Electrosynthesis with Perfluorinated Ionomer Membranes in Chlor

14

Electrosynthesis

with

Perfluorinated

Ionomer

Membranes in Chlor-Alkali Cells

RONALD L. DOTSON and KENNETH E. WOODARD

Downloaded by UNIV OF PITTSBURGH on May 4, 2015 | http://pubs.acs.org Publication Date: February 4, 1982 | doi: 10.1021/bk-1982-0180.ch014

Olin Corporation, P. O. Box 248, Charleston, T N 37310

The field of e l e c t r o c h e m i c a l science has been q u i e t l y r e v o l u t i o n i z e d during t h i s past decade by development and a p p l i c a t i o n of a new family of p e r f l u o r i n a t e d ionomer ion-exchange membrane separators i n concert with new cell designs and s t a b l e electrode systems f o r e l e c t r o s y n t h e s i s (1) ( 2 ) . These new membranes are much more than s t r u c t u r a l supports. The perfluorocarbon s t r u c t u r e s impart oxidative and h y d r o l y t i c resistance to the membrane materials while their cationic strength r e j e c t s anions. T h i s combination of unusual i o n i c character and e x c e p t i o n a l chemical resistance makes these m a t e r i a l s prime candidates f o r use as e l e c t r o l y t i c separators f o r electrosynthesis (3). In recent years, a number o f electrolytic processes have utilized membranes i n producing both anodic and cathodic products. By f a r , however, the most important a p p l i c a t i o n of t h i s t e c h nology has been i n the c h l o r - a l k a l i i n d u s t r y . Intense commercial and academic i n t e r e s t has been focused i n t o t h i s field during the past decade so that i o n exchange theory as a p p l i e d to membranes is in a more advanced s t a t e than any of the other i o n exchange systems. The primary examples of i n d u s t r i a l chlor-alkali e l e c t r o c h e m i s t r y are found in the production o f c h l o r i n e , c a u s t i c soda and potash, hydrogen and hypochlorite (1) ( 4 ) . The three general types of c h l o r - a l k a l i e l e c t r o l y z e r s i n use today are mercury, diaphragm and membrane c e l l s . Each one o f f e r s c e r t a i n advantages, and the f i r s t two have undergone many changes s i n c e t h e i r appearance i n i n d u s t r y over 80 years ago. During the past decade there has been a resurgence of i n t e r e s t i n the design and operation of c h l o r - a l k a l i c e l l s , and an e n t i r e l y new type of c e l l was invented. This new c e l l , the membrane c e l l , was developed i n response to the new p o l l u t i o n requirements and higher c a p i t a l , energy and operating costs required by the o l d e r types of c e l l s (5) (6) ( 7 ) .

0097-6156/82/0180-0311$13.50/0

©

1982 American Chemical Society

In Perfluorinated Ionomer Membranes; Eisenberg, A., et al.; ACS Symposium Series; American Chemical Society: Washington, DC, 1982.


312

PERFLUORINATED IONOMER

M E M B R A N E S

Membrane c e l l s use metal anodes and f l u o r i n a t e d ion-exchange membranes as separators to produce high q u a l i t y c a u s t i c and c h l o r i n e i n a two chambered c e l l where c h l o r i n e i s discharged from the anode i n a r e l a t i v e l y pure form, having low oxygen content and where the hydrogen i s evolved from the cathode i n pure form and separated from the c h l o r i n e by the membrane. The cathol y t e i n these c e l l s contains 20 to 38 weight percent c a u s t i c with 16 to 22 weight percent s a l t i n the a n o l y t e . The s u c c e s s f u l development of the membrane c e l l has depended on the s t r u c t u r a l i n t e g r i t y , dimensional s t a b i l i t y , low e l e c t r i c a l r e s i s t a n c e and h i g h current e f f i c i e n c y over a long operating l i f e of these new membranes. I n order to bring t h i s technology i n t o full c o m m e r c i a l i z a t i o n , i t has also been important to develop a fundamental understanding of the p h y s i c a l and chemical p r o p e r t i e s of these membrane systems i n t e r a c t i n g with simple e l e c t r o d e s i n the s o l u t i o n environments i n which they are used ( 8 ) . Membrane C h l o r - A l k a l i C e l l C h a r a c t e r i s t i c s The three types of e l e c t r o c h e m i c a l c e l l s used f o r the product i o n of c h l o r i n e gas, C I 2 , hydrogen, H2, and c a u s t i c , NaOH, ment i o n e d are unique i n d i f f e r e n t ways. Both f l o w i n g mercury cathode and diaphragm type c e l l s have undergone many developments d u r i n g the past four score years, and each o f f e r s c e r t a i n advantages. During the past decade, however, there has been a resurgence of i n t e r e s t i n c e l l s because of increased power, c a p i t a l and l a b o r costs and questions about p o l l u t i o n and product quality. Considerable changes have been made i n c e l l s i z e s and m a t e r i a l s of c o n s t r u c t i o n ; and an e n t i r e l y new type of c e l l , the membrane c e l l , was invented and commercialized (9) ( 1 0 ) . Membrane c e l l s are e l e c t r o c h e m i c a l c e l l s with metal e l e c t r o des set i n t o a two chambered c o n t a i n e r separated by a l e t t a b l e p e r f l u o r i n a t e d i o n exchange membrane. The s u c c e s s f u l development of the membrane c e l l has r e s u l t e d from the development of new and improved membranes having long l i f e and a unique property of the membrane c a l l e d c a t i o n p e r m s e l e c t i v i t y . This s e l e c t i v i t y permits the passage of p o s i t i v e l y charged sodium ions and water, while i t e s s e n t i a l l y r e j e c t s the passage i n both d i r e c t i o n s of n e g a t i v e l y charged i o n s , such as c h l o r i d e ( C l ~ ) , and hydroxyl i o n s , (OH"). The membrane c e l l ' s present success has depended on the membrane's a b i l i t y to operate e f f i c i e n t l y over an extended time period.


14.

Electrosynthesis

DOTSON A N D WOODARD

313

in Chlor-A Ikali Cells

The o v e r a l l e l e c t r o d e r e a c t i o n s f o r the diaphragm membrane c e l l s are both the same, and given as:

and

Electrical Power Na + CI" > 1/2 C l + e + N a (anode) OH" +tf*"+ > 1/2 H + OH" (cathode) +

+

2

e

NaCl + H 0

> 1/2 C l + 1/2 H

2


2

2

2

+ NaOH

as with the diaphragm c e l l , the cathode current e f f i c i e n c y depends on two f a c t o r s : the amount of hydroxyl back-migration across the separator from the c a t h o l y t e to the a n o l y t e , and the extent of water s p l i t t i n g i n the anolyte or membrane p o l a r i z a t i o n a t the brine - membrane phase boundaries depending on b r i n e c o n c e n t r a t i o n (6) ( 7 ) . Basic Membrane C e l l

Operation

A membrane c e l l i s s i m i l a r to a diaphragm c e l l except that the porous diaphragm i s replaced by a non-porous i o n exchange membrane, as shown i n F i g u r e 1. Saturated b r i n e i s f e d to the anode chamber, c h l o r i n e gas produced by o x i d a t i o n of the c h l o r i d e i o n , (CI") at the anode leaves the anode chamber as C L 8 » T h weak b r i n e leaves the anode chamber f o r r e s a t u r a t i o n . Sodium i o n s , ( N a ) , and water molecules are d r i v e n through the membrane as a flow of c u r r e n t by an imposed e l e c t r i c a l pressure and ions flow through the perms e l e c t i v e c a t i o n exchange membrane separator and then i n t o the cathode chamber. The i o n exchange membrane prevents passage of c h l o r i d e ions to the cathode chamber and hydroxyl ions to the anode chamber. Some of the water added t o the cathode chamber i s e l e c t o l y z e d at the cathode forming hydrogen gas and hydroxyl i o n s , and these hydroxyl ions combine wLth the sodium ions t o form sodium hydroxide. Performance of the conventional membrane c e l l depends on s e v e r a l operating v a r i a b l e s such as: c a u s t i c s t r e n g t h , b r i n e c o n c e n t r a t i o n , c e l l v o l t a g e , c e l l temperature, current d e n s i t y , b r i n e p u r i t y and pH. The current e f f i c i e n c y of these c e l l s shows a dependence on 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 . Most c e l l s operate from l-3KA/m w i t h anolyte strengths of 3-3.5N, (176-205 GPL) at 80-90°C and c a u s t i c products of 20-40 weight percent. The c e l l v o l t a g e increases d r a m a t i c a l l y with c a u s t i c s t r e n g t h . The ohmic drop of the membrane or i t s e l e c t r i c a l r e s i s t a n c e increases 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 and a l s o with b r i n e conc e n t r a t i o n but t o a l e s s e r extent w i t h b r i n e than c a u s t i c s t r e n g t h . The water t r a n s f e r c o e f f i c i e n t of a membrane depends d i r e c t l y on anolyte and c a t h o l y t e c o n c e n t r a t i o n s . The amount of water t r a n s f e r r e d across the membrane per mole of sodium t r a n s f e r r e d , c a l l e d the water t r a n s f e r c o e f f i c i e n t , decreases with i n c r e a s i n g as

e

2

+

2


In Perfluorinated Ionomer Membranes; Eisenberg, A., et al.; ACS Symposium Series; American Chemical Society: Washington, DC, 1982. 9

pure H 0

Figure 1. Basic operation process for membrane chlor-alkali cells.

pure b r i n e

c a t i o n exchange membrane



14.

