SALINE WATER CONVERSION

Scale Deposition on a Heated Surface J. T. BANCHERO and KENNETH F. GORDON 1

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Department of Chemical and Metallurgical Engineering, University of Michigan, Ann Arbor, Mich.

Scale formation was followed visually in an appa ratus which approximated conditions in evaporators producing potable water. The time required for appearance of scale was investigated with and with out boiling under a variety of solution and surface temperatures, concentrations, and flow rates.

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sults with aqueous solutions of lithium carbonate, calcium sulfate, calcium hydroxide, and sodium sul fate, all of which possess inverted solubility curves, gave gentle curves when plotted as per cent supersaturation against the logarithm of the time for scale to appear with a parameter of concentration. For a given supersaturation a lower concentration (and necessarily higher wall temperature) resulted in more rapid formation of scale than a higher con centration. The time for scale formation was inde pendent of liquid velocity between 2 and 10 feet per second and ranged from 2 to 360 minutes with supersaturations from 90 down to 5%.

Ο ne of t h e s i m p l e s t m e t h o d s of r e c o v e r i n g p o t a b l e w a t e r f r o m sea w a t e r is d i s t i l l a t i o n . T h e t e c h n o l o g y i s w e l l u n d e r s t o o d , w i t h m u c h experience a v a i l a b l e f r o m b o t h c i v i l i a n a n d a r m e d service a p p l i c a t i o n s . T h e f o r m a t i o n of a t e n a c i o u s scale w h i c h decreases t h e h e a t flux b y p r o v i d i n g a n a d d i t i o n a l t h e r m a l resistance reduces t h e e c o n o m i c a t t r a c t i o n of sea w a t e r d i s t i l l a t i o n . T h e u n d e s i r a b l e scale is r e m o v e d a n d c o n t r o l l e d b y a w k w a r d a n d e x p e n s i v e c h e m i c a l o r m e c h a n i c a l m e a n s . T h i s m a y w e l l b e c o m e a serious f a c t o r i n t h e l a r g e scale 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 sea. A c o n t a c t s t a b i l i z a t i o n m e t h o d has b e e n u s e d (2) w h e r e m u c h of t h e scale i s d e p o s i t e d i n a b e d o f c o n t a c t m a t e r i a l o u t s i d e r a t h e r t h a n i n s i d e t h e e v a p o r a t o r . T h e c i r c u l a t i o n of a s u s p e n s i o n o f seed c r y s t a l s of t h e s c a l e - f o r m i n g c o n s t i t u e n t t h r o u g h t h e sea w a t e r e v a p o r a t o r , so t h a t t h e scale w o u l d b e d e p o s i t e d o n these c r y s t a l s 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 , w a s suggested b y B a d g e r a n d B a n chero (1) a n d a p p l i c a t i o n of t h i s t e c h n i q u e h a s b e e n c h e c k e d e x p e r i m e n t a l l y b y S t a n d i f o r d , S i n e k , a n d B j o r k (6). T h e costs o f c o n t r o l o f 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 h a v e b e e n r e p o r t e d as 4 0 cents p e r t h o u s a n d g a l l o n s w i t h c i t r i c a c i d (8) Present address, D e p a r t m e n t of C h e m i c a l E n g i n e e r i n g , U n i v e r s i t y of N o t r e Notre Dame, Ind. 1

105

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

Dame,

106

ADVANCES IN CHEMISTRY SERIES

a n d 12 cents w i t h f e r r i c c h l o r i d e (5). W i t h s u l f u r i c a c i d t h e cost m i g h t d r o p t o 3 c e n t s p e r t h o u s a n d gallons. T h e d e s i r e d t o t a l cost f o r p o t a b l e w a t e r d e l i v e r e d a t a l a r g e p l a n t w o u l d b e a b o u t 5 0 cents p e r t h o u s a n d g a l l o n s . T h e S y m p o s i u m o n S a l i n e W a t e r C o n v e r s i o n (4) p r o v i d e s b a c k g r o u n d i n f o r m a t i o n . N e v i l l e - J o n e s ( 5 ) a n d B a d g e r a n d B a n c h e r o (1) c o v e r scale p r e v e n t i o n k n o w l e d g e a n d p r a c t i c e w i t h m a n y references t o t h e l i t e r a t u r e .

