effect of porous support fabric on osmosis through a

8/9/2019 Effect of Porous Support Fabric on Osmosis Through A

1/7

E L S E V I E R

Journal of Membrane Science 129 (1997) 243-249

j o u r n a l o f

M E M B R A N E

S C I E N C E

Effec t o f porous suppor t fabr ic on osmos i s through a

L o e b S o u r i r a ja n t y p e a s y m m e t ri c m e m b r a n e

Sidney Loeb , Leonid Titelman, Em manuel Korngold, Joseph Freiman

Inst i tutes for App lied Research Ben-Gu rion Universi ty of the Negev Bee r Sheva Israel

Received 30 August 1996; received in revised form 5 December 1996; accepted 6 December 1996

Abstract

C o m m e rc ia l ly a v a i l a b le a s y m m e t r i c m e m b ra n e s o f t h e L o e b -S o u r i r a j a n (L - S ) ty p e c o m p r i s e a s u p p o r t f a b r ic , b o n d e d to

the porous subs truc tu re . The in f luence o f th is fabr ic on osmotic pe rme a t ion f lux was exam ined , mos t ly with a Toray CA -3000

m e m b ra n e f ro m w h ic h , w i th c are , i t w a s p o s s ib l e to r e m o v e th e s u p p o r t f a b r ic . In o s m o s i s e x p e r im e n t s w i th 1 2 Mg C 1 2

so lu t ion on one s ide (e i the r s ide ) and 6 so lu t ion on the o the r , the pe rmea t ion f lux (J1) was o f the o rder o f 0 .01 and 0 .06

m3/

m 2 d with and w ithou t fabr ic , re spec t ive ly . These resu l ts cou ld be gen era l ized by cons ide r ing the res is t iv i ty to so lu te d if fus ion

in the non-sk in pa r t o f the mem brane . Th is res is t iv i ty te rm ave raged 104 and 17 d /m for me mbra nes with and witho u t fabr ic ,

respec t ive ly , and in fu r the r te s ts wi thou t fabr ic , i t was be tween 15 and 25 d /m over a wide range o f MgC12 concen tra t ions .

F o u r o th e r L -S m e m b ra n e s , a l l w i th s u p p o r t f a b r i c , w e re t e s t e d in o s m o s i s e x p e r im e n t s. T h e i r r e s i s t iv i ty v a lu e s w e re s im i l a r

to o r h igher than those o f the Toray mem brane w ith fabr ic , bu t , wi th one o f the four , the resu l ts were a ffec ted by switch ing the

lo c a t io n o f t h e h ig h a n d lo w c o n c e n t ra t io n s o lu tio n s . I t w a s c o n c lu d e d th a t e x i s t in g c o m m e rc ia l ly a v a i l a b le L -S m e m b ra n e s

a re no t appropria te fo r la rge -sca le osmos is app l ica t ions because the ir support fabr ic dec reases pe rmea t ion f lux excess ive ly .

Keywords A s y m m e t r i c m e m b ra n e s ; C o n c e n t ra t io n p o la r i z a t io n ; L o e b -S o u r i r a j a n m e m b ra n e s ; Me m b ra n e p re p a ra t io n a n d

s truc tu re ; Osmos is

1 Introduct ion

T h e p r a c t i c a l l a r g e - s c a l e u s e o f p r e s s u r e - r e t a r d e d

o s m o s i s ( P R O ) h a s b e e n c o n s i d e r e d b y p r e v i o u s

i n v e s t i g a t o r s [ 1 , 2 ] w h o c o n c l u d e d t h a t a m o n g t h e

c o m m e r c i a l l y a v a i l a b l e f ia t s h e e t m e m b r a n e s , t h e

L o e b - S o u r i r a j a n ( L - S ) t y p e h o l d s t h e m o s t p r o m i s e

f o r o s m o s i s b e c a u s e i t o f f e r s t h e h i g h e s t p e r m e a t i o n

f l u x f o r a g i v e n o s m o t i c d r i v i n g f o r c e .

*Corresponding author. PO Box 41, Omer, 84965 Israel. Tel.:

972-76460475; fax: 972-76467763; e-mail: sidloeb@bgumail.

bgu.ac.il.

