Seawater Desalination by Reverse Osmosis - ACS Publications

32 Seawater Desalination by Reverse Osmosis N. W A L T E R

ROSENBLATT

Downloaded by UNIV OF CALIFORNIA SAN DIEGO on March 4, 2016 | http://pubs.acs.org Publication Date: June 1, 1975 | doi: 10.1021/bk-1975-0018.ch032

Ε . I. D u Pont de Nemours & Co., Inc., Wilmington, Del. 19898

In reverse osmosis (RO), a semipermeable membrane acts as a molecular separator or filter removing the pure water ("permeate") from the saline feed stream by pressure driven transport. The process is today established in the desalination of brackish water, where bidding and selling plants of one million gpd* and more on the basis of specified water quality data is an accepted practice in the trade. The published performance record of some plants installed over the last five years is available as realistic evidence to back the warranties given by manufacturers. Research in seawater RO - supported to a large measure by the U.S. Department of the Interior (Office of Water Research and Technology)has now reached the point where reverse osmosis is moving also in this field from the R&D phase into the commercial arena. Process

Principles

The phenomenon of n a t u r a l osmosis takes p l a c e when a d i l u t e s o l u t i o n and a c o n c e n t r a t e d s o l u t i o n are s e p a r a t e d by a semipermeable membrane, the s o l v e n t m i g r a t i n g from the d i l u t e to the c o n c e n t r a t e d compart ment ( F i g u r e 1 ) . The p r o c e s s i s r e v e r s i b l e and under a h y d r a u l i c p r e s s u r e i n excess o f the osmotic p r e s s u r e , s o l v e n t (water) w i l l flow i n t o the d i l u t e compartment. C o n s i d e r a b l e work of a fundamental n a t u r e was c a r r i e d out i n the l a t e 19th and e a r l y 20th c e n t u r i e s as p a r t of the s t u d i e s on c o l l i g a t i v e p r o p e r t i e s of s o l u t i o n s , but the f i e l d was dormant

* g a l l o n s per day

519

Church; Marine Chemistry in the Coastal Environment ACS Symposium Series; American Chemical Society: Washington, DC, 1975.

520

MARINE

CHEMISTRY

by t h e l a t e 1 9 2 0 ' s . I n t e r e s t was r e k i n d l e d i n t h e 1950 s w i t h the i n t e n s i f i e d s e a r c h f o r e c o n o m i c a l m e t h o d s t o d e s a l t b r a c k i s h and s e a w a t e r . The t h e r m o dynamic e f f i c i e n c y of r e v e r s e osmosis appealed to p h y s i c a l c h e m i s t s a c t i v e i n t h e f i e l d and t h e s e a r c h f o r membranes w i t h s e m i p e r m e a b i l i t y b e g a n . The g o a l s were h i g h r e j e c t i o n of the s o l u t e , i . e . s a l t s , h i g h s o l v e n t o r w a t e r f l u x and i n a d d i t i o n , l i k e w i t h so many o t h e r g o o d t h i n g s i n n a t u r e - l o n g l i f e . The f i r s t i m p o r t a n t l e a d was t h e d i s c o v e r y by R e i d and Breton t h a t c e l l u l o s e a c e t a t e f i l m s have semipermea b i l i t y t o w a r d s s a l t s o l u t i o n s and a c t i v i t i e s i n t h i s f i e l d s u b s e q u e n t l y g a t h e r e d momentum when L o e b and S o u r i r a j a n invented asymmetric c e l l u l o s e acetate membranes. I n t h e s e membranes a t h i n r e j e c t i n g l a y e r s e v e r a l h u n d r e d A n g s t r o m s t h i c k i s s u p p o r t e d on a t h i c k e r b u t more p o r o u s s u b s t r a t e t h a t h a s little r e s i s t a n c e t o w a t e r f l o w ( F i g u r e 2) (1). With asymmetry w a t e r f l u x c o u l d be i n c r e a s e d by one"" t o two o r d e r s o f magnitude without l o s i n g r e j e c t i o n . From t h a t t i m e on, t h e p a c e q u i c k e n e d a t t h e U n i v e r s i t y o f C a l i f o r n i a and a t o t h e r p l a c e s . The f i r s t a t t e m p t s t o d e s a l t s e a w a t e r w e r e more a c r e d i t a b l e d e m o n s t r a t i o n o f p i o n e e r i n g s p i r i t than a t e c h n i c a l accomplishment. The r e a s o n s w e r e i n p a r t t h a t c e l l u l o s e d i a c e t a t e membranes d i d n o t h a v e t h e c h e m i c a l p r o p e r t i e s f o r prolonged s t a b l e performance i n d e s a l t i n g seawater on t o p o f a l l t h e o t h e r f o r m i d a b l e c h a l l e n g e s o f the marine environment. S i n c e then e f f o r t s were l a r g e l y turned towards b r a c k i s h water d e s a l t i n g , w h e r e RO h a s e s t a b l i s h e d a w e l l r e c o g n i z e d p o s i t i o n . S e v e r a l c o m p r e h e n s i v e t e x t s on t h e s t a t e o f t h e a r t i n r e v e r s e osmosis t e c h n o l o g y have been p u b l i s h e d (2, 3, 4, 5 ) . L e t ' s now t u r n t o t h e p r o b l e m s we f a c e w i t h s e a w a t e r RO: TABLE I


