* 0.2
1.06 = 0.02
0.1
72 71 70 69 63
Suit Rejection, 94.7 0.2 96.4 i 0.3 93.5 =k 0.6 93.8 0.6 95.2 + 0.1 95.0 0.8 69 95.0 =k 0.8
...
... ...
Surface Tension," Djnes Cm
... 19
*
0.2
0.9
z
0.22
1.11 =k 0.08
1.15
+ 0.03
" A s giuen in manufacturer's literature. 'Atlas Chemical Co. 'Minnesota Mining and Manufacturing Co. 'Monsanto Co. ' L'nion Carbide Corp. I Rohm and Haas Ccl.
VOL. 8 NO. 1 M A R C H 1969
85
Table 111. Results of Reverse Osmosis Tests of Membranes Dried with Anionic, Cationic, and Amphoteric Surface Active Agents
Sample
TJpe of Surface ActiLe Agent
Chemical Composition
Duponol C n Control Hyamine 35OOb
Sodium lauryl sulfate
Anionic
...
Cationic
...
...
...
Partial E a salt of LV-lauryl d-iminopropionic acid
Amphoteric pH 2.0 pH 6.0 pH 8.0
1 1 1
Control Deriphat 160'
c
n-Alkyl ( C & d dimethyl benzyl ammonium chloride
...
Control
...
95.1 0.7 95.0 L 0.8 97.5 = 0.3
1.14 =t 0.04 1.13 & 0.06 1.30 rt 0.03
Salt Rejection,
Concn , 4
...
...
cc
Membrane Const 10 G Sq C m See -Atm
4
95.5
* 0.9
1.31
* 0.02
95.5 i- 0.6 95.4 0.7 95.5 I 0.7 95.5 0.9
1.23 1.16 1.22 zr 1.31 -c
*
*
...
* 0.04 0.03 0.06 0.04
' E . I . du Pant de Yemours & Co. 'I Rohm and Haas Co. 'Genera/ Mills, Chemical Division Table IV. Effect of Immersion Time and Concentration on Desalination Properties
Sample
Immersion Concn, Time, Salt Mcn. Rejection, (c 0.5 1 2 4
Tergitol l 5 S i
6
15 15 15 0.25 1 5 15 30 60 900 15
Control
...
...
Triton X-100
1 4
96.3 96.1 95.8 97.3 0.1 97.2 =t 0.4 97.2 I 0.4 96.7 =t 0.1
0.46 0.58 0.66 0.02 0.97 1.03 i 0.02 1.03 I 0.02 1.09 0.05
= 0.8
* 0.3 =t
*
...
Control
e 0.3 = 0.4 i 0.6 i 0.9 = 0.4 = 0.5 r 0.4 i 0.4 =t 0.8
1.03 + 0.06 1.13 i 0.06 1.12 zx 0.03 1.12 = 0.03 1.18 rr 0.04 1.18 i 0.04 1.15 I 0 03 1.06 I 0.04 1.11 = 0.04 1.15 = 0.04 1.15 2 0.02 1.13 i 0.06
95.7 96.0 95.8 94.0 94.6 94.9 95.3 94.9 95.1 95.5 94.9 95.0
1 5 15 1 5 15
'C
Membrane Const, 10 G Sq C m Sec -Atm
0.6
*
*
membrane is actually longer than that given in Table IV, since, when the membrane is hung up t o dry, the surface active agent continues to diffuse until the water completely evaporates. Membranes dried with most of the surface active agents discussed above are somewhat brittle and tear easily. Therefore, tests were also made with five plasticizers (Table V ) . Glycerol and ethylene glycol also perform as surface active agents. Glycerol was used with four additional surface active agents-Lutanol M-40, Sterox AJ-100, Triton X-100, and Deriphat 17OC- and functioned very well. The membranes had good physical and very good desalination properties (99+'1 salt rejection). Ethylene glycol also worked well, but because of its toxicity is less desirable than glycerol. Kelzan (Kelco Co.) and triacetin were used with Tergitol 15S7 and gave good desalination properties, but a t the concentrations used the physical properties of the dry membranes were not as good as those obtained with glycerol. Polyethylene glycol 6000 with Tergitol I S 7 gave membranes with inferior desalination properties.
