Structure and Function of Membranes for Modern Chloralkali Cells

10 Structure and Function of Membranes for Modern Chloralkali Cells Ronald L. Dotson Olin Corporation, New Haven, CT 06511

With the advent of dimensionally s t a b l e anodes and p e r f l u o r i n a t e d i o n exchange membranes, over the past decade and a h a l f , came an a l t e r n a t i v e method f o r the manufacture of c h l o r i n e and c a u s t i c soda. This new process produces a food grade product without p o l l u t i o n . P e r f l u o r i n a t e d membranes now provide us with the key to a new era of high t e c h nology i n e l e c t r o c h e m i c a l science and technology, e s p e c i a l l y i n the manufacture of heavy chemicals. These membranes can be c h a r a c t e r i z e d by t h e i r s t r u c t u r e and f u n c t i o n . In the e a r l y 1970's a maximum i n the cathode current e f f i c i e n c y was found to appear as a f u n c t i o n of c a u s t i c s t r e n g t h , (1), u s i n g these p e r f l u o r i n a t e d membranes and t h i s germinated the development of the f i r s t commercially s u c c e s s f u l c h l o r - a l k a l i membrane c e l l s . The maximum i s thought to occur through discontinuous phase change zones l e s s than ten microns t h i c k on the cathode surface of the membrane. Here p e r c o l a t i o n can occur through t o p o l o g i c a l l y d i s t o r t e d c l u s t e r s i n high d e n s i t y films maintained under dynamic e l e c t r i c a l load. Two r e v o l u t i o n a r y developments have made a s u b t l e , but permanent change i n the technology of c h l o r i n e and c a u s t i c manufacture d u r i n g the past decade and a h a l f , dimensionally stable anodes and p e r f l u o r i n a t e d i o n exchange membranes. New c e l l concepts are now made 0097-6156/ 86/ 0302-0134S06.00/ 0 © 1986 American Chemical Society

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Membranes for Chloralkali Cells

p o s s i b l e u s i n g these components i n unique geometric electrode-membrane designs which demonstrate lower operating costs than past technology without adding p o l l u t i o n problems a s s o c i a t e d with asbestos and mercury c e l l processes. The basic o p e r a t i o n a l process f o r a s i m p l i f i e d c h l o r - a l k a l i membrane c e l l i s i l l u s t r a t e d i n "Figure 1 " . I t i s not at a l l s u r p r i s i n g that the large p r o ducers of f l u o r i n e and f l u o r i n a t e d products have been i n the vanguard of an i n t e n s i v e development e f f o r t to p r o duce the most e f f i c i e n t , l o n g - l i v e d i o n exchange membranes p o s s i b l e . Even though the complete h i s t o r y of these polymeric i o n exchange membranes spans j u s t over two and a h a l f decades, some of the s c i e n t i s t s and engineers i n t h i s f i e l d have become p r o l i f i c on both the theory and p r a c t i c a l a p p l i c a t i o n s . I t i s l a r g e l y because of the large investment i n time and manpower and the unique juncture of technology with p o l i c i e s and p o l i t i c s that the theories of i o n exchange membranes are i n a much more advanced stage than any of the other ion exchange systems. Membranes can be c h a r a c t e r i z e d by t h e i r s t r u c t u r e and f u n c t i o n , that i s how they form and how they p e r form. I t i s e s s e n t i a l that the c a t i o n exchange membranes used i n c h l o r - a l k a l i c e l l s have very good chemi c a l s t a b i l i t y and good s t r u c t u r a l p r o p e r t i e s . The combination of unusual i o n i c c o n d u c t i v i t y , high i o n i c s e l e c t i v i t y and r e s i s t a n c e to o x i d a t i v e h y d r o l y s i s , make the p e r f l u o r i n a t e d ionomer m a t e r i a l s prime candidates f o r c h l o r - a l k a l i membrane c e l l s e p a r a t o r s . Structure. The f i r s t commercially s u c c e s s f u l c h l o r i n e c a u s t i c c e l l s were developed and tested at Diamond Shamrock's T . R . Evans Research Center i n P a i n e s v i l l e , Ohio i n the e a r l y 7 0 ' s . The r e s i n formulation f o r these separators was based on the p o l y t e t r a f l u o r o e t h y l e n e backbone with short polyether side chains as shown: fCF CF >-fOCF CF>--OCF CF S0 H 2