DOTSON AND

WOODARD

Electrosynthesis in Chlor-AIkali

Cells

315

anolyte c o n c e n t r a t i o n ; however, i t changes very l i t t l e with the c a t h o l y t e concentration i n the range of 30-40% c a u s t i c . In order to keep the c a t h o l y t e c o n c e n t r a t i o n constant, the r a t e of e x t e r n a l water a d d i t i o n to the c e l l must be balanced with the r a t e of water t r a n s f e r across the membrane. The reason that the water balance i s important i s that the composition of the d e s i r e d corresponding c r y s t a l l i n e c a u s t i c hydrate, (NaOH'nl^O), formed i n the c a t h o l y t e must be maintained during c e l l o p e r a t i o n at 25,35, and 40 wt% as seen i n the f r e e z i n g point s o l u b i l i t y given i n Figure 2. In every case, the known s t r u c t u r e s of the corresponding c r y s t a l l i n e hydrates are r e t a i n e d i n concentrated solutions (1). The pH dependence of membrane c o n d u c t i v i t y i s important. The pH dependence of the membrane c o n d u c t i v i t y shows that f o r weak a c i d membranes below a c e r t a i n c r i t i c a l pH, the membrane c o n d u c t i v i t y drops d r a m a t i c a l l y . This occurs because i n the a c i d form, the membrane i s i n an u n d i s s o c i a t e d form and the p o l y e l e c t r o l y t e becomes much l e s s d i s s o c i a t e d than when i n the sodium s a l t form(19). Membrane C e l l Components Dimensionally s t a b l e e l e c t r o d e s i n t h i s system serve conductive, r i g i d , c o r r o s i o n r e s i s t a n t e l e c t r o c a t a l y s t s .

as

Anode The d i m e n s i o n a l l y s t a b l e anode i n t h i s system i s composed of an e l e c t r i c a l l y conductive substrate of t i t a n i u m , having a c o a t i n g of a defect s o l i d s o l u t i o n c o n t a i n i n g mixed c r y s t a l s of precious metal oxides. These s u b s t i t u t i o n a l s o l i d s o l u t i o n s are both e l e c t r i c a l l y conductive, e l e c t r o c a t a l y t i c , and d i m e n s i o n a l l y s t a b l e . Within the aforementioned s o l i d - s o l u t i o n host s t r u c t u r e s the valve metals i n c l u d e : t i t a n i u m , tantalum, niobium, and molybdenum; while the implanted conductive precious metal guest elements i n c l u d e : platinum, ruthenium, palladium, indium, rhodium, and osmium. There i s a c l o s e connection between the nature of the defect s o l i d and i t s c a t a l y t i c p r o p e r t i e s i n the coatings. At present, the t i t a n i u m - d i o x i d e ruthenium-dioxide s o l i d s o l u t i o n coatings are p r e f e r r e d . Cathode The cathode m a t e r i a l may be made of any conductive metal having a surface that i s capable of withstanding the c o r r o s i v e c o n d i t i o n s i n the cathode chamber of the c e l l . U s e f u l m a t e r i a l s may be s e l e c t e d from a group c o n s i s t i n g of s t a i n l e s s s t e e l , n i c k e l , s t e e l , or platinum metals with s i n t e r e d or otherwise porous coated surfaces that provide c a t a l y t i c s i t e s showing low overvoltage c h a r a c t e r i s t i c s f o r hydrogen e v o l u t i o n . The cathode may be made from f o r a m i n i f e r o u s expanded metal mesh or screen. A h i g h surface area m a t e r i a l i s desired with c o r r e c t geometrical



316


M E M B R A N E S

Figure 2. Freezing point solubilities for ice (A), ice + NaOH • 7H 0 (B), NaOH • 7H O (C), NaOH • 7H 0 + NaOH • 5H 0 (D), NaOH • 5H 0 (E), NaOH • 5H 0 + NaOH • 4H 0 (F), NaOH • 4H 0 (G, X), NaOH • 4H 0 + NaOH • 3.5H 0 (H, *), NaOH • 3.5H 0 (product) (I), NaOH • 3.5H 0 + NaOH • 2# 0 W , NflOi¥ • 2H 0 (K), NaOH • H 0 (L, M), NaOH • H 0 + NaOH (N), and NaOH(P). 2

z

2

2

2

2

2

2

2

2

2

2

2

2

2


2

14.

DOTSON AND

WOODARD

Electrosynthesis in Chlor-A Ikali Cells

317

design so as to provide good gas r e l e a s e hydrodynamics and m i n i mize e l e c t r o l y t i c r e s i s t a n c e . F a c i l e r e l e a s e of gas i s important i n the c a t h o l y t e where membrane-cathode bubble masking w i t h i n the h i g h l y viscous c a u s t i c s o l u t i o n i s found to d r a m a t i c a l l y increase voltages as claimed by U.S. Patent 4,105,514.


Membrane The separator i n the c h l o r - a l k a l i c e l l i s by f a r the most important component. I t allows the f r e e passage of e l e c t r i c a l c u r r e n t and keeps reactants and products apart by maintaining sufficient gradients between its phase boundaries. In the absence of an e l e c t r i c a l f i e l d and in dilute s o l u t i o n s , the degree of i o n i c s e l e c t i v i t y depends s o l e l y on the physico-chemical p r o p e r t i e s of the membrane, but i n the presence of a high i n t e n s i t y e l e c t r i c a l f i e l d and the r e s u l t a n t l a r g e f i e l d gradients i n concentrated s o l u t i o n s , the dynamic propert i e s of both the membrane and s o l u t i o n i n t e r a c t with the imposed electrical field to provide the anomalous perms e l e c t i v i t y observed. Membrane P r o p e r t i e s and

Processes

Membranes are c h a r a c t e r i z e d by s t r u c t u r e and f u n c t i o n ; that i s , what they are and how they perform. The most s i g n i f i c a n t primary s t r u c t u r a l p r o p e r t i e s of a membrane are i t s chemical nature; i n c l u d i n g the presence of charged species at the molecul a r l e v e l , i t s m i c r o c r y s t a l l i n e s t r u c t u r e at the m i c r o c r y s t a l l i n e l e v e l , and on the c o l l o d i a l l e v e l i t s pore s t a t i s t i c s such as pore s i z e d i s t r i b u t i o n and d e n s i t y , and degree of asymmetry (11) (12). C a t i o n exchange membranes are used i n the membrane c h l o r alkali c e l l process and must have good chemical stability. This requirement i s s a t i s f i e d by the perfluoropolymers. The types of membranes that are a v a i l a b l e f o r i n d u s t r i a l c h l o r - a l k a l i production are c l a s s i f i e d as: 1) p e r f l u o r o s u l f o n i c a c i d ; 2) perfluorosulfonamide: and 3) p e r f l u o r o c a r b o x y l i c a c i d types. Large d i f f e r e n c e s i n p e r m e a b i l i t i e s of membranes can be a t t r i b u t e d to d i f f e r e n c e s i n i n t e r c h a i n displacement and f l e x i b i l i t y r e l a t e d to p o l a r and s t e r i c e f f e c t s . The p o l a r molecules such as p o l y t e t r a f l u o r o e t h y l e n e have a stronger tendency to form r i g i d a s s o c i a t i o n s l e a d i n g to c r y s t a l formation than nonpolar molecules. Polytetrafluoroethylene polymers are highly c r y s t a l l i n e products with sharply d e f i n a b l e melting points. Oriented specimens of high s t r e n g t h may be obtained, e x a c t l y as in the crystalline condensation polymers (13). For every amorphous polymer there e x i s t s a narrow temperature r e g i o n i n which i t changes from a viscous or rubbery c o n d i t i o n a t temperatures above t h i s region, and changes to a hard and r e l a t i v e l y b r i t t l e one below i t . This transformation i s equival e n t to the s o l i d i f i c a t i o n of a l i q u i d to a g l a s s ; i t i s not n e c e s s a r i l y a phase t r a n s i t i o n (13)(14).



318

PERFLUORINATED

IONOMER

M E M B R A N E S

Not o n l y do hardness and b r i t t l e n e s s undergo r a p i d changes i n the v i c i n i t y of the g l a s s t r a n s i t i o n temperature, Tg, hut other p r o p e r t i e s such as the thermal expansion c o e f f i c i e n t , heat c a p a c i t y , and i n the case of a polar polymer the d i e l e c t r i c constant a l s o changes markedly over the i n t e r v a l of a few hundred degrees, Tg i s regarded as the b r i t t l e temperature, or the c r i t i c a l temperature f o r the g l a s s y s t a t e , or second order t r a n s i t i o n temperature, although no phase t r a n s i t i o n i s involved (14) (15). Considering phase e q u i l i b r i a i n these l i q u i d systems, when a solvent i s chosen f o r a given polymer that becomes p r o g r e s s i v e l y poorer or the c o n c e n t r a t i o n s or temperature i s lowered, event u a l l y a point i s reached below which solvent and polymer are no longer m i s c i b l e i n a l l p r o p o r t i o n s . At each lower c o n c e n t r a t i o n or temperature, mixtures of polymer and solvent over a c e r t a i n composition range w i l l separate i n t o two phases l e a d i n g t o a partitioning of species between the two phases. Membranes produced for chlor-alkali applications are g e n e r a l l y composites of two e q u i v a l e n t weights of polymer o r of one e q u i v a l e n t weight where one surface has been c h e m i c a l l y modified to change the nature of the i o n exchange grouping. These m o d i f i c a t i o n s a l t e r the dynamic p r o p e r t i e s of the membrane. I t has been found from experiments i n which NaCl and NaOH s o l u t i o n s are separated by such membranes that the side of the membrane i n contact with the NaOH s o l u t i o n dominates i n the c o n t r o l of membrane performance as found i n i t s r e s i s t a n c e and selectivity. The apparent membrane d i f f u s i o n c o e f f i c i e n t s f o r NaOH have been measured f o r such systems. D i f f u s i o n of sodium ions increases with the lower e q u i v a l e n t weights and decreased NaOH c o n c e n t r a t i o n s . Surface m o d i f i c a t i o n s can be used t o produce f i l m s with lower d i f f u s i o n rates by i n c r e a s i n g the a c t i v a t i o n energy f o r d i f f u s i o n ( 1 5 ) . The measurement and c o n t r o l of transport p r o p e r t i e s f o r i o n exchange membranes i s the key element i n o p t i m i z i n g the o p e r a t i n g c o n d i t i o n s f o r modern c h l o r - a l k a l i membrane c e l l s . Ideally, a membrane should a l l o w a l a r g e a n o l y t e - c a t h o l y t e sodium i o n f l u x under l o a d , while at the same time the hydroxide i o n and water f l u x e s are kept minimal. Under these c o n d i t i o n s , high current e f f i c i e n c y and low membrane r e s i s t a n c e can be r e a l i z e d s i m u l t a neously i n a c e l l producing concentrated c a u s t i c and c h l o r i n e gas. Water, sodium i o n , and hydroxide i o n c o n c e n t r a t i o n s have been measured w i t h i n the membrane phase as a f u n c t i o n of bulk c a u s t i c s o l u t i o n c o n c e n t r a t i o n and temperature. These i n t e r n a l membrane c o n c e n t r a t i o n s are important because of t h e i r i n f l u e n c e on the membrane polymer morphology, s t r u c t u r a l memory, p l a s t i c i t y and t h e r e s u l t a n t e f f e c t s on i t s i n t e r n a l r e s i s t a n c e , v i s c o e l a s t i c i t y and material transport. In a d d i t i o n , the s e l f - d i f f u s i o n c o e f f i c i e n t of the sodium ions i n v a r i o u s Nafion membranes has been measured as a f u n c t i o n of temperature and e x t e r n a l c a u s t i c c o n c e n t r a t i o n


14.



+

using the ^ N a r a d i o t r a c e r isotope. In t h i s d i f f u s i o n c o e f f i c i e n t i n the membranes can without the complicating problems of osmotic the r e s u l t a n t gradients i n i o n i c a c t i v i t y ever s i s membranes (15).

