Octane Enhancement in Catalytic Cracking by Using High-Silica Zeolites

Chapter 7

Octane Enhancement in Catalytic Cracking by Using High-Silica Zeolites Downloaded by UNIV OF CALIFORNIA SANTA BARBARA on October 8, 2015 | http://pubs.acs.org Publication Date: January 23, 1991 | doi: 10.1021/bk-1991-0452.ch007

S. J . Miller and C. R. Hsieh Chevron Research and Technology Company, P.O. Box 1627, Richmond, CA 94802-0627

The e f f e c t of ZSM-5 as an octane additive to a cracking catalyst was studied i n both a small fixed-bed reactor and a fluidized-bed p i l o t plant. Analyses of the products of these tests were used to determine the reaction chemistry. I t was found that by maximizing the r a t i o of isomerization a c t i v i t y to hydrogen transfer a c t i v i t y , gasoline octane was increased with a minimum of y i e l d loss. This could be accomplished by increasing the silica-to-alumina r a t i o of the additive zeolite.

Approximately 40% of the gasoline produced i n the United States i s made by f l u i d c a t a l y t i c cracking. Because of the need for higher gasoline octane, r e f i n e r s are looking for economical ways of improving octane i n t h e i r FCC units, either by changing operating conditions or cracking catalyst. The l a t t e r has primarily involved the use of catalysts containing low unit c e l l size ultrastable-Y z e o l i t e (USY) which produces fewer paraffins and more o l e f i n s than conventional rare earth catalysts (1). A second c a t a l y t i c option, which may be used i n conjunction with USY, i s the introduction of a minor amount of a catalyst designed to improve gasoline octane. Anderson et a l . (2) and Donnelly et a l . (3) have reported on the use of the z e o l i t e ZSM-5 as an additive catalyst for increasing the octane of FCC gasoline. This z e o l i t e has been reported by workers at Mobil (4) t o increase octane by cracking out low octane straight-chain paraffins and o l e f i n s i n the heavy gasoline product and by o l e f i n isomerization. Rajagopalan and Young (5.) have proposed that the ZSM-5 prevents p a r a f f i n formation by cracking carbonium ions or o l e f i n intermediates to l i g h t products. In e i t h e r case, C and C o l e f i n s increase while gasoline y i e l d decreases. Chevron researchers (6,7) have shown that gasoline loss can be reduced by using a ZSM-5 type additive of high s i l i c a t o alumina r a t i o , i n p a r t i c u l a r one of Si0 /Al 0 molar r a t i o greater than 500. In t h i s report we examine the effect of Si0 /Al 0 r a t i o on the s e l e c t i v i t y of ZSM-5 type additives and discuss the chemistry involved. 3

4

2

2

3

2

2

3

EXPERIMENTAL CATALYST PREPARATION ZSM-5 was prepared according t o procedures discussed i n the l i t e r a t u r e (8.9) and i d e n t i f i e d as such by X-ray d i f f r a c t i o n analysis. Ultrastable Y z e o l i t e was obtained from the

0097-6156/91/0452-0096$06.00/0 © 1991 American Chemical Society

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

7.

Octane Enhancement in Catalytic Cracking

MILLER AND HSIEH

97

Downloaded by UNIV OF CALIFORNIA SANTA BARBARA on October 8, 2015 | http://pubs.acs.org Publication Date: January 23, 1991 | doi: 10.1021/bk-1991-0452.ch007

