Zeolite ZSM-5 in Fluid Catalytic Cracking - American Chemical Society

Chapter 5

Zeolite ZSM-5 in Fluid Catalytic Cracking: Performance, Benefits, and Applications

Downloaded by UNIV OF CINCINNATI on November 10, 2014 | http://pubs.acs.org Publication Date: September 12, 1988 | doi: 10.1021/bk-1988-0375.ch005

P. H. Schipper, F. G. Dwyer, P. T. Sparrell, S. Mizrahi, and J. A. Herbst Paulsboro Research Laboratory, Mobil Research and Development Corporation, Paulsboro, NJ 08066 The demonstrated performance of ZSM-5 in over 35 cracking units is reviewed. The main features of ZSM-5 are its high activity and stability, favorable selectivity, metals tolerance and flexibility, particularly when used as an additive catalyst. ZSM-5 cracks and isomerizes low octane components in the naphtha produced by the faujasite cracking catalyst. As a result C and C olefins are produced and gasoline compositional changes occur which explain its increased research and motor octanes. A model was developed which predicts ZSM-5 performance in an FCC unit. Discussed is how this model calculates ZSM-5 activity, gasoline research and motor octane increases and catalyst management policies. Also discussed are ways to utilize ZSM-5 and how its use permits reoptimization of not only the FCC unit but the entire refinery. 3

4

There have been many advances i n c a t a l y t i c cracking catalysts over the past 25 years s t a r t i n g with the introduction of rare earth exchanged X and Y z e o l i t e s (REX, REY) into Thermofor C a t a l y t i c Cracking (TCC) and F l u i d C a t a l y t i c Cracking (FCC) catalysts (1-3). Another s i g n i f i c a n t cracking catalyst improvement was the introduction of CO oxidation promoters (4-6). More r e c e n t c a t a l y s t improvements have i n c l u d e d use of the hydrogen form of Y , u l t r a s t a b l e Y (USY) (jO , c h e m i c a l l y d e a l u m i n a t e d Y and s i l i c o n e n r i c h e d Y z e o l i t e s ( 8 ) . The l a t e s t advance i n c r a c k i n g c a t a l y s t s has been the i n t r o d u c t i o n of ZSM-5 which i s the f i r s t g e n e r i c z e o l i t e change i n c a t a l y t i c c r a c k i n g .

ZSM-5 i s a Mobil-proprietary, shape-selective z e o l i t e which i s used commercially i n synthetic fuels (methanol-to-gasoline), petrochemicals (xylene isomerization, toluene disproportionation, benzene alkylation) and i n petroleum r e f i n i n g (lube and 0097-6156/88/0375-0064$06.75/0 β

1988 A m e r i c a n Chemical Society

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


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65

d i s t i l l a t e dewaxing) (9). I t has also been used i n c a t a l y t i c cracking to increase gasoline Research and Motor clear Octane numbers (RON and MON) and gasoline plus alkylate y i e l d . The f i r s t f u l l - s c a l e refinery t e s t demonstration of ZSM-5 i n c a t a l y t i c cracking was made i n the TCC unit of the Neste Oy refinery i n Naantali, Finland (10). The catalyst used i n t h i s application was a composite catalyst containing both REY and ZSM-5. This commercial demonstration was a success with RON increasing up to 4 numbers. Since t h i s i n i t i a l demonstration, ZSM-5 has been used i n over t h i r t y - f i v e units (both TCC and FCC) ranging i n size from 6,000 to 90,000 barrels per day (BPD). Table I l i s t s some of the available r e s u l t s from twenty of these commercial applications. In a l l cases, ZSM-5 increased both Research and Motor Octane. The ZSM-5 content i n the unit inventory f o r these applications ranged from 0.2 to 3 wt %. The variations i n the octane response from a given concentration of ZSM-5 are due t o variations i n the base octane (octane without ZSM-5), gasoline cut point, catalyst makeup rate and regenerator temperature. In FCC applications, ZSM-5 has been used primarily i n the form of a high concentration, separate p a r t i c l e additive, since t h i s method affords maximum f l e x i b i l i t y . However, i t has also been successfully employed as a composite catalyst containing both ZSM-5 and the f a u j a s i t i c cracking component i n the same p a r t i c l e (11). Detailed r e s u l t s of demonstrations i n FCC units belonging to Oklahoma Refining i n C y r i l , Oklahoma and EniChem Anic i n Gela, I t a l y have been published (11-14). These demonstrations have shown that the target octane can be achieved within 1 to 7 days by accelerating the i n i t i a l addition of a ZSM-5 additive c a t a l y s t . In contrast, octane catalysts containing Y z e o l i t e s with reduced unit c e l l size can generally take several months to boost octane, since they require s i g n i f i c a n t turnover of c a t a l y s t inventory. Alternative methods, such as increasing r i s e r top temperature to increase octane, are not as a t t r a c t i v e as using ZSM-5 because the former increases C ~ (H +CH +C H +C H ) gas and coke make and t h i s can p o t e n t i a l l y l i m i t FCC throughput. Thus, for a given octane boost, ZSM-5 w i l l produce less wet gas than r a i s i n g r i s e r top temperatures. This allows the FCC unit to be re-optimized so that higher feed rates and/or conversions can be achieved at constant wet gas, while s t i l l improving the o v e r a l l pool octane. As a r e s u l t of these considerations and i t s commercial success, ZSM-5 usage has now taken i t s place among the least expensive octane enhancement methods available to r e f i n e r s 2

