Fluid Catalytic Cracking - ACS Publications - American Chemical Society

Chapter 3

Downloaded by UNIV OF CALIFORNIA SAN DIEGO on March 20, 2016 | http://pubs.acs.org Publication Date: September 12, 1988 | doi: 10.1021/bk-1988-0375.ch003

Hydrocarbon Cracking Selectivities with Dual-Function Zeolite Catalysts K. Rajagopalan and G. W. Young Davison Chemical Division, W. R. Grace & Company, 7379 Route 32, Columbia, MD 21044 High yield of C to C aromatics was obtained with thermally treated ZSM-5 (with a SiO2 to Al2O3 ratio of 47) during n-hexadecane cracking in a fixed bed reactor. After steam treatment, yield of C5 to C7 olefins increased at the expense of aromatics. The effect of the addition of a small concentration (1 Wt.%) of ZSM-5 to a rare earth Y fluid cracking catalyst during cracking of a commercial gas oil was investigated by measurement of yields of gasoline range hydrocarbons. The addition of either thermally treated or steam treated ZSM-5 to the catalyst reduced the concentration of normal and monomethyl paraffins in the product gasoline, thus enriching olefins and aromatics. No evidence of preferential reduction of straight chain paraffins compared to monomethyl paraffins was observed. Experiments using a catalytically cracked gasoline as feedstock suggest that ZSM-5 cracks paraffin precursors (carbonium ion or olefin intermediates) and prevents formation of gasoline range paraffins. These changes in gasoline composition due to ZSM-5 in the dual zeolite catalyst are probably responsible for increased gasoline octane number observed with that catalyst. 6

8

S y n t h e t i c Y f a u j a s i t e z e o l i t e s have been u s e d c o m m e r c i a l l y as c r a c k i n g c a t a l y s t s o v e r t h e p a s t two decades (1) and more r e c e n t l y d u a l z e o l i t e f l u i d c r a c k i n g c a t a l y s t s c o n t a i n i n g f a u j a s i t e and ZSM-5 were d i s c o v e r e d t o i n c r e a s e t h e o c t a n e number o f t h e p r o d u c t g a s o l i n e d u r i n g c a t a l y t i c c r a c k i n g o f gas o i l ( 2 ) . This concept, where ZSM-5 c o n s t i t u t e s o n l y a s m a l l f r a c t i o n (about 1 weight p e r c e n t ) o f t h e c r a c k i n g c a t a l y s t , has been t e s t e d c o m m e r c i a l l y i n Europe (3) and i n t h e U n i t e d S t a t e s ( 4 ) . Cracking of p a r a f f i n i e and o l e f i n i c h y d r o c a r b o n s by ZSM-5 c a t a l y s t s have been s t u d i e d by s e v e r a l i n v e s t i g a t o r s (5-11) o v e r a range o f t e m p e r a t u r e s (350 t o 540°C). These i n v e s t i g a t o r s u s e d e i t h e r n e a r l y p u r e ZSM-5 o r a c a t a l y s t c o n t a i n i n g 50 Wt.% ZSM-5 i n a s i l i c a m a t r i x . The 0097-6156/88/0375-0034$06.00/0 • 1988 American Chemical Society

Occelli; Fluid Catalytic Cracking ACS Symposium Series; American Chemical Society: Washington, DC, 1988.

