Evaporation of Sea Water in Long-Tube Vertical Evaporators

Evaporation of Sea Water in Long-Tube Vertical Evaporators F. C. STANDIFORD, Jr., and H. F. BJORK

Downloaded by UNIV OF NEW SOUTH WALES on September 5, 2015 | http://pubs.acs.org Publication Date: January 1, 1960 | doi: 10.1021/ba-1960-0027.ch013

W. L Badger Associates, Inc., Ann Arbor, Mich.

An inexpensive, dependable method of producing fresh water from saline sources is becoming increasingly important throughout the world as fresh water requirements grow and supplies decrease. Use of a falling-film long-tube vertical multiple-effect evaporation system in conjunction with a sludge recirculation technique has prevented scale formation from sea water evaporation at operating temperatures up to 250°F. Pilot plant test runs of over 1500 hours have been made. Pilot plant results translated to the design of a 1,000,000-gallon-per-day demonstration plant indicate fresh water costs of $1.00 per 1000 gallons. Ultimate costs for 15,000,000-gallon-per-day production will be 35 cents per 1000 gallons of fresh water.

The l a t e W . L . B a d g e r ' s 4 0 y e a r s ' experience w i t h c o m m e r c i a l e v a p o r a t o r s i n t h e c h e m i c a l i n d u s t r y h a s p r o v i d e d t h e basis f o r t h e e c o n o m i c p r o d u c t i o n o f f r e s h w a t e r f r o m sea w a t e r . F i v e y e a r s ago, those f a m i l i a r w i t h sea w a t e r e v a p o r a t i o n p r a c t i c e c o u l d p r e d i c t m i n i m u m p o s s i b l e w a t e r costs n o l o w e r t h a n a b o u t $1.60 p e r 1000 g a l l o n s . I n 1955, t h e Office of S a l i n e W a t e r , U . S . D e p a r t m e n t o f t h e I n t e r i o r , c o m m i s s i o n e d W . L . B a d g e r a n d A s s o c i a t e s t o s t u d y t h e m i n i m u m cost o f m a k i n g f r e s h w a t e r f r o m sea w a t e r b y u s i n g e v a p o r a t o r t e c h n i q u e s of t h e c h e m i c a l i n d u s t r y . B e c a u s e p r e v i o u s e s t i m a t e s of w a t e r cost h a d b e e n s e v e r a l t i m e s a b o v e t h e Office o f S a l i n e W a t e r ' s g o a l , s e v e r a l o p t i m i s t i c a s s u m p t i o n s s e r v e d as a basis f o r t h i s s t u d y : T h a t t h e h i g h e s t p e r f o r m a n c e e v a p o r a t o r c o u l d b e u s e d f o r sea w a t e r . B y p e r f o r m a n c e w a s m e a n t h e a t t r a n s f e r coefficient n o t i n B . t . u . / h r . / ° F . / s q . f t . b u t i n B . t . u . / h r . / ° F . / d o l l a r o f i n s t a l l e d cost. F o r a l o n g t i m e , t h e l o n g - t u b e v e r t i c a l ( L T V ) e v a p o r a t o r h a s best fitted t h i s d e s c r i p t i o n , a t least u n d e r f a v o r a b l e o p e r a t i n g c o n d i t i o n s , s u c h as a t r e l a t i v e l y h i g h t e m p e r a t u r e differences ( u s u a l l y ) , a n d w i t h l i t t l e scale formation. T h a t t h e e v a p o r a t o r c o u l d be m a d e as efficient as e c o n o m i c a l l y j u s t i f i a b l e . T h e r m a l efficiency of a n e v a p o r a t o r i s i n c r e a s e d b y m u l t i p l e - e f f e c t o p e r a t i o n , b y r e c o m p r e s s i o n of t h e v a p o r , b y a c o m b i n a t i o n of these, a n d b y a n u m b e r o f o t h e r d e s i g n f e a t u r e s . W h i l e sea w a t e r e v a p o r a t o r s h a d r a r e l y b e e n m a d e w i t h m o r e t h a n t h r e e effects, c o m m e r c i a l e v a p o r a t o r s o f s i x a n d s e v e n effects a r e c o m m o n a n d ten-effect e v a p o r a t o r s h a v e been u s e d . T h a t t h e L T V e v a p o r a t o r c o u l d b e k e p t free o f scale, a t l i t t l e o r n o cost f o r scale p r e v e n t i o n , a t t e m p e r a t u r e s as h i g h as 2 5 0 ° F . I n 1955, n o s a t i s f a c t o r y m e t h o d o f scale 115

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

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ADVANCES IN CHEMISTRY SERIES

p r e v e n t i o n w a s p r o v e d f o r sea w a t e r e v a p o r a t o r s , scale w a s m o s t severe a t t h e h i g h e s t temperatures, a n d about 220° F . was the highest t e m p e r a t u r e t h a t h a d been used. T h a t t h e e v a p o r a t o r c o u l d b e m a d e as l a r g e i n c a p a c i t y a s p r a c t i c a l a n d c o u l d t a k e u p as m u c h r o o m as n e c e s s a r y . A l t h o u g h a " l a r g e " e v a p o r a t o r t u r n e d o u t t o b e s m a l l i n terms of m u n i c i p a l water requirements, i t w o u l d be several orders of magnitude l a r g e r t h a n m o s t sea w a t e r e v a p o r a t o r s , w h i c h w e r e b u i l t s m a l l , c o m p a c t , a n d e a s i l y o p e r a b l e f o r s h i p b o a r d a n d m i l i t a r y use. T h a t t h e e v a p o r a t o r c o u l d b e f a b r i c a t e d p r i m a r i l y o r c o m p l e t e l y f r o m steel.


Preliminary Estimates O n t h e basis o f these a s s u m p t i o n s , p l a n t designs a n d cost e s t i m a t e s w e r e p r e p a r e d t h a t s h o w e d c o n s i d e r a b l e e c o n o m i c p r o m i s e (3). T w o b a s i c t y p e s o f flowsheet w e r e considered. One used exhaust steam f r o m a power p l a n t , a t a price, t o heat a m u l t i p l e effect L T V e v a p o r a t o r h a v i n g a c a p a c i t y o f a b o u t 17,000,000 g a l l o n s p e r d a y . W a t e r cost w a s e s t i m a t e d a t 23 cents p e r 1000 g a l l o n s w h e n t h e s t e a m cost w a s a d j u s t e d t o g i v e t h e same p o w e r cost as w o u l d b e i n c u r r e d b y a c o n v e n t i o n a l p o w e r p l a n t t h a t e x p a n d e d t h e s t e a m t o h i g h v a c u u m . T h e o t h e r t y p e o f flowsheet w a s s i m i l a r , e x c e p t t h a t the powerhouse t u r b i n e drove a v a p o r compressor instead of a generator. T h e c o m p r e s s o r s e r v e d a n u m b e r o f L T V e v a p o r a t o r bodies, m a k i n g a t h e r m o c o m p r e s s i o n e v a p o r a t o r , a n d t h e t u r b i n e e x h a u s t w a s u s e d t o heat a m u l t i p l e - e f f e c t L T V e v a p o r a t o r . T h e c a p a c i t y o f t h e p l a n t w a s also a b o u t 17,000,000 g a l l o n s p e r d a y . B e c a u s e t h e o n l y p o w e r g e n e r a t e d w a s f o r use i n d r i v i n g p l a n t a u x i l i a r i e s , a l l costs were c h a r g e a b l e t o t h e p r o d u c t i o n o f w a t e r . T a b l e I shows t h e c a p i t a l cost o f t h e p l a n t as e s t i m a t e d i n 1955 a n d T a b l e I I shows t h e a n n u a l o p e r a t i n g cost.

