DRAFT-TUBE-BAFFLE
CRYSTALLIZERS
A Study of Stationary and
Dynamicly-Behaving Crystal-Size-Distributions
by
O 00 ¥* Ch O CD O O •O co
DRAFT-TUBE-BAFFLE
CRYSTALLIZERS
A Study of Stationary and Dynamicly Behaving
Crystal-Size-Distributions
PROEFSCHRIFT
ter verkrijging van de graad van doctor in de technische
weten-schappen aan de Technische Hogeschool Delft, op gezag van de
rector magnificus prof.ir. B.P.Th. Veltman, voor een commissie
aangewezen door het college van dekanen, te verdedigen op
donderdag 17 december 1981 des namiddags om 16.00 uur
door
BARTHOLOMEUS GERARDUS MARIA DE LEER
werktuigkundig ingenieur
geboren te Rotterdam
B I B L I O T H E E K T U Delft P 1680 1395
Dit proefschrift is goedgekeurd door
de promotor prof.ir. E J . de Jong
DRAFT-TUBE-BAFFLE
CRYSTALLIZERS
A Study of Stationary and Dynamicly Behaving
Crystal-Size-Distributions
by
In the c l e a r i n g stands a boxer And a f i g h t e r by h i s trade And he c a r r i e s the reminders Of ev'ry glove that l a i d him down And cut him t i l l he c r i e d out In h i s anger and h i s shame "I am leaving, I am l e a v i n g " But the f i g h t e r s t i l l remains
ACKNOWLEDGEMENT
The author i s indebted to P.W.H. van Hagen and W.J. Bevaart f o r typing the manuscript and indebted to W. Hoogstad and B.N. Sodderland f o r drawing the f i g u r e s .
This manuscript i s printed by the Central P r i n t i n g O f f i c e o f the D e l f t U n i v e r s i t y o f Technology*
To Berty,
Was i t not always one way t r a f f i c .
SUMMARY AND CONCLUSIONS
Draft Tube B a f f l e (DTB) c r y s t a l l i z e r s are designed to make l a r g e , uniform product c r y s t a l s f o r f e r t i l i z e r and s i m i l a r a p p l i c a t i o n s where superior f i l t r a t i o n ,
c e n t r i f u g a t i o n , washing, drying and storage c h a r a c t e r i s t i c s are required. A DTB-c r y s t a l l i z e r i s t y p i f i e d by i t s forDTB-ced i n t e r n a l DTB-c i r DTB-c u l a t i o n of s l u r r y and i s a general purpose c r y s t a l l i z e r that can be u t i l i z e d f o r a d i a b a t i c c o o l i n g , i n d i r e c t c o o l i n g and medium evaporative loads. Three techniques i n a d d i t i o n to staging, are a v a i l a b l e to vary the c h a r a c t e r i s t i c s o f the suspension i n a well-mixed
c r y s t a l l i z e r : 1) Fines d e s t r u c t i o n , 2) Clear l i q u o r advance and 3) Product c l a s s i -f i c a t i o n or i n t e r n a l segregation o-f product c r y s t a l s .
Design o f D T B - c r y s t a l l i z e r s i s p r i n c i p a l l y based on past engineering experience. P i l o t plant equipment i s normally used to p r e d i c t medium s i z e and uniformity of product c r y s t a l s when beaten tracks can not be followed anymore. Generally, DTB-c r y s t a l l i z e r s are rather s e n s i t i v e t o outside disturbanDTB-ces and do sometimes pro-duce p e r i o d i c outputs. Optimalization of c r y s t a l l i z e r design and performance i s dependent on the inventiveness o f design engineers and i n d u s t r i a l workers, and i s normally r e a l i z e d by applying t r i a l and e r r o r procedures.
In t h i s work steady-state and t r a n s i e n t CSD-behaviour have been studied on basis o f the population balance concept. Aim-- D T B - c r y s t a l l i z e r which could be operated both evaporative and c o o l i n g , was used to study CSD-behaviour as function o f geo-m e t r i c a l and operating v a r i a b l e s . To t e s t experigeo-mental r e s u l t s by l i t e r a t u r e data, use was made o f the potash alum-water system. Since the D T B - c r y s t a l l i z e r was
operated i n a temperature range where the s o l u b i l i t y o f potash alum steeply i n -creases, conventional crystal/mother-liquor separation techniques could not be applied. In chapter 3 a separation technique using surface a c t i v e agents and an apolar, organic l i q u i d to separate potash alum c r y s t a l s from mother-liquor under the influence o f g r a v i t a t i o n a l f o r c e s , i s discussed. This separation technique r e s u l t e d i n r e l a t i v e l y simple sample handling procedures and equipment. Moreover, the l a t t e r technique was found to improve dry and wet-sieving c h a r a c t e r i s t i c s o f the product c r y s t a l s and was generally found to produce s u f f i c i e n t l y accurate r e s u l t s i n s i z e ranges, o f i n t e r e s t f o r the underlying work. In chapter 4 the
e f f e c t of geometrical and operating v a r i a b l e s on the c l a s s i f y i n g action o f (annular) s e t t l i n g spaces i s discussed. The present work revealed that the c l a s s i f y i n g
action of (annular) s e t t l i n g spaces i s strongly influenced by f l u i d v e l o c i t i e s perpendicular on those d i r e c t e d upwards. The l a t t e r f l u i d v e l o c i t i e s r e s u l t from a s w i r l i n the lower part of the (annular) s e t t l i n g space which i s introduced by the i n t e r n a l l y c i r c u l a t i n g s l u r r y i n the well-mixed part of the D T B - c r y s t a l l i z e r . Further, the present work revealed that D T B - c r y s t a l l i z e r s should be regarded as being composed o f a w e l l and poorly mixed volume which both depend on c r y s t a l - s i z e . The well-mixed volume decreases with increasing s i z e u n t i l a minimum e f f e c t i v e
volume has been reached. As supersaturation w i l l be more or l e s s constant throughout the c r y s t a l l i z e r volume under stationary operating c o n d i t i o n s , the annular s e t t l i n g space w i l l behave l i k e a chamber o f growth f o r c r y s t a l s present i n i t s volume. Moreover, the s w i r l i n the lower part of the annular volume w i l l introduce s i g n i -f i c a n t back-mixing. Crystals within t h i s space w i l l consequently have a change to grow and to r e t u r n to the well-mixed part of the D T B - c r y s t a l l i z e r or a change to be withdrawn from the annular volume permanently. The l a t t e r findings could
p a r t i a l l y explain both steady-state and t r a n s i e n t CSD-behaviour i n the DTB-crystal-l i z e r under study. To p r e d i c t the c DTB-crystal-l a s s i f y i n g action of annuDTB-crystal-lar s e t t DTB-crystal-l i n g spaces, a s o - c a l l e d FALL-OUT model i s introduced. Although the l a t t e r model i s f a r from perfect y e t , the r e s u l t s f o l l o w i n g from t h i s approach are promising indeed. In chapter 5 the r e l a t i o n between shape and growth-rate o f potash alum c r y s t a l s i s discussed. Experimental r e s u l t s i n d i c a t e that imperfections i n the shape of any
c r y s t a l w h i c h has t a k e n on i t s n a t u r a l c r y s t a l l i n e f o r m , w i l l r e s u l t i n a d e c r e a s e d g r o w t h - r a t e . G r o w t h - r a t e phenomena such as s i z e - d e p e n d e n t c r y s t a l growth and
g r o w t h r a t e d i s p e r s i o n may be e x p l a i n e d from t h i s s u g g e s t i o n . Other r e s u l t s i n d i -c a t e t h a t a g g l o m e r a t i o n o f -c r y s t a l s w i l l o -c -c u r a t any l e v e l o f s u p e r s a t u r a t i o n i n low t u r b u l e n t s u s p e n s i o n s and s m a l l e r s i z e - r a n g e s p r e f e r e n t i a l l y w i t h the p r o v i s o t h a t a c r i t i c a l p o p u l a t i o n d e n s i t y i s exceeded. I n c h a p t e r 6 a r e v i e w o f s t u d i e s d i r e c t e d a t u n d e r s t a n d i n g and e x p l a i n i n g s e c o n d a r y n u c l e a t i o n phenomena, i s g i v e n so f a r as r e l e v a n t t o t h e p r e s e n t work. The r e l a t i o n between t h e • c r y s t a l l i z a t i o n k i n e t i c s and t h e o p e r a t i n g c o n d i t i o n s i s d e r i v e d on a t h e o r e t i c a l b a s i s u s i n g i n f o r m a t i o n o b t a i n e d f r o m t h e l a t t e r s t u d i e s and r e s u l t s o b t a i n e d from an e x p e r i -m e n t a l s t u d y o f p a r t i c l e - i -m p e l l e r c o l l i s i o n s . The e q u a t i o n s r e s u l t i n g f r o -m t h i s a p p r o a c h , frame t h e s o - c a l l e d B i r t h Growth M u t a t i o n (BGM) c o n c e p t w h i c h can be used t o c a l c u l a t e m u t a t i o n s i n t h e c r y s t a l l i z a t i o n k i n e t i c s when a l t e r i n g t h e o p e r a t i n g c o n d i t i o n s . I n c h a p t e r 7 t h e BGM-concept i s c o n f r o n t e d w i t h e x p e r i m e n t a l r e s u l t s o b t a i n e d f r o m l i t e r a t u r e and r e s u l t s o b t a i n e d f r o m t h e D T B - c r y s t a l l i z e r under s t u d y . T h i s c o n f r o n t a t i o n r e s u l t e d i n the c o n c l u s i o n t h a t the e f f e c t o f the o p e r a t i n g c o n d i t i o n s on t h e c r y s t a l l i z a t i o n k i n e t i c s can be e x p l a i n e d i n most c a s e s u s i n g t h e BGM-concept and, t h a t i n more complex s i t u a t i o n s , c r y s t a l l i z e r r e s e a r c h s h o u l d f o c u s on C S D - e f f e c t i n g p r o c e s s e s t h a t are i n f l u e n c e d by t h e o p e r a t i n g c o n d i t i o n s ( i . e . a b r a s i o n o f c r y s t a l s ) . I n c h a p t e r 8 a new p o p u l a t i o n b a l a n c e a p p r o a c h h e n c e f o r w a r d c a l l e d t h e FRACTION-TRAJECTORY c o n c e p t , i s i n t r o d u c e d and d i s c u s s e d . The b a l a n c e e q u a t i o n s o f a b a t c h w i s e o r i n s t a t i o n a r y o p e r a t e d c o n t i -nuous c r y s t a l l i z e r can w i t h h e l p o f t h i s c o n c e p t be s o l v e d w i t h o u t b e i n g h i n d e r e d by the n e c e s s i t y o f complex m a t h e m a t i c a l and c o m p u t e r a l m a n i p u l a t i o n s o f t h e e q u a t i o n s i n v o l v e d . S i n c e t h e number o f i d e a l i z e d a s s u m p t i o n s can s t r o n g l y be r e d u c e d when u s i n g t h i s c o n c e p t , CSD-dynamics can be s i m u l a t e d more r e a l i s t i c a l l y . The f r a c t i o n - t r a j e c t o r y c o n c e p t has been a p p l i e d t o the D T B - c r y s t a l l i z e r u n d e r s t u d y . A l t h o u g h t h e q u a l i t y o f the s i m u l a t i o n may s t i l l be i m p r o v e d , e x p e r i m e n t a l CSD-dynamics c o u l d be r e p r o d u c e d s a t i s f a c t o r y .
