performance of porous cellulose acetate membranes during extended

PERFORMANCE OF POROUS CELLULOSE ACETATE MEMBRANES DURING EXTENDED CONTINUOUS OPERATION UNDER PRESSURE I N T H E REVERSE OSMOSIS PROCESS USING AQUEOUS SOLUTIONS S H O J I K I M U R A ‘ A N D S. S O U R I R A J A N

Downloaded by UNIV OF WINNIPEG on September 9, 2015 | http://pubs.acs.org Publication Date: April 1, 1968 | doi: 10.1021/i260026a008

Division of Applied Chemisty, .Vational Research Council, Ottawa, Canada

The experimental separation and product rate data obtained in the extended continuous test runs for the systems glycerol-water and sodium chloride-water show that the solute transport parameter, (Dam/KG), remains unchanged as the pure water permeability constant, A, decreases because of membrane compaction during continuous service under pressure. This observation establishes an experimental basis, and offers an analytical means of predicting solute separation as the product rate decreases because of membrane compaction during continuous operation of the reverse osmosis process. This prediction technique is illustrated for the systems glycerol-water, and sodium chloride-water using a few typical films. HE

service life of the Loeb-Sourirajan type of porous cellulose

T acetate membranes during extended continuous operation under pressure in the reverse osmosis process is discussed in the literature (Loeb, 1966). Lonsdale et al. (1964) conducted a 2-month laboratory test with recycled synthetic sea water. During this period the salt reduction factor dropped from 66 to 12, and the membrane constant decreased from about 8 X 10-6 to 4.5 X 10-6 gram/sq. cm. sec. atm. They also conducted a life test with lOy0 sodium chloride solution, and observed a considerable decrease in the membrane’s salt rejection capacity within about 1 week. Loeb and Manjikian (1964) conducted a 3-month laboratory test with sea tvater at a n operating pressure of 1500 p.s.i.g. The initial steady-state flux was about 4 x gram/sq. cm. sec., and the salt content of the product water was about 500 p.p,m.; after 3 months, gram/sq. cm. sec. and the the flux dropped to about 2 X salt content of the product increased to over 4000 p.p.m. Loeb and Manjikian (1965) also conducted a 6-month field test of the membrane with brackish water at Coalinga, Calif. They found that the flux of desalinized water dropped from 24 X lop4 to 8 X gram/sq. cm. sec. and the salt content of the product increased from 150 to 300 p.p.m. Vos et al. (1966) found that membrane life was strongly dependent on feed water pH, and the above results might probably be attributed to partial hydrolysis of the membrane material. From the point of view of process design, it would be of interest to be able to predict the variations in solute separations as the product rate slows down because of membrane compaction during continuous operation under pressure in the reverse osmosis process using the Loeb-Sourirajan type of porous cellulose acetate membranes. This paper illustrates that the Kimura-Sourirajan analysis (Kimura and Sourirajan, 1967, 1968a; Sourirajan and Kimura, 1967) of the experimental d a t a obtained in the extended continuous test runs for the systems glycerol-water and sodium chloride-water establishes a basis, and offers an analytical means, for such pre1 Present address, Department of Chemical Engineering, University of Tokyo, Tokyo, Japan

diction. This analytical technique has been applied for predicting the performance of a few typical films for the systems glycerol-water and sodium chloride-water, and the results are discussed. Experimental Details

Porous cellulose acetate membranes (designated here as CA-NRC-18 type films) made in the laboratory and reagent grade glycerol and sodium chloride were used. T h e films were cast at -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 = 39.8%) 17.0, water 13.5, and magnesium perchlorate 1.5. T h e film details, the apparatus, and the experimental procedure have been reported (Sourirajan, 1964; Sourirajan and Govindan, 1965). Membranes shrunk at different temperatures were used to give different levels of solute separation at a given set of operating conditions. T h e aqueous glycerol solution (feed) 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, and were carried out at the laboratory temperature. The reported product rates are those corrected to 25’ C. using the relative viscosity and density data for pure water. I n most of the experiments, the feed rates were maintained in the range 250 to 580 cc. per minute. In general, the solute separation, f,defined as molality of feed ( m l ) - molality of product (m3) molality of feed ( m l ) the product rate, [PR], and the pure water permeability, [PWP], in grams per hour per 7.6 sq. cm. of effective film area, were determined for each experiment at the preset operating conditions. I n all cases, the terms “product” and “product rate” refer to the membrane-permeated solu-

f=

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Table 1.

