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FREEZE CRYSTALLIZATION PROCESSES:EFFICIENCY BY

James A. H e i s t , P.E.

H e i s t E n g i n e e r i n g Corp.Wi lming ton , NC

ABSTRACT

Energy consumpt ion i n f r a c t i o n a t i n gs o l u t i o n s by d i s t i l l a t i o n and e v a p o r a t i o ncan be r educed by 70 t o 90 by us ingf r e e z e c r y s t a l l i z a t i o n p r o c e s s e s . Thethermodynamic b a s e s f o r t h e s u b s t a n t i a l l yl ow e r ene rgy r e q u i r e m e n t s i n c l u d e :

1) The phase c hange i s r e q u i r e d o n l yonce i n f r e e z e p r o c e s s e s , a s opposedt o t h e h igh r e f l u x r a t i o s needed i nmost d i s t i l l a t i o n s e p a r a t i o n s .

2) The l a t e n t h e a t o f f u s i o n i s l e s st h a n t h e h e a t o f v a p o r i z a t i o n , andt h e p r o c e s s o p e r a t e s a t a lowert e m p e r a t u r e , so t h e e n t r o p y o f t h es e p a r a t i o n i s l e s s .

3) A h e a t pump i s used i n t h e f r e e z ec r y s t a l l i z a t i o n p r o c e s s t o t r a n s f e rh e a t from t h e f r e e z e r t o t h e m e l t e r ,p r o v i d i n g a d v a n t a g e s s i m i l a r t o t h evapor compres s ion e v a p o r a t o r c y c l e .

Freeze c r y s t a l l i z a t i o n shou ld bec o n s i d e r e d an a l t e r n a t i v e t o a l ld i s t i l l a t i o n and e v a p o r a t i o n s e p a r a t i o np r o c e s s e s . The f a c t o r s which a f f e c t t h er e l a t i v e e f f i c i e n c y and economics o f t h ep r o c e s s a r e d i s c u s s e d i n t h i s p a p e r .

INTRODUCTION

Freeze c r y s t a l l i z a t i o n has been u sedh i s t o r i c a l l y i n t h o s e a p p l i c a t i o n s whereo t h e r s e p a r a t i o n p r o c e s s e s a r e i n c a p a b l eo f e f f e c t i n g t h e s e p a r a t i o n ; i . e .p r i n c i p a l l y i n i somer s e p a r a t i o n s . Yett h e ene rgy co ns um p t ion w i t h t h e f r e e z ep r o c e s s i s s u b s t a n t i a l l y l e s s t h a n t h a t o fa l t e r n a t i v e e v a p o r a t i v e p r o c e s s e s , a sshown i n Tab le 1 .

G u i d e l i n e s ca n be f o r m u l a t e d f o rp r e l i m i n a r y e v a l u a t i o n s o f newa p p l i c a t i o n s , b u t none have been foundt h a t a r e f o o l p r o o f . T h e r e f o r e , anya n a l y s i s o f a new a p p l i c a t i o n shou ldi n c l u d e a p r e l i m i n a r y d e s i g n s u f f i c i e n t t oe s t i m a t e t h e ene rgy consumpt ion o f t h eproposed f r e e z e s e p a r a t i o n p r o c e s s .

E x i s t i n g co mme rc i a l f r e e z e c r y s t a l l i z a t i o nequ ipmen t i s based on i n d i r e c t h e a tt r a n s f e r - e i t h e r t h rough a s c r a p e ds u r f a c e exchange r o r w i t h a b u i l d - u p o fc r y s t a l l i z e d m a t e r i a l on t h e s u r f a c e ( i nb a t c h o p e r a t i o n s ) . N e i t h e r i s

FLEXIBILITY

Thomas S. Bar ron , P.E.

H e i s t E n g i n e e r i n g Corp .L a f a y e t t e , CA

p a r t i c u l a r l y e f f i c i e n t , e s p e c i a l l y whencompared t o o t h e r t y p e s o f f r e e z i n gmethods , and n e i t h e r s c a l e s w e l l t oa p p l i c a t i o n s w i t h very l a r g e h e a t l o a d s .The i n a b i l i t y t o s c a l e t o l a r g e s i z e s i st h e majo r f a c t o r t h a t h a s k e p t t h i sequ ipmen t from a p p l i c a t i o n s i n t h ec h e m i c a l and p e t r o l e u m i n d u s t r i e s .

TABLE 1ENERGY USE COMPARISON

Energy Consumpt ion

A p p l i c a t i o n C o n v e n t ' l F r e e z eE v a p / D i s t C r y s t ' n

Suga r P r o d u c t i o n 3850 550( f rom Bee t s w/cogen)BTU/# w a t e r removed

F r u i t J u i c e Conc. 365 85BTU/# w a t e r removed

Seawa te r D e s a l t i n g , 100 60( M u l t i s t a g e f l a s h )BTU/# w a t e r removed

Br ine C o n c e n t r a t i o n 120 80(vapor comp, f a l l f i l m )BTU/# w a t e r removed

Black Liquor 230 50( p r e - c o n c , 15 t o 25%)BTU/# w a t e r removed

A c e t i c Acid R e f i n i n g 8800 500( a c e t a l d e h y d e r o u t e )BTU/# a c i d produced

A c r y l i c Acid R e f i n i n g 1600 190( p r o p y l e n e o x i d a t i o n )BTU/# a c i d produced

BTX F r a c t i o n a t i o n 600 150(TeEG e x t r a c t i o n )BTU/# BTX produced

Note : a l l e l e c t r i c a l power c o n v e r t e d t oBTU a t 10 ,000 BTU/kw-hr.