2

2

2

2

3

(1)

The equivalent weights f o r these r e s i n s ranged from 1 0 0 0 to 2 0 0 0 meq/g, i n both s u l f o n i c and c a r b o x y l i c a c i d forms. The s e m i c r y s t a l l i n e , supermolecular s t r u c t u r e of the organic carboxylate and the amorphous s t r u c t u r e of the sulfonate r e s i n s have been studied with x - r a y s c a t t e r i n g and mechanical r e l a x a t i o n . This work shows no trace of c r y s t a l l i n i t y i n the s u l f o n a t e s , but the s t r e s s - r e l a x a t i o n data suggests the presence of a common s t r u c t u r a l f e a t u r e , i o n - c l u s t e r e d s t r u c t u r e , with regions of high and low i o n content. In "Figure 2 " i s shown the x - r a y d i f f r a c t i o n patterns d e p i c t i n g the supermolecular s t r u c t u r e of p e r f l u o r o c a r b o x y l a t e and the sulfonate. Here i s shown the amorphous halos i n both

136

COULOMBIC INTERACTIONS IN MACROMOLECULAR SYSTEMS

Figure cells.

1.

Basic o p e r a t i o n a l process

f o r membrane c h l o r a l k a l i

ο 4.85 A

Carboxylate f i l m showing h i g h % crystallinity

Η—I—ι—ι—ι—ι—ι—I—f—* ι 52 48 44 40 36 32

ι

I 28

I

i —ι—ι—J—μ-· 2 4 20 16 1

4~—4— 14

D e g r e e s 2Θ F i g u r e 2. structures

X - r a y d i f f r a c t i o n scans showing the s u p e r m o l e c u l a r of p e r f l u o r o c a r b o x y l a t e and s u l f o n a t e m a t e r i a l s .

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of the polymers, and also the carboxyl group aggregation y i e l d i n g c r y s t a l l i n e peaks at 2.36, 3.15 and 4.85A. In "Figure 3" we see an i d e a l i z e d model f o r a random-coil network of i o n - c l u s t e r e d s t r u c t u r e of an amorphous g l a s s i n a polymer c h a i n . One polymer chain i s darkened f o r "better v i s u a l i z a t i o n . A concept of how i o n i c c l u s t e r s form i n the s u l f o n i c a c i d polymer i s presented here i n "Figure 4 . Here i s shown the r e g i o n of c l u s t e r s o l and polymer g e l proposed f o r the p e r f l u o r i n a t e d sulfonate membranes. The c l u s t e r r e g i o n i s the gate which c o n t r o l s ion flow through the separator, and i t i s modulated by c o n c e n t r a t i o n , temperature, s t r u c t u r e and current d e n s i t y i n these f i l m s . In a d d i t i o n to the i o n - c l u s t e r e d g e l morphology and m i c r o c r y s t a l l i n i t y , other s t r u c t u r a l features i n c l u d e : p o r e - s i z e d i s t r i b u t i o n , v o i d type, compaction and h y d r o l y s i s r e s i s t a n c e , c a p a c i t y and charge d e n s i t y . The func t i o n a l parameters of i n t e r e s t i n t h i s instance include permeability, diffusion coefficients, temperature-time, p r e s s u r e , phase boundary solute c o n c e n t r a t i o n s , c e l l r e s i s t a n c e , i o n i c f l u x e s , concentration p r o f i l e s , mem brane p o t e n t i a l s , transference numbers, electroosmotic volume t r a n s f e r and f i n a l l y current e f f i c i e n c y . When strong i n t e r a c t i o n s take place between the membrane s t r u c t u r e and solvated ions and s o l v e n t , the s o l u b i l i t y of the permeant species are influenced by m o d i f i c a t i o n of the s o l v a t i o n c a p a c i t y of the solvent molecules through a r e s t r i c t e d b i n d i n g caused by the close proximity of polymer substrate-pore walls and hydrated i o n s . In t h i s instance the membrane forms a polymer-solvent complex i n the t h i n c o n t r o l l i n g l a y e r next to the catholyte which r e j e c t s hydroxyl i o n s . The brine side of t h i s separator i s a h i g h l y solvated g e l . This e f f e c t becomes much more pronounced i n regions where the a l k a l i s o l u t i o n becomes most h i g h l y s t r u c t u r e d , as NaOH.3iH 0 or Κ 0 Η · 4 Η 0 . In t h i s case water associated with h y d r o p h i l i c groups f i l l s the flow channels between c r y s t a l l i t e s i n the t h i n a l k a l i n e s k i n or l a y e r f i l m thus producing a lowering of the d i e l e c t r i c constant and thereby i n t r o d u c i n g a n i s o t r o p i c microporous c h a r a c t e r i s t i c s there. Dense, impermeable p e r f l u o r i n a t e d membranes are converted i n t o permeable m a t e r i a l s through increased s w e l l i n g i n a s u i t a b l e medium. I t i s c l e a r that the s t r u c t u r e and f u n c t i o n of membranes are interdependent, M