Union Carbide Corporation. Dealuminated H-mordenite was prepared by acid extraction of H-mordenite obtained from the Norton Company. Octane additives were composed of 25 wt % z e o l i t e i n a s i l i c a alumina binder, spray-dried to conventional FCC c a t a l y s t s i z e , and treated at 1450°F i n 100% steam at 1 atm for f i v e hours before testing. An equilibrium cracking catalyst containing approximately 25 wt % z e o l i t e was used as the base catalyst for a l l additives. FIXED-BED AND FLUIDIZED-BED TESTS In the fixed-bed t e s t , a sample of cracking catalyst was contacted with an FCC feed i n a manner s i m i l a r to the standard microactivity test (MAT) prescribed by ASTM (10). One major difference was that the reactor temperature was increased from 900°F to 960°F to better simulate current commercial operations. The fluidized-bed tests were conducted i n a 0.5 BPD c i r c u l a t i n g p i l o t plant equipped with a 50-ft r i s e r reactor and an on-line debutanizer. The t o t a l l i q u i d product from each t e s t period was d i s t i l l e d externally, and the i n d i v i d u a l cuts were submitted for inspections. In both types of tests, additives were admixed with the equilibrium catalyst p r i o r to t e s t i n g . HYDROGEN TRANSFER INDEX (HTI) TEST In t h i s t e s t , 0.5 /*L pulses of 1-hexene feed were carried from a heated sampling valve into a fixed-catalyst bed i n a stainless steel reactor by a nitrogen c a r r i e r stream at 800 mL/min. (at STP). The c a t a l y s t was -250 mesh and d i l u t e d with alumina of the same mesh size plus 80-100 mesh acid-washed Alundum. Reactor pressure was controlled by an Annin valve. The e f f l u e n t stream went to the i n j e c t o r s p l i t t e r of a gas chromatograph. The reactor conditions included a c a t a l y s t temperature of 221°C and 3.45 MPa t o t a l pressure. The HTI was calculated from the product r a t i o of 3-methylpentenes to 3-methylpentane at a linear hexene conversion of 30-70%. RESULTS MICROACTIVITY TESTING S e l e c t i v i t y to C -Plus Versus Si0 /Al 0 Ratio 5

2

2

3

An additive containing a sieve of 40 S/A (Si0 /Al 0 ) was compared with one of 525 S/A. Figure 1 shows the C -plus gasoline y i e l d using the 525 S/A sieve was less than with the base c a t a l y s t alone, but s u b s t a n t i a l l y higher than when using the 40 S/A sieve at equal conversion. Furthermore, the C o l e f i n i c i t y (Figure 2), generally an indicator of l i g h t gasoline research octane, showed about the same improvement r e l a t i v e to the base for both additives. Using another binder which was higher i n s i l i c a , the 525 S/A was compared with a sieve of 1000 S/A. Figure 3 shows a continued increase i n C -plus y i e l d up to 1000 S/A. Again, a l l additives produced about the same C o l e f i n i c i t y (Figure 4), above that for the base c a t a l y s t as well as the base plus additive with no z e o l i t e . 2

2

3

5

4

5

4

E f f e c t of Additives on Selected Hydrocarbon Types The e f f e c t of the 40 S/A and 525 S/A additives on c e r t a i n hydrocarbon types was examined by adding to the t e s t feed e i t h e r an n-paraffin (n-nonane) i n the higher b o i l i n g range of gasoline or a 1-olefin (1-pentene) i n the lower b o i l i n g range. A high amount of additive (10%) was used.


98

FLUID CATALYTIC CRACKING II: CONCEPTS IN CATALYST DESIGN


$ 1 s2

8

O Base • +40 S/A • +525 S/A

co O _

51

B 49

48 73

Conversion Wt% Figure 1. E f f e c t of z e o l i t e Si0 /Al 0 r a t i o on gasoline s e l e c t i v i t y i n MAT. 2

2

3

0.520 0.515 0.510 0.505 0.500 0.495

0 c

1

O Base • +40 S/A • +525 S/A

0.490 0.485

O

0.480 [-

3

0.475

-

0.470

-

0.465

-

0.460 H 0.455 0.450 70

72

73

Conversion, Wt.% Figure 2. MAT.

E f f e c t of z e o l i t e Si0 /Al 0 2

2

3

r a t i o on C o l e f i n i c i t y i n 4


74

7. MILLER AND HSIEH


99


55 -

48 I 68

• 69

• 70

• 71

• 72

• 73

• 74

Conversion Wt% Figure 3. Effect of z e o l i t e Si0 /Al 0 r a t i o on gasoline s e l e c t i v i t y i n MAT. 2