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The f r e e z e c r y s t a l l i z a t i o n p r o c e s s has ag r e a t d e a l o f i n h e r e n t d e s i g n f l e x i b i l i t y .Thus , i t shou ld no t be t r e a t e d l i k ee v a p o r a t o r s , where a s t a n d a r d commerc i a lu n i t i s purchased and i n s t a l l e d w i t hminimal o p t i m i z a t i o n . R a t h e r , i t shou ldbe t hough t o f i n t e rms o f f r a c t i o n a ld i s t i l l a t i o n and e x t r a c t i o n t r a i n s , and

o p t i m i z e d f o r each a p p l i c a t i o n . Thef r e e z e p r o c e s s w i l l u l t i m a t e l y be a

b u i l d i n g b l o c k t echno logy, w i t h s t a n d a r dand cus tom components , as sembled i n t o acomple t e p r o c e s s t echno logy, op t imized f o re ac h g e n e r a l a p p l i c a t i o n a r e a . Uni to p e r a t i o n s t h a t make up t h e f r e e z ec r y s t a l l i z a t i o n p r o c e s s a r e d i s c u s s e dbelow.

FREEZE SEPARATION TECHNOLOGY

The U n i t O p e r a t i o n s

A g e n e r a l p r o c e s s schema t i c o f t h e f r e e z ec r y s t a l l i z a t i o n s e p a r a t i o n p r o c e s s i sshown i n F i g u r e 1 The major componentso f t h e p r o c e s s i n c l u d e :

- A F r e e z e r , o r c r y s t a l l i z e r , wherea p o r t i o n o f t h e l i q u i d i s changedt o t h e s o l i d phase by removing h e a t .

- A S e p a r a t o r where t h e c r y s t a lphase i s s e p a r a t e d from t h er ema in ing l i q u i d , and t h e c r y s t a l sa r e washed t o remove a d h e r i n g b r i n efrom t h e s u r f a c e . Leach ing o f t h ec r y s t a l pack can a l s o occur h e r e i ft h e r e i s a s o l i d s o l u t i o n i n t h ec r y s t a l s .

P R O O U C T ~ ~

CONCEN f

TRATE

CRYSTALSEPARATOR-

HASHER

C.H.

r-v-~ L ~

~ I N COMPRESSOR _ l.J

RECYCLE LIQUIDFREEZER

FEED HE T

EXCHANGERS

- - - J

R e f r i g e r a t i o n Cyc le t h a t a c t sl i k e a h e a t pump, removing h e a t fromt h e f r e e z e r and compres s ing i t t o as u f f i c i e n t p r e s s u r e t h a t i t w i l lcondense t o p r o v i d e t h e hea t neededt o mel t t h e c r y s t a l a f t e r i t hasbeen s e p a r a t e d . A Heat R e j e c t i o np o r t i o n o f t h i s c y c l e pumps t h e

ambien t h e a t l e a k i n g i n t o t h e sys t emand i n e f f i c i e n c i e s i n t h e p r o c e s sfrom t h e o p e r a t i n g t e m p e r a t u r e up t oambien t c o n d i t i o n s , where t h i s h e a ti s r e j e c t e d from t h e sys t em.

- Also i n c l u d e d i n t h e p r o c e s s a r eHea t Exchange r s t o r e c o v e r t h e

c o l d e n e rg y from t h e p r o c e s ss t r e a m s , f o r c o o l i n g th e f r e s h f eedm a t e r i a l .

A g e n e r a l o p e r a t i n g mode i s a s f o l l o w s .Feed e n t e r s t h e p r o c e s s t h rough h e a texchange r s where t h e c o l d i s r e c o v e r e dfrom t h e p r o d u c t and c o n c e n t r a t e s t r eamse x i t i n g t h e sys t em. This feed t h e n e n t e r st h e f r e e z e r o r c r y s t a l l i z e r where ap o r t i o n o f i t i s c o n v e r t e d t o a s o l i d , i nd i s c r e e t , r e l a t i v e l y s m a l l (100 t o 500microns ) c r y s t a l s . The c r y s t a l s a r epumped a s a s l u r r y from t h e f r e e z e r t o as e p a r a t o r which removes t h e l i q u i d fromt h e s l u r r y , and u s u a l l y washes t h ec r y s t a l s w i t h a s m a l l p o r t i o n o f e i t h e rmel t ed p r o d u c t o r some s u i t a b l e washm a t e r i a l t h a t w i l l no t d i s s o l v e t h ec r y s t a l . Most o f t h e l i q u i d i s r e c y c l e dt o t h e f r e e z e r where more o f t h e d e s i r e dc o n s t i t u e n t i s r e c o v e r e d . Exces sc o n c e n t r a t e i s b l e d from t h e sys t em,

