Perfluorinated Ionomer Membranes - ACS Publications - American

15

Perfluorocarboxylic Acid Membrane and Membrane Chlor-Alkali Process Developed by Asahi Chemical Industry

MAOMI SEKO, SHINSAKU OGAWA, and KYOJI KIMOTO Downloaded by UNIV OF PITTSBURGH on May 3, 2015 | http://pubs.acs.org Publication Date: February 4, 1982 | doi: 10.1021/bk-1982-0180.ch015

Asahi Chemical Industry Co., Ltd., 1-2, Yurakucho1-chome,Chiyoda-ku, Tokyo, Japan

Asahi Chemical s t a r t e d research and development of the ion-exchange membrane c h l o r - a l k a l i process i n 1966. Research was c a r r i e d out on the e f f e c t s of the type of ion-exchange group, i o n exchange c a p a c i t y , degree of c r o s s l i n k i n g , membrane s t r u c t u r e , c a u s t i c c o n c e n t r a t i o n , and many other parameters on current e f f i c i e n c y , operation voltage, e t c . In 1969, a benchs c a l e p l a n t s t a r t e d o p e r a t i o n based on a three compartment process using hydrocarbon membrane. Further study on f l u o r i n a t e d monomers and polymers s t a r t e d i n 1970, to improve the chemical stability of the membrane. A f t e r i n t e n s i v e research and development work, Asahi Chemical filed the b a s i c patents of f l u o r i n a t e d c a r b o x y l i c a c i d membrane and c a r b o x y l i c and s u l f o n i c a c i d membrane and the r e l a t e d e l e c t r o l y s i s processes i n 1974 (1 - 8). In A p r i l 1975, Asahi Chemical s t a r t e d o p e r a t i o n of a membrane c h l o r - a l k a l i plant with a c a p a c i t y of 40,000 MT/Y of c a u s t i c soda using Nafion p e r f l u o r o s u l f o n i c a c i d membrane. In 1976, t h i s membrane was replaced by p e r f l u o r o c a r b o x y l i c a c i d membrane developed by Asahi Chemical. The t o t a l c a u s t i c product i o n c a p a c i t y of plants based on Asahi Chemical's membrane c h l o r - a l k a l i technology using p e r f l u o r o c a r b o x y l i c a c i d membrane will reach 520,000 MT/Y i n 1982, at seven l o c a t i o n s i n v a r i o u s countries. General Requirements f o r Membranes f o r C h l o r - A l k a l i

Process

Ion-exchange membranes f o r the c h l o r - a l k a l i process should s a t i s f y the f o l l o w i n g requirements, some of which tend to be mutually c o n t r a d i c t o r y . - Chemical s t a b i l i t y - Physical s t a b i l i t y - Uniform s t r e n g t h and f l e x i b i l i t y - High current e f f i c i e n c y - Low e l e c t r i c r e s i s t a n c e - Low e l e c t r o l y t e d i f f u s i o n 0097-6156/82/0180-0365$ 11.50/ 0 © 1982 American Chemical Society In Perfluorinated Ionomer Membranes; Eisenberg, A., et al.; ACS Symposium Series; American Chemical Society: Washington, DC, 1982.

PERFLUORINATED

Downloaded by UNIV OF PITTSBURGH on May 3, 2015 | http://pubs.acs.org Publication Date: February 4, 1982 | doi: 10.1021/bk-1982-0180.ch015

366

IONOMER

M E M B R A N E S

In order t o s a t i s f y these requirements and to optimize the e l e c t r o c h e m i c a l p r o p e r t i e s of the membrane, the f o l l o w i n g f a c t o r s must be considered i n r e l a t i o n t o s p e c i f i c e l e c t r o l y s i s conditions. - Water content - Type of ion-exchange group - Ion exchange capacity - Polymer s t r u c t u r e - Polymer composition - P h y s i c a l s t r u c t u r e of membrane - D i s t r i b u t i o n of ion-exchange groups i n the membrane - Membrane thickness The t y p i c a l membranes are homogeneous, and are p r e f e r a b l y r e i n f o r c e d with an i n e r t m a t e r i a l . The chemical composition of the membranes are hydrolyzed copolymers of t e t r a f l u o r o e t h y l e n e (TFE) and p e r f l u o r o v i n y l ether monomer c o n t a i n i n g an i o n exchange group or i t s precursor (PVEX), represented by the f o l l o w i n g general formula (1_ - 21). CF I CFz = CF0-(CF CF0) -(CF ) -X 3

PVEX:

2

Where:

in

2

n

(1)

m = 0 or 1 η = 2 - 12 X = ion-exchange group o r i t s precursor such as S0 F, SR, S0 R, COOR, COF or CN 2

2

In preparing the membranes, the f o l l o w i n g steps must be c a r e f u l l y designed to c o n t r o l the above f a c t o r s . - Monomer s y n t h e s i s - Polymerization - Membrane f a b r i c a t i o n - Treatment t o improve the e l e c t r o c h e m i c a l p r o p e r t i e s of the membrane - Reinforcement C l a s s i f i c a t i o n of Membranes The membranes f o r the c h l o r - a l k a l i process are c l a s s i f i e d by chemical s t r u c t u r e of ion-exchange group, number and type of membrane l a y e r s , and polymer s t r u c t u r e . Ion-Exchange Group. The f o l l o w i n g f i v e ion-exchange groups have been reported i n the l i t e r a t u r e . a. s u l f o n i c a c i d group -SOaH (£, Γ1, 12, 13) b. sulfonamide group - S O 2 N H R (14, 15) c. c a r b o x y l i c a c i d group -COOH Q-8, 10, Ιβ_"21) d. phosphoric a c i d group -PO3H2 (22, 23) e. quaternary a l c o h o l group -Ç-0H (24, 25)

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


15.

SEKO E T A L .

