Catalyst Deactivation - ACS Symposium Series (ACS Publications)

9

Downloaded by UNIV OF CALIFORNIA SAN DIEGO on December 24, 2015 | http://pubs.acs.org Publication Date: January 19, 1978 | doi: 10.1021/bk-1978-0072.ch009

Catalyst Deactivation JOHN B. BUTT and RUSTOM M. BILLIMORIA Department of Chemical Engineering and Ipatieff Catalytic Laboratory, Northwestern University, Evanston, IL 60201

In the years since the First International Symposium on Chemical Reaction Engineering, where a first review on catalyst deactivation was presented (1), there has been considerable activity in this area. In particular, there have been appreciable advances in the understanding of sintering processes, and of intraparticle and fixed bed reactor behavior under conditions of catalyst deactivation. Happily, much of this has consisted of experimental information as well as analysis. The present effort makes no attempt to match in scope the previous review; we shall confine ourselves to work concerning chemical poisoning and coking as the primary mechanism of deactivation but retain the classification according to scale — individual kinetics and mechanism, intraparticle problems, and chemical reactor problems. Sintering has been admirably covered in a recent review (2), and the subject of automotive exhaust catalysis (which is almost wholly an exercise in catalyst mortality) w i l l be treated in one forthcoming (3). Mechanisms and Kinetics Kinetic networks used to depict the processes of catalyst deactivation have typically been based on models which are simultaneous, parallel, or sequential. Both Carberry (4) and Khang and Levenspiel (5) have enlarged on these to include two additional cases, independent deactivation (characteristic of sintering) and simultaneous-consecutive deactivation (characteristic of coking via participation of both reactants and products). As w i l l be seen from the examples to be given here, deactivation kinetics have almost universally been correlated in terms of separable (6) rate factors. More detailed analysis, however, indicates that such assumptions are questionable for surfaces other than those ideal in the Langmuir sense (7). The argument can be developed along the lines employed to derive adsorption isotherms for nonideal surfaces starting with the concept of a subassembly of ideal surfaces distributed according to the heat of chemisorption. The differences in separable and nonseparable 0-8412-0432-2/78/47-072-288$08.75/0 © 1978 American Chemical Society In Chemical Reaction Engineering Reviews—Houston; Luss, Dan, et al.; ACS Symposium Series; American Chemical Society: Washington, DC, 1978.

9.

BUTT A N D

BiLLiMORiA

Catalyst Deactivation

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(3)

f o r the separable c a s e . I t i s i n t e r e s t i n g t h a t t h e a n a l y s i s (7^) r e v e a l s an extreme s e n s i t i v i t y o f the v a l i d i t y o f the s e p a r a b l e a p p r o x i m a t i o n t o p r e s s u r e b u t o n l y a modest e f f e c t o f t e m p e r a t u r e . The r e a s o n t h a t s e p a r a b l e f o r m u l a t i o n s seem t o work i s p r o b a b l y t h e same t h a t L a n g m u i r - H i n s h e l w o o d r a t e e q u a t i o n s w o r k : t h e q u a l i t a t i v e r e s u l t s a r e o f s i m i l a r form f o r i d e a l and n o n i d e a l s u r faces but p h y s i c a l i n t e r p r e t a t i o n o f the parameters i s i n c o r r e c t . A s f a r a s mechanism o f d e a c t i v a t i o n — c o n s i d e r i n g o n l y chem i c a l p o i s o n i n g and c o k i n g — t h e s i t u a t i o n r e m a i n s much t h e same a s o u t l i n e d p r e v i o u s l y (1). S p e c i f i c c a s e s o f c h e m i c a l p o i s o n i n g t e n d t o be r e l a t i v e l y w e l l u n d e r s t o o d a s f a r a s d e t a i l i n g s u b s t a n c e s r e s p o n s i b l e f o r p o i s o n i n g and the n a t u r e o f the d e a c t i v a ted surfaces. A u g e r e l e c t r o n s p e c t r o s c o p y h a s become a n i m p o r t a n t t o o l f o r the i n v e s t i g a t i o n o f poisoned s u r f a c e s s i n c e the charac t e r i s t i c energies i n that spectroscopy are s u f f i c i e n t l y low to i n v o l v e o n l y the s u r f a c e and i m m e d i a t e l y a d j a c e n t l a y e r s . Progress h a s a l s o b e e n made i n u n d e r s t a n d i n g t h e p o i s o n i n g o f b i f u n c t i o n a l catalysts; n i c e e x p e r i m e n t a l e x a m p l e s a r e p r o v i d e d b y Webb a n d Macnab (8) a n d B u r n e t t a n d Hughes ( 9 ) . The f o r m e r i n v e s t i g a t e d t h e h y d r o i s o m e r i z a t i o n o f b u t è n e o n a Rh/Si02 c a t a l y s t a n d demons t r a t e d the s e l e c t i v e d e a c t i v a t i o n o f the hydrogénation f u n c t i o n b y s m a l l amounts o f m e r c u r y i n t r o d u c e d i n t o t h e s y s t e m . T h i s type o f b i f u n c t i o n a l r e a c t i o n m i g h t be t e r m e d a " p a r a l l e l " o n e : (I) η

(Si0 ) 2

where t h e p o i s o n i n g d r a s t i c a l l y a f f e c t s one r e a c t i o n b u t n o t t h e other. The r e a c t i o n s t u d i e d b y B u r n e t t a n d H u g h e s , on t h e o t h e r h a n d , was a " s e r i e s " b i f u n c t i o n a l r e a c t i o n , d i s p r o p o r t i o n a t i o n o f b u t a n e o v e r a m e c h a n i c a l m i x t u r e o f P t / A l 0 « a n d W0« , w h e r e : 9

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They showed t h a t c o k i n g o r w a t e r p o i s o n i n g o f t h e P t f u n c t i o n s h u t t h e e n t i r e r e a c t i o n down, c h a r a c t e r i s t i c o f t h e s e q u e n t i a l n a t u r e of the steps i n v o l v e d . The d e s c r i p t i o n o f t h e m e c h a n i s m ( s ) o f c o k e f o r m a t i o n r e m a i n e s s e n t i a l l y t h e same a s d e s c r i b e d b e f o r e . With increasing experim e n t a l i n f o r m a t i o n on c o k i n g i n t h e l i t e r a t u r e t h e r e a p p e a r s now t o be a t l e a s t t a c i t a g r e e m e n t t h a t i t i s n o t p a r t i c u l a r l y r e w a r d i n g t o l o o k f o r a mechanism o f coke f o r m a t i o n when t h e r e a r e p r o b a b l y a s many mechanisms a s t h e r e a r e r e a c t i o n s a n d c a t a l y s t s . In t h i s s e n s e , s i m p l e r e a c t i o n schemes may p r o v i d e a s t a r t i n g p o i n t f o r a n a l y s i s b u t may a l s o f a l l f a r s h o r t o f t h e mark i n c o n f r o n t a t i o n w i t h experiment. Individual

