High flux cellulose acetate ultrafiltration membranes

Desahkation, 26 (1978) 195-210

@

Ehevier ScientificPublishingCompany, Amsterdam - Printedin The Netherlands

HIGHFLUXCELLULOSEACETATEULTRAFILTRATIONMEMBRANES* 0.

KUTOWY, W. L..T&IA=

AND

S.

souR&kJ~

Division of Chemistry National Research Council of Canada, Ottawa (Canada) KIA OR9

(ReceivedNovember 15, 1977; in revised form April 7, 1978)

SUMhlARY

Conditions for casting high flux tubular and flat cellulose acetate ultrafiltration membranes are given. The usefulness of some of the tubular membranes cast under the above conditions fcr ultrafiltration applications in industrial water purification and reuse is illustrated Using ethanol-water mixture as the gelation medium in the temperature range -20° to 30°C, ultrafiltration membranes giving water fluxes in the range t4 to >400 m3/m2 day at the operating pressure of 689.5 x lo3 Pa (100 psig) have been obtained. LNlRODUCllON High flux ultrafiltration membranes are of practical industrial interest. This paper extends the work reported earlier (I) on cellulose acetate ultrafiltration membranes. It has been shown that the solution structure-evaporation rate approach to membrane development for reverse osmosis (2) is generally applicable for the development of cellulose acetate ultrafiltration membranes; decrease in (N/S) ratio, and solvent/polymer (S/P) ratio, increas c: in nonsolvent/solvent increase in nonsolvent/polymer (N/P) ratio in the casting solution composition tend to increase the size of pores, and increase in the temperature of the casting solution tends to increase the effective number of pores on the surface of resulting membranes in the as-cast condition. Further recent studies have shown (3) that ethanol-water gelation medium at different temperatures is particularly effective for the creation of high flux cellulose acetate ultrafiltration membranes_ The foregoing results indicate that by appropriate choice of casting solution composition and control of film casting and gelation conditions, high flux cellulose acetate ultrafiltration membranes useful for many industrial applications can be developed. This possibility is illustrated in this work. COhfPOSITION

OF FILM CASTING

SOLUTION

The composition (weight per cent) of the film casting solution used for all membranes reported in this paper was as follows: l

Issued as N.R.C. No. 16877.

196

0.

Cellulose acetate (Eastman Acetone: Water: Magnesium perchlorate:

KUTOWY,

grade 400-25):

W.

L. THAYER

AND

S. SOURIRAJAN

14.8 63.0 19.9 2.3

From the correlations of S/P, N/S and N/P with data on NaCI separation and pure water permeation rate (PWP) established earlier (Fig. 3 in reference I), it is evident that the above composition for film casting solution is particularly suitable for making ultrafiltration membranes. MEMBRANE

FORMS

Membranes were made both in tubular and flat forms. The tubular membranes were made using the apparatus described earlier (4) in which the casting tube was held stationary and the casting bob moved upwards at controlled speed during film casting. A few tubular membranes (identified in Table III with an asterisk added to membrane number) were also made by using essentially the same apparatus but by keeping the casting bob stationary and moving the casting tube down at controlled speed along a fixed guide using a spring loaded rubber wheel. The latter modification in the film casting machine is shown in Fig. 1. The flat

Fig. 1 Modification of film casting apparatus for controlled downward movement of casting tube.

60 min

1.343

(33)

(35.8)

85

1.457

71

(26.9)

1,095

62

per~rntancc i/r reverse osmosis

15 min

(40)

1.628

89

GOmin

6 air flow rate = 14 %*10-O m3/s;b composition by volume:e see Table III.

sepn. % Water ffux ma/maday (~l/ft~day)

Solute

Membrm

1°C

60 min

Gclation bath” Temp. of gelation bat11 G&ion time

air bubbled air bubbled air through SO/SO through SO/SO bubbled acetone/water acctonc/watcr through solution acetone sotution water S’)6acetonc- water water 1°C 1°C 8°C

30.5cm

air bubbled through 50/50 aectone/water solution water

3OScm

30.5cm

15.2cm

T4

Length of solvent cvapn. zone Atmosphcrc of solvent evapn. zon&

T3

Filrtl rrrcrnbcr

cusllng

DATA ON TUllULAR MEMDRANES

TABLE I

38.1cm

T6

(90)