3. RAJAGOPALAN AND YOUNG

Hydrocarbon Cracking Selectivities

35


mechanism o f o c t a n e enhancement w i t h t h e d u a l z e o l i t e c a t a l y s t was i n v e s t i g a t e d by examining t h e e f f e c t o f a d d i t i o n o f 1 Wt.% ZSM-5 t o t h e c r a c k i n g c a t a l y s t c o m p o s i t i o n on p r o d u c t s e l e c t i v i t y d u r i n g c a t a l y t i c c r a c k i n g o f a commercial gas o i l a t 500°C. Changes i n c o m p o s i t i o n o f p r o d u c t g a s o l i n e ( p a r a f f i n s , o l e f i n s , naphthenes and a r o m a t i c s ) caused b y ZSM-5 were measured. A model p a r a f f i n compound and a g a s o l i n e o b t a i n e d by c a t a l y t i c c r a c k i n g were used as feedstocks i n c e r t a i n experiments. S i n c e commercial c r a c k i n g c a t a l y s t s undergo c o n t i n u o u s h i g h temperature r e g e n e r a t i o n i n t h e p r e s e n c e o f steam ( 1 ) , t h e e f f e c t o f h y d r o t h e r m a l t r e a t m e n t o f ZSM-5 was a l s o i n v e s t i g a t e d . E x p e r i m e n t a l Methods C a t a l y s t s and P r e t r e a t m e n t . ZSM-5 was p r e p a r e d i n o u r l a b o r a t o r y and was d e t e r m i n e d t o be c r y s t a l l o g r a p h i c a l l y p u r e by X - r a y d i f fraction. Η-form o f t h e z e o l i t e was o b t a i n e d by i o n e x c h a n g i n g w i t h N H 4 N O 3 s o l u t i o n f o l l o w e d by c a l c i n a t i o n t o r e d u c e t h e sodium c o n t e n t below 500 ppm. The d e s i g n a t i o n ZSM-5 w i l l be u s e d i n t h i s m a n u s c r i p t t o denote t h e Η-form o f t h e z e o l i t e . The S 1 O 2 t o A I 2 O 3 r a t i o o f t h e z e o l i t e was a n a l y z e d as 47 and average p a r t i c l e s i z e was about .2 vim. C a t a l y s t s were p r e p a r e d by a t h o r o u g h m i x i n g o f z e o l i t e w i t h a s i l i c a - a l u m i n a s o l b i n d e r and k a o l i n c l a y d i l u e n t f o l l o w e d by d r y i n g . The b i n d e r and c l a y component o f t h e c a t a l y s t w i l l be r e f e r r e d t o as t h e " m a t r i x " . Two c a t a l y s t s c o n t a i n i n g 1 Wt.% and 25 Wt.% ZSM-5 i n t h e m a t r i x were p r e p a r e d . A " p u r e " m a t r i x c a t a l y s t t h a t d i d n o t c o n t a i n any z e o l i t e was a l s o p r e p a r e d so t h a t t h e e f f e c t o f t h e m a t r i x i n t h e ZSM-5 c a t a l y s t s c o u l d be t a k e n i n t o account. C o m m e r c i a l l y a v a i l a b l e Super-D, which c o n t a i n s r a r e e a r t h exchanged Y z e o l i t e (REY) i n t h e m a t r i x and i s m a n u f a c t u r e d by t h e D a v i s o n C h e m i c a l D i v i s i o n o f W. R. Grace & Co., was used as t h e REY catalyst. Steam p r e - t r e a t m e n t o f f l u i d c r a c k i n g c a t a l y s t s has been con v e n t i o n a l l y employed t o r e p r e s e n t t h e d e a c t i v a t i o n o c c u r r i n g i n a commercial FCC u n i t . A p p r o p r i a t e steam p r e - t r e a t m e n t methods have been d e v e l o p e d so t h a t t h e a c t i v i t y and s e l e c t i v i t y o f t h e steam pre-treated c a t a l y s t i s e q u i v a l e n t t o a commercially d e a c t i v a t e d c a t a l y s t (12). However, a u n i q u e steaming method may n o t be s u i t a b l e f o r c a t a l y s t s o f v a r y i n g c o m p o s i t i o n s ( 1 2 ) . Two steaming methods d e s i g n e d t o s i m u l a t e d e a c t i v a t i o n i n a commercial u n i t o f t h e two t y p e s o f c a t a l y s t s used i n t h i s work were employed. Super-D was t r e a t e d f o r 8 h o u r s a t 732°C w i t h a steam p r e s s u r e o f 2 atmospheres. The c a t a l y s t s c o n t a i n i n g ZSM-5 were t r e a t e d f o r 12 h o u r s a t 827°C w i t h a steam p a r t i a l p r e s s u r e o f 0.2 atmosphere. I n a d d i t i o n t o t h e above p r e - t r e a t m e n t s , t h e ZSM-5 c o n t a i n i n g c a t a l y s t s were a l s o t h e r m a l l y t r e a t e d f o r 3 h o u r s a t 704°C i n s t a t i c a i r t o r e p r e s e n t t h e e f f e c t o f f r e s h l y added ZSM-5 i n a commercial FCC u n i t . Dual z e o l i t e c a t a l y s t s were p r e p a r e d by p h y s i c a l l y b l e n d i n g Super-D w i t h t h e c a t a l y s t c o n t a i n i n g 25 Wt.% ZSM-5 so as t o a c h i e v e 1 Wt.% ZSM-5 i n t h e p h y s i c a l b l e n d . Two b l e n d s , one c o n t a i n i n g steam t r e a t e d Super-D and t h e r m a l l y t r e a t e d ZSM-5 c a t a l y s t and t h e o t h e r c o n t a i n i n g steam t r e a t e d Super-D and steam t r e a t e d ZSM-5 c a t a l y s t were p r e p a r e d . E a c h o f t h e b l e n d s was t h e n i n t i m a t e l y mixed.


36

FLUID CATALYTIC CRACKING: ROLE IN MODERN REFINING


F e e d s t o c k s and R e a c t o r . F e e d s t o c k f o r t h e e x p e r i m e n t s was chosen from among a gas o i l w i t h p r o p e r t i e s d e s c r i b e d i n T a b l e I , a g a s o l i n e o b t a i n e d by c a t a l y t i c c r a c k i n g w i t h t h e c o m p o s i t i o n d e s c r i b e d i n T a b l e VI and 99% p u r e n-hexadecane from A l d r i c h (Milwaukee, WI). A f i x e d bed r e a c t o r d e s c r i b e d i n ASTM D3907 was employed f o r c a t a l y t i c t e s t i n g . E x p e r i m e n t s were c a r r i e d out a t a r e a c t o r t e m p e r a t u r e o f 500°C, Weight H o u r l y Space V e l o c i t y o f 16 and c a t a l y s t r e s i d e n c e t i m e (13) o f 75 seconds. Material b a l a n c e f o r t h e r e a c t o r runs ranged from 95 t o 102 w e i g h t p e r c e n t .

Table I.

P r o p e r t i e s o f a Commercial

Gas O i l

Mass S p e c t r a l A n a l y s i s Paraffins, Vol.% C y c l o p a r a f f i n s ( 1 , 2, and 3 r i n g ) , V o l . % Aromatics, V o l . %

: : :

14.5 33.1 52.4

Boilinft Point D i s t r i b u t i o n Initial 10 V o l . % 50 V o l . % 90 V o l . %

: : : :

187°C 339°C 437°C 544°C

(ASTM:

S p e c i f i c G r a v i t y a t 16°C UOP

M

K

M

Factor

D-1160)

: :

.912 11.78

S u l f u r , Wt.%

:

.39

N i t r o g e n , Wt.%

:

.11

P r o d u c t A n a l y s e s . L i q u i d p r o d u c t from t h e r e a c t o r was s e p a r a t e d i n t o i t s components and a n a l y z e d w i t h t h e a i d o f a H e w l e t t - P a c k a r d (HP) 5880A gas chromatograph u s i n g a 50m D i m e t h y l S i l i c o n e F l u i d c a p i l l a r y column and a flame i o n i z a t i o n d e t e c t o r . A column t e m p e r a t u r e program r a n g i n g from 15°C t o 280°C was u s e d t o e n a b l e adequate r e s o l u t i o n . The components were i d e n t i f i e d u s i n g a HP 5990A GC/MS System. L i g h t h y d r o c a r b o n s ( C i t o C s ) were c o l l e c t e d i n a gas bag and a n a l y z e d on a HP 5710 gas chromatograph w i t h a 20% D i b u t y l m a l e a t e packed column m a i n t a i n e d a t 22°C and a flame i o n i z a t i o n d e t e c t o r . Hydrogen i n t h e gas bag was a n a l y z e d by a Gow-Mac t h e r m a l c o n d u c t i v i t y d e t e c t o r c o n n e c t e d t o a 13X m o l e c u l a r s i e v e packed column. Coke on s p e n t c a t a l y s t was measured by Carbon D e t e r m i n a t o r WR-12 (Leco C o r p o r a t i o n , S t . J o s e p h , MI). R e s u l t s and