Particles

An i n t e r e s t i n g p r o b l e m , n o t much commented u p o n i n t h e o l d e r l i t e r a t u r e , i s the r e l a t i o n s h i p between coke d e p o s i t i o n and the p h y s i c a l p r o p e r t i e s o f the c a t a l y s t . P o r e b l o c k a g e by coke depos i t i o n has been demonstrated i n s p e c i f i c i n s t a n c e s , b u t has been ignored i n e a r l i e r a n a l y t i c a l studies of coking (10,11). Swabb a n d G a t e s (12) i n v e s t i g a t e d m e t h a n o l d e h y d r a t i o n on H - m o r d e n i t e (1 a t m . , 1 0 0 - 2 4 0 ° C ) w i t h t h e o b j e c t i v e o f i n v e s t i g a t i n g t h e i n f l u e n c e o f i n t r a c r y s t a l l i n e mass t r a n s p o r t o n i n i t i a l a c t i v i t y and d e a c t i v a t i o n r a t e s . M a s s t r a n s p o r t p r o p e r t i e s were v a r i e d b y u s i n g t h r e e d i f f e r e n t s a m p l e s o f d i f f e r i n g mean p o r e length; s i g n i f i c a n t i n f l u e n c e s were f o u n d o n l y above 2 0 0 ° C . T h e r e w e r e no e f f e c t s of p o r e d i m e n s i o n on t h e d e a c t i v a t i o n r a t e s (due t o c o k e f o r m a t i o n ) o f f r e s h c a t a l y s t o v e r t h e r a n g e i n v e s t i gated. F u r t h e r s t u d i e s o f d e a c t i v a t i o n r a t e s a f t e r one and two h o u r s o f u t i l i z a t i o n a t 205°C a l s o r e v e a l e d no i n f l u e n c e on p o r e structure. T h i s w o u l d r u l e o u t i n t r a p a r t i c l e mass t r a n s p o r t a s c o n t r o l l i n g d e a c t i v a t i o n r a t e s a s w e l l a s t h e o c c u r r e n c e o f any pore blockage r e s u l t i n g from c o k i n g i n t h i s r e a c t i o n . E x p e r i m e n t s on l a r g e r s i z e p a r t i c l e s have a l s o i n v o l v e d H-mord e n i t e , b u t w i t h cumene c r a c k i n g a s t h e r e a c t i o n ( 1 3 ) . Relations b e t w e e n coke c o n t e n t , a c t i v i t y , a n d i n t r a p a r t i c l e d i f f u s i v i t y were i n v e s t i g a t e d on 1/16 i n . N o r t o n Z e o l o n e x t r u d a t e s f o r 2 3 0 - 2 5 0 ° C and s p a c e v e l o c i t i e s f r o m 0 . 2 t o 0 . 6 5 w t / w t - h r . E f f e c t i v e d i f f u s i v i t i e s w e r e d e t e r m i n e d ( w i t h SF$ v i a c h r o m a t o g r a p h y ) a s a f u n c t i o n o f r e a c t i o n t i m e and coke c o n t e n t w i t h t h e r e s u l t s shown i n Figure 1. D i f f u s i v i t y decreased t w o f o l d f o r r e a c t i o n times o f 2 hours o r l o n g e r , but remained e s s e n t i a l l y constant a f t e r t h a t . The e f f e c t i v e n e s s f a c t o r v a r i e d i n t h e r a n g e 0 . 3 - 0 . 7 d u r i n g t h e s e experiments; e r r o r i n e s t i m a t i o n o f t h i s f a c t o r u s i n g the e f f e c t i v e d i f f u s i v i t y o f t h e f r e s h c a t a l y s t was 20 t o 30% a t l o n g e r reaction times. SEM e x a m i n a t i o n o f c o k e d p a r t i c l e s r e v e a l e d t h e f o r m a t i o n o f a g l a s s y - l i k e c o a t i n g on t h e e x t e r n a l s u r f a c e , p e n e t r a t i n g a p p r o x i m a t e l y 0 . 1 t h e r a d i u s i n t o t h e i n t e r i o r , so i t was c o n c l u d e d t h a t p o r e b l o c k i n g was r e s p o n s i b l e f o r t h e d e c r e a s e in diffusivity.



9.

BUTT A N D BILLIMORIA


291

A d d i t i o n a l r e s u l t s on coke d e p o s i t i o n and pore b l o c k i n g a r e p r o v i d e d by T o e i , e t a l . (14) a n d R i c h a r d s o n ( 1 5 ) . Toei, et a l . f o u n d no e f f e c t o f c o k i n g on i n t r a p a r t i c l e d i f f u s i o n f o r d e h y d r o g e n a t i o n o f η - b u t a n e o v e r c h r o m i a - a l u m i n a (80% Y-AI2O3, 5x5 mm p e l l e t s o r 0 . 2 - 2 . 2 mm p a r t i c l e s ) . C a r e f u l e x a m i n a t i o n o f the pore s i z e d i s t r i b u t i o n a t d i f f e r e n t coke l e v e l s r e v e a l e d o n l y m i n o r a l t e r a t i o n s o f t h e m i c r o p o r e s t r u c t u r e , a n d t h e s y s t e m was modeled w i t h the normal i s o t h e r m a l , q u a s i - s t e a d y s t a t e c o n t i n u i t y e q u a t i o n s w i t h good a g r e e m e n t t o t h e e x p e r i m e n t a l d a t a . Richard son f o u n d a l i n e a r d e c r e a s e i n e f f e c t i v e d i f f u s i v i t y (Ar/He) w i t h coke c o n t e n t f o r N a l c o 471 c o b a l t m o l y b d e n a (1/8 i n . p e l l e t s ) f o u l e d i n a p i l o t u n i t under h y d r o t r e a t i n g c o n d i t i o n s (700-800°F, 1 0 0 0 - 1 5 0 0 p s i g , 2 wt% N , 0.05% S, no m e t a l s ) . The T h i e l e m o d u l u s h o w e v e r , was e s s e n t i a l l y unchanged up t o a b o u t 15 wt % c o k e on catalyst. S e v e r a l e a r l i e r w o r k e r s have r e p o r t e d d e c r e a s i n g d i f f u s i v i t y on coke f o r m a t i o n , b u t o n l y t h r e e i n v e s t i g a t i o n s ( 1 6 , 1 7 , 1 8 ) gave d i r e c t e x p e r i m e n t a l measurement and none o f t h e s e w e r e f o r z e o lites. R e c e n t s t u d i e s a p p a r e n t l y h a v e p r o v i d e d no more g e n e r a l p i c t u r e c o n c e r n i n g t h e e f f e c t o f c o k i n g on t r a n s p o r t p r o p e r t i e s t h a n was a v a i l a b l e b e f o r e ; however, on w e i g h t o f accumulated e v i d e n c e i t seems r e a s o n a b l e t h a t f o r r e a c t i o n s o f l a r g e m o l e c u l e s i n c a t a l y s t s o f f i n e p o r e s t r u c t u r e s i g n i f i c a n t changes i n d i f f u s i v i t y on c o k i n g can o c c u r . W h e t h e r t h i s i n t u r n w i l l change t h e e f f e c t i v e n e s s f a c t o r w i l l depend on o t h e r c h e m i c a l a n d p h y s i c a l parameters. We a r e unaware o f any s i m i l a r s t u d i e s d e v o t e d t o c o k i n g e f f e c t s on t h e r m a l c o n d u c t i v i t y . Recent t h e o r e t i c a l s t u d i e s o f d e a c t i v a t i o n i n i n d i v i d u a l p a r t i c l e s have f o c u s e d on b i f u n c t i o n a l c a t a l y s t s ( 1 9 , 2 0 ) , on a p p a r e n t o v e r a l l k i n e t i c s of d e a c t i v a t i o n (5), d e a c t i v a t i o n i n nonisothermal p a r t i c l e s ( 2 1 ) , and t h e e f f e c t o f n o n u n i f o r m d i s t r i b u t i o n o f t h e c a t a l y t i c f u n c t i o n w i t h i n t h e p a r t i c l e on d e a c t i v a t i o n ( 2 2 , 2 3 , 24). Some f a c t o r s a s s o c i a t e d w i t h t h e d e a c t i v a t i o n o f b i f u n c t i o n a l c a t a l y s t s have b e e n e x p l o r e d c o m p u t a t i o n a l l y b y S n y d e r a n d M a t t h e w s (19) a n d b y L e e a n d B u t t ( 2 0 ) . These c a t a l y s t s have the a d d i t i o n a l c o m p l i c a t i o n o f t h e r e l a t i v e c o m p o s i t i o n o f t h e two f u n c t i o n s , w h i c h may be e m p l o y e d a s a means t o c o n t r o l s e l e c t i v i t y or d e a c t i v a t i o n r a t e s (20). I n (19) r e a c t i o n n e t w o r k s o f t h e form: 2

Α . . * Β. .-"^ (8)

(III)

