Fluidized-Bed Reactors

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P. N . ROWE University College London, Department of Chemical and Biochemical Engineering, Torrington Place, London WC1E 7JE, England

This Plenary Lecture surveys the place of fluidised beds within the field of chemical reaction engineering and describes some developments and changes of interest over the last decade. It concludes by suggesting the direction basic research should take in the future.

The previous s i x I n t e r n a t i o n a l Symposia on CRE have included 56 plenary l e c t u r e s three of which were devoted exc l u s i v e l y to f l u i d i s e d bed r e a c t o r s and a f u r t h e r two were subs t a n t i a l l y concerned w i t h them (1-5). This i s a l e v e l of i n t e r e s t equalled only by the general subject of o p t i m i s a t i o n , s t a b i l i t y and c o n t r o l and exceeded only be plenary l e c t u r e s concerned w i t h k i n e t i c s ( j u s t exceeded; there were s i x ) . Here we are w i t h yet another t a s t e of t h i s f a s c i n a t i n g , i n t r i g u i n g , c h a l l e n g i n g , i n t e r e s t - f u l l , promising and v e r s a t i l e form of chemical r e a c t o r . There i s a l o t of j u i c e yet l e f t i n such a succulent subject although i t becomes i n c r e a s i n g l y d i f f i c u l t to t r e a t w i t h o r i g i n a l i t y a t o p i c that has been mouthed by so many. Two years ago we were t r e a t e d to a n o s t a l g i c account of the coming of age of chemical r e a c t i o n engineering by P r o f e s s o r L e v e n s p i e l who dated i t s b i r t h as 1957 and i t s p o s s i b l e conception as 1947 (6). U n c h a r a c t e r i s t i c a l l y Octave seems to have overlooked Denbigh's c l a s s i c a l paper (7) submitted i n 1943 i n which Denbigh described and defined the continuous f l o w t u b u l a r r e a c t o r and the 'continuous flow s t i r r e d tank r e a c t o r f o r each of which he showed how to c a l c u l a t e both y i e l d and s i z e of r e a c t o r needed f o r given orders of r e a c t i o n and r e q u i r e d production r a t e s . He a l s o showed how s e l e c t i v i t y would depend on the order of competing r e a c t i o n s . This and Danckwerts c l a s s i c a l paper on residence time d i s t r i b u t i o n s (8) submitted i n 1952, l a i d most o f the founda t i o n s on which chemical r e a c t i o n engineering has since b u i l t . I t i s an i n t e r e s t i n g experience to re-read these papers, e s p e c i a l l y Denbigh's. I t i s q u i t e a long paper (22 pages, he 1

0097-6156/83/0226-0049$06.00/0 © 1983 American Chemical Society

Wei and Georgakis; Chemical Reaction Engineering—Plenary Lectures ACS Symposium Series; American Chemical Society: Washington, DC, 1983.

