Heavy Water Distillation

9 Heavy Water Distillation G. M. KEYSER, D. B. McCONNELL, N. ANYAS-WEISS, and P. KIRKBY Downloaded by UNIV OF CALIFORNIA SAN DIEGO on February 4, 2016 | http://pubs.acs.org Publication Date: June 1, 1978 | doi: 10.1021/bk-1978-0068.ch009

Ontario Hydro Research Div., 800 Kipling Ave., Toronto, Canada

The m o t i v a t i o n f o r t h e c u r r e n t heavy water r e s e a r c h program a t O n t a r i o Hydro comes from o u r growing dependence on CANDU-based n u c l e a r power p l a n t s . About 15% o f t h e c a p i t a l c o s t o f t h e s e p l a n t s i s f o r t h e heavy water moderator and c o o l a n t . We hope t o be a b l e to p r o v i d e e c o n o m i c a l l y and t e c h n i c a l l y v i a b l e o p t i o n s f o r heavy water p r o d u c t i o n i n t h e 1990 s by d e v e l o p i n g a l t e r n a t i v e p r o d u c t i o n methods t o an i n d u s t r i a l l y usef u l degree by t h a t t i m e . The Heavy Water Group a t Hydro Research D i v i s i o n has a s s i s t e d i n e s t a b l i s h i n g t h e s c i e n t i f i c feasibili t y o f two heavy water p r o d u c t i o n methods : l a s e r i n d u c e d d i s s o c i a t i o n o f formaldehyde, and low-temperat u r e water d i s t i l l a t i o n u s i n g waste h e a t and hopes t o c o n t i n u e development o f b o t h t h e s e methods. In t h i s t a l k , I w i l l d e s c r i b e t h e experiments c a r r i e d o u t t o demonstrate t h e f e a s i b i l i t y o f lowtemperature d i s t i l l a t i o n w i t h a p a r a l l e l - s h e e t p a c k i n g d e v e l o p e d a t t h e Research D i v i s i o n . The c o s t e n v i r o n ment o f heavy water d i s t i l l a t i o n w i l l a l s o be d i s cussed. 1

Distillation

Systems

- Physical

To r e f r e s h your memory, I w i l l show i n F i g u r e 1 (a) a s c h e m a t i c r e p r e s e n t a t i o n o f a d i s t i l l a t i o n system. The main f e a t u r e s o f t h e system a r e upward moving v a pour streams and f a l l i n g l i q u i d f i l m s i n c l o s e p r o x i m i t y , m a i n t a i n e d by a temperature g r a d i e n t between t h e h e a t s o u r c e ( b o i l e r ) a t t h e bottom and t h e heat s i n k (condenser) a t t h e t o p o f t h e system. A c o n v e n i e n t way o f b r i n g i n g t h e c o u n t e r - f l o w i n g phases c l o s e t o each o t h e r i s t o p r o v i d e v e r t i c a l s h e e t s o f m a t e r i a l o v e r which t h e l i q u i d f l o w s as a t h i n f i l m , w h i l e t h e vapour r i s e s i n narrow c h a n n e l s between t h e s h e e t s , as ©

0-8412-0420-9/78/47-068-126$05.00/0

In Separation of Hydrogen Isotopes; Rae, Howard K.; ACS Symposium Series; American Chemical Society: Washington, DC, 1978.

9.

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Heavy

Water

127

Distillation

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shown i n F i g u r e 1 ( b ) . What makes a heavy water system " d i f f e r e n t " i s t h a t the component t o be s e p a r a t e d o u t , HDO, i s p r e s e n t i n v e r y low c o n c e n t r a t i o n , about one p a r t i n 3500, i n n a t u r a l water f e d t o the system, so t h a t enormous v o l umes o f vapour must be p a s s e d t h r o u g h the s e p a r a t i n g system t o e x t r a c t s u f f i c i e n t HDO. C o s t Components. From our e s t i m a t e (and those o f many p r i o r workers) o f the c o s t o f heavy water from a d i s t i l l a t i o n p l a n t , i f we were t o pay f o r f o s s i l f u e l s , or i f we used the heat from a n u c l e a r r e a c t o r which would o t h e r w i s e be g e n e r a t i n g e l e c t r i c i t y , the c o s t o f t h i s energy would i t s e l f amount t o w e l l o v e r the c u r r e n t c o s t o f heavy water. S i n c e t h e r m a l g e n e r a t i n g s t a t i o n s are o n l y about 40% e f f i c i e n t i n c o n v e r t i n g the t h e r m a l energy r e l e a s e d from f u e l i n t o e l e c t r i c i t y , ~ 6 0 % o f i t i s a v a i l a b l e as low grade heat (water a t about 26°C) c o s t i n g o n l y the p r i c e o f t r a n s p o r t i n g i t t o the p o i n t o f use. Our o b j e c t i v e , t h e n , has been t o u t i l i z e t h i s waste h e a t , by o p e r a t i n g between about 26°C and the l a k e water temperature which i s t y p i c a l l y 20° lower. I f we can u t i l i z e waste h e a t , then the r e m a i n i n g h i g h c o s t components a r e the m a t e r i a l , o r p a c k i n g , used t o b r i n g the l i q u i d and vapour i n t o c l o s e p r o x i m i t y , the s t r u c t u r e s (vacuum b u i l d i n g s ) used t o house the packi n g , and the h e a t exchangers and b o i l e r s f o r m a i n t a i n i n g the f l o w o f h e a t t h r o u g h the system. The r e l a t i v e magnitudes o f t h e s e components, based on D 0 a t $200/kg i s shown i n F i g u r e 2. The g r e a t e s t c o s t component, by f a r , i s the p a c k i n g f o r the columns. 2

