13 High-Flux Cellulose Acetate Membranes K. W. BÖDDEKER, H. FINKEN, and A. WENZLAFF
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GKSS—Forschungszentrum, 2054 Geesthacht, Germany
Three routes to increase the permeate flux of asymmetric cellulose diacetate membranes of the Loeb-Sourirajan type are investigated: increasing the hydrophilicity of the membranes; increasing their compaction stability; employing a swelling agent which allows for higher solvent-to-polymer ratio in the casting solution. The effect of casting solution composition on flux and rejection of formamide-modified cellulose acetate membranes is shown in Figure 1, illustrating the general capability of this membrane type as function of solvent concentration. Membranes of casting solution composition cellulose diacetate/acetone/ formamide 23/52/25 (solvent-to-polymer ratio 2.26) were used as reference membranes in this work. Increased H y d r o p h i l i c i t y E f f e c t o f H y d r o p h i l i c Bentonites. A l l membrane models imp l y a d i r e c t r e l a t i o n between f l u x and membrane water content. The gross water content of the membranes can be increased by i n c o r p o r a t i n g p r e - g e l l e d h y d r o p h i l i c bentonites i n t o the membranes. The u s e f u l bentonite c o n c e n t r a t i o n i s l i m i t e d by the f a c t that p r e - g e l a t i o n introduces water i n t o the c a s t i n g s o l u t i o n (1). Membrane P r e p a r a t i o n . The bentonite used, trade-named Bentone EW (Kronos Titan-GmbH, Leverkusen, Germany), i s a h i g h l y p u r i f i e d magnesium montmorillonite which gels i n water. Benton i t e a d d i t i o n i s by way of a f u l l y swollen g e l of 10 g of Bentone EW i n 400 g of water to which i s added 300 g of acetone to render the aqueous g e l compatible with the remainder of the c a s t i n g s o l u t i o n . 7.1 % of t h i s s l u r r y i s introduced i n t o the reference c a s t i n g s o l u t i o n (see above), r e s u l t i n g i n a compos i t i o n as f o l l o w s (wt-%): c e l l u l o s e diacetate/acetone/formamide/ water/bentonite 21.4/51.3/23.2/4.0/0.1 (solvent-to-polymer r a t i o 2.4). A c u r i n g time of about three weeks i s r e q u i r e d before
0097-6156/81/0153-0191$05.00/0 © 1981 American Chemical Society
In Synthetic Membranes:; Turbak, A.; ACS Symposium Series; American Chemical Society: Washington, DC, 1981.
192
SYNTHETIC
MEMBRANES:
DESALINATION
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membranes are prepared i n the usual manner. Annealing temperatures are somewhat higher than f o r the reference membrane at the same r e j e c t i o n . Membrane P r o p e r t i e s . The water contents are as f o l l o w s : without annealing: reference, 67.0; with bentonite, 69.2 wt-%; annealed at 70 °C: reference, 65.8; with bentonite, 68.6 wt-%; annealed at 90 °C: reference, 63.7; with bentonite, 66.3 wt-%. The net gain i n water content due to h y d r o p h i l i c bentonite i n c o r p o r a t i o n i s thus of the same order as the water l o s s on annealing. The reverse osmosis performance of the two membranes under t y p i c a l b r a c k i s h water c o n d i t i o n s i s shown i n F i g u r e 2 ( I , r e f e rence membrane; I I I , with b e n t o n i t e ) . At a r e j e c t i o n of 85 % the f l u x i s almost doubled (from 2000 to n e a r l y 4000 l/m d), the e f f e c t becoming smaller when going to higher r e j e c t i o n s . Maximum b r a c k i s h water r e j e c t i o n of the bentonite membrane i s 97 % as against 98 % f o r the reference membrane. The e f f e c t of operating pressure on f l u x f o r v a r i o u s anneal i n g treatments i s shown i n F i g u r e 3. As i s u s u a l l y observed the optimum pressure, beyond which the curves f l a t e n out, increases as the membranes become denser. Maximum f l u x of the untreated membrane at a pressure of 30 bar i s around 10 m^/m d (250 gfd) at 20 % b r a c k i s h water r e j e c t i o n . The optimum operating pressure of annealed b e n t o n i t e - c o n t a i n i n g membranes i s lowered by about 10 bar, however, the compaction behavior i s comparable to that of the reference membrane. 2
