saline water conversion - ACS Publications

The Place of Solvent Extraction in Saline Water Conversion DONALD W. HOOD and RICHARD R. DAVISON

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.ch006

TexasA& M College, College Station, Tex.

No method of desalination is likely to be best in all circumstances and the conditions most favorable for solvent extraction are investigated. In this process, saline water is contacted with an organic solvent to produce a more concentrated raffinate and an extract containing the low salinity water which separates upon heating. The solvent is recycled and the product and raffinate are stripped of the residual solvent content. Data on both the extracting and stripping steps indicate their practicality. Of the many types of compounds investigated as solvents, secondary and tertiary amines are the best in every respect, except that the resulting high pH precipitates magnesium. Low temperatures in the process minimize scale and corrosion and make possible the use of low-level heat, and by varying the solvent mixture, the operating temperature may be varied to suit ambient conditions. As for any stagewise process, the costs are less for low salinity feed. From a competitive standpoint, water of low magnesium and 5000 to 10,000 p.p.m. solids are favored.

T h e complexity of the p r o b l e m a n d the diversity of operating conditions i n saline water c o n v e r s i o n m a k e i t u n l i k e l y t h a t a n y process b a s e d o n o n e p r i n c i p l e o r p h e n o m e n o n w i l l p r o v i d e t h e m o s t efficient c o n v e r s i o n i n a l l o p e r a t i n g s i t u a t i o n s e n c o u n t e r e d . T h e a r t o f s a l i n e w a t e r c o n v e r s i o n has n o w r e a c h e d a l e v e l a t w h i c h one c a n b e g i n t o t a k e s t o c k w i t h respect t o t h e p a r t i c u l a r a d v a n t a g e s o f t h e m a n y different processes i n a n y g i v e n s i t u a t i o n . T h e final s e l e c t i o n o f a process w i l l o n l y b e possible a f t e r c a r e f u l c o n s i d e r a t i o n o f process o p e r a t i o n d a t a as a p p l i e d t o t h e c o n v e r s i o n p r o b l e m a t h a n d . S i n c e s u c h d a t a a r e a v a i l a b l e o n b u t v e r y f e w processes a t t h e p r e s e n t t i m e , i t i s o n l y possible t o p r o j e c t o n t h e basis o f t h e o r y a n d experience those p o i n t s w h i c h set a p a r t one process f r o m a n o t h e r . T h e p u r p o s e o f t h i s p a p e r i s t o p r e s e n t i n f o r m a t i o n n o w a v a i l a b l e w h i c h m a y h e l p t o l o c a t e t h e s o l v e n t e x t r a c t i o n process i n i t s r i g h t f u l p o s i t i o n i n t h e s a l i n e w a t e r c o n v e r s i o n field. S o l v e n t e x t r a c t i o n w a s p r o p o s e d as a m e t h o d f o r saline w a t e r c o n v e r s i o n b y H o o d a n d H a r w e l l i n 1953 (1), a n d h a s b e e n u n d e r i n v e s t i g a t i o n since t h a t t i m e t h r o u g h s u p p o r t o f t h e Office o f S a l i n e W a t e r a t T e x a s A & M C o l l e g e . A n u m b e r o f r e v i e w a r t i c l e s h a v e b e e n p u b l i s h e d c o n c e r n i n g t h i s process {2-5). 40

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

HOOD AND DAVISON—SOLVENT EXTRACTION

41

Process


T h e process operates o n t h e p r i n c i p l e t h a t c e r t a i n s o l v e n t s w h i c h c o n t a i n s t r o n g electronegative atoms w i t h i n t h e molecule have t h e p r o p e r t y of f o r m i n g hydrogen b o n d s w i t h w a t e r m o l e c u l e s a n d y e t because o f t h e i r h y d r o p h o b i c side c h a i n d r a w t h e water-solvent couple into t h e solvent phase. S u b s t i t u t i o n of a l k y l groups o n o r near t h e n i t r o g e n , as i n b r a n c h e d s e c o n d a r y o r t e r t i a r y a m i n e s , r e s u l t s i n e x t r e m e s e n s i t i v i t y of s o l u b i l i t y t o t e m p e r a t u r e , m a k i n g i t possible t o s e p a r a t e s o l v e n t a n d w a t e r b y a s m a l l change i n t e m p e r a t u r e . W i t h these c o n f i g u r a t i o n s of t h e s o l v e n t m o l e c u l e , w a t e r is e x t r a c t e d f r o m t h e b r i n e s o l u t i o n s w i t h t h e e x c l u s i o n of s a l t . A s c h e m a t i c of t h e e x t r a c t i o n process i s s h o w n i n F i g u r e 1. I f r e f l u x i s necessary, s l i g h t m o d i f i c a t i o n s w i l l REFLUX