PRODUCTSLURRY

FIGURE 1

FREEZE CRYSTRLLIZRTIONPROCESS DIRGRRM

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The cu rve between t h e e u t e c t i c p o i n t andt h e pure component m e l t t e m p e r a t u r e s i st h e s o l i d - l i q u i d e q u i l i b r i a l i n e f o rv a r i o u s c o m p o s i t i o n s . At a s t a r t i n gcompos i t i on X and a t t e m p e r a t u r e T i ) ,n o t h i n g ha p pe n s as t h e s o l u t i o n i s coo ledu n t i l t r e a c h e s t e m p e r a t u r e T C i ) , t h ei n i t i a l c r y s t a l l i z a t i o n t e m p e r a t u r e . Att h i s p o i n t p u r e Component I c r y s t a l s s t a r tt o form and t h e c o m p o s i t i o n o f t h er ema in ing l i q u i d t hen becomes r i c h e r w i t hComponent 2. As h e a t i s removed from t h es o l u t i o n more Component I c r y s t a l i sformed b u t t h e c r y s t a l l i z a t i o nt e m p e r a t u r e a l s o d e c r e a s e s . E v e n t u a l l yt h e c o m p o s i t i o n i s changed s u f f i c i e n t l yt h a t t h e r ema in ing b r i n e a c h i e v e s t h ee u t e c t i c compos i t i on and t hen Component 2b e g i n s t o c r y s t a l l i z e . F u r t h e r h e a tremoval produces s t o i c h i o m e t r i c q u a n t i t i e so f each c r y s t a l .

have t o c a l c u l a t e d from b i n a r y componentd a t a . We have found t h a t p o r t r a y i n g t h emore complex e q u i l i b r i a by t h e methodshown i n F i g u r e 4 i s a u s e f u lv i s u a l i z a t i o n o f w h a t happens i n t h e s ef r e e z e p r o c e s s e s . Here c r y s t a l l i z i n gt e m p e r a t u r e i s shown as a f u n c t i o n o f t h ep e r c e n t r ecove ry o f t h e f i r s t componentt h a t c r y s t a l l i z e s . E u t e c t i c p o i n t s a r eshown on t h e d iag ram and a s d e p i c t e d ,u s u a l l y r e s u l t i n a change i n t h e s l o p e o ft h e c u r v e .

TC

/

/ 1

III

907. 75 507.

RECOVERY

~ ~

IIII

T P 257. X

FIGURE

OPERATING SCHEMATIC

1007. 1 DESIGN VARIABLES7. 1 COMPOSITION1007. 2 07. 2

FIGURE 3

BINARY SLE PHASE DIAGRAM

The e u t e c t i c migh t r e p r e s e n t an uppe rl i m i t t o r e c o v e r y o f one component fromt h e m a t e r i a l . But u s u a l l y t h e s o l i dphases can be f u r t h e r s e p a r a t e d based ons i z e , d e n s i t y , o r o t h e r p r o p e r t i e s . Thust h e on ly l i m i t i n r e c o v e r y from t h ep r o c e s s i s imposed by o t h e r f a c t o r s , suchas a b u i l d up o f t r a c e m a t e r i a l s whichhave t o b l e d from t h e sys t em.

In a mUl t i - c omp onen t sys t em t h e phasee q u i l i b r i a d i ag ram i s n o t a s s i m p l e as t h eone shown i n F i g u r e 3 . T r i a n g u l a rd iag rams c an be used f o r t e r t i a r ys o l u t i o n s , b u t m o s t sys t ems canno t bep o r t r a y e d a c c u r a t e l y by even t h i s .Usua l ly phase e q u i l i b r i a i n r e a l sys t ems

A number o f o p t i o n s a r e a v a i l a b l e t o t h ep r o c e s s e n g i n e e r t o d e s i g n an e f f i c i e n tf r e e z e s e p a r a t i o n p r o c e s s . Much i s commonw i t h o t h e r e n g i n e e r i n g p r a c t i c e s , such asi n s u l a t i o n t h i c k n e s s and t y p e , p i p ep r e s s u r e d r o p s pump e f f i c i e n c i e s , andh e a t exchange r s u r f a c e a r e a . I n a d d i t i o n ,many o t h e r f a c t o r s a r e c o n s i d e r e d f o r t h ed e s i g n o f t h e f r e e z e p r o c e s s :

- I n t e g r a t i o n o f f r e e z i n g w i t h t h er e s t o f t h e p r o c e s s .

- Phase e q u i l i b r i a a l t e r a t i o n s .

- C r y s t a l l i z e r d e s i g n and s t a g i n g .

- The r e f r i g e r a t i o n c y c l e .

- M e l t e r d e s i g n .

Wash ing / l each ing p r o c e s s i n g .