3.663

28

10min

(107)

4.355

11

IO min

nir bubbled air through SO/SO a~ton~/water solution 33% EtOH33% EtOHwater WiltCr 0°C 0°C

38.1cm

TS

(200)

8.140

1

10 min

very high”

-0

10 min

air

38.1 cm

T8

309 cm

l-10

very high0

NO

10min

air bubbled through acetone 500/,EtOH- 50% EtOH- !W&EtOHwntcr water water 0°C 20°C 20°C

nir

38.1cm

T7

E %

% ” 8

d

$

r

5

i

?

F2

Solute

1“C

21“c!

(71.6)

(35.8)

pIconlpositionby volume; b see Fig, 2.

2.914

70

1.457

85

(58)

2.361

75

(89)

3.622

20

33% EtOHwater -15°C

20% EtOH-

water 7°C

air

F4

air

Metttbraneperforttlattce Ct reverse osntosis

wnter

water

G&ion bath& Temp. of gclation bath

air

air

sepn. y0 Water ilux ma/m?day (gal/fteday)

F3

Otlrer film casfittg cottditlotts

FI

Film trrttttbet

Atmosphere of solvent cvnpn. zoneL

Caslittg attd per~omtattce details

DATA ON FLAT MEMIIRANES

TABLE II

(100)

4.07

15.5

33% EtOHwater 15%

air

M

very high”

NO

21“C

air equilibrated with SO/50 acetonc/watcr solution water

F6

(134)

5.454

17

1°C

WillCC

air cquilibratcd with SO/SO acctonclwatcr solution

f7

f

I

8

?

P

k!

6 r

2

G

8

200 ILLUSTRATIVE

0.

RESULTS

KUTOWY,

W. L. THAYER

AND

S. SOURIRAJAN

ON APPLICATIONS

The tubular membranes T5, T6, T7, T8 and TlO (Table I) were tested for their possible application in the ultrafiltration treatment of machine shop oily wastewaters, water containing potato starch in suspension, water containing suspended clay particles and water containing coal tailings. The oily wastewaters (supplied by Electrohome Ltd., Kitchener, Ontario) contained emulsified cutting oils and coolants commonly used in automotive industry. Before and after ultrafiltration, the organic matter in the oily water was ether extracted and determined ~avimetrieally. The suspended matter in the starch-, clay-, or coal-laden wastewaters was quantitatively determined by turbidity measurement using a HF Instrument Model DRT 100 turbidity meter. The ultrafiltration experiments were carried out in the operating pressure range 69.8 x 10’ to 172.4 x lo3 Pa (10 to 25 psig) and temperature range 17 o to 65 “C. The results obtained, given in Table III, illustrate the practical utility of the tubular ultrafiltration membranes made in this work for industrial water purification and water reuse. EFFECT OF ETHANOL-WATER

MIXTURE

AS GELATION

MEDIUM

In view of the effectiveness of ethanol-water mixture as gelation medium for creating membranes with relatively bigger size surface pores (ultrafiltration membranes), a detailed study was made on the effect of composition and temperature of the above gelation medium on reverse osmosis performance of resulting membranes. The rest of this paper is concerned with this study for which purpose flat membranes were used. The films were made under the following casting conditions. Temperature of casting solution: 22°C (room temp.) Temperature of solvent evaporation atmosphere: 22°C Solvent evaporation atmosphere: ambient air, relative humidity, 85-90 % Solvent evaporation time: t5 s Gelation bath: water, or ethyl alcohol + water mixture, or absolute ethyl alcohol as indicated Temperature of gelation bath: -20°C to 30°C as indicated Gelation time: 10 min Leaching bath: water at 1 “C Leaching time: 1 h The films (without any prior shrinkage) were tested as before at 689.5 x lo3 Pa gauge pressure (100 psig) at laboratory temperature (22°C to 25°C) using 300 ppm MgSO,+-Hz0 feed solution at a feed flow rate of 500 x lo- ’ m3/min. The results obtained are given in Fig. 2. With pure water gelation medium, solute separations increased from 30% to 90 %, and the corresponding product rates decreased from 4.274 to 0.855 m3/m2

