Chapter 7
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Strategies for Catalytic Octane Enhancement in a Fluid Catalytic Cracking Unit G. C. Edwards, K. Rajagopalan, A. W. Peters, G. W. Young, and J. E. Creighton Davison Chemical Division, W. R. Grace & Company, 7379 Route 32, Columbia, MD 21044 Commercial catalysts can achieve octane improvements by using ultra stable faujasite of low cell size, by using an active amorphous matrix, by using small amounts of an additive containing ZSM-5, or by a combination of two or three of these techniques. An analysis of gasolines from these three types of catalysts shows markedly different compositions. Ultra stable faujasite produces an increase in olefins at the expense of paraffins over conventional rare earth stabilized catalysts of high cell size. An active amorphous s i l i c a alumina catalyst produces a large increase in olefins, but less aromatics. The effect of an additive containing ZSM-5 is discussed in other papers in this symposium. Among USY catalysts, USY hydrothermally dealuminated to a cell size of about 2.426 nm has the best selectivity for octane and coke. In practice, USY catalysts hydrothermally dealuminate during use to about the same activity and selectivity regardless of the procedure used in the i n i t i a l preparation. Previous studies have shown that a low sodium content is required for high octane. New results show that i t is not the presence of the sodium but the presence of sodium during the dealumination process that affects octane. Octane increases i f sodium is removed from the zeolite before dealumination. Octane, however, is insensitive to the sodium content of the dealuminated zeolite. To the extent that motor octane is dependent on aromatic and isoparaffin content, motor octane increases in an FCC unit will be difficult to achieve. Thermo dynamics does not allow the production of highly branched isoparaffins in an FCC unit, and blending studies show that motor octane is relatively insensitive to the aromatic content of an FCC gasoline in the range of 35-50% aromatics. However, a more aromatic gasoline may have better blending characteristics in that less aromatics from other sources will be required to achieve the same motor octane number increase. 0097-6156/88/0375-0101$06.00/0 ° 1988 American Chemical Society
Occelli; Fluid Catalytic Cracking ACS Symposium Series; American Chemical Society: Washington, DC, 1988.
FLUID CATALYTIC CRACKING: R O L E IN M O D E R N REFINING
102
G a s o l i n e q u a l i t y i s l a r g e l y d e t e r m i n e d by motor and r e s e a r c h o c t a n e numbers. T h e r e i s a s t r o n g c o r r e l a t i o n between o c t a n e and t h e s t r u c t u r e o f the Cs t o C 1 2 h y d r o c a r b o n s t y p i c a l l y p r e s e n t i n g a s o l i n e , T a b l e I . F o r p a r a f f i n s , o c t a n e d e c r e a s e s as m o l e c u l a r weight i n c r e a s e s and i n c r e a s e s w i t h degree o f b r a n c h i n g . The same i s true of o l e f i n s .
Table I.
Octanes o f S e l e c t e d O r g a n i c Compounds T y p i c a l l y Found i n G a s o l i n e 13
Paraffins Pentane Hexane 3-Methylpentane 2,2-Dimethylbutane
RON* 62 25 75 92
Olefins 1-Pentene 1- Hexene trans-2-Hexene 2- M e t h y l - 2 - p e n t e n e
91 76 93 98
77 63 81 83
>100 >100
>100 >100
Aromatics Toluene Xylenes
MON 62 26 74 93
R e s e a r c h Octane Number Motor Octane Number
Catalytic • • •
s t r a t e g i e s f o r making h i g h o c t a n e g a s o l i n e s
include:
D e c r e a s e the amount o f h i g h e r m o l e c u l a r w e i g h t , l e s s branched p a r a f f i n s . I s o m e r i z e p a r a f f i n s t o a more h i g h l y b r a n c h e d p r o d u c t . Produce more o l e f i n s o r a r o m a t i c s .
A number o f c a t a l y t i c p r o c e s s e s i n c u r r e n t use make use o f t h e s e strategies including reforming, isomerization, dimerization, a l k y l a t i o n and f l u i d c a t a l y t i c c r a c k i n g (FCC). The o b j e c t o f t h i s paper i s t o d i s c u s s t h e c a t a l y t i c s t r a t e g i e s a v a i l a b l e t o produce o c t a n e i n t h e FCC u n i t . Experimental G a s o l i n e a n a l y s e s were p e r f o r m e d by gas chromatography u s i n g a 50 m d i m e t h y l s i l i c o n c a p i l l a r y column temperature programmed t o 280°C. H y d r o t h e r m a l l y d e a l u m i n a t e d Y z e o l i t e was p r e p a r e d by h e a t i n g NH4 exchanged Y f a u j a s i t e , 2.5 S i / A l r a t i o and 0.7% NazO, a t 650°C f o r 3 h r s . i n 100% steam g i v i n g a S i / A l framework r a t i o ~5 a t 2.447 nm u n i t c e l l . C h e m i c a l l y d e a l u m i n a t e d Y z e o l i t e was p r e p a r e d s t a r t i n g from NH4"" exchanged Y (2.5 S i / A l r a t i o ) u s i n g the p r o c e d u r e o f S k e e l s +
1
Occelli; Fluid Catalytic Cracking ACS Symposium Series; American Chemical Society: Washington, DC, 1988.
7.
EDWARDS ET AL.
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Catalytic Octane Enhancement
and B r e c k ( 1 ) . The p r o d u c t had 0.7% Na20 and S i / A l ~5 a t 2.447 nra unit cell. Sodium i m p r e g n a t i o n e x p e r i m e n t s were performed u s i n g sodium carbonate. E x c e p t f o r the r e s u l t s r e p o r t e d i n T a b l e I I I , a l l c a t a l y t i c a c t i v i t y and s e l e c t i v i t y r e s u l t s were o b t a i n e d u s i n g a m i c r o a c t i v i t y t e s t (MAT), ASTM D-3907-80. C a t a l y s t s were p r e p a r e d u s i n g a s p r a y d r i e d s l u r r y o f t h e a p p r o p r i a t e z e o l i t e , c l a y and a p r o p r i e t a r y i n e r t b i n d e r . C a t a l y s t d e a c t i v a t i o n was c a r r i e d out under t h e f o l l o w i n g c o n d i t i o n s u n l e s s o t h e r w i s e s p e c i f i e d : 830°C, 12 h r s . , 0.30 atm. steam. A l l c a t a l y s t s were t e s t e d o n l y a f t e r d e a c t i v a t i o n . The A 1 magic a n g l e s p i n n i n g n u c l e a r magnetic r e s o n a n c e (MASNMR) s p e c t r a were a c q u i r e d u s i n g a 30° p u l s e a t 0.1 s e c . intervals. About 5000 t o 7000 scans were a c q u i r e d on a B r u k e r AM-400. The c h e m i c a l s h i f t i s r e l a t i v e t o aluminum n i t r a t e i n water s o l u t i o n which c o n t a i n s Α1(Η2θ)β " ". The p r o p e r t i e s o f t h e West Texas Gas O i l f e e d s t o c k used a r e given i n Table I I . 2 7
3
Table I I .
P r o p e r t i e s o f West Texas Gas
API a t 15.5°C S p e c i f i c g r a v i t y 15.5°C A n i l i n e P o i n t °C Sulfur Total Nitrogen Basic Nitrogen Conradson c a r b o n M e t a l s , ppm Ni V Fe Cu D i s t i l l a t i o n Vol.7., IBP 10 30 50 80 95 K-Factor
°C @
Wt.% Wt.% Wt.% Wt.%
1
O i l (WTGO) F e e d s t o c k
: : : : :
27.4 0.8905 93 0.38 0.07 0.019 0.07