Types of Scale


C a l c i u m s u l f a t e , w h i c h exists i n sea w a t e r i n i o n i c f o r m , has a reverse o r i n v e r t e d s o l u b i l i t y c u r v e a b o v e a b o u t 3 7 ° C . — t h a t i s , s o l u b i l i t y decreases w i t h i n c r e a s i n g t e m perature. I n d i s t i l l a t i o n t h e w a t e r closest t o t h e h e a t i n g s u r f a c e i s h o t t e s t a n d i t i s t h e r e t h a t c a l c i u m s u l f a t e i s least s o l u b l e . T h u s , c a l c i u m s u l f a t e d e p o s i t s , f o r m i n g a n a d h e r i n g f i l m t h a t increases t h e t h e r m a l resistance a n d decreases t h e heat flux. T h e scale i s c o n t i n u o u s l y d e p o s i t e d u n t i l t h e t u b e s are c l e a n e d o r b e c o m e p l u g g e d . F o r scale d e p o s i t i o n t h e l o c a l c o n c e n t r a t i o n m u s t b e a t least s a t u r a t e d i n c a l c i u m s u l f a t e . A t 100° C . t h i s o c c u r s i n c o n c e n t r a t e d sea w a t e r a t a c o n c e n t r a t i o n 3.1 t i m e s t h a t of o r d i n a r y sea water. A p l a n t h a s been successfully operated continuously w i t h o u t calcium sulfate d e p o s i t i o n b y t a k i n g o n l y p a r t o f t h e a v a i l a b l e w a t e r f r o m t h e sea w a t e r , so t h a t t h e l i q u i d i n t h e e v a p o r a t o r i s n e v e r m o r e t h a n 1.8 t i m e s t h e c o n c e n t r a t i o n of sea w a t e r a n d t h e w a l l t e m p e r a t u r e i s b e l o w a b o u t 2 5 0 ° F . (6). T h i s i m p o s e s t e c h n i c a l a n d e c o n o m i c limitations o n distillation plants. S i m i l a r considerations h o l d f o r plants distilling brackish water containing calcium sulfate. W h i l e t h e reverse s o l u b i l i t y c u r v e of c a l c i u m s u l f a t e i s o f t e n t h e m a i n reason f o r scale d e p o s i t i o n i n f r e s h w a t e r b o i l e r s a n d i n b r a c k i s h w a t e r d i s t i l l a t i o n , w h e n t h e sea w a t e r i s n o t c h e m i c a l l y t r e a t e d t h e cause i s c h e m i c a l r a t h e r t h a n p h y s i c a l . Sea w a t e r c o n t a i n s b i c a r b o n a t e i o n . O n h e a t i n g , t h e b i c a r b o n a t e i o n reacts w i t h w a t e r t o f o r m c a r b o n a t e i o n p l u s c a r b o n d i o x i d e , w h i c h t e n d s t o b e e v o l v e d as a gas as s h o w n i n t h e equations 2 H C 0 - -> C 0 ~ 3

Ca H 0 2

3

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2 O H - -> Mg(OH) J 2

T h e i n c r e a s e d a m o u n t of c a r b o n a t e i o n p r e s e n t causes t h e s u p e r s a t u r a t i o n of c a l c i u m c a r b o n a t e , w h i c h comes o u t of t h e s o l u t i o n . A s t h e c a r b o n d i o x i d e i s l e a s t s o l u b l e a t t h e t e m p e r a t u r e o f t h e h o t m e t a l s u r f a c e , t h e c a l c i u m c a r b o n a t e h a s i t s greatest supersaturation a t the surface a n d therefore tends t o deposit there. I n t u r n , the carbonate ion reacts w i t h water t o f o r m hydroxide i o n a n d m o r e carbon d i o x i d e i s e v o l v e d . T h e i n c r e a s e d c o n c e n t r a t i o n of h y d r o x i d e i o n m a k e s t h e s o l u t i o n s u p e r s a t u r a t e d w i t h respect t o m a g n e s i u m h y d r o x i d e . T h e m a g n e s i u m h y d r o x i d e w i l l h a v e t h e greatest s u p e r s a t u r a t i o n a t t h e t e m p e r a t u r e of t h e h o t m e t a l , w h e r e i t t o o w i l l d e p o s i t . B y s u i t a b l y a l t e r i n g t h e c o n c e n t r a t i o n f a c t o r o r t h e t e m p e r a t u r e of t h e sea water being distilled, either magnesium hydroxide o r calcium carbonate can be made t o b e t h e m a i n c o n s t i t u e n t of t h e scale. W h i l e o t h e r m a t e r i a l s are d e p o s i t e d , these t w o c a n m a k e u p 9 8 % of t h e scale (#), a n d c a n b e p r e v e n t e d f r o m d e p o s i t i n g b y c o n t r o l l i n g the p H w i t h acidic materials. C i t r i c acid a n d ferric chloride have been used successfully. S u l f u r i c a c i d h a s b e e n u s e d i n t h e W r i g h t s v i l l e B e a c h , N . C , sea w a t e r d i s t i l l a t i o n p i l o t p l a n t o p e r a t e d b y W . L . B a d g e r A s s o c i a t e s . T h e same p i l o t p l a n t has s h o w n t h a t t h e seed r e c y c l e t e c h n i q u e c o u l d 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 (6). I t h a s n o t y e t b e e n a p p l i e d s u c c e s s f u l l y t o 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 i n t h a t pilot plant. T h e o b j e c t i v e of t h e p r e s e n t c o n t i n u i n g i n v e s t i g a t i o n i s t o o b t a i n a b e t t e r k n o w l edge o f t h e m e c h a n i s m a n d l i m i t s of scale f o r m a t i o n o n a h e a t e d s u r f a c e t o p r o v i d e a s o u n d basis f o r d e v e l o p i n g m e t h o d s of scale p r e v e n t i o n .