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w e r e a n a l y z e d , w i t h coke f o r m a t i o n o c c u r r i n g on t h e X f u n c t i o n and t h e d e s i r e d r e a c t i o n on t h e Y f u n c t i o n . B o t h c o m p o s i t e (mechan i c a l m i x t u r e s o f X and Y ) a n d d i s c r e t e f o r m u l a t i o n s were i n v e s t i g a t e d u s i n g a two p h a s e , i s o t h e r m a l , q u a s i - s t e a d y s t a t e m o d e l , assuming a l i n e a r r e l a t i o n between coke c o n t e n t and a c t i v i t y . The major f a c t o r s e x p l o r e d were the e f f e c t s o f the r e l a t i v e magnitudes o f t h e r a t e c o n s t a n t s f o r i n d i v i d u a l s t e p s on t h e o p t i o n a l c a t a l y s t




292

formulation. I n (20) a s i m i l a r p r o b l e m was a d d r e s s e d , b u t w i t h e m p h a s i s on t h e i n t e r a c t i o n o f i n t r a p a r t i c l e d i f f u s i o n , t h e r a t e s o f d e a c t i v a t i o n , a n d c a t a l y s t f o r m u l a t i o n . B i f u n c t i o n a l forms o f t h e s e r i e s (Type I I I ) r e a c t i o n model were i n v e s t i g a t e d i n w h i c h e i t h e r t h e r e a c t a n t and i n t e r m e d i a t e o r t h e i n t e r m e d i a t e a n d p r o d u c t were r e s p o n s i b l e f o r c o k i n g o f t h e two f u n c t i o n s . The most i m p o r t a n t c o n c l u s i o n o f t h i s s t u d y was t h a t i n e i t h e r c a s e , g i v e n a s e t f o r m u l a t i o n , t h e i n c l u s i o n o f some d e g r e e o f d i f f u s i o n a l l i m i t a t i o n c o u l d be u s e d t o enhance b o t h c a t a l y s t e f f i c i e n cy ( y i e l d o f d e s i r e d p r o d u c t ) a n d l i f e — a f a c t n o t e d b e f o r e f o r some t y p e s o f c o k i n g mechanisms i n m o n o f u n c t i o n a l c a t a l y s i s (10). Khang a n d L e v e n s p i e l (5) i n v e s t i g a t e d t h e r e l a t i o n s h i p between the g l o b a l a c t i v i t y o f a p a r t i c l e , a , and the p o i n t a c t i v ity, £. The v a l u e s o f a so d e t e r m i n e d w e r e u s e d t o compute t h e o v e r a l l o r d e r of the d e a c t i v a t i o n r e a c t i o n a c c o r d i n g t o : da dt

« -y k. C ~ d ~A

a

A

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(5)

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3

R C

A

s f o r a s p h e r i c a l p a r t i c l e o f r a d i u s R, p o r o s i t y £ , a n d r e a c t a n t c o n c e n t r a t i o n a t t h e s u r f a c e jC. . A number o f p a r a l l e l , s e r i e s s a n d p o i s o n i n g schemes w e r e s t u d i e d ; i n t h e f i r s t two schemes r e a c t i o n p r o d u c t s w e r e t a k e n t o be coke p r e c u r s o r s . For p a r a l l e l d e a c t i v a t i o n a n d no d i f f u s i o n a l i n f l u e n c e , n e t d e a c t i v a t i o n o r d e r d was c l o s e t o u n i t y ; as the T h i e l e modulus i n c r e a s e d from 1 to 100 t h e n e t o r d e r v a r i e d f r o m 1 t o 3 . For series deactivation d = 1 f o r a l l v a l u e s o f the T h i e l e modulus p r o v i d i n g the r a t i o o f i n t e r m e d i a t e t o r e a c t a n t c o n c e n t r a t i o n a t t h e s u r f a c e was g r e a t e r t h a n one t e n t h t h e v a l u e o f t h e m o d u l u s . F a i l i n g t h i s , d a p proached 3. F o r p o i s o n i n g , p r o v i d i n g t h e r e was no o r s m a l l d i f f u s i o n l i m i t on t h e p o i s o n , d was n e a r u n i t y f o r any s i t u a t i o n w i t h r e g a r d to d i f f u s i o n l i m i t s on the main r e a c t i o n . F o r o t h e r com b i n a t i o n s o f d i f f u s i o n a l l i m i t s on m a i n a n d p o i s o n i n g r e a c t i o n s t h e n e t o r d e r was g e n e r a l l y g r e a t e r t h a n u n i t y , up t o a b o u t 3 . The i n t r a p a r t i c l e d e a c t i v a t i o n o f n o n i s o t h e r m a l p e l l e t s h a s b e e n a n a l y z e d b y Ray (21) u s i n g a p o r e mouth p o i s o n i n g , s l a b geometry m o d e l . Heat f l u x a t the boundary o f the a c t i v e and i n a c t i v e p o r t i o n s o f t h e p a r t i c l e ( F i g u r e 2a) was computed v i a B i s c h o f f ' s (25) a s y m p t o t i c s o l u t i o n f o r l a r g e T h i e l e m o d u l u s . A n e f f e c t i v e d i f f u s i o n a l modulus f o r t h i s case can be d e f i n e d a s : A

i

=

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(6s)

=

^

^

h

s

2

(6)

where :

K(6)

[e

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%

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δ = 0

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6

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9.

BUTT AND ΒΠΧΙΜΟΜΑ


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Chemical Engineering Science

Figure 1. (a) Voorhies correlation of coke formation, cumene cracking on H-mord 260°-350°C, 0.33 g/hr-g (13); (b) effective diffusivity (SF ) variation with coke co 6


294

CHEMICAL REACTION ENGINEERING

f

/ i + B

8

y ' a n d

Y

T

S

.

»

.

ΔΗ

RT

'

Λ

ρ

°s

„

λ

. (-*H)D c p

s

η

g A L expC-E/RTl 2

f h


- y \

0

REVIEWS—HOUSTON

s

Here

»

δ

i s the c a t a l y s t

- } d e n s i t y and A t h e p r e - e x p o n e n t i a l

Now t h e h e a t f l u x f r o m t h e a c t i v e p a r t o f t h e p e l l e t ,

q 1

cQlTis:

factor. at

1

y)

(7)

and a n e f f e c t i v e a c t i v i t y f a c t o r c a n b e d e f i n e d a s t h e r a t i o o f t h i s value to that f o r the undeactivated p a r t i c l e . This r a t i o , 1 f o r some p o r t i o n o f t h e c a t a l y s t l i f e was f o u n d t o b e : 3s

Pp

e* - (1 + 6 )
1 (Case 4 ) , t h e o p p o s i t e h o l d s a n d d e a c t i v a t i o n i s accelerated. A s i n d i c a t e d b y C a s e s 8 a n d 9 , some p a r a m e t e r s e t s r e s u l t i n m u l t i p l e steady s t a t e s . These a r e c h a r a c t e r i z e d b y l a r g e 6 and Y (60 i n t h e s e e x a m p l e s ) a n d (Ps/Pp) < 1 ; s u c h r e s u l t s i n d i c a t e T h a t m u l t i p l i c i t y c a n be i n d u c e d i n a d e a c t i v a t i n g p a r t i c l e even when t h e f r e s h c a t a l y s t shows no s u c h p o s s i b i l i t y . However, (Ps/Pp) < 1 seems p h y s i c a l l y i m p r o b a b l e . I f d e a c t i v a t i o n by cok i n g c l o s e s o f f a p o r t i o n o f t h e p o r e s t r u c t u r e , t h e n (Bs/^p) > 1> w h i l e p o i s o n i n g s h o u l d have l i t t l e e f f e c t on t h e r a t i o . This m u l t i p l i c i t y would r e q u i r e a type o f d e a c t i v a t i o n l e a d i n g t o an i n c r e a s e i n p o r o s i t y a s decay p r o c e e d s . I n many i n d u s t r i a l a p p l i c a t i o n s t h e p e r f o r m a n c e o f c a t a l y s t s i n which the a c t i v i t y d i s t r i b u t i o n i s nonuniform w i t h i n the p e l l e t i s s u p e r i o r t o those w i t h a u n i f o r m d i s t r i b u t i o n . Mars and G r o g e l s (26) s t u d i e d t h e performance o f such a c a t a l y s t f o r S

s



9.

