13 Catalytic Detritiation of Water M. L. ROGERS, P. H . LAMBERGER, R. E . ELLIS, and T. K. MILLS Monsanto Research Corp., Mound Laboratory*, Miamisburg, O H 45342
Mound L a b o r a t o r y has, d u r i n g t h e p a s t f o u r y e a r s , been a c t i v e l y i n v o l v e d i n the development o f methods t o c o n t a i n and c o n t r o l t r i t i u m d u r i n g i t s p r o c e s s i n g and t o r e c o v e r i t from waste s t r e a m s . I n i t i a l b e n c h - s c a l e r e s e a r c h was d i r e c t e d m a i n l y toward removal o f t r i t i u m from gaseous e f f l u e n t s t r e a m s . The gaseous e f f l u e n t i n v e s t i g a t i o n has p r o g r e s s e d t h r o u g h t h e development s t a g e and has been implemented i n r o u t i n e o p e r a t i o n s . A t e s t l a b o r a t o r y embodying many o f t h e r e s u l t s o f t h e r e s e a r c h phase has been d e s i g n e d and i t s c o n s t r u c t i o n has been completed. As t h e program a t Mound L a b o r a t o r y has p r o g r e s s e d , the scope o f t h e e f f o r t has been expanded t o i n c l u d e r e s e a r c h c o n c e r n e d w i t h h a n d l i n g t r i t i a t e d l i q u i d s as well. A program i s p r e s e n t l y under way t o i n v e s t i g a t e the d e t r i t i a t i o n o f aqueous wastes u s i n g the Combined E l e c t r o l y s i s C a t a l y t i c Exchange (CECE) p r o c e s s , a p r o c e s s v e r y s i m i l a r t o the CECE-TRP p r o c e s s d i s c u s s e d a t t h i s symposium by Dr. Hammerli. As i n the CECE-TRP p r o c e s s , the key t o o u r CECE p r o c e s s i s a p r e c i o u s m e t a l h y d r o p h o b i c c a t a l y s t d e v e l o p e d by Dr. John B u t l e r a t Chalk R i v e r . Dr. B u t l e r , who d i s c u s s e d t h i s t y p e o f c a t a l y s t e a r l i e r i n the symposium, made t h e c a t a l y s t f o r our system. Catalytic
Exchange
The c a t a l y t i c c e n t e r s around two arranged i n s e r i e s and lower s e c t i o n s
exchange s e c t i o n o f the p r o c e s s c a t a l y s t - p a c k e d columns, which a r e t o o p e r a t e as i f they were t h e upper o f a s i n g l e column. (See F i g u r e 1.)
*Mound L a b o r a t o r y is o p e r a t e d by Monsanto R e s e a r c h C o r p o r a t i o n f o r t h e U.S. Energy R e s e a r c h and Development A d m i n i s t r a t i o n under C o n t r a c t No. EY-76-C-04-0053. ©
0-8412-0420-9/78/47-068-171$05.00/0
SEPARATION
172
O F HYDROGEN ISOTOPES
Each column i s 7.5 m i n l e n g t h and has an i n s i d e diame t e r o f 2.5 cm, which r e s u l t s i n a s u p e r f i c i a l c r o s s s e c t i o n a l a r e a o f 5 cm and a g r o s s column volume o f 3800 cm . The p a c k i n g i n each column c o n s i s t s o f 4300 g o f 0.6-cm d i a m e t e r c a t a l y s t s p h e r e s . L i q u i d r e d i s t r i b u t i o n r i n g s a r e p l a c e d a t 45-cm i n t e r v a l s t h r o u g h out the l e n g t h o f each column t o l i m i t c h a n n e l i n g i n the packed s e c t i o n s . In o p e r a t i o n , the l i q u i d t o be d e t r i t i a t e d i s s u p p l i e d a t a p p r o x i m a t e l y 5 cm /min. I t i s combined w i t h the l i q u i d stream e x i t i n g the upper column, and i n t r o d u c e d a t the t o p o f the lower column. The 10 cm / min l i q u i d stream from the bottom o f the lower column i s r o u t e d t o the e l e c t r o l y s i s s e c t i o n o f the p r o c e s s where i t i s used as f e e d . C u r r e n t l y , the l i q u i d stream e n t e r i n g a t the top o f the upper column i s made up o f d i s t i l l e d water, s u p p l i e d from o u t s i d e the p r o c e s s . U l t i m a t e l y , however, t h i s stream i s e x p e c t e d t o be a r e f l u x stream, p r o v i d e d by a recombiner o r f u e l c e l l