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Chemical and Catalytic Reactor Modeling Downloaded from pubs.acs.org by UNIV OF SYDNEY on 10/06/15. For personal use only.
The Effect of Gas and Liquid Velocities and Solid Size on Solid Suspension in a Three-Phase Bubble Column Reactor BARRY W. BRIAN and PAUL N. DYER Air Products and Chemicals, Inc., Allentown, PA 18105
In the design of upflow, three phase bubble column reactors, i t is important that the catalyst remains well distributed throughout the bed, or reactor space time yields will suffer. The solid concentration profiles of 2.5, 50 and 100 µm silica and iron oxide particles in water and organic solutions were measured in a 12.7 cm ID bubble column to determine what conditions gave satisfactory solids suspension. These results were compared against the theoretical mean solid settling velocity and the sedimentation diffusion models. Discrepancies between the data and models are discussed. The implications for the design of the reactors for the slurry phase Fischer-Tropsch synthesis are reviewed. Three phase, upflow, bubble column reactors are used in the process industry because of their simplicity and good liquid mixing characteristics, without the need for mechanical agitation. While current applications include hydrogénation and fermentation, slurry bubble columns have recently been the subject of renewed study for use in two areas of hetergeneous catalysis, SRC coal liquefaction and the Fischer-Tropsch reaction to produce hydrocarbons from synthesis gas. An important aspect of the design of three phase bubble columns is the variation of catalyst distribution along the reactor height, and its effect on reactor performance. Many factors influence the degree of catalyst distribution, including gas velocity, liquid velocity, solid particle size, phase densities, slurry viscosity, and, to a lesser extent, column diameter, solid shape and chemical affinity between the solid and liquid phases. Three phase bubble column reactors are of interest for Fischer-Tropsch synthesis because of their following characteristics : 0097-6156/84/0237-0107$06.00/0 © 1984 American Chemical Society
CHEMICAL AND CATALYTIC REACTOR MODELING
Chemical and Catalytic Reactor Modeling Downloaded from pubs.acs.org by UNIV OF SYDNEY on 10/06/15. For personal use only.
108
(a) a high heat a x i a l d i s p e r s i o n and i s o t h e r m i c i t y , (b) e x c e l l e n t c o n t r o l o f the r e a c t i o n exotherm, l e a d i n g to lower y i e l d s of CH^, (c) the a b i l i t y t o accept CO r i c h syngas and produce higher mole c u l a r weight products, without the coking or plugging t h a t i s inherent with other r e a c t o r types. As p a r t o f the work undertaken by APCI under contract t o the DOE, to develop a s l u r r y phase F i s c h e r - T r o p s c h process to produce s e l e c t i v e l y t r a n s p o r t a t i o n f u e l s , a study o f the hydrodynamics of t h r e e phase bubble column r e a c t o r s was begun u s i n g c o l d flow modelling techniques ( l ) . Part of t h i s study i n c l u d e s the measurement of s o l i d c o n c e n t r a t i o n p r o f i l e s over a range of i n d e pendent column operating v a l u e s . Roy, et a l {2_) e m p i r i c a l l y determined the gas v e l o c i t y needed t o completely suspend a given amount of s o l i d i n a 5 cm i d χ 1.52 m L u c i t e column using c o a l and quartz s l u r r i e d i n water, a l c o h o l , or o i l . The degree o f suspension was found to depend on p h y s i c a l p r o p e r t i e s as w e l l as gas holdup, volume f r a c t i o n , bubble diameter, and the contact angle between the s o l i d and liquid. Cova (3.) measured the s o l i d c o n c e n t r a t i o n p r o f i l e s of a Raney n i c k e l c a t a l y s t with an average diameter of 15.7 ym i n a k.6 cm i d r e a c t o r , u s i n g water and acetone as the l i q u i d s . He developed a sedimentation d i f f u s i o n model, assuming s o l i d and l i q u i d d i s p e r s i o n c o e f f i c i e n t s were equal, and s l u r r y s e t t l i n g v e l o c i t i e s were independent of s o l i d c o n c e n t r a t i o n . The model was then a p p l i e d t o data f o r Raney n i c k e l i n 6.35 and hk.