BUTT AND

BELLIMORIA

295


acetylene hydrogénation and F r i e d r i c h s e n (27) r e p o r t s a p p l i c a t i o n s i n o x i d a t i o n of o-xylene to p h t h a l i c anhydride, to c i t e two examples. Recently Shadman-Yazdi and P e t e r s e n (22), Corbett and Luss (23) and Becker and Wei (24) have a l l i n v e s t i g a t e d the d e a c t i v a t i o n of c a t a l y s t s w i t h s p a t i a l d i s t r i b u t i o n o f the c a t a l y t i c f u n c t i o n . These s t u d i e s are s i m i l a r i n method and o b j e c t i v e but d i f f e r considerably i n the types of a c t i v i t y d i s t r i b u t i o n cons i d e r e d ; a summary of the models and d e a c t i v a t i o n mechanisms i n v e s t i g a t e d i s given i n Table 1., I n (22) the s e n s i t i v i t y of the d e a c t i v a t i o n w i t h respect to the d i s t r i b u t i o n parameter a was i n v e s t i g a t e d . I n l i n e w i t h p r i o r r e s u l t s (10), a = 0 gave r i s e to core poisoning a t higher v a l u e s of the T h i e l e modulus; of > 0 l e d to uniform poisoning — the r e s u l t of compensating v a r i a t i o n s i n kg and Cg w i t h i n the p a r t i c l e . An a n a l y t i c a l s o l u t i o n presented Tn terms~of the e f f e c t i v e n e s s f a c t o r f o r 0 was:

η

( θ )

=

Y

(9)

where : 2m-p - Μ 1 k

p=

l/(« 2), +

6

=

h (l-w)%, A

a

m

«

(

π

Γ

7

^ .

)

-p

and I.ρ i s a modified Bessel f u n c t i o n of the f i r s t k i n d of order £. The parameter w appearing i n 6^ was determined as a f u n c t i o n of £ by numerical i n t e g r a t i o n of:

- Σ οη/ρΛ χ + « I

S =

m=0 Figure 3 gives a p l o t of Ή (Θ) vs. θ_ f o r s e v e r a l types o f a c t i v i t y distribution. Clearly, — i n c r e a s i n g # enhances the long-term performance of the c a t a l y s t f o r t h i s s e r i e s r e a c t i o n i n a manner reminiscent of the response of the p a r a l l e l scheme to i n c o r p o r a t i o n of a c e r t a i n amount of d i f f u s i o n a l r e s i s t a n c e (10). In Figure 4 are given some r e s u l t s obtained n u m e r i c a l l y by Corbett and Luss (23) f o r i m p u r i t y poisoning of the s e r i e s reac t i o n scheme f o r cases of small and l a r g e d i f f u s i o n a l r e s i s t a n c e . The e f f e c t i v e n e s s f a c t o r i n these p l o t s i s computed w i t h respect to the i n i t i a l volume-averaged r a t e constant, which was the same f o r a l l the d i f f e r e n t d i s t r i b u t i o n s . The d i f f e r e n t i a l s e l e c t i v i t y , £, appearing i n the f i g u r e i s defined as: Α


iΙ·ι··Ι s p e c i e s . k : CS - " Y S . k . j j : YS ~ * C S ; : S cS; K : S * V s . K : S * 2 S .

Wong, e t a l . ( S 3 ) , P r i c e and B u t t (54)

2

Μ»! .» !»!*)

Wojciechowski, (67,68,69,70.

catalytic cracking

UVekaan, et a l . . (62,63,64,65,66) , >

1·butène dehydrogenatIon

2

SIO,,

dehydration of Al 0 2-aethyl-3 butene-2ol to isoprene

Ouatez a n d Froment (50)

(61_)

7.

et a l .

Grecο,

chlorlnation of tetrachloroethane

(60)

Prasad and llorlaswaay

2

M

theoretical Investigation

3

(59)

2

H/g

Sadana a n d Ooriaawaay

3

200

e

497-592 C, 1 a t a K.. 0.32-0.71 a t a

2

Cr 0 /Al 0

ai»

n-butane dchydrogena t Ion

488-560°C, 1 a t a 0 . 6 - 3 . 6 aec ft.T. x „ - 0.14

Uchlda, et a l . (57) Otake, et a l . (58)

(56)

2

( β / 1 0 , 16/32 a n d 40/60 a e s h )

3

550"C, 1 a t a Ρ - 0.11-0.56 4

et a l .

2

Studies ÇataIyat

Cr 0 /Al 0

geactor

n-butane dchyd rogena t i o n

loci,

i-pentane dehydrogenattoi

6.

2.

(55)

Catalyst Deactivation - Integral

Noda, et a l .

Autlior

Table 3.



1

k

c

r

MBB

Time o u s t r e s a (hyperbolic)

Contact tiae (exp)

'c '

Contact tlae ( l i n e a r , exp)

tlae exp,

* * "

Contact (linear,

'

algebraic)

Forst o f D e a c t i v a t i o n Kinetics

L

-

c

r

r

r

c

4

C

c

• k )fj

• K.. P > \ 4

3

A

B

2

K

2

c

"

B

exp(-E,/rr)C a

4

** Û])

)

( 1 Λ exp(Q/RT)C^)

a

C

-k2*|MC.2*/*1Γ»1Γ*3

1

. » ο - -(k.«k ) sy 2 ' - kjsyj - k *y

B

-skgK (P -

acidity

· Lewis a c i d i t y

"(I

s(k

- .k,C

kj. - B r o n s t e d

k

B

r

r

Forte o f M a l u Reaction Kinetics

(

t

-

(

B

«

s -

c

i

c

exp)

(1 < C t „ ) " " C

y

than

tlae

s - e

unity

unity

(better

s - I - crC

Activity vs. Coke Content

b)

1.1

kcal/aole

430 -U » 0.0259 500*-C - 0.0144

3 6 0 * C •• 0 . 0 8 0 6

Data g i v e n a s f ( T ) ·

V

(C^")

kcal/aole, (dUnc)

3 2 . 8keal/stole, 21.0

a)

kcal/aole

10.6 kcal/aole

-

8.9

12.6 kcal/aole

-

A c t i v a t i o n Energy f o r Coke F o r m a t i o n No,

(345*C) (271-C)

(4aa,

Yes (67) — No ( 6 9 )

Ye*

Yes

aal

Effects

industrial)

No ( 0 . 4 - 0 . 7

No

Yes

aesh

Diffusion

t o r < 16/32

Intraparticle



CHEMICAL REACTION ENGINEERING REVIEWS—

Chemical Engineering Science

Figure 11. Experimental and computed temperature profiles for afixedbed reactor, parallel poisoning, (a) Hot spot migration, nonisothermal. Profiles at 60 min intervals (1 = 0 min). 4.3% C H , thiopnenelC = 5.65 X 10' . X T — rnole fractions, B fraction sites remain ing (53); (b) active front migration, adiabatic. Profiles at 30 min intervals. 1.4% C H , 0.032% thiophene. Solid lines computed (54). 6

s

B

A

6

6

6

β

6


9.