C H E M I C A L REACTION ENGINEERING

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would never get away w i t h that l e n g t h today) and gives an imp r e s s i o n of being r a t h e r tedious and t a k i n g a long time to come to the p o i n t . This i s a poignant reminder of how deeply we have accepted h i s ideas and now take f o r granted t h i s imaginative and f r u i t f u l approach. The heart of h i s i d e a i s the b e a u t i f u l l y l o g i c a l combination of b a s i c laws of chemistry w i t h simple models of how the r e a c t a n t s come i n t o contact w i t h each other. The PFR and CSTR are r e a d i l y imagined and widely a p p l i c a b l e d e s c r i p t i o n s of how r e a l l a r g e s c a l e r e a c t o r s a c t u a l l y behave. These models of the patterns of r e a c t a n t c o n t a c t i n g form the b a s i s f o r much of present day chemical r e a c t i o n engineering. The reason f o r me reminding you of these foundations i s to draw a t t e n t i o n to the s c a r c i t y of f u r t h e r models beyond these two b a s i c types. The bubbling gas f l u i d i s e d bed i s one of the very few a d d i t i o n a l models a l b e i t a much more complicated one and n e c e s s a r i l y more l i m i t e d i n a p p l i c a t i o n . This i s why i t i s such a f a s c i n a t i n g subject to chemical r e a c t i o n engineers. To r e t u r n f o r a moment to the t r a d i t i o n a l models, they have been expanded, elaborated upon and mixed i n a l l p o s s i b l e (and some impossible) proportions and, w i t h the w i l l i n g a i d of modern computational methods, developed to a l e v e l of complexity that i s i n some cases out of touch w i t h r e a l i t y and c e r t a i n l y no longer p h y s i c a l l y imaginable but t h i s process of p a r a l y s i s by a n a l y s i s i s the f a t e of many o r i g i n a l l y simple i d e a s . F o r t u n a t e l y f l u i d i s e d bed r e a c t o r models are s t i l l reasonably c l o s e l y r e l a t e d to what a c t u a l l y happens and by and l a r g e the models can s t i l l be imagined as p h y s i c a l r e a l i t i e s . At t h i s stage I should f o r the b e n e f i t of those l e s s f a m i l i a r w i t h f l u i d i s e d beds spend a few minutes d e s c r i b i n g the p r i n c i p a l features that govern t h e i r behaviour as r e a c t o r s ( 9 ) . A l l powders bubble when f l u i d i s e d by gas at v e l o c i t i e s i n excess of a minimum value and look r a t h e r as i n F i g u r e 1 which i s a photograph of a two-dimensional bed sandwiched between glass p l a t e s . I t i s the bubbles that cause p a r t i c l e mixing and b r i n g about the h i g h heat t r a n s f e r r a t e s that can occur between the bed and w a l l s or immersed s u r f a c e s . They are r e s p o n s i b l e f o r the high degree of u n i f o r m i t y , e s p e c i a l l y of temperature and p a r t i c l e composition, w i t h i n the bed and without them the system would behave more or l e s s as a packed bed. Bubbling leads immediately to the concept of two phases w i t h p a r t of the gas f l o w i n g i n t e r s t i t i a l l y amongst c l o s e l y spaced p a r t i c l e s and the r e s t f l o w i n g i n the form of bubbles. The i n t e r s t i t i a l f l o w remains constant at the minimum f l u i d i s a t i o n value although f i n e powders may expand a l i t t l e and permit a r a t h e r l a r g e r flow. The bubble flow i s t h e r e f o r e e a s i l y evaluated from Q

B

=

Q

-

and the f a c t that

Qi =

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Fluidized-Bed Reactors

Figure 1. Bubbles i n a two-dimensional f l u i d i z e d bed.


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The bubbles coalesce r e a d i l y so that they may be small and numerous at the bottom and large and few at the top w h i l s t maint a i n i n g s u b s t a n t i a l l y constant flow. Average bubble s i z e i n creases w i t h height as i n d i c a t e d i n F i g u r e 2. These simple f a c t s a l l o w an immediate c o n s i d e r a t i o n of r e a c t o r v e s s e l s i z i n g (10). P r o d u c t i o n requirements w i l l determine the r e a c t a n t gas feed r a t e and bed diameter f i x e s v e l o c i t y and thence the bubbling r a t e . Heat t r a n s f e r c o n s i d e r a t i o n s and blower c a p a c i t y are l i k e l y to set l i m i t s to the bed height or aspect r a t i o and bed diameter must accommodate the bubbles. What would be a bubble i n a l a r g e r e a c t o r at a given height and v e l o c i t y would be a s l u g i n a narrow one as Figure 3 i n d i c a t e s . The slugging bed g e n e r a l l y gives b e t t e r g a s / s o l i d c o n t a c t i n g but must be modelled d i f f e r e n t l y from the bubbling bed and r e q u i r e s greater free-board. In the past p i l o t p l a n t s have operated i n the slugging regime and reduced performance was experienced when s c a l i n g up simply by i n c r e a s i n g diameter but no one should make that mistake today. P a r t i c l e mixing i s caused by the bubbles, p a r t l y be shear displacement or d r i f t but a l s o by the bulk t r a n s p o r t of p a r t i c l e s i n the bubble wake. Bubbles may a l s o cause segregation i f there are d i f f e r e n t kinds of p a r t i c l e s present. U n l i k e other kinds of mixers, segregation i s i n s e n s i t i v e to p a r t i c l e s i z e d i f f e r e n c e but p a r t i c u l a r l y s e n s i t i v e to d e n s i t y d i f f e r e n c e . I n a b i n a r y system of p a r t i c l e s segregation increases approximately as p a r t i c l e d e n s i t y r a t i o to the power 5/2 but w i t h p a r t i c l e s i z e r a t i o only to the power 1/5 (11). This can cause problems i n , f o r example, c o a l combustion where char has a markedly lower d e n s i t y than ash and a l s o i n some ore r e d u c t i o n processes using coke. Chemical r e a c t o r models i n v a r i a b l y s t a r t from the two-phase theory (12). The i n t e r s t i t i a l f l o w i s assumed to be i n good and continuous contact w i t h s o l i d s w h i l s t some by-passing occurs i n the bubble phase. There i s , however, very l i t t l e a x i a l or r a d i a l mixing of the gas. There may be some exchange between the two phases and F i g u r e 4 d e p i c t s t h i s k i n d of model. A major reason f o r by-passing i n the bubble phase i s the formation of clouds or s p h e r i c a l v o r t i c e s centred on the bubbles, a s i t u a t i o n that occurs when bubbles r i s e f a s t e r than i n t e r s t i t i a l gas which i s commonly the case. Gas i s o b l i g e d to f l o w upwards through the bubble because of pressure d i f f e r e n c e but as i t r e - e n t e r s the dense phase ahead i t i s caught i n a stream of p a r t i c l e s f l o w i n g downwards r e l a t i v e to the bubble and, when the f l o w i s f a s t enough, i t i s trapped and dragged downwards u n t i l pressure d i f f e r e n c e pushes i t back i n t o the bubble. This i s i l l u s t r a t e d i n Figure 5. Only that p o r t i o n of the cloud or v o r t e x that extends beyond the bubble i s i n contact w i t h p a r t i c l e s andtherefore able to r e a c t . In many r e a l i s t i c s i t u a t i o n s t h i s p r o p o r t i o n can be very s m a l l . At any i n s t a n t only t h i s proport i o n of the t o t a l cloud i s r e a c t i n g so that the e f f e c t i v e r a t e constant i s reduced. The whole i s c i r c u l a t i n g as a v o r t e x w i t h