2

2

(1.2)·

Function. The chemical, thermal and mechanical s t a b i l i t y and ion-exchange behavior of the p e r f l u o r i n a t e d r e s i n s have been found to depend on the r e s i n s t r u c t u r e , degree of c r o s s l i n k i n g and a l s o on the nature and number of f i x e d i o n i c groups. The degree of c r o s s l i n k i n g through i o n i c linkages i n the c l u s t e r s and covalent l i n k i n g i n the polymer backbone, e s t a b l i s h e d a mesh width f o r the


One

chain

F i g u r e 3. I d e a l i z e d model of a r a n d o m - c o i l network of c l u s t e r e d s t r u c t u r e f o r an o r g a n i c g l a s s .

F i g u r e 4. I o n i c a c i d membrane.

clusters

formed i n p e r f l u o r i n a t e d

ion-

sulfonic

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matrix and a l s o s w e l l i n g a b i l i t y of r e s i n and i o n i c m o b i l i t i e s f o r the counterions i n the mesh. Many of the p e r f l u o r i n a t e d m a t e r i a l s are s t r a i g h t chain polymers l i n k i n g through i o n i c entanglement alone. The average mesh width of the h i g h l y c r o s s l i n k e d r e s i n s i s of the order of only a few angstrom u n i t s . Shown i n "Figure 5" i s a conception of the network of interconnected channels i n the macro g e l - s o l network. This d e p i c t s the approximate spacing between f u n c t i o n a l groups, where the mesh width of weakly c r o s s l i n k e d and f u l l y swollen r e s i n s range between 10 to 100 angstrom u n i t s i n s i z e , (4,j5). The s e v e r e l y d i s t o r t e d geometry e x i s t i n g across c h l o r - a l k a l i c e l l membranes operating under dynamic load can be treated u s i n g p e r c o l a t i o n theory. Percolation theory t r e a t s the degree of interconnectedness present i n condensed matter i n terms of a p e r c o l a t i o n t r a n s i t i o n that develops from i n c r e a s i n g connectedness, i d e n t i t y and occupation w i t h i n the network. The model accounts f o r the presence of a sharp phase t r a n s i t i o n at which l o n g range c o n n e c t i v i t y suddenly appears as a f u n c t i o n of d e n s i t y , occupation and concentration and produces a second-order phase t r a n s i t i o n i n the amorphous s o l i d phase. Depicted i n "Figure 5" i s the concept of a c r i t i c a l volume f r a c t i o n f o r p e r c o l a t i o n i n the context of a two-dimensional honeycomb l a t t i c e . At the p e r c o l a t i o n t r a n s i t i o n p o i n t i n the network, the u n d e r l y i n g s t r u c t u r e becomes t o p o l o g i c a l l y d i s t o r t e d and leads to anomalous s e l e c t i v i t y i n the t h i n permeable f i l m s . During e l e c t r o l y s i s ions w i t h i n a membrane which i s permeable to them do not remain permanently hydrated as the e l e c t r i c current d r i v e s them through the t h i n , i o n i c a l l y c r o s s l i n k e d s t r u c t u r e of the microfine r e s i n m a t r i x . Movement of the hydrated cations toward the cathode occurs simultaneous with water s t r i p p i n g on the anodic side of the s e p a r a t o r . The a c t i v i t y c o e f f i c i e n t of water depends on molar concentrations of s o l u t i o n s on both sides of the s e p a r a t o r . The t r a n s i t i o n from h y d r a t ed cations to cations with negative h y d r a t i o n d i s p l a c e s the a c t i v i t y versus c o n c e n t r a t i o n curve to higher molar v a l u e s , as shown i n "Figure 6". Data i n t h i s f i g u r e shows a t y p i c a l f u n c t i o n a l p l o t of sodium transport number versus c a u s t i c and brine concentrations at 2KA/M and 90°C with a p e r f l u o r o s u l f o n i c a c i d - c a r b o x y l i c a c i d membrane system. The s u l f o n i c a c i d p o r t i o n i s very much t h i c k e r than the carboxyl l a y e r on the cathode s i d e , (6). The existence of the maximum i n the curve at the proper combination of a l l v a r i a b l e s i s thought to be due to a r a p i d v i s c o s i t y increase at the c r i t i c a l molar conc e n t r a t i o n thereby generating a p e r c o l a t i o n t r a n s i t i o n zone at the cathode surface of the separator f i l m . At t h i s p o i n t the hydroxyl m o b i l i t y decreases on the m i c r o s c a l e w i t h i n the micron t h i c k cathodic membrane phase boundary corresponding to s t r u c t u r a l phase transformat i o n s w i t h i n the c o n s t r i c t e d c r i t i c a l pore volume as p i c t u r e d i n "Figure 5".