0376-7388/97/ 17.00 1997 Elsevier Science B.V. All rights reserved.

P I I S 0 3 7 6 - 7 3 8 8 ( 9 6 ) 0 0 3 5 4 - 7

T h e s e i n v e s t i g a t o r s a l s o f o u n d t h a t t h e p o r o u s s u b -

s t r u c tu r e o f s u c h m e m b r a n e s i s t h e p r i n c i p a l h i n d r a n c e

t o o s m o t i c f l o w . H o w e v e r , t h e i r w o r k w a s a l w a y s

c o n f i n e d t o m e m b r a n e s w i t h o u t t h e w o v e n o r n o n -

w o v e n p o r o u s s u p p o r t f a b r i c w h i c h i s u n i v e r s a l l y

a p p l i e d to d a y t o a ll c o m m e r c i a l l y a v a i l a b l e L - S m e m -

b r a n e s b y b o n d i n g t o t h e s u b s t r u c t u r e s i d e .

S i n c e t h e p o r o u s s u b s t r u c t u r e i s t h e p r i n c i p a l r e s i s-

t a n c e t o o s m o t i c p e r m e a t i o n t h r o u g h a m e m b r a n e

w i t h o u t a s u p p o r t f a b r i c , o n e m u s t a s k w h e t h e r t h e

a d d i t i o n o f a f a b r i c o n t h e p o r o u s s u b s t r u c t u r e s i d e

w o u l d a c t a s a n e x c e s s i v e a d d i t i o n a l r e s is t a n c e . T h e


2/7

244 S. Loeb et al./ Journal of Membrane Science 129 1997) 243-249

X = O

P L o w

11 Low C 2 LOW

J l _ w a t e r f l u x

J 2 ~ou p l ed )

= J 1 C 2 ~ w

P L o w

d

Porous Subs t ruc ture ps

~ X

T r a n s i t i o n

I n t e r f a c e

r PHI4

q

v

T r a n s o o r t i n s k in

J ~

= A A P - A H )

Jz

= B (CzHi - CzLow)

[ [ H i J C 2 H i

] [ t r ( = ] [ L o w )

C Z t r (= C z L o w )

A ) R E V E R S E O S M O S I S

11 -Osmot ic Pressure

J1 -Water F lux

J2 -So lu te F lux

C -So lu te Concent ra t ion

ps -Porous Subs t ruc ture

t r -T rans i t ion In te r face

A -Water Permeat ion

Constan t , RO

B -So lu te Permeat ion

Con stant , RO

t -Porou s Subs t ruc ture

thickness

t approx imate ly equa ls

~ x , t h e t o t a l m e m b r a ne

thickness H k o w ,2 ~ t c w

T r a n s i t i o n

I n t e r f a c e

X - - O

I w a t e r u x

z couplu ) = J 1Cz ps

J z d i f f us i on )

=

- D ~ ( d C z m / d x ) / / ~

l i p s C z p s

Porous Subs t ruc ture ~ s

~ X

B )

OS MOS IS

l - [Hi, C2 Hi

T r a n s p o r t i n s k i n

J 1 i d e l = A I - [H i - ] - [Low)

J l a c t u a l )

= A ] - [Hi -

Ht r )

~ r ] t r , C 2 t r

~ H t r ( I d e a l

)

= H L ow

I L C z t r ( I d e a l ) = C z L o w

D zps - D i f f u s i v i t y o f s o l u t e in p o r o u s s u b s t r u c t u r e .

K - R e s i s t i v i t y o f s o l u t e t o d i f f u s io n i n p o r o u s s u b s t r u c t u r e . K = t / D z p s

Fig. 1. Transport conditions in A) reverse osmo sis and B) osmo sis, both through an asym metric membrane.


3/7

S . L o e b et a l . /J o u r n a l o f M e m b r a n e S c i e n c e 1 2 9 1 9 9 7 ) 2 4 3 - 2 4 9 245

p u r p o s e o f t h i s p a p e r i s t o p r o v i d e i n f o r m a t i o n r e l e -

van t to th i s ques t ion .

