TECHNICAL REPORTS SERIES No. 87

Design and Operation

of Evaporators

for

Radioactive Wastes

v > " J S INTERNATIONAL ATOMIC ENERGY AGENCY, VIENNA, 1968

DESIGN AND OPERATION OF EVAPORATORS FOR RADIOACTIVE WASTES

T h e f o l l owing States are Members o f the International A t o m i c Energy A g e n c y :

AFGHANISTAN GERMANY, FEDERAL NORWAY ALBANIA REPUBLIC OF PAKISTAN ALGERIA GHANA PANAMA ARGENTINA GREECE PARAGUAY AUSTRALIA GUATEMALA PERU AUSTRIA HAITI PHILIPPINES BELGIUM HOLY SEE- POLAND BOLIVIA HUNGARY PORTUGAL BRAZIL ICELAND ROMANIA BULGARIA INDIA SAUDI ARABIA BURMA INDONESIA SENEGAL BYELORUSSIAN SOVIET IRAN SIERRA LEONE

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REPUBLIC OF KUWAIT TURKEY COSTA RICA LEBANON UGANDA CUBA LIBERIA UKRAINIAN SOVIET SOCIALIST CYPRUS LIBYA REPUBLIC CZECHOSLOVAK SOCIALIST LUXEMBOURG UNION OF SOVIET SOCIALIST

REPUBLIC M A D A G A S C A R REPUBLICS DENMARK MALI UNITED ARAB REPUBLIC DOMINICAN REPUBLIC MEXICO UNITED KINGDOM OF GREAT ECUADOR M O N A C O BRITAIN AND NORTHERN EL SALVADOR MOROCCO IRELAND ETHIOPIA NETHERLANDS UNITED STATES OF AMERICA FINLAND NEW ZEALAND URUGUAY FRANCE NICARAGUA VENEZUELA GABON NIGERIA VIET-NAM

YUGOSLAVIA

The A g e n c y ' s Statute was approved on 23 October 1956 by the C o n f e r e n c e on the Statute o f the IAEA held at United Nations Headquarters, New York ; it entered into f o r ce on 29 July 1957. The Headquarters o f the A g e n c y are situated in V i e n n a . Its principal o b j e c t i v e is " t o a c ce l e ra te and enlarge the contribution o f a t o m i c energy to p e a c e , health and prosperity throughout the w o r l d " .

© IAEA, 1968

Permission to reproduce or translate the information contained in this publ icat ion m a y be obtained by writing to the International A t o m i c Energy A g e n c y , Karntner Ring 11, A - 1 0 1 0 Vienna I , Austria.

Printed by the IAEA in Austria

May 1968

TECHNICAL REPORTS SERIES No. 87

DESIGN AND OPERATION OF EVAPORATORS

FOR RADIOACTIVE WASTES C o m p i l e d b y

Y . Y a m o m o t o ( U n i v e r s i t y o f T o k y o )

a s s i s t e d b y

N. M i t s u i s h i ( U n i v e r s i t y o f Kyushu)

and S. K a d o y a

( E b a r a M a n u f a c t u r i n g C o . L td , T o k y o )

I N T E R N A T I O N A L A T O M I C E N E R G Y A G E N C Y V I E N N A , 1968

DESIGN AND OPERATION OF EVAPORATORS FOR RADIOACTIVE WASTES (Technical Reports Series, N o . 8 7 )

ABSTRACT. A manual deal ing with the appl icat ion o f evaporators to the treatment o f r a d i o a c t i v e wastes. This book is the second of three commiss ioned by the IAEA on the three principal techniques for concentrating radioactive wastes, namely c h e m i c a l precipitation, evaporation and i on -exchange , and was prepared by Mr. F . N . Biowder,

Contents: Introduction; Design and description o f evaporator types and associated equipment ; Oper -ational procedures; Disposal of evaporator condensates and concentrates; Costs; Bibliography; Appendices I, II and III.

(115 p p . , 16 X 24 c m , paper-bound, 61 figures) (1968) Price: U S $ 2 . 5 0 ; £ 1 . 0 . 1 0

DESIGN A N D O P E R A T I O N O F E V A P O R A T O R S F O R R A D I O A C T I V E W A S T E S

I A E A , V I E N N A , 1968 S T I / D O C / l O / 8 7

FOREWORD

It is often di f f i cult to determine what is the m o s t e f f ec t ive and e c o n o m i -c a l m e t h o d of r e m o v i n g r a d i o a c t i v e c o m p o n e n t s f r o m l iqu id w a s t e s . T o g i v e d e v e l o p i n g M e m b e r States s o m e g u i d a n c e , the In ternat i ona l A t o m i c E n e r g y A g e n c y has c o m m i s s i o n e d b o o k s on the three p r i n c i p a l t e chn iques u s e d in concentra t ing r a d i o a c t i v e l iquid w a s t e s , n a m e l y c h e m i c a l p r e c i p i -tation, evaporat ion and ion exchange . The p r e s e n t manual dea ls with e v a -p o r a t i o n , wh i ch i s o f t en m o r e su i tab le than the o t h e r two t e c h n i q u e s f o r treating i n t e r m e d i a t e - and h i g h e r - a c t i v i t y w a s t e s . " O p e r a t i o n and Contro l of Ion-Exchange P r o c e s s e s f o r Treatment of Radioact ive W a s t e s " , was pub-l ished in 1967 as No. 78 in the A g e n c y ' s Technica l Repor ts Ser ies . The third book , " C h e m i c a l T r e a t m e n t o f R a d i o a c t i v e W a s t e s " , w i l l be pub l i shed in the T e c h n i c a l R e p o r t s S e r i e s in the n e a r fu ture .

The o rgan i za t i on and p r e p a r a t i o n of the p r e s e n t b o o k w a s d i r e c t e d by M r . Frank N. B r o w d e r of the IAEA as P r o j e c t O f f i c e r . It p resents the a d -vantages and l imi ta t i ons of evapora t i on as a m e a n s of t reat ing r a d i o a c t i v e w a s t e s , d e s c r i b e s the c h a r a c t e r i s t i c s of v a r i o u s types of e v a p o r a t o r s , and ind i ca tes how to c o p e with p r o b l e m s e n c o u n t e r e d in the i r o p e r a t i o n . The in format ion is b a s e d on actual operat ing e x p e r i e n c e at a number of nuc lear insta l lat ions .

CONTENTS

1. INTRODUCTION 1

1 . 1 . Decontaminat ion f a c t o r . 1 1 . 2 . V o l u m e reduct i on 2 1 . 3 . T y p e s of w a s t e s suitable f o r evaporat ion 2 1 . 4 . C o r r o s i o n 3 1 . 5 . F o a m i n g 3 1 . 6 . Scal ing and salting 3 1 . 7 . G e n e r a l l imi tat ions 4

2. DESIGN AND DESCRIPTION OF E V A P O R A T O R T Y P E S AND ASSOCIATED E Q U I P M E N T 4

2 . 1 . T y p e s of e v a p o r a t o r s 4 2 . 1 . 1 . Co i l o r pot type 2 . 1 . 2 . N a t u r a l - c i r c u l a t i o n type 2 . 1 . 3 . F o r c e d - c i r c u l a t i o n type 2 . 1 . 4 . V a p o u r - c o m p r e s s i o n type 2 . 1 . 5 . M u l t i p l e - e f f e c t type 2 . 1 . 6 . W i p e d - f i l m type 2 . 1 . 7 . Other types

2 . 2 . Heat t r a n s f e r in e v a p o r a t o r s 23 2 . 2 . 1 . The a r e a of h e a t - t r a n s f e r s u r f a c e 2 . 2 . 2 . T e m p e r a t u r e d i f f e r e n c e A T 2 . 2 . 3 . H e a t - t r a n s f e r c o e f f i c i e n t s

2 . 3 . T y p e s of aux i l iary equipment 25 2 . 3 . 1 . Mist s e p a r a t o r s 2 . 3 . 2 . C o n d e n s e r s 2 . 3 . 3 . P r e h e a t e r s

2 . 4 . Instrumentat ion and c o n t r o l of evaporat ion unit 26 2. 5. P r e - t r e a t m e n t and combinat ion with other waste

t reatment 28 2 . 5 . 1 . Adjustment of the pH value 2 . 5 . 2 . F i l t ra t ion 2 . 5 . 3 . T r e a t m e n t by act ivated c h a r c o a l 2 . 5 . 4 . Combinat ion with o ther p r o c e s s e s

3. O P E R A T I O N A L P R O C E D U R E S 29

3 . 1 . P r o c e d u r e s f o r var i ous evapora to r types 29 3 . 1 . 1 . E v a p o r a t o r s with s u b m e r g e d heating s u r f a c e s 3 . 1 . 2 . F i l m e v a p o r a t o r s

3 . 2 . Operat ional p r o b l e m s 31 3 . 2 . 1 . Entrainment 3 . 2 . 2 . F o a m i n g 3 . 2 . 3 . Scal ing 3 . 2 . 4 . C o r r o s i o n 3 . 2 . 5 . P r e s e n c e of o rgan i c o r potentially explos ive

m a t e r i a l s 3 . 2 . 6 . O v e r a l l sa fety

3 . 3 . Maintenance 65 3 . 4 . Manpower r e q u i r e m e n t s f o r var i ous evaporator types . . . . 67 3 . 5 . Re lat ive m e r i t s and l imitat ions of var ious

e v a p o r a t o r types 71

4. DISPOSAL OF E V A P O R A T O R CONDENSATES

AND C O N C E N T R A T E S 71

5. COSTS 72

5 . 1 . Capita l c o s t s 72 5 . 1 . 1 . Cost of s o m e exist ing rad ioac t ive waste e v a p o r a t o r s

5 . 1 . 2 . Calculat ion of e x - f a c t o r y p r i c e of var i ous types of rad ioac t ive waste evapora to r plant

5 . 2 . Operat ional c o s t s 76 5 . 2 . 1 . B a s i s of ca lculat ion 5 . 2 . 2 . Result of ca lculat ion 5 . 2 . 3 . Work ing t ime of the plant

B I B L I O G R A P H Y 78

A P P E N D I X I 81

A P P E N D I X II 95

A P P E N D I X III 109

1. INTRODUCTION

With the d e v e l o p m e n t of n u c l e a r industry throughout the w o r l d , the amount of r a d i o a c t i v e waste l iqu ids is i n c r e a s i n g and the p r o b l e m of c o n -centrat ing and r e m o v i n g r a d i o a c t i v e m a t e r i a l s f r o m these waste l iquids has b e c o m e m o r e and m o r e impor tant . Although rad i oac t i ve waste l iquids v a r y g r e a t l y in act iv i ty l e v e l , c o m p o s i t i o n and p h y s i c a l p r o p e r t i e s , e f f o r t s to d e v e l o p sa fe and e c o n o m i c a l m e t h o d s of t reat ing them have been s u c c e s s f u l . The m o s t c o m m o n m e t h o d s of t reatment a r e c h e m i c a l p re c ip i ta t i on , ion e x c h a n g e and e v a p o r a t i o n . B e c a u s e of its capabil i ty of a high d e g r e e of s e p a r a t i o n f o r m o s t r a d i o a c t i v e m a t e r i a l s and its inherent ly high c o s t , e v a p o r a t i o n i s g e n e r a l l y conf ined to the treatment of i n t e r m e d i a t e - o r h i g h - a c t i v i t y w a s t e s .

