34 Modeling of a Trickle-Bed Reactor: The Hydrogenation
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of 2-Butanone on a Ruthenium Catalyst A. G E R M A I N , M. C R I N E , P. M A R C H O T , and G . A . L ' H O M M E Laboratoire de Chimie Industrielle et de Génie Chimique, Université de Liège, Rue A . Stévart, 2, B-4000 Liège, Belgique
It is generally accepted now that the trickle-bed reactor operating mechanisms depend strongly on the vo latility of the processed liquid. When its vapor pres sure is very low, the chemical reaction can only occur on the irrigated catalyst particles and the reactor production increases with the catalyst wetting effici ency i.e. with the liquid flow rate ( L . F . R . ) . In that case the rather empirical models which have been deve lopped (1,2) allow the reactor design estimation or at least the correlation and extrapolation of experimental measurements. Such low volatilities are frequently met in hydrotreating heavy petroleum fractions. But when the processed liquid is volatile, the chemical reaction can occur on the non irrigated particles as on the ir rigated ones. Because of the comparatively weaker transfer resistance in the gaseous phase, the more ef ficient catalyst particles are the non irrigated ones. So it happens that the highest specific reaction rates are observed at the lowest wetting efficiency i.e. at the lowest L . F . R . T h a t paradoxical behaviour appeared during various experimental investigations a b o u t the operation of trickle-bed reactors with test chemical r e a c t i o n s (3_,£, 5_, 6_,7J · these s t u d i e s , the produc t i o n i n c r e a s e a t l o w L.F.R. i s e x p l a i n e d b y t h e a p p e a rance o f d r y c a t a l y s t p a r t i c l e s on which t h e r e a c t i o n r a t e c a n be f a s t e r , f o r t h e d i f f u s i o n i n s i d e t h e i r p o res i s e a s i e r . Y e t , t i l l now, a q u a n t i t a t i v e t r e a t m e n t o f t h e s e phenomena i s l a c k i n g . I n a d d i t i o n , s e v e r a l o b s e r v a t i o n s remain c o m p l e t e l y u n e x p l a i n e d : d r y zones f o r m a t i o n even w i t h p r e w e t t e d beds, s t a t i o n n a r y o p e r a t i o n slow s e t t l i n g , h y s t e r e s i s . . . T h a t i s t h e r e a s o n why we h a v e c h o s e n t o i n v e s t i g a t e t h e hydrogénation o f 2 - b u t a n o n e i n t o 2 - b u t a n o l on a r u t h e n i u m s u p p o r t e d c a t a l y s t . I n deed t h i s c h e m i c a l r e a c t i o n c a n be s t u d i e d o v e r a l a r g e I
©
n
0-8412-0401-2/78/47-065-411$05.00/0
In Chemical Reaction Engineering—Houston; Weekman, V., et al.; ACS Symposium Series; American Chemical Society: Washington, DC, 1978.
CHEMICAL REACTION
412
ENGINEERING—HOUSTON
range o f t e m p e r a t u r e u s i n g a ketone aqueous s o l u t i o n which enables t omodify s t r o n g l y the r e a c t a n t v o l a t i l i ty.
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Experimental
Set-up
A s i m p l i f i e d diagram o f the experimental apparatus i s g i v e n o n F i g u r e 1. I t c o n s i s t s o f a j a c k e t e d r e a c t o r t h r o u g h which hydrogen and t h e k e t o n e aqueous s o l u t i o n f l o w c o c u r r e n t l y down. The r e a c t o r i s p r o v i d e d w i t h a n a x i a l t h e r m o c o u p l e w e l l and a s l i d i n g t h e r m o c o u p l e , and w i t h s a m p l i n g v a l v e s a l l o w i n g t h e i n l e t and o u t l e t f l o w s a n a l y s i s . A n i n e r t b e d , r e a l i z e d w i t h a l u m i n a pel l e t s s i m i l a r t o the c a t a l y s t p e l l e t s , ensures the pre h e a t i n g and a good d i s t r i b u t i o n o f t h e l i q u i d . S i x s m a l l 1 / 1 6 p i p e s p r o v i d e a good l i q u i d s p r e a d i n g a t the t o p o f t h e c o l u m n . The e s s e n t i a l r e a t o r f e a t u r e s a n d the e x p e r i m e n t a l c o n d i t i o n s v a r i a t i o n range a r e g i v e n i n T a b l e 1. A l l t h e e x p e r i m e n t s w e r e r e a l i z e d o n a com p l e t e l y p r e w e t t e d b e d o b t a i n e d b y f l o o d i n g . So i t i s p o s s i b l e t o r e c o r d w e l l r e p r o d u c i b l e measurements, w i t h o u t any h y s t e r e s i s e f f e c t . An o p e r a t i n g s t a t i o n n a r y s t a t e i s reached d u r i n g t h e hour which f o l l o w s t h e s e t t l i n g o f t h e f l o w r a t e s and o f t h e h e a t i n g f l u i d t e m p e r a t u r e . F u r t h e r d e t a i l s a r e g i v e n by Germain ( 8 J . M
Table I. EXPERIMENTAL
VARIABLES
a. BED PARAMETERS DIAMETER, m
0.040
HEIGHT OF LOWER ALUMINA LAYER, m
0.114
HEIGHT OF UPPER ALUMINA LAYER, m
0.086
HEIGHT OF CATALYST LAYER, m
0.250
WEIGHT OF CATALYST LAYER, kg
0.280
b
*
CATALYST
(0,5% Ru on γ - Α Ι ^ ) COATED CYLINDER
BULK FORM BULK DIMENSIONS, HEIGHT, mm DIAMETER, mm PELLET APPARENT DENSITY, kg.m"*3 PELLET POROSITY, % c.