1

SALT CONCENTRATION AND IN SEAWATER

OSMOTIC PRESSURE RO

Salt Concentration

Osmotic Pressure (25°C)

Feed

3.45%

369

psia

Concentrate or Reject (30% r e c o v e r y of p r o duct water)

5.15%

567

psia

Average

4.3%

460

psia



ROSENBLATT

Seawater

Figure 1.

Figure 2.

Desalination

Reverse osmosis (RO) principles

Skin structure of asymmetric cellulose acetate membrane (1)



522

MARINE

CHEMISTRY

Standard seawater w i t h a c o n c e n t r a t i o n o f 3·45% s a l t has an osmotic p r e s s u r e of more than 350 p s i . Assuming t h a t 30% of the incoming feed water i s r e covered as product w a t e r , t h e c o n c e n t r a t e stream l e a v i n g the module has a c o n c e n t r a t i o n of 5.15% s a l t and mo re than 550 p s i osmotic p r e s s u r e . In r e a l i t y the t r u e salt concentration at the membrane s u r f a c e i s h i g h e r as a r e s u l t of a phenomenon c a l l e d c o n c e n t r a t i o n p o l a r i z a t i o n ( F i g u r e 3 ) . S a l t and water m i g r a t e to the membrane s u r f a c e , the water passes and the s a l t i s r e j e c t e d and accumulates. U l t i m a t e l y the s a l t r e t u r n s from the boundary to the main strearn as a r e s u l t of the c o n c e n t r a t i o n d i f f e r e n c e . The h i g h e r the membrane f l u x the l a r g e r the accumulât i o n p r o blem . T u r b u l e n t flow o r a narrow gap between membranes i n a laminar flow c o n f i g u r a t i o n w i l l a i d i n d e p l e t i n g the boundary l a y e r . The adverse e f f e c t s of conc e n t r a t i o n p o l a r i z a t i o n or a c c u m u l a t i o n of s a l t i n the boundary l a y e r are : © h i g h e r osmotic p r e s s u r e l e a d i n g to reduced water f l u x , © higher s a l t concentration d i f f e r e n c e a c r o s s the membrane between the b r i n e s i d e and t h e product s i d e l e a d i n g to i n c r e a s e d trans-membrane m i g r a t i o n of s a l t , ® i n c r e a s e d r i s k of p r e c i p i t a t i n g sparingly soluble salts. To a c h i e v e r e a l i s t i c f l u x l e v e l s and p r o d u c t i v i t i e s i n seawater RO, the a p p l i e d p r e s s u r e s a c r o s s the membrane a r e 800-1500 p s i . The problems of c o n c e n t r a t i o n p o l a r i z a t i o n were r e c o g n i z e d e a r l y and w e l l u n d e r s t o o d from t h e o r e t i c a l a n a l y s e s of membrane p e r m e a b i l l t y . Another problem encountered i n RO p r a c t i c e was a p p r e c i a t e d f u l l y o n l y when more f i e l d e x p e r i e n c e was g a i n e d . Any s u r f a c e i n a marine environment i s r a p i d l y covered w i t h d e p o s i t s t h a t w i l l i n t e r f e r w i t h water t r a n s p o r t a c r o s s the membrane and w i t h the e f f e c t i v e removal of the accumulated s a l t s from the membrane s u r f a c e . Sources of these d e p o s i t s a r e 1) b i o l o g i c a l , 2) p a r t i c u l a t e matter i n the seawater,or 3) c o r r o s i o n p r o d u c t s generated by the metals used i n the cons t r u c t i o n of the RO p l a n t .



32.

