Fluid Catalytic Cracking II - American Chemical Society

Chapter 8

Monitoring Fluid Cracking Catalyst Deactivation Profile by Equilibrium Catalyst Separation

Downloaded by UNIV OF TEXAS AT DALLAS on April 25, 2017 | http://pubs.acs.org Publication Date: January 23, 1991 | doi: 10.1021/bk-1991-0452.ch008

R. A. Beyerlein, G. A. Tamborski, C. L. Marshall, B. L. Meyers, J. B. Hall, and B. J. Huggins Amoco Corporation, Naperville, IL 60566

Successful separation of a precoked, equilibrium USY cracking catalyst into fractions of increasing age reveals a deactivation profile that i s largely controlled by zeolite dealumination. Precoking i s found to aid the density/age separation only for the youngest fractions. The observed change i n Ni/V ratio with increasing "Ni age" is indicative of vanadium migration. Catalytic activity, assessed by cumene cracking on separated fractions and also by analysis of residual coke on catalyst fractions, shows a sharp decline with increasing density (age). This rapid loss of initial activity coincides with zeolite dealumination which i s largely completed as a slow rate of zeolite destruction is established. Subsequent loss of c r y s t a l l i n i t y has little additional effect on activity. The associated loss of microporosity leads to an apparent increase i n skeletal density with increasing age. Lattice imaging studies by TEM on a "young" fraction showed extensive regions of c r y s t a l l i n i t y with minimal evidence of crystallite fracturing. By contrast, similar investigations of an "old" fraction, combined with i n situ compositional analysis, revealed small "islands" of c r y s t a l l i n i t y within a "sea" of disordered material that was once crystalline. Fracture lines at crystallite boundaries are absent. Instead, the small USY crystallites within the "old" fraction are in intimate mixture with the collapsed zeolite. Laboratory evaluation o ffluidized-bed c a t a l y t i c cracking (FCC) c a t a l y s t s i s c r i t i c a l l y dependent upon t h e method c h o s e n t o s i m u l a t e c a t a l y s t d e a c t i v a t i o n i n t h e c o m m e r c i a l FCU. Conventional l a b o r a t o r y d e a c t i v a t i o n s (high-temperature steam

0097-6156/91/0452-0109$09.75/0 © 1991 American Chemical Society

Occelli; Fluid Catalytic Cracking II ACS Symposium Series; American Chemical Society: Washington, DC, 1991.


110

FLUID CATALYTIC CRACKING II: CONCEPTS IN CATALYST DESIGN

treatment) are t y p i c a l l y designed to simulate only the i r r e v e r s i b l e loss of a c t i v i t y that r e s u l t s from hydrothermal deactivation of the z e o l i t i c component i n the FCC regenerator. The combination of high temperature and steam i n the regenerator causes framework dealumination (rapid) and c r y s t a l l i n e z e o l i t e destruction (slower). Dealumination both lowers a c t i v i t y and produces important changes i n s e l e c t i v i t y , while z e o l i t e destruction leads primarily to a loss i n a c t i v i t y . Z e o l i t e destruction i s slower for dealuminated z e o l i t e s which have increased hydrothermal s t a b i l i t y , but i s accelerated by deposition of metals, p a r t i c u l a r l y vanadium and sodium from the feed. In addition, these metals are powerful poisons which neutralize a c i d s i t e s , thereby reducing a c t i v i t y and compromising s e l e c t i v i t y . Nickel species, which accompany vanadium as feed contaminants, are powerful dehydrogenation catalysts and lead to increased production of hydrogen and coke with reduced gasoline y i e l d s . A l l of t h i s i s complicated by the fact that the c i r c u l a t i n g equilibrium c a t a l y s t inventory i n the FCU contains a wide spectrum of ages and l e v e l s of deactivation, owing to the 1% to 3% d a i l y fresh c a t a l y s t make-up rate. Laboratory steam deactivations represent a s i g n i f i c a n t compromise i n the e f f o r t to simulate equilibrium c a t a l y s t . Since hydrothermal deactivation of FCC catalysts i s not rapid i n commercial practice, deactivation of the fresh c a t a l y s t i n the laboratory requires accelerated techniques. The associated temperatures and steam p a r t i a l pressures are often i n substantial excess of those encountered i n commercial units. In some instances, the e f f e c t of contaminant metals i s measured by an independent t e s t not a f f i l i a t e d with steam deactivation. In subsequent y i e l d s testing, interactions between d i f f e r e n t modes of deactivation may be overlooked. F i n a l l y , single mode deactivation procedures can not reproduce the complex p r o f i l e of ages and l e v e l s of deactivation present i n equilibrium c a t a l y s t . Density Separation of Precoked Catalyst. In order to monitor and understand c a t a l y s t deactivation i n the commercial u n i t and to supply guidance for developing improved laboratory deactivation procedures, we have c a r r i e d out a separation by density of equilibrium cracking c a t a l y s t from an Amoco FCU. The density separation i s based, i n part, on the a c t i v i t y of the equilibrium c a t a l y s t for coke deposition from isobutene. Each c a t a l y s t p a r t i c l e develops a coke l e v e l proportional to i t s i n t r i n s i c a c t i v i t y l e v e l . The coked c a t a l y s t p a r t i c l e s are then separated into fractions by immersion i n progressively more dense mixtures of carbon tetrachloride and tetrabromoethane. Since coked p a r t i c l e s exhibit lower s k e l e t a l density, the higher a c t i v i t y and presumably younger c a t a l y s t p a r t i c l e s are sequestered into the l i g h t e r f r a c t i o n s . A recent report (1) indicates that t h i s separation method produces fractions of increasing age, the heaviest fractions being the oldest. In the present work, i t i s found that the separation by density i s based only i n part on differences i n coking a c t i v i t y .



