Addition of shape selective zeolites to catalytic cracking units

Shape selective zeolites, such as ZSM-5, are added to the equilibrium catalyst in a catalytic cracking unit in large amounts. Sufficient fresh zeolite is added to result in an unacceptable increase in production of wet gas (C.sub.3 /C.sub.4) by the catalytic cracking unit, if operated at normal charge rates and operating conditions. To compensate, conditions in the catalytic cracking unit are adjusted, such as by reducing feed rate, catalyst/oil ratio, or cracking reaction temperature, to reduce the production of "light ends" to an amount which can be tolerated by the catalytic cracking unit. Within a few days to a few weeks, excessive production of wet gas will diminish so that unit operation may be returned to normal. Resumption of normal cracking operations using a catalyst inventory containing a relatively large amount of shape selective zeolite additive results in significant increases in the octane number of the catalytically cracked gasoline.

BACKGROUND OF THE INVENTION 
The invention is an improvement in the catalytic cracking process. 
Catalytic cracking of hydrocarbons with zeolite-containing catalyst is a 
well-known process. 
In fluidized catalytic cracking, the hydrocarbons contact fluidized 
catalyst and are cracked to lighter products. 
The catalyst is deactivated by coke deposition, necessitating regeneration 
of coked catalyst in a regenerator. 
Zeolitic materials, both natural and synthetic, have been demonstrated in 
the past to have catalytic properties for various types of hydrocarbon 
conversions. Certain zeolites are ordered, porous crystalline 
aluminosilicates having a definite crystalline structure within which 
there are a large number of smaller cavities which may be interconnected 
by still smaller channels. These materials are known as molecular sieves. 
Many methods have been developed for preparing a variety of synthetic 
aluminosilicates. These aluminosilicates have come to be designated by 
letter or other symbol, e.g., Zeolite A (U.S. Pat. No. 2,882,243), Zeolite 
X (U.S. No. 2,882,244), Zeolite Y (U.S. No. 3,130,007), and ZSM-5 (U.S. 
No. 3,702,886), merely to name a few. 
ZSM-5 is a particularly interesting zeolite. Much work has been reported in 
the patent literature on adding ZSM-5 to the conventional cracking 
catalyst in a catalytic cracking unit. 
In U.S. Pat. No. 3,758,403, from 2-1/2 to 10 wt. % ZSM-5 catalyst was added 
to a conventional cracking catalyst containing 10 percent REY, the 
remainder being Georgia clay. Examples were given showing use of 1.5, 2.5, 
5 and 10 wt. % ZSM-5 added to the conventional cracking catalyst. 
The shape selective zeolites, e.g., ZSM-5, increase production of LPG 
(C.sub.3 /C.sub.4 olefins) in cracking units and cause some loss of 
gasoline yield, and an increase in octane number. 
ZSM-5 catalyst, especially virgin catalyst, has exceedingly high activity. 
Researchers have attempted to take advantage of the super activity of 
fresh ZSM-5 catalysts by adding only small amounts of it to FCC catalyst. 
Such work is shown in U.S. Pat. No. 4,309,280. This patent taught adding 
very small amounts of powdered, neat ZSM-5 catalyst, characterized by a 
particle size less than 5 microns. 
This patent taught that adding as little as 0.25 weight percent ZSM-5 
powder to the circulating catalyst inventory in an FCC unit would increase 
wet gas or LPG production by 50 percent. 
Because of the high initial activity of the ZSM-5 catalyst, and increased 
wet gas production due to the addition of large amounts of ZSM-5, refiners 
are reluctant to start up a unit with, or add quickly to a unit, a 
relatively large amount of virgin ZSM-5 catalyst. 
If only very small amounts of ZSM-5 catalysts are added, there will be no 
problem with excessive wet gas production, but the effect of adding ZSM-5 
may not be apparent for days or weeks, by which time normal variations in 
feed, or unit operation, may obscure the octane enhancing affect of ZSM-5 
addition. 
A good way to add a modest amount of ZSM-5 to an FCC unit is disclosed in 
EP 0168979, published European Application No. 85304367.7 (F-2955 EPO). 
The method involves adding ZSM-5 catalyst to the equilibrium catalyst in 
an FCC unit in a programmed manner so that an immediate boost in gasoline 
product octane number, typically 178 -2 octane number, is achieved. 