2

3

O510 &505 0.500 0.495 ^

0.490

O C

0.485

g

0.480

OBase • +525 S/A A+1000 S/A • +Matrix

O 0.475 ^

0.470 0.465 0.460 &4S5 70

73

71

Conversion Wt% Figure 4. E f f e c t of z e o l i t e Si0 /Al 0 r a t i o on C o l e f i n i c i t y i n MAT. 2

2

3

4


100

FLUID CATALYTIC CRACKING H: CONCEPTS IN CATALYST DESIGN

With 5% nC i n feed, conversion of the nonane was less than 5% with e i t h e r the 40 S/A or 525 S/A additives. If less than 10% additive had been used, the conversion of nC due to the additive would have been almost undetectable. Since the rate of cracking of lower b o i l i n g paraffins i s l i k e l y to be lower than that of nC , those components should also be unaffected. This suggests that any octane enhancement observed with the additives has l i t t l e to do with cracking of n-paraffins. With 1% 1-pentene i n feed, and using the 525 S/A additive, there was an increase i n C branched o l e f i n s and p a r a f f i n s , as well as an increase i n C normal o l e f i n s and paraffins. Other carbon numbers remained unaffected (Table I ) . 9

9

9

5


5

Table I. Relative Yields of C -C Paraffins and Olefins Upon Addition of 1% 1-Pentene to Feed i n MAT 5

iC nC

5

5

t and C-2-C = iso-C = 5

5

ic

6

nC n-C = iso-C = 6

6

6

6

1.2 1.2 1.1 1.1 1.1 1.0 1.0 1.0 1.0

E f f e c t of Additives on Iso/Normal P a r a f f i n Composition The e f f e c t of 40 S/A and 525 S/A additives on the iso/normal p a r a f f i n r a t i o s of the product gasoline was studied using both fresh and steam-deactivated samples. Iso/normal C -C p a r a f f i n ratios versus carbon number are shown i n Figure 5. If a major function of the additive i s to crack away n-paraffins, then one would expect the iso/normal r a t i o to be higher with additive than i n the base case. While t h i s i s found for C and C , with fresh additive the opposite was observed with C and C. Since C and C paraffins are more prone to crack than C and C , i t i s u n l i k e l y that an n-paraffin mechanism i s involved i n increasing iso/normal r a t i o s . 4

4

7

7

5

6

6

7

4

5

HYDROGEN TRANSFER INDEX TEST Relative hydrogen transfer a c t i v i t y can be determined using an HTI t e s t (11), where the index i s a measure of the degree of saturation i n the reaction product. The test determines the product r a t i o of 3-methylpentenes to 3-methylpentane derived from a 1-hexene feed. While the branched products come mainly from oligomerization followed by cracking, the results should be relevant here as well. The higher the index, the lower the r e l a t i v e hydrogen transfer activity. Figure 6 shows the HTI values for a number of catalysts as a function of l i n e a r hexene conversion. Ultrastable Y was very active for H-transfer, with dealuminated H-mordenite somewhat l e s s . Although i t s S i 0 / A l 0 r a t i o was near that of the mordenite, ZSM-5 of 78 S/A catalyzed even less H-transfer, possibly r e s u l t i n g from the l i m i t e d space i n the ZSM-5 c a v i t i e s for forming the bimolecular 2

2

3



7.

MILLER AND HSIEH

0.6 L J 4


1 5

1 6

Carbon Number Figure 5.

Iso/normal r a t i o of paraffins r e l a t i v e to base.


101

L 7



Figure 6.

Hydrogen transfer

index test r e s u l t s .


7.

MILLER AND HSIEH


103

H-transfer reaction complex (12). ZSM-5 of 2200 S/A had the highest index with very l i t t l e H-transfer even at high conversion. PILOT PLANT TEST Comparisons were made using a c i r c u l a t i n g p i l o t plant f o r the following cases:


a. b. c.

Base case (equilibrium cracking c a t a l y s t ) . Base plus 40 S/A. Base plus 525 S/A.