8.

BEYERLEIN ETAL.

Monitoring FCC Deactivation Profile

111

Measured N i l e v e l s on c a t a l y s t are expected to supply an independent age marker, as i t has been shown that Ni tends not to migrate following deposition on the c a t a l y s t (2). In f a c t , metals (Ni and V) deposition from the feed onto the c a t a l y s t i s expected to a s s i s t the separation by density/age/activity as older c a t a l y s t p a r t i c l e s exhibit higher metals l e v e l s which contribute to an increase i n density. Except for the oldest f r a c t i o n s , which contain the highest metals l e v e l s , the older portion of c a t a l y s t tends to make less coke as the increase i n a c t i v i t y due to increasing metals content i s overwhelmed by the loss of a c t i v i t y due to c r y s t a l l i n e z e o l i t e destruction (1). The present study shows that only the youngest, most active fractions are density-separated on the basis of a c t i v i t y for coking from isobutene. For the majority of the separated f r a c t i o n s , the gradual increase i n s k e l e t a l density with increasing age (Ni l e v e l ) appears to be associated with the gradual loss of z e o l i t e c r y s t a l l i n i t y that occurs with increasing time i n the u n i t . Catalyst History Catalyst Changeover. Metals Content. The equilibrium sample of a USY octane c a t a l y s t , Catalyst A, was withdrawn from an Amoco FCU. Results of metals analyses on the equilibrium c a t a l y s t and on i t s parent are given i n Table I. Catalyst A was introduced to the u n i t during a five-month period over Catalyst B, which was i d e n t i c a l to Catalyst A i n a l l respects, except for a low l e v e l of contaminant rare earth. Catalyst B had, i n turn, been introduced over a rare earth-containing catalyst, Catalyst C, eight months p r i o r to withdrawal of the equilibrium sample. Catalyst h i s t o r y and rare earth contents are summarized i n Table I I . Table I. Equilibrium and Fresh USY Catalyst

Elemental

Fresh Catalyst A

Inspections* Equilibrium Catalyst A

570 Ni, ppm (XRF) 30 Fe 3110 (XRF) 2140 V (XRF) 440 1 a n d t h a t c o n d e n s e d p o l y a r o m a t i c s w i t h s i m i l a r H/C ( e . g . , p h e n a n t h r e n e , a n t h r a c e n e ) e x h i b i t d e n s i t i e s i n t h e r a n g e 1 t o 1 . 3 , a s k e l e t a l d e n s i t y o f 1.2 g / c c i s a s s u m e d f o r c o k e o n c a t a l y s t ( 1 4 , 1 5 ) . T h e n f o r e a c h 1% a d d i t i o n i n c o k e , f o r a n o m i n a l c a t a l y s t d e n s i t y o f 2.4, 0.99 2.4 or

d

and

Ad

a v e

+

0.01 1.2

-

1 d

m

- 2.3162 0.0238 g / c c f o r a d d i t i o n o f 1 w t % c o k e .