4

2

4

2

(is=iz). In addition to i t s octane enhancement a b i l i t y described above, ZSM-5 also increases the feed to a l k y l a t i o n , methyl t e r t i a r y butyl ether (MTBE) and t e r t i a r y amyl methyl ether (TAME) units. Since the products from a l l these processes contain high Research and Motor Octane components, ZSM-5 provides the r e f i n e r additional f l e x i b i l i t y i n h i s downstream processing whenever the need exists to increase o v e r a l l gasoline pool octane. In addition, the o v e r a l l r e f i n e r y can be rebalanced to take


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


advantage of the benefits a r i s i n g from the use of ZSM-5; p a r t i c u l a r l y , the higher f r a c t i o n of reformate and alkylate i n the o v e r a l l gasoline pool. One example i s to reform the FCC heart cut, i . e . , the mid-boiling range, naphtha. ZSM-5 enhances reforming of an FCC heart cut gasoline by increasing the octane of the remaining FCC gasoline and also increasing the f r a c t i o n of alkylate and reformate i n the o v e r a l l pool gasoline. In t h i s paper, we discuss how ZSM-5 works to increase octane i n an FCC unit, the development of a model t o predict ZSM5 performance, and methods t o optimize the FCC unit and o v e r a l l refinery with ZSM-5. Table I . Summary of Commercial Performance of ZSM-5 i n C a t a l y t i c Cracking

Refinerv A Β C D £ F 6 H I J Κ L M Ν 0 Ρ Q R S Τ

< RON—> Base Δ 91.5 +1.5 93.4 +0.9 91.8 +1.0 86.0 +4.5 87.3 +1.7 87.6 +1.6 89.8 +2.2 88.5 +1.5 91.5 +1.2 92.6 +1.9 +0.7 93.4 92.5 +0.9 92.5 +1.4 92.5 +1.0 +1.0 92.0 +0.7 92.5 92.7 +1.2 +0.8 91.4 88.6 +1.6 91.0 +1.2

Δ +0.6 +0.3 +0.3 +2.2 +0.6 +1.2 N/A N/A +0.5 +0.7 +0.3 +1.1 +0.9 +0.4 +0.4 +0.5 +0.5 N/A N/A +0.7

Estimated ZSM-5 i n In ventory, wt % 2.8 2.0 0.2 2.2 0.9 0.3 1.2 0.8 0.5 1.5 2.4 0.2 2.2 2.4 0.2 0.2 3.0 1.5 2.2 2.2

How ZSM-5 Works ZSM-5 increases octane by c a t a l y t i c a l l y cracking and isomerizing low octane components i n the gasoline b o i l i n g range t o higher octane components plus propylene and butylène. ZSM-5 accomplishes t h i s s e l e c t i v e l y , without increasing coke y i e l d or C ~ gas make. In addition, conversion and l i g h t cycle o i l (ECO)/main column bottom (MCB) s e l e c t i v i t i e s remain unchanged. In contrast, increasing octane by r a i s i n g reactor top temperature s i g n i f i c a n t l y increases C ~ gas y i e l d s which p o t e n t i a l l y l i m i t FCC unit throughput or severity. Several commercial examples of the y i e l d and octane s h i f t s for ZSM-5 addition are i l l u s t r a t e d i n Tables I I through VI. In 2


5.