Although the method works well in practice, it requires careful monitoring 
of ZSM-5 addition over a period of many days, and results in only a modest 
amount of ZSM-5 catalyst being added to the equilibrium catalyst. The 
upper limit on ZSM-5 addition taught in the published European Application 
is typically enough ZSM-5 to achieve an immediate 2.0 octane number gainm 
with the possibility being left open to add enough ZSM-5 to go to a 3.0 
octane number gain. The limiting factor is the ability of the wet gas 
compressor of the catalytic cracking unit to deal with the enhanced wet 
gas production associated with adding large amounts of ZSM-5. 
A good way of adding ZSM-5 to a moving bed catalytic cracking unit is 
disclosed in published European Application No. EP 0167325A3. The make-up 
catalyst may contain 2 or 3 times the amount of ZSM-5 sought for the 
equilibrium catalyst. 
Accordingly, refiners have not been able to add as much ZSM-5, or other 
shape selective zeolite additives which behave in a similar manner, as 
desired because of the constraints placed on such addition by the unit's 
wet gas compressor. 
A way has now been discovered to overcome this restraint, and add 
relatively large amounts of shape selective zeolites, such as ZSM-5, to a 
catalytic cracking unit without exceeding the compressor limits. 
BRIEF SUMMARY OF THE INVENTION 
Accordingly, the present invention provides A method of adding an additive 
zeolite having a constraint index of 1-12 to a catalytic cracking unit 
wherein a heavy feed is added at a given feed rate to a catalytic cracking 
unit and contacts a source of hot, regenerated equilibrium catalytic 
cracking catalyst to produce cracked products including gasoline having an 
octane number, C.sub.3 /C.sub.4, and heavy products boiling above the 
gasoline boiling range, and wherein the severity of the catalytic cracking 
operation is controlled to achieve a predetermined conversion of feed to 
desired products characterized by adding at least 0.5 wt. % additive, 
based on zeolite content of the additive and the total weight of the 
equilibrium catalyst in the catalytic cracking unit, to the catalytic 
cracking unit within a 24 hour period to produce an unacceptable increase 
in C.sub.3 /C.sub.4 production; adjusting at least one of the feed rate 
and the severity of the catalytic cracking operation to reduce the 
production of C.sub.3 /C.sub.4 to an acceptable level; aging in the 
catalytic cracking unit the zeolite additive, said aging reducing the 
cracking activity of the additive zeolite and thereby reducing the 
production of C.sub.3 /C.sub.4 ; and increasing at least one of the feed 
rate and the severity of the catalytic cracking operation to normal levels 
without producing unacceptable amounts of C.sub.3 /C.sub.4, and recovering 
a gasoline boiling range product having an increased octane number. 
In another embodiment the present invention provides a method of adding 
ZSM-5 catalyst to an FCC unit containing an inventory of equilibrium 
catalyst comprising adding within a 48 hour period at least 2.0 wt. % 
ZSM-5 zeolite having a high initial cracking activity to the equillibrium 
catalyst inventory, based on the weight of ZSM-5 zeolite on a matrix-free 
basis and on the total weight of the equilibrium catalyst inventory, said 
addition being in an amount sufficient to cause an increase of at least 20 
mole % of the volume of C.sub.3 /C.sub.4 normally produced during 
catalytic cracking; changing the operation of the catalystic cracking unit 
to reduce the production of C.sub.3 /C.sub.4 said additive zeolites and 
continuing the cracking operation for at least 48 hours and reducing the 
high initial cracking activity to a reduced activity; and thereafter 
restoring operation of the catalytic cracking unit to normal operation and 
recovering gasoline product having an increased octane no. as compared to 
gasoline product produced before the addition of ZSM-5. 
In a more limited embodiment the present invention provides method of 
adding ZSM-5 zeolite to a fluidized catalytic cracking unit comprising 
adding 0.5-2.5 wt. % ZSM-5 zeolite, on a matrix free basis, in a 48 hour 
period; reducing at least one of the fresh feed rate and reaction severity 
to reduce the production of C.sub.3 /C.sub.4 ends; and thereafter 
increasing over at least a one week period at least one of the reaction 
severity and fresh feed rate to restore operation of the catalytic 
cracking unit to normal. 
DETAILED DESCRIPTION 
SHAPE SELECTIVE ZEOLITE 
Any zeolite having a constraint index of 1-12 can be used herein. Details 
of the constraint Index test procedures are provided in J. Catalysis 67, 
218-222 (1981) and in U.S. No. 4,711,710 (Chen et al), both of which are 
incorporated herein by reference. 
Preferred shape selective zeolites are exemplified by ZSM-5, ZSM-11, 
ZSM-12, ZSM-23, ZSM-35, ZSM-38, ZSM-48, ZSM-57 and similar materials. 
