SALINE WATER CONVERSION

Sea Water Conversion by the Multistage Flash Evaporation Method Downloaded by UNIV OF CALIFORNIA SAN DIEGO on September 5, 2015 | http://pubs.acs.org Publication Date: January 1, 1960 | doi: 10.1021/ba-1960-0027.ch015

D. B. BRICE and C. R. TOWNSEND Research Division, The Fluor Corp., Ltd., Whittier, Calif.

The cost of converting sea water into potable water is sufficiently low to make it a potentially important source of supplemental water for many parts of the world. At present, multistage flash evaporation in large-capacity plants is the most economical process. In many areas, because of the tremendous quantities of heat required, nuclear fuels are the only feasible source of energy for large-capacity sea water conversion plants. Based on today's technology, the cost of water produced by a single-purpose multistage flash evaporator using a nuclear steam generator was estimated to be in the range of 38 to 42 cents per thousand gallons. If optimistic predictions of future advances in both the evaporator plant and the nuclear steam generator are realized, an ultimate water cost of 24 to 31 cents per thousand gallons will be possible within the next decade.

For m a n y y e a r s , f r e s h w a t e r has been o b t a i n e d f r o m t h e ocean f o r s h i p b o a r d use b y u t i l i z i n g t h e p r i n c i p l e of f l a s h e v a p o r a t i o n . M o r e r e c e n t l y , m o d e s t - s i z e d l a n d - b a s e d m u l t i s t a g e flash e v a p o r a t i o n p l a n t s h a v e b e e n c o n s t r u c t e d a n d are p r o d u c i n g p o t a b l e w a t e r . E n g i n e e r i n g e v a l u a t i o n s of t h e m u l t i s t a g e flash s y s t e m h a v e s h o w n t h a t i t c a n e c o n o m i c a l l y s u p p l y f r e s h w a t e r t o l a r g e p o p u l a t i o n centers, a l t h o u g h l a r g e - c a p a c i t y sea w a t e r c o n v e r s i o n p l a n t s of a n y k i n d h a v e n o t y e t b e e n b u i l t . I n t h e n o t t o o d i s t a n t f u t u r e , h o w e v e r , t h e i n s t a l l e d c a p a c i t y of l a n d - b a s e d m u l t i s t a g e flash p l a n t s w i l l p r o b a b l y d w a r f t h i s y e a r ' s e s t i m a t e d c a p a c i t y of a b o u t 6,000,000 g a l l o n s a d a y . W a t e r administrators throughout the w o r l d are greatly concerned about supplem e n t i n g w a t e r s u p p l i e s . M a n y of t h e m w o u l d l i k e t o t a p t h e ocean as a source f o r future fresh water. A . L . M i l l e r , D i r e c t o r o f t h e Office o f S a l i n e W a t e r , s u m m a r i z e d t h e p r o b l e m i n a recent speech. " I t i s h a r d t o r e a l i z e / ' h e s a i d , " a s w e s t a n d o n t h e t h r e s h o l d o f space, t h a t w i t h i n a f e w y e a r s o u r n u m b e r one d o m e s t i c p r o b l e m m a y b e t h e p r o v i s i o n of a d e q u a t e s u p p l i e s of p l a i n o r d i n a r y w a t e r . T h e p r e d i c t e d increase i n w a t e r use i n t h e c o m i n g decades m a k e s i t u n m i s t a k a b l y c l e a r t h a t w e w i l l n e e d m o r e w a t e r t h a n c a n b e p r o v i d e d f r o m r e a d i l y a v a i l a b l e n a t u r a l sources of s u p p l y . T h e d a y of t h e w a t e r w i t c h i s o v e r . W e m u s t t u r n t o scientific a n d t e c h n o l o g i c a l r e s e a r c h t o d e v e l o p a n e w s o u r c e of s u p p l y t h a t c a n p r o v i d e a n e v e r - g r o w i n g p e r c e n t a g e of t o m o r r o w ' s w a t e r . " 147

In SALINE WATER CONVERSION; Advances in Chemistry; American Chemical Society: Washington, DC, 1960.

148

ADVANCES IN CHEMISTRY SERIES

T h e m u l t i s t a g e flash e v a p o r a t o r process c o u l d p l a y a v i t a l role i n m a k i n g D r . M i l l e r ' s p r e d i c t i o n of a n e w source of s u p p l y come t r u e . I t w o u l d c o n v e r t sea w a t e r t o f r e s h o n a l a r g e scale.