Table V. Reverse Osmosis Tests of Membranes Dried with a Surface Active Agent and a Plasticizer
Sample Sterox DJ Control Tergitol 15s;"
Concn., 5;
Plasticizer
0.01 0.05 0.10
Glycerol
...
4
.
.
I
Ethylene glycol
30 30 30
... 2 5 10 20 60
Control
...
...
...
Tergitol 15Si
0.5
Glycerol
Control
...
...
20 30 40 60
Tergitol 15S7
4
Control
...
None Glycerol
...
Samples dried at 11 0" C.
86
Plasticizer Concn.,
I&EC PRODUCT RESEARCH A N D DEVEtOPMENT
... ... 20 40 60
...
Membrane Const., Salt Rejection, C C
1 0 '~G. Sq. Cm.-See.-Atm
98.8 =t 0.1 98.6 = 0.1 98.5 = 0.2 98.2 i 0.8
1.14 i 0.03 1.17 0.07 1.15 0.08 1.11 =t 0.08
97.2 96.9 97.2 96.8 96.8 i 0.4
1.16 1.22 1.13 1.23 1.33 1.08 i. 0.05
96.1 =t 0.2 95.4 0.9 94.3 + 0.3 95.0 ~f0.5 95.0 i 0.8
1.34 i 0.09 1.38 i 0.04 1.26 + 0.05 1.36 + 0.05 1.13 i 0.06
99.1 99.3 99.1 99.1 99.1
1.10 1.13 1.14 1.17 0.94
97.6
*
* 0.2
*
*
* 0.04
~~~~~~~~~~~
~
~
Table VII. Reverse Osmosis Tests of Membranes Dried with an Antifoam Agent
Surjactant
Surjactant Concn. CC
Tergitol 15S7
0.01
Control Tergitol 15S7
Control
I
.
.
0.01
...
Plasticizer and Antifoam
Glycerol Antifoam BO Glycerol Antifoam B
...
Glycerol Antifoam H-1OU Glycerol Antifoam H-10
...
Plasticizer and Antifoam Concn.
Salt Rejection,
30L 100 p.p.m. 305 200 p.p.m.
...
30% 50 p.p.m. 30“; 100 p.p.m.
...
Membrane Const., lo-’ G. Sq. Cm. Sec.-Atm .
Cc
97.4
* 0.7
1.26 =t0.02
97.4
* 0.5
1.17 i 0.13
98.1
* 0.5
1.18
* 0.06
98.5 i- 0.2
1.12
i 0.06
98.6 i 0.1
1.21
* 0.05
98.2
1.11 i. 0.08
I
0.8
DOLLChemical Co Table VIII. Storage Test on Dried Membranes
membrane in rows 2 and 3 is substantial and shows that the surface active agent is essential to eliminate curling a t low glycerol concentrations. The membrane immersed in pure glycerol is clear because the refractive indexes of glycerol and cellulose acetate are virtually identical. The last membrane in row 3 is one wet with water. At high concentrations of glycerol or ethylene glycol, much of the decrease in curling is due to the simple replacement of water with these materials. I n these cases the liquid exchange process is extensive. If the dry membranes are placed directly in the reverse osmosis cell, the product water initially contains surface active agents and the water foams very easily. Two antifoam agents were examined; the results with Tergitol 1587 are given in Table VII. Both antifoam agents substantially reduce the foaming tendency. Dry membranes have been stored in covered unstoppered bottles for various periods of time, rewet, and tested in reverse osmosis (Table VIII). Membranes containing surface agents and plasticizers can be stored dry for a t least 300 days and still retain their desalination properties. Discussion
The potential advantages in having the modified cellulose acetate membrane dry include easier handling-e.g., the membrane need not be kept immersed or wet with water during incorporation into systems and during storage. The storage life of the dry membrane, in comparison with the wet membrane, should be increased, since a membrane that is wet with water is subject to hydrolysis, which is dependent on the activity of water in the membrane, and thus a loss in desalination properties (VOS et al., 1966a,b). Membranes that are stored wet are subject to bacterial growth unless they are disinfected; this should be less of a problem if the membranes are dry by suitable desiccation. I n addition, dry membranes might be shaped and sealed in ways that cannot be used with wet membranes. During ambient drying, the membrane porosity is grossly reduced, apparently because the surface-tension forces between the water and the membrane are so great that as the water globules within the membrane decrease in size, the pores around them collapse. This loss of porosity is indicated by shrinking and curling of the irreversible membrane upon drying as well as by a substantial loss in desalination properties (Table I and Figure 1). If the 88
I B E C PRODUCT RESEARCH A N D DEVELOPMENT
Storage Time, Membrane 0.5cc Tergitol 15S7, 20‘c glycerol
Days 1
71 140 197 297
Control
...