which would i n t u r n be f e d by the hydrogen p r o d u c t from the upper column. The hydrogen stream from t h e e l e c t r o l y s i s s e c t i o n i s f e d t o the bottom o f the lower column and t h e n i s r o u t e d t o t h e bottom o f the upper column. Upon e x i t i n g t h e upper column, i t i s c u r r e n t l y v e n t e d t o the a t mosphere t h r o u g h a s t a c k . As mentioned p r e v i o u s l y , however, i t i s a n t i c i p a t e d t h a t t h i s stream w i l l l a t e r be f e d t o e i t h e r a f u e l c e l l o r a recombiner t o p r o v i d e r e f l u x t o the p r o c e s s and a d e t r i t i a t e d l i q u i d p r o d u c t . C o n t r o l o f the c a t a l y t i c exchange s e c t i o n o f the p r o c e s s c o n s i s t s o f f i v e l o o p s , s e r v i c e d by a s i n g l e microprocessor. In each o f the l o o p s t h e c o n t r o l element i s a m e t e r i n g v a l v e , d r i v e n by a s t e p p i n g motor. F o r the gas stream p a s s i n g through the columns, the s e n s i n g element i s a t u r b i n e meter l o c a t e d downstream from the columns. The two l i q u i d f e e d s t o the p r o c e s s are sensed by h o t - w i r e anemometers, and the l e v e l s m a i n t a i n e d i n the bottom o f each column a r e m o n i t o r e d by c a p a c i t i v e l e v e l s e n s o r s . A l t h o u g h not used i n the c o n t r o l o f the p r o c e s s , t r i t i u m c o n c e n t r a t i o n s a r e m o n i t o r e d a t v a r i o u s p o i n t s by l i q u i d s c i n t i l l a t i o n c o u n t e r s f o r l i q u i d streams and i o n chambers f o r gas streams. 2
3
3
3
Electrolysis The p r i n c i p a l items o f equipment i n the e l e c t r o l y s i s s e c t i o n are f o u r G e n e r a l E l e c t r i c e l e c t r o l y s i s stacks. (See F i g u r e 2.) Each s t a c k c o n s i s t s o f e i g h t c e l l s , each o f which has an a c t i v e a r e a o f 46.45 cm .
13.
ROGERS
E T
Catalytic
AL.
Detritiation
of
173
Water
DEPLETED H TO STACK 2
DISTILLED WATER
ENRICHED H FROM ELECTROLYSIS 2
TRITIATED LIQUID FEED
ENRICHED LIQUID T O ELECTROLYSIS
Figure 1.
Catalytic exchange section
PHASE SEPARATOR
ENRICHED LIQUID _ FROM CATALYTIC EXCHANGE
OXYGEN TO EX HAUST
Ho TO PHASE SEPARATOR
ft ι
I I ι
cb
-
DEIONIZER
\ |ELECTROLYSIS| CELLS
Figure 2. Electrolysis section
CATALYTIC EXCHANGE
174
SEPARATION O F
HYDROGEN
ISOTOPES
C u r r e n t d e n s i t y i s 1.076 amp/cm f o r the maximum r a t e d c u r r e n t o f 50 amps. Rated s t a c k v o l t a g e i s 16.8 VDC. O p e r a t i n g a t t h e s e c o n d i t i o n s , each s t a c k e l e c t r o l y z e s 2.45 cm water/min t o produce a p p r o x i m a t e l y 3000 cm hydrogen/min and 1500 cm oxygen/min. The maximum p r e s s u r e i s 100 p s i g and the upper l i m i t temperature i s 66°C. Proper s t a c k o p e r a t i o n r e q u i r e s t h a t f e e d water r e s i s t i v i t y be m a i n t a i n e d a t g r e a t e r than 500,000 ohmcm, and t h a t a water c i r c u l a t i o n r a t e o f a p p r o x i m a t e l y 400 cm /min p e r 8 - c e l l s t a c k be m a i n t a i n e d . Feed t o the e l e c t r o l y s i s s e c t i o n i s s u p p l i e d from a f e e d tank, which a l s o p r o v i d e s r e c i r c u l a t i o n capacity. From the f e e d tank, the water i s pumped through a d e i o n i z e r , and then t o the c e l l s . From the c e l l s , the hydrogen stream p a s s e s through a c o o l e r and a phase s e p a r a t o r , and i s r e t u r n e d t o the c a t a l y t i c exchange s e c t i o n . The oxygen stream, a f t e r b e i n g r o u t e d through a c o o l e r and a phase s e p a r a t o r , i s c u r r e n t l y s e n t t o the a i r d e t r i t i a t i o n system f o r p u r i f i c a t i o n and d i s p o s a l . I t i s a n t i c i p a t e d , however, t h a t t h i s stream w i l l u l t i m a t e l y be f e d t o a recombiner o r a f u e l c e l l s u p p l y i n g r e f l u x t o the c a t a l y t i c exchange columns. The water removed from both the oxygen and hydrogen streams i n the phase s e p a r a t o r s i s mixed w i t h the f r e s h f e e d upstream from the d e i o n i z e r , and r e c i r c u l a t e d through the system. 3