Cells

319

way, a true s e l f now be determined flow of water and present i n d i a l y -


D i f f u s i o n the Fundamental Process D i f f u s i o n c o e f f i c i e n t s provide two kinds of information. F i r s t , t h e i r absolute magnitudes, combined with membrane sodium i o n concentrations, are u s e f u l i n d i c a t o r s of the temperature dependence of i o n i c s e l f - d i f f u s i o n and thereby they y i e l d the a c t i v a t i o n energy f o r d i f f u s i o n . They thereby provide i n s i g h t i n t o the nature of the d i f f u s i o n mechanism (16). When a c t i v a t i o n energies are measured f o r various types of r e l a t e d membranes, the i n f l u e n c e of d i f f e r e n t membrane s t r u c t u r a l design features can thus be separated and determined d i r e c t l y . Measurements of the 120, 214 and 295 duPont Nafion f i l m s and a l s o f u l l y converted ethylene diamine f i l m s are considered to be typical. The 120 polymer i s a homogeneous f i l m 10 m i l s t h i c k of 1200 equivalent weight, (ew), p e r f l u o r o s u l f o n i c a c i d r e s i n . The 214 and 295 f i l m s are each of 7 m i l s t h i c k r e s i n of 1150 ew having one surface f a c i n g the cathode that has been chemically modified to 1.5 m i l s depth, and having T-24 backing. The 295 f i l m s are the same as the 214 except that they are modified to a 1.5 m i l depth and have T-900 backing (17) ( 1 8 ) . R e s u l t s show that the water uptake decreases and c a u s t i c conc e n t r a t i o n i s r e l a t i v e l y constant f o r these m a t e r i a l s as the c a u s t i c c o n c e n t r a t i o n of the s o l u t i o n i n c r e a s e s . The temperature dependence of these p r o p e r t i e s i s not pronounced. The s e l f d i f f u s i o n c o e f f i c i e n t of Na+ i n these membranes i s s t r o n g l y dependent on both temperature and c a u s t i c c o n c e n t r a t i o n . Below c e r t a i n temperatures, dependent on c a u s t i c c o n c e n t r a t i o n s , EDA t r e a t e d Nafion becomes impermeable to sodium i o n d i f f u s i o n . At h i g h e r temperatures, d i f f u s i o n proceeds by a d i f f e r e n t process w i t h a c t i v a t i o n energies of 7 to 12 kcal/mol depending on the separator m a t e r i a l . The a c t i v a t i o n energies are i n s e n s i t i v e to c a u s t i c c o n c e n t r a t i o n , but the absolute magnitudes of sodium i o n d i f f u s i o n c o e f f i c i e n t s are very concentration dependent. Also, d i f f e r e n c e s i n the a c t i v a t i o n energy f o r 214 and 295 Nafion" can be c o r r e l a t e d w i t h d i f f e r e n c e s i n membrane voltage drops found i n operating c e l l s . An o v e r a l l c o n c l u s i o n from t h i s work i s that the f a b r i c backing i n these m a t e r i a l s i s an important f a c t o r i n increasing the e l e c t r i c a l membrane r e s i s t a n c e (19) ( 2 0 ) . Several processes occur simultaneously w i t h i n the membrane phase of an operating c e l l . Sodium, c h l o r i d e and hydroxide ions a l l migrate under the combined e f f e c t s of c o n c e n t r a t i o n and e l e c t r i c a l p o t e n t i a l gradients with sodium ions as the major current c a r r i e r . The flow of sodium ions i n a f i e l d i s accompanied by a net e l e c t roosmo t i c flow of water i n the same d i r e c t i o n . C h l o r i d e i o n f l u x i s much s m a l l e r than that of sodium



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and hydroxide ions, since the membrane presents an e f f e c t i v e b a r r i e r to i t , and the e l e c t r i c a l p o t e n t i a l across the separator opposes the t r a n s p o r t of the c h l o r i d e . I n t e r a c t i o n s occur among c a t i o n s , anions, water and the membrane m a t r i x . The magnitude of these i n t e r a c t i o n s depends on the membrane p r o p e r t i e s as w e l l as the water and e l e c t r o l y t e s o r p t i o n , combined w i t h c a p i l l a r y transport through the t h i n - f i l m q u a s i - l a t t i c e of imbibed solution. These polymer-solvent i n t e r a c t i o n s define the o v e r a l l o p e r a t i o n a l p r o p e r t i e s of the membrane such as i t s s e l e c t i v i t y , r e s i s t a n c e and operating p r o p e r t i e s . The r e l a t i v e magnitudes of these i n t e r a c t i o n s d i f f e r from those observed i n e l e c t r o l y t e s o l u t i o n s due to the presence of f i x e d charges and polymer i n the membrane phase (19). I o n i c t r a n s p o r t through these p e r f l u o r i n a t e d ionomers i s cons i d e r e d now to be e s s e n t i a l l y a d i f f u s i o n a l process whenever no flow of current i s imposed. This d i f f u s i o n can be defined as a r a t e process with an average energy b a r r i e r f o r d i f f u s i o n that must be exceeded before transport can occur. This approach i s u s e f u l because t h i s a c t i v a t i o n energy provides a convenient indec a t o r of the minimum energy requirements f o r i o n transport through the membrane, and t h i s provides a mechanism f o r d i f f u s i o n there. Ionic d i f f u s i o n c o e f f i c i e n t s and the r e s u l t a n t a c t i v a t i o n energies are thereby r e l a t e d to the operating c h a r a c t e r i s t i c s of the membrane under current flow or load c o n d i t i o n s . A selfd i f f u s i o n c o e f f i c i e n t can be obtained without imposing conc e n t r a t i o n gradients of water and ions across the membrane, and so that i t i s an unambiguous measure of the d i f f u s i o n a b i l i t y of an i o n through a separator (18) (19) (20) (21) ( 2 2 ) . The thermodynamic d i f f u s i o n c o e f f i c i e n t , D , i s defined as: T

D=D (1 + dlny/dlnC) T

here y i s the a c t i v i t y c o e f f i c i e n t , a/C, and: D = RTU T

(1.)

(2.)

where U i s the m o b i l i t y , and: 2

D = X /2

T

(3.)

and (3.) c o n s t i t u t e s a new d e f i n i t i o n of the d i f f u s i v i t y i n terms of the mean molecular jump d i s t a n c e X , and the mean time per jump, T , and (3.) a l s o can be given as: 2

D =X k

(4.)

Here, (4.) g i v e s the d i f f u s i v i t y i n terms of molecular properties. In t h i s case, k i s the Absolute Reaction Rate constant given f o r a s o l u t i o n which i s homogeneous, i n which cond u c t i n g holes are d i s t r i b u t e d at random along with the solute molecules across the t h i n f i l m q u a s i - l a t t i c e . The s p e c i f i c rate


14.

Electrosynthesis in Chlor-A

DOTSON AND WOODARD

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321

constant from Absolute Reaction Rate Theory (A.R.R.T) i s given as: k = k'T/h (F*/F)

exp (-E /k'T)

(5.)

A

where F* and F are the p a r t i t i o n f u n c t i o n s f o r the system and E the a c t i v a t i o n energy per molecule at 0°K, and k the normal Boltzmann's constant, so that: 1

A

D=D exp(-E /RT) o

(6.)

A


Given i n terms of v i s c o s i t y , experimental r e s u l t s confirm the t h e o r e t i c a l E i n s t e i n formula r e l a t i n g the d i f f u s i o n c o e f f i c i e n t to v i s c o s i t y : 2

D = X /2

- RT/N(l/6irn r )

T

(7.)

where X i s the average molecular displacement i n time x and n the v i s c o s i t y (22) (23) and r the r a d i u s of the f i n e c a p i l l a r y across the t h i n f i l m q u a s i - l a t t i c e . D i f f u s i o n Related to Flux D i f f u s i v i t y i s defined as the F i c k ' s law c o e f f i c i e n t which i s based on an analogy w i t h other p h y s i c a l phenomena, such as heat t r a n s f e r and e l e c t r i c a l conduction. The drag on the ions and molecules being driven through a solution and producing r e s i s t a n c e to flow i s caused by the v i s c o s i t y of the medium. For d i f f u s i o n rates which are not extermely h i g h , the mean v e l o c i t y of d i f f u s i n g molecules i s p r o p o r t i o n a l to the force a c t i n g on them: 2

v (m/s) = U (m /V-s) f (V/m)

(8.)

here v i s the net v e l o c i t y of the i o n or molecule, U i s the prop o r t i o n a l i t y contant c a l l e d the m o b i l i t y , and f i s the d r i v i n g f o r c e a c t i n g on the p a r t i c l e , c a l l e d the p o t e n t i a l gradient or e l e c t r i c f i e l d strength. The c o n c e n t r a t i o n , C, times the v e l o c i t y , v, gives the f l u x , J , as: J = Cv = - RTU {dC/dx} * - D{dC/dx}

(9.)

g i v e n as F i c k ' s F i r s t law. The c o e f f i c i e n t , D, i s the diffusivity. I t i s more convenient to express the product Cv i n terms of the molecular f l u x , J , and area of s o l u t i o n transferred: 2

2

3

J(moles/m s) - -D(m /s) {dC/dx} (moles/m )(1/m)

(10.)

E x p e r i m e n t a l l y , the q u a n t i t y J i s measured by the average time rate of change of concentration per unit area. In any case, d i f f u s i v i t y depends on the concentrations e s t a b l i s h e d across the b a r r i e r f i l m s and the d i f f u s i o n coef-


322

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f i c i e n t i s very u s e f u l because i t r e l a t e s d i r e c t l y to the m o b i l i t y , U, which can be determined from d i f f u s i o n experiments(22)(23).


D i f f u s i o n through Q u a s i - L a t t i c e Films C r y s t a l s t r u c t u r e s of many simple m e t a l l i c o x i d e s , i n c l u d i n g strong a l k a l i s , are considered to c o n s i s t of hexagonal or cubic c l o s e - p a c k i n g s t r u c t u r e s of two types: 1.) Voids surrounded by f o u r oxygen i o n s , t e t r a h e d r a l v o i d s , and 2.) Voids surrounded by s i x oxygen i o n s , the o c t a h e d r a l v o i d s . In the c l o s e - packed s t r u c t u r e s there are two t e t r a h e d r a l s i t e s and one o c t a h e d r a l s i t e per oxygen i o n (24) (25). Even though most of the simple MO oxides have the h a l i d e s t r u c t u r e s where the metal ions are o c t a h e d r a l l y coordinated by the oxygen ions there are a few MO oxides where the metal ions are t e t r a h e d r a l l y coordinated (26). The a l k a l i metal oxides, L I 2 O , Na20, K 0 and Rb 0 possess the a n t i - f l u o r i t e s t r u c t u r e w i t h oxygen ions considered as close-packed and cations occupying a l l of the tetrahedral sites. The s t r u c t u r e s of a number of concentrated aqueous s o l u t i o n s have been examined by x-ray d i f f r a c t i o n by Finbak and co-workers, i n c l u d i n g n i t r i c a c i d , s u l f u r i c a c i d and sodium hydroxide. 2

2

The x-ray r a d i a l d i s t r i b u t i o n curves obtained f o r a 38 weight percent aqueous s o l u t i o n of NaOH i s i n t e r p r e t e d as i n d i c a t i n g a t e t r a h e d r a l arrangement of water molecules that surrounds the Na+-0H a t bond d i s t a n c e s of 2.03 A. At 38 weight percent c a u s t i c , a NaOH-3.6H 0 composition i s found at 536 GPL (25). In these very concentrated s o l u t i o n s , formation of Na+OH" i o n - p a i r s i s assumed to e x i s t . Even though i t may not be g e n e r a l l y agreed t h a t i t i s j u s t i f i a b l e to draw d e t a i l e d c o n c l u s i o n s about s t r u c t u r e s of i o n i c s o l u t i o n s from t h e i r x-ray s c a t t e r i n g patterns, i t i s p o s s i b l e to obtain information about the s t r u c t u r e and immed i a t e environment of c e r t a i n ions i n t h i s manner (25) (27). 2

2

E l e c t r i c a l Conductivity Whenever an e l e c t r i c f i e l d , E, i s a p p l i e d q u a s i - c r y s t a l system, such as found i n these f o r c e i s exerted on the charged p a r t i c l e s i n a an i o n or a defect has a charge, Q i , then the i o n or defect f i l m i s given as (21) ( 2 8 ) :

across a t h i n f i l m m e t a l l i c oxides, a q u a s i - c r y s t a l . If f o r c e , F i , on t h i s

F i (joules/m) = Q ( c o u l ) E (V/m) i

1

(11.)

where: coulombs = amp-sec, and joules = watt-sec=coulomb-volt This force causes a d i r e c t i o n a l transport of the charged part i c l e s i n the c r y s t a l , or q u a s i - c r y s t a l f i l m , i n a d d i t i o n to t h e i r random thermal motion. In t h i s case, Qi i s the net charge contained w i t h i n a mobile, c o l l e c t i v e Gaussian surface. Ions cross i n t e r f a c i a l boundaries such as membranes and c r e a t e a net


14.