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67


a l l cases, the same type ZSM-5 additive c a t a l y s t was used. Also, operating conditions and feedstock remained constant f o r each example. In the examples i n Table I I , the octane gains are accompanied by an increase i n C and o l e f i n s (potential alkylate feed) and a decrease i n gasoline y i e l d . The increase i n p o t e n t i a l a l k y l a t e more than compensates f o r the l o s s i n gasoline y i e l d so that o v e r a l l FCC plus p o t e n t i a l a l k y l a t e gasoline y i e l d increases. The increase i n a l k y l a t e increases the r e f i n e r y pool motor octane because a l k y l a t e has a higher motor octane blending number. Table I I also i l l u s t r a t e s that as the base octane increases, more gasoline plus p o t e n t i a l a l k y l a t e i s produced f o r approximately the same FCC gasoline octane increase. As shown i n the three examples, ZSM-5 does not a f f e c t the d i s t i l l a t e or bottoms s e l e c t i v i t y ; nor does i t a f f e c t C ~ or coke y i e l d . We can gain an understanding of the gasoline compositional s h i f t s which occur with ZSM-5 by studying the examples shown i n Table I I I . The gasoline octane increase with ZSM-5 r e s u l t s from a decrease i n concentration of the C_+ p a r a f f i n s and o l e f i n s , and an increase i n the concentration of Doth l i g h t o l e f i n s and aromatics. The concentration of the l i g h t o l e f i n s i s s e l e c t i v e l y increased t o the s i n g l e and multiple branched l i g h t o l e f i n s , a l l high octane species (Table IV), with an actual reduction i n the concentration of l i n e a r C_ and C o l e f i n s . A larger increase i s observed f o r s i n g l e brancned o l e i i n s when compared t o multiple branched o l e f i n s . To understand the reaction pathways, the y i e l d s h i f t s f o r the three examples i l l u s t r a t e d i n Table I I I were calculated on a fresh feed basis (Table V). These data show that the predominant reaction i s the loss of C + p a r a f f i n s and o l e f i n s . Approximately 2.5 wt % C_+ p a r a f f i n s plus o l e f i n s were l o s t f o r a +1.5 Research Octane numter increase. ZSM 5 i s s e l e c t i v e t o cracking both single branched and l i n e a r p a r n f f i n s , and single branched and l i n e a r o l e f i n s (9) which have very low Research and Motor Octanes, as i l l u s t r a t e d below:


3

2

&

?

n-octene methyl-hexanes

RON 57 42-52

MON 58 46-55

In general, the higher the i n i t i a l concentration of C_+ o l e f i n s i n the gasoline, the larger the rate of cracking of tne C + olefins. The products of the ZSM-5 reaction i n the gasoline b o i l i n g range are mainly C ~C s i n g l e branched o l e f i n s which have high Research and Motor Octane, as i l l u s t r a t e d below: ?

5

Q

2M2C = 4

RON 100

MON 83


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F L U I D C A T A L Y T I C C R A C K I N G : R O L E IN M O D E R N

REFINING

The data i n Table I I I show that the concentration of aromatics i n the ZSM-5 gasoline f r a c t i o n increases. As the data i n Table V show, the increase i s not due to aromatics formation, but rather due to the concentration of the aromatics i n a smaller amount of gasoline. As discussed previously, the ZSM-5 cracking products which occur outside the gasoline b o i l i n g range are mainly CL and C o l e f i n s . The incremental C o l e f i n y i e l d shows a higher percentage of isobutylene, s i m i l a r to the d i s t r i b u t i o n s found i n the C and C o l e f i n s (Table IV). 4