Perfluorocarboxylic

367

Membrane

Groups d and e are not y e t u t i l i z e d i n commercial membranes, probably because of d i f f i c u l t i e s i n the monomer s y n t h e s i s . Table I shows a comparison of the membranes c u r r e n t l y i n use which c o n t a i n s u l f o n i c a c i d , sulfonamide, c a r b o x y l i c a c i d , and both c a r b o x y l i c and s u l f o n i c a c i d groups. The strong a c i d i t y and high h y d r o p h i l i c i t y of the p e r f l u o r o s u l f o n i c a c i d group r e s u l t i n a membrane of high water content and low e l e c t r i c r e s i s t a n c e . Since the f i x e d - i o n c o n c e n t r a t i o n i n the s u l f o n i c a c i d membrane i s a l s o low, current e f f i c i e n c y i s l e s s than 80% with c a u s t i c concentrations of 17% or more (26). The chemical s t a b i l i t y of p e r f l u o r o s u l f o n i c a c i d group i s e x c e l l e n t . Because of i t s low pKa v a l u e , the membrane can be exposed to s o l u t i o n s of pH 1. The weak a c i d i t y and r e l a t i v e l y low h y d r o p h i l i c i t y of the p e r f l u o r o c a r b o x y l i c a c i d group r e s u l t s i n a very high current e f f i c i e n c y of over 96%, although i t s e l e c t r i c r e s i s t a n c e i s h i g h (1^, 3^ _5, J7, 1Ό, 26-29). The membrane can be exposed to f a i r l y a c i d i c s o l u t i o n as the pKa value i s around 2. I t s chemical s t a b i l i t y i s q u i t e good under e l e c t r o l y s i s c o n d i t i o n s . Perfluorosulfonamide has also been proposed as ion-exchange group having very weak a c i d i t y (27, 30, 31). I t i s necessary to keep the membrane of t h i s type i n an a l k a l i n e s o l u t i o n i n order to maintain d i s s o c i a t i o n of the ion-exchange group. Another drawback of t h i s membrane i s i t s rather poor chemical s t a b i l i t y due to i t s tendency to be hydrolyzed during electrolysis. Some patent a p p l i c a t i o n s report membranes which contain ion-exchange groups of d i f f e r e n t types or exchange c a p a c i t i e s , thus achieving b e t t e r performance. These membranes can be c l a s s i f i e d i n t o those composed of a homogeneous mixture of d i f f e r e n t ion-exchange groups and those w i t h a m u l t i l a y e r s t r u c t u r e of ion-exchange groups d i f f e r i n g i n type or i n i o n exchange c a p a c i t y . To o b t a i n the former, ion-exchange groups of d i f f e r e n t types can be incorporated by t e r p o l y m e r i z a t i o n , blending (mixing), o r impregnation (2, 3, 4^ J5, 8_» 19, 32, 33). These membranes show f a i r l y high current e f f i c i e n c y and f a i r l y low e l e c t r i c r e s i s t a n c e but do not f u l l y u t i l i z e the m e r i t s of each of the groups, and thus do not e x h i b i t the h i g h l y s u p e r i o r c h a r a c t e r i s t i c s of m u l t i l a y e r membranes. The l a t t e r are obtained by l a m i n a t i o n , chemical treatment, or c o a t i n g to i n c o r p o r a t e two or more l a y e r s , with each l a y e r c o n t a i n i n g an ion-exchange group of a s p e c i f i c type or exchange capacity (2_, 2* 1» 6_, 2> 11, 13.» 14, 15, 18-22, 25 34). Figure 1 shows the general methods f o r preparation of p e r f l u o r o c a r b o x y l i c a c i d monolayer and m u l t i l a y e r membranes. 9

M u l t i l a y e r Membranes. Various types of m u l t i l a y e r membranes have been developed to o b t a i n a combination of high current e f f i c i e n c y and low e l e c t r i c r e s i s t a n c e . For high current



Ο

96 low high >3 >1 applicable < 0.5%

96 high high >3 >3 impossible >2% short low large

88 very high low >10 >10 impossible >2% short low large

75 low very high >1 >1 applicable < 0.5% long high large

Current efficiency % (8N NaOH)

Electric resistance

Chemical stability

Handling condition (pH)

pH of anolyte

Life of anode

Current density

Necessary number of cells

0

2

in product C l

2

Neutralization of O H " by HCI

small

high

long

5S

low/high

low

very low

high

Water content

w

>

W

W

M

Ο

α ο

H W

2 2 >

τι r

low/high

low

very low

high

Hydrophilicity

M

2-3/

w

5β

W

Ο

Η m ο ο

>

2

Ο

G

r

w

Ν)

15.

SEKO E T A L .

Perfluorocarboxylic

Membrane

373


m u l t i l a y e r membrane i s the higher current density which can be a p p l i e d because of i t s lower e l e c t r i c r e s i s t a n c e as w e l l as the higher current e f f i c i e n c y , which allows a reduction i n the number of c e l l s needed. R e l a t i o n between Ion Exchange Capacity and Pendant S t r u c t u r e of PVEX Monomer. Generally speaking, the p h y s i c a l s t r e n g t h of the membrane depends on various f a c t o r s such as the r a t i o of copolymers having ion-exchange groups i n the membrane, the TFE content of the copolymers, the molecular weight and molecular weight d i s t r i b u t i o n of the copolymers, and the type of r e i n f o r c i n g m a t e r i a l used. The d e s i r e d i o n exchange c a p a c i t y of the membrane i s obtained by a d j u s t i n g the r a t i o of the copolymers i n the membrane and the TFE content of each copolymer. The most important f a c t o r f o r a d j u s t i n g both p h y s i c a l s t r e n g t h and i o n exchange capacity i s the TFE content. The i o n exchange c a p a c i t y of p e r f l u o r o c a r b o x y l i c a c i d membranes r e p o r t e d l y f a l l s i n the range from 0.5 to 4 meq/gram dry r e s i n 12.» 16-21). At a given i o n exchange c a p a c i t y , a g r e a t e r molecular weight i n the PVEX monomer r e s u l t s i n a decrease i n the TFE content of the copolymer, and a lowering of the p h y s i c a l s t r e n g t h of the membrane. Conversely, the a t t a i n a b l e i o n exchange c a p a c i t y decreases when the molecular weight of PVEX monomer i s increased i n order to o b t a i n s u f f i c i e n t p h y s i c a l s t r e n g t h . In cases where PVEX monomer with m = 1, η = 3 and X = COOCH3 i n formula (1) i s u t i l i z e d f o r p e r f l u o r o c a r b o x y l i c a c i d membrane, the highest i o n exchange c a p a c i t y i s reported t o be approximately 1.3 meq/gran dry r e s i n (10). A much higher i o n exchange c a p a c i t y can be a t t a i n e d i n cases where m = 0 α-8_' 10_). Membranes w i t h a high i o n exchange c a p a c i t y are d e s i r a b l e f o r the production of concentrated c a u s t i c soda, and i t i s t h e r e f o r e e s s e n t i a l to use PVEX monomer w i t h m = 0 as a comonomer. Although i t i s d e s i r a b l e to use the PVEX monomer w i t h η = 2 and X = SO2F i n formula (1) as a comonomer to o b t a i n membrane c o n t a i n i n g s u l f o n i c a c i d group (9_, 11, 12, 18, 21, 35 - 38), which i s u s e f u l f o r the p r e p a r a t i o n of m u l t i l a y e r membrane with both c a r b o x y l i c and s u l f o n i c a c i d groups by chemical treatment, i t i s reported that when m = 0 t h i s PVEX monomer undergoes the c y c l i z a t i o n r e a c t i o n shown below during v i n y l i z a t i o n , and a l s o undergoes a c y c l i z a t i o n r e a c t i o n during p o l y m e r i z a t i o n under c e r t a i n c o n d i t i o n s (13, 20, 40). This makes i t s s y n t h e s i s i m p r a c t i c a l , and may cause a low molecular weight of the r e s u l t a n t polymer. CF3


