Molecular sieve abstract
A process is provided for converting feedstock hydrocarbon compounds
over a catalyst composition which comprises a large-pore molecular
sieve material and an additive catalyst composition comprising crystalline
material having the structure of ZSM-5 and a silica/alumina mole
ratio of less than about 30. An embodiment of the present invention
comprises an improved catalytic cracking process to produce high
octane gasoline, increased alkylate and potential alkylate, and
increased lower olefins, especially propylene.
Molecular sieve claims
What is claimed is:
1. A process for converting feedstock hydrocarbon compounds to
product hydrocarbon compounds having a lower molecular weight than
the feedstock hydrocarbon compounds which comprises contacting said
feedstock at catalytic cracking conversion conditions with a catalyst
composition comprising a large-pore molecular sieve material having
pore openings greater than about 7 Angstroms and an additive catalyst
composition comprising crystalline material having the structure
of ZSM-5 and a silica/alumina mole ratio of less than about 30
said crystalline material having, as-synthesized, a formula on an
anhydrous basis
wherein x is a number greater than about 0.1 M is alkali or alkaline
earth metal, R is n-propylamine, and y is a number less than about
30 said additive catalyst composition having an Alpha Value of
greater than about 30.
2. The process of claim 1 wherein the large-pore molecular sieve
material is selected from the group consisting of zeolites X, Y,
REX, REY, USY, REUSY, dealuminated Y, ultra-hydrophobic Y, silicon-enriched
dealuminated Y, ZSM-20 Beta, L, silicoaluminophosphates SAPO-5
SAPO-31 SAPO-37 SAPO-40 pillared silicates, pillared clays, and
combinations thereof.
3. The process of claim 2 wherein the large pore molecular sieve
material comprises REY, USY or REUSY.
4. The process of claim 1 wherein the catalyst composition and
additive catalyst composition comprise matrix material selected
from the group consisting of silica, alumina, titania, zirconia,
magnesia, kaolin, bentonite, and combinations thereof.
5. The process of claim 1 wherein the catalyst composition comprises
up to about 6 wt. % of the crystalline material having the structure
of ZSM-5.
6. The process of claim 1 wherein the additive catalyst composition
comprises from about 5 wt. % to about 80 wt. % of the crystalline
material having the structure of ZSM-5 and from about 20 wt. % to
about 95 wt. % matrix.
7. The process of claim 6 wherein the additive catalyst composition
comprises from about 1.5 wt. % to about 5.5 wt. % elemental phosphorus
based on weight of matrix.
8. The process of claim 1 wherein the additive catalyst composition
has been synthesized by a method comprising:
(i) forming a reaction mixture hydrogel having a pH of from about
10 to about 14 and containing sources of alkali or alkaline earth
metal (M) cations; an oxide of aluminum; an oxide of silicon; n-propylamine
directing agent (R); and water, said reaction mixture having a composition
in terms of mole ratios, within the following ranges:
(ii) maintaining the reaction mixture until the crystals of ZSM-5
structure are formed,
(iii) recovering the ZSM-5 crystals from the reaction mixture,
(iv) ammonium exchanging the recovered ZSM-5 crystals,
(v) deagglomerating the ammonium-exchanged crystals,
(vi) slurrying a matrix material with the deagglomerated ZSM-5
crystals at a pH of from about 2 to about 12 to yield a ZSM-5/matrix
material comprising from about 5 to about 80 wt. % ZSM-5 and from
about 20 to about 95 wt. % matrix,
(vii) drying the ZSM-5/matrix material, and
(viii) converting the dried ZSM-5/matrix material to the protonic
form having an Alpha Value of greater than about 30.
9. The process of claim 8 wherein said step (viii) comprises steps
of (1) contacting the dried ZSM-5/matrix material with mineral,
carboxylic, or dicarboxylic acid, and (2) calcining the acid treated
ZSM-5/matrix material at a temperature of from about 200.degree.
C. to about 550.degree. C. for from about 1 minute to about 48 hours.
10. The process of claim 8 wherein said step (viii) comprises steps
of (1) ammonium exchanging the dried ZSM-5/matrix material, and
(2) calcining the ammonium exchanged ZSM-5/matrix at a temperature
of from about 200.degree. C. to about 550.degree. C. for from about
1 minute to about 48 hours.
11. The process of claim 8 wherein said step (viii) comprises calcining
the dried ZSM-5/matrix material at a temperature of from about 200.degree.
C. to about 550.degree. C. for from about 1 minute to about 48 hours.
12. The process of claim 1 wherein said conversion conditions include
an average reactor temperature of from about 450.degree. C. to about
510.degree. C., a catalyst/oil volume ratio of from about 2 to about
7 and a space velocity of from about 1 to about 2.5.
13. The process of claim 8 wherein said conversion conditions include
an average reactor temperature of from about 450.degree. C. to about
510.degree. C., a catalyst/oil volume ratio of from about 2 to about
7 and a space velocity of from about 1 to about 2.5.
14. The process of claim 1 wherein said conversion conditions include
a riser top temperature of from about 500.degree. C. to about 595.degree.
C., a catalyst/oil weight ratio of from about 3 to about 12 and
a catalyst residence time of from about 0.5 to about 15 seconds.
15. The process of claim 8 wherein said conversion conditions include
a riser top temperature of from about 500.degree. C. to about 595.degree.
C., a catalyst/oil weight ratio of from about 3 to about 12 and
a catalyst residence time of from about 0.5 to about 15 seconds.
16. A process for cracking feedstock hydrocarbon compounds to product
comprising gasoline, alkylate, potential alkylate and propylene
which comprises contacting said feedstock at cracking conditions
including a temperature of from about 200.degree. C. to about 870.degree.