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g'4

increasing gas flow

Ο

Ο Ο ο

ο ο

^ ο ο

»

* ° Ο

ο ° ο|

tut Ο

Figure 2. Bubbles s i z e increases w i t h bed height and flow r a t e .

Figure 3. A bubble i n a l a r g e bed becomes a s l u g i n a s m a l l one.


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interstitial

" exchange

phase

Figure h.

ENGINEERING

bubble phase

B a s i c two-phase bubble model.

particle •

flow

V

induced •

Figure 5· clouds.

gas circulation

P a r t i c l e flow induces gas c i r c u l a t i o n and so forms


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upward v e l o c i t y through the empty space of the bubble of s i m i l a r magnitude to minimum f l u i d i s a t i o n v e l o c i t y . With f i n e p a r t i c l e s t h i s i s small so that l i t t l e c i r c u l a t i o n occurs during the r e s i dence time of a bubble i n the bed. This i s the very s i t u a t i o n where the cloud cannot penetrate f a r i n t o the dense phase so the o v e r a l l e f f e c t i s poor c o n t a c t i n g o f bubble phase gas w i t h p a r t icles. Severe by-passing i n the bubble phase and very poor gas mixing can make the f l u i d i s e d bed an extremely poor chemical r e a c t o r but a t l e a s t the major reasons f o r t h i s are now understood. The prospects o f success f o r a given process can be judged a t an e a r l y stage and design can minimize by-passing u s u a l l y by choosing p a r t i c l e s i z e a p p r o p r i a t e l y . What development and progress has occurred during the l a s t ten years or so? Experience has accumulated and a number of f l u i d i s e d bed processes are operated s u c c e s s f u l l y . I n d u s t r i a l companies are r e l u c t a n t to d i s c l o s e too many d e t a i l s o f a successf u l and p r o d u c t i v e r e a c t o r but I have seen beds as l a r g e as 17 m i n diameter very uniformly and s t a b l y f l u i d i s e d which could be shut down and r e - s t a r t e d without t r o u b l e . During the l a s t decade much i n t e r e s t has centred on the behaviour of l a r g e p a r t i c l e s (diameters i n m i l l i m e t e r s r a t h e r than microns) as r e q u i r e d i n f l u i d i s e d c o a l combustion. The t o p i c has generated two l a r g e conferences r e c e n t l y i n the U.K. alone (13, 14) and many more i n the U.S.A. This i s the subject of the next Plenary Lecture so I w i l l only mention some f e a t u r e s b r i e f l y . Large p a r t i c l e s are necessary i n t h i s a p p l i c a t i o n because high f l o w r a t e s are r e q u i r e d since the f l u i d i s i n g a i r i s a l s o combustion a i r and a high i n t e n s i t y of heat generation i s aimed f o r . I n order f u l l y t o convert oxygen to C O 2 , c o a l conce n t r a t i o n i n the bed i s low (a few percent) and most of i t i s ash or a chosen r e f r a c t o r y granular m a t e r i a l such as sand. Limestone may be a d d i t i o n a l l y included to adsorb S O 2 . These p a r t i c l e s must be f a i r l y l a r g e t o avoid e l u t r i a t i o n . Furthermore, the bed i s shallow to minimise the pressure drop the fans are r e q u i r e d to overcome. There i s l i k e l y t o be a l o t o f heat exchanger surface w i t h i n the bed. Of course, gas v e l o c i t y and hence the need f o r l a r g e p a r t i c l e s can be reduced by i n c r e a s i n g absolute pressure. These c o n d i t i o n s have not been widely studied i n the l a b o r a t o r y because of the obvious d i f f i c u l t i e s of p r o v i d i n g high a i r v e l o c i t i e s (a few meters/sec) on a s c a l e that i s l a r g e compared w i t h the s i z e of the p a r t i c l e s . C e r t a i n l y i n t e r s t i t i a l gas f l o w w i l l f a r exceed bubble r i s e v e l o c i t y and there i s no p o s s i b i l i t y of cloud formation as shown i n Figure 5. Instead gas w i l l f l o w s t r a i g h t through the bubbles and the o v e r a l l p a t t e r n w i l l approximate to plug flow. Because o f the l a r g e d i f f e r e n c e i n d e n s i t y between c o a l char and other bed m a t e r i a l the former w i l l tend to f l o a t . This i s a c o m p l i c a t i o n imposed upon what would otherwise be a w e l l mixed p a r t i c l e bed. E f f i c i e n t in-bed combustion r e q u i r e s uniform d i s p e r s i o n of c o a l but the process of p a r t i c l e