PERFLUORINATED IONOMER

374

M E M B R A N E S

For these reasons, the PVEX monomer w i t h m = 1 i s g e n e r a l l y u t i l i z e d i n commercial a p p l i c a t i o n s , and the i o n exchange c a p a c i t y i s t h e r e f o r e l i m i t e d to around 0.9 meq/gram dry r e s i n (9, 41). A s a h i Chemical's Japanese patent a p p l i c a t i o n s c l a i m a method to overcome these d i f f i c u l t i e s , i n which PVEX monomer represented by the f o l l o w i n g general formula are u t i l i z e d as a comonomer (13, 20). CF

2


Where:

= CF0-(CF CP0) -(CF ) -X 2

m

2

n

(2)

m = 0 or 1 η = 3 - 5 X = precursor of s u l f o n i c a c i d group such as S0 F, SR o r S0 R 2

2

Under s u i t a b l e c o n d i t i o n s , even w i t h m = 0 these PVEX monomers do not c y c l i z e during monomer s y n t h e s i s nor during p o l y m e r i z a t i o n , because the f u n c t i o n a l end group or the s i z e of the r i n g which would form upon c y l i z a t i o n i s d i f f e r e n t from that i n the case of η = 2. This f a c i l i t a t e s monomer s y n t h e s i s and the formation of a polymer with high molecular weight, and allows the use of PVEX monomer w i t h m = 0 as the main f u n c t i o n a l comonomer i n the membrane p r e p a r a t i o n . Consequently, p h y s i c a l l y strong membrane w i t h high i o n exchange c a p a c i t y can be prepared s i n c e the polymer contains s u f f i c i e n t amount of TFE. F i g u r e 3 shows the r e l a t i o n between e l e c t r i c c o n d u c t i v i t y and i o n exchange c a p a c i t y of membranes produced from the PVEX monomers w i t h m = 0 and 1 which are i n d i c a t e d by formula ( 2 ) . General P r o p e r t i e s of P e r f l u o r o c a r b o x y l i c Acid Membranes As membranes employed i n the c h l o r - a l k a l i i n d u s t r y are g e n e r a l l y of the n o n - c r o s s i i n k e d type, t h e i r p r o p e r t i e s a r e i n f l u e n c e d s i g n i f i c a n t l y by the c o n d i t i o n s i n which they are u t i l i z e d . The extreme temperature, c o n c e n t r a t i o n and c u r r e n t d e n s i t y to which they are subjected i n the c h l o r - a l k a l i process are not encountered i n other a p p l i c a t i o n s such as e l e c t r o d i a l y s i s . C l a r i f i c a t i o n of the membrane p r o p e r t i e s i s t h e r e f o r e both p r a c t i c a l l y necessary and t h e o r e t i c a l l y i n t e r e s t i n g , and a p p l i c a t i o n of i o n c l u s t e r theory has been attempted (42, 43). The water content, e l e c t r i c r e s i s t a n c e , c u r r e n t e f f i c i e n c y and mechanical p r o p e r t i e s a r e i n f l u e n c e d by v a r i o u s f a c t o r s . Water Content. Figure 4 shows the r e l a t i o n between the water content of p e r f l u o r o c a r b o x y l i c a c i d membrane prepared by chemical treatment and the i o n exchange c a p a c i t y with v a r y i n g e x t e r n a l s o l u t i o n c o n c e n t r a t i o n . As the c o n c e n t r a t i o n of the e x t e r n a l s o l u t i o n i n c r e a s e s , the membrane shrinkage increases and the water content i s t h e r e f o r e decreased. The i n f l u e n c e of


SEKO E T A L .

Perfluorocarboxylic

Membrane


15.


375


376

PERFLUORINATED IONOMER M E M B R A N E S

Ο

I

I

I

L_

4

6

8

10

J

2

Concentration of NaOH (N) Figure 4. Water content, equivalent weight and concentration of NaOH, for perCF 3

fluorocarboxylic acid. \-OCF

CFOCF COOH 0.001855 treatment. Line represents: W = j q iq^SC 2

2

-(1100

2

-S0

I uv

2

or S0

3

-0(CF ) _ C00H 2

C.

n

i

D e s u l f o n y l a t i o n r e a c t i o n (18, 35, 38) P e r f l u o r o s u l f o n y l c h l o r i d e or s u l f o n i c a c i d group can be d e s u l f o n y l a t e d under v a r i o u s c o n d i t i o n s , to prepare a c a r b o x y l i c a c i d group.

-0(CF ) S0 C1 2

2

n

UV, heat, p e r o x i d e s , or o x i d i z i n g agent > - (CF ) COOH * n-i F + 0 0

2

-0(CF ) S0 H 2

n

2

>

3

2

-OCCF^^COOH

D.