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__

Here , a s i n v i r t u a l l y a l l a p p l i c a t i o n s ,P r o c e s s I n t e g r a t i o nt h e r e i s an a d v a n t a g e t o i n t e g r a t i n g t h e

EXTRRCTSTRIPPER

RRFFINRTE

BENZENE

• B25 BTU

BENZENE• BTX COLUMN

TOLUENECOLUMN

375-400· F

FIGURE 5

4 ~ 2 5 BTU

XYLENE

C9RROMRTICSBTX FRACTIONATION

Y DISTILLATION

The impor t ance o f s e l e c t i n g t h e r i g h t p a r to f t h e p r o c e s s i n which t o p l a c e t h ef r e e z e s e p a r a t i o n p r o c e s s i s i l l u s t r a t e dby BTX s e p a r a t i o n s . T h i s a p p l i c a t i o ni n v o l v e s t h e f r a c t i o n a t i o n o f t h e benzenet o l u e n e and xy lene found i n bo th p t r o u ~ f r a c t i o n s and i n t h e b y - p r o d u c t s t r e a mfrom o l e f i n s p l a n t s . BTX s can compr i sefrom 25 t o 75 o f t h e s t r e a m s from c ruded i s t i l l a t i o n s , from r e f o r m i n g , and fromo l e f i n p y r o l y s i s f u r n a c e s . The o t h e rcomponents a r e p r i n c i p a l l y p a r a f i n s andc y c l o p a r a f i n s , a s w e l l a s some h e a v i e ra r o m a t i c s .

The p a r a f i ~ s and a r o m a t i c s have very c l o s eb O l l l n g p O l n t s , so f r a c t i o n a l d i s t i l l a t i o ni s no t p r a c t i c a l . E x t r a c t i o n w i t ht e t r a m e t h y l e n e s u l f o n e o r t e t r a e t h y l e n eg l y c o l (TeEG) i s t h e u s u a l method used t os e p a r a t e t h e a r o m a t i c s from t h e p a r a f i n s .The a r o m a t i c s a r e t hen f r a c t i o n a t e d i n a

s e r i e s o f columns a s shown i n F i g u r e 5 .Inc luded i n F i g u r e 5 i s t h e ene rgyconsumed i n each o f t h e r e f i n i n g s t e p s .Note t h a t h a l f o f t h e e n e rg y i s used i ns e p a r a t i n g t h e a r o m a t i c s from t h ep a r a f f i n s . The o t h e r h a l f i s used t os e p a r a t e t h e a r o m a t i c s i n t o t h e benzene ,t o l u e n e , and (mixed) xy lene f r a c t i o n s .

f r e e z e s e p a r a t i o n p r o c e s s a s e a r l y i n t h emain p r o d u c t i o n p r o c e s s a s p o s s i b l e . Tha l t e r n a t i v e s t r e a m s f o r p r o c e s s i n g byf r e e z e c r y s t a l l i z a t i o n a r e shown i n F i g u r e6 . As i s shown, t h e r e i s r e l a t i v e l yl i t t l d i f f e r e n c e i n t h e ene rgy r e q u i r e d

f o r f r e e z e r e f i n i n g any o f t h e p r o c 8 s ss t r e a m s . However, t h e f u r t h e r back i n t ot h e c o n v e n t i o n a l r e f i n i n g p r o c e s s t h ef r e e z e p r o c e s s i s p l a c e d , t h e more ene rgyr e d u c t i o n i s e f f e c t e d by r e p l a c i n g t h ecomponents i n t h e c o n v e n t i o n a l r e f i n i n gt r a i n .

Phase E q u i l i b r i a

Another s e t o f d e s i g n v a r i b l e s t h a t can beused t o min imize e n e rg y consumpt ion i nf r e e z e s e p a r a t i o n p r o c e s s i n g a r e t h e s o l i dl i q u i d e q u i l i b r i a t h a t gove rn t h et e m p e r a t u r e s and c o n c e n t r a t i o n s a t whicht h e v a r i o u s s o l i d c r y s t a l ) p h a s e s form.The t e m p e r a t u r e and c o n c e n t r a t i o n a t whicha p r o d u c t b e g i n s t o c r y s t a l l i z e froms o l u t i o n can be a l t e r e d by e i t h e r chang ingt h e s o l u t i o n i t i s i n e g . , t h r o u g h ap r e l i m i n a r y s o l v e n t e x t r a c t i o n s t e p ) , o rby u s i n g a r e f r i g e r a n t o r o t h e r m a t e r i a lwhich a l t e r s t h e p h a s e e q u i l i b r i a o f t h eo r i g i n a l s o l u t i o n .