BUTT A N D BILLIMORIA


311


A Case H i s t o r y : C o k i n g i n C a t a l y t i c C r a c k i n g V a r i o u s members o f t h e M o b i l R e s e a r c h a n d D e v e l o p m e n t C o r p . have t r e a t e d us o v e r t h e y e a r s t o a number o f d i s c u s s i o n s on v a r i o u s a s p e c t s o f t h e i r l u m p i n g , d e a c t i v a t i o n and r e a c t o r models f o r catalytic cracking. Some o f t h i s work was c o n s i d e r e d i n t h e p r e v i o u s r e v i e w ( 1 ) , a n d we have done o u r own t y p e o f l u m p i n g i n t h e f o r m o f e n t r y 8 i n T a b l e 3 f o r work t h r o u g h 1 9 7 1 . The s t a t e o f t h e a r t t o t h a t p o i n t may be s u m m a r i z e d , p e r h a p s t o o c o n c i s e l y , by t h e r e s u l t s g i v e n i n F i g u r e s 12a and b . Good c o r r e l a t i o n o f t h e a c t i v i t y o f a l a r g e range o f c a t l y s t s f o r f e e d s t o c k s v a r y i n g w i d e l y i n c o m p o s i t i o n was o b t a i n e d s i m p l y on t h e b a s i s o f a r o m a t i c t o naphthene w e i g h t r a t i o . G r o s s , e t a l . (75) s u b s e q u e n t l y e x t e n d e d t h i s work i n a t h o r o u g h s t u d y o f t h e c r a c k i n g o f t h r e e d i f f e r e n t f e e d s t o c k s on z e o l i t e c a t a l y s t s i n b o t h f i x e d a n d f l u i d i z e d b e d s . T h e i r r e s u l t s c o n f i r m e d t h a t c a t a l y s t d e c a y was i n d e p e n d e n t o f t h e r e a c t o r t y p e , t h a t g a s o l i n e s e l e c t i v i t y was e s s e n t i a l l y t h e same i n b o t h r e a c t o r s , and t h a t t h e f i x e d b e d was more e f f i c i e n t t h a n the f l u i d b e d . S u c h b e h a v i o r was a l l p r e d i c t e d by t h e a p p r o a c h e s d e v e l o p e d p r e v i o u s l y , so t h e w o r k o f G r o s s , e t a l . s t a n d s a s a r i g o r o u s t e s t o f those r e a c t o r and c a t a l y s t decay m o d e l s . The i n t e r e s t i n g p o i n t i n F i g u r e 12 i s t h e f a i l u r e o f t h e c o r r e l a t i o n f o r t h e two f e e d s i n d i c a t e d a s PC 32 a n d PA 3 7 . These d i f f e r e d from a l l other feeds used i n the c o r r e l a t i o n i n t h a t they were r e c y c l e s t o c k s , a n d f u r t h e r e x p e r i m e n t a t i o n r e v e a l e d t h a t t h e c o r r e l a t i o n f a i l e d as w e l l f o r s t o c k s c o n t a i n i n g p o i s o n s such as b a s i c n i t r o g e n compounds. T h i s was i n v e s t i g a t e d b y V o l t z , e t a l . (76) u s i n g t h e l u m p i n g , d e c a y a n d r e a c t o r m o d e l s o f p r i o r w o r k . MCGO, MCGO p l u s q u i n o l i n e , FCC f r e s h , and FCC w i t h v a r y i n g amounts o f r e c y c l e were s t u d i e d . MCGO p l u s q u i n o l i n e ( 0 . 1 % wt) o n l y s l i g h t l y r e d u c e d t h e c a t a l y s t decay c o n s t a n t a t 9 0 0 ° F b u t r e d u c e d the r a t e c o n s t a n t s f o r o v e r a l l c r a c k i n g and g a s o l i n e f o r m a t i o n by a b o u t 50%; some d e c r e a s e i n g a s o l i n e s e l e c t i v i t y was a l s o n o t e d . The a d d i t i o n o f r e c y c l e s t o c k t o FCC f r e s h f e e d a l s o h a d p r o n o u n c e d e f f e c t s : a t 50% r e c y c l e a d d i t i o n t h e r a t e c o n s t a n t s f o r b o t h o v e r a l l c r a c k i n g and g a s o l i n e f o r m a t i o n decreased s h a r p l y w h i l e t h e g a s o l i n e c r a c k i n g r a t e c o n s t a n t i n c r e a s e d by a f a c t o r o f 10. I n b o t h cases the a l t e r a t i o n o f feed s t o c k s had p r o f o u n d e f f e c t upon the a c t i v a t i o n energy o f the g a s o l i n e c r a c k i n g r e a c t i o n , i n c r e a s i n g f r o m a n o m i n a l v a l u e o f 10 k c a l / m o l e f o r MCGO t o a b o u t 40 k c a l / m o l e f o r MCGO p l u s 0 . 1 % q u i n o l i n e , a n d 2 3 . 5 k c a l / m o l e f o r FCC p l u s 30% r e c y c l e s t o c k . C l e a r l y the a c t i o n o f q u i n o l i n e i s to p o i s o n the a c i d i c s i t e s active for cracking; t h e d e a c t i v a t i o n model a n t i c i p a t e d c o k i n g o n l y a n d i s c l e a r l y n o t c a p a b l e o f c o r r e l a t i n g r e s u l t s due t o i m p u r i t y p o i s o n i n g . Recycle stocks d i f f e r from f r e s h feed p r i m a r i l y i n the f a c t t h a t t h e i r a r o m a t i c content i s h i g h e r ; a l s o these a r o m a t i c s i n g e n e r a l do n o t h a v e s u b s t i t u e n t g r o u p s , w h i c h i n some way may a c c o u n t f o r t h e i r d i f f e r e n t r e a c t i v i t y i n c r a c k i n g .



312


R e c e n t l y J a c o b s , e t a l . ( 7 7 ) h a v e s e t f o r t h a l u m p i n g scheme w h i c h accounts i n d e t a i l f o r the i n d i v i d u a l reactions of p a r a f f i n s , naphthenes, a r o m a t i c r i n g s , and a r o m a t i c s u b s t i t u e n t groups i n b o t h l i g h t and heavy f r a c t i o n s . T h i s i s shown i n F i g u r e 1 3 . The major a l t e r a t i o n , a s i d e from the i n c r e a s e d c o m p l e x i t y , from the p r i o r t h r e e lump scheme ( T a b l e 3) i s w i t h r e s p e c t t o a r o m a t i c r i n g s w i t h no s u b s t i t u e n t g r o u p s : t h e s e do n o t f o r m g a s o l i n e a n d can c r a c k u l t i m a t e l y o n l y t o the C lump. In addition, a l l reac t i o n s a r e t r e a t e d as f i r s t order w i t h an i n h i b i t i o n term f o r the a d s o r p t i o n o f heavy a r o m a t i c r i n g s . The d e a c t i v a t i o n f u n c t i o n i s s t i l l r e t a i n e d a s s e p a r a b l e b u t i s more c o m p l e x : s

=

— (P)

m

* (1 + B t ) Y

(23)

c

where α , β a n d γ a r e d e a c t i v a t i o n p a r a m e t e r s , Ρ i s o i l p a r t i a l p r e s s u r e , and catalyst residence time. The e f f e c t o f n i t r o g e n i s a l s o t r e a t e d as an a d s o r p t i o n i n h i b i t i o n and the r e s u l t a n t f u n c t i o n used a s a s c a l a r m u l t i p l i e r on t h e r a t e c o n s t a n t m a t r i x . The o v e r a l l scheme i s d e m o n s t r a t e d t o p r o v i d e e x c e l l e n t c o r r e l a t i o n o f r e s u l t s w i t h a seemingly e n d l e s s range o f f e e d s t o c k s , c a t a l y s t s , and r e a c t i o n c o n d i t i o n s . A n o t h e r s u b s t a n t i a l body o f w o r k d e a l i n g w i t h c r a c k i n g r e a c t i o n s has been r e p o r t e d o v e r t h e p a s t s e v e r a l y e a r s by Wojciechow s k i and coworkers. R e s u l t s w i t h cumene c r a c k i n g o v e r L a - Y z e o l i t e c a t a l y s t a r e summarized i n e n t r y 9 o f T a b l e 3 . Catalyst deactiva t i o n i n these s t u d i e s i s a l s o treated as separable, but an hyper b o l i c f u n c t i o n o f time on stream i s used f o r c o r r e l a t i o n o f activity. The u s e o f t h i s t y p e o f c o r r e l a t i o n f o r a number o f a p p l i c a t i o n s was s u m m a r i z e d i n a 1974 r e v i e w ( 7 0 ) . Since that t i m e , i n a d d i t i o n t o r e c e n t w o r k o n cumene c r a c k i n g (69) t h e model h a s been a p p l i e d t o a n e x t e n s i v e s e r i e s o f s t u d i e s on t h e c r a c k i n g o f gas o i l d i s t i l l a t e s ( 7 1 , 7 2 , 7 3 , 7 4 ) , a s w e l l as b e i n g employed i n t h e c o r r e l a t i o n o f J a c o b s , e t a l . ( 7 7 ) . A n o t h e r Case H i s t o r y :