Table I. Capital Cost of 17,350,000-Gallon-per-Day Combination Thermocompression-10-Effect Sea Water Evaporation Plant Boiler Turbine-generator T u r b i n e - v a p o r compressor Evaporators H e a t exchangers Pumps Instruments T o t a l installed process e q u i p m e n t

$1,020,000 170,000 925,000 2,425,000 991 > 999 138,000 305,000 ^υ,υυυ $6,014,000

Site development Office, shop, a n d l a b Weatherproofing Engineering Contingencies T o t a l p l a n t cost

70,000 135,000 300,000 481,000 600,000 $7,600,000

P i p i n

P

g

Table II. Annual Operating Cost of 17,350,000-Gallon-per-Day Combination Thermocompression-10-Effect Sea Water Evaporation Plant Interest, 3 % of t o t a l c a p i t a l Insurance, 1 % of t o t a l c a p i t a l D e p r e c i a t i o n , 5 % of t o t a l c a p i t a l M a i n t e n a n c e , 3 % of equipment cost L a b o r , $2.50 per m a n - h o u r F u e l , $0.30 per m i l l i o n B . t . u . Total

$

228,000 76,000 380,000 180,000 138,000 991,000 $1,993,000

E q u i v a l e n t t o $0.328 per 1000 gallons or $106.90 per acre-foot P l a n t s of t h i s size were c o n s i d e r e d a b o u t t h e l a r g e s t t h a t c o u l d b e b u i l t as single u n i t s . S m a l l e r p l a n t s c o u l d e a s i l y b e b u i l t , b u t t h e r e d u c e d efficiencies o f p u m p s , t u r b i n e s , a n d c o m p r e s s o r s a n d t h e i n a b i l i t y t o reduce l a b o r costs m a d e t h e w a t e r cost


STANDIFORD AND BJORK—EVAPORATION IN LONG-TUBE VERTICAL EVAPORATORS

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f r o m s m a l l p l a n t s a p p r e c i a b l y h i g h e r . F i g u r e 1 i s a n e s t i m a t e p r e p a r e d i n 1955 o f t h e w a t e r cost f r o m v a r i o u s sizes o f these c o m b i n a t i o n m u l t i p l e - e f f e c t , t h e r m o c o m p r e s s i o n L T V p l a n t s . T h e s e e s t i m a t e s w e r e p r e p a r e d b e f o r e t h e Office o f S a l i n e W a t e r ' s " s t a n d a r d estimating p r o c e d u r e " was available, b u t later estimates made o n t h e basis of t h i s p r o c e d u r e g a v e s u b s t a n t i a l l y t h e same w a t e r costs, i f t h e a s s u m p t i o n s l i s t e d a b o v e were true.

Pilot Plant Investigation T h e cost e s t i m a t e s w e r e so p r o m i s i n g t h a t t h e Office of S a l i n e W a t e r financed a p i l o t p l a n t p r o g r a m t o test these o p t i m i s t i c a s s u m p t i o n s . T h i s p i l o t p l a n t i s l o c a t e d o n t h e site o f t h e I n t e r n a t i o n a l N i c k e l C o . T e s t S t a t i o n a t W r i g h t s v i l l e B e a c h , N . C . I t w a s designed b y W . L . B a d g e r Associates, I n c . , a n d w a s erected a n d h a s been operated for the past 2 / years b y t h e m under subcontract f r o m the W h i t i n g Corp. ( 1 ) . T h e pilot plant contains a n L T V evaporator, donated b y the Swenson E v a p o r a t o r C o . , D i v i s i o n o f t h e W h i t i n g C o r p . , t h a t has t u b e s o f t h e d i m e n s i o n s t h a t w o u l d p r o b a b l y b e u s e d i n a f u l l scale p l a n t . T h e r e a r e s e v e n o f these t u b e s , e a c h 2 i n c h e s i n o u t s i d e d i a m e t e r b y 2 4 feet l o n g , a n d i n s u l a t e d f r o m e a c h o t h e r a n d f r o m t h e s h e l l , so t h a t different t u b e m a t e r i a l s c a n b e t e s t e d w i t h o u t t h e c o m p l i c a t i o n o f g a l v a n i c c o r r o s i o n . T h e L T V e v a p o r a t o r i s i n s t r u m e n t e d so t h a t i t c a n b e r u n u n d e r a n y d e s i r e d c o n d i t i o n s of feed r a t e , feed t e m p e r a t u r e , b o i l i n g p o i n t , a n d s t e a m flow. T h i s m a k e s i t possible t o d u p l i c a t e o p e r a t i n g c o n d i t i o n s t h a t w o u l d b e m e t i n a n y effect o f a m u l t i p l e - e f f e c t evaporator or i n a thermocompression evaporator. 1

2

B e c a u s e t h e feed t o o n e effect i n t h i s t y p e of e v a p o r a t o r i s t h e p a r t i a l l y c o n c e n t r a t e d sea w a t e r d i s c h a r g e d f r o m a n o t h e r effect, i t w a s n e c e s s a r y t o h a v e a source of p a r t i a l l y concentrated sea water f o r t h e pilot p l a n t . R a t h e r t h a n t r y i n g t o store p r e v i o u s l y c o n c e n t r a t e d s e a w a t e r , w i t h t h e possible r e s u l t t h a t some o f t h e p o t e n t i a l scale m i g h t d e p o s i t o n s t o r a g e , sea w a t e r i s c o n c e n t r a t e d c o n t i n u o u s l y i n one o r t w o f o r c e d - c i r c u l a t i o n e v a p o r a t o r s w h i c h c a n b e o p e r a t e d single o r d o u b l e effect, as n e c e s s a r y , a n d c a n p r o v i d e sea w a t e r feed t o t h e L T V a t a n y d e s i r e d c o n c e n t r a t i o n . H e a t T r a n s f e r . T h e first series o f tests were m a d e t o m e a s u r e h e a t t r a n s f e r coefficients a n d t h e r e b y c o n f i r m t h e o p e r a t i n g c o n d i t i o n s t h a t h a d b e e n p r e d i c t e d for t h e v a r i o u s effects i n t h e p r o d u c t i o n p l a n t s . T h e L T V e v a p o r a t o r c a n b e b u i l t t o o p e r ate i n one o f t w o w a y s . M o s t s u c h e v a p o r a t o r s h a v e t h e l i q u i d feed a t t h e b o t t o m . T h e l i q u i d rises i n t h e t u b e s , i s h e a t e d , a n d b e g i n s t o b o i l , a n d t h e v a p o r f o r m e d creates s u c h h i g h v e l o c i t i e s i n t h e b o i l i n g s e c t i o n t h a t h i g h t r a n s f e r coefficients a r e o b t a i n e d i n t h e



118

b o i l i n g s e c t i o n . T h e o t h e r m e t h o d of o p e r a t i o n i n v o l v e s feeding l i q u i d t o t h e t o p s of t h e t u b e s as a film, so t h a t b o i l i n g t a k e s p l a c e f o r p r a c t i c a l l y t h e f u l l l e n g t h o f t h e t u b e . T h e p i l o t p l a n t e v a p o r a t o r w a s b u i l t so t h a t i t c o u l d b e o p e r a t e d e i t h e r w a y . T h e first tests u s e d r i s i n g flow t h r o u g h t h e t u b e s . I t w a s f o u n d t h a t , u n d e r c o n d i t i o n s o f l o w t e m p e r a t u r e difference a n d w i t h feed p r a c t i c a l l y a t t h e v a p o r h e a d b o i l i n g p o i n t , as w o u l d b e e x p e c t e d i n t h e p r o d u c t i o n p l a n t , h e a t t r a n s f e r coefficients were l o w . T h e p r i m a r y c o n t r o l l i n g v a r i a b l e s w e r e t e m p e r a t u r e a n d t e m p e r a t u r e difference. E v e n a t h i g h t e m p e r a t u r e differences b e t w e e n s t e a m a n d b o i l i n g l i q u i d , h e a t t r a n s f e r coefficients w e r e o n l y a b o u t 150 B . t . u . / h r . / s q . f t . / F . a t l o w t e m p e r a t u r e a n d a b o u t 500 i n t h e same u n i t s a t h i g h t e m p e r a t u r e . P r e s s u r e d r o p m e a s u r e m e n t s i n d i c a t e d t h a t m o s t of t h e t u b e l e n g t h w a s filled w i t h n o n b o i l i n g a n d t h e r e f o r e r e l a t i v e l y s l o w - m o v i n g liquid a n d that this was most pronounced at l o w temperature a n d l o w temperature difference. C o n s e q u e n t l y , a l l s u b s e q u e n t w o r k w a s d o n e w i t h f a l l i n g - f i l m o p e r a t i o n t o e l i m i n a t e t h i s h y d r o s t a t i c h e a d t h a t p r e v e n t e d b o i l i n g i n m o s t of t h e t u b e l e n g t h .


