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UBC Theses and Dissertations

Simultaneous electrosynthesis of alkaline hydrogen peroxide and sodium chlorate Kalu, Eric Egwu 1987

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SIMULTANEOUS ELECTROSYNTHESIS OF A L K A L I N E H Y D R O G E N PEROXIDE AND SODIUM CHLORATE by ERIC EGWU KALU B.Sc.(Hons), Chem. Eng., University of Lagos, Nigeria, 1984 A THESIS SUBMITTED IN PARTIAL FULFILMENT OF THE REQUIREMENTS FOR THE DEGREE OF MASTER OF APPLIED SCIENCE in THE FACULTY OF GRADUATE STUDIES DEPARTMENT OF CHEMICAL ENGINEERING We accept this thesis as conforming to the required standard THE UNIVERSITY OF BRITISH COLUMBIA December 1987 o ERIC EGWU KALU, 1987 In presenting this thesis in partial fulfilment of the requirements for an advanced degree at the University of British Columbia, I agree that the Library shall make it freely available for reference and study. I further agree that permission for extensive copying of this thesis for scholarly purposes may be granted by the head of my department or by his or her representatives. It is understood that copying or publication of this thesis for financial gain shall not be allowed without my written permission. The University of British Columbia 1956 Main Mall Vancouver, Canada V6T 1Y3 Date v | % 3" DE-6(3/81) - i i -ABSTRACT S i m u l t a n e o u s e l e c t r o s y n t h e s i s o f a l k a l i n e h y d r o g e n p e r o x i d e and s o d i u m c h l o r a t e i n t h e same c e l l was i n v e s t i g a t e d . The a l k a l i n e h y d r o g e n p e r o x i d e was o b t a i n e d by t h e e l e c t r o r e d u c t i o n o f o x y g e n i n NaOH on a f i x e d c a r b o n bed w h i l e t h e c h l o r a t e was o b t a i n e d by t h e r e a c t i o n o f a n o d i c e l e c t r o g e n e r a t e d h y p o c h l o r i t e and h y p o c h l o r o u s a c i d i n an e x t e r n a l r e a c t o r . An a n i o n m e m b r a n e , p r o t e c t e d on t h e a n o d e s i d e w i t h an a s b e s t o s d i a p h r a g m was u s e d as t h e s e p a r a t o r b e t w e e n t h e two c h a m b e r s o f t h e c e l l . The e f f e c t s o f s u p e r f i c i a l c u r r e n t d e n s i t y ( 1 . 2 - 2 . 4 kA m - 2 ) , s o d i u m h y d r o x i d e c o n c e n t r a t i o n ( 0 . 5 - 2 . 0 M) and c a t h o l y t e f l o w ( 0 . 1 x l f j - 6 - 0 . 5 x 1 0 - 6 m 3 s _ 1 ) on t h e c h l o r a t e And p e r o x i d e c u r r e n t e f f i c i e n c i e s w e r e m e a s u r e d . The e f f e c t o f p e r o x y t o h y d r o x y m o l e r a t i o on t h e c h l o r a t e c u r r e n t e f f i c i e n c y was m e a s u r e d t o o . The c e l l was o p e r a t e d a t f i x e d a n o l y t e f l o w o f 2 . 0 x 1 0 ~ 6 m 3 s _ 1 , i n l e t and o u t l e t t e m p e r a t u r e s o f 2 7 / 3 3 ° C ( a n o d e s i d e ) , 2 0 / 2 9 ° C ( c a t h o d e s i d e ) , c e l l v o l t a g e s o f 3 . 0 - 4 . 2 V ( c u r r e n t d e n s i t y o f 1 . 2 - 2 . 4 kA -m " 2 ) and a f i x e d t e m p e r a t u r e o f 7 0 ° C i n t h e a n o l y t e t a n k . D e p e n d i n g on t h e c o n d i t i o n s , a l k a l i n e p e r o x i d e s o l u t i o n and s o d i u m c h l o r a t e w e r e c o g e n e r a t e d a t p e r o x i d e c u r r e n t e f f i c i e n c y b e t w e e n 20% and 8 6 % , c h l o r a t e c u r r e n t e f f i c i e n c y b e t w e e n 5 1 . 0 % and 8 0 . 6 % and p e r o x i d e c o n c e n t r a t i o n r a n g i n g f r o m 0 . 0 6 9 M t o 0 . 8 0 M . The c o g e n e r a t i o n o f t h e two c h e m i c a l s was c a r r i e d o u t a t b o t h c o n c e n t r a t e d ( 2 . 4 - 2 . 8 M) and d i l u t e ( 0 - 0 . 5 M) c h l o r a t e s o l u t i o n s . A r e l a t i v e i m p r o v e m e n t on t h e c u r r e n t e f f i c i e n c i e s a t c o n c e n t r a t e d c h l o r a t e was o b s e r v e d . A c h l o r i d e b a l a n c e i n d i c a t e d n e g l i g i b l e c h l o r i d e l o s s t o t h e c a t h o l y t e . The r e s u l t s a r e i n t e r p r e t e d i n t e r m s o f t h e e l e c t r o c h e m i c a l and c h e m i c A l k i n e t i C s aNd t h e h y d r o d y n a m i c s o f t h e c e l l . - i i i -TABLE OF CONTENTS P a g e ABSTRACT i i TABLE OF CONTENTS i i i L I S T OF TABLES v i L I S T OF FIGURES v i i ACKNOWLEDGEMENTS i x CHAPTER 1 - INTRODUCTION 1 CHAPTER 2 - BACKGROUND 6 2 . 1 P r e v i o u s Work 6 2 . 2 S o d i u m C h l o r a t e i . s 6 2 . 3 H y d r o g e n P e r o x i d e 8 2 . 4 The E l e c t r o c h e m i s t r y o f S o d i u m C h l o r a t e P r o d u c t i o n . . 9 2 . 4 . 1 K i n e t i c s o f C h l o r a t e F o r m a t i o n 13 2 . 4 . 2 S e c o n d a r y R e a c t i o n s A f f e c t i n g C h l o r a t e C u r r e n t E f f i c i e n c y 14 2 . 4 . 3 A n o d i c S i d e R e a c t i o n s 14 2 . 4 . 4 B u l k P a r a s i t i c R e a c t i o n s i n C h l o r a t e E l e c t r o s y n t h e s i s 16 2 . 4 . 5 C a t h o d i c S i d e R e a c t i o n s . 16 2 . 4 . 6 S u p p r e s s i o n o f C a t h o d i c S e c o n d a r y R e a c t i o n s . . 17 2 . 4 . 7 E n e r g y C o n s u m p t i o n i n C h l o r a t e C e l l 19 2 . 4 . 8 E n e r g y C o n s e r v a t i o n i n C h l o r a t e C e l l s 20 2 . 5 E l e c t r o c h e m i s t r y o f H y d r o g e n P e r o x i d e 20 2 . 5 . 1 E l e c t r o c h e m i c a l R e d u c t i o n o f Oxygen i n A l k a l i n e S o l u t i o n 21 2 . 5 . 2 P a t h w a y s f o r Oxygen R e d u c t i o n 21 2 . 5 . 3 H y d r o g e n P e r o x i d e P r o d u c t i o n i n T r i c k l e Bed E l e c t r o c h e m i c a l R e a c t o r s 29 2 . 6 C o u p l i n g o f C a t h o d e R e a c t i o n o f 0 1 o m a n - W a t k i n s o n P e r o x i d e C e l l w i t h Anode R e a c t i o n o f a C h l o r a t e C e l l 31 2 . 6 . 1 P r o c e s s D e s c r i p t i o n 31 2 . 6 . 2 P r o b l e m s I n h e r e n t i n t h e C o u p l e d P r o c e s s 3 5 2 . 6 . 3 D i r e c t i o n o f F l o w o f A n o l y t e and C a t h o l y t e S t r e a m s 38 CHAPTER 3 - OBJECT 40 - i v -Page CHAPTER 4 - A P P A R A T U S , METHODS AND ACCURACY 41 4 . 1 A p p a r a t u s 41 4 . 2 M e t h o d s . . . 48 4 . 3 A c c u r a c y 53 CHAPTER 5 - RESULTS AND DISCUSSION 55 5 . 1 G e n e r a l C o n s i d e r a t i o n s 55 5 . 1 . 1 P r e l i m i n a r y E x p e r i m e n t s . 57 5 . 1 . 2 S c r e e n i n g t h e P r o c e s s V a r i a b l e s 62 5 . 2 F a c t o r i a l E x p e r i m e n t s 73 5 . 3 Low C h l o r a t e R u n s 75 5 . 3 . 1 E f f e c t o f S u p e r f i c i a l C u r r e n t D e n s i t y 75 5 . 3 . 2 E f f e c t o f C a t h o l y t e F l o w 83 5 . 3 . 3 E f f e c t o f NaOH C o n c e n t r a t i o n 86 5 . 3 . 4 E f f e c t o f P e r o x y - H y d r o x y M o l e R a t i o 91 5 . 4 H i g h o r S t r o n g C h l o r a t e C o n c e n t r a t i o n Runs 96 5 . 4 . 1 C h l o r a t e C u r r e n t E f f i c i e n c y . . 96 5 . 4 . 2 P e r o x i d e C o n c e n t r a t i o n and C u r r e n t E f f i c i e n c y 97 5 . 5 C e l l V o l t a g e 97 5 . 6 L e n g t h o f T i m e o f U s e and S t a b i l i t y o f Membrane 98 5 . 6 . 1 T y p i c a l C e l l V o l t a g e V a r i a t i o n w i t h T i m e 102 5 . 6 . 2 P r o d u c t C u r r e n t E f f i c i e n c y w i t h M e m b r a n e U s a g e 102 5 . 7 HC1 A d d i t i o n 104 5 . 7 . 1 C h l o r i d e B a l a n c e 108 5 . 7 . 2 W a t e r T r a n s p o r t A c r o s s t h e M e m b r a n e 109 CHAPTER 6 - CONCLUSIONS 110 CHAPTER 7 - RECOMMENDATIONS 113 REFERENCES 115 NOMENCLATURE 119 APPENDICES 1 . A u x i l i a r y E q u i p m e n t S p e c i f i c a t i o n s 120 2 . T a b u l a t e d E x p e r i m e n t a l R e s u l t s , A n a l y t i c a l T e c h n i q u e a n d S a m p l e C a l c u l a t i o n 124 3 . P r o c e s s M o d e l s 141 - v -LIST OF TABLES T a b l e 1 D i m e n s i o n s o f p e r o x y - c h l o r a t e c e l l 47 T a b l e 2 P r o p e r t i e s o f g r a p h i t e f e l t ( c a t h o d e b e d ) 49 T a b l e 3 Range o f e x p e r i m e n t a l v a r i a b l e s 52 T a b l e 4 E s t i m a t e s o f e x p e r i m e n t a l a c c u r a c y 54 T a b l e 5 P r e l i m i n a r y e x p l o r a t o r y e x p e r i m e n t s r e s u l t s ( f i r s t 4 h o u r s ) 59 T a b l e 6 P r e l i m i n a r y e x p l o r a t o r y e x p e r i m e n t a l r e s u l t s ( 6 t h h o u r - 1 0 t h h o u r ) 60 T a b l e 7 M a g n i t u d e s o f l e v e l s o f f a c t o r s 67 T a b l e 8 V a r i a b l e s s c r e e n i n g e x p e r i m e n t a l r e s u l t s 68 T a b l e 9 V a r i a b l e s and e f f e c t s f o r C 1 0 3 - 71 T a b l e 10 V a r i a b l e s and e f f e c t s f o r p e r o x i d e 72 T a b l e 11 L i s t o f i n d e p e n d e n t and d e p e n d e n t v a r i a b l e s 75 T a b l e 12 C h l o r a t e c u r r e n t e f f i c i e n c y and c o n c e n t r a t i o n a t l o w c h l o r a t e . r u n s 76 T a b l e 13 P e r o x i d e c u r r e n t e f f i c i e n c y and c o n c e n t r a t i o n a t l o w c h l o r a t e r u n 77 T a b l e 14 C h l o r a t e c u r r e n t e f f i c i e n c y and c o n c e n t r a t i o n a t h i g h c h l o r a t e r u n s 78 T a b l e 15 P e r o x i d e c u r r e n t e f f i c i e n c y and c o n c e n t r a t i o n f o r h i g h c h l o r a t e r u n s 79 T a b l e 16 C h l o r a t e c u r r e n t e f f i c i e n c y as a f u n c t i o n o f p e r o x y - h y d r o x y m o l e r a t i o 92 T a b l e 17 V a r i a t i o n o f c e l l v o l t a g e w i t h a n o l y t e f l o w 101 T a b l e 18 C e l l v o l t a g e v a r i a t i o n w i t h t i m e . . . 103 T a b l e 19 E f f e c t o f l e n g t h o f t i m e o f u s e o f membrane on p r o d u c t c u r r e n t e f f i c i e n c y 105 T a b l e 20 A t y p i c a l c h l o r i d e b a l a n c e r e s u l t 108 - vi -LIST OF FIGURES Page Figure 1 Simple chlorate cell 10 Figure 2 Trickle bed electrochemical cell for generation of alkaline hydrogen peroxide 30 Figure 3 Schematic diagram of a membrane peroxy-chlorate cell showing electrode reactions and transport processes.... - 32 Figure 4 Possible flow configurations for the anolyte and catholyte streams using membrane as cell separator.... 39 Figure 5 Apparatus for electrochemical cogeneration of sodium chlorate and alkaline peroxide solutions 42 Figure 6 Photographs of the apparatus for simultaneous electrosynthesis of alkaline peroxide and sodium chlorate 43 Figure 7 Electrochemical reactor used for cogeneration of alkaline peroxide and sodium chlorate 45 Figure 8 Side and top elevations of peroxy-chlorate cell 46 Figure 9 Preliminary exploratory experimental results 61 Figure 10 Plackett-Burman design for 12 experiments 65 Figure 11 Effect of superficial current density on chlorate current efficiency 80 Figure 12 The effect of current density on the peroxide current efficiency 81 Figure 13 Effect of catholyte flow on chlorate current efficiency 85 Figure 14 Effect of NaOH concentration on chlorate current efficiency 88 Figure 15 Effect of NaOH concentration on peroxide current efficiency 90 Figure 16 Effect of H02~/0H~ mole ratio on chlorate current efficiency at 0.5 M caustic 93 - vi i -Page Figure 17 Effect of rl02"/0H- mole ratio on chlorate current efficiency at 1.0 M caustic 94 Figure 18 Effect of H02"/0H" mole ratio on chlorate current efficiency at 2.0 M caustic 95 Figure 19 Effect of superficial current density on cell voltage for 0.5 M NaOH concentration 99 Figure 20 Effect of superficial current density on cell voltage for 2.0 M NaOH concentration 100 - v i i i -ACKNOWLEDGMENTS My s i n c e r e t h a n k s t o P r o f e s s o r C o l i n W. Oloman f o r h i s a d v i c e , h e l p and e n c o u r a g e m e n t i n t h i s w o r k . My g r a t e f u l a c k n o w l e d g e m e n t i s due t o my l o v i n g w i f e , N g o z i K a l u f o r h e r e n c o u r a g e m e n t , t h o u g h t f u l n e s s and i n t e r e s t i n t h e p r o j e c t . A l s o a c k n o w l e d g e d i s t h e i n v a l u a b l e work done and i d e a s g e n e r a t e d by t h e s t a f f o f t h e w o r k s h o p and s t o r e s o f C h e m i c a l E n g i n e e r i n g D e p a r t m e n t a n d P u l p and P a p e r C e n t r e . F u r t h e r t h a n k s a r e due t o t h e N a t i o n a l S c i e n c e and E n g i n e e r i n g R e s e a r c h C o u n c i l o f C a n a d a and PAPRICAN f o r f i n a n c i n g t h i s p r o j e c t . - 1 -CHAPTER 1 INTRODUCTION S o d i u m c h l o r a t e and h y d r o g e n p e r o x i d e a r e s t r o n g o x i d i z i n g a g e n t s u s e d i n b l e a c h i n g p r o c e s s e s . W h i l e s o d i u m c h l o r a t e i s u s e d i n t h e p u l p and p a p e r m i l l s f o r g e n e r a t i n g c h l o r i n e d i o x i d e ( a b l e a c h i n g c h e m i c a l ) , a l k a l i n e h y d r o g e n p e r o x i d e i s g a i n i n g p o p u l a r i t y as b o t h a b r i g h t e n i n g and e x t r a c t i v e a g e n t f o r wood p u l p . A l t h o u g h h y d r o g e n p e r o x i d e was o r i g i n a l l y m a n u f a c t u r e d by an e l e c t r o c h e m i c a l r o u t e , i t i s t o d a y w h o l l y p r o d u c e d by a u t o - o x i d a t i o n o f a n t h r o q u i n o n e (a c h e m i c a l r o u t e ) and s o l d i n s t a b i l i z e d w a t e r c o n c e n t r a t i o n o f up t o 90% by v o l u m e . F o r a c o m p l e t e a c c o u n t o f i t s m a n u f a c t u r e and d e t a i l e d p r o p e r t i e s , r e f e r e n c e i s made t o ( 1 ) . On t h e o t h e r h a n d , s o d i u m c h l o r a t e i s c o m m e r c i a l l y made by t h e e l e c t r o l y s i s o f r e l a t i v e l y c o n c e n t r a t e d n e u t r a l s o d i u m c h l o r i d e s o l u t i o n i n an u n d i v i d e d c e l l . F o r a c o m p l e t e d e s c r i p t i o n o f c h l o r a t e p r o c e s s p r i n c i p l e , s e e r e f e r e n c e s ( 2 , 3 and 6 0 ) . The u s e o f a l k a l i n e h y d r o g e n p e r o x i d e as a c o m p l i m e n t o f c h l o r i n e d i o x i d e , c h l o r i n e and h y p o c h l o r i t e i n t h e b l e a c h i n g s e q u e n c e s i s on t h e i n c r e a s e . The p e r o x i d e u s e i s n o t l i m i t e d t o o n l y m e c h a n i c a l p u l p s ( a s a b r i g h t e n i n g a g e n t ) , i t s u s e i n c h e m i c a l p u l p b l e a c h i n g i n one o r more s e q u e n c e s i s g r o w i n g ( 4 ) . When p e r o x i d e i s u s e d i n an i n t e r m e d i a t e s t a g e o f c h e m i c a l o r s e m i - c h e m i c a l p u l p i n g , t h e b l e a c h l i q u o r ' s c o m b i n e d e x t r a c t i v e and o x i d a t i v e e f f e c t s c o n t r i b u t e p r i m a r i l y t o - 2 -d e l i g n i f i c a t i o n , although s i gn i f i cant brightening may also occur. When applied in a f i na l stage, the dominant action i s oxidative and aimed at imparting a high and stable brightness to the pulp. The use of ox id iz ing agents such as hydrogen peroxide and hypochlorite in the a l k a l i treatment of the extraction stage of a mult ip le bleaching sequences i s known to improve the effect of the a l ka l i [5 ] . Also recent work [6] , has shown that the combined use of peroxide and oxygen in the extraction stage results in greater de l i g n i f i c a t i on and higher brightness than with oxygen extraction stage alone. Hydrogen peroxide use in the extraction stage not only increases the de l i g n i f i c a t i on rate leading to a 55-60% ef f luent load el iminat ion (7), i t also allows a m i l l to decrease the amount of chlorine dioxide use, shorten the bleaching sequences, reduce the operating cost and improve the pulp out-put [6]. 1 kg H 2 0 2 saves about 1.8 kg C10 2. The hydrogen peroxide to a l ka l i ra t io required in the de l i gn i fcat ion or extraction stage i s lower than that required for a bleaching stage. This work i s aimed at generation of a l ka l i peroxide to a l k a l i r a t i o required for a de l i gn i f i c a t i on and extraction stages of pulp bleaching process. The demand and use of a lka l ine peroxide in the pulp m i l l industry i s constrained by the high cost of hydrogen peroxide ($1.0/kg, USA). The disposal of chromate eff luents (for pulp mi l l s generating t he i r NaC103 requirement or using purchased NaC10a solutions or unpurified so l id NaC103) const i tute a problem to the environment. The problems associated with sulphate disposal could be solved by the subst i tut ion of - 3 -h y d r o c h l o r i c a c i d f o r s u l p h u r i c a c i d . The r e s u l t i n g N a C l i n s u c h a s u b s t i t u t i o n c o u l d be r e c y c l e d f o r N a C 1 0 3 g e n e r a t i o n i f t h e c h l o r a t e i s g e n e r a t e d on s i t e . A t l o w a l k a l i n e p e r o x i d e t o c a u s t i c c o n c e n t r a t i o n r a t i o r e q u i r e d f o r p u l p e x t r a c t i o n , an on s i t e g e n e r a t i o n o f t h e p e r o x i d e c a n be c o n s i d e r e d a s an a l t e r n a t i v e method o f o b t a i n i n g c h e a p e r a l k a l i n e , p e r o x i d e i n p u l p m i l l s . E l e c t r o c h e m i c a l r e d u c t i o n o f o x y g e n i n an a l k a l i n e s o l u t i o n n o t o n l y p r o d u c e s p e r o x y l i o n s , t h e r e i s a l s o an i n c r e a s e i n h y d r o x y l i o n c o n c e n t r a t i o n ( i . e . , p r o c e s s i s n o t n e t a l k a l i c o n s u m i n g p r o c e s s ) . When t h i s i s c o n s i d e r e d t o g e t h e r w i t h a n o d i c o x i d a t i o n o f c h l o r i d e , w h i c h r e q u i r e s h y d r o x y l i o n f o r c h l o r a t e f o r m a t i o n , a s i m u l t a n e o u s p r o d u c t i o n o f a l k a l i n e p e r o x i d e on t h e c a t h o d e c h a m b e r and c h l o r a t e i n t h e anode chamber, o f a d i v i d e d c e l l seems an i d e a w o r t h y o f i n v e s t i g a t i o n . T h e r e i s l i t t l e c o n s i d e r a t i o n i n t h e l i t e r a t u r e r e g a r d i n g t h e s i m u l t a n e o u s p r o d u c t i o n o f p e r o x i d e and c h l o r a t e i n t h e same c e l l . R e c e n t l y , C a r l i t company o f J a p a n [ 8 ] o b t a i n e d a p a t e n t u s i n g an o r g a n i c compound i n t h e c a t h o d e o f a d i v i d e d c e l l t o make p e r o x i d e and a l k a l i m e t a l h a l o g e n a c i d s a l t i n t h e a n o d e c h a m b e r . The p r i n c i p l e o f a s i m u l t a n e o u s p r o d u c t i o n o f a l k a l i n e p e r o x i d e and c h l o r a t e i n t h e same c e l l c o u l d be e x t e n d e d and a p p l i e d t o o t h e r e l e c t r o c h e m i c a l s y s t e m s 0 2 + H 2 0 + 2e H 0 2 _ + OH" (1) C I " + 6 0 H " -»• C 1 0 3 - + 3 H 2 0 + 6e (2) - 4 -where one of the electrode reaction products is cheaper than the other electrode product. The reason that l i t t le work has been previously done on the simultaneous peroxide and chlorate synthesis might have been the obvious problems of such a system. The problems to overcome include the identification of an ideal long lasting separator that would selectively allow hydroxyl ion passage or transfer from the cathode to the anode side and at the same time be stable in the anode side oxidative chlorate environment. The identification of a long lasting cathode for oxygen reduction is also required to be achieved. The opposing process factor requirements for the primary products of interest may be a reason too. The present work was undertaken to explore the possibility of making alkaline peroxide and sodium chlorate simultaneously in the-same electrochemical cell using an anion selective membrane or just an ordinary diaphragm separator. The use of oxygen depolarized cathode in chlorate synthesis could mean an energy reduction per ton of chlorate. The success and adaptation of the present process would eliminate the use of chromate in chlorate generation at pulp mill sites for those mills generating their chlorate requirements. Such a process would not only reduce the cost of production of chlorate that appears in form of chromate prices and environmental clean up exercises in the pulp mills, it could also mean lower energy per ton of chlorate produced. The reaction stoichiometry on the cathode gives a product solution in which the minimum ratio by weight of sodium hydroxide to hydrogen peroxide is about 2:1. This is a constraint in the utilization of the - 5 -generated peroxide for bleaching purposes as the traditional bleaching sequence requires caustic to peroxide ratio of 1:1. Thus the alkaline peroxide generated in this sytem seems well suited for applications such as peroxide addition during caustic extraction and for some bleaching sequences where peroxide is used along with chlorine dioxide or oxygen. - 6 -CHAPTER 2 BACKGROUND 2 . 1 Previous Work The s i m u l t a n e o u s p r o d u c t i o n o f s o d i u m c h l o r a t e and h y d r o g e n p e r o x i d e i n t h e same c e l l h a s n o t been c o m m e r c i a l i z e d . R e c e n t l y ( 1 9 8 6 ) , a p r o c e s s o f s u c h a n a t u r e was d o c u m e n t e d ( 8 ) . The i d e a o f i n t e r e s t commonly e x p r e s s e d w i t h r e g a r d t o c h l o r a t e p r o d u c t i o n and o x y g e n d e p o l a r i z e d c a t h o d e i s f o r e n e r g y r e d u c t i o n o f t h e c h l o r a t e m a n u f a c t u r i n g p r o c e s s . In s u c h a p r o c e s s , t h e p e r o x i d e f o r m e d i s d e s t r o y e d by t h e u s e o f h e a v y m e t a l c a t a l y s t s ( 9 ) . As p o i n t e d o u t e a r l i e r o n , t h e l a c k o f i n f o r m a t i o n on s i m u l t a n e o u s p r o d u c t i o n o f a l k a l i n e p e r o x i d e and c h l o r a t e may be t h e r e s u l t o f t h e c o m p l e x c h e m i s t r y o f t h e p r o c e s s and t h e b e l i e f i n t h e i m p r a c t i c a l i t y o f p r e v e n t i o n o f p e r h y d r o x y l i o n f r o m c o m i n g i n t o c o n t a c t w i t h h y p o c h l o r i t e , t h e r e s u l t o f w h i c h c o u l d l e a d t o n o t a c h i e v i n g a c u r r e n t e f f i c i e n c y o f e c o n o m i c i m p o r t a n c e . The f o l l o w i n g i s a r e v i e w o f t h e i n d i v i d u a l p r o c e s s e s f o r e l e c t r o s y n t h e s i s o f s o d i u m c h l o r a t e and a l k a l i n e p e r o x i d e s o l u t i o n s . 2.2 Sodium Chlorate The p r e p a r a t i o n o f s o d i u m c h l o r a t e was f i r s t r e p o r t e d by B e r z e l i u s i n 1808 (10) w h i l e i n 1847 K o l b e (41) c o n f i r m e d t h e e l e c t r o c h e m i c a l c o n v e r s i o n o f c h l o r i d e t o c h l o r a t e . The f i r s t c o m m e r c i a l e l e c t r o c h e m i c a l p r o d u c t i o n o f c h l o r a t e was s e t up i n F r a n c e 19 y e a r s l a t e r ( 2 ) . The c h l o r a t e c e l l u s e d p l a t i n u m a n o d e s . I n S w e d e n , C a r l s o n - 7 -( 2 ) r e p o r t e d i n 1894 t h e u s e o f g r a p h i t e a n o d e s i n s t e a d o f p l a t i n u m . M u l l e r s u g g e s t e d i n 1898 t h a t t h e u s e o f a d i a p h r a g m was u n n e c e s s a r y i n c h l o r a t e c e l l s i f c h r o m a t e i s a d d e d t o t h e e l e c t r o l y t e ( 2 ) . Wagner i n 1954 ( 1 2 ) e x p l a i n e d t h e r o l e o f d i c h r o m a t e i n c h l o r a t e s y n t h e s i s . The m e c h a n i s m o f e l e c t r o c h e m i c a l c h l o r a t e f o r m a t i o n was p o s t u l a t e d by F o r e s t e r and h i s s t u d e n t s i n 1899 ( 1 3 ) . A c c o r d i n g t o t h i s p o s t u l a t i o n ( w h i c h has s t o o d t h e t e s t o f t i m e ) , d i s c h a r g e o f c h l o r i d e i o n s i s f o l l o w e d by a s e r i e s o f c h e m i c a l r e a c t i o n s . Oxygen e v o l u t i o n on t h e a n o d e t h r o u g h h y p o c h l o r i t e d i s c h a r g e i s r e s p o n s i b l e f o r l o s s e s i n c u r r e n t e f f i c i e n c y . O t h e r p r i n c i p a l c o n t r i b u t o r s i n t h e t h e o r y and p r a c t i c e o f c h l o r a t e s y n t h e s i s i n c l u d e K n i b b s and P a l f r e e m a n ( 1 4 ) , de V a l e r a ( 1 5 ) , N a g a i and T a k a i ( 1 6 ) , B e c k ( 3 ) , J a c k s i c e t a l ( 1 7 , 1 8 ) , I b l and L a n d o l t ( 1 9 ) , T i l a k ( 2 0 ) and a h o s t o f o t h e r s . E v e r s i n c e t h e c o m m e r c i a l i z a t i o n o f e l e c t r o c h e m i c a l p r o d u c t i o n o f c h l o r a t e , a c t i v e s t u d i e s and i n v e s t i g a t i o n i n t o ways o f i m p r o v i n g c h l o r a t e c e l l p e r f o r m a n c e h a v e been t h e o r d e r o f t h e d a y . S u c h s t u d i e s i n c l u d e t h o s e a i m e d a t r e d u c i n g e n e r g y c o n s u m p t i o n p e r t o n o f c h l o r a t e , i n c r e a s i n g c u r r e n t e f f i c i e n c y , a c h i e v i n g h i g h c h e m i c a l c h l o r a t e f o r m a t i o n , e t c . The d e s i g n and u s e o f c h l o r a t e e l e c t r o l y z e r s w i t h e x t e r n a l o r s e p a r a t e r e a c t o r s came as a c o n s e q u e n c e o f s u c h r e s e a r c h . O p e r a t i n g t h e e l e c t r o l y z e r s a t h i g h t e m p e r a t u r e s r e d u c e s power c o n s u m p t i o n as t h e o p e r a t i n g v o l t a g e i s l o w e r t h a n a t l o w t e m p e r a t u r e s . M a t e r i a l s c a p a b l e o f b e i n g u s e d a t h i g h t e m p e r a t u r e and c o r r o s i v e e n v i r o n m e n t o f t h e c h l o r a t e c e l l a r e r e l a t i v e l y e x p e n s i v e . - 8 -A n o t h e r d i r e c t i o n o f e n e r g y r e d u c t i o n i n c h l o r a t e c e l l c o n t e m p l a t e d ( t h a t d o e s n o t n e c e s s a r i l y i n v o l v e a v e r y h i g h t e m p e r a t u r e o f o p e r a t i o n ) i s t h e u s e o f c a t h o d e v o l t a g e r e d u c t i o n . S u g g e s t i o n s h a v e b e e n made on t h e u s e o f a i r o r o x y g e n d e p o l a r i z e d c a t h o d e s ( 9 , 21) o r c o u p l i n g o f a c h l o r a t e c e l l w i t h a f u e l c e l l i n o r d e r t o a c h i e v e a h i g h e r e n e r g y e f f i c i e n c y i n t h e c h l o r a t e c e l l . A l t h o u g h none o f t h e s e have b e e n s u c c e s s f u l l y c o m m e r c i a l i z e d , a n y b r e a k t h r o u g h i n t h e u s e o f a i r d e p o l a r i z e d c a t h o d e c o u l d mean a d e c r e a s e i n t h e power c o n s u m p t i o n p e r t o n o f c h l o r a t e p r o d u c e d . The u s e o f an o x y g e n d e p o l a r i z e d c a t h o d e r e d u c e s t h e d e c o m p o s i t i o n v o l t a g e o f c h l o r i d e ( t o c h l o r a t e ) f r o m 2 . 1 9 V t o 0 . 9 6 V . T h i s work i s a i m e d a t e x p l o r i n g t h e p r i n c i p l e o f t h e o x y g e n d e p o l a r i z e d c a t h o d e f o r c h l o r a t e p r o d u c t i o n u s i n g a d i v i d e d c e l l . 2 . 3 Hydrogen Peroxide T h e e l e c t r o r e d u c t i o n o f o x y g e n a t a c a t h o d e , t h o u g h r e c o g n i z e d s i n c e 1882 as a s o u r c e o f h y d r o g e n p e r o x i d e ( 2 2 ) has n o t been u s e d on a c o m m e r c i a l s c a l e . A j o i n t v e n t u r e by Dow and Huron c h e m i c a l s L t d . l e d t o a p i l o t p l a n t (1000 k g / d a y ) o f p e r o x i d e s o l u t i o n i n O n t a r i o i n 1986 ( 2 3 ) . P r i n c i p a l w o r k e r s i n t h e f i e l d o f o x y g e n r e d u c t i o n on a c a t h o d e f o r p e r o x i d e p r o d u c t i o n i n c l u d e F i s c h e r and P r i e s s i n 1913 ( 2 4 ) , B e r l f r o m 1935 - 1943 ( 2 5 ) , M i z u n o ( 1 9 4 8 - 5 0 ) ( 2 6 ) , G r a n g a a r d - b e t w e e n 1966 - 1970 ( 2 7 ) , Oloman and W a t k i n s o n ( 2 8 , 2 9 , 3 0 ) b e t w e e n 1 9 7 4 - 1 9 7 9 , and Brown e t a l [ 3 3 ] . F i s c h e r and P r i e s s ( 2 4 ) s t u d i e d t h e e l e c t r o r e d u c t i o n o f o x y g e n i n a c i d s o l u t i o n on a m e r c u r y c a t h o d e a t o x y g e n p r e s s u r e s o f up t o 100 a t m . A h y d r o g e n p e r o x i d e o f 2 . 3 % wt c o n c e n t r a t i o n a t a c u r r e n t d e n s i t y - 9 -of 0.023 kA m~2 with a power consumption of 2.8 kW per kg of hydrogen peroxide was produced. In 1939, E. Berl reported hydrogen peroxide formation by reduction of oxygen in alkali solution on a cathode of active carbon. The reaction was carried out at near atmospheric pressure using a diaphragm. A 2.5% wt hydrogen peroxide in 50% potassium hydroxide was obtained (25). Mizuno reinvestigated Berl's work again between 1948-1950 in Japan (26), while later studies of reduction of oxygen on active carbon led to a series of patents by Grangaard (27). The Grangaard's cel l produced 0.5% solution of hydrogen peroxide in 2% sodium hydroxide with a power cosumption of 13.82 x 10 kJ per kg of peroxide. Between 1971-1976, Oloman and Watkinson (29-32) reported methods of reduction of oxygen on carbon cathodes for peroxide production including the use of tr ickle beds and cell pressurization (30). In one of their earl ier works, an alkaline peroxide solution of 3.7% wt hydrogen peroxide at 54% current efficiency and Na0H/H 20 2 ratio of 3.2 kg/kg was reported made at a power consumption of 18.24 x 10 3 kJ/kg H 2 0 2 , a reactor inlet pressure of 9.2 atm. abs, and oxygen and electrolyte flow of 3.33 x 10"5 m3 s"1 and 3.33 x 10"8 m3 s _ 1 respectively at a current density of 300 A m"2(30). Brown et al (33) also reported producing a stream of 1.5% wt peroxide in 1.0 M sodium hydroxide at a current efficiency of 67%, cel l voltage of 2V and current density of 300 A m~ 2, at ambient temperature and pressure. 2 .4 The Electrochemistry of Sodium Chlorate Production Sodium chlorate is produced industrial ly by the electrolysis of. a near neutral (pH = 6.5) concentrated solution of sodium chloride in an - 10 -Simple Chlorate Cell Cell voltage 3 to 4V NaCI 70to310gpl NaCI03 0 to 650 gpl NaOCI 1 to 6 gpl Na2Cr207 0.5 to 7 gpl Temperature 30° to 90° C pH 5.5 to 7.0 Figure 1 - 11 -undivided cel l (see F i g . l ) . The overall chemical reaction in chlorate production is NaCl + 3H20 ->• NaC103 + 3H 2 (3) The desirable reactions that lead to chlorate formation are: 2C1" + C l 2 + 2e (anode) E ° 2 9 8 = 1.359V SHE (4) 2H20 + 2e + H 2 + 20H~ (Cathode) E ° 2 9 8 = -0.83V SHE (5) .... After reaction (4), the following reactions subsequently occur in the vic inity of the anode ( i f the pH condition is satisfactory; pH = 6.5): C l 2 + H20 + H0C1 + H + + CI" (6) H0C1 H + + CIO" (7) 2HC10 + CIO" * C10 3- + 2C1- + 2H+ (8) Thus 6 Faradays are required to produce 1 kg-mol of chlorate or 1512 Ah/kg of chlorate at 100% efficiency. The overall reaction of chlorate production (eqn. 3) is an endothermic reaction with an overall heat of reaction (AH) of 937.22 kJ/kg-mol. Thus the thermoneutral voltage of the reaction is calculated using Gibb's equation to be 2.189 V -ZFE = AG (9) E = -AG/ZF (10) However in practice, chlorate cel ls operate at total cell voltages of between 3-4V at temperatures of greater or equal to 80°C. Under the - 12 -o p e r a t i n g c o n d i t i o n s , c u r r e n t e f f i c i e n c y o f 9 3 - 9 6 % i s a c h i e v e d w i t h s u b s e q u e n t h e a t g e n e r a t i o n . The h e a t g e n e r a t e d c o u l d be c a l c u l a t e d f r o m Q h = [ ( 1 0 0 / C E ) ( 3 2 . 