PRRRFFINS

~ l

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CRUDE BTX [25 WTYDE-PARIFFINED

@ EXTRACT [60 WTYTETRAETHYLENE GLYCOL)EUTECTIC POINT

230

240

220

210 0-xWt

200

190

180

(CONVENTIONAL REFINING)

•

•

•FIGURE 68TX FREEZE REFINING

The f i r s t o f t h e s e i s i l l u s t r a t e d i nF i g u r e 7 by t h e same BTX a p p l i c a t i o nd i s c u s s e d above. As t h e v a r i o u s BTXs o l u t i o n s a r e coo led , benzene i s t h e f i r s tcomponent t o r each i t s s o l u b i l i t y l i m i tand b e g i n t o c r y s t a l l i z e . X y l e n e monomersand t hen t o l u e n e fo l l ow i n t h ed e - p a r a f f i n e d BTX s o l u t i o n whi le some o ft h e p a r a f f i n s may b e g i n t o c r y s t a l l i z eb e f o r e t h e t o l u e n e i n t h e c rude BTX. Thei n i t i a l c r y s t a l l i z i n g t e m p e r a t u r e i s af u n c t i o n o f t h e o t h e r components i n t h es o l u t i o n . The p a r a f f i n s d e p r e s s t h ei n i t i a l c r y s t a l l i z i n g t e m p e r a t u r e bu t t h ee x t r a c t a n t (TeEG i n t h e example shown)a c t u a l l y i n c r e a s e s t h e c r y s t a l l i z i n gt e m p e r a t u r e even t hough t h e f r a c t i o n o fben ze ne i n t he s o l u t i o n i s s u b s t a n t i a l l yl ower. Note t h a t t h e TeEG a l s o d e c r e a s e st h e s o l u b i l i t y o f t h e xy lenes i n t h es o l u t i o n . Thus , f o r a g i v e n o p e r a t i n gt e m p e r a t u r e t h e e f f e c t o f a s u i t a b l ephase a l t e r a t i o n m a t e r i a l i s t o i n c r e a s et h e amount o f produc t r ecove red andd e c r e a s e t h e ene rgy consumpt ion .

The power r e q u i r e m e n t s shown i n F i g u r e 6a r e f o r r ecove ry t o an o p r t i n ~

t e m p e r a t u r e o f 200 degrees Ke lv in . Thed i f f e r e n c e i n ene rgy consumpt ion betweenF i g u r e 6 and Tab le 1 r e f l e c t s f u l lr ecove ry o f t he BTX s . Wi th t h e TeEGa b s o r b e n t s t u d i e d h e r e t h e a b s o r b e n tc r y s t a l l i z e s and i n c r e a s e s t h e ene rgyr e q u i r e d a l t h o u g h t h e consumpt ion i ss t l l l e s s t han f o r c o n v e n t i o n a lf r a c t i o n a l d i s t i l l a t i o n .

R e f r i g e r a n t s can a l s o be s e l e c t e d t o a l t e rt h e f r e e z i n g t e m p e r a t u r e o f a s o l u t i o n .In o r g a n i c s o l u t i o n s t h e r e f r i g e r a n t i schosen t o i n c r e a s e t h e a c t i v i t yc o e f f i c i e n t o f t h e c r y s t a l l i z i n g m a t e r i a lwhich i n t u r n i n c r e a s e s t s r e l a t i v ec r y s t a l l i z i n g t e m p e r a t u r e . I n w a t e rs o l u t i o n s t h e i c e c r y s t a l l i z i n gt e m p e r a t u r e can be r a i s e d by a s much as 30d e g r e e s C e n t i g r a d e by r e f r i g e r a n t s t h a tform h y d r a t e s w i t h t h e w a t e r . Thish y d r a t e i s a c r y s t a l l i n e s t r u c t u r e c a l l e da c l a t h r a t e and i s formed o f wate r andt h e r e f r i g e r a n t .

.B . 6 .4 .2FRACTION BTX RECOVERYI f

1 .8 .6 .4 .2FRACTION BENZENE RECOVERY

FIGURE 78TX SOLID-LIQUID EQUILIBRIA

DIf\.GRAM

The h y d r a t e phase d i ag ram i s shown i nF i g u r e 8 . The s a l i e n t f e a t u r e s o f t h ed iag ram a r e :

- A c r i t i c a l t e m p e r a t u r e Tc, abovewhich t h e h y d r a t e c r y s t a l w i l l mel t ,r e g a r d l e s s o f p r e s s u r e .

- A c r i t i c a l p r e s s u r e Pc, whichcor re sponds t o t h e maximumt e m p e r a t u r e w i t h h y d r a t e c r y s t a lf o r m a t i o n . Below t h i s c r i t i c a lp r e s s u r e t h e maximum t e m p e r a t u r e a twhich h y d r a t e c r y s t a l s formd e c r e a s e s c o r r e s p o n d i n g t o l i n e AB.

- A normal f u s i o n t e m p e r a t u r e T f ,where w a t e r n o r m a l l y forms i c ec r y s t a l s .

842

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WIXJ

(I)(I)WIXn

HYDRATE

VAPOR REFVIIPOR REF + HIITER+HYDRIITE D

AGENT/

/c // I / LIQUID REF

+HATERLIQUID REF / 1 /+HYDRATE / J

/ S ' /S - / IS

IQUID IREF +ICE I

Tf TcTEMPERATURE

FIGURE 8HYDRATE PHASE

EQUILIBRIA DIAGRAM

- F r e e z i n g p o i n t d e p r e s s i o n l i n e s ,A 'B ' and A' ' 8 , which show t h ee f f e c t s o f i m p u r i t y l e v e l s on t h ec r y s t a l l i z i n g t e m p e r a t u r e o f bo tht h e h y d r a t e and i c e .