1 . 1 . D E C O N T A M I N A T I O N F A C T O R

An o v e r a l l decontaminat i on f a c t o r of m o r e than 104 (in many c a s e s m o r e than 10 5 ) be tween condensate (d ist i l late ) and thick l iquor ( concentrate ) is g e n e r a l l y e x p e c t e d f r o m s i n g l e - e f f e c t e v a p o r a t o r s s o far as non-vo la t i l e r a d i o a c t i v e contaminants are c o n c e r n e d , and this f i gure can be i m p r o v e d by the use of d e - e n t r a i n m e n t d e v i c e s . H o w e v e r , this f i gure may be r e d u c e d by the p r e s e n c e of vo la t i l e r a d i o a c t i v e m a t e r i a l s such as iod ine , ruthenium, t r i t ium and r a d i o a c t i v e vo la t i l e o r g a n i c m a t e r i a l s . When a solution containing iod ine i s evapora ted under ac id i c condi t ions , a substantial f r a c t i o n of the i od ine m a y v o l a t i l i z e l . R a d i o - i o d i n e is used frequent ly in t r a c e r s tudies and a p p e a r s in the c ondensate in the f o r m of i od ide . Ruthenium in the f o r m of ruthenium t e t r o x i d e i s vo la t i l e . This, compound can be f o r m e d under s t r o n g o x i d i z i n g condi t ions as in bo i l ing nitr ic ac id and by the p r e s e n c e of o z o n e o r permanganate ions . T h e r e i s a l so s o m e e v i d e n c e that ruthenium d is t i l s m o r e r e a d i l y f r o m dilute ac id containing f e r r i c sulphate, su lphur i c ac id and s o d i u m nitrate . T r i t i u m , in.the f o r m of t r i t ia ted w a t e r , cannot be r e m o v e d f r o m waste e c o n o m i c a l l y by e v a p o r a t i o n , but f or tunate ly the c oncent ra t i on of t r i t ium in waste is usual ly l ow and can be a c c e p t e d by the e n v i r o n m e n t , s ince there are no r e c o n -centrat i on e f f e c t s . In c a s e s w h e r e vo la t i l e rad ionuc l ides r e d u c e the o v e r a l l decontaminat i on , thorough p u r i f i c a t i o n of the condensate except f r o m trit iated w a t e r and n o n - e l e c t r o l y t i c o r g a n i c s can be ach ieved by pass ing it through an i o n - e x c h a n g e c o l u m n .

A s pointed out by Cle l land [10], the decontaminat ion f a c t o r r e q u i r e d can v a r y e n o r m o u s l y a c c o r d i n g to s p e c i f i c s i tuat ions , be ing dependent upon the amount of act iv i ty a r i s i n g and the a l lowable rate of d i s c h a r g e of act iv i ty to the e n v i r o n m e n t . In s o m e c a s e s high decontaminat ion f a c t o r s must b e guaranteed , f o r e x a m p l e when the dist i l late is d i s c h a r g e d into a

1 Volat i l izat ion o f iodine may b e prevented by the addition of a complex ing agent. It is known that the presence o f microgram quantities of mercury reduces iodine liberation from waste solutions by the formation of a HglJ" c o m p l e x [ 3 ] ,

1

l

d r i n k i n g - w a t e r s o u r c e containing a b i o l o g i c a l r e c o n c e n t r a t i o n m e c h a n i s m as part of a f o o d chain. A dilution f a c t o r of about 104 can usual ly be e x p e c t e d in the env i ronment , but this f i gure may be part ial ly o f f set by the p r e s e n c e of a r e c o n c e n t r a t i o n m e c h a n i s m , which could i n c r e a s e the decontaminat ion r e q u i r e m e n t s by 10 o r 102, and the need f o r a safety fac tor and a l lowance f o r other l o c a l d i s c h a r g e s cou ld i n c r e a s e the o v e r a l l f i gure by another f a c t o r of 10. Of c o u r s e , in m o r e favourab le c i r c u m s t a n c e s l o w e r decontaminat ion p e r f o r m a n c e cou ld b e ac ceptab le .

1 . 2 . V O L U M E REDUCTION

V o l u m e reduc t i on i s another important f a c t o r in the treatment of r a d i o a c t i v e l iquid w a s t e s . High v o l u m e reduct i on i s d e s i r a b l e f r o m the point of v iew of e c o n o m i c s in o r d e r to m i n i m i z e the s i z e and cos t of s torage f a c i l i t i e s f o r the c o n c e n t r a t e . The m a x i m u m v o l u m e reduct ion that can be ach ieved depends on the amount and the p r o p e r t i e s of d i s s o l v e d so l ids in the was te . If the concentra t i on of d i s s o l v e d s o l i d s in the thick l iquor exceeds the l imi t at which these c r y s t a l l i z e in the c o l d e r part of the b l ow-down pipe, p lugging o f this d i s c h a r g e pipe o c c u r s , thus determining the m a x i m u m concent ra t i on f a c t o r . Once such plugging o c c u r s , l a b o r i o u s work is n e c e s s a r y to r e m o v e the b l o c k a g e . T o avoid such trouble the concentrat ion of thick l i quor should be w e l l c o n t r o l l e d and the b l o w - d o w n pipe should be des igned to be as shor t and straight as p o s s i b l e . It i s n o r m a l p r a c t i c e , having a s c e r t a i n e d the concentra t i on at which the solut ion c r y s t a l l i z e s at ambient t e m p e r a t u r e , to set s o m e sa fe ty f a c t o r on this depending upon the operat ing p a r a m e t e r s of the evaporat ion s y s t e m . F o r example , if the e v a p o r a t o r has a v e r y s m a l l l iquor h o l d - u p and it is re la t ive ly easy by m a l - o p e r a t i o n to o v e r - c o n c e n t r a t e , a l a r g e safety f a c to r (say 2) might be appl ied [10], If a c c identa l o v e r - c o n c e n t r a t i o n i s c o n s i d e r e d to be unlikely, a c l o s e r approach to the c r y s t a l l i z a t i o n point can be planned.

V o l u m e reduct i on is not usual ly r e s t r i c t e d by the p r e s e n c e of smal l amounts of suspended s o l i d s and when the waste i s known to contain these , a pert inent des ign i s p o s s i b l e . The use of a f i l t e r b e f o r e evaporat ion is r e c o m m e n d e d . Higher v o l u m e reduct i on r e s u l t s in higher activity l eve l s in the thick l i q u o r and n e c e s s i t a t e s m o r e substantial b i o l o g i c a l shielding. It a l s o r e s u l t s in h igher l e v e l s of act iv i ty in the dist i l late and when an e c o n o m i c a l l y d e s i r a b l e v o l u m e reduc t i on r e s u l t s in an unacceptable act ivity l e v e l in the d is t i l late , use of a m u l t i p l e - e f f e c t s y s t e m may be one so lut ion. As the use of a m u l t i p l e - e f f e c t s y s t e m resu l t s in higher c ons t ruc t i on and operat ing c o s t s , a s c rupu lous e c o n o m i c study may be n e c e s s a r y to f ind the op t imum v o l u m e reduct i on .

1 . 3 . T Y P E S O F WASTES SUITABLE F O R E V A P O R A T I O N

As a l ready pointed out, evaporat ion is m o s t suitable f o r p r o c e s s i n g l iquids which (1) have a high total s o l i d s concentrat ion and, (2) r e q u i r e a high decontaminat ion f a c t o r . The v o l u m e s of such so lut ions are general ly

2

s m a l l c o m p a r e d with those o f l o w - a c t i v i t y w a s t e s . If l o w - a c t i v i t y waste i s to be t rea ted by evapora t i on , p r o c e s s i n g c o s t s tend to b e c o m e high b e c a u s e o f t h e inherent ly high c o s t ; n e v e r t h e l e s s , even in the treatment of l o w - l e v e l w a s t e s , evapora t i on m a y s o m e t i m e s be attract ive if the total v o l u m e to be p r o c e s s e d i s not l a r g e , s ince the operat ion is re la t ive ly s i m p l e c o m p a r e d with o ther p r o c e s s e s . H o w e v e r , when a waste l iquid containing s e v e r e l y s c a l i n g , f o a m i n g o r c o r r o s i v e m a t e r i a l s is to be e v a p o r a t e d , p r e - t r e a t m e n t s o r combinat ion with other waste treatment methods m a y be d e s i r a b l e and a c a r e f u l and pertinent des ign of the e v a p o r a t o r b e c o m e s n e c e s s a r y .

1 . 4 . CORROSION

C o r r o s i o n and e r o s i o n a r e g e n e r a l l y m o r e s e v e r e in e v a p o r a t o r s than in other types of equipment b e c a u s e of the n e c e s s a r y concentrat ion d i f f e r e n c e s , the f requent p r e s e n c e of s o l i d s in suspens ion and the high v e l o c i t i e s of vapour and l iquid .

Many f a c t o r s can in f luence the c o r r o s i v e attack in an evaporat ion plant and idea l ly the c h o i c e of m a t e r i a l should be b a s e d on actual t r ia l s using s a m p l e s of the l iqu ids to be evaporated . It i s , h o w e v e r , not ususal ly p o s s i b l e to do this and one m a y not be able to f o r e c a s t the ways in which the f e e d to the plant might change during the l i f e of the plant. Since a d e c i s i o n on the m a t e r i a l to be u s e d must be m a d e e a r l y in the p r o j e c t , a sa fe ty f a c t o r should be appl ied to the c o r r o s i o n rate which wi l l r e f l e c t an a s s e s s m e n t of the uncer ta in t i e s . In Sect ion 3. 2 . 4 . s o m e typica l c o n s t r u c t i o n m a t e r i a l s a r e l i s t ed .

1 . 5 . F O A M I N G

In waste l iquid t rea tment , f o a m i n g i s o f ten a m a j o r p r o b l e m because of the p r e s e n c e of s o a p s and detergents in vary ing concentrat i ons . When such c o n c e n t r a t i o n s a r e too high and the waste is not suitable f o r evaporat ion, d i s c h a r g e by di lution (a f ter s e g r e g a t i o n f r o m other was tes ) may be the s i m p l e s t m e t h o d of handling un less the act ivity content is too high. Even with t r a c e l e v e l s of f o a m i n g agents , s e r i o u s f oaming o c c u r s under v igorous bo i l ing condi t i ons in an e v a p o r a t o r and as this l eads to an operat ing f l ow rate c o n s i d e r a b l y b e l o w the des igned throughput, p r o p e r m e a s u r e s as d e s c r i b e d in Sect ion 3 . 2 . 2 . 2 . b e c o m e n e c e s s a r y .

1 . 6 . SCALING AND SALTING

E v a p o r a t o r s depend ent i re ly upon good heat t r a n s f e r f o r their operat ion, and s c a l i n g r e s u l t s in o v e r a l l r e s i s t a n c e to heat t r a n s f e r i n c r e a s i n g with t i m e . T o k e e p the des igned throughput, the r e m o v a l of s c a l e i s s o m e t i m e s important enough to r e q u i r e shut -down. Methods of r e m o v a l a r e d e s c r i b e d in Sect ion 3 . 2 . 3 .

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1 . 7 . G E N E R A L LIMITATIONS

As a l ready ment ioned , s e v e r e l y f oaming , s ca l ing o r c o r r o s i v e l iquids a r e not suitable f o r evaporat i on if not pre t rea ted . Liquids containing e x p l o s i v e m a t e r i a l s are a l s o unsuitable f o r evaporat ion . Limitat ions due to these c a u s e s depend on the content of each component . There are a l so c o s t l imi ta t i ons . In planning a waste l iquid p r o c e s s i n g fac i l i ty , one must know the kinds and act iv i ty l e v e l s of var i ous ef f luents and est imate which of them r e q u i r e high decontaminat ion . The decontamination fac tor requ i red • can v a r y wide ly a c c o r d i n g to s p e c i f i c condi t ions , be ing dependent on the amount of act ivity a r i s ing and the a l lowable rate of d i s c h a r g e of activity to the env i ronment . V e r y l ow concentra t i ons of rad ioac t ive mater ia l s have to be guaranteed when t reated waste l iquids are d i s charged to a dr inking water s o u r c e containing a b i o l o g i c a l r e concent ra t i on m e c h a n i s m . The m i n i m u m decontaminat ion f a c t o r r e q u i r e d is s o m e t i m e s ca lculated b a s e d on I C R P drinking water t o l e r a n c e s f o r the genera l publ ic .

If methods other than evaporat i on can be adopted as the resul t of the above examinat ion , a c a r e f u l c o s t e s t imat ion should be c a r r i e d out to s e l e c t the cheapest method .

