Synthetic Membranes: Volume I - ACS Publications - American

24 Hollow-Fiber Reverse-Osmosis Composite Membranes: Process and Properties R. B. DAVIS, D. K. SCHIFFER, and C. E. KRAMER Downloaded by UNIV OF CALIFORNIA SAN DIEGO on June 2, 2015 | http://pubs.acs.org Publication Date: May 21, 1981 | doi: 10.1021/bk-1981-0153.ch024

Albany International Research Co., Route 128 at U.S. 1, Dedham, MA 02026

Albany International Research Co. has developed an advanced hollow fiber composite reverse osmosis membrane and module under the name of Quantro II™. This composite membrane is comprised of a porous hollow fiber substrate on which has been deposited a rejection barrier capable of fluxes of commercial importance at high rejection of dissolved salts at elevated temperatures. Resistance to active chlorine has been demonstrated. Proprietary processes have been developed for spinning of the fiber, establishment of the rejection barrier and processing of the fiber to prepare modules of commercial size. Prototype modules are currently in field trials against brackish and seawater feed solutions. Applications under consideration for this membrane include brackish and seawater desalination as well as selected industrial concentration processes. Earlier publication concerning work of Albany International Research Co. described elements of processes involved in the preparation of the Quantro II. That publication identified an earlier candidate rejection barrier as a sulfonated, furanbased polymeric material. The instability of the furan membrane in field evaluations and its sensitivity to oxidizing agents including chlorine recommended a substitution of alternative rejection barrier chemistry. Recent work resulted in the rejection barrier of Quantro II which is an alternative highly sulfonated polymeric composition. Specific chemistry involved in the preparation of the rejection barrier will be published after patent coverage. The following presents details of unit processes in the preparation of this membrane and results of laboratory and field evaluation. The final portion of this paper will discuss certain elements of the potting procedure and testing necessary to the identification of successful potting materials. (1)

Membrane Unit Processes Dope Preparation. A porous w a l l substrate f i b e r i s prepared by the e x t r u s i o n o f a q u a s i - s o l u t i o n , the p r i n c i p a l component o f 0097-6156/81/0153-0367$05.00/0 © 1981 American Chemical Society

In Synthetic Membranes:; Turbak, A.; ACS Symposium Series; American Chemical Society: Washington, DC, 1981.

Downloaded by UNIV OF CALIFORNIA SAN DIEGO on June 2, 2015 | http://pubs.acs.org Publication Date: May 21, 1981 | doi: 10.1021/bk-1981-0153.ch024

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which i s a p o l y s u l f o n e . Several p o l y s u l f o n e m a t e r i a l s have been surveyed. At l e a s t two are acceptable f o r t h i s process. These are Udel® (Union Carbide Corp.) and Victrex® (ICI Americas). A f i b e r spinning dope i s prepared by mixing the p o l y s u l f o n e , s o l v e n t and a nonionic s u r f a c t a n t . A p r e f e r r e d solvent i s d i methylformamide (DMF); a p r e f e r r e d s u r f a c t a n t i s an a l k y l a r y l polyether a l c o h o l . The dope i s prepared such that i t s bulk v i s c o s i t y i s t y p i c a l l y between 9000 and 12000 p o i s e s . Successful dopes demand c o n t r o l of the mode of a d d i t i o n , degree of polymer comminution, the temperature p r o t o c o l and a g i t a t i o n r a t e . The dope i s t y p i c a l l y f i l t e r e d through a 10 micron f i l t e r under pressure p r i o r to spinning. D r y - j e t Wet Spinning. Substrate f i b e r i s prepared i n a p r o p r i e t a r y process by an apparatus developed and b u i l t at Albany I n t e r n a t i o n a l Research Co. F i l t e r e d dope i s expressed from a m u l t i h o l e hollow f i b e r s p i n n e r e t . Each o r i f i c e of the spinneret contains a centered needle f o r the d e l i v e r y of an i n e r t f l u i d , t y p i c a l l y dry n i t r o g e n . The dope passes through an a i r gap of approximately one centimeter and i n t o an aqueous c o a g u l a t i o n bath. A c r i t i c a l element of t h i s process i s that the c o a g u l a t i o n bath i s so arranged as to permit the passage of the coagulating stream through s e v e r a l meters of c o a g u l a t i o n bath before i t i s touched by any s o l i d . T h i s permits the establishment of a s u r face of uniform p h y s i c a l and chemical c h a r a c t e r i s t i c s . A f t e r extensive washing i n the c o a g u l a t i o n bath, f i b e r i s wound i n t o packages by equipment common i n the handling of t e x t i l e f i b e r s but modified to accomodate the s e n s i t i v e nature of a 50% dense polymeric s t r u c t u r e . F i b e r i s maintained wet u n t i l subsequent use. The c o a g u l a t i o n bath i s comprised of an aqueous s o l u t i o n of DMF and s u r f a c t a n t . Temperature c o n t r o l i s maintained with v a r i a t i o n l e s s than 1°C. Deionized water i s used i n the p r e p a r a t i o n of the bath. F i b e r thus prepared contains l e s s than 2% DMF. V i r t u a l l y a l l of the s u r f a c t a n t i n the dope i s r e t a i n e d i n the fiber. E x t r a c t i o n of S u r f a c t a n t . The i n f l u e n c e of s u r f a c t a n t on the p h y s i c a l p r o p e r t i e s of the f i b e r r e q u i r e i t s removal p r i o r to the establishment of the reverse osmosis r e j e c t i o n membrane. S u r f a c t a n t i s removed i n a pressure e x t r a c t i o n apparatus by the r e c i r c u l a t i o n of hot aqueous a l c o h o l . S u r f a c t a n t i s removed to concentrations l e s s than 2% on the weight of dry f i b e r . Preparation of R e j e c t i o n B a r r i e r . Subsequent to the f i e l d t r i a l s of the furan membrane, many a l t e r n a t i v e s u l f o n a t e d mater i a l s were surveyed. The present and p r e f e r r e d r e j e c t i o n b a r r i e r of Quantro II i s prepared by the d e p o s i t i o n i n and on the surface of the substrate f i b e r a complex s o l u t i o n of a s u l f o n a t e d p o l y meric m a t e r i a l prepared i n our l a b o r a t o r y . This sulfonated p o l y meric m a t e r i a l i s c o d i s s o l v e d i n aqueous a l c o h o l with i n g r e d i e n t s



