Performance of Porous Cellulose Acetate Membranes for the Reverse

V(t! = total volume of oxygen evolved a t time t , cc. (STPI p

= packing density of potassium superoxide, gram/

cc. SUBSCRIPT

0 = initial value literature Cited

Petrocelli, A. W., Capotosto, A., Aerospace Med. 35, 440 (1964). Petrocelli, A. W., Krausa, D. L., J . Chem. Educ. 40, 146 (1963). Vannenberg, N.-G., “Progress in Inorgaqic Chemistry,” F. Cotton, ed., Interscience, New York, 1962. Vol’nov, I. I., “Peroxides, Superoxides and Ozonides of Alkali and Alkaline Earth Metals,” Nauka, MOSCOW, 1964 (in Russian).

Downloaded by FLORIDA STATE UNIV on September 9, 2015 | http://pubs.acs.org Publication Date: January 1, 1970 | doi: 10.1021/i260033a002

George, P., J . Chem. SOC.70, 2367 (1955). Mel’nikvov, A. Kh., Firsova, T. P., Russ. J. Inorg. Chem. 7, 633 (1962). Petrocelli, A. W., Aerospace Med. 36, 1187 (1965).

RECEIVED for review March 28, 1966 RESUBMITTED August 7, 1969 ACCEPTED August 19, 1969

PERFORMANCE OF POROUS CELLULOSE ACETATE“ MEMBRANES

FOR THE REVERSE OSMOSIS SEPARATION OF MIXTURES OF ORGANIC LIQUIDS J.

K O P E C E K ’ A N D

s.

S O U R I R A J A N

Division of Chemistry, National Research Council of Canada, Ottawa, Canada The reverse osmosis technique is applicable for the separation of binary mixtures of alcohols and/or hydrocarbons, including azeotropic and isomeric mixtures. The pqrous structures of the cellulose acetate membranes used were affected by the composition of the feed solution in contact w i t h them; hydrocarbon liquids tended to collapse their porous structures on continued contact. Hydrogen bonding and solubility parameter may offer valid criteria of preferential sorption for nonelectrolyte binary feed mixtures containing components whose solubilities are governed primarily by either polar or dispersion forces.

of work (Sourirajan, 1964) on the reverse osmosis separation of binary mixtures of some organic liquids using the Loeb-Sourirajan type porous cellulose acetate membranes illustrates the applicability of the technique for the separation of azeotropic mixtures, substances belonging to the same homologous series, isomeric substances, and mixtures of other organic liquids. EXTENSION

Experimental Details

Reagent grade organic compounds and porous cellulose acetate membranes (designated here as CA-NRC-18 type films) made in the laboratory, were used. The films were cast a t -10°C in accordance with the general method described earlier (Loeb and Sourirajan, 1963,1964; Sourirajar1 and Govindan, 1965) using the following composition (weight per cent) for the film casting solution: acetone 68.0, cellulose acetate (acetyl content = 39.8%) 17.0, water 13.5, and magnesium perchlorate 1.5. Membranes shrunk at different temperatures were used to give different levels

’ Present address, Institute of Macromolecular Prague, Czechoslovakia

Chemistry,

Ind. Eng. Chem. Process Des. Develop., Vol. 9,No. 1, January 1970

of membrane porosity and performance a t preset operating conditions. The experiments were carried out a t the laboratory temperature in the pressure range 250 to 1000 psig, using the reverse osmosis cell shown in Figures 1 and 2. The cell was a stainless steel pressure chamber consisting of two detachable parts. The film was mounted on a stainless steel porous plate embedded in the lower part of the cell through which the membrane-permeated liquid was withdrawn at atmospheric pressure. The upper part of the cell contained the feed solution under pressure in contact with the membrane. The two parts of the cell were clamped and sealed tight using rubber O-rings. Compressed nitrogen gas was used to pressurize the system. About 250 cc of feed solution were used each time. The feed solution was kept well stirred by means of a magnetic stirrer fitted in the cell about % inch above the membrane surface. The quantity of liquid removed by membrane permeation was small compared to the amount of feed solution in the pressure chamber. The compositions of the feed and the membrane-permeated 5

Table I. Data on Shrinkage Temperature and Pure Ethyl Alcohol Permeability at 25' C for Films Used Film type. CA-NRC-18 Operating pressure. 1000 psig Film area. 9.6 sq cm

Film

No. 2 3 7

6 c


SECTI ON A A'