SAMENVATTING EN KON KLUS IE S
D r a f t Tube B a f f l e (DTB) k r i s t a l l i s a t o r e n z i j n ontworpen om g r o t e , u n i f o r m e p r o dukt k r i s t a l l e n t e p r o d u c e r e n , van b e l a n g v o o r k u n s t m e s t en s o o r t g e l i j k e t o e p a s -s i n g e n waar hoge e i -s e n aan de f i l t r e e r - o f c e n t r i f u g e e r b a a r h e i d al-smede de wa-s-, d r o o g - en o p s l a g e i g e n s c h a p p e n worden g e s t e l d . D T B - k r i s t a l l i s a t o r e n , w e l k e een b r e e d t o e p a s s i n g s g e b i e d kennen en w e l k e g e s c h i k t z i j n v o o r a d i a b a t i s c h e k o e l i n g , i n d i r e k t e k o e l i n g en m i d d e l m a t i g e d a m p b e l a s t i n g e n , worden gekenmerkt d o o r een g e f o r c e e r d e , i n t e r n e c i r c u l a t i e van s l u r r i e . De e i g e n s c h a p p e n van de s l u r r i e i n een goed g e r o e r d e k r i s t a l l i s a t o r kunnen, n a a s t h e t kreëren van v e r b l i j f t i j d door s e r i e s c h a k e l i n g , worden v e r a n d e r d onder t o e p a s s i n g van de v o l g e n d e d r i e t e c h n i e k e n : 1) K i e m v e r n i e t i g i n g , 2) A f t a p van moederloog en 3) P r o d u k t k l a s s i f i k a t i e o f
i n t e r n e s e g r e g a t i e van p r o d u k t k r i s t a l l e n .
Het ontwerp van een D T B k r i s t a l l i s a t o r i s v o o r n a m e l i j k g e b a s e e r d op, i n h e t v e r -l e d e n opgedane, t e c h n i s c h e e r v a r i n g . G e w o o n -l i j k w o r d t , i n d i e g e v a -l -l e n waar de gebaande wegen n i e t meer kunnen worden g e v o l g d , van een p r o e f o p s t e l l i n g g e b r u i k gemaakt om de g r o o t t e en de u n i f o r m i t e i t van de p r o d u k t k r i s t a l l e n t e kunnen v o o r s p e l l e n . D T B k r i s t a l l i s a t o r e n w e l k e t a m e l i j k g e v o e l i g z i j n v o o r e x t e r n e v e r s t o -r i n g e n , v e -r t o n e n i n sommige g e v a l l e n een o s c i l l a t o -r i s c h g e d -r a g . Het o p t i m a l i s e -r e n van k r i s t a l l i s a t o r e n , n a a r ontwerp en qua p r e s t a t i e , i s a f h a n k e l i j k van de i n v e n t i v i t e i t van o n t w e r p e r s en i n d u s t r i e e l werkzame t e c h n i c i , en w o r d t v e e l a l g e r e a -l i s e e r d m i d d e -l s v a -l -l e n en o p s t a a n .
Het s t a t i o n a i r en dynamisch gedrag van k r i s t a l g r o o t t e v e r d e l i n g e n i s i n d i t werk b e s t u d e e r d v i a een aanpak, w e l k e g e b r u i k maakt van de p o p u l a t i e b a l a n s . Een DTB-k r i s t a l l i s a t o r met een t o t a a l volume van c a . 1 m^, welDTB-ke z o w e l verdampings- a l s k o e l i n g s g e w i j s kon worden b e d r e v e n , i s g e b r u i k t om h e t gedrag van de k r i s t a l g r o o t t e v e r d e l i n g a l s f u n k t i e van de p r o c e s k o n d i t i e s en de g e o m e t r i e van de k r i s t a l l i s a -t o r -t e kunnen b e s -t u d e r e n . He-t k a l i u m a l u i n - w a -t e r sys-teem i s g e b r u i k -t -t e n e i n d e de e x p e r i m e n t e l e r e s u l t a t e n t e kunnen t o e t s e n a a n , i n de l i t e r a t u u r b e s c h i k b a r e ,
gegevens. Een k o n v e n t i o n e l e k r i s t a l - m o e d e r l o o g s c h e i d i n g kon n i e t worden t o e g e p a s t , daar de D T B k r i s t a l l i s a t o r werd b e d r e v e n i n een t e m p e r a t u u r g e b i e d , waar de o p l o s -b a a r h e i d van k a l i u m a l u i n s t e r k toeneemt. Een t e c h n i e k , waarmee k a l i u m a l u i n k r i s t a l l e n , onder i n v l o e d van de z w a a r t e k r a c h t , van de moederloog kunnen worden g e s c h e i d e n onder g e b r u i k m a k i n g van o p p e r v l a k t e a k t i e v e s t o f f e n en een a p o l a i r e , o r g a n i s c h e v l o e i s t o f , wordt i n h o o f d s t u k 3 b e s p r o k e n . Deze s c h e i d i n g s t e c h n i e k r e s u l t e e r d e , met b e t r e k k i n g t o t h e t nemen en b e h a n d e l e n van m o n s t e r s , i n r e l a t i e f eenvoudige p r o c e d u r e s en s i m p e l e a p p a r a t u u r . B o v e n d i e n b l e e k , b i j t o e p a s s i n g van de l a a t s t g e n o e m d e t e c h n i e k , een v e r b e t e r i n g i n de d r o o g - en n a t z e e f e i g e n s c h a p p e n van de p r o d u k t k r i s t a l l e n op t e t r e d e n , w a a r d o o r i n h e t algemeen v o l d o e n d e nauw-k e u r i g e r e s u l t a t e n werden v e r nauw-k r e g e n i n h e t , v o o r h e t o n d e r l i g g e n d e wernauw-k van b e l a n g z i j n d e , k r i s t a l g r o o t t e g e b i e d . Het e f f e k t van de p r o c e s k o n d i t i e s en de g e o m e t r i e van de k r i s t a l l i s a t o r op de k l a s s i f i c e r e n d e w e r k i n g van ( a n n u l a i r e ) b e z i n k r u i m t e n wordt b e s p r o k e n i n h o o f d s t u k 4. Het h u i d i g e werk b r a c h t aan h e t l i c h t , dat de k l a s s i f i c e r e n d e w e r k i n g van ( a n n u l a i r e ) b e z i n k r u i m t e n s t e r k beïnvloed w o r d t door de, i n deze r u i m t e n o p t r e d e n d e , d w a r s s t r o m i n g . De v l o e i s t o f s n e l h e d e n i n de dwars-r i c h t i n g z i j n h e t g e v o l g van een w e dwars-r v e l i n h e t o n d e dwars-r s t e g e d e e l t e van de ( a n n u l a i dwars-r e ) b e z i n k r u i m t e , w e l k e door de i n t e r n c i r c u l e r e n d e s l u r r i e i n h e t goed gemengde ge-d e e l t e van ge-de D T B - k r i s t a l l i s a t o r w o r ge-d t geïntroge-duceerge-d. Het h u i ge-d i g e werk b r a c h t v e r d e r aan h e t l i c h t , d a t h e t volume van een D T B - k r i s t a l l i s a t o r moet worden opge-v a t a l s z i j n d e s a m e n g e s t e l d u i t een goed en s l e c h t gemengd opge-volume, w e l k e b e i d e k r i s t a l g r o o t t e a f h a n k e l i j k z i j n . Het goed gemengde volume neemt a f met toenemende k r i s t a l g r o o t t e , t o t d a t een minimum e f f e k t i e f volume i s b e r e i k t . Daar de o v e r v e r z a -d i g i n g , o v e r h e t g e h e l e k r i s t a l l i s a t o r volume genomen, min o f meer k o n s t a n t i s onder s t a t i o n a i r e b e d r i j f s o m s t a n d i g h e d e n , z a l de a n n u l a i r e b e z i n k r u i m t e a l s g r o e i
kamer v o o r de, i n z i j n volume a a n w e z i g e , k r i s t a l l e n f u n g e r e n . Bovendien z a l , door de w e r v e l i n h e t o n d e r s t e g e d e e l t e van de a n n u l a i r e b e z i n k r u i m t e , a a n z i e n l i j k e t e r u g m e n g i n g o p t r e d e n . K r i s t a l l e n i n deze r u i m t e hebben dan ook een kans om t e g r o e i e n en t e r u g t e k e r e n n a a r h e t goed gemengde g e d e e l t e van de D T B k r i s t a l l i s a -t o r dan w e l een kans om d e f i n i -t i e f aan deze r u i m -t e -t e worden o n -t -t r o k k e n . He-t s t a t i o n a i r en dynamisch g e d r a g van de k r i s t a l g r o o t t e v e r d e l i n g i n de DTBproefo p s t e l l i n g kDTBproefon g e d e e l t e l i j k v a n u i t deze b e v i n d i n g e n wDTBproeforden v e r k l a a r d . Om de k l a s -s i f i c e r e n d e w e r k i n g van a n n u l a i r e b e z i n k r u i m t e n t e kunnen v o o r -s p e l l e n , w o r d t een methode, w e l k e de naam FALL-OUT model meekreeg, v o o r g e s t e l d . Hoewel d i t model
v e r r e van v o l m a a k t i s , z i j n hiermee v e e l b e l o v e n d e r e s u l t a t e n v e r k r e g e n . De r e l a t i e t u s s e n de vorm en de g r o e i s n e l h e i d van k a l i u m a l u i n w o r d t b e s p r o k e n i n h o o f d s t u k 5. U i t e x p e r i m e n t e l e r e s u l t a t e n v a l t a f t e l e i d e n , d a t de g r o e i s n e l h e i d van k r i s t a l l e n , welke hun n a t u u r l i j k e k r i s t a l v o r m hebben aangenomen, a f n e e m t , wanneer h i e r i n onvolmaaktheden o p t r e d e n . G r o e i v e r s c h i j n s e l e n z o a l s l e n g t e - a f h a n k e l i j k e k r i s t a l g r o e i en g r o e i d i s p e r s i e kunnen v a n u i t deze s u g g e s t i e worden v e r k l a a r d . U i t a n d e r e r e s u l t a t e n v a l t a f t e l e i d e n , dat a g g l o m e r a t i e van k r i s t a l l e n , ongea c h t h e t o v e r v e r z ongea d i g i n g s n i v e ongea u , b i j v o o r k e u r z ongea l o p t r e d e n i n h e t k l e i n e d e e l t j e s g r o o t t e g e b i e d en i n l a a g t u r b u l e n t e o p l o s s i n g e n , i n d i e n een k r i t i s c h e p o p u l a t i e -d i c h t h e i -d w o r -d t o v e r s c h r e -d e n . I n h o o f -d s t u k 6 w o r -d t een o v e r z i c h t gegeven van -d i e s t u d i e s , w e l k e r e l e v a n t e i n f o r m a t i e v o o r h e t h u i d i g e werk b e v a t t e n en w e l k e t o t d o e l hadden om s e c o n d a i r e n u c l e a t i e v e r s c h i j n s e l e n t e b e s t u d e r e n en t e v e r k l a r e n . De r e l a t i e t u s s e n de k r i s t a l l i s a t i e k i n e t i e k en de p r o c e s k o n d i t i e s w o r d t op