2. DESIGN AND DESCRIPTION OF EVAPORATOR TYPES AND ASSOCIATED EQUIPMENT

No genera l i za t i on can be m a d e which wi l l s e r v e as a re l iab le guide in the s e l e c t i o n of the op t imum type of evapora to r and its aux i l iar ies . The f a c t o r s that have to be c o n s i d e r e d in the se lec t i on of the type of evaporator m o s t suitable f o r r a d i o a c t i v e waste l iquid p r o c e s s i n g a r e : (1) c h a r a c t e r -i s t i c s of the f e e d , such as c o m p o s i t i o n and amount of d i s s o l v e d mater ia l s ( m o s t of which a r e n o n - r a d i o a c t i v e ) , and their sa l t ing 2 , s ca l ing , c o r r o d i n g and f o a m i n g c h a r a c t e r i s t i c s ; (2) amount of l iquid to be t reated per hour, its f luctuat ions and number of work ing hours per day; (3) vo lume reduc t i on and decontaminat ion fa c to r r e q u i r e d ; (4) e a s e of maintenance; (5) s p a c e l imi tat i ons ; and (6) e c o n o m i c l imitat ions .

2 . 1 . T Y P E S OF E V A P O R A T O R S

2 . 1 . 1 . Co i l o r pot type

Th i s type of e v a p o r a t o r is now s e l d o m used in the c h e m i c a l industry, but in p r o c e s s i n g r a d i o a c t i v e was tes it is f requent ly used e spec ia l l y f o r s m a l l insta l lat ions . Even in a b ig l a b o r a t o r y , f o r instance at Chalk R iver

2 Rapid build-up of a normally soluble material (a material having a solubility that increases with increase in temperature) on the heating surface and evaporator walls in the zone o f evaporation. It is aggravated by small fluctuations in the operating conditions and by any condition that will encourage crystal nucleation, rather than by the deposition of materials on previously formed crystals.

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[34], this type of e v a p o r a t o r has been used s ince 1958 f o r the p r o c e s s i n g of l o w - a c t i v i t y w a s t e s . F igure 1 i l lus t rates the s imp l i f i ed f l o w - s h e e t of the Chalk R i v e r e v a p o r a t o r cons truc ted of type 347 s ta in less s tee l . The e v a p o r a t o r has a h o l d - u p vo lume of 900 l i t r e s and a throughput rate of 900 l i t r e s p e r hour . A 9 1 - c m d iameter co lumn containing two d e -entra inment d e v i c e s surmount the e v a p o r a t o r . The f i r s t dev i ce i s an i m p i n g e m e n t plate and the s e c o n d is a w i r e - m e s h d e m i s t e r unit.

Concentrate to s t o r a g e

receivers

F I G . l . Simplified flow sheet for low-activity waste evaporator (From Ref. [ 3 1 ] ) .

P o t - t y p e e v a p o r a t o r s a r e s i m p l y c o n s t r u c t e d and m a y be use fu l f o r s i m p l e b a t c h w i s e e v a p o r a t i o n . In the United States, even at b ig power r e a c t o r stat ions [53], p o t - t y p e e v a p o r a t o r s ranging in capac i ty f r o m 2. 7 to 4 5 . 4 l i t r e s / m i n have b e e n o p e r a t e d on a batch b a s i s with a d i s t i l l a t e -t o - c o n c e n t r a t e r a t i o be tween 10 to 1 and 50 to 1. In a s m a l l instal lat ion with s m a l l heat l oad a j a cke ted kettle (the s i m p l e s t po t - type evapora tor ) m a y be u s e d . A t y p i c a l kettle i s shown in F ig . 2.

A p ipe i s c o n n e c t e d to the b o t t o m of the jacket f o r condensate outlet and another f o r d i s c h a r g e of thick l i quor f r o m the kett le . An inlet f o r s t e a m and an outlet f o r n o n - c o n d e n s e d g a s e s are p r o v i d e d near the top o f the j a cke t . The o v e r a l l heat t r a n s f e r c o e f f i c i e n t in such a kettle may v a r y f r o m 900 to 1200 k c a l / m 2 h ° C . When v e r y s m a l l quantit ies are to b e e v a p o r a t e d , d i r e c t f i r e o r e l e c t r i c a l heating m a y be u s e d instead of s t e a m .

E v a p o r a t o r s us ing s t e a m in tubes which a r e c o i l e d , U - s h a p e d o r d e f o r m e d in s o m e o ther m a n n e r are a l s o c o m m o n in waste t reatment . This type of e v a p o r a t o r i s e s p e c i a l l y suitable f o r s c a l e - f o r m i n g l iquids when d e s i g n e d to p e r m i t c o l d s h o c k i n g o r c o m p l e t e withdrawal of the c o i l f r o m the she l l f o r r e m o v a l of s c a l e . At Harwe l l [6, 54], a s i m p l e s i n g l e - s t a g e

5

De-entrainment column

m mm dim

H Vapour

Vent

Cooling c o i l

Thick l i q a o r

FIG. 2. Steam-jacketed kettle.

ACID AND WATER CONNECTIONS FOR DECONTAMINATION

SOLENOID VALVES CONTROLLED BY LEVEL PROBES IN KETTLE

FOAM BREAKING COIL

PREHEATER

HEATING COIL

FIG. 3. Harwell waste evaporator. (From Technology of Radioactive Waste Management Avoiding Environmental Disposal, Technical Reports Series, No.27,IAEA, Vienna (1964) 27).

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e v a p o r a t o r heated by an internal s t e a m c o i l (113 m total length and 2. 5 c m in d i a m e t e r ) at a s t e a m p r e s s u r e of a p p r o x i m a t e l y 4 . 2 k g / c m 2 has been u s e d (F ig . 3). The s t e a m c o i l i s mounted on a t r o l l e y and can be withdrawn f o r i n s p e c t i o n o r c l ean ing as d e s i r e d . Actua l ly , no t rouble has been e x p e r i e n c e d at H a r w e l l with s c a l i n g . This fact i s explained as due to p r i o r c h e m i c a l t r ea tment of the f e e d [7],

FIG. 4 . Horizontal - tube evaporators.

(a) Swenson evaporator.by courtesy o f Whiting International, New York, USA. (b) From Encyclopedia o f C h e m i c a l T e c h n o l o g y , V o l . 5 p . 9 3 7 , by courtesy o f A m . Inst. c h e m . Engrs. .

2. 1. 2. N a t u r a l - c i r c u l a t i o n type

2 . 1 . 2 . 1 . H o r i z o n t a l - t u b e e v a p o r a t o r s

Th i s type of e v a p o r a t o r was wide ly u s e d f o r m o r e than 50 y e a r s in the c h e m i c a l industry but nowadays i s s e l d o m u s e d except f o r b o i l e r f e e d - w a t e r p r e p a r a t i o n . H o w e v e r , in w a s t e - l i q u i d t rea tment it is s o m e t i m e s used when there are s e v e r e h e a d r o o m l imi ta t i ons . F i g u r e 4 i l lus trates two typ ica l e x a m p l e s of these e v a p o r a t o r s in which the s t e a m is ins ide and the l iquid outs ide the tubes .

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M o s t e v a p o r a t o r s of this type have hor i zonta l c y l i n d r i c a l she l l s , b e c a u s e these have the l a r g e s t rat io of v a p o u r - l i q u i d disengaging sur face to she l l d i a m e t e r , r e su l t ing in l ow entrainment . The hor izonta l s t ra ight -tube type can be c o n s t r u c t e d re la t ive ly cheaply but i s unsuitable f o r s c a l i n g l iqu ids . H o w e v e r , when the tube spac ing is des igned l a r g e r than in the v e r t i c a l type and d e f o r m e d tubes, are used , r e m o v a l of s c a l e can be a c c o m p l i s h e d m o r e e a s i l y after draining the she l l and spray ing co ld w a t e r on the tubes whi le st i l l sub jec ted to heating, making this type suitable f o r s e v e r e l y s c a l i n g l iqu ids . The h o r i z o n t a l - t u b e evapora to r i s .not g e n e r a l l y adapted f o r f o a m y l iquids , s i n c e there i s no means of breaking the f o a m . A c c o r d i n g to P e r r y ' s Handbook [37] the m e r i t s and disadvantages of h o r i z o n t a l - t u b e e v a p o r a t o r s are :

Advantages 1. V e r y l ow h e a d r o o m r e q u i r e m e n t 2. L a r g e v a p o u r - l i q u i d d isengaging area 3. Re la t ive ly l ow c o s t in s m a l l capac i ty s tra ight - tube type 4 . Good h e a t - t r a n s f e r c o e f f i c i e n t s 5. Easy s e m i a u t o m a t i c d e s c a l i n g in the bent - tube type

Disadvantages 1. Unsuitable f o r f o a m i n g l iquids 2. Unsuitable f o r sa l t ing l iquids 3. Unsuitable f o r s c a l i n g l iquids in the s t ra ight - tube type 4 . High c o s t in the bent - tube type.

B e s t appl i cat ions of h o r i z o n t a l - t u b e e v a p o r a t o r s 1. L i m i t e d h e a d r o o m 2. Smal l capac i ty 3. N o n - s c a l i n g , non - sa l t ing l iquids in the s tra ight - tube type 4. S e v e r e l y s c a l i n g l iquids in the bent - tube type

2 . 1 . 2 . 2 . V e r t i c a l - t u b e e v a p o r a t o r s

The v e r t i c a l - t u b e e v a p o r a t o r o r ca landr ia i l lustrated in F ig . 5 i s now wide ly used . The body i s n o r m a l l y a cy l inder and the heating s u r f a c e is p r o v i d e d by shor t tubes , usual ly 2 to 3 in. (5 to 10 c m ) in d i a m e t e r , through the s t e a m c h e s t at the b o t t o m of the v e s s e l . Th i s type of evapora to r g ives high h e a t - t r a n s f e r c o e f f i c i e n t s at high t e m p e r a t u r e d i f f e r e n c e s , but as the t e m p e r a t u r e d i f f e r e n c e s b e c o m e low, heat t r a n s f e r b e c o m e s p o o r . Liquid c i r c u l a t i o n past the heating s u r f a c e i s induced by the pumping act ion of w a t e r vapour f o r m e d in the tubes .

The re turn p a s s a g e f r o m above the s t eam chest to the bot tom s p a c e is usua l ly a c y l i n d r i c a l downtake. Its c r o s s - s e c t i o n a l area should be of the s a m e o r d e r of magnitude as the total c r o s s - s e c t i o n a l area of the tubes . Th i s a r r a n g e m e n t i s the m o s t c o m m o n l y used standard v e r t i c a l e v a p o r a t o r , but a var ia t i on without downtake ca l l ed the " b a s k e t - t y p e " is a l s o u s e d . B e c a u s e the natural r e c i r c u l a t i o n rate i s many t i m e s the f eed ra te , the l iquid enter ing the bo t tom s e c t i o n i s of the same, concentrat ion

8

as the produc t r e m o v e d in cont inuous opera t i on . The v e l o c i t y of the liquid enter ing the heating tubes has been m e a s u r e d in the range of 1 to 3 f t / s (0. 3 to l m / s ) [12], but the c i r c u l a t i n g v e l o c i t y and a lso heat t r a n s f e r rate a r e s t r o n g l y a f f e c t e d by the l iquid l e v e l . A c c o r d i n g to P e r r y ' s Handbook [38], "h ighes t h e a t - t r a n s f e r c o e f f i c i e n t s a r e ach ieved when the l e v e l , as ind icated by an e x t e r n a l gage g l a s s , i s only about hal fway up the tubes. A s l ight r e d u c t i o n in l e v e l b e l o w the o p t i m u m r e s u l t s in i n c o m p l e t e wetting of the tube wa l l s with a consequent i n c r e a s e d tendency to foul and a rapid r e d u c t i o n in c a p a c i t y " .

When s c a l i n g o r sa l t ing l iquids a r e to be evaporated it i s c u s t o m a r y to o p e r a t e with the l iquid l e v e l a p p r e c i a b l y h igher than the opt imum. To r e m o v e s c a l e , the tubes are s u b j e c t e d to c leaning by m e c h a n i c a l d e v i c e s which l i m i t the length of the tubes . The standard v e r t i c a l tube evaporator i s v e r s a t i l e and r e l a t i v e l y inexpens ive .

FIG. 5. Vert i ca l - tube evaporators (Swenson evaporator, by courtesy o f Whiting International, New York, USA).