BANCHERO AND GORDON—SCALE DEPOSITION ON HEATED SURFACES

107

E x p e r i e n c e i n d i c a t e d t h a t results f r o m l a b o r a t o r y b e n c h e x p e r i m e n t s m i g h t n o t b e d i r e c t l y c o m p a r a b l e t o those of p r o d u c t i o n p l a n t s . I t w a s necessary t o d e s i g n a n e x p e r i m e n t a l s y s t e m r e s e m b l i n g tubes of a n o p e r a t i o n a l e v a p o r a t o r . I t is hoped that future w o r k w i l l allow a n acceptable correlation between simpler l a b o r a t o r y bench runs a n d the experimental system used.

Apparatus


Scale d e p o s i t i o n w a s i n v e s t i g a t e d w i t h a n d w i t h o u t b o i l i n g i n a n a p p a r a t u s d e s i g n e d t o a p p r o x i m a t e t o some e x t e n t c o n d i t i o n s i n c o m m e r c i a l e v a p o r a t o r t u b e s , y e t i n w h i c h o b s e r v a t i o n of scale f o r m a t i o n i s possible.

Figure I. A s seen i n diameter, surface. O n the solution Appropriate that shown. the / - i n c h 1

4

Test section

i n F i g u r e s 1 a n d 2, t h e e q u i p m e n t consists o f a c o p p e r c y l i n d e r 3.8 i n c h e s i n w h i c h a quarter-inch-diameter helical groove was c u t o n t h e external t h e o u t s i d e , t h e r e i s a c l o s e - f i t t i n g p r e c i s i o n - b o r e glass t u b e , t h r o u g h w h i c h flowing i n t h e g r o o v e c a n b e seen a n d t h e scale d e p o s i t i o n f o l l o w e d v i s u a l l y . s a f e t y s h i e l d i n g o f steel p l a t e a n d s a f e t y p l a t e glass i s u s e d r a t h e r t h a n A n inflated spiral gasket, resting i n a / X / i n c h groove parallel t o s o l u t i o n g r o o v e , m a i n t a i n s t h e s o l u t i o n flow i n i t s h e l i c a l p a t h , p r e v e n t i n g 3

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Figure 2.