RAW WATER SOLVENT

ίο §1 PHASE SEPARATION TANK

Ï WATER

REFLUX BRINE STEAM STRIPPERl STEAM \STRIPPER

WATER PRODUCT

[BRINE

Figure I. Schematic of solvent extraction process for saline water conversion be r e q u i r e d . F e e d w a t e r enters a t t h e t o p of t h e e x t r a c t o r ( o r i n t e r m e d i a t e l y , i f r e f l u x is r e q u i r e d ) a n d t h e s o l v e n t i s c o n t a c t e d i n c o u n t e r c u r r e n t flow b y t h e s o l v e n t e n t e r i n g the e x t r a c t o r a t t h e b o t t o m . T h e extract, w h i c h contains appreciable water a n d a l i t t l e s a l t , leaves t h e e x t r a c t o r a n d flows t h r o u g h t h e h e a t exchange s y s t e m . T h e e x t r a c t is d i v i d e d i n t o t w o s t r e a m s : O n e exchanges h e a t w i t h t h e s t r i p p e d s o l v e n t , a n d t h e o t h e r w i t h t h e s e p a r a t e d w a t e r . H o t e x t r a c t f r o m t h e e x c h a n g e r enters t h e e x t r a c t heater f r o m w h i c h i t emerges a t a s e p a r a t i o n t e m p e r a t u r e T - 2 b e f o r e e n t e r i n g t h e s e p a r a t o r . T h e s t r i p p e d s o l v e n t flows b a c k t o t h e e x c h a n g e r s a n d t h r o u g h a cooler f r o m w h i c h i t i s r e c y c l e d t o t h e e x t r a c t o r . T h e w a t e r f r o m t h e s e p a r a t o r flows t h r o u g h the exchangers a n d t h r o u g h the p r o d u c t solvent recovery u n i t . I f reflux i s used, p a r t of t h e p r o d u c t w a t e r s t r e a m i s r e m o v e d p r i o r t o s o l v e n t r e m o v a l a n d r e t u r n e d t o t h e e x t r a c t i o n c o l u m n . M a n y m o d i f i c a t i o n s o f t h i s b a s i c process h a v e b e e n c o n s i d e r e d ,


42

ADVANCES IN CHEMISTRY SERIES


w i t h p a r t i c u l a r reference t o t y p e o f e x t r a c t o r s , sources o f h e a t , a n d t h u s m o d i f i c a t i o n of h e a t exchangers a n d s o l v e n t r e c o v e r y processes. I n essence, h o w e v e r , a l l consist o f t h e t h r e e basic p a r t s : e x t r a c t i o n , s e p a r a t i o n , a n d s o l v e n t r e c o v e r y . I n F i g u r e 2, t h e s o l u b i l i t y c u r v e o f a t y p i c a l l y g o o d s o l v e n t i s s h o w n . I n t h i s c u r v e t h e w e i g h t p e r cent of w a t e r i n t h e s o l v e n t i s p l o t t e d vs. t e m p e r a t u r e . I f o n e chooses a n e x t r a c t i o n t e m p e r a t u r e o f 3 8 ° C , t h e s o l v e n t w i l l d i s s o l v e a p p r o x i m a t e l y 3 0 % water. I f a lower temperature is used a n d thus a higher water content, t h e

o l 0 0

ι

ι

10

20

ι ι ι 1 30 40 50 60 WATER , WEIGHT PER CENT

1 70

1 80

1 90

1 100

Figure 2. Typical solubility diagram of typical solvent useful in saline process s o l v e n t dissolves w a t e r w h i c h c o n t a i n s i n c r e a s i n g a m o u n t s o f s a l t . E x p e r i e n c e h a s s h o w n t h a t a w a t e r c o n t e n t o f t h e e x t r a c t i n excess of 35 t o 4 0 % u s u a l l y causes excess s o l u t i o n o f s a l t i n t h e s o l v e n t p h a s e f o r o p t i m u m o p e r a t i o n . F i g u r e 3 shows t h e effect of s o l v e n t w a t e r c o n t e n t o n t h e s e l e c t i v i t y o f w a t e r o v e r s a l t f o r some s e c o n d a r y a n d t e r t i a r y a m i n e s . I f t h e s o l v e n t s r e p r e s e n t e d were t h e n h e a t e d t o 7 5 ° C , a l l e x c e p t 8 % o f t h e w a t e r w o u l d phase o u t . A f t e r phase s e p a r a t i o n o f w a t e r a n d s o l v e n t , t h e s o l v e n t c a n b e r e c y c l e d t h r o u g h t h e e x t r a c t i o n phase t o m a k e t h e process c o n t i n u o u s .