Downloaded by UNIV OF CALIFORNIA SAN DIEGO on September 5, 2015 | http://pubs.acs.org Publication Date: January 1, 1960 | doi: 10.1021/ba-1960-0027.ch015

B a s e d o n our evaluations, a single-purpose p l a n t using nuclear energy t o operate m u l t i s t a g e flash e v a p o r a t o r s w o u l d r e s u l t i n t h e m o s t e c o n o m i c a l p r o d u c t i o n of p o t a b l e w a t e r f r o m t h e ocean. T h e r e a r e , of course, s p e c i a l e c o n o m i c s i t u a t i o n s w h e r e t h e l o g i c a l choice w o u l d b e a c o m b i n e d p o w e r a n d w a t e r p l a n t t h a t w o u l d u t i l i z e f o s s i l fuels, b u t i n g e n e r a l , a s i n g l e - p u r p o s e p l a n t w o u l d b e t h e m o s t a d v a n t a g e o u s . A s i n g l e p u r p o s e m u l t i s t a g e flash e v a p o r a t o r p l a n t t h a t uses a n u c l e a r s t e a m g e n e r a t o r i s s h o w n i n F i g u r e 1. HIGH PRESSURE HOT WATER k

STEAM

.

Figure I.

•

1

Fresh water produced by multistage flash evaporation

I t w a s e s t a b l i s h e d , o n t h e basis of c u r r e n t t e c h n o l o g y , t h a t t h e p r a c t i c a l l i m i t of c a p a c i t y of a m u l t i s t a g e flash e v a p o r a t o r p l a n t ( c o n s i s t i n g of s e v e r a l vessels i n series as s h o w n i n F i g u r e 2 ) w o u l d b e a p p r o x i m a t e l y 25,000,000 t o 30,000,000 g a l l o n s a d a y . T h e e c o n o m i c s of w a t e r p r o d u c t i o n discussed b e l o w are b a s e d o n t w o s u c h u n i t s o p e r a t i n g i n p a r a l l e l , n o m i n a l l y p r o d u c i n g 50,000,000 gallons of p o t a b l e w a t e r a d a y . T h e c a p a c i t y of t h e p l a n t w a s d e t e r m i n e d b y t h e s m a l l e s t e c o n o m i c size of n u c l e a r s t e a m generator, w h i c h w o u l d provide energy f o r evaporation a n d f o r d r i v i n g a m a j o r i t y of the p u m p s .

Steam Generation T h e s t e a m g e n e r a t o r selected f o r t h e o p t i m i z a t i o n s t u d y ( s h o w n s c h e m a t i c a l l y i n F i g u r e 1) i s a 3 7 0 - t h e r m a l m e g a w a t t ( n e t t o t h e e v a p o r a t o r ) p r e s s u r i z e d l i g h t - w a t e r r e a c t o r . T h e selection of t h e r e a c t o r t y p e a n d d e t a i l s of i t s design (3) are o u t s i d e t h e scope of t h i s p a p e r . H o w e v e r , t h e s t e a m i s e s t i m a t e d t o cost 37 cents p e r m i l l i o n B . t . u . (3), i n c l u d i n g a l l costs associated w i t h t h e n u c l e a r s t e a m g e n e r a t o r , T h e sea w a t e r c o n v e r s i o n p l a n t w a s o p t i m i z e d t o c o n s u m e 350 t m w i n t h e b r i n e h e a t e r s . T h e b a l a n c e of t h e e n e r g y was r e q u i r e d f o r t h e s t e a m t u r b i n e p u m p d r i v e r s a n d s t e a m j e t ejectors. A n u c l e a r process h e a t r e a c t o r w a s selected f o r t h i s a p p l i c a t i o n f o r e c o n o m i c r e a sons. S t u d i e s (1, 3) i n d i c a t e d t h a t t h e e c o n o m i c s o f s t e a m g e n e r a t i o n f r o m n u c l e a r e n e r g y a r e f a v o r a b l e w h e r e a l a r g e a m o u n t of r e l a t i v e l y l o w - t e m p e r a t u r e t h e r m a l e n e r g y i s r e q u i r e d i n a s i n g l e - p u r p o s e p l a n t . T h e source o f e n e r g y u s e d t o p r o d u c e t h e s t e a m r e q u i r e d i s i m m a t e r i a l as f a r as t h e sea w a t e r e v a p o r a t o r s a r e c o n c e r n e d .