Calcium sulfate scale deposited on hot upper end of groove

s h o r t c i r c u i t i n g o r b y p a s s i n g . B y m e a n s of a n i n t e r n a l h e l i x ( n o t s h o w n ) a c o u n t e r c u r r e n t s t r e a m of h o t w a t e r u n d e r p r e s s u r e heats t h e s o l u t i o n b e i n g i n v e s t i g a t e d . T h e i n t e r n a l h e l i x i s a q u a r t e r - i n c h , s e m i c i r c u l a r g r o o v e c u t o n t h e o u t s i d e of a 3 . 1 - i n c h diameter copper hollow cylinder w h i c h is shrunk-fit inside the 3.8-inch cylinder. T h e i n t e r n a l , h o t - w a t e r h e l i x g r o o v e rests d i r e c t l y u n d e r t h e i n f l a t a b l e gasket g r o o v e , so t h a t 18 t h e r m o c o u p l e w e l l s of / - i n c h d i a m e t e r c o u l d b e c u t t o t h e surface of t h e s c a l i n g s o l u t i o n g r o o v e a n d sealed w i t h / - i n c h - l o n g p l u g s s h r u n k - f i t a n d finished t o g i v e a s m o o t h s u r f a c e . T h e s u r f a c e t e m p e r a t u r e of t h e h o t - w a t e r g r o o v e i s o b t a i n e d t h r o u g h e i g h t t h e r m o c o u p l e wells c u t i n the i n t e r n a l c y l i n d e r . T h e r m o c o u p l e s l e d t h r o u g h t h e h o l l o w i n t e r n a l c y l i n d e r a l l o w m e a s u r e m e n t of t h e a p p r o p r i a t e c o p p e r s u r f a c e t e m p e r a t u r e s a l o n g t h e w h o l e p a t h l e n g t h of e a c h g r o o v e . T h e s o l u t i o n u n d e r s t u d y flows i n a 3 7 - f o o t - l o n g h e l i c a l p a t h of s e m i c i r c u l a r cross s e c t i o n w h i c h has a c o p p e r c i r c u l a r edge a n d a s t r a i g h t edge of glass. Thermocouples i n t h e w a l l a n d t h e flowing s t r e a m a l l o w g o o d m e a s u r e m e n t of the v a r i o u s t e m p e r a t u r e s . T h e s o l u t i o n of i n t e r e s t flows u p w a r d s i n t h e e x t e r n a l h e l i c a l g r o o v e , w h i l e t h e h o t w a t e r flowing c o u n t e r c u r r e n t l y a n d i n t e r n a l l y s u p p l i e s t h e heat t o t h e s o l u t i o n . T h u s t h e s o l u t i o n enters c o l d a t t h e b o t t o m a n d leaves h o t a t t h e t o p . I t s e x i t t e m p e r a t u r e is fixed b y t h e i n l e t h o t w a t e r t e m p e r a t u r e a n d r e l a t i v e flow r a t e s . T h e h o t w a t e r also d e t e r m i n e s t h e t e m p e r a t u r e a t t h e m e t a l w a l l of t h e s o l u t i o n h e l i x a n d , hence, t h e s u p e r saturation there. 1

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109

T h e e q u i p m e n t c o n t a i n s t h r e e s e p a r a t e fluid s y s t e m s : t h e s c a l i n g s o l u t i o n c i r c u l a tion system, the heating water circulation system, a n d the constant temperature c i r c u l a t i o n s y s t e m . T h e s o l u t i o n i s p r e p a r e d i n a 5 0 0 - g a l l o n stainless steel t a n k w i t h a h e a t i n g j a c k e t . I t i s p u m p e d b y a v a r i a b l e speed, M o y n o , stainless steel s l u r r y p u m p t h r o u g h a stainless steel C u n o A u t o c l e a n filter, t h e n t h r o u g h a r o t a m e t e r t o t h e h e l i x test s e c t i o n . A needle v a l v e a f t e r t h e h e l i x test s e c t i o n a l l o w s c o n t r o l of t h e p r e s s u r e i n t h e test s e c t i o n . F r o m t h e test s e c t i o n , t h e s o l u t i o n goes t h r o u g h a cooler t o t h e spent solution t a n k a n d either is discarded o r returned t o t h e 500-gallon t a n k . A 5 5 g a l l o n stainless steel t a n k of d i s t i l l e d w a t e r w h o s e t e m p e r a t u r e i s c o n t r o l l e d b y a h e a t i n g c o o l i n g c o i l i s c o n n e c t e d t o t h e i n l e t of t h e s l u r r y p u m p , so t h a t t h e test s e c t i o n m a y be b r o u g h t t o t h e r m a l e q u i l i b r i u m b e f o r e t h e s a l t s o l u t i o n i s r u n t h r o u g h i t . A f t e r a r u n , t h i s d i s t i l l e d w a t e r c a n b e u s e d t o d i s s o l v e t h e scale i n t h e test s e c t i o n i f necessary. T h e c o n s t a n t t e m p e r a t u r e c i r c u l a t i o n s y s t e m consists of a n o t h e r 5 5 - g a l l o n d r u m o f water m a i n t a i n e d a t tho desired temperature b y a thermostat. W a t e r is p u m p e d f r o m t h e d r u m t h r o u g h t h e j a c k e t of t h e l a r g e s o l u t i o n t a n k a n d t h e test s e c t i o n . T h i s a l l o w s close t e m p e r a t u r e c o n t r o l of t h e s o l u t i o n e n t e r i n g t h e test s e c t i o n . T h e h o t w a t e r u s e d t o c o n t r o l t h e t e m p e r a t u r e i n t h e h e l i c a l test s e c t i o n i s h e a t e d b y steam i n a shell a n d coil heat exchanger. T h e h o t water temperature i n the exc h a n g e r o u t l e t l i n e c o n t r o l s t h e s t e a m flow. F r o m t h e e x c h a n g e r t h e h o t w a t e r flows t o a 1 2 - g a l l o n h i g h p r e s s u r e surge t a n k , w h e r e t h e fine c o n t r o l of t h e t e m p e r a t u r e i s o b t a i n e d b y e l e c t r i c a l h e a t e r s . T h e w a t e r i s t h e n p u m p e d t o t h e h e l i x test s e c t i o n through a rotameter a n d returned to the heater.