Coking o f N i c k e l

Catalysts

The m e c h a n i s m o f c o k e f o r m a t i o n o n v a r i o u s t y p e s o f n i c k e l c a t a l y s t s d u r i n g CO o r CH^ d e c o m p o s i t i o n , o r d u r i n g s t e a m r e f o r m i n g o f h y d r o c a r b o n s , has been a t o p i c o f i n t e n s i v e i n v e s t i g a t i o n d u r i n g t h e p a s t few y e a r s . R o s t r u p - N i e l s e n (78) i n v e s t i g a t e d t h e d e c o m p o s i t i o n r e a c t i o n s i n t h e range 4 5 0 - 7 0 0 ° C ; one would e x p e c t t h a t coke o r i g i n a t i n g from these decompositions c o u l d be con t r o l l e d thermodynamically by o p e r a t i n g w i t h excess steam; however t h e r e has been u n c e r t a i n t y o v e r t h e y e a r s c o n c e r n i n g c h e m i c a l e q u i l i b r i u m i n these reactions (79,80,81). H o f e r , e t a l . (82) h a d f o u n d v i a e l e c t r o n m i c r o s c o p y t h a t t h e c o k e d e p o s i t e d on N i was i n the form o f t u b u l a r , w h i s k e r - l i k e t h r e a d s ; subsequent s t u d i e s ( 8 3 , 8 4 ) h a v e r e v e a l e d two s t r u c t u r e s , N i ^ C ( c a r b i d i c ) a n d g r a p h i t e



9.

BUTT AND


BiLLiMORiA

313

Industrial and Engineering Chemistry Process Design and Development Quarterly

Figure 12. Correlation of cracking kinetics with feed aromatic/naphthene (65). ( Overall gas-oil cracking; (b) gasoline formation.

Pf

- Wt. % paraffinic molecules, {mass spec analysis), 4 3 0 ° - 6 5 0 ° F

Ν/

= Wt. % naphthenic molecules, (mass spec analysis), 4 3 0 ° - 6 5 0 ° F

CΛ , * Wt. % carbon atoms among aromatic rings, (n-d-M method), e

430° - 6 5 0 F A(

= Wt. % aromatic substituent groups ( 4 3 0 ° - 6 5 0 ° F )

P

= Wt. % paraff inic molecules, (mass spec analysis), 6 5 0 ° F

n

N

n

C

A

A

n

+

= Wt. % naphthenic molecules, (mass spec analysis), 6 5 0 ° F h

+

= Wt. % carbon atoms among aromatic rings, n - d - M method, 6 5 0 ° F

+

+

= Wt. % aromatic substituent groups ( 6 5 0 ° F )

G

= G lump ( C - 4 3 0 ° F)

C

= C lump ( C to C

5

1

C

Ai

+

9

c

Ah

+

p

(

+

N

(

N

h + h

+

+

A

A

/ h

=

s

L

H

F

4

0

F

• COKE)

American Institute of Chemical Engineers Journal

Figure 13. A ten lump model for the kinetics of catalytic cracking


314


( w h i s k e r s ) w i t h t h e g r a p h i t e s t r u c t u r e d e c o m p o s i n g above a b o u t 400°C. R o s t r u p - N i e l s e n (78) f o u n d t h a t e q u i l i b r i u m c o n s t a n t s v a r i e d f r o m c a t a l y s t t o c a t a l y s t , b u t were c o r r e l a t e d w i t h N i crystallite size; f u r t h e r , the dimensions of the t u b u l a r g r a p h i t e s t r u c t u r e s were s i m i l a r t o t h e a s s o c i a t e d c r y s t a l l i t e . Steam r e f o r m i n g o f h y d r o c a r b o n s o n s u p p o r t e d N i i s a n i n t e r e s t i n g and v e r y c o m p l i c a t e d system. The m a j o r r e a c t i o n s t o b e considered a r e : + nH 0

nCO + ( n + j ) H


2

CO + H —> 2

CO + 3 H — * 2

C0 + H 2

2

(VI)

2

CH + H 0 4

2

w i t h coke d e p o s i t i o n p o s s i b l y v i a : CH

4

—y

2C0

C + 2H C + 2H

CmHm

2

(VII)

2

p o l y m e r —*

coke

Many w o r k e r s have d e a l t w i t h c a t a l y t i c p r o p e r t i e s a s t h e y a f f e c t VI and V I I . F o r c o k i n g i t i s g e n e r a l l y agreed t h a t b o t h m e t a l and a c i d i c f u n c t i o n s a r e i n v o l v e d ; the i n c o r p o r a t i o n o f a l k a l i o r the use o f l e s s a c i d i c s u p p o r t s r e t a r d s coke f o r m a t i o n ( 8 5 , 8 6 , 8 7 ) . K i n e t i c s o f coke f o r m a t i o n a r e r e p o r t e d t o be n e a r l y f i r s t o r d e r i n hydrocarbon ( f o r n-hexane) (88), and n o r m a l l y decrease w i t h i n c r e a s i n g steam/hydrocarbon r a t i o and i n c r e a s e w i t h i n c r e a s i n g u n saturation (86,89). A maximum i n coke f o r m a t i o n r a t e i n t h e r e g i o n 5 0 0 - 6 0 0 ° C h a s b e e n o b s e r v e d ( 8 6 , 8 8 ) , b u t t h e r e i s some d i s a g r e e m e n t c o n c e r n i n g t h e e f f e c t o f h y d r o g e n p a r t i a l p r e s s u r e on coking rates (86,90). T h e r e a r e some i n t e r e s t i n g a s p e c t s t o coke f o r m a t i o n i n t h e steam r e f o r m i n g r e a c t i o n . F i r s t i s the observation o f an i n d u c t i o n p e r i o d f o r c o k e f o r m a t i o n , shown i n F i g u r e 14a f o r n - h e p t a n e a t 5 0 0 C ( 8 6 ) , s t r o n g l y d e p e n d e n t u p o n steam/C r a t i o . T h i s has been noted f o r o t h e r hydrocarbons as w e l l ( 9 1 ) . Correlation of coke on c a t a l y s t i n t h i s i n s t a n c e can be p r o v i d e d b y : Q

C

c

=

k (t - t ) c

0

(24)

with the i n d u c t i o n time. F i g u r e 14b shows t h e s t r o n g i n h i b i t i o n o f coke f o r m a t i o n w i t h i n c r e a s e i n s t e a m / c a r b o n ( d e c r e a s e i n k ) , w i t h an accompanying i n c r e a s e i n i n d u c t i o n t i m e . An i n c r e a s e i n hydrogen p a r t i a l pressure i n c r e a s e d c o k i n g r a t e (86). The c o r r e l a t i o n o f E q . (24) i s o b v i o u s l y q u i t e d i f f e r e n t f r o m t h e f a m i l i a r V o o r h i e s f o r m , a n d t h e t h e r m a l dependence much g r e a t e r than that observed f o r V o o r h i e s - t y p e systems. Rostrup-Nielsen c



9.