2 . C o m p a r i s o n o f t r a n s p o r t i n r e v er s e o s m o s i s

a n d o s m o s i s b o t h t h r o u g h a n a s y m m e t r i c

m e m b r a n e w i t h o u t a su p p o r t f a b r ic

F i g . 1 A s h o w s t h e p r e s s u r e a n d s o l u t e g r a d i e n t s

a c r o ss a n L - S m e m b r a n e i n r e v e r s e o s m o s i s R O ) . T h e

w a t e r p e r m e a t i o n f l u x , J i , i s t a k e n f r o m t h e h i g h

c o n c e n t r a t i o n s o l u t io n , f l o w s t o t h e l e f t f r o m t h e s k i n

i n t o t h e p o r o u s s u b s t r u c t u r e , a n d p a s s e s t h r o u g h i t

w i t h n o c h a n g e i n t h e s o l u t e c o n c e n t r a t i o n , C 2 , a n d

l i tt l e c h a n g e i n p r e s s u r e . H e n c e , t h e p o r o u s s u b s t r u c -

t u r e t h i c k n e s s p l a y s o n l y a m i n o r r o l e i n d e t e r m i n i n g

t h e m a g n i t u d e o f J1 .

I n o s m o s i s , t h e s i t u a t io n h a s b e e n d e s c r i b e d b y L e e

e t a l . [ 2 ] a s in te r na l po la r iza t ion . As shown in

F ig . 1 B 1

t h e w a t e r p e r m e a t i o n f l u x ,

J1

i s t a k e n f r o m

t h e l o w c o n c e n t r a t i o n s o l u t io n , a n d f l o w s t o t h e r i g h t

in the por ous subs t r uc tur e , ca r r y ing the so lu te wi th i t .

A t a n y g i v e n p o i n t i n t h e p o r o u s s u b s t r u c tu r e , w h e r e

the so lu te c onc en t r a t io n i s C2ps , the so lu te f lux , J2,

t ends to f low to the r igh t , w hi le co uple d to J1 , a t a ra te

J2 = J1C2ps. T h e s o l u t e f lu x i s la r g e l y s t o p p e d b y t h e

sk in a t the t r ans i t ion in te r f ace , caus ing the so lu te

concen t r a t ion the r e , C2 , to r i s e . However , th i s r i s e

c a u s e s b a c k d i f f u s i o n o f t h e s o l u t e t o t h e l e f t a t a ra t e

J2 --- - - D2ps) dC2 /dx ) wh ere

D2ps

i s t h e d i f f u s i v i t y o f

the so lu te in the por ous subs t r uc tur e .

T h e s o l u t e c o n c e n t r a t i o n , C2, cont inu es to r i s e un t i l

a s t ea dy s ta te i s r each ed a t 7 r tr and C2 t r . Sinc e 7r tr is

g r ea te r than 7fLow, the sk in t r an spor t equa t ion s in

F ig . 1B show tha t J1 ac tua l ) i s l e s s than

J1

ideal) .

J1

a c t u a l ) c a n o n l y a p p r o a c h c l o s e l y t o J 1 i d e a l) i f t h e

s o l u t e b a c k d i f f u s i o n r a t e i s h i g h o r t h e d i f f u s i o n

th ickness , t , i s low. This i s equ iva len t to s ay ing tha t

the solute res is t ivi t y K - -- -

/ D 2 p s )

in the subs t r uc tur e

i s l o w . T h u s , i n o s m o s i s , t h e p o r o u s s u b s t r u c t u r e

c l e a r l y h a s a l a r g e e f f e c t i n d e t e r m i n i n g t h e w a t e r

p e r m e a t i o n f lu x , J r .

T h e e x a c t e q u a t i o n , d e v e l o p e d b y L e e e t a l . [ 2] , i s

g i v e n i n A p p e n d i x A f o r t h e c a s e i n w h i c h t h e l o w

c o n c e n t r a t i o n s o l u t i o n i s o n t h e p o r o u s s u b s t r u c t u r e

1Fig. 1B also gives nom enclature use d throughou t this paper

with the exception of Kfab o be defined n this section.