Procedure T o e s t a b l i s h t h e final s t e a d y - s t a t e t e m p e r a t u r e s , a r u n w a s s t a r t e d b y h a v i n g d i s t i l l e d w a t e r a t t h e t e m p e r a t u r e o f t h e i n l e t s o l u t i o n flowing t h r o u g h t h e o u t e r g r o o v e s w i t h h e a t i n g w a t e r p a s s i n g t h r o u g h t h e i n t e r n a l h e l i x . A t t i m e zero t h e d i s t i l l e d w a t e r w a s t u r n e d off a n d t h e s o l u t i o n of k n o w n c o n c e n t r a t i o n t u r n e d o n a t t h e s a m e t e m p e r a t u r e a n d flow r a t e as t h e d i s t i l l e d w a t e r . B y o b s e r v a t i o n t h e e l a p s e d t i m e u n t i l t h e b e g i n n i n g o f scale f o r m a t i o n w a s n o t e d . W i t h t e m p e r a t u r e s a n d c o n c e n t r a t i o n s k n o w n , the per cent s u p e r s a t u r a t i o n a t the exit e n d of the helix could b e calculated a n d , as t h e scale a l w a y s o c c u r r e d a t t h e e x i t e n d , i t w a s t a k e n t o b e t h e p e r c e n t s u p e r s a t u r a tion for the r u n . A t y p i c a l d e p o s i t o f c a l c i u m s u l f a t e c a n b e seen i n F i g u r e 2 . O n c o n t i n u i n g t h e r u n t h e scale d e p o s i t i n c r e a s e d , c a u s i n g v e r y h i g h p r e s s u r e d r o p s , a n d i n s o m e cases t h e e q u i p m e n t b e c a m e p l u g g e d w i t h scale. T h e i n i t i a l s t e p w a s t o s t u d y s y s t e m s w i t h reverse s o l u b i l i t y c u r v e s t o l e a r n t h e g e n e r a l p a t t e r n o f t h e onset o f s c a l i n g w h i c h w o u l d b e o f v a l u e f o r u n d e r s t a n d i n g t h e sea w a t e r s y s t e m . C a l c i u m s u l f a t e , l i t h i u m c a r b o n a t e , s o d i u m s u l f a t e , a n d c a l c i u m h y d r o x i d e h a v e reverse s o l u b i l i t y c u r v e s i n w a t e r , a r e r e a d i l y a v a i l a b l e , a n d a r e s o l u b l e t o a n e x t e n t t h a t n e i t h e r v i s u a l o b s e r v a t i o n o f scale n o r c h e m i c a l a n a l y s i s w o u l d b e a problem. T h e b e h a v i o r of s o l u t i o n s of e a c h s u b s t a n c e w a s e x p l o r e d i n t h e h e l i x , l i t h i u m c a r b o n a t e s o l u t i o n b e i n g t h e l a s t u s e d . T h e effect o f c o n c e n t r a t i o n l e v e l w a s t h e n e x amined w i t h the solution i n the equipment, l i t h i u m carbonate. I t was discarded a n d replaced w i t h calcium sulfate, w h i c h is being studied more intensely. I t is hoped t h a t t h e r e s u l t s of these r u n s w i l l b e c o r r e l a t e d w i t h those f r o m p i l o t p l a n t s a n d o p e r a t i o n a l p l a n t s d i s t i l l i n g sea w a t e r .