0

T h e tests u n d e r f a l l i n g - f i l m c o n d i t i o n s g a v e h e a t t r a n s f e r coefficients t h a t were p r a c t i c a l l y t h e same as those u s e d i n p r e p a r i n g t h e o r i g i n a l p l a n t designs a n d cost e s t i m a t e s . T h e coefficient d e p e n d e d m a i n l y o n b o i l i n g t e m p e r a t u r e a n d v a r i e d f r o m 3 5 0 t o 400 a t a b o u t 100° F . t o 700 t o 800 a t 2 5 0 ° F . S u b s e q u e n t tests u n d e r c o n d i t i o n s t h a t w o u l d b e m e t i n a 12-effect e v a p o r a t o r o p e r a t i n g b e t w e e n a n i n i t i a l b o i l i n g p o i n t o f 2 5 0 ° F . a n d a final b o i l i n g p o i n t of 1 2 5 ° F . g a v e coefficients t h a t v a r i e d as s h o w n i n

750

3SOl 100

ι

ι

IIO

120

ι

ι

ι

ι

ι

ι

ι

ι

ι

ι

130

140

150

160

170

180

190

200

210

220

ι — ι — I 230

240

250

TEMPERATURE

F i g u r e 2 (2). T h e s e coefficients were d e t e r m i n e d w h e n t h e f o l l o w i n g t u b e s w e r e i n s t a l l e d in the evaporator: 2 c o p p e r , 1 A d m i r a l t y , 1 A m p c o G r a d e 8, 2 a l u m i n u m b r a s s , a n d 1 c u p r o n i c k e l . E a c h t u b e w a s 2 i n c h e s i n o u t s i d e d i a m e t e r a n d 24 feet l o n g , a n d h a d a 0 . 1 0 9 - i n c h w a l l . T h e d a s h e d c u r v e o f F i g u r e 2 shows t h e h e a t t r a n s f e r coefficients t h a t w e r e a s s u m e d i n p r e p a r i n g t h e o r i g i n a l e s t i m a t e s . I n these tests, t e m p e r a t u r e differences



119


w e r e o b t a i n e d f r o m p r e s s u r e m e a s u r e m e n t s u s i n g m a n o m e t e r s o r c a l i b r a t e d gages a n d w e r e c o r r e c t e d f o r b o i l i n g p o i n t e l e v a t i o n of t h e c o n c e n t r a t e d s e a w a t e r . A r e a s w e r e b a s e d o n t h e i n s i d e d i a m e t e r of the t u b e s . T h e a m o u n t o f heat t r a n s f e r r e d w a s c a l c u lated b o t h f r o m steam consumption (measured b y a calibrated d r i p tank) corrected for s u p e r h e a t , c o n d e n s a t e s u b c o o l i n g , a n d c a l i b r a t e d h e a t losses f r o m t h e s t e a m chest a n d f r o m e v a p o r a t i o n r a t e ( m e a s u r e d b y a n o t h e r c a l i b r a t e d d r i p t a n k ) , sensible heat c h a n g e , a n d c a l i b r a t e d h e a t losses f r o m t h e v a p o r side. M a t e r i a l b a l a n c e s u s u a l l y c h e c k e d w i t h i n 3 % a n d heat balances w i t h i n 8 % . Scale P r e v e n t i o n . T h e scale n o r m a l l y f o r m e d o n h e a t t r a n s f e r surfaces of sea w a t e r e v a p o r a t o r s consists o f c a l c i u m c a r b o n a t e , m a g n e s i u m h y d r o x i d e , a n d / o r c a l c i u m s u l f a t e . T h e first t w o f o r m as a r e s u l t o f t h e b r e a k d o w n o f b i c a r b o n a t e i n sea w a t e r , w h i c h i s i n i t i a l l y s a t u r a t e d w i t h c a l c i u m c a r b o n a t e . C a l c i u m s u l f a t e scale f o r m s p u r e l y as a r e s u l t o f i t s i n v e r t e d s o l u b i l i t y c u r v e . S e a w a t e r i s n o t s a t u r a t e d w i t h c a l c i u m s u l f a t e a n d a n e c o n o m i c a l l y r e a s o n a b l e a m o u n t of f r e s h w a t e r c a n b e r e c o v e r e d f r o m sea w a t e r w i t h o u t exceeding s a t u r a t i o n w i t h c a l c i u m s u l f a t e . H o w e v e r , a t t h e s t a r t of t h i s i n v e s t i g a t i o n , t h e s o l u b i l i t y of c a l c i u m s u l f a t e i n sea w a t e r w a s n o t a c c u r a t e l y e n o u g h k n o w n t o t e l l w h e t h e r 30, 5 0 , o r 8 0 % o f t h e w a t e r c o n t e n t c o u l d b e r e m o v e d a t v a r i o u s t e m p e r a t u r e s w i t h o u t e n c o u n t e r i n g c a l c i u m s u l f a t e scale. T h e o r i g i n a l p l a n t designs a n d cost e s t i m a t e s w e r e n o t m a d e w i t h o u t h a v i n g p l a n s f o r c o m b a t i n g scale f o r m a t i o n . T w o r a t h e r i n e x p e n s i v e p o s s i b i l i t i e s w e r e p r o p o s e d . O n e i n v o l v e d t h e u s e of a c i d t o p r e v e n t c a l c i u m c a r b o n a t e a n d m a g n e s i u m h y d r o x i d e scale. I f t h e c a r b o n d i o x i d e t h a t i s lost w h e n b i c a r b o n a t e d e c o m p o s e s i s r e p l a c e d b y n o n v o l a t i l e a c i d , t h e p H c a n b e k e p t l o w e n o u g h t o p r e v e n t f o r m a t i o n o f these a l k a l i n e scales. T h i s m e t h o d w a s a d o p t e d a t a b o u t t h e same t i m e b y t h e m i l i t a r y , u s i n g c i t r i c a c i d . T h e cost of u s i n g t h i s a c i d a m o u n t e d t o a b o u t 5 0 cents p e r 1000 g a l l o n s of d i s t i l l e d w a t e r — m o r e t h a n t h e t o t a l cost o f w a t e r f r o m t h e p r o p o s e d p l a n t s . H o w e v e r , i t w a s felt t h a t i n l a r g e l a n d - b a s e d p l a n t s , s u f f i c i e n t l y close c o n t r o l c o u l d b e a t t a i n e d t o p e r m i t t h e use o f c h e a p s u l f u r i c a c i d . E v e n i f a l l t h e b i c a r b o n a t e b r o k e d o w n t o c a r b o n d i o x i d e , t h e cost o f a c i d r e q u i r e d a m o u n t e d t o o n l y a b o u t 2 cents p e r 1000 g a l l o n s . T h i s m e t h o d o f scale p r e v e n t i o n w o u l d n o t i n t r o d u c e e n o u g h s u l f a t e i o n , c o m p a r e d t o t h a t a l r e a d y p r e s e n t i n t h e sea w a t e r , t o affect t h e s o l u b i l i t y of c a l c i u m s u l f a t e a p p r e c i a b l y . T h e chief d i s a d v a n t a g e s o f t h i s m e t h o d , besides a c i d cost, were t h a t i t c o u l d n o t b y i t s e l f p r e v e n t c a l c i u m s u l f a t e scale f o r m a t i o n a n d t h a t i t m i g h t r e q u i r e o p e r a t i o n a t p H ' s s l i g h t l y b e l o w n e u t r a l i t y , t h u s i n c r e a s i n g c o r r o s i v e tendencies. T h e o t h e r m e t h o d of scale p r e v e n t i o n p r o p o s e d , a n d i n c o r p o r a t e d i n t h e o r i g i n a l p l a n t designs a n d cost e s t i m a t e s , i n v o l v e d a seeding t e c h n i q u e . I f s o m e o f t h e s c a l i n g i n g r e d i e n t is g o i n g t o p r e c i p i t a t e , i t w i l l d e p o s i t o n t h e h e a t i n g s u r f a c e i f n o o t h e r s u r f a c e is a v a i l a b l e . H o w e v e r , i t w o u l d p r e f e r t o deposit o n c r y s t a l s o f i t s o w n k i n d . B y p r o v i d i n g seed c r y s t a l s of the s c a l i n g i n g r e d i e n t i n s u s p e n s i o n i n t h e l i q u i d , i t w a s h o p e d t h a t a l l p r e c i p i t a t i o n c o u l d b e i n d u c e d t o o c c u r o n these seeds r a t h e r t h a n o n t h e h e a t i n g s u r f a c e . T h i s p r a c t i c e h a d b e e n c o m p l e t e l y successful f o r t h e p r e v e n t i o n of c a l c i u m s u l f a t e scale f o r m a t i o n i n t h e s a l t i n d u s t r y . I n p r a c t i c e , t h i s m e t h o d of scale p r e v e n t i o n w o u l d i n v o l v e i n c o r p o r a t i o n o f scale solids i n t h e sea w a t e r t o m a k e a d i l u t e s l u r r y a n d r e c o v e r y o f these solids f r o m t h e w a s t e sea w a t e r c o n c e n t r a t e . T h e seeds w o u l d b e c o n t i n u o u s l y a d d e d t o b y p r e c i p i t a t i o n o f t h e s c a l i n g i n g r e d i e n t i n t h e sea w a t e r , t h e r e b y m a k i n g u p f o r m i n o r m e c h a n i c a l losses of s o l i d s . T h u s t h e o n l y cost f o r t h i s m e t h o d of scale p r e v e n t i o n w o u l d b e t h e cost o f e q u i p m e n t r e q u i r e d t o s e p a r a t e t h e solids f r o m t h e c o n c e n t r a t e d sea w a t e r a n d r e t u r n t h e m t o t h e feed. I t w a s h o p e d t h a t t h i s m e t h o d w o u l d w o r k b o t h f o r c a l c i u m s u l f a t e , w h i c h deposits b y one m e c h a n i s m , a n d f o r c a l c i u m c a r b o n a t e a n d m a g n e s i u m h y d r o x i d e , w h i c h deposit b y a n o t h e r m e c h a n i s m . T h i s m e t h o d o f scale p r e v e n t i o n w o u l d h a v e t h e a d d i t i o n a l advantage that evaporation w o u l d be conducted under alkaline conditions, where c o r r o s i o n s h o u l d b e less severe. A s t h e a c i d m e t h o d of scale p r e v e n t i o n h a d a l r e a d y b e e n p r o v e d b y o t h e r s , o n l y a few trials were made t o determine t h e a p p r o x i m a t e l i m i t i n g conditions f o r L T V evaporators. W e e k - l o n g tests w e r e m a d e u s i n g s u l f u r i c a c i d t o d e t e r m i n e t h e e x t e n t