t h e o r e t i s c h e gronden a f g e l e i d onder g e b r u i k m a k i n g v a n , u i t voornoemde l i t e r a t u u r , v e r k r e g e n i n f o r m a t i e e n , r e s u l t a t e n v e r k r e g e n u i t een e x p e r i m e n t e e l onderzoek n a a r h e t o p t r e d e n van b o t s i n g e n t u s s e n d e e l t j e s en r o e r w e r k e n . De, door deze benade r i n g , v e r k r e g e n v e r g e l i j k i n g e n vormen de b a s i s van h e t model, dat de naam B i r t h Growth M u t a t i o n (BGM) k o n c e p t h e e f t meegekregen en d a t g e b r u i k t kan worden-om
v e r a n d e r i n g e n i n de k r i s t a l l i s a t i e k i n e t i e k t e n g e v o l g e van v e r a n d e r i n g e n i n de p r o c e s k o n d i t i e s t e b e r e k e n e n . Het BGM-koncept wordt i n h o o f d s t u k 7 g e k o n f r o n t e e r d met, i n de l i t e r a t u u r omschreven, e x p e r i m e n t e l e r e s u l t a t e n en r e s u l t a t e n , w e l k e v e r k r e g e n z i j n u i t de D T B - p r o e f o p s t e l l i n g . Deze k o n f r o n t a t i e r e s u l t e e r d e i n de k o n k l u s i e , d a t h e t e f f e k t van de p r o c e s k o n d i t i e s op de k r i s t a l l i s a t i e k i n e t i e k i n de meeste g e v a l l e n kan. worden v e r k l a a r d , wanneer van h e t BGM-koncept g e b r u i k w o r d t gemaakt e n , dat h e t onderzoek aan k r i s t a l l i s a t o r e n z i c h , i n meer komplexe s i t u a t i e s , moet r i c h t e n op d i e p r o c e s s e n , welke beïnvloed worden door de p r o c e s -k o n d i t i e s en wel-ke i n g r i j p e n op de vorm van de -k r i s t a l g r o o t t e v e r d e l i n g ( b i j v . s l i j t a g e van k r i s t a l l e n ) . Een nieuwe methodische aanpak van de p o p u l a t i e b a l a n s , w e l k e de naam FRACTION-TRAJECTORY k o n c e p t meekreeg, wordt i n h o o f d s t u k 8 b e s p r o k e n De b a l a n s v e r g e l i j k i n g e n van een l a d i n g s g e w i j s o f e e n , n i e t - s t a t i o n a i r b e d r e v e n , k o n t i n u e k r i s t a l l i s a t o r k u n n e n , met b e h u l p van d i t k o n c e p t , worden o p g e l o s t z o n d e r t e worden g e h i n d e r d door de noodzaak van een, i n w i s k u n d i g en computer t e c h n i s c h o p z i c h t , m o e i l i j k e b e w e r k i n g van de b e t r o k k e n v e r g e l i j k i n g e n . Daar h e t a a n t a l v e r e e n v o u d i g i n g e n , b i j h e t g e b r u i k van d i t k o n c e p t , s t e r k kan worden v e r -m i n d e r d , z a l h e t dyna-misch gedrag van de k r i s t a l g r o o t t e v e r d e l i n g -meer r e a l i s t i s c h kunnen worden g e s i m u l e e r d . Het voornoemd k o n c e p t i s aangewend om h e t g e d r a g van de k r i s t a l g r o o t t e v e r d e l i n g i n de D T B - p r o e f o p s t e l l i n g t e s i m u l e r e n . Hoewel de k w a l i t e i t van de s i m u l a t i e nog s t e e d s k a n worden v e r b e t e r d , b l e e k h e t m o g e l i j k om h e t e x p e r i m e n t e e l gemeten dynamisch g e d r a g van de k r i s t a l g r o o t t e v e r d e l i n g v o l d o e n d e n a u w k e u r i g t e r e p r o d u c e r e n .
CONTENTS
Acknowledgement
Summary and Conclusions
Samenvatting en ¥orilusies
Chapter 1 Introduction 1
Chapter 2 Experimental Equipment and Procedures 5
2.1 P r o c e s s Flow Diagram 5 2.2 D T B - C r y s t a l l i z e r 8 2.3 Acknowledgement 10
Chapter 2 Sample Handling and CSD Analysis 13
3.1 I n t r o d u c t i o n 13 3.2 Sample H a n d l i n g 13 3.3 T h e o r e t i c a l Background 14 3.4 S e p a r a t i o n E f f i c i e n c y • - 16 3.5 P r a c t i c a l A p p l i c a t i o n 20 3.6 CSD A n a l y s i s 22 3.6.1 D r y - S i e v i n g 22 3.6.2 W e t - S i e v i n g 23 3.6.3 R e p r e s e n t a t i o n o f S i e v e Data 23 3.7 D i s c u s s i o n 23 3.8 Acknowledgement 25 Chapter 4 Classification 27 4.1 I n t r o d u c t i o n 27 4.2 E x p e r i m e n t a l Equipment and P r o c e d u r e s 28 4.3 E x p e r i m e n t a l R e s u l t s 33 4.3.1 D T B - C r y s t a l l i z e r 33 1 F i n e s R e c y c l e - R a t e 33 2 I m p e l l e r - S p e e d 36 3 H e i g h t o f S e d i m e n t a t i o n 36 4 E f f e c t i v e Number o f W i t h d r a w a l Tubes 40 4.3.2 D e v i c e w i t h R e c t a n g u l a r C r o s s - S e c t i o n a l A r e a s 40 1 C l e a r e n c e and A n g l e o f E n t r a n c e 40 2 F i n e s W i t h d r a w a l - R a t e and H e i g h t o f S e d i m e n t a t i o n 44 3 Flow-Rate o f t h e Feed 44 4 G l a s s - B a l l o t i n e i n Water 45 5 Flow P a t t e r n 48 x i i i
4.4 The F a i l - O u t Model 50 4.5 Temperature Response E x p e r i m e n t s 58
4.6 D i s c u s s i o n 63 4.7 C o n c l u d i n g Remarks 64
4.8 Acknowledgement 65
Chapter 5 Shape and Growth of Potash Alton Crystals 67
5.1 Growth-Rate 67 5.2 D i r e c t Growth-Rate Measurements 67 5.3 Growth-Rate f r o m CSD 72 5.4 A b r a s i o n o f C r y s t a l s 78 5.5 Shape o f P o t a s h Alum C r y s t a l s 79 5.5.1 D T B - c r y s t a l l i z e r 79 5.5.2 B a t c h C o o l i n g C r y s t a l l i z e r 82 1 E x p e r i m e n t a l Equipment 82 2 E x p e r i m e n t a l P r o c e d u r e and R e p r e s e n t a t i o n 82 o f R e s u l t s 3 E x p e r i m e n t a l R e s u l t s 84 A. C o o l i n g - R a t e 84 B. C r y s t a l - M a s s i n S u s p e n s i o n 86 C. I m p e l l e r - S p e e d • 88 D. I m p u r i t i e s 91 E. Seeded S o l u t i o n s 93 5.6 C o n c l u d i n g Remarks 97 5.7 Acknowledgement 98
Chapter 6 The Birth Growth Mutation Concept 101
6.1 I n t r o d u c t i o n 101 6.2 C o n t a c t N u c l e a t i o n 101 6.2.1 Impact Energy 101 6.2.2 A r e a o f C o n t a c t 102 6.2.3 C r y s t a l Face 102 6.2.4 Time between C o n t a c t s 102 6.2.5 S u p e r s a t u r a t i o n 102 6.3 B i r t h Models f o r S t i r r e d V e s s e l C r y s t a l l i z e r s 103
6.4 The B i r t h Growth M u t a t i o n Concept 104
6.4.1 Impact E n e r g y 104 6.4.2 A r e a o f C o n t a c t 105 6.4.3 Time between C o n t a c t s 105 6.4.4 P r o d u c t i o n o f N u c l e i i n S t i r r e d V e s s e l 105 C r y s t a l l i z e r s 6.4.5 S u p e r s a t u r a t i o n 106 x i v
6.4.6 P a r t i c l e Impact E n e r g i e s i n a S t i r r e d V e s s e l 107 6.4.7 P a r t i c l e V e l o c i t i e s i n a S t i r r e d V e s s e l 108 6.4.8 P a r t i c l e - O b j e c t C o l l i s i o n s 113 6.4.9 N u c l e i P r o d u c t i o n 117 6.4.10 C r y s t a l l i z a t i o n K i n e t i c s 119 6.4.11 Growth-Rate Critérium 120 6.4.12 N u c l e a t i o n - R a t e 122
Chapter 7 Confrontation of the BGM-Conoept with Experimental 127 Results 7.1 E v a p o r a t i v e C M S M P R - C r y s t a l l i z e r 127 7.1.1 C r y s t a l l i z a t i o n K i n e t i c s 127 7.1.2 E x p e r i m e n t a l R e s u l t s 131 7.1.3 C S D - A f f e c t i n g P r o c e s s e s 133 7.1.4 Turn-Over Time 137 7.2 E v a p o r a t i v e D T B - C r y s t a l l i z e r 139 7.2.1 Growth-Rate Critérium 139 7.2.2 Feed C o n c e n t r a t i o n 141 7.2.3 C l e a r - L i q u o r Advance 143 7.2.4 Heat-Load and Temperature D i f f e r e n c e 147
7.2.5 P r o d u c t W i t h d r a w a l - R a t e and I m p e l l e r - S p e e d 149 7.2.6 F i n e s W i t h d r a w a l - R a t e 158 7.2.7 C l a s s i f i c a t i o n 162 7.3 C o n c l u d i n g Remarks 164 7.4 Acknowledgement 165 Chapter 8 CSD Dynamics 167 8.1 I n t r o d u c t i o n 167 8.2 P a r t i c l e S i z e D i s t r i b u t i o n s 168 8.3 B a l a n c e E q u a t i o n s 168 8.4 M e t h o d o l o g i e s f o r S o l v i n g B a l a n c e E q u a t i o n s 171 8.5 F r a c t i o n T r a j e c t o r y Concept 173 8.5.1 Moments o f t h e CSD 175 8.5.2 I n t r o d u c t i o n o f N u c l e i 176 8.5.3 S i m u l a t i o n A c c u r a c y 178 8.6 E x p e r i m e n t a l CSD-Dynamics 180 8.7 S i m u l a t e d CSD-Dynamics 184 8.7.1 A n n u l a r S e t t l i n g Space 184 8.7.2 C r y s t a l l i z a t i o n K i n e t i c s 185 8.7.3 I n i t i a l CSD 187 xv
