Note: Descriptions are shown in the official language in which they were submitted.
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T~TM~T OF ~Or.TT~ TO T~PROV~ TTS RUT~N~ S~T.~l~lvl~l~Y
This invention relates to a process for treating a
medium-pore zeolite to improve its butene selectivity in
catalytic cracking.
In fluidized catalytic cracking (FCC) processes, a
relatively heavy hydrocarbon feedstock, e.g., a gas oil,
admixed with a suitable cracking catalyst to provide a
fluidized suspension, is cracked in an elongated reactor,
or riser, at elevated temperatures to provide a mixture of
lighter hydrocarbon products. The 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 hydrocarbon product 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 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.
The use of medium pore zeolites, such as ZSM-5, in
conjunction with a large pore zeolite cracking catalyst of
the X or Y faujasite variety is described in U.S. Patent
Nos. 3,894,931; 3,894,933; and 3,894,934.
It has now been found that pretreatment of a medium
pore zeolite catalyst by steaming and acid treatment has an
~ unexpected effect on its butene selectivity in catalytic
cracking processes.
The invention therefore includes a process for
increasing the butene selectivity of a cracking catalyst
comprising a crystalline aluminosilicate zeolite having a
Constraint Index of 1 to 12, which process comprises
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steA~;ng said zeolite after calcination and contacting the
steA ~~ zeolite with an acidic solution.
The invention further includes a process for the
catalytic cracking of a hydrocarbon by contact of said
hydrocarbon with a catalytic cracking catalyst, comprising
a calcined medium pore crystalline aluminosilicate zeolite,
wherein said zeolite has been pretreated by st~A~;ng and
contacting the steamed zeolite with an acidic solution.
The process of the present invention involves
treatment of a medium pore zeolite, such as ZSM-5, with an
acidic solution after st~A ;ng to improve butene
selectivity in catalytic cracking. The zeolite is calcined
prior to st~A~;ng.
A convenient measure of the extent to which a zeolite
provides control of access to molecules of varying sizes to
its internal structure is the Constraint Index of the
zeolite. Zeolites which provide a highly restricted access
to and egress from its internal structure have a high value
for the Constraint Index, and zeolites of this kind usually
have pores of small size, e.g. less than 5 Angstroms. On
the other hand, zeolites which provide relatively free
access to the internal zeolite structure have a low value
for the Constraint Index, and usually have pores of large
size, e.g. greater than 8 Angstroms. The method by which
Constraint Index is determined is described fully in U.S.
Patent No. 4,016,216.
The crystalline aluminosilicate zeolites useful herein
are medium pore zeolites. Medium pore zeolites generally
have a Constraint Index of 1-12.
Constraint Index (CI) values for some typical medium
pore zeolites useful in the process of the invention are:
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CT (~t test tf~E?eratllre)
ZSM-5 6-8.3 (371~C-316~C)
ZSM-ll 5-8.7 (371~C-316~C)
ZSM-12 2.3 (316~C)
ZSM-22 7.3 (427~C)
ZSM-23 9.1 (427~C)
ZSM-35 4.5 (454~C)
ZSM-38 2 (510~C)
ZSM-48 3.5 (538~C)
ZSM-50 2.1 (427~C)
TMA Offretite 3.7 (316~C)
Clinoptilolite 3.4 (510~C)
Zeolite Beta 0.6-2.0 (316~C-399~C)
Preferred zeolites for use in the present process
include ZSM-5, ZSM-ll, ZSM-12, ZSM-22, ZSM-23, ZSM-35 and
ZSM-48. ZSM-5 is particularly preferred.
ZSM-5 is described in greater detail in U.S. Patent
Nos. 3,702,886 and Re. 29,948.
ZSM-ll is described in greater detail in U.S. Patent
No. 3,709,979.
ZSM-12 is described in U.S. Patent No. 3,832,449.
ZSM-22 is described in U.S. Patent No. 4,556,477.
ZSM-23 is described in U.S. Patent No. 4,076,842.
ZSM-35 is described in U.S. Patent No. 4,016,245.
ZSM-48 is more particularly described in U.S. Patent
No. 4,234,231.
The zeolite is calcined in air or other inert gas at
temperatures ranging from 200~C to 900~C for periods of
time from 1 to 48 hours or more.
It has been found in accordance with the present
invention that catalysts of improved selectivity for
butenes are obtained by subjecting the calcined zeolite to
a steam treatment followed by acid treatment. Preferably,
the starting zeolite has a silica/alumina ratio of less
than about 200:1.
