Modification of Fluid Catalytic Cracking Catalysis by the Addition of

Chapter 5

Modification of Fluid Catalytic Cracking Catalysis by the Addition of ZSM-5 Downloaded via TUFTS UNIV on July 21, 2018 at 20:29:12 (UTC). See https://pubs.acs.org/sharingguidelines for options on how to legitimately share published articles.

Gasoline Over-Cracking Studies 1

2

D. J. Rawlence and J. Dwyer 1

Crosfield Catalysts, Warrington WA5 1AB, Cheshire, United Kingdom Department of Chemistry, University of Manchester Institute of Science and Technology, Sackville Street, Manchester, United Kingdom

2

Over-cracking of FCC gasoline with either ZSM-5 or REHY results, in both cases, in a preferential loss of heavier olefin components. The major differences between the two zeolites is the increased C3/C4 ratio with ZSM-5 which has been assigned to pore size effects, and enhanced bimolecular hydrogen transfer reactions with REHY, resulting in a higher paraffin/olefin ratio. Typical carbenium ion reaction mechanisms are used to explain the general differences in yields obtained for the two zeolite structures. Addition o f ZSM-5 t o the FCC c i r c u l a t i n g inventory r e s u l t s i n a s l i g h t reduction i n gasoline y i e l d but an enhancement o f gasoline q u a l i t y i n terms o f octane r a t i n g [1]. In addition, increased y i e l d s o f L.P.G. components are observed, notably propene and butenes which, subsequently, can be fed t o a l k y l a t i o n reactors. T y p i c a l l y , the ZSM-5 i s contained i n a separate microspheroidal p a r t i c l e which i s p h y s i c a l l y blended with conventional z e o l i t e Y based cracking c a t a l y s t s p r i o r t o addition t o the FCC u n i t . Concentration o f the ZSM-5 c r y s t a l i n the FCC u n i t can be regulated conveniently by t h i s approach and i s , generally, i n the range 0.5-3.0 weight % o f the t o t a l c a t a l y s t inventory. The pore opening i n ZSM-5 i s smaller than f o r z e o l i t e Y and access o f the complex gas o i l molecules i n t o the pores w i l l be r e s t r i c t e d . As a r e s u l t , ZSM-5 has l i t t l e e f f e c t on the primary cracking o f gas o i l and, when allowance i s made f o r the s l i g h t l o s s i n conversion a r i s i n g from d i l u t i o n o f the a c t i v e z e o l i t e Y concentration, there i s no s i g n i f i c a n t change i n coke, bottoms, or l i g h t gas y i e l d s .

0097-6156y9iy0452-0056$06.75/0 © 1991 American Chemical Society Occelli; Fluid Catalytic Cracking II ACS Symposium Series; American Chemical Society: Washington, DC, 1991.

5. RAWLENCE AND DWYER

Modification of FCC Catalysis

57

The major r o l e o f ZSM-5 i s t o react with converted products and, i n p a r t i c u l a r , c e r t a i n gasoline components. The shape s e l e c t i v i t y o f the z e o l i t e allows only l i n e a r and mono-methyl p a r a f f i n s and o l e f i n s ready access t o a c t i v e s i t e s w h i l s t other structures, such as aromatics and multi-branched p a r a f f i n s , w i l l be r e s t r i c t e d [2]. Octane enhancement with ZSM-5 a r i s e s from t h i s shape s e l e c t i v i t y as the most accessible hydrocarbons a r e a l s o octane depressants, w h i l s t those structures precluded from the z e o l i t e structure e.g. aromatics have high octane r a t i n g s . The gain i n octane therefore r e s u l t s from the combined e f f e c t o f reducing the concentration o f low octane compounds together with the r e l a t i v e increase i n concentration o f d e s i r a b l e hyoxocarbons as a r e s u l t o f the o v e r a l l l o s s i n gasoline y i e l d . I n addition, isomerisation o f l i g h t l i n e a r o l e f i n s can occur, leading t o further octane enhancement [3]. T h i s paper examines the o v e r a l l reaction between ZSM-5 and gasoline i n more d e t a i l by comparing the compositional changes that a r i s e a f t e r cracking o f gasoline over a deactivated ZSM-5 c a t a l y s t . I n addition, gasoline w e r - c r a c k i n g over a REHY c a t a l y s t i s studied i n p a r a l l e l t e s t s , allowing comparisons t o be made between the shape s e l e c t i v i t y and hydrogen t r a n s f e r properties o f the respective z e o l i t e structures.