VIEW B

Figure 1. Details of reverse osmosis cell

PRESSURE

dAUGE

NITROGEN T A N K U N D E R PRESSURE

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W

c

a

[L

I-

40

0 2 P

0 [L

a

20

0 I

2

3

4

XI nc,+ X2nc2

Figure 3. Product rate data for alcohol systems

8

was the magnitude of separation under otherwise identical experimental conditions. The product rate data shown in Figure 3 are particularly interesting. I n Figure 3, the abscissa represents the average carbon number for the mixture; XI and nc1 are the mole fraction and carbon number, respectively, of component 1 in the feed mixture, and X Pand na are the corresponding numbers with respect to component 2. For example, pure EtOH, pure n-PrOH, and mole ratio 1 to 1 EtOH-n-PrOH mixture are represented by 2, 3, and 2.5, respectively, in the x-axis in Figure 3. The product rate data for the pure components and their binary mixtures fall on a unique line for each membrane studied. This indicates that the changes in the porous structure of the membrane as a result of polymer-solution interaction are completely reversible, and the sorption characteristic of the membrane for the alcohol solutions is a linear function of the mole fraction of their components in the feed mixture. System Ethyl Alcohol-p-Xylene. Cellulose acetate membranes have a preferential sorption for EtOH from EtOHp-xylene mixtures. Figure 4 gives the separation and product rate data for this system for three membranes shrunk a t different temperatures. A membrane shrunk a t lower temperature has bigger (average) surface pores, and gives higher product rate and lower separation a t a given Ind. Eng. Chem. Process Des. Develop., Vol. 9,No. 1, January 1970

-- ,

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operating pressure. The curves showing separation data us. mole per cent ethyl alcohol in feed are not symmetrical, and the corresponding product rate data correlations are not straight lines. The feed mixtures containing lower mole fractions of alcohol are relatively more enriched in alcohol in the product. These observations indicate that the size of the pores on the membrane surface and/or the preferential sorption characteristics of the membrane material are affected by the composition of the feed solution. An increase in the mole fraction of that component in the feed mixture, for which the membrane has a less tendency for sorption, tends to increase the number of physical cross links in the membrane structure, reduce the average pore size, and collapse the pore structure of the membrane to different extents depending on the chemical nature of the components of the feed mixture with respect to that of the membrane material. With feed solutions containing more than about 25 mole 70 EtOH, the separation and product rate data obtained with each film were reproducible whatever the order in which the experiments were carried out, which indicates that the changes in the pore structure of the membrane brought about by changes in the composition of the feed solution are reversible. When the alcohol content in the feed solution was less than about 25 mole 70, irreversible changes in the porous structure of the membrane took place on continued contact with the feed solution. As in the case of aqueous solution systems (Sourirajan and Ind. Eng. Chem. Process Des. Develop., Vol. 9, No. 1, January 1970

PRESSURE p.s.i.g.

Figure 6. Effect of pressure on performance for system ethyl alcohol-p-xylene

Govindan, 1965; Sourirajan and Kimura, 19671, increase of operating pressure increased both separation and product rate (Figure 51, relatively more for the membrane whose surface pores were smaller (Figure 6). System Ethyl Alcohol-+Heptane. Cellulose acetate membranes have a preferential sorption for EtOH from EtOH-n-heptane mixtures. Figure 7 gives the separation and product rate data for four membranes of different pore structures obtained by shrinkage a t different temperatures. The curves showing the separation data us. mole per cent ethyl alcohol in feed, and those representing the corresponding product rate data, are similar to those for the system EtOH-p-xylene. The performance of porous cellulose acetate membranes for the reverse osmosis separation for the system EtOH-n-heptane is far more favorable than that for the system EtOH-p-xylene. For example, even with a film such as 5 in Figure 7 (whose pure ethyl alcohol permeability a t 1000 psig = 171.1 grams per hour per 9.6 sq cm of of film area), the separation and product rate data can be considered significant for industrial exploitation. With membranes having smaller surface pores, the separations are even more favorable; the product rates are lower. I n this system also, with feed solutions containing more than about 25 mole % ethyl alcohol, the separation and product rate data for different feed concentrations are reproducible whatever the order in which the experiments were carried out. This again indicates that, under those conditions, the changes in the porous structure of the membrane brought about by the changes in the composition of the feed solution were reversible. 9

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Figure 7. Effect of porosity of membrane surface on performance for system ethyl alcohol-n-heptane

Figure 8. Effect of pretreatment pressure on performance for system ethyl alcohol-n-heptane