(a) Standard type (b) Basket ..type

2 . 1 . 2 . 3 . E v a p o r a t o r s with ex te rna l hea ter ( thermos iphon)

Th i s type of e v a p o r a t o r , i l lus t rated in F i g . 6, has the m e r i t of giving h igher r e c i r c u l a t i o n r a t e s and can be des igned to p r o v i d e a l o w e r s t o rage s p a c e f o r l i q u o r than the standard type .

2 . 1 . 3 . F o r c e d - c i r c u l a t i o n type

In the f o r c e d - c i r c u l a t i o n e v a p o r a t o r , l iquid is r e c i r c u l a t e d by a pump through the heating tubes at r e a s o n a b l y high v e l o c i t i e s with re la t ive ly

9

FIG. 6. Evaporator with external heater (thermosiphon). (From TAMISfi , M . , Energie n u c l . ' 5 No .2 (1963) 85).

FIG. 7. Forced-c irculat ion evaporator (Swenson evaporator, by courtesy of Whiting International, New York, USA).

l i t t le bo i l ing p e r p a s s . C i r c u l a t i o n i s maintained r e g a r d l e s s of the e v a p o r a t i o n r a t e .

Th i s type of e v a p o r a t o r i s sui table f o r p r o c e s s i n g s ca l ing l iquids b e c a u s e high l iquid v e l o c i t i e s in the heating tubes e f f e c t i v e l y d e c r e a s e s c a l e depos i t i on . P r e s e n t p r a c t i c e tends to instal l the heating e lement

10

as a s e p a r a t e unit. F i g u r e 7 i l lus t ra tes an e v a p o r a t o r of this type. The e x t e r n a l hea ter , e s p e c i a l l y a v e r t i c a l one , has the advantage of a l lowing g r e a t e r e a s e of c l ean ing o r r e p l a c e m e n t of tubes . M o r e o v e r , one can e x p e c t an i m p r o v e d a v e r a g e c o e f f i c i e n t of heat t r a n s f e r s i n c e it is p o s s i b l e t o insta l l the heating e l e m e n t (hor i zonta l o r v e r t i c a l ) f a r be low the l iquid l e v e l to prevent b o i l i n g on the heating s u r f a c e and at the s a m e t ime great ly d e c r e a s e the ra te of s c a l e depos i t i on . The l iquid v e l o c i t i e s past the heating tubes n o r m a l l y range f r o m 4 to l O f t / s (1 to 3 m / s ) , and this i s l imi ted only by the pumping p o w e r needed and by a c c e l e r a t e d c o r r o s i o n and e r o s i o n r a t e s at the h i g h e r v e l o c i t i e s .

S e l e c t i o n of a f o r c e d - c i r c u l a t i o n e v a p o r a t o r depends on the e c o n o m i c ba lance of the c o n s t r u c t i o n c o s t , the c o s t of e n e r g y to c i r c u l a t e the liquid and the i m p r o v e d h e a t - t r a n s f e r c o e f f i c i e n t obta inable . Another important f a c t o r to be c o n s i d e r e d in r a d i o a c t i v e waste t reatment i s e a s e of maintenance . D i s a s s e m b l y of the c i r c u l a t i o n pump, which is m o s t f requent ly a centr i fugal pump, m u s t be done by d i r e c t m a i n t e n a n c e , and the rad ioac t iv i ty o ften m a k e s the w o r k t r o u b l e s o m e . A c c o r d i n g to P e r r y ' s Handbook, m e r i t s and d i sadvantages of f o r c e d - c i r c u l a t i o n e v a p o r a t o r s a r e :

Advantages 1. High h e a t - t r a n s f e r c o e f f i c i e n t s 2. P o s i t i v e c i r c u l a t i o n . 3. R e l a t i v e f r e e d o m f r o m sa l t ing , s ca l ing and foul ing

Disadvantages 1. High c o s t 2. P o w e r r e q u i r e d f o r c i r c u l a t i o n pump

F r e q u e n t d i f f i c u l t i e s with f o r c e d - c i r c u l a t i o n e v a p o r a t o r s a r e : 1. P lugg ing of tube in lets by salt depos i t s detached f r o m equipment

w a l l s 2 . P o o r c i r c u l a t i o n due to h igher than expec ted h e a d ' l o s s e s 3. Salting due to bo i l ing in tubes 4 . C o r r o s i o n and e r o s i o n

2 . 1 . 4 . V a p o u r - c o m p r e s s i o n type

The h ighest heat e c o n o m i e s in operat ing e v a p o r a t o r s can be achieved by r e - u s e of the e n e r g y of v a p o r i z a t i o n to p r o v i d e a heat s o u r c e f o r further e v a p o r a t i o n . In v a p o u r - c o m p r e s s i o n e v a p o r a t o r s the e n e r g y potential of l o w - p r e s s u r e v a p o u r a r i s i n g f r o m the e v a p o r a t o r i s i n c r e a s e d by c o m p r e s s i n g the v a p o u r , t h e r e b y making the latent heat of condensat ion ava i lab le at a h igher t e m p e r a t u r e and resu l t ing in high total heat e f f i c i e n c y . This type of e v a p o r a t o r has been u s e d at M o l in B e l g i u m [55] , R i s^ in D e n m a r k [56] and in F r a n c e [4] . F i g u r e 8 i l lus t ra tes an example . This type of e v a p o r a t o r u t i l i z e s its own v a p o u r , a f t e r c o m p r e s s i o n , as the heating m e d i u m in the s a m e e v a p o r a t o r , resu l t ing in a reduct ion of the e n e r g y r e q u i r e m e n t f o r e v a p o r a t i o n . This is not the l east expens ive type of e v a p o r a t o r , h o w e v e r , and the use of vapour c o m p r e s s i o n r e q u i r e s a c o m p l e t e e c o n o m i c ba lance to d e t e r m i n e its f eas ib i l i t y . Two types of vapour c o m p r e s s i o n a r e c u r r e n t l y u s e d , m e c h a n i c a l c o m p r e s s i o n by a b l o w e r and c o m p r e s s i o n by a s t e a m i n j e c t o r .

11

disengaging section

FIG. 8. Vapour-compression evaporators.

(a) Mechanica l - compress ion evaporator (al l dimensions in m m ) . (By courtesy o f CEN, Mo l -Donk , Belgium). (b) Mechanica l - compress ion evaporator. (From Ref. [ 4 ] ) .

M e c h a n i c a l c o m p r e s s i o n . Centr i fugal c o m p r e s s o r s o r R o o t s - t y p e p o s i t i v e - d i s p l a c e m e n t b l o w e r s a r e c o m m o n l y used in l iquid waste p r o c e s s i n g . T h e . b l o w e r o r c o m p r e s s o r i s dr iven by an e l e c t r i c m o t o r o r s t eam turbine. E l e c t r i c i t y as the e n e r g y s o u r c e m a y be e c o n o m i c a l when e l e c t r i c p o w e r i s l ow in c o s t and fue l i s e x p e n s i v e . The f eed to this type of evapora to r should be preheated as near to its bo i l ing t e m p e r a t u r e as p o s s i b l e , b e c a u s e , to keep the c o m p r e s s o r c o s t and p o w e r r e q u i r e m e n t s within r e a s o n , the e v a p o r a t o r must w o r k with a f a i r l y n a r r o w t e m p e r a t u r e d i f f e r e n c e , usual ly

12

f r o m about 5 to 10°C [39] . This m e a n s that 'a l a r g e heating s u r f a c e is n e e d e d , par t ia l l y o f f se t t ing the advantages of vapour c o m p r e s s i o n . Great c a r e must be taken to keep entra inment at a m i n i m u m in this type of e v a p o r a t o r s i n c e the l iquid entra ined e v a p o r a t e s on c o m p r e s s i o n and l e a v e s the d i s s o l v e d s o l i d and r a d i o a c t i v i t y behind, making maintenance t r o u b l e -s o m e . When a R o o t s - t y p e b l o w e r i s u s e d , the c l e a r a n c e between the i m p e l l e r and the c a s i n g is quite s m a l l (a few thousandths of an inch) and the use o f a f i l t e r o r w i r e m e s h b e f o r e and a f ter the suct ion and d e l i v e r y of the c o m p r e s s o r is r e c o m m e n d e d to avo id damage by so l id p a r t i c l e s . C a r e m u s t a l s o be taken to prevent overheat ing of the vapour in the b l o w e r . F r e n c h e x p e r i e n c e shows that vapour t e m p e r a t u r e of about 100°C at the suct ion and about 145°C in the b l o w e r is o p t i m u m [4] . A s power r e q u i r e m e n t s v a r y w i d e l y with the c o m p r e s s i o n ra t i o , determinat ion of the m o s t e c o n o m i c condi t i on i s impor tant . A m e c h a n i c a l vapour c o m -p r e s s i o n e v a p o r a t o r usua l ly r e q u i r e s m o r e heat than is avai lable f r o m the c o m p r e s s e d v a p o u r , and this extra heat can be obtained f r o m the condensate o r the produc t by heat exchange . Upon start ing the e v a p o r a t o r , m a k e - u p heat is n e c e s s a r y , and this is furn ished in v a r i o u s w a y s depending on the method of dr iv ing the c o m p r e s s o r . Operat ion under r e d u c e d p r e s s u r e m a k e s evapora t i on c o s t l y s i n c e at l o w e r t e m p e r a t u r e s the vo lume of vapour to be c o m p r e s s e d i n c r e a s e s , resul t ing in h igher c o s t .

At the R i s ^ R e s e a r c h Es tab l i shment [56] , a m e c h a n i c a l v a p o u r -c o m p r e s s i o n f o r c e d - c i r c u l a t i o n s y s t e m has been used . The consumpt ion of e n e r g y in this plant i s about t w o - t h i r d s of that in a s i m i l a r plant without f o r c e d c i r c u l a t i o n . The c oncent ra te f r o m ' t h i s e v a p o r a t o r , containing about 10% s o l i d s , i s next fed to an e l e c t r i c a l l y heated evapora to r which p r o d u c e s a s ludge containing 100 kg of s o l i d s in 100 l i t r e s .

V a p o u r c o m p r e s s i o n by s t e a m i n j e c t o r . This type ut i l i zes a s t eam i n j e c t o r in p l a c e of a c o m p r e s s o r . T h e r e f o r e , this operat ion is appl i cable only when h i g h - p r e s s u r e s t e a m is ava i lab le . The e f f i c i e n c y of a s team i n j e c t o r i s l o w , be ing on ly 25 to 35%, and b e c o m e s w o r s e when the in j e c t o r is o p e r a t e d withi a l a r g e d i f f e r e n c e in p r e s s u r e between the h igh- and l o w -p r e s s u r e s t e a m . Its o p e r a t i o n a l s o b e c o m e s unstable when it i s used at s t e a m f l ow r a t e s and p r e s s u r e s d i f f e rent f r o m those f o r which it was d e s i g n e d . Consequent ly , 2 o r 3 j e t s a r e used in p a r a l l e l when wide v a r i a t i o n s in evapora t i on rate a r e e x p e c t e d . The b a s i c a l l y low e f f i c i e n c y of an i n j e c t o r r e q u i r e s a de terminat i on of the p r o p e r suct ion and c o m -p r e s s i o n r a t i o to g ive as high e f f i c i e n c y as p o s s i b l e . Dra inage f r o m the e v a p o r a t o r i s a m i x t u r e of s t e a m f r o m both the b o i l e r and the e v a p o r a t o r , and as the la t ter s t e a m is usua l ly contaminated , s o m e pur i f i ca t i on t rea t -ment m a y be n e c e s s a r y when it i s to be r e u s e d as b o i l e r - f e e d water .

The e f f i c i e n c y of a s t e a m i n j e c t o r is l o w e r than that of a mechan i ca l c o m p r e s s o r , but b e c a u s e of its low f i r s t c o s t , abi l i ty to handle l a r g e v o l u m e s of vapour and the m e r i t of having no m o v a b l e par ts , s t eam i n j e c t o r s a r e u s e d to i n c r e a s e the e c o n o m y of e v a p o r a t o r s that must operate at low t e m p e r a t u r e s . The s t e a m - i n j e c t o r v a p o u r - c o m p r e s s i o n evaporator is des igned to have a heat input l a r g e r than needed to ba lance the s y s t e m , resul t ing in e x c e s s heat which is r e m o v e d by venting s o m e of the vapour at the suct ion of the i n j e c t o r and d i s c h a r g i n g it into the c o n d e n s e r .