Solvent Chemistry D u r i n g t h i s i n v e s t i g a t i o n a b o u t 4 0 0 s o l v e n t s h a v e been s t u d i e d f o r s u i t a b i l i t y i n t h e s o l v e n t e x t r a c t i o n process. I n c l u d e d i n t h i s g r o u p h a v e b e e n esters, f a t t y a c i d s , aldehydes, ketones, alcohols, glycerol ethers, ethylene g l y c o l , p r o p y l e n e g l y c o l , thioethers, e t h a n o l a m i n e s , a m i n o ethers, p h o s p h a t e a n d p h o s p h i t e esters, m o r p h o l i n e d e r i v a t i v e s , a large v a r i e t y of substituted aromatic a n d cyclic compounds, a n d p r i m a r y , secondary, a n d t e r t i a r y a m i n e s . O f t h i s v a s t a r r a y o f c o m p o u n d s , i t a p p e a r s t h a t o n l y those c o m p o u n d s c o n t a i n i n g s t r o n g e l e c t r o n e g a t i v e g r o u p s s u c h as o x y g e n , n i t r o g e n , a n d p o s s i b l y p h o s p h o r u s , h a v e p r o p e r t i e s u s e f u l i n t h i s process. O f these, t h e n i t r o g e n - c o n t a i n i n g c o m p o u n d s a n d p a r t i c u l a r l y t h e a m i n e s a r e f o u n d t o h a v e b y f a r t h e best s o l v e n t p r o p e r t i e s . T h e s o l u b i l i t y c u r v e s f o r some o f t h e a m i n e s a r e s h o w n i n F i g u r e 4 . T h i s figure p o i n t s o u t r a t h e r d r a m a t i c a l l y t h e effect o f o n e c a r b o n a t o m i n a c o m p o u n d a n d t h e effect of s u b s t i t u t i o n i n t h e a l i p h a t i c c h a i n w i t h respect t o t h e n i trogen atom. M e t h y l p e n t y l a m i n e a n d m e t h y l b u t y l a m i n e are n o t p a r t i c u l a r l y useful because, i n t h e case o f t h e f o r m e r , t h e s o l u b i l i t y c u r v e i s t o o p e r p e n d i c u l a r a n d , i n the l a t t e r , insufficient w a t e r i s lost b y h e a t i n g w i t h i n n o r m a l t e m p e r a t u r e s . A l t h o u g h t h e m e t h y l p e n t y l a m i n e i s s o m e w h a t b e t t e r t h a n m e t h y l b u t y l a m i n e , t h e s o l u b i l i t y does n o t change s u f f i c i e n t l y w i t h t e m p e r a t u r e f o r a g o o d s o l v e n t ; h o w e v e r , e t h y l i s o p r o p y l and e t h y l sec-butylamine w h i c h contain a m e t h y l group a l p h a t o t h e nitrogen show g r e a t i m p r o v e m e n t i n s o l u b i l i t y p r o p e r t i e s . I n t h e case o f e t h y l i s o p r o p y l a m i n e , a n almost ideal solvent s i t u a t i o n results except t h a t t h e operating temperature is m u c h



43


higher t h a n is desirable. Conversely, the e t h y l sec-butylamine requires a n extraction t e m p e r a t u r e t h a t i s t o o l o w f o r a n effective s o l v e n t . T h e r e a r e a n u m b e r o f a m i n e s w h i c h h a v e s o l v e n t p r o p e r t i e s s i m i l a r t o those s h o w n . T h e final d e c i s i o n w i t h respect t o s e l e c t i o n i s b a s e d o n t h e c o m p a r a t i v e cost of s y n t h e s i s , t h e s e l e c t i v i t y o f w a t e r o v e r s a l t , d e n s i t y , v i s c o s i t y , ease o f phase s e p a r a t i o n , m o l e c u l a r w e i g h t , a n d o t h e r p h y s i c a l a n d c h e m i c a l p r o p e r t i e s d e s i r a b l e i n t h e process.

60 70 WATER, MOLE PER CENT

Figure 3. Effect of solvent content on the selectivity of water over salt for some amines 1. n-Methyl-l, 3-dimethylbutylamine 2. η-Ethyl sec-butylamine 3. η-Ethyl rerr-butylamine 4. n, n-Dimethyl rerr-butyl amine

•

0.1% NaCl

X 1.0% NaCl A 5.0% NaCl • 10.0% NaCl

!·!* \

( \ salt + water / water (

s

a

l

t

ï

\ salt + water /solvent K+ = Na + K-= Cli n o u r extensive investigation, p r a c t i c a l l y a l l s t r u c t u r a l configurations of t h e amines w h i c h c o n t a i n five a n d s i x c a r b o n a t o m s w e r e o b t a i n e d c o m m e r c i a l l y o r s y n t h e s i z e d . T h e properties of m a n y of the amines have been found suitable, b u t the authors have b e e n u n a b l e t o find a s o l v e n t w h i c h h a p p e n e d t o f a l l i n t h e r i g h t o p e r a t i n g t e m p e r a t u r e range. F u r t h e r s t u d y indicated that mixtures of solvents b o t h o f w h i c h h a d near ideal p r o p e r t i e s b u t were d i f f i c u l t t o use i n t h e process because o f o p e r a t i n g t e m p e r a t u r e , w o u l d give solubility curves of almost ideal characteristics w h e n m i x e d i n t h e right p r o p o r t i o n s . D a t a o b t a i n e d f o r t w o o f these a r e s h o w n i n F i g u r e 5. T h e s e d a t a s h o w t h a t a n e x t r a c t i o n t e m p e r a t u r e f r o m 1 8 ° t o 5 5 ° C . i s possible b y use o f m i x t u r e s o f