0.lLcSterox DJ, 30Lc glycerol, 0.01‘; Antifoam H-10
1 60 118 218
Control
...
Cl Releetion, r
96.7 97.5 97.7 97.1 97.4 97.4
+ 0.2
.Zilembrane Const., 10-j G. Sq. Cm.Sec.-Atm.
0.5 0.3 0.4
1.43 i 0.06 1.62 0.02 1.67 0.03 1.63 0.08 1.33 i 0.33 1.28 I 0.08
97.6 i 0.1 97.7 I 0.1 97.1 i 0.6 97.8 0.6 0.5 97.7
1.49 I 0.05 1.42 0.11 1.49 0.09 1.22 0.11 1.26 0.10
* 0.3 * 0.1 & 3
I
*
* * * * *
*
interaction between the cellulose acetate and the water in the membrane were sufficiently reduced, the water could be removed from the membrane without collapsing the pores. A method of reducing the interaction is to reduce the surface tension of the pore water-i.e., the surface tension of the system, cellulose acetate-water-air. A possible method of determining this is the maximum surface tension that a liquid can have and still spread on the cellulose acetate. This surface tension, for homologous series of liquids, is a characteristic of the cellulose acetate, which Zisman (1962, 1964) called the critical surface tension. For “dry” cellulose acetate. the critical surface tension can be determined from the contact angle data of Bartell and Ray (1952). For a 40 weight ‘C acetyl cellulose acetate (the cellulose acetate in this work was 39.8 weight % acetyl), the critical surface tension is about 40 dynes per cm. The critical surface tension of the water-wet membrane, however, can be expected to be higher than this. If a surface tension lower than 40 dynes per cm. were the only requirement, it would be predicted that an aqueous solution of FC-170, which has a surface tension of only 19 dynes per cm., would perform very well, but a solution of glycerol would not perform well. Experimental results show the reverse to be true-Le., FC-170 does not allow evaporation of the water in the membrane without changes in properties, whereas glycerol does. The surface tension, as measured with the DuNouy
tensiometer, is thus not a completely reliable guide as to which surface active agent will allow evaporation of the water in the membrane with retention of the desalination and physical properties of the membrane. A second effect coulld be operating-namely, “coating” of the cellulose acetate so that the interface of the pore water and the cellulose acetate is changed, thus reducing the capillary shrinkage forces on the cellulose acetate. This could be part of the reason why glycerol worked so well and FC-170 did not (Figure 1). There must be some adsorption of the material by the cellulose acetate; many of the surface active agents tested appear capable of filling this role. T h a t the combination of glycerol and a surface active agent--e.g., Sterox DJ-can yield better results than either alone may be attributable to the coating action of the glycerol supplementing the surface tension effect of the surface active agent. A third effect should also be considered as part of the drying-Le., the surface active agent may act as a humectant. Conclusions
Water can be evaporated from modified cellulose acetate membranes with no loss in desalination or physical properties by soaking the membrane in a surface active agent before drying. Surface active agents representative of all major types can apparently be used successfully. Dry membranes have several potential advantages which could make the process worthy of larger scale study and experimentation.