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2 . 1 . 5 . M u l t i p l e - e f f e c t type

The c o s t of s t e a m is a m a j o r i tem in the operat ing c o s t of a waste evaporat ion plant. A m u l t i p l e - e f f e c t e v a p o r a t o r , which ut i l i zes the vapour f r o m one e f f e c t as the heating med ium f o r another e f f e c t in which boil ing takes p lace at a l o w e r t e m p e r a t u r e and p r e s s u r e , i s another method of i n c r e a s i n g the ut i l izat ion of e n e r g y . This method of evaporat ion is m o r e suited to l a r g e plants . A s e m i - c o n t i n u o u s c y c l e may s o m e t i m e s by employed , but m o s t e v a p o r a t o r s of this type operate on a ful ly continuous b a s i s . Although it is t h e o r e t i c a l l y p o s s i b l e to r e u s e the vapour as many t imes as d e s i r e d thereby i m p r o v i n g the s team e c o n o m y approx imate ly in propor t i on to the n u m b e r of e f f e c t s , the capital c o s t a l s o i n c r e a s e s in propor t i on to the n u m b e r of e f f e c t s . S ince the p r i m a r y p u r p o s e of m u l t i p l e - e f f e c t e v a p o r a t o r des ign is to p r o c e s s the rad i oac t i ve waste at the l owes t total c o s t , an e c o n o m i c ba lance must d e t e r m i n e the opt imum number of e f f e c t s . An a c c u r a t e e c o n o m i c ba lance m a y be obtained by detai led heat and m a t e r i a l ba lances together with an ana lys i s of the inf luence of changes in operat ing condi t ions on h e a t - t r a n s f e r p e r f o r m a n c e . The i tems to be c o n s i d e r e d a r e :

1. Amount of e f f luent to be treated p e r day 2. P h y s i c a l and c h e m i c a l p r o p e r t i e s ; so l id content and act ivity

of e f f luent to be t reated ; v o l u m e reduct ion requ i red 3. T e m p e r a t u r e of the f eed 4 . Steam p r e s s u r e and t e m p e r a t u r e to f i r s t e f f e c t (heat s o u r c e )

v e r s u s c o s t o r avai labi l i ty 5. P r e s s u r e of f inal e f f e c t v e r s u s c o o l i n g - w a t e r t emperature and

i ts c o s t ; h e a t - t r a n s f e r p e r f o r m a n c e 6. N u m b e r of e f f e c t s v e r s u s s t e a m and water c o s t 7. Distr ibut ion of heating s u r f a c e between e f f e c t s (normal ly they

should be equal to r e d u c e the c o s t ) v e r s u s evaporator c o s t (to es t imate the e v a p o r a t o r c o s t , the type of evaporator must be d e t e r m i n e d )

8. Continuity of opera t i on ; tube l i f e ; e v a p o r a t o r l i f e ; down- t ime f o r maintenance p e r y e a r

9. Cos t f o r a u x i l i a r i e s 10. W a g e s , overhead c h a r g e s , e tc .

FIG. 9. (a) Horizontal - type w i p e d - f i l m evaporator. (By courtesy o f Kontro C o . I n c . , Petersham, Mass . , USA).

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T A B L E I. S P E C I F I C A T I O N S O F S T A N D A R D H O R I Z O N T A L W I P E D - F I L M E V A P O R A T O R ( B y c o u r t e s y o f K o n t r o C o . I n c . , P e t e r s h a m , M a s s . , USA)

N o . 1 2 3 4 5 6 7 • 8 9 10 11

C a p a c i t y ( k g / h a ) 200 380 480 570 790 9 0 0 1120 1250 1450 1560 1850

Heat ing area ( m 2 ) 0 . 5 1 1 . 5 2 3 4 5 6 7 8 10

M a x . pressure in j a c k e t t> ( k g / c m 2 g a u g e )

5 10

5 10

5 10

5 10

5 10

5 10

5 10

5 10

5 10

5 10

5 10

HP o f m o t o r

For ordinary l iquids 3 5 7 . 5 7 . 5 10 10 15 15 20 20 30

For high v i s c o u s 5 10 15 15 20 2 0 30 30 40 40 50 l iquids

a A m o u n t o f water e v a p o r a t e d at 100 m m H g abs. using 5 k g / c m 2 g a u g e saturated steam as a heat s ource k T w o m o d e l s are a v a i l a b l e as shown in the tab le

In ca l cu la t ing the e c o n o m i c balance of a m u l t i p l e - e f f e c t evapora tor , repet i t i on of in t r i ca te ca l cu la t i ons seeking the m o s t e f f i c i ent operat ing t e m p e r a t u r e s and concentra t i ons in the e v a p o r a t o r is r e q u i r e d , but p r o p e r j u d g e m e n t can r e d u c e the n u m b e r of t r i a l s . The genera l p r o c e d u r e f o r de te rmin ing operat ing condi t ions and heating a r e a s i s explained in s e v e r a l t ex tbooks and handbooks .

2 . 1 . 6 . W i p e d - f i l m type

This type of e v a p o r a t o r might be c l a s s e d as a f o r c e d - c i r c u l a t i o n ' mach ine i n a s m u c h as it u s e s m e c h a n i c a l e n e r g y to i m p r o v e heat t rans fer . A s c h e m a t i c d i a g r a m of a w i p e d - f i l m e v a p o r a t o r is i l lustrated in F i g s 9(a), ( b ) a n d ( c ) .

The heating s u r f a c e c o n s i s t s of a s ingle ( ve r t i ca l type) o r tapered (hor i zonta l type) c y l i n d e r of l a r g e d i a m e t e r in which is rotated an agitating-b lade o r a s e r i e s of w i p e r s , e i ther maintaining a f ixed c l o s e c l e a r a n c e f r o m the wal l o r r id ing on the f i l m of l iquid on the wal l . In F i g . 9(a) a f i xed but ad justable c l e a r a n c e - the rotat ing shaft is m o v a b l e r ight and l e f t - is kept be tween the heating s u r f a c e and the end of the rotat ing b lades . The l iquid waste f ed into the c y l i n d e r i s agitated v i g o r o u s l y by the rotating b l a d e s , b e c o m i n g f i l m y by centr i fuga l f o r c e , and f l o w s along the heating s u r f a c e . The rotat ing b lades a l s o s e r v e to b r e a k down f o a m and throw entrainment out of the c e n t r a l vapour p a s s a g e . The heat i s t r a n s f e r r e d f r o m the outs ide of the c y l i n d e r and the vapour f o r m e d i s exhausted f r o m the vapour out let , pass ing through the s p a c e s between the b l a d e s . The continuous f o r m a t i o n o f the f i l m p e r m i t s much higher concentrat ion of thick l i quor than can be handled in o ther types of e v a p o r a t o r s . As i l lustrated in F i g s 9(a) , (b) and ( c ) , two v a r i e t i e s (hor izonta l and v e r t i c a l ) a r e avai lable . The advantages of this type of e v a p o r a t o r a r e : (1) s ince ho ld -up vo lume is s m a l l , the r e s i d e n c e t i m e can be g rea t ly d e c r e a s e d c o m p a r e d with . o ther types of e v a p o r a t o r ; (2) h igher heat t r a n s f e r c o e f f i c i e n t s can be e x p e c t e d ; and (3) the f eed rate can be changed f r o m z e r o to its m a x i m u m throughput, i . e . the r e s i d e n c e t ime is adjustable in the hor izonta l type. H o w e v e r , the d isadvantages a r e the high c o s t of cons t ruc t i on and l imitat ion of s i z e (about 20 m 2 i s the m a x i m u m heating area' f o r the hor izonta l type) . A c c o r d i n g to the Kontro C o . Inc . of P e t e r s h a m , M a s s . , USA, the.standard s p e c i f i c a t i o n s a r e as shown in Tab le I.

2 . 1 . 7 . Other types

2 . 1 . 7 . 1 . F a l l i n g - f i l m type

The c o n s t r u c t i o n of this type of evapora to r i s a var ia t ion of the convent ional " l o n g tube e v a p o r a t o r " , one of the natural c i r cu la t i on types c o m m o n l y used in the c h e m i c a l industry . The s c h e m a t i c d iagram is shown in F i g . 10.

The f e e d f l o w s down the tube wal l s as a f i l m . F r i c t i o n l o s s e s a c c o m p a n y i n g the f low of l iquid in the heating tubes can be d e c r e a s e d great ly in this type c o m p a r e d with e v a p o r a t o r s with s u b m e r g e d heating

17

2

tubes , s i n c e f r i c t i o n l o s s e s ex is t e ssent ia l l y f o r vapour alone f lowing in a p r a c t i c a l l y empty tube. Heat t rans fe r c o e f f i c i e n t s . i n the fal l ing f i lm e v a p o r a t o r a r e high and o f ten higher ( e s p e c i a l l y at e levated temperatures ) than in f o r c e d - c i r c u l a t i o n e v a p o r a t o r s . M o r e o v e r , these high coe f f i c i en ts a r e a c h i e v e d at r e l a t i v e l y l ow f i l m v e l o c i t i e s and with much l e s s l o s s in work ing t e m p e r a t u r e d i f f e r e n c e — due to the d e c r e a s e d e f f e c t of f r i c t i on and a c c e l e r a t i o n - than with f o r c e d - c i r c u l a t i o n e v a p o r a t o r s . High entrainment separat ion e f f i c i e n c y can be expec ted as entrainment o c c u r s ma in ly in the tubes t h e m s e l v e s , and s p r a y f r o m the f i l m on one s ide of the wal l i m p i n g e s on the f i l m on the other s i d e . Short r e s i d e n c e t ime and low c o s t p e r unit a r e a a r e a l s o important advantages of this type of e v a p o r a t o r .

The pr inc ipa l d i f f i cu l ty with f a l l i n g - f i l m e v a p o r a t o r s is the achievement of u n i f o r m feed d istr ibut ion to the top of all the tubes. Means of distribution inc lude s p r a y s and p e r f o r a t e d plates above the top tube sheets o r o r i f i c e s inser ted at the inlet to each tube. A port ion of tube that is underfed may bo i l and d r y , and this t rouble b e c o m e s m o r e s e r i o u s as the requ i red v o l u m e reduct ion b e c o m e s l a r g e r . One way to avoid such troubles is by

f — Feed

Blowdown

FIG. 10. Fal l ing - f i lm evaporator.

r e c i r c u l a t i n g the l iquid , which i n c r e a s e s the l iquid loading . The fa l l ing -f i l m e v a p o r a t o r is suitable f o r f oaming l iquids of high radioact iv i ty and a s m a l l por tab le e v a p o r a t o r of this type with a capac i ty of 150 l i t r e s / h at a c o s t of 2 ^ / l i t r e has b e e n produced c o m m e r c i a l l y [33] .