8.7.4 B a l a n c e E q u a t i o n s 188 8.7.5 S a l t s y s t e m 190 8.7.6 S i m u l a t i o n P r o c e d u r e and A c c u r a c y 192 8.7.7 S i m u l a t i o n R e s u l t s 192 8.8 C o n c l u d i n g Remarks 199 8.9 Acknowledgement 200 Appendices 2 03 I C l a s s i f i c a t i o n R e s u l t s 3 A. A n n u l a r S e t t l i n g Space o f 1 m D T B - C r y s t a l l i z e r B. C l a s s i f y i n g D e v i c e w i t h R e c t a n g u l a r C r o s s -S e c t i o n a l Areas I I C r y s t a l l i z a t i o n R e s u l t s A. S t e a d y - S t a t e CSD's B. Dynamic CSD's List of Symbols 209 Literature Cited 215 x v i
CHAPTER 1 INTRODUCTION C r y s t a l l i z a t i o n i s commonly r e f e r e d t o as a t e c h n i q u e f o r p u r i f i c a t i o n and r e c o v e r y o f s o l i d m a t e r i a l s . The u n i t o p e r a t i o n o f c r y s t a l l i z a t i o n has a c t u a l l y p u t i n p r a c t i c e by i n v e n t o r s , d e s i g n e n g i n e e r s and i n d u s t r i a l w o r k e r s . T h e i r i n v e n t i v e n e s s h a s b r o u g h t f o r t h t h e c r y s t a l l i z e r c o n f i g u r a t i o n s p r e s e n t l y a v a i l -a b l e . I n dependence on t h e s o l i d m -a t e r i -a l b e i n g c r y s t -a l l i z e d , c r y s t -a l l i z e r c a p a c i t i e s have due t o t h e i r e f f o r t s , been r a i s e d t r e m e n d o u s l y . To g i v e an im-p r e s s i o n , u n i t s h a v i n g c a im-p a c i t i e s o f 1000 t o 5000 m e t r i c t o n s o f c r y s t a l l i n e
m a t e r i a l p e r day, a r e n o t an e x c e p t i o n anymore. B u t t i m e was a t t h e i r s i d e . T h e i r p r o c e d u r e i s t h a t o f t r i a l and e r r o r . T h e i r knowledge i s b a s e d on e x p e r i e n c e more t h a n on i n t e l l e c t u a l h a b i t s o f t h o u g h t . B u t i s i t p o s s i b l e t o d e s i g n and o p e r a t e a c r y s t a l l i z e r o b s e r v i n g t h e range o f p r o d u c t s i z e s , c o e f f i c i e n t s o f v a r i a t i o n , c r y s t a l shapes and even p r o d u c t t r a n s i e n t s , w i t h o u t s p e c u l a t i n g on t h e complex p h y s i c a l p r o c e s s e s t h a t g e n e r a t e such a p r o d u c t d i s t r i b u t i o n a n d , how f a r does e x p e r i e n c e r e a c h when t h e b e a t e n t r a c k s can n o t be f o l l o w e d anymore? Who f i r s t r e c o g n i z e d t h e i m p o r t a n c e o f t h e l a t t e r p r o c e s s e s and p r e d i c t e d c r y s t a l - s i z e d i s t r i b u t i o n s , may r e m a i n one o f h i s t o r y ' s s e c r e t s .
Moyers and Randolph (1973) c l a i m t h a t t h e f i r s t a n a l y t i c a l work i n t h i s a r e a s h o u l d be a t t r i b u t e d t o t h e i r f e l l o w A m e r i c a n R.B. P e e t who worked f o r t h e A m e r i c a n P o t a s h Compagny on c r y s t a l l i z a t i o n o f S e a r l e s Lake b r i n e s . P e e t (1931) s p e c u l a t e d on t h e C r y s t a l S i z e D i s t r i b u t i o n (CSD) w h i c h s h o u l d be p r o d u c e d i n a n u n s e e d e d , c o n t i n u o u s f l o w , w e l l - m i x e d b o r a x c r y s t a l l i z e r . He d e r i v e d a n e x p r e s s i o n f o r t h e p r o d u c t w e i g h t d i s t r i b u t i o n ( w h i c h was r e c o g n i z e d a s a gamma f u n c t i o n ) and even i n t e g r a t e d t h e d i s t r i b u t i o n t o o b t a i n k i n e t i c p a r a m e t e r s i n t e r m s o f moments o f the d i s t r i b u t i o n . P e r h a p s w i t h o u t r e a l i z i n g t h e s i g n i f i c a n c e o r g e n e r a l i t y o f t h e w o r k , Peet c l e a r l y l a i d o u t t h e r u d i m e n t s o f t h e Mixed S u s p e n s i o n M i x e d P r o d u c t Removal (MSMPR) c r y s t a l l i z e r . Bransom e t a l . (1949) d e t a i l t h e f i r s t p u b l i s h e d use o f t h e M S M P R - c r y s t a l l i z e r t o o b t a i n q u a n t i t a t i v e n u c l e a t i o n and g r o w t h - r a t e s . K i n e t i c p a r a m e t e r s o f t h e s a l t i n g - o u t o r g a n i c - w a t e r s y s t e m were ob-t a i n e d f r o m a s e d i m e n ob-t a ob-t i o n measuremenob-t o f mass d i s ob-t r i b u ob-t i o n . No m e n ob-t i o n was made o f t h e p o s s i b l e e n g i n e e r i n g s i g n i f i c a n c e o f t h i s t e c h n i q u e t o o b t a i n d e s i g n o r i e n t e d k i n e t i c s o r o f p r e d i c t i n g CSD i n o t h e r c o n f i g u r a t i o n s . Saeman (1956) p u b l i s h e d t h e f i r s t e n g i n e e r i n g o r i e n t e d s t u d y o f CSD i n c o n t i n u o u s mixed-magma c r y s t a l l i z e r s . T h i s s t u d y d e r i v e d t h e e x p o n e n t i a l p o p u l a t i o n and gamma w e i g h t d i s t r i b u t i o n o f t h e mixed r e m o v a l c r y s t a l l i z e r and compared them w i t h t h e c l a s s i -f i e d r e m o v a l c a s e . E n g i n e e r i n g a n a l y s i s o -f CSD d a t e s t o t h i s work o -f Saeman. The v a l i d i t y o f k i n e t i c measurements o b t a i n e d f r o m a n M S M P R - c r y s t a l l i z e r depends upon a c h i e v i n g t h e c o n s t r a i n t s assumed i n t h e d e r i v a t i o n o f t h e e x p e r i m e n t a l p o p u l a t i o n d i s t r i b u t i o n as w e l l as sample p r o c e d u r e s and t r e a t m e n t . S e p a r a t i o n o f c r y s t a l s f r o m m o t h e r l i q u o r and a c c u r a t e d e t e r m i n a t i o n o f c r y s t a l s i z e d i s t r i b u -t i o n , e s p e c i a l l y i n s m a l l e r s i z e r a n g e s , c a n be -t r o u b l e s o m e . A l -t h o u g h -t h e MSMPR-c o n MSMPR-c e p t has a t t r a MSMPR-c t e d MSMPR-c o n s i d e r a b l e a t t e n t i o n i n t h e p a s t d e MSMPR-c a d e s , MSMPR-behaviour i s s e l d o m r e p r e s e n t a t i v e o f t h e i n d u s t r i a l s i t u a t i o n . The p r o d u c t d i s t r i b u t i o n o f an i n d u s t r i a l c r y s t a l l i z e r may s t r o n g l y be i n f l u e n c e d by phenomena such as a g g l o m e r a t i o n , b r e a k a g e , a t t r i t i o n , c l a s s i f i c a t i o n and s e g r e g a t i o n o f s o l i d s a s w e l l as m a l d i s t r i b u t i o n o f s u p e r s a t u r a t i o n . The s u c c e s s o f t h e MSMPR-concept l i e s i n t h e f a c t t h a t i t i s easy t o g r a s p . S t u d i e s i n v e s t i g a t i n g t h e e f f e c t o f c r y s t a l -l i z e r s c a -l e , o p e r a t i n g c o n d i t i o n s and m a t e r i a -l s o f c o n s t r u c t i o n , become -l e s s complex when u s i n g w e l l - d e f i n e d c r y s t a l l i z e r c o n f i g u r a t i o n s i n w h i c h t h e above mentioned n o n - i d e a l i t i e s a r e s u r p r e s s e d e f f e c t i v e l y o r do not o c c u r a t a l l . Moreo v e r , t h e MSMPRcMoreoncept h a s i n i t i a t e d s t u d i e s d i r e c t e d a t i s Moreo l a t i Moreo n and q u a n t i z a -t i o n o f -t h e v a r i o u s -t y p e s o f n u c l e a -t i o n mechanisms i n i n d u s -t r i a l c r y s -t a l l i z e r s and b e s i d e t h e s e d e v e l o p m e n t s , promoted c o m p a r a t i v e , e n g i n e e r i n g o r i e n t e d s t u d i e s . The s i m p l i c i t y o f t h e MSMPRconcept t o g e t h e r w i t h i t s many e x p e r i m e n t a l v e r i f i c a
-t i o n s , has been -t h e s i n g l e mos-t i m p o r -t a n -t f a c -t o r i n -t h e a c c e p -t a n c e o f a n a l y -t i c a l CSD m o d e l i n g and a n a l y s i s . The p o p u l a t i o n , o r numbers, b a l a n c e i s t h e s t a r t i n g p o i n t f o r much o f t h e work i n the a r e a o f a n a l y s i s and p r e d i c t i o n by c r y s t a l -s i z e d i -s t r i b u t i o n i n c r y -s t a l l i z a t i o n -s y -s t e m -s . P a r t i c l e k i n e t i c -s and m e c h a n i c -s , t o g e t h e r w i t h an u n i f i e d t h e o r y o f CSD, p e r m i t a n a l y s i s e v a l u a t i o n and p r e d i c t i o n o f c r y s t a l l i z e r p e r f o r m a n c e a t l e v e l s o f o p e r a t i o n r a n g i n g from b e n c h - s c a l e t o c o m m e r c i a l p r o d u c t i o n . C r y s t a l l i z i n g systems p r e s e n t complex a n a l y s i s problems s i n c e t h e y a r e c h a r a c t e r i z e d by t h e i r mode o f o p e r a t i o n , by t h e t y p e o f c r y s t a l -l i z e r geometry emp-loyed by how t h e s o -l i d and -l i q u i d phases a r e m a n i p u -l a t e d t o c o n t r o l c r y s t a l - s i z e d i s t r i b u t i o n and by w h e t h e r o r n o t t h e s y s t e m i s c a s c a d e d . The mode o f o p e r a t i o n i s t h e t e h c n i q u e employed t o g e n e r a t e s u p e r s a t u r a t i o n (and n o r m a l l y a s o l i d phase) from s o l u t i o n . The most common t e c h n i q u e s f o r c r e a t i n g s u p e r s a t u r a t i o n i n c l u d e : 1) I n d i r e c t c o o l i n g . 2) E v a p o r a t i o n o f s o l v e n t .