C. with a catalyst composition comprising a large-pore molecular
sieve material having pore openings greater than about 7 Angstroms
and an additive catalyst composition comprising crystalline material
having the structure of ZSM-5 and a silica/alumina mole ratio of
less than about 30 said crystalline material having, as-synthesized,
a formula on an anhydrous basis
wherein x is a number greater than about 0.1 M is alkali or alkaline
earth metal, R is n-propylamine, and y is a number less than about
30 said additive catalyst composition having an Alpha Value of
greater than about 30.
17. The process of claim 16 wherein the large pore molecular sieve
material comprises REY, USY or REUSY.
18. The process of claim 16 wherein the catalyst composition and
additive catalyst composition comprise matrix material selected
from the group consisting of silica, alumina, titania, zirconia,
magnesia, kaolin, bentonite, and combinations thereof.
19. The process of claim 16 wherein the catalyst composition comprises
up to about 6 wt. % of the crystalline material having the structure
of ZSM-5.
20. The process of claim 16 wherein the additive catalyst composition
comprises from about 5 wt. % to about 80 wt. % of the crystalline
material having the structure of ZSM-5 and from about 20 wt. % to
about 95 wt. % matrix.
21. The process of claim 20 wherein the additive catalyst composition
comprises from about 1.5 wt. % to about 5.5 wt. % elemental phosphorus
based on weight of matrix.
22. The process of claim 16 wherein the additive catalyst composition
has been synthesized by a method comprising:
(i) forming a reaction mixture hydrogel having a pH of from about
10 to about 14 and containing sources of alkali or alkaline earth
metal (M) cations; an oxide of aluminum; an oxide of silicon; n-propylamine
directing agent (R); and water, said reaction mixture having a composition
in terms of mole ratios, within the following ranges:
(ii) maintaining the reaction mixture until the crystals of ZSM-5
structure are formed,
(iii) recovering the ZSM-5 crystals from the reaction mixture,
(iv) ammonium exchanging the recovered ZSM-5 crystals,
(v) deagglomerating the ammonium-exchanged crystals,
(vi) slurrying a matrix material with the deagglomerated ZSM-5/crystals
at a pH of from about 2 to about 12 to yield a ZSM-5 matrix material
comprising from about 5 to about 80 wt. % ZSM-5 and from about 20
to about 95 wt. % matrix,
(vii) drying the ZSM-5/matrix material, and
(viii) converting the dried ZSM-5/matrix material to the protonic
form having an Alpha Value of greater than about 30.
Molecular sieve description
FIELD OF THE INVENTION
The present invention relates to a process for converting, e.g.,
cracking, a hydrocarbon feed over a particular catalyst combination
to produce conversion product, e.g., a high octane gasoline fraction,
more alkylate and gasoline plus potential alkylate, and, most significantly,
substantially more lower olefins, especially propylene, when compared
to prior art processes. The catalyst combination for use herein
includes an additive catalyst comprising molecular sieve material
having the structure of ZSM-5. The particular ZSM-5 structure used
is synthesized by a method which provides crystals having high activity,
propylene selectivity, and improved processing and handling characteristics
for the present process.
BACKGROUND OF THE INVENTION
There is a growing need for higher octane in the refinery gasoline
pool, particularly since the phase-out of lead additives for gasoline
both in the U.S. and abroad. Decreases in octane sensitivity, i.e.,
the difference between research and motor octane, are especially
desirable. Increased alkylate and potential alkylate are also needed
from today's gasoline manufacturing processes. Some C.sub.3 and
C.sub.4 olefins are useful by-products of such a manufacturing process;
increases in these olefins are desired. These light olefins are
used to make ethers and/or alcohols.
Most options available to FCC operators have limited potential.
Use of shape-selective cracking additives, or large-pore cracking
catalyst containing such additives, appeared to have only limited
potential to increase yields of light olefins.
Pyrolysis units or thermal crackers produce large amounts of olefins,
but little gasoline. A high severity, shape-selective cracking process
is also available. However, like the closely related pyrolysis process,
the high severity process makes large amounts of olefins and relatively
small yields of highly aromatic, low octane gasoline.
In efforts to solve these problems, a number of processes have
been developed. For example, U.S. Pat. No. 3758403 teaches the
benefits of adding ZSM-5 to conventional large-pore cracking catalyst
formulations. Example 2 of the patent uses a catalyst consisting
of 5 wt. % ZSM-5 10 wt. % REY, and 85% clay. With a gas oil feedstock,
the catalyst produced 11.42 vol. % propylene, and a total yield
of alkylate and C.sub.5.sup.+ gasoline of 89.1 vol. %. Example 3
of the patent uses a catalyst consisting of 10 wt. % ZSM-5 10 wt.
% REY, and 80% clay. Although the ZSM-5 content doubled, propylene
yields increased from 11.4 vol. % to only 13.6 vol. %. The total
yield of alkylate and gasoline declined slightly, from 89.1 vol.
% to 88.6 vol. %.
U.S. Pat. No. 3847793 teaches a slightly different approach.
The ZSM-5 which could be in the same particle with the large-pore
zeolite, or in a separate additive, is used to convert olefins to
aromatics. A riser reactor with an enlarged upper portion is used,
along with injection of a coking fluid near the top of the riser,
to deactivate the large-pore catalyst while leaving the ZSM-5 catalyst
active. Gasoline boiling range material could be injected into the
top of the riser for conversion. Table 2 of the patent shows that
this approach reduced the mono-olefin content of an FCC gasoline
from 14.0 wt. % to 2.9 wt. %. The discussion of Example 2 reports
that ZSM-5 was effective for converting propylene to aromatics over
a wide range of catalyst silica/alumina ratios.