9 4 ) E ] - AH ( J / k g - m o l ) ( 1 1 ) T h i s e x c e s s h e a t p r o d u c e d i s n o r m a l l y removed u s i n g some c o o l i n g m e d i u m . As s t a t e d i n e q u a t i o n s ( 4 ) , ( 6 ) , (7) and (8) - t h e r e a c t i o n s f a v o u r c h l o r a t e f o r m a t i o n . A f t e r t h e d i s c h a r g e o f c h l o r i d e i o n s on t h e a n o d e , t h e c h l o r i n e f o r m e d a t a n o d e d i s s o l v e s i n t h e b o u n d a r y l a y e r . C l 2 ( g ) = C l 2 ( a q ) The a q u e o u s o r d i s s o l v e d c h l o r i n e t h e n h y d r o l y z e s f o r m i n g H0C1 and 0 C 1 " a s i n e q u a t i o n ( 6 ) . An e x p r e s s i o n f o r t h e c o n c e n t r a t i o n o f H0C1 f o r m e d was g i v e n by Y o k o t a ( 4 4 ) a s : C H Q C 1 = E x p [ - 1 3 . 8 3 8 5 - 2 . 3 0 2 5 9 x l O " 3 ( T - 2 7 3 ) + 5 . 9 2 1 0 6 x 1 0 " 5 C N a C l + 2 - 3 0 2 5 9 x P H ) ( 1 2 ) w h e r e T = t e m p e r a t u r e i n K e l v i n C.j = C o n c e n t r a t i o n o f i i n m o l e s / m 3 Some o f H0C1 f o r m e d d i s s o c i a t e s i n t o H + and O C T - and i n t h e b u l k s o l u t i o n , t h e H0C1 c o m b i n e s w i t h 0C1 - t o f o r m c h l o r a t e . The e q u i l i b r i u m c o n s t a n t s o f t h e r e a c t i o n s a t some g i v e n t e m p e r a t u r e s a r e : C l 2 ( 9 ) * = = ^ C l 2 ( a q ) ( 6 . 1 8 x 1 0 - 2 mol . / K g s o l v e n t / a t m a t 2 5 ° C ) CI 2 ( a q ) + H 2 0 s = * H + + H0C1 + C I " - 13 -( 2 . 4 x 1 0 * ( m o l / m 3 ) 2 a t 2 5 ° C ) ( r e f . 3 6 ) H0C1 H + + O C l -( 2 . 7 x 1 0 - 5 m o l / m 3 ) ( 3 6 , 3 7 ) 2H0C1 + O C l - C 1 0 3 - + 2H+ + 2 C T ( 7 6 . 0 x 10 1* ( m o l / m 3 ) 2 a t 7 0 ° C ) ( 3 7 ) R e a c t i o n ( 8 ) i s a s l o w r e a c t i o n whose o p t i m a l r a t e o c c u r s a t a pH v a l u e e q u a l t o p K - l o g 2 w h e r e pK = - l o g ( e q u i l i b r i u m c o n s t a n t ) f o r r e a c t i o n ( 8 ) . 2.4.1 Kinetics of Chlorate Formation Two m e c h a n i s m s a r e p r e s e n t e d i n t h e l i t e r a t u r e f o r p u r e l y c h e m i c a l c o n v e r s i o n o f h y p o c h l o r i t e t o c h l o r a t e : - The r e a c t i o n s e q u e n c e f i r s t p r o p o s e d by F o e r s t e r ( 1 3 ) and c o n f i r m e d by many o t h e r s ( 1 4 , 3 7 , 3 8 , 3 9 ) as g i v e n i n e q u a t i o n s ( 4 ) , ( 6 ) , ( 7 ) and ( 8 ) . - T h e r e a c t i o n m e c h a n i s m p r o p o s e d by L i s t e r ( 4 0 ) . W h e r e a s F o e r s t e r ' s m e c h a n i s m i s a t h i r d o r d e r r e a c t i o n s i n v o l v i n g 2 m o l e s o f H0C1 a n d 1 m o l e o f O C l - , L i s t e r ' s p r o p o s a l i s b i m o l e c u l a r as shown i n e q u a t i o n s ( 1 3 ) and ( 1 4 ) . 2H0C1 > C1GV + C I " + 2 H + ( 1 3 ) C 1 0 2 " + H0C1 + C 1 0 3 - + C I " + H + ( 1 4 ) R e a c t i o n ( 1 4 ) i s t h e r a t e c o n t r o l l i n g r e a c t i o n i n t h e m e c h a n i s m p r o p o s e d by L i s t e r . R e c e n t l y , T i l a k e t a l . ( 2 0 ) showed t h a t F o e r s t e r ' s m e c h a n i s m c o r r e s p o n d t o c h l o r a t e c e l l c o n d i t i o n s r a t h e r t h a n L i s t e r ' s . The r a t e f o r c h e m i c a l c h l o r a t e f o r m a t i o n u s i n g F o e r s t e r ' s m e c h a n i s m i s e x p r e s s e d - 14 -a s : d ^ C 1 0 2 d t 3 = K C C 1 0 - C H C 1 0 ( 1 5 ) The d e p e n d e n c e o f K ( a p p a r e n t r a t e c o n s t a n t ) on t e m p e r a t u r e was d e t e r m i n e d by K n i b b s and P a l f r e e m a n ( 1 4 ) f r o m 3 0 ° C t o 8 0 ° C . The r a t e o f f o r m a t i o n o f c h l o r a t e i s o b t a i n e d by m u l t i p l y i n g t h e r i g h t h a n d s i d e o f e q n . ( 1 5 ) by t h e v o l u m e o f t h e r e a c t o r , V . Thus a h i g h y i e l d o f c h l o r a t e i s o b t a i n e d by an e x t e r n a l r e a c t o r o f l a r g e v o l u m e . A c u r r e n t e f f i c i e n c y o f l e s s t h a n 100% i s t h e norm i n c h l o r a t e c e l l s i n i n d u s t r i a l p r a c t i c e . 2.4.2 Secondary Reactions Affecting Chlorate Current Efficiency S e v e r a l s i d e r e a c t i o n s o c c u r s i m u l t a n e o u s l y w i t h t h e f a v o u r a b l e r e a c t i o n s a t t h e e l e c t r o d e s and i n t h e b u l k o f t h e s o l u t i o n . T h e s e s i d e r e a c t i o n s d e c r e a s e t h e c h l o r a t e c u r r e n t e f f i c i e n c y . T h e s e s i d e r e a c t i o n s c o u l d be c o n s i d e r e d as e i t h e r a n o d i c , c a t h o d i c o r b u l k p a r a s i t i c r e a c t i o n s . 2.4.3 Anodic Side Reactions On t h e i n t r o d u c t i o n o f DSA ( D i m e n s i o n a l l y S t a b l e A n o d e s ) , t h e c h l o r a t e c u r r e n t e f f i c i e n c y was i m p r o v e d f r o m t h e 8 0 - 8 5 % r a n g e o f c a r b o n a n o d e s t o 9 3 - 9 6 % r a n g e . The p r i m a r y r e a c t i o n s t h a t a c c o u n t f o r t h i s e f f i c i e n c y l o s s i n DSA o p e r a t e d c h l o r a t e c e l l s a r e : ( a ) o x y g e n e v o l u t i o n ( b ) h y p o c h l o r i t e o x i d a t i o n The o x y g e n e v o l u t i o n i n c h l o r a t e c e l l d e p e n d s on t h e m a t e r i a l o f - 15 -the anode, the pH of the solution and the chloride concentration of the solution. The evolution of oxygen varies from 1.0% to 3% on noble metal based electrodes in the pH range 5.5 - 6.5. If chloride and chlorate ion concentrations are too low, the oxygen evolution will occur excessively. The oxygen evolution is represented by equation (16). 2H20 - 4H+ + 0 2 + 4e (16) ( E ° at 25°C = 1.23V SHE) The oxidation of CIO- at the anode has been a topic of much investigation for the past 80 years and i t is s t i l l of interest today. The interest in the oxidation l ies in i t being an alternative route to chemical chlorate formation. The reaction i s : 60C1" + 3H20 2C10 3 _ + 6H+ + | - 0 2 + 4 CI" + 6e (17) ( E ° at 25°C is 0.46 V) The hypochlorous acid formed in the immediate v ic ini ty of the anode and the resulting hypochlorite ions are oxidized at the electrode to chlorate ions as soon as formed (eqn. 17). If a l l chlorate is formed by reaction (17), the maximum efficiency attainable in chlorate cell would be 66.7%. In order to avoid this route to chlorate formation, the use of an external chemical reactor was introduced (17,18). Reaction (17) is minimized because of reaction (8) which is favoured by the larger external reactor that reduces the concentration of CIO - . The oxidation of chlorate can occur to a l i t t l e extent in the c e l l . C103- + H20 C I O 4 - + 2H+ + 2e (18) - 16 -2.4.4 Bulk P a r a s i t i c Reactions i n Chlorate Electrosynthesis Reactions (19) and (20) are two common bulk reactions whose occurrence lower the chlorate efficiency. These two reactions are pH dependent. Reaction (19) occurs i f an acid is added to the cell solution while the decomposition of 0C1 - (reaction (20)) can occur in presence of heavy metals. The decomposition of chlorate to form chlorine dioxide (the bleaching agent in pulp mill industries) is favoured at pH less than 1.0. C10 3 - + H + > C10 2 + H20 (21) Lister (40) studied reaction (21) carefully and found that at 40°C, the reaction proceeds at a rate of 1.0 x 10""2 g-mol/m3.sec in pure acid solutions at pH less than 1. 2.4.5 Cathodic Side Reactions The primary cathode reaction is hydrogen evolution 2H+ + 2e + H 2 (22) (E° at 25°C = 0.0 V) However, the thermodynamic and kinetic considerations favor 0C1 - and C I O 3 - reduction reactions (eqn. 23 and 24). HC1 + H0C1 + CI.2 + H20 (19) 20C1 - + 2C1 " + 0 2 (20) - 17 -O C l " + H 2 0 + 2e + C I " + 2 0 H - ( 2 3 ) C I O 3 - + 3 H 2 0 + 6 e > C I " + 6 0 H - ( 2 4 ) S o d i u m d i c h r o m a t e i s a d d e d i n t h e e l e c t r o l y t e ( 1 2 ) t o s u p p r e s s r e a c t i o n s ( 2 3 ) and ( 2 4 ) . Some o t h e r f u n c t i o n s a t t r i b u t e d t o d i c h r o m a t e i n c l u d e : (1) S u p p r e s s i n g t h e c o r r o s i o n o f Fe C a t h o d e ( 4 1 - 4 3 ) ( 2 ) B u f f e r i n g t h e e l e c t r o l y t e i n t h e r a n g e 5 - 7 ( 3 8 ) ( 3 ) I n h i b i t i n g t h e 0 2 e v o l u t i o n r e a c t i o n a t a n o d e ( 2 1 ) The r o l e o f d i c h r o m a t e as c o r r o s i o n s u p p r e s s o r h a s b e e n d i s p u t e d by V i s w a n a t h a n e t a l . ( 2 0 ) . A c c o r d i n g t o t h e m , t h e r e i s no d i f f e r e n c e b e t w e e n t h e o p e n c i r c u i t v o l t a g e o f a t y p i c a l c h l o r a t e c e l l s o l u t i o n i n t h e a b s e n c e o r p r e s e n c e o f d i c h r o m a t e . The b u f f e r i n g a c t i o n o f c h r o m a t e i s t r a c e d t o t h e e q u i l i b r u m r e a c t i o n . C r 2 0 7 = + H 2 0 + 2 C r 0 . + = + 2H+ ( 2 5 ) The r e m o v a l o f t h e c h r o m a t e f r o m t h e f i n a l p r o d u c t c o n s t i t u t e p a r t o f t h e s e p a r a t i o n s t e p s t h a t a r e c a r r i e d o u t b e f o r e s h i p p i n g t h e c h l o r a t e . 2.4.6 Suppression of Cathodic Secondary Reactions The c a t h o d e c u r r e n t l o s s e s due t o c h l o r a t e and h y p o c h l o r i t e r e d u c t i o n a r e g o v e r n e d by d i f f u s i o n o f i o n s t o w a r d s t h e e l e c t r o d e s u r f a c e and c o u l d be s u p p r e s s e d by c o n c e n t r a t i o n p o l a r i z a t i o n a t h i g h e n o u g h c u r r e n t d e n s i t y ( 4 4 ) . To r e a c h t h e c a t h o d e , t h e s e i o n s a p p r o a c h - 18 -t h e c a t h o d e by d i f f u s i o n and c o n v e c t i o n a g a i n s t an a d v e r s e p o t e n t i a l g r a d i e n t . In t h e p r e s e n c e o f e x c e s s o f s u p p o r t i n g e l e c t r o l y t e , t h e a d v e r s e p o t e n t i a l g r a d i e n t i s s m a l l and t h e t r a n s p o r t o f a n i o n s i s a f f e c t e d o n l y s l i g h t l y . In t h e a b s e n c e o f c o n v e c t i o n ( e . g . , p a s s a g e a c r o s s m e m b r a n e ) , a m a t e r i a l b a l a n c e f o r any s p e c i e s i c o u l d be w r i t t e n a s : N. = -Z .U.FC.V<t. - D . V C . ( 2 6 ) 1 1 1 1 1 1 w h e r e Ni = f l u x o f s p e c i e s i ( m o l e s / m s ) Z i = v a l e n c e o f s p e c i e s i U i = m o b i l i t y o f i ( m 2 • m o l e / J . s e c ) = D i / R T Cn- = c o n c e n t r a t i o n s o f s p e c i e s i ( m o l / m 3 ) V<|> = p o t e n t i a l g r a d i e n t (V/m) D i = d i f f u s i v i t y o f s p e c i e s i ( m 2 / s ) By i n c r e a s i n g t h e l o c a l c u r r e n t d e n s i t y , t h e p o t e n t i a l g r a d i e n t t e r m i s i n c r e a s e d , t h u s s u p p r e s s i n g t h e a n i o n t r a n s p o r t a t i o n t o t h e c a t h o d e . M u l l e r ( 4 5 ) f i r s t s u g g e s t e d t h e i n t r o d u c t i o n o f d i c h r o m a t e i n t h e i n d u s t r i a l c h l o r a t e p r o c e s s t o s u p p r e s s c a t h o d e h y p o c h l o r i t e r e d u c t i o n and t o a c h i e v e a s u i t a b l e pH b u f f e r i n g e f f e c t as w e l l as i n h i b i t i r o n c o r r o s i o n . The c h r o m e a c t i o n was t h e o r e t i c a l l y e x p l a i n e d as c o n s i s t i n g o f t h e f o r m a t i o n o f a t h i n c a t h o d e s u r f a c e l a y e r . T h i s p r o v i d e s h y p o c h l o r i t e i o n c o n c e n t r a t i o n p o l a r i z a t i o n and t h u s p r e v e n t s i t s r e d u c t i o n . Wagner (12) showed t h e o r e t i c a l l y t h a t t h e r e e x i s t s a c r i t i c a l - 19 -p o t e n t i a l d i f f e r e n c e a c r o s s t h e c a t h o d e d i f f u s i o n b o u n d a r y l a y e r o f t h e o r d e r o f o r g r e a t e r t h a n K T / ^ |e = 0 . 0 2 5 / |Z. | ( w h e r e k i s B o l t z m a n c o n s t a n t , T i s t e m p e r a t u r e i n K e l v i n , Z-j i s h y p o c h l o r i t e i o n v a l e n c y o r t h a t o f a n y o t h e r r e d u c i b l e a n i o n and e i s t h e e l e c t r o n c h a r g e ) , p r o v i d i n g t h e a d v e r s e p o t e n t i a l g r a d i e n t f o r n e g a t i v e l y c h a r g e d h y p o c h l o r i t e i o n so t h a t t h e i r r e d u c t i o n i s e f f e c t i v e l y s u p p r e s s e d . -W i t h a r e l a t i v e l y l o w c u r r e n t d e n s i t y o r p o t e n t i a l g r a d i e n t , c h r o m a t e r e d u c t i o n t a k e s p l a c e and w i l l c e a s e a f t e r a s u f f i c i e n t c a t h o d e l a y e r o f c h r o m i u m o x i d e i s f o r m e d . U n d e r s u c h c o n d i t i o n s , an a n i o n a p p r o a c h i n g t h e c a t h o d e w i l l be r e p u l s e d by an a d v e r s e p o t e n t i a l g r a d i e n t . Thus an e f f e c t i v e d i a p h r a g m i s f o r m e d on t h e c a t h o d e w i t h o u t e x c e s s i v e IR d r o p o r e l e c t r o d e p o l a r i z a t i o n . 2.4.7 Energy Consumption in Chlorate Cells The c h l o r a t e s y n t h e s i s i s one o f t h e m o s t e n e r g y i n t e n s i v e e l e c t r o c h e m i c a l p r o c e s s e s o f c o m m e r c i a l i m p o r t a n c e . The e n e r g y c o n s u m p t i o n o f a c h l o r a t e c e l l i s a f u n c t i o n o f c u r r e n t e f f i c i e n c y , C E , e x p r e s s e d as a f r a c t i o n and c e l l v o l t a g e , E i n v o l t s : P = 4 . 9 4 x 1 0 6 x E/CE ( k J / t o n ) ( 2 7 ) The c u r r e n t e f f i c i e n c y i s a f u n c t i o n o f o p e r a t i n g c h a r a c t e r i s t i c s and c e l l d e s i g n . The c e l l v o l t a g e i s made up o f t h e r m o d y n a m i c d e c o m p o s i t i o n v o l t a g e s o f t h e anode and c a t h o d e , a n o d i c and c a t h o d i c o v e r v o l t a g e s , o h m i c d r o p b e t w e e n a n o d e and c a t h o d e due t o g a s / e l e c t r o l y t e m i x t u r e and o h m i c d r o p a c r o s s t h e h a r d w a r e . - 20 -2.4.8 Energy Conservation i n Ch lorate C e l l s The need for reducing the energy requirement in a chlorate cell is demonstrated by the fact that about 50% of the production costs is energy related while only 30% accounts for capital investments. Some ways of achieving this reduction in energy include a high temperature cell operation and thermodynamic voltage reduction. The operation of a high temperature chlorate cell is constrained by the high cost of materials of construction that will withstand the high temperature and the oxidative environment of chlorate cel l . The theoretical decomposition voltage for chlorate electrosynthesis at 25°C is 2.19 V(SHE). Most commercial operations run at 3-4 volts at current densities of 2-3 kA m - 2 per ce l l . The reduction in the thermodynamic decomposition voltage could be achieved by the use of an air (oxygen) depolarized cathode (9,21). However, to contemplate such reductions, one must evaluate the impact such a reduction will have on capital and operating costs of the ce l l . Traditionally, the hydrogen obtained in a chlorate cell is used as a boiler fuel. The present work is aimed at depolarizing the cathode using oxygen or air and at the same time obtaining an alkaline peroxide by product from the cathode and chlorate from the anode. It is hoped that the alkaline peroxide may be useful to the pulp mills. 2.5 E lectrochemistry of Hydrogen Peroxide Hydrogen peroxide can be prepared,electrochemically either through the electrolysis of persulphuric acid or through the reduction of oxygen in alkaline or acid solutions. - 21 -2.5.1 Electrochemical Reduction of Oxygen i n A l k a l i n e Solution D i s s o l v e d o x y g e n i n an e l e c t r o l y t e c a n be r e d u c e d a t a c a t h o d e t o h y d r o g e n p e r o x i d e o r t o w a t e r . V a r i o u s c e l l s u s i n g 0 2 e l e c t r o d e s h a v e b e e n c o n t e m p l a t e d and some a r e s t i l l b e i n g i n v e s t i g a t e d . The v o l t a g e l o s s e s a s s o c i a t e d w i t h some o f s u c h c e l l s r e s u l t i n s u b s t a n t i a l e n e r g y l o s s e s and t h u s s e r i o u s l y c o m p r o m i s e t h e a p p l i c a b i l i t y o f many s u c h c e l l s . L i m i t e d c e l l l i f e due t o p r e c i p i t a t i o n o f s o d i u m p e r o x i d e s a n d h i g h c o s t a r e f u r t h e r p r o b l e m s a s s o c i a t e d w i t h o x y g e n c o n s u m i n g c a t h o d e s . The t r i c k l e bed e l e c t r o c h e m i c a l r e a c t o r d e v e l o p e d by Oloman and W a t k i n s o n ( 3 1 ) , d o e s n o t seem t o s u f f e r f r o m N a H 0 2 p r e c i p i t a t i o n d e t e r i o r a t i o n . Oxygen e l e c t r o c h e m i s t r y p l a y s an i m p o r t a n t r o l e i n c o r r o s i o n p r o c e s s e s . I n t h e p r e s e n c e o f a i r , o x y g e n r e d u c t i o n i s o f t e n t h e c a t h o d i c p r o c e s s w h i c h d r i v e s t h e p o t e n t i a l o f i r o n and f e r r o u s a l l o y s i n t o t h e p a s s i v a t i o n r a n g e w h e r e c o r r o s i o n i s i n h i b i t e d . I t i s t h u s i m p o r t a n t t o u n d e r s t a n d t h e e l e c t r o c h e m i s t r y o f 0 2 . Oxygen c a t h o d e s d e v i a t e f r o m t h e r m o d y n a m i c p o t e n t i a l by 0 . 3 t o 0 . 4 V i n a l k a l i n e e l e c t r o l y t e s a t o p e r a t i n g t e m p e r a t u r e s o f 60 t o 8 0 ° C . T h i s p o l a r i s a t i o n i s m a i n l y a s s o c i a t e d w i t h t h e i r r e v e r s i b i l i t y o f t h e o v e r a l l 0 2 r e d u c t i o n r e a c t i o n s i n a l k a l i s o l u t i o n - t h e s t o i c h i o m e t r y o f w h i c h i s g i v e n by L a t i m e r ( 4 6 ) a s f o l l o w s : 0 2 + 2 H 2 0 + 4e *=2s 4 0 H " E ° = 0 . 4 0 1 V ( 2 8 ) 2.5.2 Pathways f o r Oxygen Reduction Two m e c h a n i s t i c c o n s i d e r a t i o n s f o r 0 2 r e d u c t i o n a r e c o n s i d e r e d i n most w o r k s v i z : - 22 -(i) The direct 4 electron pathway and ( i i ) The peroxide pathway (the series processes) (i) The Direct 4-Electron Pathway The 4-electron pathway involves a series of steps in which 0 2 is reduced to OH- or water without hydrogen peroxide being produced in the solution phase (eqn. 28). The reduction process involves adsorbed peroxide intermediate which does not appear in the solution phase ( i . e . , does not desorb). The distinctions between this pathway and the peroxide pathway are dependent on the purity of the electrolyte, electrode potential and temperature. This pathway is predominant on clean platinum (47). ( i i ) The Peroxide Pathway In alkaline solution, the peroxide pathway of oxygen is believed to proceed as follows (47): H20 + 0 2 + 2e_ ^ = H 0 2 - ( a d s ) + Oh" or H20 + 0 2 + 2e" ^ H 0 2 " + OH" (29) (E° = -0.076VSHE) The peroxide is further electroreduced to OH" or catalyt ical ly decomposed i .e.: Peroxide reduction: H20 + H0 2- + 2e + 30H- E° = 0.88V (30) or HOH + H02-(ads) •»• 30rT - 23 -P e r o x i d e c a t a l y t i c d e c o m p o s t i o n 2 H 0 2 _ * 2 0 H - + 0 2 2 H 0 2 " ( a d s ) > 2 0 H - + 0 2 ( 3 1 ) Net R e a c t i o n : 0 2 + 2 H 2 0 + 4 e " > 4 0 H " E ° = + 0 . 4 0 1 ( 3 2 ) Thus t h e n e t r e a c t i o n o f p e r o x i d e p a t h w a y i s same as t h e f o u r e l e c t r o n p a t h w a y . The H 0 2 _ i s t h e p e r h y d r o x y l i o n w h i c h i s f o r m e d i n a l k a l i n e a q u e o u s s o l u t i o n when h y d r o g e n p e r o x i d e i s d i s s o l v e d . H 2 0 2 -»• H+ + H 0 2 - ( 3 3 ) The d i s s o c i a t i o n c o n s t a n t o f t h i s r e a c t i o n i s g i v e n by K = [ H + ] [ H 0 2 - ] / [ H 2 0 2 ] = 2 . 4 x 1 0 ~ 9 mol m " 3 a t 2 5 ° C ( 3 4 ) The m e c h a n i s m o f t h e o x y g e n e l e c t r o d e has been w i d e l y s t u d i e d i n t h e p a s t . M e c h a n i s t i c c o m p i l a t i o n s ( w h i c h d e p e n d on r e a c t i o n c o n d i t i o n s ) h a v e r e s u l t e d t o r e p o r t e d T a f e l b e h a v i o u r d i f f e r e n c e s f r o m one i n v e s t i g a t i o n t o a n o t h e r . A p p e l b y ( 4 8 ) , r e p o r t e d t h r e e T a f e l r e g i o n s f o r o x y g e n r e d u c t i o n w h i l e many o t h e r s r e p o r t e d t w o . T h e r e i s a l w a y s a l o w s l o p e r e g i o n o f a p p r o x i m a t e l y 60 m v / d e c a d e and a h i g h r e g i o n r a n g i n g f r o m 120 mv t o o v e r 200 m v / d e c a d e . The o r i g i n o f t h e s e l a r g e r T a f e l s l o p e s i s n o t w e l l u n d e r s t o o d . D a v i e s e t a l . ( 4 9 ) h o w e v e r e s t a b l i s h e d t h a t t h e r e d u c t i o n o f 0 2 t o H 0 2 _ as w e l l as t h e r e v e r s e a n o d i c p r o c e s s do n o t i n v o l v e a c t u a l s p l i t - 24 -o f t h e O2 bond b u t r a t h e r a m o d i f i c a t i o n o f t h e bond o c c u r s . The e l c t r o c h e m i c a l r e a c t i o n o r d e r s o f r e a c t i o n ( 2 9 ) a r e g i v e n a s +1 f o r 0 2 and 0 o r +1 f o r H + i n a c i d o r a l k a l i n e s o l u t i o n r e s p e c t i v e l y by V e t t e r ( 5 0 ) . In r e a c t i o n ( 3 0 ) , t h e o r d e r i s +1 f o r H 2 0 2 ( t h e n e u t r a l m o l e c u l e ) and 0 f o r H + o v e r t h e w h o l e pH r a n g e . The e q u i l i b r i u m c o n s t a n t o f p e r o x i d e c a t a l y t i c d e c o m p o s i t i o n r e a c t i o n ( e q n . 31) i s c a l c u l a t e d u s i n g d a t a f o r t h e f r e e e n e r g i e s o f f o r m a t i o n ( 3 2 ) a s : K = [ 0 2 ] [ 0 H - ] 2 / [ H 0 2 - ] 2 - 1 0 1 6 a t m . a t 2 5 ° C ( 3 5 ) U s i n g t h e a b o v e d a t a f o r t h e t h e r m o d y n a m i c s o f 0 2 r e d u c t i o n w i t h t h e N e r n s t e q u a t i o n , Oloman ( 3 2 ) p r e d i c t e d an e q u i l i b r i u m c o n c e n t r a t i o n o f 1 0 - 1 6 M H 0 2 - at 2 5 ° C , pH=14 and o x y g e n p a r t i a l p r e s s u r e o f 1 a t m . U n d e r t h e same c o n d i t i o n s , c h e m i c a l e q u i l i b r i u m o f 1 0 - 8 M was c a l c u l a t e d . Thus t h e p r e d i c t i o n f r o m t h e r m o d y n a m i c s shows c l e a r l y t h a t p e r o x i d e i s n o t a c c u m u l a t e d i n t h i s p r o c e s s . The o n l y p o s s i b l e means o f k n o w i n g t h e f e a s i b i l i t y o f m a k i n g H 2 0 2 i s t o l o o k i n t o t h e k i n e t i c s o f t h e r e a c t i o n . The k i n e t i c s o f o x y g e n r e d u c t i o n on a c a t h o d e a r e d e p e n d e n t on many f a c t o r s . S u c h f a c t o r s i n c l u d e : ( a ) C a t h o d e s u r f a c e c o m p o s i t i o n ( b ) E l e c t r o l y t e c o m p o s i t i o n and p u r i t y ( c ) Mass t r a n s f e r o f a c t i v e s p e c i e s b e t w e e n c a t h o d e and c a t h o l y t e ( d ) C u r r e n t d e n s i t y ( e ) T e m p e r a t u r e o f c a t h o l y t e - 25 -a . C a t h o d e s u r f a c e c o m p o s t i o n The n a t u r e o f c a t h o d e s u r f a c e d e t e r m i n e s t o a l a r g e e x t e n t t h e k i n e t i c s o f o x y g e n r e d u c t i o n . The work o f Y e a g e r (47) g i v e s i l l u s t r a t i o n s o f c a t a l y s t s ( s u r f a c e s ) on w h i c h t h e p e r o x i d e p a t h w a y i s c l e a r l y p r e d o m i n a n t as i n c l u d i n g c a r b o n , g r a p h i t e and g o l d ( i n a l k a l i n e e l e c t r o l y t e s ) , w h i l e t h e 4 - e l e c t r o n p a t h w a y p r e d o m i n a t e s o n c l e a n , p l a t i n u m and c e r t a i n t r a n s i t i o n m e t a l m a c r o c y l i c s s u c h as a d s o r b e d i r o n t e t r a s u l p h o n a t e d p h t h a l o c y n a n i n e . I n t h e p r e s e n c e o f i m p u r i t i e s , t h e s e r i e s p a t h w a y c a n become p r e d o m i n a n t e v e n on c l e a n p l a t i n u m . M o s t e f f o r t i n o x y g e n r e d u c e d c a t h o d e s t u d i e s i s g e a r e d t o w a r d s d e v e l o p i n g an e f f e c t i v e c a t a l y s t f o r t h e r e d u c t i o n . The 4 - e l e c t r o n p a t h w a y i s o f i m p o r t a n c e t o t h e f u e l c e l l i n d u s t r y and c h l o r - a l k a l i p l a n t s . However i n t h e p r e s e n t s t u d i e s , t h e o b j e c t i v e i s t o a l l o w t h e a c c u m u l a t i o n o f p e r o x i d e i . e . p e r o x i d e p a t h w a y i s t h e m e c h a n i s m o f i m p o r t a n c e i n t h e p r e s e n t w o r k . A c c o r d i n g t o Y e a g e r (47), t r a n s i t i o n m e t a l o x i d e s a n d m a c r o c y c l i c c o m p l e x e s a p p e a r more l i k e l y c a n d i d a t e c a t a l y s t s f o r t h e 4 - e l e c t r o n p a t h w a y and t o d a y most o x y g e n e l e c t r o d e i n v e s t i g a t o r s a r e i n t h e a r e a o f m a c r o c y c l i c m o l e c u l e s . b . E l e c t r o l y t e c o m p o s i t i o n The p e r o x i d e p a t h w a y i s t h e p r e d o m i n a n t p r o c e s s i n 0 2 r e d u c t i o n i n p o r o u s c a r b o n ( g r a p h i t e ) e l e c t r o d e s . U n d e r t h e s e c o n d i t i o n s t h e n , t h e e l e c t r o d e p o t e n t i a l w i t h i n t h e e l e c t r o d e r e s p o n d s t o l o c a l 0 2 , O H - a n d H 2 0 c o n c e n t r a t i o n s t h r o u g h t h e N e r n s t e q u a t i o n i . e . , F - F R T i n [ 0 H - ] [ H O 9 - ] ( 3 6 ) E l - E 0 , i - W I n [ H 2 0 j [o2] ^b> - 26 -c _ c R T i [OH"] 3 t 2 = E o , 2 - 2F 1 n LH2O-KH2OJ ( 3 7 ) w h e r e t h e q u a n t i t i e s i n b r a c k e t s c o r r e s p o n d t o t h e a c t i v i t i e s o f t h e i n d i c a t e d s p e c i e s . E q u a t i o n ( 3 6 ) shows t h a t an i n c r e a s e i n o x y g e n p r e s s u r e a n d a d e c r e a s e i n H 0 2 " a n d OH" i o n c o n c e n t r a t i o n f a v o u r s p e r o x i d e p r o d u c t i o n . On t h e o t h e r h a n d , e q u a t i o n ( 3 7 ) shows t h a t - a d e c r e a s e i n OH" c o n c e n t r a t i o n s f a v o u r s p e r o x i d e r e d u c t i o n . P a r k e t a l . ( 5 4 ) u s i n g KOH ( w i t h p l a t i n u m e l e c t r o d e ) showed t h a t t h e r a t e o f h y d r o g e n p e r o x i d e p r o d u c t i o n a n d i t s d e c o m p o s i t i o n a p p e a r t o i n c r e a s e a f t e r p a s s i n g t h r o u g h a maximum i n 0 . 3 M KOH a s t h e KOH c o n c e n t r a t i o n • i n c r e a s e s . I t i s s u g g e s t e d as p e r h a p s due t o t h e f o r m a t i o n o f an i n t e r m e d i a t e p r o d u c t o f s u p e r o x i d e i o n i . e . , 0 2 + e " -> 0 2 " ( 3 8 ) w h i c h i s s t a b i l i z e d i n n o n p r o t i c m e d i u m . R e a c t i o n ( 2 9 ) i s d e c r e a s e d by an i n c r e a s e i n pH b e c a u s e t h e r a t e o f t h e r e a c t i o n i s d e t e r m i n e d by t h e c o n c e n t r a t i o n o f t h e n e u t r a l h y d r o g e n p e r o x i d e m o l e c u l e b u t n o t by H 0 2 " i o n ( 3 2 ) . 01oman ( 3 2 ) p r e s e n t e d i n g r a p h i c a l f o r m t h e a c t u a l r e l a t i o n s h i p b e t w e e n t h e c o n c e n t r a t i o n o f n e u t r a l s p e c i e s o f H 2 0 2 and t h e pH o f t h e s o l u t i o n . c . M a s s T r a n s f e r Oxygen has a l o w s o l u b i l i t y i n c o n c e n t r a t e d c a u s t i c , f o r e x a m p l e 2 x 1 0 - 5 M i n 9 M NaOH (30% by w e i g h t ) a t 8 5 ° C . A t 2 0 ° C , t h e s o l u b i l i t y o f o x y g e n i n d i l u t e w a t e r s o l u t i o n s o f e l e c t r o l y t e s i s a p p r o x i m a t e l y - 27 -1 . 3 x 1 0 - 3 m o l e p e r l i t r e . C o n s e q u e n t l y , t o a c h i e v e p r a c t i c a l c u r r e n t d e n s i t i e s i n 0 2 r e d u c t i o n , an e f f e c t i v e mass t r a n s f e r b e t w e e n t h e c a t h o d e and t h e c a t h o l y t e i s i m p o r t a n t . V a r i o u s means o f a c h i e v i n g h i g h mass t r a n s f e r r a t e ( i . e . , i n c r e a s e i n mass t r a n s f e r c o e f f i c i e n t ) h a v e been e m p l o y e d - s u c h a s t h e u s e o f p o r o u s e l e c t r o d e s , g a s b u b b l i n g , e l e c t r o d e v i b r a t i o n , f a s t f l o w o f e l e c t r o l y t e , eddy p r o m o t e r s , e t c . The l i m i t i n g c u r r e n t d e n s i t y , i | _ , l i m i t s t h e p r o d u c t i v i t y o f t h e c e l l as i t i s d e p e n d e n t on t h e maximum o x y g e n t r a n s f e r r a t e t o t h e c a t h o d e . T h i s l i m i t i n g c u r r e n t d e n s i t y i s g i v e n by \ = 2 F K d C b , ( 3 9 ) w h e r e i ^ = o x y g e n t r a n s f e r l i m i t e d c u r r e n t d e n s i t y (A m - 2 ) C D = b u l k o x y g e n c o n c e n t r a t i o n (mol m - 3 ) F = F a r a d a y s number ( 9 6 5 0 0 c o u l o m b s / g m e q u i v ) = o x y g e n t r a n s f e r c o e f f i c i e n t (m s _ 1 ) T h e o x y g e n t r a n s f e r c o e f f i c i e n t d e p e n d s on t h e h y d r o d y n a m i c c o n d i t i o n s ( i n c l u d i n g p a r t i c l e s i z e ) i n t h e r e a c t o r . The l i m i t i n g c u r r e n t d e n s i t y i s t h u s d e p e n d e n t on t h e h y d r o d y n a m i c c o n d i t i o n s i n t h e r e a c t o r . When t h e o p e r a t i n g c u r r e n t d e n s i t y e x c e e d s t h e l i m i t i n g c u r r e n t d e n s i t y , a l t e r n a t i v e r e a c t i o n s ( e . g . , r e d u c t i o n o f p e r o x i d e o r H 2 l i b e r a t i o n ) t a k e p l a c e . I t i s a l s o known t h a t e v e n b e l o w t h e l i m i t i n g c u r r e n t d e n s i t y a l t e r n a t e r e a c t i o n s do o c c u r . The e x t e n t o f t h e s e r e a c t i o n s i s a c o m p l e x i n t e r a c t i o n o f t h e r e a c t i o n t h e r m o d y n a m i c s , i n t e r f a c i a l c o n c e n t r a t i o n o f t h e r e a c t i v e s p e c i e s and t h e e l e c t r o d e - 28 -k i n e t i c s . I m p r o v e m e n t i n t h e v a l u e o f K<j g e n e r a l l y p r o m o t e s 0 2 d i s s o l u t i o n and t h e s u p p r e s s i o n o f p e r o x i d e r e d u c t i o n , w h i l e i n c r e a s e i n c u r r e n t d e n s i t y on t h e e l e c t r o d e l o w e r s t h e i n t e r f a c i a l 0 2 c o n c e n t r a t i o n t h u s r a i s i n g t h e ( c a t h o d i c ) p o t e n t i a l and e n h a n c i n g t h e r a t e o f p e r o x i d e r e d u c t i o n ( s e e Oloman ( 3 2 ) ) . ( d ) C u r r e n t D e n s i t y The e f f e c t o f c u r r e n t d e n s i t y on a l k a l i n e p e r o x i d e e l e c t r o g e n e r a t i o n i s v e r y i m p o r t a n t as c o u l d be d e d u c e d f r o m t h e t w o c o n s e c u t i v e l o s s r e a c t i o n s o f p e r o x i d e p r o d u c t i o n , v i z : H 0 2 - + H 2 0 + 2e + 3 0 H - E ° = 0 . 8 8 V ( 4 0 ) H 0 2 " + OH" — ^ . 0 2 + H 2 0 + 2e E ° = 0 . 