3
3
3
Preliminary
Results
The CECE system was r e c e n t l y o p e r a t e d c o n t i n u o u s l y f o r a p e r i o d o f a p p r o x i m a t e l y 48 h r . Water was f e d both t o the top o f the f i r s t column and i n between the columns, each a t a r a t e o f 3 cm /min. The top f e e d had a c o n c e n t r a t i o n o f 0.028 y C i / l i t e r w h i l e the mid-column f e e d , which can be c o n s i d e r e d the "hot" f e e d , had a t r i t i u m c o n c e n t r a t i o n o f 304 y C i / l i t e r . The t o t a l e l e c t r o l y s i s stream, 7.2 l i t e r s / m i n , was r e t u r n e d t o the bottom o f the second column. The d e p l e t e d hydrogen stream e x i t s the system a t the top o f t h e f i r s t column, where a p o r t i o n o f t h i s stream i s o x i d i z e d t o water, c o l l e c t e d i n an e t h y l e n e g l y c o l " b u b b l e r " , and a n a l y z e d f o r t r i t i u m u s i n g l i q u i d s c i n t i l l a t i o n counting. A t the end of t h i s 48-hr r u n , the water formed from t h i s d e p l e t e d hydrogen had a c o n c e n t r a t i o n o f 0.006 y C i / l i t e r , w h i l e the e l e c t r o l y s i s water l o o p had been e n r i c h e d t o 386 y C i / l i t e r . These r e s u l t s a r e summarized i n F i g u r e 3. 3
ROGERS
E T
Catalytic
AL.
H 0 3 cm /min 0.028 MCi/liter
Detritiation
of
Water
2
3
H (depleted in T ) 7.2 liter/min (6 cm /min H 0) burn ^ 100 cm /min to water and sample 2
2
3
2
3
After ^28 ^39 ^42 ^46
hr hr hr hr
0.021 juCi/liter 0.012 MCi/liter 0.011 MCi/liter 0.006 MCi/liter
Equilibrium value 0.004
μΟ\/\\Χβτ
7.5-m Stripping Section
HTO 3 cm /min 304 MCi/liter 3
7.5-m Enriching Section
HTO
Electrolysis Cells 0 to Stack via Air Detritiation System 2
HTO in Cell Loop 386 MCi/liter After 48 hr Equilibrium Value ν 2 Ci/liter Figure 3.
Preliminary results combined electrolysis catalytic exchange sys tem (CECE)
176
Future
SEPARATION
O F HYDROGEN ISOTOPES
Plans
The p i l o t CECE system w i l l c o n t i n u e t o be operated with goals of obtaining operating experience, d e t e r m i n i n g s c a l e - u p parameters, and i m p r o v i n g system dependability. Mound L a b o r a t o r y a l s o t e n t a t i v e l y p l a n s t o b u i l d a l a r g e r system b e g i n n i n g i n l a t e 1 9 7 8 . T h i s system would be used t o t r e a t aqueous t r i t i a t e d waste f o r ERDA. Abstract A pilot-scale system has been used a t Mound L a b o r a t o r y t o i n v e s t i g a t e the catalytic detritiation of w a t e r . A hydrophobic, precious metal c a t a l y s t i s used t o promote the exchange o f tritium between liquid water and gaseous h y d r o g e n . T h i s c a t a l y s t was d e v e l o p e d a t C h a l k R i v e r N u c l e a r L a b o r a t o r i e s and i s operated at 6 0 ° C . The catalyst i s packed in two columns, each 7.5 m l o n g by 2 . 5 cm i.d. Water flow i s 5 - 1 0 c m / m i n and c o u n t e r c u r r e n t hydrogen flow i s 9 0 0 0 12,000 cm /min. The equipment, e x c e p t f o r the columns, is housed i n an inert atmosphere glovebox and i s computer controlled. The hydrogen i s o b t a i n e d by electrolysis of a p o r t i o n o f the water s t r e a m . E n r i c h e d gaseous tritium i s withdrawn f o r f u r t h e r enrichment. T h i s paper d e s c r i b e s the system and o u t l i n e s its o p e r a t i o n , i n c l u d i n g e x p e r i m e n t a l d a t a . 3
3
RECEIVED August 10, 1977