Reactor conditions were 950°F and 5 c a t a l y s t / o i l . The conversion below 430°F was 60 wt %. Before use, the additives were steam deactivated f o r f i v e hours at 1450°F i n 100% steam. The incremental differences i n y i e l d and measured octanes produced by the additives are shown graphically i n Figures 7 and 8, respectively. These show that both additives reduced gasoline y i e l d , mainly 265-430°F, with 525 S/A giving the least reduction. Most of the y i e l d loss i n both cases was accompanied by gains i n C -C o l e f i n s . These also show an octane gain/wt % y i e l d loss i n the C -430°F gasoline of 0.6 (1.0/1.7) f o r 40 S/A and 1.0 (0.9/0.9) for 525 S/A. Compositional information on the product C -C fractions and calculated research octanes are given i n Table I I . The C -C f r a c t i o n s a l l show higher iso/normal p a r a f f i n r a t i o s with the additives than i n the base case. The additives also give higher o l e f i n i c i t y , p a r t i c u l a r l y i n the C -C range, and higher iso/normal o l e f i n r a t i o s with reduction especially i n the r e l a t i v e amounts of normal 1-olefins. These factors a l l result i n higher octanes. 3

4

5

3

7

4

3

Table I I .

3

4

4

4

4

4

4

4

4

C =/C P 5

5

i-c =/c = ic =/c = 5

5

5

5

IC /nC C P RON C = RON 5

5

5

5

C =/C P 6

6

i-c =/c = ic =/c = 6

6

6

6

IC /nC C P RON C = RON 6

6

6

6

C =/C P 1-C =/C = 7

7

7

7

ic =/c = 7

7

IC /nC C A/C C P RON C = RON 7

7

7

7

7

7

+40

S/A

+525

S/A

5.05

4.89

1.56 0.23 0.24 3.83 1.44 0.11 0.54 6.97 88.4 98.8

1.92 0.21 0.28 4.06 1.72 0.085 0.62 7.39 88.6 98.9

1.83 0.22 0.27 4.08 1.59 0.089 0.60 7.78 88.8 98.9

1.01 0.067 0.49 6.06 70.0 93.9

1.07 0.049 0.58 6.41 70.1 94.5

1.11 0.050 0.57 6.72 70.4 94.5

0.90 0.074 0.597 9.6 0.225 53.1 84.6

0.92 0.073 0.685 10.8 0.232 54.0 85.1

0.91 0.069 0.680 12.0 0.223 54.7 85.6

4.18

3

C =/C P 1-C =/C = I-C =/C = I-C /nC

5

Product Inspections at 60% Conversion and 9 5 0 ° F Base

C =/C P

7



104


EUD 40 S/A

525 S/A

C5-265T

265°-430°F

Figure 7. Incremental p i l o t plant y i e l d s at 950 F and 60% conversion.

Dmn 40

S/A

525 S / A

C5-265RON

C5-265 MON

265-430 RON

265-430 MON

Figure 8. Incremental p i l o t plant octanes at 950°F and 60% conversion


7.


MILLER AND HSIEH

105

Looking at what happens within each carbon number group, one finds that for 40 S/A, the main contributor to octane enhancement i n the C 's i s increased o l e f i n i c i t y . While the o l e f i n i c i t y also increases with 525 S/A, nearly half the C octane increase with that additive comes from a higher iso/normal p a r a f f i n r a t i o . The o l e f i n i c i t y enhancement i s of a lesser degree i n the C 's and e s s e n t i a l l y n i l i n the C 's. Nevertheless, higher iso/normal r a t i o s contribute to octane improvement i n both carbon number f r a c t i o n s . Relative y i e l d s per volume of feed versus the base case are shown for each carbon number i n the C -265°F gasoline i n Figure 9. This shows that the 40 S/A additive increases C while reducing C -C . With 525 S/A, the C -C stays about the same, with some reduction i n C . Because the octane r a t i n g i n the C -265°F gasoline decreases with an increase i n carbon number, increasing C *s at the expense of higher b o i l i n g components increases octane even without a change i n the octane within an i n d i v i d u a l carbon number group. For the 40 S/A additive, i n fact, about half the octane increase comes from t h i s e f f e c t . Producing t h i s s h i f t , however, also involves a y i e l d l o s s . Most of the t o t a l gasoline y i e l d loss, both for 40 S/A and 525 S/A, i s found i n the heavy gasoline (265-430°F). Table III shows t h i s loss to be spread throughout the carbon number range of t h i s f r a c t i o n , due i n large part to a reduction i n p a r a f f i n s as indicated by a higher r a t i o of aromatics to p a r a f f i n s . This increased aromaticity contributes to higher octane. 5

5

6

7

5


5

6

8

5

7

8

5

5

Table I I I .