4

5

e


% of Incremental C

4

Olefins Using ZSM-5

i C = 1 C= cis-2-C = trans-2-C = 4

4

4

40 20 20 20

This increase i n isobutylene y i e l d increases feed f o r downstream MTBE processing. In general, the higher the concentration of C_+ o l e f i n s and paraffins i n the base gasoline, the lower i s the fcase Research and Motor Octane. Thus, without detailed information about the gasoline composition, the base octane can be used to characterize the gasoline. As observed i n Figure 1, when the base octane (both Research and Motor) i s lower, less ZSM-5 i s required to achieve a given octane increase. This observation i s not s u r p r i s i n g because the lower base octane gasolines contain a higher concentration of low octane components. At any given a c t i v i t y , ZSM-5's upgrading rate depends on the amount (concentration) of low octane components available. In addition, f o r a given octane boost, the increase i n gasoline plus p o t e n t i a l alkylate y i e l d also depends on the base octane. As the base octane increases, the formation of gasoline plus alkylate increases (Figure 2). This r e s u l t s from a lower concentration of C + paraffins and o l e f i n s i n the high base octane gasolines, as these components contribute less to the o v e r a l l octane. To obtain a 1 number octane increase f o r an i n i t i a l l y high octane gasoline, a greater s h i f t i n the C + paraffins and olefins'"would be required. Consequently, more l i g h t o l e f i n s f o r a l k y l a t i o n are produced. ZSM-5 increases the octane throughout the entire gasoline b o i l i n g range (Figure 3). The magnitude of the octane boost depends on the base gasoline composition and the cut point of the gasoline. In the example i l l u s t r a t e d i n Figure 3, ZSM-5 increases the octane of the 120*°C gasoline f r a c t i o n by 2 to 2.5 MON and 2 to 3 RON and the 120"°C f r a c t i o n by about 0.5 to 1.0 MON and 1 to 2.5 RON. This gives an o v e r a l l gasoline octane boost of +1 MON and +1.5 RON. The octane of the 120 C f r a c t i o n increases because the concentration of low octane components (C + paraffins and o l e f i n s ) decreases. The octane of the front end (120"°C) f r a c t i o n increases because the concentration of high octane components, f o r example, single branched C and C olef ins, increases. 7

?

+o

7

g

e



5.

SCHIPPER ET

AL.


69

In general, the octane boost i s greatest i n the f r a c t i o n of the gasoline with the lowest base octane (Table VI). Examples 1 and 3 i n Table VI show the base octane to be higher i n the l i g h t e r portion of the gasoline. In these examples, the boost i n octane increases with the gasoline cut point. The base octane i n Example 2 i s higher i n the heavier portion of the gasoline. In t h i s example, the octane boost decreases with gasoline cut point. The base octane i n Example 4 i s e s s e n t i a l l y constant with respect to b o i l i n g range. ZSM-5 has been used successfully i n commercial operations when processing high b o i l i n g range feedstock and resids. This i s p r i n c i p a l l y due to i t s a b i l i t y to maintain a c t i v i t y despite the presence of a high concentration of feed metals. ZSM-5's excellent metals tolerance has been demonstrated commercially at equilibrium c a t a l y s t metals levels up to 10,000 ppm n i c k e l plus vanadium and 6,000 ppm sodium with very l i t t l e detrimental e f f e c t . Laboratory tests show that ZSM-5 i s f a r less affected by metals than Y-zeolite c a t a l y s t s . Metals were introduced, as follows : - Nickel: Nickel naphthenate impregnation followed by a i r calcination. - Vanadium: Steam treatment i n presence of vanadium pentoxide. - Sodium: Sodium n i t r a t e impregnation. E s s e n t i a l l y no change i n octane enhancing performance of the ZSM5 catalyst was observed after the catalyst was treated with these metals (Table V I I ) . Table I I .

ZSM-5/FCC Performance Examples

Base Octane, RON MON Y i e l d S h i f t s . % Fresh Feed C5+ Gasoline, v o l % LCO, v o l % MCB, v o l % iC4, v o l % nC4, v o l % C4=, v o l % C3, v o l % C3=, v o l % C2 Minus, wt % Coke, wt % Alkylate (C = & C =), v o l % Gaso+Potential Alkylate, v o l % 3

Octane S h i f t s FCC Gasoline, RON MON

87.3 78.2

91.5 80.6

92.7 80.3

-1.6

-2.3

-3.7

+0.3

+0.2

+0.3

+1.1 +0.8

+1.2 +0.1 +1.6

+1.5 +0.5 +2.6

+3.2 +1.6

+4.7 +2.4

+6.9 +3.2

+1.7 +0.6

+1.2 +0.3

+1.5 +0.5


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Table I I I .