2
Increased Compaction S t a b i l i t y E f f e c t of O r g a n o p h i l i c Bentonites. Membrane compaction r e duces the i n t e g r a l product water output. By i m p l i c a t i o n , membrane s t a b i l i z a t i o n i s a means to increase the f l u x . S t a b i l i z a t i o n , along with some f l u x improvement, can be achieved by doping the membranes with o r g a n o p h i l i c bentonites ( 2 ) . Membrane P r e p a r a t i o n . The bentonite used, trade-named T i x o g e l VZ (Siid-Chemie AG, Munchen, Germany) i s a quarternary ammonium compound of montmorillonite o r i g i n , the ammonium moiet i e s c o n t a i n i n g hydrocarbon chains, which g e l s i n p o l a r organic s o l v e n t s . The optimal bentonite c o n c e n t r a t i o n , guided by i t s e f f e c t on s a l t r e j e c t i o n as shown i n F i g u r e 4, i s 0.1 wt-% of the c a s t i n g s o l u t i o n . Bentonite a d d i t i o n i s i n the dry s t a t e by thoroughly mixing 1 g of T i x o g e l VZ with 230 g of c e l l u l o s e d i acetate, then adding acetone followed by formamide according to the reference c a s t i n g s o l u t i o n composition. Curing time i s again three weeks. Annealing temperatures are somewhat lower than f o r the reference membrane to a t t a i n the same r e j e c t i o n . The water content of the doped membrane does not d i f f e r from that of the reference membrane.
In Synthetic Membranes:; Turbak, A.; ACS Symposium Series; American Chemical Society: Washington, DC, 1981.
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13.
BODDEKER
ET AL.
High-Flux
Cellulose
Acetate
Membranes
193
1500-i
L5
1
1
1
1
ui
L9
51
53
1H
Acetone %
Figure 1. Effect of casting solution composition on flux and rejection of formamide-modified cellulose diacetate membranes
I Reference membrane; III: with hydrophilic addititn
5.000 ppm NaCl 25°C 60 bar
2000
1000-
85
Figure 2. Effect of hydrophilic bentonite incorporation on the reverse osmosis performance of asymmetric cellulose diacetate membranes: I, reference membrane; III, with hydrophilic bentonite.
In Synthetic Membranes:; Turbak, A.; ACS Symposium Series; American Chemical Society: Washington, DC, 1981.
SYNTHETIC
194
MEMBRANES:
5000ppm
NaCl
25'C
l7m*d
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DESALINATION
9000 untreated
6000-
70 °C/5min
3000
Figure 3. Effect of operating pressure on flux of cellulose diacetate membranes with hydrophilic bentonite incorporation at various annealing levels
40
20
P bar
60
5000 ppm NaCl 25 *C
l/m2d
60 bar
100
98
96
-94
500
Figure 4. Flux and refection of cellulose diacetate membranes doped with organophilic bentonite as a function of bentonite concentration
92
01
0.2
0 3
Bentonite concentration wt - %
In Synthetic Membranes:; Turbak, A.; ACS Symposium Series; American Chemical Society: Washington, DC, 1981.
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13.
BODDEKER
ETAL.
High-Flux
Cellulose
Acetate
Membranes
195
I Reference membrane. II : with organophilic additive F l/m d 2
5.000 ppm NaCl 25° C 60 bar
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3000-
2000-
Figure 5. Effect of organophilic bentonite incorporation on the reverse osmosis performance of asymmetric cellulose diacetate membranes: I, reference membrane; II, with organophilic bentonite.
1000-
95
I Reference membrane IV NH modified membrane 3
F l/rn^d
5000-
20
60
80
§r
Figure 6. Low-pressure reverse osmosis performance of ammonia-modified cellulose diacetate membranes (IV) compared with formamide-modified reference membranes (I)
In Synthetic Membranes:; Turbak, A.; ACS Symposium Series; American Chemical Society: Washington, DC, 1981.
