Separation of hydrogen isotopes

Separation of Hydrogen Isotopes H o w a r d K . Rae, EDITOR Publication Date: June 1, 1978 | doi: 10.1021/bk-1978-0068...

Author: Howard K. Rae

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Separation of Hydrogen Isotopes H o w a r d K . Rae,

EDITOR

Publication Date: June 1, 1978 | doi: 10.1021/bk-1978-0068.fw001

Atomic Energy of Canada, Limited

A symposium co-sponsored by the Physical Chemistry Division of the Chemical Institute of Canada and the Canadian Society for Chemical Engineering at the 2nd Joint Conference of the Chemical Institute of Canada and the American Chemical Society, Montreal, May 30-June 1,

1977.

ACS SYMPOSIUM SERIES 68

AMERICAN

CHEMICAL

WASHINGTON, D. C.

SOCIETY 1978

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


Library of Congress CIP Data Main entry under title: Separation of hydrogen isotopes. (ACS symposium series; 68 ISSN 0097-6156) Includes bibliographies and index. 1. Deuterium oxide—Congresses. 2. Hydrogen—Isotopes—Congresses. 3. Isotope separation—Congresses. 4. Heavy water reactors—Congresses. I. Rae, Howard K . , 1925. II. Chemical Institute of Canada. Physical Chemistry Division. III. Canadian Society for Chemical Engineering. IV. Series: American Chemical Society. ACS symposium series; 68. TK9350.S46 ISBN 0-8412-0420-9

661'.08 ASCMC8

68

78-760 1-184 1978

Copyright © 1978 American Chemical Society A l l Rights Reserved. The appearance of the code at the bottom of the first page of each article in this volume indicates the copyright owner's consent that reprographic copies of the article may be made for personal or internal use or for the personal or internal use of specific clients. This consent is given on the condition, however, that the copier pay the stated per copy fee through the Copyright Clearance Center, Inc. for copying beyond that permitted by Sections 107 or 108 of the U.S. Copyright Law. This consent does not extend to copying or transmission by any means—graphic or electronic—for any other purpose, such as for general distribution, for advertising or promotional purposes, for creating new collective works, for resale, or for information storage and retrieval systems. The citation of trade names and/or names of manufacturers in this publication is not to be construed as an endorsement or as approval by ACS of the commercial products or services referenced herein; nor should the mere reference herein to any drawing, specification, chemical process, or other data be regarded as a license or as a conveyance of any right or permission, to the holder, reader, or any other person or corporation, to manufacture, reproduce, use, or sell any patented invention or copyrighted work that may in any way be related thereto. PRINTED IN T H E UNITED

STATES

OF

AMERICA


ACS Symposium Series


Robert F. G o u l d , Editor

Advisory Board Kenneth B. Bischoff

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Donald G. Crosby

John B. Pfeiffer

Jeremiah P. Freeman

Joseph V. Rodricks

E. Desmond Goddard

F. Sherwood Rowland

Jack Halpern

Alan C. Sartorelli

Robert A. Hofstader

Raymond B. Seymour

James P. Lodge

Roy L. Whistler

John L. Margrave

Aaron Wold



FOREWORD The ACS SYMPOSIUM SERIES was founded in 1974 to provide

a medium for publishing symposia quickly in book form. The format of the SERIES parallels that of the continuing ADVANCES IN CHEMISTRY SERIES except that in order to save time the papers are not typeset but are reproduced as they are submitted by the authors in camera-ready form. As a further means of saving time, the papers are not edited or reviewed except by the symposium chairman, who becomes editor of the book. Papers published in the ACS SYMPOSIUM SERIES are original contributions not published elsewhere in whole or major part and include reports of research as well as reviews since symposia may embrace both types of presentation.


Publication Date: June 1, 1978 | doi: 10.1021/bk-1978-0068.pr001

PREFACE '"phis volume describes process development and plant performance -•- for the separation of deuterium from hydrogen, tritium from hydrogen, and tritium from deuterium. All of the important processes that are recognized today are included although not necessarily in equal detail or appropriate relative emphasis. Nonetheless, this volume forms a valuable, up-to-date report on the status of hydrogen isotope separation. Obviously heavy water production is essential to a program of nuclear power production that is based on natural uranium-fueled and heavy water-moderated reactors. Canada and India have selected this route to nuclear power, and both are establishing an industrial heavy water production capability. The Canadian design of a heavy water power reactor, the C A N D U design, requires 0.85 Mg of heavy water per electrical M W of installed capacity. The heavy water is not consumed by reactor operation so that the demand for heavy water is set by the rate at which new nuclear power stations are built. A small make-up of less than 1% of the inventory per year is needed to replace losses by leakage. With 4000 M W of nuclear-electric generating capacity in operation and with 15,000 M W committed for operation by 1988, Canada has a very substantial demand for heavy water. Several large production plants are in operation and more are being built. Thus, it is not surprising that one-half of the papers in this volume are from Canada; the chapters report on plant performance and on the development of alternative processes to the established water-hydrogen-sulfide exchange method. Tritium is produced by neutron capture in deuterium and by uranium fission. Thus, heavy water in reactors contains a gradually increasing concentration of T D O , and aqueous wastes from any water-cooled reactor and from fuel reprocessing contain T H O . Tritium recovery from these sources is desirable to minimize the release of tritium to the biosphere, and such tritium separation plants will become increasingly common. Tritium separation on a much larger scale will be necessary when fusion reactors are developed successfully. Atomic Energy of Canada, Limited Ontario, Canada December, 1977

HOWARD K . R A E

vii


1 Selecting Heavy Water Processes H . K . RAE

Publication Date: June 1, 1978 | doi: 10.1021/bk-1978-0068.ch001

Chalk River Nuclear Laboratories, Atomic Energy of Canada Ltd., Chalk River, Ontario, Canada K0J 1J0

Hundreds o f methods have been proposed f o r the p r o d u c t i o n o f heavy water, b u t o n l y a few show any real promise. This paper will d i s c u s s the important characteristics which define relative process a t t r a c t i v e n e s s and will compare s e v e r a l chemical exchange and distillation processes. In this way it will show why the GS ( G i r d l e r - S u l f i d e ) process has dominated heavy water p r o d u c t i o n f o r 25 y e a r s . I t will a l s o review the c u r r e n t s t a t u s o f s e v e r a l promising alternatives. A brief review o f heavy water process development and heavy water p r o d u c t i o n will s e t the background. Deuterium was first separated by the fractional e v a p o r a t i o n o f hydrogen in 1932 by Urey, Brickwedde and Murphy (1). Over the next few years water electrolysis, water distillation and hydrogen distillation were i n v e s t i g a t e d as hydrogen-deuterium s e p a r a t i o n methods. During World War II in the USA, and t o l e s s e r extent in Germany, there was a very l a r g e e f f o r t t o evaluate and develop methods f o r heavy water p r o d u c t i o n ( 2 , 3 ) . Two p r o m i s i n g chemical exchange p r o cesses were defined—water-hydrogen and w a t e r - h y d r o g e n - s u l f i d e . The former was the b a s i s o f the first heavy water p r o d u c t i o n a t a reasonable c o s t from industrial s c a l e p l a n t s . The latter eventu ally became known as the GS p r o c e s s , and the b a s i s o f all l a r g e heavy water p l a n t s . Throughout the f i f t i e s and the s i x t i e s hundreds o f methods were c o n s i d e r e d , dozens were i n v e s t i g a t e d in the l a b o r a t o r y and in p i l o t p l a n t s , but o n l y a handful were used in p r o d u c t i o n (4_) . Major research and development to i n v e s t i g a t e heavy water p r o cesses (A) was done in n e a r l y a l l the c o u n t r i e s t h a t b u i l t p r o t o type heavy water power r e a c t o r s : Canada, F r a n c e , Germany, I n d i a , I t a l y , Sweden, S w i t z e r l a n d and U n i t e d Kingdom. Despite this l a r g e e f f o r t i n v o l v i n g hundreds o f man-years by chemists, p h y s i c i s t s and e n g i n e e r s , no other method has reached the stage where it can challenge the GS process as the major source o f heavy water.

©

0-8412-0420-9/78/47-068-001$10.00/0


2

SEPARATION O F

HYDROGEN

ISOTOPES


Heavy Water Plants So f a r p l a n t o p e r a t i n g experience has been obtained with f i v e processes, as shown in Table I. Monothermal water-vapour-hydrogen exchange, with the e n r i c h e d hydrogen r e f l u x p r o v i d e d by e l e c t r o l y s i s , was used a t T r a i l in Canada (5) and in Norway (5); the l a t t e r p l a n t i s s t i l l in o p e r a t i o n . An improved v e r s i o n o f this process using exchange with l i q u i d water i s d e s c r i b e d by Hammerli (<S) and r e f e r r e d to as combined e l e c t r o l y s i s and chemical exchange (CECE). The o r i g i n a l p l a n t in Norway simply used e l e c t r o l y s i s to produce heavy water. Water-hydrogen exchange u n i t s were being added to the p l a n t in 1943 d u r i n g the German occupation o f Norway when it was destroyed by A l l i e d r a i d s ; the p l a n t was r e b u i l t and expanded a f t e r the war. Water distillation was used as a p r o d u c t i o n method b r i e f l y in three s m a l l p l a n t s during World War II (2). In the USA in 1950 there was a need f o r a l a r g e heavy water p r o d u c t i o n c a p a c i t y and the GS process was chosen as the p r e f e r red method (7). Two 500 Mg/a p l a n t s were b u i l t . One t h i r d of the Savannah R i v e r p l a n t i s s t i l l o p e r a t i n g a f t e r twenty-five y e a r s . The GS process was adopted in Canada in the m i d - s i x t i e s to supply the CANDU* power r e a c t o r program. Current o p e r a t i n g c a p a c i t y in Canada i s 1600 Mg/a in three p l a n t s and three more are being b u i l t (8). In I n d i a a 100 Mg/a GS p l a n t i s a t an advanced stage o f c o n s t r u c t i o n . F i v e small hydrogen distillation p l a n t s have been operated. The p l a n t in I n d i a , of 13 Mg/a c a p a c i t y , continues in o p e r a t i o n (9). The s i z e of the Russian p l a n t has not been reported; it a l s o probably continues in o p e r a t i o n . A 25 Mg/a ammonia-hydrogen p l a n t operated f o r s e v e r a l years in France (10) . The first o f three Indian ammonia-hydrogen p l a n t s , each about 65 Mg/a, (11,12) i s being commissioned. Heavy Water Production The cumulative world production o f heavy water to May 1977 i s 11 Gg (excluding USSR and China). This i s d i s t r i b u t e d by process as f o l l o w s : GS - 93.2%, H2O/H2 exchange - 4.4%, H D i s t i l l a t i o n - 1.3%, NH /H exchange - 0.9% and H 0 D i s t i l l a t i o n - 0.2%. By country, 59% has been produced in the USA, 35% in Canada, 4% in Norway, 1% in India and 1% in France. By about 1982 the Canadian heavy water p r o d u c t i o n c a p a c i t y will be 4000 Mg/a. This will c o n s i s t o f the three o p e r a t i n g p l a n t s — P o r t Hawkesbury in Nova S c o t i a with a nominal design c a p a c i t y o f 400 Mg/a, Bruce A in Ontario o f 800 Mg/a and Glace Bay in Nova S c o t i a o f 400 Mg/a—and three p l a n t s now being b u i l t 2

3

2

2

*Canada Deuterium Uranium


1.

RAE

Selecting

Heavy

Water

3

Processes

B r u c e Β a n d D, e a c h o f 800 Mg/a n o m i n a l c a p a c i t y , a n d L a P r a d e in Quebec o f 800 Mg/a. T h i s c a p a c i t y i s e x p e c t e d t o meet demands u n t i l a b o u t 1990 (8).


General Process

Characteristics

Before c o n s i d e r i n g the v a r i o u s i n d i v i d u a l processes and c o m p a r i n g t h e i r c h a r a c t e r i s t i c s , it i s i m p o r t a n t t o l o o k a t t h e g e n e r a l nature o f the deuterium-hydrogen s e p a r a t i o n problem which d e r i v e s from the low d e u t e r i u m c o n t e n t o f a l l p o t e n t i a l s o u r c e materials. The c o n s e q u e n c e s o f this f o r a n y p r o c e s s c a n be r e a d i l y s e e n in t e r m s o f i t s a f f e c t o n t h e d e s i g n o f GS h e a v y water p l a n t s . T h i s v e r y l o w v a l u e o f t h e d e u t e r i u m - t o - h y d r o g e n r a t i o in n a t u r e *of a b o u t 150 ppm i s t h e m a i n f a c t o r r e s p o n s i b l e f o r t h e h i g h c o s t o f heavy water. I t i s necessary t o process a t l e a s t 8000 m o i s o f f e e d p e r m o l o f p r o d u c t f o r a l l p r o c e s s e s , a n d f o r t h e GS p r o c e s s t h e r a t i o o f f e e d t o p r o d u c t i s n e a r l y 40,000 t o 1. R e a c t o r g r a d e h e a v y w a t e r i s 99.75 m o l % D2O. Thus, the o v e r a l l c o n c e n t r a t i o n r a t i o from feed t o p r o d u c t i s about 3 x l O . T h i s means t h a t h u n d r e d s o f s e p a r a t i v e e l e m e n t s in s e r i e s a r e n e e d e d t o go f r o m n a t u r a l w a t e r t o r e a c t o r g r a d e h e a v y w a t e r . The c o m b i n a t i o n o f a v e r y l a r g e f e e d f l o w a n d a v e r y l a r g e number o f s e p a r a t i v e e l e m e n t s means t h a t h e a v y w a t e r p l a n t s a r e v e r y l a r g e in r e l a t i o n t o most o t h e r c h e m i c a l p l a n t s . A s a consequence heavy water i s an e x t r e m e l y c a p i t a l i n t e n s i v e p r o d u c t . F i g u r e 1 shows t h e G l a c e Bay Heavy Water P l a n t . The first s t a g e t o w e r s a r e a b o u t 7 m in d i a m e t e r a n d 60 m h i g h . F e e d w a t e r goes t o s i x o f t h e s e t o w e r s in p a r a l l e l a t a t o t a l f l o w r a t e o f a b o u t 40,000 m /d ( 1 0 US g a l l o n s / d a y ) . T o t a l h e a t r e j e c t i o n b y t h e p l a n t i s 300 t h e r m a l MW, m a i n l y b y c o o l i n g t o w e r s a t t h e r i g h t o f the photograph. The e f f l u e n t w a t e r i s c o o l e d b y s p r a y m o d u l e s in the e f f l u e n t channel a t the l e f t o f the photograph. The c o m b i n a t i o n o f a d i l u t e f e e d a n d a l a r g e o v e r a l l c o n c e n t r a t i o n r a t i o means t h a t c a s c a d i n g c o n f e r s v e r y i m p o r t a n t a d v a n tages. F i g u r e 2 i l l u s t r a t e s three cascade arrangements f o r a t y p i c a l GS p r o c e s s d e s i g n . I n e a c h c a s e t h e p r o d u c t i s 2 0 % D2O. I n t h e t w o - s t a g e a r r a n g e m e n t t h e first s t a g e g i v e s a t e n - f o l d enrichment. I n the t h r e e - s t a g e arrangement the first stage gives a f o u r - f o l d enrichment w h i l e the second stage p r o v i d e s a f u r t h e r s i x - f o l d enrichment. The r e l a t i v e t o w e r v o l u m e s show a 30% decrease f o r the two-stage case and a f u r t h e r 10% r e d u c t i o n f o r t h e t h r e e - s t a g e c a s e . The change in h e a v y w a t e r i n v e n t o r y i s more d r a m a t i c . I n the s i n g l e stage case the i n v e n t o r y approaches t h e a n n u a l p l a n t c a p a c i t y . F o r two s t a g e s this i s r e d u c e d b y a n o r d e r o f magnitude and f o r t h r e e stages i s reduced f u r t h e r by a f a c t o r o f two. A d d i n g more s t a g e s i s n o t w o r t h w h i l e s i n c e e a c h s t a g e adds e x t r a e q u i p m e n t a n d i n c r e a s e s p l a n t c o m p l e x i t y . I n a l l c a s e s t h e first s t a g e d o m i n a t e s p l a n t c o s t s . Thus, when c o m p a r i n g p r o c e s s e s it i s g e n e r a l l y o n l y this first stage 6

3

7


4

SEPARATION OF HYDROGEN ISOTOPES

Table I HEAVY WATER PRODUCTION PLANTS

First Plant

Operating Capacity 1977 Mg/a

19 34

30

Number o f P l a n t s Process

H 0/H 2

Country

P

e r a

. 9

l n

Being Built

NOR.,* CAN

2

H2O D i s t n


Shut Down

GS H2 D i s t n

USA

1944

USA, CAN, IND

1952

1800

W. GER, RUS FR, SWIT, IND

1958

>13

NH3/H2

FR,

* electrolysis

1968

IND

o n l y u n t i l 1948.

Figure 1.

Glace Bay heavy water plant


1.

RAE

Selecting

Heavy

Water

5

Processes


t h a t needs t o be c o n s i d e r e d . F i g u r e 3 i l l u s t r a t e s a t h r e e - s t a g e GS p r o c e s s d e s i g n s h o w i n g t h e r e l a t i v e m o l a r f l o w r a t e s o f w a t e r a n d h y d r o g e n s u l f i d e in e a c h s t a g e a n d t h e v a r i o u s i n t e r s t a g e c o n n e c t i o n s . The r e l a t i v e tower s i z e s a r e a l s o i l l u s t r a t e d . Because o f t h e v e r y l a r g e f l o w s r e q u i r e d in t h e first s t a g e it i s n e c e s s a r y t o have s e v e r a l l a r g e t o w e r s in p a r a l l e l ; t h r e e a r e shown in this e x a m p l e . The p r o d u c t f r o m t h e t h i r d GS s t a g e i s 2 0 % D2O. I t i s b r o u g h t t o r e a c t o r grade by water distillation w h i c h i s a much s i m p l e r p r o cess o p e r a t i n g a t low p r e s s u r e w i t h a low p o t e n t i a l f o r leakage. T h i s f i n a l e n r i c h m e n t u n i t r e p r e s e n t s l e s s t h a n 5% o f t h e t o t a l p l a n t i n v e s t m e n t . N e a r l y a l l h e a v y w a t e r p l a n t s have b e e n designed w i t h d i f f e r e n t p r o c e s s e s f o r p r i m a r y enrichment and f i n a l enrichment (13). Types o f P r o c e s s e s Table I I i s a p a r t i a l l i s t o f the types o f processes t h a t have been c o n s i d e r e d f o r heavy w a t e r p r o d u c t i o n . D i s t i l l a t i o n i s one o f t h e s i m p l e s t . However, t o w e r volume i s e x c e s s i v e f o r a l l p o t e n t i a l working substances except hydrogen itself. The most p r o m i s i n g p r o c e s s e s a r e b a s e d o n c h e m i c a l e x c h a n g e . I r r e v e r s i b l e p r o c e s s e s l i k e d i f f u s i o n have h i g h e n e r g y c o s t s and v e r y l a r g e membrane o r b a r r i e r a r e a s a r e n e e d e d . B o t h e l e c t r o l y s i s and g r a v i t a t i o n a l processes, w h i l e o f f e r i n g f a i r l y h i g h s e p a r a t i o n f a c t o r s , a r e v e r y e n e r g y i n t e n s i v e a n d f o r this r e a s o n unattractive. Adsorption processes are not p a r t i c u l a r l y s e l e c t i v e f o r deuterium and t h e r e f o r e r e q u i r e v e r y l a r g e volumes o f a d s o r b e n t . B i o l o g i c a l p r o c e s s e s have t h e same s h o r t c o m i n g s a s a d s o r p t i o n ,

Table I I POSSIBLE HEAVY WATER PROCESSES P r o c e s s Type Distillation C h e m i c a l Exchange Diffusion Electrolysis Gravitational Adsorption Biological Crystallization S e l e c t i v e Photochemical

Status S i z e E x c e s s i v e E x c e p t H2 Most P r o m i s i n g B a r r i e r o r Membrane C o s t s E x c e s s i v e ; High Energy E x c e s s i v e Energy E x c e s s i v e Energy H i g h A d s o r b e n t Volume E x c e s s i v e Volume I m p r a c t i c a l on Large S c a l e Promising i f S e l e c t i v i t y A p p r o a c h e s 10



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

SINGLE STAGE

TWO STAGES

THREE STAGES

RELATIVE TOWER VOLUME

1

0.7

0.6

RELATIVE PROCESS INVENTORY

1

0.12

Figure 2.

0.05

GS process cascades

Figure 3.

GS flowsheet


1.

RAE

Selecting

Heavy

Water

7

Processes


and h e n c e r e q u i r e t h e p r o d u c t i o n o f e x c e s s i v e amounts o f deuterium-depleted organisms. C o u n t e r c u r r e n t c r y s t a l i z a t i o n systems ( i c e - w a t e r ) c a n con c e i v a b l y h a v e v e r y l a r g e numbers o f s e p a r a t i n g e l e m e n t s in a r e a s o n a b l e volume a n d h a v e a l o w e n e r g y r e q u i r e m e n t ; h o w e v e r , a r e a s o n a b l e p r o c e s s i n g r a t e c a n o n l y be a c h i e v e d w i t h an i m practically small crystal size (14). S e l e c t i v e photochemical processes are a t too e a r l y a stage to assess p r o p e r l y . I f s u c c e s s i s t o be a c h i e v e d f o r d e u t e r i u m hydrogen s e p a r a t i o n , a v e r y h i g h s e l e c t i v i t y i s needed t o o f f s e t t h e d i s a d v a n t a g e o f t h e low n a t u r a l abundance o f d e u t e r i u m . The r e m a i n d e r o f t h e p a p e r will r e v i e w distillation and c h e m i c a l e x c h a n g e p r o c e s s e s in more d e t a i l . Separation Factor One o f t h e m o s t i m p o r t a n t p a r a m e t e r s in d e f i n i n g t h e a t t r a c t i v e n e s s o f a process i s the s e p a r a t i o n f a c t o r . It is d e f i n e d as α = (D/H) /(D/H) Β A

where A a n d Β a r e e n r i c h e d a n d d e p l e t e d s t r e a m s f r o m a s e p a r a t i n g d e v i c e , a r e t w o p h a s e s in p h y s i c a l e q u i l i b r i u m , o r a r e two com p o u n d s in c h e m i c a l e q u i l i b r i u m . The s e p a r a t i o n f a c t o r i s f r e q u e n t l y t h e first p a r a m e t e r t o be d e t e r m i n e d in s t u d y i n g a new h e a v y w a t e r p r o c e s s . Values r a n g e f r o m u n i t y (no s e p a r a t i o n ) t o a b o u t 30 (low t e m p e r a t u r e e l e c t r o l y s i s o f ammonia). Distillation T a b l e I I I compares f o u r distillation p r o c e s s e s based on h y d r o g e n , methane, ammonia a n d w a t e r . I n e a c h c a s e optimum c o n d i t i o n s have been s e l e c t e d . For water the pressure i s h i g h e r than i s used f o r f i n a l enrichment. T h i s i s because the p r o c e s s , as o p t i m i z e d f o r p r i m a r y e n r i c h m e n t , i s b a s e d o n v a p o u r recom p r e s s i o n t o conserve energy. The l o w s e p a r a t i o n f a c t o r s f o r t h e

Table I I I D I S T I L L A T I O N PROCESSES Temperature k Hydrogen Methane Ammonia Water

24 112 2 39 378

Pressure kPa 250 100 100 120

Separation Factor 1.5 1.00 35 1.036 1.024


8

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O F HYDROGEN

ISOTOPES

three substances other than hydrogen are an overwhelming d i s advantage. This can be seen by comparing tower volumes. The r e l a t i v e tower volume i s p r o p o r t i o n a l t o TP" (ot-1) " m~ f o r a given gas v e l o c i t y and HETP (height e q u i v a l e n t to a t h e o r e t i c a l p l a t e ) where Τ i s absolute temperature, Ρ i s pressure, α i s s e p a r a t i o n f a c t o r and m i s the number o f hydrogen atoms p e r molecule. C l e a r l y ( a - l ) ~ dominates this expression. F i g u r e 4 shows this r e l a t i v e tower volume as a f u n c t i o n o f p r e s s u r e . There i s a three t o four orders o f magnitude d i f f e r e n c e in tower volume between the compounds o f hydrogen and molecular hydrogen itself. The c a p i t a l c o s t a s s o c i a t e d with the former as d i s t i l l a t i o n working substance would be f a r too high f o r these t o be a t t r a c t i v e heavy water production processes. However, as already i n d i c a t e d , water distillation i s used e x t e n s i v e l y f o r f i n a l en richment from a D/H r a t i o o f about 0.1. Hydrogen distillation, on the other hand, i s a p o t e n t i a l l y a t t r a c t i v e process. As will be d i s c u s s e d l a t e r , it i s c l o s e t o being competitive with the GS process. 1

2

1


2

Chemical Exchange The GS process i s the archetype o f chemical exchange pro cesses. Deuterium i s t r a n s f e r r e d from a water molecule t o a hydrogen s u l f i d e molecule and v i s a - v e r s a : H 0 + H D S ^ ± H D O + H S. 2

2

The s e p a r a t i o n f a c t o r i s roughly equal t o the e q u i l i b r i u m con s t a n t f o r this r e a c t i o n . At 303 Κ α = 2.33 and a t 403 Κ it i s 1.82. T h i s d i f f e r e n c e i s the b a s i s o f the b i t h e r m a l concentra t i o n process t o be d e s c r i b e d l a t e r . The l a r g e flows and l a r g e energy input have already been d i s c u s s e d q u a l i t a t i v e l y . F o r the GS process the feed r a t e p e r kg D 0 i s 35 Mg and the energy r e q u i r e d per kg D 0 i s 25 GJ o f thermal energy and 700 kWh o f e l e c t r i c a l energy. The three other chemical exchange processes d i s c u s s e d in this paper are water-hydrogen, ammonia-hydrogen and aminehydrogen: 2

2

H 0 + HD ^ HDO + H , NH + H D ^ N H D + H , CH NH + HD^CH NHD + H . 2

2

3

3

2

2

3

2

2

In each case the gas i s hydrogen and the other component i s the liquid. F i g u r e 5 i l l u s t r a t e s the simplest chemical exchange flow sheet - that f o r monothermal exchange as used in Norway o r a t Trail. The l i q u i d feed i s enriched in deuterium as it flows down the tower and encounters p r o g r e s s i v e l y r i c h e r gas. A t the bottom o f the tower the l i q u i d i s t o t a l l y converted t o the gas,


1.

RAE

Selecting

E

Heavy 10

Water

9

Processes

E

ο- 1 0 ' h

S

10 h 4

< oc

1

0

AMMONIA

3

Ρ Η , Ο = Pc D

10

2


Q

2

Ο ÛC

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ONE

ATM -H

10

D

HYDROGEN

OC

0.1 1

10

10

2

10

3

10

4

10

5

PRESSURE, kPa Figure 4.

Relative tower volume for

Figure 5.

distilhtion

Simple monothermal process



10


e x c e p t f o r t h a t f r a c t i o n which forms the f e e d t o h i g h e r s t a g e s in t h e c a s c a d e . T h i s phase c o n v e r t e r i s analogous t o a r e b o i l e r in distillation. P h a s e c o n v e r s i o n i s e x p e n s i v e in m o s t e x c h a n g e systems. However, w a t e r e l e c t r o l y s i s u s e d f o r c o m m e r c i a l h y d r o gen p r o d u c t i o n c a n be a s p e c i a l c a s e . Then h e a v y w a t e r r e c o v e r y becomes a b y p r o d u c t o p e r a t i o n . O t h e r h y d r o g e n p r o d u c t i o n t e c h n i q u e s c a n f u n c t i o n in a s i m i l a r manner ( 1 5 ) . Ammonia-hydrogen e x c h a n g e i s most r e a d i l y a d a p t e d t o monothermal o p e r a t i o n because the heat o f f o r m a t i o n from hydrogen and n i t r o g e n i s r e l a t i v e l y l o w . I n this c a s e ammonia s y n t h e s i s gas i s t h e d e u t e r i u m s o u r c e , so t h e f e e d i s a g a s . The ammonia e x c h a n g e l i q u i d i s n o t o n l y c r a c k e d in t h e p h a s e c o n v e r t e r a t t h e b o t t o m o f t h e e x c h a n g e c o l u m n as in F i g u r e 5, b u t i s a l s o s y n t h e s i z e d f o r r e f l u x a t t h e t o p o f t h e c o l u m n - in this f o r m the monothermal f l o w s h e e t i s c o m p l e t e l y analogous t o a d i s t i l l a t i o n process. S u c h a m o n o t h e r m a l ammonia-hydrogen e x c h a n g e p r o c e s s was first u s e d a t M a z i n g a r b e in F r a n c e and i s t h e b a s i s o f two p l a n t s in I n d i a ( 1 1 ) . P r a c t i c a l monothermal p r o c e s s e s a r e u n f o r t u n a t e l y r a r e because phase c o n v e r s i o n i s g e n e r a l l y too complex o r too c o s t l y . I n s t e a d , t h e b i t h e r m a l a r r a n g e m e n t i s u s e d as i l l u s t r a t e d in F i g u r e 6 f o r t h e GS p r o c e s s . The p h a s e c o n v e r t e r o f F i g u r e 5 i s r e p l a c e d by a h o t t o w e r . I n this i l l u s t r a t i o n , and in s e v e r a l o f t h e GS p l a n t s , t h e h o t and c o l d t o w e r s a r e c o m b i n e d in a single vessel: t h e y a r e t h e n r e f e r r e d t o as h o t and c o l d t o w e r sections. I n t h e GS p r o c e s s t h e w a t e r i s e n r i c h e d in d e u t e r i u m in t h e c o l d s e c t i o n and i s d e p l e t e d in t h e h o t s e c t i o n . The w a t e r c o n t a c t s t h e same gas a t t h e t o p o f t h e c o l d s e c t i o n as a t t h e b o t t o m o f t h e h o t s e c t i o n , and t h e f l o w s a r e c o n t r o l l e d s o gas and w a t e r t e n d t o a p p r o a c h e q u i l i b r i u m w i t h e a c h o t h e r a t t h e s e two l o c a t i o n s . T h e r e f o r e , the deuterium-to-hydrogen r a t i o in t h e d e p l e t e d e f f l u e n t w a t e r a p p r o a c h e s a v a l u e e q u a l t o o t h / o t c (~0.8 f o r t h e GS p r o c e s s ) o f t h a t in t h e f e e d . The c o u n t e r c u r r e n t gas and w a t e r f l o w s , i f a p p r o p r i a t e l y c o n t r o l l e d a t c l o s e t o t h e c o r r e c t f l o w r a t i o , c a u s e a n e t t r a n s p o r t o f d e u t e r i u m up t h e h o t s e c t i o n and down t h e . c o l d s e c t i o n t o p r o v i d e e n r i c h e d gas and w a t e r a t t h e c e n t r e o f t h e t o w e r . Here a s m a l l e n r i c h e d s t r e a m i s f e d f o r w a r d t o a n o t h e r t o w e r w o r k i n g in a h i g h e r c o n c e n t r a t i o n r a n g e (a h i g h e r s t a g e ) and i s m a t c h e d by a p a r t i a l l y d e p l e t e d r e t u r n i n g stream. The n e t t r a n s p o r t o f d e u t e r i u m t o the h i g h e r stage i s c o n t r o l l e d t o equal the e x t r a c t i o n r a t e from the feed water. The c o n n e c t i o n b e t w e e n t h e first s t a g e and t h e h i g h e r s t a g e c a n be gas as in F i g u r e 6, o r w a t e r o r b o t h . The h o t t o w e r o f t h e b i t h e r m a l p r o c e s s c a n be t h o u g h t o f as a n a l o g o u s t o an i m p e r f e c t r e b o i l e r , and so p r o v i d e s gas t o t h e c o l d tower a t a c o n s i d e r a b l y lower deuterium c o n c e n t r a t i o n than i s a c h i e v e d by the phase c o n v e r t e r o f the monothermal p r o c e s s . As a c o n s e q u e n c e , n o t o n l y does t h e h o t t o w e r and i t s a s s o c i a t e d h e a t i n g and c o o l i n g e q u i p m e n t o f t h e b i t h e r m a l p r o c e s s r e p l a c e the phase c o n v e r t e r o f the monothermal p r o c e s s , b u t the


1.

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Selecting

Heavy

Water

11

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b i t h e r m a l p r o c e s s r e q u i r e s a much l a r g e r c o l d t o w e r . The m o n o t h e r m a l a n d b i t h e r m a l p r o c e s s a r r a n g e m e n t s a r e com p a r e d i n F i g u r e 7 i n t h e f o r m o f M c C a b e - T h i e l e d i a g r a m s . The e q u i l i b r i u m l i n e s h a v e s l o p e s o f 1/ot. The o p e r a t i n g l i n e f o r the monothermal case has a s l o p e o f u n i t y , e q u a l t o t h e l i q u i d to-gas f l o w r a t i o (L/G). The l a r g e d i v e r g e n c e b e t w e e n t h e o p e r a t i n g and e q u i l i b r i u m l i n e s a l l o w s a s u b s t a n t i a l enrichment (e.g. 5 times t h e f e e d c o n c e n t r a t i o n ) t o be reached w i t h o n l y a few t h e o r e t i c a l t r a y s . The d i f f e r e n c e i n d e u t e r i u m c o n c e n t r a t i o n b e t w e e n t h e f e e d (F) a n d t h e w a s t e (W), i . e . t h e r e c o v e r y f r o m t h e f e e d , c a n be l a r g e ; a s a f r a c t i o n i t a p p r o a c h e s (α-1)/α. F o r t h e b i t h e r m a l c a s e L/G, t h e s l o p e o f t h e o p e r a t i n g l i n e s , m u s t be b e t w e e n l / a a n d 1/oth. C o n s e q u e n t l y , b o t h h o t and c o l d c o l u m n s a r e s e v e r e l y p i n c h e d a n d many t h e o r e t i c a l t r a y s a r e r e q u i r e d f o r t h e same f i v e - f o l d e n r i c h m e n t . The r e c o v e r y , as a l r e a d y n o t e d , i s s m a l l ; l e s s t h a n ( a - a h ) / o t . Thus, t h e p e n a l t y f o r c i r c u m v e n t i n g t h e need f o r phase c o n v e r s i o n i s a l a r g e i n c r e a s e i n f l o w s a n d a n e v e n l a r g e r i n c r e a s e i n column v o l u m e . N o n e t h e l e s s , b i t h e r m a l i s g e n e r a l l y t h e more a t t r a c t i v e a r r a n g e m e n t t o s e l e c t b e c a u s e f e a s i b l e r e a c t i o n s f o r a monothermal process a r e r a r e and r e q u i r e l a r g e energy i n p u t s . A l t h o u g h t h e s i m p l e f l o w s h e e t s d i s c u s s e d so f a r have l i m i t e d r e c o v e r i e s s e t b y t h e above r e l a t i o n s h i p s , i t i s p o s s i b l e t o add s t r i p p i n g s e c t i o n s t o t h e process and achieve h i g h r e coveries approaching u n i t y i n the l i m i t . T h i s means a d d i n g more c o l u m n volume a s shown i n F i g u r e 8, f o r a g a s - f e d b i t h e r m a l p r o cess. T h i s i s t h e arrangement o f t h e amine-hydrogen p r o c e s s w h i c h h a s been s t u d i e d e x t e n s i v e l y b y A t o m i c Energy o f Canada L i m i t e d ( 1 6 ) . To s t r i p t h e g a s t o a l o w d e u t e r i u m c o n t e n t r e q u i r e s a l i q u i d s u f f i c i e n t l y l e a n i n deuterium. This i s pro d u c e d b y r e c y c l i n g some l e a n g a s t o t h e h o t t o w e r a n d l e n g t h e n i n g the h o t tower c o r r e s p o n d i n g l y . I n t h e simple case w i t h o u t s t r i p p i n g , t h e gas feed would e n t e r t h e bottom o f t h e h o t tower and n o t r e q u i r e a g a s r e c y c l e . A n o t h e r v e r s i o n o f a g a s - f e d b i t h e r m a l flowsheet w i t h s t r i p p i n g i s d e s c r i b e d by N i t s c h k e ( 1 2 ) . F i g u r e 9 d e s c r i b e s the s e p a r a t i o n f a c t o r as a f u n c t i o n o f temperature f o r t h e f o u r systems b e i n g c o n s i d e r e d . The l a r g e r a b s o l u t e v a l u e s f o r t h e hydrogen-based systems a r e e v i d e n t , as a r e t h e p o s s i b i l i t i e s o f much l a r g e r v a l u e s o f o t / a n . non-aqueous s y s t e m s c a n r e a c h v e r y l o w t e m p e r a t u r e s w i t h a c o n sequent l a r g e i n c r e a s e i n s e p a r a t i o n f a c t o r . In this respect t h e amine s y s t e m (17) i s p a r t i c u l a r l y a t t r a c t i v e . Relative to t h e GS s y s t e m , t h e much l a r g e r s e p a r a t i o n f a c t o r s f o r t h e h y d r o g e n - b a s e d s y s t e m s mean t h a t r e c o v e r i e s a r e h i g h e r , f l o w s per u n i t product lower and fewer t h e o r e t i c a l t r a y s a r e r e q u i r e d . A l l these advantages c o u l d l e a d t o s u b s t a n t i a l l y s m a l l e r tower volume f o r t h e h y d r o g e n - b a s e d p r o c e s s e s , b u t t h i s c a n o n l y b e a s s e s s e d a f t e r r a t e s o f exchange a r e c o n s i d e r e d which r e l a t e actual trays to theoretical trays.


c

c

c

T

n

c


e

t

w

o


12


Figure 6.