- The vapor p r e s s u r e l i n e CAD, o f t h eh y d r a t i n g a g e n t , which a l s o i s t h er e f r i g e r a n t i n t h e f r e e z e p r o c e s s .

R e f r i g e r a n t s e l e c t i o n t o i n c r e a s e t h ec r y s t a l l i z i n g t e m p e r a t u r e o f t h ecomponents i n t h e s o l u t i o n has two p r i m a r ya d v a n t a g e s :

1 ) There i s l e s s d i f f e r e n c e betweent h e c r y s t a l fo rma t ion t e m p e r a t u r e( i n t h e impure s o l u t i o n ) and t hem e l t i n g a n d / o r h e a t r e j e c t i o nt e m p e r a t u r e s .

2) The 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 ei n c r e a s e s c r y s t a l g rowth r a t e s andd e c r e a s e s t h e v i s c o s i t y o f t h es o l u t i o n .

The e f f e c t o f t h e lower t e m p e r a t u r e

d i f f e r e n c e i s t o d e c r e a s e o v e r a l l ene rgycon sum pt ion by up t o 10%. The advan tageo f t h e l owe r v i s c o s i t y i s i n g r e a t e rp r o d u c t i o n i n t h e s e p a r a t o r s , w i t h l e s sp r o d u c t l o s s , which improves t h e economicso f t h e p r o c e s s .

In some a p p l i c a t i o n s t h e v i s c o s i t yach ieved a t low t e m p e r a t u r e s and h ighc o n c e n t r a t i o n can a c t u a l l y l i m i t u l t i m a t ec o n c e n t r a t i o n by f r e e z i n g . This i se s p e c i a l l y p r e v a l e n t i n t h e food i n d u s t r y .

Since m os t f oo d i n d u s t r y s e p a r a t i o nr e q u i r e m e n t s a r e f o r w a t e r r emova l , whichi s now u s u a l l y done i n m u l t i p l e e f f e c te v a p o r a t o r s (MEE's) , t h i s i s an i d e a l a r e ao f a p p l i c a t i o n f o r h y d r a t e f r e e z es e p a r a t i o n p r o c e s s e s .

An example o f h y d r a t e use i s suga rr e f i n i n g . I n t h e p r o d u c t i o n o f s u g a r f romcane and b e e t s , a t h i n j u i c e i s producedwhich i s c o n c e n t r a t e d , p u r i f i e d , and t h e nc r y s t a l l i z e d . F r e e z i n g and r e v e r s eosmosis (RO) a r e c u r r e n t l y b e i n gi n v e s t i g a t e d f o r w a t e r r emova l . To d a t ei t has been found t h a t RO can o n l yc o n c e n t r a t e t o 30% s u g a r , and n o n - h y d r a t ef r e e z e p r o c e s s e s h a v e b e e n l i m i t e d t oabou t 45% suga r b e f o r e t h e v i s c o s i t y i st o o h i g h .

A h y d r a t e p r o c e s s t h a t i n c r e a s e s t h ec r y s t a l l i z i n g t e m p e r a t u r e by 30 d e g r e e s F'w i l l a l l o w an i n c r e a s e i n suga rc o n c e n t r a t i o n t o o v e r 60% a t t h e same

v i s c o s i t y . This c o n d i t i o n w i l l be v e r ynea r t o t h e e u t e c t i c c r y s t a l l i z i n gt e m p e r a t u r e , w h e r e suga r w i l l a l s oc r y s t a l l i z e from t h e s o l u t i o n a s h y d r a t ec r y s t a l s a r e formed. The e n e rg y r e u t i o ~ shown i n Tab le 1 p re sumes e i t h e r RO o rf r e e z e p r e - c o n c e n t r a t i o n , f o l l o w e d byh y d r a t e f r e e z e c r y s t a l l i z a t i o n .

S t a g i n g

Through t h e use o f p r o c e s s s t a g i n g , e n e r g ~ consumpt ion can be r educed s i g n i f i c a n t l y .I n l a r g e f a c i l i t i e s , where m u l t i p l eequ ipmen t t r a i n s would b e used f o rf l e x i b i l i t y and r e l i a b i l i t y , t h e c o s tp e n a l t i e s a r e min ima l .

A good example o f t h e e f f e c t o f s t a g i n g one n e rg y consumpt ion i s p r o v i d e d i n t h e c a s eo f a c r y l i c a c i d m a nu fa ct ur e. T hiscommodi ty chemica l i s produced i n p l a n t so f t y p i c a l l y o v e r 100 m i l l i o n pound p e ry e a r c a p a c i t y . The r e a c t i o n p r o d u c t s a r ea b s o r b e d i n t o w a t e r , p r o d u c i n g a 20%s o l u t i o n . In c o n v e n t i o n a l r e f i n i n g t h ea c r y l i c a c i d i s e x t r a c t e d from t h e w a t e rand t h e n r e f i n e d .

