Separation of Hydrogen Isotopes

5 Heavy Water Production by Isotopic Exchange between Hydrogen and Methylamine M. BRIEC, J. RAVOIRE, M. ROSTAING, and E . ROTH

Downloaded by UNIV OF TEXAS AT AUSTIN on August 27, 2017 | http://pubs.acs.org Publication Date: June 1, 1978 | doi: 10.1021/bk-1978-0068.ch005

Commissariat à l'Energie Atomique, Division de Chimie, Centre D'Études Nucléaires De Saclay, République Française B. LEFRANCOIS Sté. Chimique des Charbonnages

Isotopic exchange between water and hydrogen sulphide was the f i r s t chemical process used on a large scale for the production of heavy water, and is still employed in the big Canadian plants. Isotopic exchange between liquid ammonia and hydrogen, discovered by Wilmarth and Dayton(1) which offers a much higher separation factor than the former process, has been studied in French CEA laboratory, since 1956, and developed using available sources of hydrogen in the ammonia industry. However, to extract deuterium from hydrogen, especially from NH3 synthesis gases, Bar-Eli and Klein (2) then J. Ravoire and E . Rochard (3 and 4)studied the exchange between hydrogen and a primary or secondary amine, catalysed by an alkaline derivative of the amine used. Kinetic measurements showed that the exchange is much faster than that between hydrogen and ammonia, the separation factors being similar (7 at - 6 0 ° C and 3.5 at +30° C). The hydrogen-monomethylamine system (MMA) can thus be used for productions of the same order as those of the NH -H exchange process, as shown by detailed studies on a laboratory, p i l o t unit and plant project scale. 3

2

Laboratory Studies I t is easy t o imagine the use o f the amine-H exchange in a dual-temperature process, t u r n i n g t o account the v a r i a t i o n in the separation f a c t o r α with temperature. Once the α values were determined the p o i n t s t o be s t u d i e d were as f o l l o w s : 2

1. Exchange K i n e t i c s . The exchange is c a t a l y s e d by a l k a l i n e methylamides, the most e f f i c i e n t being C H 3 N H K ; the chemical r a t e is much f a s t e r than in the case o f ammonia under the same temperature and c a t a l y s t c o n c e n t r a t i o n c o n d i t i o n s .

© 0-8412-0420-9/78/47-068-071$05.00/0

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

72

SEPARATION O F H Y D R O G E N ISOTOPES

2. S o l u b i l i t y o f Hydrogen. Hydrogen is more s o l u b l e in monomethylamine (MMA) than in ammonia (2.5 times a t -50° C ) . T h i s helps t o speed up the exchange r a t e .


3. Thermal S t a b i l i t y o f the C a t a l y s t . Potassium methylamide is s t a b l e a t low temperatures, but in s o l u t i o n a t high temperature i t is thermolysed and converted t o a potassium s a l t of Ν,Ν'-dimethylformamidine, s o l u b l e in MMA. By long duration t e s t s in p i l o t u n i t s , the s t a b i l i t y was proved s a t i s f a c t o r y under present working c o n d i t i o n s . 4. S o l u b i l i t y o f the C a t a l y s t . I t s s o l u b i l i t y decreases as the temperature r i s e s . In the presence o f hydrogen the con c e n t r a t i o n o f potassium methylamide is l i m i t e d by the formation of potassium hydride: CH NHK + H 3

2

*

>

CH NH 3

2

+ HK.

The methylamide concentration a t e q u i l i b r i u m is i n v e r s e l y p r o p o r t i o n a l t o the hydrogen pressure and is about 0.1 mol/kg amine f o r a hydrogen pressure o f 50 bars. The e q u i l i b r i u m depends l i t t l e on temperature; i t is s e t up slowly and the hydride formation r a t e is bound up with the nature o f the con tainer walls. 5. Adjuvant E f f e c t o f Trimethylamine (TMA). I t was discovered t h a t a n o n - c a t a l y t i c a d d i t i v e t o the c a t a l y s t can speed up the exchange r a t e ; thus, a m u l t i p l i c a t i o n f a c t o r o f 1.7 was obtained by a d d i t i o n o f about 7% (molar) o f TMA t o the MMA. P i l o t Plant

Tests

The c h i e f problem was t o f i n d a h i g h l y e f f i c i e n t contact system. F i n e l y p e r f o r a t e d p l a t e s , with s u i t a b l e heights o f l i q u i d , are s a t i s f a c t o r y s i n c e a 60 cm spacing is enough f o r the p l a t e s t o f u n c t i o n w e l l . P r e l i m i n a r y D r a f t o f a Factory (in c o l l a b o r a t i o n with l a Société Chimique des Charbonnages) 1. D e s c r i p t i o n o f the Process. The r e s u l t s presented here concern research on the p r e l i m i n a r y d r a f t o f a 60 Τ p e r year heavy water p l a n t attached t o a 1000 Τ per day ammonium s y t h e s i s u n i t ; the exchange takes place by a dual temperature process, as mentioned above, t o avoid any chemical transformation o f the amine. As shown on Figure 1, the synthesis mixture (N + 3 H ) , f r e e d o f oxygenated i m p u r i t i e s by hydrogénation (A), dehydration (B) and f i n a l l y by washing in the tower (C), t r a n s f e r s i t s deuterium t o a c u r r e n t o f MMA in a low-temperature TT tower (the only tower t o c o n t a i n a nitrogen-hydrogen mixture, enabling 2


2


B R i E C ET AL.