400 Ton P l a n t C o n c e p t i o n F i g u r e 3 shows our p r e s e n t c o n c e p t i o n o f a lowtemperature d i s t i l l a t i o n p l a n t c a p a b l e o f p r o d u c i n g 400 tons o f heavy water per y e a r . I t i s d i v i d e d i n t o s e v e r a l hundred u n i t s housed i n s e p a r a t e towers, each o p e r a t i n g i n d e p e n d e n t l y to d i s t i l l the heavy water out o f c l e a n l a k e water and b r i n g i t t o a c o n c e n t r a t i o n o f about 1%. The subsequent c o n c e n t r a t i o n s t e p t o 99.8% f o r r e a c t o r use c o s t s o n l y 15% t o 20% t h a t o f p e r f o r m i n g the f i r s t s t e p , and i s c a r r i e d o u t i n a s e p a r a t e d i s t i l l a t i o n "finishing" unit. Experimental Packing The major gap i n the p l a n t c o n c e p t was the p a c k i n g . Heavy Water d i s t i l l a t i o n p a c k i n g s f o r the low-tempera-


SEPARATION OF HYDROGEN ISOTOPES H E A T SINK AT T E M P E R A T U R E T

2


DIFFUSIVE DEUTERIUM TRANSPORT FROM VAPOR T O LIQUID

FALLING LIQUID

RISING VAPOUR

HEAT SOURCE A T T E M P E R A T U R E T,

(a)

(b)

Figure 1.

Elements of a distilhtion system

COMPONENT WASTE

RELATIVE COST $ / k g D2O 6.5

HEAT

COOLING

20

V A C U U M BUILDINGS AND PLUMBING

5.5

CONDENSERS AND BOILERS

14

PACKING AND SUPPORTING STRUCTURES OPERATION AND MISCELLANEOUS*

96

58 200

•WATER CLEANUP, COMMISSIONING

Figure 2.

INSTRUMENTATION,

CONSTRUCTION LABOR,

Rehtive component costs in heavy water distilhtion


9.

KEYSER ET AL.

Heavy

Water

129

Distillation


GENERATING STATION (3000 MW)

COOLING W A T E R FROM L A K E 35 - 5 0 ° F 150 ppm 0O 125 rn /s

REJECT HEAT TO LAKE * ~ 55 - 7 0 ° F 149 ppm

X

9

3

60°F

90°F

HEATING OUT

COOLING

HEATING IN

IN

COOLING

OUT

DISTILLATION PLANT

S E V E R A L HUNDRED TOWERS W O R K I N G V O L U M E - 3 0 000 m F O R 400 T O N N E S / Y E A R 3

\\%

D 0 2

τ

FINISHER

Τ PRODUCT (45 Kg/h ) REACTOR GRADE

Figure 3.

0 0 £

Concept for a heavy water distillation plant using waste heat from a generating station. 400 tonne/year D 0 plant. 2


SEPARATION OF HYDROGEN ISOTOPES


130

t u r e and p r e s s u r e regime had not been developed com mercially. A t h e o r y o f the p r o c e s s p r e d i c t e d t h a t a u s e f u l degree o f s e p a r a t i v e power c o u l d be a c h i e v e d u s i n g simple p a r a l l e l s h e e t s c a r r y i n g downward-moving water f i l m s , w i t h upward-moving vapour between them. The p a c k i n g developed a t the Research D i v i s i o n i s based on t h i s concept, and i s shown s c h e m a t i c a l l y i n F i g u r e 4. A s e r i e s of tubes c a r r i e s condensed water t o the tops of the s h e e t s t o f l o w downward over t h e i r s u r f a c e s , w h i l e vapour moves up between them. The s h e e t s used t o date have been n y l o n f a b r i c under t e n s i o n , w i t h a s u r f a c e treatment t o improve the u n i f o r m i t y o f the water f i l m . The performance of t h i s module i s expected t o depend on the two mass and d e u t e r i u m f l o w parameters, the v e l o c i t y , and the d i f f u s i o n time i n each phase. P a c k i n g Performance -

Experimental

The b a s i c mechanism which causes i s o t o p i c s e p a r a t i o n i n d i s t i l l a t i o n systems i s the d i f f e r e n c e i n v a pour p r e s s u r e of H 0 and HDO, a t the same temperature. In d i s t i l l a t i o n , the r a t i o o f the e q u i l i b r i u m vapour p r e s s u r e s of H 0 and HDO i s c a l l e d a, as shown i n F i g u r e 5. 2

2

P

a(T) =

H- 0 , HDO 1

t y p i c a l l y , α i s around 1.08 i n t h i s temperature r e g i o n , r i s i n g a t lower temperatures towards 1.10.