E f f e c t o f Metals on C a t a l y s t . N i c k e l a n d vanadium a r e assumed t o b e p r e s e n t a s t h e o x i d e s N i O a n d V 0 w i t h d e n s i t i e s o f 6.7 a n d 4.4 g / c c . F o r a n o m i n a l c a t a l y s t d e n s i t y o f 2.5, t h e e s t i m a t e d d e n s i t y f o r a 0.1% change i n V c o n c e n t r a t i o n i s 2

Q

d

- 0.001 x 1.63^ 2.5

a v e

A

a v e

( 0 . 0 0 1 x 1.63) 4.4

-

1_ d

a v e

- 2 . 5 0 1 7 6 a n d A d - 0.0018 g / c c f o r a 1 0 0 0 ppm change i n V c o n c e n t r a t i o n . T h e f a c t o r 1.63 accounts f o r t h e vanadium oxide/vanadium weight ratio. S i m i l a r l y , f o r a 0.1% change i n N i concentration,

( 1 - 0 . 0 0 1 x 1.27^ 2.5 d

+

4

+

( 0 . 0 0 1 x 1.27) 6.7

-

1 d

a v e

- 2 . 5 0 1 9 9 a n d A d - 0 . 0 0 2 0 g / c c f o r a 1 0 0 0 ppm change i nN i c o n c e n t r a t i o n . T h e f a c t o r 1.27 accounts f o r the n i c k e l o x i d e / n i c k e l weight ratio. Then f o r an i n c r e a s e i nm e t a l s c o n t e n t

( N i + V ) - 1 0 0 0 ppm, t h e n e t d e n s i t y i n c r e a s e i s e s t i m a t e d a s A d - 0.0019 f o r A ( N i + V ) 1 0 0 0 ppm.


8.

141


BEYERLEIN ETAL.

S i m i l a r l y , i f the e f f e c t o f r a r e e a r t h i s modeled as i f r a r e e a r t h were p r e s e n t as RE 0 , t h e e s t i m a t e d d e n s i t y change f o r a 0.5% c h a n g e i n RE c o n t e n t i s 2

1


or

d

3

- 0.005 x 1.17 2.5 a v e

-

+

0.005 x 1.17 6.75

-

1 d

a v e

2.50924

and A d - 0 . 0 0 9 2 g / c c f o r a 0.5% i n c r e a s e i n r a r e e a r t h c o n t e n t . S i m i l a r c a l c u l a t i o n s show t h a t i f i r o n a n d t i t a n i u m a r e assumed t o be p r e s e n t as t h e o x i d e s a-Fe203 and T i 0 2 , w i t h d e n s i t i e s o f 5.24 g / c c a n d 4.2 g / c c , r e s p e c t i v e l y , t h e e s t i m a t e d A d ' s a r e AdFe AdTi Effect of Excluded

( 5 0 0 0 ppm) ( 5 0 0 0 ppm)

-

0.0094 g / c c 0.0068 g / c c

Volume

As d i s c u s s e d i n t h e t e x t , about 12.5% o f the t o t a l m i c r o p o r e v o l u m e o f z e o l i t e Y, 0.36 c c / g , i s a s s o c i a t e d w i t h t h e s m a l l cages. A s s u m e t h a t t h e same p r o p o r t i o n h o l d s f o r u l t r a s t a b l e Y w h i c h , due t o o c c l u d e d m a t e r i a l i n t h e m i c r o p o r e , t y p i c a l l y shows a m e a s u r e d m i c r o p o r e v o l u m e o f a b o u t 0.27 c c / g . A c c o u n t i n g f o r the excluded volume i n the ( i n a c c e s s i b l e ) s m a l l cages, the t o t a l m i c r o p o r e v o l u m e i s 0 . 2 7 / 0 . 8 7 5 - 0.31 c c / g . O b s e r v i n g t h a t t h e Xr a y d e n s i t y o f u l t r a s t a b l e Y i s a b o u t 1.3 g / c c , t h e n e t v o i d v o l u m e i s 0.31 c c / g x 1.3 g / c c « 0.40 c c / c c . Assuming that the s m a l l cages o f c r y s t a l l i n e z e o l i t e a r e never f i l l e d and t h a t the e x c l u d e d volume i n t h e l a r g e cages i s g i v e n by (1-F) where F i s a packing f r a c t i o n , the observed density before z e o l i t e d e s t r u c t i o n