3) A d i a b a t i c vacuum c o o l i n g and 4) S a l t i n g - o u t by a d d i t i o n o f a component t o r e d u c e t h e s o l u b i l i t y o f t h e c r y s t a l l i z i n g s u b s t a n c e .
The d i r e c t c o o l i n g mode, w h e r e i n h e a t i s removed t h r o u g h a h e a t e x c h a n g e r w a l l , i s u t i l i z e d when p r o d u c t s o l u b i l i t y i s s t r o n g l y t e m p e r a t u r e dependent. By con-t r a s con-t , con-t h e e v a p o r a con-t i o n con-t e c h n i q u e i s s u i con-t a b l e f o r s o l u con-t i o n s w i con-t h s m a l l o r n e g a con-t i v e t e m p e r a t u r e dependence. A d i a b a t i c c o o l i n g i s used f o r c r y s t a l s w i t h i n t e r m e d i a t e t y p e s o f s o l u b i l i t y c u r v e s , w h i l e s a l t i n g - o u t i s n o t e n c o u n t e r e d n o r m a l l y e x c e p t i n l a b o r a t o r y u n i t s . A l t h o u g h a l a r g e number o f c r y s t a l l i z e r c o n f i g u r a t i o n s may n o r m a l l y be e n c o u t e r e d i n i n d u s t r i a l p r a c t i c e , t h e p r e s e n t s t u d y w i l l f o c u s on s o - c a l l e d D r a f t T_ube B a f f l e ( D T B ) - c r y s t a l l i z e r s . A D T B - c r y s t a l l i z e r i s t y p i f i e d by h i g h i n t e r n a l c i r c u l a t i o n o f s l u r r y and i s a g e n e r a l p u r p o s e c r y s t a l l i z e r t h a t can be u t i l i z e d f o r a d i a b a t i c c o o l i n g , i n d i r e c t c o o l i n g and medium e v a p o r a t i v e l o a d s . D T B - c r y s t a l l i z e r s a r e d e s i g n e d t o make l a r g e , u n i f o r m p r o d u c t c r y s t a l s f o r f e r t i l i z e r and s i m i l a r a p p l i c a t i o n s where s u p e r i o r f i l t r a t i o n , c e n t r i f u g a t i o n , w a s h i n g , d r y i n g and s t o r a g e c h a r a c t e r i s t i c s a r e r e q u i r e d . Three t e c h n i q u e s i n condensate c l e a r l i q u o r ~
Figure 1.1 Evaporative DTB-crystallizer
a d d i t i o n t o s t a g i n g , a r e a v a i l a b l e t o v a r y t h e c h a r a c t e r i s t i c s o f t h e c r y s t a l l i -z e r s u s p e n s i o n : 1) F i n e s d e s t r u c t i o n . 2) C l e a r l i q u o r advance and 3) P r o d u c t c l a s s i f i c a t i o n o r i n t e r n a l s e g r e g a t i o n o f p r o d u c t c r y s t a l s . D T B - c r y s t a l l i z e r s a r e s u p p l i e d w i t h an i n t e r n a l b a f f l e ( a n n u l a r s e t t l i n g s p a c e ) t o f a c i l i t a t e t h e r e m o v a l o f s m a l l e r s i z e d c r y s t a l s ( f i n e s ) from t h e w e l l - m i x e d volume o f t h e c r y s t a l l i z e r . The s l u r r y w i t h d r a w n f r o m t h e t o p - s e c t i o n o f t h e a n n u l a r s e t t l i n g s p a c e , i s a f t e r p a s s i n g a h e a t e x c h a n g e r i n w h i c h t h e l a t t e r f i n e s a r e d i s s o l v e d , r e c y c l e d t o t h e w e l l - m i x e d p a r t o f t h e c r y s t a l l i z e r (= f i n e s d e s t r u c t i o n ) . F i n e s d e s t r u c t i o n i n c r e a s e s t h e medium s i z e o f t h e p r o d u c t c r y s t a l s and a l l o w s t h e s u p p l y o f a d d i t i o n a l h e a t . The i n c r e a s e i n t h e medium s i z e o f t h e p r o d u c t c r y s t a l s i s n o r m a l l y a t t e n d e d by a s i m u l t a n e o u s i n c r e a s e i n t h e c o e f f i -c i e n t o f v a r i a t i o n r e d u -c i n g t h e u n i f o r m i t y o f t h e p r o d u -c t -c r y s t a l s . P r o v i s i o n f o r c l e a r l i q u o r advance a l l o w s some c o n t r o l o f p r o d u c t s l u r r y c o n c e n t r a t i o n s when t h e f l o w o f t h e p r o d u c t i s k e p t c o n s t a n t and some c o n t r o l o f t h e medium s i z e o f t h e p r o d u c t c r y s t a l s when t h e f l o w o f t h e f e e d i s k e p t c o n s t a n t . C l a s s i -f i e d p r o d u c t r e m o v a l i s n o r m a l l y r e q u i r e d t o r e d u c e t h e c o e -f -f i c i e n t o -f v a r i a t i o n . I n i n d u s t r i a l c r y s t a l l i z e r s p r o d u c t c r y s t a l s a r e commonly w i t h d r a w n f r o m an
e l u t r i a t i o n o r s e t t l i n g l e g i n w h i c h a low upwards d i r e c t e d f l u i d v e l o c i t y i s main-t a i n e d by f e e d i n g a s m a l l p o r main-t i o n o f main-t h e f i n e s d e s main-t r u c main-t i o n f l o w main-t o main-t h e l o w e r p a r main-t o f t h e c l a s s i f y i n g d e v i c e . P r e f e r e n t i a l w i t h d r a w a l o f l a r g e r s i z e d c r y s t a l s w i l l improve t h e u n i f o r m i t y o f t h e p r o d u c t c r y s t a l s b u t may be a t t e n d e d by a s i g n i f i -c a n t r e d u -c t i o n i n t h e i n h e r e n t s t a b i l i t y o f t h e -c r y s t a l l i z a t i o n p r o -c e s s . D e s i g n and o p e r a t i o n o f i n d u s t r i a l D T B c r y s t a l l i z e r s i s p r i n c i p a l l y b a s e d on p a s t e n g i -n e e r i -n g e x p e r i e -n c e . P i l o t p l a -n t equipme-nt I s commo-nly used t o p r e d i c t medium s i z e and u n i f o r m i t y o f p r o d u c t c r y s t a l s when b e a t e n t r a c k s c a n n o t be f o l l o w e d anymore. A l t h o u g h t h e above d e s i g n p r o c e d u r e may r e d u c e t h e f i n a n c i a l r i s k s o f f a i l u r e , l a c k o f knowledge may r e s u l t i n u n n e c e s s a r y h i g h c a p i t a l i n v e s t m e n t s , a t o o s m a l l o r t o o h i g h c a p a c i t y , o f f - s p e c i f i c a t i o n p r o d u c t , waste o f energy and t r o u b l e s o m e o p e r a t i o n . S i n c e d e s i g n o f i n d u s t r i a l D T B - c r y s t a l l i z e r s i s more an a r t t h a n a s c i e n c e , t h e r e l a t i o n between m a n u f a c t u r e r and c l i e n t may b e s t be c h a r a c t e r i z e d by t h e s a y i n g : I n t h e kingdom o f b l i n d men one-eyed i s k i n g . Assuming k i n e t i c s can be modeled, t h e p o p u l a t i o n b a l a n c e c o n c e p t may be marked as t h e one and o n l y way f o r a n a l y s i s and p r e d i c t i o n o f CSD i n i n d u s t r i a l c r y s t a l -l i z e r s . To a n a -l y z e a c r y s t a -l -l i z e r s y s t e m , b o t h s t e a d y - s t a t e and t r a n s i e n t CSD-b e h a v i o u r must CSD-be c o n s i d e r e d . I n l i t e r a t u r e a l i m i t e d numCSD-ber o f p a p e r s can CSD-be f o u n d w h i c h d e a l w i t h s t e a d y - s t a t e and t r a n s i e n t CSD-behaviour i n p a r t i c u l a r D T B - c r y s t a l l i z e r c o n f i g u r a t i o n s . However, t h e r e l i a b i l i t y o f t h e d e s i g n and s c a l e - u p r u l e s as w e l l as t h e methods p r o p o s e d i n l i t e r a t u r e t o a n a l y s e and p r e d i c t CSD, i s r a t h e r p o o r . T h i s p o o r r e l i a b i l i t y may be a t t r i b u t e d t o t h e f o l l o w i n g s h o r t c o m i n g s : 1) The CSD i n a D T B - c r y s t a l l i z e r i s s t r o n g l y i n f l u e n c e d by t h e c l a s s i f y i n g a c t i o n o f t h e a n n u l a r s e t t l i n g s p a c e . The c l a s s i f i c a t i o n p r o c e s s e s o c c u r i n g n e a r and w i t h i n t h e a n n u l a r volume, a r e complex o f n a t u r e and s t r o n g l y I n f l u e n c e d by g e o m e t r i c a l and o p e r a t i n g v a r i a b l e s . M o r e o v e r , t h e r e s i d e n c e t i m e o f s m a l l e r s i z e d p r o d u c t c r y s t a l s may s t r o n g l y be i n c r e a s e d i n t h e p r e s e n c e o f an a n n u l a r volume. The e f f e c t o f t h e g e o m e t r i c a l and o p e r a t i n g v a r i a b l e s on t h e c l a s s i f y i n g a c t i o n o f t h e a n n u l a r s e t t l i n g s p a c e n o r t h e e f f e c t o f t h e a n n u l a r volume on t h e r e s i d e n c e t i m e o f t h e p r o d u c t c r y s t a l s were t h o r o u g h l y s t u d i e d a t d a t e o f t h i s work. 2) A l t h o u g h t h e r e l a t i o n between t h e c r y s t a l l i z a -t i o n k i n e -t i c s i n a M S M P R - c r y s -t a l l i z e r and f o r i n s -t a n c e , -t h e o p e r a -t i n g c o n d i -t i o n s has n o t been e x p l a i n e d on a t h e o r e t i c a l b a s i s y e t , r a p i d p r o g r e s s has been made i n t h e p a s t decade. I n l i t e r a t u r e s e m i - e m p i r i c a l r e l a t i o n s have been p r o p o s e d t o c o r r e l a t e t h e c r y s t a l l i z a t i o n k i n e t i c s w i t h o p e r a t i n g c o n d i t i o n s , c r y s t a l l i z e r s c a l e and m a t e r i a l s o f c o n s t r u c t i o n . A l t h o u g h a c o n s i d e r a b l e amount o f s k e p t i c i s m c o n c e r n i n g t h e g e n e r a l i t y o f s u c h r e l a t i o n s , s h o u l d n o r m a l l y be o b s e r v e d , t h e l a t t e r developments have c e r t a i n l y c o n t r i b u t e d t o a b e t t e r u n d e r s t a n d i n g o f CSD-behaviour i n MSMPR-equipment. Y e t , l a c k o f f u n d a m e n t a l i n s i g h t has hampered the u n d e r s t a n d i n g o f CSD-behaviour i n more complex c r y s t a l l i z e r c o n f i g u r a t i o n s . 