ZSM-5 is described in U.S. No. 3,702,886, U.S. Pat. Re. No. 29,948 and in 
U.S. No. 4,061,724 (describing a high silica ZSM-5 as "silicalite"). 
ZSM-11 is described in U.S. No. 3,709,979. 
ZSM-12 is described in U.S. No. 3,832,449. 
ZSM-23 is described in U.S. No. 4,076,842. 
ZSM-35 is described in U.S. No. 4,016,245. 
ZSM-38 is described in U.S. No. 4,046,859. 
ZSM-57 is described in U.S. No. 4,046,859. 
These patents are incorporated herein by reference. 
Preferably relatively high silica shape selective zeolites are used, i.e., 
with a silica/alumina ratio above 20/1, and more preferably with a ratio 
of 70/1, 100/1, 500/1 or even higher. 
ZSM-5 zeolite is preferred, because it is a well known and a proven zeolite 
additive for use in catalytic cracking units. 
Quite a lot of work has been done on making zeolites, such as ZSM-5, with 
different materials, ranging from all silica to materials containing 
silica and some other tetravalent metal such as boron, iron, gallium, etc. 
In lieu of, or in addition to, being incorporated into the zeolite 
framework, these and other catalytically active elements can also be 
deposited upon the zeolite by any suitable procedure, e.g., impregnation. 
As used herein a named zeolite is defined by its crystal structure. The 
term ZSM-5 refers to a material which has substantially the same crystal 
structure as shown in U.S. Pat. No. 3,702,886. Substitution of different 
cations, or changing the silica/alumina, or silica/boron ratio, may result 
in minor modifications of the X-ray diffraction patterns of the 
crystalline material so produced, but it is still ZSM-5, and contemplated 
for use herein. 
FLUID BED CATALYTIC CRACKING 
This is a very common process, and a detailed description thereof is not 
believed necessary. Somewhat oversimplified, catalyst contacts oil in a 
fluidized state, cracking the oil and coking the catalyst. Coke is burned 
from the catalyst in a fluidized regeneration zone to regenerate the 
catalyst, with regenerated catalyst being recycled to react further with 
hydrocarbon feed. 
More details of the fluidized catalytic cracking process are disclosed in 
U.S. Pat. Nos. 4,309,279 and 4,309,280. 
MOVING BED CATALYTIC CRACKING 
Moving bed catalytic cracking is a well known, mature process and a 
detailed discussion thereof is not believed to be necessary. 
It is more difficult to add shape selective zeolite additives to moving bed 
catalytic cracking units that to FCC units. This is because catalyst 
circulation ibn moving bed units is much slower. Catalyst might take 
several hours to complete one complete cycle through reaction and 
regeneration. The catalyst passes through the unit as a moving bed, not as 
a fluidized bed, so there is much less opportunity for mixing of catalyst 
to occur. Catalyst mixing will occur, but it takes days for good mixing of 
catalyst to be achieved, whereas almost complete mixing of fluidizable 
additive catalyst with the equilibrium catalyst in an FCC unit occurs 
within 30-60 minutes of additive addition. 
Heretofore, the best method known of adding ZSM-5 to a moving bed catalytic 
cracking unit was to use a makeup catalyst containing an enhanced amount 
of ZSM-5 relative to the equilibrium catalyst, e.g., the makeup catalyst 
would contain twice as much as the desired amount of ZSM-5 sought in the 
equilibrium catalyst. This method is effective, but many weeks of 
operation are needed to oobserve the affects of ZSM-5 or other shape 
selective zeolite. 
SHAPE SELECTIVE ZEOLITE ADDITION 
In general terms, the refiner will add enough of the additive zeolite to 
significantly increase the additive zeolite content of the equilibrium 
catalyst. 
Enough additive zeolite should be added to increase the zeolite additive 
content by at least 1.0 weight percent, expressed as weight percent 
zeolite additive (matrix free basis) to the total weight of equilibrium 
catalyst in the unit. Preferably, 1.5, or 2.0 weight percent, or even more 
of the zeolite additive is added to the unit within a 24-hour period. 
For ease in practicing the present invention, a very good addition regime 
is to add the desired amount of e.g., ZSM-5 over a 1 to 2 day period. 
Addition of 2.0 weight percent additive zeolite to the unit over a two day 
period, perhaps added every 4 or 8 hours, provides a fast, efficient way 
to get large amounts of zeolite additive into the catalytic cracking unit 
in way that can be easily carried out by refinery personnel. 