EXPERIMENTAL A conventional m i c r o - a c t i v i t y (MAT) reactor was employed i n these studies [4]. Reactor temperature was 516°C and operating conditions were s e t a t 4.08 catalyst/feed r a t i o , 11.8 WHSV. The feed used was an FOC gasoline sample obtained from a European r e f i n e r y and was subsequently d i s t i l l e d t o give an approximate b o i l i n g range o f 35-190°C. The gasoline feed was cracked over a f i x e d bed (4.69g) o f the relevant c a t a l y s t and l i q u i d and gaseous products were c o l l e c t e d i n conventional MAT receivers. Gaseous products were analysed on a dual-column Hewlett Packard 5890 gas dircmatograph where hydrogen and nitrogen were quantified on a 13X column l i n k e d t o the TCD detector. Individual hydrocarbons i n the number range CI - CJ were quantified on a 50m alumina-PIOT column carbon l i n k e d t o an FID detector. The l i q u i d f r a c t i o n from each t e s t was analysed d i r e c t l y on a PIONA analyser supplied by A n a l y t i c a l Control BV. Results from both analyses were combined t o give carbon d i s t r i b u t i o n i n the range 0-10 by several hydrocarbon types namely i s o , normal and cyclo p a r a f f i n s , i s o , normal, and c y c l o o l e f i n s , and mono-ring aromatics. For convenience these groups are abbreviated t o IP, NP, CP, 10, NO, CO, and AR respectively, and s u f f i x e d by the relevant carbon number i . e . IP-5 equates t o t o t a l C5 i s o - p a r a f f i n s .

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

58

FLUID CATALYTIC CRACKING II: CONCEPTS IN CATALYST DESIGN Coke on c a t a l y s t was measured by a IECO 244 Carbon/Sulphur analyser. Cracking reactions were duplicated and the r e s u l t s were averaged. In a l l cases, mass balances were i n the range 98.0-100.5% and r e s u l t s were normalised t o 100% using a feed weight correction. The REHY c a t a l y s t employed was a commercial Quantum 2000 sample with a rare earth content of 1.27 wt%. The ZSM-5 c a t a l y s t was prepared on a p i l o t p l a n t spray dryer from 25% wt% z e o l i t e , 25% wt% s i l i c a s o l , and 50 wt% k a o l i n c l a y . The ZSM-5 sample used i n t h i s study analysed a t 30:1 silica-alumina r a t i o . Both c a t a l y s t s were e l u t r i a t e d , pre-heated a t 538°C i n a i r f o r 3 hours, and f l u i d bed steam deactivated a t 816°C f o r 5 hours i n 100% steam. A f t e r t h i s treatment, the u n i t c e l l s i z e of the REHY component i n Quantum 2000 reduced from 2.453 nm t o 2.427 nm.

EFFECT OF ZSM-5 ADDITION Laboratory scale FCC evaluation studies are u s u a l l y conducted i n f i x e d bed reactors such as MAT, the r e s u l t s from which can provide a r e l i a b l e and rapid means of ranking c a t a l y s t performance [4]. Depending upon the conditions employed, the e f f e c t o f added ZSM-5 can a l s o be predicted [5] and can give the same trends as those experienced i n ccsnmercial reactors. For example, the e f f e c t o f 2.5 wt% addition o f ZSM-5 on gas o i l cracking y i e l d s with Quantum 2000 i s described i n Table 1. In t h i s example, a 4% reduction i n gasoline y i e l d occurs, predominantly from 105°C+ material. The L.P.G. composition indicates an enhancement o f propene, butenes, and iso-butane, i n agreement with commercial r e s u l t s and, furthermore, the r e l a t i v e increase i n the i n d i v i d u a l butenes are s i m i l a r t o those reported by Schipper e t a l [1]. From d e t a i l e d PIGNA analyses of the gasoline f r a c t i o n s [5] i t was apparent t h a t i s o - p a r a f f i n s , p a r t i c u l a r l y i n the C7-C9 range, were the most reactive towards ZSM-5. However the gasoline composition produced from such reactors tends t o be p a r a f f i n r i c h , and o l e f i n d e f i c i e n t when compared t o commercial samples. Commercial FCC gasolines have, f o r example, a low concentration o f n-paraffins which, although they can be e a s i l y cracked by ZSM-5, are u n l i k e l y t o be the major source o f propene and butenes as has been claimed [6]. However, such gasolines have much higher l e v e l s o f n - o l e f i n s which a l s o react r e a d i l y with ZSM-5 [7] and could therefore contribute s i g n i f i c a n t l y t o the o v e r a l l e f f e c t on L.P.G./gasoline composition induced by ZSM5. In order t o investigate more f u l l y the o r i g i n s o f the L.P.G. components and the changes i n gasoline composition, a s e r i e s o f ever-cracking studies were conducted using a commercial FCC gasoline as feed. By comparing the r e s u l t s obtained with ZSM-5


RAWLENCE AND DWYER


TABLE 1 EFFECT OF ZSM-5 ON GAS-OIL CRACKING YIELD (Wt% Feed)