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found necessary to the development o f high f l u x and r e j e c t i o n . Extracted f i b e r as a m u l t i f i l a m e n t strand i s conducted from an aqueous storage bath, around a d r i v e n r o l l and i n t o such a s o l u tion. I t then passes i n t o a two-stage f o r c e d hot a i r tower. In order to develop both high f l u x and r e j e c t i o n two stages o f heating are employed. Solvent i s removed at temperatures l e s s than 100°C. The f i n a l c u r i n g a i r temperature i s t y p i c a l l y up to 150°C. A f t e r e x i t i n g the tower, yarn i s passed around a second d r i v e n r o l l and wound up as a package. The l a r g e r l i n e a r v e l o c i t y o f the second r o l l r e l a t i v e to the f i r s t imparts an o v e r a l l s t r e t c h of a few percent to the yarn d u r i n g membrane formation. Bundle P r e p a r a t i o n . Packages of m u l t i f i l a m e n t yarns are backwound to prepare bundles necessary f o r the manufacture o f a reverse osmosis module. A p r o p r i e t a r y winder f o r t h i s o p e r a t i o n has been designed and c o n s t r u c t e d at Albany I n t e r n a t i o n a l Research Co. T h i s d e v i c e i s capable o f h e l i c a l l y winding m u l t i filament yarns i n t o bundles around a mandrel. T h i s i s done i n a manner such that the r e s u l t i n g bundle has uniform c y l i n d r i c a l dimensions and uniform f i b e r d e n s i t y . T h i s minimizes channeling and optimizes exposure of membrane surface area. A c r i t i c a l element of the winding equipment i s that i t succ e s s f u l l y d e l i v e r s f i b e r whose membrane p r o p e r t i e s cannot t o l e r ate more than a few percent e x t e n s i o n . Ceramic or metal guides, t y p i c a l of t e x t i l e winding equipment, cause such f r i c t i o n that t h i s extension i s exceeded with v a r i a b l e adverse e f f e c t s on membrane p r o p e r t i e s . Another c r i t i c a l element i n winding i s that a uniform loop length of yarn i s maintained during the development of the bundle. The reverse osmosis c h a r a c t e r i s t i c s of a hollow f i b e r membrane o f p e r m e a b i l i t y t y p i c a l of Quantro II are s e r i o u s l y a f f e c t e d by the pressure drop of permeate down the lumen o f the f i b e r ( ) . Therefore i t i s d e s i r a b l e that the loop l e n g t h remain constant and r e l a t i v e l y short throughout the advancing r a d i a l growth of the bundle. To accomplish t h i s , the bundle winding apparatus changes the h e l i x angle i n a constant manner i n order to d e l i v e r a uniform loop length of yarn. 2