Figure 8 shows the effect of pressure used in the initial pressure treatment of the film on its subsequent performance in reverse osmosis. The initial pressure treatment affects the over-all porous structure of the membrane. Figure 9 shows the effect of operating pressure on separation and product rate for the system EtOH-n-heptane. Again, both separation and product rate increased with operating pressure. Further, if the product is treated simply as a mixture of the feed solution and the preferentially sorbed alcohol, the plot of the latter us. operating pressure is essentially a straight line in the pressure range 250 to 1000 psig. System p-Xylene-n-Heptane. This system is of interest in view of the low tendency for the sorption of cellulose acetate material to either component in the feed solution. Three membranes with different initial porous structures (as shown by their pure alcohol permeability values, Table I) were tested with the above system. The product was enriched in p-xylene and the Yxyleneus. Xxylene graph was essentially symmetrical; the product rates were low (less than 3 grams per hour per 9.6 sq cm of film area) in all cases. Separation increased with operating pressure (Figure 10). The levels of separation obtained with the

three films tested were not too different from each other, even though their initial porous structures were very different (Figure 11). These observations indicate the collapse of the porous structure of the film with the above feed system, and the tendency of the film to approach some kind of a limiting structure not necessarily identical in all cases. Criteria of Preferential Sorption. The edects of various intermolecular forces (such as dispersion, repulsion, and polar forces, and dipole interactions) on solubility of nonelectrolytes have been reviewed (Burrell, 1955; Hildebrand, 1949; Small, 1953). Since solution and sorption are related phenomena, and one may consider solution simply as the limit of sorption, the physicochemical criteria of solubility based on intermolecular forces may lead to valid criteria of preferential sorption in reverse osmosis. This is suggested by a consideration of the cohesive energy density concept which is based principally on dispersion forces, and the polar attraction of molecules based principally on the hydrogen bonding forces. The solubility parameter gives a quantitative measure of the cohesive energy density; no such measure is available for the hydrogen-bonding forces which may, however, be

10

Ind. Eng. Chem. Process Der. Develop., Vol.

9,No. 1, January 1970

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Figure 10. Effect of operating pressure on separation for system p-xylene-n-heptane

OPERATING PRESSURE p. s.i.g.

Figure 9. Effect of operating pressure on performance for system ethyl alcohol-n-heptane

arbitrarily represented by the iodine-bonding number of Small (1953). At least with respect to nonelectrolyte binary mixtures containing components whose solubilities are governed primarily by the hydrogen-bonding or dispersion forces, one may expect to be able to predict the direction of separation in reverse osmosis. Since cellulose acetate itself has a hydrogen-bonded structure, the membranepermeated product may be expected to be enriched in the more hydrogen-bonded component of the feed mixture. The data on the separation of the mixtures of alcohols (Table V) justify this suggestion; the product is always enriched in the lower of the homologous series, which is also the more hydrogen-bonded one in the mixture (Small, 1953). When the feed mixture consists of components whose solubilities are governed primarily by dispersion forces, the solubility parameter of the component with respect to that of the membrane material may offer a valid criterion for preferential sorption. Since a decrease in the difference between the solubility parameters of any two substances indicates an increase in their mutual tendency for solubility, the membrane-permeated product in reverse osmosis may be expected to be enriched in that component for which (6, - 6,) is lower. where 6, and 6, are the values of the solubility parameters for the membrane material and the component in the feed mixture, respectively. The reverse osmosis data for the systems p-xylene-n-heptane and benzene-toluene (Figures 10 and 11, and Table 111) justify this expectation. The solubility parameters for cellulose acetate, p-xylene, n-heptane, benzene, and toluene are 10.9, 8.8, 7.5, 9.2, Ind. Eng. Chern. Process Des. Develop., Vol. 9,No. 1, January 1970

/ 2'

0

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100

- X Y L E N E IN FEED

Figure 11. Effect of initial porosity of membrane surface on separation for system p-xylene-n-heptane

and 8.9, respectively, a t 25°C (Burrell, 1955). Since the solubility parameters for o- and rn-xylenes are 9.0 and 8.8, respectively (Burrell, 1955), the reverse osmosis data in Tables I11 and IV for the systems p-xylene-benzene, p-xylene-toluene, p-xylene-o-xylene, and m-xylene-oxylene would appear as exceptions to this solubility parameter principle. I t is probable that such exceptions arise more from uncertainties in the values of the solubility parameter than from the principle itself. Further, the contribution of the other intermolecular forces for the solubility of nonelectrolyte substances cannot be ignored. 11

Consequently, precise numerical data on the various interfacial forces in membrane-solution systems may lead to some valid criteria of preferential sorption in reverse osmosis.