Q-2000

Q-2000/ZSM-5

Conversion ( to 221°C)

68.00

68.00

Coke 350°C 35-221°C 105-221°C 35-105°C L.P.G. Dry Gas

2.10 9.25 49.45 26.91 22.54 14.74 1.73

2.13 9.70 45.40 23.71 20.69 18.66 1.84

Bottoms Gasoline Gasoline Gasoline

H2-C4 GAS YIELDS (Wt% Feed) Hydrogen

0.053

0.048

Methane

0.52

0.53

Ethane Ethene

0.52 0.62

0.54 0.68

Propane Propene

0.82 4.34

0.88 5.92

Iso-Butane N-Butane 1-Butene C-Butene-2 T-Butene-2 Iso-Butene

3.56 0.73 1.23 1.14 1.63 1.29

4.12 0.81 1.54 1.44 2.06 1.81

MAT CONDITIONS Feed Catalyst/Feed WHSV Reactor Temperature

: Kuwait Waxy Distillate : 3.50 : • 13.7 : 516°C

Catalyst Deactivation : 5 hours, 816°C, 100% Steam


60

FLUID CATALYTIC CRACKING II: CONCEPTS IN CATALYST DESIGN with those using REHY the r e l a t i v e e f f e c t s o f hydrogen t r a n s f e r compared t o shape s e l e c t i v i t y could a l s o be studied.

PARENT GASOLINE (IMPOSITION FIONA analysis by weight % o f the parent gasoline i s reported i n Table 2A and includes the low l e v e l of C11+ material not completely analysed by PICNA. I t i s more convenient t o eliminate t h i s f r a c t i o n and express the remainder as moles per 100 moles CIO- feed (Table 2B) and, on t h i s basis, the average molecular weight o f the parent gasoline feed i s computed a t 101.8, with a c a l c u l a t e d density o f 0.7542 g/ml and a U/C atomic r a t i o o f 1.88.

OVER-VIEW OF GASOLENE CRACKING Results from the o v e r - c o c k i n g o f the parent gasoline over z e o l i t e s ZSM-5 and Y are summarised i n Table 3. I t i s evident that ZSM-5 i s more a c t i v e f o r gasoline cracking as 23.7 wt% L.P.G. + dry gas was produced compared with 18.2 wt% f o r REHY. As i s commonly observed, a lower coke y i e l d was a l s o produced with ZSM-5. The REHY c a t a l y s t produced more saturated l i g h t gas, most l i k e l y due t o the s l i g h t l y a c t i v e matrix components, present i n the cxxnmercial sample but absent from the experimental ZSM-5 c a t a l y s t . However, t o t a l dry gas y i e l d s were s i m i l a r f o r both z e o l i t e s due t o the enhanced ethene y i e l d s observed with ZSM-5. S i m i l a r l y , REHY produced more saturated L.P.G., notably i s o butane, p o s s i b l y as a r e s u l t o f hydrogen t r a n s f e r reactions involving o l e f i n s . Propene and t o t a l butenes a l s o increase, but there i s a r e l a t i v e l y low gain i n iso-butene. A considerable amount o f propene i s formed with ZSM-5, accounting f o r 40% o f converted product, and w h i l s t t o t a l butenes are n i c k e r than f o r REHY, the enhanced production o f iso-butene i s the major contributing factor. Again, with ZSM-5, the r e l a t i v e gains i n the i n d i v i d u a l butenes i s s i m i l a r t o those reported by Schipper e t a l [1]. The analysis o f product gasoline indicates an increased aromatic content with the ZSM-5 but t h i s a r i s e s s o l e l y from a concentration e f f e c t fl.61. The gain i n aromatics with REHY, however, i s greater than can be explained by concentration alone and suggests a d d i t i o n a l aromatic formation from more extensive hydrogen t r a n s f e r reactions. In both cases, there i s l i t t l e reaction with c y c l o - p a r a f f i n s but c y c l o - o l e f i n s v i r t u a l l y disappear with REHY. ZSM-5 a l s o reacts with c y c l o - o l e f i n s , and a s i m i l a r r e s u l t has been observed with g a s - o i l cracking [6].


5.