Gasoline Compositional S h i f t s with ZSM-5

Example Base Research Octane Δ RON

A 92.0 +1.5

Β 92.7 +1.5

C 91.4 +1.3

+1.0 -1.3

0 -0.9

-0.7 -2.5

+2.0 -5.6 +1.3 +3.4

+2.6 -3.2 0 +1.5

+2.0 -0.6 +0.6 +1.2

S h i f t s with ZSM-5. wt % Paraffins

V Oleiins

V Naphthenes Aromatics

Table IV.

Light O l e f i n D i s t r i b u t i o n Indicates Increased Branching with ZSM-5 % of Incremental Olefins Increase

Example

Β

C

s

V2

and 3M1C = 2M2C = Straight Chain C = g

V

50 68 -18

34 80 -14

75 0 -175

127 11 -38

s

Single Branched C = Multiple Branched C = Straight Chain C = g

4


5.

SCHIPPER E T AL.

71



3

R E L A T I V E ZSM-5 ACTIVITY

Figure

1. Impact

89

o f base octane on ZSM-5 o c t a n e b o o s t .

91

93

BASE OCTANE

F i g u r e 2. Impact

o f base o c t a n e on ZSM-5 y i e l d - o c t a n e r e s p o n s e .


F L U I D C A T A L Y T I C C R A C K I N G : R O L E IN M O D E R N


72


REFINING

5.

S C H I P P E R E T AL.


73

Table V. ZSM-5 Reactions - Y i e l d S h i f t s Based on Fresh Feed

Example Base Research Octane ARON

A 92.0 +1.5

Β 92.7 +1.5

C 91.4 +1.3

+0.1 -0.6

-0.5 -0.6

-0.6 -1.7

+0.3 -2.2 +0.1 +0.3 -2.0

+0.4 -2.0 -0.2 -0.1 -3.0

+0.7 -0.6 +0.1 -0.2 -2.3

1.5

2.0

0.6

Y i e l d S h i f t s . Wt % Fresh Feed Paraffins


Olefins

Naphthenes Aromatics Total 0/P of Base Gasoline

Table VI. E f f e c t of FCC Gasoline Cut Point and Base Octane on ZSM-5 Octane Boost

Gasoline 90%. °C 91 170

Base RON 94.3 91.5

ZSM-5 RON Boost +0.5 +1.5

2

155 195

90.4 91.5

+2.3 +1.2

3

155 185

94.4 93.3

+0.7 +1.0

4

155 195

92.8 93.0

+1.7 +1.5

Example 1

Table VII.

Metal/Content Base Ni/2000 ppm V/10000 ppm Na/5000 ppm

Metals Tolerance of ZSM-5 Laboratory Data

ARON +1.2 to +1.6 +1.8 +1.7 +1.0

AGasoline, V o l % -2 to -2.5 -3 -2 -1


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FLUID C A T A L Y T I C C R A C K I N G : R O L E IN M O D E R N REFINING

Predicting ZSM-5 Performance A model which i s consistent with the chemistry described previously has been developed to predict the performance of ZSM-5 i n an FCC unit. Application of t h i s model allows the user to take f u l l advantage of ZSM-5's f l e x i b i l i t y f o r s p e c i f i c applications. The model has been used i n many commercial applications t o determine the catalyst makeup rate required to achieve a given octane boost. I t has also been used to t a i l o r the catalyst makeup strategy to obtain a desired octane boost i n a given period of time.


•

Model Development

The model i s divided into two parts; the c a l c u l a t i o n of ZSM-5 concentration required to achieve a given a c t i v i t y i n the unit ( a c t i v i t y maintenance) and the impact of base gasoline composition on the gasoline octane boost f o r ZSM-5 of a given activity. The a c t i v i t y maintenance of ZSM-5 follows the same functional form as that f o r normal REY/USY cracking catalysts (18). Using t h i s format, ZSM-5 a c t i v i t y (the increase i n C plus C. o l e f i n s ) can be expressed i n terms of i t s age, the regenerator temperature and steam p a r t i a l pressure according to the following equation: 3

= -K,, P

M w

a

N

where Kj, = K

Do

exp (-E /RT)

(1)

