Coulombic Interactions in Macromolecular Systems - American

11 The Chloralkali Electrolysis Process Permselectivity and Conductance of Perfluorinated Ionomer Membranes

Downloaded by UNIV OF PITTSBURGH on May 14, 2016 | http://pubs.acs.org Publication Date: March 28, 1986 | doi: 10.1021/bk-1986-0302.ch011

H . L. Yeager and J. D. Malinsky Department of Chemistry, University of Calgary, Calgary, Alberta, Canada T2N 1N4

Perfluorinated ionomer membranes have been developed for use as separators in chlor-alkali electrolysis cells. Using an automated test apparatus, the current efficiency and voltage drop of such a high performance membrane were evaluated as a function of several cell parameters. Results are plotted as three dimensional surfaces, and are discussed in terms of current theories of membrane permselectivity. The development o f perfluorinated ionomer membranes f o r use i n t h e p r o d u c t i o n o f c h l o r i n e and sodium h y d r o x i d e has p r o v e d t o be a l e a d i n g s u c c e s s i n t h e f i e l d o f membrane t e c h n o l o g y . I n f a c t , t h e major i n d u s t r i a l p r o c e s s o f b r i n e e l e c t r o l y s i s , which has been employed f o r decades u s i n g b o t h diaphragm and mercury cathode c e l l s , has been r e v o l u t i o n i z e d by t h e s e polymer membranes. The modern, h i g h performance c h l o r - a l k a l i membrane must p o s s e s s t h e f o l l o w i n g capabilities: h i g h p h y s i c a l s t r e n g t h and c h e m i c a l s t a b i l i t y , l a r g e i o n i c c o n d u c t a n c e , and low p e r m e a b i l i t y t o h y d r o x i d e i o n - even when i n c o n t a c t w i t h h o t NaOH s o l u t i o n s o f up t o 15 M c o n c e n t r a t i o n . These performance g o a l s have now l a r g e l y been a t t a i n e d by c o n t i n u e d improvements t h r o u g h s e v e r a l g e n e r a t i o n s o f m a t e r i a l s . Currently, commercial p e r f l u o r i n a t e d ionomer m a t e r i a l s f o r t h i s a p p l i c a t i o n c o n s i s t o f membranes w i t h c a r b o x y l a t e o r mixed c a r b o x y l a t e - s u l f o n a t e f u n c t i o n a l i t y ; t h e l a t t e r membranes o f t e n have l a y e r e d s t r u c t u r e s w i t h t h e c a r b o x y l a t e l a y e r exposed t o t h e c a u s t i c c a t h o l y t e s o l u t i o n . F a b r i c r e i n f o r c e m e n t i s used i n some i n s t a n c e s t o improve s t r e n g t h . W h i l e t h e s e membranes e x h i b i t sodium i o n t r a n s p o r t numbers as h i g h as 0.98 mol F " ( i . e . o n l y 2% o f t h e e l e c t r o l y s i s c u r r e n t i s c a r r i e d by h y d r o x i d e i o n t h r o u g h t h e membrane) no comprehensive t h e o r e t i c a l t r e a t m e n t o f t h i s u n u s u a l l y h i g h p e r m s e l e c t i v i t y has y e t emerged. The v a r i a t i o n o f p e r m s e l e c t i v i t y as a f u n c t i o n o f v a r i o u s c e l l parameters i s a l s o o f i n t e r e s t , n o t o n l y f o r p r a c t i c a l r e a s o n s but a l s o because o f t h e i n s i g h t t h a t may be g a i n e d i n t o t h e n a t u r e of hydroxide i o n r e j e c t i o n . This research i s d i r e c t e d a t the l a t t e r problem, t h a t i s t h e c h a r a c t e r i z a t i o n o f membrane p e r m s e l e c t i v i t y 1

0097-6156/86/0302-Ό 144506.00/0 © 1986 American Chemical Society

Eisenberg and Bailey; Coulombic Interactions in Macromolecular Systems ACS Symposium Series; American Chemical Society: Washington, DC, 1986.

145

Chloralkali Electrolysis Process

11.