120


to w h i c h e q u i l i b r i u m p H c o u l d b e exceeded. T h e tests w e r e m a d e u n d e r t h e f o l l o w i n g c o n d i t i o n s , w h i c h w e r e a t t h e t i m e t h o u g h t t o r e p r e s e n t t h e m o s t severe t h a t w o u l d b e encountered w i t h o u t calcium sulfate depositing:


B o i l i n g point, ° F . T e m p e r a t u r e difference, ° F . F e e d temperature, ° F . F e e d concentration

192 8 to 10 190 2 . 3 5 times n o r m a l sea water

I n t h e .first r u n , a c i d feed r a t e w a s g r a d u a l l y r e d u c e d w i t h o u t i m m e d i a t e evidence of scale f o r m a t i o n a n d t h e n w a s c u t off e n t i r e l y . H e a t t r a n s f e r coefficients s t a r t e d t o d r o p i m m e d i a t e l y a n d a t t h e e n d of t h e w e e k t h e t u b e s w e r e c o a t e d w i t h c a l c i u m c a r b o n a t e scale. I n t h i s first r u n , t h e p H e n t e r i n g t h e e v a p o r a t o r w a s 8.3 a n d l e a v i n g i t w a s 8.4. L a n g e l i e r , C a l d w e l l , a n d L a w r e n c e h a v e m e a s u r e d t h e e q u i l i b r i u m p H a b o v e w h i c h a sea w a t e r c o n c e n t r a t e i s s u p e r s a t u r a t e d w i t h respect t o c a l c i u m c a r b o n a t e a n d m a g n e s i u m h y d r o x i d e ( 4 ) . U n d e r t h e c o n d i t i o n s o f t h i s test, t h e f o l l o w i n g c o n d i t i o n s were e n c o u n t e r e d :

C o n c e n t r a t i o n factor Total alkalinity, p.p.m. C a C 0 Equilibrium p H , C a C 0 Equilibrium p H , M g ( O H ) Actual p H

3

3

2

L T V Feed

L T V Blowdown

2.46 159 6.8 7.8 8.3

2.88 145 6.8 7.8 8.3

I t w a s e v i d e n t t h a t , u n d e r c o n d i t i o n s o f t h i s test, c a l c i u m c a r b o n a t e scale w o u l d f o r m i f t h e e q u i l i b r i u m p H w e r e exceeded b y 1.5 p H u n i t s . I n t h e s e c o n d test, t h e a c i d free r a t e w a s a d j u s t e d t o m a i n t a i n a n L T V b l o w d o w n p H o f 7.5. T h e a c i d w a s f e d t o t h e f o r c e d - c i r c u l a t i o n e v a p o r a t o r p r e c e d i n g t h e L T V , so t h a t t h e a d d i t i o n a l t i m e a v a i l a b l e p e r m i t t e d e v e n i n g o u t f l u c t u a t i o n s i n a c i d feed r a t e . U n d e r these c o n d i t i o n s , some o f t h e t o t a l a l k a l i n i t y w a s lost a n d t h e L T V o p e r a t e d under the following scaling environment:

C o n c e n t r a t i o n factor Total alkalinity, p.p.m. C a C 0 Equilibrium p H , C a C 0 Equilibrium p H , M g ( O H ) Actual p H 3

2

3

L T V Feed

L T V Blowdown

2.31 73.7 7.2 7.85 7.51

2.70 83.1 7.1 7.8 7.52

W h e n o p e r a t e d u n d e r these c o n d i t i o n s f o r a w e e k , n o scale w a s e v i d e n t , e i t h e r b y a decrease i n h e a t t r a n s f e r coefficients o r b y e x a m i n a t i o n a t t h e e n d o f t h e r u n . T h u s , i t i s a p p a r e n t t h a t scale c a n b e p r e v e n t e d b y t h i s m e c h a n i s m w h e n t h e e q u i l i b r i u m p H is exceeded b y s o m e t h i n g m o r e t h a n 0.3 t o 0.4 a n d less t h a n 1.5 p H u n i t s . A l l h e a t t r a n s f e r tests w e r e m a d e w i t h use o f a c i d f o r scale p r e v e n t i o n . I n these tests, t h e p H was k e p t b e l o w t h e e q u i l i b r i u m p H i n o r d e r t o b e o n t h e safe side, a n d n o s c a l i n g w a s ever detected. A s e v i d e n t f r o m t h e a b o v e d a t a , t h e p H r e q u i r e d i s n o t so l o w as to i n v o l v e operation under acidic conditions, where accelerated corrosion m i g h t be expected. T h e n e x t series of tests w a s m a d e t o p r o v e o u t t h e seeding m e t h o d o f scale p r e v e n t i o n . T h e first test w a s m a d e u n d e r t h e same c o n d i t i o n s as f o r t h e a c i d t r i a l s . O n c e t h e m e c h a n i c a l p r o b l e m s o f r e c y c l i n g t h e seeds i n t h e p i l o t p l a n t w e r e s o l v e d , i t w a s f o u n d possible t o p r e v e n t scale f o r m a t i o n c o m p l e t e l y , i f t h e e v a p o r a t i n g l i q u i d c o n t a i n e d 0 . 5 % c a l c i u m c a r b o n a t e s o l i d s . T h e solids w e r e m a d e i n i t i a l l y b y s l o w l y a d d i n g s o d a a s h t o sea w a t e r .




121

The first trials at higher temperature were unsuccessful m preventing scale forma tion. These trials were made under conditions such that calcium sulfate scale might be expected (concentration factor of 3.3, boiling points of 205° and 255° F.). A slurry of calcium carbonate was tried first, in the hope that this would also serve as seeds for the calcium sulfate. Heat transfer coefficients dropped rapidly and a heavy scale was found on all tubes at the end of the run. The next trials therefore used a mixed slurry of calcium carbonate and calcium sulfate. Two crystal forms of calcium sulfate might be expected as scale—hemihydrate and anhydrite. Both were tried as seeds, using purchased materials ( U . S. Gypsum Hydrocal White for hemihydrate and the Snow White filler for anhydrite). Trials with hemihydrate resulted mainly in the cementing shut of all drains, stagnant pockets, etc., where the temperature was below 180° to 190° F., the transition temperature of hemihydrate to gypsum in these solutions. Trials with a mixed slurry of calcium carbonate and anhydrite always resulted in scale forma tion, although the scaling rate was tantalizingly low in some runs.