149

BRICE AND TOWNSEND—MULTISTAGE FLASH EVAPORATION

STAGES 9-16

CONDENSATE

i

STAGES 17-24

*

STAGES 25-32

I


STAGES 41-48

STAGES 33-40

i

V

\

1 FROM GOMILUONGPD]^^ FRESH WATER ^TO STORAC

25 MILLION GPO DISTRIBUl 218 MILLION GPO

35 MILLION GPD

Figure 2. Typical process flow diagram for multistage flash evaporator sea water conversion plant Capacity 25,000,000 gallons per day

Process Description F i g u r e 2 shows t h e flow of s e a w a t e r , b r i n e , a n d condensate t h r o u g h a t y p i c a l m u l t i s t a g e flash e v a p o r a t o r . I n t h e specific p l a n t s h o w n , t h e s e a w a t e r i s p u m p e d f r o m t h e ocean t h r o u g h t u b e s i n t h e i n t e g r a l d e a e r a t o r a n d t h r o u g h t h e f o u r l o w e s t t e m p e r a t u r e stages of t h e e v a p o r a t o r p l a n t before i t i s d e a e r a t e d . Chemicals used t o c o n t r o l scale m a y b e a d d e d e i t h e r t o t h e sea w a t e r b e f o r e i t i s d e a e r a t e d , o r t o t h e combined r e c y c l e - m a k e - u p stream, before i t is p u m p e d t h r o u g h t h e tubes i n t h e higher t e m p e r a t u r e stages a n d b r i n e h e a t e r . T h e exact p o s i t i o n of t h e a d d i t i o n of scale c o n t r o l c h e m i c a l s depends o n t h e m e t h o d e m p l o y e d . T h e d e a e r a t e d s e a w a t e r serves a s t h e m a k e - u p t o t h e p l a n t . I t is m i x e d i n t h e d e a e r a t o r w i t h t h e s l i g h t l y c o n c e n t r a t e d b r i n e . These t w o s t r e a m s — n o w combined—are p u m p e d t h r o u g h the remaining tubes of t h e e v a p o r a t o r s a n d t h e b r i n e h e a t e r b e f o r e b e i n g i n t r o d u c e d i n t o t h e shell side o f t h e h i g h e s t t e m p e r a t u r e stage. T h e s h e l l - s i d e b r i n e t h e n cascades f r o m stage t o stage as a r e s u l t of t h e p r e s s u r e d i f f e r e n t i a l m a i n t a i n e d . I n e a c h stage some o f t h e w a t e r flashes f r o m t h e b r i n e s o l u t i o n . I t i s condensed o n t h e t u b e s of t h e e v a p o r a t o r a n d c a u g h t i n t r o u g h s p o s i t i o n e d b e l o w t h e t u b e s . T h e condensate also cascades f r o m stage t o stage. F i n a l l y , t h e shell-side b r i n e a n d t h e condensate r e a c h t h e l o w e s t p r e s s u r e stage. A t t h i s p o i n t , t h e condensate i s p u m p e d f r o m t h e s y s t e m as p r o d u c t . T h e brine i n excess of t h a t r e q u i r e d f o r r e c y c l e is p u m p e d f r o m t h e s y s t e m a n d d i s c h a r g e d t o t h e ocean as b l o w d o w n . T h e r e m a i n d e r of t h e b r i n e i s m i x e d w i t h t h e m a k e - u p a n d r e cycled through the system. A n e j e c t o r s y s t e m is r e q u i r e d t o r e m o v e i n e r t s f r o m t h e p l a n t a t t h e lowest p r e s s u r e p o i n t i n t h e s y s t e m . F o r t h e p l a n t s h o w n i n F i g u r e 2, t h i s p o i n t i s t h e d e a e r a t o r . S u i t a b l e i n s t r u m e n t s a r e r e q u i r e d i n t h e p l a n t t o c o n t r o l l i q u i d flows, t e m p e r a t u r e s , a n d levels. T h e process a n d c o n t r o l s h a v e b e e n d e s c r i b e d i n d e t a i l (3).

Mechanical Design C e r t a i n m e c h a n i c a l designs, s u c h as single l e v e l c o n s t r u c t i o n a n d condenser t u b e s t h a t r u n c o n t i n u o u s l y t h r o u g h s e v e r a l stages, h a v e b e e n i n c o r p o r a t e d i n t h e p l a n t because of economies t h a t c a n b e r e a l i z e d b y u s i n g t h i s t y p e of c o n s t r u c t i o n . I n t h e d e s i g n s h o w n i n F i g u r e 2, t u b e s r u n t h r o u g h f o u r stages. T u b e sheets a r e e m p l o y e d