3) Over r e c e n t y e a r s CSD-behaviour has m a i n l y been s t u d i e d under s t a t i o n a r y
-o p e r a t i n g c -o n d i t i -o n s a l t h -o u g h m-ore i n f -o r m a t i -o n c a n be g a i n e d f r -o m b a t c h - w i s e or i n s t a t i o n a r y o p e r a t e d c o n t i n u o u s c r y s t a l l i z e r s . L i m i t a t i o n s o f mathematics however, have so f a r h i n d e r e d t h e u n d e r s t a n d i n g o f CSD-dynamics a n d hampered the a p p l i c a t i o n o f s i m u l a t i o n t e c h n i q u e s as a n a l y t i c a l a i d i n r e s e a r c h and i n d u s t r i a l p r a c t i c e . I d e a l i z e d a s s u m p t i o n s n e c e s s a r y t o overcome t h e l a t t e r l i m i t a t i o n s , have o f t e n p o s e d d i f f i c u l t i e s when a p p l y i n g t h e p o p u l a t i o n b a l a n c e c o n c e p t t o more complex c r y s t a l l i z e r c o n f i g u r a t i o n s .
T e c h n i q u e s a v a i l a b l e a t d a t e o f t h i s work t o s i m u l a t e CSD-dynamics, a r e b e s i d e t h e i r l i m i t e d a p p l i c a b i l i t y , r a t h e r i n a s c e s s i b l e and e x p e n s i v e due t o complex m a t h e m a t i c a l and c o m p u t e r a l m a n i p u l a t i o n s . The l a t t e r s h o r t c o m i n g s may by way o f a c o h e r e n t e x p e r i m e n t a l a n d t h e o r e t i c a l s t u d y , be removed. To r e a l i z e t h i s o b j e c t i v e , u s e was made o f a 1 m3 D T B - c r y s t a l l i z e r w h i c h c o u l d be o p e r a t e d b o t h
e v a p o r a t i v e and c o o l i n g . S i n c e t h e s u c c e s s o f any r e s e a r c h programm may depend on t h e a b i l i t y t o t e s t e x p e r i m e n t a l r e s u l t s by l i t e r a t u r e d a t a , use was made o f t h e p o t a s h alum-water s y s t e m . S i n c e t h e D T B - c r y s t a l l i z e r under s t u d y was o p e r a t e d i n t h e t e m p e r a t u r e range o f 35 - 40 °C, a s p e c i a l t e c h n i q u e t o s e p a r a t e p o t a s h alum c r y s t a l s f r o m m o t h e r - l i q u o r had t o be d e v e l o p e d . A l t h o u g h an i n t e g r a l s t u d y o f b o t h t h e c l a s s i f y i n g a c t i o n o f t h e a n n u l a r s e t t l i n g space and t h e
b e h a v i o u r o f t h e CSD as f u n c t i o n o f g e o m e t r i c a l and o p e r a t i n g v a r i a b l e s s h o u l d be p r e f e r e d , t h e r e s e a r c h programm was s p l i t t e d up i n two phases. I n t h e f i r s t i n s t a n c e t h e D T B - c r y s t a l l i z e r was o p e r a t e d as a c l a s s i f y i n g d e v i c e . F o r a f i x e d c r y s t a l l i z e r geometry, a g i v e n s e t o f o p e r a t i n g c o n d i t i o n s and s t a t i o n a r o p e r a -t i o n , -t h e c l a s s i f y i n g a c -t i o n o f -t h e a n n u l a r s e -t -t l i n g space was s -t u d i e d under n o n - c r y s t a l l i z i n g c o n d i t i o n s by s a m p l i n g t h e r e c y c l e d f l o w o f t h e f i n e s and t h a t o f t h e p r o d u c t . By a n a l y z i n g b o t h samples s i z e d e p e n d e n t c l a s s i f i c a t i o n c o e f f i -c i e n t s -c o u l d be -c a l -c u l a t e d from t h u s o b t a i n e d d a t a . By v a r y i n g g e o m e t r i -c a l and o p e r a t i n g v a r i a b l e s s y s t e m a t i c a l l y , t h e dependence o f t h e c a l c u l a t e d c l a s s i f i c a -t i o n c o e f f i c i e n -t s on -t h e v a r i a b l e under s -t u d y c o u l d be d e -t e r m i n e d . I n a d d i -t i o n t o t h e l a s t p i e c e o f equipment a c l a s s i f y i n g d e v i c e w i t h r e c t a n g u l a r c r o s s -s e c t i o n a l a r e a -s wa-s u-sed f o r t h e -same p u r p o -s e -s a-s m e n t i o n e d above. M o r e o v e r , t h e l a t t e r d e v i c e was used t o g e t an i m p r e s s i o n o f t h e f l o w p a t t e r n w i t h i n ( a n n u l a r ) s e t t l i n g s p a c e s . The l a t t e r s t u d y h a d t o r e s u l t i n a t h e o r e t i c a l model c a p a b l e o f e x p l a i n i n g and p r e d i c t i n g t h e c l a s s i f y i n g b e h a v i o u r o f a D T B - c r y s t a l l i z e r . A f t e r c o m p l e t i n g t h e f i r s t p a r t o f t h e e x p e r i m e n t a l programm, t h e s t e a d y - s t a t e b e h a v i o u r o f t h e CSD i n t h e 1 m3 e v a p o r a t i v e D T B - c r y s t a l l i z e r was s t u d i e d as f u n c t i o n o f g e o m e t r i c a l and o p e r a t i n g v a r i a b l e s . N o n - i d e a l phenomena s u c h as a b r a s i o n and a g g l o m e r a t i o n o f c r y s t a l s were f o u n d t o a f f e c t t h e CSD i n t h e DTB-e v a p o r a t o r undDTB-er s t u d y . ThDTB-esDTB-e DTB-e x p DTB-e r i m DTB-e n t a l f i n d i n g s i n i t i a t DTB-e d a s t u d y d i r DTB-e c t DTB-e d a t u n d e r s t a n d i n g and e x p l a i n i n g t h e o c c u r e n c e o f n o n - i d e a l l y shaped p o t a s h alum c r y s t a l s i n t h e p r o d u c t o f t h e D T B - e v a p o r a t o r and a s t u d y d i r e c t e d a t m e a s u r i n g p a r t i c l e - i m p e l l e r c o l l i s i o n f r e q u e n c i e s i n an a g i t a t e d v e s s e l as f u n c t i o n p a r t i c l e s i z e , i m p e l l e r - s p e e d , s o l i d - l i q u i d d e n s i t y d i f f e r e n c e and l i q u i d v i s c o s i t y . A l l t h e s e s t u d i e s had t o r e s u l t i n a b e t t e r u n d e r s t a n d i n g o f s t e a d y - s t a t e CSD-behaviour i n p a r t i c u l a r D T B - c r y s t a l l i z e r c o n f i g u r a t i o n s . The t r a n s i e n t b e h a v i o u r o f t h e CSD i n t h e 1 m3 D T B - e v a p o r a t o r has been s t u d i e d e x p e r i m e n t a l l y by d i s t u r b i n g a s t e a d y - s t a t e CSD d e v e l o p e d under g i v e n o p e r a t i n g c o n d i t i o n s and by a n a l y z i n g t h e r e s p o n s e o f t h e p r o d u c t d i s t r i b u t i o n . I n o r d e r t o reduce t h e number o f i d e a l i z e d a s s u m p t i o n s n e c e s s a r y t o overcome t h e l i m i t a t i o n s o f mathematics when s o l v i n g t h e b a l a n c e e q u a t i o n s o f an e v a p o r a t i v e D T B - c r y s t a l l i z e r , a new p o p u l a t i o n b a l a n c e a p p r o a c h had t o be d e v e l o p e d . Comparison o f e x p e r i m e n t a l and t h e o r e t i c a l r e s u l t s s h o u l d p r o d u c e d e c i s i v e answers w i t h r e s p e c t t o t h e q u a l i t y o f t h e t h e o r e t i c a l a p p r o a c h .
The u n d e r l y i n g work i s d i v i d e d i n t o f o u r major p a r t s d e a l i n g w i t h : 1) C l a s s i f i c a -t i o n n e a r and w i -t h i n ( a n n u l a r ) s e -t -t l i n g s p a c e s . 2) Shape and grow-th o f p o -t a s h alum c r y s t a l s . 3) The r e l a t i o n between t h e c r y s t a l l i z a t i o n k i n e t i c s and t h e o p e r a t i n g c o n d i t i o n s i n p a r t i c u l a r . 4) CSD-dynamics.