A d d i t i o n r e a c t i o n (21_, 37_) In accordance w i t h the above r e a c t i o n s , pendant c a r b o x y l i c a c i d has one l e s s carbon atom than the o r i g i n a l pendant. Some patent a p p l i c a t i o n s report a method to prepare the pendant c o n t a i n i n g c a r b o x y l i c a c i d i n which the number of carbon atoms equal to o r g r e a t e r than that i n the o r i g i n a l , by a p p l i c a t i o n of an a d d i t i o n r e a c t i o n with the s u l f o n y l c h l o r i d e group or -CF X(X=I o r B r ) . Reaction A i s most appropriate f o r the preparation of m u l t i l a y e r membrane because of i t s extremely high s e l e c t i v i t y . 2


15.

Perfluorocarboxylic

SEKO E T AL.

393

Membrane

Lamination and Coating ( 2 , 3, 4 , 6 , 8 , 1 9 , 3 3 ) . M u l t i l a y e r p e r f l u o r o c a r b o x y l i c and s u l f o n i c a c i d membrane ( C 0 0 H | S 0 H ) i s prepared by h y d r o l y z i n g a laminated o r coated f i l m c o n t a i n i n g a c a r b o x y l i c a c i d e s t e r group i n one l a y e r of the membrane and a s u l f o n y l f l u o r i d e or s u l f o n i c a c i d group i n the other l a y e r i n the membrane. M u l t i l a y e r ((COOH + S 0 H ) | S 0 H ) membrane can be obtained by h y d r o l y z i n g a laminated or coated f i l m c o n t a i n i n g a mixture or blend of copolymers with c a r b o x y l i c a c i d e s t e r and s u l f o n y l f l u o r i d e . 3


3

3

Terpolymerization ( 2 , 3, 4 , 6 , 8 , 3 3 ) . Membranes c o n t a i n ing both p e r f l u o r o c a r b o x y l i c and s u l f o n i c a c i d groups can be prepared by h y d r o l y z i n g a f i l m formed by t e r p o l y m e r i z a t i o n of TFE and two PVEX monomers c o n t a i n i n g a c a r b o x y l i c e s t e r group and s u l f o n y l f l u o r i d e group. Impregnation and Blending ( 2 , 3, 4 , 6 , 8 , 1 9 ) . Membrane with mixed p e r f l u o r o c a r b o x y l i c and s u l f o n i c a c i d groups can be prepared by impregnating p e r f l u o r o s u l f o n i c a c i d membrane with a PVEX monomer c o n t a i n i n g a c a r b o x y l i c a c i d ester group, and polymerizing the monomer i n the membrane, and then h y d r o l y z i n g the r e s u l t a n t f i l m . Porous f i l m of PTFE o r T e f l o n PFA can be u t i l i z e d as a base m a t e r i a l i n place of p e r f l u o r o s u l f o n i c a c i d membrane. The same membrane with mixed c a r b o x y l i c and s u l f o n i c a c i d groups can be obtained by h y d r o l y z i n g the f i l m formed by blending the two copolymers, i n which one i s a copolymer of TFE and PVEX monomer c o n t a i n i n g a c a r b o x y l i c a c i d e s t e r group and the other i s a copolymer of TFE and PVEX monomer c o n t a i n i n g a s u l f o n y l f l u o r i d e group. A s a h i Chemical's Membranes The t y p i c a l p e r f l u o r o c a r b o x y l i c a c i d membrane developed by Asahi Chemical i s a m u l t i l a y e r membrane prepared by chemical treatment. The s t r u c t u r e of the membrane i s optimized f o r high current e f f i c i e n c y and low e l e c t r i c r e s i s t a n c e . The thickness of the c a r b o x y l i c a c i d l a y e r i s i n the range of 2 to 1 0 microns. The chemical s t r u c t u r e of the membrane i s as f o l l o w s ( 7 2 ) . -(CF -CF ) 2

2

0-(CF CP0)

- ( C F ) -COOH

2

2

m -(CF -CF ) ζ 2

2

0-

( C F

2

C F 0 )

(m =

0

M

- ( C F

or

1,

2

)

£

- S 0

η =

3

H

1-4,

I =

2-5)


394


M E M B R A N E S

The performance and l i f e of the membrane are e x c e l l e n t . The f o l l o w i n g are t y p i c a l performance data f o r A s a h i Chemical's membrane. membrane

current e f f i c i e n c y

membrane f o r 21% NaOH membrane f o r 30% NaOH

e l e c t r i c resistance 25°C i n 0.1N NaOH

cell

voltage*

96%

4.2 ohm

2.98 V

96%

3.9 ohm

3.12 V

2


* i n laboratory c e l l a t 40 Amperes/dm and 90°C Membrane C h l o r - A l k a l i Process Developed by Asahi Chemical P e r f l u o r o c a r b o x y l i c a c i d membrane d i f f e r s g r e a t l y from conventional asbestos diaphragm i n many respects, the most important of which are i t s high c a t i o n p e r m s e l e c t i v i t y , e f f e c t i v e gas impermeability, low water t r a n s p o r t , small pore diameter, h i g h mechanical s t r e n g t h , ease of handling, and r e l a t i v e l y h i g h membrane cost. I t a l s o d i f f e r s from p e r f l u o r o s u l f o n i c a c i d and sulfonamide membrane i n current e f f i c i e n c y , e l e c t r i c r e s i s t a n c e , and chemical s t a b i l i t y and other c h a r a c t e r i s t i c s . The o v e r a l l membrane process should be s p e c i f i c a l l y designed f o r optimum u t i l i z a t i o n of the c a r b o x y l i c a c i d membrane c h a r a c t e r i s t i c s i n i n d u s t r i a l a p p l i c a t i o n s . In p a r t i c u l a r , the basic e l e c t r o l y s i s c o n d i t i o n s described below a r e s p e c i f i c to the membrane process, and must be considered i n the design and s e l e c t i o n of the b r i n e p u r i f i c a t i o n , e l e c t r o l y z e r , anode, cathode, and evaporation process. L i m i t i n g Current Density. In the membrane process, boundary layers form a t both sides of the membrane due to i t s c a t i o n permselectivity. Such boundary l a y e r s do not occur i n the diaphragm process. For the boundary l a y e r a t the surface of the membrane f a c i n g the a n o l y t e , the f o l l o w i n g b a s i c equation i s e s t a b l i s h e d (73).