ROSENBLATT

Seawater

Desalination

523

E x p e r i e n c e has shown t h a t the d e c r e a s e i n membrane p r o d u c t i v i t y w i t h time from f o u l i n g and other causes can be c o r r e l a t e d on a l o g / l o g p l o t . The p r o d u c t i v i t y l o s s w i t h time f o r s e v e r a l l e v e l s of l o g a r i t h m i c f l u x d e c l i n e rate are i l l u s t r a t e d i n F i g u r e 4 and the importance of e f f e c t i v e f o u l i n g cont r o l i s evident. We have seen cases where membrane p r o d u c t i v i t y dropped at a r a t e c o r r e s p o n d i n g to 20% r e s i d u a l p r o d u c t i v i t y a f t e r one y e a r . E f f e c t i v e f o u l i n g c o n t r o l can m a i n t a i n a l o g a r i t h m i c f l u x dec l i n e r a t e b e t t e r than 0.05, c o r r e s p o n d i n g to 75% of the i n i t i a l p r o d u c t i v i t y a f t e r the f i r s t year and 70% a f t e r t h r e e years and we b e l i e v e based on our experience that t h i s i s a r e a l i s t i c goal. Interestingly, f o u l i n g i n seawater RO i n t e r f e r r e d l e s s w i t h s a l t r e j e c t i o n than w i t h p r o d u c t i v i t y . The p r o d u c t water (permeate) from a d e s a l t i n g p l a n t i s expected to conform w i t h U.S. P u b l i c H e a l t h S t a n d a r d s , i . e . to c o n t a i n not more than 500 ppm t o t a l d i s s o l v e d s o l i d s (TDS). The l i m i t f o r c h l o r i d e i o n s i s 250 ppm. These water q u a l i t y s p e c i f i c a t i o n s c a l l f o r a s a l t r e j e c t i o n c a p a b i l i t y above 98.5%. In p r a c t i c e the r e j e c t i o n has to be b e t t e r than 99% to compensate f o r the i n c r e a s e d s a l t c o n c e n t r a t i o n r e s u l t i n g from product water r e c o v e r y and from concentration polarization. As much as RO appeals t h e r m o d y n a m i c a l l y , the p r o c e s s poses a c o s t e f f i c i e n c y problem because i t i s a r e l a t i v e l y slow r a t e p r o c e s s . F o r example, the water g e n e r a t i n g c a p a c i t y of good f i l m membranes f o r seawater d e s a l i n a t i o n i s 10-15 g a l l o n / s q . f t . / d a y . In comparison a w e l l e n g i n e e r e d e v a p o r a t o r w i t h a heat t r a n s f e r c o e f f i c i e n t of 1000 BTU / ( h r . ) ( s q . f t . ) (°F) g e n e r a t e s 24 l b s . or 3 g a l l o n s o f water per square f o o t per day per °F or 30 g a l l o n s a t a 10°F driving force. Since seawater RO has to be conducted i n s i d e a p r e s s u r i z e d space d e s i g n e d f o r 800 to 1500 p s i o p e r a t i o n , an o b v i o u s g o a l i s to package as much membrane area as p o s s i b l e i n t o a p r e s s u r e v e s s e l . Engineering

of Membrane D e v i c e s

We can now compile a l i s t of the problems t h a t have to be c o n s i d e r e d i n the development of RO d e v i c e s f o r seawater d e s a l t i n g : 1.

Support of the t h i n f r a g i l e membrane a g a i n s t the d i f f e r e n t i a l p r e s s u r e of 800 to 1500 p s i i n the case of seawater.


524

MARINE

CHEMISTRY

MEMBRANE

BULK AQUEOUS


WATER

SALT SOLUTION

REMOVAL

BULK CONC

FLOW

Figure 3.

Ί

,

,

-j-

,

,

,

, ,

Concentration

,

,

,

polarization

,

1·

I ϊ

ι

1

τ

1 1I i

TIME , days

Figure 4.

Effect of fouling control on productivity


I I


32.

ROSENBLATT

Seawater

525

Desalination

2.

Compactness - h i g h p r o d u c t i v i t y per u n i t volume.

3.

Low c o n c e n t r a t i o n

4.

Fouling control. P r e v e n t i o n and f o u l a n t removal by cleaning.

5.

E f f e c t i v e s e a l i n g of the membrane to prevent b y - p a s s i n g of feed i n t o p r o d u c t .

polarization.