Discussion

E f f e c t o f Steam Treatment. X-ray d i f f r a c t i o n analyses i n d i c a t e d t h a t ZSM-5 r e t a i n e d i n e x c e s s o f 90% o f i t s c r y s t a l l i n i t y a f t e r t h e steam t r e a t m e n t d e s c r i b e d i n t h e methods s e c t i o n . Unit c e l l c o n s t a n t o f t h e REY z e o l i t e i n Super-D d e c l i n e d from 24.65& t o 24.38a due t o t h e steam t r e a t m e n t . Independent measurements





37

i n d i c a t e d t h a t RE i o n exchange r e s u l t s i n a l a t t i c e e x p a n s i o n o f 0.04A. Thus, t h e u n i t c e l l s h r i n k a g e due t o steaming cannot be f u l l y e x p l a i n e d by p o s t u l a t i n g r a r e e a r t h m i g r a t i o n o u t s i d e t h e zeolite. I t i s reasonable t o i n f e r that dealumination of the f a u j a s i t e framework o c c u r r e d d u r i n g t h e steam p r e t r e a t m e n t . Unit c e l l s h r i n k a g e o f f a u j a s i t e h a s a l s o been o b s e r v e d d u r i n g u s e i n a commercial u n i t ( 1 4 ) . The e f f e c t o f steam t r e a t m e n t o f ZSM-5 on i t s c r a c k i n g a c t i v i t y and s e l e c t i v i t y was measured w i t h e x p e r i m e n t s u s i n g n-hexadecane feed. W i t h t h e t h e r m a l l y t r e a t e d ZSM-5 c a t a l y s t , c o n c e n t r a t i o n o f t h e u n c o n v e r t e d n-hexadecane i n t h e p r o d u c t was n o t m e a s u r a b l e w h i l e 50% o f t h e f e e d was u n c o n v e r t e d w i t h t h e steam t r e a t e d ZSM-5 c a t a l y s t ( T a b l e I I ) . The lower l i m i t o f c o n v e r s i o n w i t h t h e thermally treated c a t a l y s t corresponding to d e t e c t i o n l i m i t of n-hexadecane i s 99.99%. T h i s lower l i m i t s u g g e s t s a t l e a s t an o r d e r o f magnitude r e d u c t i o n i n a p p a r e n t f i r s t o r d e r r a t e c o n s t a n t o f t h e ZSM-5 c a t a l y s t upon steam t r e a t m e n t . The s m a l l r e d u c t i o n i n c r y s t a l l i n i t y upon steaming cannot f u l l y e x p l a i n t h e d r a m a t i c a c t i v i t y loss. L o s s o f a c t i v e s i t e s due t o d e a l u m i n a t i o n o f ZSM-5 can be p o s t u l a t e d t o e x p l a i n t h e r e d u c t i o n i n a c t i v i t y .

1

TABLE I I .

E f f e c t o f Steam Treatment o f ZSM-5 C a t a l y s t on P r o d u c t S e l e c t i v i t y D u r i n g n-hexadecane C r a c k i n g

Pretreatment:

3 Hours a t 704°C in Static Air

Steaming f o r 12 Hours a t 827°C w i t h Steam p a r t i a l p r e s s u r e = .2 atm.

Y i e l d -r F r a c t i o n a l C o n v e r s i o n , Wt.% Feed Paraffins Plus Ci

Olefins 0.10 1.4 19.8 29.4 22.0 10.0 3.2

:

11.5

0.6

0.4 0.3 0.3 0.2 0.3 100 0.12 1.10

3.0 1.8 1.8 1.5 0.7 50 0.01 0.10

: :

Cs C4 C5 Ce C7 Ce-Ce

Aromatics

Olefin/Paraffin Ratio Cs C4 Cs Ce C? C o n v e r s i o n , Wt.% H 2 , Wt.% Feed Coke, Wt.% Feed

1

: : : :