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segregation i s q u i t e w e l l understood (15) and the system can be designed to be reasonably uniform under operating c o n d i t i o n s . S a t i s f a c t o r y models e x i s t f o r p r e d i c t i o n of combustor performance although there i s some d i f f i c u l t y i n c o r r e c t l y e s t i m a t i n g the temperature of a burning c o a l p a r t i c l e . The major design problems are of the broadly mechanical k i n d such as c o a l feeding and i n i t i a l d i s t r i b u t i o n , heat exchanger design and ash removal. At the other end of the spectrum there i s c o n t i n u i n g i n t e r e s t i n the behaviour of f i n e p a r t i c l e s (approximately l e s s than 100 ym) because c a t a l y s t powders are g e n e r a l l y i n t h i s range. F l u i d bed c r a c k i n g c a t a l y s t i s the best known example. Geldart (16) made an important c o n t r i b u t i o n when he proposed a c l a s s i f i c a t i o n of powders and l a b e l l e d t h i s f i n e m a t e r i a l Type A. The c h a r a c t e r i s t i c f e a t u r e of them i s that they expand u n i f o r m l y at v e l o c i t i e s not g r e a t l y i n excess of U f as i l l u s t r a t e d i n F i g u r e 6. Only at some higher v e l o c i t y , U ^ do they begin to bubble. Values of U b/U £ can be as high as 3 or 4 and the bed volume can double i n extreme cases before bubbling begins. I f the dense phase expands t h i s immediately a f f e c t s the r e a c t o r model because more gas w i l l then flow v i a the favourable i n t e r s t i t i a l phase. Most models r e a d i l y a l l o w f o r t h i s change given that the t r u e d i v i s i o n of f l o w can be p r e d i c t e d . Unh a p p i l y i t has so f a r only been p o s s i b l e to measure t h i s d i v i s i o n experimentally at f a i r l y low flow r a t e s , w e l l below those employed i n commercial r e a c t o r s . C e r t a i n l y at v e l o c i t i e s up to about 15 cm/s much more gas flows i n t e r s t i t i a l l y through Geldart type A powders than minimum f l u i d i s a t i o n f l o w (17). I n d u s t r i a l r e a c t o r s g e n e r a l l y operate a t very high v e l o c i t i e s (of order 1 m/s) much i n excess of t e r m i n a l f a l l i n g v e l o c i t y f o r at l e a s t the f i n e s t powder f r a c t i o n s . Powder i s c o n t i n u a l l y e l u t r i a t e d and returned to the bed v i a cyclones. Under these c o n d i t i o n s there i s disagreement as to whether or not bubbles r e t a i n t h e i r i d e n t i t y and such beds have been described as " t u r b u l e n t " or " f a s t f l u i d i s e d " . What l i t t l e evidence there i s supports the continued existence of bubbles but now i n a much d i s t u r b e d or heterogeneous dense phase and w i t h a l e s s d e f i n i t e shape. U n t i l more i s known about t h i s p h y s i c a l s i t u a t i o n i t i s not easy to see how the bubbling bed r e a c t o r models should be modified c o r r e c t l y to d e s c r i b e t h i s f l o w regime. I t i s too simple to assume that average p a r t i c l e s i z e i s an adequate index of powder type i n G e l d a r t s c l a s s i f i c a t i o n . C e r t a i n l y type A powders depend s t r o n g l y on d i s t r i b u t i o n of s i z e and p a r t i c u l a r l y on the p r o p o r t i o n at the lowest end of the range. There i s p l e n t y of i n d u s t r i a l experience to support t h i s view and p l a n t operators u s u a l l y acknowledge the importance of m a i n t a i n i n g a c e r t a i n p r o p o r t i o n of f i n e s i n the r e a c t o r . W h i l s t the v i t a l r o l e of the f i n e s f r a c t i o n has been recognised i t i s a d i f f i c u l t dependence to study s y t e m a t i c a l l y . The two-phase bubbling bed model i s capable of many minor adjustments and has given numerous academics a l o t of fun p l a y i n g m

