SALINE WATER CONVERSION

Osmosis through α Vapor Gap Supported by Capillarity

Downloaded by UNIV OF CALIFORNIA SANTA BARBARA on September 5, 2015 | http://pubs.acs.org Publication Date: January 1, 1960 | doi: 10.1021/ba-1960-0027.ch019

GERALD L. HASSLER and J . W. McCUTCHAN Department of Engineering, University of California, Los Angeles, Calif.

The semipermeable membrane proposed for the demineralization of sea water is based on H. L. Cal endar's theory that osmosis takes place through the membrane as vapor, condensing at the opposite membrane surface. The actual membrane being used consists of two sheets of untreated cellophane separated by a water-repellent powder, such as a silicone-coated pumice powder. The vapor gap is maintained by an air pressure in excess of the pres sure on the sea water and the cellophane sheets support the capillary surfaces, which will withstand pressures up to 1500 p.s.i. A number of successful experiments are reported with over 95% desalinization. The present effort is directed toward obtain ing reproducible experimental results and better methods of fabricating the vapor gap.

T h e t e r m " o s m o s i s " i s u s e d t o describe s p o n t a n e o u s flow of w a t e r i n t o a s o l u t i o n , o r f r o m a more dilute t o a more concentrated solution, separated f r o m each other b y a s u i t a b l e m e m b r a n e . T o o b t a i n f r e s h w a t e r f r o m s e a w a t e r , t h e flow m u s t b e r e v e r s e d , f r o m t h e s o l u t i o n i n t o a f r e s h w a t e r s t r e a m . H e n c e t h e t e r m u s e d t o describe t h i s process is " r e v e r s e o s m o s i s . " C e r t a i n e x p e r i m e n t s (16) a s s o c i a t e d w i t h t h e t h e o r e t i c a l ideas o f C a l l e n d a r (2) s h o w t h a t , i n some i n s t a n c e s a t least, osmosis t a k e s p l a c e t h r o u g h e v a p o r a t i o n of t h e w a t e r a t one m e m b r a n e s u r f a c e , passage t h r o u g h t h e m e m b r a n e as v a p o r , a n d c o n d e n s a t i o n a g a i n a t t h e o p p o s i t e m e m b r a n e s u r f a c e . T h e e x p e r i m e n t s r e p o r t e d here h a v e b e e n b a s e d e n t i r e l y o n t h i s t y p e o f osmosis. I t seems p r o b a b l e t h a t i n o t h e r i n s t a n c e s , w h e n c e r t a i n t y p e s o f m e m b r a n e s are u s e d , osmosis t a k e s p l a c e w i t h o u t a change of p h a s e . F o r e x a m p l e , t h e f r e s h w a t e r ions m a y pass t h r o u g h c h a n n e l s i n t h e m e m b r a n e s too s m a l l t o p e r m i t passage of t h e solute i o n s , o r a g a i n , t h e w a t e r m a y d i s s o l v e i n t h e m e m b r a n e ( a n d p o s s i b l y p a r t o f t h e m e m b r a n e also dissolves i n w a t e r ) , w h i l e t h e s o l u t e does n o t so d i s s o l v e a n d does n o t pass. T h i s d i s t i n c t i o n i s d e v e l o p e d b y M c B a i n (10) a n d G l a s s t o n e (4). T h e o s m o t i c p r e s s u r e o f a s o l u t i o n i s defined b y G l a s s t o n e (4) as t h e excess p r e s s u r e w h i c h m u s t b e a p p l i e d t o a s o l u t i o n t o p r e v e n t t h e passage i n t o i t o f s o l v e n t w h e n t h e y are s e p a r a t e d b y a p e r f e c t l y s e m i p e r m e a b l e m e m b r a n e . A c t u a l l y n o m e m b r a n e i s 192