Osmotic Pressure, Molar Density, Kinematic Viscosity, and Diffusivity Data for the System Sodium Chloride-Water at 25' C.

NaCl Concentration Mole fraction


Molality 0 0.1 0.2 0.3 0.4 0.5 0.6 0.7 0.8 0.9 1 .o 1.2 1.4 1.6 1.8 2.0 2.2 2.4 2.6 2.8 3.0 3.2 3.4 3.6 3.8 4.0 4.2 4.4 4.6 4.8 5 .O 5.2 5.4 5.6 5.8 6.0

x

103

0 1.798 3.590 5.375 7.154 8.927 10.693 12.453 14.207 15.955 17.696 21.160 24.600 28.016 31.408 34.777 38.122 41 .444 44.743 48.019 51.273 54.505 57.715 60.903 64.070 67.216 70.340 73.443 76.526 79.589 82.631 85.653 88.655 91.638 94.601 97.545

Weight

%

Osmotic Pressure,

0 0.58 1.16 1.72 2.28 2.84 3.39 3.93 4.47 5.00 5.52 6.55 7.56 8.55 9.52 10.47 11.39 12.30 13.19 14.06 14.92 15.76 16.58 17.38 18.17 18.95 19.71 20.46 21.19 21.91 22.62 23.31 23.99 24.66 25.32 25.96

P.S.I.

x

0 67 133 199 264 331 398 466 534 603 673 814 959 1109 1262 1419 1580 1745 1915 2089 2270 2453 2641 2834 3034 3238 3446 3661 3879 4104 4333 4568 4807 5054 5304 5560

5.535 5.535 5.535 5.535 5.534 5.534 5.534 5.534 5.533 5.532 5.530 5.526 5.526 5.526 5.524 5.521 5.517 5.515 5.512 5.507 5.504 5.500 5.497 5,492 5.488 5.484 5.479 5.475 5.472 5.467 5.463 5.458 5.453 5.447 5.443 5.438

tions. Unless otherwise specified, the [ P R ] and [PWP] data refer to 7.6 sq. cm. of film area. The concentrations of the solute in the feed and product solutions were determined by refractive index measurements using a Bausch & Lomb refractometer or by specific resistance measurements using a conductivity cell. T h e accuracy of the separation data is within 1%, and that of [PR] and [PWP] data is within 3% in all cases. Results and Discussion

Osmotic Pressure, Molar Density, Kinematic Viscosity, and Diffusivity Data for the System Sodium ChlorideWater. Table I gives the osmotic pressures, A, molar densities, c, kinematic viscosities, Y , and solute diffusivities, D, of aqueous sodium chloride solutions at 25' C. in theconcentration range 0.1 to 6.0 molal. T h e osmotic pressures were calculated from the relation (Robinson and Stokes, 1959a)

The osmotic coefficient, @, diffusivity, D ,density, and viscosity data were taken from the literature (Robinson and Stokes, 1959b, 1 9 5 9 ~ ; International Critical Tables, 1928, 1929). Similar data for the system glycerol-water have been reported (Sourirajan and Kimura, 1967). Basic Equations and Correlations. T h e Kimura-Sourirajan analysis gives rise to the following basic equations relating the pure water permeability constant, A , the transport of solvent water, N E ,the solute transport parameter, (DAM/Ka), and the mass transfer coefficient, k : 198

l & E C PROCESS D E S I G N A N D D E V E L O P M E N T

Molar Density, MoleslCc.

A =

102

Kinematic Viscosity, Sq. Cm./Sec.

x

102

0.8963 0.9009 0,9054 0.9100 0.9147 0.9193 0.9242 0.9290 0.9338 0.9389 0,9440 0,9567 0,9685 0.9802 0.9923 1 ,0044 1 ,0206 1.0365 1 ,0523 1.0683 1.0840 1.1047 1,1252 1,1457 1.1660 1.1862 1.2108 1.2350 1.2591 1.2832 1.3070

D,

Sq. Cm./Sec.

x

106

1.610 1.483 1.475 1.475 1.475 1.475 1.475 1.475 1.477 1.480 1.483 1.483 1.492 1.497 1.505 1.513 1.521 1.530 1.539 1.548 1.556 1.565 1.570 1.575 1.580 1.585 1.589 1.594 1.593 1.593 1.592