(20) 25 25

137.9 172.4 172,4

T6

Cut~insoil en~~llsion #2 oil content N 10% increasedslightly during the experiment. pw = 6.5

(10) (20) (20) (20) (20) (20) (20) (20)

(20)

13709

68.9 137.9 137.9 137.9 137.9 137.9 137.9 137.9

(201

137.9

Cutting oil emulsion# 1 oil content: 1.2 wt.% 10.0wt*(j$ 24.0 wt.% Pure water permeationrate

cm Go)

137.9 137.9

Cutting oil emulsion#2: oil concn. w 10% by wt.; concentrate and membrane permeated product recycled. $1 = 6.5 Cufting oil emulsion#2: initial oil concn. N 12% by wt,; final oi1 concn. ~24% by wt.; pH = 6.5 Pure water p~rnI~lion ram after soap cleaning 100ppm MES~~-~~~

(201 (20)

137.9 137‘9

Pure water permeationrate

TS

45 62 46 17 34 33 48 65

30 64

34

29

34

26 49

25 34

ULTRAFlLTRA’llON APPLlCATiONf OF YUDULAR MEhi,1BRANES FOR WATER PURIFICATION AND RLUSE

TABLE 111:

90 90 90

95 96 98 -

13.7 11.9

-

99

99

9s 95

-

0.472 1.034 0.830 0,659 0.944 0.745 1.038 0.961

0.549 0.952

0,928

0.867 0.696

1,176

0.606 0.928 0.790

(11.6) (25.4) (20.4) (1642) (23.2) (18.3) (25.5) (236)

(13.5) (23.4)

(22.8)

(21.3) (17.1)

(28+9)

Cutting oil emulsion # I, 7.57 X 1O-3m3 (20 gal) concentrated to 3.41 x 10-a m3 (9 enl). oil content 0.32 wt.% 0.5 wt.% 0064 wt.% 1404 wt?/ 28.8 wt.% 36.4 wt.% 45 wt.% 48 wt.% 50 wt.% 18 h run; feed flow mtc, 4.163 rn3(1100 gal)/h. Pure water permeation rate

T8

Water containing 5 wt,% Alberta clay (bcntonitc)

Water containing 3 wt.% potato starch in suspension

Pure water permeation rate

T7

Details of j&l solrrtiorr

75.8 103.4 137.9 75.8 103.4 137.9 75.8 103.4 144.8

137.9 137.9 137.9 137.9 137.9 137.9 137.9 137.9 137.9

137.9 137.9 137.9

Pa X lo3

Operalirtg gauge pressure

(20) (11) 05) (21)

‘(1:;

(11) (15) (20)

Ii;

(20)

f&l

3,232 4.803 6.431 3.215 4.579 5.881 3.716 5.230 6.866

loo 100 100 100 100 100 38 40 40 40 40 40 44 48 50

0.655 1,372 1.322 0.724 1.136 0.684 1.172 0.626 a.224 93 93 93 9408 97.0 98.2 99.3 99.9 100

29 59 58 32 62 30 53 59 49

Waler flax niJ/m2ffay

0,554 0.810 1,408 6

Seprr. of sol~rfeor saspetrded matter, O/4

15 30 60

Operalirrs tearp. “C

(16.1) (33.7) (32.5) (17.8) (27.9) (16.8) (28.8) (15.4) (5.5)

(I 3.6) (19-9) (34.6)

fsaWadO)

3 r

3

!

P

Water slurry consistingof coal tailingsmade up of 50% con1and 50% ash; solids content (wt.%) in f&d: 0 10 20 30 35 40 45 50

3 wt.%

200 grams/l.892 x lo+ mrJ(5 gal)

137.9 137#9 137.9 137.9 137.9 137.9

13709

13709

137.9

137.9

75.8 103.4 137.9 75.7 103.4

(11)

7508 103.4 137.9

g; (20) (20) w-9

(20) (20) (20)

11 15 20 11 15 20 20

(15) (20)

(PSk)