150


o n l y a t t h e ends, a n d baffles are u s e d i n s t e a d of t u b e sheets f o r t h e o t h e r stage-to-stage s e p a r a t i o n . T h e holes i n t h e baffles are designed f o r a close fit t o t h e t u b e s . T h e y c a n also b e sealed t o p r e v e n t stage-to-stage s t e a m l e a k a g e a l o n g t h e t u b e s ( 4 ) . A p l a n t t h u s c o n s t r u c t e d w o u l d e l i m i n a t e t h e n e e d f o r i n d i v i d u a l w a t e r boxes f o r e a c h stage a n d f o r a m a j o r i t y of t h e t u b e sheets, t h e r e b y effecting e c o n o m y i n c a p i t a l costs. A l s o , i n a l a r g e - c a p a c i t y p l a n t , t h e d i a m e t e r of t h e vessels i s l a r g e . F o r t h e p l a n t s h o w n i n F i g u r e 2 ( w h i c h r e p r e s e n t s a c a p a c i t y of a p p r o x i m a t e l y 25,000,000 g a l l o n s a d a y ) , t h e e v a p o r a t o r vessels are 3 0 feet i n d i a m e t e r . T h e vessels are h o r i z o n t a l c y l i n d e r s d e s i g n e d t o t a k e f u l l a d v a n t a g e of t h e s h a p e t o save m e t a l , b o t h i n t h e w a l l s a n d i n t h e stiffeners r e q u i r e d u n d e r t h e c o n d i t i o n s of o p e r a t i o n . T h e s e vessels w i l l h a v e t o b e f i e l d - e r e c t e d because of t h e i r l a r g e size. H o w e v e r , c e r t a i n c o m p o n e n t s s u c h as t h e t u b e b u n d l e s c a n b e s h o p - f a b r i c a t e d t o a v o i d field r o l l i n g o f t h e t u b e s . E x c e p t f o r t h e r e m o t e l y l o c a t e d sea w a t e r p u m p , a l l t h e p u m p s i n t h e p r o c e s s a r e a ( s h o w n i n F i g u r e 2) a r e d r i v e n b y s t e a m t u r b i n e s . B e c a u s e l a r g e q u a n t i t i e s of r e l a t i v e l y l o w - p r e s s u r e s t e a m are r e q u i r e d i n t h e process, t h e use of s t e a m - d r i v e n t u r b i n e s i n s t e a d o f e l e c t r i c m o t o r s r e s u l t s i n a s a v i n g s of s e v e r a l cents p e r t h o u s a n d g a l l o n s of product. Because of this, t h e economic analysis a n d o p t i m i z a t i o n presented below h a v e b e e n b a s e d o n t h e u s e of s t e a m - d r i v e n t u r b i n e d r i v e r s f o r t h e p u m p s l o c a t e d i n t h e process a r e a .

Design Variables T h e d e s i g n v a r i a b l e s c o n s i d e r e d i n t h e o p t i m i z a t i o n of a l a r g e - c a p a c i t y p l a n t a r e s h o w n i n F i g u r e 3. T h e r e l a t i o n s h i p b e t w e e n t h e stage t e r m i n a l t e m p e r a t u r e difference ( T T D ) , n u m b e r of stages, a n d p e r f o r m a n c e r a t i o ( p o u n d s of w a t e r p r o d u c e d p e r p o u n d of s t e a m condensed) i s r e a d i l y a p p a r e n t u p o n e x a m i n a t i o n of F i g u r e 3. U n l i k e a

2

4

6

8

10

12

TERMINAL TEMPERATURE DIFFERENCE, ° F

Figure 3.

Relation of design variables

m u l t i p l e - e f f e c t d i s t i l l a t i o n s y s t e m , i t i s p o s s i b l e i n t h e m u l t i s t a g e flash s y s t e m t o select t h e n u m b e r of stages a n d t h e p e r f o r m a n c e r a t i o i n d e p e n d e n t l y . T h e r e i s , o f course, a p r a c t i c a l l i m i t a t i o n t o t h e n u m b e r of stages t h a t c a n b e u s e d . T h e n u m b e r d e p e n d s t o a c o n s i d e r a b l e e x t e n t o n t h e o v e r - a l l t e m p e r a t u r e difference b e t w e e n t h e i n c o m i n g b r i n e t o t h e first stage a n d the b l o w d o w n t o t h e ocean. B e c a u s e a g i v e n stage-to-stage p r e s s u r e d i f f e r e n t i a l i s r e q u i r e d f o r s a t i s f a c t o r y r e g u l a t i o n o f s h e l l - s i d e b r i n e flow, a m i n i m u m t e m p e r a t u r e difference i s r e q u i r e d f r o m stage t o stage, d e p e n d i n g o n t h e a b s o l u t e p r e s s u r e . T h e n u m b e r o f stages c a n b e i n c r e a s e d as i m p r o v e m e n t s i n m e t h o d s of scale p r e v e n t i o n p e r m i t h i g h e r b r i n e t e m p e r a t u r e s . F o r t h e c o n d i t i o n o f 2 2 0 ° F . b r i n e i n l e t a n d 9 0 ° F . b l o w d o w n (as i l l u s t r a t e d i n F i g u r e 3 ) , t h e p r a c t i c a l l i m i t a t i o n i s p r o b a b l y o n t h e o r d e r o f 6 0 stages.