YEAGER AND MALINSKY

and and

r e s i s t a n c e as a f u n c t i o n o f s o l u t i o n c o n c e n t r a t i o n , current density.

temperature,

Experimental A l a b o r a t o r y membrane b r i n e e l e c t r o l y s i s c e l l , d e s i g n e d f o r automated o p e r a t i o n , was c o n s t r u c t e d (^,^2). T h i s system e n a b l e s t h e measurement o f t h e sodium i o n t r a n s p o r t number o f a membrane under s p e c i f i c s e t s o f c o n d i t i o n s u s i n g a r a d i o t r a c e r method. I n such an e x p e r i ment, t h e sodium c h l o r i d e a n o l y t e s o l u t i o n i s doped w i t h Na radiot r a c e r , a timed e l e c t r o l y s i s i s p e r f o r m e d , and t h e f r a c t i o n o f c u r r e n t c a r r i e d by sodium i o n t h r o u g h t h e membrane i s d e t e r m i n e d by the amount o f r a d i o a c t i v i t y t h a t has t r a n s f e r r e d t o t h e sodium h y d r o x i d e c a t h o l y t e s o l u t i o n . The v o l t a g e drop a c r o s s t h e membrane d u r i n g e l e c t r o l y s i s i s s i m u l t a n e o u s l y measured, so t h a t t h e o v e r a l l performance o f t h e m a t e r i a l c a n be e v a l u a t e d . A b l o c k diagram o f t h e a p p a r a t u s i s shown i n F i g u r e 1. The system i s c o n s t r u c t e d t o u s e t h r e e sodium c h l o r i d e a n o l y t e and f o u r sodium h y d r o x i d e c a t h o l y t e c o n c e n t r a t i o n s . The s t a r r e d a n o l y t e compartments r e f e r t o s e p a r a t e s o l u t i o n s which have been doped w i t h radiotracer. These s o l u t i o n s a r e used o n l y f o r d e t e r m i n a t i o n s o f t r a n s p o r t number; t h e n o n r a d i o a c t i v e b r i n e s o l u t i o n s a r e used f o r system f l u s h i n g and membrane e q u i l i b r a t i o n s . S o l u t i o n s a r e s e l e c t e d and pumped i n t o t h e c e l l , under computer c o n t r o l , t h r o u g h an a l l T e f l o n pump-valve system. The s o l u t i o n s a r e h e a t e d d u r i n g t h e s e t r a n s f e r s t o e n s u r e r a p i d a t t a i n m e n t o f e x p e r i m e n t a l temperature i n the c e l l . The b r i n e system i s d e s i g n e d t o e n a b l e t h e r e t u r n o f r a d i o t r a c e r s o l u t i o n s t o t h e i r s t o r a g e v e s s e l s a f t e r each u s e . T h i s s e r v e s t o r e d u c e consumption o f r a d i o a c t i v e s o l u t i o n s . I n p r a c t i c e , s o l u t i o n s a r e f i r s t pumped i n t o t h e t e s t c e l l , and then c i r c u l a t e d f o r a p e r i o d o f s e v e r a l hours t o c o n d i t i o n t h e memb r a n e . Next, an e l e c t r o l y s i s i s performed t o f u r t h e r c o n d i t i o n t h e material. These s o l u t i o n s a r e then d i s c a r d e d , f r e s h c a t h o l y t e and r a d i o a c t i v e a n o l y t e a r e added, and e l e c t r o l y s i s i s conducted a t a g i v e n membrane c u r r e n t d e n s i t y . A sample o f a n o l y t e and t h e e n t i r e c a t h o l y t e s o l u t i o n a r e then pumped t o a sample c o l l e c t o r f o r w e i g h i n g and d e t e r m i n a t i o n o f r a d i o a c t i v i t y . Other e x p e r i m e n t s may t h e n be r e p e a t e d a t o t h e r c u r r e n t d e n s i t i e s , o r t h e sequence r e p e a t e d w i t h new s o l u t i o n c o n c e n t r a t i o n s . Thus t w e l v e d i f f e r e n t c o m b i n a t i o n s o f a n o l y t e and c a t h o l y t e c o n c e n t r a t i o n s a r e u s e d . I n t h i s i n v e s t i g a t i o n , a sample o f N a f i o n NX-90209 c h l o r - a l k a l i membrane was used ( E . I . du Pont de Nemours and Co., Polymer P r o d u c t s Department, W i l m i n g t o n , D E ) . T h i s membrane has s u l f o n a t e and c a r b o x y l a t e polymer l a y e r s and i s r e i n f o r c e d w i t h an open weave fabric.