Based on U.S. Pat. No. 3847793 large amounts of ZSM-5 should
efficiently convert propylene into aromatics. This would reduce
light olefin production, and perhaps exacerbate problems of producing
gasoline without exceeding aromatics and/or benzene specifications.
Based on U.S. Pat. No. 3758403 use of large-pore cracking catalyst
with large amounts of ZSM-5 additive gives only modest increase
in light olefin production. A 100% increase in ZSM-5 content (from
5 wt. % ZSM-5 to 10 wt. % ZSM-5) increased the propylene yield less
than 20%, and decreased slightly the potential gasoline yield (C.sub.5
+ gasoline plus alkylate).
Because refiners must retain the ability to use the many types
of commercially available large-pore cracking catalysts available
today, the normal practice is to use additive catalysts, with 10
to 50 wt. %, more usually 10 to 25 wt. % ZSM-5 in an amorphous support,
to their FCC units. Such additives have physical properties which
allow them to circulate with the large-pore cracking catalyst.
U.S. Pat. No. 4309280 teaches adding very small amounts of powdered,
neat ZSM-5 catalyst, characterized by a particle size below 5 microns.
Adding as little as 0.25 wt. % ZSM-5 powder to the FCC catalyst
inventory increased LPG production 50%. Small amounts of neat powder
behaved much like larger amounts of ZSM-5 disposed in larger particles.
A way to add a modest amount of ZSM-5 to an FCC unit is disclosed
in U.S. Pat. No. 4994424 incorporated herein by reference. ZSM-5
additive is added to the equilibrium catalyst in a programmed manner
so an immediate boost in octane number, typically 1/2-2 octane number,
is achieved.
U.S. Pat. No. 4927523 incorporated herein by reference, teaches
a way to add large amounts of ZSM-5 to a unit without exceeding
wet gas compressor limits. Large amounts are added and cracking
severity is reduced in the FCC unit for several days.
Recent work on ZSM-5 additives has been directed at stabilizing
the additives with phosphorus or making them more attrition resistant.
Phosphorus stabilized ZSM-5 additive is believed to retain activity
for a longer time. Phosphorus stabilization thus reduces the makeup
rate of ZSM-5 additive required. U.S. Pat. No. 5110776 teaches
a method for preparing FCC catalyst comprising modifying the zeolite,
e.g., ZSM-5 with phosphorus. U.S. Pat. No. 5126298 teaches manufacture
of an FCC catalyst comprising zeolite, e.g., ZSM-5 clay, and phosphorus.
Phosphorus treatment has been used on faujasite-based cracking catalysts
for metals passivation (see U.S. Pat. Nos. 4970183 and 4430199);
reducing coke make (see U.S. Pat. Nos. 4567152; 4584091; and
5082815); increasing activity (see U.S. Patents 4454241 and
4498975); increasing gasoline selectivity (See U.S. Pat. No. 4970183);
and increasing steam stability (see U.S. Pat. Nos. 4765884 and
4873211).
One concern regarding use of ZSM-5 additive, even with phosphorus
stabilization, is that refiners fear dilution of the large-pore
cracking catalyst by addition of large amounts of ZSM-5 e.g., over
2 or 3 wt. % ZSM-5 crystal, or use of more than 5 or 10 wt. % additive,
will seriously impair conversion since ZSM-5 has difficulty cracking
the heavier molecules in gas oil feeds. Most refiners operate with
significantly smaller amounts of ZSM-5 than the upper limits recited
above.
Another concern is how well the unit will respond when pushed to
make even more olefins. The consensus is that small amounts of ZSM-5
additive make large amounts of olefins in an FCC unit operating
at low severity, but the increase in yields of light olefins attributable
to ZSM-5 declines as severity increases. As reported in Elia, M.F.
et al.,"Effect of Operation Conditions on the Behaviour of
ZSM-5 Addition to a RE-USY FCC Catalyst", Applied Catalysis,
73 195-216 202 (1991), working at low severity produces an increase
in light olefinic compounds, mostly branched, in the C.sub.5 -C.sub.6
range. At the same time, an increase in light branched alkanes results
and the aromatics and naphthenes contents are almost not affected.
Elia et al. report that when the cracking occurs at higher temperatures,
an increase in the C.sub.7 -C.sub.8 aromatics and naphthenes is
observed, but a much smaller increase in the lighter compounds results.
The poor response to unusually large concentrations of ZSM-5 was
reported in U.S. Pat. No. 3758403 while Elia et al. have shown
the unfavorable response of ZSM-5 to high severity FCC operation.
In summary, most refiners operating cracking units would prefer
more light olefins, e.g., propylene and butylene. Based on the teachings
of U.S. Pat. No. 3758403 use of ever increasing amounts of ZSM-5
and large-pore zeolite in a common particle produces rapidly diminishing
returns from the incremental amounts of ZSM-5. Based on the state
of the art on the use of separate additives in the catalytic cracking
process, use of large amounts of additive comprising ZSM-5 would
also produce diminishing returns at high severity. Today most refiners
tend to use more severe operation to increase conversion, and improve
gasoline yield and octane.
Based on the pyrolysis work reported in U.S. Pat. No. 4980053
use of large amounts of separate ZSM-5 additive at high severity
reduces both conversion and gasoline yield, and would produce a
highly aromatic gasoline.
Accordingly, it is an object of the present invention to provide
an improved cracking process using an improved additive catalyst.
It is a further object of the invention to provide for the use
of an improved additive catalyst composition to impart an octane-enhancing
property in the present catalytic cracking process, and to enhance
production of light olefins, e.g., propylene.