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

193

HASSLER AND McCUTCHAN—OSMOSIS THROUGH VAPOR GAP

p e r f e c t i n t h i s respect, b u t c o p p e r f e r r o c y a n i d e , C u F e ( C N ) , h a s b e e n f o u n d t o b e most selective. D a t a collected w i t h membranes of this t y p e p l a y e d a n i m p o r t a n t p a r t i n t h e f o r m u l a t i o n o f p r e s e n t - d a y s o l u t i o n t h e o r y — s o m u c h so t h a t t h e a u t h o r s h a v e u s e d t h i s t h e o r y w i t h o u t h e s i t a t i o n t o c o m p u t e o s m o t i c pressures of s o l u t i o n s w h o s e o s m o t i c pressures h a v e n e v e r b e e n p r e c i s e l y m e a s u r e d . S u c h a s o l u t i o n i s sea w a t e r . T h e copper ferrocyanide m e m b r a n e is " l e a k y " t o solutions of strong electrolytes. Some d a t a h a v e b e e n o b t a i n e d o n w e a k s o l u t i o n s of s t r o n g e l e c t r o l y t e s b y t h e T o w n e n d m e t h o d (16), b u t n o one has m a d e precise m e a s u r e m e n t s o n t h e o s m o t i c p r e s s u r e o f sea w a t e r .


2

6

R e c e n t w o r k b y R e i d (18), B r e t o n (1), a n d L o e b (9) shows p r o m i s e t h a t c e l l u lose acetate m a y b e u s e d f o r s u c h o s m o t i c m e a s u r e m e n t s . H o w e v e r , t h e i r o b j e c t i v e w a s t o s t u d y t h e d e s a l i n i z a t i o n process a n d n o t t h e e q u i l i b r i u m m e a s u r e m e n t o f osmotic pressure. T h e o s m o t i c p r e s s u r e ( O P ) of s o l u t i o n s expressed i n a t m o s p h e r e s i s u s u a l l y c a l c u l a t e d f r o m t h e f r e e z i n g - p o i n t d e p r e s s i o n , AT, i n degrees C e n t i g r a d e , i n a c c o r d a n c e with the relation OP

= 12.06ΔΓ -

0.021(Ar)

2

w h e r e AT i s o b t a i n e d f r o m t h e I n t e r n a t i o n a l C r i t i c a l T a b l e s (8). B y t a k i n g a v e r a g e c o n c e n t r a t i o n s o f t h e v a r i o u s salts i n sea w a t e r , t h e o s m o t i c p r e s s u r e is c a l c u l a t e d t o be a p p r o x i m a t e l y 2 5 a t m . ( 3 7 0 p . s . i . ) .

Proposed Osmotic Membrane for Sea Water T h e s e m i p e r m e a b l e m e m b r a n e p r o p o s e d a t U C L A i n 1950 (7) i s b a s e d o n C a l e n d a r ' s t h e o r y t h a t osmosis t a k e s place t h r o u g h e v a p o r a t i o n o f t h e w a t e r a t one m e m b r a n e s u r f a c e , passage t h r o u g h t h e m e m b r a n e as v a p o r , a n d c o n d e n s a t i o n a t t h e o p p o site m e m b r a n e s u r f a c e : T h e scheme i s F r e s h water channel •— — * Diffusion gap Salt water channel

„ -u ι C a p i l l a r y membrane C a p i l l a r y membrane

Pi

:

T )

P P

2 3

ft > Λ » Λ P m u s t b e > P , i n o r d e r t h a t w a t e r c o n t a i n i n g s a l t w i l l n o t flow t h r o u g h t h e m e m b r a n e b u t , r a t h e r , p u r e w a t e r w i l l e v a p o r a t e f r o m t h e t o p o f t h e c a p i l l a r y surfaces. I f P — P = o s m o t i c pressure o f t h e s a l t s o l u t i o n , n o flow b y d i f f u s i o n w i l l o c c u r across t h e g a p . F o r p r e s s u r e differences less t h a n t h i s , Δ Ρ < ( P — Pi), n o r m a l o s m o t i c flow w o u l d o c c u r f r o m t h e f r e s h w a t e r s t r e a m i n t o t h e s a l t w a t e r . T h u s , i f w e are t o reverse t h i s process, Δ Ρ m u s t be g r e a t e r t h a n ( P — P ), i n o r d e r t h a t w a t e r w i l l d i s t i l l f r o m t h e s a l t w a t e r side t o t h e f r e s h w a t e r side. B e f o r e e q u i p m e n t c a n b e designed, a d e c i s i o n m u s t b e m a d e c o n c e r n i n g pressures l*i J Τ*2> d P . T h r e e pressure a r r a n g e m e n t s h a v e b e e n c o n s i d e r e d b a s e d o n t h e a s s u m p t i o n t h a t t h e o s m o t i c pressure of sea w a t e r is 2 5 a t m . ( T a b l e I ) . 2