s i d e. I f t h e n o t - a l w a y s - p r e c i s e a s s u m p t i o n ,

C2A C2B = 7 1 A / T r B , i s m a d e a n d t h e e x p e r i m e n t a t i o n

i s r e s t r i c t e d t o t h e c o n d i t i o n o f a p p r e c i a b l e o s m o t i c

p r e s s u r e o n b o t h s i d e s o f t h e m e m b r a n e , t h e L e e

e q u a t io n c a n b e re d u c e d t o E q . A 3 ) o f A p p e n d i x A :

K = 1 / J ) ln T rH i /T rL o w )

I t a p p e a r s f r o m f u r t h e r a n a l y s i s t h a t t h i s e q u a t i o n

w o u l d a p p l y w h e t h e r t h e l o w c o n c e n t r a t i o n s o l u t i o n i s

o n t h e s k i n s i d e o r th e p o r o u s s u b s t r u c t u r e s i d e o f th e

m e m b r a n e . I t s h o u l d a l s o b e n o t e d t h a t t h e o s m o t i c

d r i v i n g f o r c e i s e s s e n ti a l l y t h e l o g a r i t h m o f t h e r a t io o f

h i g h t o l o w o s m o t i c p r e s s u r e . I n R O , t h e o s m o t i c

d r i v i n g f o r c e i s t h e d i f f e r e n c e b e t w e e n h i g h a n d

l o w o s m o t i c p r e s s u r e s .

T h e a b o v e d i s c u s s i o n a n d t h e a n a l y s i s i n A p p e n -

d i x A a r e fo r t h e a s y m m e t r i c m e m b r a n e w i t h o u t a

suppor t f abr ic , in which case the so lu te r e s i s t iv i ty

te r m , K, r e f e r s to so lu te d i f f us ion in the por ous sub-

s t r uc tur e . When the suppor t f abr ic i s inc luded , the r e

a r e , a s show n in F ig . 2 , th r ee d i s t inc t r eg ion s f o r so lu te

d i f f u s io n s p e c i fi c a l ly : m o s t o f t h e s u b s t r u c tu r e , m o s t

o f t h e s u p p o r t f ab r i c , a n d a n o v e r l a p r e g i o n c o m b i n i n g

the f i r s t two.

T h e m e t h o d s o f te s t i n g a n d d a t a a n a l y s i s u s e d h e r e

a r e a l s o a p p r o p r i a t e f o r a n o v e r a l l s o l u t e r e s i s t i v i t y

~

J~

1 5 5

SuDDor t

Fabric

3 0

o v e r

l a p

S k i n ~

C e l l u l o s e a c e t a t e p a r t

of membrane skin a n d ]

porous s u b s t r u c t u r e I

1 3 5

O v e r a e n g t h

2 6 0 v

Dimensions in microns

Fig. 2. Edgew ise dimensions of the Toray CA-3000 membrane.


4/7

246 S . Loe b e t a l . / J our n a l o f M e m br ane Sc ie nc e 129 1997) 243-249

t e r m t o b e u s e d w i t h t h e m e m b r a n e s h a v i n g a s u p p o r t

f a b r i c . Th i s r e s i s t i v i t y t e r m w i l l be c a l l e d K f a b .

3 . D e s c r ip t i o n o f T o r a y C A 3 0 0 0 m e m b r a n e

T h e T o r a y C A 3 0 0 0 m e m b r a n e i s a n R O c e ll u lo s e

a c e t a t e a s y m m e t r i c m e m b r a n e s u p p o r t e d b y a w o v e n

f a b r i c . I t w a s c h o s e n f o r e x t e n s i v e t e s ts b e c a u s e t h e

s u p p o r t f a b ri c c o u l d , w i t h c a re , b e r e m o v e d f r o m t h e

r e st o f t h e m e m b r a n e . T h i s e n a b l e d c o m p a r i s o n o f

p e r f o r m a n c e w i t h a n d w i t h o u t t h e f ab r ic . I n a n e d g e -

w i s e v i e w , t h e m e m b r a n e a n d s u p p o r t m a t e r i a l h a v e

t h e d i m e n s i o n s s h o w n i n F i g . 2 .