RAWLENCE AND DWYER

61


TABLE 2 PARENT GASOLINE ANALYSIS TABLE 2A - WEIGHT % OF HYDROCARBON TYPES PARAFFINS

c#

iso

0 1 2 3 4 5 6 7 8 9 10

0.00 0.00 0.00 0.00 0.22 4.01 3.87 2.78 2.03 1.71 1.42

TOT

n

AROMATICS

OLEFINS

cyclo

iso

0.00 0.00 0.00 0.00 0.34 0.81 1.00 0.57 0.39 0.30 0.25

0.00 0.00 0.00 0.00 0.00 0.13 1.86 2.32 1.81 1.08 0.52

0.00 0.00 0.00 0.00 0.33 3.23 4.26 4.82 3.72 2.74 2.08

16.04 3.66

7.72

n

TOTAL

cyclo

0.00 0.00 0.00 0.00 1.37 3.72 3.35 1.18 0.43 0.33 0.34

0.00 0.00 0.00 0.00 0.00 0.00 2.13 2.79 2.14 0.84 0.28

0.00 0.00 0.00 0.00 0.00 0.00 0.42 4.46 6.75 6.57 10.91

0.00 0.00 0.00 0.00 2.26 11.01 17.78 18.93 17.26 13.58 15.80

21.18 10.72

8.18

29.11

96.62 0.00 3.38

Coke Un-analysed C11+

100.00

TABLE 2B

- MT>TFS PRE?

mn

MDTFS

PARAFFINS

c#

iso

0 1 2 3 4 5 6 7 8 9 10

0.00 0.00 0.00 0.00 0.39 5.65 4.57 2.82 1.81 1.36 1.01

TOT

FRFT>

OLEFINS

cyclo

iso

0.00 0.00 0.00 0.00 0.60 1.14 1.18 0.58 0.35 0.24 0.18

0.00 0.00 0.00 0.00 0.00 0.18 2.20 2.35 1.61 0.86 0.37

0.00 0.00 0.00 0.00 0.60 4.68 5.15 4.99 3.37 2.21 1.51

17.61 4.26

7.57

n

n

AROMATICS

TOTAL

cyclo

0.00 0.00 0.00 0.00 2.48 5.39 4.05 1.22 0.39 0.27 0.25

0.00 0.00 0.00 0.00 0.00 0.00 2.57 2.89 1.94 0.68 0.20

0.00 0.00 0.00 0.00 0.00 0.00 0.55 4.92 6.46 5.56 8.26

0.00 0.00 0.00 0.00 4.06 17.06 20.26 19.78 15.93 11.16 11.79

22.51 14.05

8.28

25.75

100.03


62

FLUID CATALYTIC CRACKING II: CONCEPTS IN CATALYST DESIGN

TABLE 3 SUMMARY OF GASOLINE OVER-CRACKING

YIELD (Wt% Feed) C5-10 Gasoline L.P.G. Dry Gas Coke

PARENT

ZSM-5

94.36 2.26 0.00 0.00

73.36 22.45 23 58

ZEOLITE Y 78.32 16.92 1.37 1.46

H2-C4 GAS COMPOSITION (Wt% Feed) Hydrogen

0.00

0.028

0.051

Methane

0.00

0.18

0.44

Ethane Ethene

0.00 0.00

0.13 0.89

0.38 0.58

Propane Propene

00 ,00

43 23

0.70 4.83

Iso-Butane N-Butane 1-Butene C-Butene-2 T-Butene-2 Iso-Butene

22 ,34 ,42 39 56 33

19 64 82 70 42 4.01

16 04 41 32 70 66

C5-C10 GASOLINE ANALYSIS (Wt% Gasoline) Average Mol.Wt Total Total Total Total Total Total Total

IP NP CP IO NO CO AR

101.8

94.6

94.4

16.8 3.5 8.2 22.1 9.9 8.7 30.9

26. 4. 9. 9. 8. 2. 38.

36..3 4. .7 8. .2 .0 6. 4.9 0.6 39.3


5.

RAWLENCE AND DWYER


63

RESULTS AND DISCUSSION More d e t a i l e d r e s u l t s of gasoline ever-cracking with ZSM-5 o r Y are given i n Tables 4 and 5 where, f o r each hydrocarbon type and carbon number, the d i f f e r e n c e i n moles i . e . moles product - moles (parent) are presented. From these, d e l t a mole d i s t r i b u t i o n s f o r t o t a l hydrocarbon and f o r p a r a f f i n s and o l e f i n s can be compared (Figures 1-5). The impression from the t o t a l hydrocarbon d i s t r i b u t i o n (Figure 1) i s t h a t both z e o l i t e s behave i n a s i m i l a r manner by producing C3-C5 products from C6+ hydrocarbons, the most s i g n i f i c a n t d i f f e r e n c e being the r a t i o o f C3/C4 y i e l d s . However, as i s apparent from Figures 2-5, there are marked d i f f e r e n c e s i n p a r a f f i n s and o l e f i n s f o r each z e o l i t e . C l e a r l y the losses and gains o f a p a r t i c u l a r species present i n the product gasoline as compared t o the parent feed gasoline can represent the balance o f complex reactions. However, under the r e a c t i o n conditions employed, i t i s not l i k e l y that there w i l l be appreciable generation o f C6+ hydrocarbons other than as intermediates, so that an examination o f reactant losses i n t h i s region provides a reasonable comparison o f reactant conversion over the two z e o l i t e s . Figure 2 shows that there i s a l i m i t e d conversion of higher p a r a f f i n s on e i t h e r z e o l i t e . There i s some evidence t o suggest that C7/C8 p a r a f f i n conversion i s higher with ZSM-5 and C9/C10 with REHY. However, i t would appear t h a t ZSM-5 i s not, under the present conditions, p a r t i c u l a r l y e f f e c t i v e i n p a r a f f i n modification i n the presence of a r e l a t i v e l y high concentration of o l e f i n s .