Exchange tower with gas feed forward

Figure 7.

McCabe-Thiele

diagrams



1.

RAE

Selecting

Heavy

Figure

ι

ι

Water

8.

Gas-fed bithermal stripping

I

ι ι

13

Processes

1 1 ι

-

\H -CH

" Η

2

2

- Ν Η ^ Λ

:

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ι

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process with

I

ι ι ι ι

11 1 1

I

1 1 11

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2

H

ν

H

2

- H

2

-

0

H

2

S - H

2

0

o°c 1 1 1 1 200

11

250

1 ι

ι I 300

ι

ι ι

ι

1

100°C I 1 I I I I

350

400

I

I

I

450

TEMPERATURE,°K Figure 9.

Liquid^gas separation factors



14

SEPARATION O F

HYDROGEN ISOTOPES

Because exchange r a t e s are g e n e r a l l y f a s t a t h o t tower con d i t i o n s f o r a l l four processes being c o n s i d e r e d , the important mass t r a n s f e r and k i n e t i c l i m i t a t i o n s o c c u r i n t h e c o l d t o w e r . These w i l l now be d i s c u s s e d i n some d e t a i l . The f i r s t s t e p i n c o m p a r i n g r a t e s o f d e u t e r i u m e x c h a n g e i s t o d e f i n e p r a c t i c a l p r o c e s s c o n d i t i o n s f o r e a c h s y s t e m , and t h i s i s done i n T a b l e IV. B e c a u s e o f t h e low s o l u b i l i t y o f h y d r o g e n , g i v e n as t h e r e c i p r o c a l o f H e n r y ' s Law c o n s t a n t i n t h e T a b l e , the hydrogen-based processes operate a t high p r e s s u r e . The t e m p e r a t u r e r a n g e s a r e p r i m a r i l y d i c t a t e d by v a p o u r p r e s s u r e f o r t h e u p p e r v a l u e and by a minimum p r a c t i c a l e x c h a n g e r a t e f o r t h e l o w e r v a l u e . The amine p r o c e s s i s r e s t r i c t e d i n h o t t o w e r t e m p e r a t u r e by 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 (16_). The GS p r o c e s s i s r e s t r i c t e d i n c o l d t o w e r t e m p e r a t u r e by t h e f o r m a t i o n of a s o l i d hydrate. Each o f the hydrogen-based p r o c e s s e s r e q u i r e s a c a t a l y s t - homogeneous f o r t h e non-aqueous o n e s and heterogeneous for'water-hydrogen. Even so t h e v a l u e s o f t h e k i n e t i c r a t e c o n s t a n t s w h i c h can be a c h i e v e d a r e low (18, 1 9 ) . The v a l u e g i v e n i n T a b l e IV f o r w a t e r - h y d r o g e n i s f o r t h e c a t a l y s t as a c o l l o i d a l s l u r r y (20) ; h i g h e r e f f e c t i v e r a t e s can be a c h i e v e d w i t h a f i x e d - b e d c a t a l y s t as w i l l be d i s c u s s e d l a t e r . F o r the purposes o f comparison exchange r a t e s are e v a l u a t e d h e r e f o r g a s - l i q u i d c o n t a c t i n g on a s i e v e t r a y , even t h o u g h t h i s i s n o t t h e most p r a c t i c a l c o l d t o w e r c o n t a c t i n g e q u i p m e n t f o r a l l four processes. As i l l u s t r a t e d s c h e m a t i c a l l y i n F i g u r e 10, t h e gas f l o w s up t h e t o w e r t h r o u g h a m u l t i t u d e o f h o l e s i n e a c h t r a y . The l i q u i d f l o w s a c r o s s e a c h t r a y , o v e r a w e i r and down t o t h e n e x t t r a y ; t h e gas p r e s s u r e d r o p a c r o s s a t r a y h o l d s t h e l i q u i d on t h e t r a y . Thus, t h e r e i s c o n t i n u o u s , m u l t i - c o n t a c t , c o u n t e r c u r r e n t o p e r a t i o n . The g a s - l i q u i d m i x i n g on e a c h t r a y g e n e r a t e s a l a r g e i n t e r f a c i a l a r e a b e t w e e n them i n t h e f o r m o f b u b b l e and d r o p l e t s u r f a c e s . The gas and l i q u i d t h e n must s e p a r a t e b e f o r e each phase passes t o i t s subsequent t r a y .

Table

IV

CHEMICAL EXCHANGE PROCESSES GS

7 230 330

2

H 0/H 2

and a p r a c t i c a l v a l u e o f

catalyst

Catalyst Rate Constant* s" Gas S o l u b i l i t y * mol/(m .MPa) *at c o l d tower temperature concentration.

7 220 315

3

15 10

2 30 3 400

3

NH /H

10 300 440 Pt/C 1 8

P r e s s u r e MPa Temperature Κ

1

Amine

-

5000 830

CH3NHK

130 24

NH2K


2

1.

RAE

Selecting

Heavy

Water

15

Processes

I n s i m p l e t e r m s t h e t o w e r volume i s g i v e n b y : Volume = GRTN/(PK a) G where G i s the gas f l o w i n m o l s / s , R i s the gas law c o n s t a n t (MPa.m /(mol.Κ)), Τ i s t h e t e m p e r a t u r e i n Κ, Ν i s t h e number o f t h e o r e t i c a l t r a y s , Ρ i s t h e p r e s s u r e i n MPa, K G i s t h e o v e r a l l mass t r a n s f e r c o e f f i c i e n t i n m/s a n d a i s t h e i n t e r f a c i a l a r e a p e r u n i t volume ( m / m ) . The o v e r a l l mass t r a n s f e r c o e f f i c i e n t can be r e l a t e d t o t h e g a s p h a s e mass t r a n s f e r c o e f f i c i e n t , k^, and t h e l i q u i d p h a s e mass t r a n s f e r c o e f f i c i e n t , k , b y : 3

2

3


L

1__ Κ G

=

1_ k G

H k RT L 3

where H i s t h e H e n r y ' s Law c o n s t a n t (MPa.m /mol). For a s o l u b l e gas l i k e hydrogen s u l f i d e the v a l u e o f H i s s m a l l ( T a b l e IV) a n d t h e g a s p h a s e r e s i s t a n c e i s i m p o r t a n t a n d p o s s i b l y c o n t r o l l i n g . F o r h y d r o g e n H i s l a r g e r b y two o r d e r s o f magnitude and k g i s a l s o i n c r e a s e d ; t h u s , o n l y the l i q u i d phase r e s i s t a n c e need be c o n s i d e r e d . The v a l u e o f k L depends o n b o t h mass t r a n s f e r a n d c h e m i c a l r e a c t i o n s o t h a t t h e k i n e t i c r a t e constant, k , enters i n t o i t s determination. I t i s beyond the scope o f t h i s paper t o t r e a t the e v a l u a t i o n o f k i n any d e t a i l . F o l l o w i n g the e x p o s i t i o n o f A s t a r i t a (21), F i g u r e 11 shows how k L , t h e mass t r a n s f e r c o e f f i c i e n t f o r t h e l i q u i d phase f o r combined a b s o r p t i o n and c h e m i c a l r e a c t i o n , i s r e l a t e d t o kg, the l i q u i d phase c o e f f i c i e n t f o r p h y s i c a l ab s o r p t i o n w i t h o u t r e a c t i o n , as a f u n c t i o n o f k . I n e x p r e s s i n g t h i s f u n c t i o n t h e r a t i o o f k / s i s u s e d where s i s t h e f r a c t i o n a l rate o f surface renewal ( s " ) . A t y p i c a l value o f s for a sieve t r a y i s 50 s " . I f D i s the d i f f u s i v i t y o f the d i s s o l v e d gas ( m / s ) , t h e n k g = (Ds)°« . The p a r a m e t e r i n t h i s f i g u r e , e s ° * / a D - , a l l o w s f o r t h e e f f e c t o f t h e volume f r a c t i o n o f l i q u i d c o n t a i n e d i n t h e t o t a l u n i t v o l u m e , ε, w h i c h i s i m p o r t a n t i n the slow r e a c t i o n regime. I n t h e s l o w r e a c t i o n r e g i m e i n F i g u r e 11, t h e e x c h a n g e r e a c t i o n occurs i n the bulk l i q u i d . k i s l e s s than k g and equals ek /a. I n t h e c a s e o f t h e e j e c t o r c o n t a c t o r (15) where a i s very l a r g e and s i s l i k e l y a l s o i n c r e a s e d r e l a t i v e t o a s i e v e t r a y , t h e low v a l u e o f e s ° - / a D - c o u l d b r i n g b o t h t h e w a t e r - h y d r o g e n a n d t h e ammonia-hydrogen s y s t e m s i n t o t h e s l o w r e a c t i o n regime. When k s k g t h e e x c h a n g e r a t e i s c o n t r o l l e d p r i m a r i l y b y physical absorption. This i s a t r a n s i t i o n r e g i o n o r the d i f f u s i o n r e g i m e w h e r e k < s . The r e a c t i o n o c c u r s p r i m a r i l y i n t h e bulk l i q u i d , but s u f f i c i e n t l y r a p i d l y that p h y s i c a l absorption i s the r a t e c o n t r o l l i n g s t e p . Both the s l u r r y - c a t a l y z e d w a t e r h y d r o g e n s y s t e m a n d t h e ammonia-hydrogen s y s t e m a r e i n t h i s r e g i o n f o r c o n t a c t i n g on a s i e v e t r a y . r

L

r

r

1

1

2

5

5

0

5

L

r

5

0

5

L

r


16



COLD

SECTION O F E X C H A N G E

WATER

Figure 10.

Figure 11.

TOWER

E N R I C H E D IN D E U T E R I U M

Sieve tray operation

Cold tower mass transfer coefficients


1.

RAE

Selecting

Heavy

Water

17

Processes

The f a s t r e a c t i o n r e g i m e i n F i g u r e 11 b e g i n s when k > k^, and c h e m i c a l r e a c t i o n e n h a n c e s t h e mass t r a n s f e r r a t e . Once k / s e x c e e d s a b o u t 3, t h e n k]^ i s e q u a l t o ( k D ) - . Here t h e r e a c t i o n zone i s l i m i t e d t o f r e s h e l e m e n t s o f l i q u i d b r o u g h t t o the i n t e r f a c i a l s u r f a c e by t u r b u l e n t e d d i e s ; d i f f u s i o n and r e a c t i o n o c c u r s i m u l t a n e o u s l y a t the s u r f a c e w h i l e the b u l k l i q u i d i s a l w a y s a t e q u i l i b r i u m . The a m i n e - h y d r o g e n p r o c e s s i s c l e a r l y i n t h i s regime. F o r a n y g i v e n c o n t a c t o r s y s t e m t h e f a s t r e a c t i o n r e g i m e ends once t h e g a s p h a s e mass t r a n s f e r r a t e b e g i n s t o c o n t r o l ; t h i s i s t h e i n s t a n t a n e o u s r e a c t i o n r e g i m e . A s shown i n F i g u r e 11 t h e GS p r o c e s s i s a l m o s t i n t h i s r e g i m e . The v a l u e o f k f o r t h i s s y s t e m g i v e n i n T a b l e IV i s o n l y a p p r o x i m a t e ; no r e l i a b l e measurement h a s b e e n made. However, 5000 s " p r o b a b l y i s a lower l i m i t . A s d i s c u s s e d above t h e l o w v a l u e o f H f o r t h e H2O-H2S s y s t e m means t h e g a s p h a s e r e s i s t a n c e must be c o n s i d e r e d e v e n a t modest v a l u e s f o r k . P i l o t p l a n t d a t a f o r GS s i e v e t r a y e f f i c i e n c y a r e c o n s i s t e n t w i t h kgH/RTkg e q u a l t o 6. The d o t t e d l i n e i n F i g u r e 11 r e p r e s e n t s KçH/RTk^ when k H / R T k ° = 6 f o r v a r i o u s v a l u e s o f k and hence k . U s i n g t h e d a t a i n T a b l e I V a n d F i g u r e 11, t h e o v e r a l l v o l u m e t i c mass t r a n s f e r c o e f f i c i e n t s , Kça, a n d t h e t o w e r v o l u m e s have b e e n e s t i m a t e d a s shown i n T a b l e V a s s u m i n g a * 300 m" f o r L

0

r

5

r

r


1

r

G

r

L

1

H2O-H2S a n d H2O-H2,

1

a n d a * 600 m"

f o r NH3-H2 a n d C H N H - H . 3

2

2

F o r t h e p r o c e s s e s b a s e d on h y d r o g e n t h e low v a l u e s o f Kça a r e c o m p e n s a t e d f o r b y t h e f e w e r number o f t h e o r e t i c a l t r a y s , t h e h i g h e r p r e s s u r e and the lower gas f l o w r a t e . Thus, f o r t h e amine p r o c e s s t h e t o w e r volume i s l e s s t h a n f o r t h e GS p r o c e s s . However, t h e h i g h e r p r e s s u r e f o r t h e f o r m e r r e s u l t s i n a v e s s e l w e i g h t w h i c h i s s i m i l a r t o t h a t f o r t h e GS p r o c e s s . Kâ c a n be enhanced by u s i n g mechanical energy t o i n c r e a s e the i n t e r f a c i a l area very c o n s i d e r a b l y , as i n the e j e c t o r c o n t a c t o r (15), and so t o r e d u c e t o w e r volume s i g n i f i c a n t l y a t t h e e x p e n s e o f c o m p l e x and c o s t l y i n t e r n a l s . The o v e r a l l e x c h a n g e r a t e i s o f c o u r s e d e p e n d e n t o n c a t a l y s t c o n c e n t r a t i o n a n d on t h e f o r m o f c a t a l y s t u s e d . F o r t h e w a t e r h y d r o g e n p r o c e s s t h e v a l u e s o f Kça g i v e n i n T a b l e V a r e f o r a hydrophobic p l a t i n u m c a t a l y s t supported on the s u r f a c e o f a p a c k i n g i n a t r i c k l e - b e d r e a c t o r (22) . T h i s new c a t a l y s t d e v e l o p e d b y A t o m i c E n e r g y o f Canada L i m i t e d p r o v i d e s a h i g h e r v o l u m e t r i c mass t r a n s f e r r a t e t h a n t h e s l u r r y c a t a l y s t r e f e r r e d to e a r l i e r . The b i t h e r m a l p r o c e s s a r r a n g e m e n t r e q u i r e s a h i g h p r e s s u r e (10 MPa) s o t h a t Kâ i s l o w a n d t h e t o w e r volume i s large. F o r t h e m o n o t h e r m a l CECE a r r a n g e m e n t t h e p r e s s u r e i s l o w e r (1.5 MPa) a n d Kça i s a n o r d e r o f m a g n i t u d e h i g h e r g i v i n g a somewhat l a r g e r v a l u e f o r PKça; t h e much s m a l l e r t o w e r volume r e s u l t s m a i n l y f r o m t h e r e d u c t i o n i n Ν a n d G. The same a d v a n t a g e f o r t h e m o n o t h e r m a l f l o w s h e e t c a n a l s o be s e e n f o r t h e ammoniahydrogen system.


18

SEPARATION O F

HYDROGEN ISOTOPES


T u r n i n g now t o c o n s i d e r h o t t o w e r v o l u m e s , t h e r e i s no l o n g e r a p r o b l e m o f low exchange r a t e s . F o r t h e GS p r o c e s s t h e r a t e s i n t h e c o l d and h o t t o w e r s a r e r o u g h l y s i m i l a r , b o t h c o n trolled by mass t r a n s f e r i n t h e gas p h a s e . F o r the hydrogenb a s e d p r o c e s s e s t h e r a t e s o f e x c h a n g e a r e much f a s t e r i n t h e h o t tower than i n the c o l d tower. Thus, i n p r i n c i p l e the h o t tower v o l u m e s f o r t h e s e p r o c e s s e s c o u l d be s m a l l e r t h a n f o r t h e GS p r o c e s s . However, i n p r a c t i c e t h e b i t h e r m a l d e s i g n i n t h e s e c a s e s i s o p t i m i z e d t o c o n c e n t r a t e s e p a r a t i v e work i n t h e h o t t o w e r and so l i m i t demands on t h e c o l d t o w e r ; as a r e s u l t b o t h t e n d t o have a b o u t t h e same v o l u m e . The v a l u e s o f Ν u s e d i n T a b l e V r e f l e c t t h i s o p t i m i z i n g p r o c e s s , and so t h e r e l a t i v e c o l d tower volumes a r e r e p r e s e n t a t i v e o f the r e l a t i v e t o t a l tower volumes f o r t h e b i t h e r m a l f l o w s h e e t s . Transfer Processes A s p e c i a l c a t e g o r y o f c h e m i c a l exchange r e a c t i o n i s h i g h temperature p r o c e s s e s which t r a n s f e r deuterium from a source m a t e r i a l a v a i l a b l e as l a r g e s i n g l e s t r e a m s , s u c h as w a t e r o r methane, t o h y d r o g e n . Thus, the v e r y l a r g e s c a l e p o s s i b l e w i t h t h e s e s o u r c e m a t e r i a l s c a n be c o m b i n e d w i t h t h e i n h e r e n t a d vantages o f u s i n g a hydrogen-based p r o c e s s w h i c h can c o n c e n t r a t e d e u t e r i u m more e f f i c i e n t l y . H y d r o g e n i t s e l f i s an a b u n d a n t s o u r c e ( w o r l d p r o d u c t i o n e q u i v a l e n t t o 30,000 Mg D20/a b u t i t i s d e r i v e d f r o m a v a s t number o f i n d i v i d u a l u n i t s , w i t h a t y p i c a l l a r g e u n i t b e i n g e q u i v a l e n t t o o n l y 70 Mg D20/a. The s t e a m - h y d r o g e n p r o c e s s i l l u s t r a t e d i n F i g u r e 12 l i n k s a w a t e r f e e d t o t h e amine-hydrogen p r o c e s s . Water i s e v a p o r a t e d i n t o d e p l e t e d h y d r o g e n and t h e m i x t u r e i s h e a t e d t o 870 Κ where the exchange r e a c t i o n o c c u r s o v e r a n i c k e l o x i d e c a t a l y s t t o t r a n s f e r d e u t e r i u m from t h e steam t o the hydrogen. The s t e a m i s c o n d e n s e d and t h e h y d r o g e n r e t u r n e d t o t h e b i t h e r m a l a m i n e hydrogen u n i t . Two s t e a m - h y d r o g e n e q u i l i b r a t i o n s i n s e r i e s a r e r e q u i r e d , and e v e n so t h e d e u t e r i u m c o n t e n t o f t h e r e p l e n i s h e d h y d r o g e n i s s i g n i f i c a n t l y b e l o w n a t u r a l . F l o w s a r e l a r g e and energy requirements are h i g h ; the steam-hydrogen t r a n s f e r p r o c e s s by i t s e l f i s e q u i v a l e n t i n c o s t p e r k g o f h e a v y w a t e r e x t r a c t e d t o more t h a n h a l f t h e c o s t o f t h e h e a v y w a t e r p r o d u c e d by t h e GS p r o c e s s . A l t h o u g h some i m p r o v e m e n t s i n t h i s t r a n s f e r p r o c e s s may be p o s s i b l e , i t w i l l be d i f f i c u l t t o a c h i e v e a c o m p e t i t i v e combination w i t h amine-hydrogen o r o t h e r hydrogen-based p r o c e s s . A s i m i l a r t r a n s f e r p r o c e s s w h i c h c a n be u s e d t o e x t r a c t d e u t e r i u m f r o m methane (2_3) has b e e n s t u d i e d a t t h e G u l f R e s e a r c h and D e v e l o p m e n t Company. D e p l e t e d h y d r o g e n i s m i x e d w i t h methane f e e d and t h e e x c h a n g e r e a c t i o n i s c a r r i e d o u t o v e r a c a t a l y s t a t a b o u t 1000 K. The two components a r e s e p a r a t e d by l i q u e f y i n g t h e methane. The r e p l e n i s h e d h y d r o g e n f e e d s a d i s t i l l a t i o n u n i t which separates the deuterium. T h i s p r o c e s s com b i n a t i o n i s i l l u s t r a t e d i n F i g u r e 13. A g a i n two e q u i l i b r a t i o n s


RAE

Selecting

Heavy Water

Processes

Table V RELATIVE COLD TOWER VOLUMES Gas F l o w Theoretical k m o l / m o l D2O Trays H 0 -H2S

74

2

CH3NH2

-

H

24

2

;

Kça s^ 1

Relative Volume 1

27

0.9

1

10

0.04

0.6

5 10

0.008 0.008

0.9 4

0.08* 0.008^

0.3 5

NH3-H2

14 31


monothermal bithermal H 0 - H CECE bithermal 2

2

*For a 4 - f o l d 2

3

9 50 enrichment.

V o l u m e = GNT/PK a; s e e T a b l e I V f o r Ρ a n d Τ e x c e p t f o r CECE w h i c h h a s Ρ =1.5 MPa. G

3

Weighted average f o r s t r i p p i n g and e n r i c h i n g (see F i g u r e 8 ) .

*For t h e f i x e d - b e d

columns

hydrophobic platinum c a t a l y s t .

Figure 12.

Steam-Η

-amine process

2


20



with countercurrent flow o f hydrogen and methane between them are needed. Energy consumption i s h i g h , most o f i t f o r r e f r i g e r a t i o n . The methane-hydrogen p a r t o f the process i s probably s i m i l a r i n cost t o steam-hydrogen t r a n s f e r . I t should be noted t h a t s e l e c t i v e photochemical processes based on the use o f l a s e r s w i l l r e q u i r e a s i m i l a r t r a n s f e r step to l i n k the s e p a r a t i o n process t o an abundant feed. Depending on the molecules i n v o l v e d , high temperature may not be needed t o achieve a s e p a r a t i o n f a c t o r near u n i t y f o r the t r a n s f e r r e a c t i o n , but unusually low p r e s s u r e s may be a requirement. C e r t a i n l y , t h i s can be a c o s t l y o p e r a t i o n because o f the l a r g e volume o f m a t e r i a l t o be handled and the n e c e s s i t y t o recover the r e c i r c u l a t i n g deuterium c a r r i e r from the waste stream with a h i g h efficiency . Process

Comparison

While the s e p a r a t i o n f a c t o r i s a key p r o p e r t y f o r r a n k i n g the economic a t t r a c t i v e n e s s o f p r o c e s s e s , energy consumption i s almost o f equal importance. A convenient c h a r t to i l l u s t r a t e t h i s i s shown i n Figure 14. An expanded s c a l e o f l o g ( a - l ) i s used f o r the s e p a r a t i o n f a c t o r . E l e c t r i c a l o r mechanical energy i s converted t o e q u i v a l e n t thermal energy u s i n g 40% e f f i c i e n c y and added t o the a c t u a l thermal energy requirement t o give t o t a l e q u i v a l e n t thermal energy i n GJ/kg. The GS process r e q u i r e s 30 GJ/kg which i s e q u i v a l e n t to about 5 b a r r e l s o f o i l per kilogram. The energy consumption f o r a process i s d i f f i c u l t t o d e f i n e without a d e t a i l e d d e s i g n . I t depends on the degree o f energy recovery which i n turn depends on such f a c t o r s as the i n g e n u i t y of the designer, the r e l a t i v e cost o f energy and c a p i t a l equipment, and how the p l a n t i s t o be f i n a n c e d . T o t a l energy i f o f t e n under-estimated i n p r e l i m i n a r y process e v a l u a t i o n s because o f the s i m p l i f y i n g assumptions that are u s u a l l y made. Processes i n the uneconomic region o f Figure 14 have too low a s e p a r a t i o n f a c t o r (water c r y s t a l l i z a t i o n ) , too high an energy consumption (hydrogen d i f f u s i o n ) o r both (water d i s t i l l a t i o n ) . E l e c t r o l y s i s ranks h i g h e s t i n s e p a r a t i o n f a c t o r and h i g h e s t i n energy consumption unless i t i s undertaken f o r hydrogen production. In t h a t case about one t h i r d o f the deuterium cont a i n e d i n the feed water can be recovered as a hydrogen stream enriched t o three times n a t u r a l a t p r a c t i c a l l y zero c o s t (9_) . The G u l f process (23) i s d e s c r i b e d i n F i g u r e 13; while i t s energy consumption i s high, i t has many i n h e r e n t advantages which are l i k e l y t o permit an a t t r a c t i v e l y low c a p i t a l c o s t . However, not only i s i t s energy consumption about twice t h a t o f the GS process, but t h i s energy i s p r i m a r i l y mechanical and t h e r e f o r e expensive. Under e s p e c i a l l y favourable circumstances the G u l f process might become competitive with GS.



Selecting

Heavy

Water

Processes

Figure 13.

C,-H

2

process

Figure 14


22

SEPARATION

O F HYDROGEN

The CECE process i s shown twice - a t the high energy con sumption f o r the case where there i s no market f o r the e l e c t r o l y t i c hydrogen and a t the low energy consumption where the heavy water i s a by-product o f the hydrogen p r o d u c t i o n . The l a t t e r i s a p a r t i c u l a r l y favourable s i t u a t i o n . The value o f α used i n F i g u r e 14 f o r the s e p a r a t i o n f a c t o r of the b i t h e r m a l exchange processes i s α / α ^ i n order t o c h a r a c t e r i z e each o f them by a s i n g l e v a l u e . The GS process has a lower s e p a r a t i o n f a c t o r and a h i g h e r energy consumption than s i x other processes i n Figure 14. I t even has the d i s t i n c t l y u n d e s i r a b l e f e a t u r e o f using hydrogen s u l f i d e which i s c o r r o s i v e , smelly and t o x i c . Yet the GS process dominates heavy water p r o d u c t i o n and i s s t i l l the p r e f e r r e d method o f p r o d u c t i o n . Why? Table VI compares i t with two o f i t s p o t e n t i a l competitors: - the GS process uses an abundant feed and t h e r e f o r e enjoys the advantage o f a l a r g e s c a l e ; hydrogen-fed p l a n t s are l i m i t e d to l e s s than 100 Mg/a c a p a c i t y and the hydrogenbased processes are expensive t o l i n k to water o r methane, - while the GS process energy consumption i s higher than the other two, i t i s not e x c e s s i v e , - the GS process enjoys f a s t mass t r a n s f e r r a t e s , b u t so does hydrogen d i s t i l l a t i o n , - the temperature which c o n t r o l s GS process energy i n p u t i s moderate, whereas both hydrogen d i s t i l l a t i o n and aminehydrogen r e q u i r e expensive r e f r i g e r a t i o n , - the GS process pressure i s not high enough t o i n c u r a large cost penalty, - while the GS process s e p a r a t i o n f a c t o r i s not as h i g h as f o r the other processes, i t i s reasonable. The major reason f o r the success o f the GS process i s the s c a l e effect. α


ISOTOPES

Table VI PROCESS COMPARISON GS Feed Energy GJ/kg D 2 O Mass T r a n s f e r Temperature* Κ Pressure MPa Separation F a c t o r

H0 30 Fast 400 2 1.3 2

Amine

Hydrogen Distillation

H2

H2

11 Slow 220 7 2.2

22 Fast 24 0.25 1.5

C o n t r o l l i n g major energy i n p u t



1.

RAE

Selecting

Heavy Water Processes

Figure 15.

Key factors in process evaluation


23

24

SEPARATION O F HYDROGEN ISOTOPES


The k e y f a c t o r s i n p r o c e s s e v a l u a t i o n a r e o u t l i n e d i n F i g u r e 15. The i n h e r e n t p r o c e s s c o n d i t i o n s a n d p r o p e r t i e s o f p r e s s u r e , t e m p e r a t u r e , mass t r a n s f e r r a t e and s e p a r a t i o n f a c t o r a r e a l l i m p o r t a n t . P r e s s u r e and t e m p e r a t u r e c a n be v a r i e d t o o p t i m i z e the c o n d i t i o n s , b u t u s u a l l y o n l y a r e l a t i v e l y narrow range i s p r a c t i c a l . These c o n d i t i o n s s e t t h e v a l u e s o f α a n d Kça. Knowing α i t i s p o s s i b l e t o s e l e c t t h e r e c o v e r y , t h e f l o w r a t e , G, a n d t h e number o f t r a n s f e r u n i t s o r t h e o r e t i c a l t r a y s , N. The p r e s s u r e and t e m p e r a t u r e s e t t h e a l l o w a b l e v e l o c i t y , V. Then G, N, Kça and V combine t o d e f i n e t o w e r v o l u m e . The m a j o r p a r a m e t e r s d e t e r m i n i n g e n e r g y c o n s u m p t i o n a r e f l o w and t e m p e r a t u r e , b u t as a l r e a d y n o t e d t h e d e t a i l e d p r o c e s s f l o w s h e e t i s important here. Summary The GS p r o c e s s i s t h e o n l y l a r g e - s c a l e i n d e p e n d e n t p r o c e s s w i t h a d i r e c t w a t e r f e e d and t h i s i s l a r g e l y r e s p o n s i b l e f o r i t s p r e - e m i n e n t p o s i t i o n i n heavy w a t e r p r o d u c t i o n . Water-hydrogen exchange i s o n l y a t t r a c t i v e i n c o m b i n a t i o n w i t h e l e c t r o l y t i c hydrogen p r o d u c t i o n - t h e combined e l e c t r o l y s i s and c a t a l y t i c exchange (CECE) p r o c e s s . T h r e e h y d r o g e n - b a s e d p r o c e s s e s , a m i n e - h y d r o g e n , ammoniah y d r o g e n and h y d r o g e n d i s t i l l a t i o n , a r e a l l c l o s e t o b e i n g comp e t i t i v e w i t h t h e GS p r o c e s s . Of t h e s e , a m i n e - h y d r o g e n i s l i k e l y t o be t h e c h e a p e s t . H y d r o g e n d i s t i l l a t i o n l i n k e d t o n a t u r a l gas as t h e d e u t e r i u m s o u r c e , w h i l e d i s t i n c t l y e n e r g y i n t e n s i v e , may be a n e a r c o m p e t i t o r u n d e r some c i r c u m s t a n c e s . Hydrogen i s a p o t e n t i a l l y abundant heavy w a t e r s o u r c e , b u t i n d i v i d u a l p l a n t s are small. However, h y d r o g e n - b a s e d h e a v y w a t e r p l a n t s a r e b e i n g b u i l t a n d more w i l l be c o m m i t t e d . N o n e t h e l e s s , t h e GS p r o c e s s w i l l c o n t i n u e as t h e d o m i n a n t one f o r a n o t h e r 10 o r 20 y e a r s . Nomenclature a D (D/H) G H G KQ

~ -

k kg

-

k

-

k

L

L

r

-

i n t e r f a c i a l a r e a p e r u n i t volume o f m i x e d p h a s e s , m" d i f f u s i v i t y o f d i s s o l v e d g a s , m /s atom r a t i o o f d e u t e r i u m t o h y d r o g e n gas f l o w r a t e , mol/s H e n r y ' s Law c o n s t a n t , MPa.m /kmol 9 p h a s e mass t r a n s f e r c o e f f i c i e n t , m/s o v e r a l l mass t r a n s f e r c o e f f i c i e n t b a s e d on gas p h a s e c o n c e n t r a t i o n s , m/s l i q u i d p h a s e mass t r a n s f e r c o e f f i c i e n t , m/s l i q u i d p h a s e mass t r a n s f e r c o e f f i c i e n t f o r p h y s i c a l a b s o r p t i o n , m/s psuedo f i r s t o r d e r r a t e c o n s t a n t f o r c o n v e r s i o n o f m o n o d e u t e r a t e d d i s s o l v e d gas t o m o n o d e u t e r a t e d s o l v e n t , s " l i q u i d f l o w r a t e , mol/s 2

3

a s


1

1.

Selecting

RAE

m Ν Ρ R s Τ V

α ε

Water

-

25

Processes

number o f hydrogen atoms i n the molecule number o f t h e o r e t i c a l t r a y s p r e s s u r e , MPa gas law constant = 8.2 χ 10"" m .MPa/ (kmol .K) f r a c t i o n a l r a t e o f surface renewal, s " temperature, Κ gas v e l o c i t y based on the a c t i v e area o f the t r a y , i . e . tower cross s e c t i o n a l area minus t o t a l downcomer area, m/s - deuterium s e p a r a t i o n f a c t o r - volume f r a c t i o n o f l i q u i d on a t r a y

Subscripts : Publication Date: June 1, 1978 | doi: 10.1021/bk-1978-0068.ch001

Heavy

3

3

1

c - c o l d tower;

h - h o t tower

Literature Cited (1) (2) (3) (4) (5) (6)

(7)

(8)

(9)

(10) (11)

(12)

Urey, H . C . , Brickwedde, F . G . , Murphy, G.M., Phys. Rev., (1932), 39, 164. Murphy, G.M., Urey, H . C . , Kirshenbaum, I . , "Production of Heavy Water", McGraw-Hill Book Co., Inc., New York, 1955. Clusius, K . , et al., "Nuclear Physics and Cosmic Rays, Part II", FIAT Rev. Ger. S c i . , (1948), p.182-188. Lavrencic, D . , Comitato Nazionale Energia Nucleare Report RT/ING(74)9, (1974), Rome. Benedict, M . , Pigford, T . H . , "Nuclear Chemical Engineering", McGraw-Hill Book Co., Inc., New York, 1957. Hammerli, Μ., Stevens, W.H., Butler, J.P., "Separation of Hydrogen Isotopes", p.110, American Chemical Society, Washington, 1978. Bebbington, W.P., Proctor, J.F., Scotten, W.C., Thayer, V.R., Third United Nations International Conference on the Peace ful Uses of Atomic Energy, (1964), Proceedings 12, 334 United Nations, Geneva. Dahlinger, Α . , Lockerby, W.E., Rae, H . K . , IAEA Inter national Conference on Nuclear Power and i t s Fuel Cycle, (1977), Salzbury, Austria, Paper IAEA-CN-36/183; Atomic Energy of Canada Limited Report 5710 (1977). Gami, D . C . , Rapial, A . S . , Third United Nations Inter national Conference on the Peaceful Uses of Atomic Energy, (1964), Proceedings 12, 421 United Nations, Geneva. Roth, E., Bedhome, Α . , Lefrancois, B . , LeChatelier, J., Tillol, Α . , Energie Nucleaire, (1968), 10, 214. Roth, Ε . , Traourouder, R., V i r a t e l l e , J., Lefrancois, B . , Fourth United Nations Conference on the Peaceful Uses of Atomic Energy, (1971), Proceedings 9, 69, United Nations, Geneva. Nitschke, E., Ilgner, H . , Walter, S., "Separation of Hydrogen Isotopes", p. 77, American Chemical Society, Washington, 1978.