140


F i g u r e 6. Three d i m e n s i o n a l p l o t o f sodium t r a n s p o r t number versus a n o l y t e - c a t h o l y t e concentrations.

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Only a f t e r viewing the membrane as a t h i n f i l m semiconductive phase can one begin to s e r i o u s l y evaluate its potentialities. I t i s a multidimensional problem, and i n the c h l o r - a l k a l i c e l l s the water t r a n s p o r t i s c o n t r o l l e d by brine c o n c e n t r a t i o n while c a u s t i c s t r e n g t h c o n t r o l s the cathode e f f i c i e n c y . The membrane provides a low energy pathway for the phase change and s e p a r a t i o n process. The f l u x of water, s a l t and ions through the membrane can be defined i n a net flow equation. The net equation, n e g l e c t i n g convection and surface coverage, i s : dN/dt = ( d N / d t ) + ( d N / d t ) + ( d N / d t ) D

0

EM

+(dN/dt)

ET

(2)

where : (dN/dt) = s a l t f l u x , and i o n f l u x under d i f f u s i o n a l c o n t r o l , k ( C - C ) , mol/time-area. n

s

s l

s 2

(dN/dt) = water f l u x under pressure c o n t r o l , osmotic f l u x , Q

k ( w

A P - Û M ) .

(dN/d t ) „ = ele ctromigra t i o n , k (dE/dx). E

M M

E

( d N / d t ) „ =charge t r a n s f e r at k (exp(5KzFtl/RT]) h

electrodes,

l

ET

The increased d e n s i t y of the t h i n cathodic b a r r i e r i n a c h l o r - a l k a l i membrane provides the second phase of matter r e q u i r e d as the s e p a r a t i n g agent f o r a l k a l i and chloride. This d e n s i t y change i n the cathodic f i l m s e l e c t s a d i f f e r e n t , more c o n s t r i c t i v e innerchannel geometry at the c r i t i c a l p e r c o l a t i o n volume p o i n t , where s e l e c t i v i t y i s maximized as manifested by a maximum i n the p l o t of sodium t r a n s p o r t versus c o n c e n t r a t i o n , c u r r e n t , temperature and p r e s s u r e . This d e n s i t y changes e x p o n e n t i a l l y between p o i n t s x and x i n "Figure 7", along the thickness a x i s . This b a r r i e r f i l m i s l e s s than 10 microns i n depth but s t r o n g l y modulates the i n t r i n s i c conduction as i t increases voltage and the s e l e c t i v i t y at the maximum i n c u r r e n t e f f i c i e n c y . Equation ( 2 ) i s the base r e l a t i o n s h i p electrochemi c a l engineers u t i l i z e to describe and optimize the c e l l s and make compromises among the competing f a c t o r s such as: space-time y i e l d , energy consumption, product q u a l i t y and m a t e r i a l s of c o n s t r u c t i o n . 1