Hydrogen

and

Methyhmine

Isotopic

A

methanation

TF2

cold

BB'

dehydration

TC2

h o t tower

(2nd

stage) stage)

CC

1

TT

purification transfer

tower

DD'

deamination

TEP

s t r i p p i n g tower

(1st stage)

TEN

enrichment

TC

hot toner ( 1 s t sta^e)

tower

TH

humidification

tower

Exchange

(2nd

r

compressor(2nd

F

f i n i s h i n g plant

abed

hydrogen

efgh

liquid

stage)

loop (1st stage)

loop (1st stage)

(1st stage)

tower

(1st stage)

Figure 1.

Heavy water production unit


74

SEPARATION O F

HYDROGEN ISOTOPES


p u r e h y d r o g e n t o be u s e d in t h e r e s t o f t h e s y s t e m ) . After r e c u p e r a t i o n of the methylamine t h e gas is s e n t t o t h e ammonia s y n t h e s i s u n i t . The l i q u i d is f e d t h r o u g h a s e r i e s o f h o t and c o l d t o w e r s in w h i c h i t a c c u m u l a t e s d e u t e r i u m a g a i n s t t h e c o u n t e r - f l o w o f a h y d r o g e n l o o p . The p r o c e s s i n v o l v e s two e n r i c h m e n t s t a g e s b e f o r e a f i n i s h i n g p l a n t w h i c h can be e i t h e r a h y d r o g e n r e c t i f i c a t i o n o r a w a t e r r e c t i f i c a t i o n a f t e r e x c h a n g e b e t w e e n t h e w a t e r and t h e amine. 2. D e t e r m i n a t i o n o f W o r k i n g C o n d i t i o n s . The p r e s s u r e and t e m p e r a t u r e c o n d i t i o n s were d e t e r m i n e d on t h e b a s i s o f p h y s i c o - c h e m i c a l f a c t o r s s u c h as t h e r m a l s t a b i l i t y o f t h e c a t a l y s t , s o l u b i l i t y in t h e p r e s e n c e o f h y d r o g e n , f a c t o r s r e l a t i v e t o a s s o c i a t i o n w i t h ammonia s y s t h e s i s s u c h as t h e a v a i l a b l e p r e s s u r e o f f e e d - g a s , and e c o n o m i c c o n s i d e r a t i o n s . The c h o i c e o f t h e p r e s s u r e is t h e r e s u l t o f many c o m p r o m i s e s ; t h e p r e s s u r e must be l o w enough t o a l l o w a r e a s o n a b l e c a t a l y s t c o n c e n t r a t i o n , so as t o o b t a i n good p l a t e e f f i c i e n c i e s and t h e use o f more c o n v e n t i o n a l m a t e r i a l t h a n in h i g h p r e s s u r e ammonia s y n t h e s i s ; on t h e o t h e r hand i t must be f a i r l y h i g h in o r d e r t o a v o i d undue gas r e c o m p r e s s i o n and t o m a i n t a i n t h e l o w MMA v a p o u r p r e s s u r e w h i c h a f f e c t s t h e e n e r g y consump t i o n (amine s a t u r a t i o n o f t h e gas and c o n d e n s a t i o n o f t h i s amine b e t w e e n t h e h o t a n d c o l d t o w e r s ) . The h i g h e r t e m p e r a t u r e c h o s e n is l o w enough t o r e t a i n s a t i s f a c t o r y b e h a v i o u r o f t h e catalyst. I n c h o o s i n g t h e c o l d t e m p e r a t u r e i t was n e c e s s a r y t o d e c i d e b e t w e e n l o w e r f l o w s a t l o w e r t e m p e r a t u r e on t h e one hand and l e s s f a v o u r a b l e k i n e t i c s , h i g h e r c o o l i n g c o s t s and more e l a b o r a t e s t e e l s on t h e o t h e r . A f t e r a d e t a i l e d e c o n o m i c s t u d y a c o l d t e m p e r a t u r e o f -50° C was a d o p t e d . 3. Economy o f t h e P r o c e s s . T a b l e I shows t h e m a i n s p e c i f i c a t i o n s o f the u n i t : dimensions o f the i s o t o p i c exchange towers, d i s t r i b u t i o n of investments according t o the d i f f e r e n t i t e m s , and p r i n c i p a l c o n s u m p t i o n s . Where i n v e s t m e n t s a r e c o n c e r n e d t h e two m a i n i t e m s o f e x p e n s e a r e t h e h e a t t r a n s f e r e q u i p m e n t ( c o o l i n g s t a t i o n + e x c h a n g e r s + i n s u l a t i o n ) and t h e exchange towers equipped w i t h t h e i r p l a t e s . The c a p i t a l c o s t f o r t h i s p r o c e s s r e p r e s e n t s $460 p e r k g o f h e a v y w a t e r p e r y e a r . The p r e s s u r e b e l o w t h a t o f t h e s y n t h e s i s c a n i n v o l v e an i n c r e a s e o f gas r e c o m p r e s s i o n c o s t s and o f c e r t a i n c o o l i n g costs. I n s p i t e o f a l l t h e s e f a c t o r s the energy expenses are lower than those o f o t h e r p r o c e s s e s , H S - H 0 f o r example, u n d e r t h e e c o n o m i c c o n d i t i o n s p u b l i s h e d f o r t h e P o r t Hawkesbury and B r u c e s i t e s {5) (where 60% o f t h e e n e r g y is r e c o v e r e d ) . The c o n s u m p t i o n s f o r o u r p r o c e s s a r e 0.7 Τ s t e a m (60 p s i ^ k g D 0 and 700 k W h A g a g a i n s t 11 Τ s t e a m (330 p s i ) / k g D 0 and 550 kWh/kg. 2