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The steam treatment is conducted at elevated
temperatures in the range of 425 to 870~C (800 to 1600~F)
and preferably in the range of 540 to 815~C (1000 to
1500~F) and at a water partial pressure of 13 to 200 kPa
(100 to 1500 torr). The treatment may be accomplished with
an atmosphere comprising 5 to 100% steam. The ste~m;ng is
carried out for a period in the range of from 0.5 to 12
hours.
Following steaming the zeolite is contacted with an
acidic solution. Typical inorganic acids which can be
employed include mineral acids such as hydrochloric,
sulfuric, nitric and phosphoric acids, peroxydisulfonic
acid, dithionic acid, sulfamic acid, peroxymonosulfuric
acid, amidodisulfonic acid, nitrosulfonic acid,
chlorosulfonic acid, pyrosulfonic acid,and nitrous acid.
Representative organic acids which may be used include
formic acid, oxalic acid, trichloroacetic acid and
trifluoroacetic acid. Hydrochloric acid and nitric acid
are preferred.
The acid treatment is conducted at a temperature in
the range of 35 to 120~C (100 to 250~F). The acid
treatment is carried out for a period in the range of 1 to
36 hours. The treatment may be accomplished with an acidic
solution having a pH less than 4.
It may be desired to incorporate the zeolite with
another material which is resistant to the temperatures and
other conditions employed in the present process. Such
materials include active and inactive materials and
synthetic or naturally occurring zeolites as well as
inorganic materials such as clays, silica and/or metal
oxides such as alumina. The latter may be either naturally
occurring or in the form of gelatinous precipitates or gels
including mixtures of silica and metal oxides. Use of a
material in conjunction with the zeolite, i.e., combined t
therewith or present during its synthesis, which itself is
catalytically active may change the conversion and/or
selectivity of the catalyst. Inactive materials suitably
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serve as diluents to control the amount of conversion so
that products can be obtained economically and orderly
without employing other means for controlling the rate of
reaction. These materials may be incorporated into
naturally occurring clays, e.g., bentonite and kaolin, to
improve the crush strength of the catalyst under commercial
operating conditions. Said materials, i.e., clays, oxides,
etc., function as binders for the catalyst. It is
desirable to provide a catalyst having good crush strength
because in commercial use, it is desirable to prevent the
catalyst from breaking down into powder-like materials.
These clay binders have been employed normally only for the
purpose of improving the crush strength of the catalyst.
Naturally occurring clays which can be composited with
zeolite crystals include the montmorillonite and kaolin
family, which families 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. Binders useful
for compositing with the zeolite also include inorganic
oxides, notably alumina.
In addition to the foregoing materials, the crystals
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
compositions such as silica-alumina-thoria, silica-alumina-
zirconia, silica-alumina-magnesia and silica-magnesia-
zirconia. It may also be advantageous to provide at least
a part of the foregoing matrix materials in colloidal form
so as to facilitate extrusion of the bound catalyst
component(s).
The relative proportions of finely divided crystalline
material and inorganic oxide matrix vary widely, with the
crystal content ranging from 1 to 90 percent by weight and
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more usually, particularly when the composite is prepared
in the form of beads, in the range of 2 to 80 weight
percent of the composite.
The cracking catalyst comprising the medium pore
zeolite pretreated in accordance with the invention will
also normally contain a large pore cracking catalyst having
a constraint index less than 1 such as zeolite X or Y. The
large pore zeolite may be combined with the same matrix as
the medium pore zeolite (so that a single catalyst particle
contains both zeolites) or may be combined with a separate
matrix (so that each zeolite is contained by a separate
catalyst particle).
Hydrocarbon charge stocks undergoing cracking in
accordance with this invention comprise hydrocarbons
generally and, in particular, petroleum fractions having an
initial boiling range of at least 200~C (400~F), a 50%
point range of at least 260~C (500~F) and an end point
range of at least 315~C (600~F). Such hydrocarbon
fractions include gas oils, residual oils, cycle stocks,
whole top crudes and heavy hydrocarbon fractions derived by
the destructive hydrogenation of coal, tar, pitches,
asphalts and the like. As will be recognized, the
distillation of higher boiling petroleum fractions above
about 400~C (750~F) 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.
Catalytic cracking, in which the catalysts of the
invention are employed, embraces operational conditions
including temperature ranges of 400~F (204~C) to 1200~F
(649~C) and reduced, atmospheric or super atmospheric
pressures. The catalytic cracking process may be operated
batchwise or continuously. The catalytic cracking process
can be either fixed bed, moving bed or fluidized bed. The
hydrocarbon chargestock flow may be either concurrent or
countercurrent to the catalyst flow. The process of the
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invention is particularly applicable to fluid catalytic
cracking (FCC) processes.