0.17 5.1 26.7 26.9 14.2 2.7 1.3

C2

25 Wt.% ZSM-5, 75 Wt.% m a t r i x



38


O b s e r v a t i o n s r e g a r d i n g a c t i v i t y and s e l e c t i v i t y o f steam t r e a t e d ZSM-5 i n t h i s work c o u l d t h u s a p p l y t o d e a l u m i n a t e d ZSM-5. F o r t h e sake o f b r e v i t y , t h e y i e l d d a t a f o r a l l t h e i n d i v i d u a l components a r e n o t r e p o r t e d i n T a b l e I I and subsequent t a b l e s . The y i e l d o f u n r e p o r t e d components ( u s u a l l y Ce+ o l e f i n s and naphthenes) c a n be c a l c u l a t e d as 100 minus p e r c e n t a g e y i e l d o f t h e r e p o r t e d components. R e s u l t s shown i n T a b l e I I i n d i c a t e t h a t t h e r m a l l y t r e a t e d ZSM-5 produced a h i g h y i e l d o f Ce t o Ce a r o m a t i c s , C3 and C4 h y d r o c a r b o n s . Steam t r e a t m e n t o f ZSM-5 r e d u c e d c r a c k i n g a c t i v i t y and i n c r e a s e d t h e s e l e c t i v i t y f o r Cs t o Ci a l i p h a t i c s a t t h e expense o f a r o m a t i c s . The o l e f i n t o p a r a f f i n r a t i o i n t h e p r o d u c t a l s o i n c r e a s e d upon s t e a m i n g . O l e f i n s have been s u g g e s t e d t o be i n t e r m e d i a t e s d u r i n g f o r m a t i o n o f a r o m a t i c s w i t h ZSM-5 ( 6 ) . Hence, as t h e a c t i v i t y o f ZSM-5 was r e d u c e d t h r o u g h steam t r e a t m e n t , consumption o f o l e f i n s t o form a r o m a t i c s d i d n o t t a k e p l a c e . These r e s u l t s a r e thus c o n s i s t e n t w i t h o b s e r v a t i o n s o f Anderson e t a l . ( 6 ) . Gas O i l C r a c k i n g by t h e D u a l Z e o l i t e C a t a l y s t . A f t e r t h e steam p r e - t r e a t m e n t , t h e gas o i l c r a c k i n g a c t i v i t y o f REY was about 100 times g r e a t e r than t h e a c t i v i t y o f t h e "pure" m a t r i x c a t a l y s t . Thus, t h e dominant c o n t r i b u t o r o f c r a c k i n g s e l e c t i v i t y o f Super D i s e x p e c t e d t o be t h e REY z e o l i t e p r e s e n t i n t h e c a t a l y s t . Yields of Cs t o C7 p a r a f f i n isomers measured d u r i n g t h e c r a c k i n g o f t h e gas o i l a t 500°C by t h e steam t r e a t e d REY c a t a l y s t a r e shown i n T a b l e I I I . The y i e l d o f branched p a r a f f i n s was g e n e r a l l y f i v e t o t e n times h i g h e r t h a n t h e y i e l d o f n o r m a l p a r a f f i n s and t h e major (>50%) p o r t i o n o f t h e b r a n c h e d p a r a f f i n y i e l d c o n s i s t e d o f monomethyl p a r a f f i n s .

Table I I I .

Catalyst

Y i e l d o f P a r a f f i n Isomers D u r i n g C r a c k i n g o f t h e Gas O i l by t h e REY C a t a l y s t

Pretreatment

55.8

Wt.% C o n v e r s i o n Product Y i e l d s

Steam t r e a t e d f o r 8 h r s a t 732°C Steam p r e s s u r e = 2 atmospheres.

(Wt.% Feed)

n-Pentane Isopentane n-Hexane

.20 1.2 .32

2- M e t h y l p e n t a n e

1.7

3- M e t h y l p e n t a n e

1.1

2,3-Dimethylbutane n-Heptane

.33 .23

2- Methylhexane

1.5

3- Methylhexane

1.1


3.

RAJAGOPALAN AND YOUNG

T a b l e IV.

Catalyst

C r a c k i n g o f t h e Gas O i l b y D u a l Z e o l i t e

Description

: REY


C o n v e r s i o n , Wt.%

:

39


Catalyst

1

REY C a t a l y s t + 1% T h e r m a l l y T r e a t e d ZSM-5 2

Catalysts

REY C a t a l y s t + 1% Steam Treated ZSM-5 3

55.8

54.4

50.3

.04 .33 .67 2.4 .9 2.7 3.7 37.4

P r o d u c t Y i e l d s . Wt.% Feed H2

Methane Ethane + E t h y l e n e Propylene Propane C4 O l e f i n s C4 P a r a f f i n s C s Gasoline

: : : : : : : :

.04 .33 1.1 2.2 .9 2.6 3.6 43.2

.04 .32 1.5 6.1 2.2 4.1 6.0 31.9

C5-C12

:

18.8

+

Paraffins

Coke

:

11.5

15.6

2.1

2.4

2.2

G a s o l i n e C o m p o s i t i o n , Wt.% n-hexane Methylpentanes 2,3 D i m e t h y l Butane

: : :

.92 7.9 .94

.97 5.9 .73

.83 7.3 .86

n-heptane Methylhexanes

: :

.65 7.3

.64 4.1

.63 6.8

n-Ce-Ci2

:

2.7

2.9

Total Paraffins Cs-C? O l e f i n s Ce-Ce A r o m a t i c s

: : :

43.4 7.2 11.9

36.0 8.4 16.2

41.6 8.0 12.2

C9-C11

:

20.9

21.8

20.8

Aromatics

1

Steam t r e a t e d f o r 8 h o u r s a t 732°C. Steam p r e s s u r e = 2 atmospheres.

2

Heat t r e a t e d

3

Steam t r e a t e d f o r 12 h o u r s a t 827°C. Steam p a r t i a l p r e s s u r e = .2 atmosphere.

2.5

f o r 3 h o u r s a t 704°C i n s t a t i c a i r .