2

2

2



1000 T/day N2 + 3H2 (H2) = 1,4

piping + valves e l e c t r i c i t y + control compressors + pumps b u i l d i n g , engineering H2preparation tanks + catalyst preparation 2nd stage dehydration + deamination c o o l i n g s t a t i o n + exchanger + insulation towers + p l a t e s 18, 46% 17, 34%

1 1 , 30% 13, 60%

r

10, 00%

7, 18% 4,44% 5, 59%

1 1 , 98%

Coolant water:

Steam

o f which

Electricity

1164 m /hour

2292 : 1 s t stage com pressors 2245 : c o o l i n g s t a t i o n 6 tons/hour

5893 kWh

Consumptions

75 73,8 51 55,5 2 χ 49,8 28

2,35 2,44 2,44 2,94 1,22 1,47 Principal

h e i g h t (m)

130 ppm

Diameter (m)

Deuterium content o f the hydrogen:

t r a n s f e r tower s t r i p p i n g tower enrichment tower hot tower c o l d tower hot tower

D i s t r i b u t i o n o f investments (%)

2nd tower

1 s t stage:

1000 (25 f o r the 1 s t stage)

Tower dimension

Enrichment:

Loop gas flow-rate/supply gas f l o w - r a t e

Feeding r a t e :

Main s p e c i f i c a t i o n s o f the heavy water p r o d u c t i o n p l a n t (production: 58,5 T/year)

TABLE I


76

SEPARATION OF HYDROGEN ISOTOPES

Conclusion The exchange r a t e s in the H -MMA system are extremely f a s t in view o f the a c t i v i t y o f the c a t a l y s t and the s o l u b i l i t y of hydrogen. The development o f a s u f f i c i e n t l y s t a b l e and s o l u b l e c a t a l y s t means t h a t the c o n s t r u c t i o n o f heavy water p l a n t s is f e a s i b l e under f i n a n c i a l l y competitive c o n d i t i o n s . The production o f appreciable q u a n t i t i e s o f heavy water from the hydrogen used f o r synthesis o f ammonia is now p o s s i b l e in many c o u n t r i e s . I f the use o f hydrogen in the future as an energy v e c t o r is developed, the MMA-hydrogen process w i l l p r o f i t from the s c a l e e f f e c t in the b u i l d i n g o f u n i t s , and the low p r i c e s o f thermal and e l e c t r i c a l s u p p l i e s a v a i l a b l e a t the proximity o f hydrogen chemical-nuclear generators.


2

Literature Cited (1) (2) (3) (4) (5)

Wilmarth, W . , . and Dayton, J.C., JACS (1953) 75.4553. B a r - E l i , Ε . , and Klein, F . S . , J. Chem. Soc. (1962) 3083. Rochard, E., and Ravoire, J., Rapport C . E . A . (1969) 3835. Rochard, Ε., and Ravoire, J., J. Chem. Phys. (1971) 1183. Rae, H . K . , "Heavy Water", Atomic Energy of Canada Limited Report No. 3866, Geneva, 1971.

RECEIVED October 17, 1977


Separation of Hydrogen Isotopes

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