3

1

3

3

3

a

R

x

3

Table I. Condition F r e s h water Gap Sea water

Pressure Data A

-25 0.01 + 0.01

Pressure, Atmospheres Β -24 1 + 1

C 1 25 + 25

T h u s a n y pressure d i f f e r e n t i a l i n excess of t h e 2 5 - a t m . o s m o t i c p r e s s u r e s h o u l d p r o d u c e some y i e l d o f f r e s h w a t e r . T h e q u e s t i o n t h e n b e c o m e s one o f r a t e o f p r o d u c t i o n . A n a n a l y s i s m a d e as follows shows t h a t t h e l i m i t i n g f a c t o r , so f a r as r a t e i s


194

ADVANCES IN CHEMISTRY SERIES

c o n c e r n e d i n a w e l l designed s y s t e m , w i l l b e t h e d i f f u s i o n o f t h e w a t e r v a p o r across t h e g a p . W a l k e r et al. (17) p r e s e n t S t e f a n ' s e q u a t i o n f o r t h e d i f f u s i o n o f v a p o r t h r o u g h a gas f i l m . W h e n t h e r e i s n o d i f f u s i o n o f a i r , t h e e q u a t i o n b e c o m e s : ^ 4

dA

-D dP P dZ m

A

B

T h i s e q u a t i o n shows u s t h a t t h e p r a c t i c a l p r o b l e m i s t o d e s i g n a n a r r o w g a p , t h u s m a k i n g Ζ s m a l l , o r t o decrease t h e a i r p r e s s u r e i n t h e g a p , P ( T o w n e n d ' s a p p r o a c h ) , w h i c h necessitates a p a r t i a l v a c u u m i n t h e g a p a n d p u l l i n g t e n s i o n o n t h e f r e s h w a t e r column i n the equipment.


B

T h e o r e t i c a l l y t h i s i s possible, a n d — 1 5 0 - a t m . t e n s i o n has been d e m o n s t r a t e d e x p e r i m e n t a l l y o n w a t e r i n B e r t h e l o t (3) t u b e s . H o w e v e r , n e g a t i v e pressures i n T o w n end's e x p e r i m e n t s w i t h d i l u t e s o l u t i o n were less t h a n 1 a t m . a n d i t i s felt t h a t m o r e b a s i c r e s e a r c h o n l i q u i d tensions i s necessary b e f o r e schemes A a n d Β of T a b l e I c a n be c o n s i d e r e d f o r s o l u t i o n s as c o n c e n t r a t e d as s e a w a t e r . T o w n e n d m e a s u r e d o n l y e q u i l i b r i u m a n d h a d n o need f o r r a p i d v a p o r t r a n s f e r . T h i s leaves a r r a n g e m e n t C , T a b l e I , i n w h i c h t h e n a r r o w a i r g a p f u n c t i o n s as a s e m i p e r m e a b l e m e m b r a n e , as t h e m o s t p r o m i s i n g . T h e choice o f m a t e r i a l w i t h c a p i l l a r i e s s m a l l e n o u g h t o s u p p o r t t h e p r e s s u r e necessary f o r reverse osmosis o f sea water is a research project i n itself. T h e d e s i r a b l e p r o p e r t i e s f o r t h e a u t h o r s ' e q u i p m e n t are a i r e n t r y pressures g r e a t e r t h a n 7 5 0 p.s.i., s t a b i l i t y ( m a n y p l a s t i c f i l m s d e t e r i o r a t e w i t h t i m e ) , l o w a i r p e r m e a b i l i t y o f t h e w e t t e d f i l m , a v a i l a b i l i t y , a n d l o w cost. C e l l o p h a n e seems t o c o m e closest t o s a t i s f y i n g these r e q u i r e m e n t s . T h e d e c i s i o n t o u s e c e l l o p h a n e w a s b a s e d o n t h e w o r k o f R i c h a r d s (15). whose classic studies o n h i g h v a l u e s o f v a p o r p r e s s u r e a n d l o w s a t u r a t i o n s i n soils l e d h i m t o test m a n y p l a s t i c sheets before d e c i d i n g t o u s e V i s k i n g c a s i n g i n h i s h i g h p r e s s u r e equipment.