B U T T A N D BILLIMORIA

Catalyst

Deactivation

315

(86) r e p o r t s a n a c t i v a t i o n e n e r g y o f 40 k c a l / m o l e compared t o t h e 10 k c a l / m o l e o r l e s s f o r coke f o r m a t i o n o n c r a c k i n g c a t a l y s t s (1^). A second f a c t o r i s the r e l a t i v e l y s m a l l d i m u n i t i o n i n a c t i v i t y o n coke f o r m a t i o n i n t h e s e c a t a l y s t s , d e m o n s t r a t e d b y s p e c i f i c e x p e r i m e n t s i n (86) f o r two c a t a l y s t s c o n t a i n i n g 4 . 5 a n d 1 1 . 0 wt % coke o n c a t a l y s t s . The r e a s o n f o r t h i s i s n o t c l e a r , b u t may r e s i d e i n t h e t e n d e n c y f o r c a r b o n t o s e g r e g a t e on n i c k e l s u r f a c e s ( 9 2 , 9 3 ) o r be due t o t h e f a c t t h a t a c a r b i d i c p h a s e s u c h a s N i ^ C i s the a c t i v e c a t a l y t i c surface under r e a c t i o n c o n d i t i o n s (84). Coke f o r m a t i o n r e a c t i o n s r e p o r t e d above i n V I I may be more c o m p l e x t h a n r e a l i z e d p r e v i o u s l y . I n v e r y r e c e n t work o n s t e a m r e f o r m i n g o f n - h e x a n e , B e t t , e t a l . (94) c o n c l u d e t h a t c a r b o n f o r m a t i o n may r e s u l t f r o m i n t e r a c t i o n o f u n r e a c t e d h e x a n e a n d t h e products of secondary c r a c k i n g r e a c t i o n s , or from unstable i n t e r mediates i n the c r a c k i n g p r o c e s s . A s e l e c t i o n of other thoughts c o n c e r n i n g t h e mechanism o f c o k e f o r m a t i o n i n r e f o r m i n g on N i would i n c l u d e the works o f Whalley, e t a l . (95), P r e s l a n d and W a l k e r ( 9 6 ) , a n d Renshaw, e t a l . (97). Carbon d e p o s i t i o n on n i c k e l c a t a l y s t s f o r r e a c t i o n s o t h e r than steam r e f o r m i n g has a l s o been s t u d i e d e x t e n s i v e l y i n r e c e n t years. The i n t e r a c t i o n o f l i g h t h y d r o c a r b o n s on N i f o i l s a t 4 0 0 6 0 0 ° C h a s b e e n i n v e s t i g a t e d b y L o b o a n d Trimm ( 9 8 ) , and t h e p y r o l y s i s o f o l e f i n s on N i f o i l s a t 4 0 0 - 7 5 0 ° C by R o s t r u p - N i e l s e n a n d Trimm ( 9 9 ) . I n the p y r o l y s i s r e a c t i o n s the c o k i n g r a t e s pass t h r o u g h a maximum, a s w i t h s t e a m r e f o r m i n g , i n t h e r a n g e 5 0 0 - 6 0 0 ° C . The k i n e t i c s o f r e a c t i o n a r e c o m p l e x ; b e l o w a b o u t 500°C t h e a p p a r e n t a c t i v a t i o n e n e r g y i s 32 t 2 k c a l / m o l e f o r a l l o l e f i n s and the r e a c t i o n i s zero o r d e r . Above 600°C t h e a c t i v a t i o n e n e r g y i s c a . -44 k c a l / m o l e and the r e a c t i o n o r d e r i s u n i t y f o r b o t h o l e f i n and h y d r o g e n . S i m i l a r r e s u l t s have b e e n r e p o r t e d b y D e r b y s h i r e a n d Trimm ( 1 0 0 ) . A number o f r e a s o n s i n c l u d i n g r e g a s i f i c a t i o n ( 8 5 , 1 0 1 ) , coke d e a c t i v a t i o n (101), p o i s o n i n g by h y d r i d e f o r m a t i o n , and c o m p e t i t i v e a d s o r p t i o n - s u r f a c e d i f f u s i o n phenomena ( 9 9 , 1 0 0 ) have b e e n s e t f o r t h t o e x p l a i n t h e s e k i n e t i c s . F i n a l l y , there i s a r a t h e r unique type of c o k i n g of supported N i under c e r t a i n c o n d i t i o n s which leads to c a t a s t r o p h i c d e s t r u c t i o n o f the p h y s i c a l s t r u c t u r e ( i . e . , r e d u c t i o n t o d u s t ) . Kiovsky (91) h a s r e p o r t e d t h i s phenomenon i n some d e t a i l f o r l o w s u r f a c e a r e a (~ 10 m /g) s u p p o r t e d N i (10% w t ) , u s e d f o r p r o d u c t i o n o f a n n e a l i n g gas v i a 0^ + C H ^ . I n one t e s t , a c o m m e r c i a l N o r t o n N C - 1 0 0 f o r m u l a t i o n m t h e f o r m o f 1 i n d i a m e t e r r i n g s was comp l e t e l y destroyed w i t h i n e i g h t hours under p a r t i a l o x i d a t i o n cond i t i o n s w i t h i n l e t t e m p e r a t u r e o f 1 0 1 0 ° C , 3/1 a i r / C H ^ mole r a t i o a n d 400 h r " s p a c e v e l o c i t y . O p e r a t i o n a t h i g h e r SV (800 h r " ) r e s u l t e d i n no d e s t r u c t i o n . P h y s i c a l d e g r a d a t i o n was a c c o m p a n i e d by coke f o r m a t i o n , b u t a n i n d u c t i o n p e r i o d was o b s e r v e d i n a l l cases before d e s t r u c t i o n o c c u r r e d ; t h e c a t a l y s t c o u l d be comp l e t e l y r e g e n e r a t e d by a i r o x i d a t i o n i f t i m e on s t r e a m were l e s s t h a n t h e i n d u c t i o n p e r i o d . Some t y p i c a l d a t a a r e shown i n F i g u r e 15a f o r e x p e r i m e n t s a t 730 h r " , 4 9 0 ° C , w i t h a t o l u e n e - s a t u r a t e d 2

1

1

1



316

C H E M I C A L REACTION ENGINEERING

REVIEWS—HOUSTON

Journal of Catalysis

Figure 14. (a) Mean coke content of catalyst vs. time on stream, η-heptane reformi 500°C. 1.) H O/C — 1.3, 2.) H O/C — 1.5, 3.) H O/C — 2.0; (b) coke correlation co stants, Equation 24, and steam/carbon ratio. Catalyst: 23.8% Ni on MgO with Al, 0.07% Na, 2 m /g Ni, 20 m? I g BET (W). t

t

t

2

American Chemical Society

Figure 15. Destruction of supported Ni by coke formation during partial oxidatio hydrocarbons, (a) Survivors vs. time at two air/HC ratios; (b) effect of average p ameter upon survival rate (91)



9.