26

(13) (14) (15) (16)

(17)

(18)


(19) (20) (21) (22)

(23)

Nitschke, E., Atomwirtschaft, (1973), 18, 297. Kuhn, W., Thurkauf, Μ., Helv. Chem. Acta, (1958), 41, 938. Wynn, N.P., "Separation of Hydrogen Isotopes", p. , American Chemical Society, Washington, 1978. Holtslander, W . J . , Lockerby, W.E., "Separation of Hydrogen Isotopes", p. 4 0 , American Chemical Society, Washington, 1978. Rolston, J.H., Butler, J.P., Denhartog, J., "A Deter mination of the Isotopic Separation Factor Between Hydrogen and Liquid Methylamine", to be published. Bourke, P.J., Lee, J.C., Trans. Inst. Chem. Engrs., (1961), 39, 280. Kalra, H . , Otto, F . D . , Can. J. Chem. Eng., (1974), 52, 258. Becker, E.W., Hubener, R., Kessler, R., Chemie Ing. Tech., (1958), 30, 288. Astarita, G . , "Mass Transfer with Chemical Reaction", Elsevier, 1967. Butler, J.P., Rolston, J.H., Stevens, W.H., "Separation of Hydrogen Isotopes", p. 9 3 , American Chemical Society, Washington, 1978. Pachaly, R.W., "Process for Obtaining Deuterium from Hydrogen-Containing Compounds and the Production of Heavy Water Therefrom", Canadian Patent 943742, (1974).

RECEIVED December 16, 1977


2 Bruce Heavy Water Plant Performance G. D. DAVIDSON


Bruce Heavy Water Plant, Ontario Hydro, Box 2000, Triverton, Ontario, CanadaN0G2T0

To satisfy the Canadian demand f o r heavy water which is used as a moderator and heat t r a n s p o r t medium in CANDU r e a c t o r s , Atomic Energy o f Canada L i m i t e d c o n t r a c t e d the Lummus Company o f Canada L i m i t e d in 1969 t o d e s i g n and c o n s t r u c t Bruce Heavy Water P l a n t ' A ' a t the Bruce N u c l e a r Power Development. The 9.5 square k i l o m e t e r (2300 a c r e s ) s i t e had p r e v i o u s l y been p u r c h a s e d by O n t a r i o Hydro f o r the Douglas P o i n t N u c l e a r G e n e r a t i n g S t a t i o n . I t is l o c a t e d in the County o f Bruce on the e a s t e r n shore o f Lake Huron, midway between the towns o f K i n c a r d i n e and P o r t Elgin, a p p r o x i m a t e l y 240 k i l o m e t e r s (150 m i l e s ) northwest o f T o r o n t o . O n t a r i o Hydro was r e s p o n s i b l e f o r commissioning and o p e r a t i n g the p l a n t which has a d e s i g n c a p a c i t y o f 96.6 k g / h o f 99.75% purity heavy water (D2O). Commissioning o f P l a n t ' Α ' commenced in 1971 and proceeded w i t h o u t any major difficulty t h r o u g h 1972 and 1973. On 28 June, 1973, O n t a r i o Hydro p u r c h a s e d the p l a n t from AECL and d e c l a r e d the p l a n t in-service. D e s i g n p r o d u c t i o n c a p a c i t y was r e a c h e d in April, 1974 after e l e v e n months o f o p e r a t i o n . Production rates and c a p a c i t y f a c t o r s have s t e a d i l y been i n c r e a s e d such t h a t the official capacity was i n c r e a s e d t o 100.6 k g / h in 1976. As a r e s u l t o f t h e e a r l y o p e r a t i n g s u c c e s s o f BHWP A, O n t a r i o Hydro announced t h e c o n s t r u c t i o n o f t h r e e a d d i t i o n a l and e s s e n t i a l l y i d e n t i c a l heavy water p l a n t s a t Bruce (BHWP B, C and D ) . A c u t b a c k in t h e N u c l e a r G e n e r a t i o n c o n s t r u c t i o n program in 1976 r e s u l t e d in one o f t h e s e , BHWP C, b e i n g c a n c e l l e d and the s c h e d u l e f o r a n o t h e r , BHWP D, b e i n g d e f e r r e d by two y e a r s . ©

0-8412-0420-9/78/47-068-027$05.00/0


28

SEPARATION O F H Y D R O G E N

ISOTOPES

Bruce Heavy Water Plant uses the proven Dual Temperature Hydrogen Sulphide - Water Exchange Process to separate deuterium. This process has been in use on a commercial scale in North America f o r 25 years. Following a b r i e f d e s c r i p t i o n of the production process, BHWP A performance from in-service to the end of 1976 is described in terms of the following key performance c r i t e r i a : Employee Safety, Care of the Environment, R e l i a b i l i t y and Manpower Development. F i n a l l y , commissioning progress to date of BHWP Β is described.


Production Process Water is pumped from Lake Huron (see "Figure 1") with a deuterium i s o t o p i c content of 148 mg/kg. I t is f i l t e r e d to remove s o l i d s and preheated to 29°C. The pH is lowered to 3.8 - 4.2 by the addition of sulphuric acid to decompose carbonates, the water is degassed of oxygen and carbon dioxide, and then the pH is raised to 6.0 - 6.5 by the addition of caustic to avoid corrosion to carbon s t e e l processing equipment. This treated water is then fed to the Enriching Units. Each of the heavy water plants consists of two i d e n t i c a l enriching units and one f i n i s h i n g u n i t . Each Enriching Unit can be divided broadly into three sections: the absorption and desorption section, the extraction section and the enriching section. The absorption and desorption section serves two functions: removal of H2S from the depleted water be fore i t is returned to the lake, and removal of H2S in the purge gas before i t is f l a r e d . As the feed water passes through t h i s section, i t is p a r t i a l l y saturated with H2S. In the extraction section (see "Figure 2") water passes counter-current to a r e c i r c u l a t i n g stream of hydrogen sulphide (H2S) gas in three large sieve tray towers operating in p a r a l l e l which are the f i r s t stage. By making use of a two temperature exchange reaction (see "Figure 3") between hydrogen sulphide (H2S) and water - at 30°C deuterium is concentrated in the l i q u i d phase and a t 130°C deuterium is concentrated in the gas phase. The three f i r s t stage towers (see "Figure 2") each have two d i s t i n c t process temperature sections the top being a cold section and the bottom being a hot section. Deuterium is c a r r i e d forward to the second stage only in the gas phase while anything not extracted from the water is returned to the lake from the f i r s t stage a f t e r i t has been stripped of H2S and cooled. I f H2S in the e f f l u e n t is not within the


2.

DAVIDSON

Bruce

Heavy Water Plant

29

Performance

Water Treating

D

2

0

Enrichingj 20-30% D

2

0

Finishing 99.73% Product


Treating

Candu Reactors

Figure 1.

BHWP-AD 0

Figure 2.

2

production

Enriching section

30°C H

2

0

(i> +

P D S (g) ^

±

P D O

(£) +

H

2

S

(g)

130°C

Figure 3.

GS process




30

r e g u l a t o r y l i m i t s i t is d i v e r t e d t o a e r a t e d lagoons where the gas is removed and the water is s e n t t o t h e lake. One p a r t in 7000 coming o u t o f t h e l a k e is deuterium. But i t is o n l y e c o n o m i c a l t o remove about 20% or one p a r t in 35000. The second and t h i r d s t a g e s o n l y e n r i c h o r conc e n t r a t e t h e d e u t e r i u m which has been e x t r a c t e d . The second s t a g e is a s i n g l e tower whose o p e r a t i o n is s i m i l a r t o t h e f i r s t s t a g e towers. However, t h e r e is no h e a t i n g s e c t i o n a t the bottom o f the h o t tower as the gas is a l r e a d y hot coming forward from the f i r s t stage. To m a i n t a i n l i q u i d c i r c u l a t i o n in the second s t a g e , water is pumped from t h e bottom o f the h o t tower, t h r o u g h a c o o l e r t o t h e top o f the c o l d tower. The t h i r d s t a g e c o n s i s t s o f two s e p a r a t e towers - one c o l d tower, t h e o t h e r a hot tower. T h i r d s t a g e o p e r a t i o n a l s o d i f f e r s from the p r e v i o u s two. Enriched l i q u i d from the bottom o f t h e c o l d tower is t a k e n f o r ward f o r f u r t h e r p r o c e s s i n g . The H 2 S in t h i s e n r i c h i n g u n i t l i q u i d p r o d u c t is s t r i p p e d out and r e t u r n e d t o t h e t h i r d s t a g e gas l o o p . E n r i c h e d water a t 20 - 30% D 0 c o n c e n t r a t i o n from b o t h e n r i c h i n g u n i t s is f e d t o t h e f i n i s h i n g u n i t . The f i n i s h i n g u n i t is a t h r e e s t a g e , f o u r tower steam vacuum d i s t i l l a t i o n system t h a t c o n c e n t r a t e s t h e p r o d u c t from t h e e n r i c h i n g u n i t s i n t o the f i n a l r e a c t o r grade heavy water. A l l f i n i s h i n g u n i t towers c o n t a i n sieve trays. The heavy water is then t r e a t e d in e i t h e r o f two p o t a s s i u m permanganate b a t c h k e t t l e s t o o x i d i z e o r g a n i c impurities. The p r o d u c t is e i t h e r drummed o r s h i p p e d in b u l k t o CANDU r e a c t o r s . T r a n s l a t e d i n t o r e a l i t y , the process u n i t s a r e shown in t h e a e r i a l view o f BHWP (see " F i g u r e 4 " ) . BHWP A water i n t a k e is shared w i t h the Douglas P o i n t Nuclear Generating S t a t i o n . The f i g u r e shows t h e b u i l d i n g h o u s i n g t h e sand f i l t e r s , t h e d e g a s s i n g towe r s , the main s w i t c h y a r d , t h e steam s u p p l y from Douglas P o i n t N u c l e a r G e n e r a t i n g S t a t i o n and t h e Bruce Steam P l a n t , t h e H S s t o r a g e b u l l e t s , t h e f l a r e (145 m or 475 f e e t above g r a d e ) , t h e l a g o o n s , t h e a b s o r p t i o n and d e s o r p t i o n s e c t i o n c o n s i s t i n g o f an Absorber tower, Purge tower and an E f f l u e n t S t r i p p e r tower, t h e t h r e e f i r s t s t a g e towers, the second s t a g e tower, the t h i r d s t a g e and t h e f o u r tower, t h r e e s t a g e F i n i s h i n g U n i t . The f i g u r e a l s o shows the bank o f heat exchangers a c r o s s the f r o n t o f each u n i t , used t o a c h i e v e t h e two p r o c e s s temperatures and o p t i m i z e steam u t i l i z a tion . 2

2


2.

DAVIDSON


Employee

Bruce

Heavy

Water

Plant

Performance

31

Safety

One m a j o r d i s a d v a n t a g e o f t h e D u a l T e m p e r a t u r e Hydrogen S u l p h i d e - Water Exchange Process i s t h e t o x i c i t y o f the H2Sgas and the p o t e n t i a l s a f e t y hazards i t poses. The g r e a t e s t d a n g e r f r o m t h e i n h a l a t i o n o f hydrogen sulphide i s from i t s acute e f f e c t s ; i t i s n o tcumulative i n a c t i o n . Exposure t o moderate c o n c e n t r a t i o n s causes headaches, d i z z i n e s s , nausea and vomiting i n that order. C o n t i n u e d e x p o s u r e may c a u s e l o s s o f consciousness, r e s p i r a t o r y f a i l u r e and death i f the gas c o n c e n t r a t i o n i s h i g h enough. Hydrogen S u l phide i s a l s o flammable and i n c e r t a i n mixtures w i t h a i r i t c a nbe e x p l o s i v e . From t h e o u t s e t , O n t a r i o Hydro o p e r a t i n g s t a f f e s t a b l i s h e d rigorous s a f e t y p o l i c i e s and procedures f o r a l l a s p e c t s o f c o m m i s s i o n i n g a n d o p e r a t i n g P l a n t A, w i t h s p e c i a l emphasis on the h a n d l i n g o f H2S and equipment c o n t a i n i n g i t . S a f e t y p e r f o r m a n c e a t B r u c e Heavy Water P l a n t i s p r e s e n t e d i n terms o f "H2S I n c i d e n t s " (see " F i g u r e 5") o r t h e number o f e m p l o y e e s t e m p o r a r i l y a f f e c t e d b y t h e t o x i c o l o g i c a l p r o p e r t i e s o f h y d r o g e n s u l p h i d e , number of l o s t time accidents and l o s t time accident frequency (see " F i g u r e 6"). "H2S I n c i d e n t s " are d e f i n e d as "sub-acute" o r "acute". B r i e f l y , a "sub-acute" i n c i d e n t i s one i n w h i c h a p e r s o n e x p o s e d t o H y d r o g e n S u l p h i d e shows s i g n s of b e i n g a f f e c t e d b u t does n o tr e q u i r e r e s u s c i t a t i o n or a s s i s t a n c e t o e x i t from the area a f f e c t e d by H2S, w h i l e an "acute" i n c i d e n t i s one i n which a person overcome by Hydrogen S u l p h i d e r e q u i r e s r e s u s c i t a t i o n and/or a s s i s t a n c e t o e x i t from the area a f f e c t e d by Hydrogen S u l p h i d e . Only one H2S I n c i d e n t has r e s u l t e d i na l o s t time a c c i d e n t ( i n 1975) when t h e a f f e c t s o f H 2 S c a u s e d a n employee t o f a l l and b r u i s e h i s r i b s . I n t h e t w o y e a r p e r i o d f r o m May 1 9 7 2 t o May 1 9 7 4 , w h i c h c o v e r e d t h e b u l k o f c o m m i s s i o n i n g , t h e r e was n o t a s i n g l e l o s t time accident. The two l o s t t i m e a c c i d e n t s i n 1976 were b o t h b a c k i n j u r i e s c a u s e d b y improper l i f t i n g techniques. F a c t o r s c o n t r i b u t i n g t o BHWP s a f e w o r k p e r f o r m a n c e ( s e e " F i g u r e 7") h a v e b e e n g r o u p e d u n d e r t h e b r o a d h e a d i n g s o f Management P o l i c i e s , S a f e t y T r a i n i n g , General T r a i n i n g , Procedures, Employee R e l a t i o n s , P l a n t I n t e g r i t y , P e r s o n a l P r o t e c t i v e Equipment and S a f e t y Equipment. A l l a r eimportant t o a successful s a f e t y program. A t no t i m e d u r i n g the h i s t o r y o f B r u c e Heavy Water




Figure 4. Aerial view of BHWP

1973

74

75

76

Figures. BHWP—History of H S incidents t


2.

3

Bruce

DAVIDSON

Heavy

Water

Plant

Performance

33

·

Lost T i m e

ι:·:·:·:·:.·ι :

Accidents

Frequency (/10

6

Rate

Manhours)


1 ·

Manhours

1973

74

75

76

772,575

737,906

965,870

1,230,626

Figure 6.

1.

Management

BHWP—Safety

4.

Policies

- the b u d d y s y s t e m

environment

- training

- work on H2S systems

- procedural approach

- control of work - equipment inspections.

- plant m o d i f i c a t i o n c o n t r o l - equipment

maintenance

5.

Safety

- communications.

orientation 6.

- emergency procedures

Plant

Integrity

- b u d d y training

- isolating circuits a n d devices

- r e s c u e training

- equipment design

- p r o t e c t i o n training - fire,

- e m e r g e n c y p o w e r , s t e a m , w a t e r a n d air - equipment inspections.

c h e m i c a l , electrical, etc. - W o r k Protection C o d e

7.

Personal Protective

- t a n k c a r repairs

- conventional

- v e s s e l entry

- breathing

- first aid

- rescue stations.

- regular m e e t i n g s . 3.

Relations

- Personal Hygiene

Training

- new employee

Employee - Medicals

- event reporting. 2.

Procedures - plant a c c e s s control

- n u m b e r 1 priority - work

performance

8.

Safety

Equipment

equipment

Equipment

General Training

- fire f i g h t i n g

- science fundamentals

- H2S detectors and monitors

- e q u i p m e n t / s y s t e m s principles

- gas detectors

- plant s y s t e m s

- rescue vehicle

- field skills.

- survey vehicles - first aid r o o m .

Figure 7.

Factors contributing to safe work performance


34

SEPARATION

P l a n t has i t s o p e r a t i o n p r e s e n t e d and s a f e t y o f the p u b l i c . Care o f t h e

O F HYDROGEN

a r i s k t o the

health

Environment

Bruce Heavy Water P l a n t has two major e n v i r o n mental c o n s i d e r a t i o n s - H 2 S e m i s s i o n s t o atmosphere and H 2 S e m i s s i o n s t o water. On the f i r s t o f t h e s e , Hydrogen S u l p h i d e i s an odourous m a t e r i a l . I f concent r a t i o n s r e a c h more than 2 0 0 yg/m (micrograms per metre cubed) H 2 S can be s m e l l e d by most p e o p l e and i s c o n s i d e r e d by v i r t u a l l y a l l as an u n a c c e p t a b l e odour. The p l a n t i s s i t u a t e d i n a r u r a l and summer r e s o r t area. C o m p l a i n t s o f odour s t a r t e d t o be r e c e i v e d s h o r t l y a f t e r H S was i n t r o d u c e d i n t o the u n i t s i n 1973. By y e a r end, a t o t a l o f 56 c o m p l a i n t s were r e g i s t e r e d (see " F i g u r e 8"). A t a s k f o r c e was formed to r e c t i f y t h i s u n s a t i s f a c t o r y s i t u a t i o n . Several a c t i o n s were t a k e n : e f f o r t s were d i r e c t e d toward r e d u c i n g the amount o f H 2 S r e l e a s e d ; s t e p s were taken t o i n s u r e a c o m b u s t i b l e m i x t u r e o f s t a c k gas a t a l l t i m e s ; d r a i n water t o be s t r i p p e d o f H 2 S was m i n i m i z e d ; r e l e a s e s o f steam o r n i t r o g e n and o t h e r i n e r t s t o t h e f l a r e were accompanied by i n c r e a s e d propane f l o w s t o the f l a r e . In 1974, t h r e e odour r e p o r t s came i n , i n 1975 f o u r odour r e p o r t s were r e c e i v e d and i n 1976 t h e r e were two. R e l a t i o n s w i t h t h e p u b l i c a r e no l o n g e r a problem as f a r as H 2 S odours a r e c o n c e r n e d . An H 2 S r e c o v e r y system t o m i n i m i z e H 2 S and p r o pane usage i s b e i n g b u i l t as p a r t o f t h e p l a n t expansion. As a r e s u l t , H 2 S r e l e a s e s t o the f l a r e w i l l be f u r t h e r r e d u c e d . On the second e n v i r o n m e n t a l c o n s i d e r a t i o n , H 2 S e m i s s i o n s t o water, a l l water streams c o n t a i n i n g H 2 S must be s t r i p p e d o f i t b e f o r e the water i s r e t u r n e d to the l a k e . " F i g u r e 8" shows the number o f times the r e g u l a t o r y l i m i t f o r hydrogen s u l p h i d e i n water was exceeded by y e a r . U n s t a b l e f l o w s i n the towers were q u i t e f r e q u e n t i n 1973. T h i s i n s t a b i l i t y caused the e f f l u e n t s t r i p p e r operating conditions to f l u c t u a t e d e t e r r i n g i t s e f f e c t i v e n e s s . P h y s i c a l changes t o tower t r a y w e i r s and the a d d i t i o n o f a n t i f o a m t o t h e p r o c e s s water s t e a d i e d o p e r a t i n g c o n d i t i o n s and t h e r e f o r e improved the e f f e c t i v e n e s s o f the s t r i p p e r . Replacement o f u n r e l i a b l e i n s t r u m e n t a t i o n and b e t t e r p r o c e s s c o n t r o l d u r i n g s t a r t u p s and shutdowns have improved performance. A continuous H2S-in-water m o n i t o r which has p r o v e n r e l i a b l e w i t h q u i c k r e s p o n s e was i n s t a l l e d on the s t r i p p e r o u t l e t . This monitor p l u s o t h e r e f f l u e n t s t r i p p e r p r o c e s s parameters o f 3


ISOTOPES

2


2.

DAVIDSON

Bruce

Heavy

Water

Plant

Performance

35

f e e d temperature, l e v e l and p r e s s u r e have a l l been interlocked to automatically d i v e r t e f f l u e n t to the lagoons i f a p r e s e t c o n c e n t r a t i o n o f H 2 S i s exceeded o r t h e measured v a r i a b l e s exceed d e f i n e d l i m i t s .


Reliability The r e l i a b i l i t y o f BHWP A has g e n e r a l l y been good f o r a p l a n t o f i t s s i z e w i t h m i n i m a l redundancy i n i t s o p e r a t i n g equipment. " F i g u r e 9" shows downtime e x p e r i e n c e t o d a t e . P l a n n e d u n i t t u r n - a r o u n d s have been r e d u c e d from 10 weeks i n 1973 t o 4 weeks i n 1975. An outage p l a n n e d f o r September, 1976 was advanced t o t a k e advantage o f an unplanned outage i n J u l y . Another f a c t o r o f r e l i a b i l i t y i s production r a t e . " F i g u r e 10" shows i n s t a n t a n e o u s p r o d u c t i o n r a t e s i n k i l o g r a m s p e r hour t h a t t h e p l a n t was o p e r a t i n g , i . e . shutdown p e r i o d s have been e x c l u d e d . The o r i g i n a l Des i g n C a p a c i t y was 48.3 kg/h f o r each e n r i c h i n g u n i t . F o l l o w i n g s t a r t up i n 1973, t r a y f l o o d i n g problems caused by foaminess o f t h e p r o c e s s and a d e f i c i e n c y i n t r a y d e s i g n were e x p e r i e n c e d . C o r r e c t i o n o f these problems by t h e a d d i t i o n o f a n t i f o a m t o t h e feedwater and m o d i f i c a t i o n o f t h e t r a y s r e s u l t e d i n d r a m a t i c improvements i n p r o d u c t i o n r a t e s . P r o d u c t i o n r a t e s have c o n t i n u e d t o improve, r e s u l t i n g i n a r e - r a t i n g o f t h e p l a n t c a p a c i t y i n 1976 t o 100.6 kg/h. C a p a c i t y F a c t o r s (based on D e s i g n C a p a c i t y f o r 1973 t o 1975 and based on Demonstrated C a p a c i t y f o r 1976) and a n n u a l p r o d u c t i o n a r e shown i n " F i g u r e 11". These do n o t i n c l u d e about 99 Mg t h a t were " e n r i c h e d " t o 20% i s o t o p i c p u r i t y d u r i n g 1974/1975/1976, o f which 31 Mg were " f i n i s h e d " t o r e a c t o r grade elsewhere i n 1975 and 42 Mg were " f i n i s h e d " elsewhere i n " 1976. I f t h e s e " f i n i s h e d " q u a n t i t i e s were c o m p l e t e l y c r e d i t e d t o BHWP A, t h e apparent C a p a c i t y F a c t o r s i n 1975 and 1976 would be 71.7% and 90.9% r e s p e c t i v e l y . Manpower Development The purpose o f t r a i n i n g i s t o e n s u r e t h a t each p o s i t i o n i n t h e p l a n t i s f i l l e d by a p e r s o n w i t h a p p r o p r i a t e knowledge and s k i l l s so t h a t t h e p l a n t i s o p e r a t e d s a f e l y , e f f e c t i v e l y and e f f i c i e n t l y . T r a i n i n g i s therefore a part o f the plant's p e r f o r mance o b j e c t i v e s . Formal t r a i n i n g i s p r o v i d e d by l e c t u r e d c o u r s e s and demonstrated s k i l l s i n t h e c l a s s r o o m and t h e f i e l d . F o r each j o b , t h e l e v e l and p r o f i c i e n c y which must be


36

SEPARATION

O F HYDROGEN

Odour

ΓΤ!Τ·!·!·!·!·!·!:!:Ι

i.'.',.„'.'.'.'.'.'|

ι

ISOTOPES

Reports

•!


H2S E m i s s i o n s t o W a t e r

mam Figure 8.

%

rnmrn

74

1973

75

76

Environmental performance to date

Downtime

Enriching

Finishing

Units

Planned E$:$:$:3 F o r c e d

Unit

V////A T o t a l

Outages

Outages

Outages

ρ

m ϋ m

S 1973

74 Figure 9.

75

76

BHWP—Reliability



2.

DAVIDSON

Bruce

Heavy

Water

Plant

Performance

37

a t t a i n e d are s p e c i f i e d i n each o f the f o l l o w i n g : Management, S c i e n c e Fundamentals, Equipment and System P r i n c i p l e s , F i e l d S k i l l s , S p e c i f i c P l a n t Systems, AECB Operating License, Protection T r a i n i n g . T r a i n i n g i s p r o v i d e d by the N u c l e a r T r a i n i n g De partment o f O n t a r i o Hydro a t R o l p h t o n , by the Manpower Development Department o f O n t a r i o Hydro a t O r a n g e v i l l e , by the Bruce T r a i n i n g C e n t r e a t the s i t e and by the BHWP T r a i n i n g S e c t i o n . BHWP T r a i n i n g S e c t i o n i s s t a f f e d by e i g h t T r a i n i n g T e c h n i c i a n s , two e n g i n e e r s and a T r a i n i n g O f f i c e r . BHWP o p e r a t e s a f i v e s h i f t system i n s t e a d o f t h e t r a d i t i o n a l f o u r , so t h a t o p e r a t o r s , m a i n t a i n e r s and c o n t r o l t e c h n i c i a n s on s h i f t spend t e n t o f o u r t e e n p e r c e n t o f t h e i r time i n t r a i n i n g . I n 1975 t h e r e were 6,822 man c o u r s e s g i v e n and i n 1976 t h e r e were 11,400 man c o u r s e s g i v e n a t BHWP. Commissioning P r o g r e s s Report - BHWP Β A s s e m b l i n g a commissioning team o f e n g i n e e r s , o p e r a t o r s , m a i n t a i n e r s and c o n t r o l t e c h n i c i a n s f o r P l a n t Β began i n 1975. A nucleus o f personnel with o p e r a t i n g e x p e r i e n c e i n P l a n t A i s b e i n g augmented w i t h new employees. The commissioning group i s p r e p a r i n g commissioning e s t i m a t e s and p r o c e d u r e s , and i n s p e c t i n g and w i t n e s s i n g equipment t e s t s as the p l a n t i s being b u i l t . T h i s group i s commissioning and o p e r a t i n g the v a r i o u s p l a n t systems as t h e y a r e t u r n e d o v e r from c o n s t r u c t i o n . The a i r compressor system f o r P l a n t Β and t h e u t i l i t i e s f o r F i n i s h i n g U n i t F2 were t u r n e d o v e r from c o n s t r u c t i o n and commissioned i n the l a s t q u a r t e r o f 1976. The l a s t t u r n o v e r o f the F i n i s h i n g U n i t p r o c e s s system was on 6 J a n u a r y , 1977. P r e - s t a r t u p checks o f equipment and i n s t r u m e n t a t i o n were completed by 18 F e b r u a r y , 1977. O p e r a t i o n o f F2 on d e m i n e r a l i z e d water f o l l o w e d and t h i s program was completed by the end o f A p r i l . Equipment i n s p e c t i o n s f o l l o w i n g t h i s run and minor d e f i c i e n c y c o r r e c t i o n s were completed by 10 May, 1977 and s t a r t u p on i n t e r m e d i a t e p r o d u c t from P l a n t A commenced. F i r s t r e a c t o r grade p r o d u c t from F2 was produced on 23 May, 1977. The p l a n (see " F i g u r e 12") shows l a s t t u r n o v e r o f E n r i c h i n g U n i t E4 equipment f o r commissioning 1 May, 1978. E n r i c h i n g U n i t E3 l a s t t u r n o v e r i s s c h e d u l e d f o r 15 December, 1978. T h i s p l a n a l s o shows P l a n t D F i n i s h i n g U n i t F4 l a s t t u r n o v e r f o r commissioning a t the end o f September, 1979 w i t h E n r i c h i n g U n i t s 7 and


38

SEPARATION

OF

HYDROGEN

ISOTOPES

Extraction kg D 0 / h 2

60· Hot Tower

Clarifier Tests Alum Addition June 74

40
CO Su


NiTscHKE ET AL.

6.

UHDE

Process

83


s y n t h e s i s u n i t and r e t u r n e d t o i t a f t e r p a s s i n g t h e p u r i f i c a t i o n unit. C a r b o n monoxide and a l s o o x y g e n , i f t h e l a t t e r is p r e s e n t in t h e g a s , a r e removed b y s c r u b b i n g w i t h p o t a s s i u m amide solution. A special design o f the trays eliminates the r i s k o f c l o g g i n g . The r e a c t i o n p r o d u c t s f o r m e d in t h i s s t a g e a r e r e moved o u t s i d e t h e c o l u m n and t h e p o t a s s i u m amide s o l u t i o n is f e d to t h e t o p a g a i n . T r a n s f e r Column. The g a s t h e n p a s s e s t h r o u g h t h e t r a n s f e r c o l u m n where t h e d e u t e r i u m is t r a n s f e r r e d t o t h e ammonia a n d t h e s y n g a s is r e t u r n e d t o t h e ammonia s y n t h e s i s u n i t . I n washing t r a y s a t t h e t o p o f t h e t r a n s f e r column e n t r a i n e d p o t a s s i u m amide is removed. The e n r i c h e d ammonia l e a v i n g t h e b o t t o m o f t h e t r a n s f e r c o l u m n is p r e s s u r i z e d t o t h e p r e s s u r e o f t h e downstream f i r s t stage. F i r s t S t a g e a n d S t r i p p i n g System. The f i r s t s t a g e c o n s i s t s o f t h e b i t h e r m a l s e c t i o n p r o p e r a n d t h e s t r i p p i n g s y s t e m . The o p e r a t i o n o f t h e i n d i v i d u a l s e c t i o n s o f t h e f i r s t stage might b e s t be e x p l a i n e d w i t h t h e a i d o f t h e M c C a b e - T h i e l e diagrams ( F i g u r e 4 ) . H e r e , N ( 0 ) is t h e c o n c e n t r a t i o n o f t h e l i q u i d ammonia f e d f r o m t h e b o t t o m o f t h e t r a n s f e r c o l u m n t o t h e t o p o f t h e c o l d column o f t h e e n r i c h m e n t s y s t e m . In t h e c o l d c o l u m n , d e u t e r i u m is t r a n s f e r r e d f r o m t h e g a s t o t h e l i q u i d ; in o t h e r w o r d s , we a r e g o i n g up t h e w o r k i n g l i n e . A t t h e b o t t o m o f t h e c o l d c o l u m n , d e u t e r i u m - b e a r i n g h y d r o g e n is removed a n d t r a n s f e r r e d t o t h e n e x t s t a g e a n d a c o r r e s p o n d i n g s t r e a m is r e t u r n e d ; h o w e v e r , w i t h a l o w e r d e u t e r i u m c o n c e n t r a tion. The c o n c e n t r a t i o n c h a n g e s f r o m N ( z ) t o N ( z ) a n d we f o l l o w the w o r k i n g l i n e o f t h e h o t column as f a r as t o t h e bottom o f t h e h o t c o l u m n . Here t h e g a s s t r e a m is s p l i t . On t h e McCabeT h i e l e d i a g r a m , t h i s means a change in t h e s l o p e o f t h e w o r k i n g line. I n t h e h o t s t r i p p i n g c o l u m n d e u t e r i u m c o n t i n u e s t o be t r a n s f e r r e d t o t h e gas and t h e l o w e s t deuterium c o n c e n t r a t i o n N ( Z ) is f i n a l l y r e a c h e d a t t h e b o t t o m o f t h e h o t s t r i p p i n g column. P r i o r t o e n t e r i n g t h e c o l d s t r i p p i n g column, t h e ammonia, w h i c h is b e i n g r e t u r n e d t o t h e t o p o f t h e t r a n s f e r column, is removed. T h i s a l t e r s t h e s l o p e o f t h e w o r k i n g l i n e a g a i n a n d in t h e c o l d s t r i p p i n g c o l u m n we come b a c k t o t h e feeding point. T h i s d i a g r a m a l s o shows t h a t t h e s l o p e o f t h e w o r k i n g l i n e is l i m i t e d b y t h e s l o p e o f t h e two e q u i l i b r i u m l i n e s a t t h e t e m p e r a t u r e o f t h e h o t a n d c o l d c o l u m n . When d e s i g n i n g t h e p l a n t , t h e s l o p e o f t h e w o r k i n g l i n e s must be d e t e r m i n e d in s u c h a way t h a t o n l y a minimum o f t r a y s is r e quired. As t h e t r a y e f f i c i e n c y in t h e h o t and c o l d c o l u m n s is d i f f e r e n t , t h i s minimum r e f e r s t o t h e number o f a c t u a l t r a y s ; in o t h e r w o r d s , t h e c o l u m n volume o f t h e s y s t e m is t o be m i n i m i z e d . 1

2


In Separation of Hydrogen Isotopes; Rae, H.; ACS Symposium Series; American Chemical Society: Washington, DC, 1978. Figure 4.