T h e c h a r a c t e r is t ic p e r f o r m a n c e o f t h e m e m b r a n e i n

R O , a c c o r d i n g t o T o r a y , i s : f r a c t i o n a l N a C 1 r e j e c t i o n ,

R , 0 .98 ( 98 ) ; w a t e r f l ux , J1 0 . 9 0 m 3 / m 2 d. D a t a w e r e

o b t a i n e d a t a fe e d c o n c e n t r a t i o n o f 1 5 0 0 m g /1 N a C 1 ,

a n d o p e r a t i n g p r e s s u r e o f 3 0 k g / c m 2 ( o r 2 9 .0 3 a t m o r

2 9 . 4 1 b a r o r 4 2 6 . 8 p s i ) .

N a C 1 a t 1 5 0 0 m g / 1 h a s 1 . 2 a t m o s m o t i c p r e s s u r e .

T h e n , i n R O :

A = J I / A P

-- ATr) = 0 .9/ (29 .03 - 1.2) ----0 . 0 3 2 8 m 3

/ m 2 d a t m .

Bm J 2 / C 2 H i -C2 Lo w ) . B i s t h e s o l u t e p e r m e a t i o n

c o n s t a n t , i n r n / d . I f C 2 h a s t h e u n i t s k g o f s o l u t e

p e r m 3 th e n J 2 i s k g o f s o l u t e p e r m 2 p e r d a y . I t

c a n b e s h o w n [ 2 ] t h a t :

B = (1 - R ) A ) A P - A ~ r )

R

= (1 -

R ) J 1 _

( 1 - 0 . 9 8 ) ( 0 . 9 0 )

- - = 0 . 0 1 8 4 m / d

R 0 . 9 8

4 . T e s t a p p a r a t u s a n d p r o c e d u r e

T h e o s m o s i s t e s t a p p a r a tu s i s s h o w n c o n c e p t u a l l y i n

F i g . 3 . A s a b a t c h d e v i c e , c o n d i t i o n s w e r e c h a n g i n g

c o n s t a n t l y d u r i n g t h e c o u r s e o f a te s t . S p e c i f ic a l l y , t h e

d i l u t e s o l u t i o n w a s c o n c e n t r a t i n g a n d t h e o t h e r w a s

d i l u t i n g . H e n c e , t h e o s m o t i c d r i v i n g f o r c e ,

l n( TrH i/ Tr Low ) , w a s c on t i nua l l y d i m i n i s h i n g , p r od uc i ng

a c o r r e s p o n d i n g d e c r e a s e i n th e w a t e r p e r m e a t i o n f l u x ,

J 1 , i n a c c o r d a n c e w i t h E q . ( A 3 ) i n A p p e n d i x A :

J1 ~ ln(TrHi/TrLow)

T h e t e s t s w e r e s h o r t e n o u g h s o t h a t t h e a r i t h m e t i c

a ve r a ge va l ue s w e r e a c c e p t a b l e , i . e . l n ( Tr n i , a ve /

~ rL ow ,av e). J 1 w a s d e t e r m i n e d o f c o u r s e f r o m t h e c h a n g e

i n b u r e t t e l e v e l s . T h e t e s t s r e p o r t e d h e r e i n w e r e a l l

m a d e w i t h a p p r e c i a b l e m a g n e s i u m c h l o r i d e c o n c e n -

t r at io n s o n b o t h s i d e s o f th e m e m b r a n e s . T h e c o n -

_ ~ B u r e t t e s . ~

Level

{ - - 0 0 -

Rise

Level Fall

As ym m etric ~J j

Membrane 13

Condit ions tes ted:

1) Low co ncent rat ion solut ion on porous substructure s ide or on sk in side

2) Solut ion pai rs 6-12% (as shown) or 6-18 , 12-18, 12-24% MgCIz

3) Mem brane with o r witho ut fabric.

Fig. 3. Osm osis test cell.


5/7

S . L o e b e t a l ./ J o u r n a l o f M e m b r a n e S c ie n c e 1 2 9 1 9 9 7 ) 2 4 3 - 2 4 9 247

centrations and therefore osmotic pressures were read-

ily known from density measurements of solutions

both before and after the tests.

opposite side of the membrane, with or without fabric.

This conclusion could also be examined by the test

procedures, i.e. by simply switching the solutions

from one side to the other.

5 . Purp oses o f tes ts

The primary purpose of the tests was to determine

the effect of the support fabric on osmotic water

permeation flux. The value of the solute resistivity

term, K or Kfab), was also determined for each test.

