Chemical Reaction Engineering-Houston

17 Interpretation of Catalyst Deactivation by Fouling from Interactions of Pore Structure and Foulant Deposit Geometries

Downloaded by UNIV OF ARIZONA on March 15, 2017 | http://pubs.acs.org Publication Date: June 1, 1978 | doi: 10.1021/bk-1978-0065.ch017

C. C. H U G H E S and R E G I N A L D

MANN

Department of Chemical Engineering, University of Manchester Institute of Science and Technology, Manchester, England

Deactivation is a term of general designation and encompasses several d i s t i n c t , though sometimes poorly defined processes, that give r i s e to a lowering of catalyst a c t i v i t y . Poisoning, ageing, sintering and fouling are particular examples of deactivation and these terms ought in principle to clearly indicate and discriminate the mechanisms involved. In practice, probably due to the potential complexity if these processes take place simultaneously, there is often some overlap and confusion i n their use. Levenspiel 1 has referred to fouling as being primarily rapid, accompanied by deposition and a physical blocking of surface. He then defines poisoning as a slow modification of a c t i v i t y by chemisorption on the active s i t e s , the poison being characterised by difficulty of removal. It i s our view that rate of loss of a c t i v i t y is not a sufficiently meaningful discriminant. Instead, we propose that poisoning should refer to active site deactivation by monolayer type adsorption at the site, and thereafter no further poison adsorption takes place at that location. In this way, a very great loss of a c t i v i t y can take place with the adsorption of very small amounts of poison. I f , however, successive adsorption on the surface can take place, such that significant amounts of material accumulate, then t h i s represents fouling of the catalyst. The c l a s s i c a l treatments of a c t i v i t y loss by poisoning by Thiele 2 and Wheeler 3t support the above d i s t i n c t i o n s , since the poison was not considered to have any influence upon the pore geometry or effective d i f f u s i v i t y . Uniform, non-uniform and anti-selective poisoning do give r i s e to a wide spectrum of deactivation behaviour, but the non-comprehensive capability of a theory of poisoning to explain deactivation when significant accumulation takes place, requires that new approaches be made. This is confirmed by several more recent observations. Thus, Butt's 4 measurements of a very non-uniform coke p r o f i l e , indicate that ultimate penetration of a coke type foulant into a catalyst p a r t i c l e i s very quickly attained, and sybsequent deposition occurs entirely within an outer s h e l l . ©

0-8412-0401-2/78/47-065-201$05.00/0

Weekman and Luss; Chemical Reaction Engineering—Houston ACS Symposium Series; American Chemical Society: Washington, DC, 1978.


202

CHEMICAL

REACTION

ENGINEERING—HOUSTOI

There remains an uncoked c e n t r a l c o r e , apparently never contactée by r e a c t a n t . The work o f Rostrup-Nielsen 5 and L e v i n t n e r 6 a l s o suggests t h a t pore mouth c l o s u r e by coke plugs may be t h e o r e t i c a l l y r e q u i r e d , and a s i m p l i f i e d theory has been r e c e n t l y proposed by Newson 7· There i s t h e r e f o r e a good d e a l o f evidence t o support t h e i d e a t h a t the e f f e c t o f f o u l a n t d e p o s i t s on a c t i v i t y , s e l e c t i v i t y and p e l l e t macroscopic p r o p e r t i e s i s s e n s i t i v e t o both the pore s t r u c t u r e and the f o u l a n t deposit s t r u c t u r e . Such an approach should improve upon the more e m p i r i c a l l y based methods used by Voorhies 8 and Wojciechowski 9» which have t r a d i t i o n a l l y attemptedTto describe d e a c t i v a t i o n when f o u l i n g occurs. O u t l i n e o f the Theory The nature o f the i n t e r a c t i o n o f the pore s t r u c t u r e and f o u l a n t deposit geometries i s determined from two b a s i c assumptions. F i r s t l y , t h a t t h e pore s t r u c t u r e may be represente( by a set o f i d e a l i s e d p a r a l l e l s i d e d non i n t e r s e c t i n g pores o f v a r i a b l e r a d i u s , but each o f a c e r t a i n l e n g t h L. This i s the so c a l l e d ' p a r a l l e l bundle' model. Secondly, t h a t the f o u l a n t accumulates by simultaneous p e n e t r a t i o n and t h i c k e n i n g , g i v i n g r i s e t o successive l a y i n g down o f f o u l a n t . We c a l l t h i s t h e 'wedge l a y e r i n g ' model o f f o u l a n t d e p o s i t i o n . The q u a l i t a t i v e f e a t u r e s o f the subsequent i n t e r a c t i o n are depicted i n F i g . 1. The s m a l l e s t pore A b l o c k s f i r s t as t h i c k e n i n g proceeds and t h e r e a f t e r the remaining surface w i t h i n pore A i s rendered i n a c c e s s i b l e and thus c a t a l y t i c a l l y i n a c t i v e . I f the d e s i r e d n o n - f o u l i n g r e a c t i o n i s t a k i n g place without d i f f u s i o n i n f l u e n c e , the l o s s i n a c t i v i t y i s equal t o t h i s l o s s o f area i n pore A. T h i s i s shown i n F i g . 2 . As p e n e t r a t i o n and t h i c k e n i n g continue a t the same r e l a t i v e r a t e , a d d i t i o n a l l o s s e s i n a c t i v i t y take p l a c e , as the remaining l a r g e r pores become plugged. I t i s c l e a r t h a t i n the absence o f a theory o f p l u g g i n g , the a c t i v i t y l o s s would be erroneously i n t e r p r e t e d as being caused by p o i s o n i n g w i t h d i f f u s i o n a l r e s i s t a n c e . T o t a l f o u l a n t content as a f u n c t i o n o f p e l l e t a c t i v i t y i s perhaps the most important c h a r a c t e r i s t i c i n a n a l y s i n g f o u l i n g behaviour, s i n c e i t i s f a i r l y simply observed by experiment. F i g . 3 shows some q u a l i t a t i v e aspects. I t i s c l e a r t h a t a t any given f o u l a n t content two p o s s i b i l i t i e s e x i s t f o r t h e d i s t r i b u t i o n o f f o u l a n t t h a t g i v e r i s e t o the same a c t i v i t y . Thj i s i l l u s t r a t e d i n F i g s . 3 ( a ) , ( c ) . I f the f o u l a n t has a s m a l l t h i c k n e s s and l a r g e p e n e t r a t i o n as i n ( a ) , t h i s can be viewed as a p o i s o n i n g mode o f d e a c t i v a t i o n . On the other hand, the same amount o f f o u l a n t could be present as a l a r g e t h i c k n e s s s m a l l p e n e t r a t i o n wedge as i n F i g . 3 ( c ) , and t h i s would be a pore mouth plugging mode o f d e a c t i v a t i o n . A f u r t h e r o b s e r v a t i o n i s t h a t a t a c e r t a i n l e v e l o f the parameter β defined by β = rate o f foulant thickening/rate o f foulant penetration