REACTIONS OF OIEFINS For both z e o l i t e s , the major reactants are the branched and l i n e a r C6+ o l e f i n s . The i s o - o l e f i n s present i n the FCC gasoline are considered t o be predominantly mono-branched which, using C8 i s o - o l e f i n (IO-8) as an example, react t y p i c a l l y as i n Scheme 1. A previous report indicates that the conversion o f 10-8 a t 450°C gives mainly C4 products with l i t t l e C3 + C5 [8]. T h i s suggests that the increased r a t e o f beta s c i s s i o n associated with the c o c k i n g o f the C8 c a t i o n isomer (III, Scheme 1) i s the preferred route t o cracked products. This i s i n keeping with the accepted order o f r e a c t i v i t y f o r cracking (A > B > C), and s i m i l a r c o c k i n g schemes can be postulated f o r the other C6+ o l e f i n s . [8,9]. The major products, f o r both z e o l i t e s are C2-C5 o l e f i n s (Figures 3-5) and C1-C6 p a r a f f i n s . Results show c l e a r l y the enhanced bimolecular hydrogen t r a n s f e r with REHY as compared t o ZSM-5 since i n d i v i d u a l C1-C7 p a r a f f i n s are c o n s i s t e n t l y higher


64


TABLE 4 GASOLINE OVER-CRACKING WITH ZSM-5 DELTA MDTKS PER 1QQ MDTiES FRRD

c#

iso

0 1 2 3 4 5 6 7 8 9 10

+0.00 +0.00 +0.00 +0.00 +3.45 +3.54 +2.25 +0.10 -0.12 -0.20 -0.24

TOT

+8.78 +4.28 -0.44

n

cyclo

+1.62 +1.14 +0.44 +0.99 +0.53 +0.17 +0.13 -0.07 -0.05 -0.04 -0.04

+0.00 +0.00 +0.00 +0.00 +0.00 -0.01 -0.20 +0.14 -0.09 -0.17 -0.11

AROMATICS

OLEFINS

PARAFFINS iso

n

-KD.00 + 0.00 +0.00 + 0.00 +0.00 + 3.23 +0.00 +22.31 +6.67 + 8.28 -0.14 + 2.25 -1.40 - 3.05 -4.63 - 1.15 -3.24 - 0.32 -2.21 - 0.23 -1.51 -0.25 -6.46

TOTAL

cyclo +0.00 +0.00 +0.00 +0.00 +0.00 +0.00 -0.21 +1.11 +0.87 +0.03 -2.08

+0.00 +0.00 +0.00 +0.00 +0.00 +0.00 -1.43 -2.12 -1.60 -0.68 -0.20

-0.28

+31.08 -6.03

+ 1.62 + 1.14 + 3.67 +23.30 +18.93 + 5.80 - 3.90 - 6.62 - 4.54 - 3.49 - 4.44 31.47

TABLE 5 GASOLINE OVER-CRACKING WITH REHY DELTA MOLES PER 100 MOLES FEED

OLEFINS

PARAFFINS

c#

iso

0 1 2 3 4 5 6 7 8 9 10

+0.00 +0.00 +0.00 +0.00 +6.72 +9.30 +5.88 +1.37 +0.32 -0.17 -0.35

n +2.44 +2.79 +1.01 +1.61 +1.23 +0.51 +0.19 +0.01 -0.03 -0.04 -0.04

cyclo +0.00 +0.00 +0.00 +0.00 +0.00 +0.01 +0.11 -0.08 -0.41 -0.41 -0.24

TOT - -23.07 +9.68 -1.02

iso

n

-KD.00 + 0.00 +0.00 + 0.00 +0.00 + 2.10 +0.00 +11.67 +2.41 + 5.89 -1.33 - 0.94 -2.95 - 3.29 -4.52 - 1.11 -3.27 - 0.36 -2.21 - 0.24 -1.51 - 0.25

AROMATICS

TOTAL

cyclo +0.00 +0.00 +0.00 +0.00 +0.00 +0.00 -2.25 -2.76 -1.91 -0.68 -0.20

-13.3''+13.48 -7.79

+0.00 +0.00 +0.00 +0.00 +0.00 +0.00 -0.22 +1.40 +2.53 +1.04 -2.52

+ 2.44 + 2.79 + 3.12 +13.29 +16.25 + 7.55 - 2.53 - 5.68 - 3.13 - 2.71 - 5.11

+2.23

+26.26



0 ' 1

1

1

4 ' 5

1

B "7

1

Figure 1 :

>

B ' 3 'la'

8

0

1

2

3

5

B

7

8 CARBON NUMBER

4

Delta mole d i s t r i b u t i o n s f o r t o t a l hydrocarbons.