^ cm i d bubble columns, i n both cocurrent and countercurrent flow. Imafuku, et a l (k) measured the s o l i d c o n c e n t r a t i o n p r o f i l e s of s e v e r a l s o l i d s i n 5» 10 and 20 cm columns, i n water and aqueous g l y c e r i n e . The s o l i d p a r t i c l e s ranged from 6h to l80 μια i n diameter, and i n c l u d e d g l a s s spheres, i r o n s i l i c a t e and copper powders. They measured s o l i d d i s p e r s i o n c o e f f i c i e n t s , confirming Cova s assumption of equal l i q u i d and s o l i d d i s p e r s i o n c o e f f i c i e n t s , and developed an e m p i r i c a l c o r r e l a t i o n between the ob served t e r m i n a l s e t t l i n g v e l o c i t y i n a three phase system, and the c a l c u l a t e d t e r m i n a l s e t t l i n g v e l o c i t y o f a s i n g l e p a r t i c l e i n a stagnant l i q u i d . Kato, et a l (5_) measured s o l i d c o n c e n t r a t i o n p r o f i l e s , s o l i d d i s p e r s i o n c o e f f i c i e n t s and t e r m i n a l s e t t l i n g v e l o c i t i e s f o r g l a s s spheres i n water, u s i n g 6 . 6 , 12.2 and 2 1 Λ cm bubble columns. They developed a dimensionless, e m p i r i c a l c o r r e l a t i o n f o r the s o l i d d i s p e r s i o n c o e f f i c i e n t s which agreed with t h e i r observed values t o w i t h i n ±20$. Sivasubramanian, et a l (6_) and Moujaes, et a l (7_) measured s o l i d c o n c e n t r a t i o n p r o f i l e s of sand i n water and ethanol/water, using 12.7 and 30.5 cm bubble columns. They developed a s o l i d s accumulation model, which c o r r e l a t e d s u c c e s s f u l l y with an a c t u a l 30.5 cm solvent r e f i n e d c o a l d i s s o l v e r . 1
Chemical and Catalytic Reactor Modeling Downloaded from pubs.acs.org by UNIV OF SYDNEY on 10/06/15. For personal use only.
7.
BRIAN AND
Gas and
DYER
Liquid Velocities and Solid Size Effects
109
The -work described i n t h i s paper extends the understanding of s o l i d concentration p r o f i l e s i n three phase bubble column r e a c t o r s , with emphasis on the Fischer-Tropsch s y n t h e s i s , by: 1) u t i l i z i n g Cg-Cn hydrocarbon l i q u i d as the s l u r r y base, 2) u s i n g s o l i d s s i m i l a r to Fischer-Tropsch bulk and supported c a t a l y s t s , and 3) modelling operating c o n d i t i o n s t y p i c a l of s l u r r y phase FischerTropsch r e a c t o r s , i n c l u d i n g the use of i n t e r n a l heat t r a n s f e r elements and a v a r i a t i o n i n gas d i s t r i b u t o r p l a t e . The l i m i t a t i o n s of the s e d i m e n t a t i o n - d i f f u s i o n model f o r d e s c r i b i n g s o l i d d i s p e r s i o n i n these systems are discussed. Theoretical S o l i d concentration p r o f i l e s are produced from a balance of g r a v i t a t i o n a l with buoyancy and k i n e t i c energy t r a n s f e r f o r c e s . For a s i n g l e p a r t i c l e i n a stagnant l i q u i d , the s e t t l i n g v e l o c i t y , V , i s given by (.8): p
13
Ρ
(1)
f
where f , the Fanning f r i c t i o n f o r spheres, i s a f u n c t i o n of Reynolds number, 2U/Re
18.5
f =
Re Re 3/5
O.kh
(2a)