Addtion every 4-8 hours in this manner will result in a number of step 
changes in C.sub.3 /C.sub.4 production. The plant operators can be 
instructed to lower fresh feed rate, lower riser top temperature (in the 
case of FCC), adjust catalyst/oil ratios, reduce feed preheat, leave more 
carbon on regenerated catalst, or take such other measures as are 
necessary to reduce production of C.sub.3 /C.sub.4 to an amount that can 
be tolerated by the wet gas compressor of the catalytic cracking unit. 
After two days, catalyst addition can be stopped, and unit operating 
conditions increased in severity to the maximum permitted by the unit's 
wet gas compressor over the next 1-2 weeks. 
An ideal time to practice the present invention would be when the catalytic 
cracking unit is already operating, or scheduled for operation, in a 
relatively low severity mode of operation. Thus, if catalytic cracking 
conditions are adjusted to maximize production of fuel oil, this minimizes 
somewhat the amount of gasoline and light ends that are produced. Such low 
severity operation is also characterized by a relative lower octane number 
gasoline product, which increases the need for zeolite additives such as 
ZSM-5 which increase the octane number of the gasoline. 
ACID ACTIVITY 
The acid activity, or cracking activity, of the shape selective zeolites 
will vary greatly with silica/alumina ratio. In general, the more aluminum 
present, the more active acid sites on the catalyst. Conventional ZSM-5 
catalyst loses activity fairly rapidly in an FCC unit, probably due to a 
slight steaming effect that occurs in the FCC regenerator. In TCC units 
the activity decline occurs, but not so rapidly. 
Various modifiers, e.g., Ag, may be added to the ZSM-5 catalyst to improve 
its steam stability. Such modifiers form no part of the present invention. 
If modified ZSM-5 catalysts are used, it may be necessary to adjust 
somewhat the rate of addition of the modified ZSM-5 catalyst. 
EQUILIBRIUM CATALYST 
If the equilibrium catalyst has been severly deactivated, the addition of 
ZSM-5 will have a bigger effect than if a more active equilibrium catalyst 
was present in the unit. 
Most, but not all, FCC units operate with a catalyst comprising a large 
pore zeolite, typically a rare earth exchanged Y, or ultrastable Y zeolite 
in an amorphous matrix. Typically the FAI activity of this catalyst will 
be 40 to 70. 
The fluid activity index (FAI) is a method used to measure the relative 
cracking activity of fluidized catalyst. The test is carried out in a 
fixed fluidized bed containing 180 g of the catalyst using a standardized 
feed, Light East Texas gas oil, with 3% water (based on feed) added. The 
standard test conditions are atmospheric pressure, 850.degree. 
F./454.degree. C., a weight hourly space velocity of 6, a catalyst-to-oil 
ratio of 2, and a time on stream of 5 minutes. The product is a gasoline 
having a 90% boiling point of 180.degree. C. Conversion is defined as 100% 
minus LV% cycle oil product. 
OPERATING CONDITIONS 
The severity of the operation may have an effect upon target octane. 
Addition of a given amount of ZSM-5 catalyst to a unit operating at 
relatively low severity conditions may produce a different effect than 
addition of the same amount of ZSM-5 catalyst to a unit operating under 
very severe conditions. 
FEEDSTOCK 
Any conventional cracking unit feed may continue to be used as feed to the 
FCC unit during the practice of the present invention. The octane no. 
response of different feeds to ZSM-5 catalyst may be different.

COMISON EXAMPLE 1 
The following example represents the prior art method of adding ZSM-5, 
namely adding enough to see a 1.5 octane number gain. The example is taken 
from EP 0 167 325 A3. 
ZSM-5 ACTIVITY DECAY 
The most important variable in determining a ZSM-5 addition rate is the 
rate of activity decay of the ZSM-5 catalyst added to the FCC unit. 
If the catalyst activity did not decline, but remained undiminished, there 
would be no need to practice the present invention. Addition of a given 
amount of ZSM-5 catalyst on the first day of operation would, without 
further ZSM-5 addition, achieve the desired boost in product octane 
number. Such operation, as desirable as it may be, has never been attained 
so after about one day of operation more fresh ZSM-5 catalyst must be 
added to compensate for the loss in activity of the ZSM-5 catalyst already 
in the FCC unit. 
A typical ZSM-5 addition rate necessary to achieve a constant increase in 
octane will be given. The precise addition rates are dependent upon all of 
the variables discussed above. The basic operating conditions for this 
exercise are listed below. 
1. Equilibrium catalyst 13% REY in a silica/alumina matrix average particle 
size 40 to 120 microns, 58 FAI activity. 
2. ZSM-5 additive--the additive consisted of 25 wt. % HZSM-5 (which had not 
been subjected to steaming or calcining) in a silica/alumina matrix (75 
wt. %). The FAI activity of the additive composite (which is a mixture of 
ZSM-5 and silica/alumina) was around 68. 