McCabe-Thiele

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I n t h e f i r s t s t a g e t h e r e a r e two s u p e r i m p o s e d gas c y c l e s . However, o n l y one c i r c u l a t i n g c o m p r e s s o r is s u f f i c i e n t as t h e d i f f e r e n t p r e s s u r e d r o p s in b o t h c y c l e s c a n be u s e d f o r c o n t r o l l i n g t h e gas r a t e s . When b e i n g t r a n s f e r r e d f r o m t h e h o t c o l u m n t o t h e c o l d c o l u m n and v i c e v e r s a , t h e gas and l i q u i d s t r e a m s are h e a t e d and c o o l e d r e s p e c t i v e l y . I t is o b v i o u s t h a t t h i s must be done in a way t o c o n s e r v e t h e h e a t in o r d e r t o r e d u c e t h e energy consumption. F i g u r e 5 shows t h e e n t h a l p y / t e m p e r a t u r e d i a g r a m f o r s y n t h e s i s gas w h i c h is s a t u r a t e d w i t h ammonia and t h e a r r a n g e m e n t of the h e a t exchanges f o r c o o l i n g t h e s y n t h e s i s gas from the h o t column. Downstream o f a d i r e c t h e a t e x c h a n g e r , w h i c h is a r r a n g e d i n s i d e t h e h o t c o l u m n , t h e h e a t is t r a n s f e r r e d t o l i q u i d ammonia a t a r e l a t i v e l y h i g h t e m p e r a t u r e w h i c h t h e n t r a n s f e r s t h i s h e a t t o t h e gas f o r s a t u r a t i o n . The gas t h e n f l o w s t h r o u g h a water c o o l e r and t h e n t h r o u g h a gas/gas exchanger. The r e m a i n i n g gap t o come down t o t h e t e m p e r a t u r e o f t h e c o l d c o l u m n is p r o v i d e d by t h e r e f r i g e r a t i o n p l a n t and t r a n s f e r r e d f r o m the gas in two h e a t e x c h a n g e r s , c o n n e c t e d in s e r i e s . These o p e r a t e a t d i f f e r e n t t e m p e r a t u r e s in o r d e r t o r e d u c e t h e e n e r g y consumption f o r the r e f r i g e r a t i o n compressor. Altogether, a l m o s t 70% o f t h e h e a t r e l e a s e d d u r i n g c o o l i n g is u t i l i z e d . The a r r a n g e m e n t o f t h e h e a t e x c h a n g e r s and t h e d e t e r m i n a t i o n o f t h e w o r k i n g r a n g e is n o t a r b i t r a r y , b u t is d e t e r m i n e d by t h e e n e r g y c o s t s and t h e c a p i t a l i n v e s t m e n t f o r h e a t e x c h a n g e r s , t a k i n g i n t o account the s p e c i a l c o n d i t i o n s a t the s i t e . T h i s p r o b l e m o f o p t i m i z a t i o n was r e s o l v e d s i m u l t a n e o u s l y w i t h t h e p r o b l e m o f m i n i m i z i n g t h e c o l u m n v o l u m e by s e t t i n g up a p p r o p r i a t e computer programs. T h i s work was done in a v e r y c l o s e c o o p e r a t i o n w i t h the I n s t i t u t e f o r Nuclear Technology a t K a r l s r u h e U n i v e r s i t y (4). The c o o p e r a t i o n is n o t c o n f i n e d t o t h i s s u b j e c t b u t a l s o t o s o l v i n g o t h e r p r o b l e m s , in p a r t i c u l a r the p e r f o r m a n c e o f p i l o t p l a n t t e s t s w h i c h s u p p l i e d t h e r e s u l t s on w h i c h t h e d e s i g n o f t h e p r o c e s s is b a s e d ( 3 ) , C5) . Second and T h i r d S t a g e . The a r r a n g e m e n t o f t h e s e c o n d and t h i r d s t a g e is a n a l o g u e t o t h e f i r s t s t a g e . I n e a c h s t a g e t h e d e u t e r i u m c o n t e n t is i n c r e a s e d by a f a c t o r a r o u n d 9. Ammonia w h i c h h a s b e e n e n r i c h e d t o 15-20% d e u t e r i u m c a n t h e n be t a k e n o u t b e t w e e n t h e h o t and c o l d c o l u m n o f t h e t h i r d and l a s t s t a g e . Since the streams are s m a l l e r , i . e . i n v e r s e l y p r o p o r t i o n a l t o the e n r i c h m e n t f a c t o r , t h e h e a t r e c o v e r y s y s t e m b e t w e e n h o t and c o l d c o l u m n is made s i m p l e r . F i n a l C o n c e n t r a t i o n . A f t e r c a r e f u l e x a m i n a t i o n Uhde d e c i d e d f o r a w a t e r d i s t i l l a t i o n as f i n a l c o n c e n t r a t i o n . W a t e r d i s t i l l a t i o n is a w e l l - p r o v e n p r o c e s s and c a n be o p e r a t e d u n d e r n o r m a l p r e s s u r e and t e m p e r a t u r e . F o r t h i s , i t is n e c e s s a r y t o t r a n s f e r t h e d e u t e r i u m f r o m the ammonia t o w a t e r . The e n r i c h e d ammonia w h i c h h a d b e e n




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s e p a r a t e d f r o m t h e c a t a l y s t t h r o u g h e v a p o r a t i o n , is t h u s b r o u g h t i n t o contact w i t h water. Both r e a c t i o n p a r t n e r s a r e separated a g a i n in a d i s t i l l a t i o n s e c t i o n a t t o p and b o t t o m o f t h e c o l u m n , t h e ammonia is f e d b a c k t o t h e t h i r d s t a g e . The e n r i c h e d w a t e r is f e d t o a s i n g l e - s t a g e d i s t i l l a t i o n c o l u m n . The h e a v y w a t e r w i t h a c o n t e n t o f o v e r 9 9 . 8 % D 0 c a n f i n a l l y be w i t h d r a w n f r o m the bottom o f t h e d i s t i l l a t i o n column. B a s e d on t h i s c o n s i d e r a t i o n t h e f i n a l c o n c e n t r a t i o n h a s b e e n d e s i g n e d and s u p p l i e d by S u l z e r B r o s . , W i n t e r t h u r .


2

O p e r a t i o n o f t h e P l a n t . The d i f f e r e n t s e c t i o n s o f t h e p l a n t - t h e gas p u r i f i c a t i o n s e c t i o n w i t h t r a n s f e r column, t h e f i r s t s t a g e w i t h s t r i p p i n g s y s t e m , t h e s e c o n d and t h i r d s t a g e and l a s t l y , t h e f i n a l p u r i f i c a t i o n s e c t i o n , c a n be o p e r a t e d i n d e p e n d e n t l y ; t h a t means i f one o f them h a s t o be s h u t down, t h e o t h e r s c a n still be o p e r a t e d . This reduces the process c o n t r o l p r o b l e m s and i n c r e a s e s p l a n t r e l i a b i l i t y . I t is a c h a r a c t e r i s t i c f e a t u r e o f the process t h a t the steady s t a t e c o n d i t i o n s , i . e . t h e f i n a l c o n c e n t r a t i o n p r o f i l e in t h e c o l u m n s , a r e r e a c h e d o n l y a f t e r a p e r i o d o f a few d a y s . Also therefore e a c h s e c t i o n is p r o v i d e d w i t h a s e p a r a t e c o l l e c t i n g t a n k f o r t h e l i q u i d h o l d up in o r d e r t o a v o i d m i x i n g o f l i q u i d s w i t h d i f f e r e n t i s o t o p e c o n c e n t r a t i o n s , and so t o f a c i l i t a t e r e - s t a r t i n g . Engineering

Problems

Apart from q u e s t i o n s d e a l i n g w i t h t h e process d e s i g n e n g i n e e r i n g t h e r e were a l s o s e v e r a l m e c h a n i c a l e n g i n e e r i n g p r o b l e m s t o be s o l v e d . The p l a n t is d e s i g n e d f o r much t h e same p r e s s u r e a n d t e m p e r a t u r e r a n g e as an ammonia s y n t h e s i s p l a n t ; c o n s e q u e n t l y , t h e same s o r t o f t e c h n i q u e s c a n be u s e d for the design. This a p p l i e s p a r t i c u l a r l y t o the heat exchangers as w e l l as t h e c h o i c e o f o t h e r equipment. The h i g h - p r e s s u r e c o l u m n s i n v o l v e new p r o b l e m s . The d i a m e t e r o f t h e c o l d c o l u m n in t h e f i r s t s t a g e is 2 m and t h e d i a m e t e r o f t h e c o r r e s p o n d i n g h o t c o l u m n s is l a r g e r still due t o t h e g r e a t e r g a s and l i q u i d l o a d . The c o l u m n s a r e up t o 40 m h i g h . The c o l d c o l u m n s o f t h e e n r i c h m e n t s y s t e m a n d o f t h e t r a n s f e r s e c t i o n a r e d i v i d e d in two p a r t s in s e r i e s . A t an o p e r a t i n g p r e s s u r e o f 325 k g / c m t h e c o l u m n w e i g h t is c o n s i d e r a b l e . The l a r g e s t c o l u m n , w h i c h h a s a d e a d w e i g h t o f 270 t , must be t r a n s p o r t e d t o t h e p l a n t s i t e in one p i e c e and e r e c t e d t h e r e . T h e r e f o r e t h e t r a n s p o r t a t i o n c o n d i t i o n s s h o u l d be r e v i e w e d c a r e f u l l y b e f o r e d e c i d i n g o n t h e l o c a t i o n f o r a plant of this kind. A l l towers are equipped w i t h s i e v e t r a y s , which p r o v i d e a good mass t r a n s f e r b e t w e e n t h e p h a s e s . A c c o r d i n g t o t h e s p e c i a l f e a t u r e s o f the system w i t h regard t o the k i n e t i c s o f the r e a c t i o n and t h e h y d r o d y n a m i c c h a r a c t e r i s t i c s o f t h e f l u i d s , t h e d e s i g n o f t h e s i e v e t r a y s d i f f e r s from t h e d e s i g n n o r m a l l y used 2


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d i s t i l l a t i o n or a b s o r p t i o n towers. O x y g e n - b e a r i n g s u b s t a n c e s a r e c o m p l e t e l y removed f r o m t h e gas due t o t h e p r e s e n c e o f p o t a s s i u m amide. I t is w e l l known t h a t hydrogen o f extreme p u r i t y reduces the n o t c h impact s t r e n g t h of high-grade s t e e l s . T e s t s have shown, h o w e v e r , t h a t s u c h a r i s k is u n l i k e l y u n d e r t h e c o n d i t i o n s p r e v a i l i n g h e r e . For s a f e t y r e a s o n s s t e e l s a r e p r e f e r r e d , w h i c h h a v e a r e l a t i v e l y low s t r e n g t h and good d u c t i l i t y , e v e n a t low t e m p e r a t u r e s , a s is t h e case f o r higher n i c k e l - a l l o y e d s t e e l s . P o t a s s i u m amide in ammonia a c t s v e r y s t r o n g l y on o r g a n i c m a t e r i a l s , so e v e n m a t e r i a l s l i k e T e f l o n c o r r o d e w i t h i n a short time. A v a r i e t y o f m a t e r i a l s were e x a m i n e d in a u t o c l a v e t e s t s in o r d e r t o f i n d m a t e r i a l s w h i c h a r e r e s i s t a n t t c p o t a s s i u m amide a t t h e p r e v a i l i n g o p e r a t i n g c o n d i t i o n s . L a t e r t h e s e m a t e r i a l s a r e t e s t e d u n d e r o p e r a t i n g c o n d i t i o n s in a s p e c i a l facility. Even t o d a y t h i s is done t o t e s t m a t e r i a l f o r pump s e a l s and s t u f f i n g b o x e s w h i c h a r e new on t h e m a r k e t . A l l t h e s e q u e s t i o n s have b e e n e x a m i n e d v e r y c a r e f u l l y , f i r s t on a l a b o r a t o r y s c a l e and t h e n on a p i l o t - p l a n t s c a l e in o r d e r t o f i n d a r e l i a b l e s o l u t i o n f o r the design of a commercial p l a n t . I n t e g r a t i o n o f Heavy W a t e r P l a n t and Ammonia

Synthesis

T h i s p r o c e s s is b a s e d on h y d r o g e n as t h e s o u r c e o f d e u t e r i u m . T h i s h y d r o g e n can be o b t a i n e d f o r example f r o m t h e g a s i f i c a t i o n u n i t o f t h e ammonia p l a n t . The h e a v y w a t e r p l a n t is s t r i p p i n g t h e d e u t e r i u m f r o m t h e s y n g a s , w h i c h is t h e n r e t u r n e d t o t h e ammonia s y n t h e s i s l o o p . T h i s c o u p l i n g is o f v i t a l i m p o r t a n c e f o r t h e o p e r a t i o n o f ammonia p l a n t s . I t must be c e r t a i n t h a t t h e p r o d u c t i o n and a l s o t h e o n - s t r e a m t i m e o f t h e ammonia p l a n t are not a f f e c t e d . F i g u r e 6 shows t h e scheme o f i n t e g r a t i o n o f a h e a v y w a t e r p l a n t w i t h an ammonia p l a n t . 1.

2.

The s y n t h e s i s gas p r o d u c e d in t h e g a s i f i c a t i o n s e c t i o n is p u r i f i e d in t h e u s u a l way and f e d t o t h e s y n t h e s i s gas c o m p r e s s o r , where i t is b r o u g h t up t o t h e p r e s s u r e o f t h e ammonia s y n t h e s i s u n i t and a d m i x e d t o t h e s y n t h e s i s gas c y c l e as f e e d g a s . A f t e r the l a s t stage o f the syngas c o m p r e s s o r , t h e s y n t h e s i s gas is d i v e r t e d t o t h e h e a v y w a t e r p l a n t . The s y n t h e s i s gas w h i c h is d e p l e t e d o f d e u t e r i u m is t h e n r e t u r n e d t o t h e ammonia s y n t h e s i s u n i t . The h e a v y w a t e r p l a n t in f a c t c o n s t i t u t e s a b y - p a s s t o t h e ammonia s y n t h e s i s u n i t . The s y n t h e s i s gas p a s s e s o n l y t h r o u g h t h e gas p u r i f i c a t i o n c o l u m n and t h e t r a n s f e r column o f t h e h e a v y w a t e r p l a n t r e s u l t i n g in a c e r t a i n p r e s s u r e d r o p . T h i s p r e s s u r e d r o p w o u l d have t o be made up by t h e s y n t h e s i s gas c o m p r e s s o r . However, i t is b e t t e r t o i n s t a l l an a d d i t i o n a l b o o s t e r comp r e s s o r w h i c h compensates f o r t h i s p r e s s u r e drop. In the


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SEPARATION OF

HYDROGEN ISOTOPES

e v e n t o f f a i l u r e o f t h e NH s y n t h e s i s u n i t , t h e gas p u r i f i c a t i o n c o l u m n and t h e t r a n s f e r c o l u m n can be o p e r a t e d in a closed c i r c u i t . A l s o in t h i s c a s e t h e r e is no n e e d f o r a v a l v e in t h e b y - p a s s l i n e , t h u s e s t a b l i s h i n g an open bypass. I n t h e c a s e o f f l u c t u a t i o n in t h e gas f l o w t h e h e a v y water p l a n t runs under c o n s t a n t c o n d i t i o n s ; o n l y the d i r e c t i o n f l o w in t h e b y - p a s s l i n e w i l l be c h a n g e d . The f l o w can s l o w l y be a d j u s t e d t o new c o n d i t i o n s a c c o r d i n g l y . The same is t h e c a s e i f t h e r e a r e d i s t u r b a n c e s in t h e h e a v y water p l a n t . The s y n t h e s i s gas w h i c h is r e t u r n e d t o t h e s y n t h e s i s u n i t is s a t u r a t e d w i t h ammonia a t a dew p o i n t o f -28° C c o r r e sponding t o the temperature a t the top o f the c o l d t r a n s f e r c o l u m n . A t a p r e s s u r e o f 200 k g / c m in t h e s y n t h e s i s u n i t , t h e ammonia c o n t e n t o f t h i s s t r e a m is t h u s a p p r o x i m a t e l y 1.2%. I f t h i s gas is a d m i x e d t o t h e r e c y c l e gas o f t h e s y n t h e s i s u n i t u p s t r e a m o f t h e f i r s t s e p a r a t o r , i t has no e f f e c t on t h e s y n t h e s i s c y c l e . However, i f t h e gas is f e d t o t h e r e c y c l e gas d i r e c t l y u p s t r e a m o f t h e c o n v e r t e r , as is t h e c a s e sometimes in s y n t h e s i s u n i t s where n i t r o g e n s c r u b b i n g is u s e d f o r gas p u r i f i c a t i o n , t h e gas e n t e r i n g t h e c o n v e r t e r w o u l d have a s l i g h t l y h i g h e r ammonia c o n t e n t and t h i s w o u l d r e s u l t in a c o r r e s p o n d i n g d e c r e a s e in t h e N H 3 c o n v e r s i o n r a t e in t h e c o n v e r t e r . T h i s can be compens a t e d by i n c r e a s i n g t h e p r e s s u r e o f t h e s y n t h e s i s l o o p , b u t i t is b e t t e r t o compensate t h i s h i g h e r ammonia c o n t e n t by s l i g h t l y i n c r e a s i n g t h e c o l d d u t y o f t h e l a s t s e p a r a t o r o f t h e ammonia s y n t h e s i s u n i t , in o r d e r t o k e e p t h e d e s i g n c o n d i t i o n s of the converter constant. The t e m p e r a t u r e o f t h e r e t u r n gas is l o w e r t h a n t h a t o f t h e f e e d g a s , b e c a u s e t h e h e a t e x c h a n g e r must have a c e r t a i n temperature gradient. F u r t h e r m o r e , t h e gas c o n t a i n s no t r a c e s o f e i t h e r CO o r o x y g e n w h i c h u s u a l l y have a d e t r i m e n t a l e f f e c t on t h e ammonia c a t a l y s t . The l a s t two f a c t o r s m e n t i o n e d have a f a v o u r a b l e e f f e c t on t h e o p e r a t i o n o f t h e ammonia s y n t h e s i s u n i t , a l t h o u g h t h e b e n e f i t s c a n n o t be d e f i n e d q u a n t i t a t i v e l y . An e s s e n t i a l p o i n t o f t h e i n t e g r a t i o n c o n c e r n s t h e g a s i f i c a t i o n s e c t i o n . The gas p u r i f i c a t i o n s h o u l d be d e s i g n e d in s u c h a way as t o k e e p t h e c o n t e n t o f o x y g e n - b e a r i n g i m p u r i t i e s as low as p o s s i b l e in o r d e r t o r e d u c e t h e l o a d on t h e gas p u r i f i c a t i o n s e c t i o n o f t h e h e a v y w a t e r p l a n t . However, t h e d e u t e r i u m c o n t e n t in t h e gas is more i m p o r t a n t and s h o u l d , o f c o u r s e , be k e p t as h i g h as p o s s i b l e . The d e u t e r i u m c o n t e n t is d e t e r m i n e d by t h e d e u t e r i u m c o n t e n t o f t h e f e e d s t o c k s u s e d f o r t h e ammonia, e.g. methane and w a t e r ; b u t , as we h a v e s e e n f r o m numerous measurements o f the d e u t e r i u m content o f samples taken a t v a r i o u s p a r t s o f ammonia p l a n t s , i t c a n be n e g a t i v e l y i n f l u e n c e d by p r o c e s s e s w i t h i n t h e g a s i f i c a t i o n s e c t i o n . A d e u t e r i u m e x c h a n g e can 3

3.


2

4.

5.

6.



NiTscHKE ET AL.

Figure 7.

UHDE

Process

Model of a heavy water plant (capacity 631/yr)



92


take p l a c e between the hydrogen and the excess steam i n the CO s h i f t conversion,the deuterium being t r a n s f e r r e d t o the steam. The hydrogen thus becomes depleted o f deuterium and the steam becomes e n r i c h e d . This steam i s condensed upstream o f the CO2 scrubbing u n i t and i s discharged from the p l a n t . The condensate i s u s u a l l y not returned t o the p l a n t and the deuterium contained i n i t would be l o s t and t h i s would r e s u l t i n a p r o p o r t i o n a t e r e d u c t i o n i n the p r o d u c t i o n o f the heavy water p l a n t . These l o s s e s can be avoided i f t h i s enriched condensate i s transformed t o steam and t h i s steam i s then used as process steam i n the steam reformer. However, t h i s i n v o l v e s a g r e a t e r load on the steam system o f the ammonia p l a n t which cannot be r e a l i z e d i n the case o f e x i s t i n g ammonia p l a n t s . This problem can be avoided by an isotope exchange o f the enriched condensate with the process steam which i s being fed t o the steam reformer o r t o the s h i f t conversion. The e q u i l i b r i u m between water and water vapour i s n e a r l y equal t o u n i t y ; t h a t means the condensate l e a v i n g the exchange system has the normal deuterium c o n c e n t r a t i o n again. These p o i n t s show t h a t an ammonia p l a n t and a heavy water p l a n t can be i n t e g r a t e d without any p a r t i c u l a r d i f f i c u l t i e s . Conclusion A heavy water p l a n t with a c a p a c i t y o f 63 t D 0 p e r year w i l l be b u i l t a t T a l c h e r / I n d i a . The p l a n t i s coupled w i t h a 900 t per day ammonia p l a n t and w i l l be put on stream beginning 1978. F i g u r e 7 i s a photo o f the model. A comparison o f the p r o d u c t i o n costs based on the c a l c u l a t i o n f o r t h i s p l a n t shows with regard t o the investment as w e l l as t o the o p e r a t i n g c o s t s t h a t t h i s process can compete with other processes o r may even be s u p e r i o r . 2

Literature (1) (2) (3)

(4)

(5) (6)

Cited

Lumb, P.B.; J. B r . N u c l . Energy Soc. (1976) 15, pp. 35-46. Becker, E . W . ; IAEA R e v . , S e r i e s No. 21 (1962) V i e n n a . W a l t e r , S., N i t s c h k e , E.; "Tecnica ed Economia d e l l a Produzione d i Acqua Pesante", p . 173, Comitato Nazionale Energia Nucleare, Rome (1971). Schindewolf, U., Lang, G.; "Tecnica ed Economia d e l l a Produzione d i Acqua Pesante", p . 75, Comitato Nazionale Energia N u c l e a r e , Rome (1971). Walter, S., Schindewolf, U . ; Chem. Ing. Techn. (1965), 37, pp. 1185-91. Schindewolf, U., Hornke, S . ; Chem. Ing. Techn. (1969), 41, p p . 645-648.

RECEIVED October 28, 1977


7 Novel Catalysts for Isotopic Exchange between Hydrogen and Liquid Water J. P. BUTLER, J. H . ROLSTON, and W. H . STEVENS


Atomic Energy of Canada Ltd., Chalk River Nuclear Laboratories, Physical Chemistry Branch, Chalk River, Ontario K0J 1J0

Canada's atomic power program r e q u i r e s l a r g e q u a n t i t i e s o f heavy water t h a t a r e c u r r e n t l y produced by t h e G i r d l e r - S u l p h i d e o r GS p r o c e s s . This process i s based on exchange o f hydrogen i s o t o p e s between hydrogen s u l p h i d e gas and l i q u i d w a t e r as g i v e n by t h e f o l l o w i n g c h e m i c a l exchange r e a c t i o n : HDS

g

+ H 0,. s liq

N

HDO . + H S l i q g

z 2

n

(1)

z 2

At Chalk R i v e r we have a s t r o n g i n t e r e s t i n t r y i n g t o d e v e l o p new and more e f f i c i e n t p r o c e s s e s f o r t h e p r o d u c t i o n o f heavy w a t e r . A v e r y a t t r a c t i v e p r o c e s s and one w i t h many i n h e r e n t p o t e n t i a l advantages i s based on t h e h y d r o g e n - l i q u i d w a t e r i s o t o p i c exchange reaction. HD + H 0 2

l i q

N

^

H

D

0

H

l i q

+ 2

(2>

The hydrogen-water i s o t o p i c exchange r e a c t i o n i s more a t t r a c t i v e p r i m a r i l y because i t has a h i g h e r s e p a r a t i o n f a c t o r , a , d e f i n e d i n terms o f t h e d e u t e r i u m (D) to p r o t i u m (H) atom r a t i o s i n t h e two s p e c i e s a t equilibrium. α

= (D/H)

l i q

j/(D/H)

(3)

g

A t low d e u t e r i u m c o n c e n t r a t i o n s , α i s e q u a l t o t h e e q u i l i b r i u m c o n s t a n t f o r b o t h o f t h e s e exchange reactions. Figure 1 gives the separation f a c t o r f o r the h y d r o g e n - l i q u i d water (1) and t h e hydrogen s u l p h i d e l i q u i d water (2) exchange r e a c t i o n s i n t h e temperature range 0-200°C. Over t h e e n t i r e temperature range, t h e s e p a r a t i o n f a c t o r , a , f o r t h e H - H 0 system i s about a f a c t o r o f 2 h i g h e r than t h a t f o r t h e H S - H 0 2

2

2

©

2

0-8412-0420-9/78/47-068-093$05.00/0




Figure 1.

Separation factor, a, as a function of temperature for the hydrogen-water and hydrogen sulfide—water systems


7.

BUTLER ET

AL.

Novel

Catalysts

for

Isotopic

Exchange

95

exchange r e a c t i o n . As a r e s u l t , a p r o c e s s based on the H 2 - H 2 O r e a c t i o n w i l l have a h i g h e r r e c o v e r y o f d e u t e r i u m from n a t u r a l w a t e r and, as a consequence, a s m a l l e r p l a n t w i l l be r e q u i r e d . T h e o r e t i c a l r e c o v e r i e s o f 50% are p o s s i b l e f o r a b i t h e r m a l p r o c e s s based on the hydrogen-water r e a c t i o n compared t o 21% f o r the GS p r o c e s s .


C a t a l y s t Development A l t h o u g h the hydrogen-water i s o t o p i c exchange r e a c t i o n i s v e r y d e s i r a b l e f o r the s e p a r a t i o n o f hydrogen i s o t o p e s , the d i f f i c u l t y has been t o o b t a i n a c a t a l y s t t h a t can o p e r a t e e f f i c i e n t l y i n the p r e s e n c e o f l i q u i d w a t e r (3) . E f f e c t i v e n o b l e metal c a t a l y s t s f o r the vapour exchange r e a c t i o n have been known f o r many y e a r s b u t t h e s e c a t a l y s t s l o s e t h e i r a c t i v i t y i n contact with water. Our new approach i s t o use a c a t a l y s t s u i t a b l e f o r the vapour exchange r e a c t i o n , such as h i g h l y d i s p e r s e d p l a t i n u m metal d e p o s i t e d on γ-alumina, and t o c o a t the c a t a l y s t body w i t h a v e r y t h i n l a y e r o f a water r e p e l l e n t m a t e r i a l , such as a s i l i c o n e polymer, t o p r e v e n t w e t t i n g (4) . To i l l u s t r a t e the e f f e c t i v e ness o f the s i l i c o n e w e t p r o o f i n g , F i g u r e 2 shows what happens when c a t a l y s t p e l l e t s o f p l a t i n u m on alumina, a commercial c a t a l y s t produced by E n g e l h a r d I n d u s t r i e s , a r e immersed i n water. The u n t r e a t e d p e l l e t s r e l e a s e a stream o f s m a l l b u b b l e s o f a i r as w a t e r immediately e n t e r s the p o r e s o f the alumina. The p e l l e t s become j e t b l a c k , i n d i c a t i n g t h a t the s u r f a c e i s completely wetted. In c o n t r a s t , the s i l i c o n e t r e a t e d p e l l e t s shown i n F i g u r e 3 have l a r g e gas b u b b l e s a d h e r i n g t o t h e i r s u r f a c e and the s u r f a c e remains l i g h t grey i n c o l o u r , which i s c h a r a c t e r i s t i c o f the w e t p r o o f i n g a c t i o n o f the s i l i c o n e . These s i l i c o n e t r e a t e d , o r w e t p r o o f e d , c a t a l y s t s are q u i t e e f f e c t i v e f o r the h y d r o g e n - l i q u i d water i s o t o p i c exchange r e a c t i o n . S u b s e q u e n t l y , c a t a l y s t s were p r e p a r e d by d e p o s i t i n g p l a t i n u m on porous p o l y t e t r a f l u o r o e t h y l e n e i n an attempt t o p r o v i d e an improved h y d r o p h o b i c environment f o r t h e p l a t i n u m c r y s t a l l i t e s (5) . A more s u c c e s s f u l approach has been t o d e p o s i t p l a t i n u m on h i g h s u r f a c e a r e a carbon and t o bond the p l a t i n i z e d carbon t o a v a r i e t y o f column p a c k i n g s o r c a r r i e r s u s i n g T e f l o n as b o t h the b o n d i n g and the w e t p r o o f i n g agent. These l a i t t e r c a t a l y s t s have proven v e r y e f f e c t i v e f o r the h y d r o g e n - l i q u i d w a t e r exchange r e a c t i o n .


96



Figure

2.

Untreated 0.5% Pt-Al O Engelhard mersed in water 2

s

catalyst im-



BUTLER E T A L .

Figure 3.

0.5%

Novel

Catalysts for Isotopic

Exchange

Pt-Al 0 Engelhard catalyst treated with Dow 773 Silicone and immersed in water 2

3

Corning


98



C a t a l y s t Performance To measure the performance, o r the a c t i v i t y , o f t h e s e c a t a l y s t s we have used a s i n g l e pass t r i c k l e bed r e a c t o r shown d i a g r a m m a t i c a l l y i n F i g u r e 4. A known q u a n t i t y o f c a t a l y s t i s packed i n a 2.5 cm d i a m e t e r g l a s s column t o depths v a r y i n g from 5 t o 75 cm. About 15 cm o f n o n - c a t a l y t i c p a c k i n g i s p l a c e d d i r e c t l y below the c a t a l y s t bed t o a i d i n e s t a b l i s h i n g u n i f o r m gas flow p a t t e r n s through the bed. The e n t i r e p a c k i n g i s s u p p o r t e d on a s t a i n l e s s s t e e l s c r e e n (mesh s i z e , 40 χ 40). N a t u r a l w a t e r (D/H = 144 ppm) i s i n t r o d u c e d a t the top o f the column through a w a t e r d i s t r i b u t o r and flows downward through the c a t a l y s t bed assembly. P u r i f i e d d r y hydrogen, e n r i c h e d i n d e u t e r i u m (D/H = 300 ppm), flows c o u n t e r c u r r e n t l y upward through the h u m i d i f i e r where i t becomes s a t u r a t e d w i t h w a t e r vapour which i s i n i s o t o p i c e q u i l i b r i u m w i t h the l i q u i d water l e a v i n g the c a t a l y s t bed. The gas and water vapour then c o n t i n u e upward t h r o u g h the c a t a l y s t bed. The d e u t e r i u m c o n c e n t r a t i o n i n the i n l e t and o u t l e t hydrogen gas streams i s measured w i t h an on l i n e mass s p e c t r o m e t e r {6) . F o r the o p e r a t i n g con d i t i o n s d e s c r i b e d , the e n r i c h e d hydrogen gas becomes d e p l e t e d i n d e u t e r i u m i n p a s s i n g through the column w h i l e the n a t u r a l water f e e d i s e n r i c h e d . For example, f o r a g i v e n c a t a l y s t o p e r a t i n g a t 25°C where the s e p a r a t i o n f a c t o r , a, i s 3.81, the hydrogen gas s t r e a m l e a v i n g the top o f the column may have a D/H = 150 ppm, w h i l e i f e q u i l i b r i u m had been e s t a b l i s h e d between the two phases the c o n c e n t r a t i o n would be 37.8 ppm. The a c t i v i t y o f the v a r i o u s c a t a l y s t s f o r the i s o t o p i c exchange r e a c t i o n i n the hydrogen-water system i s d e t e r m i n e d by the nearness o f approach t o i s o t o p i c e q u i l i b r i u m between the two phases a f t e r p a s s i n g through the column. T h i s degree o f approach i s e x p r e s s e d i n terms o f a column e f f i c i e n c y , η , d e f i n e d as the r a t i o o f the a c t u a l change i n the d e u t e r i u m c o n c e n t r a t i o n o f the hydrogen t o the change t h a t would have o c c u r r e d had the hydrogen gas stream r e a c h e d e q u i l i b r i u m w i t h the f e e d water. With m i x t u r e s o f hydrogen and water a t low d e u t e r i u m i s o t o p i c c o m p o s i t i o n , the column e f f i c i e n c y , η , f o r the c a t a l y s t bed i s g i v e n by the e x p r e s s i o n η = F r a c t i o n a l Approach t o E q u i l i b r i u m Between the Two Phases


7.

BUTLER ET AL.

Novel

Catalysts

for Isotopic


NATURAL WATER FEED (D/H)=l44ppm > WATER DISTRIBUTOR

—

PACKED CATALYST BED

—

NON-CATALYTIC PACKING

—

H

2°LIQ Ψ

Exchange

H -(D/H)=l50ppm 2

(AT EQUIL= 37.8ppm)

T=

25°C

a= 3.806

H - SATURATED WITH H 0 H (D/H)= 300ppm 2

ι

2

V A P

2

PACKED BED HUMIDIFIER

—

PURIFIED^DRY ENRICHED

H

2

(D/H)=300ppm

Figure 4. Trickle bed reactor used for hydrogenliquid water isotopic exchange studies


99

100


(D/H),

-

(D/H)

(D/H)^ "

(D/H)


b

t

(4) eq

where (D/H) i s the atom r a t i o and the s u b s c r i p t s , b and t , r e f e r t o the i n l e t and o u t l e t hydrogen gas streams a t the bottom and top r e s p e c t i v e l y o f the exchange column. The s u b s c r i p t , eq, r e f e r s t o the d e u t e r i u m c o n c e n t r a t i o n o f the hydrogen i n e q u i l i b r i u m w i t h the l i q u i d w a t e r . From a measurement o f the column e f f i c i e n c y , η , the o v e r a l l t r a n s f e r o f d e u t e r i u m from hydrogen t o l i q u i d w a t e r can be calculated. F o r hydrogen and water f l o w i n g c o u n t e r c u r r e n t l y i n a packed column, the t r a n s f e r o f d e u t e r i u m from the hydrogen t o the w a t e r i s c h a r a c t e r i z e d by an o v e r a l l gas-phase mass t r a n s f e r c o e f f i c i e n t , Tya, e x p r e s s e d i n gram atoms p e r u n i t t i m e , p e r u n i t volume o f bed, f o r u n i t d i s p l a c e m e n t o f the d e u t e r i u m mole f r a c t i o n from the e q u i l i b r i u m value. F o r a s m a l l t e s t r e a c t o r where the concen t r a t i o n o f d e u t e r i u m i n the water does n o t v a r y s i g n i f i c a n t l y t h r o u g h o u t the r e a c t o r , the r a t e o f d e u t e r i u m t r a n s f e r from the gas t o the l i q u i d phase i n a s m a l l volume element o f the r e a c t o r , dV, o f c r o s s s e c t i o n a l a r e a A and h e i g h t dh, i s g i v e n by the following d i f f e r e n t i a l equation: -G-A-dy = T a ( y - y*)A-dh y

(5)

where G i s the molar f l o w r a t e o f hydrogen p e r second p e r u n i t a r e a , y i s the a c t u a l mole f r a c t i o n o f d e u t e r i u m i n the hydrogen a t a g i v e n h e i g h t , h , and y* i s the v a l u e a t e q u i l i b r i u m w i t h the l i q u i d w a t e r . I n t e g r a t i n g e q u a t i o n 5 o v e r the h e i g h t o f the c a t a l y s t bed, h, g i v e s (6) Τ a = = g £-ln(l - n) y S i n c e gas flow r a t e s are n o r m a l l y measured i n terms o f volume, we have found i t c o n v e n i e n t t o use a volume t r a n s f e r c o e f f i c i e n t , Κ a g i v e n by the e x p r e s s i o n K a y

= ~

I -ln(l

-

n)

(7)

where F

- s u p e r f i c i a l hydrogen flow r a t e , m-s a t STP h - h e i g h t o f the c a t a l y s t bed, m Κ a - gas-phase volume t r a n s f e r c o e f f i c i e n t , m (of H at STP)-s" -m" (of c a t a l y s t bed). Y

1

3

2


:

BUTLER ET AL.

7.