CRRBON NUMBER

2 ' 3

9 >

10

8}

! F i g u r e

2

:

D e l t a

mole

d i s t r i b u t i o n s

f o r

t o t a l

p a r a f f i n s .

2

8

s

η

s


Delta mole d i s t r i b u t i o n s f o r t o t a l o l e f i n s . Figure 3 : Occelli; Fluid Catalytic Cracking II ACS Symposium Series; American Chemical Society: Washington, DC, 1991.

Delta mole d i s t r i b u t i o n s f o r i s o - o l e f i n s . Figure 4 : Occelli; Fluid Catalytic Cracking II ACS Symposium Series; American Chemical Society: Washington, DC, 1991.

F i g u r e

5

:

D e l t a

mole

d i s t r i b u t i o n s

f o r

n - o l e f i n s .

I

i I

w

η



B- Isom

(2)

(D

C- Cracking

B- Cracking

(3)

A- Cracking

•H*

Scheme (1)


5.

RAWLENCE AND DWYER


71

f o r reactions over REHY, with the converse being true f o r o l e f i n s . The product d i s t r i b u t i o n f o r ZSM-5 favours C3 hydrocarbons (23 moles) compared t o 13 moles with REHY. This trend has a l s o been observed i n the cracking o f several p a r a f f i n s [10] and t h i s i s assigned t o cage e f f e c t s . This explanation may a l s o apply here as the r a t i o o f C3/C4 (1.00 f o r ZSM-5 and 0.65 f o r REHY) w i l l r a t i o t o 1.54, a value s i m i l a r t o that derived from the cracking o f Cn hydrocarbons [10]. A s t r u c t u r a l as opposed t o a compositional e f f e c t f o r the enhanced C3/C4 r a t i o i n ZSM-5 i s supported by r e s u l t s from a p a r a l l e l s e r i e s o f t e s t s on gasoline cracking using a US-Y o f s i m i l a r s i l i c a - a l u m i n a r a t i o t o t h a t o f ZSM-5 described here [11]. The C3/C4 r a t i o from t h i s study was i d e n t i c a l t o that observed f o r REHY. Results f o r C4 products (Table 6) i n d i c a t e that, w h i l s t the t o t a l C4 y i e l d i s s l i g h t l y higher with ZSM-5 (due t o the greater conversion compared t o REHY), the r a t i o o f p>araffins t o o l e f i n s i s s i g n i f i c a n t l y lower, and i s p a r t i c u l a r l y so with the i s o cx3mpounds. This r e f l e c t s the greater f a c i l i t y f o r hydrogen t r a n s f e r i n REHY, as opposed t o ZSM-5, which can be explained i n terms o f the influence o f s i t e proximity [12], o r the e f f e c t s o f sorption preference [13] both o f which favour hydrogen t r a n s f e r i n more alumina-rich z e o l i t e s .

TABLE 6 YIELDS OF C4 products

( A MOLES) REHY

iso PARAFFINS OLEFINS TOTALS TOTAL C4 PARAFFIN/OIEFIN i-BCJTANE/i BOTENE

3.45 6.67 10.12

normal 0.53 8.28 8.81 18.93 0.27 0.52

iso 6.72 2.41 9.13

normal 1.23 5.89 7.12 16.25 0.96 2.79

Size r e s t r i c t i o n s i n ZSM-5 are expected t o i n h i b i t hydrogen t r a n s f e r processes [14]. However, the y i e l d o f iso-butane shows that t h i s process i s not e n t i r e l y excluded i n ZSM-5 since a l t e r n a t i v e explanations f o r substantial y i e l d s o f t h i s product, f o r example d i r e c t attack o f alkane chains by protonic hydrogen o r by carbenium ions, seem l e s s l i k e l y i n view o f the l i m i t e d reaction o f higher alkanes (Figure 2) under the present conditions. S i z e r e s t r i c t i o n s on bimolecular hydrogen t r a n s f e r involving l a r g e r branched o l e f i n i c carbenium ions such as those generated from c y c l o - o l e f i n s are expected t o be more s i g n i f i c a n t with ZSM-5, however, and w i l l be discussed l a t e r .


