Chapter 17
Fluid Catalytic Cracking Downloaded from pubs.acs.org by UNIV OF CALIFORNIA SAN DIEGO on 11/11/15. For personal use only.
Catalytic Cracking of Heavy Oils J.-E. Otterstedt, B. Gevert, and J. Sterte Department of Engineering Chemistry 1, Chalmers University of Technology, Fack, S-412 96, Göteborg, Sweden The effects of composition of heavy oils derived from petroleum and biomass, on their response to cracking over catalysts of various composition were investigated. The contribution to the conversion from different types of cracking was estimated and the effect of temperature on the product distribution was studied. Heavy fractions of Wilmington crude contained more aromatics and polars compared with a conventional HVGO. The conversion of the Wilmington fractions increased with boiling point range. The zeolitic contribution to the conversion decreased while the matrix contribution remained constant and the contribution from thermal cracking increased. The low H/C-ratio of F C C feed derived from liquefied biomass led to low conversion and poor gasoline selectivity. Addition of alumina to the matrix resulted in a catalyst more active for heavy oil cracking but with a poor selectivity. Alumina-montmorillonite catalysts showed activities for heavy oil cracking comparable to that of a conventional, zeolite based, cracking catalyst. Effects of matrix composition and zeolite type on the heavy oil cracking performance are discussed.
Of the many factors which influence product yields in a fluid catalytic cracker, the feed stock quality and the catalyst composition are of particular interest as they can be controlled only to a limited extent by the refiner. In the past decade there has been a trend towards using heavier feedstocks in the FCC-unit. This trend is expected to continue in the foreseeable future. It is therefore important to study how molecular types, characteristic not only of heavy petroleum oil but also of e.g. coal liquid, shale oil and biomass oil, respond to cracking over catalysts of different compositions. Heavy oils have high specific viscosities and contain components, boiling above 5 2 5 ° C , which are not necessarily distillation bottoms but can also be vacuum gasoils boiling above the normal range of such oils ( 3 5 0 - 5 2 5 ° C ) . The hydrogen to carbon ratio is generally lower for heavy oils and their contents of heteroatoms and metals, such as vanadium, nickel and iron, are higher. Heavy oils contain large molecules of which some have polar character (resins) or are large clusters (asphaltenes). Normally, synthetic oils derived from coal, shale
0097-6156/88/0375-0266$06.00/0 © 1988 American Chemical Society
Fluid Catalytic Cracking Downloaded from pubs.acs.org by UNIV OF CALIFORNIA SAN DIEGO on 11/11/15. For personal use only.
17. OTTERSTEDT E T AL.
Catalytic Cracking ofHeavy Oils
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or b i o m a s s also h a v e t h e c h a r a c t e r i s t i c s of h e a v y oils s i n c e t h e y m a y c o n t a i n large a m o u n t s of m e t a l s , have l o w hydrogen to c a r b o n r a t i o s and high s p e c i f i c d e n s i t i e s (see T a b l e I). In t h e f l u i d c a t a l y t i c c r a c k i n g ( F C C ) p r o c e s s t h e m e t a l s a n d t h e a s h w i l l be deposited on the c a t a l y s t , causing c a t a l y s t d e a c t i v a t i o n and c r a c k i n g to higher p r o d u c t i o n of c o k e and gas. T h e higher sulfur c o n t e n t r e s u l t s i n higher sulfur e m i s s i o n s w h i l e t h e high n i t r o g e n c o n t e n t s l o w e r the c o n v e r s i o n , as p a r t of t h e nitrogen compounds are basic and neutralize the acid sites on the c a t a l y s t . D e a c t i v a t i o n of the c a t a l y s t is r e f l e c t e d by a higher t e n d e n c y for p r o d u c t i o n of gas and c o k e on t h e c a t a l y s t . T h e e f f e c t s of t h e i n c r e a s e d c o k e on c a t a l y s t are: - an increase in regeneration temperature which can cause catalyst d e a c t i v a t i o n , m e t a l l u r g i c a l p r o b l e m s and higher e m i s s i o n s of N O - a lower catalyst to o i l ratio, required in order to m a i n t a i n heat balance i n the reactor - a risk for i n a d e q u a t e r e g e n e r a t i o n i f t h e c a p a c i t y of t h e a i r b l o w e r s is insufficient. - h i g h gas v e l o c i t i e s i n t h e r e g e n e r a t o r