0 8 V ( 4 1 ) From t h e w o r k s o f Oloman and W a t k i n s o n ( 3 1 ) , M c l n t y r e and P h i l l i p s [ 3 4 ] and Brown e t a l [ 3 3 ] , i t i s known t h a t i n c r e a s e i n c u r r e n t d e n s i t y l e a d s t o l o s s o f c u r r e n t e f f i c i e n c y i n p e r o x i d e p r o d u c t i o n . T h i s was e x p l a i n e d as d u e t o t h e i n c r e a s e i n t h e c a t h o d e o v e r v o l t a g e ( w i t h i n c r e a s e i n c u r r e n t d e n s i t y ) w h i c h r e s u l t s i n a more n e g a t i v e c a t h o d e p o t e n t i a l . As t h e c a t h o d e becomes more n e g a t i v e , t h e r a t e o f r e a c t i o n ( 4 0 ) i n c r e a s e s . B e c a u s e r e a c t i o n ( 4 0 ) i s k i n e t i c a l l y s l o w , i t i s p o s s i b l e t o i s o l a t e p e r o x i d e a t e v e n h i g h e r c u r r e n t d e n s i t i e s . The e f f e c t o f t h i s l o s s m e c h a n i s m c a n be r e d u c e d by o p e r a t i n g a t l o w e r c u r r e n t d e n s i t i e s o r by i n c r e a s i n g t h e s p e c i f i c s u r f a c e a r e a o f t h e c a t h o d e . - 29 -( e ) T e m p e r a t u r e The r a t e o f o x y g e n r e d u c t i o n on a c a t h o d e i s t e m p e r a t u r e d e p e n d e n t . H o w e v e r , t h e r e i s r e l a t i v e l y l i t t l e d a t a on t h e t e m p e r a t u r e d e p e n d e n c y o f t h i s r e d u c t i o n . The s o l u b i l i t y o f o x y g e n i n a q u e o u s s o l u t i o n i s t e m p e r a t u r e d e p e n d e n t , t h u s an i n c r e a s e i n t e m p e r a t u r e r e d u c e s t h e amount o f s o l u b l e o x y g e n and h e n c e t h e amount o f o x y g e n " t r a n s f e r r e d t o t h e e l e c t r o d e . The d i s s o c i a t i o n ( c h e m i c a l ) o f p e r o x i d e w i l l a l s o be a f f e c t e d by t e m p e r a t u r e - t h u s t h e r a t e o f t h e e l e c t r o -c h e m i c a l p r o c e s s w i l l be a f f e c t e d by t h e t e m p e r a t u r e . I n t h e d o c u m e n t e d e l e c t r o c h e m i c a l s y n t h e s i s o f p e r o x i d e ( 3 1 , 3 3 , 3 4 ) , no a t t e m p t was made t o i n v e s t i g a t e t h e i n f l u e n c e o f t e m p e r a t u r e on p e r o x i d e c o n c e n t r a t i o n o r c u r r e n t e f f i c i e n c y . 2.5.3 Hydrogen Peroxide Production i n T r i c k l e Bed Electrochemical  Reactors Oloman and W a t k i n s o n ( 2 8 ) d e v e l o p e d a t r i c k l e bed e l e c t r o c h e m i c a l r e a c t o r f o r t h e p r o d u c t i o n o f d i l u t e a l k a l i n e p e r o x i d e s o l u t i o n s by r e d u c t i o n o f o x y g e n . The t r i c k l e - b e d e l e c t r o c h e m i c a l r e a c t o r c o n s i s t e d o f a m e t a l c a t h o d e p l a t e , a t h i n bed o f g r a p h i t e p a r t i c l e s , a p o r o u s n o n - c o n d u c t i n g d i a p h r a g m ( c o u l d be a c a t i o n i c membrane) and a m e t a l a n o d e p l a t e c o m p r e s s e d i n a s a n d w i c h a s shown i n F i g 2 . The g r a p h i t e b e d s w e r e c o n t a i n e d i n a s b e s t o s / n e o p r e n e g a s k e t s and t h e e l e c t r o d e p l a t e s h e l d b e t w e e n b o l t e d m i l d s t e e l c h a n n e l s t o h o l d i n t e r n a l p r e s s u r e s o f up t o 2000 k P a . The O l o m a n - W a t k i n s o n t r i c k l e - b e d c e l l c o u l d h a v e a s i n g l e e l e c t r o l y t e f e e d as shown i n F i g 2 o r w i t h some m i n o r m o d i f i c a t i o n s a - 30 -Oxygen + Electrolyte Peroxide solution Anode plate Gaskets Porous diaphragm Cathode bed Cathode plate F i g . 2 T r i c k l e bed e l e c t r o c h e m i c a l c e l l f o r g e n e r a t i o n o f a l k a l i n e h y d r o g e n p e r o x i d e . (from ref. 28) - 31 -s e p a r a t e a n o l y t e and c a t h o l y t e f e e d s . The a n o d e e l e c t r o d e and t h e c a t h o d e c u r r e n t f e e d e r e l e c t r o d e w e r e 316 s t a i n l e s s s t e e l . Oxygen g a s was g e n e r a t e d on t h e a n o d e . 2.6 Coupling of Cathode Reaction of OToman-Hatkinson Peroxide C e l l with  Anode Reaction of a Chlorate C e l l T h e m o t i v a t i o n i n t h i s work i s t o e x p l o r e t h e f e a s i b i l i t y o f m a k i n g a c h e a p a l k a l i n e p e r o x i d e and s o d i u m c h l o r a t e i n t h e same e l e c t r o -c h e m i c a l c e l l . The i n c o r p o r a t i o n o f s u c h a p r o c e s s i n a p u l p m i l l w o u l d n o t o n l y make c h e a p p e r o x i d e a v a i l a b l e b u t w o u l d remove t h e p r o b l e m s a s s o c i a t e d w i t h c h r o m i u m e f f l u e n t i n C 1 0 2 g e n e r a t i o n i n p u l p m i l l s . The f i x e d - b e d c e l l d e v e l o p e d by Oloman a n d W a t k i n s o n [ 3 0 ] i s a c a n d i d a t e c e l l f o r s u c h a p r o c e s s . The 0 1 o m a n - W a t k i n s o n c e l l i s a d i v i d e d c e l l w i t h a c a t h o d e o f f i x e d c a r b o n bed f o r g e n e r a t i n g a l k a l i n e p e r o x i d e and an a n o d e e l e c t r o d e o f s t a i n l e s s s t e e l a t w h i c h 0 2 i s g e n e r a t e d . The s e p a r a t o r u s e d c o u l d be a d i a p h r a g m o r c a t i o n m e m b r a n e . In c o n v e r t i n g t h i s c e l l i n t o a p e r o x y - c h l o r a t e c e l l , t h e a n o d e e l e c t r o d e s h o u l d be a DSA and t h e s e p a r a t o r c o u l d be a d i a p h r a g m o r a n a n i o n s p e c i f i c m e m b r a n e . An a n i o n s p e c i f i c membrane i s p r e f e r r e d . 2.6.1 Process Description F i g u r e 3 shows s c h e m a t i c a l l y t h e b a s i c r e a c t i o n s i n a membrane p e r o x y - c h l o r a t e c e l l . An e x p l a n a t i o n o f t h e b a s i c p r o c e s s p r i n c i p l e r e q u i r e s an u n d e r s t a n d i n g o f t h e i o n t r a n s p o r t a c r o s s t h e s e p a r a t o r . The a n i o n e x c h a n g e membrane w h i c h d i v i d e s t h e c e l l i n t o an a n o d e and a c a t h o d e c h a m b e r i s h y d r o d y n a m i c a l l y i m p e r m e a b l e b u t p e r m i t s t h e p a s s a g e - 32 -o f n e g a t i v e l y c h a r g e d a n i o n s . A l l i o n s i n t h e c e l l (when i n o p e r a t i o n ) a r e u n d e r a c o m b i n e d i n f l u e n c e o f e l e c t r i c f i e l d ( m i g r a t i o n a l ) , c o n c e n t r a t i o n p o t e n t i a l ( d i f f u s i o n ) and c o n v e c t i o n f o r c e s . T h e s e c o m b i n e d f o r c e s a r e r e s p o n s i b l e f o r i o n t r a n s p o r t i n s o l u t i o n a n d a c r o s s t h e m e m b r a n e . An a n a l y s i s o f t h e i o n movement i n t h e r e a c t o r c o u l d be g i v e n u s i n g t h e N e r n s t - E i n s t e i n e q u a t i o n : N i = " Z i D i C ( 4 2 ) Membrane + 0 2 +HzO + 2e -> H0 2 +OH" Cathode a2(g) + H 2 O - * H 0 C l + H+ + Cr 6 0 C 1 - +3H zO 6 4 2 C 1 0 3 " +6H++ 3/20 2 + 4Cr Anode 2Ci:-» a 2 +2e Mixture of Brine oxygen and feed caustic feed (Anolyte) (Catholyte) F i g . 3 S c h e m a t i c d i a g r a m o f a membrane p e r o x y - c h l o r a t e c e l l s h o w i n g e l e c t r o d e r e a c t i o n s and t r a n s p o r t p r o c e s s e s . - 33 -T h e a d d i t i o n o f a w a t e r f l u x t e r m ( C-JV ) i s a c c o r d i n g t o Koh ( 5 1 ) and K r u i s s i n k ( 5 2 ) . On t h e a p p l i c a t i o n o f t h i s e q u a t i o n t o t h e a n o l y t e s i d e a n i o n s ( C I - , C I O 3 - , C I O - and C l 3 _ ) , t h e e l e c t r i c a l ( o r m i g r a t i o n a l ) f l u x [DC(jpjr) ^ ] and w a t e r f l u x t e r m s ( C ^ v ) f a v o u r t h e t r a n s p o r t a t i o n o f t h e s e a n i o n s t o w a r d s t h e a n o d e w h i l e t h e d i f f u s i o n t e r m (D ^ ) f a v o u r t h e movement o f t h e a n i o n s t o w a r d s t h e c a t h o d e . The c o n s e q u e n c e o f t h e s e o p p o s i n g f l u x e s i s t h a t t h e n e t f l u x o f a n o l y t e a n i o n s c o u l d be t o w a r d s t h e a n o d e i n s t e a d o f t h e c a t h o l y t e . [The n e t f l u x d e p e n d s on r e l a t i v e m a g n i t u d e o f DVC ( t o w a r d t h e c a t h o l y t e ) and ZFCDV<j) ( t o w a r d t h e a n o d e ) - h e n c e d e p e n d s on c u r r e n t d e n s i t y and c o n c e n t r a t i o n o f C I O - , C I 0 3 " , C I - and C I 3 - ] . T h e r e f o r e , t h e n e t f l u x o f a n o l y t e a n i o n s i n t o t h e c a t h o l y t e w o u l d be l o w e r t h a n t h e f l u x t o w a r d s t h e a n o d e s i d e ( i f o t h e r c o n d i t i o n s f a v o u r t h i s ) . F o r i n s t a n c e , t h e n e t m i g r a t i o n a l f l u x f o r C I " i o n s i n 3 . 0 M NaCl a t 8 0 ° C i s 4 . 7 5 x IQ-1* m o l e s / m 2 s ( a s s u m i n g a p o t e n t i a l d r o p o f 0 . 8 V a c r o s s a membrane o f t h i c k n e s s 2 . 5 4 x 1 0 - 5 m ) . The C I - s e l f d i f f u s i o n c o e f f i c i e n t i n t h e membrane i s assumed t o be 1 5 . 3 x 1 0 " 1 1 m 2 s _ 1 ( 5 3 ) . U n d e r t h e same c o n d i t i o n s , t h e c o n c e n t r a t i o n f l u x i s - 1 . 8 1 x 1 0 - 5 mol ( m 2 s ) _ 1 . Thus t h e n e t c h l o r i d e f l u x t o w a r d s t h e a n o d e w i l l be 4 . 5 7 x 10" 4 mol ( m 2 s ) - 1 . On c o n s i d e r i n g t h e a n i o n s i n t h e c a t h o l y t e , t h e e l e c t r i c a l f l u x t e r m , w a t e r f l u x t e r m and t h e d i f f u s i o n f l u x d r i v e t h e a n i o n s t o w a r d s - 34 -t h e a n o d e . I t i s t h u s p o s s i b l e t o h a v e OH" i o n s t r a n s p o r t f r o m t h e c a t h o l y t e i n t o t h e a n o l y t e f o r n e u t r a l i z a t i o n o f t h e H + i o n s t h a t r e s u l t f r o m t h e C l 2 gas h y d r o l y s i s . The c a t i o n t r a n s p o r t a c r o s s t h e membrane i s p r e v e n t e d . E q u a t i o n s (1) and ( 2 ) show t h e u n e q u a l b a l a n c e i n h y d r o x y l i o n g e n e r a t i o n ( b y t h e c a t h o d e r e a c t i o n s ) and h y d r o x y l i o n c o n s u m p t i o n ( b y t h e c h l o r a t e f o r m a t i o n ) . F o r s u c c e s s f u l s y n t h e s i s o f c h l o r a t e , t h e b a l a n c e b e t w e e n t h e h y d r o x y l r e q u i r e m e n t s i n t h e a n o d e a n d t h e h y d r o x y l i o n g e n e r a t i o n i n t h e c a t h o d e c o u l d be s u p p l i e d by t h e c a u s t i c ^ e l e c t r o l y t e u s e d i n t h e c a t h o d e . The s u c c e s s f u l p r o d u c t i o n o f c h l o r a t e and p e r o x i d e i n t h e same c e l l w i l l d e p e n d on t h e s e l e c t i v e t r a n s p o r t o f O H - i o n s o v e r t h e H 0 2 ~ i o n s a c r o s s t h e s e p a r a t o r f r o m t h e c a t h o l y t e t o t h e a n o l y t e . As shown i n e q u a t i o n ( 4 2 ) , t h e s e l e c t i v e t r a n s p o r t o f OH" i o n s o v e r H 0 2 _ i o n w o u l d d e p e n d on b o t h t h e d i f f e r e n c e s b e t w e e n t h e i r d i f f u s i o n c o e f f i c i e n t s and c o n c e n t r a t i o n s . A c c o r d i n g t o Oloman [ 2 3 ] , t h e h y d r o x y l i o n h a s a d i f f u s i o n c o e f f i c i e n t t h a t i s a b o u t f o u r t i m e s t h a t o f p e r h y d r o x y l i o n i n 1 .0 M NaOH. W i t h a l o w r a t i o o f H 0 2 ~ c o n c e n t r a t i o n t o 0H~ c o n c e n t r a t i o n ( p u l p m i l l r e q u i r e m e n t ) , t h e s e l e c t i v e t r a n s p o r t o f 0H~ o v e r H 0 2 ~ a c r o s s t h e membrane i s l i k e l y t o be f a v o u r e d . Y e a g e r e t a l [ 5 3 ] r e p o r t e d t h e s e l f d i f f u s i o n c o e f f i c i e n t s o f Na+ i n c a t i o n membrane f o r c o n c e n t r a t e d s o d i u m c h l o r i d e ( 3 . 0 M ) . T h e i r r e p o r t e d r e s u l t s show t h a t t h e d i f f u s i o n c o e f f i c i e n t f o r N a + i s l e s s t h a n i t s v a l u e i n w a t e r ( a t 2 5 ° C ) a s c o m p a r e d t o i t s v a l u e i n p e r f 1 u o r i n a t e d s u l p h o n a t e and c a r b o x y l a t e m e m b r a n e . I t i s l i k e l y t h a t - 35 -OH" and H 0 2 " i o n s w o u l d h a v e l o w e r d i f f u s i o n c o e f f i c i e n t s i n a n i o n i c membrane t h a n i n w a t e r . I f s u c h i s t r u e , OH" i o n s w i l l be t h e m a j o r c u r r e n t c a r r i e r i n t h e p r o c e s s . 2.6.2 Problems Inherent i n the Coupled Process T h e s u c c e s s o f a c o u p l e d s y n t h e s i s o f t h i s n a t u r e d e p e n d s o n . t h e o v e r c o m i n g o f many e c o n o m i c and t e c h n i c a l p r o b l e m s t h a t a r e i n h e r e n t i n s u c h a s y s t e m . T h e e c o n o m i c f e a s i b i l i t y o f t h i s p r o c e s s w o u l d d e p e n d on t h e r a t i o o f t h e p r o d u c t s o f i n t e r e s t . Thus t h e kg o f H 2 0 2 p r o d u c e d p e r kg o f c h l o r a t e p r o d u c e d a n d t h e c u r r e n t e f f i c i e n c i e s a c h i e v e d a t s u c h r a t i o s w o u l d d e t e r m i n e t h e e c o n o m i c f e a s i b i l i t y o f t h e p r o c e s s . In a d d i t i o n t o t h i s , t h e p e r o x i d e and c h l o r a t e c o n c e n t r a t i o n s m u s t be o f c o m m e r c i a l i n t e r e s t t o w a r r a n t t h e p r o c e s s t o be c o n s i d e r e d a s s u c c e s s f u l . The m a j o r t e c h n i c a l p r o b l e m s t o o v e r c o m e i n t h e s u c c e s s f u l a p p l i c a t i o n o f t h i s p r o c e s s p e r t a i n t o t h e p e r f o r m a n c e o f t h e s e p a r a t o r u s e d b e t w e e n t h e c a t h o d e and t h e a n o d e c h a m b e r s . The r e q u i r e m e n t o f a good s e p a r a t o r t o u s e f o r t h i s p r o c e s s i n c l u d e t h e f o l l o w i n g : H i g h C o n d u c t i v i t y : Any good s e p a r a t o r t o u s e must be a b l e t o a l l o w t h e p a s s a g e o f e l e c t r i c c u r r e n t . T h u s s u c h a s e p a r a t o r must a l l o w a v o l t a g e d r o p o f l e s s t h a n 1 V a c r o s s i t a t a c u r r e n t d e n s i t y o f 2 - 3 kA m ~ 2 . S i n c e e l e c t r i c c u r r e n t i s c a r r i e d by i o n s i n e l e c t r o l y t e s , i f an i o n e x c h a n g e membrane i s t h e s e p a r a t o r u s e d , t h e m o l e c u l a r d e s i g n o f s u c h a membrane w o u l d h a v e t o i n v o l v e m a t e r i a l s t h a t w i l l g i v e t h e - 36 -membrane a h i g h c o n d u c t i v i t y , i . e . , a l o w e r power c o n s u m p t i o n a c h i e v e m e n t o r l o w membrane r e s i s t a n c e . S e l e c t i v i t y : The s u c c e s s o f t h i s p r o c e s s d e p e n d s on t h e s e l e c t i v e p a s s a g e o f 0 H ~ i o n s o v e r t h e H 0 2 " i o n s i n t o t h e a n o l y t e . P e r h y d r o x y l i o n ( H 0 2 " ) r e a c t s w i t h O C l " i o n s . T h i s r e a c t i o n i s a c h l o r a t e e f f i c i e n t l i m i t i n g p r o c e s s a s h y p o c h l o r i t e ( O C l " ) i s an i n t e r m e d i a t e i o n i n c h l o r a t e s y n t h e s i s . T h u s , t h i s p r o c e s s r e q u i r e s a s e p a r a t o r t h a t w i l l s e l e c t i v e l y a l l o w OH" i o n s b u t w i l l b l o c k t h e p a s s a g e o f a n y o t h e r i o n s - e i t h e r a n i o n s o r c a t i o n . H o w e v e r , t h e p r e s e n t t e c h n o l o g y on s e p a r a t o r p r e c l u d e s a " o n e i o n " s p e c i f i c s e p a r a t o r - t h u s s u c h r e q u i r e m e n t i s t o o s t r i n g e n t t o be m e t . The o n l y way t h i s p r o b l e m i s r e s o l v e d i s t h e l o w e r m o b i l i t y o f H 0 2 ~ i o n s a n d t h e r e l a t i v e c o n c e n t r a t i o n o f H 0 2 " c o m p a r e d t o t h e 0 H ~ i o n s . F o r a n i o n m e m b r a n e s , t h e i r p e r m s e l e c t i v i t i e s d e p e n d u p o n t h e v a r i o u s c o m p o n e n t s s u c h a s p o l y m e r b a c k b o n e c o m p o s i t i o n s , c h a r g e d e n s i t i e s , m o r p h o l o g i e s and c r o s s l i n k i n g l e v e l s . M o s t a n i o n - e x c h a n g e p o l y m e r i c membranes n o t o n l y h a v e t r a n s p o r t s e l e c t i v i t i e s b e t w e e n a n i o n s and c a t i o n s , b u t a l s o h a v e s e l e c t i v i t i e s t o w a r d s v a r i o u s a n i o n s . T h i s i s an a d v a n t a g e i n p r o c e s s e s w h e r e c e r t a i n a n i o n s a r e t o be e x c l u d e d f r o m b e i n g t r a n s p o r t e d a c r o s s t h e m e m b r a n e s . D i m e n s i o n S t a b i l i t y : The s e p a r a t o r m u s t be a b l e t o m a i n t a i n i t s s i z e and f l a t n e s s u n d e r e x t r e m e t e m p e r a t u r e c o n d i t i o n s o r p r e s s u r e . F o r t h e p r e s e n t p r o c e s s , a c e l l t e m p e r a t u r e o f 6 0 ° C may be r e q u i r e d . - 37 -C h e m i c a l S t a b i l i t y : I f t h e s e p a r a t o r t o be u s e d c o n t a i n s a t o m i c g r o u p s w h i c h c a n be a t t a c k e d by C l 2 , C I O " o r C I 0 3 " i o n s e v e n a t h i g h t e m p e r a t u r e , i t c a n n o t be u t i l i z e d ( e . g . , C-H b o n d s ) . The c h e m i c a l and t h e r m a l s t a b i l i t y o f i o n e x c h a n g e membranes w e r e p r o b l e m s t h a t c o n t r i b u t e d t o t h e l a t e e n t r y o f p e r m s e l e c t i v e membranes i n t o t h e c h l o r - a l k a l i i n d u s t r y . U n t i l t h e u s e o f p e r f l u o r i n a t e d c h a i n s w e r e i n t r o d u c e d , i o n - e x c h a n g e membranes t r i e d i n c h l o r - a l k a l i i n d u s t r y t e n d e d t o d e g r a d e i n t h e p r e s e n c e o f o x i d i z i n g a g e n t s s u c h a s c h l o r i n e . A n i o n i c membranes n o r m a l l y u s e q u a t e r n a r y a m i n e s a s t h e e x c h a n g e s i t e s . T h e s e s p e c i e s a r e q u i t e u n s t a b l e c h e m i c a l l y u n d e r common i n d u s t r i a l c o n d i t i o n s . T h e r e i s t h e r e f o r e l i m i t e d a p p l i c a t i o n o f a n i o n membranes i n e l e c t r o s y n t h e s i s p r o c e s s e s . C r o s s - l i n k i n g o f a n i o n p o l y m e r i c m a t e r i a l s t e n d t o s t r e n g t h e n them c h e m i c a l l y . I n o r d e r t o u s e an a n i o n membrane i n t h i s p r o c e s s , a d i a p h r a g m ( a s b e s t o s o r p o l y m e r i c m a t e r i a l ) c o u l d be u s e d on t h e a n o d e s i d e o f t h e a n i o n m e m b r a n e . The i d e a i s t o p r e v e n t a d i r e c t c o n t a c t b e t w e e n g e n e r a t e d c h l o r i n e and t h e m e m b r a n e . The c h e m i c a l s t a b i l i t y o f t h e membrane i s t h e m a j o r p r o b l e m i n t h e s u c c e s s o f t h i s p r o c e s s . S e p a r a t o r L i f e : The s e p a r a t o r l i f e s p a n i s an i m p o r t a n t f a c t o r w o r t h y o f c o n s i d e r a t i o n i n c h o o s i n g any m e m b r a n e . In t h e c h l o r - a l k a l i i n d u s t r y , w h e r e membrane c e l l s a r e i n c o m m e r c i a l u s e , t h e a v e r a g e l i f e s p a n o f a membrane i s t w o y e a r s . B a s e d on t h a t , one c a n s u g g e s t same l i f e s p a n f o r any s e p a r a t o r t o be a c c e p t a b l e i n t h i s p r o c e s s . - 38 -C a p i t a l and E n e r g y C o s t s : The e c o n o m i c s o f t h i s p r o c e s s d e p e n d on b o t h t h e c a p i t a l c o s t s a n d e n e r g y r e q u i r e m e n t . The c a p i t a l c o s t s f o r t h e p r o c e s s must be c o m p a r e d w i t h t h e c o m b i n e d c a p i t a l c o s t s o f a s e p a r a t e c h l o r a t e c e l l a n d p e r o x i d e c e l l . S i m i l a r l y , t h e e n e r g y r e q u i r e m e n t o f a p e r o x y - c h l o r a t e c e l l m u s t be c o n s i d e r e d i n c o n t r a s t t o t h e c o m b i n e d e n e r g y r e q u i r e m e n t o f a c h l o r a t e c e l l and p e r o x i d e c e l l . I f t h e c o u p l e d p r o c e s s p r o d u c t i o n o f c h l o r a t e and p e r o x i d e i s c h e a p e r r e l a t i v e t o s e p a r a t e p r o d u c t i o n s , o n e w o u l d c o n s i d e r t h e c o u p l i n g an e c o n o m i c a l l y v i a b l e p r o c e s s . Of c o u r s e o t h e r e c o n o m i c v a r i a b l e s s u c h a s t h e o p e r a t i n g c o s t s m u s t be i n c l u d e d i n a n y e c o n o m i c v i a b i l i t y a n a l y s i s o f t h e p r o c e s s . H 0 ? ~ a n d O C l " R e a c t i o n s ( E f f i c i e n c y L o s s R e a c t i o n s ) : The r e a c t i o n o f H 0 2 " w i t h O C l " a s shown i n e q u a t i o n ( 4 3 ) c o n s t i t u t e c u r r e n t e f f i c i e n c y l o s s r e a c t i o n s i n b o t h t h e a n o d e and c a t h o d e c h a m b e r s . H 0 2 " + H + + O C T + 0 2 + H 2 0 + C I " ( 4 3 ) The t r a n s p o r t o f e i t h e r H 0 2 " o r O C T a c r o s s t h e membrane l e a d s t o t h e r e a c t i o n a s shown i n e q u a t i o n ( 4 3 ) . 2.6.3 Direction of Flow of Anolyte and Catholyte T h e u s e o f a membrane a s a s e p a r a t o r i n t h i s p r o c e s s o f f e r s p o s s i b i l i t i e s f o r p r o c e s s v a r i a t i o n s . One d i f f e r e n c e b e t w e e n t h e u s e o f d i a p h r a g m and a membrane i s t h e mass f l o w c o n t r o l i n t h e e l e c t r o c h e m i c a l r e a c t o r . T h e r e a r e two p o s s i b l e f l o w d i r e c t i o n s f o r t h e a n o l y t e a n d c a t h o l y t e s t r e a m s a s shown i n F i g u r e 4 - C o - c u r r e n t and c o u n t e r - c u r r e n t f l o w s . - 39 -HOj =0 — (Catholyte) OCI"=Ca (Anolyte) HOj =0 — (Catholyte) OCI"=Cb + AC (Anolyte) HOj^O OCr=C a +AC a HO2 * Q ocr=Cb (a) (b) F i g . 4 P o s s i b l e f l o w c o n f i g u r a t i o n s f o r t h e a n o l y t e a n d c a t h o l y t e s t r e a m s u s i n g membrane as c e l l s e p a r a t o r ( a ) c o - c u r r e n t f l o w ( b ) c o u n t e r - c u r r e n t f l o w . S i n c e t h e r e a c t i o n b e t w e e n OCT~ a n d H 0 2 " i o n s c o n s t i t u t e e f f i c i e n c y l o s s t o b o t h c h l o r a t e and p e r o x i d e c u r r e n t e f f i c i e n c i e s and t h e H 0 2 ~ t r a n s p o r t i n t o t h e a n o l y t e d e p e n d s o n H 0 2 " c o n c e n t r a t i o n s i n t h e c a t h o l y t e , t h e s y s t e m c u r r e n t e f f i c i e n c i e s w o u l d d e p e n d t o some e x t e n t on t h e f l o w a r r a n g e m e n t c h o s e n . S i m m r o c k ( 5 5 ) showed t h a t i n a c h l o r - a l k a l i membrane e l e c t r o l y s i s , t h e c u r r e n t e f f i c i e n c y i n c r e a s e d by 5 t o 6% by t h e r e p l a c e m e n t o f a c o n v e n t i o n a l f l o w a r r a n g e m e n t w i t h a c o u n t e r - c u r r e n t s e r i e s f l o w . The c o u n t e r - c u r r e n t a r r a n g e m e n t ( b ) c o u l d h a v e h i g h e r c u r r e n t e f f i c i e n c i e s d e p e n d i n g on t h e e l e c t r o l y t e c o n c e n t r a t i o n w h i c h a f f e c t s e l e c t r o l y t e r e s i s t a n c e , t h e membrane r e s i s t a n c e and o v e r v o l t a g e s . In t h i s w o r k , t h e c o - c u r r e n t s t r e a m f l o w i s u s e d . - 40 -CHAPTER 3 OBJECT The aim of this work is to investigate the poss ibi l i ty of making alkaline hydrogen peroxide and sodium chlorate in the,same c e l l . The work involves the design and construction of the equipment and i t s use in exploratory experiments involving the search for a suitable separator between the anode and cathode chambers. An examination of several variables that could affect the process wil l be undertaken. Further examination of major effects is carried out in more de ta i l . The variables whose effects on chlorate and peroxide current efficiencies are examined in more detail are; 1. catholyte flow 2. sodium hydroxide concentration 3 . superficial current density. - 41 -CHAPTER 4 APPARATUS, METHODS AND ACCURACY One e l e c t r o c h e m i c a l r e a c t o r was u s e d i n t h i s w o r k . I t was a l a b o r a t o r y u n i t made t o o p e r a t e i n a c o n t i n u o u s manner w i t h e l e c t r o l y t e f e e d r a t e s f r o m 8 3 . 3 3 x 1 0 - 9 t o 3 3 . 3 3 x 1 0 - 7 m 3 s _ 1 . The r e a c t o r was b u i l t t o o p e r a t e a t a t m o s p h e r i c p r e s s u r e . 4 . 1 A p p a r a t u s A l i n e d i a g r a m o f t h e e q u i p m e n t u s e d i s i l l u s t r a t e d i n F i g 5 and t h e p h o t o g r a p h s o f t h e a p p a r a t u s a r e shown i n F i g s 6 and 7 . The e l e c t r o c h e m i c a l r e a c t o r c o n s i s t e d o f a c a t h o d e bed s a n d w i c h e d b e t w e e n a f l a t f e e d e r e l e c t r o d e p l a t e , an a n i o n membrane and an anode c h a m b e r . The c e l l i s f e d f r o m t h e t o p w i t h s e p a r a t e a n o l y t e and c a t h o l y t e s o l u t i o n s ( c o - c u r r e n t f l o w i n t h e c e l l ) . The r e a c t o r e m p l o y s an anode m o n o p o l e c h l o r i n e e l e c t r o d e 3 x 1 0 ~ 3 m t h i c k ( D S A , E l e c t r o d e C o r p o r a t i o n , U . S . A . ) . The a n o d e c h a m b e r was c o n t a i n e d i n a 3 x 10" m t h i c k n e o p r e n e g a s k e t w i t h PVC mesh membrane s u p p o r t (mesh s i z e 1 0 , C o l e P a r m e r , U . S . A . ) . The a n i o n membrane was p r o t e c t e d on t h e a n o d e s i d e by an a s b e s t o s d i a p h r a g m o f t h i c k n e s s 0 . 8 x 1 0 - 3 m. ( A l b i o n I n d . P r o d . L t d . , V a n c o u v e r ) . The a n i o n membrane was ' R A I P O R E 1 1035 ( R - 1 0 3 5 , E l e c t r o s y n t h e s i s C o . I n c . , N . Y . U . S . A . ) and was 2 . 5 4 x 1 0 ~ 5 m ( f i l m ) i n t h i c k n e s s . The c a t h o d e bed was a f i x e d bed o f g r a p h i t e f e l t ( G r a p h i t e f e l t G r a d e G F , C a r b o r u n d u m C o r p . N . Y . , U . S . A . ) o f d i m e n s i o n s : l e n g t h 23 x - -APPARATUS FOR ELECTROCHEMICAL C06ENERATI0N OF SODIUM CHLORATE AND ALKALINE PEROXIDE SOLUTIONS D.C. POWER SUPPLY E THERMOSTATED ANOLYTE REACTOR F i g . 5 - 43 -F i g . 6 P h o t o g r a p h s o f t h e a p p a r a t u s f o r s i m u l t a n e o u s e l e c t r o s y n t h e s i s o f a l k a l i n e p e r o x i d e and s o d i u m c h l o r a t e . - 44 -1 0 " 2 m , w i d t h 3 6 . 2 x 1 0 - 3 m and 3 x 1 0 - 3 m t h i c k (when c o m p r e s s e d ) . The c a t h o d e bed was p r e t r e a t e d by s o a k i n g i n 5 N n i t r i c a c i d f o r 2 h o u r s o r more and t h e n washed w i t h d i s t i l l e d w a t e r . The c a t h o d e f e e d e r p l a t e was a 0 . 8 x 1 0 - 3 m t h i c k 316 s t a i n l e s s s t e e l . The c a t h o d e bed was c o n t a i n e d i n a 3 x 1 0 ~ 3 m t h i c k n e o p r e n e r u b b e r g a s k e t . The c e l l was h e l d t o g e t h e r by p l e x i - g l a s s c o m p r e s s i o n p l a t e s o f t h i c k n e s s , 2 5 . 4 x 1 0 - 3 m . Some i m p o r t a n t d i m e n s i o n s o f t h e r e a c t o r a r e g i v e n i n T a b l e 1 . F i g u r e 8 shows t h e s i d e and t o p e l e v a t i o n s o f t h e e l e c t r o c h e m i c a l r e a c t o r . Some r e l e v a n t t e c h n i c a l s p e c i f i c a t i o n s o f t h e g r a p h i t e a r e l i s t e d i n T a b l e 2 . The e q u i p m e n t c o n s i s t s o f an a n o l y t e h o l d i n g v o l u m e o r e x t e r n a l r e a c t o r , a c a t h o l y t e f e e d t a n k , two c i r c u l a t i o n o r M a r c h m e t e r i n g pumps (model 2 1 0 - 1 0 R , C o l e P a r m e r , U . S . A . ) , an a n o l y t e pH c o n t r o l l e r ( c h e m c a d e t pH m e t e r / c o n t r o l l e r , model N o . 5 6 5 2 - 0 0 , C o l e P a r m e r C o m p a n y , U . S . A . ) , two c h e m i c a l f e e d pumps ( C h e m f e e d , model C - 1 5 3 0 S P , C o l e P a r m e r ) , two t u b e c o o l e r s o f t i t a n i u m , an o x y g e n t a n k and a D . C . p o w e r s u p p l y ( S o r e s e n Power s u p p l i e s , model DCR 4 0 - 2 5 B ) . The a n o l y t e was pumped f r o m a 2 . 2 l i t r e r e a c t i o n t a n k t h e r m o s t a t e d t o a c o n t s t a n t t e m p e r a t u r e , t h e n p a s s e d t h r o u g h a pH p r o b e c o n n e c t e d t o t h e pH m e t e r / c o n t r o l l e r f o r t h e m o n i t o r i n g and c o n t r o l o f t h e a n o l y t e t a n k p H . The pH v a l u e o f t h e o u t f l o w f r o m t h e a n o l y t e t a n k ( o r a n o l y t e f e e d t o t h e c e l l ) was c o n t r o l l e d and m a i n t a i n e d c o n s t a n t t o ± 0 . 3 pH u n i t by a d d i n g 1 . 0 M HC1 ( o r 2 . 1 2 M HC1 f o r some r u n s ) o r 1 . 0 M NaOH t o t h e t a n k f r o m t h e a u t o m a t i c pH c o n t r o l l e r . The a n o l y t e p a s s e d t h r o u g h t h e a n o d e c h a m b e r o f t h e c e l l and was r e c y c l e d t o t h e a n o l y t e e x t e r n a l r e a c t o r . The c a t h o l y t e was d e l i v e r e d f r o m a 20 l i t r e t a n k and p a s s e d o n c e t h r o u g h t h e c e l l and was s a m p l e d b e f o r e d i s p o s a l t o t h e d r a i n s . - 4 5 -F i g . 7 E l e c t r o c h e m i c a l r e a c t o r u s e d f o r c o g e n e r a t i o n o f a l k a l i n e p e r o x i d e and s o d i u m c h l o r a t e . F i g . 8 S i d e and t o p e l e v a t i o n s o f a p e r o x y - c h l o r a t e c e l l (1) (2) <3)(4)(5)(6)(7)(8) (9) (10) (11) (12) 127 m m Inlet port 6 . 3 5 m m N P T Items (1) C a t h o d e c h a m b e r c o m p r e s s i o n p l a t e • 25.4 mm plexiglass (2) Neoprene gasket - 3 m m (3) C a t h o d e current feeder - 0 . 8 m m s s (4) G r a p h i t e cathode b e d (5) G a s k e t • 3 m m neoprene (6) A n i o n e x c h a n g e m e m b r a n e - R - 1 0 3 5 (7) Diaphragm - a s b e s t o s (8) A n o d e c h a m b e r (9) G a s k e t • 3 m m neoprene (10) .Anode plate • 2 m m D S A (11) G a s k e t • 3 m m neoprene (12) A n o d e c h a m b e r c o m p r e s s i o n - 25.4 mm - plexiglass Outlet port 6 . 3 5 m m N P T (1) (2) (3) (4) (5) (6) (7) (8)(9)(10)(11)(12) 25.4 m m 2 5 . 4 m m p* 127.0 m m w Top Elevation Side Elevation - 47 -T a b l e 1 D i m e n s i o n s o f p e r o x y - c h l o r a t e c e l l C o m p r e s s i o n p l a t e s 4 3 . 2 w i d e x l O - 2 m l o n g x 1 2 . 7 x 1 0 " 2 m x 2 . 5 4 x 1 0 - 2 m t h i c k C h l o r i n e o r DSA e l e c t r o d e ( a n o d e ) R u 0 2 / T i 0 2 c o a t e d p l a t i n i z e d e l e c t r o d e ( E l e c t r o d e C o r p o r a t i o n ) 4 8 . 3 w i d e x 1 0 - 2 m l o n g x 1 2 . 7 x 1 0 ~ 2 m x 3 x l O " 3 m t h i c k E f f e c t i v e s u p e r f i c i a l a r e a o f membrane 8 3 . 3 x 1Q-* m Anode c h a m b e r d i m e n s i o n s 23 x l O " 2 m l o n g x 3 . 6 2 x 1 0 " 2 m w i d e x 3 x 1 0 " m t h i c k ( i n t h e d i r e c t i o n o f c u r r e n t f l o w ) C a t h o d e bed d i m e n s i o n s ( c o m p r e s s e d ) 23 x w i d e 1 0 - 2 m l o n g x 3 . 6 2 x 1 0 " 2 m x 3 x l O " 3 m t h i c k PVC mesh membrane s u p p o r t (mesh s i z e - 10) 23 x 1 0 - 2 m x 3 . 6 x 1 0 - 2 m (2 u s e d ) - 48 -B o t h t h e a n o l y t e and c a t h o l y t e w e r e p r e - c o o l e d b e f o r e t h e c e l l by p a s s i n g t h r o u g h w a t e r c o o l e d t i t a n i u m c o o l e r s e a c h o f w h i c h c o n s i s t e d o f an o u t e r c o p p e r t u b e ( 1 . 2 7 x 1 0 - 2 m d i a m e t e r and 3 0 . 5 x 1 0 - 2 m l e n g t h ) and an i n n e r 0 . 6 3 5 x 1 0 - 2 m. t i t a n i u m t u b e . The w a t e r was f e d c o u n t e r c u r r e n t l y t o t h e e l e c t r o l y t e i n t h e i n n e r t u b e . The c e l l was n o t c o o l e d . The f l o w s o f t h e a n o l y t e and t h e c a t h o l y t e t o t h e r e a c t o r w e r e m e a s u r e d by r o t a m e t e r s and c o n t r o l l e d m a n u a l l y t h r o u g h n e e d l e v a l v e s . The o x y g e n g a s was d e l i v e r e d f r o m an o x y g e n g a s c y l i n d e r . The o x y g e n f l o w was c o n t r o l l e d by a n e e d l e v a l v e and m e a s u r e d by a r o t a m e t e r . The g a s was m i x e d w i t h t h e s o d i u m h y d r o x i d e ( c a t h o l y t e ) i n a Tee b e f o r e e n t e r i n g t h e c e l l . P o w e r f o r t h e c e l l was o b t a i n e d f r o m a d i r e c t c u r r e n t p o w e r s u p p l y w i t h a maximum power o u t p u t o f 1 . 