m

m

m

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v a r i a t i o n s on the b a s i c tune. There are more models than data against which to t e s t them and s u i t a b l e data f o r t h i s purpose are not e a s i l y obtained. Whatever the model chosen i t i s commonly observed that the r e a c t i o n r a t e i s unusually h i g h i n the bottom few centimeters of the bed and t h i s i s o f t e n r e f e r r e d to as the distributor effect. The d i s t r i b u t o r p l a t e i n a f u l l s c a l e r e a c t o r i s u s u a l l y d r i l l e d w i t h a number of holes s e v e r a l m i l l i m e t e r s i n diameter and the gas v e l o c i t y through these i s h i g h (tens of m/s). I t i s easy to imagine that t h i s produces a spout or j e t of gas above which a stream of bubbles occurs. Powder could be entrained i n t o the j e t as i n a spouted bed and i t has been imagined that t h i s p o t e n t i a l l y good g a s / s o l i d s c o n t a c t i n g arrangement i s r e s p o n s i b l e for a h i g h r a t e of chemical conversion (18). Although a t t r a c t i v e t h i s model i s q u i t e wrong simply because gas does not form a j e t above the d i s t r i b u t i o n o r i f i c e but enters the bed i n the form of bubbles j u s t as, f o r example, a i r blown i n t o water (19) - F i g u r e 7. Here I must pause to emphasise the importance of i d e n t i f y i n g c o r r e c t l y the mechanism by which events occur which can o n l y be done by s u i t a b l e e x p e r i mental o b s e r v a t i o n . The evidence comes from X-ray ciné photographs taken at 50 frames/s w i t h an exposure time of 1 ms which c o n d i t i o n s a l l o w the very r a p i d events to be c l e a r l y followed. As Figure 8 shows, the bubble forms as a near p e r f e c t sphere above the h o l e , detaches and r i s e s and w i t h i n a very short d i s t a n c e , assumes i t s c h a r a c t e r i s t i c shape w i t h an indented base as the wake forms. E s s e n t i a l l y the same bubbling occurs i f the hole i s covered with a bubble cap, i f gas enters downwards through a pipe b u r i e d i n the bed or i f gas enters h o r i z o n t a l l y through a hole i n the w a l l . In t h i s l a s t case there i s no observable l a t e r a l p e n e t r a t i o n and bubbles r i s e v e r t i c a l l y from the o r i f i c e . This mode of entry has been observed at gas v e l o c i t i e s up to 70 m/s and through holes as l a r g e as 16 mm diameter. A l l t h i s has been seen w i t h a v a r i e t y of p a r t i c l e s but not very l a r g e ones ( d measured i n mm) where there i s reason to t h i n k behaviour may be d i f f e r e n t . One reason f o r the p e r s i s t e n c e of a wrong concept of how gas enters i s the f a c t that i t does so d i f f e r e n t l y i n a two-dimensiona l f l u i d i s e d bed, an arrangement chosen because of the ease of o b s e r v a t i o n . In t h i s case i t i s q u i t e p o s s i b l e to e s t a b l i s h a permanent j e t s t a b i l i s e d by the w a l l s . In a narrow slab bed p a r t i c l e s can only flow towards the hole over a r e s t r i c t e d r e g i o n and cannot always move f a s t enough to c l o s e the hole p e r i o d i c a l l y . There i s no such d i f f i c u l t y when p a r t i c l e s can approach w i t h i n the f u l l 360°. P o s s i b l y a j e t could be e s t a b l i s h e d i n a c y l i n d r i c a l bed by e r e c t i n g s u i t a b l e b a f f l e s to hinder p a r t i c l e f l o w but t h i s would have to be a d e l i b e r a t e arrangement. I would l i k e to spend a few minutes over t h i s d e t a i l because i t leads to a simple and s a t i s f y i n g model f o r chemical r e a c t i o n i n the d i s t r i b u t o r zone. When an i n s o l u b l e gas enters a l i q u i d p