The a c r y l i c a c i d f r e e z e r e f i n i n g p r o c e s si s gove rned by t h e s o l i d - l i q u i d e q u i l i b r i ao f F i g u r e 9 . I n a f r e e z e r e f i n i n g p r o c e s st h e w a t e r s o l u t i o n would be c o n c e n t r a t e dt o t h e e u t e c t i c (42% a c i d a t - 19 d e g r e e sF ) , and t h e n a c r y l i c a c i d and w a t e r wouldbe c r y s t a l l i z e d t o g e t h e r . Freeze r e f i n i n ghas an a d d i t i o n a l advan tage o f an improvedy i e l d , s i n c e t h e d imer f o r m a t i o n o f t h ep r e s e n t vacuum d i s t i l l a t i o n r e f i n i n gm e t ho d s w o ul d no t o c c u r . Ta b l e 2 showst h e e f f e c t o f s t a g i n g on e n e rg yconsumpt ion , w i t h an e q u a l f r e e z i n g p o i n tl ower ing i n each s t a g e .

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20

15

10

5

w 0

-5

-10

-15

-20 20 30 40 50 SO 70 80

W /. ~ T R

FIGURE 9

ACRYLIC ACID - W TER SLE

TABLE 2STAGING EFFECTS ON ENERGY CONSUMPTION

D e s i g nC o n d i t i o n

E n e rg ykW

R e d u c t i o n%

1 S t a g e 1100

2 S t a g e 770 30

3 S t a g e 670 40

B a s i s : 1 0 0 M i l l i o n pound p e r y e a r20 A c r y l i c Acid i n w a t e r

C y s t a l l i z e r / R e f r i g e r a t i o n Cyc le

The c r y s t a l l i z e r and t h e r e f r i g e r a t i o nc y c l e a r e o f t e n d i c t a t e d by t h e p r o c e s sn e c d s o f t h e a p p l i c a t i o n , r a t h e r t h a n b ye n e rg y c o n s i d e r a t i o n s . St i l l t h e y h a v ea n impac t on e n e r g y e f f i c i e n c y o f t h ep r o c e s s .

The p a i r i n g o f c r y s t a l l i z e r d e s i g n andr e f r i g e r a t i o n c y c l e a r e d e t e r m i n e d b y t h emethod by w h i c h h e a t i s removed from t h ep r o c e s s s O l u t i o n . The re a r e e s s e n t i a l l ytwo ways t o r emove h e a t :

1 ) I n d i r e c t l y , t h r o u g h a h e a t t r a n s f e rs u r f a c e .

2) D i r e c t l y , by b o i l i n g a r e f r i g e r a n tt h a t i s i n d i r e c t c o n t a c t w i t h t h ep r o c e s s s o l u t i o n .

I n d i r e c t h e a t t r a n s f e r a l l o w s t h e c h o i c eo f t h e o p t i m a l r e f r i g e r a n t f o r t h et e m p e r a t u r e r a n g e b e i n g c o v e r e d a s w e l la s p r o v i d i n g o p t i m a l c o n d i t i o n s f o r t h ec o m p r e s s o r. The predominan t f a c t o r i s ,h o w e v e r t h e i n c r e a s e d t e m p e r a t u r ed i f f e r e n c e n e e d e d f o r t r a n s f e r o f h e a tt h r o u g h t h e e x c h a n g e r . Wi t h e x p e n s i v e ,s c r a p e d s u r f a c e e x c h a n g e r s , t h e economica lc h o i c e i s f o r v e r y h i g h t e m p e r a t u r ed i f f e r e n c e s ( o f t e n o v e r 30 d e g r e e s F ) ,

which can doub le t h e e n e r g y consumpt ion o ft h e f r e e z e p r o c e s s .

I n a d i r e c t c o n t a c t p r o c e s s e s , t h er e f r i g e r a n t e i t h e r c a n b e an i m m i s c i b l es e c o n d p h a s e t h a t i s t h o r o u g h l y mixed w i t ht h e p r o c e s s f l u i d , o r i t c a n b e o n e o f t h ec o n s t i t u e n t s i n t h e p r o c e s s f l u i d i t s e l f .Wa t e r, f o r i n s t a n c e , h a s b e e n u s e d i n t h em o s t s u c c e s s f u l d e s a l t i n g p i l o t p l a n t s i nt h e p a s t , a c t i n g a t i t s t r i p l e p o i n t .Here w a t e r i s b o i l e d a t a b o u t 3 . 5 Hga b s o l u t e p r e s s u r e . The h e a t removedp r o d u c e s i c e a t a r a t i o o f a b o u t 7 poundso f i c e formed p e r pound o f w a t e r b o i l e d .

I n a d i r e c t c o n t a c t p r o c e s s , t h e v a p o r sa r e e i t h e r :

- C o m p r e s se d b y m e c h a n i c a lc o m p r e s s i o n ;

- C o m p r e s s e d by t h e r m a l compres s ion ;o r

- A b s o r b e d i n t o a s u i t a b l e m a t e r i a l ,which i s t h e n r e g e n e r a t e d .

The method i s s e l e c t e d b a s e d on t h ec o n d i t i o n s o f t h e r e f r i g e r a n t . Fori n s t a n c e , t h e t h e r m a l c o m p r e s s i o n a n da b s o r p t i o n p r o c e s s e s a r e u s u a l l y u s e d o n l yf o r t h o s e r e f r i g e r a n t s t h a t a c t a t low

p a r t i a l p r e s s u r e s .