151



T h e m o s t i m p o r t a n t design v a r i a b l e i s t h e t e r m i n a l t e m p e r a t u r e difference. T h i s v a r i a b l e h a s t h e strongest influence o n t h e condenser s u r f a c e r e q u i r e d i n t h e e v a p o r a t o r s a n d o n t h e h e a t e c o n o m y of t h e p l a n t . T h e n u m b e r o f stages also has a n effect, b u t i t i s c o n s i d e r a b l y less t h a n t h e effect of t h e t e r m i n a l t e m p e r a t u r e difference. A l s o , t h e r e l a t i o n s h i p s s h o w n i n F i g u r e 3 are f o r a b l o w d o w n c o n c e n t r a t i o n of t w i c e t h a t o f i n c o m i n g sea w a t e r . H o w e v e r , v a r i a t i o n i n b l o w d o w n c o n c e n t r a t i o n has o n l y a m i n o r effect o n t h e e c o n o m i c s . T h e p e r f o r m a n c e r a t i o o r h e a t e c o n o m y i s a r e s u l t of t h e selection of d e s i g n v a r i a b l e s p r e v i o u s l y discussed, a n d i s n o t a v a r i a b l e as s u c h . L i n e s o f c o n s t a n t c a p i t a l cost p e r d a i l y g a l l o n o f c a p a c i t y are also i n c l u d e d i n F i g u r e 3. C a p i t a l costs h a v e been b a s e d o n p l a n t c a p a c i t i e s i n a range o f 25,000,000 t o 60,000,000 g a l l o n s a d a y a n d a v e l o c i t y i n t h e e v a p o r a t o r t u b e s o f 5 feet p e r second. T h e s e lines o f c o n s t a n t c a p i t a l cost p e r d a i l y g a l l o n a r e a r e s u l t of cross p l o t t i n g t h e r e s u l t s o b t a i n e d i n t h e o p t i m i z a t i o n study.

Optimization T h e o p t i m i z a t i o n o f t h e l a r g e - c a p a c i t y m u l t i s t a g e flash e v a p o r a t o r w a s b a s e d o n t h e c o n s u m p t i o n of t h e 3 7 0 t h e r m a l m e g a w a t t s of e n e r g y a v a i l a b l e f r o m t h e n u c l e a r s t e a m g e n e r a t o r . I t w a s n e c e s s a r y t o d e t e r m i n e t h e c a p i t a l cost f o r v a r i o u s a s s u m e d t e r m i n a l t e m p e r a t u r e differences a n d n u m b e r s of stages. Added to the amortized c a p i t a l cost w e r e a l l o t h e r costs n e c e s s a r y f o r o p e r a t i o n of a c o m p l e t e p l a n t , s u c h a s steam, labor, utilities, materials, and overhead. R e s u l t s are s h o w n g r a p h i c a l l y i n F i g u r e 4 f o r a b r i n e t e m p e r a t u r e o f 2 2 0 ° F . , c o n denser t u b e v e l o c i t y o f 5 feet p e r second, b l o w d o w n t e m p e r a t u r e o f 9 0 ° F . , a n d b r i n e c o n c e n t r a t i o n o f t w i c e sea w a t e r . A s c a n b e seen, a m i n i m u m w a t e r cost f o r these c o n d i t i o n s i s o b t a i n e d w i t h a 50-stage p l a n t o p e r a t i n g w i t h a t e r m i n a l t e m p e r a t u r e difference o f a b o u t 4 ° F . S i m i l a r c a l c u l a t i o n s w e r e m a d e f o r a b l o w d o w n c o n c e n t r a t i o n of 1.5 t i m e s sea w a t e r a n d f o r a o n c e - t h r o u g h s y s t e m . B y cross p l o t t i n g , i t w a s t h e n possible t o d e t e r m i n e t h e o p t i m u m b l o w d o w n salt c o n c e n t r a t i o n f o r t h e p l a n t . I t w a s a b o u t 1.7 t i m e s sea w a t e r . H o w e v e r , t h e c u r v e i s a l m o s t flat i n t h e r a n g e of 1.5 t o 2.0 t i m e s sea w a t e r . 20 STAGES

1.4

30 STAGES

40 STAGES 50 STAGES

STAGES

1.0

2

4 6 8 10 TERMINAL TEMPERATURE DIFFERENCE, °F

Figure 4. Relative water cost as a function of terminal temperature difference for several numbers of stages Plant capacity 25,000,000 to 60,000,000 gallons per day


