SPECIFICATION, SELECTIVITY, AND PERFORMANCE OF POROUS CELLULOSE ACETATE MEMBRANES IN REVERSE OSMOSIS J.
P. A G R A W A L A N D S . S O U R I R A J A N
Division of Applied Chemistry, National Research Council of Canada, Ottawa, Canada
The general specifications of the Loeb-Sourirajan type porous cellulose acetate membranes are given i n terms of the pure water permeability constant, A, and the solute transport parameter, ( D A M / K ~, ) for sodium chloride a t different operating pressures. A scale of membrane selectivity i s presented for 12 inorganic and two organic solutes in terms of their relative values of (DAMIKG) for several membranes covering a wide range of surface porosities. Using the data on membrane specification and selectivity, and the applicable mass transfer coefficient, k , obtained from a generalized correlation, the performance data for different membranes, solution systems, and operating conditions have been calculated. These data relate membrane specification to membrane performance expressed i n simple terms of solute separation and product rate. The effects ), concentration, feed flow rate, operating temperature, of variation of ( D A M / K ~ feed A-factor (membrane compaction), and chemical nature of solute on membrane performance are illustrated and discussed. The order of solute separation w i t h respect to any two solutes does not always correspond to that of their relative
, the former i s a function of the latter and the operating values of ( D A M I K ~ )and conditions of pressure, feed concentration, and the mass transfer coeficient, k .
A LOEB-SOURIRAJAN type porous cellulose acetate membrane can be specified for reverse osmosis application with aqueous feed solutions in terms of the pure water permeability constant, A , and the solute transport parameter, ( D A M ~ K 8a t) ,a given operating pressure (Sourirajan and Kimura, 1967). For several inorganic and organic solutes, (DA.uI K6) is independent of feed concentration and feed flow rate (Kimura and Sourirajan, 1967; Ohya and Sourirajan, 1969b; Sourirajan and Kimura, 1967); this report is concerned with such solutes only. Membrane selectivity for such solutes can be expressed in terms of their relative values of (DA.M; K6) (Ohya and Sourirajan, 196913). Provided the mass transfer coefficient, h , applicable for the high pressure side of the membrane is available or can be calculated, the performance of a membrane, expressed in terms of solute separation and product rate, can be predicted from membrane specifications (Kimura and Sourirajan, 1967; Sourirajan and Kimura, 1967). This paper relates membrane specification to membrane performance, so that the significance of the specifying parameters can be recognized readily in practical terms of cause and effect. Experimental Details
Reagent grade solute substances and porous cellulose acetate membranes (CA-NRC-18 type films) made in the laboratory were used. These films were cast a t -10°C. in accordance with the general method described earlier (Loeb and Sourirajan, 1963, 1964; Sourirajan and Govindan, 1965) using the following composition (weight per cent) for the film casting solution: acetone 68.0, cellulose acetate (acetyl content = 39.89) 17.0, water 13.5, and magnesium perchlorate 1.5. The film details, the
apparatus, and the experimental procedure have been reported (Sourirajan, 1964a,b; Sourirajan and Govindan, 1965). Membranes shrunk a t different temperatures were used to give different levels of solute separation a t a given set of operating conditions. The aqueous feed solution was pumped under pressure past the surface of the membrane held in a stainless steel pressure chamber provided with two separate outlet openings, one for the flow of the membrane-permeated solution, and the other for that of the concentrated effluent. A porous stainless steel plate, specified to have pores of average size equal to 5 microns, was mounted between the pump and the cell to act as a filter for dust particles which might otherwise clog the pores on the membrane surface. Unless