Potting. S e v e r a l r e s i n formulations have been surveyed as candidates f o r the p o t t i n g of Quantro II f i b e r bundles. This t o p i c i s t r e a t e d i n d e t a i l below. C r i t i c a l elements i n the choice o f formulation i n c l u d e i t s v i s c o s i t y , absence o f v o l a t i l e s , pot l i f e , maximum exotherm l i m i t and i t s adhesion to the f i b e r ( 3 ) . The o b j e c t i v e o f the p r o p r i e t a r y p o t t i n g procedure i s to s e a l a l l f i b e r s i n a mass of cured epoxy such that the transmembrane pressure may be brought to ground through the pot without f a i l u r e due to creep or shear along the f i b e r s u r f a c e . In t h i s procedure one face o f the pot i s maintained f l a t against a base p l a t e of the module. Resin i s d e l i v e r e d to the bundle mass through a c e n t r i f u g a l system which f o r c e s the v i s c o u s formulation through the f i b e r mass. Speed o f d e l i v e r y i s so regulated that



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a i r i s not entrapped by o c c l u s i o n . The exotherm develops s u f f i c i e n t r e a c t i o n to r e s u l t i n a mechanically t r a c t a b l e "green state". F i b e r ends are exposed by e x c i s i o n of a p o r t i o n of the pot and f i b e r bundle at s e v e r a l p o i n t s i n the circumference of the pot. A s u f f i c i e n t l y high and deep s l i c e at frequent enough i n t e r v a l s w i l l guarantee the exposure of each f i b e r loop at l e a s t once. Due to the p h y s i c a l texture of p o l y s u l f o n e f i b e r i t was found necessary to s l i c e the f i b e r i n tension i n order to avoid smearing of the polymer with lumen c l o s u r e . T h i s may be accomp l i s h e d through the expedient of advancing the cure of the epoxy r e s i n to a green s t a t e at which i t i s mechanically s t a b l e enough to be removed from the mold and s l i c e d . Apparatus f o r s l i c i n g r e s i n and f i b e r has been designed and constructed. The s l i c e d bundle i s r e i n s e r t e d i n a mold and the epoxy cure i s completed by advancing the temperature. Module Assembly. A f t e r the f i n a l cure of epoxy the potted bundle i s assembled i n t o a module i n which the feed s o l u t i o n and permeate are separated by O-ring s e a l s i n a pressure s h e l l . Des i g n of the module i s p r o p r i e t a r y . Patent a p p l i c a t i o n s have been filed. Prototype modules capable of d e l i v e r i n g s e v e r a l hundred g a l l o n s a day of permeate are i n t e s t i n various f a c i l i t i e s against b r a c k i s h and sea water. Membrane P r o p e r t i e s Many Quantro II membranes varying incrementally i n composit i o n have been under t e s t f o r 18 months at Albany I n t e r n a t i o n a l Research Co. Tests are performed on experimental samples of f i b e r . Approximately 16 inches of m u l t i f i l a m e n t yarn are t y p i c a l l y subjected to v a r i o u s feeds and c o n d i t i o n s . Such a yarn sample i s embedded i n epoxy which i s sealed i n t o a pressure system. Several t e s t f a c i l i t i e s are i n operation to provide v a r i o u s feeds and c o n d i t i o n s . A t y p i c a l t e s t loop i n c l u d e s a pump capable of pressure development to 1000 p s i g and s u f f i c i e n t v a l v i n g and p i p i n g to permit m u l t i s t a t i o n i n s t a l l a t i o n of f i b e r samples. Feed s o l u t i o n s are d e l i v e r e d from r e s e r v o i r s which are thermostated and i s o l a t e d i n order to maintain constancy of temperature and compos i t i o n . The f a c i l i t y i s so e s t a b l i s h e d that both permeate and concentrate are returned continuously to the r e s e r v o i r . Test feeds include s y n t h e t i c b r a c k i s h water t y p i c a l of southwest United States at approximately 3500 ppm, and a t y p i c a l s y n t h e t i c seawater at approximately 35,000 ppm. Test c o n d i t i o n s include b r a c k i s h water at 400 p s i g , 25°C and 760 p s i g , 25°C. Seawater t e s t i n g i s done at 1000 p s i g at 25°C and at 50°C. C h l o r i n e s e n s i t i v i t y i s evaluated through the use of a feed of b r a c k i s h water, doped to include a nominal 100 ppm h y p o c h l o r i t e ion at pH 7.5 to 8.5. C h l o r i n e c o n c e n t r a t i o n i s monitored d a i l y and adjusted to the nominal.