44


these t w o a m i n e s . I n t h e desert s o u t h w e s t t h e feed w a t e r w i l l h a v e a t e m p e r a t u r e b e t w e e n 2 5 ° a n d 3 5 ° C , a n d i n o r d e r t o a l l o w f o r h e a t o f s o l u t i o n of t h e w a t e r i n solvents, a n extraction temperature between 3 5 ° a n d 4 5 ° C . w o u l d result. F o r this condition, a combination of one p a r t t r i e t h y l a m i n e a n d t w o parts m e t h y l d i e t h y l a m i n e provides a near-ideal solvent. T r i e t h y l a m i n e is t h e best solvent of the t w o a n d there fore, o p e r a t i o n a t c o l d e r t e m p e r a t u r e s w o u l d b e s o m e w h a t a d v a n t a g e o u s . A feed w a t e r t e m p e r a t u r e o f 1 2 ° t o 1 5 ° C . w o u l d a l l o w use of t r i e t h y l a m i n e as a single s o l v e n t . A s e c o n d i m p o r t a n t p o i n t of c o n s i d e r a t i o n i s t h e s h o r t t e m p e r a t u r e r a n g e r e q u i r e d f o r t h i s s y s t e m o f s o l v e n t s . T h e 1 t o 2 m i x t u r e of m e t h y l d i e t h y l a m i n e - t r i e t h y l a m i n e dissolves 3 5 % b y w e i g h t o f w a t e r a t 4 0 ° C . a n d a t a s e p a r a t i o n t e m p e r a t u r e o f 5 5 ° C . i t dissolves o n l y 1 0 % w a t e r . A feed w a t e r t e m p e r a t u r e o f 3 0 ° C . w o u l d b e d e s i r a b l e a n d t o h e a t t o t h e s e p a r a t i o n t e m p e r a t u r e b y m e a n s o f h e a t e x c h a n g e r s , a source of heat a t 6 5 ° C . w o u l d be a d e q u a t e .

i

ι

ι

T)

10

20

? 0

ι

ι

ι

ι

30 40 50 60 WATER , WEIGHT PER CENT

ι

ι

70

80

l_

90

Figure 4. Effect of structure on water solubility X—X •—· •—• A—A •—τ

n-Methylbutylamine n-Methylamylamine n-Ethylpropylamine n-Ethylisopropylamine η-Ethyl sec-butylamine

I n o r d e r t o test t h e l a b o r a t o r y d a t a o b t a i n e d , a s m a l l e x t r a c t o r s y s t e m w a s u s e d w i t h those s o l v e n t s h a v i n g s u i t a b l e p r o p e r t i e s , w h i c h were o b t a i n a b l e i n sufficient q u a n tities for testing, using n a t u r a l waters or sodium chloride solutions. T h e extraction s y s t e m consists o f a 2 - i n c h p a c k e d c o l u m n a p p r o x i m a t e l y 4 feet h i g h t o w h i c h w a t e r a n d s o l v e n t w e r e f e d c o u n t e r c u r r e n t l y . A n a n a l y s i s o f t h e r e s u l t i n g e x t r a c t feed a n d brine was made to determine the m a t e r i a l balance for the system. T h e data obtained f r o m t h i s c o l u m n u s i n g d i i s o p r o p y l a m i n e as s o l v e n t a r e s h o w n i n T a b l e I . T h e feed c o n c e n t r a t i o n w a s 2000 p . p . m . o f s o d i u m c h l o r i d e . T h e p r o d u c t c o n t a i n e d 4 9 0 p . p . m . , of w h i c h p a r t w a s t h e a m i n e h y d r o c h l o r i d e . I n p r a c t i c e , t h i s w o u l d b e r e p l a c e d i n t h e s o l v e n t r e c o v e r y s y s t e m b y a n e q u i v a l e n t a m o u n t o f s o d i u m t o g i v e t h e t o t a l salt c o n t e n t i n d i c a t e d . Sufficient d a t a h a v e b e e n o b t a i n e d t o i n d i c a t e t h a t t h e c a l c u l a t i o n s



45


\

J 10

I 20

I I I I 30 40 50 60 WATER , WEIGHT PER CENT

I 70

I 80

I 90

I 100

Figure 5. Solubility diagram of methyldiethylamine, triethylamine, and mixtures of the two •—A •—• X—X •—• •—·

Triethylamine Methydiethylamine 1-2 mixture I -3 mixture I-1 mixture

b a s e d o n l a b o r a t o r y d a t a a r e c o m p a t i b l e w i t h those o b t a i n e d i n p r a c t i c a l e x t r a c t i o n studies. S i m i l a r runs w i t h this c o l u m n using a c t u a l b r a c k i s h water obtained f r o m brine wells gave comparable results. I n this experiment, n o reflux was used a n d the c o l u m n g a v e a b o u t t h r e e t h e o r e t i c a l stages. I t is expected t h a t t h e m i x e d solvent system p r e v i o u s l y i n d i c a t e d w o u l d g i v e b e t t e r r e s u l t s t h a n d i i s o p r o p y l a m i n e , b u t sufficient quantities of solvent have n o t been available f o r such testing.

Table I.

Production of Fresh Water from Salt Water with Diisopropylamine Sodium Meq.

Chloride P.p.m.

Meq.

P.p.m.

F e e d — S a l t , 2000 p . p . m . 33.3

800

33.3

1200

P r o d u c t — S a l t , 491 p . p . m . 6.0 144 2 . 4 (amine as sodium)

8.4

325

R a f f i n a t e — S a l t , 7150 p . p . m . 128

3090

115

4060

Solvent Recovery F r o m t h e t i m e of i n c e p t i o n o f t h i s process, m u c h c o n c e r n h a s b e e n i n d i c a t e d c o n c e r n i n g t h e p r o b l e m o f s o l v e n t r e c o v e r y f r o m t h e p r o d u c t w a t e r a n d raffinate. T h i s c o n c e r n stems f r o m t w o i m p o r t a n t c o n s i d e r a t i o n s : first, t h e cost of s o l v e n t losses; a n d second, t h e possible t o x i c i t y o f t h e r e m a i n i n g s o l v e n t . T o i n v e s t i g a t e s o l v e n t r e c o v e r y


46


o n a scale u s e f u l i n p l a n t s c a l e - u p , a l a b o r a t o r y s y s t e m w a s c o n s t r u c t e d w h i c h uses a s t e a m s t r i p p e r c o n s i s t i n g of a 6 - i n c h p a c k e d c o l u m n a p p r o x i m a t e l y 6 feet t a l l . W a t e r containing amine was fed t h r o u g h the stripper countercurrently t o the steam a n d t h e d a t a s h o w n i n T a b l e I I were o b t a i n e d . T h e s e d a t a s h o w t h a t p r o d u c t w a t e r c o n t a i n i n g

Table II.