Since K is a property dependent primarily on the

nature of the solute and the porous medium through

which it diffuses, K should be relatively constant for a

given solute whatever the value o f J1, 7rni, and 7fLow n

a given test, i.e. K should be an appropriate parameter

for evaluating osmosis membranes. Hence, another

purpose of these tests was to determine the constancy

of K with varying values of J1, 71Hi, and 7fLow.

Another tentative conclusion reached in the devel-

opment ofEq. A3) was that it should be valid whether

the low concentration solution is on the skin side or the

6 . Tes t resul ts w i th Toray ce l lu lose aceta te

m e m b r a n e C A - 3 0 0 0

The following conclusions can be gleaned from the

tests reported in Table 1:

First, the detrimental effect of the support fabric on

the water permeat ion flux is very clear. In the tests

with the 12/6 concentration pair the average permea-

tion flux was ,- d/6 as high with the fabric as without it

(0.01 vs. 0.06 m3/ m 2 d).

Approximately

th i s same

value is seen in the 17/104 ratio for

K/Kfab

Second, in all the tests made without the support

fabric, the term K was relatively constant at values

between 15 and 25 d/m, regardless of appreciable

varia tion in J t, 7rm, and 7fLow, and their ratio. Thus,

Eq. A3) is supported and therefore the expectation

Table 1

Effect of support fabric on osmosis through the Toray CA-3000 membrane

Test dates

(November 1995) a

Approximate initial MgC12 solutions

on each side of the membrane

Concentration ratio Osmotic pressure

C2m/C2Low

( / ) 7rrn/rrLow atm/atm)

Water permeation

flux J1 ( m3/mz d)

Resistivity to MgC12

diffusion (d/m)

With fabric, Without fabric,

Kfab K

Solution in contact

with skin side,

Hi or Low

concentration

1 12/6 140/50 0.012 101

22 0.0097 112

20 0.0097 98

Ave. 104

8 12/6 140/50 0.060

5 0.056

16 0.051

2 0.071

12 0.064

9 18/6 290/50 0.094

13 18/12 290/140 0.020

23 0.031

15 24/12 540/140 0.059

18

15

18

15

18

Ave. 17

17

25

23

20

Hi

Low

Hi

Hi

Low

Hi

Hi

Hi

Hi

i

Hi

Hi

a Tests on 8, 9, 12, 13, 15 and 16 November were all made with the sa me membrane .


6/7

248 S. Loeb et al . /Journal o f Membrane Science 129 1997) 243-249

t h a t th e t e r m K i s d o m i n a n t a n d g e n e r a l i n d e t e r m i n i n g

t h e o s m o t i c p e r m e a t i o n f l u x .

T h i r d , t h e t e s t s o n 2 2 n d a n d 5 t h N o v e m b e r w e r e

d o n e w i t h t h e l o w c o n c e n t r a t i o n s o l u t i o n o n t h e s k i n

s i d e. T h e a p p r o x i m a t e e q u a l i t y o f K a n d K f ab v a l u e s

w i t h t h o s e o f t h e o t h e r t e s t s in t h e i r r e s p e c t i v e g r o u p s

s u p p o r t e d t h e b e l i e f t h a t t h e s i d e o f th e m e m b r a n e i n

c o n t a c t w i t h t h e l o w c o n c e n t r a t i o n s o l u t i o n i s n o t

i m p o r t a n t ( b u t s e e t h e f o l l o w i n g s e c t i o n ) .

7 Te s ts w i t h o t h e r c e ll u l o se a c e t a te m e m b r a n e s

O s m o s i s te s t s w i t h 6 a n d 1 2 M g C 1 2 s o l u t i o n s w e r e

m a d e o n s e v e ra l o t h er c o m m e r c i a l l y a v a i l ab l e c e ll u -

l o s e a ce t a t e m e m b r a n e s . A l l o f t h e s e w e r e b a c k e d w i t h

w o v e n o r n o n - w o v e n f a b r i c . N o e f f o r t w a s m a d e t o

r e m o v e t h e f a b r i c .