TEMPERATURE^

The inability to control calcium sulfate scale formation led to a more thorough investigation of solubility limits of calcium sulfate in sea water concentrates (δ). The current "best guess" as to solubility of the different crystal forms of calcium sulfate is shown in Figure 3. Anhydrite is so inert that it is almost never encountered as scale in evaporators. The main difference between this solubility diagram and previous estimates (4) is a much higher solubility limit for gypsum or hemihydrate at tempera tures below 212° F. Figure 3 indicates that concentration factors of more than 4 can be achieved if the evaporator is operated in such a manner that high concentrations are reached only at low temperatures. This corresponds to recovery of over 80% of the water under conditions such that calcium sulfate solubility is not exceeded and hence no scale can form from this source. Such conditions can be achieved by use of a forward-feed evaporator in which unconcentrated sea water enters the first, or hottest, effect and is then passed from effect to effect until concentrated sea water is pumped from the last, or coolest, effect and discarded. Typical conditions for a 12-effect forward-feed evaporator are plotted in Figure 3. Also plotted in this diagram are the equilibrium pH's for the acid method of preventing calcium carbonate and magnesium hydroxide scale, assuming that no carbonate alkalinity has been lost. Forward-feed operation results in relatively mild p H conditions—much milder than for a backwardfeed evaporator, where the high concentrations are reached at high temperatures. In SALINE WATER CONVERSION; Advances in Chemistry; American Chemical Society: Washington, DC, 1960.

122



B y a d o p t i n g a f o r w a r d - f e e d flowsheet, i t b e c a m e possible t o i g n o r e t h e c a l c i u m s u l f a t e scale p r o b l e m . H o w e v e r , i t w a s s t i l l necessary t o p r o v e t h a t t h e a l k a l i n e scales c o u l d b e p r e v e n t e d a t t e m p e r a t u r e s u p t o 2 5 0 ° F . T h e first t r i a l w a s m a d e a t 2 5 0 ° F . w i t h r a w sea w a t e r feed a n d a c a l c i u m c a r b o n a t e s l u r r y . T h e e v a p o r a t o r s c a l e d r a p i d l y a n d a n a l y s i s s h o w e d t h e scale t o b e m a g n e s i u m h y d r o x i d e . T h e n e x t r u n w a s t h e r e f o r e made under the same conditions, except t h a t a m a g n e s i u m h y d r o x i d e s l u r r y was used. T h i s s l u r r y was made before the r u n started b y slowly adding caustic soda t o the i n i t i a l dilute c a l c i u m carbonate s l u r r y u n t i l t h e solids contained about 5 0 % magnesium h y d r o x i d e . D u r i n g t h e 200 h o u r s o f t h e r u n , t h e m a g n e s i u m h y d r o x i d e c o n t e n t o f t h e solids g r a d u a l l y i n c r e a s e d t o o v e r 9 5 % a n d n o scale f o r m e d i n t h e t u b e s .

Table

Summary of Operating Conditions for All Sludge Runs

Evap. Feed Temp., Temp., °F. °F.

Run No.

Disch. Concn. Factor

Temp. Diff., °F.

Hr. Operation

Sludge Used

3

6.5 46

CaC0

3

22

5.5

CaC0

3

44

?

2.75 2.75 3.3

7.5 13 11

CaC0 CaC0 CaC0

210 150 162

None None CaS0

4

3.3

14

CaC0 + CaS0 CaC0 + CaSCV CaC0 + CaS(V CaC0 + CaS0 «

52

CaS0

4

LWBB-1 -2

190 190

192 192

2.75 2.75

8 8

CaC0 CaC0

-3

190

192

2.75

7

-4

190

192

2.75

-5 -6 -7

190 187 194

192 192 204

LWBC-1

250

255

3

3 3 3

3

4

-2

250

255

3.3

14

-3

250

255

3.3

14

-4

250

255

3.3

10

Reason for Termination

Tvpe of Scale Trace, 2 alloy tubes

200

205

11

3.0

Descaling

3

33

3

26

CaS0

4

33

CaS0

4

16

CaS0

4

3

Plugged piping Descaling D e s c a l i n g , low solids

CaCO* + CaS0 ° CaC0 + CaS0

CaS0 . V2H 0

Descaling, plugged p i p i n ( N o scale-steel tube) P l u g g e d orifices

4

-2

195

200

13

3.5

3

4

LWBE-1

250

250

13

3.5

-2

250

250

13

3.3

168

&

4

2

CaC0 + CaS0 CaC0 + CaS0

141

CaS0

154

CaS0 . 7 H 0

3

4

4

&

3

4

6

2

225 250

250 250

1.0 1.2

9.5 9.5

CaC0 Mg(OH)

2

133 200

Mg(OH) None

LWBG-1

250

250

1.7

7

Mg(OH)

2

170 141

CaS0 . V H 0 None

2

238 167 160

None None None

Mg(OH)2

593

CaS0 . V H 0 None

3

250

LSBH-1 LWBI-1 LWBJ-1

130 236 203

120 220 186

LWBG-3a

250

250

1.7 3.0-5.7 2.0 2.3 1.6

7

Mg(OH)

13 8 7

Mg(OH) Mg(OH) Mg(OH)

7-8

2

2 2

a 6

250

250

1.6

7-8

Mg(OH)

2

1001

2

4

2

-3b

2

4

2

250

Hurricane

4

2

LWBF-1 -2

-2

B u r s t filter cartridges P l u g g e d feed controller

a

4

LWBD-1

M e c h . trouble B u r s t filter cartridges

2

P o w e r failure

Descaling, overconcentrated

C a S 0 solids were C a S 0 . V H 0 ( H y d r o c a l W h i t e , U . S. G y p s u m C o . ) . C a S 0 solids were C a S 0 (insoluble) (Snow W h i t e filler, U . S. G y p s u m C o . ) . 4

4

4

4

2

2



123


S u b s e q u e n t r u n s were m a d e , u s i n g t h i s same m a g n e s i u m h y d r o x i d e s l u r r y , a t l o w e r t e m p e r a t u r e s a n d h i g h e r c o n c e n t r a t i o n f a c t o r s a n d a l l w e r e successful as l o n g as c o n d i t i o n s were u n d e r t h e h e m i h y d r a t e a n d g y p s u m s o l u b i l i t y c u r v e o f F i g u r e 3. A f i n a l r u n of 1594 h o u r s ' d u r a t i o n w a s m a d e t o p r o v e t h i s m e t h o d o f scale p r e v e n t i o n u n d e r t h e m o s t severe c o n d i t i o n s t h a t m i g h t b e e n c o u n t e r e d — 2 5 0 ° F . a n d a c o n c e n t r a t i o n f a c t o r j u s t u n d e r t h e h e m i h y d r a t e s o l u b i l i t y c u r v e . I n t h e first 593 h o u r s , p o o r c o n t r o l a l l o w e d c o n c e n t r a t i o n s t o exceed h e m i h y d r a t e s o l u b i l i t y a t t i m e s a n d a h e m i h y d r a t e scale f o r m e d s l o w l y . T h i s w a s r e m o v e d b y r i n s i n g w i t h sea w a t e r a n d t h e r u n c o n t i n u e d u n d e r m o r e c a r e f u l c o n t r o l f o r a n o t h e r 1001 h o u r s w i t h n o scale f o r m a t i o n w h a t s o e v e r . T a b l e I I I s u m m a r i z e s t h e r u n s t h a t p r o v e d t h e efficacy o f t h e seeding m e t h o d o f scale c o n t r o l f o r t h e d e m o n s t r a t i o n p l a n t (1). T h i s w o r k i s b e i n g c o n t i n u e d a t s t i l l higher temperatures ( u p to 300° F . ) , b u t i t has been found impossible t o keep below the c a l c i u m sulfate s o l u b i l i t y curve. Consequently, t h e a t t a c k is again being concent r a t e d o n use o f seeding f o r p r e v e n t i o n of c a l c i u m s u l f a t e scale. I f t h i s w o r k i s s u c cessful, i t s h o u l d p e r m i t d e v e l o p m e n t of e v a p o r a t o r s t h a t c o u l d o p e r a t e a t s t i l l h i g h e r t e m p e r a t u r e s , t h e r e b y m a k i n g p r a c t i c a l use o f m o r e effects w i t h a consequent s a v i n g i n s t e a m ; design o f e v a p o r a t o r s t o c o n c e n t r a t e sea w a t e r t o o r b e y o n d t h e p o i n t w h e r e i t i s s a t u r a t e d w i t h s o d i u m c h l o r i d e , m a k i n g possible t h e r e c o v e r y of b y - p r o d u c t s ; a n d design of e v a p o r a t o r s f o r b r a c k i s h w a t e r s h i g h i n c a l c i u m s u l f a t e . C o r r o s i o n . T h e tubes i n the L T V evaporator were installed w i t h Swenson r u b b e r grommet p a c k i n g c o m m o n l y used f o r K a r b a t e tubes i n acid evaporators. T h i s b o t h insulated t h e tubes f r o m galvanic corrosion a n d allowed nondestructive r e m o v a l for w e i g h i n g . C o r r o s i o n r a t e s w e r e e s t i m a t e d b o t h f r o m w e i g h t loss o f t h e t u b e s a n d f r o m I n t e r n a t i o n a l N i c k e l C o . test spools l o c a t e d b o t h i n t h e v a p o r space a n d i m m e r s e d i n the l i q u i d i n the b l o w d o w n t a n k . T h e first set of c o r r o s i o n r e s u l t s w a s p r o m i s i n g , e v e n t h o u g h t h e d a t a w e r e t a k e n d u r i n g t h e t i m e t h e first h e a t t r a n s f e r r u n s w e r e m a d e , u s i n g excess a c i d ( p H ' s d o w n t o 3 ) , a n d t h e t i m e t h e tests o f t h e a c i d m e t h o d o f scale p r e v e n t i o n w e r e u n d e r w a y . C o r r o s i o n r a t e s f r o m I N C O test s p o o l d a t a w e r e as f o l l o w s ( i n i n c h e s p e r y e a r ) : Material