6Z

- 2 x 4

( 0 . 1 2 5 x 0 + 0.875#F) x 0.40

g / c c + 1.3

x 0.60

g/cc

w h e r e t h e s o l v e n t medium d e n s i t y 6 ^ - 2 . 4 g / c c . The o b s e r v e d d e n s i t y o f t h e mesoporous m a t e r i a l t h a t r e s u l t s f o l l o w i n g z e o l i t e destruction i s d ^ - 2.4

( 0 . 1 2 5 + 0 . 8 7 5 ) 0.40

The c a t a l y s t d e n s i t y of excluded volume, i s

A d , - R, (c£

- 6^)

g / c c + 1.3 x 0.60

g/cc

i n c r e a s e , A£, a s s o c i a t e d w i t h

- 2.4 R, ( 0 . 1 2 5 +

the e f f e c t

( 1 - F ) x 0 . 8 7 5 ) x 0.40

w h e r e R, - f r a c t i o n o f c a t a l y s t c r y s t a l l i n i t y

g/cc

destroyed.



142

FLUID CATALYTIC CRACKING II: CONCEPTS IN CATALYST DESIGN

U p o n s t e a m t r e a t m e n t a t 815°C f o r 5 h o u r s , t h e c r y s t a l l i n i t y o f t h e f r e s h c a t a l y s t , as d e t e r m i n e d f r o m m i c r o p o r e v o l u m e , d e c r e a s e s f r o m 34 t o 2 4 % ( T a b l e V I ) . T h i s r e q u i r e s R, — 0.1. The a s s o c i a t e d m e a s u r e d d e n s i t y i n c r e a s e i s 0.04 g/cc. Setting A d , - 0.04 g / c c g i v e s t h e r e s u l t F = 0.67. I n other words, a p a c k i n g f r a c t i o n o f 67% i n t h e l a r g e c a g e s a c c o u n t s f o r t h e d e n s i t y i n c r e a s e t h a t was o b s e r v e d u p o n s t e a m t r e a t m e n t o f t h e fresh catalyst. The a b o v e e s t i m a t e f o r p a c k i n g f r a c t i o n / e x c l u d e d v o l u m e i n t h e l a r g e c a g e s i s b a s e d on n o n - s e l e c t i v e a d s o r p t i o n . The estimated value i s decreased i f , f o r a given packing f r a c t i o n , t h e r e i s s e l e c t i v e a d s o r p t i o n by the z e o l i t e o f the l i g h t e r component, c a r b o n t e t r a c h l o r i d e . S e l e c t i v e s o r p t i o n gives r i s e to ambiguity w i t h i n the model j u s t described. For example, suppose t h a t the d e n s i t y o f the s o l v e n t m e d i u m i s 2.4 g / c c . I t i s not p o s s i b l e to d i f f e r e n t i a t e between an a c t u a l l a r g e - c a g e p a c k i n g f r a c t i o n o f u n i t y i n combination w i t h a s e l e c t i v e l y adsorbed solvent density of 1.61 g / c c a n d a n a c t u a l p a c k i n g f r a c t i o n o f 0.67, together with a n o n - s e l e c t i v e l y a d s o r b e d s o l v e n t d e n s i t y o f 2.4 g/cc. A s s u m i n g n o n - s e l e c t i v e a d s o r p t i o n and o b s e r v i n g t h a t the e q u i l i b r i u m c a t a l y s t F r a c t i o n A e x h i b i t s a c r y s t a l l i n i t y o f 28% w i t h a m i c r o p o r e v o l u m e o f 0.075 c c / g , t h e e x p e c t e d i n c r e a s e i n d e n s i t y due t o a g i v e n c h a n g e i n m i c r o p o r e v o l u m e , A m p o r e , i s :