0 kVA and c a p a b l e o f e i t h e r v o l t a g e o r c u r r e n t c o n t r o l s up t o 4 0 . 0 v o l t s o r 25 a m p e r e s . A l l p a r t s i n c o n t a c t w i t h t h e a n o l y t e a r e made o f e i t h e r p o l y p r o p y l e n e , p o l y e t h e l e n e , t e f l o n , g l a s s o r p l e x i g l a s s w h i l e f o r t h e c a t h o l y t e , p o l y p r o p y l e n e , s t a i n l e s s s t e e l , g l a s s o r p l e x i g l a s s w e r e u s e d . No c o r r o s i o n was o b s e r v e d d u r i n g t h e e x p e r i m e n t a l p r o g r a m . The s p e c i f i c a t i o n s o f t h e p a r t s o f t h e a p p a r a t u s a r e g i v e n i n A p p e n d i x 1 . 4 . 2 Methods The a n o l y t e s o l u t i o n was p r e p a r e d f r o m 9 9 . 0 p e r c e n t p u r e s o d i u m c h l o r i d e (BOH c o m p a n y ) and d e i o n i z e d w a t e r . The c a t h o l y t e s o d i u m h y d r o x i d e was o f same p u r i t y and f r o m t h e same s o u r c e and was p r e p a r e d i n l i k e m a n n e r . - 49 -T a b l e 2 P r o p e r t i e s o f g r a p h i t e f e l t ( c a t h o d e b e d ) F e l t t h i c k n e s s ( b e f o r e c o m p r e s s i o n ) m 6.35 x 10-3 m S u r f a c e a r e a ( B . E . T . a d s o r p t i o n ) m 2 g _ 1 0.23 P o r o s i t y 0.94 C a r b o n c o n t e n t ( w t . f r a c t i o n ) 0.99 F i b r e d i a m urn 10.0 - 50 -A f t e r e x p l o r a t o r y e x p e r i m e n t s ( i n v o l v i n g t h e i d e n t i f i c a t i o n o f t h e s e p a r a t o r and p r o c e s s c o n d i t i o n s ) i n d i c a t e d t h a t s o d i u m c h l o r a t e and a l k a l i n e h y d r o g e n p e r o x i d e c o u l d be made s i m u l t a n e o u s l y i n t h e same c e l l , a s e r i e s o f e x p e r i m e n t a l r u n s were c a r r i e d o u t . The e x p e r i m e n t a -t i o n c o n s i s t e d o f r u n s i n w h i c h m e a s u r e m e n t s w e r e made on t h e c a t h o l y t e f e e d r a t e s , a n o l y t e f l o w , t h e pH o f t h e a n o l y t e i n t h e e x t e r n a l r e a c t o r , t h e t e m p e r a t u r e o f t h e a n o l y t e t a n k , t h e t e m p e r a t u r e o f f e e d s i n t o and o u t o f t h e c e l l , t h e o x y g e n f l o w , t h e c u r r e n t t o t h e c e l l , t h e v o l t a g e d r o p a c r o s s t h e c e l l , t h e c o n c e n t r a t i o n o f p e r o x i d e i n t h e c a t h o l y t e e x i t i n g f r o m t h e c e l l , t h e c o n c e n t r a t i o n o f c h l o r a t e i n t h e a n o l y t e t a n k , t h e c o n c e n t r a t i o n s o f a c t i v e c h l o r i n e ( h y p o c h l o r i t e and h y p o c h l o r o u s a c i d ) i n t h e a n o l y t e l e a v i n g t h e c e l l ( f o r some r u n s ) and i n t h e a n o l y t e t a n k , t h e i n i t i a l and f i n a l s o d i u m c h l o r i d e c o n c e n t r a t i o n i n t h e a n o l y t e t a n k , t h e s o d i u m c h l o r i d e c o n c e n t r a t i o n i n t h e c a t h o l y t e , t h e i n i t i a l and f i n a l a n o l y t e s o l u t i o n v o l u m e and t h e d u r a t i o n o f r u n o r o p e r a t i o n . To c a r r y o u t an e x p e r i m e n t a l r u n , t h e c a t h o d e and anode f e e d t a n k s w e r e f i r s t c h a r g e d r e s p e c t i v e l y w i t h a s o l u t i o n o f s o d i u m h y d r o x i d e and s o d i u m c h l o r i d e ( i n some r u n s a m i x t u r e o f s o d i u m c h l o r i d e and c h l o r a t e was u s e d as t h e a n o l y t e s o l u t i o n ) o f known c o n c e n t r a t i o n s . The a n o l y t e l i n e was f l u s h e d w i t h t h e a n o l y t e s o l u t i o n i n t o t h e d r a i n s t o make s u r e t h a t no c h l o r a t e l e f t i n t h e l i n e ( f r o m p r e v i o u s e x p e r i m e n t a t i o n ) i s r e c i r c u l a t e d . The a n o l y t e t a n k was t h e r m o s t a t e d and h e a t e d t o t h e o p e r a t i n g t e m p e r a t u r e . The a n o l y t e r a t e t h r o u g h t h e c e l l was s e t a t 2 . 0 x 1 0 " 6 m 3 s - 1 and t h e o x y g e n gas f l o w was k e p t a t 8 . 5 x 1 0 - 6 m 3 p e r s e c o n d ( S T P ) . The - 51 -o x y g e n was m i x e d w i t h t h e s o d i u m h y d r o x i d e b e f o r e t h e c a t h o l y t e f l o w s i n t o t h e c e l l . The c u r r e n t t h r o u g h t h e c e l l was t h e n s e t w i t h t h e p o w e r s u p p l y i n t h e c u r r e n t c o n t r o l m o d e . S a m p l e s o f t h e c a t h o l y t e p r o d u c t were t a k e n a t i n t e r v a l s o f t i m e f o r a n a l y s i s o f p e r o x i d e c o n c e n t r a t i o n . A c o n v e n t i o n a l t i t r a t i o n m e t h o d u s i n g p o t a s s i u m p e r m a n g a n a t e ( 0 . 1 N ) a s t i t r a n t and s u l p h u r i c a c i d f o r t h e a c i d i f i c a t i o n o f s a m p l e b e f o r e t i t r a t i o n was u s e d . The m e t h o d i s a c c u r a t e e n o u g h f o r t h i s p u r p o s e [ 3 2 ] , S a m p l e s o f t h e a n o l y t e p r o d u c t w e r e t a k e n and t h e c o n c e n t r a t i o n s o f a c t i v e c h l o r i n e (sum o f c o n c e n t r a t i o n s o f h y p o c h l o r i t e , h y p o c h l o r o u s a c i d and m o l e c u l a r c h l o r i n e ) and c h l o r a t e w e r e a n a l y z e d . A p o t e n t i o m e t r i c t i t r a t i o n m e t h o d was u s e d and t h e t e c h n i q u e i s b a s e d on a m o d i f i e d m e t h o d d u e t o N o r k u s and P r o k o p c h i k [ 5 6 ] . The d e t a i l e d t e c h n i q u e i s g i v e n i n A p p e n d i x 2 . A s a m p l e e a c h o f t h e a n o l y t e and c a t h o l y t e was o b t a i n e d t o s e r v e f o r t h e d e t e r m i n a t i o n o f t h e c h l o r i d e i o n c o n c e n t r a t i o n s by u s i n g a c o n v e n t i o n a l t i t r a t i o n m e t h o d o f s i l v e r n i t r a t e a s t i t r a n t and p o t a s s i u m b i c h r o m a t e as i n d i c a t o r . The c a t h o l y t e was f i r s t n e u t r a l i z e d u s i n g 1 . 0 M n i t r i c a c i d . A t o t a l o f 51 r u n s w e r e c a r r i e d o u t . E i g h t o f t h e s e r u n s w e r e a t h i g h c h l o r a t e c o n c e n t r a t i o n s . A h i g h c h l o r a t e c o n c e n t r a t i o n i s meant a n y c o n c e n t r a t i o n g r e a t e r t h a n o r e q u a l t o 1 . 0 M . The r a n g e s o f o p e r a t i n g v a r i a b l e s c o v e r e d * i n t h i s work a r e g i v e n i n T a b l e 3 . S a m p l e c a l c u l a t i o n s , i n c l u d i n g t h e c h l o r i d e b a l a n c e i n t h e s y s t e m a r e shown i n A p p e n d i x 2 . *A11 p r e s s u r e s a r e g a u g e p r e s s u r e s . - 52 -Table 3 Range of experimental variables Variable Units Range Catholyte flow 3 -1 cm s 0.10 - 0.50 NaOH Concentration M 0.5 - 2.0 Superficial Current Density kA m"2 1.2 - 2.4 Current A 10.0 - 20.0 Temperature °C 18.0 - 70.0 pH 6.0 - 7.0 Pressure kPa 0 - 82.7 - 53 -4 . 3 A c c u r a c y A l i s t o f t h e q u a n t i t i e s m e a s u r e d , t h e m e t h o d u s e d arid an e s t i m a t e o f t h e a c c u r a c y o f e a c h m e t h o d i s shown i n T a b l e 4 . M e a s u r e m e n t s w e r e made w i t h t h e most c o n v e n i e n t t e c h n i q u e s . The maximum d e g r e e o f u n c e r t a i n t y i n t h e c a l c u l a t e d q u a n t i t i e s a r e as f o l l o w s : P e r o x i d e c o n c e n t r a t i o n ± 2 % C u r r e n t e f f i c i e n c y ( p e r o x i d e ) ± 7 % S e p a r a t o r c u r r e n t d e n s i t y ± 3 % C h l o r a t e c o n c e n t r a t i o n ±5% C h l o r a t e c u r r e n t e f f i c i e n c y ± 1 0 % A c t i v e c h l o r i n e c o n c e n t r a t i o n ±3% S o d i u m c h l o r i d e c o n c e n t r a t i o n . ± 2 % T o t a l m o l e s o f c h l o r a t e ±3% - 54 -T a b l e 4 E s t i m a t e s o f e x p e r i m e n t a l a c c u r a c y Q u a n t i t y Range M e t h o d A c c u r a c y C u r r e n t 10 - 25 amps Ammeter ± 3% S o d i u m h y d r o x i d e C o n e . 0 . 5 - 2 . 0 m o l e s / l i t r e T i t r a t i o n ± 1 .5% C a t h o l y t e f l o w 0 . 1 - 0 . 5 c m 3 / s R o t a m e t e r ± 1 0 % Oxygen f l o w 2 . 5 - 8 . 5 c m 3 / s R o t a m e t e r ± 3% S o d i u m c h l o r i d e C o n e . 0 . 5 - 4 . 0 m o l e s / l i t r e T i t r a t i o n ± 2 . 3 % A n o l y t e f l o w 0 . 5 - 3 . 0 c m 3 / s R o t a m e t e r ± 4% A n o l y t e t a n k t e m p . 40 • - 7 0 ° C T h e r m o m e t e r ± T C A n o l y t e t a n k pH 5 -- 7 . 5 pH m e t e r ± 0 . 0 5 pH P r e s s u r e 0 - 8 2 . 7 k P a P r e s s u r e g a u g e ± 1 . 4 k P a C e l l t e m p e r a t u r e C e l l v o l t a g e 19 • 2 . 9 - 3 5 ° C - 4 . 7 v T e m p . g a u g e T h e r m o m e t e r V o l m e t e r ± T C ± 0 . 2 % A n o l y t e v o l u m e 1 . 5 - 2 . 5 l i t r e s G r a d u a t e d c y l i n d e r ± 2 . 0 % P e r o x i d e , c h l o r a t e a n d c h l o r i d e t i t r e 1 - 50 ml 50 ml b u r r e t t e ± 0 . 0 5 ml - 55 -CHAPTER 5 RESULTS AND DISCUSSION 5.1 General Considerations In the simuTtaneous electrosynthesis of sodium chlorate and a lka l ine hydrogen peroxide, current e f f i c i enc ie s or rate of production of the products could be used as a measure of the system performance. In th i s work, current e f f i c iency is used. The current e f f i c iency is defined here as J r j r . . , Rate of production of the desired species current efficiency = 100 x R a t e 0 f Production of the desired Species in stoichiometric equivalence to the tota l current in the process. i .e. CE = 100 ipp- (44) where M = moles of desired product F = Faraday's number (coul/g-equiv) I = current (A) T = total time of c e l l operation in seconds Z = stoichiometric number of electrons transferred per mole of product CE = current e f f i c iency (%) - 56 -The basic anode process in this system is chloride ion oxidation, accompanied by intermediate hydrolysis of elemental chlorine which yields hypochlorous species (hypochlorite and hypochlorous acid) or available chlorine. The active chlorine as an intermediate product could be converted to chlorate in two parallel ways, viz—the chemical route in the external reactor and the electrochemical route in the c e l l . In evaluating the sodium chlorate current efficiency, an assumption is made that a l l the chlorate produced is through the chemical route. The chemical route is the combination of equations (4), (6) and (7) or 2HC10 + CIO" + 2H20 C10 3- + 2H30+ + 2C1" (45) The total moles of electrons required to produce one mole of chlorate through the chemical route as in equation (45) is 6 moles of electrons. Thus, the chlorate efficiency is expressed as ouu r IN , . C ^ In a similar way, the cathodic reduction of oxygen to hydrogen peroxide has the peroxide current efficiency expressed in the form: n H0 2 = -^ 7-^ - x 100 (47) where Q = flow rate of catholyte in cm3/s C = concentration of peroxide in moles/cm3 I = current in amperes - 57 -The c u r r e n t e f f i c i e n c y as a f i g u r e o f m e r i t o f t h e p r o c e s s i s d e t e r m i n e d by t h e c h e m i s t r y o f t h e p r o c e s s , o p e r a t i n g c o n d i t i o n s and t h e d e s i g n o f t h e e e l 1 . 5.1.1 Preliminary Experiments P r i o r t o s c r e e n i n g o f t h e p r o c e s s v a r i a b l e s and m a i n i n v e s t i g a t i o n s , e x p l o r a t o r y e x p e r i m e n t s were c a r r i e d o u t t o i n v e s t i g a t e w h e t h e r a l k a l i n e p e r o x i d e and c h l o r a t e c o u l d be p r o d u c e d s i m u l t a n e o u s l y i n t h e same e l e c t r o c h e m i c a l c e l l . The s e a r c h f o r s u i t a b l e s e p a r a t o r i n c l u d e d t h e t r i a l o f : ( i ) P o r o u s a s b e s t o s d i a p h r a g m c o n v e c t i v e d i f f u s i o n m i t i g a t e d i t s u s e f u l n e s s ( i i ) C e l g a r d m i c r o p o r o u s p o l y p r o p y l e n e ( t y p e 5 5 1 1 ) ( C e l a n e s e C o r p . USA) t h e p o l y p r o p y l e n e p r o d u c t c o u l d n o t s t a n d h i g h t e m p e r a t u r e and l o n g o p e r a t i o n i n c h l o r i n e e n v i r o n m e n t . ( i i i ) I o n i c s MA-3475 a n i o n membrane ( E l e c t r o s y n t h e s i s C o . , I n c . USA) has f a b r i c b a s e b u t was n o t c h e m i c a l l y s u i t a b l e f o r u s e i n t h e p r o c e s s . ( i v ) RAIPORE - R - 1 0 3 5 a n i o n membrane ( E l e c t r o s y n t h e s i s C o . , I n c . USA) b a c k e d by a s b e s t o s d i a p h r a g m . T h i s c o m b i n a t i o n o f a n i o n membrane w i t h an a s b e s t o s d i a p h r a g m was f o u n d s u i t a b l e f o r t h i s w o r k . In one o f t h e e x p l o r a t o r y e x p e r i m e n t s ( u s i n g R - 1 0 3 5 - a s b e s t o s d i a p h r a g m ) , t h e c o n c e n t r a t i o n s o f c h l o r a t e and p e r o x i d e and t h e i r c u r r e n t e f f i c i e n c i e s w e r e f o l l o w e d f o r 10 h o u r s . The 10 h o u r s r u n was d i v i d e d i n t o t w o - - 4 h o u r s and 6 h o u r s o f d i f f e r e n t a n o l y t e a v e r a g e - 58 -f l o w s . I n t h e f i r s t 4 h o u r s , t h e a v e r a g e a n o l y t e f l o w was k e p t a t 0 . 6 c m 3 s _ 1 w h i l e i n t h e n e x t 6 h o u r s t h e a v e r a g e a n o l y t e f l o w was k e p t a t 3 . 4 c m 3 s " 1 . The c a t h o l y t e f l o w was k e p t c o n s t a n t a t 0 . 2 c m 3 s _ 1 . E x p e r i m e n t a l c o n d i t i o n s and r e s u l t s o b t a i n e d i n t h i s p r e l i m i n a r y e x p e r i m e n t a r e r e c o r d e d i n T a b l e s 5 and 6 . C o n s i d e r i n g f i r s t t h e c h l o r a t e s i d e , we f i n d t h a t 1 . The c h l o r a t e c o n c e n t r a t i o n i n c r e a s e s o r r i s e s w i t h t i m e o f o p e r a t i o n ( s e e F i g . 9) 2 . The c h l o r a t e e f f i c i e n c y a t t h e a v e r a g e a n o l y t e f l o w o f 0 . 6 c m 3 s _ 1 ( f i r s t 4 h o u r s o f t h e r u n ) h a s an a v e r a g e v a l u e o f 5 7 . 2 % w i t h t h e i n d i v i d u a l c a l c u l a t e d v a l u e s v a r y i n g b e t w e e n 70% t o 4 9 % . 3 . The a v e r a g e v a l u e when t h e a v e r a g e a n o l y t e f l o w was 3 . 4 c m 3 s - 1 was 5 7 . 4 % w i t h t h e i n d i v i d u a l v a l u e s v a r y i n g b e t w e e n 60% and 5 6 % . On t h e c a t h o d e s i d e t h e f o l l o w i n g o b s e r v a t i o n s a r e m a d e : ( a ) I n t h e f i r s t 4 h o u r s , t h e p e r o x i d e c o n c e n t r a t i o n was a l m o s t c o n s t a n t a t 0 . 4 7 M w i t h an a v e r a g e e f f i c i e n c y o f 7 2 . 0 % ( T a b l e 5 ) . ( b ) The c o n c e n t r a t i o n o f p e r o x i d e i s s e e n t o i n c r e a s e o n i n c r e a s i n g t h e a n o l y t e f l o w — t h e a v e r a g e c o n c e n t r a t i o n b e t w e e n t h e 5 t h and 1 0 t h h o u r s i s 0 . 5 8 M w i t h an a v e r a g e c u r r e n t e f f i c i e n c y o f 8 4 . 0 % . The c e l l v o l t a g e h a s an a v e r a g e v a l u e o f 5 . 9 6 V a t a l o w a n o l y t e f l o w o f 0 . 6 c m 3 s " 1 b u t f a l l s t o an a v e r a g e o f 3 . 7 V when t h e a n o l y t e f l o w was i n c r e a s e d t o 3 . 4 c m 3 s " 1 . The a n o l y t e t a n k pH was m a n u a l l y c o n t r o l l e d and l i e s b e t w e e n 5 . 3 a n d 5 . 7 5 p H . In t h e s e p r e l i m i n a r y r u n s , t h e f e e d s w e r e n o t c o o l e d b e f o r e e n t e r i n g i n t o t h e c e l l — t h u s h i g h - 59 -T a b l e 5 P r e l i m i n a r y e x p l o r a t o r y e x p e r i m e n t a l r e s u l t s ( O t h h o u r - 4 t h h o u r ) Time ( m i n s ) A n o l y t e F l o w ( c m 3 s - 1 C h l o r a t e C o n e , x 1 0 3 (M) C h i o r a t e C u r r e n t E f f i c i e n c y (%) C a t h o l y t e F l o w ( c m 3 s " 1 ) P e r o x i d e C o n e . (M) P e r o x i d e C u r r e n t E f f i c i e n c y (%) 30 0 . 5 3 2 7 . 1 7 0 . 0 0 . 2 0 0 . 4 5 6 9 . 5 90 0 . 5 7 6 8 . 8 5 9 . 0 0 . 2 0 0 . 4 7 7 2 . 6 135 0 . 6 1 8 3 . 1 2 ' 4 7 . 5 0 . 2 0 0 . 4 7 7 2 . 6 180 0 . 6 8 1 4 0 . 8 6 0 . 4 0 . 2 0 0 . 4 7 7 2 . 6 240 0 . 6 5 1 5 3 . 3 4 9 . 3 0 . 2 0 0 . 4 7 7 2 . 6 C o n d i t i o n s : NaOH c o n e = 2 . 0 M O x y g e n f l o w = 8 . 5 c m 3 s " 1 ( S T P ) C a t h o l y t e c e l l s i d e t e m p ( ° C ) = 1 8 / 4 8 ( i n l e t / o u t l e t ) C a t h o l y t e s i d e p r e s s u r e ( k P a ) = 0 / 0 A n o l y t e c e l l s i d e t e m p ( ° C ) = 3 0 / 6 2 A n o l y t e c e l l p r e s s u r e ( k P a ) = 0 / 0 A n o l y t e t a n k pH = 5 . 5 A n o l y t e t a n k temp ( ° C ) = 7 0 C e l l v o l t a g e ( V ) = 5 . 9 A v e r a g e a n o l y t e t a n k v o l = 2 l i t r e s C u r r e n t s u p p l y = 25A (CD = 3 kA m - 2 ) - 60 -T a b l e 6 P r e l i m i n a r y e x p l o r a t o r y e x p e r i m e n t a l r e s u l t s ( 6 t h h o u r - 1 0 t h h o u r ) Time ( m i n s ) A n o l y t e F l o w ( c m 3 s _ 1 C h l o r a t e C o n e , x 1 0 3 (M) C h i o r a t e C u r r e n t E f f i c i e n c y (%) C a t h o l y t e F l o w ( c m 3 s " 1 ) P e r o x i d e C o n e . (M) P e r o x i d e C u r r e n t E f f i c i e n c y (%) 300 3 . 2 0 2 3 3 . 5 0 6 0 . 0 0 . 1 8 0 . 5 1 7 1 . 0 3 6 0 3 . 5 0 2 6 9 . 5 5 7 . 8 0 . 2 0 0 . 5 8 8 9 . 6 4 2 0 3 . 4 0 3 1 1 . 5 5 7 . 3 0 . 2 0 0 . 5 8 8 9 . 6 4 8 0 3 . 5 0 3 5 8 . 1 5 7 . 6 0 . 2 0 0 . 5 8 8 9 . 6 540 3 . 4 0 3 9 4 . 8 5 6 . 4 0 . 1 8 0 . 5 2 7 2 . 3 6 0 0 3 . 3 0 4 3 1 . 3 5 5 . 5 0 . 2 0 0 . 5 8 8 9 . 6 C o n d i t i o n s : NaOH c o n e = 2 . 0 M Oxygen f l o w = 8 . 5 c m 3 s _ 1 ( S T P ) C a t h o l y t e c e l l s i d e t e m p ( ° C ) = 2 0 / 4 6 C a t h o l y t e s i d e p r e s s u r e ( k P a ) = 0 / 0 A n o l y t e c e l l s i d e t e m p ( ° C ) = 5 6 / 5 9 A n o l y t e c e l l p r e s s u r e ( k P a ) = 1 3 . 8 / 0 A n o l y t e t a n k pH = 5 . 6 A n o l y t e t a n k temp ( ° C ) = 70 C e l l v o l t a g e (V) = 3 . 7 A v e r a g e a n o l y t e t a n k v o l = 2 l i t r e C u r r e n t s u p p l y = 25A ( 3 kA m - 2 ) - 61 Preliminary exploratory experimental results 500 °o 400, Conditions Catholyte flow rate = 0.20 c m 3 s" 1 Anolyte flow rate = 3.4 cm3s-i Current = 25 A (CD = 3 KAM ' 2) O Chloride • Chlorate NaOH cone. = 2.00 (M) Cell voltage = 3.7 V Temp. (°C) - Anolyte side = 56/59 - Catholyte side = 20/46 4.0 o rz o O CD T5 O 300 360 420 480 Time (min) 540 600 Figure 9 - 62 -v a l u e s o f i n l e t and o u t l e t t e m p e r a t u r e s ( f o r t h e a n o l y t e s i d e ) w e r e o b s e r v e d w h i l e t h e c a t h o l y t e e x i t t e m p e r a t u r e was b e t w e e n 4 6 a n d 4 8 ° C . A p r e s s u r e d r o p o f l e s s t h a n 6 . 9 k P a i n t h e r e a c t o r on t h e a n o l y t e s i d e a t l o w a n o l y t e f l o w was o b s e r v e d w h i l e an a v e r a g e o f 1 3 . 8 k P a d r o p i n p r e s s u r e was r e c o r d e d when t h e a n o l y t e f l o w was i n c r e a s e d . T h e r e was a s m a l l p r e s s u r e d r o p ( l e s s t h a n 6 . 9 k P a ) on t h e c a t h o l y t e s i d e o f t h e c e l l . T h i s p r e l i m i n a r y e x p e r i m e n t d e m o n s t r a t e s t h a t a l k a l i n e h y d r o g e n p e r o x i d e and c h l o r a t e c a n be made s i m u l t a n e o u s l y i n t h e same c e l l a t r e a s o n a b l e c u r r e n t e f f i c i e n c i e s . 5.1.2 Screening the Process Variables I n s e l e c t i n g t h e p r o c e s s v a r i a b l e s o r f a c t o r s t o be i n v e s t i g a t e d f o r t h e i r r e l a t i v e i m p o r t a n c e , t h e k n o w l e d g e o f f a c t o r s t h a t a f f e c t c h l o r a t e and a l k a l i n e p e r o x i d e s y n t h e s i s i n d e p e n d e n t l y was t a k e n i n t o c o n s i d e r a t i o n . In v i e w o f t h e k n o w l e d g e , t h e f o l l o w i n g f a c t o r s w e r e c o n s i d e r e d a s p o s s i b l e e x p e r i m e n t a l p l a n f a c t o r s . 1 . NaOH c o n c e n t r a t i o n 2 . C a t h o l y t e f l o w 3 . Oxygen f l o w 4 . T e m p e r a t u r e o f t h e c a t h o l y t e i n l e t i n t o c e l l 5 . A n o l y t e f l o w 6 . NaCl c o n c e n t r a t i o n 7 . N a C 1 0 3 c o n c e n t r a t i o n 8 . A n o l y t e t a n k t e m p e r a t u r e 9 . A n o l y t e s t r e a m c e l l i n l e t t e m p e r a t u r e - 63 -1 0 . A n o l y t e s i d e p r e s s u r e 11 C a t h o l y t e s i d e p r e s s u r e 1 2 . C u r r e n t o r c u r r e n t d e n s i t y 1 3 . S e p a r a t o r 1 4 . pH o f a n o l y t e t a n k s o l u t i o n 1 5 . E l e c t r o l y t e f l o w d i r e c t i o n s ( c o - c u r r e n t o r c o u n t e r c u r r e n t f l o w s ) I n c o n s i d e r i n g t h e a b o v e f a c t o r s , t h e r e a r e bound t o be some i n t e r a c t i o n s and an e f f i c i e n t m e t h o d o f i n v e s t i g a t i n g t h e r e l a t i v e i m p o r t a n c e o f e a c h f a c t o r w o u l d be a c o m p l e t e f a c t o r i a l d e s i g n . T h i s w o u l d h o w e v e r i n v o l v e 2 1 5 ( o r 3 2 7 8 8 0 0 ) e x p e r i m e n t s f o r 15 f a c t o r s a t 2 l e v e l s o f e a c h ! B a s e d on p r e l i m i n a r y e x p e r i m e n t s a n d e q u i p m e n t l i m i t a t i o n s , t h e 15 f a c t o r s w e r e r e d u c e d t o 9 . The 9 f a c t o r s c h o s e n t o i n v e s t i g a t e f o r p o s s i b l e e f f e c t on t h e e f f i c i e n c i e s a r e : 1 . C a t h o l y t e (NaOH) c o n c e n t r a t i o n 2 . Oxygen f l o w 3 . C a t h o l y t e f l o w 4 . NaCl c o n c e n t r a t i o n 5 . N a C 1 0 3 c o n c e n t r a t i o n 6 . A n o l y t e f l o w 7 . A n o l y t e t a n k t e m p e r a t u r e 8 . C u r r e n t o r c u r r e n t d e n s i t y 9 . A n o l y t e t a n k pH To c a r r y o u t a f a c t o r i a l e x p e r i m e n t f o r 9 f a c t o r s a t 2 l e v e l s o f e a c h r e q u i r e s 2 9 o r 512 e x p e r i m e n t s . A l t h o u g h s u c h w o u l d h a v e b e e n i n v a l u a b l e i n o b t a i n i n g i n f o r m a t i o n r e l a t i n g t o i n t e r a c t i o n s , a more - 64 -practical method of screening the variables would be sought. In this work, the Plackett-Burman (P-B) (57) design is chosen at two levels of each factor. According to this design, N experiments could be used to study N-l variables. For the present problem, N=12 is chosen. To choose a number that cannot be expressed in powers of 2 i . e . , N # 2 n , where n is a whole number, we limit the power of this design. If N were a factor of 2 n , then possible factors interactions could be obtained. Although in the original P-B paper (57), use of dummy variables was not suggested as binding, in the present work, 2 dummy variables were used to relate the uncertainty of our results with experimental errors, confounding or interactions. The P-B design for 12 experiments is shown in Fig. 10. In the figure, the following letters are used to represent the factors of interest. A - NaOH concentration (M) B - Catholyte flow (cm3/s) C - Dummy D - Oxygen flow (cm3/s) E - NaCl concentration (M) F - Anolyte flow (cm3/s) G - N a C 1 0 3 concentration (M) H - Anolyte tank temp (°C) I - Anolyte tank pH J - Dummy K - current (A) - 65 -F i g u r e 10 P I a c k e t t - B u r m a n D e s i g n f o r 12 e x p e r i m e n t s Run # A B C D E F G H I J K Random O r d e r 1 + + - + + + - - - + - 3 2 + - + + + - - - + - + 12 3 - + + + - - - + - + + 4 4 + + + - - - + - + + - 11 5 + + - - - + - + + - + 1 6 + - - - + - + + - + + 6 7 - - - + - + + - + + + 7 8 - - + - + + - + + + - 8 9 - + - + + - + + + - - 9 10 + - + + - + + + - - - 10 11 + + - + + + - - - + 5 12 - - - - - - - - - - - 2 - 66 -I n F i g 1 0 , (+) p l u s s i g n s t a n d s f o r h i g h l e v e l and ( - ) m i n u s s i g n f o r l o w l e v e l o f f a c t o r i n q u e s t i o n . M a g n i t u d e o f t h e L e v e l s T a b l e 7 shows t h e m a g n i t u d e s - b o t h l o w and h i g h - o f t h e f a c t o r l e v e l s u s e d . In c h o o s i n g t h e l e v e l s , c o n s i d e r a t i o n was g i v e n t o ( i ) -r e p o r t e d e x p e r i m e n t a l r e s u l t s o f i n d i v i d u a l a l k a l i n e and c h l o r a t e s y n t h e s i s ( i i ) p r e l i m i n a r y r e s u l t s o f work a l r e a d y c a r r i e d o u t ( i i i ) I n d u s t r i a l p r a c t i c e s f o r t h e m a n u f a c t u r e o f t h e c h l o r a t e o f s o d i u m . An i n s t a n c e o f ( i ) , Oloman and W a t k i n s o n ( 2 8 ) , g e n e r a t e d a l k a l i n e p e r o x i d e o f 0 . 8 M c o n c e n t r a t i o n a t 6 0 % , c u r r e n t e f f i c i e n c y i n 2 . 0 M c a u s t i c w i t h p r e s s u r i z e d c e l l w h i l e Brown e t a l [ 3 3 ] c l a i m e d a 1 . 5 % wt p e r o x i d e i n 1 . 0 M c a u s t i c a t 67% c u r r e n t e f f i c i e n c y (no p r e s s u r i z a t i o n ) . Thus a l o w l e v e l o f 0 . 5 M and a h i g h l e v e l o f 2 . 0 M c a u s t i c was u s e d . D e s i g n and O r d e r o f E x p e r i m e n t a t i o n As shown i n F i g 1 0 , two l e v e l s o f e a c h f a c t o r were c o n s i d e r e d and t w e l v e e x p e r i m e n t s c a r r i e d o u t . E a c h e x p e r i m e n t was c a r r i e d o u t f o r a s p e c i f i c d u r a t i o n o f t i m e and t h e c u r r e n t e f f i c i e n c y o f t h e p r o d u c t o f i n t e r e s t c a l c u l a t e d . The e x p e r i m e n t s were c o m p l e t e l y r a n d o m i z e d as shown i n t h e l a s t c o l u m n o f F i g 1 0 . The e f f i c i e n c y c a l c u l a t i o n o b t a i n e d f r o m t h e e x p e r i m e n t a l r e s u l t s a r e as shown i n T a b l e 8 „ - 67 -T a b l e 7 M a g n i t u d e s o f l e v e l s o f f a c t o r s F a c t o r Low L e v e l ( - ) H i g h L e v e l (+) A - NaOH C o n e . (M) 0 . 5 2 . 0 B - C a t h o l y t e f l o w ( c m 3 / s ) 0 . 1 0 1 . 0 C - Dummy - -D - 0 2 f l o w ( c m 3 / s , S T P ) 2 . 0 1 0 . 0 E - N a C l Cone (M) 0 . 5 5 . 0 F - A n o l y t e f l o w ( c m 3 / s ) 0 . 5 3 . 0 G - N a C 1 0 3 c o n e (M) 0 . 0 0 . 2 H - A n o l y t e t a n k temp ( ° C ) 4 0 . 7 0 . I - A n o l y t e t a n k pH 5 . 0 7 . 5 J - Dummy - -K - C u r r e n t ( A ) 5 . 0 2 5 . 0 - 68 -T a b l e 8 V a r i a b l e s c r e e n i n g e x p e r i m e n t a l r e s u l t s Run # A B C D E F G H • I J K % C 1 0 3 -C u r r e n t E f f i c i e n c y H 2 0 2 % C u r r e n t E f f i c i e n y 3 + + - + + . + - - - + - 8 . 5 0 7 3 . 3 12 + - + + + - - - + - + 6 2 . 0 8 4 8 . 2 8 4 - + + + - - - + - + 5 . 4 3 1 9 . 3 0 11 + + + - - - + - + + - 6 7 . 7 6 12.21 1 + + - - - + - + + - + 7 4 . 3 1 1 0 . 1 3 6 + - - - + - + + - + + 4 1 . 7 7 5 4 . 0 4 7 - - - + - + + - + + + 6 0 . 4 0 2 0 . 5 0 8 - - + - + + - + + + - 4 0 . 9 8 2 . 0 9 - + - + + - + + + - - 4 9 . 8 4 8 2 . 4 10 + - + + - + + + - - - 5 6 . 5 7 5 0 . 1 8 5 - + + - + + + - - - + 5 0 . 2 1 3 . 0 2 6 . 7 5 3 6 . 6 7 - 69 -C o m p u t a t i o n o f T e s t S t a t i s t i c s Each o f t h e v a r i a b l e s a c c o r d i n g t o P-B d e s i g n shown i n F i g . 1 0 , ( d u r i n g t h e 12 r u n s ) a p p e a r s a t i t s h i g h l e v e l s i x t i m e s and a t i t s l o w l e v e l s i x t i m e s . The e f f e c t o f a v a r i a b l e on t h e r e s p o n s e ( e f f i c i e n c y ) i s s i m p l y t h e d i f f e r e n c e b e t w e e n t h e a v e r a g e v a l u e o f t h e r e s p o n s e f o r t h e s i x r u n s a t h i g h l e v e l and t h e a v e r a g e v a l u e o f t h e r e s p o n s e f o r t h e s i x r u n s a t t h e l o w l e v e l , i . e . , F _ RO a t (+) RO a t ( - ) A 6 5" [ ™ > w h e r e E = E f f e c t o f A RO = r e s p o n s e o r r e s u l t ( e f f i c i e n c y ) The e f f e c t s o f t h e dummy v a r i a b l e s a r e c a l c u l a t e d i n l i k e ma nner as f o r r e a l v a r i a b l e s . In t h e a b s e n c e o f i n t e r a c t i o n s and t h e l e v e l s a r e r e p r o d u c e d p e r f e c t l y w i t h no e r r o r i n r e s p o n s e m e a s u r e m e n t , t h e e f f e c t shown by a dummy v a r i a b l e w i l l be z e r o . A d e v i a t i o n o f a dummy e f f e c t f r o m z e r o i s assumed t o be a m e a s u r e o f t h e l a c k o f e x p e r i m e n t a l p r e c i s i o n p l u s any a n a l y t i c a l e r r o r i n r e s p o n s e m e a s u r e m e n t . The v a r i a n c e o f an e f f e c t was e s t i m a t e d u s i n g t h e f o l l o w i n g P - B d e r i v e d e q u a t i o n V ( 4 9 ) e f f n w h e r e V e f f v a r i a n c e o f an e f f e c t e f f e c t shown by a dummy n number o f dummy v a r i a b l e s - 70 -T h e r e l a t i o n s h i p b e t w e e n t h e v a r i a n c e o f an e f f e c t and t h e s t a n d a r d e r r o r o f an e f f e c t i s g i v e n b y : S - E e f f • < Veff>' / 2 <50> From t h e r e s p o n s e s o f t h e 12 r u n s , t h e e f f e c t o f e a c h o f t h e v a r i a b l e s was c a l c u l a t e d and t h e s t a n d a r d e r r o r o f t h e e f f e c t s o b t a i n e d . The r e s u l t s o b t a i n e d a r e shown i n T a b l e 9 f o r C 1 0 3 ~ a n d T a b l e 10 f o r p e r o x i d e . The s i g n i f i c a n c e o f e a c h e f f e c t was d e t e r m i n e d by a t - t e s t : t = £S£ct ( 5 1 ) ^ e f f The number o f dummies p r o v i d e d t h e number o f d e g r e e s o f f r e e d o m f o r e n t e r i n g t h e t a b u l a r v a l u e s o f t . The r e s u l t s i n T a b l e 9 shows t h a t a t 90% c o n f i d e n c e l e v e l , t h e a n o l y t e t a n k pH i s an i m p o r t a n t f a c t o r i n t h e c h l o r a t e e f f i c i e n c y . I t i s a l s o f o u n d t h a t a t l o w e r c o n f i d e n c e l e v e l s , t h e N a C 1 0 3 c o n c e n t r a t i o n a n d NaOH c o n c e n t r a t i o n seem t o be i m p o r t a n t i n t h e c h l o r a t e e f f i c i e n c y . On t h e o t h e r hand T a b l e 10 f o r t h e p e r o x i d e e f f i c i e n c y shows t h a t t h e c u r r e n t i s an i m p o r t a n t f a c t o r . A t l o w e r c o n f i d e n c e l e v e l s , t h e N a C l c o n c e n t r a t i o n seem t o be i m p o r t a n t . I t i s i n t e r e s t i n g t o o b s e r v e t h a t no one f a c t o r was i m p o r t a n t f o r t h e two p r o d u c t s up t o t h e same c o n f i d e n c e l e v e l . T h a t many o f t h e f a c t o r s seem n o t t o be i m p o r t a n t up t o t h 90% c o n f i d e n c e l e v e l may be a r e s u l t o f l a c k o f e x p e r i m e n t a l p r e c i s i o n , - 71 -T a b l e 9 V a r i a b l e s and e f f e c t s f o r C 1 0 3 V a r i a b l e Name L e v e l E f f e c t R e l a t i v e S i g n i f i c a n c e Low H i g h ( - ) t o (+) t - T e s t P r o b . C o n f . L e v e l NaOH c o n c e n t r a t i o n (M) 0 . 5 2 . 0 1 6 . 2 4 1 . 6 1 0 0 . 2 6 74% C a t h o l y t e f l o w r a t e ( c m 3 / s ) 0 . 1 1 . 0 - 2 . 0 7 0 . 2 0 5 Dummy - - 6 . 9 0 0 . 6 8 3 O x y g e n f l o w r a t e ( c m 3 / s ) 2 . 