Vapor

Liquid

Steel Cast iron Admiralty Copper A l u m i n u m brass 90/10 cupronickel Aluminum

0.016-0.027 0.009-0.016 0.001 0.001 0.001 0.001 0.001

0.010-O.021 0.008-0.030 0.001-0.003 0.002-0.004 0.001-0.002 0.001-0.003 0.004-0.006

T h e n e x t sets of spools were e x p o s e d d u r i n g t h e e a r l y sludge r u n s , w h i c h w e r e m a d e u n d e r a l k a l i n e c o n d i t i o n s w h e r e c o r r o s i o n s h o u l d h a v e b e e n m u c h less severe. T h e r e s u l t s w e r e as f o l l o w s : Material

Vapor

Liquid

Steel Cast iron Admiralty Copper A l u m i n u m brass 90/10 cupronickel Aluminum

0.057-0.062 0.016-0.021 0.002 0.002-0.003 0.001 0.001

0.030-0.044 0.031-0.032 0.001-0.003 0.002-0.011 0.001 0.001

Perforated

T h e f a c t t h a t steel a n d cast i r o n suffered b y f a r t h e greatest c o r r o s i o n u n d e r s u p posedly the mildest conditions i n d i c a t e d a n extraneous influence. T h i s could o n l y have been atmospheric corrosion; t h e evaporator was shut d o w n a n d t h e samples were exposed t o t h e a i r f a r oftener w h e n t h e e a r l y sludge r u n s were m a d e — r u n s w h i c h s o m e times lasted only a day o r two each.


124



A set o f spools exposed d u r i n g t h e 1 5 9 4 - h o u r d e m o n s t r a t i o n r u n s h o w e d t h e f o l lowing corrosion rates: Material

Vapor

Liquid

Steel Cast iron Admiralty Copper A l u m i n u m brass 9 0 / 1 0 cupronickel

0.0164 0.0147 0.0018 0.0016 0.0015 0.0009

0.0031 0.0022 0.0015 0.0019 0.0002 0.0003

E v e n t h o u g h t h e test spools were e x p o s e d t o t h e a t m o s p h e r e a f t e r 508, 593, a n d 1093 h o u r s , c o r r o s i o n rates f o r steel a n d cast i r o n w e r e a c c e p t a b l y l o w , e s p e c i a l l y f o r samples i m m e r s e d i n the l i q u i d . C o r r o s i o n rates f o r t h e t u b e s t h e m s e l v e s were e v e n l o w e r , p r e s u m a b l y because t h e y were c o n t i n u o u s l y c o a t e d w i t h f l o w i n g l i q u i d w h e n t h e e v a p o r a t o r w a s i n o p e r a t i o n . T h e o n l y t u b e m a t e r i a l s t h a t s h o w e d a n y change i n w e i g h t g r e a t e r t h a n t h e e x p e r i m e n t a l e r r o r w e r e t h e steel a n d a l u m i n u m t u b e s . T h e a l u m i n u m t u b e s s h o w e d s u c h p o o r c o r r o s i o n resistance t h a t some t u b e s f a i l e d d u r i n g t h e s h o r t t i m e of t h e tests. T h e steel t u b e s s h o w e d w e i g h t losses e q u i v a l e n t t o 0.01 i n c h p e r y e a r d u r i n g t h e l a t e r sludge r u n s . S t e e l t u b e s u s e d d u r i n g t h e 1 5 9 3 - h o u r d e m o n s t r a t i o n r u n s h o w e d n o w e i g h t loss. T h e r e w a s some evidence o f p i t t i n g of t h e t u b e s u s e d d u r i n g t h e first 593 h o u r s , b u t n o p i t s deeper t h a n 0.001 i n c h w e r e f o u n d i n t u b e s u s e d d u r i n g t h e l a s t 1001 h o u r s . These good results could be a t t r i b u t e d t o the p r a c t i c a l l y continuous operation after the c a l c i u m s u l f a t e scale p r o b l e m o f t h e first 593 h o u r s w a s o v e r c o m e . T h e s e c o r r o s i o n d a t a i n d i c a t e t h a t d e a e r a t i o n of t h e sea w a t e r i s e s s e n t i a l t o l o n g life o f steel sea w a t e r e v a p o r a t o r s . W h e r e reasonable c o r r o s i o n a l l o w a n c e s c a n b e m a d e , as i n p i p i n g , v a p o r heads, etc., steel i s t h e m o s t p r a c t i c a l m a t e r i a l o f c o n s t r u c t i o n . I t s only u n c e r t a i n a p p l i c a t i o n is f o r the evaporator tubes, w h i c h m u s t be made t h i n f o r good heat conduction. O n l y long runs i n a continuously operating plant c a n prove w h e t h e r o r n o t steel is t h e m o s t s a t i s f a c t o r y m a t e r i a l f o r t u b e s .