It is a particular object of the invention to provide for the use
of an improved additive catalyst composition in hydrocarbon cracking
to result in product rich in high octane gasoline, alkylate, gasoline
plus potential alkylate, and petrochemical grade lower olefins,
e.g., propylene.
SUMMARY OF THE INVENTION
These and other objects are achieved by the present invention which
provides a process for converting feedstock hydrocarbon compounds
to product hydrocarbon compounds of lower molecular weight than
the feedstock hydrocarbon compounds which comprises contacting the
feedstock at conversion conditions with catalyst comprising a large-pore
molecular sieve and additive catalyst, the additive catalyst comprising
an improved formulation of crystals having the structure of ZSM-5.
More particularly, the invention provides a hydrocarbon cracking
process which uses a catalyst composition comprising a large-pore
molecular sieve, such as, for example, USY, REY or REUSY, and an
additive catalyst comprising ZSM-5 having been synthesized and formulated
in a special way to provide product significantly improved in gasoline
quality, alkylate and potential alkylate quantity, and valuable
lower olefin, e.g., propylene, quantity and selectivity.
The large-pore molecular sieve catalyst composition may be prepared
by combining a slurry of the large-pore molecular sieve, e.g., USY,
REY or REUSY, and a slurry comprising matrix material The combined
slurries may be dewatered, reslurried, homogenized, and spray dried.
The additive catalyst composition will be prepared by 1) synthesizing
crystals having the structure of ZSM-5 from a particular, critical
reaction mixture; 2) recovering the specially synthesized ZSM-5
crystals; 3) ammonium exchanging the recovered ZSM-5; 4) deagglomerating
and slurrying the ammonium-exchanged ZSM-5 such as by ball milling;
5) slurrying the ZSM-5 with matrix material, such as silica, alumina,
silica-alumina, or clay and, if desired, phosphorus to make a ZSM-5/matrix
composition; 6) drying the product ZSM-5/matrix composition, such
as by spray drying to form a fluid powder; and 7) converting the
dried ZSM-5/matrix composition to the protonic form. This conversion
may be accomplished by, for example, acid treatment, ammonium exchange,
and/or calcination. If acid treatment or ammonium exchange is performed,
calcination will follow.
In the process for cracking a hydrocarbon feedstock, the feedstock
is contacted under catalytic cracking conditions with a catalyst
composition comprising a large-pore molecular sieve and the additive
catalyst to yield the improved product. The additive catalyst provides
up to about 6 wt. % ZSM-5 crystals, for example from about 0.01
wt. % to about 6 wt. % ZSM-5 crystals, preferably from about 0.3
wt. % to about 4.5 wt. %, based on total catalyst inventory.
Advantageously, the use of the present improved additive catalyst
in the cracking process results in a high octane gasoline product,
higher amounts of potential alkylate which can be subsequently processed
to yield a high octane gasoline, and a dramatically increased amount
of lower olefins, with selectivity for petrochemical grade propylene.
DETAILED DESCRIPTION
It has been found that the use of a minor amount of up to about
6 wt. %, e.g., from about 0.01 wt. % to about 6 wt. %, usually from
about 0.3 wt. % to about 4.5 wt. % of total catalyst inventory of
specially synthesized ZSM-5 crystal in an additive catalyst, prepared
in a special way and having certain physical properties due to its
manufacture, along with cracking catalyst in a fluidized-bed cracking
process leads to an unexpected shift in product composition as compared
with the same process using the cracking catalyst alone or with
a different additive catalyst. The product gasoline fraction octane
is essentially the same as provided by commercial ZSM-5 additives.
However, the yield shift produced by the present process results
in an increase in C.sub.3 and C.sub.4 olefins, especially propylene,
which shift is valuable to the refiner.
In catalytic cracking, high molecular weight hydrocarbons are converted
to lower molecular weight hydrocarbons of suitable volatility to
permit their use as liquid fuels. The combustion characteristics
of gasoline are assessed empirically by assigning the fuel an octane
rating. This is generally defined as a comparison with a primary
reference which is the percentage of iso-octane (224-trimethylpentane)
in an n-heptane/iso-octane mixture to which the gasoline under examination
is equivalent in terms of combustion behavior when considering the
octane ratings of n-heptane and iso-octane to be zero and 100 respectively.
Both RON and MON can be tested on the same single-cylinder, four-stroke
engine of standardized design. RON signifies the research octane
number, MON signifies the motor octane number, and the terms are
used to describe the knocking characteristics of gasoline, that
is, its combustion behavior. For a measurement of RON, the engine
speed used is 600 rpm which yields results comparable to an automobile
engine operated at low speed. For a measurement of MON, the engine
speed is 900 rpm which approximates higher speed cruising conditions.
Generally, higher octane numbers are found by the research method
compared to the motor method for the same gasoline sample. The average
of the RON and MON, known as the road octane number, gives an indication
of typical performance in an engine. The higher the octane, the
better the combustion behavior in a spark-ignition engine. It has
been found that road octane number correlates much more closely
to the motor octane number than the research octane. Generally,
aromatics and branched paraffinic and olefinic hydrocarbons have
higher octane values than acyclic or linear paraffinic hydrocarbons.
In conjunction with catalytic cracking to produce gasoline product,
alkylate and potential alkylate may result from the cracking process.
This indirectly leads to product of increased octane because high
octane, highly branched paraffinic gasoline blending stocks are
produced principally by alkylation of C.sub.3 and C.sub.4 olefins
with isobutane. Unlike cracking, alkylation makes larger branched
hydrocarbons from smaller hydrocarbons and these larger branched
hydrocarbons are inherently higher in octane.