40


The e f f e c t o f a d d i n g 1% ZSM-5 t o t h e REY c r a c k i n g c a t a l y s t on p r o d u c t y i e l d s i s d e s c r i b e d by t h e d a t a i n T a b l e IV. Two c a s e s where ZSM-5 was e i t h e r t h e r m a l l y t r e a t e d o r steam t r e a t e d were considered. C o n s i s t e n t w i t h e a r l i e r f i n d i n g s (2, 3 ) , a d d i t i o n o f 1% ZSM-5 t o t h e c r a c k i n g c a t a l y s t i n c r e a s e d t h e y i e l d o f C3 and C4 h y d r o c a r b o n s a t t h e expense o f Cs+ g a s o l i n e . Analysis of t h e g a s o l i n e c o m p o s i t i o n i n d i c a t e d t h a t t h e a d d i t i o n o f ZSM-5 t o t h e c a t a l y s t e n r i c h e d o l e f i n s and a r o m a t i c s ( e s p e c i a l l y Ce t o Cs a r o m a t i c s ) i n t h e g a s o l i n e a t t h e expense o f p a r a f f i n s . W i t h 1% t h e r m a l l y t r e a t e d ZSM-5, about 20% r e d u c t i o n i n t h e c o n c e n t r a t i o n o f p a r a f f i n s was o b s e r v e d i n t h e p r o d u c t g a s o l i n e . The r e d u c t i o n i n t h e y i e l d o f g a s o l i n e range p a r a f f i n s ( a s a p e r c e n t o f f e e d ) due t o 1% t h e r m a l l y t r e a t e d ZSM-5 was about 40%. P a r a f f i n y i e l d r e d u c t i o n t o a l e s s e r e x t e n t was o b s e r v e d w i t h t h e steam t r e a t e d ZSM-5. Thus, e n r i c h m e n t o f o l e f i n s and a r o m a t i c s i n the g a s o l i n e occurred w i t h t h e a d d i t i o n o f e i t h e r t h e r m a l l y t r e a t e d o r steam t r e a t e d ZSM-5. The d a t a i n T a b l e IV and F i g u r e 1 i n d i c a t e t h a t most o f t h e p a r a f f i n c o n c e n t r a t i o n r e d u c t i o n t o o k p l a c e i n t h e Ce t o C* range and i n v o l v e d monomethyl p a r a f f i n s . The b l e n d i n g o c t a n e numbers f o r 2-methyl and 3-methyl hexanes a r e r e p o r t e d as 40 t o 56 by ASTM (15) and a r e lower t h a n t h e b l e n d i n g numbers f o r l i g h t (Cs t o C 7 ) o l e f i n s and Ce t o C u a r o m a t i c s . Thus, r e d u c t i o n i n c o n c e n t r a t i o n o f these branched p a r a f f i n s i s expected t o improve t h e r e s e a r c h o c t a n e number o f t h e g a s o l i n e .

T a b l e V.

Cracking

o f t h e Gas O i l by 1% ZSM-5 C a t a l y s t

Catalyst Description

:

1% ZSM-5 99%

Matrix

C a t a l y s t Pretreatment

:

Conversion,

:

34.5

: : : : : :

11.5 .39 .16 .06 .40 0

Wt.%

P r o d u c t Y i e l d s . Wt.% Feed Ci-C4 n-hexane Methylpentanes 2,3 D i m e t h y l b u t a n e n-heptane Methylhexanes n-Ce-Ci2

:

Total Paraffins Cs O l e f i n s Ce O l e f i n s C? O l e f i n s Ce-Ce A r o m a t i c s C e - C n Aromatics

: : : : : :

100% M a t r i x

3 h o u r s a t 704°C i n S t a t i c A i r 24.1

.72

2.3 2.2 1.5 .47 5.7 10.0


7.5 .10 0 0 .16 0 .80

1.5 .25 .54 .45 1.8 6.4




ul

C a r b o n Number Figure

1.

P a r a f f i n y i e l d as a f u n c t i o n o f c a r b o n number f o r t h r e e c a t a l y s t s d u r i n g gas o i l c r a c k i n g . REY C a t a l y s t REY C a t a l y s t + 1 Wt.% t h e r m a l l y t r e a t e d ZSM-5 REY C a t a l y s t + 1 Wt.% steam t r e a t e d ZSM-5


41

42



To c l a r i f y t h e mechanism by which g a s o l i n e c o m p o s i t i o n i s changed ( r e d u c t i o n i n c o n c e n t r a t i o n o f t h e p a r a f f i n s and an i n c r e a s e i n c o n c e n t r a t i o n o f o l e f i n s and a r o m a t i c s ) by ZSM-5 i n t h e d u a l z e o l i t e c a t a l y s t , e x p e r i m e n t s were c a r r i e d o u t on c a t a l y s t s c o n t a i n i n g o n l y ZSM-5 d i s p e r s e d i n t h e m a t r i x . The c a t a l y s t c o n t a i n i n g 1 Wt.% ZSM-5 was u s e d i n o r d e r t o d u p l i c a t e t h e c o n c e n t r a t i o n o f ZSM-5 i n t h e d u a l z e o l i t e c a t a l y s t s t u d i e d earlier. E x p e r i m e n t s w i t h t h e " p u r e " m a t r i x c a t a l y s t were u s e d t o d e f i n e t h e c o n t r i b u t i o n o f ZSM-5. Gas O i l and G a s o l i n e C o n v e r s i o n by ZSM-5. R e s u l t s o b t a i n e d d u r i n g c r a c k i n g o f t h e gas o i l i n d i c a t e d t h a t 1 Wt.% t h e r m a l l y t r e a t e d ZSM-5 e x h i b i t e d a h i g h e r a c t i v i t y f o r c o n v e r s i o n o f heavy p a r a f f i n s ( C 2 0 " " ) t h a n 10 Wt.% REY ( F i g u r e 2 ) . The p a r a f f i n s were c o n v e r t e d m a i n l y t o Ce t o C u g a s o l i n e range a r o m a t i c s and l i g h t hydrocarbons (Table V). Smaller concentration of n - p a r a f f i n s and l i g h t o l e f i n s (Cs and Ce) were a l s o p r o d u c e d . Y i e l d s o f monomethyl b r a n c h e d p a r a f f i n s were lower than t h a t o f t h e c o r r e s p o n d i n g η-paraffin u n l i k e r e s u l t s o b t a i n e d ( T a b l e I I I ) d u r i n g c r a c k i n g w i t h t h e REY c a t a l y s t . 1

Table VI.

Conversion

Description

Feed o r P r o d u c t

o f Components i n G a s o l i n e by 1% ZSM-5 a t 500°C

Feed Catalytically Cracked Gasoline Composition.