B U T T A N D BILLIMORIA

317

methane a t two d i f f e r e n t a i r / h y d r o c a r b o n r a t i o s . Survivors are d e f i n e d a s t h e number o f c a t a l y s t p i e c e s i n t h e o r i g i n a l c h a r g e r e t a i n i n g a t l e a s t 80% o f t h e i r o r i g i n a l m a s s ; t y p i c a l coke l e v e l s a r e o n l y on t h e o r d e r o f 3 - 5 wt % w h i l e d e s t r u c t i o n i s o c c u r r i n g and i t i s c l e a r t h a t the k i n e t i c s o f the p r o c e s s a r e relatively rapid. I n i t i a l c r u s h s t r e n g t h was n o t w e l l c o r r e l a t e d w i t h s u r v i v a l r a t e , b u t t h e i n d u c t i o n p e r i o d was i n c r e a s e d a n d r a t e o f d e s t r u c t i o n decreased by r e d u c i n g c a t a l y s t e x t e r n a l s u r f a c e a r e a p e r volume and r e d u c i n g median pore d i a m e t e r . The l a t t e r e f f e c t i s shown i n F i g u r e 15b f o r two c a t a l y s t s ; "new", w i t h p o r e d i a m e t e r 5μ,, a n d " s t a n d a r d " , 1 7 u . The a b i l i t y o f coke f o r m a t i o n to d e s t r o y the p h y s i c a l s t r u c t u r e of the c a t a l y s t i s s u r p r i s i n g , but hardly e q u i v o c a l . Acknowledgments T h i s work was s u p p o r t e d v i a t h e g e n e r o u s h e l p o f t h e C e n t r a l R e s e a r c h D e p a r t m e n t , Dow C h e m i c a l , U . S . A . F i g u r e s 1 , 2 , 3 , 4 , 6, 11 by p e r m i s s i o n o f Pergamon P r e s s , L t d . , F i g u r e s 1 2 , 15 by p e r m i s s i o n o f t h e A m e r i c a n C h e m i c a l S o c i e t y , F i g u r e s 9 , 10 by p e r m i s s i o n o f E l s e v i e r P u b l i s h i n g C o . , F i g u r e 13 b y p e r m i s s i o n o f t h e A m e r i c a n I n s t i t u t e o f C h e m i c a l E n g i n e e r s , a n d F i g u r e 14 by p e r m i s s i o n o f Academic P r e s s , I n c . Appendix Two o c c u p a t i o n a l h a z a r d s a s s o c i a t e d w i t h t h e w r i t i n g o f r e v i e w s such as t h i s a r e the i n a d v e r t e n t o v e r s i g h t o f p a r t i c u l a r l y germane l i t e r a t u r e a n d t h e p u b l i c a t i o n o f r e l e v a n t w o r k i n t h e p e r i o d b e t w e e n c o m p l e t i o n a n d p u b l i c a t i o n o f t h e r e v i e w . We have examples o f b o t h h e r e . I n t h e t e x t we commented upon t h e p o s s i b l e i n a d e q u a c i e s o f the separable form of r e p r e s e n t a t i o n f o r d e a c t i v a t i o n k i n e t i c s . I n f a c t , B a k s h i a n d G a v a l a s (1A) h a v e d e m o n s t r a t e d t h i s e x p e r i m e n t a l l y f o r methanol and e t h a n o l d e h y d r a t i o n on S i 0 2 / A l 2 0 a t 1 5 0 - 2 2 5 ° C , p o i s o n e d by n - b u t y l a m i n e . The k i n e t i c s o f r e a c t i o n were c o r r e l a t e d b y : k

(-r )

-

T

K

A ? A

%

^A;

ι + K C * + A

A

Vw

(1A)

f o r b o t h the f r e s h and d e a c t i v a t e d c a t a l y s t , but the a d s o r p t i o n c o n s t a n t s K . a n d K« v a r i e d w i t h e x t e n t o f d e a c t i v a t i o n w h i c h i s n o t as the s e p a r a b l e f o r m u l a t i o n would have i t . Such c h a n g e s were i n t e r p r e t e d as a m a n i f e s t a t i o n o f the n o n u n i f o r m i t y o f s u r f a c e s i t e s , a s we have s u g g e s t e d i n E q . ( 1 ) . Kam, e t a l . (2A) have a l s o u s e d a r a t e e x p r e s s i o n o f t h e f o r m o f E q . (1A) i n a t h e o r e t i c a l analysis of isothermal, i n t r a p a r t i c l e fouling i n v o l v i n g c o m b i n e d s e r i e s and p a r a l l e l f o u l i n g . The r e s u l t s o f t h e c o m b i n e d mechanism a r e c o n t r a s t e d w i t h t h o s e p e r t a i n i n g t o t h e i n d i v i d u a l steps alone. A


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An example o f s t r u c t u r e s e n s i t i v i t y i n d e a c t i v a t i o n i s f o u n d i n t h e work o f O s t e r m a i e r , e t a l . (3A) f o r 2-15 nm Ρ ί / Α ^ Ο β a n d P t b l a c k i n ammonia o x i d a t i o n a t 3 6 8 - 4 7 3 ° K . The e f f e c t s o f c r y s t a l l i t e s i z e a n d t e m p e r a t u r e i n d e a c t i v a t i o n were i n v e s t i g a t e d ; i t was f o u n d t h a t t h e e x t e n t o f d e a c t i v a t i o n i n c r e a s e d w i t h d e c r e a s i n g t e m p e r a t u r e , a n d t h e r e was a d i f f e r e n c e i n t h e A r r h e n i u s b e h a v i o r between s i n t e r e d and u n s i n t e r e d m a t e r i a l s . Deactivation was more s e v e r e w i t h s m a l l e r c r y s t a l l i t e s , b u t t h e s u r f a c e c o u l d be c o m p l e t e l y r e a c t i v a t e d by at 673°K. I t was s u g g e s t e d t h a t P t O was t h e d e a c t i v a t e d s u r f a c e , a n d a n e x c e l l e n t c o r r e l a t i o n o f a c t i v i t y was p r o v i d e d b y : dN dt W

i

t

h

N

2 i y r

=

l + N °

K

(2A)

p

t

< > 3A

where Ν i s d e n s i t y o f s i t e s , Κ a r a t e c o n s t a n t f o r d e a c t i v a t i o n , and 1^ an i n i t i a l s i t e d e n s i t y . I n r e c e n t w o r k Hegedus a n d Summers (4A) h a v e i n v e s t i g a t e d t h e p o i s o n i n g o f n o b l e m e t a l s s u p p o r t e d on AI2O3 by l e a d and p h o s p h o rous i n the o x i d a t i o n o f c a r b o n monoxide and h y d r o c a r b o n s . The p a r a m e t e r s s t u d i e d were p o r e s t r u c t u r e , s u p p o r t a r e a a n d impregnation depth. C o r r e l a t i o n s are given for poison penetra t i o n and e f f e c t i v e n e s s as a f u n c t i o n o f time o f o p e r a t i o n . As i n the case o f c o k i n g v i a the p a r a l l e l mechanism, the o v e r a l l c a t a l y s t l i f e c a n be o p t i m i z e d by m a n i p u l a t i o n o f t h e m a c r o p o r e s t r u c ture. D e s i g n i n g f o r a g i v e n d i f f u s i o n a l l i m i t a t i o n i n the f r e s h c a t a l y s t r e t a r d s the r a t e o f the main r e a c t i o n b u t a l s o r e t a r d s p e n e t r a t i o n o f p o i s o n i n t o t h e c a t a l y s t m a t r i x , h e n c e an optimum may be s o u g h t f o r maximum t i m e - a v e r a g e d effectiveness. F i n a l l y , M i k u s , e t a l . (5A) have r e p o r t e d s t u d i e s o f f i x e d b e d r e a c t o r t r a n s i e n t s f o r CO o x i d a t i o n on Ρ ί / Α ^ Ο β ( 0 . 1 % P t ) w i t h CS2 p o i s o n . Their experimental r e s u l t s are i n q u a l i t a t i v e a g r e e m e n t w i t h t h e c o m p u t a t i o n s o f B l a u m (52) a n d E r v i n and L u s s (6A), b u t no s i m u l a t i o n s a r e r e p o r t e d i n t h e p a p e r . The t e m p e r a t u r e p r o f i l e s r e p o r t e d have a c r u i o u s shape f o r what i s r e p o r t e d t o be a n a d i a b a t i c r e a c t o r . β

Literature Cited 1. Butt, J.B., Adv. Chem., (1972), 109, 259. 2. Wanke, S.E. and Flynn, P.C., Catal. Rev.-Sci. and Eng., (1975), 12, 93. 3. Shelef, M., Otto, K. and Otto, N.C., Adv. Catal., (1978), 27. 4. Carberry, J.J., "Chemical and Catalytic Reaction Engineering", McGraw-Hill, New York, 1976. 5. Khang, S.J. and Levenspiel, O., Ind. Eng. Chem. Fundls., (1973), 12, 185. 6. Szepé, S. and Levenspiel, O., Proc. European Fed., 4th Chem. Reaction Eng., Brussels, 1968; Pergamon Press, 1971.



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319

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Ervin, M.A. and Luss, D., AIChE J 1 . , (1970), 16, 979.

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