Pa X 103

Operulitg gauge pressure

30 30 30 30 30 30 30 30

40 37 37 40 41 41 40

40 40 40

Operatitrg tertIp* “C

100 100 100 100 100 100 100

100 100 100 100 100 100 100

-

Seprr.of solrrteor stspettded matter, o/o

7.326 6.838 6.593 6.227 6.064 5.698 4.640 2.523

* 1.156 1.636 2.165 1.193 1.701 2.320 2.198

1.254 1.856 2.552

n13/tt+day

Water flux ’

(153) (149) (140) (114) (62)

(180) (168) (162)

(28.5) (40.2) (53.2) (29.3) (41.8) (57.0) (54.0)

(30.8) (45.6) (62.7)

(saUSzday)

Note 1. All solute separation and wrttcrflux dota wcrc taken at essentiallysteady flux experimentalconditions. Note. 2. Solute separation data for cutting oil feed solution representovcrnll separation of ail orgnnicswhich included some low molcculur weight emulsifyingngcntsand rust inhibitors.

TlO

Pure water permeation rate

T8’

Water containing potato starch in suspension,starch content: 100 grams/l.892 x 10-am3 (5 gal)

Derails of feed sohtiott

Metttbratte nranber

TABLE III (confirrrred)

204

0. KUTOWY,

W. L. ‘IBAYER AND S. SOURIRAJAN

BATH TEMP.

1

0 I. 0

.

I

hwi_F~*&i& I. 45

I 66

I

*

&k *

I 63 5

ETHYL ALCOHOL (before

I

I

b&L *

I 93

mlxhg)

I

#

I

4i-~o I 100

I

Vol. %

Fig. 2. Effect of temperature and ethyl alcohol concentration in the gelation medium on membrane performance. Operating pressure, 689.5 x 103 Pa (100 psig); feed solution, 300 ppm MgSOtI&O. 1 gal/f@ day = 0.0407 ms/m2 day.

day (IO5 to 21 gaI/ft2 day) with increase in the temperature of the gelation bath from 0” to 30°C. These results showed that for the particular composition of the casting solution studied, an increase in the temperature of the gelation water tended to favor the formation of smaller size pores on the surface of resulting membrane. The same tendency was observed earlier to a much less pronounced extent with membranes made from cellulose acetate-acetone-formamide casting solution (3)_ These results however do not necessarily contradict those of Kesting et al. (6) who reported that an increase in the temperature of gelation water tended to favor the formation of bigger size pores on the surface of resulting membrane. In Kesting’s membrane making procedure, the cast film was partially evaporated before immersion in the gelation medium; the shortest evaporation period reported

HIGH

FLUX

CA ULTRAFILTRATION

hlEhU%RANES

205

in their work was 1 min. Further, their casting solution composition was also different. In the membrane making procedure used in this work, the lilm was immersed in the gel&ion medium immediately after casting. Consequently, the rate of solvent removal from the film surface during film formation and develop ment, was controlled by the temperature of the gelation medium. Further, during this solvent removal step, the temperature of the casting solution on the film surface was closer to that of the gelation medium. The smaller average size of pores on the surface of resulting membrane obtained with increase in the temperature of the gelation medium is understandable on the basis that the temperature of the casting solution on the film surface during membrane formation and development was higher, and hence the size of the supermolecular polymer aggregates in the casting solution on the film surface was smaller and also the solvent removal rate during film formation and development was higher (7). l&sting’s conclusion (6) that ultragel tends to change to microgel with increase in the temperature of the gelation medium may still be valid with respect to the membrane structure underneath t&e surface layer. The data on the effect of ethyl alcohol concentration in the gelation medium on the reverse osmosis performance of the resulting membranes are similar to those obtained with the other casting solution compositions reported earlier (3). Fig. 2 shows that at each temperature of the gelation bath, the product rate data at the test pressure (689.5 x lo3 Pa (100 psig)) fall in at least four distinct regions. For example, at the gelation temperature of O”C, in region 1, the product rate decreased from 4.884 to 1.302 m”/m’ day (120 to 32 gal/f? day) and the corresponding solute separation increased from 30 to 85% with increase in alcohol concentration in the gelation medium from 0 to X,,, = 0.1 where XEtoHrepresents the mole fraction of ethyl alcohol in the gelation medium. In region 2, the product rate increased from 1.302 to 8.140 m3/mZ day (32 to 200 gal/f? day) and the corresponding solute separation decreased from 850,; to practically zero with increase in alcohol concentration in the gelation medium from X,,,, = 0.1 to -0.23. In region 3, the product rate decreased from 8.140 to 4.070 m3/m2 day (200 to 100 gal/ft’ day) with further increase in alcohol concentration in the = 0.23 to 0.52; in region 4, product rate again gelation medium from X,,u increased from 4.070 to 18.32 m3/m2 day (100 to 450 gal/ft2 day) with still further increase in alcohol concentration in the gelation medium from X,,,, = 0.52 to 1.O. In both the regions 3 and 4, the corresponding solute separations for magnesium sulfate were practically zero. At the gelation temperatures of 10°C and 20°C the changes in product rate and solute separation data were simi!ar in direction but difherent in magnitude to those obtained at the gelation temperature of 0°C. At the gelation temperature of 3O”C, in region 1, the product rate decreased from 0.855 to 0.488 m3/m2 day (21 to 12 gal/ft’ day) and the corresponding solute separation also decreased from 90 to 83% with increase in alcohol concentration