The present process not only provides a high octane product and
product alkylate and potential alkylate, but significantly more
light olefins, especially propylene. The increase in propylene product
at the expense of other olefins is an unexpected, very valuable
occurrence. The propylene is high quality, petrochemical grade,
and may be used for manufacture of valuable ethers and/or alcohols,
or as an alkylating agent.
The presently required improved additive catalyst provides high
selectivity to propylene as the light olefin product. In other respects,
it provides comparable catalytic performance compared to the best
of presently used ZSM-5 FCC additive catalysts. Further, the presently
used improved additive catalyst provides catalyst usage improvements,
such as ease of handling, loading, and processing, found desirable
by refiners.
Feeds
The feedstock, that is, the hydrocarbons to be cracked, may include
in whole or in part, a gas oil (e.g., light, medium, or heavy gas
oil) having an initial boiling point above 204.degree. C., a 50%
point range of at least 260.degree. C. and an end point range of
at least 315.degree. C. The feedstock may also include vacuum gas
oils, thermal oils, residual oils, cycle stocks, whole top crudes,
tar sand oils, shale oils, synthetic fuels, heavy hydrocarbon fractions
derived from the destructive hydrogenation of coal, tar, pitches,
asphalts, hydrotreated feedstocks derived from any of the foregoing,
and the like. As will be recognized, the distillation of higher
boiling petroleum fractions above about 400.degree. C. must be carried
out under vacuum in order to avoid thermal cracking. The boiling
temperatures utilized herein are expressed in terms of convenience
of the boiling point corrected to atmospheric pressure. Resids or
deeper cut gas oils with high metals contents can also be cracked
using the invention.
Process
The present invention provides a process for converting feedstock
hydrocarbon compounds to product hydrocarbon compounds of lower
molecular weight than the feedstock hydrocarbon compounds. In particular,
the present invention provides a process for catalytically cracking
a hydrocarbon feed to a mixture of products comprising gasoline,
alkylate, potential alkylate, and propylene in the presence of a
cracking catalyst under catalytic cracking conditions. Catalytic
cracking units which are amenable to the process of the invention
operate at temperatures from about 200.degree. C. to about 870.degree.
C. and under reduced, atmospheric or superatmospheric pressure.
The catalytic process can be either fixed bed, moving bed or fluidized
bed and the hydrocarbon flow may be either concurrent or countercurrent
to the catalyst flow. The process of the invention is particularly
applicable to the Fluid Catalytic Cracking (FCC) or Thermofor Catalytic
Cracking (TCC) processes. In both of these processes, the hydrocarbon
feed and catalyst are passed through a reactor and the catalyst
is regenerated. The two processes differ substantially in the size
of the catalyst particles and in the engineering contact and transfer
which is at least partially a function of catalyst size.
The TCC process is a moving bed and the catalyst is in the shape
of pellets or beads having an average particle size of about one-sixty-fourth
to one-fourth inch. Active, hot catalyst beads progress downwardly
cocurrent with a hydrocarbon charge stock through a cracking reaction
zone. The hydrocarbon products are separated from the coked catalyst
and recovered, and the catalyst is recovered at the lower end of
the zone and regenerated.
Typically preferred TCC conversion conditions include an average
reactor temperature of from about 450.degree. C. to about 510.degree.
C.; catalyst/oil volume ratio of from about 2 to about 7; reactor
space velocity of from about 1 to about 2.5 vol./hr./vol.; and recycle
to fresh feed ratio of from 0 to about 0.5 (volume).
The process of the invention is particularly applicable to Fluid
Catalytic Cracking. In fluidized catalytic cracking processes, the
catalyst is a fine powder of abut 10 to 200 microns. This powder
is generally suspended in the feed and propelled upward in a reaction
zone. A relatively heavy hydrocarbon feedstock, e.g., a gas oil,
is admixed with a suitable cracking catalyst to provide a fluidized
suspension and cracked in an elongated reactor, or riser, at elevated
temperatures to provide a mixture of lighter hydrocarbon products.
The gaseous reaction products and spent catalyst are discharged
from the riser into a separator, e.g., a cyclone unit, located within
the upper section of an enclosed stripping vessel, or stripper,
with the reaction products being conveyed to a product recovery
zone and the spent catalyst entering a dense catalyst bed within
the lower section of the stripper. In order to remove entrained
hydrocarbons from the spent catalyst prior to conveying the latter
to a catalyst regenerator unit, an inert stripping gas, e.g., steam,
is passed through the catalyst bed where it desorbs such hydrocarbons
conveying them to the product recovery zone. The fluidizable catalyst
is continuously circulated between the riser and the regenerator
and serves to transfer heat from the latter to the former thereby
supplying the thermal needs of the cracking reaction which is endothermic.
Gas from the FCC main-column overhead receiver is compressed and
directed with primary-absorber bottoms and stripper overhead gas
through a cooler to the high pressure receiver. Gas from this receiver
is routed to the primary absorber, where it is contacted by the
unstabilized gasoline from the main-column overhead receiver. The
net effect of this contacting is a separation between C.sub.3 +
and C.sub.2 - fractions in the feed to the primary absorber. Primary
absorber off-gas is directed to a secondary or sponge absorber,
where a circulating stream of light cycle oil from the main column
is used to absorb most of the remaining C.sub.5 + material in the
sponge absorber feed. Some C.sub.3 and C.sub.4 materials are also
absorbed. The sponge-absorber rich oil is returned to the FCC main
column. The sponge-absorber overhead, with most of the valuable
C.sub.4 + material removed but including H.sub.2 S, is sent to the
fuel gas or other process streams.