0 1 0 . 0 - 6 . 4 8 0 . 6 4 2 NaCl c o n c e n t r a t i o n (M) 1 . 0 5 . 0 - 2 . 9 9 0 . 2 9 6 A n o l y t e f l o w r a t e ( c m 3 / s ) 0 . 5 3 . 0 9 . 5 4 0 . 9 4 5 0 . 4 5 55% N a C 1 0 3 c o n c e n t r a t i o n (M) 0 0 . 2 2 1 . 4 3 2 . 1 2 3 0 . 1 8 82% A n o l y t e t a n k temp ( ° C ) 40 7 0 . 0 2 . 1 9 0 . 2 1 7 A n o l y t e t a n k pH 5 . 5 7 . 5 3 1 . 0 1 3 . 0 7 0 0 . 1 0 91% Dummy - - - 1 2 . 5 0 1 . 2 4 0 0 . 3 5 C u r r e n t ( A ) 5 . 0 2 5 . 0 1 0 . 6 5 1 . 0 5 5 0 . 4 0 60% - 72 -T a b l e 10 V a r i a b l e s and e f f e c t s f o r p e r o x i d e V a r i a b l e Name L e v e l E f f e c t R e l a t i v e S i g n i f i c a n c e Low H i g h ( - ) t o (+) t - T e s t P r o b . C o n f . L e v e l NaOH c o n c e n t r a t i o n (M) 0 . 5 2 . 0 1 0 . 7 2 1 . 0 0 2 0 . 4 2 58% C a t h o l y t e f l o w r a t e ( c m 3 / s ) 0 . 1 1 . 0 - 5 . 2 1 0 . 4 8 7 Dummy - - - 0 . 3 3 5 0 . 0 3 1 Oxygen f l o w r a t e ( c m 3 / s ) 2 . 0 1 0 . 0 5 . 9 7 5 0 . 5 5 9 NaCl c o n c e n t r a t i o n (M) 1 . 0 5 . 0 2 2 . 3 2 8 2 . 0 8 7 0 . 1 8 82% A n o l y t e f l o w r a t e ( c m 3 / s ) 0 . 5 3 . 0 - 1 2 . 3 0 8 1 . 1 5 0 0 . 3 7 63% N a C 1 0 3 c o n c e n t r a t i o n (M) 0 0 . 2 2 . 1 1 8 0 . 1 9 8 A n o l y t e t a n k temp ( ° C ) 40 7 0 . 0 7 . 3 3 8 0 . 6 8 6 A n o l y t e t a n k pH 5 . 5 7 . 5 1 3 . 1 8 2 1 . 2 3 2 0 . 3 5 65% Dummy - - 1 5 . 1 2 5 1 . 4 1 4 0 . 2 9 C u r r e n t ( A ) 5 2 5 . 0 - 4 0 . 2 6 2 3 . 7 6 4 0 . 0 7 93% - 73 -i n t e r a c t i o n s o r c o n f o u n d i n g a m o n g s t t h e f a c t o r s . As p o i n t e d o u t e a r l i e r o n , t h e a i m o f t h i s v a r i a b l e s c r e e n i n g i s t o r e d u c e t h e number o f f a c t o r s t o be i n v e s t i g a t e d . From t h e s e r e s u l t s , i t was d e c i d e d t o i n v e s t i g a t e f u r t h e r t h e e f f e c t s o f t h e f o l l o w i n g on t h e c o u p l e d p r o c e s s : 1 . NaOH c o n c e n t r a t i o n - a t a 74% c o n f i d e n c e l e v e l o f b e i n g i m p o r t a n t i n C 1 0 3 " e f f i c i e n c y 2 . C u r r e n t - 93% c o n f i d e n c e l e v e l o f b e i n g i m p o r t a n t i n p e r o x i d e s y n t h e s i s 3 . N a C 1 0 3 c o n c e n t r a t i o n - 82% c o n f i d e n c e l e v e l o f b e i n g r e l e v a n t i n c h l o r a t e s y n t h e s i s 4 . C a t h o l y t e f l o w - I t was n o t shown t o be i m p o r t a n t t o any a p p r e c i a b l e d e g r e e i n b o t h p e r o x i d e and c h l o r a t e e f f i c i e n c y . However r e s u l t s f r o m p r e l i m i n a r y e x p e r i m e n t s showed t h a t c a t h o l y t e a t r a t e s l o w e r t h a n 1 . 0 cm 3 s " 1 do a f f e c t t h e p e r o x i d e a n d c h l o r a t e c u r r e n t e f f i c i e n c i e s . 5.2 Factorial Experiments F u r t h e r e x p e r i m e n t s w e r e c a r r i e d o u t t o e l u c i d a t e t h e e f f e c t s o f c a t h o l y t e f l o w r a t e , s o d i u m h y d r o x i d e c o n c e n t r a t i o n a n d c u r r e n t d e n s i t y o n t h e c o u p l e d s y n t h e s i s . The f a c t o r i a l e x p e r i m e n t s w e r e d i v i d e d i n t o two g r o u p s o f c h l o r a t e c o n c e n t r a t i o n s , v i z : ( a ) l o w c h l o r a t e c o n c e n t r a t i o n run ( b ) s t r o n g c h l o r a t e s o l u t i o n r u n . A l o w c h l o r a t e c o n c e n t r a t i o n i s meant a c h l o r a t e c o n c e n t r a t i o n l e s s t h a n 1 . 0 M w h i l e a h i g h o r s t r o n g c h l o r a t e s o l u t i o n r e f e r s t o any c h l o r a t e c o n c e n t r a t i o n g r e a t e r t h a n 1 . 0 M . - 74 -T a b l e 11 shows t h e l i s t o f i n d e p e n d e n t and d e p e n d e n t v a r i a b l e s and t h e l e v e l s a t w h i c h t h e f a c t o r i a l e x p e r i m e n t s w e r e p e r f o r m e d . The r e s u l t s o f t h e f a c t o r i a l e x p e r i m e n t s a t t h r e e l e v e l s f o r t h e l o w c h l o r a t e r u n s a r e p r e s e n t e d i n T a b l e s 12 a n d 1 3 w h i l e t h o s e o f t w o l e v e l r u n s f o r t h e s t r o n g c h l o r a t e s o l u t i o n s a r e shown i n T a b l e s 14 and 1 5 . Some o f t h e u s e f u l r e s u l t s o b t a i n e d f r o m t h e f a c t o r i a l e x p e r i m e n t a l r u n s ( t h r e e and two l e v e l s i n c l u s i v e ) a r e c o n t a i n e d i n T a b l e s A-C i n A p p e n d i x 2 . The p r i n c i p a l f e a t u r e s o f t h e s e r e s u l t s a r e d i s c u s s e d b e l o w . 5.3 Low Chlorate Concentration Runs 5.3.1 Sup e r f i c i a l Current Density Effects T h e r e s u l t s i n T a b l e s 12 and 13 i l l u s t r a t e t h e e f f e c t o f c u r r e n t d e n s i t y ( i . e . , c u r r e n t d e n s i t y b a s e d on t h e a c t i v e a r e a o f m e t a l c u r r e n t f e e d e r p l a t e s o r t h e s e p a r a t o r ) . The p r i n c i p a l f e a t u r e s o f t h e s e r e s u l t s a r e : i . A t any g i v e n c a u s t i c c o n c e n t r a t i o n and f l o w , t h e c h l o r a t e e f f i c i e n c y i n c r e a s e s w i t h i n c r e a s e i n c u r r e n t d e n s i t y ( s e e F i g . 11 f o r t y p i c a l r e s u l t ) , i i . The c h l o r a t e e f f i c i e n c y i n c r e a s e s w i t h i n c r e a s e i n c a t h o l y t e f l o w r a t e a t any g i v e n c u r r e n t d e n s i t y , i i i . The p e r o x i d e e f f i c i e n c y , a t any g i v e n c a u s t i c c o n c e n t r a t i o n and c a t h o l y t e f l o w f a l l s w i t h i n c r e a s e i n c u r r e n t d e n s i t y ( s e e F i g . 12 f o r t y p i c a l r e s u l t s ) , i v . The p e r o x i d e e f f i c i e n c y a l s o r i s e s w i t h r i s e i n c a t h o l y t e f l o w a t a n y g i v e n c u r r e n t d e n s i t y . - 75 -T a b l e 11 L i s t o f i n d e p e n d e n t and d e p e n d e n t v a r i a b l e s I n d e p e n d e n t L e v e l s C a t h o l y t e c o n c e n t r a t i o n (M) C u r r e n t d e n s i t y ( A ) C a t h o l y t e f l o w ( c m 3 s _ 1 ) D e p e n d e n t V a r i a b l e C h l o r a t e c u r r e n t e f f i c i e n c y P e r o x i d e c u r r e n t e f f i c i e n c y C h l o r a t e c o n c e n t r a t i o n P e r o x i d e c o n c e n t r a t i o n Low C 1 0 3 ~ r u n s H i g h C 1 0 3 - r u n s 0 . 5 , 1 . 0 a n d 2 . 0 1 . 2 , 1 . 8 and 2 . 4 0 . 1 , 0 . 3 and 0 . 5 1 . 0 and 2 . 0 1 . 2 a n d 2 . 4 0 . 1 a n d 0 . 5 T a b l e 12 C h l o r a t e c u r r e n t e f f i c i e n c y and c o n c e n t r a t i o n a t l o w c h l o r a t e c o n c e n t r a t i o n r u n s C u r r e n t D e n s i t y k A / m 2 C a t h o l y t e F l o w R a t e c m 3 / s C a t h o l y t e (NaOH C o n c e n t r a t i o n (M) 0 . 5 1 . 0 2 . 0 C I O 3 " c u r r e n t E f f i c i e n c y (%) C103~ C o n e . I n i t i a l / f i n a l (M) C I O 3 " c u r r e n t E f f i c i e n c y (%) C I O 3 - C o n e . I n i t i a l / f i n a l (M) C 1 0 3 " c u r r e n t E f f i c i e n c y (%) C I O 3 - C o n e . I n i t i a l / f i n a l (M) 1 . 2 ( 1 0 A ) 0 . 1 0 . 3 0 . 5 5 3 . 0 5 7 . 4 6 8 . 5 0 . 0 / 0 . 0 4 9 0 . 2 0 9 / 0 . 2 6 3 0 . 2 5 8 / 0 . 3 2 2 5 2 . 7 6 2 . 6 7 0 . 3 0 . 3 7 6 / 0 4 2 5 0 . 0 / 0 . 0 5 8 0 . 0 / 0 . 0 6 6 5 3 . 2 5 9 . 7 7 0 . 3 0 . 0 / 0 . 0 3 3 0 . 2 8 9 / 0 . 3 2 7 0 . 0 / 0 . 0 4 4 1 . 8 ( 1 5 A ) 0 . 1 0 . 3 0 . 5 6 0 . 2 6 8 . 0 7 3 . 5 0 . 0 / 0 . 0 8 4 0 . 0 2 9 / 0 . 1 2 4 0 . 3 7 6 / 0 . 4 7 9 6 1 . 5 6 7 . 5 7 2 . 0 0 . 3 8 2 / 0 . 4 7 7 0 . 0 5 8 / 0 . 1 5 2 0 . 0 / 0 . 0 6 7 6 5 . 0 7 0 . 0 7 4 . 0 0 . 0 3 2 / 0 . 0 4 6 0 . 2 3 5 / 0 . 3 2 7 0 . 0 4 2 / 0 . 0 9 0 2 . 4 ( 2 0 A ) 0 . 1 0 . 3 0 . 5 6 9 . 1 7 2 . 1 7 8 . 0 0 . 0 / 0 . 1 2 9 0 . 1 1 0 / 0 . 2 4 4 0 . 2 2 5 / 0 . 3 7 0 6 9 . 0 7 2 . 5 7 6 . 0 0 . 4 7 5 / 0 . 6 0 4 0 . 2 0 0 / 0 . 3 3 5 0 . 0 5 5 / 0 . 1 5 3 6 2 . 6 6 7 . 0 7 2 . 9 0 . 0 9 9 / 0 . 1 9 0 0 . 1 5 8 / 0 . 2 4 1 0 . 1 0 2 / 0 . 1 9 3 *A11 t h e c o n c e n t r a t i o n s a r e n o r m a l i z e d t o 2 . 0 l i t r e s s o l u t i o n . C o n d i t i o n s : A v e r a g e c e l l v o l t a g e s : 1 . 2 kA m - 2 - 3 . 0 0 V ; 1 . 8 kA m" 3 . 4 6 V ; 2 . 4 kA m " 2 - 4 . 1 V C e l l temp ( ° C ) C e l l p r e s s . ( k P a ) r a n g e Oxygen f l o w ( c m 3 s , STP) A n o l y t e t a n k pH A n o l y t e f l o w ( c m 3 s " 1 ) A n o l y t e t a n k temp ( ° C ) NaCl c o n e . (M) N a C 1 0 3 c o n e . (M) A n o l y t e S i d e 2 7 / 3 3 0 . 0 - 5 8 . 6 / 0 . 0 6 . 5 ± 0 . 2 2 . 0 7 0 . 0 3 . 5 - 4 . 2 0 . 0 - 0 . 5 C a t h o l y t e S i d e 2 2 / 3 0 0 . 0 - 4 4 . 8 / 0 . 0 8 . 5 0 . 1 - 3 x l O " 3 T a b l e 13 P e r o x i d e c u r r e n t e f f i c i e n c y and c o n c e n t r a t i o n a t l o w c h l o r a t e c o n c e n t r a t i o n r u n s C a t h o l y t e (NaOH C o n c e n t r a t i o n (m) C u r r e n t D e n s i t y k A / m 2 C a t h o l y t e F l o w R a t e c m 3 / s 0 . 5 1 . 0 2 . 0 P e r o x i d e C u r r e n t E f f . (%) P e r o x i d e C o n e . (M) P e r o x i d e C u r r e n t E f f . (%) P e r o x i d e C o n e . (M) P e r o x i d e C u r r e n t E f f . (%) P e r o x i d e C o n e . (M) 1 . 2 ( 1 0 A ) 0 . 1 0 . 3 0 . 5 2 9 . 5 5 0 . 4 6 2 . 0 0 . 1 5 3 0 . 0 8 7 0 . 0 6 4 5 1 . 8 6 7 . 6 6 6 . 6 0 . 2 6 8 0 . 1 1 5 0 . 0 6 9 8 6 . 1 7 5 . 4 6 6 . 2 0 . 4 4 6 0 . 1 3 0 0 . 0 6 9 1 . 8 ( 1 5 A ) 0 . 1 0 . 3 0 . 5 2 0 . 0 3 4 . 7 4 8 . 3 0 . 1 5 5 0 . 0 9 0 . 0 7 5 3 2 . 0 4 4 . 4 6 0 . 8 0 . 2 4 9 0 . 1 1 5 0 . 0 9 7 5 7 . 7 5 4 . 0 4 4 . 0 0 . 4 4 9 0 . 1 4 0 0 . 0 6 8 2 . 4 ( 2 0 A ) 0 . 1 0 . 3 0 . 5 1 5 . 9 2 5 . 4 3 9 . 5 0 . 1 4 5 0 . 0 8 7 0 . 0 8 2 2 8 . 4 4 1 . 3 5 6 . 5 0 . 2 7 5 0 . 1 4 3 0 . 1 1 7 5 2 . 7 4 5 . 2 3 7 . 7 0 . 5 2 0 . 1 5 6 0 . 0 7 7 C o n d i t i o n s : A v e r a g e c e l l v o l t a g e s : 1 . 2 kA m ~ z - 3 . 0 0 V ; 1 . 8 kA m " 2 - 3 . 4 6 V ; 2 . 4 kA m ~ z - 4 . 1 V .-2 C e l l temp ( ° C ) C e l l p r e s s . ( k P a ) r a n g e Oxygen f l o w ( c m 3 s " 1 , STP) A n o l y t e t a n k pH A n o l y t e f l o w (cm s ) A n o l y t e t a n k t e m p ( ° C ) NaCl c o n e . (M) N a C 1 0 3 c o n e . (M) A n o l y t e S i d e C a t h o l y t e S i d e 2 7 / 3 3 2 2 / 3 0 0 . 0 - 5 8 . 6 / 0 . 0 0 . 0 - 4 4 . 8 / 0 . 0 8 . 5 6 . 5 ± 0 . 2 2 . 0 7 0 . 0 3 . 5 - 4 . 2 0 . 1 - 3 x 1 0 - 3 0 - 0 . 5 T a b l e 14 C h l o r a t e c u r r e n t e f f i c i e n c y and c o n c e n t r a t i o n a t h i g h c h l o r a t e c o n c e n t r a t i o n r u n s C a t h o l y t e (NaOH C o n c e n t r a t i o n ) (M) C u r r e n t D e n s i t y k A / m 2 C a t h o l y t e F l o w R a t e 3 -1 cm3 s 1 . 0 2 . 0 C 1 0 3 - C u r r e n t E f f i c i e n c y (%) • C 1 0 3 " C o n c . I n i t i a l / F i n a l (M) CIO3 C u r r e n t E f f i c i e n c y (%) CIO3-C o n c . I n i t i a l / F i n a l (M) 0 . 1 6 1 . 1 2 . 7 0 0 / 2 . 7 3 8 7 7 . 3 2 . 8 9 0 / 2 . 9 4 2 * 1 . 2 ( 1 0 A ) 0 . 5 6 9 . 0 2 . 7 0 0 / 2 . 7 4 4 6 7 . 0 2 . 7 2 3 / 2 . 7 8 8 0 . 1 6 6 . 4 2 . 5 2 3 / 2 . 5 8 9 7 2 . 8 2 . 8 9 0 / 3 . 0 2 1 2 . 4 ( 2 0 A ) 0 . 5 8 0 . 6 2 . 9 0 3 / 3 . 0 1 5 5 1 . 0 2 . 8 3 6 / 2 . 8 8 3 *A11 t h e c o n c e n t r a t i o n s a r e n o r m a l i z e d t o 2 . 0 l i t r e s s o l u t i o n . C o n d i t i o n s : A v e r a g e c e l l v o l t a g e s : 1 . 2 kA m ' 2 - 3 . 1 V ; 2 . 4 kA i r r 2 - 4 . 2 V 3" -1 C e l l temp ( ° C ) C e l l p r e s s . ( k P a ) Oxygen f l o w ( c m 3 s A n o l y t e f l o w ( c m 3 s A n o l y t e t a n k pH NaCl c o n e . (M) N a C 1 0 3 c o n e . (M) STP) A n o l y t e S i d e 2 6 / 3 1 0 . 0 - 4 4 . 8 / 0 . 0 2 . 0 6 . 5 ± 0 . 2 2 . 5 - 3 . 2 2 . 4 - 2 . 8 9 C a t h o l y t e S i d e 2 0 / 2 7 0 . 0 - 4 8 . 3 / 0 . 0 8 . 5 0 - 4 x 1 0 - 3 T a b l e ' 15 P e r o x i d e c u r r e n t e f f i c i e n c y and c o n c e n t r a t i o n a t h i g h c h l o r a t e c o n c e n t r a t i o n r u n s C a t h o l y t e (NaOH C o n c e n t r a t i o n ) (M) C u r r e n t D e n s i t y k A / m 2 C a t h o l y t e F l o w R a t e c m 3 s " 1 1 . 0 2 . 0 P e r o x i d e C u r r e n t E f f i c i e n c y (%) P e r o x i d e C o n e . (M) P e r o x i d e C u r r e n t E f f i c i e n c y (%) P e r o x i d e C o n e . (M) 0 . 1 6 0 . 0 0 . 3 1 1 7 3 . 0 0 . 3 7 8 1 . 2 ( 1 0 A ) 0 . 5 7 7 . 2 0 . 0 8 0 9 6 . 5 0 . 1 0 0 0 . 1 3 6 . 2 0 . 3 7 5 7 8 . 2 0 . 8 1 0 2 . 4 ( 2 0 A ) 0 . 5 6 3 . 7 0 . 1 3 2 6 1 . 4 0 . 1 2 7 C o n d i t i o n s : A v e r a g e c e l l v o l t a g e s : 1 . 2 kA K\~ - 3 . 1 V 2 . 4 kA m " 2 - 4 . 2 V C e l l temp ( ° C ) C e l l p r e s s . ( k P a ) Oxygen f l o w (cm s" A n o l y t e f l o w ( c m 3 J A n o l y t e t a n k pH NaCl c o n e . (M) N a C 1 0 3 c o n e . (M) STP) A n o l y t e S i d e 2 6 / 3 1 0 . 0 - 4 4 . 8 / 0 . 0 2 . 0 6 . 5 ± 0 . 2 2 . 5 - 3 . 2 2 . 4 - 2 . 8 9 C a t h o l y t e S i d e 2 0 / 2 7 0 . 0 - 4 8 . 3 / 0 . 0 8 . 5 0 - 4 x l O " 3 - 80 -100.0 Effect of superficial current density on chlorate current efficiency 90.0 80 .0 70 .0 60 .0 50 .0 Conditions O Cath. flow rate = 0.10 cm 3 s "1 • Cath. flow rate = 0.30 cm 3 s "1 • Cath. flow rate = 0.50 cm 3 s "1 • Fitted values Anolyte flow rate = 2.0 c m 3 s" 1 NaOH cone. = 1.00 (M) NaCl cone. = 4.2-3.8 (M) Temp. (°C) - Anolyte side = 27/33 - Catholyte side = 20/29 Low chlorate run (0-0.5M) 0.0 0.5 1.0 1.5 2.0 2.5 Current Density (KAM " 2) Figure.11 81 -1 0 0 . 0 Effect of superficial current density on peroxide current efficiency o c CD O k= L U c CD =3 o CD - g "x o CD Q_ 8 0 . 0 6 0 . 0 4 0 . 0 2 0 . 0 0 .0 Conditions O Cath. flow rate = 0.10 cm 3 s "1 • Cath. flow rate = 0.30 cm 3 s "1 • Cath. flow rate = 0.50 cm 3 s "1 Fitted values Anolyte flow rate = 2.0 cm 3 s "1 NaOH cone. = 0.50 (M) NaCI cone. = 4.2-3.8 (M) Temp. (°C) - Anolyte side = 28/33 - Catholyte side = 22/30 Low chlorate run (0-0.5M) 1 I 0 .0 0 .5 1.0 1.5 Current Density (KAM "2) 2 . 0 2 .5 Figure 12 - 82 -The e f f e c t o f c u r r e n t d e n s i t y on c h l o r a t e c u r r e n t e f f i c i e n c y c a n be e x p l a i n e d i n p a r t as due t o c h a n g e i n m o l e s o f h y p o c h l o r i t e . An i n c r e a s e o f c u r r e n t d e n s i t y t e n d s t o i n c r e a s e t h e number o f m o l e s o f h y p o c h l o r i t e t o be c o n v e r t e d t o c h l o r a t e i n t h e c h e m i c a l r e a c t o r . An i n c r e a s e i n c u r r e n t d e n s i t y a l s o i n c r e a s e s t h e r a t e o f g a s e v o l u t i o n and t h u s a c c e l e r a t e s t h e mass t r a n s f e r . The r e s u l t o f t h i s i s an i n c r e a s e i n t h e r a t e o f t h e e l e c t r o c h e m i c a l c h l o r a t e f o r m a t i o n w h i c h may d e c r e a s e t h e o v e r a l l c u r r e n t e f f i c i e n c y . A l s o an i n c r e a s e i n c u r r e n t d e n s i t y i n c r e a s e s t h e e l e c t r i c a l t r a n s f e r e n c e f l u x o f t h e OH- and H 0 2 ~ a n i o n s i n t o t h e a n o l y t e . The i n c r e a s e i n H 0 2 _ f l u x i n t o t h e a n o l y t e d e c r e a s e s h y p o c h l o r i t e c o n c e n t r a t i o n . The r e s u l t s o f T a b l e 12 i n d i c a t e t h a t a t any g i v e n c a t h o l y t e f l o w and NaOH c o n c e n t r a t i o n , t h e c h l o r a t e c u r r e n t e f f i c i e n c y l o s s due t o p e r o x i d e t r a n s p o r t i n t o t h e a n o l y t e i s o v e r c o m p e n s a t e d by e f f i c i e n c y i n c r e a s e due t o g e n e r a t i o n o f h y p o c h l o r i t e . The e f f e c t s o f s u p e r f i c i a l c u r r e n t d e n s i t y on p e r o x i d e c u r r e n t e f f i c i e n c y and c o n c e n t r a t i o n a r e i l l u s t r a t e d i n T a b l e 1 3 . The t a b l e shows t h a t an i n c r e a s e i n c u r r e n t d e n s i t y r e s u l t s i n a d e c r e a s e i n p e r o x i d e c u r r e n t e f f i c i e n c y and an i n c r e a s e i n p e r o x i d e c o n c e n t r a t i o n a t any g i v e n c a t h o l y t e f l o w and NaOH c o n c e n t r a t i o n . I n c r e a s e i n t h e c u r r e n t d e n s i t y r e s u l t s i n a more n e g a t i v e c a t h o d e p o t e n t i a l . T h i s n e g a t i v e c a t h o d e p o t e n t i a l i n c r e a s e s t h e r a t e o f p e r o x i d e r e d u c t i o n , i . e . , H 0 2 " + H 2 0 + 2e + 3 0 H - E ° = 0 . 0 8 8 V ( 3 0 ) An i n c r e a s e i n c u r r e n t d e n s i t y d o e s n o t o n l y r e s u l t t o h i g h e r p e r o x i d e - 83 -c o n c e n t r a t i o n b u t a l s o r e s u l t s i n HO2" m i g r a t i o n t o t h e a n o l y t e . T h e s e r e s u l t i n l o s s i n p e r o x i d e c u r r e n t e f f i c i e n c y . At 2 . 0 M NaOH, c u r r e n t e f f i c i e n c y d e c r e a s e s f r o m 86% (H02~ c o n c e n t r a t i o n o f 0 . 4 4 6 M) a t 1 . 2 kA n r 2 t o 5 3 . 0 % ( p e r o x i d e c o n c e n t r a t i o n o f 0 . 5 2 0 M) a t 2 . 4 kA n r 2 f o r a c a t h o l y t e f l o w o f 0 . 1 x c m 3 s " 1 . T h i s i s an i m p r o v e m e n t i n e f f i c i e n c y o v e r t h e r e s u l t s o f Oloman and W a t k i n s o n [ 2 8 ] . They o b t a i n e d an 8 4 . 0 % e f f i c i e n c y a t 0 . 5 kA m ~ 2 ( p e r o x i d e c o n c e n t r a t i o n = 0 . 3 0 M) and 60% a t 1 . 5 kA n r 2 ( p e r o x i d e c o n c e n t r a t i o n = 0 . 7 0 M) u n d e r a p r e s s u r e o f 9 3 0 k P a , o x y g e n f l o w o f 3 2 . 0 c m 3 s - 1 and c a t h o l y t e f l o w o f 0 . 1 7 c m 3 s - 1 . 5.3.2 Effect of Catholyte Flow The i n f l u e n c e o f c a t h o l y t e f l o w on t h e p r o c e s s i s shown i n T a b l e 12 f o r t h e c h l o r a t e s i d e and i n T a b l e 13 f o r t h e p e r o x i d e s i d e . T a b l e 12 i n d i c a t e s t h a t t h e c h l o r a t e c u r r e n t e f f i c i e n c y i n c r e a s e s w i t h r i s e i n c a t h o l y t e f l o w a t f i x e d c u r r e n t d e n s i t y . T h i s c o u l d be e x p l a i n e d as due t o l o w e r p e r o x i d e c o n c e n t r a t i o n t h a t r e s u l t s w i t h h i g h e r c a t h o l y t e f l o w . A l o w e r p e r o x i d e c o n c e n t r a t i o n ( a t a c o n s t a n t c u r r e n t d e n s i t y ) means a l o w e r p e r o x i d e m i g r a t i o n a l f l u x and d i f f u s i o n a l f l u x i n t o t h e a n o l y t e t h u s l e a d i n g t o a l o w e r h y p o c h l o r i t e l o s s i n t h e a n o l y t e . W i t h s u c h l o w e r l o s s i n h y p o c h l o r i t e , more c h l o r a t e i s f o r m e d g i v i n g r i s e t o a h i g h e r c h l o r a t e c u r r e n t e f f i c i e n c y w i t h a r i s e i n c a t h o l y t e f l o w . I t i s a l s o s e e n i n T a b l e 12 t h a t f o r t h e 0 . 5 M and 1 . 0 M NaOH c o n d i t i o n s , t h e c h l o r a t e c u r r e n t e f f i c i e n c y r e l a t i o n s h i p w i t h t h e c a t h o l y t e f l o w i s i n t h e o r d e r : v a l u e s o f c h l o r a t e e f f i c i e n c y a t 2 . 4 kA n r 2 > 1 . 8 kA > 1 . 2 kA n r 2 i . e . , t h e c h l o r a t e c u r r e n t e f f i c i e n c y - 84 -i n c r e a s e s w i t h c u r r e n t d e n s i t y a t a n y g i v e n c a t h o l y t e f l o w . H o w e v e r , t h i s t r e n d i s b r o k e n a t 2 . 0 M NaOH. T a b l e 12 a n d F i g . 13 i n d i c a t e t h a t t h e v a l u e s o f c h l o r a t e c u r r e n t e f f i c i e n c y a t 2 . 4 kA m~2 a t a n y g i v e n c a t h o l y t e f l o w l i e s b e t w e e n t h e v a l u e s o f t h e e f f i c i e n c y a t 1 . 8 kA m~ and 1 . 2 kA m - 2 . T h u s , t h e maximum c u r r e n t e f f i c i e n c y f o r t h e c h l o r a t e s i d e i s o b t a i n e d a t 1 . 8 kA m " 2 . T h i s b e h a v i o u r c o u l d be a t t r i b u t e d t o t h e c o m b i n e d i n f l u e n c e o f p e r o x i d e 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 e f f e c t s . T a b l e 12 s h o w s t h a t t h e p e r o x i d e c o n c e n t r a t i o n a t 2 . 0 M NaOH and 2 . 4 0 kA m ~ 2 i s h i g h e r t h a n t h e v a l u e s o b t a i n e d a t 1 . 8 kA m ~ 2 . I t i s l i k e l y t h a t t h e i n c r e a s e i n c h l o r a t e c u r r e n t e f f i c i e n c y d u e t o i n c r e a s e i n c u r r e n t d e n s i t y ( a t 2 . 0 M NaOH) i s u n d e r c o m p e n s a t e d by t h e l o s s o f e f f i c i e n c y d u e t o h i g h e r p e r o x i d e c o n c e n t r a t i o n . The e f f e c t s o f c a t h o l y t e r a t e on t h e p e r o x i d e c u r r e n t e f f i c i e n c y and c o n c e n t r a t i o n a r e d e m o n s t r a t e d i n T a b l e 1 3 . A t 0 . 5 M a n d 1 . 0 M NaOH, i t i s s e e n t h a t t h e p e r o x i d e e f f i c i e n c y i n c r e a s e s w i t h i n c r e a s i n g c a t h o l y t e f l o w w h i l e t h e p e r o x i d e c o n c e n t r a t i o n f a l l s . H o w e v e r , a t 2 . 0 M NaOH, t h e t r e n d i n c u r r e n t e f f i c i e n c y f o u n d f o r t h e l o w e r c a u s t i c c o n c e n t r a t i o n i s r e v e r s e d . T h u s , i n c r e a s i n g t h e c a t h o l y t e f l o w l o w e r s t h e c u r r e n t e f f i c i e n c y . F u r t h e r , i t i s o b s e r v e d t h a t i n t h e r e g i o n o f o p e r a t i o n , t h e p e r o x i d e c u r r e n t e f f i c i e n c y d e c r e a s e s w i t h i n c r e a s e i n c u r r e n t d e n s i t y a t any g i v e n c a t h o l y t e f l o w r a t e . 01oman a n d W a t k i n s o n [ 3 1 ] o p e r a t e d a p e r o x i d e c e l l o f t h e t y p e u s e d h e r e ( w i t h a c a t i o n i c membrane and o x y g e n g e n e r a t i o n a t t h e a n o d e a t a c a u s t i c c o n c e n t r a t i o n o f 2 . 0 M) and o b s e r v e d a r i s e i n p e r o x i d e c u r r e n t e f f i c i e n c y w i t h i n c r e a s i n g c a t h o l y t e f l o w . A l t h o u g h t h e y o p e r a t e d t h e 85 -1 0 0 . 0 Ef fect of ca tho ly te f low o n ch lo ra te cur rent e f f i c iency 9 0 . 0 8 0 . 0 7 0 . 0 6 0 . 0 5 0 . 0 Conditions O Current density = 1.20 (KAM' 2 ) • Current density = 1.80 (KAM' 2 ) Current density = 2.40 (KAM Fitted values -2 ,3S-1 Anolyte flow rate = 2.0 cm' NaOH cone. = 2.0 (M) NaCl cone. = 3.8-3.5 (M) Temp. (°C) - Anolyte side = 28/33 - Catholyte side = 22/30 Low chlorate run (0-0.5M) 1 0.0 0 .12 0 .24 0 . 3 6 0 .48 Catho ly te F l ow Ra te ( c m 3 s ~ 1 ) 0 .60 F i y u r e 13 - 86 -t r i c k l e bed c e l l a t a c u r r e n t d e n s i t y r a n g e o f 0 . 4 - 1 . 6 kA m ~ 2 , t h e o b s e r v a t i o n made h e r e c o n t r a s t s w i t h t h e i r r e s u l t s a t 2 . 0 M NaOH c o n c e n t r a t i o n . T h i s o b s e r v a t i o n u n d e r l i e s t h e i n t e r a c t i n g e f f e c t o f t h e a n o l y t e s i d e r e a c t i o n s t o t h e c a t h o l y t e r e a c t i o n s . The f l o w o f t h e c a t h o l y t e a f f e c t s t h e p e r o x i d e l o s s m e c h a n i s m s h e n c e i t s i m p o r t a n c e i n t h e p e r o x i d e c u r r e n t e f f i c i e n c y . At l o w e r f l o w o f t h e c a t h o l y t e , t h e r e i s a h i g h e r p e r o x i d e c o n c e n t r a t i o n . P e r o x i d e l o s s t h r o u g h m i g r a t i o n and d i f f u s i o n i n t o t h e a n o l y t e and l o s s due t o r e d u c t i o n a t t h e c a t h o l y t e a r e f a v o u r e d by h i g h e r p e r o x i d e c o n c e n t r a t i o n . Thus t h e l o w e r c u r r e n t e f f i c i e n c y o b t a i n e d a t l o w e r c a t h o l y t e f l o w may be a c o n s e q u e n c e o f t h e a b o v e l o s s e s . The o t h e r l o s s r e a c t i o n t h a t o c c u r s i n c l u d e t h e d e c o m p o s i t i o n o f p e r o x i d e . 2 H 2 0 2 ^ 2 H 2 0 + 0 2 ( 5 2 ) T r a c e m e t a l i o n s a c t as c a t a l y s t f o r r e a c t i o n ( 5 2 ) . I t i s l i k e l y t h a t t h e d e c r e a s e i n p e r o x i d e c u r r e n t e f f i c i e n c y w i t h h i g h e r c a t h o l y t e f l o w a t 2 . 0 M c a u s t i c may be due t o p e r o x i d e c o n c e n t r a t i o n e f f e c t on e f f i c i e n c y as a l r e a d y p o i n t e d o u t . A t 2 . 0 M NaOH, t h e p e r o x i d e c o n c e n t r a t i o n i s h i g h e r t h a n t h e c o n c e n t r a t i o n o b t a i n e d a t any l o w e r NaOH c o n c e n t r a t i o n u n d e r t h e same o p e r a t i n g c o n d i t i o n s . 5.3.3 Effect of NaOH Concentration The e f f e c t s o f NaOH c o n c e n t r a t i o n on t h e c h l o r a t e c u r r e n t e f f i c i e n c y a r e shown i n T a b l e 1 3 . The f o l l o w i n g f e a t u r e s a r e o b s e r v e d : a . A t c u r r e n t d e n s i t y o f 1 . 2 kA n r 2 , t h e c h l o r a t e c u r r e n t e f f i c i e n c y seems t o r e m a i n c o n s t a n t w i t h i n c r e a s e i n NaOH c o n c e n t r a t i o n . - 87 -b . A t 1 . 8 kA m - 2 , a s l o w i n c r e a s e i n c h l o r a t e c u r r e n t e f f i c i e n c y i s o b s e r v e d w i t h r i s e i n NaOH c o n c e n t r a t i o n . c . A t 2 . 4 kA m ~ 2 , t h e c h l o r a t e c u r r e n t e f f i c i e n c y i s o b s e r v e d t o f a l l w i t h NaOH c o n c e n t r a t i o n ( F i g 1 4 ) . T h i s c o n t r a s t s w i t h t h e o b s e r v a t i o n s made i n ( a ) and ( b ) a b o v e . The o b s e r v a t i o n made i n ( a ) a b o v e c o u l d be e x p l a i n e d i n p a r t by e l e c t r i c a l t r a n s f e r e n c e f l u x t e r m a t 1 . 2 kA m " 2 and d i f f u s i o n f l u x . The l o s s o f c h l o r a t e i n t e r m e d i a t e s by t h e t r a n s p o r t o f p e r o x i d e i n t o t h e a n o l y t e f r o m t h e c a t h o l y t e seems t o r e m a i n c o n s t a n t w i t h i n c r e a s e i n NaOH c o n c e n t r a t i o n . A l t h o u g h T a b l e 13 shows t h a t t h e p e r o x i d e c o n c e n t r a t i o n i n c r e a s e s w i t h NaOH c o n c e n t r a t i o n , t h e o b s e r v a t i o n i n d i c a t e s t h a t t h e h i g h e r d i f f u s i o n c o e f f i c i e n t o f OH - i o n s a n d c o n c e n t r a t i o n a f f e c t t h e a n o l y t e s i d e more t h a n t h e p e r o x i d e e f f e c t . The c h l o r a t e c u r r e n t e f f i c i e n c y t h e n r e m a i n s i n d i f f e r e n t t o t h e NaOH c o n c e n t r a t i o n a t 1 . 2 kA m - 2 . At 1 . 8 kA m - 2 , t h e r e i s an i n c r e a s e i n C I O - m o l e s g e n e r a t e d as w e l l a s i n c r e a s e i n m i g r a t i o n o f H 0 2 " and O H - i o n s i n t o t h e a n o l y t e . The i n c r e a s e i n c h l o r a t e e f f i c i e n c y due t o C I O - m o l e s g e n e r a t e d and 0 H _ m i g r a t i o n s i s l o w e r e d by H 0 2 ~ m i g r a t i o n i n t o t h e a n o l y t e . T h i s e x p l a i n s t h e o b s e r v a t i o n s made i n ( b ) . The o b s e r v a t i o n made i n ( c ) i s a c c o u n t e d f o r by t h e i n c r e a s e d i m p o r t a n c e o f m i g r a t i o n and c o n c e n t r a t i o n s o f p e r o x i d e a t 2 . 4 kA n n 2 . The e f f e c t o f NaOH c o n c e n t r a t i o n on t h e p e r o x i d e c u r r e n t e f f i c i e n c y and c o n c e n t r a t i o n a r e shown i n T a b l e 1 3 . The r e s u l t s i n d i c a t e t h e f o l l o w i n g : - 88 -100.0 Effect of N a O H concentrat ion on chlorate current ef f ic iency 90 .0 80 .0 70 .0 60 .0 50 .0 Conditions O Cath. flow rate = 0.10 cm 3 s "1 • Cath. flow rate = 0.30 cm 3 s "1 • Cath. flow rate = 0.50 cm 3 s *1 Fitted values Anolyte flow rate = 2.0 cm 3 s "1 NaCI cone. = 4.2-3.5 (M) Current density = 2.40 (KAM "2) Temp. (°C) - Anolyte side = 28/33 - Catholyte side = 22/30 Low chlorate run (0-0.5M) J_ 0.0 0.5 1.0 1.5 N a O H Concent ra t ion (M) 2.0 2.5 F i g u r e 14 - 89 -( i ) The p e r o x i d e c o n c e n t r a t i o n and c u r r e n t e f f i c i e n c y i n c r e a s e w i t h i n c r e a s e i n NaOH c o n c e n t r a t i o n , ( i i ) The o b s e r v a t i o n i n ( i ) a b o v e w i t h r e s p e c t t o e f f i c i e n c y seems t o be c o n s i s t e n t w i t h l o w c a t h o l y t e f l o w . As t h e c a t h o l y t e r a t e i n c r e a s e s , t h e e f f i c i e n c y d o e s n o t r i s e as r a p i d l y as i t d o e s a t l o w e r r a t e . In f a c t , i t i s o b s e r v e d t h a t a t a f l o w o f 0 . 5 c m 3 s - 1 , t h e c u r r e n t e f f i c i e n c y f a l l s w i t h i n c r e a s e i n c a u s t i c c o n c e n t r a t i o n . The same t r e n d i s o b s e r v e d f o r t h e p e r o x i d e c o n c e n t r a t i o n . An e x a m i n a t i o n o f T a b l e 13 and F i g u r e 15 i n d i c a t e s t h a t a c r i t i c a l c a u s t i c c o n c e n t r a t i o n c o u l d be d e f i n e d . A c c o r d i n g t o F i g u r e 1 5 , b e l o w a NaOH c o n c e n t r a t i o n o f 1 . 5 M , t h e p e r o x i d e c u r r e n t e f f i c i e n c y i n c r e a s e s w i t h c a t h o l y t e f l o w . H o w e v e r , a t NaOH c o n c e n t r a t i o n s g r e a t e r t h a n 1 . 5 M , t h e e f f i c i e n c y o f p e r o x i d e f a l l s w i t h i n c r e a s e i n c a t h o l y t e f l o w . T h e s e e f f e c t s a r e o b s e r v e d a t a l l c u r r e n t d e n s i t i e s e x p e r i m e n t e d w i t h . H o w e v e r , t h e e f f e c t s a r e n o t w e l l u n d e r s t o o d . T h i s b e h a v i o u r m i g h t w e l l be a r e s u l t o f many o t h e r p a r a m e t e r i n t e r a c t i o n s . As o b s e r v e d by Oloman and W a t k i n s o n [ 2 8 ] f o r a p e r o x i d e c e l l , ( c u r r e n t d e n s i t y 0 . 2 - 0 . 8 kA m - 2 ) p e r o x i d e c o n c e n t r a t i o n and c u r r e n t e f f i c i e n c y i n c r e a s e w i t h i n c r e a s e i n NaOH c o n c e n t r a t i o n . I t i s s u g g e s t e d t h a t h i g h i n i t i a l NaOH c o n c e n t r a t i o n c o u l d p r o t e c t t h e p e r o x i d e f r o m s u b s e q u e n t c a t h o d e r e a c t i o n s . I t m i g h t w e l l be t h a t a t h i g h c a t h o l y t e f l o w ( a t h i g h NaOH c o n c e n t r a t i o n ) t h e e f f e c t s o f f l o w and NaOH c o n c e n t r a t i o n on p e r o x i d e l o s s t o t h e a n o l y t e become more p r o n o u n c e d t h a n t h e i n f l u e n c e o f o t h e r f a c t o r s t h a t f a v o u r t h e p e r o x i d e c u r r e n t e f f i c i e n c y . - 90 -100.0 Effect of N a O H concent ra t ion on pe rox ide current ef f ic iency 80 .0 -o •§ 60 .0 40 .0 20 .0 0.0 Conditions O Cath. flow rate = 0.10 cm 3 s" 1 • Cath. flow rate = 0.30 cm 3 s "1 • Cath. flow rate = 0.50 cm 3 s "1 Fitted values Anolyte flow rate = 2.0 c m 3 s" 1 NaCI cone. = 4.2-3.8 (M) Current density = 1.80 (KAM ~2) Temp. (°C) - Anolyte side = 28/33 - Catholyte side = 22/30 Low chlorate run (0-0.5M]y 0.0 0.5 1.0 1.5 2.0 NaOH Concentration (M) 2.5 Figu re 15 - 91 -5.3.4 Effect of Peroxy-Hydroxy Mole Ratio A l k a l i n e p e r o x i d e r e a c t s w i t h h y p o c h l o r i t e o r d i s s o l v e d e l e m e n t a l c h l o r i n e - i m p o r t a n t i n t e r m e d i a t e s i n t h e c h l o r a t e s y n t h e s i s . H y d r o l y s i s o f c h l o r i n e and e v e n t h e f o r m a t i o n o f O C l " r e q u i r e an a l k a l i n e e n v i r o n m e n t ( O H - i o n s ) . Thus i n a c o u p l e d p r o c e s s o f t h i s n a t u r e , i t i s p o s s i b l e t o s e e t h e e f f e c t o f t h e m o l e r a t i o o f t h e . t w o o p p o s i n g s u b s t a n c e s on t h e c h l o r a t e c u r r e n t e f f i c i e n c y . T h e s e e f f e c t s a r e i l l u s t r a t e d i n T a b l e 16 and F i g u r e s 1 6 , 17 and 1 8 . I t i s i m p o r t a n t t o p o i n t o u t t h a t p e r o x i d e m o l a r c o n c e n t r a t i o n i s n o t an i n d e p e n d e n t v a r i a b l e ( i n t h i s p r o c e s s ) t h u s i t c o u l d be a r g u e d t h a t F i g u r e s 16 t o 18 a r e s i m p l y t h e i n v e r s e o f t h e e f f e c t o f h y d r o x i d e c o n c e n t r a t i o n pn c h l o r a t e c u r r e n t e f f i c i e n c y . H o w e v e r , c o m p a r i s o n o f t h e s e t w o d o e s n o t seem t o a g r e e w i t h t h e l a t e r a r g u m e n t . The h y d r o x y l i o n m o l a r c o n c e n t r a t i o n u s e d i n c o m p u t i n g t h e m o l e r a t i o i s t h e i n i t i a l NaOH f e e d c o n c e n t r a t i o n i n t o t h e c e l l . The f o l l o w i n g o b s e r v a t i o n s c o u l d be made f r o m t h e p e r o x y - h y d r o x y m o l e r a t i o p l o t s : a . The c h l o r a t e c u r r e n t e f f i c i e n c y f a l l s w i t h r i s e i n p e r o x y - h y d r o x y m o l e r a t i o b . F o r t h e 0 . 