323

t r a n s f e r of charge to produce the steady-state p o t e n t i a l surfaces observed. The r e s u l t i n g current d e n s i t y i s given by (22) ( 2 6 ) : 2

I (amps/m ) =

1

(12.)

3

(13.)

^ ( o h m ' V * ) E(V/m)

and where: 1

a^ohm-V" ) = p (amp-s/m ) U(m2/V-s)

and where a± i s the c o n d u c t i v i t y , p i s the charge d e n s i t y , and U i s the i o n m o b i l i t y of ions w i t h i n the Gaussian surface. Here a represents the t o t a l e l e c t r i c a l c o n d u c t i v i t y , the i o n i c c o n d u c t i v i t y of i o n i i s given as a± and r e l a t e d to the t o t a l c o n d u c t i v i t y through a p r o p o r t i o n a l i t y constant, t± or the t r a n s p o r t , or transference number of species i (22) ( 2 6 ) :


9

a±/a

ti =

(14.)

In an i o n i z a b l e compound of any geometry, the t o t a l e l e c t r i c a l c o n d u c t i v i t y i s given by the sum of a n i o n i c and c a t i o n i c conductivities: a = a + Cation and,

a - a(t + t ) Anion c

a

(15.)

i t follows that: t

+ t

c

- 1

a

(16.)

The current d e n s i t y of the p a r t i c l e s of type i , 1^, i s r e l a t e d to t h e i r m i g r a t i o n or d r i f t v e l o c i t y , v^, through the relationships: I

i

=

C Q v =C Z ev 1

1

1

1

1

i

= pv

A

(17.)

where C± i s the i o n i c c o n c e n t r a t i o n , Q^, the i o n i c charge and given as the product of charge e and number of charges, Z^, and i n terms of charge d e n s i t y , p, the current d e n s i t y i s : 2

1

1

I(amp/m ) = a(ohm'" m"" ) E(V/m) = 3

2

= p(amp-s/m ) U(m /V-s) E(V/m)

(18.)

here w i t h Z^ as i o n i c valence and C^ c o n c e n t r a t i o n of p a r t i c l e s , the charge m o b i l i t y U i s defined as the v e l o c i t y i n a u n i t e l e c t r i c f i e l d (23). D i f f u s i o n Related to Transport E l e c t r i c a l t r a n s p o r t through i n d u s t r i a l membranes used i n c h l o r - a l k a l i c e l l s i s not shared e q u a l l y among a l l of the mobile


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M E M B R A N E S

components w i t h i n the conductive polymer f i l m . In a l l cases, however, the sodium i o n w i t h i n the hydrated ionomeric phase of the polymeric f i l m i n the membrane i s the major current c a r r i e r as these ions move through the membrane they drag along much water with them. A q u a n t i t a t i v e d e f i n i t i o n of the transport of water molecules and sodium ions through i o n exchange membranes i s thus found to be of fundamental importance i n a l l phases of c e l l o p e r a t i o n (16) ( 2 4 ) .


Electroosmotic C o e f f i c i e n t s The e l e c t r o o s m o t i c t r a n s p o r t c o e f f i c i e n t f o r water through Nafion 295 and 1150 membranes i s t y p i c a l and i s shown to be h i g h l y dependent on the anolyte c o n c e n t r a t i o n to the e x c l u s i o n of a l l the other v a r i a b l e s s t u d i e d . The water t r a n s p o r t c o e f f i c i e n t v a r i e s almost l i n e a r l y with anolyte c o n c e n t r a t i o n from 6 to 17 molar c a u s t i c , g i v i n g 2.9 t o 0.8 moles/F, as shown i n Figure 3. The sodium i o n transport number goes through a maximum of 0.82 eq/F in the 7 to 13 molar caustic range ( 2 7 ) . The data shows that changes i n water c o n c e n t r a t i o n and a c t i v i t y across the f i l m , as c o n t r o l l e d by anolyte c o n c e n t r a t i o n , r e g u l a t e s the c l o s e range i o n i c h y d r a t i o n of the h y d r o p h y l i c macro-molecules making up the matrix of the membrane, by changing i t s phases. These phase changes induce changes i n the d i f f u s i o n mechanism f o r water molecules, sodium and hydroxide ions passing through the polymer f i l m , e s p e c i a l l y at the t h i n f i l m c a t h o l y t e interface. When one views the membrane as a m u l t i - l a y e r t h i n f i l m d e v i c e , he begins to understand how the interphases c o n t r o l i t s e l e c t r o p h y s i c a l p r o p e r t i e s and the r e a c t i o n rates across i t s j u n c t i o n s (18) ( 2 9 ) . Voltage

Profiles

Membrane voltages are p l o t t e d versus current d e n s i t y f o r Nafion 120 and 295 f i l m s and given i n Figures 4 and 5. The curve i n Figure 4 shows a greater slope change, dE/dl, f o r the Nafion 120 than f o r the Nafion 295. The e s s e n t i a l d i f f e r e n c e between these membranes i s the degree of h y d r a t i o n of the cathod i c surface of the separator f i l m s . The Nafion 120 membrane m a t e r i a l o v e r a l l contains 30% water, and t h i s i s released i n the presence of high concentrations of c a u s t i c or s a l t , thereby forming dynamic t h i n - f i l m l a t t i c e b a r r i e r l a y e r s . The l a r g e v a r i a t i o n i n t h i s slope i m p l i e s existence of a s e l e c t i v i t y that i s s t r o n g l y dependent on c a u s t i c c o n c e n t r a t i o n , while the Nafion 295 membrane contains a t h i n chemically modified l a y e r of ethylenediamine, having 10-15% water (27). Sodium Transport The t r a n s p o r t p r o p e r t i e s of the Nafion

membranes can best be



DOTSON AND WOODARD

Cells

325


14.

NaOH, % b y W e i g h t Figure 3. Water electro-osmotic coefficient vs. anolyte concentration for Nafion 295 membrane, 80° C, 2 kA/m . Key: Q, measurements with new cell design, identical anolyte/catholytes; •, chlorate present in anolyte; A, measurements with old cell design, identical catholyte/anolytes. 2



M E M B R A N E S


326

Figure 4. Membrane cell voltage profile for Nafion 120 membrane. Key: • * 2 5 % NaOH; Q, 32% NaOH; A, 33% NaOH; O, 35% NaOH. Conditions: 85-90° C, 18-24% NaCl Anolyte DSA anode, Ni cathode, Vs inch electrode-membrane gap.



Electrosynthesis in Chlor-Alkali

Cells


14.

Figure 5. Membrane cell voltage profile for Nafion 295 membrane. Key: • , 2 2 % NaOH; O, 35% NaOH; A, 39% NaOH. Conditions: 85-90° C, 22-24% NaCl anolyte DSA anode, Ni cathode, Vs inch electrode-membrane gap.


327


328


M E M B R A N E S

explained based on the f i n e - c a p i l l a r y - p o r e membrane model. The membrane appears to be composed of separate phases formed as separate three-dimensional s t a t i s t i c a l networks generating phase boundary junctions at the membrane pore s o l u t i o n interface. Few s t u d i e s have been made on transport processes i n v o l v i n g concentrated s o l u t i o n s . In the concentrated s o l u t i o n s , i n the range of dehydrated melt formation, incompletely hydrated melts and anhydrous salt melts, various structural models are described to define t h e i r p r o p e r t i e s , i . e . the free-volume model, the l a t t i c e - m o d e l and the quasi-crystalline model. Measured and c a l c u l a t e d t r a n s p o r t phenomena do not always represent simple i o n m i g r a t i o n of i n d i v i d u a l p a r t i c l e s , but i n s t e a d we sometimes f i n d them to be complicated cooperative e f f e c t s (27). At v e r y high c o n c e n t r a t i o n s of the i o n i c s o l u t i o n s , the q u a s i l a t t i c e model of Braunstein i s u s e f u l wherein a h i g h l y conc e n t r a t e d e l e c t r o l y t e s o l u t i o n i s considered to be a s o l u t i o n of water i n fused s a l t s (24) (26). The data presented i n F i g u r e 6 g i v e s a t y p i c a l d e s c r i p t i o n of the t r a n s p o r t p r o p e r t i e s of Nafion membranes w i t h i n the framework of o p e r a t i n g data f o r a c h l o r - a l k a l i or a water e l e c t r o l y z e r system. In a l l c a s e s , one f i n d s b e t t e r s e l e c t i v i t y f o r d i s c r i m i n a t i n g against the back-flow of h y i r o x y l ions at or near the 33-38% c a u s t i c c o n c e n t r a t i o n . This gives a higher sodium to hydroxide t r a n s p o r t number r a t i o . Since we know that i n each case, the known s t r u c t u r e s of the corresponding crystalline hydrates are r e t a i n e d i n concentrated s o l u t i o n s , then they may induce c a t i o n p a r t i a l l a t t i c e s w i t h i n the membrane phase which i n t u r n provides t h i n f i l m anion p a r t i a l l a t t i c e s i n the 35-36% c a u s t i c ranges. Whenever the concentrations go above or below t h i s maximum i n the curve, the short-range t r a n s l a t i o n a l motion of i o n s , exemplified by a c t i v a t e d jumps from one e q u i l i b r i u m p o s i t i o n t o the other, has great s i g n i f i c a n c e i n r e l a t i o n to the k i n e t i c t r a n s p o r t p r o p e r t i e s of aqueous e l e c t r o l y t e s o l u t i o n s i n the space charge region of these thin film separators. The data on e l e c t r i c a l conductance of aqueous s a l t s o l u t i o n s are o f great i n t e r e s t f o r r e l a t i n g s t r u c t u r a l changes i n the e l e c t r o l y t e s o l u t i o n s t o the degree of s w e l l i n g and v a r i a b l e pore s t r u c t u r e s . If the s a l t under c o n s i d e r a t i o n forms a c r y s t a l l i n e hydrate, then f o r an isotherm not too f a r from the melting point of the hydrate a maximum i n conductance occurs c l o s e to the m e l t i n g point o f the hydrate e u t e c t i c composition. The e x i s t e n c e of these maxima i s due t o the f a c t t h a t t h i s v i s c o s i t y i n c r e a s e s r a p i d l y a t c e r t a i n concentrations causing the hydroxyl i o n m o b i l i t y t o decrease 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 s o l u t i o n q u a s i - l a t t i c e which forms i n the pores. These c o n d i t i o n s forming the maxima become imposed on the phase s t r u c t u r e of the membrane and correspond to s t r u c t u r a l transformations w i t h i n the s o l u t i o n s as shown(22)(24): TU/TI

=

l + A-fcP ion-ion term

+

B c ion-solvent term


(19.)