T

~T~^ Na

+-t

+

Na ^ "

^-00)

where: I Τ + Na tNa F D +

« = » » =

2

current density (Ampere/cm ) transport number of Na i n the membrane (Na current e f f i c i e n c y ) transport number of Na i n the anolyte Faraday constant (96500 Ampere.sec/ equivalent) d i f f u s i o n c o e f f i c i e n t of sodium c h l o r i d e a t the boundary l a y e r (cm /sec) 2


15.

Perfluorocarboxylic

SEKO E T A L .

Membrane

395

C =

c o n c e n t r a t i o n of sodium c h l o r i d e i n the bulk phase of a n o l y t e (equivalent/cm ) Co = c o n c e n t r a t i o n of sodium c h l o r i d e a t the surface of membrane (equivalent/cm ) δ = thickness of the boundary l a y e r (cm) 3

The l e f t s i d e of the equation represents the r a t e of sodium c h l o r i d e removal from the boundary l a y e r due to the d i f f e r e n c e between the t r a n s p o r t number of N a i o n i n the membrane and t h a t i n the a n o l y t e . The r i g h t s i d e of the equation represents the rate of sodium c h l o r i d e supply to the boundary l a y e r caused by d i f f u s i o n from the bulk phase. At a c e r t a i n c u r r e n t d e n s i t y ( I = I o ) , Co approaches zero and the f o l l o w i n g equation i s e s t a b l i s h e d .


+

1

Io = -£—D-F»-

At a higher current d e n s i t y than t h i s l i m i t i n g current d e n s i t y ( I o ) , the supply of N a i o n to the boundary l a y e r becomes i n s u f f i c i e n t f o r the t r a n s p o r t of e l e c t r i c c u r r e n t , and water therefore decomposes to hydrogen i o n and hydroxyl i o n f o r the t r a n s p o r t of e l e c t r i c current i n the boundary l a y e r . Under these c o n d i t i o n s , e l e c t r i c current i s c a r r i e d through the membrane not only by N a i o n but a l s o by t h i s hydrogen i o n . This r e s u l t s i n higher operating v o l t a g e and lower current efficiency. The operating current d e n s i t y must t h e r e f o r e be lower than the l i m i t i n g current density over the e n t i r e membrane s u r f a c e . T h i s r e q u i r e s c a r e f u l design of the c e l l to ensure uniform current d i s t r i b u t i o n throughout the membrane and uniform d i s t r i b u t i o n of the e l e c t r o l y t e c o n c e n t r a t i o n throughout the cell. A b i p o l a r c o n f i g u r a t i o n , i n which the e l e c t r i c current i n the i n d i v i d u a l c e l l i s unaffected by the number of c e l l s i n the e l e c t r o l y z e r , i s p r e f e r a b l e f o r t h i s purpose ( 2 6 - 2 9 ) . The b i p o l a r c o n f i g u r a t i o n , moreover, i s h i g h l y p r e f e r a b l e f o r m i n i m i z a t i o n of the e l e c t r o d e gap, f a c i l i t a t e s d e t e c t i o n of any v a r i a t i o n i n performance due to membrane manufacturing or c e l l c o n s t r u c t i o n through measurement of the operating v o l t a g e of each c e l l , and allows automatic t r i p - o f f of one e l e c t o l y z e r independently of others ( 7 4 ) . For uniform d i s t r i b u t i o n of e l e c t r o l y t e c o n c e n t r a t i o n i n and among c e l l s , f o r c e d , continuous c i r c u l a t i o n of a l a r g e amount of e l e c t r o l y t e i s h i g h l y p r e f e r a b l e to the conventional drop-wise supply used i n the mercury and diaphragm process. Forced c i r c u l a t i o n a l s o allows e f f e c t i v e removal of the heat generated by e l e c t r o l y s i s . I f the boundary l a y e r ( 6 ) i s narrow, a lower sodium c h l o r i d e c o n c e n t r a t i o n i n the anolyte can be used a t a given current d e n s i t y . This r e s u l t s i n a high r a t e of sodium c h l o r i d e +

+



396


M E M B R A N E S

u t i l i z a t i o n i n the anolyte stream, and a consequent r e d u c t i o n i n b r i n e p u r i f i c a t i o n c o s t . I t has been reported that δ can be reduced by l o c a t i n g the membrane near the anode, w i t h appropriate turbulence a t the boundary l a y e r thus provided by the e v o l v i n g c h l o r i n e gas (75). The anode c o n f i g u r a t i o n must promote both e f f e c t i v e d i f f u s i o n of sodium c h l o r i d e i n t o the boundary l a y e r and uniform d i s t r i b u t i o n of c u r r e n t through the membrane. C a u s t i c soda of high p u r i t y can be obtained by o p e r a t i o n at a current d e n s i t y s l i g h t l y below I o , s i n c e t h i s r e s u l t s i n a very low sodium c h l o r i d e c o n c e n t r a t i o n near the membrane (Co) and thus e f f e c t i v e l y prevents d i f f u s i o n of sodium c h l o r i d e through the membrane and i n t o the c a t h o l y t e . A boundary l a y e r w i t h a gradient i n c a u s t i c soda concen t r a t i o n a l s o forms a t the s u r f a c e of the membrane f a c i n g the c a t h o l y t e based on a s i m i l a r p r i n c i p l e , r e s u l t i n g i n a c a u s t i c soda c o n c e n t r a t i o n on the membrane surface which i s higher than that i n the bulk phase. Since t h i s tends to reduce the c u r r e n t e f f i c i e n c y and e l e c t r i c c o n d u c t i v i t y of the membrane, i t i s necessary to minimize the boundary l a y e r t h i c k n e s s or reduce the c a u s t i c soda c o n c e n t r a t i o n i n the bulk phase. I t i s a l s o e s s e n t i a l to p u r i f y the b r i n e with ion-exchange r e s i n of high s e l e c t i v i t y , i n order to prevent p r e c i p i t a t i o n of metal ions as hydroxides i n the membrane and the boundary l a y e r (74). In the diaphragm process, these phenomena do not occur because the diaphragm has no c a t i o n p e r m s e l e c t i v i t y . In the s o l i d polymer e l e c t r o l y t e (SPE) c e l l process, the membrane and the e l e c t r o d e are bonded together, and i t i s d i f f i c u l t to reduce the boundary l a y e r t h i c k n e s s on both surfaces of the membrane. This process a l s o r e q u i r e s very t h i n e l e c t r o d e s which must be made h i g h l y porous without i n c r e a s i n g t h e i r ohmic r e s i s t a n c e or reducing t h e i r mechanical s t r e n g t h . The optimum current d e n s i t y of the membrane process ( i n c l u d i n g the SPE c e l l process) i s higher than that of the diaphragm process, because of the r e l a t i v e l y high cost of the p e r f l u o r o ionomer membrane and i t s greater s e n s i t i v i t y to i m p u r i t i e s , which r e q u i r e s the use of more expensive m a t e r i a l for equipment. C a u s t i c Soda Concentration. The maximum e l e c t r i c conduc t i v i t y of c a u s t i c soda s o l u t i o n occurs a t a c o n c e n t r a t i o n of about 20% a t the o r d i n a r y e l e c t r o l y s i s temperature, and the membrane c o n d u c t i v i t y tends to d e c l i n e s h a r p l y w i t h c a u s t i c soda c o n c e n t r a t i o n i n the c a t h o l y t e exceeding 20% (26). The boundary l a y e r e f f e c t described i n the previous s e c t i o n a l s o makes r e l a t i v e l y low concentrations p r e f e r a b l e . With i n c r e a s i n g c o n c e n t r a t i o n of c a u s t i c soda, moreover, the a l l o w a b l e concentra t i o n of m u l t i v a l e n t c a t i o n i n the b r i n e must be decreased e x p o n e n t i a l l y because the s o l u b i l i t y products of m u l t i v a l e n t c a t i o n hydroxides a r e constant, and o p e r a t i o n a l d i f f i c u l t i e s