PWPI M , X 7.6 X 3600 X P

(5) Equations 3,4, and 5 have been derived (Kimura and Sourirajan, 1967) ; they follow from a straightforward application of film theory and a simple pore diffusion model. T h e parameter (DAM/K6)plays the role of a mass transfer coefficient with respect to solute transport through the membrane; hence it is treated as a single quantity for purposes of analysis. I t must, however, be understood that (DAM/K6)is not a single factor, but is a combination of several interrelated factors, none of which is, or need be, precisely known for chemical engineering calculations based on the above analysis. When it can be assumed that the molar density, c, of the solution remains essentially constant, Equations 4 and 5 become identical to those given earlier (Sourirajan and Kimura, 1967). From the [PWP], [PR], and f data, thevalues and k can be calculated for every experiment. of A , (DAIW/K6), Kimura and Sourirajan (1967, 1968a) and Sourirajan and Kimura (1967) have shown that the values of the interconnected parameters A and (DA.,,/K6)specify a particular membrane-solution system with reference to this separation process. Both A and (DA.M/K6)are dependent on the porous structure


tinuous operation under pressure in the reverse osmosis process, the dense microporous surface structure of the film may be expected to remain intact, but the spongy porous structure of the interior of the film will become more densely compacted, T h e result is an increase in resistance to fluid flow and a decrease in A . I t is of practical interest to determine the re) A decreases entirely lationship between A and ( D A M / K 8 when because of membrane compaction during continuous operation under pressure. This was done experimentally from the extended continuous test run data for the systems glycerolwater and sodium chloride-water. Experimental Results with the System Glycerol-Water. A 30-day-long continuous test run was conducted with the system glycerol-water a t a n operating pressure of 1500 p.s.i.g. using five different films (87, 88, 89, 90, and 91) covering a wide range (51 to 98%) of solute separations. The feed concentrations were maintained in the range 0.50 to 0.52M, and the feed flow rates in the range 580 to 640 cc. per minute during the run. T h e initial characteristics of the films used are specified in Table I1 in terms of A and (DAlf/K6). During the continuous extended operation under pressure both product rate and solute separation changed with time. [PR] values decreased rapidly during the first 24 hours, and slowly thereafter. T h e values off also decreased with decrease in [PR]. A set of typical data is given in Table 111. At different time intervals during the test run the experimental [PR] and f values were determined for each film; the corresponding feed concentrations and feed flow rates were also noted. From the feed concentration, [PR], and f data, Ns, X A 1 , and X A , can be determined; the mass transfer coefficient applicable for the conditions of the experiment can be sbtained from the correlation of the experimental k data given earlier as a function of feed flow rate and feed concentration (Souriraj a n and Kimura, 1967). Therefore, by using Equations 3, 4, and 5, one can calculate the values of A and (DAM/K6)corresponding to every set of [PR] a n d f data obtained during the continuous test run. These calculations were performed with the experimental data obtained with films 87, 88, 89, 90, and 91 during the 30-day-long test run (Figure 1). T h e results showed that even though the values of A decreased signifTable It. Specifications of Porous Cellulose Acetate icantly during the test period (finally reaching about 57% Membranes Used of the original value for film 87 and 137, of the original value A X 106, Film for film 91), (DAM/K6)remained essentially constant throughG. Mole Shrinkout for each film tested. HzO Operating (D A . M / ~ ~ ) age Experimental Results with the System Sodium ChloridePressure, Film Temp., Sq. Cm. x 705, P.S.I.G. 2VO. c. Solute See. Atm. Cm./Sec. Water. A continuous test run extending for 72 hours was 500 1.142 1 90 NaCl 0.90 conducted with the system sodium chloride-water a t a n 1500 0.97 1 0.90 90 NaCl 88 NaCl 2 500 1.754 6.00 operating pressure of 1500 p.s.i.g. using six different films (94, 2 1500 1.46 6.00 88 NaCl 2,329 5 00 3 86 NaCl 23.37 1500 3 1.87 86 NaCl 20.00 4 500 3,086 84 NaCl 97.43 5 500 3,246 117.1 82 NaCl Table 111. Variation of Solute Separation with Product Rate 2.37 82 5 1500 75.0 NaCl in Reverse Osmosis during Continuous Operation under 1500 2.93 80 NaCl 6 140.0 Pressure for the System Glycerol-Water 600 3.773 87 NaCl F1 18.67 F2 600 2,670 88 NaCl 4.423 Film type CA-NRC-18 1500 1.295 90 NaCl F3 0.891 Feed molality 0.5 molal NaCl 600 F4 2.670 89 5.825 Feed rate 580 cc./minute Glycerol 1500 85 0.6736 92.5 0.918 Operating pressure 1500 p.s.i.g. 1500 87 0.6674 92 Glycerol 1.896 88.5 1500 88 1.4417 Glycerol 10,112 Values after Several Days 1500 2.4491 85 Glycerol 89 61.924 Initial Values of Continuous Operation 1500 1.8909 86.5 Glycerol 90 25.311 Film [PRI 9 [PRI 9 82 1500 3.1579 Glycerol 91 154,434 N o* g./hr. f g.,Jhr. f 80 1500 1.873 NaCl 94 55.13 1500 105 86 NaCl 1.494 1.832 87 28.39 0.978 16.50 0.971 80.5 1500 106 NaCl 2.962 41.60 88 58.57 0.930 25.50 0.890 79.5 1500 107 3.215 NaCl 60.58 90 75.51 0.855 22.07 0.745 108 78.5 NaCl 1500 5.356 476.9 89 97.19 0.721 20.32 0.490 109 78.5 NaCl 1500 4.747 335.8 91 127.45 0.510 30.67 0.348