Demonstration Plant T h e p i l o t p l a n t w o r k has p r o v e d t h a t t h e i n i t i a l a s s u m p t i o n s were n o t as o p t i m i s t i c as w e h a d f e a r e d n o r as i m p o s s i b l e as o t h e r s h a d p r e d i c t e d . I t w a s s h o w n t h a t : T h e cheapest t y p e o f c o m m e r c i a l e v a p o r a t o r c o u l d b e u s e d f o r sea w a t e r . T h i s L T V evaporator could be operated under such conditions that the high heat t r a n s f e r coefficients i n i t i a l l y a s s u m e d c o u l d b e a t t a i n e d . T h i s t y p e o f e v a p o r a t o r c o u l d b e k e p t free of scale, a t l i t t l e o r n o cost, a n d a t t e m p e r a t u r e s as h i g h as 2 5 0 ° F . T h e e v a p o r a t o r c o u l d be b u i l t p r i m a r i l y o f steel. E a r l y i n 1959, t h e Office o f S a l i n e W a t e r ' s d e m o n s t r a t i o n p l a n t p r o g r a m w a s i n a u g u r a t e d . T h i s process, w h i c h s h o w e d t h e greatest e c o n o m i c p r o m i s e , w a s chosen f o r t h e first o f t h e five d e m o n s t r a t i o n p l a n t s , w h i c h i s n o w u n d e r c o n s t r u c t i o n n e a r F r e e p o r t , T e x . W . L . B a d g e r A s s o c i a t e s , I n c . , p r o v i d e d t h e process d e s i g n a n d a r c h i t e c t e n g i n e e r i n g services f o r t h i s p l a n t (2), w h i c h w i l l h a v e a c a p a c i t y o f 1,000,000 g a l l o n s p e r d a y a n d w i l l use a 12-effect f a l l i n g - f i l m L T V e v a p o r a t o r . E v a p o r a t o r o p e r a t i n g c o n d i t i o n s w i l l b e as s h o w n i n F i g u r e 3. O p e r a t i n g c o n d i t i o n s w i l l b e b e l o w t h e s o l u b i l i t y c u r v e o f c a l c i u m s u l f a t e h e m i h y d r a t e , so t h a t o n l y t h e a l k a l i n e scales n e e d b e d e a l t w i t h . P r o v i s i o n s h a v e b e e n m a d e t o use t h e seeding m e t h o d as t h e b a s i c m e a n s of scale p r e v e n t i o n f o r t h e a l k a l i n e scales. T h e s e p r o v i s i o n s i n c l u d e t h e i n s t a l l a t i o n o f a t h i c k e n e r - c l a r i f i e r t o r e m o v e solids f r o m t h e c o n c e n t r a t e d sea w a t e r b l o w d o w n a n d a m i x e r t o r e i n c o r p o r a t e these solids i n t h e feed. B e c a u s e p i l o t p l a n t w o r k i n d i c a t e d t h a t d e a e r a t i o n of t h e sea w a t e r w a s i m p o r t a n t f o r c o r r o s i o n p r e v e n t i o n , a d e a e r a t o r h a s b e e n i n c o r p o r a t e d i n t h e flowsheet. T h i s d e a e r a t o r w i l l use a s m a l l p a r t o f t h e v a p o r f r o m t h e e l e v e n t h effect t o s t r i p o u t o x y g e n f r o m sea w a t e r feed t h a t h a s b e e n p a r t i a l l y p r e h e a t e d , so t h a t i t w i l l b e a t i t s b o i l i n g point under conditions i n the deaerator.




125

A f o r w a r d - f e e d e v a p o r a t o r i s n o t efficient w h e n t h e feed i s c o l d , unless t h e feed i s p r e h e a t e d i n some m a n n e r a l m o s t t o t h e b o i l i n g p o i n t i n t h e first effect. O t h e r w i s e , a c o n s i d e r a b l e p r o p o r t i o n o f t h e p r i m e s t e a m m u s t b e u s e d t o p r e h e a t t h e feed a n d t h u s is u n a v a i l a b l e t o e v a p o r a t e w a t e r . I n t h e d e m o n s t r a t i o n p l a n t , feed w i l l b e p r e h e a t e d b y t w o sets o f h e a t e x c h a n g e r s . O n e set w i l l o b t a i n h e a t b y c o o l i n g o f t h e condensate f r o m e a c h effect o f t h e e v a p o r a t o r . T h e first o f these w i l l c o o l first-effect c o n d e n s a t e , t h e second w i l l c o o l c o m b i n e d f i r s t - a n d second-effect condensate, a n d so o n t o t h e l a s t one, w h i c h w i l l h a n d l e t h e c o m b i n e d condensate f r o m a l l effects, c o o l i n g t h e d i s t i l l e d w a t e r as close as p r a c t i c a l t o t h e i n c o m i n g s e a w a t e r t e m p e r a t u r e . T h e s e condensate coolers w i l l n o t p r o v i d e a l l o f t h e h e a t needed f o r p r e h e a t i n g t h e f e e d ; t h e r e m a i n d e r w i l l b e s u p p l i e d b y c o n d e n s i n g v a p o r b l e d f r o m e a c h effect o f t h e e v a p o r a t o r i n a n o t h e r series o f heat e x c h a n g e r s . T h e sea w a t e r feed w i l l pass a l t e r n a t e l y t h r o u g h a condensate cooler, a v a p o r condenser, a n o t h e r condensate cooler, etc., so t h a t h e a t c a n be r e c o v e r e d a t t h e h i g h e s t t e m p e r a t u r e l e v e l p o s s i b l e . F i g u r e 4 i s a m o d e l o f t h e p l a n t ,

Figure 4. Demonstration plant for sea water conversion s h o w i n g d i s p o s i t i o n of e v a p o r a t o r s , h e a t e x c h a n g e r s , c l a r i f i e r - t h i c k e n e r , d e a e r a t o r , a n d o t h e r a u x i l i a r y e q u i p m e n t . A s i m p l i f i e d flow d i a g r a m o f t h e p l a n t i s s h o w n i n F i g u r e 5. T h i s d e m o n s t r a t i o n p l a n t w i l l n o r m a l l y o p e r a t e a t a first-effect b o i l i n g p o i n t o f 2 5 0 ° F . , a last-effect b o i l i n g p o i n t o f 120° F . , a n d a d i s c h a r g e sea w a t e r c o n c e n t r a t i o n f a c t o r of 4. T h e p r i m a r y c o n t r o l o f t h e process i s a c c o m p l i s h e d b y a u t o m a t i c c o n t r o l of s t e a m flow r a t e , sea w a t e r flow r a t e , a n d last-effect v a c u u m . N o c o n t r o l i s needed f o r t e m p e r a t u r e o r p r e s s u r e i n t h e i n d i v i d u a l effects a n d h e a t e x c h a n g e r s , since these achieve their own levels, influenced o n l y b y the p r o p o r t i o n i n g of t h e equipment. T h e d e m o n s t r a t i o n p l a n t i s r a t h e r h e a v i l y i n s t r u m e n t e d t o p e r m i t close s u r v e i l l a n c e of o p e r a t i n g c o n d i t i o n s a n d c a r r y i n g o u t o f s p e c i a l tests. T o increase t h e v a l u e o f t h e d e m o n s t r a t i o n p l a n t , features h a v e b e e n i n c o r p o r a t e d t o p e r m i t o p e r a t i o n u n d e r o t h e r t h a n d e m o n s t r a t i o n c o n d i t i o n s . I t w i l l b e possible t o o p e r a t e t h e e v a p o r a t o r a t first-effect t e m p e r a t u r e s u p t o 3 0 0 ° F . , t h u s a l m o s t d o u b l i n g p l a n t o u t p u t i f c a l c i u m s u l f a t e scale c a n b e p r e v e n t e d , a n d t o u s e t h e a c i d m e t h o d o f scale p r e v e n t i o n i n p l a c e o f t h e sludge m e t h o d . P r o v i s i o n s h a v e b e e n m a d e f o r l a t e r installation of a v a p o r compressor, w h i c h w o u l d convert t h e p l a n t t o a combination multiple effect-thermocompression system. T h i s would add about 1 5 % to plant output and w o u l d p e r m i t performance evaluation of v a p o r compressors i n sea water service.



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^PRODUCT " ^ D I S T I L L E D WATER 355.854W 99.6F

Figure 5. Flowsheet for demonstration plant Steam Sea water Condensate F. Temp., F. W. Flow, lb./hr. 0