H o p e f u l l y , t h e e x p e r i m e n t a l and t h e o r e t i c a l r e s u l t s p r e s e n t e d h e r e i n , w i l l c o n t r i -b u t e t o a -b e t t e r u n d e r s t a n d i n g o f s t e a d y - s t a t e and t r a n s i e n t CSD--behaviour i n D T B - c r y s t a l l i z e r s .
-CHAPTER 2
EXPERIMENTAL EQUIPMENT AND PROCEDURES
I n t h i s c h a p t e r p a r t o f t h e equipment and p r o c e d u r e s used t o r e a l i z e t h e f o r e -mentioned o b j e c t i v e s , w i l l be d i s c u s s e d . In f i g u r e 2 . 1 a s c h e m a t i c p r o c e s s f l o w d i a g r a m o f t h e e x p e r i m e n t a l equipment i s shown. The p r o c e d u r e d u r i n g c r y s t a l l i z a -t i o n i s b r i e f l y d i s c u s s e d below. Feed i s p r e p a r e d i n an a g i -t a -t e d -t a n k w i -t h a n o m i n a l volume o f a p p r o x i m a t e l y 2.5 m3. The t a n k i s s p l i t t e d - u p i n two s e c t i o n s
w i t h h e l p o f a c o n i c a l shaped d i v i d e r . The b o t t o m s e c t i o n w h i c h i s a g i t a t e d by a 4 - b l a d e d f l a t d i s c t u r b i n e , i s steam h e a t e d and t e m p e r a t u r e c o n t r o l e d . A l l r e t u r n i n g s t r e a m s e n t e r t o t h i s compartment. The t a n k i s f i l l e d w i t h compagnies w a t e r a f t e r w h i c h a s u r p l u s on p o t a s h alum o f i n d u s t r i a l q u a l i t y i s s u p p l i e d . Even when f l u c t u a t i o n s i n t h e t e m p e r a t u r e o f t h e f e e d p r e p a r a t i o n t a n k o c c u r , t h e mother l i q u o r w i t h i n t h i s tank w i l l be s a t u r a t e d due t o t h e l a t t e r s u r p l u s . The c o n i c a l shaped d i v i d e r c r e a t e s a q u i e s c e n t zone w i t h i n t h e f e e d p r e p a r a t i o n t a n k . S e t t l i n g o f c r y s t a l s i s promoted by a s m a l l t u r b i n e i m p e l l e r p o s i t i o n e d h a l f w a y t h e c i l i n d r i c a l i n l e t t o t h e c l a r i f i e d volume. The l a t t e r volume i s by way o f an o v e r f l o w , c o n n e c t e d w i t h a 0.6 m3 d i s s o l u t i o n t a n k . The l a t t e r t a n k
w h i c h i s steam h e a t e d and t e m p e r a t u r e c o n t r o l e d , i s a g i t a t e d by a marine t y p e i m p e l l e r . To d i s s o l v e f i n e s e n t r a i n e d from t h e f e e d p r e p a r a t i o n t a n k , a tempe-r a t u tempe-r e o f 5 t o 10 °C above t h e s a t u tempe-r a t i o n t e m p e tempe-r a t u tempe-r e o f t h e f e e d i s m a i n t a i n e d w i t h i n t h i s t a n k . U n d e r s a t u r a t e d mother l i q u o r w i t h d r a w n from t h e 0.6 m3 d i s s o l u -t i o n -t a n k i s r e s p e c -t i v e l y f i l -t e r e d , d e a i r a -t e d and f e d -t o -t h e b o -t -t o m s e c -t i o n o f t h e D T B - c r y s t a l l i z e r . The l i q u i d l e v e l i n t h e d e a i r a t o r i s c o n t r o l e d w i t h a l e v e l s w i t c h o p e n i n g a c o n t r o l v a l v e i n t h e s u c t i o n l i n e o f t h e c i r c u l a t i o n pump a t low l e v e l and c l o s i n g t h e same v a l v e a t h i g h l e v e l . A s p a r e s e t f i l t e r s can be s w i t c h e d on when t h e f i l t e r s used d u r i n g o p e r a t i o n , become c l o g g e d . The l i q u i d l e v e l i n t h e D T B - c r y s t a l l i z e r i s m a n u a l l y c o n t r o l e d by a d j u s t i n g t h e number o f r e v o l u t i o n s o f t h e f e e d i n g pump. The f l o w - r a t e o f t h e f e e d w h i c h i s measured w i t h a m a g n e t i c f l o w i n d i c a t o r , can be v a r i e d f r o m 0 t o 35 1/min. P r o -d u c t i s o k i n e t i c l y w i t h -d r a w n from t h e t o p - s e c t i o n o f t h e D T B - c r y s t a l l i z e r , i s s u p p l i e d t o t h e f e e d p r e p a r a t i o n t a n k where i t can be r e u s e d a g a i n . The f l o w -r a t e o f t h e p -r o d u c t w h i c h i s by hand measu-red w i t h a 10 1 b u c k e t and a s t o p w a t c h , can be v a r i e d f r o m 0 t o 35 1/min by a d j u s t i n g t h e number o f r e v o l u t i o n s o f t h e p r o d u c t pump. The CSD o f t h e p r o d u c t w h i c h i s r e p r e s e n t a t i v e o f t h e CSD i n t h e w e l l - m i x e d volume o f t h e D T B - c r y s t a l l i z e r , i s d e t e r m i n e d by s a m p l i n g t h e l a t t e r
f l o w and by a p p l y i n g t h e sample h a n d l i n g and CSD a n a l y z i n g t e c h n i q u e t o be d i s -c u s s e d i n -c h a p t e r 3. The -c r y s t a l l i z e r under s t u d y i s p r o v i d e d w i t h an i n t e r n a l b a f f l e t o f a c i l i t a t e t h e r e m o v a l o f s m a l l e r s i z e d c r y s t a l s from t h e w e l l - m i x e d volume o f t h e c r y s t a l l i z e r . F i n e s w i t h d r a w n f r o m t h e a n n u l a r s e t t l i n g s p a c e a r e f e d t o a 0.6 m3 f i n e s d i s s o l u t i o n t a n k . The t e m p e r a t u r e w i t h i n t h i s tank can be
r a i s e d w i t h 4 °C a t a f i n e s w i t h d r a w a l - r a t e o f 90 1/min and i s c o n t r o l e d by ad-j u s t i n g t h e f l o w o f t h e h e a t i n g w a t e r . The h e a t i n g w a t e r i s w i t h d r a w n f r o m and r e c y c l e d t o a 2.5 m3 w a t e r t a n k w h i c h i s steam h e a t e d and t e m p e r a t u r e c o n t r o l e d ,
and w h i c h i s a g i t a t e d by a 4 b l a d e d f l a t d i s c t u r b i n e . The t e m p e r a t u r e o f t h e h e a t i n g w a t e r v a r i e d between 70 and 75 °C. The o u t l e t f l o w o f t h e f i n e s d i s s o l u -t i o n -t a n k w h i c h i s a g i -t a -t e d by a m a r i n e -t y p e i m p e l l e r a -t 500 R.P.M., i s a f -t e r p a s s i n g a a-LAVAL p l a t e t y p e h e a t e x c h a n g e r , r e c y c l e d t o t h e b o t t o m s e c t i o n o f t h e D T B - c r y s t a l l i z e r ( f i n e s d e s t r u c t i o n ) . The f l o w - r a t e i n t h e f i n e s d e s t r u c t i o n l o o p w h i c h i s measured w i t h h e l p o f a m a g n e t i c f l o w i n d i c a t o r , can be v a r i e d from 0 t o 166.7 1/min by a d j u s t i n g t h e number o f r e v o l u t i o n s o f t h e f i n e s c i r c u -l a t i o n pump. The t e m p e r a t u r e o f t h e r e c y c -l e d f -l o w i s c o n t r o -l e d by a d j u s t i n g t h e f l o w - r a t e o f t h e c o o l i n g w a t e r s u p p l i e d t o t h e l a t t e r h e a t e x c h a n g e r . The c o o l i n g
-DE-AIRATOR
WATER DAMP CONDENSOR
w a t e r i s w i t h d r a w n from and r e c y c l e d t o a s m a l l w a t e r t a n k w h i c h i s t e m p e r a t u r e c o n t r o l e d by a d j u s t i n g t h e companies w a t e r s u p p l y . S u r p l u s o f w a t e r i s d r a i n e d . A s m a l l p a r t o f t h e u n d e r s a t u r a t e d mother l i q u o r w i t h d r a w n f r o m t h e 0.6 m3 f i n e s
d i s s o l u t i o n t a n k , may be r e c y c l e d t o t h e f e e d p r e p a r a t i o n t a n k ( c l e a r l i q u o r a d v a n c e ) . The f l o w - r a t e o f t h e c l e a r l i q u i d w h i c h i s measured w i t h a m a g n e t i c f l o w i n d i c a t o r , can be v a r i e d from 0 t o 14- 1/min w i t h h e l p o f a n e e d l e t y p e v a l v e . The vapour space o f t h e D T B - c r y s t a l l i z e r i s c o n n e c t e d w i t h t h e c o n d e n s o r . The w a t e r damp i s c o n d e n s a t e d by s u p p l y i n g companies w a t e r t o a s p i r a l c o o l e r i n t h e t o p - s e c t i o n o f t h e l a t t e r d e v i c e . Non-condensable gases a r e removed w i t h h e l p o f a w a t e r r i n g vacuumpump. The c r y s t a l l i z e r t e m p e r a t u r e i s k e p t c o n s t a n t by c o n t r o l i n g t h e v a p o u r p r e s s u r e above t h e b o i l i n g zone by means o f t h e t h r o u g h -put o f c o o l a n t i n t h e c o n d e n s o r . By r e g i s t r a t i n g t h e c o n d e n s a t e l e v e l as f u n c t i o n o f t i m e , t h e v a p o u r p r o d u c t i o n - r a t e can be c a l c u l a t e d . A 2.5 m3 s t o r a g e t a n k i s
used t o c o l l e c t a l l s l u r r y and m o t h e r - l i q u o r w i t h i n t h e c r y s t a l l i z e r equipment a t t h e end o f one o r more e x p e r i m e n t a l r u n s . The l a t t e r tank w h i c h i s steam h e a t e d and t e m p e r a t u r e c o n t r o l e d , i s a g i t a t e d by a 4- b l a d e d f l a t d i s c t u r b i n e . When s t a r t i n g - u p , a pump i s used t o t r a n s p o r t t h e s l u r r y from t h e s t o r a g e t o t h e f e e d p r e p a r a t i o n t a n k . The t e m p e r a t u r e o f a l l i n - and o u t g o i n g streams o f t h e D T B - c r y s t a l l i z e r a r e measured and r e g i s t r a t e d w i t h h e l p o f a 24- c h a n n e l r e c o r d e r . Moreover, t h e t e m p e r a t u r e i n t h e d e a i r a t o r and a l l t a n k s a r e measured and r e g i s t r a -t e d w i -t h h e l p o f -t h e l a -t -t e r r e c o r d e r . I n a l l c r y s -t a l l i z a -t i o n e x p e r i m e n -t s r e p o r -t e d o f i n t h i s work, t h e p o t a s h alum-water system i s u s e d . M a t e r i a l s i n c o n t a c t w i t h p o t a s h alum a r e s t a i n l e s s s t e e l 316 o r s y n t h e t i c s . The p r o d u c t d i s c h a r g e i s sampled t o measure CSD and s l u r r y d e n s i t y . I n h a v i n g c o n s t a n t t e m p e r a t u r e s and c r y s t a l s i n s u s p e n s i o n , t h e CSD i s assumed t o be a s t e a d y - s t a t e d i s t r i b u t i o n a f t e r a minimum c r y s t a l l i z a t i o n d u r a t i o n o f 10 t i m e s t h e mean r e s i d e n c e t i m e o f t h e p r o d u c t .