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196
SYNTHETIC
MEMBRANES:
DESALINATION
Membrane P r o p e r t i e s . The reverse osmosis performance of the bentonite-doped membrane under b r a c k i s h water c o n d i t i o n s i s compared to that of the reference membrane i n Figure 5 ( I , reference membrane; I I , with o r g a n o p h i l i c b e n t o n i t e ) . At low s a l t r e j e c t i o n the bentonite membrane again shows a higher i n i t i a l f l u x than the reference membrane, the performance of the two becoming i d e n t i c a l at the high r e j e c t i o n l i m i t . The o b j e c t i v e of employing o r g a n o p h i l i c bentonite i s f l u x s t a b i l i z a t i o n . In terms of the membrane compaction slope the s t a b i l i z i n g e f f e c t i s exemplified by the f o l l o w i n g f i g u r e s (brackish water c o n d i t i o n s ) : reference, -0.10; bentonite-doped, -0.06. In a f i e l d t e s t over 1300 hours on w e l l water of 5200 ppm TDS at a pressure of 60 bar, s t a r t i n g with an i n i t i a l f l u x of 1780 l/m d and 95 % r e j e c t i o n , a compaction slope of -0.058 was found; under the same c o n d i t i o n s the reference membrane had a compaction slope of -0.094. 2
Increased
Solvent-to-Polymer
Ratio
Ammonia as Swelling Agent. As i s i n d i c a t e d i n Figure 1 the f l u x of asymmetric c e l l u l o s e acetate membranes increases with i n c r e a s i n g solvent p r o p o r t i o n i n the c a s t i n g s o l u t i o n , accompanied by an unavoidable l o s s i n r e j e c t i o n . By using anhydrous ammonia as s w e l l i n g agent i n place of formamide, d i l u t e c e l l u l o s e acetate s o l u t i o n s are a c c e s s i b l e f o r the p r e p a r a t i o n of membranes showing correspondingly high f l u x values ( 3 ) . Membrane P r e p a r a t i o n . D r i e d c e l l u l o s e d i a c e t a t e i s d i s s o l v e d i n acetone i n the weight r a t i o of 1 to 3 o r 4. Gaseous ammonia i s d i r e c t e d at room temperature over the s o l u t i o n surface i n a r o t a r y evaporator, the ammonia being r e a d i l y absorbed by the polymer sol u t i o n . Optimal ammonia concentration i s 5 to 6 wt-%, a t y p i c a l c a s t i n g s o l u t i o n composition i s c e l l u l o s e diacetate/acetone/ ammonia 18.8/75.2/6.0 (solvent-to-polymer r a t i o 4 ) . Casting i s at room temperature. The p r e c i p i t a t i o n bath i s maintained at pH 4 through c o n t r o l l e d a d d i t i o n of h y d r o c h l o r i c a c i d to compensate for the a l k a l i n e i n t a k e . Membrane P r o p e r t i e s . The performance range of ammonia-modif i e d membranes i n low pressure o p e r a t i o n i s i n d i c a t e d i n F i g u r e 6 along with the performance of the reference membrane ( I , reference membrane; IV, ammonia-modified membrane). The lower boundary of the performance range r e f e r s to a solvent-to-polymer r a t i o of 3, the upper boundary to a r a t i o of 4. While the s a l t r e j e c t i o n t o wards u n i v a l e n t ions of the ammonia-modified membrane i s l i m i t e d to below 80 %, the maximum low pressure f l u x i s over 15 m^/m^d (approaching 400 gfd) at a sodium c h l o r i d e r e j e c t i o n of the order of 10 %. This membrane thus e x h i b i t s the f l u x c a p a b i l i t y of an u l t r a f i l t r a t i o n membrane while r e t a i n i n g the features of reverse osmosis membranes, v i z . asymmetry and pressure r e s i s t a n c e .
In Synthetic Membranes:; Turbak, A.; ACS Symposium Series; American Chemical Society: Washington, DC, 1981.
13.
BODDEKER E T A L .
Literature
High-Flux
Cellulose
Acetate
Membranes
197
Cited
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1. Böddeker, K. W.; Kaschemekat, J.; Willamowski, M., Abstract, National Meeting American Chemical Society, 1975, 169, 97. 2. Finken, H . , Proc. 7th Int. Symposium Fresh Water from the Sea, 1980, 2, 125. 3. Boddeker, K. W.; Kaschemekat, J.; Woldmann, H . , Proc. 4th Int. Symposium Fresh Water from the Sea, 1973, 4, 65. RECEIVED
December 4, 1980.
In Synthetic Membranes:; Turbak, A.; ACS Symposium Series; American Chemical Society: Washington, DC, 1981.