Ad

-

(%

crystallinity 100

reduction)

0.04 R,

g/cc

or Ad

=

Ampore 0.075

x

0.28

(0.4)

Ampore 0.075

x

0.112

g/cc

q u a n t i t y — z — - 0.4 g / c c r e p r e s e n t s t h e R x t h a t would occur upon complete d e s t r u c t i o n of the

where the

density neat

increase

zeolite.

Acknowledgments The a u t h o r s g r a t e f u l l y a c k n o w l e d g e J o h n F. N e w b u r y f o r p e r f o r m i n g t h e s t e a m t r e a t m e n t s o n f r e s h c a t a l y s t , J o h n P. C o m p a n i k f o r c o l l e c t i n g t h e d a t a o n cumene c r a c k i n g , F r a n k S. M o d i c a a n d Dr. E u g e n e H. H i r s c h b e r g f o r many h e l p f u l d i s c u s s i o n s , a n d D r . J a m e s A. K a d u k f o r g e n e r o u s a s s i s t a n c e a n d many h e l p f u l d i s c u s s i o n s c o n c e r n i n g the X-ray d i f f r a c t i o n analyses.


8. BEYERLEIN ETAL.


REFERENCES

1. 2. 3. 4. 5.


6. 7. 8. 9. 10. 11.

12.

13.

14.

15.

Palmer, J . L . ; Cornelius, E. B. Applied Catalysis 1987, 35. 217-35. Kugler, E. L.; Leta, D. P. J . Catal. 1988, 109, 387-395. Johnson, M. F. I. J . Catal 1978, 52, 425-31. Breck, D. W. Zeolite Molecular Sieves: R. E. Krieger: Malabar, Florida, 1984; p 637. The effective size of 1,1,2,2-tetrabromoethane was determined using CHEM-X molecular modeling. Barrer, R. M. In Zeolites and Clay Minerals as Sorbents and Molecular Sieves. Academic Press: London, 1978; p 121. Kerr, G. T. Zeolites 1989, 9, 350-51. Keyworth, D. A . ; Turner, W. J.; Reid, T. A. O i l & Gas J . 1988, 86 (11) 65-8. Rawlence, D. J.; Gosling, K. Applied Catalysis 1988, 43, 213-37. McDaniel, C. V . ; Maher, P. K. In Zeolite Chemistry and Catalysis: Rabo, J. A . , Ed.; ACS Monograph 171; American Chemical Society: Washington, DC, 1976; p 323. Occelli, M. L.; Stencel, J . M. "Surface-Metals Interaction in Fluid Cracking Catalysts During the Upgrading of VanadiumContaminated Gas Oils," in Proceedings of the International Zeolite Conference. Wurzburg, Germany, September 1988. Mauge, F.; Courcelle, J . C.; Engelhard, Ph.; Gallezot, P.; Grosmangin, J . In New Developments in Zeolite Science and Technology: Murikami, Y . ; Iijima, A . ; Ward, J. W., Eds.; Stud. Surf. Sci. Catal. 28; Kodansha, Tokyo; Elsevier, Amsterdam, 1986; pp 803-9. Gallezot, P.; Feron, B . ; Bourgogne, M.; Engelhard, Ph. Zeolites: Facts. Figures. Future: Jacobs, P. A . ; van Santen, R. A . , Eds.; Stud. Surf. Sci. Catal. 49: Elsevier, Amsterdam, 1989; pp. 1281-90. Guisnet, M.; Magnoux, P.; Canaff, C. New Developments in Zeolite Science and Technology: Murikami, Y . ; Iijima, A . ; Ward, J. W., Eds.; Stud. Surf. Sci. Catal. 28: Kodansha, Tokyo; Elsevier, Amsterdam, 1986; pp 701-07. Magnoux, P.; Cartraud, P.; Mignard, S.; Guisnet, M. J . Catal. 1987; 106, pp 235-41.

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Fluid Catalytic Cracking II - American Chemical Society

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