Novel

Catalysts

for

Isotopic

Exchange

101

The r a t e o f the o v e r a l l i s o t o p i c exchange r e a c t i o n i s f i r s t o r d e r i n the approach o f the d e u t e r i u m concent r a t i o n o f any o f the s p e c i e s t o i t s f i n a l e q u i l i b r i u m v a l u e and thus Kya i s a f i r s t o r d e r r a t e c o n s t a n t and d i m e n s i o n a l l y i t has the u n i t s o f s " . We have s t u d i e d the e f f e c t o f many parameters on the a c t i v i t y o f the c a t a l y s t , Kya, and F i g u r e 5 shows the e f f e c t o f hydrogen gas flow r a t e and temperature f o r a 0.37% P t - C - T e f l o n c a t a l y s t on 6.1 mm c e r a m i c spheres. F o r t h i s c a t a l y s t , Kya i n c r e a s e s a p p r o x i mately as the 0.3 power o f the hydrogen flow r a t e i n the range 0.05 t o 1.4 m/s a t STP. We a r e d e a l i n g h e r e w i t h a v e r y f a s t i s o t o p i c exchange r e a c t i o n : f o r example, a t a temperature o f 25°C and a hydrogen flow r a t e o f 1.0 m/s a t STP, Kya = 1.2. This i s equivalent t o a h a l f - t i m e f o r the exchange r e a c t i o n o f 0.18 s. The h a l f - t i m e i s c a l c u l a t e d u s i n g the a c t u a l hydrogen f l o w r a t e which i s 3.1 times the s u p e r f i c i a l flow r a t e f o r t h i s p a r t i c u l a r c a t a l y s t bed assembly (the volume f r a c t i o n o f gas i n the o p e r a t i n g column i s 0.32). The s o l i d p o i n t s i n F i g u r e 5 were o b t a i n e d u s i n g e n r i c h e d hydrogen (D/H = 250 ppm), where the d e u t e r i u m i s t r a n s f e r r e d from the hydrogen gas t o the l i q u i d w a t e r , w h i l e the open p o i n t s were o b t a i n e d u s i n g e n r i c h e d water (D/H = 1130 ppm),where d e u t e r i u m i s t r a n s f e r r e d from the l i q u i d water t o the hydrogen gas. From the d a t a no d i f f e r e n c e can be d e t e c t e d i n the r a t e o f d e u t e r i u m t r a n s f e r f o r the n e t r e a c t i o n o c c u r r i n g i n either direction. The e f f e c t o f temperature on the a c t i v i t y o f the c a t a l y s t i s a l s o i l l u s t r a t e d i n F i g u r e 5. F o r an i n c r e a s e i n temperature from 25° t o 60°C, K a i n c r e a s e s by a f a c t o r o f 2.4. The temperature c o e f f i c i e n t o f Kya f o r t h i s c a t a l y s t i n the range 15° t o 60°C i s e q u i v a l e n t t o an A r r h e n i u s a c t i v a t i o n energy o f 21 k J - m o l " o r about 5 k c a l - m o l " . The e f f e c t o f p l a t i n u m m e t a l s u r f a c e a r e a on the a c t i v i t y o f the c a t a l y s t i s shown i n F i g u r e 6 where Kya, the volume t r a n s f e r r a t e c o n s t a n t , i s p l o t t e d a g a i n s t the p l a t i n u m m e t a l s u r f a c e a r e a , measured by hydrogen c h e m i s o r p t i o n , and e x p r e s s e d as m p e r cm of packed bed. A l t h o u g h Kya i n c r e a s e s w i t h m e t a l a r e a , the i n c r e a s e i s not d i r e c t l y p r o p o r t i o n a l t o the a r e a . F o r m e t a l areas below 0.06 m -cm" , Kya i n c r e a s e s as the 0.75 power o f the m e t a l s u r f a c e a r e a and a t h i g h e r m e t a l areas the r a t e o f i n c r e a s e o f Kya i s l e s s . The curve appears t o be a p p r o a c h i n g a maximum v a l u e . These r e s u l t s i n d i c a t e t h a t a n o t h e r r e a c t i o n b e s i d e s the c a t a l y t i c one i s l i m i t i n g the o v e r a l l exchange reaction rate.


1

y

1

1

2

2

3


3

102

SEPARATION

O F HYDROGEN

-

-

6

*

0

^ ^ ^ - P - 4 5


C

°

e

C

^0 • ^

-

2

5

^

OOo-—°*

-

CARRIER

-

6 . 1 mm C E R A M I C

SPHERES

CATALYST BED

0. 5

HEIGHT

-

20.3

cm

AREA

-

4.80

cm*

PRESSURE

-

106 k P a

H 0 2

FLOW -

2. 1 k g - m (60

•

m

ο

•

1 0.2

A

1 0 . 4 HYDROGEN

Figure

^

5.

ENRICHED Η ENRICHED

2

- °/H

Η 0 2

0 . 6 . 0.8 FLOW

RATE

= 2 5 0 ppm

- D/H

1

1

· $•'

2

g/m i n )

= 1 1 3 0 ppm

1

1

1.0

1.2

- m/s ( A T

1.4

STP)

Effect of hydrogen gas flow rate and temperature on the activity, K a , of α 0.37% Pt-C-Teflon y

catalyst


ISOTOPES


7.

BUTLER ET AL.

Figure 6.

Novel Catalysts

for Isotopic

Exchange

103

Dependence of the transfer rate constant, K a, on the platinum metal area o) Pt-C-Teflon catalysts y


104

SEPARATION

O F HYDROGEN

ISOTOPES

Mechanism o f the Exchange R e a c t i o n As a f i r s t a p p r o x i m a t i o n , t h e o v e r a l l r a t e o f transfer of d e u t e r i u m between streams o f hydrogen and l i q u i d water o v e r these w e t p r o o f e d c a t a l y s t s can be c o n s i d e r e d i n terms o f two t r a n s f e r s t e p s . The f i r s t corresponds t o the c a t a l y t i c r a t e o f t r a n s f e r o f d e u t e r i u m from an e n r i c h e d hydrogen stream t o water vapour and t h e second c o r r e s p o n d s t o the t r a n s f e r r a t e from water vapour t o l i q u i d as shown i n e q u a t i o n s [8] and [ 9 ] .


HD HDO

vap

+ H zo O

vap

+

Ηz

2

Ο

η

.

liq

ν

^

N

s

HDO

vap

HDO,.

l i q

+

+ Hz

(8)

H 0 z

(9)

2

2

vap

The c a t a l y t i c r e a c t i o n [8] o c c u r s on a c t i v e c a t a l y s t s i t e s w h i l e t h e v a p o u r - l i q u i d t r a n s f e r r e a c t i o n [9] o c c u r s on any s u r f a c e . The l a t t e r t r a n s f e r s t e p can be c o n s i d e r e d as a c o n d e n s a t i o n - e v a p o r a t i o n r e a c t i o n . Techniques have been d e v e l o p e d so t h a t , i n a t r i c k l e bed r e a c t o r , t h e o v e r a l l r a t e and t h e i n d i v i d u a l t r a n s f e r r a t e s ( r e a c t i o n s [8] and [9]) can be d e t e r mined s i m u l t a n e o u s l y (7) . The magnitudes o f t h e s e s e p a r a t e d r a t e s a r e b o t h about a f a c t o r o f 2 l a r g e r than v a l u e s from d i r e c t measurements made on these two r e a c t i o n s i n d e p e n d e n t l y . These r e s u l t s have l e d to t h e c o n c l u s i o n t h a t t h e o v e r a l l mechanism f o r t h e exchange r e a c t i o n i s n o t as s i m p l e as f i r s t assumed and t h a t t h e r e i s a t h i r d p r o c e s s independent o f t h e w a t e r vapour i n v o l v e d i n the t r a n s f e r o f deuterium from hydrogen gas t o l i q u i d w a t e r . The e x a c t n a t u r e of t h i s t h i r d p r o c e s s i s n o t y e t known. S t u d i e s have been made o f t h e e f f e c t o f some 20 d i f f e r e n t parameters on t h e a c t i v i t y o f t h e c a t a l y s t s and t h e s e w i l l be d i s c u s s e d i n f u t u r e p u b l i c a t i o n s . Improvements i n C a t a l y s t Performance I n n o v a t i o n s r e s u l t i n g i n improved c a t a l y s t p e r formance a r e summarized i n T a b l e 1. The a c t i v i t y , Kya, o f each c a t a l y s t i n a t r i c k l e bed r e a c t o r i s g i v e n f o r a column o p e r a t i n g a t 25°C, a t about 1 atmosphere p r e s s u r e (0.106 MPa) and a s u p e r f i c i a l hydrogen gas flow r a t e o f 1.0 m/s a t STP. A l s o l i s t e d i n T a b l e 1 i s t h e s p e c i f i c a c t i v i t y o f each c a t a l y s t , Kya*, d e f i n e d as t h e o v e r a l l t r a n s f e r r a t e per u n i t c o n c e n t r a t i o n o f platinum i n the c a t a l y s t bed,



0.14% P t - C - T e f l o n

0.087% P t - C - T e f l o n 0.39% P t - C - T e f l o n Ordered Packing

6.

7. 8.

0.98 2.40

1.20

0.58

0.20

0.22

0.005

0.017

Kâ m^'S 1 ·itT3

C a l c u l a t e d from d a t a g i v e n i n r e f e r e n c e

0.4% P t - C - T e f l o n

5.

Teflon

0.4% P t - P o r o u s

3

4.

2

0.5% P t - A l 0 S i l i c o n e Treated

3

3.

2

0.05% P t - A l 0 Untreated

2.

Taylor

Pt-Charcoal,

1.

Catalyst

(8) .

a

2.18 2.84

1.44

0.146

0.064

0.046

0.0011

0.0022

K

Performance

a

y * m3*s~Mkg Pt)"^-

Improvement i n C a t a l y s t

TABLE 1


1980 2580

1300

133

58

42

1.0

2.0

Rel. Specific Activity

106

SEPARATION


/

O F HYDROGEN

ISOTOPES

Κ a* = (10) kg P t p e r m bed y The s p e c i f i c a c t i v i t y g i v e s a measure o f how e f f e c t i v e l y t h e p l a t i n u m i s b e i n g used i n a g i v e n c a t a l y s t . The l a s t column g i v e s t h e s p e c i f i c a c t i v i t y r e l a t i v e t o c a t a l y s t #2, an u n t r e a t e d , 0 . 5 % P t c a t a l y s t on 3.2 mm p e l l e t s o f γ-alumina. C a t a l y s t #1, p l a t i n u m on powdered c h a r c o a l , was p a t e n t e d by T a y l o r f o r t h i s exchange r e a c t i o n i n 1954 (8) . Treatment o f c a t a l y s t #2 w i t h s i l i c o n e (Cat. #3) improved t h e performance by a f a c t o r o f 40. In an attempt t o f u r t h e r improve t h e h y d r o p h o b i c environment o f t h e p l a t i n u m c r y s t a l l i t e s , p l a t i n u m was d e p o s i t e d on porous T e f l o n (Cat. #4) and t h i s enhanced the a c t i v i t y s l i g h t l y . Depositing p l a t i n u m on h i g h s u r f a c e a r e a carbon and b o n d i n g t h i s t o a c a r r i e r w i t h T e f l o n (Cat. #5) made a s i g n i f i c a n t improvement i n the s p e c i f i c a c t i v i t y . By d e c r e a s i n g the p l a t i n u m l o a d i n g and i n c r e a s i n g i t s d i s p e r s i o n on the carbon b l a c k (Cat. #6) t h e r e l a t i v e s p e c i f i c a c t i v i t y was i n c r e a s e d by a f a c t o r o f 10. By f u r t h e r d e c r e a s i n g t h e p l a t i n u m l o a d i n g t o 0.0 87% (Cat. #7) a f u r t h e r i n c r e a s e was o b t a i n e d . This l a t t e r c a t a l y s t has a s p e c i f i c a c t i v i t y about 2000 times h i g h e r than the u n t r e a t e d 0.5% P t - A l 0 c a t a l y s t , #3, and i s about 1000 times more a c t i v e than the c a t a l y s t p a t e n t e d by T a y l o r (8) f o r t h i s exchange r e a c t i o n (Cat. #1). In c a t a l y s t #8, the p l a t i n i z e d carbon was bonded w i t h T e f l o n t o c o r r u g a t e d m e t a l s h e e t s a r r a n g e d i n an o r d e r e d f a s h i o n i n t h e exchange column. A l t h o u g h t h i s c a t a l y s t has a h i g h e r p l a t i n u m c o n t e n t , 0.39%, t h e c o n c e n t r a t i o n o f p l a t i n u m p e r u n i t volume o f b e d i s e s s e n t i a l l y the same as c a t a l y s t #6. However as a r e s u l t o f t h e improved g e o m e t r i c c o n f i g u r a t i o n o f t h e c a t a l y s t , b o t h t h e a c t i v i t y and t h e s p e c i f i c a c t i v i t y are t w i c e t h a t o f c a t a l y s t #6. S i n c e the h e i g h t o f the column r e q u i r e d f o r a g i v e n s e p a r a t i o n i s i n v e r s e l y p r o p o r t i o n a l t o Kya, t h e s p e c t a c u l a r improvements i n c a t a l y s t a c t i v i t y r e p o r t e d here make a hydrogen-water exchange p r o c e s s f o r d e u t e r i u m e n r i c h ment l o o k v e r y p r o m i s i n g . 3

2

Process

3

F e a t u r e s o f Hydrogen-Water Exchange

P r o c e s s e s based on the hydrogen-water exchange r e a c t i o n p o t e n t i a l l y have many a t t r a c t i v e f e a t u r e s f o r t h e s e p a r a t i o n o f hydrogen i s o t o p e s r e l a t i v e t o the GS p r o c e s s .


7.

BUTLER ET AL.

Novel Catalysts

for Isotopic

Exchange

107

ADVANTAGES OF HYDROGEN-WATER PROCESSES


1. 2. 3. 4. 5. 6. 7. 8. 9.

HIGHER SEPARATION FACTOR HIGHER RECOVERY LOWER WATER AND GAS FLOWS SMALLER EXCHANGE COLUMNS SIMPLE CHEMISTRY NON-CORROSIVE SYSTEM NON-TOXIC LOW ENVIRONMENTAL IMPACT, POLLUTION-NIL USES ONLY ONE EXCHANGE REACTION

These g e n e r i c advantages a r e g e n e r a l l y a p p l i c a b l e t o any p r o c e s s based on the hydrogen-water exchange r e a c t i o n and a r e p a r t i c u l a r l y r e l e v a n t t o t h e Combined Electrolysis C a t a l y t i c Exchange (CECE) p r o c e s s . The CECE p r o c e s s which c o u p l e s a hydrogen-water c a t a l y t i c exchange column t o the hydrogen gas stream from an e l e c t r o l y s i s c e l l o f f e r s a v e r y e f f e c t i v e system f o r the enrichment o f deuterium. The p r o c e s s i s d i s c u s s e d i n more d e t a i l by Hammerli, e t a^L. (9) i n the next paper. S i n c e t h e s e p a r a t i o n f a c t o r , a , f o r the hydrogen-water exchange r e a c t i o n i s h i g h e r than t h a t f o r the GS system, a h i g h e r deuterium r e c o v e r y i s p o s s i b l e , 70% from n a t u r a l water compared t o 19% f o r the GS p r o c e s s . H i g h e r r e c o v e r y p e r m i t s lower water and gas flow r a t e s i n t h e p l a n t . The v e r y a c t i v e c a t a l y s t s t h a t have been d e v e l o p e d mean t h a t the exchange columns w i l l be q u i t e s m a l l . A l t h o u g h most o f t h e s e p a r a t o r y work i s a c c o m p l i s h e d i n these columns, t h e i r volume would o n l y be about 4% o r 1/25 o f t h e exchange volume i n c u r r e n t GS p l a n t s f o r t h e same p r o d u c t i o n o f heavy water. The c h e m i s t r y o f the hydrogen-water system i s s i m p l e and o p e r a t i n g c o n d i t i o n s o f the CECE p r o c e s s are n e a r ambient. The system i s n o n - c o r r o s i v e , an i m p o r t a n t f e a t u r e s i n c e many o f the shut-downs i n GS p l a n t s r e s u l t from c o r r o s i o n . The system i s nont o x i c and n o n - p o l l u t i n g and would have a v e r y low e n v i r o n m e n t a l impact. The CECE p r o c e s s y i e l d s two v a l u a b l e p r o d u c t s - heavy water and l a r g e q u a n t i t i e s o f hydrogen, a v e r y u s e f u l c h e m i c a l . Electrolytic oxygen may a l s o be c o n s i d e r e d as an a d d i t i o n a l product. F i n a l l y the p r o c e s s u t i l i z e s o n l y one exchange r e a c t i o n f o r t h e complete enrichment o f d e u t e r i u m from n a t u r a l water t o r e a c t o r grade heavy w a t e r , 99.8% D 0 . T h i s removes many o f the comp l e x i t i e s o f o t h e r heavy water p r o c e s s e s which r e q u i r e d i f f e r e n t processes a t various stages i n the enrichment. 2


108



The major d i s a d v a n t a g e s o f t h e p r o c e s s a r e t h a t i t r e q u i r e s a l a r g e amount o f e l e c t r i c a l power f o r t h e e l e c t r o l y s i s c e l l s and i t r e q u i r e s a s t a b l e p l a t i n u m c a t a l y s t which i s somewhat e x p e n s i v e . However a c t i v e c a t a l y s t s have been d e v e l o p e d w i t h low p l a t i n u m l o a d i n g s so t h a t the p l a t i n u m r e p r e s e n t s o n l y about 30% o f the c o s t o f t h e f i n i s h e d c a t a l y s t . Further, the amount o f c a t a l y s t r e q u i r e d i s s m a l l and i t has been e s t i m a t e d t h a t t h e c a t a l y s t c o s t w i l l be o n l y a v e r y s m a l l p e r c e n t a g e o f the t o t a l c a p i t a l c o s t o f a heavy water p l a n t . L i k e w i s e c a t a l y s t replacement i s n o t e x p e c t e d t o add s i g n i f i c a n t l y t o t h e o p e r a t i n g costs. Acknowledgements We thank J. den H a r t o g and F.W. Molson f o r t h e i r a s s i s t a n c e i n t h e development o f these w e t p r o o f e d catalysts. We e x p r e s s o u r thanks t o W.M. T h u r s t o n f o r the development o f an a u t o m a t i c mass s p e c t r o m e t e r system f o r o n - l i n e a n a l y s i s o f d e u t e r i u m i n hydrogen gas streams, a t n e a r n a t u r a l abundance. Only w i t h h i s equipment and a s s i s t a n c e i n the a n a l y s i s o f thousands o f gas samples has t h i s r e s e a r c h been feasible. Abstract Catalytic i s o t o p i c exchange between hydrogen and liquid water o f f e r s many i n h e r e n t p o t e n t i a l advantages f o r the s e p a r a t i o n o f hydrogen i s o t o p e s which i s o f g r e a t importance i n the Canadian n u c l e a r program. Active catalysts for isotopic exchange between hydrogen and water vapour have l o n g been a v a i l a b l e , but these c a t a l y s t s are essentially i n a c t i v e i n the presence o f liquid w a t e r . New, water r e p e l l e n t p l a t i n u m c a t a l y s t s have been p r e p a r e d b y : 1) t r e a t i n g supported c a t a l y s t s with silicone, 2) d e p o s i t i n g p l a t i n u m on i n h e r e n t l y hydrophobic polymeric s u p p o r t s , and 3) t r e a t i n g platinized carbon with T e f l o n and b o n d i n g to a carrier. The activity of these c a t a l y s t s f o r i s o t o p i c exchange between c o u n t e r - c u r r e n t streams o f liquid water and hydrogen s a t u r a t e d w i t h water vapour has been measured in a packed trickle bed integral reactor. The performance o f t h e s e h y d r o p h o b i c c a t a l y s t s i s compared w i t h non-wetproofed catalysts. The mechanism o f the overall exchange r e a c t i o n i s briefly discussed.


7.

BUTLER ET AL.

Literature 1. 2. 3.

4.


5. 6. 7.

8. 9.

Novel

Catalysts

for Isotopic

Exchange

109

Cited

R o l s t o n , J.H., den H a r t o g , J. and B u t l e r , J.P., J. Phys. Chem., (1976), 80, 1064. P o h l , H.Α., J. Chem. & E n g . D a t a , (1961), 6, (4), 515. Murphy, G.M., U r e y , H . C . and Kirshenbaum, I., Editors, " P r o d u c t i o n o f Heavy Water, N a t i o n a l Energy S e r i e s " , C h a p t e r 2, p . 16, M c G r a w - H i l l , New Y o r k , 1955. S t e v e n s , W . H . , Canadian P a t e n t No. 907,292, August 15, 1972. Rolston, J.H., den H a r t o g , J. and B u t l e r , J.P., U . S . P a t e n t No. 4 , 0 2 5 , 5 6 0 , May 24, 1977. T h u r s t o n , W . M . , Rev. Sci. I n s t r u m e n t s , (1970), 41, 963 and (1971), 42, 700. Butler, J.P., den H a r t o g , J., G o o d a l e , J.W. and Rolston, J.H., "Proceedings o f the S i x t h I n t e r n a t i o n a l Congress on Catalysis, London 1976", Vol. 2, p . 747, The C h e m i c a l S o c i e t y , London, 1977. T a y l o r , H.S., U . S . P a t e n t No. 2 , 6 9 0 , 3 8 0 , September 28, 1954. Hammerli, Μ . , S t e v e n s , W . H . , and Butler, J.P., P r o c e e d i n g s o f the Symposium on " S e p a r a t i o n o f Hydrogen I s o t o p e s " , M o n t r e a l 1977, p . 110 , American Chemical S o c i e t y , Washington, 1977.



8 Combined Electrolysis Catalytic Exchange (CECE) Process for Hydrogen Isotope Separation M. HAMMERLI, W. H . STEVENS, and J. P. BUTLER


Atomic Energy of Canada Ltd., Chalk River Nuclear Laboratories, Chalk River, Ontario, Canada K0J 1J0

In t h e p r e c e d i n g p a p e r ( J ) a n o v e l p l a t i n u m - c a r b o n - T e f l o n c a t a l y s t for e f f i c i e n t s e p a r a t i o n o f the hydro gen i s o t o p e s i n t h e p r e s e n c e o f l i q u i d w a t e r has been discussed. T h i s p a p e r d e a l s w i t h the Combined E l e c t r o l y s i s C a t a l y t i c E x c h a n g e (CECE) p r o c e s s e s w h i c h use t h i s c a t a l y s t f o r s e p a r a t i n g t h e h y d r o g e n i s o t o p e s , n a m e l y t h e CECE-HWP ( - H e a v y W a t e r p r o c e s s ) and t h e CECE-TRP ( - T j i t i u m R e c o v e r y p r o c e s s ) . The i s o t o p i c s e p a r a t i o n p r i n c i p l e s i n h e r e n t i n the CECE p r o c e s s e s are: (a) the e q u i l i b r i u m i s o t o p e e f f e c t i n r e a c t i o n s o f the type : H D

and

gas

+

i t s isotopic

H

*

0

l i q u i d ^ % a s

+

H

D

0

l i q u i d

analogues, and

™

(b) the k i n e t i c isotope e f f e c t (2) i n h e r e n t i n t h e e l e c t r o l y t i c hydrogen e v o l u t i o n r e a c t i o n . There are thus two r e l a t i v e l y large isotopic separation factors i n v o l v e d i n t h e s e p r o c e s s e s w h i c h , as we m i g h t e x p e c t , l e a d t o many o f t h e i r a d v a n t a g e s , d i s c u s s e d later. Both s e p a r a t i o n f a c t o r s favour c o n c e n t r a t i o n o f the heavier isotope i n the l i q u i d water r e l a t i v e to the hydrogen g a s . T h e CECE-HWP i s a n e w , y e t o l d p r o c e s s , f o r i t u t i l i z e s t h e same b a s i c p r i n c i p l e s as C a n a d a ' s first i n d u s t r i a l heavy w a t e r p l a n t o p e r a t e d by C o n s o l i d a t e d M i n i n g a n d S m e l t i n g Company f r o m 1 9 4 3 t o 1956 a t T r a i l , B.C. (J3 ) . The T r a i l p r o c e s s was a l s o a c o m b i n a t i o n o f e l e c t r o l y s i s and hydrogen-water exchange, but the e x c h a n g e r e a c t i o n h a d t o be c a r r i e d o u t i n t h e gas phase because l i q u i d water p o i s o n e d the c a t a l y s t . In the e x c h a n g e c o l u m n , d e u t e r i u m was t r a n s f e r r e d from hydrogen to water vapour i n a heated c a t a l y s t bed, and then a p a i r o f bubble cap t r a y s promoted d e u t e r i u m ©

0-8412-0420-9/78/47-068-110$05.00/0



8.

HAMMERLI ET AL.

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Exchange

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exchange between the w a t e r vapour and l i q u i d w a t e r . T h i s s e q u e n c e w a s r e p e a t e d many t i m e s t h r o u g h o u t t h e column. Heavy w a t e r p r o d u c e d by t h e T r a i l p r o c e s s was too c o s t l y because o f the s i z e and c o m p l e x i t y o f t h e exchange columns and t h e e x c e s s i v e energy r e q u i r e d f o r the steam preheaters t o r e p e a t e d l y v a p o u r i z e any l i q u i d w a t e r i n the gas s t r e a m . I t s h o u l d be n o t e d t h a t t h e c a t a l y s t beds o c c u p i e d o n l y 4 p e r c e n t o f t h e t o t a l volume o f the t o w e r s . If a catalyst c o u l d be d e v e l o p e d t h a t w o u l d remain a c t i v e i n the presence of l i q u i d w a t e r , the need f o r t h e c o m p l e x T r a i l - t y p e c o l u m n s c o u l d be e l i m i n a t e d , a n d p a c k e d l i q u i d - v a p o u r c o n t a c t o r s c o u l d be u s e d . Becker and c o - w o r k e r s (£) developed a s l u r r y c a t a l y s t cons i s t i n g o f p l a t i n u m on f i n e l y d i v i d e d c h a r c o a l f o r t h e hydrogen l i q u i d water exchange r e a c t i o n . The e x c h a n g e r a t e f o r t h e i r c a t a l y s t d e p e n d e d p r i m a r i l y on t h e d i s s o l v e d hydrogen c o n c e n t r a t i o n so t h a t t h e c a t a l y s t c o u l d o n l y be u s e d a t v e r y h i g h p r e s s u r e s , 20 M P a . Low c a t a l y s t a c t i v i t y , h i g h i n v e n t o r y o f p l a t i n u m and l o s s e s of the f i n e l y d i v i d e d c a t a l y s t are t h e main reasons the process proved uneconomic. With Stevens' i n v e n t i o n of the wetproofed catalyst (5>) , i t b e c a m e p o s s i b l e t o e f f e c t a v e r y efficient d e u t e r i u m exchange between hydrogen and l i q u i d w a t e r a t low p r e s s u r e s , t h i s c a t a l y s t allows the very close c o u p l i n g o f t h e two e x c h a n g e r e a c t i o n s . Preheaters and b u b b l e cap t r a y s become u n n e c e s s a r y and t h e e x c h a n g e c o l u m n c a n be o p e r a t e d a t l o w e r ( a m b i e n t ) temperatures where the s e p a r a t i o n f a c t o r i s l a r g e r ( 6 ) . Thus t h e v o l u m e o f t h e c o l u m n f o r t h e CECE-HWP i s r e d u c e d b y a f a c t o r o f a b o u t 20 r e l a t i v e to the T r a i l process. B e c a u s e t h e w e t p r o o f e d c a t a l y s t i n t h e CECE-HWP has d r a s t i c a l l y a l t e r e d t h e e x c h a n g e column o f t h e T r a i l - t y p e p r o c e s s , t h e d e s i g n a t i o n CECE w i l l only apply t o processes i n c o r p o r a t i n g t h i s type o f c a t a l y s t , e v e n t h o u g h t h e same b a s i c i s o t o p e s e p a r a t i o n p r i n c i p l e s a p p l y to b o t h . B e f o r e d i s c u s s i n g t h e CECE-HWP i n m o r e d e t a i l , i t is worth mentioning that this process involves three p r o d u c t s , H , D 0 and 0 , o f w h i c h t h e f i r s t two have considerable commercial value at present. The l a r g e amount o f e l e c t r i c a l e n e r g y r e q u i r e d t o p r o d u c e one k i l o g r a m o f r e a c t o r g r a d e h e a v y w a t e r can be o f f s e t s o m e w h a t b y t h e v a l u e o f t h e h y d r o g e n , e i t h e r as a c h e m i c a l o r as a f u e l (equivalent t o ^ 2 0 MWh/kg D 0 ) . N e v e r t h e l e s s , i t m u s t be u n d e r s t o o d a t t h e o u t s e t that t h e m a j o r d i s a d v a n t a g e o f t h e CECE-HWP i s i t s r e l a tively l a r g e s p e c i f i c e n e r g y r e q u i r e m e n t ( ^ 5 0 MWh/kg D 0), t h e energy b e i n g o f the h i g h e s t grade and c o s t . 2

2

2

2

2


112

SEPARATION OF

H Y D R O G E N ISOTOPES

T h e CECE-HWP w i l l b e c o m e i n c r e a s i n g l y m o r e attractive f o r the l a r g e s c a l e p r o d u c t i o n of D 0 from n a t u r a l water, to the degree t h a t the c o s t of f o s s i l fuels escalates f a s t e r t h a n t h e c o s t o f CANDU* n u c l e a r p o w e r . 2


The

CECE-HWP

B a s i c D e s c r i p t i o n of the P r o c e s s . I n t h e CECE-HWP (see F i g u r e 1) t h e e l e c t r o l y t i c h y d r o a e n , already depleted in deuterium r e l a t i v e to the e l e c t r o l y t e by v i r t u e o f the k i n e t i c i s o t o p e e f f e c t i n h e r e n t i n the hydrogen e v o l u t i o n r e a c t i o n , s t e a d i l y loses most of its r e m a i n i n g d e u t e r i u m as i t m o v e s up t h e c a t a l y s t column in c o u n t e r - c u r r e n t flow w i t h the feed water t r i c k l i n g down i n t o t h e e l e c t r o l y s i s c e l l . The w a t e r becomes e n r i c h e d i n d e u t e r i u m a c c o r d i n g t o r e a c t i o n 1 as it p a s s e s down t h e c a t a l y s t b e d . The o v e r a l l deuterium p r o f i l e i s one i n w h i c h t h e d e u t e r i u m c o n c e n t r a t i o n in the w a t e r i n c r e a s e s a l o n g t h e l e n g t h o f the column from top to b o t t o m , w h i l e i n the gas phase t h e deuterium concentration decreases from the bottom to top. The d e h u m i d i f i e r i s n e c e s s a r y i f t h e depleted h y d r o g e n g a s m u s t be d r y . This unit also transfers the d e u t e r i u m i n t h e w a t e r v a p o u r c a r r i e d by t h e hydroaen gas to the f e e d w a t e r . The s c r u b b e r b e t w e e n t h e c a t a l y s t c o l u m n and t h e e l e c t r o l y s i s c e l l serves the f o l l o w i n g f u n c t i o n s : (a) removes e n t r a i n e d e l e c t r o l y t e i n t h e hydrogen g a s , (b) a d j u s t s the h u m i d i t y of the h y d r o g e n gas to t h e c o n d i t i o n s p r e v a i l i n g i n t h e c o l u m n , w h i c h n e e d n o t be t h e same as t h o s e i n t h e cell, (c) t h e r m o s t a t s t h e h u m i d i f i e d gas t o t h e c o l u m n t e m perature, and (d) t r a n s f e r s deuterium from the w a t e r vapour e n t r a i n e d i n t h e h y d r o g e n gas to the l i q u i d water. The l a t t e r f u n c t i o n i s v e r y i m p o r t a n t s i n c e the deuterium c o n c e n t r a t i o n of the water vapour in the h y d r o g e n gas s t r e a m l e a v i n g t h e e l e c t r o l y s i s cell c o u l d e a s i l y be a b o u t 3 t o 10 t i m e s l a r g e r t h a n that in the w a t e r at the b o t t o m o f the e x c h a n q e c o l u m n . T h i s d i f f e r e n c e i s m a i n l y d e p e n d e n t on t h e m a g n i t u d e o f t h e e f f e c t i v e e l e c t r o l y t i c H/D s e p a r a t i o n f a c t o r , which i n t u r n d e p e n d s on t h e c a t h o d e m a t e r i a l a n d t h e o p e r a ting conditions of the e l e c t r o l y s i s cell. Any t y p e o f e l e c t r o l y s i s c e l l i n c o r p o r a t i n g a s e p a r a t o r between the anode and c a t h o d e compartments may b e u s e d i n t h e C E C E - H W P . If the c e l l contains a l i q u i d e l e c t r o l y t e , s u c h as 2 5 w e i g h t % s o l u t i o n o f K O H , t h e s a l t o f t h e e l e c t r o l y t e m u s t be r e m o v e d f r o m t h e w

*CANada

Deuterium

I U r a n i urn


Combined

HAMMERLI ET AL.

Electrolysis

FEED WATER

Catalytic

Exchange

HYDROGEN GAS TO f s T Ô R AGF ÔVENÊRGY '

[l48 ppm]

DEHUMIDIFIER

CONVERTER ETC.


1 H /H 0 DEUTERIUM EXCHANGE CATALYST TOWER 2

OXYGEN GAS TO STORAGEJDR ENERGY CONVERTER ETC.

2

~! SCRUBBER

DRYER

H0 2

τ. ,H 2

1

ELECTROLYS IS CELLS ANODE COMPARTMENT , 1

CATHODE COMPARTMENT

SALT OF ELECTROLYTE

ELECTROLYTE SEPARATOR DEUTERIUM ENRICHED WATER TO HEAVY WATER PLANT Figure 1.

Combined Electrolysis Catalytic Exchange-Heavy "Process (CECE-HWP)

Water


114



p r o d u c t s t r e a m b e f o r e i t c a n be f e d t o t h e n e x t h i g h e r stage. Electrolyte r e m o v a l may b e s t b e a c c o m p l i s h e d b y f l a s h e v a p o r a t i o n (_7) * w h i c h w o u l d a l s o remove s i m u l t a n e o u s l y the excess heat generated in the c e l l . It i s i m p o r t a n t t o dry the c o - l i b e r a t e d oxygen b e c a u s e t h e f i r s t s t a g e p r o d u c t c o u l d e a s i l y be a b o u t 20 t i m e s r i c h e r i n d e u t e r i u m t h a n t h e f e e d w a t e r a t the top o f the column ( 8j , so t h a t l o s s e s w o u l d o t h e r w i s e be i n t o l e r a b l e . Results from L a b o r a t o r y U n i t . The s m a l l labora t o r y u n i t used to demonstrate the p r i n c i p l e s of the CECE-HWP c o n s i s t s o f a c a t a l y s t c o l u m n , 36 cm l o n g a n d 2 . 5 4 cm i n d i a m e t e r , a n d a G . E . e l e c t r o l y s i s module containing 2-unit c e l l s . The a p p a r a t u s i s shown s c h e m a t i c a l l y in Figure 2. Because the G.E. e l e c t r o l y s i s module c o n t a i n s a s o l i d polymer electrolyte (SPE), the e l e c t r o l y t e s e p a r a t o r (see F i g u r e 1) i s n o t r e q u i r e d . The e l e c t r o l y s i s c e l l was o p e r a t e d a t a c u r r e n t o f 2 χ 2 0 = 40 A . With o p e r a t i n g c o n d i t i o n s such that the feed water r a t e i s a d j u s t e d t o e q u a l t h e e l e c t r o l y s i s r a t e and no p r o d u c t i s w i t h d r a w n ( t o t a l r e f l u x ) , one w o u l d e x p e c t to see a l i n e a r i n c r e a s e i n the deuterium concentration i n t h e c e l l - w a t e r c i r c u i t as a f u n c t i o n o f e l e c t r o l y s i s time p r o v i d i n g the f o l l o w i n g c o n d i t i o n s p e r t a i n : (a) t h e c e l l c u r r e n t remains constant, (b) t h e d e u t e r i u m c o n c e n t r a t i o n s i n both t h e feed w a t e r and t h e e f f l u e n t h y d r o g e n r e m a i n c o n s t a n t , and (c) t h e r e are no e x t r a n e o u s d e u t e r i u m l o s s e s i n t h e system. C o n d i t i o n (a) i s e a s i l y s a t i s f i e d w i t h a c o n s t a n t c u r r e n t power s u p p l y . The d e u t e r i u m c o n c e n t r a t i o n i n the f e e d w a t e r i s a l s o easy to keep c o n s t a n t , b u t t h a t i n t h e e f f l u e n t h y d r o g e n gas i s a f u n c t i o n o f the c o l u m n p a r a m e t e r s i n c l u d i n g c a t a l y s t a c t i v i t y , as w e l l as t h e d e u t e r i u m c o n c e n t r a t i o n i n t h e e l e c t r o l y t i c hydrogen. However, the c a t a l y s t bed parameters were c h o s e n so t h a t t h e column o p e r a t e d a t n e a r 100% e f f i c i e n c y f o r a p r o d u c t c o n c e n t r a t i o n up t o a b o u t 0 . 1 % . T h i s c a n be s e e n b y t h e a l m o s t h o r i z o n t a l l i n e f o r t h e d e u t e r i u m c o n c e n t r a t i o n i n t h e d e p l e t e d h y d r o q e n gas i n F i g u r e 3. The d e v i a t i o n o f t h e d e u t e r i u m b u i l d - u p i n the w a t e r from the t h e o r e t i c a l l i n e i s thus mainly a t t r i b u t a b l e to l o s s e s a s s o c i a t e d w i t h incomplete d r y i n g o f the oxygen g a s . These l o s s e s naturally b e c o m e l a r g e r as t h e d e u t e r i u m c o n c e n t r a t i o n increases in the c e l l water. The s l o p e o f t h e c a l c u l a t e d l i q u i d p h a s e l i n e i n F i g u r e 3 i s a f u n c t i o n o f t h e volume o f w a t e r i n the



Laboratory CECE-HWP demonstration unit. The unit incorporates a General Electric solid polymer electrolyte electrolysis module.