15.

SEKO E T A L .

Perfluorocarboxylic

397

Membrane

occur as a r e s u l t . The advanced p e r f l u o r o c a r b o x y l i c a c i d membrane now a v a i l a b l e has a c u r r e n t e f f i c i e n c y of over 90% i n a broad range of c a u s t i c soda c o n c e n t r a t i o n s , and minimum power consumption i s achieved at a c a u s t i c soda c o n c e n t r a t i o n of approximately 20%-30%, where the e l e c t r o l y s i s v o l t a g e i s lowest. The h i g h p u r i t y of the c a u s t i c soda obtained by the membrane process e l i m i n a t e s the need f o r a c a u s t i c soda evaporator i n cases where i t i s to be s u p p l i e d to customers such as pulp m i l l s which u t i l i z e a d i l u t e c a u s t i c soda. T h i s i s i n marked c o n t r a s t to the diaphragm process which i n e v i t a b l y r e q u i r e s evaporation to separate sodium c h l o r i d e . For the general t r a d e , i n which c a u s t i c soda a t 50% c o n c e n t r a t i o n i s r e q u i r e d , a conventional m u l t i p l e e f f e c t evaporator i s g e n e r a l l y u t i l i z e d to concentrate the c a t h o l y t e . C a u s t i c soda from the membrane process contains a very s l i g h t amount of sodium c h l o r i d e which does not cause c o r r o s i o n of the evaporator m a t e r i a l s or p r e c i p i t a t i o n of sodium c h l o r i d e , and thus allows e a s i e r and more s t a b l e evaporator o p e r a t i o n than i n the diaphragm process. Although as p r e v i o u s l y d e s c r i b e d i t i s p r e f e r a b l e to operate the c e l l at around 20% - 30% of c a u s t i c soda c o n c e n t r a t i o n to minimize e l e c t r o l y s i s power consumption, h i g h e r c o n c e n t r a t i o n s of c a t h o l y t e are g e n e r a l l y p r e f e r a b l e to minimize steam consumption i n the evaporation process. However, A s a h i Chemical has developed a heat recovery evaporator which g r e a t l y reduces the need f o r e x t e r n a l stream supply and t h e r e f o r e permits a s i g n i f i c a n t r e d u c t i o n i n the t o t a l energy consumption of e l e c t r i c i t y and steam (74,76). The heat recovery evaporator i s a m u l t i s t a g e , m u l t i - e f f e c t evaporator which i s d i f f e r e n t from c o n v e n t i o n a l m u l t i p l e e f f e c t evaporator or m u l t i s t a g e f l a s h evaporator. Asahi Chemical s heat recovery evaporator can concentrate the c a t h o l y t e from 21% to about 40% without steam by u t i l i z i n g heat generated during e l e c t r o l y s i s . To o b t a i n product c a u s t i c soda of 50% concentrat i o n , a s m a l l amount of steam i s s u p p l i e d to the f i n i s h i n g evaporator. f

Operating Pressure. In the diaphragm process, a small d i f f e r e n c e i n h y d r a u l i c pressure i s a p p l i e d between anolyte and c a t h o l y t e to t r a n s p o r t a n o l y t e through the diaphragm to the c a t h o l y t e compartment. I t i s t h e r e f o r e i m p r a c t i c a l to p r e s s u r i z e the a n o l y t e , s i n c e t h i s would cause c h l o r i n e gas to d i s s o l v e i n the a n o l y t e and thus mix w i t h the c a t h o l y t e . In the membrane process, however, p r e s s u r i z e d o p e r a t i o n does not cause mixing of a n o l y t e and c a t h o l y t e due to the dense s t r u c t u r e of the membrane, and v a r i o u s advantages such as reduced operating v o l t a g e and membrane v i b r a t i o n can be gained by p r e s s u r i z i n g both c h l o r i n e gas and hydrogen gas (77). Figure 11 shows the r e l a t i o n between o p e r a t i n g v o l t a g e and o p e r a t i n g temperature at v a r i o u s o p e r a t i n g pressures (77).




1.0 ata

Membrane: Nafion 315 Current density: 40 A / d m Anolyte cone: 2.5 Ν Catholyte cone: 17% Electrode gap: 3 mm

M E M B R A N E S

2

1.3 ata 1.5 ata

2.0 ata 2.5 ata

3.0 ata

ι 70

1

1

80

90

1

1

100

110

τ 120

1

130

~ C

Operating temperature Figure 11. Cell voltage vs. operating pressure.


15.

SEKO E T A L .