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The membrane composition of Quantro I I has been under continuous research f o r 19 months. E a r l y samples were put on t e s t in order to e s t a b l i s h a long-term data base f o r t h i s chemistry. While that e a r l y composition has been modified to achieve higher f l u x and r e j e c t i o n , i t s d u r a b i l i t y can be judged from Table I. Table I p l o t s data obtained d a i l y over the time p e r i o d i n d i c a t e d . Curves on a l l t a b l e s have been smoothed to avoid d a i l y f l u c t u a tions. I t should be understood that f l u c t u a t i o n s of as much as 0.2% r e j e c t i o n and 0.2 gfd i n f l u x are r o u t i n e l y experienced and are not shown. Table I d i s p l a y s the r e s u l t s over s e v e r a l thousand hours of exemplary samples tested a t zero l e n g t h , zero recovery. B r a c k i s h water r e j e c t i o n under c o n d i t i o n s s p e c i f i e d are i n excess of 99% at high pressure and greater than 98.5% at lower pressure such as 400 p s i . Flux values d i s p l a y e d are those t y p i c a l of e a r l y samp l e s i n the development of Quantro I I . At low pressure a g a i n s t b r a c k i s h water such values were 1-2 g f d . Recent values of samp l e s t e s t e d under i d e n t i c a l c o n d i t i o n s have e x h i b i t e d r e j e c t i o n s of 94+% at a f l u x of 5-7 gfd at 400 p s i . Table II d i s p l a y s s e v e r a l thousand hours data on v a r i o u s samples of Quantro I I tested a g a i n s t s y n t h e t i c seawater at 1520°C. Routine samples e x h i b i t a f l u x i n excess of 1 gfd at r e j e c t i o n of 98-99%. I t i s p o s s i b l e with composite membrane prepar a t i o n to trade r e j e c t i o n f o r f l u x w i t h i n c e r t a i n l i m i t s . The o b j e c t i v e of present work i s to maintain a r e j e c t i o n of 99+ against seawater while modifying formulation i n order to achieve a higher f l u x . As a r e s u l t of p r i o r f i e l d experience with the furan system, a q u a l i f y i n g t e s t has been employed at Albany I n t e r n a t i o n a l Research Co. to measure the d u r a b i l i t y . Table I I I d i s p l a y s data of t y p i c a l samples t e s t e d a g a i n s t s y n t h e t i c seawater at 1000 p s i g maintained at a temperature of 50°C. Samples of c e l l u l o s i c seawater membrane and polyamide membrane were found to f a i l i n seve r a l hours of c h a l l e n g e by these c o n d i t i o n s . Quantro I I has e x h i b i t e d a d e c l i n e i n r e j e c t i o n of a few percent over 8000 hours. Flux has d e c l i n e d implying a change i n the p h y s i c a l chemistry of the r e j e c t i o n b a r r i e r . P o l y s u l f o n e composite membranes i n our experience have not e x h i b i t e d l o n g term compaction. The d e c l i n e i n f l u x i n Table I I I i n d i c a t e s , however, that the higher temperature and pressure i n the presence of seawater may be compacting Quantro I I . The r e s u l t s , however, are e q u i v o c a l . More t e s t i n g w i l l help to i d e n t i f y the magnitude of t h i s change. A c o n t i n u i n g major o b j e c t i v e i n the search f o r a l t e r n a t i v e commercial membranes i s s t a b i l i t y a g a i n s t a c t i v e c h l o r i n e . Cont r o l of b i o l o g i c a l growth upstream of the membrane appears des i r a b l e i n most p r a c t i c a l water systems. Removal of c h l o r i n e p r i o r to membrane treatment i s expensive and p o t e n t i a l l y hazardous should the removal system f a i l . The polymer chosen f o r the 1


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Table I

Feed:

3500 ppm B r a c k i s h ,

760 p s i -> 400 p s i , 25°C


Rejection

Flux

Flux (gfd) 3

--2

-•1

1000

3000

5000 Time

7000

(hrs)

Rejection

Flux (gfd)

2483-24-5-2

(%)

99

--1 98

—t

1000

3000

1 5000 Time

1

• 7000

1

»9000

(hrs) Flux (gfd)

Rejection 2483-24-6-1

(%)

760 p s i

>

400 p s i

-2 99

98

H

1 2000

h-

H 4000 Time

1 6000

1

\— 8000

(hrs)


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Table I I

Feed:

