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Pergamon

Chemical En ineerin9 Science, Vol. 52, No. 15, pp. 2471-2482, 1997

( 1997 Elsevier Science Ltd. All rights reserved

Printed in Great Britain

P I I :

S 0 0 0 9 - 2 5 0 9 ( 9 7 ) 0 0 0 6 6 - 3

0009-2509/97 17.00 + 0.00

Circulat ing f lu id ized-bed reactors

D a i z o K u n i i * a n d O c t a v e L e v e n s p i e l * *

1 - 2 5 - 1 6 N a k a m a c h i , M e g u r o k u , T o k y o 1 5 3 , J a p a n ; * C h e m i c a l E n g i n e e r i n g D e p a r t m e n t ,

Or e g o n S t a t e U n i v e r s i t y , C o r v a l l i s , OR , U . S . A .

( A c c e p t e d 1 5 M a y 1 9 9 6 )

A b s t r a c t - - T h i s p a p e r d e v e l o p s a fl o w a n d c o n t a c t i n g m o d e l t o r e p r e s e n t a C F B . B e s t e s t im a t e s

o f c o n t a c t i n g e ff ic ie n ci es a r e p r e s e n t e d f o r t h e t u r b u l e n t , f a s t f l u id i z e d , a n d p n e u m a t i c t r a n s p o r t

r e g im e s o f t h e C F B . M a t e r i a l b a l a n c e s a r e p r e s en t e d , e n d i n g u p w i t h c o n v e r s i o n e q u a t i o n s f o r

f i rs t - o rd e r s o l i d - c a t a ly z e d g a s - p h a s e r e a c ti o n s . F o u r e x a m p l e s s h o w h o w t o u s e t h i s m o d e l a n d

s h o w i t s p r e d i c t io n s . 1 9 97 E ls e v i e r S c ie n c e L t d

K e y w o r d s : C F B r e a c t o r s ; c o n v e r s i o n ; c i r c u l a t i n g s o l i d s r e a c t o r s .

1. INTRODUCTION

L e t u s f i rs t s k e t c h t h e c o n t a c t i n g r e g i m e s e n c o u n t e r e d

w h e n a b e d o f s o li d s is f lu i d i z e d b y g a s a t p r o g r e s s -

ive ly h ighe r veloc it ie s . Bec ause co n ta c t in g d if fe r s in

these co n ta c t in g reg imes , d i f fe ren t r ea c to r mo de ls qSsdsCr

m u s t b e u s e d f o r p r e d i c t i n g r e a c t o r b e h a v i o r ; s ee

Fig. 1.

This pa pe r dea l s w i th c i rcu la t ing so l ids reac to rs , ds~r

T h e s e c o n t a i n v e r y f in e p a r ti c l e s w h i c h a r e f l u i d i z e d a t

a r a t h e r h i g h g a s v e l o c i t y , a r e b l o w n o u t o f t h e b e d dp ~-

a n d r e a c t o r , a n d h a v e t o b e r e p l a c e d b y f r e s h s o l i d s .

W e ca l l these c i rcu la t ing f lu id ized bed s (CFB) . q52dscr

W e m u s t f i r s t f i n d w h a t c o n t a c t i n g r e g i m e i s i n -

v o l v e d , a n d t h e n d e a l w i th t h e r e a c t o r a n d i ts c o n v e r -

s i o n e q u a t i o n s .

r e l a ti o n s h i p b e t w e e n d s . a n d d p . T h e b e s t w e c a n s a y

f o r p re s s u r e d r o p c o n s i d e r a t i o n s i s

f o r i r r e g u l a r p a r t i c l e s w i t h n o ( 4)

s e e m i n g lo n g e r o r s h o r t e r

d i m e n s i o n

f o r i r r e g u l a r p a r t i c l e s w i t h o n e

s o m e w h a t l o n g e r d i m e n s i o n b u t

w i t h l e n g t h r a t i o n o t g r e a t e r ( 5)

t h a n 2 : 1 ( e gg s , f o r e x a m p l e )

f o r ir r e g u l a r p a r t i c l e s w i t h o n e

s h o r t e r d i m e n s i o n b u t w i t h ( 6)

l e n g t h r a t i o n o t l e s s t h a n 1 : 2

(p il lows , for examp le )

2. CHARACTERIZATION F PARTICLES

T h e e q u i v a l e n t s p h e r i c a l p a r t i c l e d i a m e t e r i s d e -

f ined as

d i a m e t e r o f a s p h e r e ) ( ~ _ ) 1 / 3

dsph = wh ich has the s am e = (1)

v o l u m e a s t h e p a r t i c le , V

an d the pa r t i c le sp her ic i ty q~s i s de f ined as

= ( s u r f a c e o_f a_ s p h e r e ' ]

qS~ \ surfa ce of pa rt ic le J . . . . . . lume (2)

F r o m t h e a b o v e t w o e x p r e s s io n s w e d e f in e t h e p a r t i c le

s i z e t o b e u s e d f o r s u s p e n d e d s o l i d r e a c t o r s a s

dp = ~b~dsph. (3)

F o r f i n e p a r t i c l e s w e e v a l u a t e t h e s i ze b y s c r e e n a n a l y -

s is , w h i c h g i v e s d sc r. U n f o r t u n a t e l y , t h e r e i s n o g e n e r a l

:Corresponding author.

3. GAS/SOLIDCONTACTINGREGIMES

F o r g i v e n p a r t i c l e s (d p a n d p s ) a n d g i v e n s u p e r f i c ia l

g a s v e l o c i t y t h r o u g h t h e b e d ( U o ), w e f i r st n e e d t o f i n d

w h a t c o n t a c t i n g r e g i m e i s i n v o l v e d - - p a c k e d b e d ,

b u b b l i n g f l u i d iz e d b e d ( B F B ), o r c i r c u l a t i n g f l u i d iz e d

b e d ( C F B ) , w i t h i t s s u b r e g i m e s - - t u r b u l e n t f l u i d i z e d ,

f a s t f l u id i z e d , o r p n e u m a t i c t r a n s p o r t . T o d o t h i s f i rs t

e v a l u a t e t h e d i m e n s i o n l e s s m e a s u r e s o f p a r t i c l e s iz e

a n d g a s v e l o c i t y . T h e s e a r e d e f i n e d a s

d V p ~ ( p ~ - p g ) g ] , / 3

= Y (7)

= u - . ( 8 )

#(p~ - pg)g

3 .1 . M i n i m u m f l u i d iz i n g v e l o c it y

T h e s o l i d s w i l l b e s u s p e n d e d w h e n t h e p r e s s u r e

d r o p e x c e e d s th e w e i g h t o f s o li d s. T h i s h a p p e n s w h e n

t h e g a s v e l o c i t y e x c e e d s t h e m i n i m u m f l u i d i z i n g v e l o -

c i t y . T h i s v e l o c i t y i s g i v e n b y E r g u n ( 1 9 5 2 ) , a n d i n

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