14.


329

330

PERFLUORINATED

IONOMER

MEMBRANES

Where no i s the s o l v e n t v i s c o s i t y and n s o l u t i o n v i s c o s i t y , C i s the bulk c o n c e n t r a t i o n and A i s a constant dependent on i n t e r i o n i c a t t r a c t i o n and B a constant dependent on i o n - s o l v e n t i n t e r a c t i o n and i s a f u n c t i o n of the i o n i c m o b i l i t y . The c o n c e n t r a t i o n of non-exchange e l e c t r o l y t e i n the pore system of a membrane i s determined by a d i s t r i b u t i o n e q u i l i b r i u m , dependent on the width of the pore. The higher the c o n c e n t r a t i o n of the outside s o l u t i o n , the g r e a t e r i s the concentration of non-exchange e l e c t r o l y t e i n the pore system of a p o o r l y hydrated i o n i c membrane f i l m .


Reactor Design E l e c t r o c h e m i c a l r e a c t o r d e s i g n i s i d e a l l y a good compromise between c a p i t a l and power c o s t s . The power consumption of a c e l l or r e a c t o r i s the most important s i n g l e f a c t o r needed to evaluate i t s performance. Both the power production and chemical process i n d u s t r i e s , ( C P I ) , i n v o l v e heat and e l e c t r i c a l energy i n a s i m i l a r fundamental way, and so are governed by the second law of thermodynamics. The second law a c t u a l l y imposes an absolute n a t u r a l l i m i t a t i o n on the e f f i c i e n c y of any energy t r a n s f o r mation, and t h e r e f o r e i t p r o v i d e s a r e l i a b l e standard w i t h which to compare and c o n t r o l p r a c t i c a l operations (30) (31) ( 3 2 ) . Power Costs Power c o s t s f o r aqueous c h l o r - a l k a l i c e l l s amount to about 50% of the t o t a l operating c o s t s and almost 75% f o r water e l e c t r o l y s i s . Molten s a l t sodium and aluminum processes are even more power i n t e n s i v e than c h l o r - a l k a l i c e l l s . Energy E f f i c i e n c y Power consumption i s thus seen t o be the primary c r i t e r i o n of o v e r a l l c e l l performance, where the energy consumption per mole of product, i s given (33): Wj = ( E I t ) / N j = watts/mole

(20.)

where i s the moles of product, E the t o t a l c e l l p o t e n t i a l , I i s the amperage, and t the time of the current f l o w , so that the minimum e l e c t r i c a l energy expended f o r the o v e r a l l process i n terms of watts (30): W(watts) = N^(moles)Wj(watts/mole)

jAGj=Minimum e l e c t r i c a l energy expended i n the process. (21.)

From thermodynamics, a t constant temperature we r e c a l l that the AG.j =* AH.j - TASj relationship exists,


(22.)



14.

Cells

331

where AGj i s the free energy a v a i l a b l e f o r a r e v e r s i b l e process, AHj the t o t a l enthalpy change and ASj the entropy change, so that W

j

+

N

j

T

A

S

j

=

and 23.) i s obtained that:

N

j

A

H

j

^

a f t e r s u b s i t u t i n g 20.),

^

21.) and 22.), so

E l j t = NjAHj + Q


2 3 -

(24.)

where the energy balance f o r the c o n d i t i o n E>E° e x i s t s , and Q i s the q u a n t i t y of heat removed from the reactor at constant temperature, NjTASj, so that the t o t a l e l e c t r i c a l pressure or voltage imposea across the electrodes i n the reactor during the process shown i n Figure 7, i s given as (30) (34): E = (NjAHj-K))/Ijt

(25.)

The general basis f o r c a l c u l a t i n g the power usage i n e l e c t r o c h e m i c a l processes i s the o v e r a l l energy e f f i c i e n c y : E.E.

= V.E. X C.E.

(26.)

where E.E. i s the t o t a l energy e f f i c i e n c y , and V.E. the voltage e f f i c i e n c y f o r the given process. The voltage e f f i c i e n c y p o r t i o n of equation 26.) deals with the e l e c t r i c a l pressure e f f e c t s imposed across the e l e c t r o d e s , and i s c a l c u l a t e d by: V.E.

= (E°/E) x 100

(27.)

with E the a c t u a l bus to bus c e l l voltage and E° the t h e r modynamic r e v e r s i b l e c e l l p o t e n t i a l . The current e f f i c i e n c y port i o n of the o v e r a l l energy e f f i c i e n c y i n 26.) above deals with the flow of current and i s c a l c u l a t e d : C.E.

= Eq.(Produced)/ nF(Passed)

(28.)

where Eq.(Produced) i s the equivalents of product produced and nF(passed) i s the t o t a l Faradays of charge passed (34). Voltage Balance The t o t a l c e l l p o t e n t i a l E f o r r e a c t i o n can be resolved i n t o the r e s p e c t i v e c e l l components, as given i n Table 1., as f o l l o w s (35) (36): E

=

E° + n'a + n

f

+ E (internal) c

I R

+

E (external) I R

(29.)

As seen i n Table 1., as above, I i s the t o t a l net current f l o w , R the e x t e r n a l c i r c u i t r e s i s t a n c e and R^ the i n t e r n a l c e l l e x

n


PERFLUORINATED

332

IONOMER

M E M B R A N E S


r e a c t o r volume element

electrode electrode

energy input EIT

energy out t o products

1_—.

energy accumulates and c o l l e c t s here i n the element

Qc

Q

P

energy disappears or d i s s i p a t e s here i n the element

Figure 7. Energy balance for a reactor volume element.



r

1

( C a u s t i c p l u s Bubble Resistance-Internal)

IRC^

IRC/

IRC«;

E

E

e

B

a

s

e

Plate

E

(Separator Resistance-Internal) C e l l Voltage-TOTAL

Resistance-External)

(

(Cathode C o n t a c t to B a s e - E x t e r n a l )

A n o (

IRC2 ( * Contact t o Base-External)

R

3

8

5

0.520 '

0.060

-

0.080

-

4.79

0.06

0.05

0.09

-

0.35 (3mm)

4.58

0.06

0.05

0.09

0.30 (2mm)

0.650 4.17

0.040

0.080

0.160

0.29

0.150 0.495

0.12 0.85

0.18 0.85

0.160 0.350

(Anode O v e r p o t e n t i a l ) (Cathode O v e r p o t e n t i a l )

0.390

1.359 0.950

1.37 1.74

1.47 1.74

2

Membrane C e l l 2 KA/m 37% C a u s t i c (427 Membrane)

1.359 0.930

E

E

J

2

Mercury C e l l a t 10 KA/m 50% C a u s t i c Metal Graphite Anodes Anodes

Cells

(Anode Decomposition) (Cathode Decomposition)

Component

2

Diaphragm C e l l a t 2.3 KA/m 12%NaCl/16%NaOH

T y p i c a l V o l t a g e Breakdown Diaphragm, Membrane and Mercury

I R B ( B r i n e Gap p l u s Bubble Resistance-Internal)

E

E

f:n'c

n'a

E°a E°c

Voltage

for

TABLE I


334


M E M B R A N E S

r e s i s t a n c e . The i n t e r n a l r e s i s t a n c e i s made up of r e s i s t a n c e i n separator and e l e c t r o l y t e s . The anode and cathode over potent i a l s are n and n r e s p e c t i v e l y . Table 1 gives the t y p i c a l v o l t a g e breakdown f o r diaphragm, membrane and mercury c e l l s , w i t h E° the t o t a l decomposition p o t e n t i a l determined from e q u i l i b r i u m thermodynamics• The e l e c t r o d e o v e r p o t e n t i a l has s e v e r a l components p o s s i b l e : a

c

n

?

-

l±r\i

Total t


These components of n^ are: t r a n s i t i o n ; c o n c e n t r a t i o n ; d i f f u s i o n ; r e a c t i o n ; c r y s t a l l i z a t i o n and r e s i s t a n c e . Process Development E l e c t r o c h e m i c a l processes are now becoming important in modern technology. There are c e r t a i n basic fundamental design f a c t o r s to consider f o r any e l e c t r o l y t i c f a c i l i t y . These are comprised of four u n i t s : c e l l feed preparation, e l e c t r o l y t i c c e l l r e a c t i o n s , e l e c t r i c a l power s u p p l i e s and f i n a l l y product recovery equipment. The technology and costs f o r c e l l feed prep a r a t i o n and product recovery equipment are w e l l e s t a b l i s h e d i n process engineering and not considered f u r t h e r here. The focus of t h i s s e c t i o n i s the e l e c t r o c h e m i c a l r e a c t o r . Reactor Design A r e a c t o r of any type must be optimized i n operating p e r f o r mance f o r best y i e l d s of products, and the minimum power and reactant consumption f o r any system can be evaluated using the b a s i c law of transport as shown i n Figure 7, and the f o l l o w i n g equation (40):

Mass+ Energy + Momentum input

Mass + Energy + _ Momentum output

Generation or Depletion

External Influences

Rate of Accumulation

Critical

(31.)

Parameters

The design of e l e c t r o c h e m i c a l r e a c t o r s impacts c a p i t a l and prod u c t i o n costs very s i g n i f i c a n t l y . Much e f f o r t has thus been expended i n the e l e c t r o c h e m i c a l process i n d u s t r y during the past two decades toward reducing power consumption i n order to meet prod u c t i o n goals w i t h much more expensive power and raw m a t e r i a l c o s t s .



14.

Electrosynthesis



335

E l e c t r o c h e m i c a l processes are by t h e i r very nature more s p e c i f i c , but q u i t e c a p i t a l i n t e n s i v e and thus consume l a r g e amounts of energy i n i t s most valuable form as e l e c t r i c current (7)(34)(40). The e s s e n t i a l task of the e l e c t r o c h e m i c a l engineer deals with the process o p t i m i z a t i o n , that i s the idea of d e f i n i n g the best economics i n terms of 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 . The removal of r e a c t i o n products i s the second f a c t o r that the engineer must consider i n order t o get the r e a c t o r scaled-up to the next pre-pilot stage of development successfully. Product r e c y c l i n g must be examined at t h i s stage a f t e r a s u i t a b l e r e a c t o r system i s s e l e c t e d , designed and proven (28)(30)(34). Reactor scale-up i s an extremely important step a f t e r benchs c a l e s t u d i e s have been conducted. In design development, reactants are charged i n t o the batchtype r e a c t o r one at a time a t the beginning of e l e c t r o l y s i s while products are removed at the end of the run. A continuous flow system can be evaluated as a n a t u r a l extension of the batch system. C r i t i c a l design parameters f o r these systems can be cast i n t o i d e a l i z e d , q u a n t i t a t i v e design equations i n order to d e f i n e such f a c t o r s as r e a c t o r volume-flow, e l e c t r o d e o v e r p o t e n t i a l , and hold-up time, as functions of reactor design (40). We r e a l i z e that whenever the o v e r p o t e n t i a l on an e l e c t r o d e i s g r e a t e r than ~60 m i l l i v o l t s , the reverse r e a c t i o n can be neglected and the k i n e t i c equation can be s i m p l i f i e d as f o l l o w s : I = QA { k C f

- k C } s

0

r

= n F A { k C f

0

OA k C

r

exp ( -

f

a

G

=

n Fnj /RT)}

(32.)