Figure 3. Deuterium concentration in the electrolysis cell water circuit as a function of electrolysis time. The feed water flow rate equals the electrolysis rate, and the product flow rate is zero. The dashed line is calculated assuming no deuterium loss from the system (see text).


8.

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c e l l - w a t e r c i r c u i t , the deuterium concentration i n the feed water, the operating temperature o f the column, and t h e r a t e o f e l e c t r o l y s i s . Thus i t h a s no f u n d a mental significance. It i s worth e m p h a s i z i n g that both t h e w a t e r and h y d r o g e n l i n e a r f l o w r a t e s a r e a b o u t two o r d e r s o f magnitude s m a l l e r than t h e optimum flow rates f o r t h e 2 . 5 4 cm d i a m e t e r c o l u m n , b e c a u s e o f t h e l o w c u r r e n t , 40 A , i n t h e e l e c t r o l y s i s c e l l . L i n e a r hydroaen flow r a t e s o f 1 to 2 m - s " at S . T . P . appear optimum f o r e f f i c i e n t u t i l i z a t i o n o f the novel c a t a l y s t Q ) for a column o p e r a t i n g at near a t m o s p h e r i c p r e s s u r e . This c o l u m n w o u l d r e q u i r e an e l e c t r o l y s i s c u r r e n t o f 4 , 3 6 5 A to 8,730 A t o produce t h e r e q u i r e d l i n e a r hydrogen flow rates. These f i g u r e s p o i n t to t h e f a c t t h a t t h e e l e c t r o l y s i s p l a n t w i l l be c o n s i d e r a b l y l a r g e r t h a n t h e c o r r e s p o n d i n g c a t a l y s t c o l u m n i n an o p t i m u m C E C E - H W P pi ant.


1

De u t e r i urn R e c o v e r y . In t h e CECE-HWP, t h e theoretical f r a c t i o n o f t h e d e u t e r i u m w h i c h c a n be r e c o v e r e d from t h e f e e d w a t e r i s g o v e r n e d s o l e l y by t h e v a l u e o f t h e e q u i l i b r i u m c o n s t a n t o f r e a c t i o n 1, and hence t h e t e m p e r a t u r e o f t h e top o f t h e c o l u m n . I t may be e x p r e s s e d i n t e r m s o f t h e s e p a r a t i o n f a c t o r f o r t h e column, aç, which i s i d e n t i c a l to the e q u i l i b r i u m cons t a n t , as f o l 1 o w s : Theoretical

Recovery

=

D

p

-

D /a p

c

(2)

w h e r e Dp i s t h e d e u t e r i u m c o n c e n t r a t i o n i n t h e f e e d wate r. In p r a c t i c e , i t i s p r u d e n t t o c h o o s e a d e u t e r i u m c o n c e n t r a t i o n i n the e f f l u e n t hydrogen gas which i s a b o u t 10% l a r g e r than t h e c a l c u l a t e d e q u i l i b r i u m v a l u e , Dp/aQ. Then c a t a l y s t r e q u i r e m e n t s , w h i c h w o u l d otherwise approach i n f i n i t y , are reasonable. The p r a c t i c a l range of deuterium recoveries f o r t h e CECE-HWP w i l l probably be i n t h e r a n g e 6 6 - 7 5 % c o r r e s p o n d i n g t o a t e m p e r a t u r e range o f about 60°C t o 15°C r e s p e c t i v e l y . This i s a h i g h r e c o v e r y f o r a d e u t e r i u m p r o d u c t i o n p r o c e s s (9). S y n e r q i s t i c Ε f fe c t s . Several synergistic effects r e s u l t from combining e l e c t r o l y s i s with H /H 0 deuterium exchange r e l a t i v e t o e i t h e r cascaded conventional e l e c t r o l y s i s alone or cascaded H /H 0 exchange alone. In t h e l a t t e r c a s e , a b i t h e r m a l e x c h a n g e p r o c e s s m u s t be u s e d , s i m i l a r i n p r i n c i p l e t o t h e p r e s e n t GirdlerS u l p h i d e ( G S ) p r o c e s s (9) b a s e d on t h e H S / H 0 e x c h a n g e 2

2

2

2

2

2


118

SEPARATION O F

HYDROGEN ISOTOPES

re a c t i o n . The s y n e r g i s t i c e f f e c t s o f t h e CECE-HWP a r e s u m m a r i z e d i n T a b l e s I and II. I n T a b l e I, e l e c t r o l y s i s a l o n e , o p e r a t i n g a s an i d e a l c a s c a d e , i s c o m p a r e d to e l e c t r o l y s i s i n the CECE-HWP a s s u m i n g t h e e l e c t r o l y t i c H/D s e p a r a t i o n f a c t o r , CL£ is 6 for both. This value is certainly attainable in p r a c t i c e . For e x a m p l e , the electrolytic h e a v y w a t e r U p g r a d i n a P l a n t a t CRNL h a s o p e r a t e d for s e v e r a l y e a r s w i t h an e f f e c t i v e a p o f 8 t o 9 a t cell t e m p e r a t u r e s f r o m 35°C t o 25°C r e s p e c t i v e l y . However, these c e l l s incorporate mild steel cathodes and operate at lower temperatures than i s the current p r a c t i c e in commercial c e l l s . S i n c e the t r e n d is to w a r d s h i g h e r o p e r a t i n g t e m p e r a t u r e s and o t h e r cathode m a t e r i a l s * , i n c l u d i n g p r e c i o u s m e t a l s , an a£ o f s a y , 3, m i g h t be m o r e r e a l i s t i c i n t h e f u t u r e . A n y v a l u e o f ap less than 6 w o u l d , of c o u r s e , i n c r e a s e the differences in Table I. The f a c t o r s w h i c h l e a d t o a r e d u c t i o n i n catalyst r e q u i r e m e n t s f o r t h e CECE-HWP v e r s u s t h e b i t h e r m a l H / H 0-HWP are t a b u l a t e d i n T a b l e II. S i n c e the s p e c i f i c c a t a l y s t r e q u i r e m e n t (amount o f c a t a l y s t r e q u i r e d p e r unit of D 0 production per unit of time) is directly p r o p o r t i o n a l to t h e gas f l o w r a t e but t h e deuterium p r o d u c t i o n i s d i r e c t l y p r o p o r t i o n a l to the l i q u i d flow r a t e , i n c r e a s i n g t h e L/G r a t i o ( L a n d G a r e t h e m o l a r w a t e r and gas f l o w r a t e s r e s p e c t i v e l y ) decreases the catalyst requirement d i r e c t l y . D a t a i n T a b l e II serve to i l l u s t r a t e the e f f e c t s of the d i f f e r e n t parameters on t h e c a t a l y s t v o l u m e f o r a g i v e n D 0 p r o d u c t i o n rate and are n o t n e c e s s a r i l y optimum v a l u e s . For the b i t h e r m a l H /H 0-HWP, the d e u t e r i u m r e c o v e r y i s l i m i t e d by t h e d i f f e r e n c e i n t h e practical c o l d and hot column t e m p e r a t u r e a t t a i n a b l e , s i n c e it i s b a s e d on t h e t e m p e r a t u r e c o e f f i c i e n t o f α ς . For a g i v e n d e u t e r i u m c o n c e n t r a t i o n i n the f e e d , the specific v o l u m e o f w a t e r w h i c h m u s t be p r o c e s s e d p e r u n i t o f D 0 p r o d u c e d i s d i r e c t l y d e p e n d e n t on t h e recovery. The t h i r d p a r a m e t e r w h i c h i s c o m p a r e d i n T a b l e II i s the number of t r a n s f e r u n i t s ( N . T . U . ) r e q u i r e d f o r a deuterium c o n c e n t r a t i o n in the product of 0.1% [ 1 0 0 0 ppm D/(H + D ) ] w h e n a£ = 6 , a n d t h e f e e d w a t e r con t a i n s 1 4 8 ppm d e u t e r i u m , w h i c h c o i n c i d e s w i t h the Great Lakes w a t e r s ( 1 0 ) . The s p e c i f i c c a t a l y s t volume f o r a g i v e n c a t a l y s t i s d i r e c t l y d e p e n d e n t on t h e N.T.U.'s (£). It i s i m p o r t a n t to p o i n t out that only


9

2

2

2

2

2

2

2

* I r o n o r m i l d s t e e 1 c a t h o d e s y i e l d t h e l a r g e s t aE values f o r a given set of operating conditions relat i ve t o o t h e r m e t a l s .



a

F

No.

of

Reflux,

F

Stages

n

Parameter

Cascade 1.7

^17

Ideal

Electrolysis

Compared

CECE-HWP

3

1.0

CECE-HWP

Cascade)

CECE-HWP

OF THE

I

Size of electrolysis plant not affected

the

(Ideal

in

EFFECTS

Size of electrolysis plant d i r e c t l y dependent on ac

Conventional

SYNERGISTIC

Table

in plant volume increases a s α^· d e c r e a s e s

complicated Catalyst somewhat

Less

costs

reduction

Remarks

Electrolysis

energy

^70%

to



for

the

cold

FACTOR

^1

=

6

MPa

7.4

^66

1

CECE-HWP

E

the

Bithermal

since = 265

the ppm

^24

^2

Λ

^15

deuterium instead of

1 .66

2

2.3

C a t a l y s t R e d u c t i on Factor

versus

CECE-HWP

CECE-HWP

THE

^ T h e N . T . U . ' s f o r t h e CECE-HWP a r e r e d u c e d by t h i s f a c t o r c o n c e n t r a t i o n i n t h e c o l u m n l i q u i d p r o d u c t n e e d s o n l y be 1 0 0 0 ppm i f a is 6 instead of unity.

on1y

REDUCTION

Calculated

TOTAL column

12.3*

^33

Nil

Separation

2

0.43

2

H /H 0-HWP

Electrolytic

ppm

OF

II

Requirements f o r the H2/H2O-HWP

Bithermal

Catalyst

7 MP a

1000

in

EFFECTS

Pressure

for

Recovery

N.T.U.

%

L/G

Ρ aramete r

Reduction

SYNERGISTIC

Table



8.

HAMMERLI ET AL.

Combined

Electrolysis

Catalytic

Exchange

121

catalyst f o r t h e c o l d column i n the b i t h e r m a l process is considered here. A d d i t i o n a l c a t a l y s t would of c o u r s e be r e q u i r e d f o r t h e h o t c o l u m n i n t h e b i t h e r m a l p r o c e s s b u t n o t i n t h e CECE-HWP. On t h e o t h e r h a n d , e l e c t r o l y s i s c e l l s a r e r e q u i r e d i n t h e CECE-HWP. The b i t h e r m a l p r o c e s s must o p e r a t e a t h i g h p r e s s u r e s t o a v o i d an e x c e s s i v e l y l a r g e h o t c o l u m n a s a r e s u l t o f t h e vapour l o a d a t t h e hot column t e m p e r a ture (^200°C). On t h e o t h e r h a n d , t h e c a t a l y s t column i n t h e CECE-HWP c o u l d o p e r a t e a t a b o u t a t m o s p h e r i c p r e s s u r e , a l t h o u g h 1 t o 2 M P a may b e p r e f e r a b l e i n t h e 1st stage. S e v e r a l p r e s s u r e e f f e c t s combine t o produce t h e a p p r o x i m a t e f a c t o r shown i n T a b l e II. A s s u m i n g f o r t h e m o m e n t t h a t ot£ i s u n i t y , a v a l u e w h i c h w o u l d be a p p r o a c h e d i n a h i g h temperature (~1000°C) e l e c t r o l y s i s c e l l , t h e c a t a l y s t requirements f o r t h e C E C E - H W P w o u l d be a b o u t 1 5 t i m e s l e s s t h a n that f o r only t h e c o l d column of the b i t h e r m a l H /H 0-HWP. I f aE i s 6 , t h e o v e r a l l catalyst reduction factor is approximately 24. In the above a n a l y s i s , t h e 2nd and 3 r d s t a g e s have been i g n o r e d . This i s j u s t i f i a b l e s i n c e t h e s e s t a g e s w o u l d t o g e t h e r r e q u i r e o n l y 10% a d d i t i o n a l c a t a l y s t o r l e s s d e p e n d i n g on t h e d e u t e r i u m c o n c e n t r a t i o n o f t h e 1 s t s t a g e p r o d u c t (8;). 2

Applications

o f t h e CECE-HWP

2

and CECE-TRP

T h e d e v e l o p m e n t o f b o t h t h e CECE-HWP a n d t h e CECE-TRP i s b a s e d m a i n l y on a b o u t 7 y e a r s o f l a b o r a t o r y r e s e a r c h (]_). The r e s u l t s f r o m t h i s w o r k h a v e l e d t o commitments o f s m a l l p i l o t p l a n t s f o r both p r o c e s s e s . T h e CECE-HWP p i l o t p l a n t w i l l b e l o c a t e d a t C h a l k River Nuclear Laboratories (CRNL). The CECE-TRP p i l o t plant i s a c o o p e r a t i v e p r o j e c t b e t w e e n CRNL a n d t h e E n e r g y Research and D e v e l o p m e n t A d m i n i s t r a t i o n (ERDA) o f t h e U.S. G o v e r n m e n t , and i s l o c a t e d at t h e Mound L a b o r a t o r y , Miamisburg, Ohio. P r e l i m i n a r y r e s u l t s f r o m t h e Mound p i l o t p l a n t have been r e p o r t e d by Rogers e t a l . ( Π ) . D e s p i t e t h e s u c c e s s a c h i e v e d i n t h e c a t a l ys t H J e v e l o p m e n t p r o g r a m , we h a v e n o t y e t p r e p a r e d a p l a t i n u m carbon-TefIon catalyst (J_) w i t h s u f f i c i e n t a c t i v i t y t o make a f u l l s c a l e b i t h e r m a l H - H 0 h e a v y w a t e r p l a n t economically feasible. With our present catalysts the amount r e q u i r e d w o u l d be t o o l a r g e . Furthermore, a s a t i s f a c t o r y h y d r o p h o b i c c a t a l y s t f o r the h o t column has n o t been d e v e l o p e d . Our present c a t a l y s t s are s u f f i c i e n t l y a c t i v e , however, to perform e f f e c t i v e l y i n t h e CECE-HWP w h e r e t h e c a t a l y s t r e q u i r e m e n t s a r e r e l a t i v e l y s m a l l , o n l y a b o u t 4% o f t h a t r e q u i r e d i n t h e c o l d column o f a b i t h e r m a l p l a n t . Several small scale 2

2


122


a p p l i c a t i o n s o f t h e CECE-HWP a n d t h e C E C E - T R P that c o u l d now b e u s e d i n t h e n u c l e a r p o w e r i n d u s t r y are outlined below. (1) U p g r a d i n g o f D 0 - T h e h e a v y w a t e r u s e d i n CANDU reactors as m o d e r a t o r a n d c o o l a n t t h a t h a s b e c o m e d o w n g r a d e d w i t h l i g h t w a t e r c a n be r e - e n r i c h e d t o reactor g r a d e , 9 9 . 8 % D 0 , by t h e C E C E - H W P . (2) F i n a l E n r i c h m e n t S t a g e f o r GS P l a n t s - T h e GS system produces 10-20% D 0 which i s c u r r e n t l y e n r i c h e d to r e a c t o r g r a d e by w a t e r d i s t i l l a t i o n . T h i s c o u l d be done more e f f e c t i v e l y by t h e CECE-HWP. (3) Recovery o f T r i t i u m f r o m t h e D 0 i n CANDU R e a c t o r s T r i t i u m b u i l d s up i n t h e h e a v y w a t e r d u r i n g t h e operat i o n o f CANDU r e a c t o r s . The r e c o v e r y of this tritium w o u l d r e d u c e t h e r a d i a t i o n f i e l d i n some a r e a s of the p o w e r s t a t i o n and i t s r e m o v a l may b e c o m e an e n v i r o n mental n e c e s s i t y . T r i t i u m may a l s o b e c o m e a s a l e a b l e commodity f o r the d e v e l o p m e n t o f e x p e r i m e n t a l nuclear fusion reactors. (4) T r i t i u m Recovery f r o m L i g h t W a t e r - T r i t i u m c a n be recovered from l i g h t water wastes f r o m ERDA contractors in the U.S.A. As a l r e a d y n o t e d , p i l o t s t u d i e s o f this a p p l i c a t i o n a r e b e i n g made a t t h e M o u n d L a b o r a t o r y (11). This a p p l i c a t i o n w i l l a l s o be r e q u i r e d i n n u c l e a r fuel reprocessing plants since t r i t i u m is a product of the fission process. The d i s a d v a n t a g e o f t h e h i g h e n e r g y requirement for t h e f u l l s c a l e p r o d u c t i o n o f h e a v y w a t e r by t h e CECEHWP d o e s n o t a p p l y t o t h e s e a p p l i c a t i o n s , p r i m a r i l y b e c a u s e o f t h e much s m a l l e r l i q u i d f l o w s . F o r t h e CECE-HWP a p p l i c a t i o n s 1 a n d 2 , t h e overall separation required is considerably less (enrichment f a c t o r of ^10) than t h a t r e q u i r e d to p r o d u c e pure D 0 from natural water (enrichment f a c t o r 'WOOO ) . Thus the e l e c t r o l y s i s energy r e q u i r e d i s r e d u c e d by a f a c t o r of a b o u t 7 0 0 f r o m 50 MWh t o 70 kWh p e r k i l o g r a m o f D 0. F o r the CECE-TRP a p p l i c a t i o n s 3 and 4, t h e water f l o w s w i l l be a t l e a s t a t h o u s a n d t i m e s s m a l l e r than f o r a f u l l s c a l e heavy w a t e r p l a n t . Furthermore, the energy requirements f o r the CECE-TRP are s m a l l e r than f o r other processes c u r r e n t l y being c o n s i d e r e d , such as w a t e r and h y d r o g e n d i s t i l l a t i o n , because the separation factors are larger. L a r g e r s c a l e a p p l i c a t i o n s o f t h e CECE-HWP, i f pilot plant studies warrant these, would i n c l u d e : (1) Small D 0 Plant - A s m a l l ('WO Mg/a) complete D 0 p l a n t w o u l d be e c o n o m i c a l l y f e a s i b l e w h e r e a m a r k e t e x i s t s f o r the e l e c t r o l y t i c h y d r o g e n and the heavy water. (2) S i m u l t a n e o u s D 0 and P e a k P o w e r P r o d u c t i o n - The CECE-HWP c o u l d be c o u p l e d w i t h e i t h e r H / 0 gas turbine2

2

2


2

2

2

2

2

2

2

2


8.

HAMMERLI ET AL.

Combined

Electrolysis

Catalytic

Exchange

123

generators or fuel c e l l s through appropriate H and 0 s t o r a g e f a c i l i t i e s to produce D 0 ( d u r i n g the off-peak periods) and peak p o w e r . This load l e v e l l i n g scheme (8.) i s b e i n g c o n s i d e r e d b y O n t a r i o H y d r o , Canada's l a r g e s t e l e c t r i c u t i l i t y , as o n e a l t e r n a t i v e for their future load-level l i n g requi rements. (3) E l e c t r o l y t i c H as N a t u r a l G a s S u p p l e m e n t or C h e m i c a l - I t m i g h t be f e a s i b l e t o u s e e x c e s s hydro e l e c t r i c power, w h e r e v e r i t e x i s t s , to produce hydrogen and D 0 u s i n g t h e CECE-HWP. T h e h y d r o g e n c o u l d be u s e d as a n a t u r a l gas s u p p l e m e n t i n a p i p e l i n e and a f e a s i b i l i t y s t u d y o f a 1 0 0 MW p r o t o t y p e p l a n t i s underway. A l t e r n a t i v e l y t h e h y d r o g e n c o u l d b e u s e d as a c h e m i c a l , e . g . hydrogénation o f bitumen or chemical intermediate, e . g . m e t h a n o l p r o d u c t i o n w h i c h c o u l d s e r v e as a g a s o line supplement. 2

2

2

2


2

A l l of the above p o t e n t i a l a p p l i c a t i o n s must a w a i t p i l o t p l a n t s t u d i e s b e f o r e p l a n t s c a n be c o m m i t t e d . Furthermore, the r e l a t i v e economics of electrolytic hydrogen versus hydrogen produced from f o s s i l fuels w i l l p l a y a m a j o r r o l e i n any d e c i s i o n t o p r o c e e d in each of these a p p l i c a t i o n s . The c u r r e n t v a l u e o f the D 0 by-product w i l l a l s o be i m p o r t a n t a n d t o a l e s s e r e x t e n t , the v a l u e and m a r k e t a b i l i t y o f the co-liberated oxygen. I t n e e d h a r d l y be s a i d t h a t s u c c e s s f u l development of e f f i c i e n t , r e l a t i v e l y low c o s t electrol y z e r s i s a l s o a key r e q u i r e m e n t f o r these larger applications. Moreover, c u r r e n t emphasis to conserve a l l our non-renewable resources w i l l increase the s t i m u l u s t o d e v e l o p t h e CECE p r o c e s s o v e r t h e next decade or s o . When f o s s i l r e s o u r c e s become s u f f i c i e n t l y s c a r c e , a n d / o r e x p e n s i v e , t h e move t o w a r d s a hybrid h y d r o g e n - e l e c t r i c e c o n o m y w i l l be a c c e l e r a t e d and c o n s e q u e n t l y l a r g e r amounts o f b y - p r o d u c t heavy w a t e r c o u l d be p r o d u c e d b y t h e C E C E - H W P . 2

Summary The CECE p r o c e s s i s a v e r y e f f i c i e n t m e t h o d f o r the s e p a r a t i o n of hydrogen i s o t o p e s . The catalytic exchange columns where most o f the s e p a r a t o r y work is a c c o m p l i s h e d w o u l d be v e r y s m a l l c o m p a r e d t o t h e e x change column used i n e i t h e r a b i t h e r m a l H / H 0 p r o c e s s o r t h e p r e s e n t b i t h e r m a l H S / H 0 (GS) process. Laborat o r y s t u d i e s have d e m o n s t r a t e d the s a l i e n t f e a t u r e s of t h e CECE p r o c e s s a n d i f p i l o t p l a n t s t u d i e s a t CRNL and Mound L a b o r a t o r y are s u c c e s s f u l , the process w i l l und o u b t e d l y be u s e d i n t h e n u c l e a r p o w e r i n d u s t r y i n a number of the s m a l l s c a l e a p p l i c a t i o n s d i s c u s s e d i n this paper. As l a r g e h y d r o g e n g a s s t r e a m s become 2

2

2

2


124

SEPARATION O F HYDROGEN

ISOTOPES

a v a i l a b l e t h e p r o c e s s c o u l d a l s o be u s e d f o r t h e f u l l s c a l e p r o d u c t i o n o f l a r g e q u a n t i t i e s , 70-100 M g / a , o f heavy w a t e r . Acknowledgements


We t h a n k A . S . D e n o v a n f o r t e c h n i c a l a s s i s t a n c e i n operatinq the laboratory demonstration unit of the CECE p r o c e s s a n d f o r p e r f o r m i n g t h e d e u t e r i u m a n a l y s e s i n gas and w a t e r s a m p l e s . We a l s o t h a n k W.M. T h u r s t o n and M.W.D. James f o r c o n s t r u c t i n g and m a i n t a i n i n g t h e mass- and i n f r a r e d - s p e c t r o m e t e r s y s t e m s used f o r the d e u t e r i urn an a l y s e s .

Abstract Hydrogen i s o t o p e s can be s e p a r a t e d efficiently by a process which combines an electrolysis c e l l with a trickle bed column packed with a hydrophobic p l a t i n u m catalyst. The column e f f e c t s i s o t o p i c exchange between c o u n t e r - c u r r e n t streams o f electrolytic hydrogen and liquid water w h i l e the electrolysis c e l l contributes to i s o t o p e s e p a r a t i o n by v i r t u e of the k i n e t i c i s o t o p e e f f e c t i n h e r e n t i n the hydrogen e v o l u t i o n r e a c t i o n . The main f e a t u r e s of the CECE process f o r heavy water p r o d u c t i o n are p r e s e n t e d as w e l l as a d i s c u s s i o n of the i n h e r e n t p o s i t i v e s y n e r g i s t i c effects, and o t h e r advantages and disadvantages of the p r o c e s s . S e v e r a l p o t e n t i a l a p p l i c a t i o n s o f the process in the n u c l e a r power i n d u s t r y are d i s c u s s e d . Literature 1.

2. 3. 4.

5.

Cited

B u t l e r , J.P., R o l s t o n , J.H. and S t e v e n s , W.H., Proceedings of the Symposium on " S e p a r a t i o n of Hydrogen I s o t o p e s " , Montreal 1977, p. 93 , American Chemical S o c i e t y , Washington, 1977. Hammerli, M., M i s l a n , J.P. and Olmstead, W.J., J. E l e c t r o c h e m . Soc., ( 1 9 7 0 ) , 117, 751 (and references t h e r e i n ) . B a r l o w , E . A , Atomic Energy of Canada r e p o r t no. CRE-374, March 8, 1948; D e c l a s s i f i e d and r e i s s u e d as r e p o r t no. A E C L - 1 6 3 , February 22, 1955. B e c k e r , H . W . , Bier, K . , Hubener, R . P . and K e s s e l e r , R.W., " P r o c e e d i n g s of 2nd I n t e r n a t i o n a l Conference on P e a c e f u l Uses of Atomic E n e r g y " , ( 1 9 5 9 ) , 4, 543, U n i t e d N a t i o n s . S t e v e n s , W.H., U . S . Patent no. 3 , 8 8 8 , 9 7 4 , June 10, 1975.


8.

6. 7. 8. 9. 10.


11.

HAMMERLI ET AL.

Combined Electrolysis Catalytic Exchange

125

Rolston, J.H., den H a r t o g , J. and B u t l e r , J.P., J. Phys. Chem., (1976), 80, 1064. Winter, E.E., p r i v a t e communications. Hammerli, Μ., S t e v e n s , W.H., B r a d l e y , W.J. and Butler, J.P., Atomic Energy of Canada r e p o r t no. AECL-5512, A p r i l 1976. Rae, H . K . , Proceedings of the Symposium on " S e p a r a t i o n o f Hydrogen I s o t o p e s " , Montreal 1977, p. 1 , American Chemical S o c i e t y , Washington, 1977. Brown, R . M . , R o b e r t s o n , E . and T h u r s t o n , W.M., Atomic Energy of Canada L t d . r e p o r t no. AECL-3800, January 1971. Rogers, M.L., Lamberger, P.H., Ellis, R . E . and Mills, T.K., Proceedings of the Symposium on " S e p a r a t i o n of Hydrogen I s o t o p e s " , Montreal 1977, p. 171, American Chemical S o c i e t y , Washington, 1977.



9 Heavy Water Distillation G. M. KEYSER, D. B. McCONNELL, N. ANYAS-WEISS, and P. KIRKBY


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 ©

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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.

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Water

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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




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



Figure 5.

Earning power of two experimental packings


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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


10 Deuterium Isotope Separation via Vibrationally Enhanced Deuterium Halide—Olefin Addition Reactions J. B . M A R L I N G and J. R. S I M P S O N University of California, Lawrence Livermore Laboratory, Livermore, C A


M. M . M I L L E R Francis Bitter National Magnet Laboratory, Massachusetts Institute of Technology, Cambridge, M A

Over t h e past few y e a r s , t h e r e has been much excitement i n the s c i e n t i f i c community about the prospects f o r e f f i c i e n t sep a r a t i o n o f i s o t o p e s , p a r t i c u l a r l y uranium-235 and deuterium, using l a s e r techniques. Of t h e v a r i o u s l a s e r methods which have been suggested, those which i n v o l v e t h e use o f IR photons t o enhance t h e r a t e o f i s o t o p i c a l l y s e l e c t i v e photochemical r e a c t i o n s have r e c e i v e d much a t t e n t i o n , and t h i s paper d i s c u s s e s one e x a m p l e — v i b r a t i o n a l l y enhanced gas phase deuterium h a l i d e a d d i t i o n i n t o o l e f i n s . The i n c e n t i v e f o r u s i n g IR photons i s c l e a r ; t o quote from a recent review a r t i c l e (l_) : "The a t t r a c t i v e f e a t u r e o f v i b r a t i o n a l photochemistry f o r i s o t o p e s e p a r a t i o n i s the promise o f u s i n g low energy IR photons from an e f f i c i e n t molecular l a s e r t o get a good y i e l d o f product. Since 1 mole of photons at 3000 cm" i s 1 0 kwh and some IR l a s e r s are about 10 per cent e f f i c i e n t , p r o c e s s i n g o f b u l k chemicals might even be economic." Thus, a deuterium s e p a r a t i o n process w i t h 1% quantum e f f i c i e n c y t h a t u t i l i z e d 2000 cm" photons from a 10 per cent e f f i c i e n t CO l a s e r would r e q u i r e a l a s e r process energy o f 6 . 6 kwh/mole D, e q u i v a l e n t t o a l a s e r o p e r a t i n g cost o f 13^/mole D Ξ $13/kg D 0, assuming e l e c t r i c i t y at 20 m i l l s per kwh. The s p e c i f i c l a s e r c a p i t a l investment, assuming a p r i c e o f $20 per o p t i c a l watt o f 10 per cent e f f i c i e n t l a s e r power, would be roughly $152/kg D 0/yr., e q u i v a l e n t t o approximately $30/kg D 0 at a c a p i t a l charge r a t e o f 20 per cent/year. To put these numbers i n p e r s p e c t i v e , we note t h a t t h e current Canadian p r i c e f o r heavy water made by the H S/H 0 exchange (G-S) process i s about $150/kilogram, o f which 60 per cent i s due t o c a p i t a l charges, 25 per cent due t o energy, and 15 per cent f o r operations and maintenance (2). Since heavy water i s a much cheaper commod i t y than U-235, t h i s approximate c a l c u l a t i o n i l l u s t r a t e s t h e challenge i n developing a l a s e r deuterium s e p a r a t i o n process which i s economically c o m p e t i t i v e w i t h the e x i s t i n g technology ( 3_). In t h i s paper we i l l u s t r a t e t h e problems and prospects i n v o l v e d by examining i n some d e t a i l a s p e c i f i c deuterium separation process based on deuterium h a l i d e - o l e f i n a d d i t i o n 1

- 2

1

2

2

2

2

©

2

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Isotope

Separation

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r e a c t i o n s . I n S e c t i o n I I we describe t h e b a s i c r e a c t i o n mechan ism, w i t h p a r t i c u l a r a t t e n t i o n t o the problem o f e x c i t i n g t h e deuterium h a l i d e w i t h e x i s t i n g l a s e r s . S e c t i o n I I I i s devoted t o t h e c r u c i a l question o f the expected e f f e c t i v e n e s s o f v i b r a t i o n a l e x c i t a t i o n f o r t h i s c l a s s o f r e a c t i o n s . I n S e c t i o n IV, we focus on t h e "back-end" o f t h i s separation scheme i n the context of a p o s s i b l e flow-sheet f o r t h e o v e r a l l process. We conclude w i t h a summary o f our r e s u l t s and the i m p l i c a t i o n s t h e r e o f i n S e c t i o n V.


II.