3. FCC operating conditions, including regeneration conditions, are shown 
in Table 1. 
4. Feedstock properties are also shown in Table 1. 
TABLE I 
______________________________________ 
Operating Conditions 
______________________________________ 
Fresh Feed, TBD/m.sup.3 /hr 
15.3/101.4 
C/O, Catalyst/Oil-Weight 6.6 
Riser Top Temp., .degree.F./.degree.C. 
967/519 
Maximum regenerator Temp., .degree.F./.degree.C. 
1310/710 
Oil/Preheat Temp., .degree.F./.degree.C. 
735/391 
Catalyst Activity, FAI 58 
Catalyst Inventory, Tons 60 
______________________________________ 
Charge Stock Characterization 
______________________________________ 
Density, g/cc 0.927 
API 21.8 
S, wt. % 0.5 
Molecular Weight 375 
Basic Nitrogen, ppm 570 
C.sub.A, wt. % 17.3 
Conradson Carbon Residue, wt. % 
0.06 
______________________________________ 
Based on these assumptions, the wt. % additive necessary to achieve a 1.5 
octane no. boost is presented hereafter in Table 2. This is a projection 
based upon our pilot plant results and mathematical models, it does not 
represent an actual commercial scale test. 
TABLE 2 
______________________________________ 
Example of ZSM-5 Catalyst Makeup 
+1.5 Research Octane Increase 
Additive Makeup 
ZSM-5 Makeup 
Rate at % of Rate as % of 
From (Day) 
To (Day) Inventory per day 
Inventory per 
______________________________________ 
0 1 3.6 to 3.7 0.75 to 0.9 
1 3 0.96 0.192 to 0.24 
3 6 0.70 0.14 to 0.175 
6 12 0.60 0.12 to 0.15 
12 18 0.52 0.108 to 0.13 
18 30 0.46 0.092 to 0.115 
30+ 0.38 0.08 to 0.095 
______________________________________ 
The reason that additive amounts are reported in two ways is that the 
additive comprises 20 wt. % ZSM-5, with the remaining portion being a 
silica/alumina binder. The silica/alumina is not inert, it has some 
cracking activity, so for completeness both the ZSM-5 portion added, and 
the total additive (consisting of ZSM-5 plus its binder) is reported 
above. 
The addition rate takes into account the small amounts of ZSM-5 present in 
the circulating catalyst removed as additional catalyst is added. It also 
assumes that both catalysts attrit from the unit at the same rate. 
The equilibrium ZSM-5 additive catalyst has a much lower FAI activity than 
does the equilibrium catalyst, so eventually more fresh REY cracking 
catalyst must be added to maintain overall catalyst activity in the unit 
at a predetermined level. 
This dilution effect, of the REY zeolite conventional cracking catalyst 
with the additive, can be minimized either by using a more concentrated 
ZSM-5 additive catalyst, or incorporating some conventional REY zeolite 
into the additive catalyst so that the overall fAI activity of a unit is 
not changed. The additive dilution effect could be largely avoided by 
using as an additive catalyst a ZSM-5+matrix which contained, on an 
overall weight percent basis, about 12% REY. 
ILLUSTRATIVE EBODIMENT (Invention) 
The following represents a preferred manner of adding ZSM-5 to an FCC unit. 
The amount of ZSM-5 added, on a pure zeolite basis, would be 0.5-2.5% of 
inventory in 24-48 hours. For a 25% ZSM-5 zeolite additive, 2-10% of 
additive (four times as much) would be required. 
The preferred ZSM-5 addition schedule is continuous, using a catalyst pump. 
Otherwise the ZSM-5 would be added in equal slugs very 4-6 hours over a 
24-48 hour period. 
The changes in the unit that would have to be made to accomodate the slugs 
of ZSM-5 area a reduction of fresh feed rate and/or conversion (via 
decreasing riser top temperature or increasing preheat) of 5-25%. However, 
these can be restored to base levels over the next 1-2 week. When feed 
rate, riser top temperature and compressor loading have returned to base 
levels, the octane boost in the gasoline product will still persist, 
declining gradually over the next 2 months. Slugging can then be repeated 
during periods of low FCC unit feed rate/severity. 
Comparing the method of the present invention to the prior art method, the 
use of slugging addition of ZSM-5 results in significantly greater 
addition of ZSM-5, in a much shorter time, than could be achieved using 
the method of the published European Application. These enhanced ZSM-5 
levels result in more production of higher octane gasoline than could be 
achieved using the prior art method.