152



T h e t u b e b r i n e v e l o c i t y w a s selected a f t e r c o n s i d e r a t i o n of t h e h i g h e r pressure d r o p ( p u m p i n g cost) a n d m o r e r a p i d r a t e of e r o s i o n c o r r o s i o n w i t h h i g h e r t u b e b r i n e v e l o c i t y a n d l o w e r h e a t t r a n s f e r coefficients w i t h l o w e r v e l o c i t y , c o n s i s t e n t w i t h s o u n d e n g i n e e r i n g design. A s a p r e v i o u s s t u d y (2) i n d i c a t e d t h e d e s i r a b i l i t y o f h i g h e r b r i n e t e m p e r a t u r e s i n t e r m s of e c o n o m i c w a t e r p r o d u c t i o n , t h e h i g h e s t p r a c t i c a l t e m p e r a t u r e ( 2 2 0 ° F . ) w a s selected. T h e s e l e c t i o n o f a b l o w d o w n t e m p e r a t u r e o f 9 0 ° F . w a s p r i m a r i l y based u p o n t h e v a p o r v o l u m e r e q u i r e m e n t s a t t h i s t e m p e r a t u r e a n d , t o a lesser e x t e n t , o n t h e t e m p e r a t u r e o f t h e sea w a t e r , w h i c h w a s i n t h e r a n g e o f 5 7 ° t o 6 7 ° F . B e c a u s e a l a r g e n u m b e r of c a l c u l a t i o n s were r e q u i r e d t o d e t e r m i n e m a t e r i a l a n d e n e r g y balances f o r a l l t h e c o n d i t i o n s r e q u i r e d , these c a l c u l a t i o n s w e r e p r o g r a m m e d f o r s o l u t i o n o n a c o m p u t e r . D e t a i l s of t h e p r o g r a m h a v e been p u b l i s h e d (3). I t w a s w r i t t e n so t h a t flows, t e m p e r a t u r e s , pressures, salt c o n c e n t r a t i o n of t h e b r i n e , a n d c o n denser surface f o r e a c h stage w e r e p r i n t e d as c o m p u t e r o u t p u t d a t a . I t w a s t h e n p o s sible t o t a k e these d a t a , design t h e vessels, d e t e r m i n e l i n e sizes a n d condenser r e q u i r e d , a n d e s t i m a t e c a p i t a l costs f o r p l a n t s w i t h i n a c a p a c i t y r a n g e of 25,000,000 t o 60,000,000 gallons a d a y . F i g u r e 5 shows t h e r e l a t i o n s h i p of t h e s e v e r a l w a t e r cost c o m p o n e n t s as a f u n c t i o n of t h e T T D f o r a 50-stage sea w a t e r c o n v e r s i o n p l a n t w i t h a b l o w d o w n c o n c e n t r a t i o n of t w i c e t h a t of sea w a t e r . Because the blowdown concentration was maintained c o n s t a n t , the cost of c h e m i c a l s f o r scale c o n t r o l r e m a i n e d c o n s t a n t .

2

4

6

8

io

TERMINAL TEMPERATURE DIFFERENCE, F E

Figure 5. Relation of water cost components to terminal temperature difference for 50-stage sea water conversion plants Plant capacity 25,000,000 to 60,000,000 gallons per day E l e c t r i c a l e n e r g y costs as w e l l as l a b o r a n d m i s c e l l a n e o u s expenses f o r s u c h a p l a n t w e r e r e l a t i v e l y c o n s t a n t . T h e s e charges w e r e s l i g h t l y h i g h e r a t b o t h l o w a n d h i g h T T D , however. I n c r e a s e d costs r e s u l t e d f r o m : s l i g h t l y different l a b o r costs as t h e p l a n t c a p a c i t y c h a n g e d , a n d s m a l l differences i n e l e c t r i c a l l o a d w i t h T T D . O v e r t h e r a n g e i n v e s t i g a t e d , t h e o n l y costs t h a t l a r g e l y d e p e n d e d u p o n t h e s e l e c t i o n of T T D w e r e s t e a m cost ( w h i c h i n c r e a s e d ) a n d c a p i t a l cost ( w h i c h d e c r e a s e d ) . T h e s u m of a l l these costs i s s h o w n as t h e t o t a l cost c u r v e . T h e m i n i m u m is a p p r o x i m a t e l y 4 ° F . T T D . A cross p l o t o f t h e d a t a i n d i c a t e d t h a t a c o n c e n t r a t i o n o f 1.7 t i m e s sea w a t e r h a d a s l i g h t e c o n o m i c a d v a n t a g e o v e r e i t h e r 1.5 o r 2.0 t i m e s sea


153

BRICE AND TOWNSEND—MULTISTAGE FLASH EVAPORATION water. A l l three h a d a considerable advantage over a once-through because o f t h e c h e m i c a l c o n s u m p t i o n f o r a o n c e - t h r o u g h s y s t e m ) .