w h i c h w i l l i n c r e a s e t h e a m o u n t of f i n e s in the flue gas. T h e l a r g e r a m o u n t s of c o k e on t h e c a t a l y s t c a n be h a n d l e d by m o r e e f f e c t i v e s t e a m s t r i p p i n g , r e g e n e r a t i o n w i t h heat r e m o v a l , or using a t w o - s t e p r e g e n e r a t o r . I m p r o v e d t e m p e r a t u r e t o l e r a n c e of the c a t a l y s t and of t h e c o n s t r u c t i o n m a t e r i a l s in the regenerator also c o n t r i b u t e s to the handling of the coke problem. T h e e f f e c t s of heavy oils on c r a c k i n g c a n be m e t by process m o d i f i c a t i o n s , f e e d p r e t r e a t m e n t o r by t h e u s e o f s p e c i a l l y d e s i g n e d c a t a l y s t s . T h e most i m p o r t a n t p r e t r e a t m e n t steps of t h e feed are proper d e s a l t i n g and hydroprocessing. The l a t t e r step w i l l reduce the content of heteroatoms, a s p h a l t e n e s a n d m e t a l s , a n d r a i s e t h e h y d r o g e n t o c a r b o n r a t i o o f t h e f e e d . In other words, hydroprocessing s i g n i f i c a n t l y reduces the heavy o i l c h a r a c t e r of the feed o i l . T h e s e l e c t i v i t y p r o b l e m c a u s e d by n i c k e l c o n t a m i n a n t s c a n be r e a s o n a b l y w e l l handled by adding n i c k e l passivators, usually c o n t a i n i n g a n t i m o n y , to the f e e d (6). A l t h o u g h p a s s i v a t o r s h a v e b e e n r e p o r t e d t o d e c r e a s e t h e d e a c t i v a t i o n e f f e c t s of v a n a d i u m , a m o r e p r o m i s i n g w a y to handle t h i s p r o b l e m is to introduce vanadium "traps", which prevent the vanadium from migrating and destroying the z e o l i t e . C a t a l y s t s designed to have a high metals tolerance are a v a i l a b l e on t h e m a r k e t (7-9). A n o t h e r i n t e r e s t i n g w a y t o s o l v e t h e m e t a l s p r o b l e m is to r e s t o r e the a c t i v i t y by r e m o v i n g the m e t a l s d e p o s i t e d on t h e c a t a l y s t u s i n g t h e N E W D E M E T p r o c e s s (10). T h i s p r o c e s s c a n a l s o b e u s e d i n c o m b i n a t i o n w i t h p a s s i v a t o r s and/or m e t a l t o l e r a n t c a t a l y s t s . In o r d e r t o d e c r e a s e s u l f u r e m i s s i o n s , s u l f u r t r a n s f e r r i n g c a t a l y s t s c o n t a i n i n g A I 2 O 3 o r M g O ( 1 1 - 1 3 ) , c a n be u s e d . T h e large m o l e c u l e s found i n heavy oils c a n not p e n e t r a t e the z e o l i t e s t r u c t u r e but must i n i t i a l l y be c r a c k e d on the s u r f a c e of the z e o l i t e c r y s t a l s , o n t h e m a t r i x s u r f a c e or t h e r m a l l y (1^). T h e i n t e r m e d i a t e p r o d u c t s c a n t h e n b e c r a c k e d i n t h e z e o l i t e t o d e s i r a b l e p r o d u c t s . In o r d e r t o i n c r e a s e t h e a c t i v i t y i n t h e c r a c k i n g o f h e a v y c o m p o n e n t s i t is t h u s p o s s i b l e t o u s e a c a t a l y s t h a v i n g an a c t i v e m a t r i x or t o c r a c k a t higher t e m p e r a t u r e s . A t h i r d a p p r o a c h is t o substitute some or a l l of the Y - t y p e z e o l i t e , c u r r e n t l y used i n c o m m e r c i a l c r a c k i n g c a t a l y s t s , w i t h a z e o l i t i c c o m p o n e n t h a v i n g a l a r g e r p o r e - s i z e , e.g. a p i l l a r e d s m e c t i t e . T h e p o t e n t i a l o f s m e c t i t e s , p i l l a r e d or c r o s s - l i n k e d w i t h i n o r g a n i c p o l y c a t i o n s , as c r a c k i n g c a t a l y s t s w a s f i r s t d e m o n s t r a t e d b y V a u g h a n e t a l . (15). T h e m a i n o b s t a c l e f o r t h e u s e o f t h e s e m a t e r i a l s i n c o m m e r c i a l c a t a l y s t s is t h e i r l a c k of t h e r m a l and h y d r o t h e r m a l s t a b i l i t y . A t t h e x
FLUID CATALYTIC CRACKING: R O L E IN M O D E R N REFINING
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Fluid Catalytic Cracking Downloaded from pubs.acs.org by UNIV OF CALIFORNIA SAN DIEGO on 11/11/15. For personal use only.
Table I . P r o p e r t i e s o f O i l s o f D i f f e r e n t O r i g i n s Property
North Sea 6
H/C-ratio N(wt%) 0(wt%) S(wt%) Ash(wt%) Metals (ppm) Specific gravity Conradson carbon (wt%) Pour p o i n t
T i a Juana
-
Coal o i l
Shale o i l
Biomass 1.0