Hydrogen H a l i d e - O l e f i n A d d i t i o n Reactions

An i d e a l i s o t o p i c a l l y s e l e c t i v e l a s e r photochemical process would use a s i n g l e IR photon t o e x c i t e t h e fundamental mode o f the i s o t o p i c molecule o f i n t e r e s t , f o l l o w e d by a gas phase, v i b r a t i o n a l l y - e n h a n c e d b i m o l e c u l a r r e a c t i o n , l e a d i n g t o an i s o t o p i c a l l y enriched r e a c t i o n product which could e a s i l y be separated. A primary c o n s i d e r a t i o n i n t h e choice o f r e a c t i o n i s the a c t i v a t i o n energy. I t must be high enough t o minimize t h e (thermal) r e a c t i o n r a t e i n t h e absence o f v i b r a t i o n a l e x c i t a t i o n , but low enough so that t h e r e a c t i o n r a t e o f the v i b r a t i o n a l l y e x c i t e d species exceeds t h e r a t e o f scrambling r e a c t i o n s , e.g., V-V and V-T energy t r a n s f e r s , which l e a d t o the l o s s o f i s o t o p i c s e l e c t i v i t y . I n t h i s s e c t i o n we d i s c u s s a c l a s s o f r e a c t i o n s , deuterium h a l i d e - o l e f i n a d d i t i o n s , w i t h thermal e q u i l i b r i u m a c t i v a t i o n energies i n the range 15-^0 kcal/mole. The i n i t i a l step o f t h i s process i n v o l v e s s e q u e n t i a l absorp t i o n o f s e v e r a l quanta near 5 microns from a pulsed CO l a s e r t o e x c i t e DBr o r DC1 up t h e i r v i b r a t i o n a l ladders t o t h e ν >_ 3 v i b r a t i o n a l l e v e l . A l t e r n a t e l y , a pulsed DF l a s e r near h microns can s e q u e n t i a l l y e x c i t e DF or EDO t o ν > 3. I n pure DX, c o l l i sions o f the type DX(v = l ) + DX(v = l ) •> DX(v = 2) + DX(v = 0) can a l s o e x c i t e higher v i b r a t i o n a l l e v e l s , but t h i s mechanism i s not f e a s i b l e i n n a t u r a l HX c o n t a i n i n g only 0.015% DX. The v i b r a t i o n a l l y e x c i t e d DX molecule (X = B r , C l , F, OH) then w i l l p r e f e r e n t i a l l y r e a c t w i t h unsaturated hydrocarbons ( f o r example, DBr r e a c t i n g w i t h ethylene) t o y i e l d a deuterium-tagged a d d i t i o n product, e.g., e t h y l bromide-d}. These steps may be w r i t t e n DX + nhv + DX* (v=n) η >_ 3 (pulsed CO o r DF l a s e r e x c i t a t i o n ) DX* +

(R

(1)

(2)

l5

R

2

= H, CH , CH=CH , etc.) 3

2

This type o f a d d i t i o n r e a c t i o n i n t o unsaturated hydrocarbons i n t h e gas phase occurs by a homogeneous, b i m o l e c u l a r , f o u r - o r s i x - c e n t e r process (^,5.). Both the forward r e a c t i o n (2) and t h e



136

reverse r e a c t i o n (unimolecular e l i m i n a t i o n o f HX) have been very w e l l s t u d i e d i n the gas p h a s e ( 6 l ) . K i n e t i c parameters f o r the gas phase thermal a d d i t i o n r e a c t i o n are w e l l represented by the Arrhenius r a t e e x p r e s s i o n , -E /RT k(sec ) = [C]-Ae (3) 9

a

where

[c] = o l e f i n c o n c e n t r a t i o n ( m o l e s / l i t e r ) A = frequency f a c t o r ( l i t e r / m o l e - s e c ) Ε = thermal a c t i v a t i o n energy (kcal/mole) a

A r r h e n i u s parameters f o r HX a d d i t i o n i n t o simple and con jugated o l e f i n s are given i n Table I , which i l l u s t r a t e s the v a r i a t i o n o f a c t i v a t i o n energy E and frequency f a c t o r A, according t o the choice o f reagents. Examination o f Table I r e v e a l s t h a t a c t i v a t i o n energies i n the range 10-50 kcal/mole are a v a i l a b l e , depending on the choice o f reagents. Lowest a c t i v a t i o n energies occur f o r HI a d d i t i o n w i t h i n c r e a s i n g a c t i v a t i o n energy f o r l i g h t e r HX, such t h a t E ( H l ) < E ( H B r ) < E (HCl) < E (HF) < E ( H 0 ) . Table I a l s o shows t h a t a c t i v a t i o n energy decreases w i t h i n c r e a s i n g s u b s t i t u t i o n , w i t h E decreasing by about 5 kcal/mole per methyl (-CH3) group and by about 9 kcal/mole per v i n y l (-CH=CH2) group on the a- or h a l o g e n - r e c e i v i n g carbon atom. Among olefins, 2-methylpropene and 1,3-butadiene have n e a r l y i d e n t i c a l a c t i v a t i o n e n e r g i e s , but 1,3-butadiene i s a f a r s u p e r i o r reagent choice because of i t s approximately 2 0 0 - f o l d higher frequency f a c t o r (A v a l u e ) . Although HI has the lowest a c t i v a t i o n energies f o r r e a c t i o n , i t i s probably not an acceptable choice i n a l a r g e s c a l e process because of i t s tendency t o decompose. HF and H 0 are probably r e l a t i v e l y u n a t t r a c t i v e , s i n c e they have s i g n i f i c a n t l y higher a c t i v a t i o n e n e r g i e s ; t h i s l e a v e s HBr or HC1 as the most l i k e l y reagent c h o i c e . A l s o , use of HF or H 0 would r e q u i r e e x c i t a t i o n by a DF chemical l a s e r , which i s an i n h e r e n t l y more expensive and l e s s e f f i c i e n t device than the CO l a s e r , p r i m a r i l y due t o the cost of r e g e n e r a t i n g the f l u o r i n e . The CO l a s e r i s an e f f i c i e n t , well-developed device ( l l ) which i s q u i t e s u i t a b l e f o r a l a r g e - s c a l e i n d u s t r i a l process. To evaluate the s u i t a b i l i t y o f the CO l a s e r f o r e x c i t a t i o n o f DBr or DC1, a computer comparison was made o f the c a l c u l a t e d wavelengths of CO emission l i n e s and DBr or DC1 a b s o r p t i o n l i n e s . For these diatomic molecules, accurate frequencies may be gener ated from a Dunham equation f o r the energy l e v e l s :


a

a

a

a

a

a

2

a

2

2

kj j+i

*™ = Z\*K)\î


(u)

10.

MARLING ET AL.

Deuterium

Isotope

137

Separation

TABLE I A r r h e n i u s Parameters Olefins: Frequency Factor

Unsaturated Hydrocarbon

LogipA

propylene^

3


t

2-methylpropene Phenyl e t h y l e n e

X = I , B r , C l , F, OH A c t i v a t i o n Energy Ε (kcal/mole) a HBr HCl HF HI H 0 tot7

2

8.3°

ethylene

f o r HX A d d i t i o n t o

28.5

ko

50

57

U5

52

7.9

C

23.5

28.5

3k

6.5

C

18.5

23.5

28

20

2k

30

13

1,3-butadiene 2-methyl-l,3butadiene 1 5 3-pentadiene

3k

18.7

1

Q.k

h

7-7

I5

s

1

26

g

20

1

25

s

2k 20

f

ih.T

h

2-methyl-l,3 . pentadiene

l.k*

11

16

21

H-methyl-1,3-. pentadiene

6.3

10

15

20

J

U n i t s o f A are l i t e r s - m o l e f o r a l l HX compounds. b

Ε

a

l e

38

29

1

e

kg

sec \

L o g A i s constant ± 0 . 3 10

values from Reference 6.

c Reference k. d Computed from A o f back r e a c t i o n .

See Reference 7 ·

Reference 8. Computed from k i n e t i c data o f Reference 5 . Computed from k i n e t i c s o f back r e a c t i o n .

See Reference 9 ·

Reference 1 0 . Estimated Ε

value.

Probable accuracy:

Estimated v a l u e f o r L o g ^ A .

± 2 kcal/mole.

Probable accuracy: ± O.k.



138


For normal and i s o t o p i c y i e l d computed emission (Ref. 12) or even a few measured values f o r the

carbon monoxide, the c o e f f i c i e n t s Y frequencies accurate t o 0.001 cm" megahertz (13). For D ^ ^ c i , d i r e c t l y Υ „ were a v a i l a b l e ( l U ) . For D^Tci the £k ntr — Y were computed from D ^ C l u s i n g the Dunham i s o t o p e r e l a t i o n s (15) and the a p p r o p r i a t e atomic reduced masses ( l 6 ) . For D^^Br and D^lBr the e a r l i e r r e p o r t e d a b s o r p t i o n f r e q u e n c i e s (IT) were not s u f f i c i e n t l y accurate. T h e r e f o r e , accurate Y were generated f o r HBr u s i n g the accurate r o t a t i o n a l constants (l8) and recent higher l e v e l v i b r a t i o n a l constants (19.). The a p p r o p r i a t e reduced masses (l6) and Dunham i s o t o p e relation§^(15) then were used t o d e r i v e a l l Y except Y ^ f o r D Br and D Br. Y-|_Q was then f i x e d by r e q u i r i n g the computed 1-0 band t o be 0.25 cm" higher than i t s measured value (17) > according t o the c o r r e c t i o n suggested i n Ref. 20. The d e r i v e d Dunham c o e f f i c i e n t s f o r DC1 and DBr thus p r o v i d e d c a l c u l a t e d a b s o r p t i o n l i n e center frequencies f o r a computer search t o f i n d near-coincidences w i t h CO l a s e r emission f r e q u e n c i e s . For each near c o i n c i d e n c e , the a b s o r p t i o n co e f f i c i e n t was computed u s i n g a standard Lorentz l i n e shape f o r mula, equation (5)> which a l s o i n c l u d e d the temperature depen dence o f the r o t a t i o n a l l e v e l p o p u l a t i o n : -3m(m+l)/kT 1

Λ

1

α ( Δ ν )

= lV

(Λν/0. Ρ) 5Ύ

( 5 ) 2

In equation ( 5 ) , α(Δν) i s the a b s o r p t i o n ( i n cm" ) at a d i s t a n c e Av(cm~l) from DX l i n e center at a pressure o f Ρ atmospheres. The values f o r the pressure-broadened l i n e w i d t h γ(cm atm" ) depend on r o t a t i o n a l number m, and were taken from Ref. 21_ f o r DC1 broadening by HC1. The r o t a t i o n a l number i s m = J - l f o r R-branch and m = J f o r P-branch t r a n s i t i o n s . DBr values f o r γ were d e r i v e d from the r e p o r t e d values f o r HBr(22). The parameters Κ and 3 i n equation (5) were a d j u s t e d t o match the r e p o r t e d DC1 a b s o r p t i o n l i n e s t r e n g t h s ( 2 3 ) , and DBr values f o r Κ and 3 were d e r i v e d from the HBr values (22) by assuming K(DBr) = l/k K(HBr) and 3(DBr) Ξ 2 3(HBr), assumptions found t o be reasonable f o r the DC1/HC1 data (23). Equation (5) thus permits reasonably accurate e s t i m a t i o n of a b s o r p t i o n i n the 1-0 band o f DC1 and DBr by CO l a s e r l i n e s . A b s o r p t i o n s t r e n g t h of the higher v i b r a t i o n a l bands (2-1 through 5-U i n t h i s study) w i l l i n c r e a s e approximately p r o p o r t i o n a l t o ν (Réf. 2*0, but decrease due t o the d i s t r i b u t i o n o f p o p u l a t i o n over the s e v e r a l l e v e l s . As a f i r s t approximation, a b s o r p t i o n s t r e n g t h f o r higher v i b r a t i o n a l bands was thus taken t o be the same as the 1-0 band. Tables I I and I I I summarize the computed a b s o r p t i o n o f -^C-Ô l a s e r emission l i n e s by the v a r i o u s i s o t o p e s of DC1 and DBr. Data f o r Tables I I and I I I were computed f o r atmospheric pressure and room temperature (298°K), and i l l u s t r a t e the ease 1

-1


1

10.

MARLING ET AL.

Deuterium

Isotope

139

Separation

TABLE I I ABSORPTION OF

C O

DC1 T r a n s i t i o n * · ν ( cm" )


1

1-0 1-0 1-0 1-0 2-1 2-1 2-1 2-1 3-2 3-2 3-2 3-2 U-3 h-3 h-3 h-3 5-U

P(T) P(3)37 P(T)37 P(U) P(2) p(n) P(U) 3 5

3 5

3 5

3 5

3 7

2011.Ohl 2055-1^6 2008.282 20U6.609 2016.033 1909.678 1991.iih

R(3) 2077.3k0 P(2) 1963.IUO P(ll) 1858.853 P(6) 1918.799 P(2) 1960.51^ P ( 5 ) 5 1878.238 R(5) 1986.873 P(2) 1910.507 P(3) 1897.518 P ( 7 ) 5 I80U.358 p(i) 1868.160 3 5

3 5

3 5

3 5

3 7

3

3 5

3 5

3 7

3

5-U 5-U R ( 2 )

3 5

3 7

1903.939

LASER LINES BY D ^ C l and D^'Cl, a t P=l ATM.

1 2

l 6

c o LASER LINE 5-h P ( 7 ) 3-2 P ( 9 ) 3-2 P ( 2 0 ) 3-2 P ( l l ) 2 - 1 F(2k) 6-5 P ( 2 5 ) 5-U P ( 1 2 ) 2 - 1 P(10) 7-6 P ( 6 ) 1 0 - 9 P(13) 7-6 P ( 1 7 ) h-3 P ( 2 5 ) 10-9 P(8) 5-U P ( 1 3 ) 7-6 P ( 1 9 ) 7-6 P ( 2 2 ) 12-11 P(lU) 9-8 P ( 1 7 ) 9-8 P(8)

v

Vi- co

( c n r l )

ABSORPTION COEF. (cm" )

-0.0i+9 -0.012 -O.I38 -0.3^9 0.007 -0.001 0.090 0.201 0.058 -O.Okh -0.179 0.103 -O.OI7 -0.037 -0.02U -0.101 0.026 0.03U 0.058

* S u b s c r i p t 35 o r 37 r e f e r s t o D ^ C l o r D respectively.

1

11.8 l.U 1.1 12.6 5.1 3.0 2.9 8.7 3.6

3Λ 1.7 18.5 15.2 11.8 2.3 Ik.k 6.0 3.7

Cltransitions,



140

TABLE I I I ABSORPTION OF

1 2

C

l 6

0 LASER LINES BY D


TRANSITION*

v ( cm*" ) 1

B r and D

C C 0 LASER LINE 1 2

DBr

T 9

8 l

B r a t P=l ATM.

1 6

v

v

DBr- C0

( c m

"

1 )

ABSORPTION (cm" ) 1

1-0

R ( 2 ) 81

1863.999

9-8

P(l8)

0.011

3.2

1-0

R(3) 79

18T2.3T8

8-7

P(22)

0.0U8

2.5

1-0

P(9) 79

1757.851

lU-13 P(13)

1-0

P(3) 81

1813.573

11-10

2-1

R ( 6 ] 81

I8U7.I63

9-8

2-1

P(8] 81

1722.606

2-1

R ( 9 ! 79

1867.966

8-7

2-1

P ( 9 ! 79

1713.U97

15-lh

3-2

P(6) 81

1696.517

3-2

R ( 2 ; 81

3-2

P(l8)

2.1 0.057

1.9

P(22)

0.0U6

3.0

16-15 P(9)

0.003

3.8

-0.032

2.0

P(l8)

-0.0U6

2.0

16-15

P(l6)

0.008

h.3

1771.3h3

13-12

P(l6)

0.017

3.1

P ( 9 l 79

1669.172

18-17

P(10)

0.07^

1.3

h-3

p(u; 81

1669.H5

18-17

P(10)

0.017

3.7

h-3

R(3

79

1732.808

15-lU P(13)

0.0U3

2.7

5-U

P(T ) g

1598.386

21-20 P ( 9 )

0.016

u.o

5-U

P(T '81

1597.983

20-19

P(l6)

0.0^9

2.9

Br o r D

Br t r a n s i t i o n s ,

7

* S u b s c r i p t 79 o r 8 l r e f e r s t o D

P(23)

respectively.


10.

MARLING ET AL.

Deuterium

Isotope

141

Separation

i n a c h i e v i n g s e q u e n t i a l a b s o r p t i o n o f CO l a s e r quanta t o e x c i t e DC1 o r DBr up t h e i r v i b r a t i o n a l l a d d e r s t o ν - 5· Examination o f Table I I i n d i c a t e s t h a t the best matches o f 12^1 ο l a s e r emission w i t h D^^ci y i e l d a b s o r p t i o n c o e f f i c i e n t s of about 12 cm" a t 1 atmosphere. S i m i l a r l y , the best CO laser-DBr a b s o r p t i o n c o e f f i c i e n t s are about 3 cm" . At higher p r e s s u r e s , r a d i a t i o n from CO l a s e r l i n e s i s e a s i l y absorbed w i t h out need f o r CO l a s e r l i n e s e l e c t i o n . For example, a t 5 atmo spheres pressure any CO l a s e r l i n e i n the U.9—5·1 micron r e g i o n i s c a l c u l a t e d t o experience an average DC1 a b s o r p t i o n c o e f f i c i e n t of 2 cm" f o r the 1-0 band, which i n c r e a s e s t o about 10 cm" when a b s o r p t i o n from DC1 v i b r a t i o n a l l y e x c i t e d s t a t e s i s i n c l u d ed. Only the "best" matches were given i n Tables I I and I I I f o r each l e v e l o f v i b r a t i o n a l e x c i t a t i o n . Table I I shows t h a t most DC1 a b s o r p t i o n o f C " 0 l a s e r emission w i l l occur i n the U . 9 micron r e g i o n , and u s e f u l DBr a b s o r p t i o n w i l l occur i n the 5.^-6.2 micron r e g i o n . At n a t u r a l deuterium abundance, t h e 1/e a b s o r p t i o n depth would be about 5 meters f o r DC1 and 20 meters f o r DBr. Since a t y p i c a l r e a c t i o n mixture would a l s o c o n t a i n an o l e f i n a t near-atmospheric p r e s s u r e , i t would be important t o i n s u r e t h a t the o l e f i n i s s u f f i c i e n t l y t r a n s p a r e n t a t these wavelengths (a < 0.1 m e t e r " ) . This i s e s s e n t i a l t o a l l o w the CO l a s e r emission t o e x c i t e p r i m a r i l y DBr o r DC1, and not waste photons by o p t i c a l l y h e a t i n g the o l e f i n . Examination o f simple and conjugated o l e f i n a b s o r p t i o n bands (25) i n the 5-6 micron r e g i o n r e v e a l s q u i t e strong C = C s t r e t c h a b s o r p t i o n a t 6.0-6.2 micron. P o t e n t i a l l y troublesome combination band a b s o r p t i o n occurs a t 5·1*Μ and e s p e c i a l l y a t 5 · 6 μ i n 1,3-butadiene and i s o prene, probably the best reagent choices from Table I . Gas phase a b s o r p t i o n s p e c t r a were examined and showed an a b s o r p t i o n c o e f f i c i e n t ( t o base e) o f about 0 . 5 cm" A t m f o r both 1,3-butadiene and isoprene near 5 · 6 micron. This i s about 2-3 orders o f magnitude stronger than n a t u r a l l y o c c u r r i n g DBr a b s o r p t i o n , r e n d e r i n g photon e f f i c i e n t e x c i t a t i o n o f DBr i m p o s s i b l e . The s i t u a t i o n i s somewhat improved a t 5·1 micron, where the a b s o r p t i o n c o e f f i c i e n t s o f these two o l e f i n s are about 0.05 cm"" atm" , s t i l l about an order o f magnitude stronger than DC1 a b s o r p t i o n a t n a t u r a l abundance. T h i s suggests t h a t l i n e a r conjugated o l e f i n s w i l l be e x c e s s i v e l y absorbing, and prevent e f f i c i e n t CO-laser e x c i t a t i o n o f DC1 and e s p e c i a l l y DBr. Cyclopentadiene and 1 , 3 - c y c l o h e x a d i e n e should a l s o e x h i b i t low a c t i v a t i o n energies f o r hydrogen h a l i d e a d d i t i o n , s i m i l a r t o the l i n e a r conjugated o l e f i n s l i s t e d i n Table I , and o p t i c a l a b s o r p t i o n s p e c t r a are somewhat more promising. Cyclopentadiene (26) appears t r a n s p a r e n t i n the 5 . 0 - 5 . 3 μ r e g i o n , p o t e n t i a l l y s u i t able f o r use w i t h DC1, and 1 , 3 - c y c l o h e x a d i e n e (27) appears t r a n s p a r e n t i n the 5 · ^ - 5 · 7 5 y r e g i o n , making i t p o t e n t i a l l y s u i t a b l e f o r use w i t h DBr. E l i m i n a t i o n of o l e f i n absorption w i l l be e s s e n t i a l f o r e f f i c i e n t photon u t i l i z a t i o n a t low deuterium Ό

1

1


1

1

1 2

1

1

1

1

1


- 1


142

c o n c e n t r a t i o n s . Only ethylene (25) shows extreme t r a n s p a r e n c y , w i t h no d e t e c t a b l e a b s o r p t i o n from 1550 cm- t o 1750 cm"" , making i t o p t i c a l l y s u i t a b l e f o r use w i t h DBr. Non-reactive quenching o f v i b r a t i o n a l l y e x c i t e d deuterium h a l i d e w i l l be an important p r o c e s s , as i t competes w i t h r e a c t i v e a d d i t i o n o f t h e e x c i t e d s p e c i e s i n t o t h e o l e f i n . The r e a c t i o n e f f i c i e n c y φ w i l l be simply t h e r a t i o o f r e a c t i v e quenching t o t o t a l quenching, or 1

1

R [c] v

φ

=

( V Q ) [ c ] + Q [x] c

(6)

x


where [c] = Olefin concentration [x] = Hydrogen h a l i d e c o n c e n t r a t i o n R = Deuterium h a l i d e r e a c t i v i t y i n vibrational level ν Q = Quenching r a t e / u n i t c o n c e n t r a t i o n ° of olefin Q = Quenching r a t e / u n i t c o n c e n t r a t i o n of hydrogen h a l i d e In a d d i t i o n t o r e a c t i o n , t h e v i b r a t i o n a l l y e x c i t e d deuterium h a l i d e w i l l experience n o n - r e a c t i v e v i b r a t i o n a l quenching by both t h e o l e f i n and t h e hydrogen h a l i d e . Q denotes v i b r a t i o n a l quenching o f deuterium h a l i d e by hydrogen h a l i d e and has been measured f o r quenching o f t h e DX v = l l e v e l (28-31). For DBr, a value o f Q = 0.2 (μsec · atm)" was d e t e r mined (28) f o r quenching by HBr. For DC1 quenching by HC1, a value of Q = O.h {\isec · atm)"" may be i n f e r r e d (29). Nonr e a c t i v e v i b r a t i o n a l quenching by t h e o l e f i n may be 10-100 times f a s t e r than these r a t e s , based on t h e observed r a p i d quenching of HBr ( 3 0 ) , HC1 (31) and DC1 (31) by water and methane. Quenching r a t e s o f higher DX v i b r a t i o n a l l e v e l s w i l l be f a s t e r than f o r the v = l l e v e l , r i s i n g approximately p r o p o r t i o n a l t o ν (Ref. 2 9 ) . Thus, equation (6) reduces t o a simpler form when the o l e f i n c o n c e n t r a t i o n [θ] i s r a i s e d t o o p t i m i z e φ, x

1

x

1

x

φ ifc R /(R +Q ) i f [C] % [X] v

v

c

and Q » Q c χ

(7)

since v i b r a t i o n a l quenching by HX i s slow compared t o t h e expected o l e f i n quenching r a t e s . The upper l i m i t o f t h e DX r e a c t i o n r a t e w i t h a given o l e f i n i s simply the frequency A (Table I ) and i s expected t o be approached as t h e DX l a s e r s u p p l i e d v i b r a t i o n a l energy exceeds t h e r e a c t i o n - a c t i v a t i o n energy Ε . T h i s assumption was examined f o r other r e a c t i o n systems ( 3 2 ) , and i s d i s c u s s e d f u r t h e r i n S e c t i o n I I I . T h i s p l a c e s an upper l i m i t on t h e r e a c t i o n e f f i c i e n c y


10.

MARLING ET AL.

Deuterium

Isotope

143

Separation

(8)

max

when one assumes (32) t h a t A f o r a b i m o l e c u l a r r e a c t i o n i s i n d e pendent o f reagent v i b r a t i o n a l e x c i t a t i o n . The best value o f A from Table I occurs f o r 1 , 3 - b u t a d i e n e , A - 25 (μsec•atm) . I f o l e f i n n o n - r e a c t i v e v i b r a t i o n a l quenching i s comparable t o HX quenching r a t e s by methane ( 3 j 0 , 3 l ) , o r Q = 10-60 (μεβο •atm)*"" , then the maximum r e a c t i o n e f f i c i e n c y , from equation ( 8 ) , c o u l d l i e i n the range 0.1%-50%, w i t h the higher r e a c t i o n e f f i c i e n c i e s corresponding t o use o f o l e f i n s w i t h h i g h values o f A, such as 1,3-butadiene o r ethylene. The problem o f n o n - r e a c t i v e quenching o f v i b r a t i o n a l l y e x c i t e d DX by the o l e f i n i s r e l a t e d t o the problem o f e x c e s s i v e o p t i c a l a b s o r p t i o n by the o l e f i n , namely the presence o f o l e f i n energy resonances near DX a b s o r p t i o n f r e q u e n c i e s . DX v i b r a t i o n a l quenching by v-v energy t r a n s f e r processes becomes very r a p i d near energy resonance (30,33), but should drop t o acceptably low v a l u e s , i f the nearest resonances have an energy discrepancy o f more than about 1000 cm" . Lower n o n - r e a c t i v e quenching can be achieved by u s i n g f u l l y halogenated o l e f i n s (3*0, which may a l s o y i e l d h i g h o l e f i n transparency a t DX a b s o r p t i o n f r e q u e n c i e s . However, t y p i c a l f l u o r i n a t e d o l e f i n reagent c h o i c e s , such as h e x a f l u o r o - l , 3 - b u t a d i e n e (35.), hexafluoropropene (36), o r f l u o r i n a t e d ethylenes (36) have e x c e s s i v e o p t i c a l a b s o r p t i o n i n the 5-6 micron r e g i o n (35.,36) , and hence are not s u i t a b l e . But when i n the course o f t h i s work t e t r a c h l o r o e t h y l e n e o p t i c a l a b s o r p t i o n was examined i n the gas phase, i t was found t o be h i g h l y transparent i n the H.2-5-2 μ r e g i o n , and i s thus poten t i a l l y s u i t a b l e f o r use w i t h HC1. Experimental k i n e t i c data on a c t i v a t i o n energies are not r e p o r t e d , but c a l c u l a t e d values o f E f o r HF o r HC1 r e a c t i n g w i t h perhalogenated ethylene are 5 kcal/mole lower than w i t h normal ethylene (6). Some p e r f l u o r i n a t e d o l e f i n s are v e r y t o x i c ( 3T_), and p e r c h l o r i n a t e d o l e f i n s r e q u i r e o p e r a t i n g temperatures 100-200°C higher than normal o l e f i n s t o achieve u s e f u l vapor pressure. N e v e r t h e l e s s , t h e p o t e n t i a l f o r h i g h IR transparency and low v i b r a t i o n a l quenching makes t h i s c l a s s o f reagents (and t e t r a c h l o r o e t h y l e n e i n p a r t i c u l a r ) a t t r a c t i v e t o c o n s i d e r f o r a p r a c t i c a l process. There i s p r e s e n t l y very l i t t l e experimental data on l a s e r e x c i t e d HX a d d i t i o n i n t o o l e f i n s . However, r e a c t i o n was observed between 2-methylpropene and HCl(v=6) produced by i n t r a c a v i t y dye l a s e r e x c i t a t i o n o f the HC1 f i f t h overtone ( 3 8 ) . Quantum y i e l d s f o r r e a c t i o n were estimated (38.) t o l i e i n the range 0.01-0.1%, c o n s i s t e n t w i t h the very l o w frequency f a c t o r f o r t h i s o l e f i n (see Table I ) . Use o f 1,3-butadiene i n t h i s same experiement ( i n s t e a d o f 2-methylpropene) was not t r i e d , but should have y i e l d e d a r e a c t i o n quantum y i e l d o f 2-20%, based on i t s 2 0 0 - f o l d higher frequency f a c t o r . Economic a n a l y s i s o f l a s e r - r e l a t e d c o s t s i n heavy water p r o d u c t i o n by the D X - o l e f i n process l

1


c

1

a




144

d i s c u s s e d here i n d i c a t e t h a t r e a c t i o n e f f i c i e n c i e s must exceed 5% f o r t h i s process t o be economically v i a b l e (39)· Additional c o s t s due t o the "back-end" of t h i s process (see S e c t i o n IV) w i l l probably set a p r a c t i c a l lower l i m i t of 10% f o r an acceptable r e a c t i o n e f f i c i e n c y . In t h i s c o n t e x t , the c l a s s i c a l quantum e f f i c i e n c y i s the r e a c t i o n e f f i c i e n c y φ d i v i d e d by n, the average number of absorbed quanta per DX molecule. The assumption of η = 5 p l a c e s an approximate lower l i m i t of 2% on an economically acceptable quantum e f f i c i e n c y f o r deuterium h a l i d e a d d i t i o n i n t o o l e f i n s as a p o t e n t i a l photochemical route t o heavy water pro duction. In t h i s s e c t i o n we have shown t h a t the CO l a s e r permits s e q u e n t i a l e x c i t a t i o n of DBr or DC1 up the v i b r a t i o n a l ladder t o at l e a s t the ν = 5 v i b r a t i o n a l l e v e l . At n a t u r a l deuterium abundance, the l / e a b s o r p t i o n depth f o r s e l e c t e d CO l a s e r l i n e r a d i a t i o n i s about 5 meters f o r DC1 and about 20 meters f o r DBr, at a hydrogen h a l i d e o p e r a t i n g pressure of one atmosphere. At 5 atmospheres o p e r a t i n g p r e s s u r e , u n r e s t r i c t e d m u l t i l i n e opera t i o n of the CO l a s e r i s s u f f i c i e n t , but some l i n e t u n i n g (according t o Tables I I and I I I ) f a c i l i t a t e s DX a b s o r p t i o n at one atmosphere pressure. Troublesome o l e f i n o p t i c a l a b s o r p t i o n and v i b r a t i o n a l quenching may be reduced by u s i n g t e t r a c h l o r o e t h y l e n e . Unwanted o l e f i n o p t i c a l a b s o r p t i o n may a l s o be avoided by u s i n g e t h y l e n e , cyclopentadiene, or 1,3-cyclohexadiene. The l a r g e choice of Arrhenius parameters a v a i l a b l e f o r t y p i c a l reagent choices (Table I ) permits reagent o p t i m i z a t i o n f o r acceptable process r e a c t i o n e f f i c i e n c y . Experimental e v a l u a t i o n of the deuterium h a l i d e / o l e f i n process f o r heavy water p r o d u c t i o n i s i n progress at the Lawrence Livermore Laboratory. I I I . E f f e c t i v e n e s s of V i b r a t i o n a l E x c i t a t i o n In the preceding s e c t i o n , we have shown t h a t e x c i t a t i o n of l o w - l y i n g v i b r a t i o n a l l e v e l s of deuterated h a l i d e s should l e a d t o s i g n i f i c a n t enrichments v i a i s o t o p i c a l l y s e l e c t i v e a d d i t i o n r e a c t i o n s , I f the h a l i d e v i b r a t i o n and r e a c t i o n coordinates are e s s e n t i a l l y i d e n t i c a l . That v i b r a t i o n a l e x c i t a t i o n of hydrogen h a l i d e s l e a d s t o enhanced r a t e s f o r diatomic-atom exchange r e a c t i o n s of the type K f

A + BC

t

t

AB

+ C

(9)

have been e x p e r i m e n t a l l y confirmed; perhaps the most s t r i k i n g example i s the f a c t t h a t HC1 (v = 2) was found t o r e a c t w i t h bromine approximately 1 0 times f a s t e r than HC1 (ν = 0) (kO). The t h e o r e t i c a l e x p l a n a t i o n o f these r a t e enhancements runs as f o l l o w s : Since the forward, exoergic r e a c t i o n leaves the product AB i n a h i g h l y v i b r a t i o n a l l y e x c i t e d s t a t e (AB ), then 1 1


10.

MARLING ET AL.

Deuterium

Isotope

145

Separation

microscopic r e v e r s i b i l i t y a t the same t o t a l energy i m p l i e s t h a t the r a t e o f the r e v e r s e , endoergic r e a c t i o n - w i l l be enhanced more e f f e c t i v e l y when energy i s put i n t o AB v i b r a t i o n r a t h e r than i n t o r e l a t i v e t r a n s l a t i o n o f AB and C. I n c o n s i d e r i n g more general r e a c t i o n s i n v o l v i n g v i b r a t i o n a l l y e x c i t e d reagents, i t i s impor t a n t t o note t h a t the r e v e r s i b i l i t y argument can be a p p l i e d inde pendent o f whether the r e a c t i o n which leads t o a v i b r a t i o n a l l y e x c i t e d product i s endoergic o r exoergic. We s t r e s s t h i s because, w h i l e i t i s g e n e r a l l y b e l i e v e d t h a t t h e endoergic r e a c t i o n s u t i l i z e v i b r a t i o n a l energy more e f f e c t i v e l y than exoergic r e a c t i o n s (hi), t h e e x i s t i n g experimental evidence, as analyzed by B i r e l y and Lyman (32.) does not show any strong c o r r e l a t i o n between the e f f e c t i v e n e s s o f reagent v i b r a t i o n i n lowering the a c t i v a t i o n energy and r e a c t i o n e x o e r g i c i t y . I t f o l l o w s t h a t i n s i g h t i n t o the e f f e c t o f v i b r a t i o n a l e x c i t a t i o n o f hydrogen h a l i d e s on the r a t e o f exoergic a d d i t i o n r e a c t i o n s can best be gleaned from t h e experimental data on the energy d i s p o s a l o f the reverse r e a c t i o n s , i . e . , unimolecular hydrogen h a l i d e e l i m i n a t i o n s from, e.g., h a l o alkanes and h a l o o l e f i n s , f o l l o w i n g chemical o r photochemical a c t i v a t i o n . There i s an extensive l i t e r a t u r e i n t h i s area: The experimental evidence i s summarized by Berry (h2) who concludes t h a t , i r r e s p e c t i v e o f the a c t i v a t i o n mechanism, the t o t a l a v a i l able energy ( E ) , o r t h e molecular complexity o f the r e a c t a n t , a l l HX e l i m i n a t i o n products acquire ^ 15-h0% o f t h e p o t e n t i a l energy (E ) a v a i l a b l e t o t h e r e a c t a n t s (defined as t h e t h r e s h h o l d energy fo? HX e l i m i n a t i o n minus t h e r e a c t i o n e n d o e r g i c i t y ) as v i b r a t i o n a l energy. The remaining energy i s channeled i n t o HX r o t a t i o n , o l e f i n product r o t a t i o n and v i b r a t i o n and r e l a t i v e t r a n s l a t i o n a l energy o f t h e r e c o i l i n g products. We note t h a t a l l t h e data per t a i n s t o experiments i n which E^ >> Ε . Of g r e a t e r relevance as f a r as e f f e c t i v e n e s s o f HX v i b r a t i o n §n t h e r a t e o f t h e i n v e r s e a d d i t i o n would be data on HX e l i m i n a t i o n s f o r which E - Ε . Τ ρ Nevertheless, t h e a v a i l a b i l i t y o f many i n t e r n a l degrees o f f r e e dom o f t h e product o l e f i n makes i t improbable t h a t r a t e enhance ments f o r HX a d d i t i o n r e a c t i o n comparable t o those f o r HX-atom exchange r e a c t i o n s can be achieved. Experiments t o t e s t t h i s conjecture are i n progress a t t h e Lawrence Livermore Laboratory. IV. "Back-End" o f the Separation Cycle


5

T

m

In common w i t h most other deuterium separation schemes, t h e economic v i a b i l i t y o f an enrichment process based on l a s e r enhanced deuterium h a l i d e a d d i t i o n r e a c t i o n s n e c e s s i t a t e s p r o v i s i o n f o r r e c y c l i n g t h e working f l u i d . That i s , i t i s not f e a s i b l e t o use, e.g., h y d r o c h l o r i c a c i d , on a once-through b a s i s as feed f o r a deuterium separation p l a n t since even i f a l l t h e D i n n a t u r a l HC1 ( n a t u r a l abundance - 1 . 5 x 10" ) were removed w i t h 100% e f f i c i e n c y , t h e HC1 feed cost alone would be 4



146

$0.1

36

kgHCl

kg mole HC1 1 mole D 0

kg HC1

(1)


2 mole DC1

$2U00/kg

20 kg

2

X

^

'

to

D_0

2

kg mole D 0 2

assuming HC1 c o s t s 10φ p e r k i l o g r a m . Thus, t h e economic v i a b i l i t y o f t h i s process depends on the a b i l i t y t o redeuterate t h e HC1 which has been depleted o f D by t h e i s o t o p i c a l l y s e l e c t i v e a d d i t i o n r e a c t i o n . The "obvious" way t o accomplish t h e r e d e u t e r a t i o n i s by i s o t o p i c exchange o f HC1 w i t h n a t u r a l water (k3). For s p e c i f i c i t y , we d i s c u s s t h i s r e f l u x o p e r a t i o n i n t h e context o f the prototype system shown i n F i g u r e 1. The deuterated product of t h e a d d i t i o n r e a c t i o n (DACl) i s t h e r m a l l y d i s s o c i a t e d t o p r o duce i s o t o p i c a l l y pure DC1 (and e v e n t u a l l y D2O through exchange with n a t u r a l water), while the o l e f i n i s r e c i r c u l a t e d t o the l a s e r i r r a d i a t i o n area. The m a t e r i a l flows i n t h e r e f l u x tower assume e q u i l i b r i u m o p e r a t i o n a t 108°C (hh). At t h i s temperature the s e p a r a t i o n f a c t o r α f o r t h e exchange r e a c t i o n DC1 + H 0 $ HC1 + HDO

(2)

2

is

(U3)

(H)HÔ 2

(I)

1.9

Ή01

Besides t h e f a c t t h a t a l l t h e hydrogen h a l i d e - w a t e r exchange r e a c t i o n s are c h a r a c t e r i z e d by an unfavorable e q u i l i b r i u m as f a r as r e f l e x i s concerned, i . e . , t h e deuterium tends t o concentrate i n the water, t h e r e a r e two other p r a c t i c a l d i f f i c u l t i e s a s s o c i a t e d w i t h t h e use o f these systems: ( l ) they are h i g h l y c o r r o s i v e , n e c e s s i t a t i n g t h e use o f s p e c i a l m a t e r i a l s , e.g., Monel, and, (2) they form constant b o i l i n g ( a z e o t r o p i c ) mixtures. The s i g n i f i c a n c e o f t h e l a t t e r i s t h a t i t i s i m p o s s i b l e by s u c c e s s i v e d i s t i l l a t i o n s a t a given pressure (or a t a given temperature) t o o b t a i n both components as pure products from a hydrogen halide-water mixture. At some p o i n t i n t h e d i s t i l l a t i o n p r o c e s s , the a z e o t r o p i c c o n c e n t r a t i o n w i l l be reached (kk) ; when t h i s a z e o t r o p i c feed i s p a r t i a l l y v a p o r i z e d , t h e vapor has t h e same composition as t h e l i q u i d and no f u r t h e r s e p a r a t i o n o f com ponents i s p o s s i b l e . One convenient way t o break t h e azeotrope and separate t h e phases a f t e r t h e i s o t o p i c exchange process has been completed i s t o make use o f t h e s o - c a l l e d " s a l t e f f e c t " i n v a p o r - l i q u i d e q u i l i b r i u m . The a d d i t i o n o f a n o n - v o l a t i l e s a l t such as c a l c i u m c h l o r i d e , CaCl2, t o t h e HCl/H 0 mixture has t h e e f f e c t o f s i m u l t a n e o u s l y i n c r e a s i n g t h e vapor pressure o f t h e HC1 v i a t h e common i o n effect, and decreasing t h e vapor pressure o f the water, thus generating vapor o f higher HC1 c o n c e n t r a t i o n than t h a t c h a r a c t e r i s t i c o f t h e azeotrope. We have not been able t o 2


10.