system

(largely


Water Cost O u r studies s h o w t h a t a 52-stage e v a p o r a t o r i s t h e o p t i m u m m u l t i s t a g e flash sea water conversion plant that can be combined w i t h a 370-thermal megawatt light-water n u c l e a r s t e a m g e n e r a t o r t h a t p r o d u c e s s t e a m a t a t o t a l cost o f 37 cents p e r m i l l i o n B . t . u . T h i s evaporator w o u l d operate w i t h a n average T T D of 4 ° F . a n d w o u l d have a p e r f o r m a n c e r a t i o o f m o r e t h a n 13 p o u n d s o f condensate p e r p o u n d o f s t e a m . I t s n o m i n a l c a p a c i t y w o u l d be 50,000,000 gallons a d a y . T h e e s t i m a t e d c a p i t a l cost of t h e e v a p o r a t o r p l a n t , c o m p l e t e e x c e p t f o r t h e n u c l e a r s t e a m g e n e r a t o r , w o u l d b e $30,700,000. T h e n u c l e a r s t e a m g e n e r a t o r w o u l d cost $11,500,000. T h e e s t i m a t e d d a i l y o p e r a t i o n cost w o u l d be $20,600. A p e r s p e c t i v e o f t h e p r o p o s e d p l a n t is s h o w n i n F i g u r e 6.

Figure 6.

Perspective of proposed plant

T a b l e I shows a d e t a i l e d b r e a k d o w n o f t h e o p e r a t i n g cost f o r t h i s p l a n t . T h e cost of s t e a m r e p r e s e n t s a b o u t h a l f of t h e w a t e r cost f o r t h e o p t i m u m p l a n t . T h e c a p i t a l charges f o r t h e e v a p o r a t o r p l a n t , w h i c h i n c l u d e s a m o r t i z a t i o n , i n t e r e s t o n w o r k i n g c a p i t a l , a n d r e a l estate, r e p r e s e n t a b o u t 3 0 % . T h e r e m a i n i n g 15 t o 2 0 % i s e q u a l l y d i v i d e d b e t w e e n t h e cost of c h e m i c a l s f o r scale c o n t r o l a n d a l l t h e o t h e r costs. T h e c o n v e r t e d w a t e r i s e s t i m a t e d t o cost a p p r o x i m a t e l y 4 2 cents p e r t h o u s a n d g a l l o n s . T h i s w a t e r cost represents a r e a l i s t i c figure f o r a l a r g e - c a p a c i t y m u l t i s t a g e flash e v a p o r a t o r t h a t c o u l d b e b u i l t t o d a y w h e n the e n e r g y i n t h e f o r m o f s t e a m costs b e t w e e n 3 5 a n d 4 0 cents p e r m i l l i o n B . t . u .

Future Improvements M o r e r e c e n t l y possible f u t u r e i m p r o v e m e n t s i n n u c l e a r s t e a m g e n e r a t i o n a n d saline w a t e r c o n v e r s i o n b y m u l t i s t a g e flash e v a p o r a t i o n h a v e been e v a l u a t e d . T h e n u c l e a r steam generator planned f o r this plant w o u l d be a heavy-water n a t u r a l - u r a n i u m type. I t w o u l d r e s u l t i n a s l i g h t l y l o w e r s t e a m cost, b o t h c u r r e n t l y a n d i n t h e f u t u r e , t h a n t h e l i g h t - w a t e r r e a c t o r u s e d i n t h e o p t i m i z a t i o n s t u d y . T h e size o f t h e w a t e r p l a n t w o u l d b e 130,000,000 gallons a d a y , u t i l i z i n g m u l t i p l e u n i t s o f the flash e v a p o r a t o r s p r e v i o u s l y d e s c r i b e d . T h i s increase i n c a p a c i t y , t o g e t h e r w i t h l o w e r s t e a m cost, w o u l d reduce t h e e s t i m a t e of p r e s e n t w a t e r cost f r o m 42 t o 3 8 cents p e r t h o u s a n d gallons. B a s e d o n t h i s s t u d y {1), w a t e r costs were p r o j e c t e d t o 1972. I t w a s c o n c l u d e d t h a t t h e cost o f w a t e r c a n b e r e d u c e d f r o m t h e p r e s e n t l e v e l of a b o u t 3 8 cents p e r t h o u s a n d gallons (see F i g u r e 7) t o t h e r a n g e of 2 4 t o 31 cents. S u c h a r e d u c t i o n w o u l d d e p e n d u p o n t h e e x t e n t of i m p r o v e m e n t s t h a t c a n b e m a d e i n t h e n e x t decade i n h e a t t r a n s f e r coefficients, i n o p e r a t i o n a t h i g h e r t e m p e r a t u r e s a s t h e r e s u l t o f i m p r o v e m e n t s


154



i n scale c o n t r o l , i n t h e p o s s i b i l i t y o f u s i n g less e x p e n s i v e m a t e r i a l s o f c o n s t r u c t i o n , a n d i n l o w e r i n g t h e cost o f s t e a m p r o d u c e d i n a n u c l e a r s t e a m g e n e r a t o r . T h e p o s s i b i l i t y f o r s u c h i m p r o v e m e n t s w a s a p p r a i s e d as r e a l i s t i c a l l y as possible.