Analyses T h e l i t h i u m c a r b o n a t e c o n c e n t r a t i o n w a s m e a s u r e d b y acidimétrie t i t r a t i o n w i t h m e t h y l orange indicator. T h e c a l c i u m sulfate a n d c a l c i u m h y d r o x i d e concentrations were determined b y t i t r a t i o n w i t h d i s o d i u m dihydrogen Versenate [the disodium salt of ( e t h y l e n e d i n i t r i l o ) t e t r a a c e t i c a c i d ] , w i t h a d d e d m a g n e s i u m c h l o r i d e . A buffer of a m m o n i u m chloride i n a m m o n i u m hydroxide was employed. T h e indicator was E r i o chrome B l a c k T . A special high p u r i t y calcium carbonate i n hydrochloric acid was u s e d as a s t a n d a r d . B e c a u s e o f t h e h i g h c o n c e n t r a t i o n o f s o d i u m s u l f a t e i t w a s c o n -


110


venient to analyze b y evaporation, ignition, and weighing, all w i t h appropriate precau tions.

Results A c o n v e n i e n t c o r r e l a t i o n of t h e h e l i x d a t a i s o n a p l o t of s u p e r s a t u r a t i o n a g a i n s t t h e l o g a r i t h m of t h e t i m e f o r s c a l i n g . F r o m t h e i n i t i a l r e s u l t s ( F i g u r e s 3 t o 6) i t i s seen t h a t , f o r a g i v e n c o n c e n t r a t i o n , t h e d a t a c o u l d b e r e p r e s e n t e d as a gentle c u r v e or a s t r a i g h t l i n e . A n a r r o w i n d i c a t e s a r u n s t o p p e d b e f o r e s c a l i n g . 100,

,


90 LITHIUM CARBONATE helix runs

80

ζο

A • • •

70

1.21 g. L i C 0 / I O O m l . soin. 1.09 0.93 0.73 2

3

1 60 3 I< CO

S so Ο CO

t-

2

υ et

40

LJ CL

30,

20

10 J

0 I

1

I

I I I I

I

I

I

JO 100 TIME FOR SCALE, MINUTES

Figure 3 .

1

I

1 1 I 1

1,000

Data for lithium carbonate

L i t h i u m C a r b o n a t e . U s i n g the h e l i x , 77 r u n s were m a d e w i t h t h e l i t h i u m c a r b o n ate s y s t e m i n w a t e r , y i e l d i n g t h e results s h o w n i n F i g u r e 3. D u r i n g a r u n a l l t e m p e r a t u r e s were h e l d c o n s t a n t , b u t were v a r i e d f r o m r u n t o r u n , w i t h t h e w a l l t e m p e r a t u r e c o v e r i n g t h e r a n g e 158° t o 2 6 1 ° F . C o n c e n t r a t i o n has a definite effect o n t h e t i m e f o r s c a l i n g , w h e n t h e d a t a are c o r r e l a t e d b y u s i n g p e r cent s u p e r s a t u r a t i o n . W i t h a l o w concentration a higher temperature is required for a given supersaturation t h a n at a h i g h e r c o n c e n t r a t i o n . T h e r e s u l t s f o r the l o w c o n c e n t r a t i o n r u n s m i g h t b e s h o w i n g t h e effect of t h e h i g h e r t e m p e r a t u r e a n d l o w e r v i s c o s i t y o f t h e s o l u t i o n . T h e d e v i a t i o n o f t h e p o i n t s a t 7 8 t o 9 0 % s u p e r s a t u r a t i o n a n d v e r y s h o r t t i m e s c o u l d b e a reflection of a t i m e l a g i n t h e s y s t e m . T h e p o i n t s a t 4 5 % s u p e r s a t u r a t i o n are t a k e n t o d e t e r m i n e t h e effect of v e l o c i t y . F o u r t e e n r u n s m a d e u n d e r a p p a r e n t l y i d e n t i c a l c o n d i t i o n s ( e x c e p t f o r v e l o c i t y ) s h o w t h a t v e l o c i t y has n o effect ( F i g u r e 7 ) . T h e s c a t t e r i s p r o b a b l y d u e to u n c o n t r o l l e d v a r i a t i o n i n t h e c o p p e r s u r f a c e . I n F i g u r e 3 t h e o p e n s y m b o l s are f o r r u n s i n w h i c h t h e r e was a c t i v e b o i l i n g , w h i l e t h e s o l i d s y m b o l s are f o r those w h e r e b o i l i n g w a s p r e v e n t e d b y m a i n t a i n i n g a n a p p r o priate back pressure o n the system. A l t h o u g h the data scatter, i t is s u r p r i s i n g t h a t n o effect of b o i l i n g i s a p p a r e n t . T h i s m a y b e m i s l e a d i n g , f o r t h e v a p o r b u b b l e s c o u l d p r e v e n t a n y s m a l l p a r t i c l e s of scale f o r m e d b y b o i l i n g f r o m b e i n g seen, t h e scale b e i n g a p p a r e n t t o t h e eye o n l y a f t e r i t h a s d e p o s i t e d i n m o d e s t a m o u n t s . T h e p e r c e n t s u p e r s a t u r a t i o n w a s c a l c u l a t e d o n t h e b a s i s of t h e c o n c e n t r a t i o n of t h e feed s o l u t i o n , w i t h o u t a t t e m p t i n g t o a c c o u n t f o r a n y change d u e t o b o i l i n g . I t i s n o t possible t o e s t i m a t e t h e l o c a l c o n c e n t r a t i o n a n d s u p e r s a t u r a t i o n a n d j u d g e i f t h e i r use w o u l d raise t h e p o i n t s