Rate, L b . / H r . Water Steam feed overhead 394 394

21.1 26.4

Solvent Recovery Water Feed Temp., ° C. 83 85

Amine Concn., P . P . M . Feed Product 3200 3200

3.3 1.6

3200 p . p . m . of d i i s o p r o p y l a m i n e was s t r i p p e d t o a c o n c e n t r a t i o n of a p p r o x i m a t e l y 2.5 p . p . m . w i t h 0.06 p o u n d of s t e a m p e r p o u n d o f p r o d u c t . T h i s r e p r e s e n t s v e r y f a v o r a b l e s o l v e n t r e c o v e r y a n d t h e s t e a m u s e d f r o m t h i s process w i l l p r o v i d e a p p r o x i m a t e l y o n e h a l f t h e heat r e q u i r e m e n t s necessary f o r t h e s e p a r a t i o n stage. I t is expected t h a t s t r i p p i n g i n a m o r e efficient c o l u m n w o u l d reduce the a m i n e c o n c e n t r a t i o n e v e n f u r t h e r . F r o m a n e c o n o m i c p o i n t of v i e w , t h i s a m o u n t o f s o l v e n t c o n s t i t u t e s a loss o f 0.02 p o u n d of a m i n e p e r 1000 gallons of w a t e r p r o d u c e d . A t a n e s t i m a t e d cost o f $0.50 p e r p o u n d f o r t h e a m i n e , t h e s o l v e n t losses w o u l d a m o u n t t o $0.01 p e r 1000 g a l l o n s o f p r o d u c t . F r o m t h e t o x i c i t y p o i n t of v i e w , i t i s n o t possible t o e s t i m a t e w h a t a d d i t i o n a l a m i n e r e m o v a l w i l l b e necessary since f e w d a t a o n c h r o n i c t o x i c i t y o f a m i n e s t o h u m a n s a r e a v a i l a b l e . S o m e d a t a o n t o x i c i t y o f a l i p h a t i c a m i n e s t o p o u l t r y h a v e been p r e s e n t e d i n t h e l i t e r a t u r e . C l a r k a n d D u b o s e (2) f o u n d t h a t c h i c k e n s f e d 25 m g . of n - o c t y l a m i n e p e r k i l o g r a m o f b o d y w e i g h t f o r 10-week p e r i o d s g a v e n o s t a t i s t i c a l e v i d e n c e o f d a m a g e . A c u t e t o x i c i t y w a s r e a c h e d o n l y w h e n t h e d i e t c o n t a i n e d 1100 m g . p e r k i l o g r a m of b o d y w e i g h t . W h i l e these d a t a are n o t d i r e c t l y a p p l i c a b l e i n e s t a b l i s h i n g t h e c h r o n i c t o x i c i t y l e v e l o f a l i p h a t i c a m i n e s t o h u m a n s , y e t a s s u m i n g s i m i l a r effects, a h u m a n w o u l d n e e d t o d r i n k a p p r o x i m a t e l y 750 l i t e r s o f p r o d u c t w a t e r p e r d a y c o n t a i n i n g 2.5 p . p . m . o f a m i n e , i n o r d e r t o r e a c h t h e l o w e r l i m i t set f o r c h i c k e n s . S h o u l d i t b e n e c e s s a r y t o reduce t h e a m i n e c o n t e n t b e l o w t h a t p r o v i d e d b y s t r i p p i n g w i t h s t e a m , l a b o r a t o r y tests h a v e s h o w n t h a t a m i n e s o l u t i o n s o f t h i s c o n c e n t r a t i o n q u i c k l y lose t h e i r a m i n e c o n t e n t w h e n a l l o w e d t o s t a n d i n a l a b o r a t o r y f o r a p e r i o d of 1 t o 2 d a y s , i n d i c a t i n g r a p i d b a c t e r i a l d e c o m p o s i t i o n . T h e s e d a t a w o u l d i m p l y t h a t storage o f t h e w a t e r f o r b r i e f p e r i o d s o r use of t r i c k l e filters w o u l d b e a d v a n t a g e o u s i n r e m o v i n g t h e l a s t traces of a m i n e i n the water product.