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2

2 . 1 . 7 . 2 . R i s i n g - f i l m type

T h e r i s i n g - f i l m e v a p o r a t o r i s the m o s t c o m m o n type o f l o n g - t u b e v e r t i c a l e v a p o r a t o r and a c c o u n t s f o r m o r e total e v a p o r a t i o n than a l l o t h e r s t e a m - h e a t e d e v a p o r a t o r s c o m b i n e d , b e c a u s e it i s n o r m a l l y the c h e a p e s t p e r unit o f c a p a c i t y . T h e e v a p o r a t o r c o n s i s t s o f a s i n g l e o n e - p a s s v e r t i c a l s h e l l - a n d - t u b e heat e x c h a n g e r w h i c h s o m e t i m e s u s e s natura l c i r c u l a t i o n o f the l i q u i d t h r o u g h an e x t e r n a l c i r c u l a t i o n p ipe . T h e l a t t e r o p e r a t i o n i s d e s i r a b l e w h e n l a r g e v o l u m e r e d u c t i o n i s r e q u i r e d . F e e d e n t e r s at the b o t t o m o f the e v a p o r a t o r , s t a r t s to b o i l par t o f the w a y up the tubes and r i s e s up the tube w a l l s as a f i l m b e c a u s e o f the l a r g e i n c r e a s e in v o l u m e a c c o m p a n y i n g v a p o r i z a t i o n and i ts v e r y high v e l o c i t y . T h e tubes a r e

from feed tank (10 000 gal)

FIG. 11. Ris ing- f i lm evaporator for high-act iv i ty wastes. (From Re f . ' [ 31 ] ) .

u s u a l l y about 2 in . (5 c m ) in d i a m e t e r but m a y be s m a l l e r than 1 in . Tube l ength m a y v a r y f r o m l e s s than 20 ft (6 m ) to m o r e than 30 ft (10 m ) . The r i s i n g - f i l m e v a p o r a t o r i s , in g e n e r a l , not r e c o m m e n d e d f o r sa l t ing o r s c a l i n g l i q u i d s , but it can b e adapted e x c e l l e n t l y to the t r e a t m e n t o f f o a m y l i q u i d s . T h i s i s due t o b r e a k u p of the f o a m when the l i q u i d - v a p o u r m i x t u r e i s e j e c t e d f r o m the t u b e s at h igh v e l o c i t y aga inst a p r o p e r l y shaped d e f l e c t o r p o -s i t i o n e d a b o v e the tube e x i t s to d i r e c t l iqu id away f r o m the v a p o u r out let . T h e r e s i d e n c e t i m e of the l i qu id i s o n l y a f ew s e c o n d s . T h e h e a t - t r a n s f e r c o e f f i c i e n t in the tubes i s d i f f i c u l t to p r e d i c t s i n c e en ter ing l iquid v e l o c i t i e s a r e u s u a l l y v e r y l o w , g iv ing p o o r heat t r a n s f e r . A s the l iquid r i s e s up the tube it i s hea ted and at the s a m e t i m e the p r e s s u r e i s r e d u c e d due to the r e d u c t i o n in s ta t i c head . A t s o m e point the l iqu id s t a r t s to b o i l and a b o v e that po int the l iqu id t e m p e r a t u r e d e c r e a s e s ( b e c a u s e of the r e d u c t i o n in s t a t i c , f r i c t i o n and a c c e l e r a t i o n h e a d s o f the l i q u i d - v a p o u r

19

m i x t u r e st i l l above ) until the v a p o u r - l i q u i d mix ture r e a c h e s the top of the tubes at e ssent ia l l y v a p o u r - h e a d t e m p e r a t u r e . T e m p e r a t u r e d i f f e rences in the non -bo i l ing zone m a y be high, but the c o e f f i c i e n t s a r e l ow . V e r y high c o e f f i c i e n t s m a y be expec ted in the bo i l ing zone , but they a r e part ial ly o f f s e t by the r e d u c e d t e m p e r a t u r e d i f f e r e n c e s .

A r i s i n g - f i l m e v a p o r a t o r f o r the p r o c e s s i n g of " h i g h - l e v e l - a c t i v i t y " l iquid w a s t e s at Chalk R i v e r Nuc lear L a b o r a t o r i e s has been in operat ion s i n c e A p r i l 1961 [31] . The evaporator is des igned f o r a throughput rate of 22 imp . g a l / h (100 l i t r e s / h ) and a v o l u m e reduct ion fa c to r of 5. The tubes a r e in the l o w e r por t ion of the e v a p o r a t o r . Two de -entra inment d e v i c e s , a ba f f l e plate and a w i r e - m e s h d e m i s t e r , a r e l ocated in the d isengaging s e c t i o n above the top of the e v a p o r a t o r tubes . In initial opera t i on , decontaminat ion f a c t o r s of about 1 to 2 X 106 w e r e achieved at. v o l u m e reduc t i ons of 2. 5 to 5. The s i m p l i f i e d f low sheet f o r a r i s i n g -f i l m e v a p o r a t o r is i l lustrated in F ig . 11.

2 . 1 . 7 . 3 . F l a s h e v a p o r a t o r

The p r i n c i p l e of f lash evaporat ion is to evaporate part of a solution by us ing its s e n s i b l e heat to f l ash the heated f eed solution into a f lash c h a m b e r at l o w e r saturat ion p r e s s u r e . The latent heat of condensation of the evaporated vapour i s r e c o v e r e d by preheat ing the feed liquid. F i g u r e 12(a) i l lus t rates the pr inc ip l e of f lash evaporat ion in three stages , showing the t e m p e r a t u r e d istr ibut ion among the three f lash c h a m b e r s .

A s i l lustrated in the f i gure , the f eed liquid is passed in s e r i e s through the c o n d e n s e r s C where it is heated in turn by r e c o v e r i n g the latent heat of vapour condensat ion . The f eed liquid is f inal ly heated to its highest . t e m p e r a t u r e T M b y an outs ide heat s o u r c e and then f lashed down to s u c c e s s i v e l y l o w e r saturation t e m p e r a t u r e s and p r e s s u r e s .

Only a s m a l l f ra c t i on , n o r m a l l y l e s s than 10%, can be f lashed off in a o n c e - t h r o u g h s y s t e m (F ig . 12(a)), s o it b e c o m e s n e c e s s a r y to r e c y c l e the thick l iquor to the f eed if a higher concentrat ion is des i r ed (Fig . 12(b)).

The heat e c o n o m y of a s ingle stage of f lash evaporat ion is w o r s e than that of a m u l t i p l e - e f f e c t s y s t e m but can be i n c r e a s e d by increas ing the n u m b e r of s tages . A s the construct ion of a f l ash evaporator is re lat ive ly s i m p l e and cheap, it b e c o m e s m o r e e c o n o m i c a l when a large number of s tages is used.3

F l a s h evaporat ion i s widely applied to desal inat ion and is suitable f o r the evaporat ion of dilute so lut ions , i. e . those having a low bo i l ing -point r i s e ( B P R ) .

2 . 1 . 7 . 4 . Spray d r y e r

The spray d r y e r , which may be included in a broad sense among the e v a p o r a t o r s , r e q u i r e s no heating sur face and is suitable f o r the e v a p o r a -tion and so l id i f i ca t i on of waste liquid of high so l ids content. Heated

3 60 stages is thought to be the maximum.

20

so lut ion i s s p r a y e d f r o m a nozz le in the f o r m of smal l drop le ts into a c h a m b e r into which is fed a hot gas suf f i c ient to comple te vapor izat ion of the f eed l iquid. Radiant heat can a l s o be used in p lace of hot gas. Heat i s t r a n s f e r r e d by d i r e c t contact of the hot gas with the liquid droplets or by radiat ion . The heat e c o n o m y of a spray .dryer is w o r s e than that of e v a p o r a t o r s with heating s u r f a c e s .

The s p r a y c a l c i n e r , which has been studied at Hanford in the s o l i d i -f i ca t ion of h i g h - l e v e l - P u r e x wastes [46], is a combinat ion of spray ca l c iner and a pot c a l c i n e r . F e e d liquid is evaporated , dr ied and calc ined to s o m e extent by radiant heat in the spray c a l c i n e r sect ion , and further ca lc ined in the pot c a l c i n e r .

2 . 1 . 7 . 5 . S u b m e r g e d - c o m b u s t i o n type

S u b m e r g e d c o m b u s t i o n evaporat ion makes use of combust ion gases bubbling through the liquid as the m e a n s of heat t r a n s f e r . It c ons i s t s s imply of a tank to hold the liquid and a burner that can be l owered into the l iquid. S ince t h e r e are no heating s u r f a c e s on which s c a l e can deposi t , this d e v i c e is w e l l suited to use with s e v e r e l y sca l ing l iquids. In s u b -m e r g e d c o m b u s t i o n evaporat ion the operat ing liquid t emperature is l ower than the bo i l ing t e m p e r a t u r e of the l iquid, which m a k e s easy the handling of highly c o r r o s i v e l iquids . The disadvantage of this method of evaporation is its incapabi l i ty of r e u s i n g the heat in the vapour , s ince the vapour is

FIG, 12 (a ) . Flash evaporator (once-through system).

C : condensor (feed liquid preheater). F: flash room. H: heater. R: condensate receiver . V: evaporation room.

21

( h e a t e r \w t ^outside heat source 7 ' 1 (main evaporator)

condenser

excess feed l iquid

A W V V W W V W A W V A condensate

A * h, > M 4 -* : -1 1 H i i T

f l a s h chamber t h i c k l iquor

vapor

u

supplementary s e c t i o n

condenser

M M / V * *

A 1-

f e e d , t

condensate. E

h O T -feed l i q u i d , f , t

q t h i c k l i q u o r

r e c y c l i n g l i q u i d , t ,¥

heater main evaporator supplementary s e c t i o n

FIG, 12 (b). Flash evaporator (recirculation system),

w, f , q , W and E: flow rates of liquid. (The construction of the main evaporator is the same as in Fig. 12(a)).

mixed with l a r g e quantities of noncondensable g a s e s . The need f o r a la rger coo l ing a r e a in the c o n d e n s e r is another disadvantage. In-tank e v a p o r a -tion by s u b m e r g e d combust i on heater has been studied at Hanford [45, 47]. A m o d i f i e d in-tank e v a p o r a t o r instal lat ion has a l so been tested at W i n d s c a l e [32], Its s e c t i o n a l v iew is i l lustrated in F ig . 13. A i r at about 350°C p a s s e s down tube C to the bo t tom of the packed co lumn D where it m e e t s hot l iquor . Water is taken up, and the air p a s s e s up through the packing , f i r s t b e c o m i n g saturated and coo l ing to about 95°C, then passing a dry se c t i on where entrained spray is r e m o v e d . A little heat is absorbed h e r e f r o m the hot a ir inlet tube, and the a ir l eaves the co lumn at 9 7 - 9 9 ° C to enter the s c r u b b e r E, of s i m i l a r p r o p o r t i o n s to those of the evaporator . H e r e it m e e t s co ld water in c o u n t e r - f l o w and l eaves by the exit pipe F f o r the c u b i c l e exhaust. Raw feed is admitted to the evaporator by a d ip - l ine in the tube B and,a f ter r e m o v a l of water, fa l l s to the bot tom and f l ows into the product tank, where it m a y build up and r e - e n t e r the evaporator at tube B f o r a s e c o n d stage of evaporat ion . It is eventually drawn into the product m e t e r H. Here it is r e c y c l e d and mixed with product in the

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m e a s u r i n g v e s s e l G by means of the a i r l i f t J and the o v e r f l o w K. It is r e m o v e d f r o m the v e s s e l batchwise at wi l l by p lac ing a f l a t - b a s e d plug o v e r the a ir vent P .

Co ld water at about 20 l i t r e s / h is a l l owed to fa l l into the funnel L and enters the s c r u b b e r by the U - t r a p M, leaving at the bot tom af ter taking heat, m o i s t u r e and vo la t i l e s f r o m the wet a ir s t r e a m . The bulk of this water p a s s e s through the outer tank, into which it is d i s charged , and o v e r the exit w e i r N, which is a l e v e l c ontro l . Make -up f o r raw feed f l o w - r a t e s b e l o w the m a x i m u m is taken automat ica l ly f r o m the outer tank by tube A . A s m a l l unit r e m o v i n g up to 20 l i t res /day of water f r o m act ive analyt i ca l waste l iquor was operated at a cos t of about 7 ^ / l i t r e .

_ ACTIVE LIQUOR G L A S S - AIR K - ^ J PACKING — WATER

FIG. 13. Mod i f i ed in-tank evaporator. (From Ref. [ 3 2 ] ) .

2 . 2 . H E A T T R A N S F E R IN E V A P O R A T O R S

E v a p o r a t o r s depend on heat t r a n s f e r f o r their operat ion . The sur face needed to t r a n s f e r heat const i tutes the m o s t expens ive part of an evaporator . The rate of heat t r a n s f e r in e v a p o r a t o r s is e x p r e s s e d by the usual equation q = U A A T , where q is the rate of heat f l o w ( k c a l / h ) ; A = the area of heat -t r a n s f e r s u r f a c e (m2); A T = o v e r a l l t e m p e r a t u r e d i f f e r e n c e between heating m e d i u m and l iquid ( ° C ) ; U is the overal l c oe f f i c i en t of heat t rans fer ( k c a l / h m 2 ° C ) and is a s t rong funct ion of A T in m o s t types of evaporators .