Liquid from the high pressure separator is sent to a stripper where
most of the C.sub.2 - is removed overhead and sent back to the high
pressure separator. The bottoms liquid from the stripper is sent
to the debutanizer, where an olefinic C.sub.3 -C.sub.4 product is
further separated for gasoline production. The debutanizer bottoms,
that is, the stabilized gasoline, is sent to treating, if necessary,
and then to storage. The C.sub.3 and C.sub.4 product olefins can
be directed to an alkylation unit to produce a high octane gasoline
by the reaction of an iso-paraffin (usually iso-butane) with one
or more of the low molecular weight olefins (usually propylene and
butylene).
The FCC conversion conditions include a riser top temperature of
from about 500.degree. C. to about 595.degree. C., preferably from
about 520.degree. C. to about 565.degree. C., and most preferably
from about 530.degree. C. to about 550.degree. C.; catalyst/oil
weight ratio of from about 3 to about 12 preferably from about
4 to about 11 and most preferably from about 5 to about 10; and
catalyst residence time of from about 0.5 to about 15 seconds, preferably
from about 1 to about 10 seconds.
Molecular Sieve Catalyst
The catalyst can contain any active component which has cracking
activity. The active component may be a conventional large-pore
molecular sieve including zeolite X (U.S. Pat. No. 2882442); REX;
zeolite Y (U.S. Pat. No. 3130007); Ultrastable Y zeolite (USY)
(U.S. Pat. No. 3449070); Rare Earth exchanged Y (REY) (U.S. Pat.
No. 4415438); Rare Earth exchanged USY (REUSY); Dealuminated Y
(DeAl Y) (U.S. Pat. No. 3442792; U.S. Pat. No. 4331694); Ultrahydrophobic
Y (UHPY) (U.S. Pat. No. 4401556); and/or dealuminated silicon-enriched
zeolites, e.g., LZ-210 (U.S. Pat. No. 4678765). Preferred are
higher silica forms of zeolite Y. Zeolite ZSM-20 (U.S. Pat. No.
3972983); zeolite Beta (U.S. Pat. No. 3308069); zeolite L (U.S.
Pat. Nos. 3216789; and 4701315); and naturally occurring zeolites
such as faujasite, mordenite and the like may also be used. These
materials may be subjected to conventional treatments, such as impregnation
or ion exchange with rare earths to increase stability. These patents
are incorporated herein by reference. These large-pore molecular
sieves have a pore opening of greater than about 7 Angstroms. In
current commercial practice most cracking catalysts contain these
large-pore molecular sieves. The preferred molecular sieve of those
listed above is a zeolite Y, more preferably an REY, USY or REUSY.
Other large-pore crystalline molecular sieves include pillared
silicates and/or clays; aluminophosphates, e.g., ALPO.sub.4 -5
ALPO.sub.4 -8 VPI-5; silicoaluminophosphates, e.g., SAPO-5 SAPO-37
SAPO-31 SAPO-40; and other metal aluminophosphates. These are variously
described in U.S. Pat. Nos. 4310440; 4440871; 4554143; 4567029;
4666875; 4742033; 4880611; 4859314; and 4791083 each
incorporated herein by reference.
The preparation of some molecular sieve-containing catalysts may
require reduction of the sodium content, as well as conversion to
the acid (protonated) form. For example, with zeolites this can
be accomplished by employing the procedure of converting the zeolite
to an intermediate ammonium form as a result of ammonium ion exchange
followed by calcination to provide the hydrogen form. The operational
requirements of these procedures are well known in the art.
The source of the ammonium ion is not critical; thus the source
can be ammonium hydroxide or an ammonium salt such as ammonium nitrate,
ammonium sulfate, ammonium chloride and mixtures thereof. These
reagents are usually in aqueous solutions. By way of illustration,
aqueous solutions of 1N NH.sub.4 OH, 1N NH.sub.4 NO.sub.3 1N NH.sub.4
Cl, and 1N NH.sub.4 Cl/NH.sub.4 OH have been used to effect ammonium
ion exchange. The pH of the ion exchange is not critical but is
generally maintained at 7 to 12. Ammonium exchange may be conducted
for a period of time ranging from about 0.5 to about 20 hours at
a temperature ranging from ambient up to about 100.degree. C. The
ion exchange may be conducted in a single stage or in multiple stages.
Calcination of the ammonium exchanged zeolite will produce its acid
form. Calcination can be effected at temperatures up to about 550.degree.
C.
The molecular sieve catalyst may include phosphorus or a phosphorus
compound for any of the functions generally attributed thereto,
such as, for example, attrition resistance, stability, metals passivation,
and coke make reduction.
To prepare the catalyst for use herein, a slurry may be formed
by deagglomerating the molecular sieve, preferably in an aqueous
solution. The slurry of the matrix material may be formed by mixing
the desired matrix components such as clay and/or inorganic oxide
in an aqueous solution. The molecular sieve slurry and the matrix
slurry are then well mixed and spray dried to form catalyst particles
of, for example, less than 200 microns in diameter.
Additive Catalyst
It is conventional to use an additive catalyst with different properties
along with the conventional catalyst to form an optional mixed catalyst
system. Commercially used additives are shape-selective zeolites.
Zeolites having a Constraint Index of 1-12 can be used for this
purpose. Details of the Constraint Index test are provided in J.
Catalysis, 67 218-222 (1981) and in U.S. Pat. No. 4711710 both
of which are incorporated herein by reference.