Limitations of the Process C e r t a i n l i m i t a t i o n s exist f o r t h i s m e t h o d of saline w a t e r c o n v e r s i o n . F i r s t , due t o the f u n d a m e n t a l p r o p e r t i e s of s o l u t i o n s o f w a t e r i n s o l v e n t s , t h e process i s m o s t a p p l i cable t o l o w saline w a t e r s o r those c o m m o n l y classified as b r a c k i s h , i n a range of 2000 to 10,000 p . p . m . of t o t a l s a l t . T h e r e are s e v e r a l reasons f o r t h i s . T h i s i s a stagewise process so t h a t t h e n u m b e r of stages a n d reflux r e q u i r e d are a f u n c t i o n of t h e r a t i o of feed t o w a t e r p r o d u c t c o n c e n t r a t i o n . A l s o , i n c r e a s i n g salt c o n c e n t r a t i o n reduces t h e s o l u b i l i t y of w a t e r i n t h e s o l v e n t . B e c a u s e i t i s necessary t o c i r c u l a t e a t least t w o t o t w o a n d a h a l f t i m e s as m u c h s o l v e n t as w a t e r p r o d u c t w i t h l a r g e r a m o u n t s f o r h i g h e r salt c o n c e n t r a t i o n , t h e p r o b l e m of heat exchange a n d e x t r a c t i o n w i t h these l a r g e q u a n t i t i e s o f s o l v e n t b e c o m e s i m p o r t a n t . T h e e c o n o m i c s of heat exchange necessitate t h a t some t h e r m a l e n e r g y b e d i s c a r d e d t o t h e e n v i r o n m e n t r a t h e r t h a n a t t e m p t t o m a k e a closed h e a t s y s t e m . H o w e v e r , since t h e process c a n u t i l i z e e x t r e m e l y l o w cost s t e a m o r even hot water for a good p o r t i o n of its heating cycle, p a r t of this l i m i t a t i o n is removed. S e c o n d l y , since t h e s o l v e n t s f o u n d t o h a v e t h e b e s t s o l v e n t p r o p e r t i e s p r o d u c e s o l u t i o n s s u f f i c i e n t l y a l k a l i n e t o p r e c i p i t a t e m a g n e s i u m h y d r o x i d e , i t i s necessary t o r e m o v e m a g n e s i u m f r o m w a t e r p r i o r t o e x t r a c t i o n , o r t o feed a v e r y l o w m a g n e s i u m c o n t a i n i n g



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water. A t h i r d l i m i t a t i o n , while a p p a r e n t l y n o t i m p o r t a n t except f r o m a psychological p o i n t o f v i e w , is t h e s m a l l q u a n t i t y o f a m i n e t h a t m a y r e m a i n i n t h e p r o d u c t w a t e r . I t is l i k e l y , t h e r e w i l l b e some resistance t o use of t h i s w a t e r b y t h e g e n e r a l p u b l i c , b u t s u c h a l i m i t a t i o n s h o u l d be m u c h less t h a n t h a t i m p o s e d b y i n i t i a t i o n o f t h e c h l o r i n a t i o n of w a t e r f o r s a n i t a r y p u r p o s e s some y e a r s a g o .


Advantages of the Process T h e process h a s s e v e r a l d i s t i n c t a d v a n t a g e s t h a t set i t s o m e w h a t a p a r t f r o m m a n y of t h e o t h e r processes. F i r s t , a n d p o s s i b l y m o s t i m p o r t a n t , i s t h e v e r s a t i l i t y o f t h e process w i t h respect t o e n e r g y sources, since e s s e n t i a l l y free h e a t m a y b e o b t a i n e d f r o m s u c h sources as e x h a u s t s t e a m , o r gases o f i n d u s t r i a l engines, c o o l i n g w a t e r f r o m c o n densers, s o l a r h e a t e r s , o r f r o m a n y o t h e r source w i t h a r e l a t i v e l y l o w t e m p e r a t u r e h e a t . U n d e r p r e s e n t c o n c e p t s o f d e v e l o p m e n t o f t h e process, a p p r o x i m a t e l y one h a l f t h e energy required w i l l need t o be l o w pressure steam. T h i s steam m a y b e produced b y flashing a l a r g e q u a n t i t y o f w a t e r a t t e m p e r a t u r e s a v a i l a b l e f r o m c o o l i n g t o w e r s . Seco n d l y , t h e process l e n d s i t s e l f w e l l t o a d a p t a t i o n t o e x i s t e n t e n v i r o n m e n t a l c o n d i t i o n s . F e e d w a t e r r a n g i n g i n t e m p e r a t u r e s f r o m 18° t o 5 5 ° C . c a n b e h a n d l e d b y a l t e r n a t i n g t h e nature of the solvent composition t o give the most economic advantage. Some a d v a n tage is r e a l i z e d , b a s e d o n p r e s e n t s o l v e n t s , i n t h e l o w e r t e m p e r a t u r e feed w a t e r . T h i r d l y , t h e process is e s p e c i a l l y a p p l i c a b l e t o b r a c k i s h w a t e r s w h i c h a r e m o r e l i k e l y t o b e l o c a t e d

IBOr-

10,000 20,000 FEED CONCENTRATION

35,000

Figure 6. The effect of feed concentration on the cost of product water at various heat costs as compared to membrane and freezing processes for feed water at 25° C , in 10,000,000-gallon-perrday plant

x-.x-. 0

Ethyl sec-butylamine 1-2 mixture of triethylamine and methyldiethylamine Ionic membranes Freezing process