Product with Matrix Catalyst

Product with 1% ZSM-5 i n Matrix Catalyst

Wt.% Feed

Methane

0

.09

Ethane + E t h y l e n e

0

.33

1.8

Ca

Olefin

0

.92

4.2

Cs

Paraffin

0

.03

C4

Olefin

C4 P a r a f f i n s C5-C7 Cs-Ci2

Olefins Paraffins

C 5 - C 7 Naphthenes Ce-Ce C9-C11

Aromatics Aromatics

Coke, Wt.% F

.60 .32 12.3 33.5 6.2

.68 .24 8.8 30.5

13.5

17.5

19.3

.43 1.4 .27 2.4 27.9

6.0

12.5

.09

5.4 15.2 19.1

.5

.4

M a t r i x c a t a l y s t as w e l l as t h e c a t a l y s t c o n t a i n i n g 1% ZSM-5 were p r e t r e a t e d f o r 3 h o u r s a t 704°C i n s t a t i c a i r .



3.



*

*

w

I F i g u r e 2.

Gas chromatographs o f p r o d u c t s from gas o i l c r a c k i n g with three c a t a l y s t s : 100% m a t r i x c a t a l y s t ( a b o v e ) , 1 Wt.% ZSM-5 i n t h e m a t r i x ( m i d d l e ) and 10 Wt.% REY i n the m a t r i x (below). Peaks marked w i t h * i n d i c a t e heavy ( C 2 o ) p a r a f f i n s . +


43


44

FLUID CATALYTIC CRACKING: R O L E IN M O D E R N REFINING

The c o m p o s i t i o n o f t h e g a s o l i n e o b t a i n e d by c a t a l y t i c c r a c k i n g and u s e d as a f e e d s t o c k f o r t h e ZSM-5 c a t a l y s t i s g i v e n i n Table VI. P r o d u c t a n a l y s e s , a l s o g i v e n i n T a b l e V I , show t h a t 8 0 % o f t h e o l e f i n s and l e s s t h a n 1 0 % o f t h e p a r a f f i n s a r e c o n v e r t e d by t h e ZSM-5 c a t a l y s t w i t h about 30% o f t h e o l e f i n c o n v e r s i o n a t t r i b u t a b l e to the matrix present i n the c a t a l y s t . This i s not s u r p r i s i n g due t o t h e well-known h i g h e r r e a c t i v i t y o f o l e f i n s . L i g h t h y d r o c a r b o n s ( C i t o C O and a r o m a t i c s ( m a i n l y Ce t o Ce) were p r o d u c e d by ZSM-5 due t o t h e t h e c o n v e r s i o n o f o l e f i n s and p a r a f f i n s . T h u s , t h e s e r e s u l t s p r o v i d e e v i d e n c e f o r c r a c k i n g o f o l e f i n s , p a r a f f i n s and c y c l i z a t i o n o f o l e f i n s by ZSM-5 a t 500°C. The steam d e a c t i v a t e d ZSM-5 c a t a l y s t e x h i b i t e d r e d u c e d o l e f i n c o n v e r s i o n and n e g l i g i b l e p a r a f f i n c o n v e r s i o n a c t i v i t y . I s o m e r i z a t i o n o f l i n e a r o l e f i n s t o b r a n c h e d o l e f i n s by ZSM-5 has been o b s e r v e d u n d e r c e r t a i n c o n d i t i o n s ( 3 ) . An a n a l y s i s o f t h e d i s t r i b u t i o n o f Cs and Ce o l e f i n isomers i n t h e f e e d g a s o l i n e as w e l l as t h e p r o d u c t from steam d e a c t i v a t e d ZSM-5 show no s t r o n g e v i d e n c e f o r d e a c t i v a t e d ZSM-5 t o p r o d u c e b r a n c h e d o l e f i n s from t h e α-olefins ( T a b l e V I I ) . The d i s t r i b u t i o n o f isomers o b t a i n e d w i t h t h e c a t a l y s t c o n t a i n i n g 1% steam d e a c t i v a t e d ZSM-5 i n a m a t r i x was e q u i v a l e n t t o t h a t o b t a i n e d w i t h t h e "pure** m a t r i x c a t a l y s t . Discussion. F i x e d bed c r a c k i n g r e a c t o r s as w e l l as commercial moving bed r e a c t o r s o p e r a t e under s t e a d y s t a t e o r p s e u d o - s t e a d y s t a t e c o n d i t i o n s (13). Observed s e l e c t i v i t y (eg., r a t i o o f y i e l d o f b r a n c h e d t o η-paraffin) i n a s t e a d y s t a t e c a t a l y t i c r e a c t o r i s i n d e p e n d e n t o f space v e l o c i t y (16, 1 7 ) . The s e l e c t i v i t y depends on i n t r i n s i c r a t e c o n s t a n t s and d i f f u s i v i t i e s o f t h e r e a c t i n g s p e c i e s which depend on t e m p e r a t u r e . Thus, t h e s e l e c t i v i t y o b s e r v a t i o n s r e p o r t e d h e r e a r e a p p l i c a b l e t o commercial FCC u n i t s o p e r a t i n g a t space v e l o c i t i e s d i f f e r e n t from t h a t employed i n t h i s s t u d y . A comparison o f t h e y i e l d o f g a s o l i n e range ( C 3 - C 1 2 ) p a r a f f i n s o b t a i n e d d u r i n g gas o i l c r a c k i n g u s i n g t h e d u a l z e o l i t e c a t a l y s t with that obtained during gasoline conversion with the s i n g l e z e o l i t e (ZSM-5) c a t a l y s t c a n be c a r r i e d o u t f o r b o t h f r e s h ( t h e r m a l l y t r e a t e d ) and aged (steam t r e a t e d ) ZSM-5. When gas o i l was c r a c k e d w i t h t h e d u a l z e o l i t e c a t a l y s t c o n t a i n i n g t h e r m a l l y t r e a t e d ZSM-5, a p p r o x i m a t e l y 4 0 % r e d u c t i o n i n t h e y i e l d o f g a s o l i n e range p a r a f f i n s compared t o t h e f a u j a s i t e c a t a l y s t was o b s e r v e d ( T a b l e I V ) . However, when t h e g a s o l i n e o b t a i n e d by c a t a l y t i c c r a c k i n g was f e d t o a c a t a l y s t c o n t a i n i n g t h e r m a l l y t r e a t e d ZSM-5 (no REY) under t h e same c o n d i t i o n s , o n l y 10% r e d u c t i o n i n t h e y i e l d o f p a r a f f i n s was o b s e r v e d ( T a b l e V I ) . Steam t r e a t e d 1% ZSM-5 c a t a l y s t d i d n o t p r o d u c e any m e a s u r a b l e r e d u c t i o n i n t h e y i e l d o f p a r a f f i n s during gasoline cracking. However, a s i g n i f i c a n t r e d u c t i o n i n t h e y i e l d o f p a r a f f i n s due t o 1% steamed ZSM-5 was o b s e r v e d d u r i n g gas o i l c r a c k i n g w i t h t h e d u a l z e o l i t e c a t a l y s t . Thus, p a r a f f i n y i e l d r e d u c t i o n o b s e r v e d w i t h t h e d u a l z e o l i t e c a t a l y s t cannot be f u l l y e x p l a i n e d by r e a c t i v i t y o f ZSM-5 t o g a s o l i n e range p a r a f f i n s . A mechanism t h a t p o s t u l a t e s p r e v e n t i o n o f p a r a f f i n f o r m a t i o n d u r i n g gas o i l c r a c k i n g w i t h t h e d u a l z e o l i t e c a t a l y s t c a n e x p l a i n t h e above d a t a . Such a p r e v e n t i o n c o u l d t a k e p l a c e b y more t h a n one r o u t e . ZSM-5 p r e s e n t i n t h e c a t a l y s t c o u l d p r e v e n t c e r t a i n s e c o n d a r y r e a c t i o n s t h a t l e a d t o t h e f o r m a t i o n o f g a s o l i n e range