1000 p s i Sea Water

a t 15°-20°C


Rejection

Flux

2569-30-3-1 2569-8-5-1 2569-8-5-1 2569-30-3-1

98

400

1200

2000 Time

2800

(hrs)

Rejection 2550-21-1 99

500

1500

2500 Time

3500

(hrs)

2550-17-1-1 99 --

98 •-

500

1500

2500 Time

3500

(hrs)


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Table I I I


Feed:

1000 p s i , Sea Water,

50°C

Rejection

Flux Flux (gfd)

Rejection 2569-15-3-1

(%) 99-

98 -

.-2

97 " -.1 96 "

500

1500

2500 Time

3500

(hrs) Flux (gfd)

Rejection 2569-25-1

(%) 99 .

"3

••2

98 '

97 500

1500

2500 Time

Flux (gfd)

Rejection (%) 98

3500

(hrs)

2483-24-4-1

4

97 •

96 -•

95

2500

4500 Time

6500

8500

(hrs)



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Quantro II r e j e c t i o n b a r r i e r was s e l e c t e d with c h l o r i n e s t a b i l i t y as an o b j e c t i v e . S u l f o n a t i o n i s an o x i d a t i o n r e a c t i o n . High l e v e l s of s u l f o n a t i o n imply the p o t e n t i a l for s t a b i l i t y against c h l o r i n e and other o x i d i z i n g agents. Table IV e x h i b i t s data recorded f o r s e v e r a l t y p i c a l examples tested under c o n d i t i o n s d e s c r i b e d . A nominal 100 ppm a c t i v e c h l o r i n e was s e l e c t e d as an a c c e l e r a t e d t e s t of c h l o r i n e s e n s i tivity. Over 2000 hours there i s a s l i g h t measurable d e c l i n e i n r e j e c t i o n with no s i g n i f i c a n t change i n f l u x . The pH of the t e s t s o l u t i o n i s t y p i c a l l y 7.5-8.5 i n order to maintain a reasonably s t a b l e c o n c e n t r a t i o n of h y p o c h l o r i t e i o n . Tests are i n progress at an a c i d pH of 5-6 i n a t e s t loop i n which the r e s e r v o i r i s not sealed under pressure. Therefore a s t a b l e c o n c e n t r a t i o n of c h l o r i n e i s d i f f i c u l t to maintain due to the e v o l u t i o n of c h l o r ine gas. I t i s p o s s i b l e f o r a change i n the r a t e of degradation to occur due to a change i n chemical mechanism of a t t a c k . This w i l l be evaluated i n the near term. Table V d i s p l a y s data recorded at the t e s t f a c i l i t y i n Roswell, New Mexico, maintained by the O f f i c e of Water Research and Technology, U. S. Department of I n t e r i o r . T h i s f a c i l i t y d e l i v e r s a feed of b r a c k i s h water p r e t r e a t e d to c o n t r o l b a c t e r i a l growth and to d e l i v e r a feed f r e e of c h l o r i n e . Modules 148 and 152 were nominally i d e n t i c a l samples. They are constructed of f i b e r bundles approximately 2 inches i n diameter, 10 inches i n length and c o n t a i n i n g approximately 25 square f e e t of membrane surface area. The increase i n p r o d u c t i v i t y over time can be exp l a i n e d by an increase i n feed temperature over the course of the test. The d e c l i n e i n r e j e c t i o n of module 148 i s not f u l l y understood. However, i t i s probable that the d e c l i n e i s s i m i l a r l y the r e s p o n s i b i l i t y of a temperature i n c r e a s e . Recent data i n d i c a t e s a s t a b i l i z a t i o n at a r e j e c t i o n l e v e l of 94%. Module 152 has been challenged by the i n j e c t i o n of nominal one ppm h y p o c h l o r i t e ion under pressure immediately upstream of the module. The pH of the feed i s maintained at approximately 5. The d e c l i n e i n r e j e c t i o n i s p o s s i b l y an i n d i c a t i o n of chemical degradation at the lower pH. However, i t has been determined that t h i s t e s t feed has a l s o included a s u b s t a n t i a l loading of s o l u b l e i r o n due to the f a c t that the i n j e c t i o n pump was s u f f e r i n g s u b s t a n t i a l c o r r o s i o n during the course of t h i s t r i a l . At approximately 1000 hours the module was found to be h e a v i l y i n volved with an i r o n oxide p r e c i p i t a t e . T h i s was subsequently t r e a t e d with aqueous c i t r i c a c i d for c l e a n i n g . The d e c l i n e i n r e j e c t i o n s t a b i l i z e d at 92%. I t i s p o s s i b l e that the combination of c h l o r i n e and s o l u b l e i r o n i n i t i a t e d a chemical i n f l u e n c e r e s u l t i n g i n data u n l i k e that generated at higher pH i n our l a b o r a t o r y . Tests w i l l continue to q u a n t i f y the impact of i r o n and c h l o r i n e on Quantro I I .