Conclusions

The reverse osmosis technique is applicable for the separation of substances in nonaqueous solutions. Hydrocarbons tend to collapse the porous structure of the cellulose acetate membranes used; hence, for feed mixtures containing hydrocarbons, some other type of membranes will have to be developed for reverse osmosis application. For a t least some organic feed solutions, such as alcohol-hydrocarbon mixtures containing more than about 25 mole 70 alcohol, porous cellulose acetate membranes of the type used in this work appear sufficiently good for consideration for industrial applications. I n view of the changing pore structure of the membrane in contact with organic feed solutions, the membrane-solutionoperating systems used could not be specified precisely as done (Sourirajan and Kimura, 1967) for aqueous solution systems and cellulose acetate membranes. Consequently, the performance data given in this paper have mainly relative significance, and do not represent limiting values obtainable in reverse osmosis systems involving organic liquid mixtures.

mounted i n ' t h e reverse osmosis cell, and A. G. Baxter and Lucien Pageau for their valuable assistance in the progress of these investigations. One of the authors (J. K.) thanks the National Research Council of Canada for the award of a postdoctoral fellowship. Literature Cited

Breitenbach, J. W., Forster, E. L., Makromol. Chem. 8, 140 (1952). Burrell, H., O f i . Dig., Federation Paint Varnish Prod. Clubs 1955, 726. Ghosh, S. K., Rawat, B. S., Indian J . Technol. 4, 62 (1966). Hildebrand, J., Chem. Reu. 44, 37 (1949). Kopetek, J., Sourirajan, S., Can. J . Chem. 47, 3467 (1969). Loeb, S., Sourirajan, S., Aduan. Chem. Ser., No. 38, 117 (1963). Loeb, S., Sourirajan, S., U. S. Patent 3,133,132 (May 12, 1964). Small, P. A., J . Appl. Chem. 3, 71 (1953). Sourirajan, S.,Nature 203, 1348 (1964). Sourirajan, S., Govindan, T. S., First International Symposium on Water Desalination, Washington, D. C., October 1965, Office of Saline Water, U. S. Department of the Interior, Vol. 1, pp. 251-74. Sourirajan, S., Kimura, S., IND. ENG. CHEM. PROCESS DESIGNDEVELOP.6, 504 (1967).

Acknowledgment

The authors are grateful to W. S. Peterson and W. L. Thayer for the design and construction of the stirrer

RECEIVED for review August 16, 1968 ACCEPTED October 9, 1969 Issued as N.R.C. No. 11126

REVERSE OSMOSIS SEPARATION OF SOME INORGANIC SALTS IN

AQUEOUS SOLUTION CONTAINING MIXED SOLUTES WITH A COMMON ION J.

P .

A G R A W A L

A N D

S .

S O U R I R A J A N

DiGision of Chemistry, National Research Council of Canada, Ottaua, Canada A simple method for predicting the performance of Loeb-Sourirajan type porous cellulose acetate membranes for low concentrations of mixed solutes in aqueous feed solution systems involves two or more inorganic salts with a common ion. The method requires only data on membrane specifications and the applicable mass transfer coefficient correlation for the corresponding single solute systems.

THE transport

equations and correlations of reverse osmosis experimental data using the Loeb-Sourirajan type porous cellulose acetate membrane for aqueous feed solutions containing one inorganic solute only have been discussed extensively (Agrawal and Sourirajan, 1969; Kimura and Sourirajan, 1967). The extension of the Kimura-Sourirajan analysis to mixed inorganic solute systems is complicated by the general nonavailability of the applicable osmotic pressure data, and the possibility of ionic interactions in such systems. Many natural waters 12

and industrial aqueous solutions contain more than one inorganic solute. The application of reverse osmosis for the separation and fractionation of such mixed solutes is of great practical interest. Hence the development of suitable methods for predicting membrane performance for such mixed solute feed systems is an area of fundamental importance in reverse osmosis transport; from this point of view, the available data on the subject (Erickson, 1966; Sourirajan, 1963) are only qualitative in scope. This paper presents some experimental data for the lnd. Eng. Chem. Process Des. Develop., Vol. 9,No. 1, January 1970

Performance of Porous Cellulose Acetate Membranes for the Reverse

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