2 2

Results

and D i s c u s s i o n

Measurements were performed u s i n g 2, 3, and 4 M N a C l a n o l y t e and 8, 10, 12, and 14 M NaOH c a t h o l y t e s o l u t i o n s . The c e l l temperature was v a r i e d between 80° and 90°C and membrane c u r r e n t d e n s i t y was v a r i e d between 3 and 8 kA m~ t o t e s t t h e e f f e c t o f t h e s e parameters on membrane p e r f o r m a n c e . F o r a g i v e n t e m p e r a t u r e and c u r r e n t d e n s i t y , v a l u e s o f t j j + were used t o c r e a t e a performance s u r f a c e , u s i n g 2

a

tensioned

cubic s p l i n e functions

0,4).

Surfaces

f o r three

different


C O U L O M B I C I N T E R A C T I O N S IN M A C R O M O L E C U L A R S Y S T E M S

146

c o m b i n a t i o n s o f t e m p e r a t u r e and c u r r e n t d e n s i t y a r e shown i n F i g u r e s 2-4. As seen f o r t h i s (and o t h e r ) h i g h p e r f o r m a n c e c h l o r - a l k a l i membranes, sodium i o n t r a n s p o r t number shows a maximum w i t h i n c r e a s i n g c a u s t i c c o n c e n t r a t i o n but i s much l e s s s e n s i t i v e to b r i n e anolyte concentration. The p o s i t i o n o f t h i s maximum changes w i t h t e m p e r a t u r e and c u r r e n t d e n s i t y . The v e r y h i g h v a l u e s o f t f l + ( a p p r o a c h i n g 0.98 under c e r t a i n c o n d i t i o n s ) and i t s v a r i a t i o n w i t h changes i n c e l l p a r a m e t e r s a r e s u b j e c t s o f g r e a t t h e o r e t i c a l and p r a c t i c a l i n t e r e s t . I t s h o u l d be n o t e d t h a t the l a c k o f h y d r o x i d e i o n m i g r a t i o n t h r o u g h the membrane i s not due t o a Donnan e x c l u s i o n p r o c e s s ; c o n s i d e r a b l e s o r p t i o n o f NaOH i s found i n t h e s e p e r f l u o r i n a t e d ionomer membranes when t h e y a r e exposed to c a u s t i c s o l u t i o n s ( 5 ) .