1958

I960

1962

1964

1966

1968

1970

1972

YEAR

Figure 7.

Projected cost of sea water conversion

Plant capacity 130,000,000 to 150,000,000 gallons per day F o r purposes of comparison, a fossil-fueled boiler p r o d u c i n g b y - p r o d u c t power w a s also i n c l u d e d i n t h e a n a l y s i s . A s s h o w n i n F i g u r e 7, s u c h a p l a n t c o u l d b e b u i l t t o d a y to p r o d u c e f r e s h w a t e r a t a n e s t i m a t e d cost o f a b o u t 35 cents a t h o u s a n d g a l l o n s , a s s u m i n g t h a t t h e p o w e r p l a n t w o u l d b e b a s e - l o a d e d t h e same as t h e w a t e r p l a n t . H o w e v e r ,

Table I. Summary of Water Cost for Optimized Plant Capacity, million gal./day Stages T e r m i n a l t e m p , difference, ° F . Performance r a t i o " , l b . / l b . C a p i t a l costs, thousands of dollars Dollars / O p e r a t i n g Costs Stream D a y E l e c t r i c power 912 Steam 11,090 Chemicals 1,723 Supplies a n d maintenance materials 246 O p e r a t i n g labor 314 M a i n t e n a n c e labor 185 P a y r o l l extras 83 Overhead 57 Amortization 5,915 Taxes a n d insurance Interest o n w o r k i n g c a p i t a l 144 R e a l estate* 48 T u b e salvage value (129) T o t a l operating cost 20,588 W a t e r cost Cents per 1000 gallons D o l l a r s per acre-foot c

d

49.3 52 4 13.65 30,700* Cents/ Thousand Gallons 1.85 22.49 3.50 0.50 0.64 0.37 0.17 0.12 12.00 0.29 0.10 (0.26) 41.77

% 4.4 53.9 8.4 1.2 1.5 0.9 0.4 0.3 28.7 0.7 0.2 (0.6) 100.0

42 136

° P o u n d s of water produced per p o u n d of steam condensed. E r e c t e d cost of complete evaporator p l a n t i n c l u d i n g i n t a k e facilities, reservoir, a n d site development. Includes b o t h c a p i t a l a n d operating costs of nuclear steam generator. E v a p o r a t o r p l a n t , interest o n money 4 % per a n n u m . $5000 per acre w i t h money at 4 % per a n n u m . b

e

d

β




155

less reduction in the future cost of water can be projected for a fossil-fueled boiler plant designed to produce both power and water than for a single-purpose nuclear steam generator plant to produce water alone, because of a n assumption that fossil fuels will continue to increase in price (as they have over the past several years). Power generation is a necessary part of the economics of low-cost water production utilizing a fossilfueled boiler. A single-purpose fossil-fueled boiler producing steam for the water plant would not be competitive with a single-purpose nuclear steam generator water plant. Although some plants could be built in which both the power generation and water production would be base-loaded, not all such plants can be base-loaded, because of the fluctuating demand for electrical energy and the inability to store it. Consequently, if one is to envision a large complex of sea water conversion and power generation, only a moderate amount of the water can be produced from combination plants at a cost competitive with a single-purpose plant. This analysis, of course, is based on very large plants to serve large population areas. There are always special considerations and conditions in any given location where the combination would be the most attractive means of water production. However, many of these plants are small—too small to consider a nuclear steam generator. Therefore, a valid comparison can be made only of plants on the order of 50,000,000 gallons a day or larger as far as nuclear steam generation is concerned.

Literature Cited (1) Brice, D . B., Dusbabek, M. R., Townsend, C . R., "Economics of Sea Water Distillation in Southern California," Metropolitan Water District of Southern California, Fluor C R R 1056 (December 1959). (2) Brice, D . B . , Dusbabek, M. R., Townsend, C . R., Office of Saline Water Research and Development Progress Rept., 19, Fluor R D R 1687, O T S N o . P B 161062 (February 1958). (3) Brice, D . B., Dusbabek, M. R., Townsend, C . R., Selleck, F . T., Office of Saline Water Research and Development Progress Rept. 34, Fluor C R R 1046, O T S N o . P B 161010 (August 1959) (4) Howe, E. D . , J. Am. Water Works Assoc. 51, 1191 (1959). RECEIVED for review July 7, 1960. Accepted July 15, 1960.


SALINE WATER CONVERSION

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