Perfluorocarboxylic

399

Membrane


With i n c r e a s i n g temperature, e l e c t r o l y t e and membrane r e s i s t a n c e tend to decrease while water vapor pressure tends to i n c r e a s e , and minimum operating v o l t a g e occurs a t a s p e c i f i c temperature. The use of metal c e l l s i s j u s t i f i e d by the advantages of p r e s s u r i z e d o p e r a t i o n , as w e l l as by t h e i r other o p e r a t i o n a l and mechanical advantages (74). Anode. Metal e l e c t r o d e s of high dimensional p r e c i s i o n are e s s e n t i a l f o r maintaining a uniform d i s t a n c e between the e l e c t r o d e and a membrane of l a r g e s i z e , and an e l e c t r o d e d i s t a n c e as small as p o s s i b l e i n the membrane c e l l . I t i s p r e f e r a b l e t o l o c a t e the membrane near the anode, to minimize the t h i c k n e s s of the boundary l a y e r at the membrane during e l e c t r o l y s i s . During o p e r a t i o n , a l k a l i migrates from the c a t h o l y t e to the anode through the membrane. I f the a l k a l i i s not n e u t r a l i z e d , oxygen i s generated at the anode a t a considerable r a t e and tends to shorten the anode l i f e . I t i s t h e r e f o r e p r e f e r a b l e to n e u t r a l i z e t h i s a l k a l i w i t h h y d r o c h l o r i c a c i d , and to u t i l i z e m u l t i l a y e r Rf-COOH/Rf-SO H membrane with high current e f f i c i e n c y . For the same reason, the anode c o a t i n g f o r the membrane process must possess s u p e r i o r a l k a l i r e s i s t a n c e , while f u l f i l l i n g the requirements f o r low c h l o r i n e overvoltage and high oxygen overvoltage. A s a h i Chemical has developed an unique anode c o a t i n g which s a t i s f i e s the requirements of the membrane process, and has used i t i n d u s t r i a l l y s i n c e 1975. This c o a t i n g i s a completely s o l i d s o l u t i o n of ruthenium, t i t a n i u m and oxygen, i n which the molar percentage of ruthenium i s a t l e a s t 50% of the t o t a l metal content and various other metal components a r e incorporated to provide high oxygen overvoltage (26, 78). 3

Cathode. Because i r o n i s an inexpensive m a t e r i a l and has a r a t h e r s m a l l hydrogen overvoltage, i r o n cathode i s g e n e r a l l y u t i l i z e d , i n the form of a mesh, p e r f o r a t e d p l a t e , or expandable metal sheet. In a d i s c u s s i o n on the hydrogen overvoltage of various metal w i r e s , i t has been shown that although i r o n cathode having a f l a t surface has a r e l a t i v e l y high hydrogen v o l t a g e , about 0.45 v o l t a t the current d e n s i t y of 25 Amperes/dm , the hydrogen overvoltage of the i r o n cathode u t i l i z e d i n an i n d u s t r i a l c e l l f o r hydrogen generation i s about 0.2 v o l t due to the f a c t that the s u r f a c e of the cathode, which i s f l a t i n the i n i t i a l period of o p e r a t i o n , i s converted to a porous surface of l a r g e e f f e c t i v e area by the d e p o s i t i o n of i r o n from the c a t h o l y t e (79). The i n c r e a s i n g cost of energy has a l s o s t i m u l a t e d research on the r e d u c t i o n of hydrogen overvoltage by u t i l i z i n g v a r i o u s metal c o a t i n g s . The performance of an a c t i v a t e d cathode having a low 2


PERFLUORINATED

400

IONOMER

M E M B R A N E S

hydrogen overvoltage tends to degrade g r a d u a l l y , probably due to the p r e c i p i t a t i o n of i r o n on the cathode. In the study of t h i s problem, i t i s u s e f u l to r e f e r to the Pourbaix diagram, which i s g e n e r a l l y used i n c o n s i d e r a t i o n of c o r r o s i o n problem. F i g u r e 12 shows the Pourbaix diagram f o r i r o n c a l c u l a t e d for s o l u t i o n of c a u s t i c soda at 90°C. The h o r i z o n t a l a x i s represents pOH c a l c u l a t e d by the equation pKw = pH + pOH a t 90°C. The thermodynamic parameters at 90°C shown i n Table X are c a l c u l a t e d from the f i g u r e s at 25°C and at 100°C (By the courtesy of P r o f e s s o r M. Takahashi, Yokohama N a t i o n a l U n i v e r s i t y ) . Because the p o t e n t i a l d i f f e r e n c e between p o i n t (A) and (D) i s 0.19 v o l t , the e q u i l i b r i u m c o n c e n t r a t i o n of HFe02 on the cathode should be about 10 mol/1. This means that the i r o n cathode w i t h a hydrogen overvoltage of about 0.2 v o l t i s i n an immunity s t a t e . I f the i r o n cathode i s not p o l a r i z e d , the e q u i l i b r i u m c o n c e n t r a t i o n of HFe02 on the i r o n i s about 10 mol/1 as determined at p o i n t (Β), and the i r o n t h e r e f o r e corrodes. With an a c t i v a t e d cathode of a small hydrogen overvoltage such as 0.12 v o l t , which corresponds to the d i f f e r e n c e between the p o t e n t i a l s a t (A) and ( E ) , i t becomes necessary to reduce the HFe0 c o n c e n t r a t i o n to l e s s than 10 ** mol/1. The above c a l c u l a t i o n s are based on e q u i l i b r i u m c o n s i d e r a t i o n s and suggest the f o l l o w i n g . 1. The hydrogen overvoltage of i r o n i s f a v o r a b l e f o r the p r o t e c t i o n of the i r o n cathode. 2. I f an a c t i v e cathode w i t h a low hydrogen overvoltage i s used, the HFe0 c o n c e n t r a t i o n i n the c a t h o l y t e must be kept low as determined by e q u i l i b r i u m c a l c u l a t i o n . I f the HFe0 c o n c e n t r a t i o n i s not low, i r o n w i l l p r e c i p i t a t e on the a c t i v e cathode, r e s u l t i n g i n a hydrogen overvoltage as high t h a t of i r o n cathode. Asahi Chemical has developed an c a t a l y t i c cathode f o r i n d u s t r i a l a p p l i c a t i o n s which has been t e s t e d i n commercial o p e r a t i o n s i n c e 1980.