111


CALCIUM S U L F A T E helix runs

• 0.216 g. Co S0 /100 ml. soin • 0.17 4

•

A

0.1 16 Open points are for boiling runs Velocity ZO-IO.Oft./sec. Wall temperature

208-285°F

•

-

\

\

ο

•

0216 ς./100 ml


•

•

_

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A

A

ill

A \ .

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ITV-

V .

0.17

0.116 •Α*

ι

1

ι

ι

ι ι ι

10

I

I

I

ι

ι

ι ι ι I

100

TIME FOR SCALE, MINUTES

Figure 4.

Data for calcium sulfate

for b o i l i n g sufficiently t o i n d i c a t e a difference b e t w e e n b o i l i n g a n d n o n b o i l i n g r u n s . S u c h a difference w o u l d s h o w b o i l i n g r u n s d e p o s i t i n g scale m o r e s l o w l y t h a n n o n b o i l i n g runs a t t h e same local p e r cent supersaturation. C a l c i u m S u l f a t e . T h e results o f t h e 65 runs f o r this system (Figure 4 ) show a s i m i l a r p a t t e r n — n a m e l y , a gentle c u r v e w i t h t h e c o o r d i n a t e s u s e d , s c a t t e r o f d a t a giving a b a n d rather t h a n a line, deviation f r o m a straight line at v e r y high supersatu r a t i o n s , a p p a r e n t l y n o great difference b e t w e e n b o i l i n g a n d n o n b o i l i n g r u n s , a n d a n effect of c o n c e n t r a t i o n w i t h t h e l o w e r c o n c e n t r a t i o n ( a n d h i g h e r t e m p e r a t u r e ) g i v i n g a l o w e r t i m e f o r scale f o r m a t i o n . H e r e t h e s u p e r s a t u r a t i o n w a s c a l c u l a t e d w i t h r e s p e c t to the h e m i h y d r a t e . T h e w a l l t e m p e r a t u r e c o v e r e d t h e r a n g e 2 0 8 ° t o 2 8 5 ° F .

SODIUM S U L F A T E helix runs 32.2-33.8 g. Na S0 /IOOml.solri| 2

•

2 ft/sec

A

6 ft/sec

•

9 ft/sec

4

Wall temperature 145-265 ° F

A

àA

10

+

A A

100

1,000

TIME FOR SCALE, MINUTES

Figure 5.

Data for sodium sulfate



112

CALCIUM HYDROXIDB helix runs 1.08-1.13 g. Co(OH) /IOO ml. soin. Velocity 6.5 ft/sec. Woll temperoture l68-238 F. 2


e

—I

I

!

I I I I 1 I 10

Figure 6. 50

_J I I I I 100 TIME FOR SCALE, MINUTES

_l

I

I I I I 1

I

Data for calcium hydroxide

Δ Δ

Lithium

Carbonate

Effect of Solution Velocity

CO "401

1.18-1.21 g. Li C03/IOOg. solution 2

Woll temperature 179.8 - I80.5 F 9

UJ30

SALINE WATER CONVERSION

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