D

The deactivation order, N, the i n i t i a l a c t i v i t y , a , the exponent, M, which describes the e f f e c t of steam p a r t i a l pressure, the a c t i v a t i o n energy, E , and the a c t i v i t y rate constant, I L , were determined from both p i l o t plant and commercial data. In a commercial unit, catalyst i s added continuously r e s u l t i n g i n an age d i s t r i b u t i o n of the catalyst i n the unit. Therefore, the o v e r a l l ZSM-5 a c t i v i t y i s a d i s t r i b u t i o n of the ZSM-5 a c t i v i t i e s and i s related to t h e i r r e l a t i v e ages. To calculate the average ZSM-5 a c t i v i t y , the residence time d i s t r i b u t i o n and rate expression are integrated over time. The steady state example i s given i n the following equation: 0

D

(1/1-N) (1-N) K,

1 τ

e

-t/r , ' dt

(2)

where a i s the average a c t i v i t y per f r a c t i o n of ZSM-5, f s M - 5 ' the inventory: Z


75


SCHIPPER ET AL.

5.

The catalyst residence time, τ depends both on the ZSM-5 and base catalyst makeup rates ( Μ ,Μ ): Ζ 5 Μ 5


T

=

β

U

ZSM-6

I + M

W

8

The model assumes that the retentions of the base cracking catalyst and ZSM-5 additive catalyst are the same. So, by knowing the a c t i v i t y rate K^, the regenerator temperature, the ZSM-5 and base makeup rates, the water p a r t i a l pressure i n the regenerator, and the catalyst inventory, the a c t i v i t y can be calculated d i r e c t l y . We now need t o r e l a t e the a c t i v i t y of ZSM-5 t o the octane boost achievable commercially. As discussed previously, the octane boost at any given ZSM-5 a c t i v i t y depends on the base octane, which i s a c h a r a c t e r i s t i c of the concentration of low octane o l e f i n s and paraffins i n the gasoline. Figure 1 i l l u s t r a t e s the r e l a t i o n s h i p between octane boost and a c t i v i t y at several d i f f e r e n t base octanes. As can be seen from the curves, as the gasoline base octane increases, more ZSM-5 a c t i v i t y i s required to achieve a given octane increase. Thus, the p o t e n t i a l octane boost which can be obtained from ZSM-5 addition i s not only a function of i t s a c t i v i t y and the f r a c t i o n of ZSM-5 i n inventory, but i t also depends on i t s base gasoline octane (gasoline octane without ZSM-5 addition).

ARON = F(S, f •

Z S M

_ , RON ); ΔΜ0Ν = F(5, f 5

0

Z

S

M

^

MON ) 0

(5)

Parametric S e n s i t i v i t y of Model

As discussed above, the potential octane boost which can be achieved from ZSM-5 addition i s a function of f i v e parameters: the regenerator temperature and steam p a r t i a l pressure (which determine the a c t i v i t y maintenance); the base and ZSM-5 c a t a l y s t makeup rates (which determine the c a t a l y s t age); and the base gasoline octane. The s e n s i t i v i t y of the model to these parameters i s discussed below. Table VIII shows the influence of base octane on the r e l a t i v e f r a c t i o n a l ZSM-5 replacement rate required to achieve a 1 RON boost. Model estimates show that half as much ZSM-5 i s required to achieve a +1 RON/+0.4 MON boost at a base octane of 88 as compared t o an application with a base octane of 92. Table IX shows the e f f e c t of regenerator temperature on the r e l a t i v e f r a c t i o n a l ZSM-5 replacement rate required to achieve a 1 RON boost. The model prediction shows that ZSM-5 i s r e l a t i v e l y i n s e n s i t i v e t o FCC regenerator temperature. The ZSM-5 addition rate required to maintain a 1 RON boost increases with increasing base catalyst makeup rate, but the increase i s less than that which would be estimated due to d i l u t i o n alone. As the base catalyst makeup rate increases, the catalyst inventory turnover rate increases, causing the average


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FLUID CATALYTIC C R A C K I N G : R O L E IN M O D E R N

REFINING

catalyst age to decrease, and the ZSM-5 catalyst a c t i v i t y to increase. Since the ZSM-5 catalyst i s now more active, a lower concentration i s required to maintain a given octane boost. Therefore, the a c t i v i t y benefit r e s u l t i n g from the decreased age p a r t i a l l y compensates f o r the increase i n base catalyst makeup rate. Figure 4 shows as the base catalyst makeup rate increases from 1 to 1.5, the required r e l a t i v e f r a c t i o n a l ZSM-5 addition rate increases from 1 to 1.25 to maintain the 1 RON increase. For t h i s example, the r e l a t i v e f r a c t i o n a l replacement rate of ZSM-5 i n the inventory decreases form 1 to 0.85.