Conventional shape-selective zeolites useful for this purpose are
exemplified by intermediate pore (e.g., less than about 7 Angstroms
pore size, such as from about 5 to less than about 7 Angstroms)
zeolites ZSM-5 (U.S. Pat. No. 3702886 and Re. 29948): ZSM-11
(U.S. Pat. No. 3709979): ZSM-12 (U.S. Pat. No. 4832449): ZSM-22
(U.S. Pat. No. 4556477): ZSM-23 (U.S. Pat. No. 4076842): ZSM-35
(U.S. Pat. No. 4016245); ZSM-48 (U.S. Pat. No. 4397827); ZSM-57
(U.S. Pat. No. 4046685); PSH-3 (U.S. Pat. No. 4439409); and
MCM-22 (U.S. Pat. No. 4954325) either alone or in combination.
In addition, the catalyst composition may include metals useful
in promoting the oxidation of carbon monoxide to carbon dioxide
under regenerator conditions as described in U.S. Pat. No. 4350614.
The additive catalyst may also include phosphorus or a phosphorus
compound for any of the functions generally attributed thereto.
The additive catalyst required for the present, improved process
is synthesized and formulated in a very special way to provide certain
physical properties. The crystal component of the additive catalyst
has the structure of ZSM-5 a silica/alumina mole ratio of less
than about 30 usually from about 20 to less than about 30 and
a high as-synthesized alkali and/or alkaline earth metal to silica
molar ratio. The as-synthesized crystal has a formula, on an anhydrous
basis and in terms of y moles of SiO.sub.2 as follows:
wherein x is greater than about 0.1 usually greater than about
0.3 most often from greater than about 0.4 to about 1.4 and y
is less than about 30 usually from about 20 to less than about
30 more usually from about 23 to less than about 30. The M and
R components are associated with the material as a result of their
presence during crystallization, described in more detail below,
and may be reduced or removed by post-crystallization methods herein
more particularly described.
The synthesis of this special ZSM-5 crystalline material requires
forming a reaction mixture hydrogel having a pH of from about 10
to about 14 preferably from about 11.5 to about 13.5 and containing
sources of alkali or alkaline earth metal (M) cations; an oxide
of aluminum; an oxide of silicon; n-propylamine directing agent
(R); and water, said reaction mixture having a composition in terms
of mole ratios, within the following ranges:
______________________________________ Reactants Useful Preferred
______________________________________ SiO.sub.2 /Al.sub.2 O.sub.3
<40 20 to 35 H.sub.2 O/SiO.sub.2 10 to 35 10 to 30 OH.sup.- /SiO.sub.2
0.1 to 0.3 0.1 to 0.2 M/SiO.sub.2 0.2 to 0.6 0.3 to 0.5 R/SiO.sub.2
0.01 to 0.6 0.02 to 0.3 ______________________________________
The reaction is maintained until crystals of the ZSM-5 structure
are formed. Reaction conditions required consist of heating the
foregoing reaction mixture to a temperature of from about 100.degree.
C. to about 200.degree. C. for a period of time of from about 10
hours to about 100 hours. A more preferred temperature range is
from about 130.degree. C. to about 180.degree. C. with the amount
of time at a temperature in such range being from about 20 hours
to about 60 hours. The solid product comprising ZSM-5 crystals is
recovered from the reaction medium, as by cooling the whole to room
temperature, filtering, and water washing.
The additive catalyst comprising this specially prepared ZSM-5
for use herein is prepared as follows:
The recovered ZSM-5 crystals are ammonium exchanged such as by
contact with, for example, ammonium nitrate, sulfate, hydroxide,
or halide, e.g., chloride, solution. The exchanged crystals may
then be washed with, for example, deionized water, and dried.
The ion-exchanged crystalline material is then deagglomerated.
This may be accomplished by ball milling an aqueous slurry of the
zeolite crystals.
The deagglomerated crystalline ZSM-5 material is the slurried with
matrix material such as, for example, silica, clay and/or alumina,
at a pH of from about 2 to about 12 preferably from about 4 to
about 6 to yield a ZSM-5/matrix material composition comprising
from about 5 to about 80 wt. % ZSM-5 and from about 20 to about
95 wt. % matrix. Phosphorus compounds, e.g., phosphoric acid, may
be added to the composition in this step of the manufacture such
that elemental phosphorus comprises from about 1.5 to about 5.5
wt. % of the matrix of the product material.
The final ZSM-5/matrix slurry is then dried, such as by spray drying
to form a fluid powder, at a temperature of, for example, from about
65.degree. C. to about 315.degree. C.
This dried ZSM-5/matrix composition is then converted to the protonic
form having an Alpha Value of greater than about 30. This conversion
may be accomplished by, for example, acid treatment, ammonium exchange,
and/or calcination. If acid treatment or ammonium exchange is performed,
calcination will follow.
Acid treatment for this purpose comprises, for example, contacting
the dried ZSM-5/matrix composition with a 0.1 to about 1 N mineral
acid such as, for example, hydrochloric acid, or a carboxylic or
dicarboxylic acid such as, for example, oxalic acid, at room temperature
or a temperature up to about 150.degree. C. The acid treated composition
may be washed with, for example, deionized water and again dried
at a temperature of, for example, from about 65.degree. C. to about
315.degree. C.
Ammonium exchange for this purpose comprises, for example, contacting
the dried ZSM-5/matrix composition with ammonium nitrate, sulfate,
hydroxide, and/or halide solution, washing the exchanged catalyst
material with, for example, deionized water, and again drying the
product catalyst material at a temperature of, for example, from
about 65.degree. C. to about 315.degree. C.