a

T h e r e f o r e the h i g h p e r m s e l e c t i v i t y s e e n f o r t h e s e membranes i n c o n c e n t r a t e d c a u s t i c media must be a t t r i b u t e d to a k i n e t i c s o u r c e . S e v e r a l t r e a t m e n t s o f the t r a n s p o r t p r o p e r t i e s o f p e r f l u o r i n a t e d ionomers have been p r e s e n t e d , u s i n g v a r i o u s approaches (6-12) . Most o f t h e s e have i n c l u d e d the u n u s u a l i o n c l u s t e r i n g (13) o f the polymer as a c e n t r a l m o r p h o l o g i c a l f a c t o r . G i e r k e ' s t r e a t m e n t s u g g e s t e d t h a t c h a n n e l s which connect i o n i c c l u s t e r s may s e r v e as e l e c t r o s t a t i c b a r r i e r s to anion t r a n s p o r t . U s i n g the P o i s s o n - B o l t z m a n e q u a t i o n t o c a l c u l a t e the magnitude o f t h i s b a r r i e r , t r e n d s i n o b s e r v e d c u r r e n t e f f i c i e n c y w i t h polymer e q u i v a l e n t weight were obtained. R e i s s and B a s s i g n a n a (8) n o t e t h a t s u c h e l e c t r o s t a t i c b a r r i e r s t o a n i o n t r a n s p o r t would a l s o s e r v e as w e l l s f o r c a t i o n t r a n s p o r t , and t h a t the t r a n s p o r t o f b o t h c a t i o n s and a n i o n s must be c o n s i d e r e d to e x p l a i n what the a u t h o r s term " s u p e r s e l e c t i v i t y " o f t h e s e polymers. C l e a r l y though, the s p a t i a l v a r i a t i o n i n the c o n c e n t r a t i o n o f f i x e d i o n exchange s i t e s w i t h i n t h e s e ionomers i s seen as the u n d e r l y i n g cause o f h i g h p e r m s e l e c t i v i t y (8,12,14). Inherent i n t h i s view i s the r e q u i r e m e n t t h a t the symmetry o f the v a r i a t i o n i s o f a form t h a t w i l l a f f e c t c a t i o n and a n i o n t r a n s p o r t t o d i f f e r i n g degrees. Datye and coworkers (12) t r e a t an i o n i c c l u s t e r as a s p h e r e w i t h an e l e c t r i c a l d i p o l e l a y e r a t the s u r f a c e . Ions o f o p p o s i t e charge would e x p e r i e n c e a d i f f e r e n t change i n p o t e n t i a l energy when t r a v e r s i n g t h i s d i p o l e l a y e r , g i v i n g r i s e to i n h e r e n t p e r m s e l e c t i v ity. T h i s g e n e r a l approach l o o k s v e r y p r o m i s i n g i n terms o f b e i n g a b l e t o g e n e r a t e a model w h i c h can e v e n t u a l l y p r e d i c t i o n i c t r a n s p o r t p r o p e r t i e s from m o l e c u l a r and s t r u c t u r a l c h a r a c t e r i s t i c s o f t h e s e ionomers. The v a r i a t i o n o f c u r r e n t e f f i c i e n c y w i t h s o l u t i o n c o n c e n t r a t i o n i n t h e c h l o r - a l k a l i environment i s an added c o m p l i c a t i n g f e a t u r e o f t h e s e membranes b e h a v i o r . K r u i s s i n k (9) has p e r f o r m e d e l a b o r a t e c a l c u l a t i o n s t o y i e l d the e f f e c t o f e l e c t r o - o s m o t i c water t r a n s p o r t on p e r m s e l e c t i v i t y , i n c l a s s i c a l terms. R e s u l t s s u g g e s t t h a t the minimum seen i n t^a+ ( a t lower NaOH c o n c e n t r a t i o n s t h a n used h e r e ) may be due to the e f f e c t s o f e l e c t r o - o s m o s i s . Since hydroxide i o n i s the t r a n s p o r t e d a n i o n i n c h l o r - a l k a l i membrane c ells, the p o s s i b i l i t y e x i s t s that n o n - c l a s s i c a l transport i n v o l v i n g proton t u n n e l i n g e v e n t s i s a l s o a c o n t r i b u t i n g f a c t o r . M a u r i t z and Gray (15) have i n v e s t i g a t e d the e x t e n t o f p r o t o n t u n n e l i n g e v e n t s i n a p e r f l u o r o s u l f o n a t e f i l m i n c o n t a c t w i t h c a u s t i c s o l u t i o n s . They show 1


11.

YEAGER A N D MALINSKY ANOLYTE 1

1* 2

CATHOLYTE STORAGE

STORAGE

2* 3

3*

DRAIN

DRAIN


147


F i g u r e 1. B l o c k diagram o f automated membrane t e s t c e l l apparatus. Reproduced w i t h p e r m i s s i o n from Réf. 1. C o p y r i g h t 1982, t h e E l e c t r o c h e m i c a l S o c i e t y , I n c .

1.00 η

Library Eisenberg and Bailey; Coulombic Interactions in Macromolecular Systems 1155 16th St., N.W. ACS Symposium Series; American Chemical Washington. D.C. Society: 20036Washington, DC, 1986.

COULOMBIC INTERACTIONS IN MACROMOLECULAR SYSTEMS

1.00


Ί

F i g u r e 4.

t

N

+ f o r N a f i o n NX-90209, 90°C, 8 kA m

2

.



11.