376

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MEMBRANES:

DESALINATION

T a b l e IV

Feed:

760 p s i , 3500 ppm B r a c k i s h


~100

ppm A c t i v e C h l o r i n e

Water,

25°C

a s 0C1"

Flux (gfd)

2582-12-2-1

4

• 3 97 • t 2 96

t • 1 200

1000 Time

1800

2600

1800

2600

1800

2600

(hrs)

2569-50-2-2

98

97

f

96

200

200

1000 Time

(hrs)

Time

(hrs)

1000



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Table V

On

Test at Roswell


377

378

SYNTHETIC

MEMBRANES:

DESALINATION


Epoxy Resin S e l e c t i o n Quantro II assembled i n a bundle must be sealed at l e a s t at one end p r i o r to assembly i n a module. In order to make use o f the membrane, f i b e r must be c u t to permit c o l l e c t i o n o f permeate. The membrane d i f f e r e n t i a l pressure must be brought to ground through the s e a l . The pot s e a l must be r e s i s t a n t over time to creep, adhesive i n t e r f a c e l o s s and chemical degradation. The p r o p r i e t a r y development by Albany I n t e r n a t i o n a l Research Co. of the p o t t i n g process and f i b e r end e x p o s u r e ^ ) d e s c r i b e s a design i n which pressure i s brought to ground through the pot at the module s e a l p l a t e . Permeate e x i t i n g from exposed f i b e r ends passes i n t o a g a l l e r y and i s removed at atmospheric pressure. The t o t a l f o r c e o f upstream pressure i s d i s t r i b u t e d over the uncut face o f the pot i n c o n t a c t with the module face p l a t e . The epoxy pot must have such dimensional s t a b i l i t y that the circumference o f the pot makes e x c e l l e n t c o n t a c t with the 0-ring s e a l of the pressure s h e l l . These m u l t i p l e demands imply numerous i n t e r a c t i v e and r e s t r i c t i v e parameters i n the c h o i c e of the epoxy system chosen for pot manufacture. Formulation Requirements. In order to penetrate the mass o f f i b e r at one end of the bundle, the formulation must have s u f f i c i e n t l y low v i s c o s i t y to move e a s i l y through the bundle comp l e t e l y wetting a l l f i b e r surface area. T y p i c a l l y , formulations of v i s c o s i t y l e s s than 8000 poises have been s u c c e s s f u l . Too low v i s c o s i t y or too r a p i d d e l i v e r y of the formulation can r e s u l t i n the o c c l u s i o n of a i r and the ultimate development o f voids with l o s s o f mechanical i n t e g r i t y . Our process demands that formulat i o n be d e l i v e r e d and p a r t i a l l y cured to an intermediate p l a t e a u termed green s t a t e . This r e q u i r e s a minimum pot l i f e o f 30 minutes a f t e r blending o f r e s i n and c u r a t i v e . The p h y s i c a l chemi s t r y o f the composite membrane r e q u i r e s that the i n i t i a l exotherm not exceed approximately 150°C. A f t e r subsidence o f temperature f o l l o w i n g i n i t i a l exotherm, the f i b e r bundle with the p a r t i a l l y cured pot i s removed from the mold for f i b e r end exposure. The mechanical i n t e g r i t y o f the potted end i n t h i s green s t a t e must be s u f f i c i e n t l y g r e a t to permit c a r e f u l handling and i n s e r t i o n i n the machine necessary for fiber-epoxy e x c i s i o n . The p r o p r i e t a r y device employed for exposure o f f i b e r ends s l i c e s the pot on a diagonal and at an angle through the lower end o f the mass. Several s l i c e s are taken u n t i l the necessary depth i s achieved i n order to assure exposure o f a l l f i b e r ends. The texture o f the pot o f the epoxy i s such as to permit smooth s c i s s i o n while p e r m i t t i n g the f i b e r to be cut i n t e n s i o n . This avoids smearing f i b e r ends c l o s e d . The s l i c e d pot i s r e i n s e r t e d i n a heated mold and at temperatures necessary for the development of u l t i m a t e p h y s i c a l p r o p e r t i e s a f i n a l cure i s achieved.



