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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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
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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
COULOMBIC INTERACTIONS IN MACROMOLECULAR SYSTEMS
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".
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COULOMBIC INTERACTIONS IN MACROMOLECULAR SYSTEMS
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 .
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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