•

Model V e r i f i c a t i o n

The model has been v e r i f i e d by accurately predicting the octane boost f o r many d i f f e r e n t commercial applications. Figure 5 shows the model predictions versus measured data f o r two commercial applications. The objective of the application at Refinery A, was to increase the Research Octane by 1 number within 1 day. The base octane f o r t h i s application was 93.4 and the regenerator temperature was 1335°F. As the data show, the model matches the commercial response very well. The objective of the application at Refinery B, was to increase the Research Octane by 1.5 numbers within the f i r s t week. The base octane f o r t h i s application was 92.7 and the regenerator temperature was 1275°F. Again, the model prediction f i t s the commercial data quite well. •

Catalyst Makeup Strategy Options

One of the main applications of the ZSM-5 performance model i s to determine the optimal catalyst makeup strategy to achieve a desired octane boost i n a required period of time. Since ZSM-5 i s used primarily as an additive, the catalyst makeup policy can be t a i l o r e d to f i t the r e f i n e r ' s needs. Figure 6 shows two example makeup strategies to boost the Research Octane i n an FCC unit by 1 number. The f i r s t example shows that the addition rate can be accelerated i n i t i a l l y to achieve the octane boost i n one day and then i t can gradually be reduced to maintain the desired octane boost. In t h i s example, one unit of r e l a t i v e f r a c t i o n a l ZSM-5 replacement rate was added the f i r s t day to achieve a 1 RON boost. The addition rate was reduced to less than 0.2 units by the t h i r d day to maintain the octane boost. The second example shows that the catalyst can be added more gradually to achieve the desired octane boost within one week or even longer i f desired. In t h i s example, 0.4 units of r e l a t i v e f r a c t i o n a l ZSM-5 replacement rate were added i n i t i a l l y giving a 0.4 RON boost after the f i r s t day. The addition rate was gradually reduced to less than 0.2 units by the eighth day to maintain the 1 number RON boost achieved after seven days of ZSM-5 addition.


5.

SCHIPPER E T AL.


φ CO

α

φ


oc

C/3 Ν

φ ce

1.2

1.4

1.6

Relative B a s e C a t a l y s t R e p l a c e m e n t

Figure

1.5

4. E f f e c t

Ί

1

o f base

catalyst

1.8

Rate

makeup

rate.

Γ

1 Ο Refinery A

ce

.5

93.4 B a s e O c t a n e 1 3 3 5 °F R e g T e m p

I

0

4

5

6

I

I

8

9

10

Time. Days 2

I

I

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I

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I

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ce

r

1 .5 0

- / I \ \ \

I

• ·_

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Refinery Β

—

92.7 B a s e O c t a n e 1275 " F R e g T e m p 8

10

12

14

16

18

_

20 '

Time. D a y s

Figure

5. Model

f i t f o r commercial

trials.


22

78

FLUID C A T A L Y T I C C R A C K I N G : R O L E IN M O D E R N


I

I

I

I

I

I

I

I

I

I

I

I

I

I

I

REFINING

I

I

δ

RON

Ν

I

ZSM5 Makeup Rate 0

•r-r-fi-ri-ri 1 Γ Τ Τ Τ Τ ' Γ ' ί Τ Τ Τ 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 Time. Days I

I

I

I

I

I

l

I

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I

I

I

ι

ι a

,^

0

1 2

RON

Η

ZSM5 Makeup Rate

3

4

5

^ i > T - T r T - r - r - T - r i - n 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 Time. Days

F i g u r e 6. Make up s t r a t e g i e s t o boost r e s e a r c h o c t a n e . Top, r a p i d octane boost and bottom, g r a d u a l octane boost.


5.

SCHIPPER ET AL.


Table VIII.

79

ZSM-5 Model - Parametric S e n s i t i v i t y E f f e c t of Base Octane

Relative F r a c t i o n a l ZSM-5 Replacement Rate

RON

0


88 90 92

0.48 0.66 1.0

Table IX.