3.2 3.2 1470 54
RANGE OF VARIABLES
TOTAL PRESSURE TEMPERATURE, Κ LIQUID SUPERFICIAL MASS FLOW RATE, k g . m " 2 . s _ 1 (L.F.R.) GAS SUPERFICIAL MASS FLOW RATE, k g . m " " 2 . s _ 1 2-BUTAN0NE CONCENTRATION, mol.ra" 3
atmospheric 269
- 326
0.135 - 1 . 1 3 2.5 10~* - 7.7 10 0.245 - 0.893
In Chemical Reaction Engineering—Houston; Weekman, V., et al.; ACS Symposium Series; American Chemical Society: Washington, DC, 1978.
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GERMAIN ET AL.
Figure 1.
2-Butanone Hydrogénation
on Ruthenium Catalyst
Butanone hydrogénation in trickle-bed reactor
In Chemical Reaction Engineering—Houston; Weekman, V., et al.; ACS Symposium Series; American Chemical Society: Washington, DC, 1978.
CHEMICAL REACTION
414
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Experimental
ENGINEERING—HOUSTON
Result
We h a v e d e t e r m i n e d t h e i n t r i n s i c r e a c t i o n r a t e e q u a t i o n f o r t h e hydrogénation o f t h e 2 - b u t a n o n e i n t o the 2 - b u t a n o l on R U - A I 2 O 3 i n a p r e l i m i n a r y k i n e t i c i n v e s t i g a t i o n using a p e r f e c t l y agitated s l u r r y reactor. A f t e r some c o r r e c t i o n s due t o a p a r t i a l d i f f u s i o n a l r e sistance a t the l i q u i d - s o l i d i n t e r f a c e , this equation takes t h e form : r
5
= 3.27 1 0 ~ e x p [ 5 3 6 7 . 9 ( 1 / 3 1 3 - 1 / T ) ] . C . p ° ' R
5
[ l ]
H
C o n t r a r y t o w h a t we o b s e r v e d f o r t h e hydrogénation of α - m e t h y l s t y r e n e on Pd-Al203 i n a c o u n t e r c u r r e n t tri c k l e - b e d r e a c t o r (_4) , t h e t e m p e r a t u r e c o n t r o l a p p e a r s very easy i n t h e p r e s e n t i n v e s t i g a t i o n . I f the l i q u i d and t h e g a s a r e p r e h e a t e d , t h e j a c k e t h e a t t r a n s f e r i s s u f f i c i e n t t o ensure the i s o t h e r m i c i t y of the c a t a l y t i c bed. Hot s p o t s o r d r y c a t a l y s t p a r t i c l e s a r e n o t o b s e r ved, c o r r o b o r a t i n g t h e assumption t h a t t h e h o t spot for m a t i o n i s r e l a t e d t o t h e d r y i n g o f some c a t a l y s t p a r t i cles. As shown on F i g u r e s 2 a n d 3, t h e L.F.R. a p p e a r s t o be a v e r y i m p o r t a n t p a r a m e t e r i n t h e o p e r a t i o n o f t h i s t r i c k l e - b e d r e a c t o r ; however i t s i n f l u e n c e on t h e r e a c t o r p r o d u c t i o n depends on t h e c a t a l y t i c bed t e m p e r a t u r e . A t l o w t e m p e r a t u r e (T