2 . 2 . 1 . The a r e a of h e a t - t r a n s f e r s u r f a c e

Unless o t h e r w i s e s p e c i f i e d , the area is m e a s u r e d on the liquid side of the s u r f a c e .

23

2 . 2 . 2 . T e m p e r a t u r e d i f f e r e n c e A T

The determinat ion of actual t emperature d i f f e r e n c e is very di f f icult , s i n c e the t e m p e r a t u r e of the liquid is not the s a m e at all parts of the heating s u r f a c e . B e c a u s e of this , A T is usually taken as that between the saturated t e m p e r a t u r e of the heating med ium at the p r e s s u r e in the heating m e d i u m s ide of the heating e l ement and the t emperature of boi l ing l iquid in equ i l i b r ium with vapour at the p r e s s u r e of the f lash chamber . T e m p e r a t u r e d i f f e r e n c e s ca l cu lated on this b a s i s are cal led "apparent t e m p e r a t u r e d i f f e r e n c e s " . Ac tua l t e m p e r a t u r e d i f f e r e n c e s a c r o s s the heating s u r f a c e a r e usual ly l o w e r than this , because the temperature of the bo i l ing liquid is h igher due to bo i l ing -po int r i s e (BPR) of the liquid.

B P R due to pure solute is i n v e r s e l y propor t i ona l to its m o l e c u l a r weight , i. e. T

2. 2. 3. 2. R e s i s t a n c e of the tube i tse l f

R e s i s t a n c e of the heating tube can be ca lculated by the equation:

^ _ ( th i ckness of tube )X (outside d i a m e t e r of tube) (mean t h e r m a l conduct iv i ty of tube m e t a l ) X (mean d iam. of tube)

but its value is usual ly neg l ig ib ly s m a l l .

2 . 2 . 3 . 3. R e s i s t a n c e of liquid f i l m

T h i s is usual ly the contro l l ing r e s i s t a n c e to heat t r a n s f e r . It depends on the type of e v a p o r a t o r used . Th i s r e s i s t a n c e in f o r c e d - c i r c u l a t i o n e v a p o r a t o r s v a r i e s wide ly with var iat ion in c i r cu la t ion rate . At low c i r c u l a t i o n r a t e s , bo i l ing m a y start c o n s i d e r a b l y b e l o w the top of the tube and the h e a t - t r a n s f e r f i l m c o e f f i c i e n t ca lculated by a Nusse l t - type equation m a y be l e s s than half the o b s e r v e d c o e f f i c i e n t . However , f o r c i rcu lat ion g r e a t e r than about 1 m / s , bo i l ing in the tubes is a lmos t s u p p r e s s e d , ' and the ca lcu lated c o e f f i c i e n t a p p r o a c h e s the o b s e r v e d value. F o r natural -c i r cu la t i on e v a p o r a t o r s the f i l m c o e f f i c i e n t s vary roughly in inverse p r o p o r t i o n to the v i s c o s i t y . The value can be ca lculated either by Co lburn-type or by N u s s e l t - t y p e equat ions . E m p i r i c a l d i m e n s i o n a l - t y p e equations m a y a l s o be used . D e p o s i t s on the liquid s ide of the tube, which may result f r o m salt ing or s ca l ing , add a r e s i s t a n c e to heat t r a n s f e r . The added r e s i s t a n c e i s p r o p o r t i o n a l to the th i ckness of. the s c a l e . Ease of m i n i -m i z i n g , avoiding o r r e m o v i n g such depos i t s is f requent ly the m o s t important f a c t o r in s e l e c t i n g the type of e v a p o r a t o r when a s e v e r e l y sca l ing liquid is to be evapora ted .

A p p r o x i m a t e values f o r o v e r a l l h e a t - t r a n s f e r c o e f f i c i e n t s U, in kcal /h m2 °C a r e :

1. N a t u r a l - c i r c u l a t i o n e v a p o r a t o r s (a) H o r i z o n t a l - t u b e and s u b m e r g e d - c o i l

U ^ 1000 at m o d e r a t e o v e r a l l t e m p e r a t u r e drop and boi l ing points in the v i c in i ty of 70°C and i n c r e a s e s to 2000 f o r a temperature drop of 40 to 4 5 ° C and bo i l ing point at a t m o s p h e r i c p r e s s u r e .

(b) Standard v e r t i c a l and basket type U== 1500 to 2500 f o r operat ion at l ow liquid l e v e l s , U ^ 750 to 1500 f o r operat i on at high l iquid l e v e l s .

2. F o r c e d - c i r c u l a t i o n e v a p o r a t o r s U can be as high as 30 000 with liquid v e l o c i t i e s . o f 4. 5 m / s .

3. R i s i n g - f i l m e v a p o r a t o r s 1000 to .3000

2 . 3 . T Y P E S OF A U X I L I A R Y E Q U I P M E N T

The m o s t important aux i l iary equipment of an evapora to r c o m p r i s e s the mis t s e p a r a t o r s , c o n d e n s e r s and p r e h e a t e r s .

25

2. 3. 1. Mist s e p a r a t o r s

De -ent ra inment d e v i c e s are e ssent ia l in evaporat ion of radioact ive was tes . V a r i o u s types of m i s t s e p a r a t o r s are d e s c r i b e d in Section 3. 2. 1. 2.

2 . 3 . 2 . C o n d e n s e r s

C o n d e n s e r s c o m m o n l y used in the c h e m i c a l industry are the sur face type, b a r o m e t r i c type and jet type. In waste treatment the activity l eve l of the dist i l late must be examined b e f o r e d i s charge into the environment, and b e c a u s e of this , s u r f a c e c o n d e n s e r s are the only type now in use , although they r e q u i r e m o r e coo l ing water and a r e m o r e expensive than other types . Surface c o n d e n s e r s a r e f o r the most part she l l -and- tube d e v i c e s with vapour outs ide the tubes and coo l ing water inside. The heat -t r a n s f e r c o e f f i c i e n t i s i m p r o v e d by i n c r e a s i n g the water ve loc i ty and this i s done by making them m u l t i p l e - p a s s . A s explained in Section 2. 2. 3. 1 . , the p r e s e n c e of inert o r n o n - c o n d e n s e d gases in vapour great ly d e c r e a s e s the h e a t - t r a n s f e r c o e f f i c i e n t , r equ i r ing that they be vented. They a r e e a s i l y vented when the p r e s s u r e in the c o n d e n s e r is above a tmospher i c , but when the p r e s s u r e is b e l o w a t m o s p h e r i c s o m e f o r m of vacuum pump must be used .

2 . 3 . 3 . P r e h e a t e r s

Tubular hea ters are wide ly used . Preheat ing of the feed liquid by the vapour is d e s i r a b l e f o r e v a p o r a t o r s of large capac i ty , but f o r s m a l l e r e v a p o r a t o r s the pro f i t is o f f s e t by the i n c r e a s e of capital cost and s m a l l e r heat l o s s .

2 . 4 . INSTRUMENTATION AND C O N T R O L OF E V A P O R A T I O N UNIT

When a l a r g e v o l u m e of rad ioac t ive waste liquid is to be evaporated, a we l l - equ ipped instrumentat ion s y s t e m o r automatic contro l sys tem may ensure e c o n o m i c and safe operat ion of the plant to meet s p e c i f i c r e -qu i rements such as reduct ion of m a n p o w e r , c ont ro l of operating conditions and min imizat i on of utility c o s t s . Even f o r s m a l l evaporators the use of s o m e c o n t r o l s , such as f o r l iquid l eve l , may be helpful .

A c a r e f u l f inanc ia l just i f i cat ion f o r an investment in instrumentation must be made on the s a m e bas i s used f o r other equipment. Instrumentation i s usual ly coupled with high c o s t of instal lation, t h e r e f o r e a des igner must s c ru t in i ze the need f o r e v e r y e lement in the instrumentation sys tem.

The n o r m a l method f o r contro l l ing an evaporator in s e m i - b a t c h w i s e operat ion is to m e a s u r e the liquid l e v e l in the sti l lpot and to contro l either the f l ow rate of the feed to keep the l eve l constant or the input of the heat-ing s team in a c c o r d a n c e with the f eed rate o r l iquid leve l . In continuous operat ion it is c o m m o n p r a c t i c e to c o n t r o l the s p e c i f i c gravity and liquid l e v e l in the st i l lpot by regulat ing the rate of d i s charg ing thick l iquor . In s e m i - b a t c h w i s e operat ion , s p e c i f i c gravi ty must be m e a s u r e d repeatedly

26

C : c o n t r o l l e r B : recorder B : b o i l i n g point r i s e FIG. 14. Liquid-level control. L : l e v e l I : i n d i c a t o r

unless the v o l u m e reduct i on can be set by knowing the concentrat ion of so l id c omponents in the f eed l iquid. If necessary , an a l a r m signal may be prov ided in the dist i l late r e c e i v e r to detect an unexpected r i s e in act iv i ty l e v e l caused by extens ive c a r r y - o v e r of thick l iquor by f oaming o r entrainment . An automatic shut-down dev i ce m a y a l so be prov ided .

F i g u r e 14 shows two s y s t e m s f o r evapora to rs where l iqu id - l eve l c o n t r o l l e r s are used . L i q u i d - l e v e l can be contro l led by regulating either the b o i l - u p o r the feed rate or both.

2 7

F i g u r e 14 (a) sugges t s a m e a n s of inventory c o n t r o l in which liquid l e v e l is c ontro l l ed by a regulat ion of the feed rate , maintaining the f l ow of s t eam constant. This c o n t r o l s y s t e m is suitable f o r s e m i - b a t c h w i s e operat ion . V a r i o u s types of LIC ( l e v e l - i n d i c a t o r - c o n t r o l l e r ) are available, but the s i m p l e s t f loat type or d i s p l a c e r - t y p e has high re l iabi l i ty and can opera te without t rouble . When density contro l of the thick l iquor is r e -quired , such a s y s t e m as i l lustrated in F ig . 14 (b) may be adopted. In the f i gure a BRC (boi l ing point r i s e c o n t r o l l e r ) is prov ided which is actual ly a kind of t h e r m o m e t e r that can contro l the e x c e s s i v e inc rease of l iquid density by regulat ing the rate of r e m o v a l of thick l iquor .

2. 5. P R E - T R E A T M E N T AND COMBINATION WITH OTHER WASTE T R E A T M E N T

P r e - t r e a t m e n t of f e e d can m a t e r i a l l y r e d u c e t roub les which may .be encountered during e v a p o r a t o r operat ion . Methods of p r e - t r e a t m e n t c o m m o n l y used a r e adjustment of pH and f i l t rat ion . P r e - t r e a t m e n t to r e d u c e f o a m i n g is a l s o in p r a c t i c e at Harwel l .

2. 5. 1. Ad justment of the pH value

The pH value of act ive waste is usual ly adjusted to between 6 and 7 by adding ac id or base . The adjustment can m i n i m i z e c o r r o s i o n when the pH value is e x t r e m e l y low, but the e f f e c t in reducing f o a m is uncertain; f o r instance Harwe l l e x p e r i e n c e shows that reduc ing the pH of the feed f r o m 10 to 4 had little e f f e c t in reduc ing f o a m even in the p r e s e n c e of ant i f oam agent [6], but another r e p o r t [19] says the. maintenance of feed pH at 6. 5 to 7. 5 is helpful in f o a m c o n t r o l and high pH leads to e x c e s s i v e f o a m i n g . At F o n t e n a y - a u x - R o s e s , Grenob le and Cadarash in F r a n c e the pH value is adjusted to about 6 [4].