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Cured Epoxy Requirements. In order to comply with m u l t i p l e needs i d e n t i f i e d above, the epoxy pot should have a compressive y i e l d s t r e n g t h o f greater than 9000 p s i . In a d d i t i o n , f o r bracki s h and seawater a p p l i c a t i o n s an a r b i t r a r y s p e c i f i c a t i o n o f zero creep at 50°C at 1000 p s i under seawater for 3000 hours has been established. A p p l i c a t i o n s at higher temperatures would o b v i o u s l y demand a higher zero creep temperature. A minimum shrinkage o f 5% or l e s s , more t y p i c a l l y 1% or l e s s , has been s p e c i f i e d i n order to maintain e x c e l l e n t s e a l t r a n s f e r ence i n the cured mass. T h i s requirement has been met through much experimentation i n v o l v i n g v a r i o u s l e v e l s o f f i l l e r . Fillers t y p i c a l l y r e s u l t i n lower shrinkage with lower bulk v i s c o s i t y . F i l l e r s a l s o present d i f f i c u l t y i n t h i s a p p l i c a t i o n due to the inherent problem presented by a f i b r o u s mass with i t s high s u r f a c e area. The present f o r m u l a t i o n has avoided the use o f f i l l e r s . To be s u c c e s s f u l the f i b e r - p o t i n t e r f a c e must s u r v i v e i n d e f i n i t e l y at temperature and pressure a g a i n s t the feed l i q u o r . The q u a l i f y i n g t e s t f o r a given formulation at Albany I n t e r n a t i o n a l Research Co. has been s u r v i v a l for greater than 3000 hours of m u l t i p l e samples tested a t 50°C at 1000 p s i g a g a i n s t s y n t h e t i c seawater. Samples are tested by p o t t i n g small numbers o f f i b e r s i n a plug of epoxy sealed with a Swagelok® f i t t i n g i n t o a p r e s sure s h e l l . T h i s i s a t e s t o f creep, shear f a i l u r e , adhesion and chemical d u r a b i l i t y . F i n a l l y , most formulations s u c c e s s f u l i n meeting other c r i t e r i a have been tested f o r heat d i s t o r t i o n temperature as a f i n a l c r i t e r i o n for judgment. A commercial device (Tinius-Olsen) has been employed f o r these t e s t s . Analysis. I t has been our o b j e c t i v e to determine c r i t e r i a for r e s i n , c u r a t i v e or f o r m u l a t i o n which would permit p r e d i c t i o n of sucess p r i o r to p o t t i n g t e s t s . Many t e s t s , both chemical and p h y s i c a l i n nature, have been executed on commercial r e s i n s y s tems. These have i n c l u d e d high pressure l i q u i d chromatography (HPLC), F o u r i e r Transform i n f r a r e d spectrometry (FTIR), g e l permeation chromatography, compressive t e n s i l e t e s t s by Instron on r e s i n plaques i n a i r and under v a r i o u s aqueous s o l u t i o n s and heat d i s t o r t i o n temperature. Over 30 commercial formulations have been surveyed i n depth. Compressive s t r e n g t h measurements permit the e x c l u s i o n o f mater i a l s o b v i o u s l y prone to f a i l under pressure. FTIR (MX-1, N i c o l e t Instrument Corp.) a n a l y s i s has i d e n t i f i e d formulations with v o l a t i l e d i l u e n t s capable o f c h e m i c a l l y modifying the composite membrane. Through the use o f FTIR i t was p o s s i b l e with an otherwise s u c c e s s f u l f o r m u l a t i o n to i d e n t i f y the presence o f b u t y l g l y c i d y l ether (BGE) as a d i l u e n t . Subsequently experimentation showed that vapor o f BGE i s capable of p l a s t i c i z i n g porous p o l y s u l f o n e with a drop i n both f l u x and r e j e c t i o n of the membrane. C o l l a b o r a t i o n with the s u p p l i e r r e s u l t e d i n s u b s t i t u t i o n of a nonv o l a t i l e g l y c i d y l ether d i l u e n t to avoid the problem.