Briefly, in the FCC process, the catalyst is in the
form of microspheres, which act as a fluid when suspended
in oil vapor or gas. The hydrocarbons contact the
fluidized catalysts and are catalytically cracked to
lighter products. Deactivation of the catalyst by coke
necessitates regeneration of the coked catalyst in the
regenerator of an FCC unit. Although the design and
construction of individual FCC units can vary, the
essential elements of an FCC unit are illustrated in U.S.
Patent No. 4,368,114.
The treatment process of the present invention results
in at least about a 3~ increase in the C4/C3 ratio and
preferably at least about a 10% increase in the C4/C3 ratio.
In refineries where butene is more valuable than propene,
the impact of the process of the present invention is
significant.
The treatment process of the present invention further
results in an increase in gasoline selectivity since there
is a correlation between C4/C3 selectivity and gasoline
selectivity. A ZSM-5 FCC additive is gasoline selective if
it causes little gasoline yield loss per octane gain.
The invention will now be more particularly described
with reference to the Examples and the accompanying
drawings, in which:
Figure 1 is a graphical illustration of the C4/C3
selectivity using the steamed ZSM-5 of Example 2 in
comparison to the steamed/acid treated ZSM-5 of Example 3.
Figure 2 is a graphical illustration of the C4/C3
selectivity using the steamed ZSM-5 of Example 5 in
comparison to the st~me~/acid treated ZSM-5 of Example 6.
~am~le 1
A steam treated catalyst is prepared by steaming a
calcined, unbound ZSM-5 with 100% steam at 760~C (1400~F)
and 1 atmosphere (100 kPa) pressure for 5 hours. The
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steamed ZSM-5 has an Si/Al ratio of about 60 and an average
particle size of about 0.5 micron.
~x~le 2
A 1:1 mixture of l-hexene and l-octene is
representative of a mixture of gasoline range (C5-C12)
olefins found in FCC feedstocks. The 1:1 hexene/octene
mixture is reacted over the catalyst of Example 1 at 540~C
(1000~F) and a pressure of about 1.1 atm (111 kPa). The
hydrocarbon feed partial pressure is about 0.4 atmospheres
(40 kPa) and a nitrogen carrier gas is also used. The flow
rate is varied to obtain conversions of the hexene/octene
feed to C5- products in the range of 20 to 50%.
Conversions, the yields of propene and butene and the C4/C3
ratio are shown below in Table 1.
~x~le 3
The catalyst of Example 1 is refluxed in 0.5 N HCl for
24 hours, filtered and dried. The catalyst is then used to
crack the hexene/octene feed mixture as set forth in
Example 2. Conversions, the yields of propene and butene
and the C4/C3 ratio are shown below in Table 1.
~x~le 4
A steam treated ZSM-5 catalyst is prepared in the same
manner as Example 1. The st~A~~~ ZSM-5 has an Si/Al ratio
of about 60 and an average particle size of about 0.1
micron.
~x~le 5
A 1:1 hexene/octene mixture is reacted over the
catalyst of Example 4 in the same manner as Example 2.
Conversions, the yields of propene and butene and the C4/C3
ratio are shown below in Table 1.
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_9_
~ le 6
The catalyst of Example 5 is refluxed in 0.5 N HCl for
24 hours, filtered and dried. The catalyst is then used to
crack the hexene/octene feed mixture as set forth in
Example 2. Conversions, the yields of propene and butene
and the C4/C3 ratio are shown below in Table 1.
TABT~
C6+ Conv. Propene Butene
s~n~le Wt%Y;el-l, Wt%Yiel~, Wt% C4/C3
Example 2 31.2 9.2 11.6 1026
Example 2 45.014.2 16.3 1~15
Example 2 48.615.6 17.5 1 12
Example 2 54.517.8 19.4 1 09
Example 3 31.7 9.1 12.0 1032
Example 3 39.211.3 14.8 1.31
Example 3 52.416.5 19.1 1c16
Example 5 18.8 5.6 7.0 1.25
Example 5 45.514.3 16.8 1.17
Example 5 52.717.2 19.1 1.11
Example 6 17.0 4.5 6.7 1.49
Example 6 40.311.5 15.7 1.37
Example 6 53.216.5 19.9 1.21
The process of the present invention maximizes the
C4/C3 ratio. Figure 1 compares C4/C3 ratios for the catalyst
of Example 1 before and after the acid treatment. Figure 2
compares the C4/C3 ratios for the catalyst of Example 4
before and after acid treatment. Table 1 shows that the
acid treatment results in an increase in the butene
selectivity. It is believed that the non-framework alumina
generated during st~A~;ng partially blocks the transport of
species in the zeolite crystal, resulting in lower butene
selectivity. The acid treatment, however, removes at least
part of this alumina with a concomitant increases in butene
selectivity.