72

FLUID CATALYTIC CRACKING II: CONCEPTS IN CATALYST DESIGN Both normal and iso-butenes are formed l a r g e l y from higher o l e f i n s but are not i n i t i a l l y i n thermodynamic equilibrium. In ZSM-5, equilibrium between the butenes i s approximated but there i s excess iso-butane over n-butane. S i m i l a r l y f o r REHY, i s o butane i s a l s o i n excess o f equilibrium but there i s a c l e a r d e f i c i e n c y i n iso-butene. Y i e l d s o f iso-butene are very susceptible t o the hydrogen t r a n s f e r function o f z e o l i t e Y [11] and the present observation may be explained i f i t i s assumed t h a t hydrogen t r a n s f e r involving the iso-C4 c a t i o n i s r e l a t i v e l y r a p i d compared t o isomerisation. In t h i s case iso-butene w i l l be depleted with REHY but much l e s s hydrogen t r a n s f e r reaction w i l l occur with ZSM-5. Excess iso-butane over equilibrium i s observed f o r both z e o l i t e s and suggests t h a t bimolecular hydrogen t r a n s f e r involving nC4 cations i s l e s s r a p i d than f o r the iso-C4 cation. We are not aware of any d e t a i l e d studies f o r these reactions but suggest, t e n t a t i v e l y , t h a t the greater s t a b i l i t y o f the iso-C4 c a t i o n may enhance i t s concentration and increase the r a t e o f (bimolecular) hydrogen t r a n s f e r as compared t o the n-C4 cation which p r e f e r e n t i a l l y desorbs t o o l e f i n s . Reference t o Figure 6 shows that, i n addition t o the greater y i e l d o f i s o - p a r a f f i n s over REHY, the d i s t r i b u t i o n s are d i f f e r e n t f o r the two z e o l i t e s . The l a r g e r C5-C7 iso-alkanes are r e l a t i v e l y more abundant with REHY suggesting t h a t hydrogen t r a n s f e r i n the l a r g e r pore z e o l i t e can take place more r e a d i l y with l a r g e r carbenium ions. In the smaller pore ZSM-5, the reduced r a t e o f hydrogen transfer, discussed above, along with reduced space, f o r bimolecular reactions involving l a r g e r ions favours more s c i s s i o n of these l a r g e r ions t o smaller ions where a l i m i t e d amount o f hydrogen t r a n s f e r can then occur t o produce l i g h t alkanes. REACTIONS OF CYCLIC STRUCTURES Cyclo-paraffins react more r e a d i l y with REHY, presumably as a r e s u l t o f pore s i z e differences between the 10 r i n g ZSM-5 and the 12 r i n g f a u j a s i t e , r e s u l t i n g i n s i z e discrimination (reactant shape s e l e c t i v i t y ) . Over both z e o l i t e s , C9-C10 c y c l o - o l e f i n s are completely converted so t h a t s i z e discrimination i s not observed. However, where conversion o f c y c l o - o l e f i n s i s not complete ( i . e . C6-C8) there i s c l e a r evidence f o r cUscximination between the two z e o l i t e s (Figure 7). Presumably t h i s high r e a c t i v i t y o f C9/C10 c y c l o - o l e f i n s over e i t h e r z e o l i t e can be explained by i n i t i a l f a c i l e attack a t outer surfaces. Reactions o f c y c l o - o l e f i n s within the z e o l i t e s can generate aromatics by bi-molecular hydrogen t r a n s f e r reactions. Such reactions are expected t o be more favourable with REHY because o f i t s greater framework aluminium content and because o f r e s t r i c t e d access t o the i n t e r - c r y s t a l l i n e pores i n ZSM-5. T h i s i s



0

i

1

I

2

3

4

I

i

5

I

6

i

7

Figure 6 :

CRRBON NUMBER

i I

9

i

>

I

10

i

-sI 0

,

1

i

2

,

4

i

,

5

i

6

i

7 CRRBON NUMBER

3

Delta mole comparisons f o r iso/n p a r a f f i n s .

8

I

8

i

>

9

1

10

in


REHY

ZSM-5

0

1

2

3

4

5

6

7

CRRBON NUMBER

Figure 7 :

8

9

10

>

Relative l o s s o f c y c l o - o l e f i n s .


5.

RAWLENCE AND DWYER


supported by the increased t o t a l aromatics observed with REHY. There are changes i n composition of the aromatics f r a c t i o n i n ZSM-5 but t h i s i s l a r g e l y accounted f o r by the dealkylation o f CIO aromatics t o C7/C8 aromatics i . e . AR-10 AR-10