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i t i s no s u r p r i s e that bubbles form because of the i n c o m p a t i b i l i - * ty of the two phases. Furthermore bubble s i z e and frequency must be equivalent t o the gas flow r a t e . When gas enters a f l u i d i s e d bed the phases are not incompatible because gas may form bubbles, f l o w i n t e r s t i t i a l l y o r t r a n s f e r between the two. I t i s a f a c t of observation that a t the moment o f detachment bubble s i z e and frequency accounts f o r only about h a l f the o r i f i c e gas flow. About two bubble diameters higher they have grown i n s i z e and now account f o r a l l the flow. Frequency i s e s s e n t i a l l y independent of f l o w r a t e and constant a t about 8/s. U n l i k e formation i n a l i q u i d the boundary of a f l u i d i s e d bed bubble can only expand by gas f l o w i n g across i t to produce the drag f o r c e that w i l l cause the p a r t i c l e s to move a p p r o p r i a t e l y . During the time that a bubble grows to the s i z e shown i n F i g u r e 9 the gas that produced i t has advanced t o f i l l the volume i n d i c a t ed by the outer broken l i n e . The annular r e g i o n above and around the bubble now contains an excess of gas and so the powder v o i d age must increase. This i s unstable and as the bubble detaches and r i s e s through the expanded dense phase the powder r e l a x e s and and r e t u r n s the excess gas t o the bubble. This appears t o be completed by the time i t has r i s e n about one diameter (of order 1/10 second) and t h e r e a f t e r i s of constant volume u n t i l i t coalesces . Considering now the consequences f o r chemical r e a c t i o n , much of the reactant gas that u l t i m a t e l y forms a bubble w i l l f i r s t enter the i n t e r s t i t i a l phase and enjoy a b r i e f moment of i n t i m a t e contact w i t h the p a r t i c l e s . L a t e r i t may form a cloud as i n F i g u r e 5 w i t h l i m i t e d access t o p a r t i c l e s . Hence the high r e a c t i o n r a t e s a s s o c i a t e d w i t h the process of bubble formation and l i m i t e d to the bottom l a y e r of the bed of thickness up to twice the i n i t i a l bubble diameter - a few centimeters a t most. As yet there i s no f l u i d dynamic model that describes i n q u a n t i t a t i v e d e t a i l the bubble formation process but i t i s b a r e l y necessary f o r a r e a c t i o n engineering model. I t i s adequate to assume that e n t e r i n g reactant gas passes i n plug f l o w through the bottom l a y e r of p a r t i c l e s , say, one i n i t i a l bubble diameter deep and t h e r e a f t e r forms bubbles. I n i t i a l bubble diameter i s r e a d i l y estimated from the known flow through the o r i f i c e and the f a c t that frequency i s about 8/s. Above t h i s d i s t r i b u t o r l a y e r the two-phase bubble model can be a p p l i e d . The other end c o n d i t i o n where the bubbling bed model i s i n a p p r o p r i a t e i s above the bed where there may be r e a c t i o n i n the f r e e board r e g i o n . With f i n e powders where there i s appreciable e l u t r i a t i o n gas and p a r t i c l e s may remain i n contact w i t h f u r t h e r opportunity f o r r e a c t i o n . This s i t u a t i o n has not a t t r a c t e d the a t t e n t i o n of many modellers but a t l e a s t one model p r e d i c t s that considerable r e a c t i o n can continue under c e r t a i n circumstances (20). The l a s t area I wish to mention i s the e f f e c t on f l u i d i s a t i o n of changing temperature and pressure. Not very much funda-