Literature Cited
Bartell, F. E., Ray, B. R., J . A m . Chem. Soc. 74, 778 (1952). Harkins, W. D., Jordan, H. F., J . A m . Chem. SOC.52, 1751 (1930). Loeb, S., Sourirajan, S., Aduan. Chem. Ser. No. 38, 117 (1963). Lonsdale, H. K., Merten, U., Riley, R. L., J . A p p l . Polymer S c i . 9, 1341 (1965). Manjikian, S., Loeb, S., McCutchan, J. W., Proceedings of the First International Symposium on Water Desalination, Washington, D. C., Oct. 3-9, 1965, Vol. 2, p. 159. Merten, U., I n d . E r g , Chem. Fundamentals 2, 229 (1963). Riley, R . L., Gardner, J. O., Merten, E., Science 143, 801 (1964). Riley, R. L., Merten, U., Gardner, J. O., Desalination 1, 30 (1966). Sourirajan, S., Govindan, T. S., Proceedings of the First International Symposium on Water Desalination, Washington, D. C., Oct. 3-9, 1965, Vol. 1, p. 251. Vos, K. D., Burris, F. O., Jr., Riley, R . L., J . A p p l . Polymer Sci. 10, 825 (1966a). Vos, K. D., Hatcher, A . P., Merten, U., IND.ENG.CHEM. PROD. RES. DEVELOP.5 , 211 (196613). Washburn, E . W., Ed., “International Critical Tables of Numerical Data,” Vol. IV, McGraw-Hill, New York, 1928. Zisman, W. A., Aduan. Chem. Ser. No. 43, 1 (1964). Zisman, W. A., “Adhesion and Cohesion, Proceedings,” P. Weiss, Ed., p. 176, Elsevier, Xew York, 1962. RECEIVED for review June 26, 1968 ACCEPTED January 4, 1969
Acknowledgment
The authors thank .R. L. Riley for helpful discussions.
Work sponsored by the U. S. Department of the Interior, Office of Saline Water, Contract KO.14-01-001-767.
GLYOXAL FROM OZONOLYSIS OF BENZENE WILLIAM
P .
K E A V E N E Y ,
R A Y M O N D
V .
RUSH,
Central Research Laboratories, Interchemical Corp., Clifton, N . J .
AND
JAMES
J.
PAPPAS
07015
Ozone reacts with benzene in low-molecular-weight carboxylic acids, or mixed solvents incorporating such acids, at a reasonable rate, with very little absorption by the solvent. Subsequent reduction by dimethyl sulfide (DMS) and gravimetric analysis with 2,4-dinitrophenylhydrazine (DNPH) have demonstrated yields of glyoxal up to 73% based on absorbed ozone. Tests for several possible by-products wer’e all negative. Various solvent systems, temperatures, conversions, catalysts, and reducing agents were utilized, and the effects of each variation noted. The ozonolysis of cyclooctatetraene w a s briefly investigated and related to that of benzene.
SINCE the end
of World War 11, glyoxal has evolved from a laboratory chemical to a large-scale commercial product which has expanded to a reported 1967 capacity of some 180,000,000 pounds. I t s most prominent use has been as a key component in crease-resistant formulations for textiles, notably with urea and formaldehyde. The industrial synthesis of glyoxal involves catalytic oxygenation of ethylene glycol, giving as the ultimate salable product a concentrated aqueous glyoxal solution containing
unreacted glycol, formaldehyde, and formic and glycolic acids as the main impurities (Bohmfalk et al., 1951). The ozonolysis of benzene has long been recognized as leading to glyoxal, as mentioned in Long’s review (1940). The lower cost of benzene as a starting material uis a uis ethylene glycol encouraged investigation of this application of ozonization as a synthetic route to glyoxal. The reaction of benzene with ozonized oxygen has been reported only rarely in the chemical literature of this VOL. 8 NO. 1 M A R C H 1 9 6 9
89