2.2 DTB-Crnis taj^^zev
I n f i g u r e 2.2 a s c h e m a t i c diagram o f t h e D r a f t _Tube B a f f l e (DTB) c r y s t a l l i z e r as used i n t h i s s t u d y , i s shown. The l a t t e r c r y s t a l l i z e r w h i c h i s c o n s t r u c t e d o f p l e x i g l a s s and s t a i n l e s s s t e e l 316, has a t o t a l volume o f a p p r o x i m a t e l y 925 l i t r e . A g i t a t i o n i s a c h i e v e d u s i n g a 0.2 m marine p r o p e l l e r w i t h s l u r r y f l o w up t h e
d r a f t t u b e . The number o f r e v o l u t i o n s o f t h e marine t y p e i m p e l l e r can under a t m o s p h e r i c c o n d i t i o n s , be v a r i e d from 0 t o 1150 R.P.M. i n u s i n g w a t e r . A b o t t o m e n t e r i n g a g i t a t o r i s used t o e l i m i n a t e t h e need o f an i n t e r n a l b e a r i n g and t o p r e c l u d e problems o f i n c r u s t a t i o n on t h e s h a f t as i t t r a v e r s e s t h e b o i l i n g zone i n t o p - d r i v e n o p e r a t i o n . S i n c e t h e e f f e c t o f any a i r l e a k a g e i s v e r y l a r g e , s e a l i n g o f the s h a f t i s a c h i e v e d by t h e use o f a double m e c h a n i c a l s e a l . S e a l w a t e r i s w i t h d r a w n from and r e c y c l e d t o a s m a l l w a t e r t a n k w h i c h i s e l e c t r i c a l l y h e a t e d . The t e m p e r a t u r e i s m a n u a l l y c o n t r o l e d by a d j u s t i n g t h e companies w a t e r s u p p l y t o t h i s t a n k . S u r p l u s o f w a t e r i s d r a i n e d . The p r o f i l e d b o t t o m w h i c h i s made o f p l e x i g l a s s , i s shaped i n s u c h a manner t h a t t h e c r o s s s e c t i o n a l a r e a p e r p e n d i c u l a r t o t h e c o n t o u r s o f t h e h e m i s p h e r i c a l bottom end o f t h e d r a f t t u b e r e -mains c o n s t a n t . The d r a f t tube i s by way o f b a f f l e s c o n n e c t e d w i t h t h e c e n t r a l p a r t o f t h e c r y s t a l l i z e r . To m i n i m i s e h e a t s h o r t - c i r c u i t i n g , t h e t o p end o f t h e d r a f t t u b e i s a d d i t i o n a l l y p r o v i d e d w i t h a s p e c i a l l y shaped p l e x i g l a s s "dummy". T h i s arrangement a l s o p r e v e n t s f l a s h i n g o f mother l i q u o r due t o a p r e s s u r e drop o v e r t h e o u t l e t o f t h e d r a f t t u b e . The l i q u i d l e v e l i s about 0.4- m above t h e t o p o f t h e dummy. P r o d u c t i s 1.3 m below t h e l i q u i d l e v e l , w i t h d r a w n from t h e DTB-c r y s t a l l i z e r w i t h h e l p o f a 12 mm r e m o v a l t u b e p o s i t i o n e d h a l f w a y t h e d i s t a n DTB-c e between t h e d r a f t t u b e and t h e c r y s t a l l i z e r w a l l . To c r e a t e i s o k i n e t i c p r o d u c t r e m o v a l c o n d i t i o n s , t h e p r o d u c t d i s c h a r g e v e l o c i t y i s t h e same i n magnitude and d i r e c t i o n as t h e f l u i d v e l o c i t y o f t h e i n t e r n a l l y c i r c u l a t i n g s l u r r y . I n u s i n g w a t e r , t h e downwards d i r e c t e d f l u i d v e l o c i t i e s n e a r t h e p o i n t o f p r o d u c t d i s -c h a r g e were measured w i t h h e l p o f a -c i l i n d r i -c a l t h r e e h o l e p i t o t t u b e . The
-3.0 •J 2.5 -2.0 1.5 -1.0 0.5 vA X( m / s ) vA X = 2.085 x 10 ( NI M p - 80) Medium: Water 0.2 m Marine p r o p e l l o r A t m o s p h e r i c C o n d i t i o n s • E x p e r i m e n t a l r e s u l t Manual f i t NI M p( R . P . M . ) — r ~ 600 00
— I —
1200 1— 1400 400 600 800 1000Figure 2.3 Axial fluid velocities
e x p e r i m e n t a l r e s u l t s w h i c h a r e g i v e n i n f i g u r e 2.3, show a l i n e a r r e l a t i o n between t h e mean a x i a l f l u i d v e l o c i t y and t h e i m p e l l e r - s p e e d . I n f o r m u l a :
a x 2.085 x 10 (N IMP 80) (2.1) w i t h vax t h e mean a x i a l f l u i d v e l o c i t y i n m/s and N j ^ p t h e i m p e l l e r - s p e e d i n R.P.M. The p r o d u c t w i t h d r a w a l - r a t e can be g i v e n a s : d>„ = — d v yP 4 ax (2.2) S i n c e t h e p r o d u c t r e m o v a l tube had a d i a m e t e r o f 12 mm i n a l l e x p e r i m e n t s r e -p o r t e d o f i n t h i s work, c o m b i n a t i o n o f e q u a t i o n (2.1) and (2.2) g i v e s : 1.415 x 10 (N IMP 80) (2.3) w i t h <j»p t h e p r o d u c t w i t h d r a w a l - r a t e i n 1/min.
S i n c e t h e number o f r e v o l u t i o n s o f t h e i m p e l l e r can e a s i l y be measured w i t h h e l p o f a speed i n d i c a t o r , t h e l a t t e r e q u a t i o n e n a b l e s t h e c a l c u l a t i o n o f t h e r a t e a t w h i c h i s o k i n e t i c p r o d u c t r e m o v a l w i l l o c c u r . From f i g u r e 2.2 and 2.3 i t can be d e d u c t e d t h a t t h e w a t e r c i r c u l a t i o n r a t e may be v a r i e d from 0 t o 2440 1/min. The 1 n r D T B - c r y s t a l l i z e r i s p r o v i d e d w i t h an i n t e r n a l b a f f l e t o f a c i l i t a t e t h e r e m o v a l o f s m a l l e r s i z e d c r y s t a l s f r o m t h e w e l l - m i x e d volume. The p r i n c i p a l d i m e n s i o n s o f t h e a n n u l a r s e t t l i n g space a r e g i v e n i n f i g u r e 2.2. The h e i g h t o f t h e c l e a r e n c e w h i c h c o u l d be v a r i e d i n p r i n c i p l e , was k e p t c o n s t a n t a t 0.35 m i n
-a l l e x p e r i m e n t s r e p o r t e d o f i n t h i s work. The s-ame -a p p l i e s t o t h e -a n g l e o f e n t r a n c e w h i c h was k e p t c o n s t a n t a t 45°. The h e i g h t o f s e d i m e n t a t i o n w h i c h i s d e f i n e d as t h e d i s t a n c e between t h e upper p a r t o f t h e c l e a r e n c e and t h e p o i n t o f w i t h d r a w a l , can be e q u a l i z e d t o a 0.36, 0.86 and 1.26 m by u s i n g one o u t o f t h r e e s e t s o f r e m o v a l t u b e s o f d i f f e r e n t l e n g t h . The r e m o v a l t u b e s o f w h i c h t h e e f f e c t i v e number c o u l d be v a r i e d from 1 t o 12, a r e a t r e g u l a r d i s t a n c e s f r o m each o t h e r , p o s i t i o n e d i n t h e c e n t r e o f a c r o s s - s e c t i o n a l a r e a o f t h e a n n u l a r volume. The l a t t e r t u b e s a r e by way o f f l e x i b l e t u b e s , c o n n e c t e d w i t h t h e c o l l e c t o r . A l l r e m o v a l t u b e s a r e p r o v i d e d w i t h a b e l l - e n d t o r e d u c e p a r t i c l e a c c e l e r a t i o n e f f e c t s n e a r t h e p o i n t o f w i t h d r a w a l .
2.3 d ^ r c ^ ^ e ^ g e w e n t
The a u t h o r w i s h e s t o acknowledge t h e e f f o r t s o f H. B i k , A. de B r u i n , A.C. G r o o t , J . J . H e e r e n s , A. Hoebeke, W. H o o g s t a d , J.H. t e r H o r s t , C.J. K u n t z e l , H.P. Lange-v e l d , B.H. Makkus, F.H.E. P i q u i l l e t , J.H.P. R o t t e Lange-v e e l , C. Lange-van d e r Snoek, B.N. S o d d e r l a n d , J.A. de V r i e s , M. van der Weegen, B.. de W o l f f , t h e C e n t r a l Workshop o f t h e Department o f M e c h a n i c a l E n g i n e e r i n g and a l l o t h e r p e r s o n s and f i r m s d i -r e c t l y o -r i n d i -r e c t l y i n v o l v e d d u -r i n g d e s i g n and c o n s t -r u c t i o n o f the e x p e -r i m e n t a l equipment and, i n v o l v e d d u r i n g t h e r e a l i z a t i o n o f the e x p e r i m e n t a l programm.
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