Thus, the behavior at d i f f u s i o n l i m i t e d current flow cond i t i o n s could be most simply represented as a f u n c t i o n of the bulk c o n c e n t r a t i o n , C , and given as ( 5 ) : G

W

= nFC (fr.)

(33.)

0

where ( f r . ) i s the flow rate through the e l e c t r o c h e m i c a l rate: r = I/(nFA) (electrochem)

= i / D

the r e a c t o r , and r ] _ here

nF { exp (-ann-J/RT)}

e

(34.)

Considering the s e c t i o n of a r e a c t o r as shown i n Figure 8, apart from i t s v e r t i c a l o r i e n t a t i o n of the e l e c t r o d e s , the only other departure from previous nomenclature i s the i n t r o d u c t i o n of terms l and l which represent the thicknesses of the anode and cathode r e s p e c t i v e l y . The e l e c t r i c a l connections are l o c a t e d at the tops of the e l e c t r o d e s which are designated by the x = 0 p o s i t ions. a

c



M E M B R A N E S


336

Figure 8. Design parameters for single-compartment parallel plate reactor (membrane chlor-alkali cell) with slow gas evolution in two dimensions.


14.

DOTSON AND

Elcctrosynthesis in Chlor-AIkali

WOODARD

Cells

337

The passage of current through an element of the e l e c t r o l y t e having dimensions Bl.,dx, i s seen i n Figure 8. If we assume that no current flows through the metal i n the z d i r e c t i o n , then cond u c t i o n i s one-dimensional only and takes place i n the x d i r e c tion. The metal current I , at a d i s t a n c e x from the top of the e l e c t r o d e i s , from Ohm's law (30)(38): a

x

=

a

- Bl a (d /dx) a

a

a

(35.)

y

where $ i s the p o t e n t i a l and a i s i t s s p e c i f i c conductance. A t o t a l voltage balance over any h o r i z o n t a l s e c t i o n must now i n c l u d e the e x t r a p o t e n t i a l c o n t r i b u t i o n s at anode and cathode due to mass and charge t r a n s f e r l i m i t a t i o n s , A$ and A

w w

je

m

ο

δ

o

m

H

>

S

2

ο

r

W

Ο

14.



Cells

341

the process and the concentrations of the m u l t i v a l e n t c a t i o n s such as the calcium, magnesium, Iron and alumlmum Ions have to be maintained as low as p o s s i b l e In the a n o l y t e . For t h i s purpose, the a d d i t i o n a l treatment of c o n v e n t i o n a l l y t r e a t e d b r i n e can be c a r r i e d out by an Ion exchange r e s i n to remove such m u l t i v a l e n t c a t i o n s , or complexed with b r i n e a d d i t i v e s . However, the Investment cost f o r the Ion exchange column i s r e l a t i v e l y l e s s due to the low c o n c e n t r a t i o n of such Ions In the c o n v e n t i o n a l l y p u r i f i e d brine. Brine P u r i f i c a t i o n - Ion Exchange


The

Ion Exchange Process

Whenever i o n exchange r e s i n s are regarded as Insoluble acids , bases or s a l t s , the process of Ion exchange can be regarded as salt formation and displacement. The I n t e r i o r of a waterswollen, strong a c i d c a t i o n exchange r e s i n may be regarded as a concentrated a c i d s o l u t i o n . Ion exchange i s an e q u i l i b r i u m process, and the exchange r e a c t i o n I n v o l v i n g two c a t i o n s , Na and Ca"*""*" can be w r i t t e n : +

2 R~Na~

+ Ca** t R

2

Ca** + 2

The f i n a l p o s i t i o n of the e q u i l i b r i u m Is then given by the values of the e q u i l i b r i u m constant and the concentrations of r e a c t i n g s p e c i e s (41). S e v e r a l new kinds of ion exchangers have been developed i n recent years that give more s p e c i f i c and s e l e c t i v e removal of d i v a l e n t brine i m p u r i t i e s such as calcium. One such r e s i n was developed by Dow, Rohm and Haas and Mitsubishi, as a c r o s s l i n k e d styrene - divlnylbenzene copolymer having iminodiacet a t e groups f o r j o i n i n g f i x e d f u n c t i o n a l group s i t e s to the metals by a c h e l a t e bond, (42) as: (CH2-CH-) —>

+

n

φ

—

CH C0 Na" 2

i 2

++ +M

\

+

CH C0 Na 2

2

2

φ

2

/

CH-N :

~(CH -ÇH-)—— CH C0 2

I

/

CH 2

2

+

\+* >-

N

\

M

+

/ CH C0 2

2


2Na

342

PERFLUORINATED

IONOMER

M E M B R A N E S

The c h e l a t e r e s i n s e l e c t i v i t y f o r the heavy metals i s s i m i l a r t o EDTA as i t attaches p r e f e r e n t i a l l y to b i v a l e n t metal ions in the presence of monovalent metal ions. R e l a t i o n s between pH and c h e l a t i n g a b i l i t y of various metal ions are given i n Figure 10. These p l o t s show that t h i s par t i c u l a r r e s i n has a maximum r a t e f o r c h e l a t i o n above 2-5 f o r the b i v a l e n t metal i o n s , but depends s t r o n g l y on the metal i o n being chelated. Care must be taken that the metal ions not be p r e c i p i t a t e d as hydroxides and thereby increase metal i o n leakage i n the column e f f l u e n t . The general s e l e c t i v i t y f o r d i v a l e n t and mono v a l e n t metal ions i s :


Hg>Cu>Pb>Ni>Cd>Zn>Co>Mn>Ca>Mg>Ba>Sr»Na The operating c y c l e of such a column i s : 1.) Removal; 2.) Backwash; 3.) Regenerate; 4.) Wash; 5.) NaOH treatment; 6.) Wash t o c l e a n e f f l u e n t . If the hardness i s not t o be removed from the b r i n e system, then i t must be sequestered by a d d i t i v e s . Brine

Additives

The build-up of Ca(0H) o r Mg(0H) a t the anolyte i n t e r f a c e of the membrane-brine system can be prevented by a d d i t i o n of c e r tain sequestering-gelling agents into the b r i n e such as phosphoric a c i d or phosphate s a l t s (43). A s t r o n g l y hydrogen bonded non-stoichiometric calcium οrthophosphate g e l i s formed at pH's greater than 5 to sequester the d i v a l e n t ions at the membrane-brine i n t e r f a c e , but the g e l d i s s o l v e s a t pHs of 2-3.5. The pH s e n s i t i v e r e v e r s i b l e nature o f the phosphate g e l p r o vides a continuously renewable surface f o r the entrapment of d i v a l e n t i m p u r i t i e s moving toward the membrane from the brine during operation and i t eliminates the need f o r expensive i o n exchange equipment f o r p u r i f y i n g the b r i n e . C e r t a i n membranes are more s u s c e p t i b l e to damage by these i m p u r i t i e s than o t h e r s . The membrane s t r u c t u r e thus determines the s e n s i t i v i t y of the f i l m s to contamination as well as i t s s e l e c t i v i t y and a b i l i t y to suppress free e l e c t r o l y t e d i f f u s i o n (44). Since the o r i g i n a l development of low e l e c t r i c a l r e s i s t a n c e membranes by Walter Juda i n the 1950 s, (45), many other commer c i a l membranes have been developed. 2

2

f

Commercial Membranes The e a r l i e s t commercial membranes tested f o r use i n c h l o r a l k a l i c e l l s were composed o f ionomeric polymers having hydrocar bon backbones with attached carboxyl and sulfonate f u n c t i o n a l groups such as the polystyrene s u l f o n i c o r carboxylate m a t e r i a l s , (46), (47), (48), ( 4 9 ) .


WOODARD


Cells


DOTSON A N D


343


344

M E M B R A N E S

The f i r s t s u c c e s s f u l c h l o r - a l k a l i c e l l membrane m a t e r i a l s (9)(10) t e s t e d i n the 1970's were adopted from p e r f l u r o s u l f o n i c a c i d m a t e r i a l s produced by duPont f o r GE f o r t h e i r f u e l c e l l program i n the 1960 s. These m a t e r i a l s had no hydrocarbon linkage i n t h e i r backbone and thus could withstand the a t t a c k of the s t r o n g l y o x i d i z i n g a n o l y t e ( l l ) . Remarkable advances i n i o n exchange membranes have been made s i n c e t h e i r i n c e p t i o n and a p p l i c a t i o n to c h l o r - a l k a l i c e l l s i n the 1970's, and since that time many patents have issued on t h e i r applications. Several companies besides duPont have developed proprietary membranes and electrolyzers for commercial application. f


duPont Membranes duPont has been a c t i v e i n developing new membranes over the past two decades and has made continuous improvements i n t h e i r m a t e r i a l s with reference to power e f f i c i e n c y and d u r a b i l i t y . F i g u r e 12 shows the e a r l i e r standard Nafion 120, 295 and the newer Nafions plotted as a function of time on line. The Nafion r e s i n i s a copolymer of t e t r a f l u o r o e t h y l e n e with perfluorosulfonyl-ethoxypropylvinyl ether, which i s converted from s u l f o n y l f l u o r i d e t o s u l f o n i c a c i d form: —

(CF CF ) -(CF -ÇF) 0 ÇF CF -CF-0CF CF S0 H 2

2

x

2

y

2

3

2

2

3

The membranes s u p p l i e d by duPont have equivalent weights ranging from 1,100 to 1,500 meq/g with thickness of 5 to 10 mils, (2)(8). Mold processing of these r e s i n s i s c a r r i e d out o n l y i n the s u l f o n y l c h l o r i d e form because i t i s t h e r m o p l a s t i c i n t h i s case (50) (51). The chemical r e a c t i o n s reported by duPont appear to i n v o l v e the r e a c t i o n of perfluoropropylene oxide and 3 - sultones to form sulfonic acid resins. The f u n c t i o n a l group monomers are generated as f o l l o w s (52): 2 C F 3 - CF0CF + C F C F S 0 2

2

2

2

—~—> Press

OHCCF(CF3)OCF CF(CF )OCF CF S0 F 2

3

2

2

—>

2

Na C0 2

CF =CFOCF CF(CF )OCF CF S0 F + C0 2

2

3

2

2

2

2

3

+ 2NaF




14.