Prediction Equation T h e process o f p r e s s u r e d i s t i l l a t i o n t h r o u g h a h o m o g e n e o u s m e m b r a n e i s b a s e d first o n t h e c o m m o n f a c t t h a t t h e v a p o r p r e s s u r e of a n y l i q u i d c a n b e i n c r e a s e d b y c o m p r e s s i n g i t o r decreased b y p l a c i n g i t u n d e r s u c t i o n , a n d second o n t h e e q u a l l y c o m m o n f a c t t h a t o n l y p u r e w a t e r v a p o r escapes f r o m w a t e r i n t o v a p o r o r a i r , l e a v i n g n o n v o l a t i l e salts b e h i n d t h e phase b o u n d a r y . I n o p e r a t i n g t h e processes o f v a p o r i z a t i o n — h e a t t r a n s f e r a n d d i f f u s i o n across a n e x t r e m e l y t h i n g a p — n o n e w p h e n o m e n a o r n e w p r o p e r t i e s o f m a t e r i a l s are r e q u i r e d . H o w e v e r , t h e n o v e l c o m b i n a t i o n o f c a p i l l a r y surfaces, p r e s s u r e , a n d e x t r e m e l y s h o r t p a t h s f o r h e a t a n d d i f f u s i o n offers a n o p p o r t u n i t y f o r i m p r o v e m e n t s i n film p r o p e r t i e s a n d m e t h o d s o f c o n s t r u c t i o n not k n o w n before. T h e f o l l o w i n g c a l c u l a t i o n as m a d e f o r t h e S a l i n e W a t e r P r o j e c t (6) shows t h e r e l a t i o n b e t w e e n p r e s s u r e a p p l i e d a n d p r o d u c t i o n r a t e . T h e d o m i n a n t f a c t o r s a r e : (1) t h e s a l t s o l u t i o n w h o s e o s m o t i c p r e s s u r e m u s t b e o v e r c o m e , (2) t h e p r e s s u r e , as a n e n e r g y source, (3) t h e d i f f u s i o n of heat a n d (4) v a p o r as resistance f a c t o r s , a n d (5) v i s c o u s losses w i t h i n t h e c e l l o p h a n e c a p i l l a r i e s . T h e v a p o r d e n s i t y , l i k e t h e v a p o r p r e s s u r e , c a n b e u s e d as a t h e r m o d y n a m i c p o t e n t i a l whose t o t a l change a r o u n d a closed p a t h i s zero. A c c o r d i n g t o t h i s a r g u m e n t , t h e effect of t h e a b o v e five f a c t o r s o n v a p o r d e n s i t y c a n b e m a t h e m a t i c a l l y expressed a n d s u m m e d t o zero. B e g i n n i n g a t t h e p r o d u c t w a t e r o u t l e t , m o v e t o s a l t w a t e r b y a d d i n g M, c o m p r e s s t h e s a l t w a t e r t o p r e s s u r e p , a n d s u b j e c t i t t o t h e t h e r m a l loss o f l a t e n t h e a t t r a n s f e r , t h e d i f f u s i o n loss o f m a s s t r a n s f e r , a n d t h e v i s c o u s loss o f p r e s s u r e i n c e l l o p h a n e a n d m a n i f o l d passages. T h i s r e t u r n s t h e p a t h t o f r e s h w a t e r a n d a closed circuit.


HASSLER AND McCUTCHAN—OSMOSIS THROUGH VAPOR GAP

195

Salt. T h e d e p a r t u r e f r o m s t a n d a r d v a p o r d e n s i t y , Δ ρ , w i l l b e g i v e n b y — A — X 1 0 M g r a m p e r cc. Pressure. T h e effect o f p r e s s u r e i s ( b y a s t r a i g h t - l i n e a p p r o x i m a t i o n o f t h e K e l v i n equation) P

ν

0.61

v

_ e

- f Δρί, = 1.31 X 10 " ρ grams per cc.