> AR-8 > AR-7

+ +

NO-2 NO-3

As t o t a l number of aromatic rings remains constant f o r ZSM-5, the converted cycle-hydrocarbons must l a r g e l y be cracked to non-cyclic smaller molecules. To an extent, t h i s must a l s o be true f o r REHY since l e s s aromatics are formed than t o t a l c y c l o hydrocarbons are l o s t . Cracking patterns f o r cyclo-paraffins over US-Y have recently been reported [9] where products from the conversion o f methycyclchexane include C2-C6 exxrpounds with C3 + C4 hydrocarbons as the major y i e l d s . The authors o f t h i s study have assumed that iscmerisation of the i n i t i a l carbenium i o n i s r e l a t i v e l y f a c i l e and that a combination o f hydrogen transfer, iscmerisation, and beta s c i s s i o n generates the products. T h i s i s envisaged i n Scheme 2 which uses the c l a s s i f i c a t i o n A, B, C f o r cracking, A, B f o r iscmerisation [8.9] and which presumes that intramolecular hydrogen t r a n s f e r i s r a p i d [8]. S i m i l a r fates can be postulated f o r isomers I I and I I I (Scheme 2A) and also f o r C8 - CIO c y c l o - o l e f i n s which would generate C4+ o l e f i n s . C5 and C6 o l e f i n s are a l s o presumed t o a r i s e from cracking o f methylcyclopentane [8] by the i n t e r a c t i o n of small carbenium ions with o l e f i n s t o form l a r g e r carbenium ions which can subsequently undergo beta s c i s s i o n . Such reactions can c l e a r l y modify product d i s t r i b u t i o n s i n the present more complicated system. Scheme 2 suggests that the r e l a t i v e l y large o l e f i n i c carbenium ions are involved i n bimolecular hydrogen t r a n s f e r reactions generating o l e f i n s which can then react further. T h i s process i s l i k e l y t o be i n h i b i t e d i n the smaller pore ZSM-5 which could r e s u l t i n more conversion v i a intramolecular reaction paths to g i v e a l l y l i c or d i - o l e f i n i c intermediates, as i n Scheme 3. Under the present conditions, involving long contact times and high conversion, dienes are not observed with e i t h e r c a t a l y s t . Low y i e l d s o f butadiene are observed during r i s e r cracking with z e o l i t e Y based c a t a l y s t s ; t o date, however, there i s no evidence f o r increased diene formation with ZSM-5. CONCLUSIONS From a study o f gasoline over-cracking with e i t h e r ZSM-5 o r REHY, the r e s u l t i n g t o t a l hydrocarbon d i s t r i b u t i o n s by carbon number are grossly s i m i l a r . In both cases, predominantly C2 - C5 products are formed from C6+ components. However, there are s i g n i f i c a n t differences i n d e t a i l , notably i n hydrocarbon-type distributions.


75


(Isomers)

iso-butane. The more e f f e c t i v e hydrogen t r a n s f e r i n REHY i s a l s o evident from the hic£i p a r a f f i n / o l e f i n r a t i o and from increased coke production. ACKNOWLEDGMENTS

The authors wish t o acknowledge Dr. G.J. E a r l , Mr. A. Howard Monks, Mr. M.R. Butler, Miss C. de Prez, and Mr. M. Woodhouse o f C r o s f i e l d Catalysts f o r the preparation and analyses o f the c a t a l y s t s and Dr. B. Booth o f U.M.I.S.T. f o r h e l p f u l discussion. LITERATURE CITED

1. 2. 3. 4. 5. 6. 7. 8. 9. 10. 11. 12. 13. 14.

Schipper P.H.; Dwyer F . G . ; Sparrell P . T . ; Mizrahi S; Herbst J . A . In Fluid Catalytic Cracking : Role in Modern Refining : American Chemical Society, 1988, P.65. Chen N.Y.; Garwood W.E., J . Catal. 1978, 58, p.453. Scherzer J. Catal.Rev.-Sci.Eng., 1989, 31(3), p.215 Rawlence D . J . ; Gosling K . Appl.Catal., 1988, 43, p.213 Rawlence D . J . ; Dwyer J., In preparation. Biswas J.; Maxwell I . E . In Zeolites : Facts, Figures, Future; Elsevier 1989, p.1263. Wojciechowski B. J.; Corma A. Catalytic Cracking; Marcel Dekker Inc., 1986, p.154. Lin L.; Gnep N.S.; Guisnet M.R. Symposium on the Hydrocarbon Chemistry of Naphtha Formation, ACS Div. Pet. Chem.Inc, American Chemical Society Miami, Sept. 1989, p.687. Weitkamp J.; Jacobs P.; Martens P.A. Appl. Catal., 1983, 8, p.123. Barthomuef D.; Merodatos C. Reference 8, p.714. Rawlence D.J. Unpublished results. Pine L.; Mather P.J.; Wachter W.A. J . Catal. 1984, 85, p.466. Corma A . ; Faraldos M.; Mifsud A. Appl. Catal. 1989, 47, p.125. Haag W.O.; Weiss P.B. Chem. Soc. Faraday Disc. 1981, p.317

RECEIVED October 5, 1990


Modification of Fluid Catalytic Cracking Catalysis by the Addition of

Recommend Documents