and then polymerized m - 1 t o η = 3-15:

with

t e t r a f l u o r o e t h y l e n e i n the

345

ratio

of

n ( C F C F ) + m(CF CF) OCF (CF ) OCF CF S0 F + 2

2

2

2

3

2

2

2

^(CF CF ) ~(CF CF) — I OCF CF(CF )OCF CF S0 F 2

2

n

2

2


—

ΚΟΗ,Δ

m

3

2

2

2

( C F C F ) — (CF -CF) ~ I OCF CF(CF ) O C F CF SO K+ 2

2

2

m

2

3

2

2

3

The Nafion membranes are produced i n t h i s way and with a f a b r i c backing such as PTFE or mixed PTFE rayon f a b r i c s . These supporting m a t e r i a l s improve the mechanical s t r e n g t h of the f i l m and keep the dimensional changes i n bounds. In general, f o r c h l o r - a l k a l i e l e c t r o l y s i s , the side of the membrane with the h i g h e s t r e s i s t a n c e , s e l e c t i v i t y and charge d e n s i t y i s p r e f e r r e d toward the cathode s i d e t o l i m i t the undesirable e f f e c t s of the back f l o w of hydroxide ions i n t o the anode chamber. The anolyte s i d e of the membrane polymer i s thus l e s s dense, l e s s s e l e c t i v e and more conductive than the c a t h o l y t e s i d e of the separator f i l m . The newer membranes provided by duPont have improved p e r f o r mance and Figure 11 shows the r e l a t i v e degradation rates f o r these m a t e r i a l s used by GE i n t h e i r f u e l c e l l s , (53). Asahi Chemical Membranes The Asahi Chemical Company of Japan has developed a p e r f l u o r o c a r b o x y l i c a c i d membrane (54) (55) (56). I t i s reported t o be formed from Nafion f i l m s wherein the S0 H groups on the cathode surface are s p l i t o f f and the adjacent CF groups t h e r e a f t e r o x i d i z e d to c a r b o x y l i c a c i d groups. 3

2

ϋύ —

(CF CF ) —(CF^F)^— ^ OCF CF(CF )OCF CF S0 H 2

2

n

2

3

(CF CF ) 2

2

m

2

2

3

(CF CF)Ji 6-CF CF(CF )OCF C0 H 2

2

3

2

2

These membranes are reported to achieve 93% cathode current e f f i c i e n c y at 21.6% c a u s t i c concentrations from the e l e c t r o l y s i s process.



346

PERFLUORINATED

IONOMER

M E M B R A N E S

R e c i p r o c a l Temperature 4

lxl0 /T(K) Figure 11. Degradation rates of perfluorinated sulfonic acid membranes.


14.


Electrosynthesis


347


1 •*»»

§ «Ο OS

Ο

^

Q

S!

l i

s:

ο v.

fi ν» 3

£


348


M E M B R A N E S

Asahi Glass Co., Membranes


A s a h i Glass Co. has r e c e n t l y d i s c l o s e d t h e i r own p e r f l u o r o carbon membrane f i l m s f o r use i n c h l o r - a l k a l i c e l l e l e c t r o l y s i s . These membrane polymers have high molecular weights to impart strong mechanical p r o p e r t i e s , and a t i g h t s t r u c t u r e t o supress unselective diffusion and r e s i s t a n c e t o s w e l l i n g . The perfluorinated s t r u c t u r e imparts stability against c h l o r i n e and strong c a u s t i c soda a t high temperatures, c o p o l y merization of t e t r a f l u o r o e t h y l e n e w i t h molecules such as perfluoro-ô-butyrolactone and hexafluoropropylene oxide as:

CF3CFOCF2 + C F C F C F 2 C 0 2 2

2

CH3OH +

Diglime 0-10°C Na2CC>3 + Δ

OHCCF(CF3)OCF2CF CF C02CH3 2

2

2N F + C 0 + CF2CFOCF2CF2CF2CO2CH3 a

2

next they polymerize w i t h t e t r a f l u o r o e t h y l e n e and a l a r g e monomer molecule (57) (58) (59) (60) (61) ( 6 2 ) . AIBN n(CF CF ) 2

+ m(CF CF)

2

+ q(CF CF)

2

2

3 C0 CH 2

3

m »

m

ο ο

Η m α

2 2 >

Ο

G

r

m

14.




3000

1.0

1.5

2.0

2.5

3.0

Anodic Current Density KA/SQ Meter Figure 18. Expected membrane performance. Level 1: performance previously demonstrated during long term commercial plant operation. Level 2: performance demonstrated in laboratory cells. Membranes in commercial cells are expected to achieve this level of performance in the future.


357

PERFLUORINATED

IONOMER

M E M B R A N E S


358

Figure 19. Principle of the Hooker-Uhde cell.


14.


Electrosynthesis in Chlor-A

Ikali Cells

359

c i t y f o r the membrane c e l l process i s p r o j e c t e d t o be approximat e l y 600,000 m e t r i c tons o f NaOH per year. The i n s t a l l e d capac i t y by e l e c t r o l y z e r system i s shown: World Membrane E l e c t r o l y z e r C a p a c i t y


E l e c t r o l y z e r System Asahi Chemical Co. Diamond Shamrock Co. Hooker Asahi Glass Co. Tokuyama Soda Ionics

(66)

Estimated 1982 Installed Capacity *470,000 MT/yr NaOH 41,000 34,000 20,000 10,000 10,600 *Assumes AKZO w i l l

be i n s t a l l e d .

Membrane c e l l p l a n t s show considerable c a p i t a l cost advantages against diaphragm cell plants f o r a l l capacities. Investment costs f o r a membrane plant a r e 15-20% lower than the diaphragm c e l l p l a n t s . Production cost comparison shows that the membrane c e l l p l a n t has a 10-15% lower net production cost per ton NaOH (72) ( 7 9 ) . Membrane c h l o r - a l k a l i c e l l s represent a very s u c c e s s f u l , commercially v e r i f i e d , economically competitive technology with a short h i s t r o y of v e r y r a p i d l y advancing technology. Membrane Developments The Nafion membranes u t i l i z e d i n the e a r l y 1970*8 produced c a u s t i c soda concentrations o f 10-15wt% a t e l e c t r o l y t i c power consumptions o f approximately 3450 KWH/MT NaOH. Advancements i n the technology of membranes by duPont, Asahi Glass Co., and Asahi Chemical Co., Tokuyama Soda Co., have achieved membranes that today can produce c a u s t i c soda concentrations of 28-40wt% with c a u s t i c current e f f i c i e n c y w e l l over 90% f o r long term operations. A s a h i Glass Co., has announced the improvement of i t s Flemion membrane, Flemion 723, which reduces e l e c t r o l y s i s power from 2500 to 2300 KWH/M Ton NaOH, operating a t 35% NaOH and 2.0 KA/M current density. C e l l voltage i s 3.23 v o l t s at 2 KA/M with 94% c u r r e n t e f f i c i e n c y (75). DuPont has r e c e n t l y announced the development o f a new high performance c h l o r - a l k a l i membrane Nafion 901X. Caustic soda i s produced a t 33 wt% with over 94% current e f f i c i e n c y . The Nafion 901X i s capable o f operating a t minimum voltage and high current e f f i c i e n c y f o r extended periods estimated t o be i n excess o f two years (76). 2

2


360


M E M B R A N E S

F u r t h e r r a p i d developments of the membrane technology a r e t o be expected, which are c e r t a i n t o f u r t h e r decrease e l e c t r o l y t i c power requirements. Membrane E l e c t r o l y z e r Developments Advancement of the membrane c e l l technology by Oronzo De Nora Impianti E l e t t r o c h i m i c i S.p.A., u t i l i z i n g the s o l i d polymer e l e c t r o l y t e (SPE) b r i n e e l e c t r o l y z e r , was f i r s t reported i n May of 1979. This new technology u t i l i z i n g s p e c i a l l y a c t i v a t e d perms e l e c t i v e membranes i s reported to have achieved c e l l voltages of l e s s than 3.15 ν a t 3.3 KA/ c u r r e n t d e n s i t y producing 28-30% c a u s t i c soda at 94% current e f f i c i e n c y , equivalent to 2200 KWH/MT NaOH. This technology i s reported t o be operating i n excess of one year i n a commercial prototype c e l l having an e l e c t r o d e area of 0.5 X 1.7 meters ( 7 7 ) . F u r t h e r development of the "Zero Gap" membrane c e l l t e c h nology by Asahi Glass Co., c a l l e d AZEC, i s reported t o have achieved at l a b o r a t o r y s c a l e an e l e c t r o l y t i c power consumption of 1950 KWH/M ton NaOH a t 2 KA/M c u r r e n t d e n s i t y and 35% NaOH and at a current density of 4 KA/M the power consumption i s 2140 KWH/M ton NaOH (78). Development of these new zero gap membrane c e l l e l e c t r o l y zers represents a major new approach i n the membrane c e l l t e c h nology and promises to provide even more r a p i d development i n t h i s q u i e t r e v o l u t i o n of the membrane c e l l c h l o r a l k a l i process.


2

2

2


14.


Electrosynthesis

in Chlor-Alkali

Cells

361


Bibliography

(1) C o u l t e r , M.V., "Modern C h l o r - A l k a l i Technology," Society of Chemical Industry, Ellis Horwood Limited, Chichester, England, 1980. (2) O'Leary, K. J . , "Membrane Chlorine Cell Design and Technology," Lecture, The E l e c t r o c h e m i c a l Technology Group of the SCI, London, June 16-17, 1976. (3) Bergner, D., "Current E f f i c i e n c i e s , Chlorate and Oxygen Formation i n Alkali Chloride E l e c t r o l y s i s According to the Membrane Method," Chemiker Zeitung, Vol.104, No 7-8, pp 215-224, 1980. (4) 0R0NZI0 DE NORA SYMPOSIUM, 0R0NZI0 dE Nora Impianti, E l e t t r o c h i m i c i a S.P.A. - Milano, 1979. (5) Dotson, R. L., "Modem E l e c t r o c h e m i c a l Technology," Chem. Eng. (Feature Article), July, 106-118, 1978. (6) Sconce, J . S., "Chlorine, ACS Monograph S e r i e s , No.154," Reinhold Publishing Co., New York, 1962. (7) Mantell, C. L., "Electrochemical Engineering," McGraw-Hill, New York, 1960. (8) Bergner, D., "Electrolytic Chlorine Generation by the Membrane Process," ( E l e k t r o l y t i s c h e Chlorerzeugung nach dem Membranverfahren), Chemikerzeitung, V o l . 101, No.10., pp 433-447, 1977. (9) Dotson, R. L. and O'Leary, K. J., "Electrolytic Production of High P u r i t y A l k a l i Metal Hydroxide," USP 4,025,405, German Ρ 2,251,660, 1977. (10) Stacy, A. J . and Dotson, R. L., "Control of Anolyte-Catholyte Concentrations i n Membrane Chlor-Alkali Cells," USP 3,773,634; B r i t i s h P. 1,369,579, and German Ρ 2,311,556, 1973. (11) F l o r y , P. J . , " P r i n c i p l e s of Polymer Chemistry," C o r n e l l University Press, Ithaca, New York, 1953. (12) Kesting, R.E., "Synthetic Polymeric Membranes," McGraw-Hill Book Company, New York, 1971. (13) Houwink, R. and Burgers, W. G., " E l a s t i c i t y , P l a s t i c i t y and the Structure of Matter," Cambridge, At the U n i v e r s i t y Press, 1937. (14) Ferry, J . D., " V i s c o e l a s t i c P r o p e r t i e s of Polymers," John Wiley & Sons, Inc., New York, 1970. (15) Eisenberg, Α., and King, Μ., "Ion-Containing Polymers, ( P h y s i c a l P r o p e r t i e s and Structure)," V o l . 2. Academic Press, New York, 1977. (16) Skelland, A.H.P., " D i f f u s i o n and Mass T r a n s f e r , " John Wiley & Sons, New York, 1974. (17) Dotson, R. L. and Yeager, H. L., "Fundamentals of Transport and Diffusion through Chlor-Alkali Cell Membranes", Presented at the Symposium of the Advances i n C h l o r - A l k a l i Technology i n London, June, 1979, Sponsored by SCI.