8

D i f f u s i o n of H e a t . I n d y n a m i c e q u i l i b r i u m , a transfer of v a p o r f r o m l i q u i d t h r o u g h a v a p o r phase t o a second l i q u i d ( t h e t w o l i q u i d s b e i n g t h e r m a l l y c o n n e c t e d o n l y across t h e t h i n g a p ) w i l l r e q u i r e reverse t r a n s f e r o f t h e h e a t o f v a p o r i z a t i o n . T h i s w i l l a c c o m p a n y a t e m p e r a t u r e difference d e t e r m i n e d b y t h e r a t i o o f h e a t flow t o t h e t h e r m a l conductance of t h e t w o heat paths. These t w o a r e t h e diffusion v a p o r g a p a n d t h e series o f s a l t w a t e r a n d p l a s t i c films. F o r t h e d i f f u s i o n g a p t h e c.g.s. a i r v a l u e 5.7 X I O i s c h o s e n f o r t h e t h e r m a l c o n d u c t i v i t y ( n e g l e c t i n g t h e s e p a r a t i n g p o w d e r ) , w h i l e f o r t h e series p o l y e t h y l e n e ( 5 0 Χ 1 0 c m . t h i c k ) , w e t c e l l o p h a n e ( 5 0 X 1 0 c m . t h i c k ) , a n d water (200 Χ 1 0 c m . t h i c k ) t h e respective t h e r m a l conductivities a r e 3.5 X I O " , 4 X I O " , a n d 14 X 1 0 " . - 5

- 4

- 4

- 4

4

4

Δ0

4

= \Z/gap

ΑΘ =

V / disk

585w 5.7 X 10 ~* 50 X 10 ~ 3.5 X 1 0 ~ 585w 0.57 + 0.0244

ΑΘ

, 50 X 10 " 200 X 10 ~ 4 X 1 0 " - " 14 X 1 0 ~

4

4

4

f

4

585w 5.7 X 1 0 - *

+

0

Q

2

4

1

4

4

4

S u c h a t e m p e r a t u r e difference w i l l cause a f u r t h e r d e p a r t u r e f r o m s t a n d a r d v a p o r density given b y -A

P

= - 1 . 0 2 X 10-e ΑΘ = - ,

v

5

g r a m per cc.

t°^-}°~

4W

7

7

X

1

+ 0.0244

0

ζ

Diffusion of V a p o r . across t h e g a p i s

T h e difference i n v a p o r d e n s i t y needed t o cause diffusion w — Ap

Χ

=

v

ζ

w h e r e t h e d i f f u s i o n coefficient, D, f o r w a t e r v a p o r t h r o u g h a i r is a f u n c t i o n o f p r e s s u r e . η = °· D

A sq. c m . per second

2 0 8

[See t h e G i l l i l a n d e q u a t i o n (12).] V i s c o u s P r e s s u r e L o s s . T h e v i s c o u s p r e s s u r e loss as d e s c r i b e d b y t h e m e a s u r e d p e r m e a b i l i t y o f c e l l o p h a n e (IS) i s AP

=

W lo.o X 1U

(one sheet of cellophane)

β

T h i s p r e s s u r e d r o p w i l l b e reflected i n t h e d e n s i t y o f v a p o r i n e q u i l i b r i u m w i t h w a t e r . S u b s t i t u t i n g t h e a b o v e a n d i n c l u d i n g t w o sheets o f c e l l o p h a n e : 1

- A

P

v

Q1

* 1.31 X 10-» ρ -

ν

1Ω~8

W

6 9 χ 10-*

=

L

9

X

1 0

~

3

W

N o w s e t t i n g t h e s u m of these five t e r m s e q u a l t o z e r o : ΔρββΗ + ΔρρΓβββιΐΓβ + Apthermal loss + Ap^Uius^jn + Δρ ί

SALINE WATER CONVERSION

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