ARON

ΔΜ0Ν

+1.0 +1.0 +1.0

+0.4 +0.4 +0.4

Impact of Regenerator Temperature on ZSM-5 Replacement Rate

Regenerator Temperature Base Base less 30°C

Relative F r a c t i o n a l Replacement Rate of ZSM-5 Required t o Achieve a +1 RON Increase 1 0.91

Optimization with ZSM-5 •

FCÇ

As mentioned previously, because of i t s unique chemistry, ZSM-5 produces less wet gas per octane gain than increasing r i s e r top temperature (Figure 7). This happens because ZSM-5 does not increase C ~ y i e l d s . Thus, using ZSM-5 allows the r e f i n e r t o reoptimize FCC conditions to again reach a wet gas l i m i t . Several strategies, which allow the r e f i n e r t o take advantage of t h i s phenomena, are i l l u s t r a t e d below. At constant octane, the r e f i n e r can: 2

*

Increase FCC feed rate up to the wet gas l i m i t . The example i n Figure 7 indicates that ZSM-5 provides about 7% less wet gas at a constant octane. This translates into about 7% additional throughput that could be charged t o the FCC at constant octane using ZSM-5. This would s i g n i f i c a n t l y increase the octane barrels produced i n the refinery.


80

F L U I D C A T A L Y T I C C R A C K I N G : R O L E IN M O D E R N R E F I N I N G

•

Increase FCC conversion by decreasing r i s e r top and preheat temperature ( i f a i r blower capacity i s available) up to the wet gas l i m i t . Table X gives an example of the y i e l d benefits for using ZSM-5 i n t h i s manner.


The r e f i n e r can also: •

Increase FCC plus p o t e n t i a l alkylate octanes (Motor/Research) at a wet gas constraint by s l i g h t l y decreasing r i s e r top temperature and increasing preheat (Table XI). Because ZSM-5 increases the f r a c t i o n of alkylate feed r e l a t i v e to FCC gasoline, the FCC plus p o t e n t i a l alkylate gasoline blend has higher octanes.

•

Decrease butadiene y i e l d while increasing p o t e n t i a l alkylate feed. Unlike increasing r i s e r top temperature, ZSM-5 does not increase butadiene y i e l d with increasing octane (Table XII) (12-14). Butadiene make can negatively impact and ultimately l i m i t downstream a l k y l a t i o n capacity. Thus, C and C products from ZSM-5 can produce more a l k y l a t e product at the same e f f e c t i v e a l k y l a t i o n unit capacity. 3

•

Refinery

Not only does ZSM-5 increase FCC gasoline Research and Motor Octane and allow f o r optimization of the FCC operation, i t s unique chemistry allows for reoptimization of downstream processes to increase pool octane. Addition of ZSM-5 i n a c a t a l y t i c cracking unit can permit a reduction i n reformer severity. At a given pool octane requirement, the higher FCC octane contribution (Research/Motor) allows f o r lower reformer severity and, consequently, higher reformate y i e l d s . At EniChem's Gela FCC unit f12-14^. when ZSM-5 was i n the FCC, the reformate severity dropped from 95.0 RON clear to 90.0 RON c l e a r while l i q u i d y i e l d increased by 4% v o l . ZSM-5 also s i g n i f i c a n t l y enhances the octane gain achievable i f a heart cut or mid-boiling range FCC gasoline i s reformed. As Figure 8 shows, ZSM-5 increases the Motor Octane throughout the entire gasoline b o i l i n g range. However, there i s s t i l l a minimum i n Motor Octane with respect to b o i l i n g point. In t h i s example, the minimum occurs at 100°C to 150°C. A heart cut taken i n t h i s b o i l i n g range (100°C to 150°C) i s an a t t r a c t i v e feed f o r reforming since the nitrogen and s u l f u r l e v e l s are low i n t h i s b o i l i n g range. LP studies were run to determine the impact on the r e f i n e r y gasoline pool of adding ZSM-5 to the FCC and then reforming a heart cut of the FCC gasoline. Enough ZSM-5 was added to increase the FCC gasoline octane by +1 RON/+0.4 MON. A 100°C to 150°C heart cut of the FCC gasoline was then reformed, and a l l additional C and C o l e f i n s were alkylated. The impact on the refinery gasoline pool octane was an increase of 0.3 numbers f o r 3

4


SCHIPPER E T AL.


10 < RISER T O P Τ LLI


5 LU C/>

Zeolite ZSM-5 in Fluid Catalytic Cracking - American Chemical Society

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