5 M and 1 . 0 M NaOH, t h e g r a p h s show t h e f o l l o w i n g t r e n d s f o r t h e a v e r a g e c h l o r a t e c u r r e n t e f f i c i e n c y 2 . 4 > 1 . 8 > 1 . 2 kA m - 2 , ( F i g 16 and 1 7 ) c . A t 2 . 0 M NaOH, t h e c h l o r a t e c u r r e n t e f f i c i e n c y f o r t h e 2 . 4 KA m " 2 l i e s b e t w e e n t h o s e a t 1 . 8 kA m - 2 and 1 . 2 kA m ~ 2 ( F i g . 1 8 ) . F i g u r e s ( 1 6 - 1 8 ) show t h a t t h e f i t t e d v a l u e s a r e l i n e a r w h i l e t h e a c t u a l e x p e r i m e n t a l p o i n t s a r e non l i n e a r . The n o n - l i n e a r i t y o f t h e T a b l e 16 C h l o r a t e c u r r e n t e f f i c i e n c y as a f u n c t i o n o f p e r o x y - h y d r o x y m o l e r a t i o ( l o w c h l o r a t e r u n ) C a t h o l y t e (NaOH C o n c e n t r a t i o n C u r r e n t D e n s i t y k A / m 2 C a t h o l y t e F l o w R a t e c m 3 / s • 0 . 5 1 . 0 2 . 0 H 0 2 _ / 0 H -R a t i o Chi o r a t e C u r r e n t E f f . (%) H 0 2 " / 0 H -R a t i o C h i o r a t e •' C u r r e n t E f f . (%) H 0 2 " / 0 H -R a t i o C h i o r a t e C u r r e n t E f f . (%) 1 . 2 ( 1 0 A ) 0 . 1 0 . 3 0 . 5 0 . 3 0 6 0 . 1 7 0 0 . 0 6 4 5 3 . 0 5 7 . 4 6 8 . 5 0 . 2 1 5 0 . 1 1 5 ' 0 . 0 7 0 5 2 . 7 6 2 . 6 7 0 . 3 0 . 2 1 5 0 . 0 6 5 0 . 0 3 4 5 3 . 2 5 9 . 7 7 0 . 3 1 . 8 ( 1 5 A ) 0 . 1 0 . 3 0 . 5 0 . 3 1 0 0 . 1 8 0 0 . 1 5 0 6 0 . 2 6 8 : 0 7 3 . 5 0 . 2 4 9 0 . 1 1 5 0 . 0 9 7 6 1 . 5 0 6 7 . 5 7 2 . 0 0 . 2 2 4 5 0 . 0 7 0 . 0 3 3 5 6 5 . 0 7 0 . 0 7 4 . 0 2 . 4 ( 2 0 A ) 0 . 1 0 . 3 0 . 5 0 . 2 9 0 0 . 1 7 4 0 . 0 8 2 6 9 . 1 7 2 . 1 7 8 . 0 0 . 2 7 5 0 . 1 4 3 0 . 1 1 7 6 9 . 0 7 2 . 5 7 6 . 0 0 . 2 6 5 0 . 0 7 8 0 . 0 3 9 6 2 . 6 6 7 . 0 7 2 . 9 C o n d i t i o n s : A v e r a g e c e l l v o l t a g e s : 1 . 2 kA m - 2 - 3 . 0 0 V ; 1 . 8 kA m - 2 - 3 . 4 6 V ; 2 . 4 kA m - 2 - 4 . 1 0 V A n o l y t e S i d e C a t h o l y t e S i d e C e l l temp ( ° C ) 27733 2 2 / 3 0 C e l l p r e s s . ( k P a ) r a n g e 0 . 0 - 5 8 . 6 / 0 . 0 0 . 0 - 4 4 . 8 / 0 . 0 Oxygen f l o w ( c m 3 s _ 1 ) - 8 . 5 A n o l y t e t a n k pH 6 . 5 ± 0 . 2 A n o l y t e f l o w ( c m 3 s " 1 ) 2 . 0 A n o l y t e t a n k temp ( ° C ) 7 0 . 0 NaCl c o n e . (M) . 0 . 0 - 0 . 5 N a C 1 0 3 c o n e . (M) 0 - 0 . 5 - 93 -100.0 Effect of H O 2 / O H mole ratio on chlorate current ef f ic iency at 0.5 (M) N a O H 90 .0 -80 .0 70 .0 60 .0 50 .0 Conditions O Current density = 1.20 (KAM ~2) • Current density = 1.80 (KAM' 2 ) • Current density = 2.40 (KAM "2) Fitted values Anolyte flow rate = 2.0 cm 3 s "1 NaOH cone. = 0.5 (M) NaCI cone. = 4.2-3.5 (M) Temp. (°C) - Anolyte side = 27/33 - Catholyte side = 20/29 Low chlorate run (0-0.5M) 0.0 0.1 0.2 0.3 P e r o x y - H y d r o x y Rat io 0.4 0.5 F i g u r e 16 - 94 -100.0 Effect of H O 2 / O H " mole ratio on chlorate current ef f ic iency at 1.0 (M) N a O H 90 .0 80 .0 70 .0 60 .0 50 .0 Conditions O Current density = 1.20 (KAM "2) • Current density = 1.80 (KAM " 2 ) Current density = 2.40 (KAM' Fitted values ) Anolyte flow rate = 2.0 cm 3 s ~1 NaOH cone. = 1.0 (M) NaCl cone. = 4.2-3.5 (M) Temp. (°C) - Anolyte side = 28/33 - Catholyte side = 22/30 Low chlorate run (0-0.5M) 0.0 0.1 0.2 0.3 P e r o x y - H y d r o x y Rat io 0.4 0.5 F I y u r e 17 - 95 -100.0 Effect of HO 2 /OH mole ratio on chlorate current efficiency at 2.0 (M) NaOH 90.0 > v o C D M — LU -4—' c CD o sz O 80.0 70.0 60.0 50.0 Conditions O Current density = 1.20 (KAM "2) • Current density = 1.80 (KAM - 2 ) • Current density = 2.40 (KAM "2) Fitted values Anolyte flow rate = 2.0 cm 3 s "1 NaOH cone. = 2.0 (M) NaCI cone. = 4.2-3.5 (M) Temp. (°C) - Anolyte side = 27/33 - Catholyte side = 22/29 Low chlorate run (0-0.5M) 0.0 0.1 0.2 0.3 Peroxy-Hydroxy Ratio 0.4 0.5 F i g u r e 18 - 96 -a c t u a l e x p e r i m e n t a l p o i n t s c o u l d be a r e s u l t o f t h e r e l a t i v e f l u x e s o f H 0 2 ~ a n d O H " . 5.4 High or Strong Chlorate Concentration Runs T a b l e s 14 and 15 show t h e e x p e r i m e n t a l r e s u l t s a t s t r o n g c h l o r a t e c o n c e n t r a t i o n . The i n v e s t i g a t i o n o f t h e e f f e c t s o f NaOH, c u r r e n t d e n s i t y and c a t h o l y t e f l o w on t h e c u r r e n t e f f i c i e n c i e s o f i n t e r e s t a t h i g h c h l o r a t e c o n c e n t r a t i o n was u n d e r t a k e n i n o r d e r t o a s s e s s t h e i n d u s t r i a l p o s s i b i l i t i e s o f t h e p r o c e s s . 5.4.1 Chlorate Current Effi c i e n c y C o m p a r i n g T a b l e s 12 a n d 14 ( l o w and h i g h c h l o r a t e r u n s r e s p e c t i v e l y ) f o r t h e c h l o r a t e c u r r e n t e f f i c i e n c y , t h e f o l l o w i n g o b s e r v a t i o n s c o u l d be m a d e : ( i ) A t 1 . 2 kA m ~ 2 , t h e c h l o r a t e c u r r e n t e f f i c i e n c i e s a t 0 . 1 c m 3 s _ 1 c a t h o l y t e r a t e ( f o r b o t h 1 . 0 a n d 2 . 0 M NaOH) i n T a b l e 14 a r e h i g h e r t h a n t h e v a l u e s o b t a i n e d a t l o w c h l o r a t e r u n s i n T a b l e 1 2 . B u t a t 0 . 5 cm 3 s - 1 c a t h o l y t e f l o w , t h e two t a b l e s g i v e s i m i l a r v a l u e s . ( i i ) A t 2 . 4 kA m ~ 2 , t h e t r e n d o b s e r v e d a t 1 . 2 kA m ~ 2 i s n o t r e p e a t e d . T h i s may be a c o n s e q u e n c e o f n o t r e p l i c a t i n g e x a c t l y a l l t h e n e c e s s a r y c o n d i t i o n s o f t h e l o w c h l o r a t e r u n f o r t h e h i g h c h l o r a t e r u n . - 97 -5.4.2 Peroxide Concentration and Current E f f i c i e n c y C o m p a r i n g t h e p e r o x i d e r e s u l t s f o r t h e l o w and h i g h c h l o r a t e r u n s ( c . f . T a b l e s 13 a n d 1 5 ) , t h e f o l l o w i n g c o u l d be i n f e r r e d : ( i ) T h e p e r o x i d e c u r r e n t e f f i c i e n c y and c o n c e n t r a t i o n a t h i g h c h l o r a t e r u n s show an i m p r o v e m e n t o v e r t h e r e s u l t s a t l o w c h l o r a t e r u n s . The r o l e o f c h l o r a t e c o n c e n t r a t i o n i n t h e p e r o x i d e c u r r e n t e f f i c i e n c y a n d c o n c e n t r a t i o n i s n o t w e l l u n d e r s t o o d a n d i t m i g h t w e l l be due t o membrane c o n d i t i o n . 5.5 Ce l l Voltage T h e e c o n o m i c o p t i m u m o f an e l e c t r o c h e m i c a l p r o c e s s i s g o v e r n e d by t h e c u r r e n t e f f i c i e n c y and t h e n e c e s s a r y e l e c t r o l y s i s v o l t a g e . I n a p e r o x y - c h l o r a t e c e l l , t h e p e r o x i d e c o n c e n t r a t i o n m u s t be i n c l u d e d a d d i t i o n a l l y a s a t h i r d f a c t o r . To d i s c u s s t h e c e l l v o l t a g e , i t must be b o r n e i n m i n d t h a t t h e c e l l v o l t a g e d e p e n d s on t h e c h a r a c t e r i s t i c e l e m e n t s o f t h e e l e c t r o l y s i s s y s t e m . Thus t h e f o l l o w i n g e l e m e n t s a r e known t o a c c o u n t f o r t h e v o l t a g e o f a c e l l , v i z : e l e c t r o d e m a t e r i a l s ( c a t a l y s t s ) , s e p a r a t o r (membrane o r d i a p h r a g m ) , e l e c t r o l y t e c o n c e n t r a t i o n s , c u r r e n t d e n s i t y , o p e r a t i n g t e m p e r a t u r e , o p e r a t i n g p r e s s u r e a n d e l e c t r o l y t e f l o w s . No a t t e m p t i s made h e r e i n d i s c u s s i n g i n d e t a i l s t h e r e l a t i v e c o n t r i b u t i o n s o f e a c h o f t h e a b o v e f a c t o r s t o t h e c e l l v o l t a g e . A s u r v e y o f A p p e n d i x 2 ( T a b l e s A - C ) h o w e v e r , shows t h a t a t a f i x e d a n o l y t e f l o w , t h e c u r r e n t d e n s i t y a n d c a t h o l y t e f l o w seem t o be t h e m o s t i m p o r t a n t f a c t o r s a f f e c t i n g t h e c e l l v o l t a g e a t t h e . g i v e n c o n d i t i o n s . - 98 -T y p i c a l r e l a t i o n s b e t w e e n t h e c e l l v o l t a g e and s e p a r a t o r c u r r e n t d e n s i t y a r e d e m o n s t r a t e d i n F i g u r e s 19 and 20 a t g i v e n c a t h o l y t e f l o w s . The t w o f i g u r e s i n d i c a t e t h a t t h e c e l l v o l t a g e i n c r e a s e s l i n e a r l y w i t h c u r r e n t d e n s i t y . The o b s e r v e d i n c r e a s e i n c e l l v o l t a g e w i t h c a t h o l y t e f l o w c o u l d be due t o membrane b u l g i n g w i t h h i g h e r c a t h o l y t e f l o w . The t e m p e r a t u r e e f f e c t on t h e c o n d u c t i v i t y o f e l e c t r o l y t e s a n d h e n c e on c e l l v o l t a g e s i s w e l l k n o w n . I n t h i s w o r k , a l o w e r c e l l v o l t a g e c o u l d be a c h i e v e d by i n c r e a s i n g t h e o p e r a t i n g c e l l t e m p e r a t u r e . O t h e r ways o f r e d u c i n g t h e c e l l v o l t a g e a r e by i n c r e a s i n g t h e w o r k i n g p r e s s u r e i n membrane e l e c t r o l y s i s o r w o r k i n g a t a h i g h e r a n o l y t e f l o w . I n c r e a s e i n w o r k i n g c e l l p r e s s u r e w o u l d n o t o n l y d e c r e a s e t h e v o l u m e t r i c amount o f g a s , b u t an i n c r e a s e o f t h e e l e c t r o l y s i s t e m p e r a t u r e w i t h o u t i n c r e a s i n g t h e e v a p o r a t i o n r a t e o f w a t e r i s a l s o a c h i e v e d . In t h i s w o r k , t h e e f f e c t o f p r e s s u r e on t h e c e l l v o l t a g e i s n o t e x p l o r e d . The e f f e c t o f a n o l y t e f l o w on t h e c e l l v o l t a g e i s i l l u s t r a t e d i n T a b l e 1 7 . The t a b l e i n d i c a t e s t h a t o p e r a t i n g a t a l o w e r a n o l y t e f l o w i n c r e a s e s t h e ^ c e l l v o l t a g e . H o w e v e r , a h i g h e l e c t r o l y t e f l o w g i v e s r i s e t o a h i g h e r p r e s s u r e l o s s i n t h e c e l l and membrane b u l g i n g . T h u s a c h o i c e o f e l e c t r o l y t e f l o w must be c h o s e n t h a t w o u l d n o t be d e t r i m e n t a l t o t h e m e m b r a n e . 5.6 Length of Time of Use and S t a b i l i t y of Membrane The membrane s t a b i l i t y a f f e c t s b o t h c u r r e n t e f f i c i e n c i e s o f p e r o x i d e and c h l o r a t e and t h e c e l l v o l t a g e . The s u c c e s s o f t h i s work d e p e n d s on t h e membrane p e r f o r m a n c e . The u s e o f an a s b e s t o s d i a p h r a g m - 99 -Effect of super f ic ia l cur rent dens i ty on the cel l vo l tage at 0.5 (M) N a O H Conditions O Cath. flow rate = 0.10 cm 3 s _ 1 • Cath. flow rate = 0.30 cm 3 s ~1 • Cath. flow rate = 0.50 cm 3 s "1 Fitted values Anolyte flow rate = 2.0 c m 3 s " 1 NaOH cone. = 0.50 (M) NaCl cone. = 3.5-4.2 (M) Temp. (°C) - Anolyte side = 27/33 - Catholyte side = 22/30 Low chlorate run (0-0.5M) 1.0 1.5 2.0 2 .5 Cur ren t Dens i ty ( K A M ~2) Fiyu re 19 - 100 -Ef fec t of s u p e r f i c i a l c u r r e n t d e n s i t y o n the ce l l v o l t a g e a t 2 . (M) N a O H Condit ions O Cath. flow rate = 0.1 O c m 3 s~ 1 Cath. (low rate = 0.30 cm 3 s Cath. flow rate = 0.50 cm • Fitted values -i 4 . 5 Anolyte flow rate = 2.0 cm " s NaOH cone. = 2. 0 (M) NaCl cone. = 3.8-3.5 (M) Temp. (°C) - Anolyte side = 28/33 - Catholyte side = 22/30 Low chlorate run (0-0.5M) 4 . 0 CD > o 3 . 5 3 . 0 2 . 5 .0 1.5 2 . 0 2 . 5 Current Density ( K A M ) Figure 20 - 101 -T a b l e 17 V a r i a t i o n o f c e l l v o l t a g e w i t h a n o l y t e f l o w A n o l y t e F l o w C e l l V o l t a g e ( c m 3 / s ) ( V ) 0 . 5 7 6 . 0 0 . 6 8 5 . 8 3 . 2 3 . 7 3 . 5 3 . 7 C o n d i t i o n s : NaOH c o n e Oxygen f l o w C a t h o l y t e f l o w C a t h o l y t e c e l l s i d e temp ( ° C ) C a t h o l y t e s i d e p r e s s u r e ( k P a ) A n o l y t e s i d e temp ( ° C ) A n o l y t e c e l l s i d e p r e s s u r e ( k P a ) A n o l y t e c e l l s i d e p r e s s u r e ( k P a ) A n o l y t e t a n k pH A n o l y t e t a n k temp ( ° C ) A v e r a g e a n o l y t e t a n k v o l C u r r e n t s u p p l y P e r o x i d e c o n e (M) P e r o x i d e c o n e (M) 2 . 0 m 8 . 5 c m 3 s " 1 (STP) 0 . 2 0 c m 3 s " 1 1 8 / 4 8 0 / 0 3 0 / 6 2 0 / 0 . 0 ( f o r f l o w < 1 . 0 c m 3 / s ) 1 3 . 8 / 0 . 0 ( f o r f l o w > 3 . 0 c m 3 / s ) 5 . 6 7 0 . 0 2 . 0 L 25A ( 3 kA m - 2 ) 0 . 4 7 ( a t l o w e r a n o l y t e f l o w ) 0 . 5 8 ( a t h i g h e r a n o l y t e f l o w ) - 102 -on t h e a n o d e s i d e o f t h e a n i o n i c membrane was t o p r o t e c t t h e membrane and p r o l o n g i t s l i f e . S i n c e no p h y s i c a l o r c h e m i c a l t e s t s were c a r r i e d o u t on t h e membrane i n o r d e r t o e v a l u a t e i t s d e t e r i o r a t i o n , t h e r e p r o d u c i b i l i t y o f p r o d u c t c u r r e n t e f f i c i e n c i e s and t h e c e l l v o l t a g e c h a n g e s w i t h t i m e w e r e u s e d as a y a r d s t i c k f o r m e a s u r i n g t h e membrane s t a b i l i t y . 5.6.1 Typical C e l l Voltage with Time The c e l l v o l t a g e v a r i a t i o n w i t h t i m e f o r an i o n s p e c i f i c p e r m e a b l e membrane c o u l d be u s e d as a q u a l i t a t i v e m e a s u r e o f r e s i s t a n c e c h a n g e s o f t h e m e m b r a n e . I f t h e f i x e d g r o u p o f a s p e c i f i c membrane i s d e g r a d e d d u e t o u s a g e , t h e r e a r e bound t o be c h a n g e s i n r e s i s t a n c e o f t h e c e l l , h e n c e t h e c e l l v o l t a g e w i l l v a r y w i t h t i m e . To be a b l e t o a s s e s s t h e e x t e n t o f a t y p i c a l membrane d e g r a d a t i o n i n t h e p r o c e s s , t h e c e l l v o l t a g e was f o l l o w e d w i t h t i m e . A t y p i c a l r e s u l t o b t a i n e d f o r a membrane t h a t had been u s e d f o r o v e r 20 h o u r s i s as shown i n T a b l e 1 8 . From t h e s e r e s u l t s , a c o n c l u s i o n c o u l d be made t h a t t h e v a r i a t i o n i n c e l l v o l t a g e w i t h t i m e d o e s n o t i n d i c a t e a s e r i o u s membrane damage - a t l e a s t i n t h e f i r s t 20 h o u r s o f i t s u s e . 5.6.2 Product Current E f f i c i e n c y with Membrane Usage Some e x p e r i m e n t a l r u n s w e r e r e p e a t e d . The r e p e t i t i o n i n v o l v e s k e e p i n g a l l t h e p r o c e s s v a r i a b l e s w i t h i n t h e same v a l u e s - e x c e p t t h a t t h e t i m e t h e membrane h a s been i n u s e was n o t k e p t c o n s t a n t i . e . , t h e age o f t h e m e m b r a n e . The o b s e r v a t i o n s made i n t h e e x p e r i m e n t a l r u n s i n c l u d e an a n o l y t e p i n k c o l o r a t i o n o f e a c h i n i t i a l ' n e w membrane' r u n . - 103 -T a b l e 18 C e l l v o l t a g e v a r i a t i o n w i t h t i m e AA BB Time ( m i n ) C e l l V o l t a g e ( V ) C e l l V o l t a g e ( V ) 1 3 . 1 3 4 . 1 9 15 3 . 1 3 4 . 2 5 30 3 . 1 2 4 . 2 4 45 3." 16 -60 3 . 1 4 4 . 1 7 90 3 . 1 7 4 . 1 8 105 - 4 . 2 0 120 3 . 1 4 4 . 1 8 180 - 4 . 1 0 C o n d i t i o n s : C u r r e n t d e n s i t y ( k A / m 2 ) NaOH c o n e (M) C a t h o l y t e f l o w ( c m 3 s " 1 ) Oxygen f l o w ( c m 3 s - 1 . STP) A n o l y t e f l o w (cm s ) A n o l y t e t a n k temp ( ° C ) A n o l y t e t a n k pH A n o l y t e s i d e c e l l temp ( ° C ) i n / o u t C a t h o l y t e s i d e c e l l temp ( ° C ) i n / o u t A n o l y t e s i d e c e l l p r e s s u r e ( k P a ) C a t h o l y t e s i d e c e l l p r e s s u r e ( k P a ) C h l o r a t e c u r r e n t e f f i c i e n c y (%) P e r o x i d e c u r r e n t e f f i c i e n c y (%) AA BB 1 . 2 2 . 4 2 . 0 2 . 0 0 . 1 0 0 . 5 4 . 5 0 4 . 5 2 . 0 2 . 0 7 0 . 0 7 0 . 0 6 . 6 6 . 5 2 4 / 2 7 . 2 3 / 3 0 19/21 1 8 / 2 3 1 7 . 2 / 0 . 0 5 8 . 6 / 0 . 0 5 5 . 2 / 0 . 0 4 1 . 4 / 0 . 0 7 7 . 3 5 1 . 0 7 3 . 0 6 1 . 0 - 104 -H o w e v e r , t h e p i n k c o l o r a t i o n o b s e r v e d i n t h e u s e o f a new membrane was n o t p r o n o u n c e d n o r was s u c h o b s e r v e d i n s u b s e q u e n t r u n s . T h i s s l i g h t c o l o r a t i o n may be due t o a w e a r i n g away o f t h e membrane - m o s t p r o b a b l y as a r e s u l t o f r e a c t i o n b e t w e e n t h e g e n e r a t e d c h l o r a t e i n t e r m e d i a t e s and membrane b a c k b o n e m a t e r i a l s . T h i s w e a r i n g d i d n o t m a n i f e s t i t s e l f i n t h e c e l l v o l t a g e r e s u l t s n e i t h e r d i d i t show i n s u b s e q u e n t r u n s u s i n g t h e same m e m b r a n e . A t y p i c a l r e p l i c a t e d r e s u l t i s shown i n T a b l e 1 9 . Run A 2 i s a r e p e a t o f A x w h i l e B 2 i s a r e p e a t o f B 1 . The r e s u l t s o f A L and A 2 do n o t v a r y s o much as t o be a t t r i b u t e d t o membrane d e g r a d a t i o n , w h i l e t h o s e o f Bi and B 2 d i f f e r . The d i f f e r e n c e b e t w e e n %i and B 2 c o u l d be due t o i n a b i l i t y t o r e p l i c a t e a l l e x p e r i m e n t a l c o n d i t i o n s p r e c i s e l y o r due t o e x p e r i m e n t a l e r r o r s . The most p r o b a b l e c o n d i t i o n s n o t r e p l i c a t e d a c c o r d i n g t o T a b l e 19 a r e t e m p e r a t u r e and p r e s s u r e . A t e m p e r a t u r e d i f f e r e n c e o f 2 ° C h o w e v e r c a n ' t a c c o u n t f o r t h e d i f f e r e n c e i n B x and B 2 c h l o r a t e c u r r e n t e f f i c i e n c y . The p r e s s u r e d i f f e r e n c e i s a l s o l o w and t h e p r e s s u r e e f f e c t s a r e n o t w e l l u n d e r s t o o d . I t c o u l d o t h e r w i s e be d e d u c e d t h a t t h e membrane l o s e s p a r t o f i t s s t a b i l i t y a f t e r 26 h o u r s o f u s e . 5.7 HC1 Addition A c c o r d i n g t o e q u a t i o n s ( 1 ) and ( 2 ) , t h e amount o f 0 2 + H 2 0 + 2e ^ H 0 2 - + OH" (1) C I - + 6 0 H - — ^ . C 1 0 3 - + 3 H 2 0 + 6e ( 2 ) O H - f u r n i s h e d by t h e p e r o x i d e f o r m a t i o n r e a c t i o n ( r x n 1) w o u l d n o t b e e n o u g h t o make a m o l e o f c h l o r a t e f o r a p a s s a g e o f 6 m o l e s o f - 105 -T a b l e 19 T h e e f f e c t o f l e n g t h o f t i m e o f u s e o f membrane on p r o d u c t c u r r e n t e f f i c i e n c i e s 1 . 2 kA m " 2 2 . 4 kA m " 2 -A i A 2 B i B 2 C I 0 3 ~ c u r r e n t e f f i c i e n c y (%) 7 3 . 0 7 7 . 3 6 0 . 0 8 0 . 6 H 0 2 " c u r r e n t e f f i c i e n c y (%) 7 8 . 0 7 3 . 0 7 0 . 0 7 0 . 0 P e r o x i d e c o n e (M) 0 . 4 0 4 0 . 3 7 8 0 . 1 4 6 0 . 1 4 6 A g e o f membrane ( h r s ) 3 . 0 2 5 . 0 1 2 . 0 2 8 . 0 s - 1 ) STP) C o n d i t i o n s NaOH c o n e (M) C a t h o l y t e f l o w ( c m 3 Oxygen f l o w (cm s " 1 . A n o l y t e f l o w ( c m 3 / s ) A n o l y t e t a n k temp ( ° C ) A n o l y t e t a n k pH A n o l y t e s i d e c e l l p r e s s ( k P a ) c a t h o l y t e s i d e c e l l p r e s s ( k P a ) A n o l y t e s i d e c e l l t e m p ( ° C ) C a t h o l y t e s i d e c e l l temp ( ° C ) C e l l v o l t a g e T i m e o f r u n ( m i n ) A] A? B, B 2 2 . 0 2 . 0 1 . 0 1 . 0 0 . 1 0 0 . 1 0 0 . 5 0 0 . 5 0 . 5 . 5 8 . 5 8 . 5 2 . 0 2 . 0 2 . 0 2 . 0 7 0 . 0 7 0 . 0 6 8 . 0 7 0 . 0 6 . 6 5 6 . 6 6 . 7 6 . 5 0 2 0 . 7 / 0 5 5 . 2 / 0 4 6 . 2 / 0 5 5 . 2 / 0 1 3 . 8 / 0 1 7 . 2 / 0 4 6 . 2 / 0 4 8 . 3 / 0 2 6 / 3 0 2 4 / 2 7 2 6 / 3 2 2 6 / 3 3 2 2 / 2 4 1 9 / 2 1 2 0 / 2 5 1 9 / 2 3 3 . 0 3 . 1 4 4 . 2 4 . 2 0 180 128 130 175 - 106 -e l e c t r i c i t y . T h u s t h e two r e a c t i o n s a b o v e g i v e a s t o i c h i o m e t r y t h a t c o u l d be i n t e r p r e t e d t o mean t h a t a s i m u l t a n e o u s s y n t h e s i s o f a l k a l i n e p e r o x i d e and c h l o r a t e i n t h e same c e l l w o u l d r e q u i r e a l k a l i n e a d d i t i o n f r o m an e x t e r n a l s o u r c e i n t o t h e a n o l y t e . In t h e p r e s e n t w o r k , s u c h was n o t t h e c a s e . The r e a s o n f o r t h i s c o u l d be ( a ) r e a c t i o n (1) i s c a r r i e d o u t i n a l k a l i n e m e d i a , h e n c e enough O H - i o n s w o u l d be a v a i l a b l e t o make up t h e 0H~ p r o d u c t i o n i n r e a c t i o n ( 1 ) ; ( b ) o t h e r s i d e r e a c t i o n s p r o d u c t i n g 0H~ i o n s o c c u r as w e l l as r e a c t i o n ( 1 ) . I n t h i s w o r k , HC1 a d d i t i o n i n t o t h e a n o l y t e was u s e d t o k e e p and c o n t r o l t h e a n o l y t e t a n k p H . The HC1 r e q u i r e m e n t v a r i e d f r o m r u n t o r u n . The r e a s o n s f o r t h i s v a r i a t i o n w e r e : ( i ) The l e n g t h o f o p e r a t i o n o f t h e c e l l f o r e a c h r u n was n o t o f t e n t h e same f o r a l l r u n s , ( i i ) In some r u n s , b e c a u s e o f t i m e l a g o f t h e c o n t r o l l e r , t h e r e q u i r e d pH i s o v e r s h o t and t h u s s m a l l amount o f NaOH had t o be a d d e d t o b r i n g t h e pH t o t h e r a n g e o f o p e r a t i o n . The o v e r s h o o t i n g o f pH c o u l d o c c u r i n e i t h e r s i d e o f t h e s e t p o i n t t h u s l e a d i n g t o a d d i t i o n o f e x t r a HC1 t h a t m i g h t n o t h a v e been n e c e s s a r y had t h e a d d i t i o n r e l i e d s o l e l y on t h e amount o f OH" f r o m t h e c a t h o d e ( f i n e c o n t r o l ) . Some t y p i c a l r e s u l t s a r e i l l u s t r a t e d i n T a b l e C i n A p p e n d i x 1 . The pH o v e r s h o o t i n g d i d n ' t o c c u r i n a l l t h e r u n s . The a d d i t i o n o f HC1 i n t o t h e a n o l y t e t a n k a c c o u n t s f o r some c h l o r a t e l o s s e s due t o t h e r e a c t i o n o f H + i o n s w i t h C I O - and C l o 3 _ . From t h e r e s u l t s , i t i s l i k e l y t h a t t h e amount o f HC1 r e q u i r e m e n t i n t h i s p r o c e s s w o u l d be l e s s t h a n t h a t r e q u i r e d t o p r o d u c e t h e same - 107 -amount o f c h l o r a t e i n an u n d i v i d e d c h l o r a t e c e l l . T h i s d e d u c t i o n c o u l d be made i n t u i t i v e l y b a s e d on t h e f a c t t h a t i n t h i s p r o c e s s , l i m i t e d t r a n s f e r o f O H - i n t o t h e a n o l y t e i s a l l o w e d w h i l e i n t h e t r a d i t i o n a l c h l o r a t e c e l l , t h e r e i s no c o n t r o l i n t h e - a m o u n t o f O H - m o v i n g i n t o t h e a n o d e s i d e o f t h e c e l l . The u s e o f HC1 t o c o n t r o l t h e a n o l y t e pH shows t h a t m a j o r c u r r e n t c a r r i e r i n t h e p r o c e s s i s O H - i o n as p r e d i c t e d by" t h e o r y . 5.7.1 Chloride Balance The membrane s e r v e s t o s e p a r a t e and t h e r e b y p r e v e n t t h e m i x i n g o f t h e a n o l y t e and c a t h o l y t e . I d e a l l y , o n l y n e g a t i v e i o n s m i g r a t e and d i f f u s e t h r o u g h t h e a n i o n e x c h a n g e membrane w h i l e t h e two p a r t s o f t h e c e l l a r e s e a l e d f r o m e a c h o t h e r . A l t h o u g h t h e a n i o n s i n t h e a n o l y t e a r e u n d e r t h e i n f l u e n c e o f n e g a t i v e e l e c t r i c p o t e n t i a l t o m i g r a t e i n t o t h e c a t h o l y t e c h a m b e r , t h e y a r e s t i l l f a v o u r e d u n d e r t h e i n f l u e n c e o f d i f f u s i o n p o t e n t i a l t o d i f f u s e i n t o t h e c a t h o l y t e . T h u s , t h e C I - , C 1 0 3 - , C I O - and CI 3 _ i o n s i n t h e a n o l y t e w o u l d t e n d t o d i f f u s e i n t o t h e c a t h o l y t e . To a s s e s s t h e l o s s o f e f f i c i e n c y due t o t h e d i f f u s i o n t e r m , a m a t e r i a l b a l a n c e was c a r r i e d o u t on t h e c h l o r i d e i n t h e s y s t e m . I n t h e m a t e r i a l b a l a n c e , no a t t e m p t was made i n a c c o u n t i n g f o r c h l o r i d e l o s s i n f o r m o f u n d i s s o l v e d c h l o r i n e g a s ( v e n t e d o u t ) and c h l o r a t e i n t h e c a t h o l y t e . A t y p i c a l c h l o r i d e b a l a n c e r e s u l t i s shown i n T a b l e 2 0 . The r e s u l t s i n T a b l e 20 i n d i c a t e t h a t n o t much o f t h e c h l o r i d e i s l o s t t o t h e c a t h o l y t e . U s i n g a p o r o u s d i a p h r a g m as a s e p a r a t o r i n t h i s s y s t e m , - 108 -T a b l e 20 A t y p i c a l c h l o r i d e b a l a n c e r e s u l t I n p u t O u t p u t M o l e s o f C I " i n N a C l 6 . 4 6 6 . 2 9 M o l e s o f C I " i n N a C 1 0 3 5 . 8 4 5 . 9 5 M o l e s o f C I " i n 0 C 1 " - 0 . 0 2 M o l e s o f C I " i n 10 c m 3 HC1 (HC1 c o n e = 1 . 0 M) 0 . 0 1 -M o l e s o f C I - i n c a t h o l y t e - 0 . 0 1 T o t a l m o l e s o f C l " 1 2 . 3 1 1 2 . 2 7 C o n d i t i o n s : (Run # D1R) D u r a t i o n o f r u n ( m i n s ) = 128 NaOH c o n e (M) = 2 . 0 Oxygen f l o w ( c m 3 s - 1 , STP) = 8 . 5 N a C l c o n e ( i n i t i a l / f i n a l ) (M) = 3 . 2 / 3 . 0 A n o l y t e f l o w ( c m 3 s " 1 ) = 2 . 0 C a t h o l y t e f l o w ( c m 3 s - 1 ) = 0 . 1 0 A n o l y t e t a n k temp ( ° C ) = 70 A n o l y t e t a n k pH = 6 . 6 0 V o l o f 1 M HC1 a d d e d ( c m 3 ) = 1 0 . 0 C a t h o l y t e C I " c o n e (M) = 9 x 1 0 " 3 C u r r e n t d e n s i t y k A / m 2 = 1 . 2 A n o l y t e s i d e c e l l temp ( ° C ) = 2 4 / 2 7 C a t h o l y t e s i d e c e l l t e m p ( ° C ) = 1 9 / 2 5 A n o l y t e t a n k v o l ( i n i t i a l / f i n a l ) (1 ) = 2 . 0 2 0 / 2 . 0 9 5 N a C 1 0 3 c o n e (M) ( i n i t i a l / f i n a l ) = 2 . 8 9 / 2 . 8 4 P e r o x i d e c o n e (M) = 0 . 3 7 8 C h l o r a t e c u r r e n t e f f i c i e n c y (%) = 7 7 . 3 P e r o x i d e c u r r e n t e f f i c i e n c y (%) = 7 3 . 0 S u p e r f i c i a l membrane a r e a ( c m 2 ) = 8 3 . 3 - 109 -more c h l o r i d e and c h l o r a t e w o u l d be e x p e c t e d t o be l o s t t o t h e c a t h o l y t e due t o c o n v e c t i o n t r a n s f e r b e t w e e n a n o d e and c a t h o d e c h a m b e r . Some u s e f u l d a t a a r e p r o v i d e d i n T a b l e C o f A p p e n d i x 2 f o r C I - m a t e r i a l b a l a n c e i n some o f t h e e x p e r i m e n t a l r u n s . 5.7.2 Water transport across the membrane I n a l m o s t a l l t h e e x p e r i m e n t a l r u n s , an i n c r e a s e i n v o l u m e o f a n o l y t e s o l u t i o n v o l u m e was n o t i c e d . T h i s i n c r e a s e i n a n o l y t e v o l u m e was a s a r e s u l t o f w a t e r t r a n s p o r t f r o m t h e c a t h o l y t e and t h e a d d i t i o n o f HC1 f o r pH c o n t r o l . The t r a n s p o r t o f w a t e r m o l e c u l e s a c r o s s t h e membrane i s a f u n c t i o n o f t h e c u r r e n t d e n s i t y , h y d r a t i o n number o f i o n s a n d membrane s t r u c t u r e . G e o r g e e t a l [ 5 8 ] o b s e r v e d t h a t w a t e r t r a n s p o r t a c r o s s an i o n e x c h a n g e membrane i s m a r k e d l y h i g h e r a t l o w c u r r e n t d e n s i t i e s t h a n a t h i g h e r c u r r e n t d e n s i t i e s . T h i s was e x p l a i n e d as due t o t h e f a c t t h a t a t h i g h c u r r e n t d e n s i t i e s , some i o n s a r e f o r c e d t h r o u g h t h e s m a l l e r p o r e s t h e r e b y p r e v e n t i n g any f r e e w a t e r m o l e c u l e f r o m p a s s i n g t h r o u g h s u c h p o r e s . O t h e r i o n s w i t h t h e i r a c c o m p a n y i n g H 2 0 p a s s t h r o u g h t h e l a r g e r p o r e s . U n d e r a l o w c u r r e n t d e n s i t y a l l o r m o s t w a t e r p a s s e s t h r o u g h t h e s m a l l e r p o r e s w i t h l i t t l e o r no i o n s b e i n g f o r c e d t h r o u g h t h e s e p o r e s . I n t h e p r e s e n t w o r k , no a t t e m p t was made a t r a t i o n a l i z i n g o r s t u d y i n g any r e l a t i o n s h i p b e t w e e n t h e amount o f w a t e r t r a n s p o r t e d a c r o s s t h e membrane and t h e e x p e r i m e n t a l c o n d i t i o n s . The o n l y o b s e r v a t i o n made i s t h a t , t h e i n c r e a s e i n s o l u t i o n v o l u m e i n t h e a n o l y t e l e a d s t o a l o w e r c h l o r a t e c o n c e n t r a t i o n s t h a n w o u l d h a v e o b t a i n e d had t h e a n o l y t e v o l u m e r e m a i n c o n s t a n t . - 110 -CHAPTER.6 CONCLUSIONS Alka l ine hydrogen peroxide and sodium chlorate were synthesized simultaneously in the same electrochemical c e l l , using a combination of an anion membrane and an asbestos diaphragm as the separator. The hydrogen peroxide was obtained by the electroreduction of oxygen in caustic on a fixed carbon bed cathode while the chlorate was obtained by the reaction of anodic electro-generated hypochlorite and hypochlorous acid in an external reactor. In the operation of a continous electrochemical reactor, the effects of current density, catholyte flow and sodium hydroxide concentration on the peroxide and chlorate current e f f i c i enc ie s were invest igated. The effects of these variables on peroxide concentration were also studied. The results show that increasing the super f i c ia l current density increases the peroxide concentration (at the expense of i t s current e f f i c iency ) and favours chlorate current e f f i c i ency . It was also found that the peroxy/hydroxy mole rat io in the catholyte affects the chlorate current e f f i c i ency—the higher th i s r a t i o , the lower the chlorate current e f f i c i ency . An increase in catholyte flow favours the current e f f i c i enc i e s of the two products of i n te re s t , but results in a lower peroxide concentration. The NaOH concentration was shown to affect the peroxide concentration pos i t ive ly ( i .e., increasing the NaOH concentration - 111 -r e s u l t s t o an i n c r e a s e i n p e r o x i d e c o n c e n t r a t i o n ) f o r NaOH c o n c e n t r a t i o n l o w e r t h a n 1 . 5 M . A c r i t i c a l NaOH c o n c e n t r a t i o n a t 1 . 5 M was d e f i n e d a s t h e o p t i m u m NaOH c o n c e n t r a t i o n a b o v e w h i c h o t h e r i n t e r a c t i n g f a c t o r s a p p e a r t o r e v e r s e t h e c a t h o l y t e r a t e e f f e c t s on a l k a l i n e h y d r o g e n p e r o x i d e p r o d u c t i o n . The s i m u l t a n e o u s s y n t h e s i s was c a r r i e d o u t a t b o t h a l o w o r weak c h l o r a t e s o l u t i o n s ( 0 - 0 . 