otherwise stated, the experiments were of the short-run type, each lasting for about 2 hours, they were carried out a t the laboratory temperature, and the reported product rates are those corrected to 25°C. using the relative viscosity and density data for pure water. A few experiments were also conducted a t temperatures in the range 5' to 36'C. For the purpose of these experiments, the cells and the feed solution were placed in an insulated box which was cooled or warmed as needed. An automatic recorder was used to record the temperature of the pure water feed flowing through each of the cells to which thermocouples were attached through Conax fittings. In each experiment, the solute separation, f , defined as molality of feed ( m l )- molality of p r o d u c t ( m i )
f=
molality of feed ( m l )
the product rate, LPR], and the pure water permeability, VOL. 8 NO. 4 OCTOBER 1 9 6 9
439
[PWP],in grams per hour per 7.6 sq. cm. of effective film area were determined a t the preset operating conditions. I n all cases the terms “product” and “product rate” refer to the membrane-permeated solutions. The concentrations of the solute in the feed and product solutions were determined by refractive index measurements using a precision Bausch & Lomb refractometer or by specific resistance measurements. T h e accuracy of the separation data is within 1% and that of [ P R ] and [ P W P ] data is within 3% in all cases. Results a n d Discussion
Solution Systems Studied. Binary aqueous solution systems involving the following solutes were studied: LiC1, NaC1, KC1, NH4C1, LiniOs, NaN03, K N 0 3 , glycerol, and urea in the concentration range 0.05 to 4.OM, and NaZSO1, MgC12, CaC12, BaC12, and MgS04 in concentrations up to 1.5M. Osmotic Pressure and Molar Density Data. These data for some of the above systems have been reported (Kimura and Sourirajan, 196813; Ohya and Sourirajan, 1969b; Sourirajan and Kimura, 1967); the data for the other systems a t 25“ C., and those for the system sodium chloride-water a t 5“, Eo,and 35°C. were calculated from the data on density, osmotic coefficient, activity, and partial molar volume of water available in the literature (Robinson and Stokes, 1959; Timmermans, 1960; Tribus et al., 1960). Basic Transport Equations and Correlations, KimuraSourirajan Analysis. This analysis (Kimura and Sourirajan, 1967) is applicable for the entire possible range ( > 0 to and
fCaCl
( D A M / K ~ for ) ~ film ~ , ~M4, for 2.OM feed solution
>
fMgcl
( D A M J K ~>) ~ (~ D~ A~V / K ~ for ) ~ film ~ , ~M2, and fhaC1 > fLlcl for 2.OM feed solution 0
2 .o FEED MOLALITY
1.0
3.0
( D ~ V I K ~ ) ~>, - ~ ( D A M I K ~for) ~films ~ ~M1, , M2, and
Figure 14. Performance data for systems sodium chloride-water and sodium nitrate-water using porous cellulose acetate membranes
and fKCl
M4,
>
for 2.OM, 3.OM, and 4.OM feed solutions for films M1, M2, and M4, respectively
fNaCl
Effect of feed concentration
Table 111. Membrane Performance Data for Different Solution Systems
Film type. CA-NRC-18 Feed concentration. 0.5M Feed rate. 300 cc./minute Operating pressure. 1500 p.s.i.g. Film area. 7.6 sq. cm.
Film M1
KaS04 BaCL CaCL MgCL LiCl NaCl KCl NHdC1 LiNO?
KNO, NafiO? Glycerol Urea
M3
M4
50 Solute Separation, %
1
10
99.9 99.8 99.7 99.4 99.2 99.0 98.9 98.8 98.4 98.2 97.8 97.0 96.5 72.5
99.5 99.1 97.0 95.7 94.8 93.0 93.4 92.0 90.1 90.8 85.0 87.1 85.8 53.4
Solute MgSOa
M2
98.3 96.9 87.1 83.4 81.6 74.3 77.4 73.1 69.0 73.2 61.2 67.1 64.3 35.3
M5
MI
M3
M2
(Da.dK6)x 10' for NaC1, Cm. Sec. 100 200 1 10
97.2 94.7 77.7 72.5 70.3 59.1 64.3 58.8 53.7 60.1 44.9 53.9 50.3
95.2 90.9 61.8 57.3 55.5 40.9 4'7.6 41.6 36.9 43.8
...
38.8
... ...
...
-
36.3 31.0 27.7 26.9 25.2 32.4 33.6 34.4 34.5 32.7 35.6 34.5 38.3 41.3
58.6 51.9 46.1 45.1 41.2 56.7 59.9 62.6 63.2 58.1 66.5 62.9 68.7 78.4
M4
M5
50 100 Product Rate, G./Hr.
200
74.3 71.2 65.9 65.9 59.0 86.8 91.9 98.8 101.0 88.6 107.4 99.0 104.4 124.8
79.8 80.2 78.4 79.9 71.2 107.7 112.9 122.9 126.8 108.6 135.4 122.2 126.4
84.7 89.9 97.7 100.2 88.7 137.3 141.8 155.7 160.4 136.3
...
152.7
... ...
...