MARLING ET AL.

Deuterium

Isotope

147

Separation

1mA

DC1 + A

1 m DAC1 DAC1

addition

dissociation


Carbon monoxide laser

13,500 m HC1 1 m DC1

1 m DC1 +

13,500

m HC1 13,501

HC1 reflux

| l m HC1

m HC1

D

2°

production

0.5m D 0 2

t

Feed: 3500 m n a t u r a l HÔ

Waste : 3500 m H 0 2

Feed: 0.5 m n a t u r a l HÔ

LEGEND : m = Mole A = Olefin DAC1 = DC1 + O l e f i n a d d i t i o n product Figure 1.

Process flow diagram for DO production by laser-augmented deuteriumchloride addition into olefins

American Chemfcaf

Society Library 1155 16th St. N. W. In Separation of Hydrogen Isotopes; Rae, H.; ACS Symposium Series; American Chemical Washington, DC, 1978. 1. D. C.Society: 20038


148

f i n d i n f o r m a t i o n i n the l i t e r a t u r e on t h e HCl/H O/CaCl^ system; however, from data on Isopropanol/HÔ/CaCl^ (^5.) and HCl/HÔ/H^SO^ mixtures (U6_)(HpS0^, w h i l e not a common i o n , be haves s i m i l a r l y t o a n o n - v o l a t i l e s a l t i n reducing the vapor pressure o f HÔ), we c o n j e c t u r e t h a t t h e c o n c e n t r a t i o n o f CaCl^ r e q u i r e d w i l l be approximately 2-k wt.%. A d e t a i l e d d i s c u s s i o n o f t h e design o f t h e HC1/H 0 exchange process i n c o r p o r a t i n g t h e equipment necessary t o generate r e deuterated, anhydrous HC1 f o r r e f l u x t o the l a s e r tower would c a r r y us too f a r a f i e l d (Vf) ; however, t h e b a s i c concept i s as f o l l o w s . The i s o t o p i c exchange takes p l a c e on a s e r i e s o f t r a y s w i t h the HC1 bubbling up and H 0 f l o w i n g down i n a countereurrent f a s h i o n . The l i q u i d stream from t h e exchange column, and a s a l t s o l u t i o n are f e d t o a concentrated s t r i p p e r which produces vapor of h i g h p u r i t y (> 99 mole % H C l ) ; t h i s i s r e c y c l e d t o t h e bottom o f t h e i s o t o p i c exchange column. The l i q u i d from t h e concentra ted s t r i p p e r , having a c o n c e n t r a t i o n lower than t h e azeotrope, passes t o an evaporator where t h e s a l t i s recovered f o r r e c y c l e to t h e concentrated s t r i p p e r , and vapor i s produced which i s subsequently s t r i p p e d t o produce two streams: a water stream which i s washed before d i s c h a r g e , and an a z e o t r o p i c HC1/H 0 mixture. The l a t t e r , mixed w i t h incoming f r e s h water, i s f e d t o the t o p o f t h e i s o t o p i c exchange column. Many v a r i a t i o n s o f t h e above are probably f e a s i b l e ; our main p o i n t here i s t o i n d i c a t e t h a t w h i l e t h e use o f hydrogen h a l i d e s i n t r o d u c e s c o m p l i c a t i o n s i n the design o f the "back-end" o f t h e s e p a r a t i o n scheme, these can be overcome. 2


2

2

V.

Conclusion

O p t i c a l p r o d u c t i o n o f heavy water i s beginning t o r e c e i v e s e r i o u s c o n s i d e r a t i o n . L a b o r a t o r y - s c a l e photochemical s e p a r a t i o n of deuterium v i a d i s s o c i a t i o n o f deuterated formaldehyde (HDCO)to y i e l d HD and CO has a l r e a d y been demonstrated i n t h e UV(3_) u s i n g a s i n g l e photon, and i n t h e IR at 10.6 microns u s i n g multi-photon a b s o r p t i o n (kO). To be economically v i a b l e , t h e former process awaits e f f i c i e n t low cost tunable u l t r a v i o l e t l a s e r s near 3^0 nm (39,^9) w h i l e the l a t t e r process r e q u i r e s s i g n i f i c a n t improve ment i n photon u t i l i z a t i o n (^ 1 0 photons are p r e s e n t l y r e q u i r e d per separated HD {kQ)). A l l photochemical deuterium enrichment processes w i l l probably r e q u i r e deuterium o p t i c a l i s o t o p i c s e l e c t i v i t y o f 1000-fold or b e t t e r f o r e f f i c i e n t photon u t i l i z a t i o n (3£,^9), about an order o f magnitude higher than has been demonstrated (3,^8). The deuterium s e p a r a t i o n process j u s t presented proposes t o u t i l i z e e x i s t i n g , e f f i c i e n t , h i g h average power CO l a s e r t e c h nology t o promote deuterium h a l i d e a d d i t i o n i n t o unsaturated hydrocarbons. Spectroscopic s t u d i e s o f CO l a s e r s and deuterium h a l i d e s have shown t h a t both DC1 and DBr can be s e q u e n t i a l l y e x c i t e d by t h e CO l a s e r near 5-6 microns t o a t l e a s t the ν = 5 9

6



10.

MARLING E T A L .

Deuterium

Isotope

149

Separation

l e v e l , as summarized i n Tables I I and I I I . A d d i t i o n i n t o unsaturated hydrocarbons should occur w i t h a c t i v a t i o n energies i n the range of 15-^0 kcal/mole u s i n g HC1 or HBr as the working gas, as i n d i c a t e d i n Table I . The e f f e c t i v e n e s s of hydrogen h a l i d e v i b r a t i o n a l energy toward l o w e r i n g the a d d i t i o n r e a c t i o n a c t i v a t i o n b a r r i e r is not known, but probably l i e s i n the range of 30-100%, suggesting t h a t perhaps 5-8 quanta of DX v i b r a t i o n a l e x c i t a t i o n w i l l be r e q u i r e d f o r u s e f u l l y f a s t r e a c t i o n t o occur. Two s i g n i f i c a n t problems t o be r e s o l v e d are photon l o s s by o l e f i n combination band absorption i n the 5 - 6 micron r e g i o n and p o t e n t i a l l y low r e a c t i o n quantum y i e l d s due t o non-reactive v i b r a t i o n a l quenching by the o l e f i n . These problems appear s o l v a b l e by s u i t a b l e o l e f i n s t r u c t u r a l design. The problems a t the "back-end" of t h e s e p a r a t i o n c y c l e , s p e c i f i c a l l y the breaking of the HX/H 0 azeotrope, a l s o appear s o l v a b l e . Research c u r r e n t l y underway w i l l examine the e f f e c t i v e n e s s of v i b r a t i o n a l c a t a l y s i s on the r e a c t i o n r a t e and r e a c t i o n quantum y i e l d as a f u n c t i o n of o l e f i n s t r u c t u r e . 2

Acknowledgement One of the authors (M.M.M.) would l i k e t o thank M. Benedict and J. E. V i v i a n f o r h e l p f u l d i s c u s s i o n .

Professors

Abstract The feasibility of a gas phase deuterium s e p a r a t i o n process is examined which would use IR l a s e r s t o augment a d d i t i o n re actions between HX (X = Br, Cl, F , OH) and unsaturated hydro carbons. High vibrational levels (V ≥ 4) of DF or HDO may be e x c i t e d by a p u l s e d DF laser. S i m i l a r h i g h vibrational excitation of DCl o r DBr may be achieved by a p u l s e d CO laser and s p e c t r o scopic d e t a i l s f o r excitation up t o V = 5 are examined. The thermal r e a c t i o n between HX and unsaturated hydrocarbons is c h a r a c t e r i z e d by activation exergies between 15 and 57 k c a l / m o l e , depending on olefin s t r u c t u r e and choice of HX. The e f f e c t i v e n e s s of HX/DX vibrational energy in l o w e r i n g the r e a c t i o n b a r r i e r is d i s c u s s e d . Primary emphasis is g i v e n t o an overall deuterium s e p a r a t i o n process utilizing HCl as a c l o s e d c y c l e working gas w i t h aqueous phase r e d e u t e r a t i o n . P r e f e r r e d olefin reagents are i n d i c a t e d compatible w i t h CO laser e x c i t a t i o n of DCl at a wave l e n g t h of 4.9-5.3 m i c r o n . Literature Cited 1. 2.

Letokhov, V . S . and Moore, C.B., Sov. J. Quant. E l e c t r o n (1976) 6, 129 and 259. Miller, A . I. and Rae, H.K., Chemistry in Canada (1975), 27, 25. We note t h a t ERDA's current (April, 1977) p r i c e f o r heavy water is $213/kg, a p r i c e i n c r e a s e due t o the


150

3.

4.


5. 6. 7. 8. 9. 10. 11. 12. 13. 14. 15. 16.

17. 18. 19. 20. 21. 22. 23.


diseconomy of small s c a l e p r o d u c t i o n . N e v e r t h e l e s s , a heavy water cost of approximately $200/kg f o r d e l i v e r y in 1980 is probably a conservative estimate. The possibility of deuterium separation via photopredissociation of formaldehyde has been demonstrated by J. B . M a r l i n g . See J. B . M a r l i n g , "Laser Isotope Separation of Deuterium," Chem. Phys. Lett., (1974) 34, 84, and "Isotope Separation of Oxygen-17, Oxygen-18, Carbon-13, and Deuterium by Ion Laser Induced Formaldehyde P h o t o p r e d i s s o c i a t i o n , " J. Chem. Phys. (1977) 66, 4200. Benson, S. W., and Bose, A . N., J. Chem. Phys. (1963), 39 3463. Gorton, P . J., and Walsh, R., J. Chem. Soc. Chem. Comm. (London) (1972), 782. Tschuikow-Roux, Ε., and Maltman, F . R., I n t . J. Chem. Kin. (1975), Vol. VII, 363. Benson, S. W., and O ' N e a l , Η. Ε., " K i n e t i c Data on Gas Phase Unimolecular R e a c t i o n s , " (1970), NSRDS-NBS 21. Kubota, Η . , Rev. Phys. Chem., Japan (1967) 37, 25, and (1967) 37, 32. Harding, C. J., M a c c o l l , Α., and Ross, R. Α., J. Chem. Soc. B, (1969), 634. Egger, K. W . , and Benson, S. W., J. Phys. Chem. (1967), 71, 1933. Boness, M. J. W . , and Center, R. E., J. A p p l . Phys. (1977), 48, 2705. See a l s o Sobolev, Ν. Ν., and Sokovikov, V . V., Sov. J. Quant. E l e c t r o n . (1973), 2, 305. Todd, T. R., C l a y t o n , C. Μ . , Telfair, W. Β., McCubbin, Τ. Κ . , Jr., and Pliva, J., J. M o l . Spect. (1976), 62, 201. Ross, A . H . M., Eng, R. S., and Kildal, Η . , Opt. Comm. (1974), 12, 433. Rank, D. Η . , Eastman, D. P., Rao, B . S., and Wiggins, Τ. Α., J. Opt. Soc. Am. (1962), 52, 1. Dunham, J. L., Phys. Rev. (1932), 41, 721. Townes, C. Η . , and Shawlow, A . L., "Microwave Spectroscopy," p. 644, McGraw-Hill Brook Company, Inc., New York, New York (1955). Keller, F . L., and N i e l s e n , A . H., J. Chem. Phys. (1954), 22, 294. Rank, D. Η . , F i n k , Uwe, and Wiggins, Τ. Α., J. M o l . Spect. (1965), 18, 170. Bernage, P., N i a y , P., Bockuet, Η . , and Houdart, R., Revue de Phys. A p p l . (1973), 8, 333. Mould, Η. Μ., Price, W. C., and W i l k i n s o n , G. R., S p e c t r o chimica A c t a (1960), 16, 479. James, T. C., and T h i b a u l t , R. J., J. Chem. Phys. (1964), 40, 534. Babrov, H. J., J. Chem. Phys. (1964), 40, 831. B e n e d i c t , W. S., Herman, R., and S i l v e r m a n , S., J. Chem. Phys. (1957), 26, 1671.


10.

MARLING ET AL.

Deuterium

Isotope

24. Smith, F. G., J. Quant. Spectrosc. R a d i a t . T r a n s f e r 13, 25.

151

Separation

(1973),

717.

26.

Rasmussen, R.S.,and Brattain, R. R., J. Chem. Phys. ( 1 9 4 7 ) , 120 and 131. Gallinella, Ε., F o r t u n a t o , Β., and Mirone, P., J. Mol. Spect.

27. 28.

DiLauro, C., and Neto, N., J. Mol. S t r u c t u r e ( 1 9 6 9 ) , 3, 219. Chen, M. Y.-D., and Chen, H. -L., J. Chem. Phys. ( 1 9 7 2 ) ,

29.

Weitz, Ε., and F l y n n , G., Ann. Rev. Phys. Chem. ( 1 9 7 4 ) , 25,

15,

(1967),

56,

24, 345.

3315.

275.

30. Hopkins, Β. M., and Chen, H. -L., J. Chem. Phys. ( 1 9 7 3 ) , 5 9 , 1495.


31.

Zittel,

P. F., and Moore, C. B.,J.Chem. Phys. ( 1 9 7 3 ) ,

58,

2004. 32. Birely, J. H., and Lyman, J. L., J. Photochem. ( 1 9 7 5 ) , 4, 2 6 9 . 33. Zittel, P. F., and Moore, C. B., J. Chem. Phys. ( 1 9 7 3 ) , 5 8 , 2922.

34. Moore, C. Β., private communication. 35. A l b r i g h t , J. C., and N i e l s o n , J. R., J. Chem. Phys. 36.

26,

(1957),

370.

N i e l s o n , J. R., C l a s s e n , H. H., and Smith, D. C., J. Chem. Phys. ( 1 9 5 0 ) , 1 8 , 485 and 8 1 2 ; ibid ( 1 9 5 2 ) , 2 0 , 1 9 1 6 . 37. C l a y t o n , J. W., Jr., Fluorine Chem. Rev. ( 1 9 6 7 ) , 1, 197. 38. B e r r y , M. J., paper presented a t t h e T h i r d Winter Colloquium on Laser Induced Chemistry, Park City, Utah, Feb. 14-16, 1977. 39. M a r l i n g , J. Β., Wood, L. L., and Daugherty, J. D., "The LaserR e l a t e d Costs of Some Approaches to Laser Isotope S e p a r a t i o n of Deuterium," Univ. Calif. Lawrence Livermore Laboratory I n t e r n a l Document, Feb., 1977. 40. Arnoldi, D., Kaufmann, Κ., and Wolfrum, J., Phys. Rev. L e t t . (1975),

34, 1 5 9 7 .

41. See, e.g., L e v i n e , R. D., and Manz, J., J. Chem. Phys. 63,

(1975),

4280.

42.

B e r r y , M. J., J. Chem. Phys. (1974), 61, 3114, and r e f e r e n c e s cited t h e r e i n . 43. B e n e d i c t , Μ., and Pigford, Τ. Η., "Nuclear Chemical Engineer i n g , " p. 454, Table II-9, McGraw-Hill, New York, New York (1957).

44. For HCl/H O the constant boiling mixture is 11.13 mole%HCl. It boils at 108.5°C under a pressure of one atmosphere. 45. Ohe, S., Japan Chem. Quart. ( 1 9 6 9 ) , 4, 2 0 . 46. Chu, J u Chin, e tal.,"Vapor L i q u i d E q u i l i b r i u m Data," 2nd Edition, p. 6 3 9 , Edwards, Ann Arbor ( 1 9 5 6 ) . 47. A d e t a i l e d f l o w sheet is a v a i l a b l e from one of the authors (M.M.M.). 48. Koren, G., Oppenheim,P.,Tal,D., Okon, Μ., and W e i l , R., A p p l . Phys. L e t t . ( 1 9 7 6 ) , 2 9 , 40. 49. Vanderleeden, J. C., "Laser S e p a r a t i o n of Deuterium," Laser Focus (June, 1 9 7 7 ) , 1 3 , 51. 2

RECEIVED September 12, 1977


11 Proposed H / D / T Separations Based on Laser-Augmented A + B = C Reactions S. H . BAUER


Department of Chemistry, Cornell University, Ithaca, NY 14853

The separationofH/D/T by laser photolysisofselected congeners in the infrared does not require high spectral resolution but does depend on the retentionofselective excitation and rapid reactionofthe species which initially absorb the photolyzing radiation. Except for the vibra tion augmentationofthree center abstraction reactions [A + B*C — AB + C], all isotope fractionations achieved to date involving polyatomics are based on fragmentationofthe irradiated species by co herent absorption of a large numberofphotons under essentially colli sion free conditions, to excited states considerably above the critical level for dissociation. A thermodynamic and kinetic analysis, which constitutes the first partofthis report indicates that the inverse process i.e. the bimolecular association should also be isotopically selective. In the second partofthis report several specific cases are proposed for testing this concept. The emphasis hereison chemical properties, not on spectroscopy, sinceitappears that the required irradiating frequen cies are either available now or shortly will be from sufficiently power ful lasers. All the systems considered are presumed to beinthe gas phase at pressures sufficiently low such that individually each species behaves ideally. Whileinsome respects the analysis appears elemen tary,itisbasic and merits formulation in a consistent manner. Thermodynamic and Kinetic Relations Consider the direct synthesisofan adduct: A + Β < " >.

C

(M)

v

(1)

C

Reference to Figure 1 provides definitionsofthe conventional symbols used. For a system whichisinstatistical equilibrium N^. ( E , T) represent a normalized Boltzmann distributionofpopulationsofthe reactants; the hashed levels represent the weighted mean energies of B

©

0-8412-0420-9/78/47-068-152$05.00/0


11.

BAUER

Proposed

H/D/T

153

Separations

the reactants, transition state, and products. The zero of energy for each species, per convention, is measured from the corresponding zeropoint level. Thus, 00

Δ

Τ =

Ε

Δ

Ε

ο

JΟ

+

00

E

< '

A; Β A; Β

Ε

N

"J

T) d E

ΔΗ° = (ΔΕ° - RT) < 0

;

E

N

C

ο

( E

C

'

T ) d E

( 2 )

Δβ° < 0


These are thermodynamically favored reactions.

K

p,eq ' Ρ

=

4

?V

μ

}

Α Ί

=

E

eq

"

L

E

X

P { - | A S ° | /

R

|ΔΕ°|/ΚΤ}

+

1

(«») -1 \ = (RT) — ·

X

2 —

-1 3 -1 The units of κ-^ and ^ * molecule cm s . Clearly, on raising the temperature, i.e. for T^ > T,both distribution functions get steeper and eq( l> eq< >For a system which is in statistical equilibrium at the temperature T, except for a steady burning of a hole at level Eg and maintenance of an overpopulation at E ^ , most of the reactant populations are unchanged, Figure 2. The magnitude of the second term in the right member of (2) is obviously increased and K^(T) is greater than Kp q(T). The reverse applies when the hole in the population is burned at fe and the over population established at Ε . These qualitative relations apply inde pendentlyofthe magnitude of E , which can be positive or zero. For example, the steady state irradiation of a mixture of B(CH3>3 and N H with a C O 2 laser should increase the magnitude of the "perturbed equili brium constant" for the reaction (1), a

K

T

κ

s

r a




Figure 2. Change in population distribution at steady state because of pumping by β -> « = (E — Ε )/1ι or + = (Ε — E )/h, assuming the statistical temperature remains at Τ a

β

Vp

σ

σ

p


11.

BAUER

Proposed H/D/T

155

Separations

The kinetic formulation provides addition details for the above analysis. To eliminate the dependence on [M] we must stipulate that X2ÎM] > κ ^ . Then the unidirectional rate of production of from the reagent is determined by the product of the individual state densities and the cross sections for association: j) P

(i) (J) A B

at stat. equil. *

P

Γ A B &

/

&

V

E

° A , BL lQ Q: e x p v - " k F ~ ) J o 7- o A

(4)

B M

B

P

A B P

i,3 -1

with


-1

r ψ « ι/L ( 1 +

(5)

skT /

where s is the equivalent number of classical oscillators assigned to the transition state (C*) and,

ν* = Π u . ( Π +

C

1=1

ι \j i =

ν* ) 3 /

Similar expressions apply to the second step in the conversion. Hence for a system at steady state, with a hole burned at E and overpopulated Q

se Β ρ ' is the molecular density at Eg at statistical equilibrium. Because the perturbed system is generated by direct pumping, the increments in densities at a and 3 are equal but of opposite sign; however, < κ£°^ and (R^-R^) > 0. The net rate (fà^-ift.j) is larger than ( ^ - i ^ ) since the reverse rate is minimally affected by pumping β — a; recall Ή^[Μ.] > κ ^ . Note that the sign of the increment in the unidirectional flux is determined by the perturbed population, again, irrespective of the magnitude of E . However, the size of the increment depends on (κ^-κ^) which is small when Eg > E (2). Clearly, to maintain the spiked distribution at the steady state, with out allowing the system temperature to rise indefinitely, one must continually remove as much energy as is pumped into it by radiation. Imagine that the reactants and the reactor s walls are initially at a uni form temperature, T < T . Upon turning on the radiation there will be a transient condition wherein, on a time scale of T(vib) (transi), the reactant temperature rises, but on a longer time scale ( T ) it levels off, due to conduction and convection, to the operating temperature Τ which a

R

T

Q

r


156


is determined by the rate of heat transfer to the walls, maintained at Τ . Thus the gas temperature tends toward, but does not attain a uniform distribution (3). The steady state populationinlevel a is established by the balance between the pumping rate and the combined loss by relaxation and reaction. The efficiency of any LIS will be determined by the selectivity and intensity of pumping (β — a) and limited by v(i) v(3) transfer efficiencies among the congeners, as well as by ν —• R, Τ pro cesses which raise the operating temperature.


Classification of Adduct Production To select suitable systems for isotope separations based on the above relations a broad range of criteria must be considered, but the principal step is to find conditions under which laser radiation does augment the rate of adduct production. In many cases the apparently simple reaction indicated in Eq. (1) proves to follow a much more complex mechanism. Toward this end it is useful to compile proposed reactions according to the magnitudes of their activation energies (E ). Values of (30-40) kcal/mole are typical for the additionofmineral acids, water and hydro gen sulfide to olefins (4); these are the inverse of (α, β) eliminations. The additionofmineral acids or water to aldehydes and ketones require (12-18) kcal/mole. Also the additionofHBr and HI to conjugated dienes are in this group. Lewis acid-base adducts and hydrogen bonded com plexes are generated with little or no activation energy (0-5 kcal/mole). A small sub-s et of the first group which is the largest and the most extensively investigated, is the addition of HF to H C = C F and other variously substituted fluoroethylenes, illustrated in Figure 3. Many lower activation energy pairs in this category are cited by Marling (5). Numerical values for the pertinent energy parameters for the HF/ethylene additions are listed in Table I. The estimated lowest activation energy is that for H F / C F ^ . Indeed, one may anticipate that excitation of H F would accelerate its addition across the double bondinview of the ob served vibrationally excited HF generated from mixtures of C H 3 and CF^ radicals. However, it appears (6) that of the 72 kcal/mole which the a

2

2

2

level], about 30 kcal/mole are statistically distributed prior to H F * elimination. Hence the application of detailed balance suggest that C H F ^ excitation may be just as essential as that of the H F to promote significant enhancement of the rate. Indeed we reached that conclusion after obtaining negative results by simply exposing mixtures of HF and various olefins to a pulsed HF laser with the cell at room temperature (Table H). Thus, the conditions outlinedinthe preceeding section are necessary but not sufficient, particularly for combinations which consist of a sizeable polyatomic acceptor and a strongly bonded intruder (HF; 2

n

n


11.

BAUER

Proposed

H/D/T

TABLE I. HF/Ethylene

x

-1

kcal mol

57

11

31

46

-93

62

24

33

38

CH FCH F

89.6

62

10

28

52

CH CF

99

69

27

30

42

92

71

-28

21

43

91

69

-26

22

43

94

72

-40

22

31

2

2

3


2

e

kcal mol

kcal mol

88

2

3

Ε

kcal mol * kcal mol

CH CH F CH CHF

Values of Parameters for Figure 3

D(C-C)

Adduct 3

157

Separations

3

CH FCF 2

3

CHF CHF 2

CHF CF 2

2

3

T A B L E II. Attempts to Augment HF Addition HF laser operation: pulsed, 2/s; & 150 mj/p ο

p(HF)/p(substrate):

3/1 to 1/3; 10 pulses

P(HF) = 0.5 torr; cell temp f\

/TTT.v

c

n

±

^

Γ

25°

^

C

(low

^

T)

Mass spectral analysis for products Compounds tested, no reaction with H F * C F 2

+ MeNH ο

Δ

GaEt + MeNH 3 2 0

κ

k 2 2

Δ

Et Ga NH Me 3 2 :

ΔΗ°

=-18.2 kcal/mole

oUU

= -19.8

Whereas both k^ and ko are presumed to be large (?^10 mole c m s and temperature insensitive, the reverse steps are much slower since and k have Arrhenius exponential factors with (-18.2/RT) and (-19.9/RT), respectively. Thus, were an equilibrium mixture of methylamine, the two Lewis acids, and the corresponding adducts in equilibrium, exposed to radiation which is selectively absorbed by EtgGa:NHDMe, the deuterium would be selectively trapped as MegBHDMe, possibly for a sufficient time to achieve separation. Other means for trapping laser shifted concentrations of dissociation products are conceivable. For example, the irradiation of matrix isolated samples followed by adjusted diffusion rates of the products (careful 12

2


1

3

162


temperature control) but these applications appear of little practical utility. Abstract Thermodynamic and kinetic analyses of adduct production:


indicate how the steady state composition and rates (forward vs reverse) can be perturbed by absorbable radiation which generates a "spiked" steady state population distribution for one of the species. Three classes of reactions were considered, wherein E ranges from (30-40) kcal/mole; (12-16) kcal/mole; and (0-5) kcal/mole. These are of particular interest for H/D/T separation, in that one of the adducts can be water, a mineral acid (which rapidly equilibrates with water) or hydrogen. Our analysis and preliminary experiments indicate that the intermediate case ( E ~ 15 kcal/mole) is particularly attractive for LIS. a

a

Literature Cited 1.

2. 3. 4.

5. 6. 7. 8.

9.

Drago, R. S., Vogel, G. C. and Needham, T . E., J. Am. Chem. Soc (1971) 93, 6014, have compiled extensive tables of enthalpies for adduct formation. Levine, R. D. and Manz, J., J. Chem. Phys. (1975) 63, 4280. Shaub, W. and Bauer, S. Η . , Int. J. Chem. Kin. (1975) 7, 509. Tschuikow-Roux, E . and Maltman, Κ. A., Int. J. Chem. Kin. (1975) 7, 363, have compiled extensive tables for the inverse processes. Marling, J. Β., Joint Conference CIC and ACS, Montreal, 1977, Physical Chemistry Division, Abst. 081. Clough, P. N., Polanyi, J. C. and Tagudu, R. Τ., Can. J. Chem. (1970) 48, 2919. Data from Petrochemical Handbood, 1976. Bell, R. P. and McDougall, Α. Ο . , Trans. Farad. Soc. (1960) 56, 1285 give useful data on hydration equilibria of aldehydes and ketones. My sincere thanks to Dr. Kuei-Ru Chien for performing measure ments on the perfluoro compounds. Lory, E . R . , Manuccia, T . and Bauer, S. H., J. Phys. Chem. (1975) 79, 545.

RECEIVED

August 31, 1977


12 Operating Experience with the Tritium and Hydrogen Extraction Plant at the Laue—Langevin Institute PH.

PAUTROT

Institute Max Von Laue-Paul Langevin, 38000, Grenoble, France M. DAMIANI


Sulzer Brothers L t d . , Dept. 6162, CH-8401 Winterthur, Switzerland

The I n s t i t u t e Max Von LAUE - P a u l LANGEVIN in GRENOBLE is a s o c i e t y founded in 1967 by a government agreement between FRANCE and GERMANY. In 1974 GREAT BRITAIN a l s o j o i n e d the I n s t i t u t e . The I n s t i t u t e o p e r a t e s the High F l u x R e a c t o r whose purpose is a l a r g e programme of n u c l e a r p h y s i c s r e s e a r c h u s i n g the n e u t r o n s produced by the r e a c t o r . T h i s High F l u x R e a c t o r has a t h e r m a l performance of 57 MW w i t h a s i n g l e heavy water c o o l e d and moderated circuit. Heavy water h o l d - u p is about 40 m . The mean f l u x of t h e r m a l n e u t r o n s in the heavy water is e q u a l t o 1,8 . 10 n / c m s e c , which l e a d s t o f o r m a t i o n of tritium w i t h a s a t u r a t i o n activity of more than 80 C u r i e per litre ( a f t e r 10 y e a r s about 30 Curie/1). T h i s is s i m i l a r t o the v a l u e e x p e c t e d in the moderator circuit of the CANDU type r e a c t o r . For s a f e t y r e a s o n s , tritium c o n c e n t r a t i o n in heavy water s h o u l d not exceed 3 C u r i e per litre and in o r d e r t o m a i n t a i n moderator efficiency of heavy w a t e r , its fractional c o n c e n t r a t i o n s h o u l d not be lower than 99,6 mol %. The installation fulfilling the mentioned d u t i e s was built by CCM/SULZER* in c l o s e c o - o p e r a t i o n w i t h the C.E.A. (Commissariat ā l'Energie A t o m i q u e ) . SULZER a l r e a d y had e x p e r i e n c e in the low temperature distillation of hydrogen from a s m a l l heavy water p l a n t in EMS S w i t z e r l a n d . The installation a t the LAUE-LANGEVIN I n s t i t u t e on the Grenoble N u c l e a r C e n t r e site is the first known p l a n t in o p e r a t i o n f o r the s i m u l t a n e o u s e x t r a c t i o n of hydrogen and tritium from heavy w a t e r . 3

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owned by the CEA for t h i s

0-8412-0420-9/78/47-068-163$05.00/0


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Design Data. - T r i t i u m c o n c e n t r a t i o n of heavy water has t o be s t a b i l i z e d a t 1,7 C i / 1 by an annual e x t r a c t i o n of 160 000 C u r i e , e q u i v a l e n t t o about 60 N l of pure t r i t i u m . - C o n t e n t of heavy water has t o be m a i n t a i n e d a t 99,6 mol % by an a n n u a l e x t r a c t i o n of 160 l i t r e s of l i g h t water, e q u i v a l e n t t o about 2 00 Nm of hydrogen. 3


P r i n c i p l e of O p e r a t i o n . F i g u r e 1 shows the mass f l o w s w i t h the c o r r e s p o n d i n g c o n t e n t s a t normal opera t i o n c o n d i t i o n s in 1976, and f u r t h e r m o r e , t h a t the p l a n t is made up of two main p a r t s : 1.

C a t a l y t i c exchange between heavy water vapour and d e u t e r i u m gas a t 200°C and about 1,2 b a r , which a l l o w s the mass t r a n s f e r of hydrogen and t r i t i u m from heavy water t o d e u t e r i u m t o r e a c h e q u i l i b r i u m a c c o r d i n g t o the f o l l o w i n g r e a c t i o n s :

HDO

+ D2

D2O

+

HD

2.

Low temperature r e c t i f i c a t i o n of the hydrogen i s o t o p e s HD/D2/DT/T2 a t about 1,5 bar in two columns w i t h e x t r a c t i o n of HD a t the top of the f i r s t column and pure t r i t i u m a t the bottom of the second column. The f l o w s h e e t F i g . 2 shows t h a t the heavy water coming from the r e a c t o r is e v a p o r a t e d , s u p e r h e a t e d , and mixed w i t h the d e u t e r i u m t o pass through the c a t a l y t i c r e a c t o r where the i s o t o p i c exchange r e a c t i o n s o c c u r ; it is then recondensed t o be s e p a r a t e d from the d e u t e r i u m and t r a n s f e r r e d t o the second and then t o the t h i r d s t a g e of the c a t a l y t i c exchange. The d e u t e r i u m coming out of the c a t a l y t i c exchange is d r i e d and p u r i f i e d b e f o r e b e i n g s e n t i n t o a d i s t i l l a t i o n column c o n t a i n i n g S u l z e r p a c k i n g s . The p a r t i a l l y t r i t i u m and hydrogen s t r i p p e d d e u t e r i u m coming out of the column r e t u r n s t o the c a t a l y t i c exchange t h r o u g h an e x p a n s i o n v e s s e l . The d e u t e r i u m h y d r i d e drawn o f f w i t h a c o n t e n t of about 80 mol % a t the head is s e n t to a burner. The t r i t i a t e d d e u t e r i u m a t the f o o t of the column is s e n t t o a second column w i t h d i x o n r i n g packing. In the m i d d l e of t h i s column, DT is w i t h drawn in a tank t o g e t t r i t i u m a c c o r d i n g t o e q u i l i b r i u m r e a c t i o n 2 DT

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