5 M) and a t s t r o n g c h l o r a t e s o l u t i o n s ( 2 . 4 -2 . 8 M) and was f o u n d t h a t t h e two p r o d u c t s c o u l d s i m u l t a n e o u s l y be made u n d e r t h e s e c h l o r a t e c o n d i t i o n s . The c e l l was o p e r a t e d a t a v e r a g e t e m p e r a t u r e o f 2 7 / 3 3 ° C f o r t h e a n o d e s i d e and 2 0 / 3 0 ° C f o r t h e c a t h o d e s i d e w i t h o p e r a t i n g c e l l v o l t a g e s o f 3 - 4 . 2 V a t s u p e r f i c i a l c u r r e n t d e n s i t i e s o f 1 . 2 0 - 2 . 4 0 kA n r 2 . A c h l o r i d e b a l a n c e o v e r t h e s y s t e m i n d i c a t e d a n e g l i g i b l e l o s s o f c h l o r i d e t o t h e c a t h o l y t e and t h e a d d i t i o n o f HC1 make up t o c o n t r o l pH o f t h e a n o l y t e s i d e o f t h e s y s t e m r e f l e c t s t h e a b i l i t y o f t h e p r o c e s s t o t r a n s f e r O H - i o n s - a c r o s s t h e membrane a t a r a t e e q u i v a l e n t t o CI 2 g e n e r a t i o n . At weak c h l o r a t e s o l u t i o n s , t h e r a n g e o f c h l o r a t e c u r r e n t e f f i c i e n c i e s o b t a i n e d a r e 53 - 78% a t t h e c u r r e n t d e n s i t y r a n g e o f 1 . 2 -2 . 4 kA n r 2 , an a n o l y t e f l o w o f 2 x 1 0 ~ 6 m 3 s _ 1 , a n o l y t e t a n k t e m p e r a t u r e o f 7 0 ° C w i t h a c a t h o l y t e f l o w b e t w e e n 0 . 1 x 1 0 " 6 m 3 / s - 0 . 5 x 1 0 ~ 6 m 3 / s . The b e s t r e s u l t s f o r t h e p e r o x i d e c u r r e n t e f f i c i e n c y u n d e r t h e same c o n d i t i o n s w e r e o b t a i n e d a t 1 . 2 kA n r 2 and t h e s e a r e : 62% ( a t 0 . 5 M NaOH, 0 . 5 x 1 0 ~ 6 m 3 s - 1 c a t h o l y t e f l o w and 0 . 0 6 4 M p e r o x i d e ) , 67% ( a t 1 . 5 M NaOH, 0 . 5 x 1 0 " 6 m 3 s " 1 c a t h o l y t e f l o w and - 112 -0 . 0 6 9 M p e r o x i d e ) , and 86% ( a t 2 . 0 M NaOH, 0 . 1 x 1 0 " 6 m 3 S"1 c a t h o l y t e f l o w and 0 . 4 4 6 M p e r o x i d e ) . The b e s t r e s u l t s f o r t h e two p r o d u c t s i n c l u d e 5 6 . 5 % and 76% c u r r e n t e f f i c i e n c y f o r p e r o x i d e and c h l o r a t e r e s p e c t i v e l y a t 2 . 4 kA m - 2 , 1 . 0 M c a u s t i c , 0 . 5 x 1 0 - 6 m 3 s _ 1 c a t h o l y t e f l o w w i t h o t h e r o p e r a t i n g c o n d i t i o n s same ( t h e p e r o x i d e c o n c e n t r a t i o n = 0 . 1 1 7 M) and 7 5 . 4 % p e r o x i d e c u r r e n t e f f i c i e n c y ( 0 . 1 3 0 M H 0 2 ~ ) and 7 0 . 3 % c h l o r a t e c u r r e n t e f f i c e n c y a t 1 . 2 kA n r 2 , 0 . 5 x 1 0 ~ 6 m 3 s - 1 c a t h o l y t e f l o w and 2 . 0 M c a u s t i c . In s t r o n g s o l u t i o n o f c h l o r a t e , some i m p r o v e d p e r o x i d e e f f i c i e n c y and c h l o r a t e e f f i c i e n c y r e s u l t s a r e o b e r v e d . A t 1 . 0 M c a u s t i c , 2 . 4 kA m - 2 and c a t h o l y t e f l o w o f 0 . 5 x 1 0 - 6 m 3 s _ 1 , t h e c h l o r a t e c u r r e n t e f f i c i e n c y and p e r o x i d e e f f i c i e n c y a r e 8 0 . 6 % and 6 3 . 7 % r e s p e c t i v e l y ( 0 . 1 3 M p e r o x i d e ) . A p e r o x i d e c u r r e n t e f f i c i e n c y o f 7 8 . 2 % ( 0 . 8 1 M) a t 2M c a u s t i c and 0 . 1 x 1 0 - 6 m 3 s _ 1 c a t h o l y t e f l o w i s o b t a i n e d w i t h a 7 2 . 3 % c h l o r a t e e f f i c i e n c y . O t h e r f a c t o r s w h i c h c a n be e x p e c t e d t o i n f l u e n c e t h e p r o c e s s , b u t w h i c h w e r e n o t i n v e s t i g a t e d a r e : T e m p e r a t u r e o f t h e e l e c t r o l y t e s a t c e l l e n t r a n c e s , p r e s s u r e e f f e c t s , pH e f f e c t s , o x y g e n f l o w , d i r e c t i o n o f f l o w o f t h e two e l e c t r o l y t e s i n t o t h e c e l l , a n o l y t e f l o w e t c . T h i s work shows t h a t s o d i u m c h l o r a t e c o u l d be g e n e r a t e d s i m u l t a n e o u s l y w i t h a l k a l i n e h y d r o g e n p e r o x i d e i n t h e same c e l l a t r e a s o n a b l e c u r r e n t e f f i c i e n c i e s . - 113 -CHAPTER 7 RECOMMENDATIONS F u r t h e r work on t h i s p r o c e s s s h o u l d be d i r e c t e d t o w a r d s s t u d y i n g t h e e f f e c t s o f some o f t h e p r o c e s s v a r i a b l e s t h a t w e r e k e p t c o n s t a n t i n t h i s w o r k . S u c h f a c t o r s i n c l u d e t h e pH o f t h e a n o l y t e , c e l l p r e s s u r e , c e l l t e m p e r a t u r e , a n o l y t e r a t e , o x y g e n r a t e , t h e e l e c t r o l y t e f l o w d i r e c t i o n s e t c . A more d e t a i l e d s t u d y i s r e q u i r e d t o f i n d t h e e f f e c t o f t h e p e r o x i d e / a l k a l i m o l e r a t i o on t h e c h l o r a t e e f f i c i e n c y . Such s t u d i e s s h o u l d be c a r r i e d o u t a t b o t h l o w e r a n d h i g h e r r a t i o s t h a n a r e r e p o r t e d i n t h i s w o r k . I t i s a l s o s u g g e s t e d t o u s e a c t u a l ( f i n a l ) c a u s t i c c o n c e n t r a t i o n i n s t e a d o f t h e i n i t i a l o r f e e d c o n c e n t r a t i o n u s e d h e r e . W i t h s u c h d a t a , i t c o u l d be p o s s i b l e t o e x t r a p o l a t e t h e c h l o r a t e e f f i c i e n c y - - p e r o x y - h y d r o x y r a t i o c u r v e t o t h e a x i s a n d t h u s be a b l e t o make r e l i a b l e i n f e r e n c e s c o n c e r n i n g o p t i m u m p e r o x y - a l k a l i r a t i o and c h l o r a t e c u r r e n t e f f i c i e n c y . A l s o s u c h d a t a c o u l d be u s e d t o d e s i g n a c e l l f o r a s p e c i f i c a l k a l i - p e r o x i d e / c h l o r a t e r e q u i r e m e n t . The f o l l o w i n g a r e a l s o s u g g e s t e d f o r f u t u r e w o r k . 1 . D i f f e r e n t d i a p h r a g m and membrane m a t e r i a l s s h o u l d be t e s t e d . 2 . E x p e r i m e n t s a t h i g h e r r e a c t o r i n l e t t e m p e r a t u r e s s h o u l d be d o n e . 3 . A m a t h e m a t i c a l model o f t h e s y s t e m s h o u l d be d e v e l o p e d . S u c h a model w o u l d be a b l e t o e l u c i d a t e and a c c o u n t f o r t h e e f f e c t s o f many o f t h e p r o c e s s v a r i a b l e s . - 114 -The p r i n c i p l e o u t l i n e d h e r e ( c o u p l i n g o f t w o p r o c e s s e s ) s h o u l d be a p p l i e d t o o t h e r s y s t e m s . A s i m u l t a n e o u s o p e r a t i o n o f t h e c a t h o d e s i d e u n d e r a l o w c u r r e n t d e n s i t y and t h e a n o d e s i d e u n d e r a h i g h c u r r e n t d e n s i t y s h o u l d be d o n e . T h i s c o u l d be a c h i e v e d by u s i n g a p e r f o r a t e d a n o d e e l e c t r o d e . The p e r f o r a t i o n s l e a d t o a s m a l l e r a c t i v e s u r f a c e . a r e a t h a n w o u l d be o b t a i n a b l e u s i n g t h e w h o l e a n o d e p l a t e s u r f a c e a r e a . H i g h c u r r e n t d e n s i t y on t h e a n o d e f a v o u r s c h l o r a t e f o r m a t i o n w h i l e l o w c u r r e n t d e n s i t y on t h e c a t h o d e f a v o u r s p e r o x i d e e f f i c i e n c y . The r e c o m m e n d a t i o n o f ( 5 ) a b o v e c o u l d s a t i s f y t h e c u r r e n t d e n s i t y r e q u i r e m e n t s o f c h l o r a t e and p e r o x i d e a t r e a s o n a b l e c u r r e n t e f f i c i e n c i e s . A l o n g t e r m run t o e x a m i n e membrane s t a b i l i t y s h o u l d be d o n e . - 115 -REFERENCES 1 . S c h u m b , W . C . , S a t t e r f i e l d , C . N . and W e n t w o r t h , R . L . , A . C . S M o n o g r a p h N o . 128 R e i n h o l d , New Y o r k ( 1 9 5 5 ) . 2 . I b l , N. & V o g t , H . M . " C o m p r e h e n s i v e T r e a t i s e o f E l e c t r o c h e m i s t r y , " V o l . 2 , p . 1 6 7 , B o c k r i s , J . O ' M . , C o n w a y , E . B . , Y e a g e r , E . & W h i t e , R . E . , E d i t o r s , P l e n u m P r e s s , New Y o r k ( 1 9 8 1 ) . 3 . B e c k , T . R . , J . E l e c t r o c h e m S o c . 1 1 6 , 1 0 3 8 - 1 0 4 1 ( 1 9 6 9 ) . 4 . C a r m i c h a e l , D . L . & A l t h o u s e , E . B . , TAPPI J o u r n a l . 1 1 , 9 0 - 9 4 ( N o v . 1 9 8 1 ) . 5 . D e l a t t r e , M . G . A P P I T A 2 8 ( 2 ) 89 ( 1 9 7 4 ) . 6 . L a c h e n a l , D . , C h o u l d e n s , d e . C . & B o u r s o n , L . , T a p p i J o u r n a l 7 , 9 0 - 9 3 ( 1 9 8 6 ) . 7 . K r u g e r , H. & S u s s , H . U . , PPI 6 , 60 ( 1 9 8 2 ) . 8 . J a p a n C a r l i t , J a p a n e s e P a t e n t 6 1 2 8 4 5 9 1 ( 1 9 8 6 ) . 9 . A n t h o n y , P . 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A I C h E S y m p o s i u m S e r i e s , 2 0 4 , V o l . 77 ( 1 9 8 1 ) . - 119 -NOMENCLATURE c o n c e n t r a t i o n o f s p e c i e s i m o l / m o x y g e n b u l k c o n c e n t r a t i o n mol n r c u r r e n t e f f i c e n c y % d i f f u s i v i t y o f s p e c i e s i m 2 s - 1 c e l l v o l t a g e V s t a n d a r d e l e c t r o d e p o t e n t i a l V (SHE) e f f e c t shown by a dummy F a r a d a y ' s number = 9 6 , 5 0 0 c o u l (gm e q u i v ) c u r r e n t A m p e r e s l i m i t i n g c u r r e n t d e n s i t y A n r 2 B o l t z m a n c o n s t a n t r a t e c o n s t a n t ( m o l / m )~ s e c -mass t r a n s f e r c o e f f i c i e n t m s _ 1 number o f m o l e s o f d e s i r e d p r o d u c t number o f dummy v a r i a b l e s f l u x o f s p e c i e s i m o l e s ( m 2 s ) " 1 e n e r g y consumed p e r t o n k J / t o n e l e c t r o l y t e f l o w m 3 s _ 1 u n i v e r s a l gas c o n s t a n t k J / k g - m o l K r e s p o n s e o r r e s u l t t e s t s t a t i s t i c ( t - t e s t ) t e m p e r a t u r e K e l v i n m o b i l i t y o f s p e c i e s i m 2 mol J e l e c t r o l y t e f l o w v e l o c i t y m s _ 1 v a r i a n c e o f an e f f e c t v a l e n c e o f s p e c i e s i p o t e n t i a l g r a d i e n t V m " 1 h e a t o f r x n k J / k g - m o l c u r r e n t e f f i c i e n c y o f i % t i m e s e c -1 1 S" - 120 -APPENDIX 1 AUXILLIARY EQUIPMENT SPECIFICATIONS - 121 -AUXILIARY EQUIPMENT SPECIFICATIONS P o w e r S u p p l y - S o r e n s e n power s u p p l i e s Model DCR 4 0 - 2 5 B V o l m e t e r r a n g e 0 - 4 0 V Ammeter r a n g e 0 - 3 0 A ( s m a l l e s t d i v i s i o n = 1A) V o l t m e t e r - D a t a T e c h n o l o g y C o r p . D i g i t a l v o l t m e t e r N o . 345 0 - 1 0 v o l t pH M e t e r / C o n t r o l l e r - C o l e P a r m e r I n s t . C o . Model 5 9 8 6 - 0 0 C o m b i n e d E l e c t r o d e P r e s s u r e G a u g e / G a u g e G u a r d s - C h e m p l e x Guage G u a r d s # GG MV 160 - pp Range 0 - 160 p s i g V a l v e s - A n o l y t e s i d e -C a t h o l y t e s i d e CHEMPLEX SYGEF 1/4" PP - S w a g e ! o c k 1/4" SS - 122 -F i t t i n g s A n o l y t e s i d e - C o l e P a r m e r PP f i t t i n g s - F a b c o PVC f i t t i n g s C a t h o l y t e s i d e - S w a g e l o c k 316 SS - F a b c o PVC f i t t i n g s - C o l e P a r m e r PP R o t a m e t e r s A n o l y t e - C o l e P a r m e r , S h i e l d e d Type ( G i l m o t ) - T y p e : R - 3 2 3 2 - 2 2 S i z e 10 F l o a t : g l a s s Tube O . D . : 3 / 1 6 " ( 0 . 4 8 mm) C a t h o l y t e - B r o o k s , t u b e s i z e R - 2 - 1 5 - D F l o a t SS and g l a s s T h e r m o m e t e r s A n o l y t e s i d e - C o l e P a r m e r , T e f l o n c o a t e d R - 8 1 7 7 - 1 9 Range 0 - 1 0 0 ° C C a t h o l y t e s i d e - F i s h e r S c i e n t i f i c C o . USA D i a l t y p e SS Range 0 - 1 2 0 ° C - 123 -Pumps F e e d Pumps - C o l e P a r m e r , M a r c h M e t e r i n g Pumps Model 2 1 0 - 1 0 R C e r a m i c and T e f l o n m a t e r i a l D o s i n g Pumps - C h e m f e e d , s u p p l i e d by C o l e P a r m e r Model C - 1530SP Max P s i - 120 Max f e e d - 0 . 6 GPH T u b i n g FABCO 1 / 4 " P P - 4 3 - 0 5 0 0 C o o l e r o r H e a t E x c h a n g e r 5 f t o f 1/4" t i t a n i u m c o n t a i n e d i n 1 / 2 " c o p p e r t u b i n g W a t e r B a t h L a b l i n e I n s t r u m e n t s Model 18100 R e a g e n t s Oxygen - C a n a d i a n L i q u i d A i r L t d . , c o m m e r c i a l g r a d e 9 9 . 5 % 0 2 S o d i u m h y d r o x i d e , BDH - 50% a n a l y t i c a l g r a d e S o d i u m c h l o r i d e , BDH - a n a l y t i c a l g r a d e S o d i u m c h l o r a t e - E r c o C h e m i c a l C o . , V a n c o u v e r W a t e r - L a b o r a t o r y , s i n g l e d i s t i l l e d w a t e r - 124 -APPENDIX 2 TABULATED EXPERIMENTAL RESULTS ANALYTICAL TECHNIQUE AND S/MPLE CALCULATION Tali I e A Some T a b u l a t e d E x p e r i m e n t a l R e s u l t s Run 1 Time (mi n s ) NaOH c o n e (M) Oxygen F l o w CIlT/ s ( S T P ) NaCl cone I n i t i a l / F i n a l (M) E l e c t r o l y t e F l o w R a t e c m 3 / s A n o l y t e Tank Temp ( ° C ) A n o l y t e Tank p l l C a t h . ci-C o n c (M) C u r r e n t ( t e n s i t y ( k A / m 2 ) C e l l P r e s s ( k P a ) i n l e t / o u t C e l l Temp ( ° C ) I n l e t / O u t A n o l y t e V o l u m c ( l i t r e s ) I n i t i a l / F i n a l A n o l y t e S i d e C c l I Sanipl e pH P r o d u c t c o n e (M) C e l l V o l t ( V ) t C u r r e n t E f f i c i e n c y Anode S i d e C a t h S i d e x 1 0 3 Anode S i d e C a t h S i d e A n o d e S i d e C a t h S i d e C 1 0 3 - H 0 2 " C I O 3 - H 0 2 " c 180 . 0 . 5 8 . 5 4 . 3 / 4 . 0 5 2 . 0 0 . 3 0 70 6 . 5 3 . 0 1 . 2 0 / 0 0 / 0 2 5 / 2 6 2 0 / 2 5 2 . 0 / 2 . 1 4 7 . 2 0 . 2 0 9 / 0 . 2 4 5 0 . 0 8 7 3 . 0 5 7 . 4 5 0 . 4 180 0 . 5 8 . 5 3 . 6 2 5 / 3 . 5 7 5 2 . 0 0 . 3 0 70 6 . 6 3 . 0 2 . 4 1 3 . 8 / 0 . 0 0 / 0 2 6 / 3 3 2 1 / 3 3 2 . 0 / 2 . 2 7 . 3 0 . 1 1 / 0 . 2 2 3 0 . 0 8 7 4 . 2 7 2 . 1 25.4 c« 180 0 . 5 8 . 5 2 . 7 3 5 / 2 . 4 3 2 . 0 0 . 5 0 70 6 . 6 2 . 0 1 . 8 1 3 . 8 / 0 . 0 1 3 . 8 / 0 . 0 3 1 / 3 5 2 2 / 3 1 2 . 1 1 0 / 2 . 1 8 1 7 . 1 5 0 . 3 7 6 / 0 . 4 6 3 0 . 0 7 5 3 . 7 5 7 3 . 5 48. 3 C 1 0 180 1 . 0 8 . 5 3 . 5 / 3 . 3 2 . 0 0 . 1 0 70 6 . 7 4 2 . 0 1 . 2 4 4 . 8 / 0 . 0 0 / 0 2 7 / 3 1 1 8 / 3 6 1 . 9 0 / 1 . 9 5 5 7 . 1 0 . 4 2 3 / 0 . 4 7 5 0 . 2 2 5 3 . 1 6 7 . 52 C 1 2 180 1 . 0 8 . 5 3 . 3 / - 2 . 0 0 . 1 0 71 6 . 6 2 . 0 2 . 4 5 5 . 2 / 0 . 0 0 / 0 27/37 1 8 / 3 0 1 . 9 0 / 2 . 0 8 5 7 . 2 0 . 0 7 5 / 0 . 5 7 9 0 . 2 7 5 4 . 1 69 28.4 180 1 . 0 8 . 5 4 . 6 / 4 . 3 2 . 0 0 . 1 71 6 . 8 3 . 0 1 . 8 5 5 . 2 / 0 . 0 0 / 0 3 0 / 3 5 2 3 / 3 3 2 . 0 / 2 0 8 5 7 . 2 0 . 0 5 8 / 0 . 1 4 6 0 . 1 1 5 3 . 4 7 6 7 . 3 44-. 4-125 1 . 0 8 . 5 4 . 3 / 4 . 0 2 . 0 0 . 5 73 6 . 6 2 . 0 2 . 4 1 4 4 . 8 / 0 . 0 4 1 . 4 / 0 . 0 27/34 1 9 / 3 0 2 . 0 / 2 . 0 2 6 7 . 0 0 . 0 5 5 / 0 . 1 5 5 0 . 1 1 7 4.4 7 6 . 0 5 6 . 5 C 2 2 120 2 . 0 8 . 5 - 2 . 0 0 . 1 0 70 6 . 7 7 . 0 1 . 2 4 3 . 4 / 0 . 0 1 3 . 8 / 0 . 0 2 8 / 3 0 2 0 / 2 3 2 . 0 / 2 . 0 6 7 . 1 0 . 0 / 0 . 0 3 2 0 . 4 4 6 3 . 0 5 3 . 2 86. 1 T a b l e A c o n t ' d Run 1 Time (m1ns) NaOH c o n e (H) Oxygen Flow cm /s (STP) NaCl cone I n i t i a l / F i n a l E l e c t r o l y t e F l o w Rate c m 3 / s A n o l y t e Tank Temp CO A n o l y t e Tank PH C a t h . ci-Conc ( « ) C u r r e n t D e n s i t y ( k A / m 2 ) C e l l P r e s s ( k P a ) 1 n l e t / o u t C e l l Temp CC) I n l e t / O u t A n o l y t e Volume ( l i t r e s ) I n i t i a l / F i n a l A n o l y t e S i d e C e l l S a m p l e pH P r o d u c t cone (M) C e l l V o l t (V) i C u r r e n t E f f i c i e n c y Anode S i d e C a t h S i d e x 1 0 3 Anode S i d e C a t h S i d e Anode S i d e C a t h S i d e C 1 0 3 - H 0 2 - C I O 3 - H 0 2 -C 2 3 1ZU 2 . 0 8 . 5 - 2 . 0 0 . 1 0 7 1 . 0 6 . 7 3 . 0 1 . 8 5 5 . 2 / 0 0 / 0 2 7 / 3 3 2 0 / 2 3 2 . 0 / 2 . 0 6 5 7 .1 0 . 0 3 2 / 0 . 0 9 9 0 . 4 4 9 3 . 4 5 7 5 . 0 5 8 . 0 C2i, 140 2 . 0 8 . 5 3 . 9 / 3 . 6 2 . 0 0 . 1 0 74 6 . 6 3 . 0 2 . 4 8 6 . 2 / 0 1 3 . 8 / 0 . 0 2 8 / 3 6 2 0 / 2 5 2 / 2 . 0 3 7 . 2 0 . 0 9 9 / 0 . 1 8 7 0 . 5 3 3 . 9 6 3 . 0 53 c 2 5 120 2 . 0 8 . 5 3 . 5 / 3 . 0 2 . 0 0 . 3 0 70 6 . 8 5 . 0 2 . 4 3 4 . 5 / 0 4 1 . 4 / 0 . 0 25/31 2 0 / 2 3 2 . 0 / 2 . 0 2 7 . 0 0 . 1 5 8 / 0 . 2 3 0 0 . 1 0 7 4 . 0 67 3 1 . 0 120 2 . 0 8 . 5 3 . 0 / 2 . 8 2 . U 0 . 3 70 6 . 7 5 - 1 . 8 6 9 . 0 / 0 4 1 . 4 / 0 . 0 2 8 / 3 3 2 0 / 2 3 2 . 0 / 2 . 0 2 5 7 .1 0 . 2 3 5 / 0 . 3 4 3 0 . 1 4 0 3 . 5 .70 54 c 2 7 123 2 . 0 8 . 5 3 . 1 / 2 . 8 5 2 . 0 0 . 3 70 6 . 6 7 . 0 1 . 2 4 3 . 4 / 0 4 3 . 4 / 0 . 0 2 6 / 2 9 18/21 3 . 1 / 2 . 8 5 7 . 0 0 . 2 8 9 / 0 . 3 2 7 0 . 1 3 0 2 . 9 7 60 7 5 . 0 c 1 9 120 2 . 0 8 . 5 4 . 5 / 4 . 0 2 . 0 0 . 5 72 6 . 6 6 . 0 1 . 2 5 5 . 2 / 0 4 1 . 4 / 0 . 0 29/31 19/26 2 / 2 . 0 9 6 6 . 9 0 / 0 . 0 4 2 0 . 0 6 8 3 . 1 7 0 . 3 6 6 . 0 C l 7 120 2 . 0 8 . 5 3 . 8 / 3 . 3 2 . 0 0 . 5 70 6 . 6 6 . 0 1 . 8 4 1 . 4 / 0 2 4 . 1 / 0 . 0 2 9 / 3 3 2 0 / 2 9 1 . 8 4 / 1 . 9 2 5 7.1 0 . 1 3 3 / 0 . 2 0 4 0 . 0 7 5 3 . 8 7 9 . 0 5 1 . 0 C 18 125 1 . 0 8 . 5 4 . 3 / 4 . 0 2 . 0 0 . 5 73 6 . 6 2 . 0 2 . 4 1 4 4 . 8 / 0 . 0 4 1 . 4 / 0 . 0 27/34 19/30 2 . 0 / 2 . 0 2 5 7 . 0 0 . 0 5 5 / 0 . 1 5 5 0.117 4 . 4 7 8 . 7 5 7 . 0 T a b l e B Some r e p e a t e d r u n s Run t Time ( m 1 n s ) NaOH c o n e (M) O x y g e n F l o w c m 3 / s ( S T P ) NaCl c o n e I n i t i a l / F i n a l (M) E l e c t r o l y t e F l o w R a t e c m 3 / s A n o l y t e Tank Temp (•c) A n o l y t e Tank PH C a t h . ci-C o n c (M) C u r r e n t D e n s i t y ( k A / m 2 ) C e l l P r e s s ( k P a ) I n l e t / o u t C e l l Temp C O I n l e t / O u t A n o l y t e V o l u m e ( l i t r e s ) I n i t i a l / F i n a l A n o l y t e S i d e C e l l S a m p l e pH P r o d u c t c o n e (M) C e l l V o l t ( V ) C u r r e n t E f f i c i e n c y A n o d e S i d e C a t h S i d e x 1 0 3 Anode S i d e C a t h S i d e A n o d e S i d e C a t h S i d e C I O 3 - H 0 2 - C I O 3 - H 0 2 " 180 0.5 8 . 5 4 . 5 / 4 . 3 2 . 0 0 0 . 3 0 80 6 . 5 3 . U 1 . 2 1 2 4 / 0 1 7 . 2 / 0 . 0 2 9 / 3 1 2 3 / 2 9 2 . 0 / 2 . 1 0 7 . 0 0 . 1 9 1 / 0 . 2 1 4 0 . 0 8 3 3 . 0 3 6 . 2 5 1 . 3 180 0.5 8.5 4 . 3 / 4 . 0 5 2 . 0 0 . 3 0 70 6 . 6 3 . 0 1 . 2 0 / 0 0 / 0 2 5 / 2 6 2 0 / 2 5 2 . 0 / 2 . 1 4 7 . 2 0 . 2 0 9 / 0 . 2 4 5 0 . 0 8 7 3 . 0 5 7 . 4 5 0 . 4 C 1 0 180 ' 1.0 8 . 6 4 . 5 2 / 4 . 2 2 . 0 0 . 1 0 70 6 . 5 2 . 5 1 . 2 1 3 . 8 / 0 . 0 0 / 0 2 1 / 2 4 2 1 / 2 4 2 . 0 / 2 . 1 2 601 0 . 3 7 6 / 0 . 4 0 1 0 . 2 2 5 3 . 0 5 3 . 0 52.0 C 1 0 R 180 1.0 8 . S 3 . 5 / 3 . 3 2 . 0 0 . 1 0 70 6 . 7 4 2 . 0 1 . 2 4 4 . 8 / 0 . 0 0 / 0 2 7 / 3 1 1 8 / 3 6 1 . 9 0 / 1 . 9 5 5 7 . 1 0 . 4 7 5 / 0 . 5 7 9 0 . 2 7 5 3 . 1 6 7 . 0 52.0 C l 7 120 1.0 8.5 4 . 7 / 4 . 3 2 . 0 0 . 5 0 70 6 . 8 3 . 0 1 . 8 1 2 4 / 0 5 5 . 2 / 0 . 0 2 7 / 3 1 1 9 / 2 7 2 . 0 / 2 . 0 7 5 7 . 2 0/ 0 . 0 5 5 0 . 0 9 7 3 . 8 6 1 . 0 6 1 . 0 C n R 120 1.0 8 . 5 3 . 8 / 3 . 3 2 . 0 0 . 5 70 6 . 6 6 . 0 1 . 8 4 1 . 4 / 0 . 0 2 4 . 0 / 0 . 0 2 9 / 3 3 2 0 / 2 9 1 . 8 4 / 1 . 9 2 5 7 . 1 0 . 1 3 2 / 0 . 2 0 4 0 . 0 7 5 3 . 8 7 9 . 0 6 1 . 0 C 2 6 120 2.0 8 . 5 3.2/3.1 2 . 0 0 . 3 70 6 . 6 4 . 0 1 . 8 4 1 . 4 / 0 . 0 4 1 . 4 / 0 . 0 2 6 / 3 1 1 8 / 2 1 2 . 0 / 2 . 0 7 . 0 0 . 2 4 2 / 0 . 2 8 9 0 . 1 4 5 3 . 5 5 1 . 0 42.0 C 2 6 R 170 2.0 8 . 5 3 . 0 / 2 . 8 2 . 0 0 . 3 70 6 . 7 5 - 1 . 8 6 9 . 0 0 . 0 4 1 . 4 / 0 . 0 2 8 / 3 3 2 0 / 2 3 2 . 0 / 2 . 0 2 5 7 . 1 0 . 2 3 5 / 0 . 3 2 3 0 . 1 4 0 3 . 5 7 0 . 0 64 T a b l e B C o n t ' d Run 1 Time (m1ns) NaOH cone ( M ) Oxygen Flow enr/s (STP) NaCl cone Init ial / Final ( M ) Electrolyte Flow Rate cmJ/s Anolyte Tank Temp C O Anolyte Tank pH Cath. ci-Conc ( H ) Current Density (kA/m2) Cell Press (kPa) 1nlet/out Cell Temp CC) Inlet/Out Anolyte Volume ( l i tres) Ini t ia l / Final Anolyte Side Cell Sample pH Product cone (M) Cell Volt (V) Current Efficiency Anode Side Cath Side x 103 Anode Side Cath Side Anode Side Cath Side C103- H0 2" cio 3 - H0 2" Oi 180 2.0 8.5 3.0/2.7 2.0 0.10 70 6.65 3.0 1.2 20.7/ 0.0 13.8/ 0.0 26/30 22/24 2.02/ 2.125 7.10 2.86/ 2.78 0.404 3.0 73.0 78 DjR 128 2.0 8.S 3.23.0 2.0 0.10 70 6.6 9.0 1.2 55.2/ 0.0 17.2/ 0.0 24/27 19/25 2.020/ 2.095 u - 2.89/ 2.84 0.378 3.14 7.3 73 D3 120 2.0 8.5 2.9/2.5 2.0 0.50 . 70 6.7 - 2.4 20.7/ 0.0 48.3/ 0.0 26/32 21/25 2.0/ 2.148 7.3 2.714/ 2.588 0.166 4.0 53.0 80 O3R1 180 2.0 8.5 3.0/2.7 2.0 0.50 70 6.5 3.0 2.4 58.6/ 0.0 41.4/ 0.0 23/30 18/23 2.0/ 2.030 - 2.836/ 2.887 0.127 4.0 51.0 61.4 Os 130 1.0 8.5 2.9/2.4 2.0 0.50 68 6.7 1.0 2.4 46.2/ 0.0 46.2/ 0.0 26/32 20/25 1.50/ 1.70 7.4 2.66/ 2.669 0.146 4.2 60 70 D 5 R 175 1.0 8.5 2.9/2.4 2.0 0.50 68 6.7 1.0 2.4 55.2/ 0.0 48.3/ 0.0 26/33 19/23 2.025/ 2.20 - 2.882/ 2.772 0.146 4.2 73 70 A p p e n d i x 2 Some u s e f u l d a t a f o r C I " m a t e r i a l b a l a n c e Run # A n o l y t e V o l (L) I n i t i a l / F i n a l NaCl Cone I n i t i a l / F i n a l (M) N a C 1 0 3 C o n e . I n i t i a l / F i n a l (M) 0 C T C o n c . I n i t i a l / F i n a l 10 3 (M) Cone o f HCl Used (M) V o l o f HCl A d d e d ( c m 3 ) C a t h o l y t e F l o w ( c m 3 / s ) 1 0 3 x C l -C o n c . i n C a t h o l y t e (M) D u r a t i o n o f Run ( m i n ) DiR 2 . 0 2 / 2 . 0 9 5 3 . 2 / 3 . 0 2 . 8 9 / 2 . 8 4 0 . 0 / 8 . 7 5 1 . 0 1 0 . 0 0 . 1 0 9 . 0 128 D3R1 2 . 0 / 2 . 0 3 0 3 . 0 / 2 . 7 2 . 8 4 / 2 . 8 9 8 . 7 5 / 9 . 5 8 1 . 0 0 . 0 0 . 5 0 3 . 0 180 D5R 2 . 0 2 5 / 2 . 2 0 3 . 2 / 2 . 8 2 . 8 8 / 2 . 7 7 0 . 0 / 1 0 . 4 2 1 . 0 3 0 . 0 0 . 5 0 3 . 0 175 D 8 1 . 5 0 / 1 . 6 0 2 . 8 / 2 . 6 2 . 7 0 / 2 . 5 7 8 5 0 . 0 / 8 . 7 5 1 . 0 2 0 . 0 0 . 1 0 1 0 . 5 0 120 D 4 2 . 0 / 2 . 1 3 6 2 . 8 / 2 . 6 2 . 7 2 3 / 2 . 6 1 0 0 . 0 / 9 . 5 0 2 . 1 2 8 0 . 0 0 . 5 0 1 . 5 0 155 - 130 -S/WPLING AND ANALYSIS On l e a v i n g t h e r e a c t o r , t h e c a t h o l y t e p r o d u c t s o l u t i o n was s a m p l e d . A ml o r more o f t h e s a m p l e was p i p e t t e d f r o m t h e m e a s u r i n g c y l i n d e r i n t o an e x c e s s o f 2 M s u l p h u r i c a c i d . The a c i d s o l u t i o n o f p e r o x i d e was t h e n t i t r a t e d w i t h 0 . 1 N ( 0 . 0 2 M) s o l u t i o n o f p o t a s s i u m p e r m a n g a n a t e , w h i c h had b e e n s t a n d a r d i z e d a g a i n s t a s o l u t i o n o f s o d i u m o x a l a t e . The e q u a t i o n f o r t h e r e a c t i o n b e t w e e n p e r o x i d e and p e r m a n g a n a t e u n d e r a c i d c o n d i t i o n i s : 2 MnO^" + 6 H + + 5 H 2 0 2 + 2 M n + + + 8 H 2 0 + 5 0 2 T h u s p e r o x i d e c o n c e n t r a t i o n i n t h e p r o d u c t s o l u t i o n i s g i v e n by P p r n v i d P r n n r - ( V o ^ e o f KMnO., u s e d ) x ( M o l a r i t y o f K M n O j P e r o x i d e c o n e ( V o l u m e o f S a m p l e ) x 2 F o r t h e a n o l y t e , j u s t b e f o r e t h e power i s s w i t c h e d o f f , a s a m p l e o f 2 ml ( f o r l o w c h l o r a t e r u n s ) o r two s a m p l e s o f 1 ml a n d 2 ml ( f o r h i g h c h l o r a t e r u n s ) a r e t a k e n f r o m t h e a n o l y t e t a n k . [ F o r c e l l s a m p l e a n a l y s i s , a s a m p l e i s t a k e n j u s t a f t e r e x i t i n g f r o m t h e c e l l . ] In t h e h i g h c h l o r a t e r u n , t h e 1 ml s a m p l e i s d i l u t e d t o 100 ml and 5 ml o f t h i s d i l u t e s o l u t i o n t a k e n f o r c h l o r a t e a n a l y s i s . The 2 ml s a m p l e o f t h e h i g h c h l o r a t e r u n i s u s e d f o r h y p o c h l o r i t e ( a c t i v e c h l o r i n e ) a n a l y s i s . F o r t h e l o w c h l o r a t e r u n , t h e 2 ml s a m p l e i s u s e d f o r b o t h c h l o r a t e and h y p o c h l o r i t e a n a l y s i s . The f o l l o w i n g p r o c e d u r e was u s e d f o r t h e h y p o c h l o r i t e a n d c h l o r a t e a n a l y s i s . A f t e r t h e a d d i t i o n o f 0 . 1 M ( 2 0 m l ) s o d i u m h y d r o g e n c a r b o n a t e - 131 -s o l u t i o n t o t h e s a m p l e , h y p o c h l o r i t e was t i t r a t e d p o t e n t i o m e t r i c a l l y w i t h a s o l u t i o n o f 0 . 0 2 5 M ( 0 . 1 N) A S 2 O 3 . A 250 ml b e a k e r c o n t a i n i n g p l a t i n u m a n d c a l o m e l e l e c t r o d e s was u s e d . The p l a t i n u m e l e c t r o d e was i m m e r s e d b r i e f l y i n c o n e , s u l f u r i c a c i d c o n t a i n i n g c h r o m a t e b e f o r e e a c h r u n . When a l a r g e p o t e n t i a l c h a n g e o c c u r s a t t h e end p o i n t o f t h e h y p o c h l o r i t e , an e x c e s s a r s e n i c ( I I I ) o x i d e was a d d e d f o l l o w e d by a b o u t 40 mg p o t a s s i u m b r o m i d e and a v o l u m e o f c h e m i c a l l y p u r e c o n c e n t r a t e d h y d r o c h l o r i c a c i d t o a c h i e v e a t l e a s t 20% HC1 i n t h e s o l u t i o n . The s o l u t i o n was a l l o w e d t o s t a n d f o r 5 m i n u t e s . A f t e r t h a t t i m e , t h e e x e s s A S 2 0 3 was t i t r a t e d p o t e n t i o m e t r i c a l l y w i t h 0 . 0 1 6 7 M ( 0 . 1 N) K B r 0 3 . The c h a n g e i n p o t e n t i a l was f o l l o w e d . A t y p i c a l p o t e n t i o m e t r i c c u r v e f o r l o w c h l o r a t e r u n i s i l l u s t r a t e d i n F i g u r e A l . F o r t h e c h l o r i d e a n a l y s i s , 1 ml s a m p l e i s d i l u t e d t o 100 ml and 5 ml s a m p l e o f d i l u t e s o l u t i o n t i t r a t e d a g a i n s t s i l v e r n i t r a t e u s i n g d i c h r o m a t e as i n d i c a t o r . F o r t h e c a t h o l y t e c h l o r i d e a n a l y s i s , t h e s a m p l e i s f i r s t n e u t r a l i z e d u s i n g n i t r i c a c i d b e f o r e t i t r a t i n g w i t h s i l v e r n i t r a t e . The e q u a t i o n s o f t h e r e a c t i o n s a r e as f o l l o w s : ( a ) R e a c t i o n o f h y p o c h l o r i t e ( C 1 0 - ) w i t h A s 2 0 3 2 C 1 0 - + A s 2 0 3 + A s 2 0 5 + 2 C 1 " ( b ) R e a c t i o n o f c h l o r a t e w i t h A r s e n i c ( I I I ) o x i d e i n a c i d e n v i r o n m e n t 2 C 1 0 3 - + 3 H 3 A s 0 3 + 6H+ + 3H3AS0, , + 2 C 1 " + 3 H 2 0 ( c ) R e a c t i o n o f A s 2 0 3 w i t h b r o m a t e i n a c i d s o l u t i o n 2 B r 0 3 " + 3 A s 3 + + 12H+ + 2 B r ~ + 3 A s 5 + + 6 H 2 0 A t t h e end p o i n t o f K B r 0 3 t i t r a t i o n , b r o m i n e i s g e n e r a t e d as f o l l o w s : B r 0 3 " + 5 B r " + 6H+ 3 B r 2 + 3 H 2 0 - 132 -Typical potentiometric titration curve 800 _ 6001-> * c a> r2 400 200 0 T o t a l A S 2O3 a d d e d - 3 5 . 0 V e q = 1 : 9 0 2 - O o " l i t r e 600 > « 550 •+—» O Q_ 500 450 11.0 K B r o 3 V e q = 1 1 . 5 7 6 • O o 3 t i t r e 12.0 13. 0.025 (M) AS2O3 Vol (cm 3) 0.0167 (M) KBro 3 Vol (cm: F i g u r e A l - 133 -( d ) R e a c t i o n o f s i l v e r n i t r a t e and c h l o r i d e C I " +• A g N 0 3 - A g C l ( s ) + N 0 3 - ( a q ) C a l c u l a t i o n o f C o n c e n t r a t i o n o f C10~ and C 1 0 3 ~ L e t t h e f i r s t e n d - p o i n t ( h y p o c h l o r i t e end p o i n t ) = V/\x ( c m 3 ) . T h i s i s t h e v o l u m e o f A s 2 0 3 u s e d i n t i t r a t i n g t h e amount o f C 1 0 -c o n t a i n e d i n t h e s a m p l e . L e t t h e s a m p l e v o l u m e be = V s (cm ) . L e t t h e t o t a l v o l u m e o f AS2O3 u s e d ( i n c l u d i n g e x c e s s ) be = V J A ( c m 3 ) . L e t t h e e n d - p o i n t i n B r o m a t e t i t r a t i o n = Vg ( c m 3 ) . L e t t h e m o l a r i t y o f A r s e n i c be = M/\ ( m o l e s / l i t r e ) . L e t t h e m o l a r i t y o f B r o m a t e be = MB ( m o l e s / l i t r e ) . M o l a r i t y o f CIO" = M C 1 Q _ ( m o l e s / l i t r e ) . M o l a r i t y o f C 1 0 3 " = MQ-JQ 3 - ( m o l e s / l i t r e ) . Then f o r ( i ) C I O - c o n c e n t r a t i o n 2 X M A X V A 1 Mr 'CIO- 3 x V s ( i i ) C 1 0 3 " c o n c e n t r a t i o n ( a ) V o l u m e o f e x c e s s AS2O3 u s e d f o r B r o m a t e t i t r a t i n g , V/\£ 3 x M Q x V D V = ° 2. AE 2 x M A - 134 -( b ) V o l u m e o f A s 2 0 3 u s e d f o r C I 0 3 ~ r e a c t i o n i s g i v e n by V C 1 0 3 - = V T A - V A 1 " V A E T h e r e f o r e M o l a r i t y o f C I 0 3 - i n s a m p l e M C I O 3 -2 X M A X V C 1 0 , -3 x V . S u b s t i t u t i n g f o r V c i O q - w e o b t a i n M CIO, 2 x M 3 T 1 T L V T A - vA1 3 X M B X V V -2 x M n -Sample Calculation Run D5R ( A p p e n d i x 2 , T a b l e B) D u r a t i o n o f Run C a t h o l y t e f l o w r a t e C u r r e n t I n i t i a l a n o l y t e t a n k v o l u m e F i n a l a n o l y t e t a n k v o l u m e T o t a l v o l u m e o f HC1 a d d e d Cone o f HC1 a d d e d = 1 7 5 m i n = 0 . 5 0 c m 3 / s = 20A = 2 . 0 2 5 l i t r e s = 2 . 2 0 l i t r e s = 30 c m 3 = 1 . 0 M I . C a t h o l y t e S a m p l e A n a l y s i s C a t h o l y t e s a m p l e v o l u m e = 5 . 0 ml P e r m a n g a n a t e c o n e . = 0 . 0 2 M P e r m a n g a n a t e t i t r e = 1 4 . 6 ml 1 & fin R P r o d u c t p e r o x i d e c o n e = g * Q * n « 0 2 x j = 0 . 1 4 6 M - 135 -P e r o x i d e c u r r e n t e f f i c i e n c y = ° ' 1 4 6 x ° - 5 x ^ x 2 x 9 6 5 0 0 x 100 = 7 0 . 5 % d U I n i t i a l C 1 0 y » c 1 ~ and C1Q-No C I O - p r e s e n t i n t h e s a m p l e , i . e . , V/\i = 0 . 1 ml i n i t i a l s a m p l e was d i l u t e d t o 100 ml and 5 ml o f t h i s d i l u t e s a m p l e u s e d f o r p o t e n t i o m e t r i c t i t r a t i o n f o r C 1 0 3 _ a n d 10 ml d i l u t e s o l u t i o n u s e d f o r C I " t i t r a t i o n . From t h e p o t e n t i o m e t r i c t i t r a t i o n c u r v e we o b t a i n e d t h e end p o i n t s : F o r i n i t i a l C 1 0 3 ~ T o t a l A s 2 0 3 a d d e d = 1 0 . 0 ml ( V J A ) E q u i v a l e n t v o l u m e o f K B r 0 3 = 1 . 3 9 5 ml ( V g ) A s 2 0 3 c o n c e n t r a t i o n = 0 . 0 2 5 M ( M A ) K B r 0 3 c o n c e n t r a t i o n = 0 . 0 1 6 7 M ( M B ) C 1 0 3 - c o n e - 2 x O f . 5 [ 1 0 . 0 - 0 - 3 X ° ' x 1 o ! o 2 5 1 ' 3 9 5 3 = 2 . 8 7 x 1 0 - 2 M T h i s i s t h e d i l u t e s o l u t i o n c o n c e n t r a t i o n T h e r e f o r e a c t u a l a n o l y t e t a n k C I 0 3 - cone' = 2 . 8 7 x 1 0 - 2 x 100 = 2 . 8 7 M I n i t i a l C I - c o n e S a m p l e v o l A g N 0 3 c o n e = 10 ml d i l s o l u t i o n = 0 . 1 M - 136 -AgNO3 t i t r e = 3 . 2 ml C I " c o n e i n d i l s o l u t i o n = ^ l O . U * 1 = 3 . 2 x 1 0 - 2 M T h e r e f o r e i n i t i a l a n o l y t e C I " c o n e = 3 . 2 x 1 0 ~ 2 x 100 = 3 . 2 M I I I . F i n a l A n o l y t e Tank C1Q- Cone S a m p l e v o l u m e = 2 . 0 ml E q u i v a l e n t AS2O3 v o l u m e = 1 . 3 1 ml ( f r o m t h e p o t e n t i o m e t r i c t i t r a t i o n c u r v e ) A s 2 0 3 c o n e = 0 . 0 2 5 M H y p o c h l o r i t e c o n e = 2 x " j 0 ^ ^ 1 ' 3 l = 1 0 . 9 2 x 1 0 - 3 M I V . F i n a l C 1 0 3 ~ and C l ~ Cone 1 ml f i n a l a n o l y t e s a m p l e was d i l u t e d t o 100 m l , 5 ml and 10 ml s a m p l e s w e r e t a k e n f o r C 1 0 3 " and C I " a n a l y s i s r e s p e c t i v e l y . From t h e p o t e n t i o m e t r i c t i t r a t i o n c u r v e , t h e e q u i v a l e n t p o i n t s o r e n d p o i n t s were o b t a i n e d . F o r f i n a l C I O 3 - : T o t a l A s 2 0 3 a d d e d = 1 0 . 0 ml E q u i v a l e n t v o l u m e o f K B r 0 3 = 1 . 6 8 ml A s 2 0 3 c o n e = 0 . 0 2 5 M K B r 0 3 c o n e = 0 . 0 1 6 7 M - 137 -T h e amount o f C I O - i n 5 ml d i l s o l u t i o n i s assumed n e g l i g i b l e n n - , n n r - - 2 x ° - 0 2 5 n n n n 3 x 0 . 0 1 6 7 x 1 . 6 8 - , C 1 0 3 c o n e - 3 x 5 > Q [ 1 0 . 0 - 0 2 x 0 . 0 2 5 3 = 2 . 7 7 x 1 0 - 2 M ( d i l s o l u t i o n ) T h e r e f o r e a c t u a l f i n a l a n o l y t e C 1 0 3 _ c o n e = 2 . 7 7 x 1 0 ~ 2 x 100 = 2 . 