YEAGER A N D MALINSKY

149


r e m a r k a b l e p a r a l l e l s between an i n f r a r e d c o n t i n u o u s a b s o r p t i o n o f t h i s polymer ( r e f l e c t i n g p r o t o n t u n n e l i n g ) and i t s p e r m s e l e c t i v i t y i n an o p e r a t i n g c h l o r - a l k a l i c e l l as a f u n c t i o n o f c a u s t i c c a t h o l y t e concentration. The e f f e c t o f i n c r e a s i n g membrane d e h y d r a t i o n w i t h s o l u t i o n c o n c e n t r a t i o n on p r o t o n t u n n e l i n g e v e n t s (and h y d r o x i d e m o b i l i t y ) i s s e e n as the l i k e l y cause o f t h i s r e l a t i o n s h i p . Thus, an o v e r a l l t h e o r y o f membrane p e r m s e l e c t i v i t y i n terms o f polymer p r o p e r t i e s may have t o t a k e i n t o account a v a r i e t y o f f a c t o r s to be a s u c c e s s f u l p r e d i c t i v e t o o l i n membrane d e s i g n . I n p r a c t i c a l terms, the v a r i a t i o n i n p e r m s e l e c t i v i t y as a f u n c t i o n o f v a r i o u s c e l l parameters means t h a t c e l l p e r f o r m a n c e must be o p t i m i z e d t o m i n i m i z e energy consumption. Not o n l y membrane p e r m s e l e c t i v i t y but membrane v o l t a g e as w e l l as o t h e r components o f c e l l v o l t a g e must be c o n s i d e r e d i n o r d e r t o o p t i m i z e c e l l power consumption p e r u n i t of p r o d u c t . Membrane v o l t a g e drop f o r N a f i o n NX-90209 i s shown a t 90°C and 3 kA n f c u r r e n t d e n s i t y i n F i g u r e 5, as a t y p i c a l c a s e . The membrane r e s i s t a n c e i n c r e a s e s monotonously w i t h i n c r e a s i n g c a u s t i c s t r e n g t h , p r o b a b l y due t o i n c r e a s i n g d e h y d r a t i o n ; i n a d d i t i o n , the r i s e i n v o l t a g e w i t h i n c r e a s i n g c u r r e n t i s v i r t u a l l y ohmic. 2

The power consumption o f a membrane c h l o r - a l k a l i c e l l w h i c h uses t h i s m a t e r i a l can be e s t i m a t e d from t h e s e v o l t a g e and t r a n s p o r t number r e s u l t s . C e l l v o l t a g e i s the sum o f membrane v o l t a g e d r o p , g a l v a n i c v o l t a g e , e l e c t r o d e o v e r p o t e n t i a l s , and s o l u t i o n and s t r u c t u r a l IR d r o p s . When t h e s e a r e summed, c e l l power consumption i s c a l c u l a t e d by: Power Consumption

- 1

(kWh

t o n n e ) = 670.1

E j^^/t^ + c e

a

E x p e r i m e n t a l l y , we f i n d t h a t t h e g a l v a n i c v o l t a g e o f t h e c e l l v a r i e s between 2.2 and 2.4 v o l t s under t h e s e r a n g e s o f c o n d i t i o n s . The o t h e r v o l t a g e components o f t h e c e l l , f o r a f i n i t e gap c o n f i g u r a t i o n w i t h low o v e r v o l t a g e anode and c a t h o d e , were e s t i m a t e d by t h e term 0.12+0.04

( I + l n I)

volts

2

where I i s c u r r e n t d e n s i t y i n kA m~ . Power consumption t r e n d s , e s t i m a t e d i n t h i s manner, a r e shown i n F i g u r e s 6-8 f o r t h r e e s e t s o f o p e r a t i n g c o n d i t i o n s . At 90°C and 3 kA m~ c u r r e n t d e n s i t y , a b r o a d minimum i s seen i n power consumpt i o n w i t h i n c r e a s i n g c a u s t i c c o n c e n t r a t i o n . At 80°C, power consumpt i o n i s lowered f o r more d i l u t e c a u s t i c c o n c e n t r a t i o n s due t o t h e s h i f t o f the maximum i n tfla+» 90°C and 8 kA m~ , power consumpt i o n r i s e s about 20-30% compared t o r e s u l t s f o r 3 kA m~ a t t h e same temperature. Values are g r e a t e s t at h i g h e r c a u s t i c s t r e n g t h s , a r e s u l t o f b o t h l a r g e membrane v o l t a g e s and reduced membrane c u r r e n t efficiencies. O v e r a l l , c e l l power consumption changes s l o w l y and m o n o t o n i c a l l y w i t h t h e s e c e l l parameters f o r t h i s h i g h p e r f o r m a n c e membrane. 2

A

t

2

2



C O U L O M B I C I N T E R A C T I O N S IN M A C R O M O L E C U L A R S Y S T E M S



11.

YEAGER AND MALINSKY


151

F i g u r e 7.

Estimated

cell

power consumption, 80°C, 3 kA m

F i g u r e 8.

Estimated

c e l l power consumption, 90°C, 8 kA m" .


2

152

COULOMBIC INTERACTIONS IN MACROMOLECULAR SYSTEMS

Literature Cited 1. 2.


3. 4. 5. 6. 7. 8. 9. 10. 11. 12. 13. 14. 15.

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Coulombic Interactions in Macromolecular Systems - American

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