206

0.

KU-TOW,

W. L. TEiAYER AND S. SOUFCIRAJAN

in the gelation medium from 0 to XE,on = 0.04. In region 2, the product rate increased from 0.488 to 48.84 m3/m2 day (12 to 1200 gal/ft2 day) and the corresponding solute separation decreased from 83 aA to practically zero with increase in alcohol concentration in the gelation medium from XEtOH= 0.04 to 0.24. In region 3, the product rate decreased from 48.84 to 22.39 m3/mz day (1200 to 550 gai/ft2 day) with further increase in alcohol concentration in the gel&ion medium from XEIOH= 0.24 to 0.50; in region 4, product rate again increased from 22.39 to ~407 m3/mz day (550 to > 10,OQOgal/f? day) with still further increase in alcohol concentration in the geiation medium from XEtOH= 0.50 to 1.O. Again, in both the regions 3 and 4, the corresponding solute separations for magnesium sulfate were practically zero. Referring to Fig. 2, at the gelation temperature of -2O”C, the data on the effect of ethyl alcohol concentration in the gelation medium on the performance of resuhing membranes are similar to those obtained at the higher gelation temperatends to decrease tures up to XEtOH = 0.W above which an increase in X,,,, product rate indicating the possible existence of a region 5 in terms of correlations described above. Carter et ai. (8) used a ceMose acetate-aceton~formamide casting solution and ethyl alcohol concentrations up to 8.7 molar (XEIOH = 0.228) in the gelation medium at 20 “C. Under the conditions of their experiments, product rate decreased

and the corresponding solute separation increased with increase in alcohol concentration- These results are qualitatively similar to those obtained in region 1 in Fig. 2. In addition, the present work shows that by using higher alcohol concentrations in the gelation medium, membrane performance data similar to those given for regions 2,3,4 and 5 in Fig. 2 can be obtained for different casting solution compositions and gelation temperatures. The decrease in prcduct rate with corresponding increase in solute separation obtained in region 1 at the gelation temperatures of O”, 10” and 20°C is also qualitatively similar to the changes observed by both Zisner and Loeb (9) and Frommer et aZ. (10). Since water is the predominant nonsolvent precipitating agent in the gelation medium in region I, foliowing Frommer et af. (ZO), the above

changes may be attributed to the decrease in water activity with increase in alcohol concentration in the geIation medium and the consequent decrease in the rate of water penetration in the membrane. The latter decrease results in finer precipitation of polymer material constituting the membrane surface and hence smaller average pore size on the membrane surface. On the other hand, an increase in the temperature of the gefation medium increases the rate of penetration of water and also that of alcohol into the membrane during its formation and development; it is reasonabIe to expect (see discussion below) that these two rates will have opposing effects on the rate of precipitation of the polymer during fIIm formation, and hence the resulting average size of pores on the membrane surface. The observed increase in the average size of pores on the membrane surface with increase in alcohol