265-430°F Gasoline Composition

Additive

40 S/A

Carbon No. Relative to Base C C C+

0.950 0.956 0.930

Wt Aromatics/Paraffins Relative to Base C* C C

1.12 1.14 1.22

8

9

10

8

9

10

•Includes C

8

525

S/A

0.966 0.965 0.946

1.04 1.06 1.18

i n C -265°F f r a c t i o n . 5

DISCUSSION Based on the findings that the ZSM-5 additives do not catalyze much conversion of gasoline range paraffins, we conclude that they primarily work on o l e f i n s produced by the large-pore cracking c a t a l y s t . We can divide these o l e f i n s into two main groups, those that are so small, e.g., C -C =, that further cracking can be neglected, and larger o l e f i n s where further cracking i s s i g n i f i c a n t . In looking at the f i r s t case for a C o l e f i n , for example, we consider the main reactions that can occur to the normal o l e f i n once i t has been protonated at a Bronsted s i t e to form a normal carbenium ion. As shown i n Figure 10, one reaction i s the isomerization of the normal carbenium ion. We can neglect the reverse reaction since the iso/normal o l e f i n r a t i o produced over the cracking c a t a l y s t i s usually well below the thermodynamic equilibrium value, due i n part to depletion of the i s o o l e f i n s by hydrogen transfer. A second reaction i s oligomerization with another o l e f i n to produce a larger branched carbenium ion. E a r l i e r work on normal 4

5

5


106


alphaolefin oligomerization over ZSM-5 type catalyst at lower temperature showed very l i t t l e skeletal isomerization of the unconverted feed o l e f i n , but substantial production of isomerized species through oligomerization and cracking (11). If the carbenium ion formed by oligomerization i s larger than C +, i t can then crack to form a d i f f e r e n t o l e f i n and a branched carbenium ion, e.g.: 5

+

+H

c = + c= —> Downloaded by UNIV OF CALIFORNIA SANTA BARBARA on October 8, 2015 | http://pubs.acs.org Publication Date: January 23, 1991 | doi: 10.1021/bk-1991-0452.ch007

4

7

+H"

i i V —>

c= + ic 5

+ 6

-—>

ic

If

ic

6

L

The MAT test with 1-pentene spiked feed did not show an increase i n carbon numbers other than C , leading to the conclusion that isomerization predominates over oligomerization at high temperatures, so that i s o o l e f i n s come mainly from isomerization, and from cracking of larger o l e f i n s which are not products of o1igomerizat ion. The main competing reaction to isomerization of the n-carbenium ion i s i t s hydrogen transfer to the n-paraffin which should then be r e l a t i v e l y unreactive. By maximizing the r e l a t i v e rates of isomerization to hydrogen transfer (k,/k ), the y i e l d of branched products, e s p e c i a l l y branched o l e f i n s , should also be maximized. The s i t u a t i o n i s more complex with a larger carbenium ion, for example, a C (Figure 11). In t h i s case, we consider not only isomerization and hydrogen transfer, but cracking as well, p a r t i c u l a r l y of the iso carbenium ion to form a smaller carbenium ion and an o l e f i n . Where the r a t i o of cracking rate to hydrogen t r a n s f e r rate i s high, the y i e l d of branched C products w i l l be reduced, rather than enhanced, due to conversion of i C to lower molecular weight species. The low iso/normal C -C p a r a f f i n r a t i o s observed i n the MAT t e s t i n g with fresh additive i s then the result of iso-C and -C carbenium ion cracking, reducing the amount of these species available for conversion to isoparaffins by H-transfer. C and C carbenium ions are too small to crack, requiring the formation of a primary ion, so that C and C isoparaff ins increase due to cracking of C -plus ions. For deactivated additive, the C -C iso/normal r a t i o s are higher than for the base catalyst, since the cracking a c t i v i t y of the additive i s reduced, and since H-transfer a c t i v i t y for, e.g., nC > nC i s lower than for the base, the 525 S/A additive having less H-transfer a c t i v i t y than the 40 S/A. Minimizing hydrogen transfer minimizes the formation of low octane n-paraffins. For optimum s e l e c t i v i t y , therefore, an additive catalyst should have a high r a t i o of isomerization a c t i v i t y compared to hydrogen transfer a c t i v i t y . The high HTI of ZSM-5 type z e o l i t e s compared to larger pore zeolites confirms the usefulness of these z e o l i t e s as octane additives. Assuming that isomerization of the normal carbenium ion i s a unimolecular reaction while H-transfer i s bimolecular (13) would explain why the HTI increases with the S i 0 / A l 0 r a t i o of the ZSM-5 type sieve. In improving gasoline octane, cracking such as: 5