Table VII.

Y i e l d s o f O l e f i n Isomers D u r i n g G a s o l i n e C o n v e r s i o n by 1% ZSM-5 a t 500°C

Description



Feed Catalytically Cracked Gasoline

Product with Matrix Catalyst

Product w i t h 1% ZSM-5 in Matrix Catalyst

Feed o r P r o d u c t C o m p o s i t i o n . Wt.% Feed Pentene

Isomers

1- Pentene

:

.27

.21

.29

2- Pentenes

:

1.8

1.0

1.2

Methylbutenes

:

2.1

2.5

2.6

Cyclopentene

:

Hexene

.20

.17

.57

.55

.19

Isomers

1-Hexene 2 & 3- Hexenes

1.6

Methylpentenes

2.3

2,3-Dimethyl 2-butene

.62

.85 2.4

.69

.63 1.0 2.4

.75

M a t r i x c a t a l y s t as w e l l as t h e c a t a l y s t c o n t a i n i n g ZSM-5 were steamed f o r 12 hours a t 827°C w i t h steam p a r t i a l p r e s s u r e = .2 atmosphere.

paraffins. These s e c o n d a r y r e a c t i o n s i n c l u d e t h e c h a i n t r a n s f e r r e a c t i o n i n v o l v i n g a carbonium i o n s u g g e s t e d by G a t e s , e t a l . (18) and t h e hydrogen t r a n s f e r r e a c t i o n i n v o l v i n g an o l e f i n , s u g g e s t e d by Thomas and Barmby ( 1 9 ) . B o t h c h a i n t r a n s f e r and hydrogen t r a n s f e r have been s u g g e s t e d t o o c c u r r e a d i l y o v e r f a u j a s i t e l e a d i n g t o t h e f o r m a t i o n o f g a s o l i n e range p a r a f f i n s . ZSM-5 c o u l d p r e v e n t t h e s e b i m o l e c u l a r hydrogen and c h a i n t r a n s f e r r e a c t i o n s by p r e f e r e n t i a l l y c a t a l y z i n g t h e monomolecular c r a c k i n g o f t h e carbonium i o n o r o l e f i n i n t e r m e d i a t e t o produce l i g h t ( C 2 t o C 4 ) hydrocarbons. These i n t e r m e d i a t e s a r e more r e a c t i v e t h a n t h e p a r a f f i n s and c a n a l s o be c r a c k e d by steam d e a c t i v a t e d ZSM-5. Thus, t h i s mechanism c a n e x p l a i n t h e r e d u c t i o n i n p a r a f f i n y i e l d