17.

HUGHES

AND M A N N

Catalyst Deactivation by Fouling

Figure 1. Fouling by "wedge layering" in a parallel bundle pore structure model

0-6 h

PORE Ap*

FOULANT PENETRATION

Figure 2. Activity hsses as foulant penetrates and accumulates


203


204

CHEMICAL

Figure 3.

REACTION

ENGINEERING—HOUSTON

Illustration of differing modes of deactivation


17.

HUGHES

Catalyst Deactivation by Fouling

AND MANN

205

which i s here assumed t o be a constant i r r e s p e c t i v e o f degree o f p e n e t r a t i o n o r t h i c k e n i n g , a pore o f a g i v e n s i z e reaches a maximum f o u l a n t content as β i n c r e a s e s from zero (pure poisoning) to i n f i n i t y (pure pore mouth p l u g g i n g ) . T h i s i s i n d i c a t e d i n F i g s . Ma) and ( b ) . T h i s c h a r a c t e r i s t i c o f maximum f o u l a n t accumulation a t some c r i t i c a l value β has been deduced w i t h respect t o pores o f a s i n g l e s i z e . lS order f o r the theory t o f i n d general a p p l i c a b i l i t y , i t requires extension t o p e l l e t s with d i s t r i b u t e d pore s i z e s . For a u n i t mass o f c a t a l y s t , i f f ( r ) d r i s the surface area contained i n pores o f s i z e between* r and r+dr, then Downloaded by UNIV OF ARIZONA on March 15, 2017 | http://pubs.acs.org Publication Date: June 1, 1978 | doi: 10.1021/bk-1978-0065.ch017

r

00

ίο

f (r)dr 8

(1)

a S *

where S i s the s p e c i f i c surface area o f the c a t a l y s t . I f pure pore moith p o i s o n i n g were t o take p l a c e under n o n - d i f f u s i o n i n f l u e n c e d c o n d i t i o n s , the reduced a c t i v i t y a t a g i v e n p e n e t r a t i o n χ i s g i v e n by (-co

(1 - τ)

Ι f (r)dr/S = 1 - f (2) Jo where L i s the l e n g t h dimension o f the p a r a l l e l pore bundle. Now, i f f o u l i n g takes p l a c e by wedge l a y e r i n g such t h a t β = h/x, pores w i l l remain unplugged, and t h e i r i n t e r i o r surface w i l l be a c c e s s i b l e and c a t a l y t i c a l l y a c t i v e , provided t h a t r>h. Therefore the reduced a c t i v i t y a t a g i v e n p e n e t r a t i o n χ w i l l be g i v e n by w

S

δ

L

-00

(1 - £) Γ Jh

(3)

f (r)dr/S 6

The dimensionless a c t i v i t y i s t h e r e f o r e i d e n t i f i e d w i t h nonf o u l e d pore surface t h a t succeeds i n remaining a c c e s s i b l e . As mentioned p r e v i o u s l y , t h i s e f f e c t due t o mouth p l u g g i n g can be s p u r i o u s l y i d e n t i f i e d as pore d i f f u s i o n a l r e s i s t a n c e accompanying p o i s o n i n g . In c a l c u l a t i n g the corresponding volume o r mass o f accumulated f o u l a n t , those pores which have a l r e a d y become sealed have t o be d i s t i n g u i s h e d from those t h a t y e t remain t o be plugged. For unplugged pores, i f f ( r ) d r i s the number f r a c t i o n o f pores s i z e d between r and r+dr, the f o u l a n t volume i n t h i s category o f pores at a p o t e n t i a l mouth t h i c k n e s s h i s g i v e n by -co V£(h) = J

£ττ(

2 Γ

2

2

- β χ )χΝί (Γ)ά Ν

Γ

W

where Ν i s the t o t a l number o f pores c o n s t i t u t i n g the p a r a l l e l bundle w i t h i n a u n i t mass o f c a t a l y s t . For the category o f plugged pores, a pore becomes plugged and t h e r e a f t e r remains plugged a t the i n s t a n t when h = βχ = r , and hence a l l the pores between 0 and h are blocked o f f f o r a



206

CHEMICAL REACTION ENGINEERING—HOUSTON

FOULANT CONTENT (a) FOR 0

Chemical Reaction Engineering-Houston

Recommend Documents