5

2

2

2

2

Oxygen Depolarized Cathode. I t was proposed by Juda i n 1964 to use oxygen at the cathode i n the membrane c h l o r - a l k a l i process i n order to reduce e l e c t r o l y s i s voltage without generating hydrogen at the cathode (80). The c e l l v o l t a g e i s t h e o r e t i c a l l y reduced by 1.23 v o l t by using the oxygen d e p o l a r i z e d cathode, but the a c t u a l r e d u c t i o n i n e l e c t r o l y s i s i s reported to be about 0.6 v o l t (81). I n i n d u s t r i a l a p p l i c a t i o n of the oxygen d e p o l a r i z e d cathode, a i r i s the p r e f e r r e d oxygen source. However, a i r w i l l cause h i g h e r e l e c t r o l y s i s v o l t a g e than pure oxygen, and n i t r o g e n from the a i r together w i t h excess oxygen w i l l remove water and heat from the cathode area. This causes l o c a l d e p o s i t i o n of sodium c h l o r i d e , sodium carbonate and other compounds. P r a c t i c a l a p p l i c a t i o n of the oxygen d e p o l a r i z e d e l e c t r o d e has been l i m i t e d to f u e l c e l l s with pure oxygen f o r s p e c i a l


Perfluorocarboxylic

SEKO E T A L .

15.

-0.9-

Membrane

401

h

g Fe(0H) ^ HFe0 " + H 2

+

2

A

) : Vol

B

lu i ζ

F

-1.1-

(Λ

\ .

Poten


-1.0-

E

°^>\

^ \

10"

3

M/1

4

\

f

10" M/1

D

5

e

10" M/1

-1.2-

\ .

d

HFe0 "= 10" 2

0.0

-1.0

|

I

I

ι

i

1

2

3

4

5

l

l

6

I

8

ι

I

10

6

M/1

pOH

I

1

12 14 NaOH (N)

Figure 12. Pourbaix diagram for Fe in caustic soda at 90° C.



kcal/mol

c

μ*

°

kcal/mol

/Λοο°ο

9 0

kcal/mol

V*25°c

2

12.49 -111.83

-85.15

-54.22

12.26

-53.84

-110.95

-90.60 -84.31

pKw 14.00

Fe (OH) (S) -117.56

2

H F e 0 ~ (aq)

-56.69

2

H 0 (liq)

Table X . Thermodynamic Parameters (79).


ο to

In Perfluorinated Ionomer Membranes; Eisenberg, A., et al.; ACS Symposium Series; American Chemical Society: Washington, DC, 1982. Figure 13. Power consumption vs. current density.

2

Current density (A/dm )


404

M E M B R A N E S



50

100 2

Current density (A/dm ) Figure 14. Comparison of total energy consumptions.


15.

SEKO E T A L .

Perfluorocarboxylic

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purposes such as s p a c e c r a f t . No f u e l c e l l i n which a i r i s u t i l i z e d as the oxygen source has yet been a p p l i e d p r a c t i c a l l y or i n d u s t r i a l l y . Some years w i l l be r e q u i r e d f o r development of the i n d u s t r i a l a p p l i c a t i o n of oxygen d e p o l a r i z e d cathode f o r the membrane c h l o r - a l k a l i process, because of the d i f f i c u l t i e s i n manufacturing cathodes of large s i z e and i n preventing contamination of the cathode and the r e s u l t i n g d e c l i n e i n c a t a l y t i c a c t i v i t y , and because of the danger of the formation of an e x p l o s i v e mixture of oxygen and hydrogen i n the event of a f a i l u r e i n the c a t a l y t i c a c t i v i t y of the cathode. This danger i s not present i n f u e l c e l l a p p l i c a t i o n s . Energy Consumption. E l e c t r i c power consumption of e l e c t r o l y s i s i s the major part of the energy consumption i n a c h l o r - a l k a l i process. The power consumption of the membrane process has r e c e n t l y been g r e a t l y reduced by v a r i o u s improvements. The l a t e s t performance of A s a h i Chemical's membrane process r e a l i z e d a t a commercial p l a n t and a l s o i n an i n d u s t r i a l s c a l e c e l l i s shown i n r e l a t i o n to c u r r e n t d e n s i t y i n Figure 13 (82). Because steam i s consumed f o r c a u s t i c soda evaporation i n the diaphragm and the membrane processes to o b t a i n 50% c a u s t i c soda, i t i s a l s o important to compare the combined energy consumption of e l e c t r i c power f o r e l e c t r o l y s i s and steam f o r c a u s t i c soda evaporation. Figure 14 shows the t o t a l energy consumption of the modern asbestos diaphragm process with metal anode and modified asbestos, the modern mercury process w i t h metal anode and A s a h i Chemical's membrane process. In t h i s comparison, i t i s assumed that one metric ton of steam i s equivalent to 250 KWH of e l e c t r i c power based on the r a t e of f u e l consumption i n the modern power generation p l a n t s . In t h i s f i g u r e , the broken l i n e s represent the power consumption f o r e l e c t r o l y s i s and the s o l i d l i n e s represent the t o t a l energy consumption. In c a l c u l a t i o n of steam consumption, the diaphragm process i s assumed to u t i l i z e t r i p l e and quadruple e f f e c t evaporators. As the optimum current d e n s i t y i s 20 - 25 Amperes/ dm f o r the diaphragm process, 100 - 130 Amperes/dm f o r the mercury process, and 30 - 50 Amperes/dm f o r the membrane process, r e f e r e n c e to F i g u r e 14 c l e a r l y shows that A s a h i Chemical's membrane process i s the most economical i n terms of t o t a l energy consumption. 2

2

Literature Cited 1. 2. 3.

Seko, Μ., (to Asahi Chemical Ind. Co., Ltd.) Japanese Patent P u b l i c a t i o n 55-1351 (Jan. 12, 1980). Seko, Μ., (to Asahi Chemical Ind. Co., Ltd.) Japanese Patent P u b l i c a t i o n 55-14 148 (Apr. 14, 1980). Seko, Μ., (to Asahi Chemical Ind. Co., Ltd.) French Patent 2 263 312 (May 11, 1979).


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R E C E I V E D November 9, 1981.


Perfluorinated Ionomer Membranes - ACS Publications - American

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