~
~~~~
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I & E C PROCESS D E S I G N A N D DEVELOPMENT
( D 4 W ’ K 6 ) Kfor C , film M2, fKc, for 4.0M feed solution (DAW K6)\a\0 for film M1, for 2.OM feed solution (Daw K6).,,CI for films M1, M2, and M4, fLaC, for 2.0 to 4.OM feed solutions
f\a\O
the above data, it seems reasonable to conclude that the order of (DA.v/K6)and that of solute separation with respect to any two solutes must be considered separat,ely. Since the values of ( D a u / K 6 )for different solutes studied above are independent of feed concentration and feed flow rate, their relative values offer a firm basis for expressing membrane selectivity. The order of solute separation is a function not only of the relative values of ( D , A . M , Kfor ~ ) different solutes but also of the operating conditions of pressure, feed concentration, and the mass transfer coefficient, h. Conclusions
This paper illustrates methods for expressing specification, selectivity, and performance of reverse osmosis membranes. The data also illustrate that while specification and selectivity of membranes can be expressed independent of the apparatus used for the reverse osmosis operation, the performance of a given membrane, in terms of solute separation and product rate, cannot be so expressed. The latter needs data on mass transfer coefficientsapplicable for the operating system, which data depend on the apparatus and experimental conditions used. Consequently, it is necessary not only to specify membranes in terms of A and ( D , d K 6 ) but also to give the mass transfer coefficient correlation applicable for the operating system, in order to calculate and/ or compare membrane performance data in reverse osmosis. Acknowledgment
The authors are grateful to A. G. Baxter and Lucien Pageau for their valuable assistance in the progress of these investigations. One of the authors (J.P. A.) thanks the National Research Council of Canada for the award of a postdoctoral fellowship. Nomenclature
pure water permeability constant, g.mole H 2 0 / s q .cm. sec. atm. molar density of bulk solution and the concentrated boundary solution on the high pressure side of the membrane, and the membrane permeated product solution on the atmospheric pressure side of the membrane, respectively, g.moles/ cc. effective diameter of membrane surface, cm. diffusivity of solute, sq. cm./sec. solute transport parameter, cm./ sec. solute separation depth of cell, cm. mass transfer coefficient, cm./sec. solute molality in feed solution solute molality in product solution molecular weight of solute molecular weight of water constant water flux through membrane, g.moles/sq. cm. Reynolds number = Q hu
Schmidt number = u / D Sherwood number = kd D operating pressure, atm. product rate, grams per hour per 7.6 sq. cm. of film area [ P W P J = pure water permeability, grams per hour per 7.6 sq. cm. of film area Q = feed flow rate, cc./sec. T = temperature, O C.
NSc = N,, = P = [PR] =
x41,
I
XA?,
x,i
= mole fraction of solute in bulk solution and
concentrated boundary solution on high pressure side of membrane, and product solution on atmospheric pressure side of membrane, respectively
GREEKLETTERS I.(.
= viscosity of water, centipoises
kinematic viscosity of feed solution, sq. cm./ sec. T ( X A ) = osmotic pressure of solution corresponding solute mole fraction X A , atm. u =
literature Cited
Erickson, D. L., Glater, J., McCutchan, J. W., Ind. Eng. Chem. Prod. Res. Develop. 5 , 205 (1966). Govindan, T. S., Sourirajan, S., IND.E N G .CHEM.PROCESS DESIGNDEVELOP. 5 , 422 (1966). Kimura, S., Sourirajan, S., A.1.Ch.E. J . 13, 497 (1967). Kimura, S., Sourirajan, S., IND. ENG. CHEM. PROCESS DESIGNDEVELOP. 7, 41 (1968a). Kimura, S.,Sourirajan, S., IND. ENG. CHEM. PROCESS 7, 197 (196813). DESIGNDEVELOP. Kimura, S., Sourirajan, S., IND. ENG. CHEM. PROCESS DESIGNDEVELOP.7, 539 ( 1 9 6 8 ~ ) . Loeb, S., Sourirajan, S.,Aduan. Chem. Ser., No. 38, 117 (1963). Loeb, S., Sourirajan, S., U.S. Patent 3,133,132 (May 12, 1964). Ohya, H., Sourirajan, S., A.l.Ch.E. J . (in press) (1969a). Ohya, H., Sourirajan, S.,IND. ENG. CHEM. PROCESS DESIGNDEVELOP. 8, 131 (196913). Robinson, R . A., Stokes, R . H., “Electrolyte Solutions,” 2nd ed., pp. 476-90, Butterworths, London, 1959. Sourirajan, S., Ind. Eng. Chem. Fundamentals 2, 51 (1963). Sourirajan, S., Ind. Eng. Chem. Fundamentals 3, 206 (1964a). Sourirajan, S., J . A p p l . Chem. 14, 506 (196413). Sourirajan, S., in “Water Resources of Canada,” C. E. Dolman, Ed., pp. 154-82, University of Toronto Press, Toronto, 1967. Sourirajan, S., Govindan, T. S., Proceedings of First International Symposium on Water Desalination, Washington, D. C., 1965, U. S. Dept. Interior, Office of Saline Water, Washington, D. C., Vol. 1, pp. 251-74. Sourirajan, S., Kimura, S., IND. ENG. CHEM.PROCESS DESIGNDEVELOP. 6, 504 (1967). Timmermans, J., “Physicochemical Constants of Binary Systems in Concentrated Solutions,” Vol. 111, p. 315, Interscience, blew York, 1960. Tribus, M., Asimow, R., Richardson, N., Gustaldo, C., Elliot, K., Chambers, J., Evans, R., “Thermodynamic and Economic Considerations in the Preparation of Fresh Water from the Sea,” Department of Engineering, University of California, Los Angeles, Rept. 59-34 (1960). RECEIVED for review June 20, 1968 ACCEPTED March 28, 1969 Issued as N.R.C. S o . 10948.
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