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CHEMICAL REACTION

F i g u r e 7· Gas enters the bed as a stream o f bubbles.

ENGINEERING

ski 1 |

F i g u r e 8. x-Ray photograph o f bubble formation at an o r i f i c e .

Figure 9· Gas flow causes the bubble boundary t o expand.


3.

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mental work has been reported on the former (21, 22) but there i s i n c r e a s i n g i n t e r e s t i n the l a t t e r (23-29). I t i s becoming f a i r l y c l e a r that when compared a t the same s u p e r f i c i a l gas v e l o c i t y , bubble s i z e decreases as pressure i n c r e a s e s . The reason f o r t h i s seems t o be that p r o p o r t i o n a t e l y more gas flows i n t e r s t i t i a l l y as pressure increases and bubbles are smaller because the reduced f l o w gives l e s s opportunity f o r growth by coalescence. Powders that normally behave as Geldart type Β ( i . e . U f = U ^ ) become type A a t q u i t e modest increases i n pressure. This i s good news from a chemical r e a c t i o n engineering point of view and the bubbling bed models continue to apply and only r e q u i r e the changed d i v i s i o n of flow to be q u a n t i f i e d . At pressures greater than about 80 bar some i n t e r e s t i n g hydrodynamic changes occur. Bubbles begin to l o s e t h e i r i d e n t i t y and a t q u i t e low gas v e l o c i t i e s the bed takes on the general appearance of a " t u r b u l e n t " o r " f a s t f l u i d i s e d bed". This again i s advantageous f o r r e a c t i o n engineering but few processes w i l l be r e q u i r e d t o operate a t such high pressures. I t u r n f i n a l l y to consider the d i r e c t i o n f u t u r e b a s i c research should take. I t i s fundamental to the s t a t e o f f l u i d i s a t i o n that p a r t i c l e s are supported by the drag of f l o w i n g gas. This f o r c e depends not only on the gas p r o p e r t i e s and v e l o c i t y but a l s o on the p a r t i c l e spacing and arrangement. I n s p i t e of the i n t e r e s t of f l u i d dynamicists l i t t l e i s known about the r e l a t i o n s h i p between p e r m e a b i l i t y , voidage and the flow c o n d i t ions and yet i t i s t h i s that decides the d i v i s i o n of gas between the phases w i t h important consequences f o r chemical r e a c t i o n . I t i s evident that p a r t i c l e s i z e and s i z e d i s t r i b u t i o n are f a c t o r s determining p e r m e a b i l i t y and that " f i n e s " are important i n t h i s respect but i t i s d i f f i c u l t to understand why change of absolute pressure should change voidage and p e r m e a b i l i t y . Understanding these things could g r e a t l y improve our a b i l i t y t o engineer f l u i d i s e d bed chemical r e a c t o r s and t h i s should be a major object of b a s i c research. However deep our knowledge of the mechanism of f l u i d i s a t i o n i t i s s a l u t a r y to pause and consider the freedom of choice a chemical r e a c t i o n engineer w i l l have. Leaving aside d e t a i l s o f mechanical design such as the d i s t r i b u t o r , b a f f l e s , heat exchang ers and m a t e r i a l s of c o n s t r u c t i o n , the v a r i a b l e s are very l i m i t e d . Gas d e n s i t y and v i s c o s i t y and p a r t i c l e d e n s i t y w i l l be determined by chemistry and thermodynamics and the only major v a r i a b l e s remaining are gas v e l o c i t y , bed height and p a r t i c l e s i z e and s i z e d i s t r i b u t i o n . The f i r s t two are l a r g e l y f i x e d by production r a t e and simple engineering c o n s i d e r a t i o n s and p a r t i c l e s i z e i s about the only t h i n g l e f t to choose. I f we knew more about how to make t h i s choice our designs might be much improved. m