T h e r e s u l t s g i v e n i n T a b l e 2 s h o w t h a t t h e K f ab

v a l u e s a r e i n th e r a n g e o f t h o s e w i t h t h e f a b r i c - b a c k e d

T o r a y m e m b r a n e o r h ig h er . A s w i t h t h e T o r a y m e m -

b r a n e , t h e r es u l t s w e r e i n d e p e n d e n t o f t h e l o c a t i o n o f

t h e 6 a n d 1 2 M g C I 2 s o l u t i o n s f o r t h e F l u i d S y s t e m s ' ,

T r i s e p ' s , a n d D e s a l i n a t i o n S y s t e m s ' m e m b r a n e s .

H o w e v e r , t he H y d r a n a u t i c s ' m e m b r a n e h a d r o u g h l y

t w i c e t h e K f ab v a l u e w h e n t h e l o w c o n c e n t r a t i o n

s o l u t i o n w a s a g a i n s t t h e s k i n a s w h e n t h e s o l u t i o n s

w e r e r e v e r s e d . T h i s a n o m a l o u s b e h a v i o r w a s n o t

e x p l o r e d f u r th e r .

8 D i s c u s s io n a n d c o n c l u s i o n s

C l e a rl y , th e p o r o u s s u p p o r t f a b ri c o n L - S m e m b r a n e s

d e c r e a s e s o s m o t i c p e r m e a t i o n e x c e s s i v e l y . H o w e v e r ,

w i t h o u t t h e f a b r ic t h e m e m b r a n e i s t o o w e a k . A c o m -

p r o m i s e w o u l d a p p e a r to b e t h e u se o f a m u c h m o r e

o p e n f a b r i c . H o w e v e r , t h e u n a n i m o u s o p i n i o n o f th e

m a n u f a c t u r e r s i s t h a t a m o r e o p e n f a b r i c w o u l d c a u s e

' b l e e d i n g ' t h r o u g h i t d u r i n g f a b r i c a t io n , a n d w o u l d

d e c r e a s e t h e r e l i a b i l i t y o f th e c o m p l e t e d m e m b r a n e .

A s t o u c h e d u p o n b y L e e e t al . [2 ] , a n L - S c a p i l l a r y

t u b e m i g h t b e a p p r o p r i a t e s i n c e it w o u l d n o t r e q u i r e

f u r t h e r s u p p o r t . T h e i n t e r n a l d i a m e t e r s h o u l d b e a t

l e a s t 1 0 0 ~ tm t o m i n i m i z e a x i a l p r e s s u r e d r o p w i t h i n

t h e c a p i l l a r y t u be . T h e w a l l t h i c k n e s s s h o u l d b e g r e a t

e n o u g h t o m a i n t a i n c a p i l l a r y r i g i d i t y b u t o t h e r w i s e ,

t h i n e n o u g h t o a c h i e v e a d e q u a t e p e r m e a t i o n f l u x s in c e

K i s p r o p o r t i o n a l t o p o r o u s s u b s t r u c tu r e t h i c k n e s s . A

m o d u l e f i l l e d w i t h c a p i l l a r y t u b i n g w o u l d b e s a t i s f a c -

t o r y f o r o s m o s i s w i t h n o n - p r e c i p i t a t i n g s o l u t io n s . H o w -

e v e r, i n o n e p o s s i b l e a p p l i c a t i o n , t h e c o n c e n t r a t i o n o f

D e a d S e a b r i n e , e x t e n s i v e p r e c i p i t a t i o n o f N a C 1 w o u l d

o c c u r a s t h e b r i n e i s d e w a t e r e d b e c a u s e i t is s a t u r a t e d

t o s t a r t . T h e r e f o r e , t h e m o d u l e w o u l d h a v e t o b e d e -

s i g n e d t o p a s s a s l u rr y o f c o n c e n t r a t e d D e a d S e a b r i n e

a n d N a C I w i t h o u t c l o g g i n g , w h i c h i s n o t a n e a s y ta s k .