of the membrane surface and hence they are different for different membranes. At a given operating pressure, the values of (DA,f/’K6) remain essentially constant for a wide range of feed concentrations and feed flow rates for the system glycerol-water, as well as for systems involving inorganic salts in aqueous solution, such as NaCl-H20. Further, the mass transfer coefficient, k , for all the above systems is essentially a function of feed flow rate and feed concentration, and the values of k are well correlated by a generalized log-log plot of IV,, us. iVsh/NSc0.33. On the basis of the above equations and correlations, a single set of experimental [PWP], [PR], and f data a t any operating pressure specifies a film in terms of A and (DAv/K6) which enable the prediction of both solute separation and product rate at that pressure for all feed concentrations and feed flow rates for which (DA.M/K6)remains constant. Membrane Specifications. Table I1 gives the specifications of all membranes used in this work in terms of A and (DAbf/K6) for the given solute at the specified operating pressure. These specifications were based on the data obtained from short-run experiments, and represent the initial characteristics of the film before being subjected to continuous pressure for extended periods of time. For information, the table also includes the temperatures at which the membranes were shrunk prior to use in the experiments; these temperatures have no precise significance from the point of view of membrane specification. Relationships between A and (DAlf/K6). O n the basis of the preferential sorption-capillary flow mechanism (Sourirajan, 1963, 1967) it is clear that when the variations in A are due to changes in the average pore size on the membrane surface, the values of (DAIr/K6)should be extremely sensitive to changes in those of A . This is confirmed by the experimental observations. I t has been shown (Kimura and Sourirajan, 1967; Sourirajan and Kimura, 1967) that, with respect to the type of films used in this work, ( D A j I / K 6 )for sodium chloride is proportional to A3.5, and (DA,lf/k6) for glycerol is proportional to A 3 . 3 5 . During conditions of extended con-

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IO-^ I

DAM

x C .?

sec


SYSTEM : GLYCEROL-WATER FEEO M O L A L I T Y : 0.50 10 0.52 FEEO RATE: 580 to 6 4 0 c d m i n .

10-5

).3

0.5

-

0.8 1.0

1.5 2.0

3.0 4.0

A X IO6 -

g.rnole H 2 0 cmT s e c . o t m .

Figure 1. Variation of (DAM/KG) with decrease in A during 30-day-long continuous test runs with the system glycerol-water

105, 106, 107, 108, and 109) covering a wide range (39.5 to 98.3%) of solute separations. T h e feed concentration was maintained at about 0.5 to 1.0 molal, and the feed flow rate was in the range 426 to 562 cc. per minute. The initial

Table IV.