We can gauge the performance by measuring the i s o t o p i c enrichment o f the water f l o w i n g out the bottom Bottonw the d e p l e t i o n o f the vapour l e a v i n g at. the top Or/op' t a k i n g the n a t u r a l l o g a r i t h m o f t h e i r r a t i o and d i v i d i n g by the n a t u r a l l o g a r i t h m of the average α v a l u e f o r the system; t h i s i s a measure of the num ber o f s t a g e s , each c a r r y i n g out a s e p a r a t i o n o f a, i n the column. The number o f such s t a g e s per metre of column h e i g h t i s a performance parameter f o r the pack ing : ~ In In [I ^Bottomjl / H l n a , where Η i s Ν (number of stages/m) « c

C

\ T"Top o p // the column h e i g h t . C u r r e n t e x p e r i m e n t a l v a l u e s o f Ν run from 2.5 t o over 3 s t a g e s per metre. U s i n g t h i s parameter, and the c o r r e s p o n d i n g mass flow r a t e , we can e s t i m a t e the t o t a l s u r f a c e a r e a o f the s h e e t s r e q u i r e d t o produce 400 tons of D 0 per year. The f i g u r e a l s o shows the " e a r n i n g power" o f our e x p e r i m e n t a l p a c k i n g and the t h e o r e t i c a l l i m i t t o i t as a f u n c t i o n o f the F-number ( r e l a t e d t o the mass 2


Heavy

Water

Distillation


KEYSER ET AL.

Figure 4.

Parallel plate packing module (without glass envelope)


132


SEPARATION O F HYDROGEN ISOTOPES

Figure 5.

Earning power of two experimental packings


9.

KEYSER ET AL.

Heavy

Water

Distillation

133

flow) i n t h e system. Each square metre o f p a c k i n g can be c o n s i d e r e d t o produce about 0 . 2 k i l o g r a m s ( - 4 0 d o l l a r s worth) o f D 0 p e r y e a r . The upper c u r v e i s t h e p r e d i c t e d maximum e a r n i n g power f o r t h i s type o f packi n g system. As you can see, o u r r e s u l t s w h i l e showing t h a t t h e system works, c a n s t i l l be c o n s i d e r a b l y improved. The t r i a n g l e s show more r e c e n t work w i t h a d i f f e r e n t m a t e r i a l ; as o u r u n d e r s t a n d i n g o f t h e d e s i r a b l e and u n d e s i r a b l e f e a t u r e s o f t h e s e m a t e r i a l s grows, we b e l i e v e we c a n come s t i l l c l o s e r t o t h e t h e o r e t i cal limit. T h i s would b r i n g our heavy water c o s t e s t i m a t e down towards $2 0 0 / k g . F u r t h e r work on a l a r g e r s c a l e would a l s o be r e q u i r e d t o study how w e l l t h e p a c k i n g u n i t c a n be s c a l e d up t o an i n d u s t r i a l l y useful size.


2

Summary I c a n summarize what we have l e a r n e d as f o l l o w s : r e c e n t work a t t h e R e s e a r c h D i v i s i o n has demonstrated the f e a s i b i l i t y o f p e r f o r m i n g heavy water s e p a r a t i o n by d i s t i l l a t i o n a t low t e m p e r a t u r e , u s i n g a newly dev e l o p e d p a c k i n g system. We b e l i e v e t h a t t h e c u r r e n t performance can be c o n s i d e r a b l y improved, and t h a t t h e c o s t o f heavy water from d i s t i l l a t i o n can be made comp a r a b l e t o t h a t from t h e c u r r e n t p r o d u c t i o n method. T e s t s on a l a r g e r s c a l e w i l l be r e q u i r e d t o demons t r a t e t h a t t h i s performance can be s u c c e s s f u l l y s c a l e d up t o an i n d u s t r i a l l y u s e f u l d e g r e e . Abstract A review of distillation as a heavy water separation method has identified a r e a s o f h i g h c o s t : energy and p a c k i n g material. A t t e m p e r a t u r e s around 2 5 ° C , l a r g e amounts o f energy are a v a i l a b l e as waste heat from t h e r m a l g e n e r a t i n g s t a t i o n s . I f a s u i t a b l e low- c o s t p a c k i n g can be d e v e l o p e d f o r use i n this temperature r e g i o n , distillation c o u l d become e c o n o m i c a l l y competitive. Results of theoretical p a c k i n g work, and e x p e r i ments on a p r o t o t y p e p a c k i n g f o r heavy water p l a n t s are d i s c u s s e d . RECEIVED August 30, 1977


Heavy Water Distillation

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