HIGH FLUX CA ULTRAFWTRATION

h¶EhlBRANES

207

concentration in region 1 at the gelation temperature of 30 “C is understandable on this basis. As the alcohol concentration in the gelation medium increases further into the regions beyond region 1 in Fig. 2, the polymer precipitating power of alcohol and its interactions with the polymer (cellulose acetate), solvent (acetone) and the nonsolvent swelling agent (aqueous magnesium perchlorate) in the membrane matrix become progressively more important. These factors could affect both the precise instant of phase inversion and also the size and distribution of nonsolvent swelling agent droplets (incipient voids) in the interdispersed phase during gelation, which ultimately determine the surface pore structure of the resulting membrane. The general increase in product rate with increase in alcohol concentration in the gelation medium in regions beyond region 1 in Fig. 2 and the existence of maxima and minima in product rates in these regions indicate that the above factors include those having opposing tendencies in the direction of change of size, number and distribution of pores on the membrane surface and the effective thickness of the membrane. The above conclusion is consistent with earlier studies. According to Kesting (II), as the alcohol concentration in the gelation medium increases, its effect on the porous structure of the membrane changes continuously from that of a swelling agent to that of a nonsolvent. As a swelling agent, alcohol tends to promote an increasing number of smaller size droplets in the interdispersed phase resulting in finer precipitation of the polymer with smaller size pores on the membrane surface, whereas alcohol as a nonsolvent tends to produce the opposite result in the final membrane; these tendencies are enhanced at higher temperatures of the gelation medium. The experimental data on product rates given in Fig. 2 are consistent with the above reasoning. The rate of precipitation of the polymer during gelation (which determines the overall porosity of the membrane including its surface structure) is controlled not only by the diffusive exchange of the solvent (acetone) and nonsolvent (water or alcohol) but also by the similar exchange of the swelling agent (aqueous magnesium perchlorate) and the gelation medium. All these rates are functions of the chemical nature and physical properties of the components involved. So et al. (Z2) pointed out the importance of solubility parameter of the components on rates of such exchange_ The solubility parameters for cellulose acetate, acetone, ethyl alcohol and water are 10.9, 9.6, 12.8 and 23.5 respectively. On the basis of these data, compared to water, ethyl alcohol has greater affinity for acetone which explains the observation of Carter et al. (8) that alcohol penetrates into the film matrix faster than water. While the available data are not sufficient to make any definite statement on the effect of solubility parameter on competing rates of exchange in a multi-component system at different temperatures, it is reasonable to conclude that the exchange rates of individual components in such a system are controlled by the total environment in the system. Consequently, increase in

0; KUTOWY,

208

W. L. THAYER

AND

S. SOURIRAJAN

alcohol concentration in the geiation medium may be expected to have varying effects on the rates of exchange of the individual components and the resulting rate of precipitation of the polymer during gelation. The experimental data presented in Fig. 2 and similar data presented earlier (3) show that control of gelation environment IS an effective means of creating a wide range of porosities on the membrane surface in the as-cast condition. EFFECT OF SOLVENT

EVAPORATION

PERIOD PRIOR TO GELATION

Kesting

(II) has pointed out that solvent evaporation time prior to immerin the gelation medium is of critical importance with respect to surface pore structure and hence the performance of resulting membranes. Fig. 3 illustrates the effect of solvent evaporation period up to 60 seconds prior to

sion of the film

IE

EVAPORATION

TIME

PRIOR TO W3ATlON.

we

Fig. 3. Efkt of solvent evaporationperiod (prior to gelation) on membraneperformance. Operating pressure,689.5 x 103 Pa (100 psi&; feed solution, 300 ppm MgSOr-HIO. 1 @/ft? day = 0.0407 m3/m2 day..