2 . 5 . 2 . F i l t ra t i on

Separation of suspended so l id m a t e r i a l s b e f o r e evaporat ion is not a lways r e q u i r e d except when a preheater i s used, but it is c o m m o n prac t i c e in m o s t evaporat i on plants . Comple te r e m o v a l of suspended so l ids is not n e c e s s a r y and is s o m e t i m e s i m p o s s i b l e s ince the so l ids in many was tes a r e c o l l o i d a l in nature . C o a r s e sand f i l t e r s , s t ra iners with c h e e s e -cloth o r other m a t e r i a l as the f i l ter m e d i u m , p r e - c o a t type f i l t e r s , vacuum f i l t e r s and p r e s s u r e f i l t e r s are used.

2. 5. 3. Treatment by act ivated c h a r c o a l

At Harwe l l act ivated c h a r c o a l has been used as a pretreatment to r e m o v e f o a m i n g m a t e r i a l s . P r e v i o u s work by the Industrial Chemis t ry Department in another connect ion has shown that act ivated c h a r c o a l t reatment is e f f e c t i v e with solut ions containing up to 3000 m g / 1 of d e -

28

tergent . F o a m i n g has thus been e l iminated completely 1 by pass ing the waste e v a p o r a t o r f e e d , at approx imate ly pH 4, through a co lumn of activated c h a r c o a l [6],

In s o m e l a b o r a t o r i e s , f o r instance at R i s^ in Denmark , d e - i o n i z e d water i s fed to al l a r e a s which drain to the rad ioac t ive waste s y s t e m to e l iminate t h e ' n e c e s s i t y f o r e laborate pre t reatment of the evaporator f e e d .

F e e d act iv i ty can be c ont ro l l ed by blending l o w e r activity wastes so that the radiat ion l e v e l of the concentrate wi l l not e x c e e d s o m e definite value.

2. 5 . 4 . Combinat ion with other p r o c e s s e s

T h e r e a r e three main p r o c e s s e s f o r treat ing rad ioac t ive liquid was tes : c h e m i c a l prec ip i ta t i on , ion exchange and evaporat ion . In many situations a combinat ion of methods is advantageous. C h e m i c a l c o n -ditioning of an e v a p o r a t o r f eed can mater ia l l y r e d u c e t roub les caused by f oaming , c o r r o s i o n and sca l ing . F o r instance , at Harwel l , the supernatant l iquor f r o m the c h e m i c a l t reatment plants f o r m e d i u m - and h igh -ac t iv i ty w a s t e s is fur ther decontaminated by evaporat ion [57]. S imi lar c o m b i n a t i o n s a r e a l s o in p r a c t i c e in many other l a b o r a t o r i e s . R a d i o a c t i v e waste f r o m the r e g e n e r a t i o n of i on -exchange r e s in is a l so concentrated by evaporat i on . The m e r i t s of c ombined methods of liquid waste t reatment a r e ful ly explained in the IAEA T e c h n i c a l Repor t s Ser ies No . 27, " T e c h n o l o g y of Rad ioac t i ve Waste Management Avoiding E n v i r o n -menta l D i s p o s a l " .

3. OPERATIONAL PROCEDURES

3 . 1 . P R O C E D U R E S F O R VARIOUS E V A P O R A T O R T Y P E S

3 . 1 . 1 . E v a p o r a t o r s with s u b m e r g e d heating s u r f a c e s

The e v a p o r a t o r i s fed by grav i ty o r by pump f r o m a f eed - tank . When a p r e h e a t e r is used , the feed p a s s e s through it b e f o r e entering-the st i l lpot . The f l o w rate is m e a s u r e d by an instrument such as an o r i f i c e m e t e r o r r o t a m e t e r and c o n t r o l l e d by a manual ly o r automat ica l ly operated va lve . The evaporat i on rate i s adjusted by m e a n s of a s team va lve contro l led e i ther manual ly o r automat i ca l l y . Liquid l eve l in the st i l lpot is contro l led by an instrument which actuates : (1) a f e e d - c o n t r o l va lve , while the s team supply i s kept constant , (2) a s t e a m - c o n t r o l valve , whi le the feed rate is kept constant , o r (3) v a l v e s f o r contro l l ing both feed and s team supply as i l lustrated in F i g . 14(b) . The latter c o n t r o l s y s t e m p r o v i d e s a safety m e a s -u r e should e i ther feed o r s team supply fa i l .

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A m e c h a n i c a l t h e r m o c o m p r e s s i o n e v a p o r a t o r r e q u i r e s s tar t -up steam and this i s furnished f r o m outs ide s o u r c e s in the s a m e manner as f o r a s i m p l e e v a p o r a t o r . T h e r m o c o m p r e s s i o n evapora to r s , either the m e c h a n i -ca l c o m p r e s s i o n type o r s t e a m - i n j e c t o r c o m p r e s s i o n type, usually requ ire m o r e heat than is ava i lab le f r o m the c o m p r e s s e d vapour . The remaining heat needed to maintain e v a p o r a t o r operat i on must be obtained f r o m out -s ide s o u r c e s , and this s t eam supply is regulated by a s team f low contro l v a l v e . V a r i o u s evaporat ion p r o c e d u r e s can be adopted, i . e . continuous, s e m i - b a t c h w i s e o r batchwise , depending on the amount of feed liquid and s i z e of the e v a p o r a t o r . D is t i l la te i s condensed , c o l l e c t e d in a holding tank, m o n i t o r e d and d i s c h a r g e d to the environment when its act ivity is be low the p e r m i s s i b l e l e v e l . If the act iv i ty l e v e l i s h igher than the p e r m i s s i b l e leve l the contents of the holding tank a r e usual ly pumped back into the feed tank and the t reatment r e p e a t e d . . F o a m i n g can be contro l led to s o m e extent by adding a n t i - f o a m agent to the feed tank ( c f . Sect ion 3 . 2 . 2 ) .

Th ick l iquor in the st i l lpot i s concentrated until an opt imum c o n c e n -trat ion is a c h i e v e d . The op t imum concentrat ion i s determined either by so l id content o r act iv i ty l e v e l . The m a x i m u m concentrat ion which can be ach ieved depends on the p r o p e r t i e s of d i s s o l v e d so l ids in the feed . In many evaporat ion plants the b o t t o m s (thick l iquor) f r o m the evaporator are fur ther treated in a c o n c e n t r a t o r which i s substantial ly a kind of evaporator . At R i s0 the bo t toms containing about 10% so l ids a r e next fed to an e l e c -t r i c a l l y heated e v a p o r a t o r which p r o d u c e s a s ludge containing 1 kg of s o l i d s p e r l i t re [56] . At Japan A t o m i c E n e r g y R e s e a r c h Institute l o w -l e v e l was tes containing about 0 . 7 to 1 g / 1 so l ids a r e concentrated to about 75 to 100 g / 1 in the p r i m a r y e v a p o r a t o r , which i s a s tandard-type , and the concentra te is further evaporated in a s t eam-heated kett le . The evaporator i s operated s e m i - b a t c h w i s e , and the total so l ids content of the f inal c o n -centrate r e a c h e s about 200 to 300 g / 1 . At Brookhaven National Laboratory m e d i u m - l e v e l wastes a r e concentrated in a s i n g l e - s t a g e v a p o u r -c o m p r e s s i o n e v a p o r a t o r up to a total s o l i d s content of about 60%. M o r e highly concentrated so lut ions might plug up the blowdown line [19]. The total so l ids content of the c o n c e n t r a t e s in the sti l lpot can be determined by B P R m e a s u r e m e n t s .

The m a x i m u m leve l of radioact iv i ty at the bottom of the stil lpot is g o v e r n e d by the kinds of rad ionuc l ides contained. At BNL m e d i u m - l e v e l e v a p o r a t o r , the act iv i ty at the bot tom reached about 150 m R / h . At this act iv i ty l e v e l in the st i l lpot , the act iv i ty l eve l at the s u r f a c e of the c o n c e n -t ra te d r u m s was about 200 m R / h [19] . In continuous evaporat ion, s p e c i f i c g rav i ty of the thick l iquor is c on t ro l l ed by means of a d i s c h a r g e contro l va lve as i l lustrated in F i g . 14(b) . Mos t e v a p o r a t o r s a r e operated at a t -m o s p h e r i c p r e s s u r e but a f ew e v a p o r a t o r s , f o r instance at the Nuclear R e s e a r c h Institute, R e z , C z e c h o s l o v a k i a , a r e operated under reduced p r e s s u r e . P r e s s u r e c o n t r o l in a vacuum evaporator i s usually obtained by regulat ing the venting of the c o n d e n s e r .

3 . 1 . 2 . F i l m - e v a p o r a t o r s

Start -up opera t i on of f i l m - e v a p o r a t o r s is a l m o s t the s a m e a's f o r e v a p o r a t o r s with s u b m e r g e d heating tubes . A s explained in Sect ion 2 . 1 . 7 ,

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the v o l u m e reduc t i on f r o m a r i s i n g - o r f a l l i n g - f i l m evaporator in a s ingle p a s s i s l imi ted to a low value; r e c i r c u l a t i o n of the concentrate b e c o m e s n e c e s s a r y when high v o l u m e reduct i on is r e q u i r e d . During re c i r cu la t i on c a r e must be taken to prevent the concentra te f l ow f r o m returning to the f eed tank through the f eed l ine . In the hor i zonta l - type w i p e d - f i l m e v a p o -r a t o r h igher v o l u m e reduc t i on in a s ing le pass can be expected s ince the r e s i d e n c e t i m e of the c o n c e n t r a t e can be regulated o v e r a wide range . Since the holdup v o l u m e in f i l m e v a p o r a t o r s i s smal l , both the liquid f low rate and s t eam supply must be w e l l regulated . E x p e r i e n c e at the Chalk R i v e r Nuclear . L a b o r a t o r i e s [31] has shown that the c o n t r o l s y s t e m , i . e . vapour f l ow used to c o n t r o l s t e a m supply, i s s a t i s f a c t o r y although with s o m e so lut ions there a p p e a r s to be a l imit to the rate of evaporat ion above which c o n t r o l is i m p o s s i b l e . This instabi l i ty has been shown to be due to f o a m i n g of the f eed so lut ion in the tubes and is probably magni f ied by the v e r y s m a l l l iquid holdup v o l u m e in the tubes . T r o u b l e s due to foaming usual ly i n c r e a s e with i n c r e a s e of s o l i d s content in the concentra te . F i lm e v a p o r a t o r s a r e g e n e r a l l y unsuited to salting o r sca l ing l iquids . Murray [31] has pointed out that the m o s t s e r i o u s cause of f requent shut-down is the a c cumula t i on of an inso lub le crud in the s y s t e m . It is mainly s i l i c eous and c o l l e c t s f i r s t on the e v a p o r a t o r tubes . This resu l t s in a d e c r e a s e in heat t r a n s f e r e f f i c i e n c y with a c o r r e s p o n d i n g i n c r e a s e in s t eam p r e s s u r e and an i n c r e a s e d tendency to an unstable operat ion . In addition, the crud f l a k e s of f the tubes and in t i m e wi l l plug one o r all of the drain l ines .

3 . 2 . O P E R A T I O N A L P R O B L E M S

3 . 2 . 1 . Entra inment

The o v e r a l l decontaminat ion f a c t o r (DF) is a function of entrainment except w h e r e vo la t i l e r a d i o a c t i v e m a t e r i a l s a r e invo lved . B e c a u s e the DF is l imi ted by c a r r y - o v e r of rad i oac t i ve contaminants into the c o n d e n -sate by entrainment , it i s n e c e s s a r y to m i n i m i z e liquid entrainment as f a r as p o s s i b l e and to r e m o v e e f f e c t i v e l y the entrained liquid drops which a r e inevitably f o r m e d .

3 . 2 . 1 . 1 . M e c h a n i s m of d rop le t f o r m a t i o n

Liquid entra inment , s m a l l l iquid d r o p s c a r r i e d o v e r by the vapour s t r e a m , a r i s e s in the burs t ing of s t e a m bubbles . Broad ly , there are two s o u r c e s of d r o p f o r m a t i o n : one d e r i v e d f r o m a dis integrat ing dome of l iquid as a bubble b u r s t s and c o n s i s t i n g of c louds of drops a f ew m i c r o n s in d i a m e t e r ; the o ther c ons i s t ing of c o m p a r a t i v e l y f e w la rge drops der ived f r o m the b