62

CHEMC IAL REACTO IN ENGN IEERN IG

L i s t of Symbols cL

bubble diameter

JD

D Κ

r e a c t o r diameter

h

bed height

Q

volumetric flow

Q_.

bubble f l o w r a t e

r a t e i n t o the bed

D

Q. ι

i n t e r s t i t i a l flow rate minimum f l u i d i s a t i o n flow r a t e

U ^

minimum bubbling flow v e l o c i t y

U ^

minimum f l u i d i s a t i o n flow v e l o c i t y

Literature Cited 1. 2. 3. 4. 5. 6. 7. 8. 9. 10. 11. 12. 13. 14. 15. 16. 17.

Pyle, D.L. Advances in Chemistry Series 109, 1972, 106. Rowe, P.N. Proc. 2nd ISCRE; Elsevier: Amsterdam, 1972, A9. van Swaaij, W.P.M. Advances in Chemistry Series 72, 1978, 193. Rudolph, P.F.H. Proc. 4th ISCRE 2; Dechema: Frankfurt, 1976, 537. Kunii, D. Chem. Eng. Science 1980, 35, 1887. Levenspiel, O. Chem. Eng. Science 1980, 35, 1821. Denbigh, K.G. Trans. Faraday Soc. 1944, 40, 352. Danckwerts, P.V. Chem. Eng. Science 1953, 2, 1. Davidson, J.F.; Harrison, D. (Editor) "Fluidisation"; Academic Press: New York, 1971, 121 et seq. Rowe, P.N. Chemistry & Industry No. 12 1978, 424. Rowe, P . N . : Nienow, A.W.; Agbim, A . J . Trans. I. Chem. Ε 1972, 50, 324. Kunii, D.; Levenspiel, O. "Fluidisation Engineering"; John Wiley: New York, 1969. "Fluidised Combustion" 1975, London: Institute of Fuel, Symp. Series N o . l . "Fluidised Combustion - systems and applications" 1980, London: Inst. of Energy, Symp. Series No.4. Nienow, A.W.; Rowe, P.N.; Cheung, L . Y . - L . Powder Tech'y 1978, 89. Geldart, D. Powder Tech'y 1973, 7, 285. Rowe, P.N.; Yates, J . G . ; Santoro, L. Chem. Eng. Science 1978, 133.


3.

18. 19. 20. 21. 22. 23. 24. 25. 26. 27. 28. 29.

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Zenz, F.A. "Fluidisation"; P i r i e , J . M . , E d . ; I. Chem. E . Symp. Series No.30, 1968, 136. Rowe, P.N.; MacGillivray, H . J . ; Cheesman, D . J . Trans. I. Chem. E. 1979, 57, 194. Yates, J . G . ; Rowe, P.N. Trans. I. Chem. E . 1977, 55, 137. B o t t e r i l l , J . S . M . ; Yeoman, Y. "Fluidization"; Grace, J . R . ; Matsen, J . M . , Eds.; Plenum Press: New York, 1980, 93. Desai, Α.; Kikukawa, H . ; Pulsifer, A.H. Powder Tech'y 1977, 16, 143. Harrison, D.; Davidson, J.F.; de Kock, J.W. Trans. I. Chem. E. 1961, 39, 201. C l i f t , R.; Grace, J . R . ; Weber, M.E. Ind. Eng. Chem. Fund. 1974, 135, 45. Carvalho, J . R . F . Guedes de; Harrison, D. Inst, of Fuel Symp. Series No.1; 1975, B1. Varadi, T . ; Grace, J.R. "Fluidisation"; C.U.P.: 1978, 55. Carvalho, J . R . F . Guedes de; King, D . F . ; Harrison, D.: i b i d . 59. Subzwari, M.P.; C l i f t , R.; Pyle, D . L . : i b i d . 50. Rowe, P.N.; MacGillivray, H . J . Inst, of Energy Symp. Series 4, 1980, IV, 1.

RECEIVED April 27,

1983


Fluidized-Bed Reactors

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