45



46

( T a b l e IV) a t t r i b u t a b l e t o 1% steam d e a c t i v a t e d ZSM-5 - a c a t a l y s t t h a t had n e g l i g i b l e a c t i v i t y f o r c r a c k i n g g a s o l i n e range p a r a f f i n s a t 500°C. T h i s mechanism c a n a l s o e x p l a i n t h e i n c r e a s e d y i e l d o f l i g h t h y d r o c a r b o n s ( C 2 t o C O when t h e d u a l z e o l i t e c a t a l y s t c o n t a i n i n g e i t h e r t h e r m a l l y t r e a t e d o r steam d e a c t i v a t e d ZSM-5 i s employed. The c o n c e n t r a t i o n s o f b o t h n o r m a l and monomethyl p a r a f f i n s i n t h e p r o d u c t g a s o l i n e were r e d u c e d by t h e a d d i t i o n o f ZSM-5 t o t h e REY c a t a l y s t , b u t p r e f e r e n t i a l r e d u c t i o n o f η-paraffins was n o t observed. T h i s r e s u l t c a n be e x p l a i n e d by t h e c o n c e p t o f c o n s t r a i n t index ( 5 ) , which i s t h e r a t i o o f r a t e constant f o r conversion of a s t r a i g h t chain p a r a f f i n to that f o r conversion of a b r a n c h e d p a r a f f i n o f t h e same c a r b o n number. The c o n s t r a i n t i n d e x o f ZSM-5 was found t o be i n d e p e n d e n t o f c r y s t a l s i z e o v e r a range s p a n n i n g two o r d e r s o f magnitude ( 9 ) . I t i s thus c o n s i d e r e d t o be a measure o f shape s e l e c t i v i t y which d e s c r i b e s t h e a p p a r e n t p o r e o p e n i n g o f t h e z e o l i t e ( 9 ) . The c o n s t r a i n t index o f ZSM-5 d e c l i n e d w i t h t e m p e r a t u r e (5) from n e a r l y 10 a t 365°C t o n e a r l y u n i t y a t 500°C. H i g h c o n s t r a i n t i n d e x i s e x p e c t e d t o r e s u l t i n p r e f e r e n t i a l c o n v e r s i o n o f s t r a i g h t c h a i n m o l e c u l e s (carbonium i o n s , o l e f i n s ) and p r e s e r v a t i o n o f b r a n c h e d m o l e c u l e s . When t h e c o n s t r a i n t i n d e x i s n e a r u n i t y , b o t h t y p e s o f m o l e c u l e s have e q u i v a l e n t r e a c t i v i t y and c o n v e r s i o n w i l l be d e t e r m i n e d by concentration (or a v a i l a b i l i t y ) of molecules. Monomethyl p a r a f f i n s a r e produced a t an o r d e r o f magnitude h i g h e r c o n c e n t r a t i o n t h a n η-paraffins by t h e REY c a t a l y s t ( T a b l e I I I ) . Thus, a d d i t i o n o f ZSM-5 t o t h e c a t a l y s t r e s u l t e d i n a s i g n i f i c a n t r e d u c t i o n o f monomethyl p a r a f f i n s i n t h e p r o d u c t g a s o l i n e . The r e s u l t s on o l e f i n isomers ( T a b l e V I I ) c a n a l s o be e x p l a i n e d by t h e o b s e r v a t i o n t h a t t h e c o n s t r a i n t i n d e x o f ZSM-5 i s approximately u n i t y under t h e c o n d i t i o n s o f t h i s study. Shape s e l e c t i v i t y o r p r e f e r e n t i a l c o n v e r s i o n o f s t r a i g h t c h a i n o l e f i n s by ZSM-5 cannot be e x p e c t e d a t 500°C. Thus, under t h e c o n d i t i o n s o f t h i s s t u d y , o l e f i n isomer d i s t r i b u t i o n was n o t s i g n i f i c a n t l y a f f e c t e d by d e a c t i v a t e d ZSM-5. A t t e m p e r a t u r e s lower t h a n t h a t employed i n t h e p r e s e n t s t u d y , i t i s c o n c e i v a b l e t h a t d i s t r i b u t i o n o f o l e f i n isomers c o u l d be a l t e r e d by steam d e a c t i v a t e d ZSM-5. Conclusions Cs t o C 7 o l e f i n s a r e i n t e r m e d i a t e s d u r i n g c o n v e r s i o n o f C i e η-paraffins t o a r o m a t i c s by ZSM-5. A d d i t i o n o f 1 Wt.% ( t h e r m a l l y t r e a t e d o r steam t r e a t e d ) ZSM-5 t o a REY f l u i d c r a c k i n g c a t a l y s t r e s u l t s i n an i n c r e a s e i n t h e y i e l d o f C3 and C4 h y d r o c a r b o n s a t t h e expense o f Cs t o C 1 2 h y d r o c a r b o n s ( g a s o l i n e ) . The g a s o l i n e p r o d u c e d from t h e d u a l z e o l i t e c a t a l y s t has a lower c o n c e n t r a t i o n o f n o r m a l , monomethyl p a r a f f i n s and a h i g h e r c o n c e n t r a t i o n o f o l e f i n s and a r o m a t i c s compared t o t h e g a s o l i n e p r o d u c e d from t h e REY c a t a l y s t . Preference of the dual z e o l i t e c a t a l y s t to monomolecular c r a c k i n g o f p a r a f f i n p r e c u r s o r s (carbonium i o n s and o l e f i n s ) a t t h e expense o f b i m o l e c u l a r r e a c t i o n s l i k e hydrogen t r a n s f e r and c h a i n t r a n s f e r c o n t r i b u t e s t o t h e p a r a f f i n r e d u c t i o n . ZSM-5 e x h i b i t s n e a r l y e q u i v a l e n t r e a c t i v i t y f o r s t r a i g h t c h a i n and monomethyl b r a n c h e d p a r a f f i n p r e c u r s o r s a t 500°C. Observed +


3.


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enhancement o f g a s o l i n e o c t a n e number (2, 3) w i t h t h e u s e o f a s m a l l c o n c e n t r a t i o n o f ZSM-5 ( e i t h e r f r e s h o r steam d e a c t i v a t e d ) p r o b a b l y due t o t h e r e d u c t i o n i n c o n c e n t r a t i o n o f p a r a f f i n s ( n o r m a l , monomethyl) and r e s u l t a n t enrichment o f o l e f i n s and a r o m a t i c s i n the g a s o l i n e . The same mechanism can be used t o e x p l a i n the changes i n g a s o l i n e c o m p o s i t i o n w i t h e i t h e r f r e s h o r steam d e a c t i v a t e d ZSM-5.

is


Acknowledgments The a u t h o r s thank D r . M. G . Sanchez f o r h i s v a l u a b l e c o n t r i b u t i o n s t o t h i s work which i n c l u d e d p r e p a r a t i o n o f ZSM-5 and d i s c u s s i o n o f t h e e x p e r i m e n t a l methods. We thank Ms. C . R. P e t r , M r . A . L . Wadsworth and Ms. G . B . L u n d q u i s t f o r t h e i r e x p e r i m e n t a l work. We a l s o acknowledge h e l p f u l d i s c u s s i o n s w i t h D r . A . W. P e t e r s and M r . J . E . C r e i g h t o n .

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18. Gates, B. C.; Katzer, J. R.; Schuit, G. C. A. Chemistry of Catalytic Processes; McGraw-Hill: New York, 1979. 19. Thomas, C. L.; Barmby, D. S. J. Catalysis 1968, 12, 341. RECEIVED March 17, 1988


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