A compact, h i g h p r e s s u r e mass exchange d e v i c e accommodating a l l these requirements was indeed a n o v e l and f o r m i d a b l e c h a l l e n g e to i m a g i n a t i v e engineering. Three c o n f i g u r a t i o n s among a l l the i d e a s proposed a t one time or another remained as the p r i n c i p a l c a n d i d a t e s : the t u b u l a r , the s p i r a l , and the h o l l o w f i b e r concept l i s t e d here i n i n c r e a s i n g order of compactness, i . e . s p e c i f i c s u r f a c e a r e a per u n i t volume: 2

3

tubular device

100 f t / f t

s p i r a l device

300

"

"

hollow f i b e r device 3300 " " The e a r l i e s t concept was the p e r f o r a t e d or porous t u b u l a r c o n f i g u r a t i o n , u s u a l l y 0.5 i n c h ID w i t h a membrane c o n t a i n e d i n s i d e the tube and supported on a l i n e r from f a b r i c , paper or non-woven m a t e r i a l s . Feed water under p r e s s u r e flows through the bore and the product water permeates through the membrane and l e a v e s v i a the p e r f o r a t i o n s (.4). T u b u l a r d e v i c e s , b e i n g r e l a t i v e l y s i m p l e , saw the most d i v e r s e e f f o r t to o p t i m i z e them t e c h n i c a l l y and e c o n o m i c a l l y by: • j u d i c i o u s s e l e c t i o n of m a t e r i a l s , e.g. sand l o g s , porous f i b e r g l a s s tubes, • compact packaging, e.g. h e l i c a l c o i l configuration, • advanced f a b r i c a t i o n t e c h n o l o g y , e.g. l a s e r d r i l l i n g of p e r f o r a t i o n s .



526

MARINE

CHEMISTRY

Another approach to a c h i e v e l a r g e s p e c i f i c areas i s the s p i r a l wound d e v i c e , wherein a f l a t membrane envelope i s formed around a f a b r i c spacer and c l o s e d on t h r e e s i d e s ( F i g u r e 5 ) . The open s i d e t e r m i n a t e s at a porous o r p e r f o r a t e d p r o d u c t water tube. The envelope or l e a f t o g e t h e r w i t h an e x t e r n a l s p a c e r f o r the feed water stream i s r o l l e d s p i r a l l y around the p r o d u c t tube and i s then i n s t a l l e d i n a p r e s s u r e vessel. The f e e d stream flows a x i a l l y through the c h a n n e l s between the s p i r a l w i n d i n g s . Water permeates through the membrane and flows r a d i a l l y i n s i d e the l e a f towards the product tube. T h i s membrane conf i g u r a t i o n i s p r e d o m i n a n t l y the r e s u l t o f work done by the Roga D i v i s i o n of G e n e r a l Atomics (now owned by U n i v e r s a l O i l P r o d u c t s ) w i t h funds p r o v i d e d by the O f f i c e of S a l i n e Water (now O f f i c e of Water Research and Technology) i n the U.S. Department of the I n t e r i o r . The most compact membrane d e v i c e s developed so f a r a r e h o l l o w f i b e r permeators. Du P o n t s h o l l o w f i b e r membranes have t h e t h i c k n e s s of a human h a i r . The f i b e r i s asymmetric w i t h a r e j ec t i n g s k i n on the o u t s i d e supported on an i n n e r porous l a y e r ( F i g u r e 6 ) . P r e s s u r i z e d s a l i n e water flows around the o u t s i d e of the f i b e r and the water permeates through the s k i n and the porous support and l e a v e s v i a the bore. The OD i s twice the ID and the f i b e r has the m e c h a n i c a l c h a r a c t e r i s t i c s of a t h i c k - w a l l e d p r e s s u r e v e s s e l and the membrane i s s e l f - s u p p o r t i n g . The f i b e r bundle c o n s i s t i n g o f s e v e r a l hundred thousand up to a few m i l l i o n f i b e r s i s s i m i l a r to a U-tube heat exchanger. The open end i s encased i n an epoxy tube sheet which s e p a r a t e s the s a l i n e water from the permeate. The feed stream e n t e r s through a c e n t r a l d i s t r i b u t o r p i p e , flows r a d i a l l y outward through the bundle and i s removed v i a the a n n u l a r flow s c r e e n a t the v e s s e l w a l l ( F i g u r e 7 ) . D e t a i l s on the c o n s t r u c t i o n and h i s t o r y of the Permasep permeator were p u b l i s h e d elsewhere (J5) . T a b l e I I summarizes the c h a r a c t e r i s t i c s of the d i f f e r e n t designs. More r e c e n t l y a new e n t r y , the " s p a g h e t t i " module h a v i n g the membrane on the o u t s i d e of a p o l y p r o p y l e n e rod w i t h l o n g i t u d i n a l grooves was announced by the U n i t e d Kingdom Atomic Energy E s t a b l i s h m e n t (JO ; but l i t t l e i s known on i t s performance i n seawater desalting. 1



32.