48


a t p o i n t s o f n e e d t h a n sea w a t e r . I n t h i s r e g a r d , t h e process c o m p e t e s f a v o r a b l y w i t h t h e i o n i c m e m b r a n e processes c u r r e n t l y i n o p e r a t i o n . I t has t h e a d v a n t a g e t h a t t h e cost o f w a t e r p r o d u c e d increases m u c h less r a p i d l y w i t h s a l t c o n t e n t o f t h e feed t h a n i t does i n t h e m e m b r a n e process. T h e s o l v e n t e x t r a c t i o n process i s t h e r e f o r e a d a p t a b l e t o a r a n g e o f feed w a t e r c o n c e n t r a t i o n b e t w e e n 5000 a n d 10,000 p . p . m . i n w h i c h f e w , i f a n y o t h e r processes, are e s p e c i a l l y a d a p t a b l e . I n F i g u r e 6, cost d a t a a r e p r e s e n t e d f o r t h e s o l v e n t e x t r a c t i o n process i n w h i c h t h e c o n c e n t r a t i o n o f s a l t vs. e s t i m a t e d cost i s p l o t t e d . I t is b e l i e v e d t h a t l o w e r cost e n e r g y t h a n $0.20 p e r 1 0 B . t . u . w i l l b e p r a c t i c a l u n d e r c e r t a i n c i r c u m s t a n c e s . E n e r g y cost r e d u c t i o n w o u l d affect t h e c u r v e i n a n a p p r o x i m a t e p r o p o r t i o n a l m a n n e r as i s i n d i c a t e d f o r t h e d a t a g i v e n f o r n - e t h y l - s e c - b u t y l a m i n e . T h e d a t a o n i o n i c m e m b r a n e s are t a k e n f r o m p i l o t p l a n t d a t a o f I o n i c s , I n c . ( 7 ) . F r o m these d a t a i t is i n d i c a t e d t h a t v e r y l o w s a l i n e w a t e r s are c o n v e r t e d a t a s o m e w h a t l o w e r cost b y t h e m e m b r a n e s y s t e m , b u t i n t h e range f r o m 5000 t o 10,000 p . p . m . t h e s o l v e n t e x t r a c t i o n process a p p e a r s t o h a v e d e c i d e d a d v a n t a g e . T h e d a t a f o r t h e f r e e z i n g process as a p p l i e d t o sea w a t e r a r e t a k e n f r o m t h e C a r r i e r C o r p . R e p o r t (8). W h i l e cost d a t a are n o t a v a i l a b l e f o r b r a c k i s h w a t e r u s i n g t h e C a r r i e r process i t is n o t e x p e c t e d t h a t m u c h a d v a n t a g e w i l l b e r e a l i z e d f r o m l o w e r feed w a t e r - s a l t c o n c e n t r a t i o n s .


6

F i n a l l y , i n those processes r e q u i r i n g h i g h t e m p e r a t u r e , t h e p r o b l e m s o f s c a l i n g a n d corrosion present formidable barriers t o economic p r o d u c t i o n of fresh water. I n solvent e x t r a c t i o n , t h e p r o b l e m o f s c a l i n g is e l i m i n a t e d b y t h e l o w t e m p e r a t u r e o f t h e o p e r a t i o n , and the solvents eliminate the g r o w t h of sliming a n d incrusting organisms. C o r r o s i o n i n t h i s s y s t e m i s e x p e c t e d t o b e m i n i m a l because o f t h e i n h i b i t o r y a c t i o n o f a m i n e s t o w a r d c o r r o s i o n o f i r o n . I n T a b l e I I I , d a t a are p r e s e n t e d o n t h e c o r r o s i o n o f m i l d steel i n the amine-water solutions. These data show t h a t even a 5 % sodium chloride solution w h i c h c o n t a i n s 2 % o f d i i s o p r o p y l a m i n e gives a c o r r o s i o n r a t e o f less t h a n 1 m i l p e r y e a r o r less t h a n 2 0 % o f t h a t r e p o r t e d f o r sea w a t e r . I n t h e l o w e r s a l t c o n t e n t w a t e r s , t h e r a t e o f c o r r o s i o n is n e g l i g i b l e .

Table III.

Corrosion of Mild Steel in Amine Water Solutions A

W t . loss/specimen, g r a m Wt. loss/sq. cm., gram T h i c k n e s s loss, m i l Thickness/yr., m i l T e s t period 64 d a y s : t e m p . 30° C A. B. C.

Β

0.0585 0.0015 3.6 X 10~ 9 X 10 ~ 0.11 0.003 0.62 0.017 , specimen area 16.3 sq. c m . 3

C 5

0.0054 3 . 3 X ΙΟ0.012 0.064

5 % s o d i u m chloride plus 2 % d i i s o p r o p y l a m i n e i n water 2 % d i i s o p r o p y l a m i n e i n water 3 0 % water i n d i i s o p r o p y l a m i n e