Variation of ( D A M / K ~with ) Decrease in A in Reverse Osmosis during Continuous Operation under Pressure with the System Sodium Chloride-Water

Film type Operating Film No.

lQ5

106

94 107 109

108

200

characteristics of the films used are specified in Table I1 in terms of A and (DAwIKa). As in the experiments with the system glycerol-water, both [PR] and f changed with time during the progress of the run. At different intervals during the test run, the experimental [PR] and f values were determined for each film. T h e generalized mass transfer correlation obtained by the diffusion current method had already been shown to be applicable for the system sodium chloride-water. Using the above correlation for obtaining the k values, and Equations 3, 4, and 5 , the values of A and (DAM/K6)were calculated for every set of [PR] and f data obtained during the continuous test run (Table IV). The experimental data for films 105 and 106 (which gave comparatively higher levels of solute separation) show that even a 40% change in the value of (DAwlK6) corresponds to less than 1% change in solute separation; similarly the data for films 109 and 108 (which gave comparatively lower levels of solute separation) show that even a n 8% change in the value of ( D A M / K 6 )corresponds to less than 1% change in solute separation. Hence, it is reasonable to conclude that within the limits of experimental error, (DAM/K6)values remained constant for each film throughout the test period. Thus the data presented in Table IV confirm that the observation that (DAIw/K8)remains constant as A decreases because of membrane compaction during extended continuous service under pressure, is also valid for the system sodium chloridewater. Probable General Validity of the Principle. For systems such as glycerol-water and sodium chloride-water, the solute transport parameter, (DAM/K6), is independent o f XA2, which is the mole fraction of solute in the concentrated boundary solution on the high pressure side of the membrane (Kimura and Sourirajan, 1967; Sourirajan and Kimura, 1967) ; the foregoing results show that this relationship between (DA.w/K6) and XA2 for the systems glycerol-water and sodium chloridewater remains unaffected during membrane compaction.

A Y 106, G. Mole HzO Sq. Cm. Sec. Atm.

(B*) x 106, Cm./Sec.

1.494 0.980 0.885 0.820 0,803 2.962 1.914 1.766 1.754 1.873 1.485 3.215 1.921 1.891 4.747 3.120 3.080 2.842 5.536 3.597 3.513 3.261

2.000 1.708 1.895 1.784 1.774 44.37 40.46 40.63 40.93 56.05 54.21 62.98 59.39 59.36 347.5 328.7 321.9 345 .O 488, G 460.2 457.7 500.9

CA-NRC-18 1500 p.s.i.g.

Feed Molality

Feed Rate, Cc./Min.

[PRI, G./Hr.

f x 702

1 ,0075 1 .0070

564 556 426 426 426 563 545 546 542 487 492 562 54s 541 558 542 552 517 561 543 554 544

35.97 24.89 22.19 20.73 20.61 72.98 53.39 50.09 49.87 72.67 5!. 66 82.22 56.38 55.69 146.25 109.11 107.55 102.80 174.37 130.83 128.37 124.56

98.3 98.0 97.5 97.5 97.5 82.1 79.8 78.9 78.7 78.1 74.0 77.8 73.3 73.5 44.4 42.4 42.9 40.4 39.5 37.9 38.0 34.5

1.0150 1.0150 1.0000 1 ,0075 1 ,0025 1 ,0075 1.0075 0.5030 0.4800 1 ,0075 1,0075 1 ,0075 1 ,0075 1.0025 1,0075 1 .0075 1 ,0075 1.0025 1.0075 1.0075

l&EC PROCESS DESIGN AND DEVELOPMENT

[PR]Final [ P R ] Initial

0.573

0.683 0.807 0.677

0.703

0.714

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TIME IN DAYS I o -

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FILM TYPE : CA-NRC-18 SYSTEM :GLYCEROL-WATER FEED MOLALITY: 0.5 M FEED RATE: 580 c o r n i n . OPERATING PRESSURE: 1500 p.s i.g

I20 91

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CALCULATED DATA

100

Q 0

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0.9

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0.4

0.3

0.2

0.1

A FACTOR Figure 2. Experimental and calculated separation data corresponding to product rates obtained during extended continuous test runs with the system glycerol-water

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Kimura and Sourirajan (1968b) have shown that for the system sucrose-water, (DA,/Kb) is a particular function of XA2, and again, this relationship is unaffected during membrane compaction. That the relationship between (DA.,

performance of porous cellulose acetate membranes during extended

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