H

+

7

7

+

7

6

7

6

7

4

4

5

5

7

6

7

+

7

2

2

7

3

iC = 8

> C= 3

+ iC = 5

can improve octane, mostly by producing higher octane, lower molecular weight i s o o l e f i n s , and by concentrating more r e f r a c t o r y high octane components ( i . e . , aromatics) i n the C -plus f r a c t i o n . This mechanism has the drawback, however, of reducing the l i q u i d C -plus y i e l d . As the additive ages, t h i s effect should diminish, p a r t i c u l a r l y involving the cracking of smaller species such as C . 7

5

+

7


MILLER AND


HSIEH

107

1.15 •40 S/A : 525 S/A

1.10

2

1.05


>1.00

> 0.95 I DC

0.90

0.85

0.80

Carbon Number Figure 9. Relative carbon number y i e l d s i n C -265°F gasoline versus base case i n p i l o t plant t e s t . 5

nC

nCs

5

iC

+

nC5

5

+

1C5

=

iCs

=

Figure 10. Normal C carbenium ion reactions. 5

nC

7

+

ki

»

iC

/ C3= + i C Figure 11. Normal C

7

4

+

7

+

\ iCy

i C

7

=

carbenium ion reactions.


108


The octane upgrade w i l l not be as great as with fresh c a t a l y s t , since a substantial f r a c t i o n of the i C carbenium ions w i l l undergo H-transfer t o form C isoparaffins which have octane numbers lower than the rest of the gasoline. Isomerization a c t i v i t y w i l l also decline during aging, but not as fast as cracking, so that there w i l l s t i l l be formation of branched products. Since t h i s involves no y i e l d loss, and since the isoparaffins and i s o o l e f i n s have higher octanes than t h e i r respective normals, the delta octane/% y i e l d loss w i l l be greater than with fresh additive. Since the r a t i o of isomerization a c t i v i t y t o H-transfer a c t i v i t y increases with Si0 /Al 0 r a t i o , the s e l e c t i v i t y for octane enhancement w i l l improve at higher r a t i o s . The t o t a l octane gain per unit of additive w i l l drop at very high Si0 /Al 0 , however, due to reduced cracking of lower octane C -plus species. This factor may be o f f s e t by increasing additive concentration. 7

7


2

2

3

2

2

3

7

SUMMARY FCC additives of the ZSM-5 type are e f f e c t i v e for increasing gasoline octane due to t h e i r low a c t i v i t y for hydrogen transfer. Octane i s increased by the cracking and isomerization of o l e f i n s produced by the base cracking catalyst. Cracking converts low octane components i n the C -plus gasoline partly t o higher octane components of lower molecular weight, and to C -minus, reducing gasoline y i e l d . Isomerization increases the concentration of i s o o l e f i n s and isoparaffins, increasing octane without y i e l d l o s s . Isomerization i s maximized and, therefore, y i e l d loss minimized, as the z e o l i t e Si0 /Al 0 r a t i o increases. While catalyst deactivation also improves s e l e c t i v i t y , the advantage for a high S i 0 / A l 0 r a t i o i s retained. 7

4

2

2

3

2

2

3

L i t e r a t u r e Cited 1. 2. 3. 4. 5. 6. 7. 8. 9. 10. 11. 12. 13.

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