I t w i l l also b e possible b y r e l a t i v e l y m i n o r p i p i n g changes t o c o n v e r t t h e f o r w a r d - f e e d e v a p o r a t o r t o b a c k w a r d feed, w h i c h m i g h t b e m o r e f a v o r a b l e i f t h e c a l c i u m s u l f a t e scale p r o b l e m c a n b e s o l v e d . E x c e p t f o r t u b e s , p u m p s h a f t sleeves, i m p e l l e r s , etc., t h e p l a n t w i l l b e b u i l t e x c l u s i v e l y o f steel a n d cast i r o n . T u b e m a t e r i a l s w i l l b e e v a l u a t e d b y t u b i n g different e v a p o r a t o r effects a n d h e a t e x c h a n g e r s w i t h steel, a d m i r a l t y m e t a l , a l u m i n u m brass, a n d 9 0 / 1 0 cupronickel. T h e copper alloy tubes w i l l b e used exclu s i v e l y i n t h e f i n a l condenser a n d i n t h e f e w h e a t e x c h a n g e r s t h a t a r e i n c o n t a c t w i t h n o n d e a e r a t e d sea w a t e r . T h e s e e x p e r i m e n t a l f e a t u r e s a d d t o t h e expense o f t h e b a s i c d e m o n s t r a t i o n p l a n t , i n b o t h o p e r a t i n g a n d c a p i t a l cost. T h e p r i n c i p a l i n c r e a s e i n o p e r a t i n g cost r e s u l t s f r o m t h e use o f 1 6 0 - p . s . i . s t e a m f r o m D o w a t 4 5 cents a t h o u s a n d p o u n d s i n s t e a d of 3 0 - p . s . i . s t e a m a t 4 0 cents. T h e h i g h e r s t e a m p r e s s u r e w a s c h o s e n t o p e r m i t tests a t t e m p e r a t u r e s u p t o 3 0 0 ° F . T h e e x t r a c a p i t a l costs r e s u l t f r o m t h e use of a l l o y t u b e s i n m o s t of t h e effects a n d h e a t e x c h a n g e r s , a n d p r o v i s i o n s f o r i n c r e a s e d p r o d u c t i o n i f 3 0 0 ° F . operation becomes possible, a n d f o r a c i d t r e a t m e n t a n d b a c k w a r d - f e e d operation. B a d g e r ' s cost e s t i m a t e w a s $1,374,000 f o r t h e e n t i r e p l a n t . O f t h i s $205,000 w a s c h a r g e a b l e t o e x p e r i m e n t a l f e a t u r e s a n d f a c t o r s of s a f e t y m a d e n e c e s s a r y b y t h e f a c t t h a t t h i s p l a n t , t h e first o f i t s k i n d , w a s t o b e b u i l t o n a c o m p e t i t i v e - b i d , g u a r a n t e e d - p e r f o r m a n c e b a s i s . T h e a c t u a l l o w b i d f o r t h e e n t i r e d e m o n s t r a t i o n p l a n t w a s $1,246,000, i n c l u d i n g a l l b u i l d i n g s , s e r v i c e s , site d e v e l o p m e n t , a n d i n i t i a l o p e r a t i o n t h r o u g h a s a t i s f a c t o r y p e r f o r m a n c e t e s t . T h e l o w b i d d e r , C h i c a g o B r i d g e a n d I r o n C o . , i s n o w i n t h e process of e r e c t i n g t h e p l a n t , w h i c h is due f o r c o m p l e t i o n i n A p r i l 1961. T h e o u t - o f - p o c k e t o p e r a t i n g cost of t h i s d e m o n s t r a t i o n p l a n t s h o u l d b e a b o u t $0.85 p e r 1000 g a l l o n s of d i s t i l l e d w a t e r , b a s e d o n s t e a m a n d p o w e r r e q u i r e m e n t s b o t h b y W . L . B a d g e r Associates, Inc., estimates a n d b y the low bidder's guarantees. T h i s water cost i n c l u d e s s t e a m a n d p o w e r costs, e s t i m a t e d c o n t r a c t m a i n t e n a n c e costs, a n d l a b o r costs t h a t i n c l u d e t h e i n c r e a s e d s u p e r v i s o r y l a b o r n e e d e d d u r i n g t h e d e m o n s t r a t i o n a n d test phases of t h e p r o g r a m , p r o j e c t e d t o e c o n o m i c c o n d i t i o n s t h a t w i l l p r o b a b l y exist i n 1963. W h e n i n t e r e s t , i n s u r a n c e , a n d d e p r e c i a t i o n costs o f t h e b a s i c p l a n t ( o n e t h a t does n o t i n c l u d e t h e test f e a t u r e s , etc.) a r e a d d e d a n d t h e l a b o r r e q u i r e m e n t i s r e d u c e d t o t h a t t o o p e r a t e a n o r m a l p r o d u c t i o n p l a n t , t h e t o t a l w a t e r cost i s a b o u t $1.04 p e r 1000 g a l l o n s , a g a i n b a s e d o n p r o j e c t e d 1963 costs. I f t h e costs s h o w n o n F i g u r e 1, w h i c h w e r e e s t i m a t e d i n 1955, w e r e p r o j e c t e d t o 1963 c o n d i t i o n s , t h e t o t a l w a t e r cost w o u l d b e



127

$0.98 p e r 1000 g a l l o n s . I f t h e d e m o n s t r a t i o n p l a n t h a d i n c l u d e d a t h e r m o c o m p r e s s i o n stage, as d i d t h e p l a n t o n w h i c h the 1955 estimates were b a s e d , t h e d e m o n s t r a t i o n p l a n t w o u l d h a v e s h o w n e v e n l o w e r costs.


Conclusions E s t i m a t e s of reasonable costs of e v a p o r a t i n g sea w a t e r t o p r o d u c e f r e s h w a t e r were m a d e i n 1955 b y W . L . B a d g e r A s s o c i a t e s , I n c . , o n t h e basis o f c e r t a i n a s s u m p t i o n s . T h e s e a s s u m p t i o n s were p r o v e d i n a p i l o t p l a n t p r o g r a m i n N o r t h C a r o l i n a c o n d u c t e d b y B a d g e r a n d S w e n s o n E v a p o r a t o r D i v i s i o n of W h i t i n g C o r p . f o r t h e Office o f S a l i n e W a t e r . T h e r e s u l t s w e r e so p r o m i s i n g t h a t t h e process w a s chosen b y Office o f S a l i n e W a t e r f o r i t s first d e m o n s t r a t i o n p l a n t . T h i s 1 , 0 0 0 , 0 0 0 - g a l l o n - p e r - d a y p l a n t w a s d e signed b y W . L . B a d g e r Associates, Inc., a n d is n o w under construction b y C h i c a g o B r i d g e a n d I r o n C o . T h e cost of w a t e r f r o m t h i s p l a n t w i l l b e a b o u t $1.00 p e r 1000 g a l l o n s , i n a g r e e m e n t w i t h t h e 1955 p r e d i c t i o n s . T h o s e same p r e d i c t i o n s s h o w e d t h a t i n a p l a n t o f reasonable size ( o v e r 15,000,000 g a l l o n s p e r d a y ) , w a t e r costs c o u l d b e b r o u g h t d o w n t o a b o u t $0.35 p e r 1000 g a l l o n s . P i l o t p l a n t w o r k i s s t i l l u n d e r w a y i n N o r t h C a r o l i n a , i n a n a t t e m p t t o increase e v e n f u r t h e r t h e o p e r a t i n g t e m p e r a t u r e s a n d sea w a t e r c o n c e n t r a t i o n s a t w h i c h scale f o r m a t i o n c a n be p r e v e n t e d . I f s u c h p r o v e s p o s s i b l e , w a t e r costs e v e n l o w e r t h a n those originally predicted should be achievable.

Acknowledgment T h e a u t h o r s a r e i n d e b t e d t o t h e Office o f S a l i n e W a t e r f o r s u p p o r t i n g t h e w o r k p r e s e n t e d i n t h e p a p e r . A l l e n C y w i n , J . J . S t r o b e l , a n d E . A . C a d w a l l a d e r of t h a t office were p a r t i c u l a r l y h e l p f u l i n t h e i r u n t i r i n g efforts w i t h t h e p r o g r a m . T h e c o o p e r a t i o n of the I n t e r n a t i o n a l N i c k e l C o . i n p r o v i d i n g space, services, a n d t e c h n i c a l assistance i s h i g h l y a p p r e c i a t e d . T h e c o n t i n u e d efforts of C . E . S e c h , J r . , J . R . S i n e k , a n d R . G . R e i m u s o f W . L . B a d g e r A s s o c i a t e s , I n c . , a r e also a c k n o w l e d g e d w i t h due a p p r e c i a t i o n . T h e Swenson E v a p o r a t o r D i v i s i o n of the W h i t i n g C o r p . is deserving of special t h a n k s for p r o v i d i n g e q u i p m e n t a n d v a l u a b l e t e c h n i c a l assistance.

Literature Cited (1) B a d g e r Associates, Inc., W. L., Office of Saline W a t e r , U. S. D e p t . C o m m e r c e , R. & D. R e p o r t 26, OTS Publ. 161290 (1959). (2) B a d g e r Associates, Inc., W. L., Office of Saline W a t e r , U. S. D e p t . I n t e r i o r , Specif. 195 (1960). (3) B a d g e r , W. L., S t a n d i f o r d , F. C., Natl. A c a d . Sci.-Natl. Research C o u n c i l , Publ. 568 (1958). (4) L a n g e l i e r , W. F., C a l d w e l l , D. H., L a w r e n c e , W. B., Ind. Eng. Chem. 42, 126-30 (1950). (5) S t a n d i f o r d , F. C., S i n e k , J. R., paper t o be presented at AIChE M e e t i n g , W a s h i n g t o n , D . C., D e c e m b e r 1960. RECEIVED for review A u g u s t 8, 1960. A c c e p t e d September 20, 1960.


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