HIGH FLUX

CA ULTRAFILTRATION

209

MEMBRANES

gelation on the performance of unshrunk membranes, using alcohol-water baths with different

alcohol

concentrations

(X,,,,

=

gelation

0, 0.088, 0.218, 0.5 and

0.854) at the gelation temperatures of 0” and 20°C. The combined effect of partial solvent evaporation and Subsequent gelation affects the number, size and size distribution of pores on the surface of the resulting membrane as well as the effective thickness of the membrane surface. As pointed out already (7) there is no precise way of determining either the size of individual pores on the membrane surface or its effective thickness. Therefore, for practical purposes, the solute separation and product rate data given in Fig. 3 may be considered simply in terms of average size of pores and the effective number of such pores on the membrane surface. Referring to data given in Fig. 3, at 0°C gelation temperature, for pure water

gelation

medium,

solute

separation

passed through

a shallow

maximum

and

product rate passed through a corresponding minimum at the same evaporation period of -20 seconds; for gelation medium at X,,,, = 0.088 (region I) solute separation passed through a minimum and the corresponding product rate passed through a maximum at different evaporation periods (37 and 25 seconds respectively); for gelation medium at XEcOH= 0.2 18 (region 2), solute separation increased and the corresponding product rate decreased with increase in evaporation period; = 0.5 (region 3) and 0.854 (region 4) product and for gelation mediums X,,,, rates decreased with increase in evaporation period and the corresponding solute separations were negligible in both regions. Similarly, at the gelation temperature of 2O”C, for pure water gelation medium, solute separation decreased and the corresponding product rate increased with increase in evaporation period; for gelation medium at X,,,, = 0.088 (region l), solute separation increased and the corresponding product rate decreased with increase in evaporation per&i; and, = 0.218 (region 2), 0.50 (region 3) al:L 0.884 for gelation mediums at X,,,, (region 4) product rate decreased with increase in evaporation period and the corresponding solute separations were negligible. The foregoing results are understandable on the basis of the combined influence of factors which have mutually opposing effects on the size and number of pores on the membrane surface. As the evaporation time increases, more nuclei are generated for the formation and development of nonsolvent droplets in the interdispersed phase. Since these droplets are incipient voids, a larger number of droplets has the potential for creating a larger number of pores on the surface of the resulting membrane. The rate of growth of such droplets and their susceptibility to coalesce are functions of droplet density and the rate of removal of solvent from membrane surface. The latter is governed both by solvent evaporation period and the following gelation conditions. Droplet generation and droplet

have opposing effects on the ultimate size and number of pores on the membrane surface. Consequently, the results shown in Fig. 3 are consistent with the observation made earlier (7) that at any given level of solute separation,

coalescence

210

0. KUTOW-Y, W. L. THAYER AND S. SOURWAN

product rate can decrease, increase, remain the same, or pass through a maximum with increase in solvent removal rate from the surface of the membrane during its formation. CONCLUSION

The porous structure of the membrane surface depends on the casting solution composition. solvent evaporation period and the total geiation environment_ By changing the latter alone, a wide range of surface porosities can be obtained for the resulting membranes. Ethanot-water gelation medium at different temperatures offers a particularly effective means of creating cellulose acetate ultrafiltration membranes of different surface porosities. REFERENCES

1. 0. KUTOWY AND S. SUURIR~UAN,J. Appi. PO&~. Sci., l9 (1975) 1449. 2. S. SOUREUS~N AND 3. KUNST, in Reverse Osmosis and Synthetic Membranes,

S. S~RIRAJAN,

Ed., Chapter 7, National Research Council Canada, Ottawa, 1977. 3. T. A. TWECXXE AND S. So URIRAJAN, J. Appl. PO&~. Sci. (in press). Desulinatiotr, 21 (1977) 209. 4. W. L. THAYER, L. PAGEW AND S. !~OURWAN, 5. S. SOURIRMAN, Rewrse Osnzosis, Academic, New York, N-Y., 1970, C&p. 1. 6. R. E. KEZXWG, M. K. BARSH AND A. L. VINCENT, J. Appt. Potym. Sci., 9 (1965) 1873. 7. L. PAGEAU AND S. SOLRUWAN, J. A&. Polym. SC& 16 (1972) 3185. AND M. T. PRICE, Desuttnuriun, 12 (1973) 177. 8. J. W. CAXTER,G. RMRAS AND S. LOEB, Proc. l%ird Intern. Symp. on Fresh Water from Sea (Dubrovnik), 9. E. ZLSNER

2 (1970) 615. 10. M. A. FROMMER, R. MATZ AND U. ROSENTHAL, Zmi. Eng. Chem. Prod. Res. Dev., 10 (1971) 193.

11. R. E. KESTING, Synfhetic

Polymeric

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