2

Summary. Membrane c e l l processes have become important to modern technology to a great extent because of the development and u t i l i z a t i o n of p e r f l u o r i n a t e d membranes. The combination of metal anodes and the p e r f l u o r i n a t e d membranes has provided a modern r e v o l u t i o n i n the area of c h l o r - a l k a l i p r o d u c t i o n .

142

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

F i g u r e 7. Density gradient across and c a u s t i c a c r o s s the b a r r i e r .

the

polymer f i l m w i t h b r i n e

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These membranes are c h a r a c t e r i z e d by t h e i r s t r u c t u r e and f u n c t i o n with r e s i n formulation based on the p o l y t e t r a f l u o r o e t h y l e n e backbone with short polyether s i d e chains terminating i n s u l f o n i c a c i d or c a r b o x y l i c a c i d groups. These f u n c t i o n a l groups can j o i n through i o n i c c r o s s - l i n k s forming i o n - c l u s t e r s . The strong i n t e r a c t i o n s set up by d e n s i f y i n g the t h i n cathodic b a r r i e r f i l m on the membrane improve the degree of s e p a r a t i o n ; thus the s e l e c t i v i t y of these f i l m s to the extent that high e f f i c i e n c i e s can be achieved at proper c o n d i t i o n s of o p e r a t i o n . One e f f e c t i v e method of i n c r e a s i n g t h i s s e l e c t i v i t y and throughput c a p a c i t y of these f i l m s i s to incorporate a substance that r e a c t s chemically with one component t r a n s m i t t e d , or that can i n t e r a c t s t r o n g l y with i t . The b a s i c idea is^to improve the sodium to c a u s t i c f l u x r a t i o by m o d i f i c a t i o n of the cathodic f i l m s , or adjust the c a u s t i c s t r e n g t h to operate at the minimum s o l u b i l i t y p o i n t f o r c a u s t i c or potassium hydroxide, NaOH3fH 0, or ΚΟΗ^Η?0. The t o p o l o g i c a l d i s t o r t i o n across these polymer f i l m s can be described by modern p e r c o l a t i o n theory, and used to define the maximum i n the sodium t r a n s p o r t versus c a u s t i c concentration curve as an optimum r e s t r i c t e d pore volume. Basic mathematical r e l a t i o n s can be developed to t r e a t the o v e r a l l process i n c h l o r - a l k a l i p r o d u c t i o n , as w e l l as many of the other membrane t a s k s . The process of "unmixing", or transforming a mix ture of substances i n t o two or more products that d i f f e r from each other i n composition i s served w e l l by the membrane processes. This separation process i s i n s t a r k c o n t r a s t to n a t u r a l f o r c e s , as C l a u s i u s so a p t l y put i t : "Die entropie der welt s t r e b t einem maximum zu". 2

Literature Cited Dotson, R. L.; O'Leary, K. J., U.S. Patent 4,025,405. Yeager, H. L.; K i p l i n g , B . ; Dotson, R. L., J . El. Chem. Soc., 127, No. 2, 1980. 3. Dotson, R. L.; Woodard, Κ. Ε., ACS Symposium Series. Washington, D.C., 1982. 4. Eisenberg, A . ; King, Μ., "Ion-Containing Polymers, (Physical Properties and Structure)", V o l . 2, Academic: New York, 1977; Flory, P. J., "Principles of Polymer Chemistry", University Press: Ithaca, N . Y . , 1953. 5. Helfferich, F . , "Ion Exchange", McGraw-Hill, N . Y . , 1962. 6. Kesting, R. Ε . , Synthetic Polymer Membranes", McGraw-Hill, N . Y . , 1971. 1. 2.

RECEIVED June 10, 1985

Structure and Function of Membranes for Modern Chloralkali Cells

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