The dried ZSM-5/matrix composition, whether acid treated or ammonium
exchanged or not, is then calcined at a temperature of from about
200.degree. C. to about 550.degree. C. for from about 1 minute to
about 48 hours. The calcined ZSM-5/matrix catalyst will have an
Alpha Value of greater than about 30 usually from greater than
about 30 to about 1200. A preferred calcination procedure in accordance
herewith would be to provide a calcined product catalyst which retains
a trace amount of carbon residue. Therefore, partial calcination
within the above conditions, e.g., at lower temperature and/or shorter
time, is preferred.
Optionally, although not necessary nor, in fact, preferred, for
the process of this invention, the calcined catalyst material may
be subjected to steaming in an atmosphere of from about 5 to about
100% steam for at least about 1 hour, e.g., from about 1 hour to
about 200 hours, at a temperature of at least about 300.degree.
C., e.g., from about 300.degree. C. to about 800.degree. C. The
resulting steamed catalyst will have an Alpha Value of from about
1 to about 10.
Matrix
The matrix, i.e., binder, materials used are resistant to the temperatures
and other conditions e.g., mechanical attrition, which occur in
various hydrocarbon conversion processes such as cracking. It is
generally necessary that the catalysts be resistant to mechanical
attrition, that is, the formation of fines which are small particles,
e.g., less than 20 .mu.m. The cycles of cracking and regeneration
at high flow rates and temperatures, such as in an FCC process,
have a tendency to break down the catalyst into fines, as compared
with an average diameter of catalyst particles of about 60-90 microns.
In an FCC process, catalyst particles range from about 10 to about
200 microns, preferably from about 20 to 120 microns. Excessive
generation of catalyst fines increases the refiner's catalyst costs.
The matrix may fulfill both physical and catalytic functions. Matrix
materials include active or inactive inorganic materials such as
clays, and/or metal oxides such as alumina or silica, titania, zirconia,
or magnesia. The metal oxides may be in the form of a gelatinous
precipitate or gel.
Use of an active matrix material in conjunction with the molecular
sieve component that is combined therewith, may enhance the conversion
and/or selectivity of the overall catalyst composition in certain
hydrocarbon conversion processes. Inactive materials may serve as
diluents to control the amount of conversion in a given process
so that products can be obtained economically and in an orderly
fashion without employing other means for controlling the rate of
reaction. These materials may be incorporated as naturally occurring
clays to improve the attrition resistance of the catalyst under
commercial operating conditions.
Naturally occurring clays which can be composited with the catalyst
include the montmorillonite and kaolin families which include the
subbentonites, and the kaolins commonly known as Dixie, McNamee,
Georgia and Florida clays or others in which the main mineral constituent
is halloysite, kaolinite, dickite, nacrite or anauxite. Such clays
can be used in the raw state as originally mined or initially subjected
to calcination, acid treatment or chemical modification.
In addition to the foregoing materials, catalysts can be composited
with a porous matrix material such as silica-alumina, silica-magnesia,
silica-zirconia, silica-thoria, silica-beryllia, silica-titania,
as well as ternary materials such as silica-alumina-thoria, silica-alumina-zirconia,
silica-alumina-magnesia, silica-magnesia-zirconia. The matrix can
be in the form of a cogel. A mixture of these components can also
be used.
In general, the relative proportions of finely divided, crystalline
molecular sieve component and inorganic oxide gel matrix vary widely,
with the molecular sieve content ranging from about 1 to about 90
percent by weight, and more usually from about 2 to about 80 weight
percent of the composite.
The large-pore molecular sieve material may comprise from about
10 to about 80 weight percent of the catalyst composition. For the
additive catalyst, the specially synthesized ZSM-5 may comprise
from about 1 to about 50 weight percent of the additive catalyst
composition.
Although neither the cracking catalyst nor the additive catalyst
need be steamed prior to use in the present process, and, in fact,
are preferably not steamed prior to use herein, they may be steamed
at a temperature of from about 300.degree. C. to about 800.degree.
C. for a time of from about 1 to about 200 hours in about 5 to about
100% steam.
In order to more fully illustrate the nature of the invention and
the manner of practicing same, the following examples are presented.
In the examples, whenever adsorption data are set forth for comparison
of sorptive capacities for water, cyclohexane and n-hexane, they
are determined as follows:
A weighed sample of the calcined adsorbant is contacted with the
desired pure adsorbate vapor in an adsorption chamber, evacuated
to 1 mm and contacted with 12 mm Hg of water vapor or 20 mm Hg of
n-hexane, or cyclohexane vapor, pressures less than the vapor-liquid
equilibrium pressure of the respective adsorbate at room temperature.
The pressure is kept constant (within about .+-.0.5 mm) by addition
of absorbate vapor controlled by a manostat during the adsorption
period, which does not exceed about 8 hours. As adsorbate is adsorbed
by the sorbant material, the decrease in pressure causes the manostat
to open a valve which admits more adsorbate vapor to the chamber
to restore the above control pressures. Sorption is complete when
the pressure change is not sufficient to activate the manostat.
The increase in weight is calculated as the adsorption capacity
of the sample in g/100 g of calcined adsorbant.
When Alpha Value is examined, it is noted that the Alpha Value
is an approximate indication of the catalytic cracking activity
of the catalyst compared to a standard catalyst and it gives the
relative rate constant (rate of normal hexane conversion per volume
of catalyst per unit time). It is based on the activity of silica-alumina
cracking catalyst taken as an Alpha of 1 (Rate Constant=0.016 sec.sup.-1).
The Alpha Test is described in U.S. Pat. No. 3354078; in the Journal
of Catalysis, 4 527 (1965); 6 278 (1966); and 61 395 (1980),
each incorporated herein by reference as to that description. The
experimental conditions of the test used herein include a constant
temperature of 538.degree. C. and a variable flow rate as described
in detail in the Journal of Catalysis, 61 395.
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