380

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MEMBRANES:

DESALINATION

FTIR has shown the c l o s e s i m i l a r i t y o f most r e s i n s based on Bis-Phenol A and has helped narrow the focus o f development on the c u r a t i v e as the p r i n c i p a l c o n t r i b u t o r to s u c c e s s f u l formulation. For present a p p l i c a t i o n s o l i g o m e r i c polyamide amines appear s u c c e s s f u l i n meeting present c r i t e r i a . However, the o n l y o b j e c t i v e a n a l y s i s o f cured r e s i n to date e x h i b i t i n g a c o r r e l a t i o n o f measured value with success i n creep r e s i s t a n c e as w e l l as adhesion i s heat d i s t o r t i o n temperature. The f o l l o w i n g p r e sents a c o r r e l a t i o n o f heat d i s t o r t i o n temperatures and adhesion for s e v e r a l formulations t e s t e d . In most cases, p a s s / f a i l c r i t e r i a was based on the m a j o r i t y o f s i x samples t e s t e d . Heat D i s t o r t i o n Temperature v s . Adhesion

Formulation A B C D F G H I J

HDT (°C) 77

92 76 72 114 67 159 157 146

Adhesion

(50°C, 1000 p s i Sea Water) +

+ + +

A f t e r approximately one man-year o f e f f o r t a t a n a l y s i s o f the p o t t i n g problem, the best advice i s s t i l l to t e s t the pot with membrane i n an a p p l i c a t i o n . An o b j e c t i v e measurement p r e d i c t i n g success o f r e s i n , d i l u e n t and formulation before and a f t e r cure i s s t i l l not secure. Summary Research e f f o r t at Albany I n t e r n a t i o n a l Research Co. has developed u n i t processes necessary f o r p i l o t s c a l e production o f s e v e r a l s p e c i e s o f reverse osmosis hollow f i b e r composite membranes. These processes i n c l u d e spin-dope p r e p a r a t i o n , a p r o p r i e t a r y apparatus f o r d r y - j e t wet-spinning o f microporous p o l y s u l fone hollow f i b e r s , c o a t i n g o f these f i b e r s with a v a r i e t y o f permselective m a t e r i a l s , bundle winding using m u l t i f i l a m e n t yarns and module assembly. Modules o f the membrane i d e n t i f i e d as Quantro I I ™ are i n f i e l d t r i a l a g a i n s t b r a c k i s h and seawater feeds. Brackish water r e j e c t i o n s o f 94+% a t a f l u x o f 5-7 g f d at 400 p s i have been measured. Seawater r e j e c t i o n s o f 99+% a t 1-2 gfd a t 1000 p s i have been measured. Membrane use r e q u i r e s s e a l i n g of some p o r t i o n o f the f i b e r bundle f o r i n s t a l l a t i o n i n a pressure shell. Much e f f o r t has been devoted to i d e n t i f i c a t i o n o f p o t t i n g m a t e r i a l s which e x h i b i t s a t i s f a c t o r y adhesion to the f i b e r while


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p e r m i t t i n g staged c u r i n g necessary t o our p r o p r i e t a r y f i b e r end exposure. Exposing f i b e r ends r e q u i r e s the c u r i n g o f epoxy pott i n g compound t o a g e l which permits e x c i s i o n of a p o r t i o n o f the pot and bundle. Changes i n surface chemistry from membrane t o membrane have required r e q u a l i f i c a t i o n o f p o t t i n g compounds. A n a l y s i s has focused on u n s u c c e s s f u l and s u c c e s s f u l formulations to i d e n t i f y c r i t i c a l elements.


Acknowledgements The authors wish to acknowledge the l e a d e r s h i p and a s s i s tance of M. J . Coplan, Corporate Senior S c i e n t i s t and D i r e c t o r . They wish t o f u r t h e r acknowledge the support and t e c h n i c a l competence o f the f o l l o w i n g co-workers: R. D. Burchesky, C. H. Park, G. Gotz, J . K. Nelson, A. M. F e j e s , F. B i l e w s k i , S. C. W i l l i a m s and R. C. Woglom. F i n a l l y , the authors wish t o acknowledge the f i n a n c i a l support o f Albany I n t e r n a t i o n a l Corp. and, s i n c e 1976, p a r t i a l support o f the O f f i c e o f Water Research and Technology, U. S. Department o f the I n t e r i o r .

(1) (2) (3)

R. B. Davis, R. D. Burchesky, M. J. Coplan, Desalination, 22(1977) 221-227. J . H. Beale, M. J. Coplan, D. B. Eagles, S. Middleman, "Advanced Quantro™ Hollow Fiber Membranes," Membrane Symposium, Clemson University, Clemson, S.C., 15 August 1977. M. J . Coplan, J . H. Beale, R. B. Davis, U.S.Patent 4,220,489, "Method of Fabricating a Hollow Filament Separator Module," September 2, 1980.

RECEIVED December 4,

1980.


Synthetic Membranes: Volume I - ACS Publications - American

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