M4

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LO QUALITYSTEAM03 F 130 F

VAPOR

MELTERCONDEN-

SERP =S 00 P =4 80mm Hg mm Hg

T=32 F

CONDENSATE

T=130 F

12S F00 F

P=3 .4Bmm Hg

ABSORBER

REFRIGERANT

p=3.SSmm HgT=26 F

T=90 F - - J - - - \ - - o

T=BO F - - - l - - +COOLINGHATER

PLATEHEAT

EXCHANGER

NOTE: T & P CONDITIONS ARETYPICAL FOR AQUEOUSSYSTEM.

FIGURE 10ABSORPTION REFRIGERATION CYCLE

An a b s o r p t i o n r e f r i g e r a t i o n c y c l e cano p e r a t e w i t h e n e rg y s u p p l i e d from a was t eh e a t s o u r c e . The c y c l e i s shown i n F i g u r e1 0 In t h e a b s o r b e r , t h e vapo r i scon dense d i n t h e a b s o r b e n t and t h ec o n d e n s i n g l a t e n t h e a t l o a d i s removed byc o o l i n g w a t e r . I n t h e g e n e r a t o r . t h ea b s o r b e n t t h a t has p r e v i o u s l y abso rbed t h ev a p o r i s h e a t e d . and t h a t abso rbed vapo r

i s e v a p o r a t e d b a c k o u t o f t h e s o l u t i o n .Tha t r e g e n e r a t e d vapor t h e n must becond en sed o n t o e i t h e r m e l t i n g c r y s t a l s o ri n t o a h e a t sump s u c h a s c o o l i n g w a t e r .

The t e m p e r a t u r e d i f f e r e n c e r e q u i r e dbetween t h e was te h e a t and t h e c o o l i n gw a t e r i s a f u n c t i o n o f t h e p r o c e s s . b u tcan b e as l i t t l a s 20 d e g r e e s F i n w a t e rsys t ems

Mel t i ng

The f i n a l d e s i g n v a r i a b l e t h a t c a nm a t e r i a l l y a f f e c t ene rgy consumpt ion i n af r e e z e p r o c e s s i s t h e method o f m e l t i n gt h e c r y s t a l l i z e d p r o d u c t . M e l t i n g i s donet o r e o v e r t h e c o l d . o r t h e e n e rg y u s e di n c r y s t a l l i z i n g t h e m a t e r i a l . I n a h e a tpump c y c l e . as i s u s e d i n f r e e z i n gp r o c e s s e s . t h e ene rgy u s e d i n t h ecom pre s so r i s a f u n c t i o n o f t h et e m p e r a t u r e d i f f e r e n c e be tween t h ee v a p o r a t o r and c o n d e n s e r . o r f r e e z e r andm e l t e r .

M e l t i n g can be done e i t h e r by i n d i r e c th e a t i n g i n a h e a t e x c h a n g e r ( u s u a l l y s h e l land t u b e u s i n g a s l u r r y o f t h e c r y s t a l andm e l t e d p r o d u c t ) . o r by d i r e c t c o n d e n s a t i o no f r e f r i g e r a n t v a p o r s o n t o t h e c r y s t a l s .The method c h o i c e i s u s u a l l y d i c t a t e d byt h e r e f r i g e r a n t c y c l e a n d / o r t h ea p p l i c a t i o n .

D i r e c t m e l t i n g c a n be done w i t h l e s s t h a na I d e g r e e F d r i v i n g f o r c e and h e a t f l u x e so f o v e r 5 0 0 0 0 BTU p e r s q u a r e f o o t p e rhour p e r d e g r e e F I n d i r e c t m e l t i n gr e q u i r e s u s u a l l y a t l e a s t a 5 deg ree Fd r i v i n g f o r c e . w i t h t h e h e a t f l u x e sd e t e r m i n e d from c o n v e n t i o n a l h e a t t r a n s f e rc o e f f i c i e n t c a l c u l a t io n s .

CONCLUSIONS

F r e e z e c r y s t a l l i z a t i o n has many i n h e r e n td e s i g n v a r i a b l e s f o r o p t i m i z i n g e n e rg yconsumpt ion f o r an a p p l i c a t i o n . Moste q u i p m e n t o p t i o n s can b e s c a l e d up t ol a r g e s i z e s w i t h v e r y f a v o r a b l ee c o n o m i e s - o f - s c a l e . The thermodynamica d v a n t a g e s o f t h e p r o c e s s f o r s e p a r a t i o na p p l i c a t i o n s . a s compared t o o t h e r phasec h a n g e o p t i o n s , i n d i c a t e t h a t f r e e z ec r y s t a l l i z a t i o n s h o u l d be c o n s i d e r e d f o r .and u l t i m a t e l y used i n many commerc i a la p p l i c a t i o n s .

ESL-IE-83-04-129

P di f th Fifth I d t i l E T h l C f V l II H t TX A il 17 20 1983