Conclusions E x t r a c t i o n o f p o t a b l e w a t e r f r o m saline w a t e r s b y m e a n s o f i m m i s c i b l e s o l v e n t s h a s b e e n s h o w n t o b e t h e o r e t i c a l l y p o s s i b l e , e x p e r i m e n t a l l y feasible, a n d e c o n o m i c a l l y a t t r a c t i v e . D a t a p r e s e n t e d s h o w t h e process t o b e e s p e c i a l l y a d a p t a b l e t o t h e c o n v e r s i o n of feed w a t e r i n t h e r a n g e o f 5000 t o 10,000 p . p . m . I t i s a d a p t a b l e t o use o f l o w - q u a l i t y h e a t s u c h as h o t w a t e r f r o m c o o l i n g t o w e r s o r l o w p r e s s u r e w a s t e s t e a m . B y u s e o f m i x e d s o l v e n t s y s t e m s , t h e process c a n b e o p t i m i z e d t o t a k e a d v a n t a g e o f seasonal changes i n t e m p e r a t u r e a n d sources o f c o l d feed w a t e r a n d l o w - l e v e l h e a t sources. T h e process, i n g e n e r a l , i s s o m e w h a t m o r e e c o n o m i c a l w h e n a c o l d source o f feed w a t e r i s available. T h e p r o b l e m s o f c o r r o s i o n a n d s c a l i n g i n h e r e n t t o m o s t o f t h e o t h e r processes a r e v i r t u a l l y eliminated i n this process; however, magnesium m u s t be removed f r o m h i g h m a g n e s i u m w a t e r s because of p r e c i p i t a t i o n a t t h e h i g h p H v a l u e s of a m i n e - w a t e r systems. E c o n o m i c recovery of solvent f r o m the water products can be readily accomplished b y s t e a m s t r i p p i n g . S m a l l r e m n a n t c o n c e n t r a t i o n s (1 t o 2 p . p . m . ) m a y b e r e m o v e d b y b a c t e r i a l d e c o m p o s i t i o n , e i t h e r o n storage o r b y use o f t r i c k l e f i l t e r s .



49

T h e s o l v e n t e x t r a c t i o n process has n o t y e t u n d e r g o n e p i l o t p l a n t i n v e s t i g a t i o n , a n d a l l t h e a b o v e estimates a r e b a s e d o n s m a l l l a b o r a t o r y o r b e n c h scale e x p e r i m e n t s . I f further testing under practical conditions substantiates the laboratory observations, i t a p p e a r s t h a t t h e s o l v e n t e x t r a c t i o n process d e f i n i t e l y has a n a r e a o f s p e c i a l i z a t i o n i n t h e o v e r - a l l saline w a t e r c o n v e r s i o n p r o g r a m .


Literature Cited (1) B o s w o r t h , C . M., C a r f a g n o , S. S., B a r d u h m , A. J., S a n d e l l , D. J., " F u r t h e r D e v e l o p m e n t of a D i r e c t - F r e e z i n g C o n t i n u o u s W a s h - S e p a r a t i o n Process f o r Saline W a t e r C o n v e r s i o n , " C a r r i e r C o r p . , Office of Saline W a t e r , Research a n d D e v e l o p m e n t P r o g r . R e p t . , N o . 32, p. 31, W a s h i n g t o n ,D.C.,1959. (2) C l a r k , S. P., D u b o s e , R . T . , J. Agr. Food Chem. 8, 47 (1960). (3) D a n k e s e , J. P., Kirkhum, T. Α., M a h e r a s , G . , Mintz, M. S., P o w e l l , J. H., Rosenberg, N. W . , " D e s i g n , C o n s t r u c t i o n a n d Field T e s t i n g C o s t A n a l y s e s o n t h e E x p e r i m e n t a l E l e c t r o d i a l y s i s D e m i n e r a l i z e r f o r B r a c k i s h W a t e r s , " Ionics, Inc., Office of Saline W a t e r , Research a n d D e v e l o p m e n t Progress R e p o r t , No. 11, W a s h i n g t o n ,D.C.,1956. (4) D a v i s o n , R. R . , H o o d , D. W., Office of Saline W a t e r , U. S. D e p t . of I n t e r i o r , Publ. 568, 408-16 (1957). (5) D a v i s o n , R. R., I s b e l l , A. F., S m i t h , W . H., Jr., H o o d , D. W., " D e v e l o p m e n t of t h e S o l v e n t D e m i n e r a l i z a t i o n of Saline W a t e r , " A n n u a l R e p t . , 1958. (6) D a v i s o n , R. R . , Jeffrey, L. M., W h i t e h o u s e , U. G., H o o d , D. W., " R e s e a r c h o n Liquid -Liquid E x t r a c t i o n for Saline W a t e r C o n v e r s i o n , " Office of Saline W a t e r Research a n d D e v e l o p m e n t P r o g r . R e p t . , No. 22, D e c e m b e r 1958. (7) D a v i s o n , R. R . , S m i t h , W . H., Jr., H o o d , D. W., J. Chem. Eng. Data 5, 420 (1960). (8) H a r w e l l , Κ . E., Futrell, M. D . , H o o d , D. W . , " D e s a l i n a t i o n b y Liquid-Liquid Extrac tion," 1st Ann. Rept., A. & M. Research F o u n d a t i o n , R e f . 54-54T, 1954. RECEIVED for review J u l y 20, 1960. A c c e p t e d A u g u s t 1, 1960. C o n t r i b u t i o n f r o m D e p a r t m e n t of Oceanography a n d M e t e o r o l o g y , A g r i c u l t u r a l a n d M e c h a n i c a l College of T e x a s , Ocean ography a n d M e t e o r o l o g y Series No. 168. W o r k sponsored b y Office of Saline W a t e r , U. S. D e p a r t m e n t of I n t e r i o r , C o n t r a c t N o . 14-01001-174.


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