PERFLUORINATED IONOMER M E M B R A N E S

362

(18) Hwang, Sun-Tak, and Kammermeyer, K a r l , "Membranes i n Separations," John Wiley & Sons, 1975. (19) Yeager, H. L., K i p l i n g , B. and Dotson, R. L., "Sodium Ion D i f f u s i o n i n Nafion Ion Exchange Membranes", J . of the E l e c t r o c h e m i c a l Society, 127, No. 2., 303-307, Feb. 1980, Olin. (20) Dotson, R. L., Yeager, H. L., Ford, J . Μ., and Bennion, D. Ν., "Parameter C o r r e l a t i o n s f o r A Multicomponent Transport Model f o r C h l o r - A l k a l i Membrane C e l l s , " Presention at the 157th Meeting of the E l e c t r o c h e m i c a l Society, St. Louis, Mo. 5-11, 16, 1980, O l i n . (21) H e l f f e r i c h , F., "Ion Exchange," McGraw-Hill, New York, 1962. (22) Tuwiner, S. B., M i l l e r , F.P., and Brown, W. E., " D i f f u s i o n and Membrane Technology," Reinhold P u b l i s h i n g Corp., 1962. (23) Lakshminarayanaiah, Ν., "Transport Phenomena i n Membranes," Academic Press, New York, 1969. (24) "Chemical Physics of Ionic S o l u t i o n s , " John Wiley & Sons, Inc. 1966. (25) Wells, A. F., " S t r u c t u r a l Inorganic Chemistry," Oxford U n i v e r s i t y Press, 1962. (26) Per Kofstad, "Nonstoichiometry, D i f f u s i o n , and Electrical C o n d u c t i v i t y i n Binary Metal Oxides," W i l e y - I n t e r s c i e n c e A D i v i s i o n of John Wiley & Sons, Inc., New York, 1972. (27) Dotson, R. L., Lynch, R. W., and Hilliard, G. E., Transport of Water Molecules and Sodium Ions through Nafion Ion Exchange Membranes," Presentation at the 158th Meeting of The E l e c t r o c h e m i c a l Society, Hollywood, F l o r i d a , 10-5,10, 1980, Olin. (28) B i r d , R. D., Stewart, W. E., and L i g h t f o o t , Ε. Ν., "Transport Phenomena," John Wiley & Sons, Inc., New York, 1960. (29) "Perfluorocarbon Ion Exchange Membranes," 152nd N a t i o n a l Meeting of The E l e c t r o c h e m i c a l Society, A t l a n t a , Georgia, Oct. 10-14, 1977. (30) P i c k e t t , D. J . , "Electrochemical Reactor Design", E l s e v i e r Scientific P u b l i s h i n g Co., New York, 1977. (31) Chem. Techn. 32(3), 119-122 (1980). (32) Chem. Techn. 31 140(1979). (33) Dotson, R. L., "The E l e c t r o n as Reagent," Chem. Tech., V o l . 8., No. 1, 1978. (34) Gerischer, Η., and Tobias, C.W.,"Advances i n E l e c t r o c h e m i s t r y and E l e c t r o c h e m i c a l Engineering," Vol.11., John Wiley & Sons, New York, 1978. (35) Bard, A. J . , and Faulkner, L. R., " E l e c t r o c h e m i c a l Methods, Fundementals and A p p l i c a t i o n s , " John Wiley & Sons, New York, 1980. (36) Erdey-Gruz, T., " K i n e t i c s of E l e c t r o d e Processes," W i l e y - I n t e r s c i e n c e , New York, 1972. (37) B o c k r i s , J . O'M., "Overpotential," J . Electrochem. Soc. V o l . 98. No. 12., 1951. (38) Kortum, G., " T r e a t i s e on E l e c t r o c h e m i s t r y , " E l s e v i e r Pub. Co., New York, 1965.


®



14.



Cells

363

(39) Hampel, C.A., "The Encyclopedia of E l e c t r o c h e m i s t r y , " Robert E . K r e i g e r P u b l i s h i n g Co., Huntington, N.Y. 1972. (40) L e v e n s p i e l , O., "Chemical Reaction Engineering," John Wiley & Sons, Inc., New York, 1972. (41) Coulson, Ε . Η., et.al., "Chemistry of Ion Exchange a S p e c i a l Study, N u f f i e l d Advanced Science, C. T i n l i n g and Co., L t d . London and Prescot, England, (1970). (42) Diaion Manual of Ion Exchange Resins (1)-(2), M i t s u b i s h i Chemical I n d u s t r i e s , L t d . , Tokyo, Japan. (43) Dotson, "USP 3,793,163 and B r i t i s h Ρ 1,375,126, (1974). (44) C. J . Molner and M. M. Dorio "Perfluorocarbon Ion Exchange Membranes," 152nd N a t i o n a l Meeting of the E l e c t r o c h e m i c a l S o c i e t y A t l a n t a , Ga., Oct 10-14, 1977. (45) Juda, W. and McRae, W. Α., USP 2,636,851 and U.K.P. 720,002. (46) Bergsma, F., Chem. Woekbl., V o l . 48, 361, 1952. (47) Juda, W., Marinsky, J . A. and Rosenberg, N. W., Ann. Rev. P h y s i c . Chem., V o l . 4, P. 373, 1953. (48) Chrysikopoulos, S., Tombalakian, A. S. and Graydon, W. F., Canad. J . Chem. Engng., V o l . 6. p. 91, 1963. (49) Kaden, H. and Schwabe, Κ., Chem. Techn., V o l . 19, P. 87, 1967 (50) Grot, W., Chemie Ing. Techn. 44. Jahry . Nr. 4, 1972. (51) Nafion Products, Ε . I. duPont de Nemours & Co. ( I n c . ) , P l a s t i c s Products & Resins Department, Wilmington, DE 19898 (52) Grot, W., Chemie Ing. Techn., 47, 617., 1975. (53) R u s s e l l , J . H., "An Update on S o l i d Polymer E l e c t r o l y t e E l e c t r o l y s i s Programs at G.E.," 3rd World Hydrogen Energy Conference, Tokyo, Japan, June 23-26, (1980). (54) Seko, M., "Commercial Operation of Ion Exchange Membrane C h l o r - A l k a l i Process," ACS Meeting New York, April 4-9, 1976. (55) Seko, M., "The Asahi Chemical Membrane C h l o r - A l k a l i Process," The C h l o r i n e I n s t i t u t e , New Orleans, Feb., 1977. (56) Seko, M., "New Development of the Asahi Chemical Membrane C h l o r - A l k a l i Process", Oronzio De Nora Symposium on C h l o r i n e Technology 15-18 May 1979, Venice, I t a l y . (57) US 4,138,373 (58) Jap 116,790 (1977) (59) Jap 81,485 (60) Jap Kokai 76,282 (61) B r i t i s h 1,522,877 (62) B r i t i s h 1,523,047 (63) U k i h a s h i , H., "A Membrane f o r E l e c t r o l y s i s , " CHEMTECH, Feb, 1980. (64) Suhara, M. and Oda, Υ., "Transport Number through the P e r f l u o r i n a t e d Cation Membrane, Flemion," 158th Meeting of the E l e c t r o c h e m i c a l S o c i e t y , Hollywood, FL., Oct. 5-10, 1980.




364

M E M B R A N E S

(65) UHDE, " A l k a l i n e Chloride E l e c t r o l y s i s by the Membrane Process," Uhde Gmb H. (66) Ogawa, Shinsaku, "Asahi Chemical Membrane Chlor A l k a l i Process," Chemical Age of India Vol 31, No. 5, May, 1980, pg 451. (67) C o u l t e r , M.O., "Modern Chlor A l k a l i Technology," Ellis Howard L i m i t e d , 1980, pgs 195-210. (68) C o u l t e r , M.O., "Modern Chlor A l k a l i Technology," Ellis Howard L i m i t e d , 1980, pg 223-234. (69) C e l l e c o , " C h l o r - a l k a l i P l a n t s , with Ion S e l e c t i v e Membrane Cells." (70) Wadsworth, A.C. 3rd; "Captive or Over-Fence Chlorine & C a u s t i c Soda f o r Bleached Pulp Production, Advantages of Membrane C e l l s , " Allied Chemical Corp. 1979. (71) Diamond Shamrock, DM-14 Membrane E l e c t r o l y z e r , ES-ECL-4A Diamond Shamrock Corp. (72) Klamp, K. Lohrberg G. "Membrane C e l l Technology-View of an Engineering Co.," Chemical Age of India V o l . 31, No. 5, May, 1980, pgs. 463-470. (73) Asahi Glass, "The Flemion Membrane C h l o r - A l k a l i Process," Asahi Glass Co., L t d . Sept. '78. (74) Hausmann, E.; W i l l , H.; B e l l o n i , A.;"Plate Type C e l l s Ion Brine E l e c t r o l y s i s , " Chemical Age of India, V o l 31, No.5, May, 1980, pg 433-440. (75) Ukihashi, H.; Oda, Y.; Asawa, T.; Morimoto, T.;"Progress of E l e c t r o l y s i s Technology with Flemion Membrane" Kyoto Symposium of Japanese Soda I n d u s t r i a l A s s o c i a t i o n , 1980. (76) duPont, NEWS CONFERENCE ANNOUNCEMENT, Japan, 1980. (77) N i d o l a , A; "Brine E l e c t r o l y s i s with an I r o n z i o DeNora Design f o r the SPE Cell;" 24th Chlorine Plant Operations Seminar, C h l o r i n e I n s t i t u t e , Feb. 1981. (78) "New Caustic Soda Process i s Devised by Asahi Glass," The Japan Economic J o u r n a l , November 25, 1980. (79) Abam Engineers, Inc. "Process Engineering and Economic Evaluations of Diaphragm and Membrane Chloride C e l l Technologies", ANL/OEPM-80-9, December, 1980, Argonne N a t i o n a l Laboratory, Argonne, Ill. Page 82.

R E C E I V E D August 7, 1981.


Electrosynthesis with Perfluorinated Ionomer Membranes in Chlor

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