ROSENBLATT

Seawater

Desalination

Figure 5. Spiral wound membrane configuration, (a) spiral element, (b) partially unrolled.

Figure 6.

Skin structure of asymmetric hollow fiber membrane

(1)



1000-1500

Tolerant

Operating Pressure, p s i (seawater)

Particulate Tolerance

Fiber

Chemical

Economical

Expensive

Cost

Sens i t i v e

800

Self-Supporting

3300

Hollow

Mechanical Chemical

Moderate

1000-1500

Supported

300

Spiral

Cleaning

Matter

Supported

100

Membrane Support

Compactness f12 membrane/ft^ v o l .

Tubular

CHARACTERISTICS OF PRINCIPAL RO DEVICES

TABLE I I


32.

ROSENBLATT


Plant

Seawater

Desalination

529

Engineering

The e s s e n t i a l elements o f an RO p l a n t a r e i n t a k e f a c i l i t i e s , p r e t r e a t m e n t equipment to c o n t r o l membrane fouling, a h i g h p r e s s u r e pump to p r o v i d e the d r i v i n g f o r c e and of course membrane modules ( F i g u r e 8 ) . The two p r i n c i p a l system concepts under c o n s i d e r a t i o n f o r seawater d e s a l t i n g were the one-pass and the two-pass p r o c e s s e s i l l u s t r a t e d i n F i g u r e 9. The two-pass approach w i t h two s t a g e s of s a l t i n t e r c e p t i o n appealed as the t e c h n i c a l l y more r e l i a b l e r o u t e , but the membranes i n the two-pass RO p l a n t s h o u l d have a t l e a s t twice the f l u x of one-pass membranes to be e c o n o m i c a l l y competitive. F i e l d t e s t s w i t h l i v e seawater to develop know-how on p l a n t e n g i n e e r i n g were c a r r i e d out a t Ocean C i t y , New J e r s e y on a s i t e o p e r a t e d by the Ocean C i t y Research C o r p o r a t i o n . The seawater was d i l u t e d w i t h r u n - o f f water from the areas b o r d e r i n g the bay, a r e p r e s e n t a t i v e c o n c e n t r a t i o n b e i n g 2.7% TDS. The seawater c o n t a i n e d r e l a t i v e l y l a r g e q u a n t i t i e s of suspended matter c o m p l i c a t i n g the cont r o l of membrane f o u l i n g . In 1968 we had l a b t e s t e d w i t h s y n t h e t i c s e a water an asymmetric p o l y a m i d e h y d r a z i d e f i l m membrane (DP-1) which produced 10-12 g f d * (>99% r e j e c t i o n ) at 1000 p s i ( F i g u r e 10) ( 8 ) . Soon t h e r e a f t e r t h i s f i l m membrane was m o d i f i e d f o r two-pass o p e r a t i o n to p r o duce more than 30 g f d * at 1000 p s i and rejection l e v e l s of 90%. Experiments were c a r r i e d out at Ocean C i t y w i t h f l a t t e s t c e l l s ( F i g u r e 11) (9) to c l a r i f y the r e l a t i v e m e r i t s o f one-pass v e r s u s twopass d e s a l i n a t i o n and to l e a r n enough about f i e l d o p e r a t i o n and f o u l i n g c o n t r o l to move towards p i l o t plant t r i a l s . The two major c l a s s e s of membrane f o u l a n t s a r e p a r t i c u l a t e matter and b i o l o g i c a l l y a c t i v e s p e c i e s which form f o u l a n t l a y e r s on the membrane. The f i r s t c a t e g o r y i s b e s t i n t e r c e p t e d by combining c o a g u l a t i o n w i t h f i l t r a t i o n through sand and/or diatomaceous earth f i l t e r s . The b i o l o g i c a l f o u l a n t s a r e conv e n i e n t l y rendered i n a c t i v e by c h l o r i n a t i o n which i s performed more e f f e c t i v e l y below the normal pH 8 of n a t u r a l seawater. S i n c e f r e e c h l o r i n e i s h a r m f u l to p o l y a m i d e h y d r a z i d e and polyamide membranes i t has to be removed c h e m i c a l l y o r w i t h a carbon treatment.

*gallon/sq. ft.-day



MARINE

Figure 7.

Cutaway drawing of Permasep permeator

SALINE WATER INTAKE Figure 8.

Schematic of RO plant

ONE-PASS PROCESS SEAWATER FEED , RO PLANT 3.5% TDS

PERMEATE

Seawater Desalination by Reverse Osmosis - ACS Publications

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