A c k n o w l e d g e m e n t s

Table 2

Solute resistivity gfab) values

cellulose acetate m embranes other

obtained with fabric-backed

than To ray CA-3000

Manufac tu rer Des igna t ion Kfab, (d /m ) Com men ta ry

Fluid Systems LP 82 Note 1

CA/CTA 96 Note 2

Trisep None 140 Note 1

140 Note 2

Desa l ina t ion CD -Yum a 380 Note

1

Systems CA/CTA 380 Note 2

Hyd ranautics CA B- 1 246, 204 Note 1

93, 136 Note 2

Note 1: Low concentration solution against skin.

Note 2: Low concentration solution against fabric.

All tests don e wit h 6 and 12 MgC12.

W e w i s h t o t h a n k t h e T o r a y , F l u i d S y s t e m s , T r i s e p ,

a n d H y d r a n a u t i c s c o m p a n i e s f o r t h e o p p o r t u n i t y t o

t e s t t h e i r m e m b r a n e s .

A p p e n d i x A

K e y e q u a t i o n s w i t h o u t s u p p o r t f a b r i c

A . ] L o w c o n c e n t r a t io n s o l u t io n o n p o r o u s

s u b s t r u c tu r e s i d e o f m e m b r a n e

E q . (1 0 ) in [2 ] i s :

J l 1 - C 2 L o w / C 2 H i ) e x p J l K ) ( A 1 )

A T ru j 1 + B / J I [ e x p ( J i K ) - 1]

I n t h i s e q u a t i o n , A a n d B a r e k n o w n f r o m t h e R O

e x p e r i m e n t s ; 7 rH i, C 2L ow , a n d C 2 Hi a r e g i v e n c o n d i t i o n s


7/7

S . L o e b e t a l . / J o u r n a l o f M e m b r a n e S c i e n c e 1 2 9 1 9 9 7 ) 2 4 3 - 2 4 9 249

i n t h e o s m o s i s e x p e r i m e n t ; a n d J 1 i s m e a s u r e d d u r i n g

t h e e x p e r i m e n t . T h e r e f o r e , th e e q u a t i o n c a n b e u s e d t o

d e t e r m i n e K , d e f i n e d a s t /D2ps.

B y u s i n g t h e r o u g h b u t s i m p l i f y in g a s s u m p t i o n t h a t

for a g iven so lu te in a so lu t ion , 7 fA/7 f =

C 2 A / C 2 B ,

Eq. A1) can be s impl i f i ed to :

l n B + AT rH i - J l ~

K = 1 ~Jr) \ B ~--A~oow J A 2 )

In our t e s ts , 7 fLow was a lways apprec iab ly h ig he r

than ze ro . A l so , ATrLow nd , th e re fore , ATrni were mu ch

higher than B or J , so tha t we can wr i t e :

K = l / J 1 I n 7rHi ~ A3)

k, 71 Low/

A . 2 L o w c o n c e n t r a t i o n s o l u t i o n o n s k i n s i d e o f

m e m b r a n e

By an ana lys i s s imi l a r to tha t i n Sec t ion A .2 above ,

we arr ive a t :

Us ing the s ame s impl i fy ing a s sumpt ions tha t l ed to

Eq. A3) , we a r r ive a t t he s ame equa t ion . There fo re ,

t h e s a m e v a l u e o f K s h o u l d b e o b t a i n e d w h i c h e v e r s id e

o f t h e m e m b r a n e i s i n c o n t a c t w i t h t h e l o w c o n c e n -

t ra t ion so lu t ion . Note tha t Eq . A3) can be wr i t t en to

emp has ize the inve rse e f fec t o f subs t ruc ture th i cknes s

t on J l :

1 In 71Hi ~ O 2p s ln 7rHi

J1 = ~ \ 7fLow// = T \ 71 Low.

eferences

[1] S. Loeb, Pressure-Retarded Osm osis Revisited: The Pros pects

for Osm ot ic Powe r a t the De a d Se a , Proc e e dings of

Eurom e m bra ne ' 95 , V ol . 2 , Unive r s i ty of Ba th , Se pte m be r

1995, pp . 11-3-11-11.

[2] K. Lee , R. Baker and H. Lonsdale , Membranes for power

gen eratio n by pressure-retarded osmosis, J. M em bran e Sci. , 8

(1981) 141-171.

K = 1 / j 1 ) l n B B + A T r H i

+ J1 + ATrLow/

A 4 )

effect of porous support fabric on osmosis through a

Documents