7 7 M F i n a l A n o l y t e C h l o r i d e C o n e . S a m p l e v o l = 10 ml d i l s o l u t i o n A g N 0 3 c o n e = 0 . 1 0 M A g N 0 3 t i t r e = 2 . 8 0 ml 2 . 8 0 x 0 . 1 0 C I " c o n e i n d i l s o l u t i o n = 10 2 . 8 x l O " 2 M T h e r e f o r e f i n a l a n o l y t e C T c o n e = 2 . 8 x 1 0 - 2 x 100 = 2 . 8 0 M V . C a t h o l y t e C h l o r i d e Cone C a t h o l y t e s a m p l e v o l u m e = 1 0 . 0 ml A g N 0 3 c o n e = 0 . 1 0 M A g N 0 3 t i t r e = 0 . 3 0 ml 0 . 3 x 0 . 1 C I " c o n e i n c a t h o l y t e 10 = 3 x l O " 3 M - 138 -C h l o r a t e c u r r e n t e f f i c i e n c y I n i t i a l t o t a l m o l e s o f c h l o r a t e i n a n o l y t e t a n k = 2 . 0 2 5 x 2 . 8 7 = 5 . 8 1 m o l e s F i n a l t o t a l m o l e s o f C 1 0 3 - i n a n o l y t e t a n k = 2 . 2 0 x 2 . 7 7 ( l i t r e s ) xlSftfSi = 6 . 0 9 m o l e s T h e r e f o r e t o t a l m o l e s o f c h l o r a t e p r o d u c e d d u r i n g t h e d u r a t i o n o f r u n = 6 . 0 9 - 5 . 8 1 = 0 . 2 8 m o l e s T h e o r e t i c a l m o l e s o f C 1 0 3 ~ p r o d u c e d i n 175 m i n a t 2 0 A 1 7 5 ( m i n ) x 60(4^) x 2 0 ( | ) 6 x 9 6 5 0 0 ( r ^ e l f ) " = 0 . 3 6 m o l e s 0 28 T h e r e f o r e c h l o r a t e c u r r e n t e f f i c i e n c y = Q ' 3 6 x 100% = 7 7 . 8 % C I " B a l a n c e ( i ) I n p u t C h l o r i d e I n i t i a l a n o l y t e v o l u m e = 2 . 0 2 5 l i t r e s I n i t i a l N a C l c o n e = 3 . 2 M - 139 -T h e r e f o r e m o l e s o f C I " i n N a C l i n p u t = 3 . 2 x 2 . 0 2 5 = 6 . 4 8 m o l e s I n p u t N a C 1 0 3 c o n e = 2 . 8 7 M T o t a l m o l e s o f C I " i n N a C 1 0 3 = 2 . 8 7 x 2 . 0 2 5 = 5 . 8 1 m o l e s V o l u m e o f HC1 a d d e d = 30 cm 3 M o l a r i t y o f HC1 u s e d = 1 . 0 M T h e r e f o r e m o l e s o f C I - i n HC1 = 30 x JQTJTJ m o l e s = 0 . 0 3 m o l e s ( i i ) O u t p u t C h l o r i d e F i n a l a n o l y t e v o l u m e = 2 . 2 0 l i t r e s F i n a l N a C l c o n e = 2 . 8 M T o t a l m o l e s o f C I " i n o u t l e t N a C l = 2 . 2 0 x 2 . 8 = 6 . 1 6 m o l e s O u t p u t N a C 1 0 3 c o n e = 2 . 7 7 2 m o l e s / l i t r e M o l e s o f C I " i n N a C 1 0 3 = 2 . 7 7 x 2 . 2 0 = 6 . 0 9 m o l e s Cone o f O C T = 1 0 . 9 2 x 1 0 " 3 m o l e s T o t a l m o l e s o f C I " i n O C T = 1 0 . 9 2 x 1 0 " 3 x 2 . 2 = 0 . 0 2 m o l e s - 140 -D u r a t i o n o f r u n = 175 m i n C a t h o l y t e f l o w r a t e = 0 . 5 0 c m 3 / s C a t h o l y t e c l " c o n e = 3 x 1 0 " 3 M T o t a l v o l u m e o f c a t h o l y t e = 0 . 5 0 x 175 x 6 0 x -JTJQQ , c m w i n i r i w S > ,cm \ (— ) (~r>W x hr* = 5 . 2 5 l i t r e s T o t a l m o l e s o f C l - i n c a t h o l y t e = 3 x 1 0 - 3 x 5 . 2 5 = 0 . 0 2 m o l e s f Summary o f C h l o r i d e B a l a n c e T a b l e D I n l e t O u t l e t M o l e s o f C l " i n N a C l 6 . 4 8 6 . 1 6 M o l e s o f C l " i n N a C 1 0 3 5 . 8 1 6 . 0 9 M o l e s o f C l " i n 0 C 1 " - 0 . 0 2 M o l e s o f C l - i n 3 0 c m 3 HCl 0 . 0 3 -M o l e s o f C l " i n c a t h o l y t e - 0 . 0 2 T o t a l m o l e s o f C l ~ • 1 2 . 3 2 1 2 . 2 9 12 29 % amount o f C l " a c c o u n t e d f o r = ^ x 100% = 9 9 . 7 6 % i . e . , % amount o f C l " u n a c c o u n t e d f o r = 0 . 2 4 % - 141 -APPENDIX 3 SYSTB< MODELS - 142 -ELEMENTARY SYSTB4 MODEL An e l e m e n t a r y p r o c e s s model i s s u g g e s t e d . The model i s o n l y g o o d f o r o b t a i n i n g t h e c h l o r a t e c u r r e n t e f f i c i e n c y k n o w i n g p e r o x i d e c o n c e n t r a t i o n and o t h e r i m p o r t a n t f a c t o r s o f t h e m o d e l . T h i s model i s a m o d i f i c a t i o n o f t h e c h l o r a t e s y s t e m model s u g g e s t e d by J a c k s i c e t a l . [ 1 8 ] . The p e r o x i d e c u r r e n t e f f i c i e n c y i s o b t a i n e d u s i n g t h e e l e c t r o c h e m i c a l e q u i v a l e n t o f p e r o x i d e r e a c t i o n . T h e P r o c e s s M o d e l Excess O z + sodium perhydroxide NaOH + NaH02 Porous cathode (-) (Anion membrane or porous diaphragm) Separator NaCl makeup Oxygen + sodium hydroxide NaOH in water Chemical Reactor L^jJ D.C. power supply Process flow diagram showing electrochemical reactor for co-generation of sodium chlorate and alkaline peroxide solution F i g u r e C I - 143 -The c e l l i s d e s c r i b e d i n t h e b o d y o f t h e t h e s i s ( s e e C h a p t e r 4 ) and as shown i n F i g u r e C l , a s e p a r a t e a n o l y t e and s e p a r a t e f e e d i s u s e d . The m a j o r e l e c t r o c h e m i c a l r e a c t i o n o f i n t e r e s t a r e : on t h e c a t h o d e s i d e i s p e r o x i d e g e n e r a t i o n and on t h e a n o d e s i d e a r e C l 2 gas g e n e r a t i o n and C I O " o x i d a t i o n . The membrane h y d r o d y n a m i c a l l y s e p a r a t e s t h e a n o l y t e and t h e c a t h o l y t e b u t a l l o w s m i g r a t i o n and d i f f u s i o n o f a n i o n s a c r o s s i t . A s s u m p t i o n s and C o n s t r a i n t s The f o l l o w i n g a s s u m p t i o n s and c o n s t r a i n t s w e r e u s e d i n d e r i v a t i o n o f t h e m o d e l : ( 1 ) E a c h o f t h e two c h a m b e r s o f t h e e l c t r o l y z e r i s c o n s i d e r e d as a b a c k m i x e d , f l o w r e a c t o r w i t h t h e membrane b o u n d a r y a c t i n g as a mass e x c h a n g e s u r f a c e b e t w e e n t h e m . The a s s u m p t i o n i s j u s t i f i e d f o r t h e a n o d e s i d e w h i c h i s an open c h a n n e l . F o r t h e c a t h o d e s i d e , i t i s n o t a j u s t i f i a b l e a s s u m p t i o n b u t w o u l d be u s e d f o r t h e s a k e o f "" s i m p l i c i t y . ( 2 ) The a n a l y s i s i s c o n f i n e d t o t h e m a j o r r e a c t i o n s t h a t l e a d t o c h l o r a t e f o r m a t i o n and l o s s and t h e H 0 2 " s p e c i e s t r a n s p o r t a c r o s s t h e m e m b r a n e . ( 3 ) A l l r e a c t i o n s b e t w e e n H 0 2 _ and C I O - o c c u r a t t h e a n o l y t e / m e m b r a n e i n t e r f a c e . ( 4 ) I s o t h e r m a l o p e r a t i o n o f t h e c e l l a t s t e a d y s t a t e i s a s s u m e d . ( 5 ) The e x t e r n a l r e a c t o r i s c o n s i d e r e d as a b a c k - m i x e d f l o w r e a c t o r . ( 6 ) The s p a t i a l d i m e n s i o n i n t h e d i r e c t i o n p a r a l l e l t o t h e c u r r e n t i s t h e o n l y i m p o r t a n t d i m e n s i o n f o r m a t e r i a l b a l a n c e s . T h i s f o l l o w s f r o m ( 1 ) . - 144 -( 7 ) The r e l a t i o n s h i p b e t w e e n t h e p o t e n t i a l g r a d i e n t d e n s i t y t h r o u g h t h e membrane i s g i v e n by i = _ 1 i i r dx w h e r e i i s t h e c u r r e n t d e n s i t y b a s e d i n t h e membrane s u r f a c e a r e a , A m - 2 r = membrane e l e c t r i c a l r e s i s t a n c e , Ohm m 2 5 = membrane t h i c k n e s s , m ^ = p o t e n t i a l g r a d i e n t , V m _ 1 and t h e c u r r e n t ( C - l ) ( 8 ) The t o t a l a v a i l a b l e c h l o r i n e i s t h e sum o f t h e h y p o c h l o r o u s a c i d and h y p o c h l o r i t e . (9) In d e r i v i n g t h e F a r a d a i c s t o i c h i o m e t r y and mass b a l a n c e e q u a t i o n s , h y p o c h l o r o u s a c i d and h y p o c h l o r i t e a r e a b l e t o t a k e p a r t i n b o t h F o e r s t e r ' s r e a c t i o n s o f t h e c h e m i c a l c o n v e r s i o n and t h e a n o d e o x i d a t i o n f o r c h l o r a t e f o r m a t i o n . ( 1 0 ) L i n e a r c o n c e n t r a t i o n g r a d i e n t i n t h e membrane i s a s s u m e d . A t q u a s i s t e a d y s t a t e , t h e m a t e r i a l b a l a n c e o f a v a i l a b l e c h l o r i n e w i t h r e s p e c t t o t h e c e l l i s g i v e n b y : dC MTT-^) = 0 ( C - 2 ) cvdx ' w h e r e Vc = v o l u m e o f t h e anode c h a m b e r , m 3 C c = c o n c e n t r a t i o n o f a v a i l a b l e c h l o r i n e , m o l e s / m 3 T = t i m e i n s e c o n d s - 145 -T h e p a r t i a l c o n t r i b u t o r s t o t h e m a t e r i a l b a l a n c e w i l l i n c l u d e t h e f o l l o w i n g : i . A v a i l a b l e c h l o r i n e g e n e r a t e d f o r c h l o r a t e f o r m a t i o n - b o t h f o r homogeneous ( c h e m i c a l ) c h l o r a t e a n d h e t e r o g e n e o u s ( e l e c t r o c h e m i c a l ) c h l o r a t e , i i . G e n e r a t e d a v a i l a b l e c h l o r i n e w h i c h i s l o s t by r e a c t i o n w i t h p e r o x i d e t h a t i s t r a n s f e r e d a c r o s s t h e m e m b r a n e , i i i . C h l o r i n e g e n e r a t e d w h i c h i s v e n t e d ( u n h y d r o l y z e d c h l o r i n e ) , i v . A v a i l a b l e c h l o r i n e g e n e r a t e d w h i c h d o e s n o t t a k e p a r t i n any o f t h e a b o v e m e n t i o n e d r e a c t i o n p r o c e s s e s . T h e e q u a t i o n s o f r e a c t i o n s f o r t h e a b o v e p r o c e s s e s a r e g i v e n a s f o l l o w s : C I 2 + 2e = = = 2 C I " ( a ) CI 2 + H 2 0 -y H0C1 + C I " + H + ( b ) H0C1 O C l " + H + ( c ) 2H0C1 .+ O C l " + C 1 0 3 - + 2 C 1 " + 2 H + ( d ) 6 0 C 1 " + 3 H 2 0 — = = » 2 C 1 0 3 - + 4 C T + 6 H + + | 0 2 + 6e ( e ) H + + H 0 2 " + O C l " -> 0 2 + H 2 0 + C I " ( f ) U s i n g , t-j a s t h e f r a c t i o n o f c u r r e n t f o r p r o c e s s i , t h e n e c e s s a r y e q u a t i o n s c o u l d be s t a t e d as f o l l o w s : F o r a q u a s i s t e a d y s t a t e m a t e r i a l b a l a n c e : - 146 -i . A n o d i c g e n e r a t i o n o f a v a i l a b l e c h l o r i n e w h i c h i s c o n v e r t e d t o c h l o r a t e by t h e two p o s s i b l e r o u t e s - homogenous and h e t e r o g e n s o u s r o u t e s I = c u r r e n t , Amps F = F a r a d a y c o n s t a n t , C o u l o u m b s / m o l e T = t i m e i n s e c o n d s t x = f r a c t i o n o f c u r r e n t ( c u r r e n t e f f i c i e n c y ) f o r r e a c t i o n ( a ) w i t h s u b s e q u e n t h y d r o l y s i s and f u r t h e r c o n v e r s i o n ( h o m o g e n e o u s o r o x i d a t i o n ( h e t e r o g e n e o u s ) ) t o c h l o r a t e . i i . A n o d i c g e n e r a t i o n o f a v a i l a b l e c h l o r i n e , w h i c h i s l o s t s u b s e q u e n t l y by r e a c t i o n w i t h p e r h y d r o x y l i o n s t h a t a r e t r a n s f e r e d a c r o s s t h e membrane ( s e e r e a c t i o n f ) . V T f f V l F 1 ( C - 3 ) w h e r e ( C - 4 ) w h e r e t 2 i s f r a c t i o n o f c u r r e n t u s e d f o r c h l o r i n e t h a t r e a c t e d w i t h H 0 2 _ - 147 -i i i . A n o d i c g e n e r a t i o n o f c h l o r i n e w h i c h i s v e n t e d o u t as c h l o r i n e g a s dC w h e r e t 3 i s f r a c t i o n o f c u r r e n t u s e d f o r v e n t e d c h l o r i n e . i v . A n o d i c g e n e r a t i o n o f a v a i l a b l e c h l o r i n e , w h i c h d i d n o t t a k e p a r t i n e i t h e r o f r e a c t i o n s ( i ) , ( i i ) o r ( i i i ) , i . e . , h y d r o l y z e d b u t no c h l o r a t e f o r m a t i o n . dC . V<rr>4 • «w) w h e r e t ^ i s f r a c t i o n o f c u r r e n t u s e d f o r c h l o r i n e t h a t d i d n o t p a r t i c i p a t e i n any f u r t h e r r e a c t i o n e x c e p t t o be h y d r o l y z e d . v . A n o d i c o x i d a t i o n o f a v a i l a b l e c h l o r i n e t o c h l o r a t e ( i s a l o s s r e a c t i o n ) s e e r e a c t i o n ( e ) d C r t T V ( _ £ ) = - i i L ( C - 7 ) c v a x ' o x F v ' w h e r e t 5 i s t h e f r a c t i o n o f t h e c u r r e n t f o r t h e a n o d i c c h l o r a t e f o r m a t i o n . - 148 -(f) C u r r e n t l o s s e s due t o a v a i l a b l e c h l o r i n e r e a c t i o n w i t h p e r h y d r o x y l i o n s d C c t o l ( c " 8 ) ( g ) C u r r e n t l o s s e s due t o v e n t e d o u t c h l o r i n e d C c t I ( h ) A v a i l a b l e c h l o r i n e c o n v e r s i o n i n s i d e t h e a n o d e c h a m b e r o f t h e c e l l , i n t o c h l o r a t e , a s s u m i n g ( a ) b a c k - m i x f l o w r e a c t o r ( b ) F o e r s t e r m e c h a n i sm t h e n dC V f f r W = " 3 f c K r c V c CMC! [ C I O " ] , . ( C - 1 0 ) w h e r e f = h y p o c h l o r i t e o r h y p o c h l o r o u s a c t i v i t y c o e f f i c i e n t K p c = r a t e c o n s t a n t ((m / m o l e ) s e c ) f o r r e a c t i o n T h e r i g h t hand s i d e o f e q u a t i o n ( C - 1 0 ) i s o b t a i n e d b a s e d on a s s u m p t i o n ( 9 ) as f o l l o w s : F o r E q n . ( d ) , t h e r a t e e q u a t i o n c o u l d be w r i t t e n as d [ C 1 0 - ] 1 d [ H C 1 0 ] d [ C 1 0 , - ] , r „ . - 149 -o r , o t h e r w i s e dC s = _ d ( [ H C 1 0 ] + [ C I O - ] ) = _ 3 d [ C 1 0 - ] ( c _ 1 2 ) dt dt di w h e r e C s = c o n c e n t r a t i o n o f t o t a l a v a i l a b l e c h l o r i n e . I n t r o d u c i n g F o e r s t e r ' s k i n e t i c l a w f o r t h e c h e m i c a l c o n v e r s i o n i n c o n c e n t r a t e d s o l u t i o n s , e q n . ( C - 1 2 ) c o u l d be w r i t t e n dC 3 f 2 K p [ H C 1 0 ] 2 [ C 1 0 - ] ( C - 1 3 ) M u l t i p l y i n g e q n . ( C - 1 3 ) by t h e v o l u m e o f t h e r e a c t o r we o b t a i n e q u a t i o n ( C - 1 0 ) . ( i ) T h e a v a i l a b l e c h l o r i n e c o n v e r s i o n t o c h l o r a t e i n s i d e t h e h o l d i n g v o l u m e ( e x t e r n a l r e a c t o r ) i s e q u a l t o t h e d i f f e r e n c e i n q u a n t i t i t e s o f a v a i l a b l e c h l o r i n e l e a v i n g t h e c e l l and e n t e r i n g t h e c e l l f r o m t h e h o l d i n g v o l u m e V ^ > R x n H " " « < C c " Ch» <C-14> w h e r e q = e l e c t r o l y t e f l o w r a t e , m 3 / s C c = c o n c e n t r a t i o n o f a v a i l a b l e c h l o r i n e i n c e l l , m o l e s / m 3 C n = c o n c e n t r a t i o n o f a v a i l a b l e c h l o r i n e i n e x t e r n a l r e a c t o r , m o l e s / m 3 - 150 -D o i n g a m a t e r i a l b a l a n c e o v e r t h e a n o d e p a r t o f t h e c e l l we h a v e : 1 1 1 + I2I + i l l + t i l _ * 5 l _ 1 1 1 _ i^L _ n f r - C ) 2F 2F r F F 2F UT q i c V - 3 f 2 K p c V J H C 1 0 ] 2 [ C 1 0 - ] c = 0 ( C - 1 5 ) A l s o tl + t 2 + t 3 + t i , + t 5 = 1 ( C - 1 6 ) f r o m ( C - 1 6 ) t 5 = 1 - t i - t 2 - t 3 - t „ ( C - 1 7 ) P u t t i n g ( C - 1 7 ) i n t o ( C - 1 5 ) , we h a v e | l i + |jtL - I + * i l + Y + ¥ " + ¥ " " 3 f J K p c V c [ C 1 0 ] [ H C 1 0 ] 2 - q ( C c - C H ) - 0 s o l v i n g f o r t i we o b t a i n t i = § + 2 ( f ) f 2 K r c V c [ H C 1 0 ] 2 [ C I O " ] + § { f ) q ( C c - C*h) - § ( t 2 + t 3 + tH) ( C - 1 8 ) S i m p ! i f i c a t i o n s R e p l a c i n g t h e c o n c e n t r a t i o n o f h y p o c h l o r o u s a c i d and h y p o c h l o r i t e ( t o t a l a v a i l a b l e c h l o r i n e ) by C s = [ H C 1 0 - ] + [ C 1 0 - ] ( C - 1 9 ) - 151 -and d e f i n i n g t h e r m o d y n a m i c d i s s o c i a t i o n c o n s t a n t o f h y p o c h l o r o u s a c i d by Ka - %nJlOjfJ0"] <C"20) R e a r r a n g i n g e q u a t i o n ( C - 2 0 ) , we d e f i n e K * as a Q + [ C 1 0 - ] Kaf a , = [Hcio] = -f^r- (c-21) From e q u a t i o n ( C - 1 9 ) [ H C 1 0 ] = C s - [ C I O " ] ( C - 2 2 ) P u t t i n g ( C - 2 2 ) i n t o ( C - 2 1 ) we o b t a i n , K * { C s - [ C I O " ] } = a H 3 0 + C C 1 0 _ ] [ K * + a H 3 0 + ] [ C 1 0 - ] = K * C s K * C K * a H n + C = = ^ [ C 1 0 " ] = a t + K * 0 r 1 ( C " 2 3 ) a H 3 0 + K 1 + — a H 3 0 + P u t t i n g ( C - 2 3 ) i n t o ( C - 2 2 ) o b t a i n K * C [ H C 1 0 ] = C - - S - r W ( C - 2 4 ) s a H 3 0 + + K S u b s t i t u t i n g i n t o ( C - 1 8 ) u s i n g ( C - 2 3 ) and ( C - 2 4 ) , o b t a i n - 152 -ti 9 p C au n + 2 K * C " ! + 2 ( f ) *l « r c V c ^ 7 7 ^ } c + a J + * H 3 0 + • — - ' "H3(r i . e . , t i = | + 2 ( f ) f c K r c V c C c K * a H 3 0 + / ( a H 3 0 + + K * ) ' + 2 / 3 ( y ) q ( C c - C h ) - | ( t 2 + t 3 + U ) ( C - 2 5 ) A s s u m i n g a b a c k - m i x f l o w r e a c t o r and d o i n g a m a t e r i a l b a l a n c e o v e r t h e h o l d i n g v o l u m e ( a n o l y t e t a n k r e a c t o r ) , i t c o u l d be shown t h a t : q ( C c - C h ) = 3 f 2 K r h V h [ H C 1 0 ] h 2 [ C 1 0 " ] h ( C - 2 6 ) w h e r e V n = a n o l y t e t a n k v o l u m e , m 3 K r n = homogenous c h l o r a t e r a t e c o n s t a n t , ( m 3 / m o l ) 2 s e c - 1 From e q u a t i o n s ( C - 1 9 ) , ( C - 2 3 ) , ( C - 2 4 ) and ( C - 2 6 ) , e q u a t i o n ( C - 2 5 ) becomes t i - | * 2 f ? K r r « < f ) " * U h ^ ' ^ • 2 f h 2 K r h V h (j). 1 3 c r c c u r - , . . , -,3 h r n h V I ' c K h ( a H , 0 + ) h C h 2 - £ ( t 2 + t 3 + t 4 ) ( C - 2 7 ) + ( a H , o ^ h ] 3 3 A p p r o x i m a t i n g e x p e r i m e n t a l v a l u e s f o r K a , V o g t [ 5 9 ] e x p r e s s e d K a i n t h e f o r m : - 153 -l o g K = 7 . 7 4 4 - 0 . 0 0 7 5 T / ° C ( C - 2 8 ) a F u r t h e r he showed t h a t e x p e r i m e n t a l d a t a o f J a k s i c e t a l ( 1 8 ) c o u l d be a p p r o x i m a t e d a s £ • - 14 . ( C - 2 9 ) K a H e n c e f r o m ( C - 2 8 ) and ( C - 2 9 ) , K * = 14K = 14 x 1 0 - ( 7 « 7 4 4 " ° - 0 0 7 5 T / ° C ) ( C - 3 0 ) a U s i n g d a t a f r o m s e v e r a l a u t h o r s c o l l a t e d by K o k o u l i n a and K r i s h t a l i k [ 3 7 ] , we c a n f i t an e q u a t i o n t o e x p r e s s K f 2 as K f 2 = 3 . 0 9 x 1 0 4 e x p ( - 8 A 2 * X 1 q 3 ) ( C - 3 1 ) w h e r e T i s i n K e l v i n K f 2 i s i n ( m o l e s / m 3 ) - 2 s e c - 1 F o r h y d r o g e n i o n c o n c e n t r a t i o n , an3o+> t h e pH o f t h e s o l u t i o n i s u s e d , i . e . , a H 3 o + = 1 0 ~ P H . To s i m p l i f y e q u a t i o n ( C - 2 7 ) f u r t h e r , assume v a l u e s o f t 3 and t 4 a r e n e g l i g i b l e . O b t a i n t 2 by e q u a t i n g f l u x o f H 0 2 ~ p a s s i n g a c r o s s t h e membrane w i t h amount o f a c t i v e c h l o r i n e l o s t by r e a c t i o n o f p e r h y d r o x y l i o n s . The f l u x o r n e t f l u x o f H 0 2 ~ i o n s t h r o u g h t h e membrane f r o m t h e c a t h o l y t e t o t h e a n o l y t e c a n be w r i t t e n a s - 154 -N H 0 2 - " V- F C H 0 2 - <&> - D H 0 2 - < ^ > V C H 0 2 - <C- 3 2> w h e r e N^Q _. = f l u x o f p e r h y d r o x y l i o n s p e r u n i t a r e a o f m e m b r a n e , '2 m o l e s / m 2 • s U H Q o _ = p e r h y d r o x y l i o n m o b i l i t y , m mol J " s '2 2 -, , _ i „_i 3 ^HO2~ = P e r 0 X 1 c * e c o n c e n t r a t i o n , mol/m = p o t e n t i a l g r a d i e n t a c r o s s t h e m e m b r a n e , V/m D H 0 2 _ = c ' " ' f f u s i o n c o e f f i c i e n t o f t h e p e r h y d r o x y l i o n i n t h e membrane ( m 2 s _ 1 ) v = e l e c t r o l y t e v e l o c i t y a l o n g t h e x - d i r e c t i o n i n s i d e t h e m e m b r a n e , m/s E q u a t i o n ( C - 3 2 ) c a n be s i m p l i f i e d by a s s u m i n g : t h a t t h e e l e c t r o l y t e v e l o c i t y i n t h e m e m b r a n e , v = 0 . D H 0 -The i o n m o b i l i t y i s e x p r e s s e d a s U^Q = R T 2 , u s i n g P l a n k - E i n s t e i n r e T a t i o n d^uri - r ° r a E x p r e s s - g p . = L ~, L ( C - 3 3 ) w h e r e C ° = b u l k p e r o x i d e c o n c e n t r a t i o n i n t h e c a t h o l y t e C a = p e r o x i d e c o n c e n t r a t i o n i n t h e a n o l y t e = 0 i . e . , assume a l l p e r o x i d e r e a c t e d i m m e d i a t e l y i t e n t e r s i n t o t h e a n o l y t e ( a s s u m p t i o n ( 3 ) ) 6 = membrane t h i c k n e s s i n m Thus ( C - 3 3 ) becomes - 155 -A s s u m e t h e p o t e n t i a l g r a d i e n t t h r o u g h t h e membrane i s g i v e n by d(j) i r dx 6 ( C - 3 5 ) w h e r e i i s t h e c u r r e n t d e n s i t y b a s e d on t h e membrane s u r f a c e a r e a , kA m - 2 r = membrane e l e c t r i c a l r e s i s t a n c e , ohm m 2 C o m b i n i n g a l l t h e s e , e q u a t i o n ( C - 3 2 ) i s r e w r i t t e n a s : M D H 0 9 - F C H 0 9 - i r D H 0 9 - C H 0 9 -^HC^" RT 6 6 ( L - J b ) I f t h e t o t a l membrane s u r f a c e a r e a = A c , t h e n t o t a l m o l e s o f H 0 2 " p a s s i n g t h r o u g h t h e membrane f r o m c a t h o l y t e t o t h e a n o l y t e i s g i v e n by [ H 0 2 - ] = A c N H ( ) 2 _ ( m o l e s / s ) ( C - 3 7 ) By a s s u m i n g m o l e s o f CIO" l o s t t h r o u g h r e a c t i o n w i t h H 0 2 - i s e q u a l t o m o l e s o f H 0 2 ~ p a s s i n g t h r o u g h membrane we o b t a i n I i i = A N 2F A N n _ t 2 = Y-^- ( C - 3 8 ) S u b s t i t u t i n g f o r ^Q2- i n ( C - 3 8 ) u s i n g ( C - 3 6 ) - 156 -t 2p2 A c V - c S o ? - i r 2F A c V - C H V t 2 " IRT 6 " 16 t 2 = 2 F A c W C H ( V ( c _ 3 9 ) S u b s t i t u t i n g t 3 = 0 , t 4 = 0 and u s i n g ( C - 3 9 ) f o r t 2 t h e n e q u a t i o n ( C - 2 7 ) b e c o m e s : i2 r 3 n - i - * ? ^ vc (f> K ; ! a ; f ; i -( K c + a H 3 0 + ) c ? f 2 K y f F v K h ( a H 3 0 + ) h C h 2 f 2 F A c D c o r F i r . i - , ! 2 f h K r h V P TTTTl^ ^ T i ' T - r 5 - D H 0 2 " C H 0 2 - L RT 1 J J ( K h + U H 3 0 + j h ( C - 4 0 ) The model e q u a t i o n ( C - 4 0 ) s t a n d s f o r t h e f o l l o w i n g : t i = T o t a l c h l o r a t e c u r r e n t e f f i c i e n c y o f t h e p r o c e s s . 2 / 3 = Maximum c h l o r a t e e f f i c i e n c y a c h i e v a b l e i f a l l c h l o r a t e i s f o r m e d t h r o u g h t h e e l e c t r o c h e m i c a l r o u t e F K r <aH 0 + } c C c 2 fl K . V. ( T ) — ^ — - — - = C h l o r a t e f o r m e d i n t h e c e l l v o l u m e , V n r n n l , \3 c K c K H 3 0 + ' c t h r o u g h t h e c h e m i c a l r o u t e . 2 f 2 K . MAT) — ^ — - — — = C h l o r a t e f o r m e d i n t h e e x t e r n a l h r h h i (K * + ( a . ) ) 3 r e a c t o r o f v o l u m e V. , t h r o u g h t h e h H3° n . c h e m i c a l r o u t e . h 2F A % D u n C ° . [ F D T r - 1 ] } = C h l o r a t e l o s t due t o r e a c t i o n o f H 0 2 ~ 3 1 6 HU 2 HU2 K l , . h r h l n r a t p i n t . P r m p H i a t . M . w i t h c l o r t e i t e r m e d i a t e s . - 157 -E x a m p l e U s e o f t h e M o d e l E q u a t i o n In u s i n g t h e m o d e l , some a s s u m p t i o n s a r e m a d e . 1 . The d i f f u s i o n c o e f f i c i e n t o f H 0 2 " i s assumed t o be 1/4 o f t h e v a l u e o f O H - i o n d i f f u s i o n c o e f f i c i e n t i n a q u e o u s s o l u t i o n . In a r e c e n t w o r k by H e r r e r a and Y e a g e r [ 5 3 ] , t h e s e l f d i f f u s i o n c o e f f i c i e n t s o f N a + , C l - , I " , and S 0 \ " i o n s i n s u l f o n a t e and c a r b o x y l a t e N a t i o n ( c a t i o n ) membranes w e r e m e a s u r e d . The m a g n i t u d e o f t h e r e p o r t e d d i f f u s i o n c o e f f i c i e n t f o r t h e N a + c a t i o n ( m a i n c u r r e n t c a r r i e r i n c a t i o n e l e c t r o l y s i s o f t h i s t y p e ) i n t h e membrane i s l o w e r t h a n t h e v a l u e i n s o l u t i o n ( 1 5 . 6 x 1 0 " 1 1 m 2 s - 1 i n 3 M NaCl c f 1 . 3 3 4 x 1 0 ~ 9 m 2 s - 1 a t i n f i n i t e d i l u t i o n ) . A v a l u e o f D H O 2 - =1•75 x 1 0 - 1 1 m 2 s _ 1 i s assumed D 0 H - = 7 . 0 x 1 0 - 1 1 m 2 s - 1 a t 2 5 ° C . 2 . A c o n s t a n t H 0 2 ~ c o n c e n t r a t i o n i n t h e c e l l i s a s s u m e d . 3 . The a v e r a g e c e l l t e m p e r a t u r e u s e d i s t h e a v e r a g e o f t h e i n l e t a n d o u t l e t t e m p e r a t u r e on t h e anode s i d e . The f o l l o w i n g a r e a l s o known q u a n t i t i e s . Membrane t h i c k n e s s , 6 = 5 . 0 8 x 1 0 - 5 m E f f e c t i v e s u p e r f i c i a l membrane s u r f a c e a r e a ; A c = 8 3 . 3 3 x 1 0 _ l t m 2 A n o l y t e c e l l v o l u m e , V c = 25 x 1 0 " 6 m 3 A n o l y t e t a n k f i n a l v o l u m e , V n = 2 . 0 2 x 1 0 " 3 m 3 E x a m p l e : C o n s i d e r r u n C 2 5 ( t a b l e C2 i n A p p e n d i x 3 ) . C e l l t e m p e r a t u r e = 2 8 ° C A n o l y t e t a n k r e a c t o r t e m p e r a t u r e = 7 0 ° C C u r r e n t d e n s i t y , i = 2 . 4 kA m - 2 C u r r e n t , I = 20A - 158 -P e r o x i d e c o n c e n t r a t i o n , C ^ Q ^ - = 107 .0 m o l e s m - 3 2 2 F K c <PH>c C c F t ! = j + 2 K p c f * V c ( f ) ^ + 2 K r h V h ( f ) x J [ K * + ( p H ) r ] 3 r P 1 K f i ( P H ) h C h 4 F A c D H 0 9 - C H 0 9 - r F i r 2 ^ [ ^ - 1 ] ( C - 4 1 ) [ K £ + ( P H ) h ] 3 3 1 6 R T I f n e g l i g i b l e c o n v e r s i o n o f a v a i l a b l e c h l o r i n e i n s i d e t h e c e l l i s a s s u m e d , i . e . , s e c o n d t e r m i n e q n . ( C - 4 1 ) i s assumed z e r o t h e n t i may be e x p r e s s e d a s t 2 + ? , f * v 4 F A c D H 0 - C H 0 9 -t i = 3 + 2 K p h f h V h (y) - 3 n [ K £ + ( p H ) h ] ; C ^ S f - 1 ] ( C - 4 2 ) t h e s e c o n d t e r m i n e q u a t i o n ( C - 4 2 ) d e f i n e s t h e y i e l d o f c h l o r a t e i n s i d e t h e h o l d i n g v o l u m e t o t h e o v e r a l l c u r r e n t e f f i c i e n c y o f t h e s y s t e m . I t c a n be e x p r e s s e d as i . e . , s e c o n d t e r m = j F ( | ) (C - C h ) ( C - 4 3 ) w h e r e C c and C n a r e t h e t o t a l a v a i l a b l e c h l o r i n e c o n c e n t r a t i o n s ( m o l e s / m ) o f i n f l o w and o u t f l o w o f t h e h o l d i n g v o l u m e , r e s p e c t i v e l y . The t h i r d t e r m i n e q u a t i o n ( C - 4 2 ) i s t h e l o s s o f c h l o r a t e c u r r e n t e f f i c i e n c y due t o p e r o x i d e m i g r a t i o n . The f i r s t t e r m i s t h e maximum c h l o r a t e e f f i c i e n c y t h a t c a n be a c h i e v e d i f a l l t h e c h l o r a t e i s f o r m e d - 159 -by a v a i l a b l e c h l o r i n e c o n c e n t r a t i o n i n c e l l , C c = 30 .6 m o l e s rrr a v a i l a b l e c h l o r i n e c o n c e n t r a t i o n i n a n o l y t e r e a c t o r , C n = 12 .6 m o l e s m membrane r e s i s t a n c e , r = 1.85 x 10" Ohm m . C e l l s a m p l e pH = 7 .0 A n o l y t e t a n k s a m p l e pH = 6 . 8 A t 2 8 ° C , t h e r e a c t i o n r a t e c o n s t a n t f o r t h e c h l o r a t e f o r m e d i n t h e c e l l , v f 2 . o no v 8 .125 X 1 0 3 \ K p c f = 3 .09 x 10 e x p ( j f J I ) = 5.85 x l O " 8 ( m o l e s / m 3 ) - 2 s e c " 1 f o r t h e r a t e c o n s t a n t i n t h e e x t e r n a l r e a c t o r , K h f ^ ( a t 7 0 ° C ) i / ^ 2 o n o „ _w „ i 8 .125 x 10 3 N K p h f = 3 .09 x 10 e x p ( ; — ^ — — ) = 1.59 x 1 0 - 6 ( m o l e s / m 3 ) - 2 s e c " 1 The e q u i l i b r i u m c o n s t a n t e x p r e s s i o n , k * * * u n * v* i / i i n - ( 7 . 7 4 4 - 0 . 0 0 7 5 x 28) - f o r t h e c e l l t e r m , K * = 14 x 10 c = 4 .094 x l O " 7 - f o r t h e a n o l y t e t a n k t e r m , (T = 7 0 ° C ) K* = 14 x 1 0 - ( 7 - 7 4 4 - n - 0 0 7 5 x 7 ° ) h = 8 .455 x l O " 7 - p H c - p H c C e l l a ^ Q + = 10 and a n o l y t e t a n k a ^ Q+ = 10 U s i n g t h e model e q u a t i o n : - 160 -o c K * ( p H ) 2 C 3 t i = f + 2 K f 2 V c ( f ) - £ + 2 K p h f 2 V h OF/I) x "* [ K * + (pH) ] 3 r n " " K f t ( P H ) h C h 4 F A c D H 0 ? - C H 0 ? - r F i r n CKh* + ( R H ) h ] 3 " 3 S u b s t i t u t i n g a l l t h e r e l e v a n t v a l u e s , t j _ = | + 2 x 5 . 8 5 x 1 0 - 8 x 25 x 1 0 " 6 ( ^ j j 0 - ) 4 . 0 9 4 x 1 0 " 7 x ( l O ' V rm 3 ,2 / 1 \ /m 3w coul \/Sec w mol\2 ^ m o l ' ^ s e c ' ^ T ~ ' ^ g - e q u i v ' ^ c o u l ^ 3 [ 4 . 0 9 4 x 1 0 - 7 + 1 0 - u ] 3 ( 3 0 - 6 ^ 3 — + 2 x 1 . 5 9 x 1 0 - 6 x 2 . 0 2 x 1 0 ~ 3 (^g 0-) x / mol\ 3 /m 3 % 3 k 3 ' ^moF m 8 . 4 5 5 x 1 Q - 7 x ( I P - 3 , 8 ) 2 x ( 1 2 . 6 ) 3 4 x 9 6 5 0 0 x 8 3 . 3 3 x 1Q-1* ( 8 . 4 5 5 x 1 0 - 7 + 1 0 - 3 * 8 ] 3 20 x 5 . 0 8 x 1 0 " 5 x 3 i n ? , 9 6 5 0 0 x 2 4 0 0 x 1 . 8 5 x 1 0 - 1 * . x i U / <• 8 . 3 1 4 x 301 " i ] t i = 6 3 . 5 % ( e x p e r i m e n t a l v a l u e = 67%) - 161 -U s i n g e x p r e s s i o n i n ( C - 4 3 ) i n ( C - 4 2 ) t o r e p l a c e t h e s e c o n d t e r m and a s s u m i n g n e g l i g i b l e c o n t r i b u t i o n by s e c o n d t e r m o f eqn ( C - 4 1 ) , t h e n , t . - f + f F f l ) ( C c • C h ) - 4 F A ' " f f t - C " V ( q f - ! ) (C-44) I n s t e a d o f e q u a t i o n ( C - 4 1 ) u s e d f o r t i a b o v e , we c a n u s e eqn ( C - 4 4 ) , w i t h t h e f o l l o w i n g : q = 2 . 0 x l O " 6 m 3 / s O t h e r f a c t o r s r e m a i n s a m e . U s i n g ( C - 4 4 ) t i = 7 5 . 1 % . Some c o m p a r i s o n s a r e made i n T a b l e CI b e t w e e n t h e c a l c u l a t e d t i v a l u e s ( u s i n g ( C - 4 4 ) o f t h e m o d e l ) and t h o s e o b t a i n e d f r o m t h e e x p e r i m e n t a l r e s u l t s . The o n l y s u g g e s t i o n f o r t h e d e v i a t i o n o f model e q u a t i o n ( C - 4 1 ) r e s u l t s f r o m t h e e x p e r i m e n t a l c a l c u l a t i o n s m i g h t be t h e u s e o f t h e a p p r o x i m a t i o n s o f V o g t ( 5 9 ) as i n e q u a t i o n s ( C - 2 9 ) and ( C - 3 0 ) . T h o s e a p p r o x i m a t i o n s may be good f o r u n d i v i d e d c h l o r a t e c e l l s . W i t h r e s p e c t t o t h e p e r o x y - c h l o r a t e s y s t e m , t h e a p p r o x i m a t i o n s V o g t u s e d d i d n o t t a k e i n t o c o n s i d e r a t i o n t h e i n t e r a c t i n g e f f e c t s o f t h e c a t h o d e c h a m b e r . - 162 -T a b l e CI C o m p a r i s o n o f e x p e r i m e n t a l and c a l c u l a t e d CI r e -c u r r e n t e f f i c i e n c i e s Run C a l c u l a t e d E x p e r i m e n t a l # t i ( C - 4 4 ) C 27 7 6 . 7 6 0 . 0 c 8 7 1 . 2 7 3 . 5 C 22 6 7 . 6 5 3 . 2 Cl8 7 4 . 8 7 8 . 7 c 6 7 6 . 5 7 2 . 1 Cl7 7 9 . 8 7 9 . 0 C25 7 5 . 1 6 7 . 0 From t h e a b o v e t a b l e , i t d o e s n ' t seem t h e model e q u a t i o n ( C - 4 4 ) d e s c r i b e s t h e s y s t e m w e l l . I t i s l i k e l y t h a t t h e c a l c u l a t e d v a l u e s owed t h e i r d e v i a t i o n s ( f r o m t h e e x p e r i m e n t a l r e s u l t s ) t o t h e assumed d i f f u s i o n c o e f f i c i e n t o f H 0 2 " i o n . Some e x p e r i m e n t a l r e s u l t s u s e f u l f o r t h e model e q u a t i o n s a r e p r e s e n t e d i n T a b l e C2 and T a b l e A o f A p p e n d i x I I . T a b l e C2 Some e x p e r i m e n t a l r e s u l t s f o r model e q u a t i o n Run # PH C I O " C o n e . (M) x 1 0 3 Temp ( ° C ) CIO3- E f f i c i e n c y C e l l S a m p l e R e a c t o r S a m p l e C e l l S a m p l e R e a c t o r S a m p l e C e l l S a m p l e R e a c t o r S a m p l e % (6 m o l e s e " ) 7 . 2 6 . 5 1 8 . 7 5 5 . 4 5 2 5 . 5 7 0 . 0 5 7 . 4 c 6 7 . 3 6 . 6 3 0 . 7 3 1 2 . 0 2 9 . 5 70 7 2 . 1 c 8 7 . 1 5 6 . 6 2 2 . 5 0 1 5 . 0 3 3 . 0 70 7 3 . 5 7 . 1 6 . 7 1 8 . 6 0 1 4 . 1 2 9 . 0 70 6 7 . 0 C 1 2 7 . 2 6 . 6 2 9 . 2 0 1 5 . 9 3 3 4 . 0 71 6 9 . 0 7 . 2 6 . 8 1 8 . 5 1 1 . 5 3 2 . 5 71 6 7 . 0 . C 1 8 7 . 0 6 . 6 3 1 . 9 1 4 . 8 3 0 . 5 73 7 6 . 0 C 2 2 7 . 1 6 . 7 2 0 . 2 1 1 . 2 2 9 . 0 70 5 3 . 2 C23 7 . 1 6 . 7 2 7 . 2 1 3 . 7 3 0 . 0 71 7 5 . 0 C2i+ 7 . 0 6 . 6 . 3 0 . 3 1 5 . 9 3 2 . 0 74 6 3 . 0 C 2 5 7 . 0 6 . 8 3 0 . 6 1 2 . 6 2 8 . 0 70 6 7 . 0 C 2 6 7 . 1 6 . 7 5 2 6 . 2 5 i 1 4 . 3 3 0 . 5 70 . 7 0 . 0 C 2 7 7 . 0 6 . 6 2 2 . 8 1 2 . 6 2 7 . 5 70 6 0 . 0 C19 7 . 1 6 . 6 2 0 . 6 1 2 . 1 3 0 . 0 7 2 . 0 7 0 . 3 " . c 1 7 7 . 1 6 . 6 2 4 . 0 6 . 5 1 3 1 . 0 ' 7 0 . 0 ' 7 9 . 0 C 1 8 7 . 0 6 . 6 3 2 . 0 1 4 . 8 3 0 . 5 7 3 . 0 7 8 . 7 

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