Note: Descriptions are shown in the official language in which they were submitted.
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DESULFURIZATION OF OLEFINIC GASOLINE
WITH A DUAL FUNCTIONAL CATALYST AT LOW PRESSURE
This invention relates to a process for desulfurizing olefinic naphtha by
olefin cracking
to produce a low sulfur, high octane product with improved gasoline yields.
More specifically,
the invention relates to a low pressure process which advantageously relies on
a dual
functional catalyst for hydrotreating and paraffin cracking in the same
reactor volume. The
operating conditions of the process provide significant olefins cracking while
minimizing olefin
saturation from the metals.
io Catalytically cracked gasoline currently forms a major part of the gasoline
product
pool in the United States and the cracking process contributes a large
proportion of the sulfur
in the gasoline. The sulfur impurities may require removal, usually by
hydrotreating, in order
to comply with product specifications or to ensure compliance with
environmental regulations.
Low sulfur levels result in reduced emissions of CO, NOx and hydrocarbons. In
addition,
other environmental controls may be expected to impose increasingly stringent
limits on
gasoline composition. Currently, the requirements of the U.S. Clean Air Act
and the physical
and compositional limitations imposed by the Reformulated Gasoline (RFG) and
EPA
Complex Model regulations will result not only in a decrease in permissible
sulfur levels but
also in limitations on boiling range, typically measured by minimum Reid Vapor
Presssure
(RVP) and T~ specifications. Limitations on aromatic content may also arise
from the
Complex Model regulations.
Cracked naphtha, as it comes from the catalytic cracker and without any
further
treatments, such as purifying operations, has a relatively high octane number
as a result of the
presence of oleflnic components. In some cases, this fraction may contribute
as much as up to
half the gasoline in the refinery pool, together with a significant
contribution to product
octane. Other unsaturated fractions boiling in the gasoline boiling range,
which are produced
in some refineries or petrochemical plants, include pyrolysis gasoline and
coker naphtha.
Pyrolysis gasoline is a fraction which is often produced as a by-product in
the cracking of
petroleum fractions to produce light unsaturates, such as ethylene and
propylene. Pyrolysis
3o gasoline has a very high octane number but is quite unstable in the absence
of hydrotreating
because, in addition to the desirable olefins boiling in the gasoline boiling
range, it also
contains a substantial proportion of diolefins, which tend to form gums after
storage or
standing. Coker naphtha is similar in containing significant amounts of sulfur
and nitrogen as
well as diolefins which make it unstable on storage.
Hydrotreating of any of the sulfur containing fractions, which boil in the
gasoline
boiling range, causes a reduction in the olefin content and consequently a
reduction in the
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2
octane number. As the degree of desulfurization increases, the octane number
of the normally
liquid gasoline boiling range product decreases. Some of the hydrogen may also
cause some
hydrocracking as well as olefin saturation, depending on the conditions of the
hydrotreating
operation.
s Naphthas and other light fractions such as heavy cracked gasoline may be
hydrotreated
by passing the feed over a hydrotreating catalyst at an elevated temperature
and a somewhat
elevated pressure in a hydrogen atmosphere. One suitable family of catalysts
which has been
widely used for this service is a combination of a Group VIII element and a
Group VI element,
such as cobalt and molybdenum, on a substrate such as alumina. After the
hydrotreating
to operation is complete, the product may be fractionated, or simply flashed,
to release the
hydrogen sulfide and collect the now sweetened gasoline.
Various proposals have been made for removing sulfur while retaining the more
desirable olefins. The sulfur impurities tend to concentrate in the heavy
fraction of the
gasoline and hydrodesulfurization processes have been employed that treat only
the heavy
15 fraction of the catalytically cracked gasoline so as to retain the octane
contribution from the
olefins which are found mainly in the lighter fraction. In one commercial
operation, the
selectivity for hydrodesulfurization relative to olefin saturation is shifted
by suitable catalyst
selection, for example, by the use of a magnesium oxide support instead of the
more
conventional alumina.
2o In any case, regardless of the mechanism by which it happens, the decrease
in octane
which takes place as a consequence of sulfur removal by hydrotreating creates
a conflict
between the growing need to produce gasoline fuels with higher octane number
and because
of current ecological considerations the need to produce cleaner burning, less
polluting fixels,
especially low sulfur fuels. This inherent conflict is yet more marked in the
current supply
25 situation for low sulfur, sweet crudes.
Aromatics are generally the source of high octane number, particularly very
high
research octane numbers, and are, therefore, desirable components of the
gasoline pool.
However, they have been the subject of severe limitations as a gasoline
component because of
possible adverse effects on the ecology, particularly with reference to
benzene. Thus, it has
30 become desirable, as far as is feasible, to create a gasoline pool in which
the higher octaves are
contributed by the olefinic and branched chain paraffinic components, rather
than the aromatic
components.
It has now been discovered that the problems encountered in the prior art can
be
overcome by the present invention, which provides a process for desulfurizing
olefinic naphtha
35 by olefin cracking at low pressure to produce a low sulfur, high octane
product with improved
gasoline yields. More specifically, the invention uses a dual fiznctional
catalyst for
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3
hydrotreating and paraffin cracking in the same reactor volume at low
pressures. Under these
conditions, olefins cracking is favored over olefin saturation from the
metals.
The present invention provides a process for reducing sulfur content of
gasoline while
substantially maintaining road octane number. The process includes contacting
a catalyticaJly
cracked olefinic gasoline stream containing organic sulfur compounds and
having an initial
boiling point in the gasoline boiling range, an initial sulfiu content, an
initial bromine number
and an initial road octane number with a dual functional catalyst. The
catalyst is made up of
an intermediate pore size zeolite having an alumina substrate and impregnated
with at least one
metal selected from the group consisting of Group VI metals of the Periodic
Table and Group
1o VIII metals of the Periodic Table. The gasoline stream contacts the
catalyst under a
combination of a pressure of from 100 to 600 psig (790.86 to 4238.35 kPaa), a
space velocity
of from 0. I to 10 LHSV and an atmosphere comprising hydrogen to convert the
sulfur
compounds to hydrogen sulfide. The hydrogen sulfide can be removed from the
gasoline
stream to provide a product gasoline with a reduced sulfur content lower than
the initial sulfur
content. The product gasoline also has a less than 5% change in the road
octane number.
The process of the present invention uses an intermediate pore size zeolite
catalyst
which can be selected from a group of several catalysts, including ZSM-5, ZSM-
11, ZSM-22,
ZSM-12, ZSM-23, ZSM-35, ZSM-48, ZSM-57, ZSM-58, MCM-22 and M-41 S. In a
preferred embodiment of the present invention, the intermediate pore size
zeolite is
2o impregnated with cobalt and molybdenum. The amounts of cobalt and
molybdenum can vary
according to several factors, such as, the composition of the feedstock, the
process operating
conditions and the desired characteristics of the product gasoline. The most
preferred ranges,
in terms of the total weight of the impregnated catalyst, are from 0. S% to
10% by weight
cobalt and from I % to 20% by weight molybdenum.
The present invention has the following process conditions: the space velocity
is from
0.1 to 10 LHSV and preferably 0.5 to 5 LHSV; the ratio of hydrogen to
hydrocarbon is 100 to
5,0~ standard cubic feet of hydrogen per barrel of hydrocarbon (1?.8 to 890
n.l.l.'' ) and
preferably 500 to 2,500 standard cubic feet of hydrogen per barrel of
hydrocarbon (89 to 445
n.l.l.'1 ); the pressure range is from 100 to 600 psig (790.86 to 4238.35
kPaa) and preferably
100 to 400 prig (790.86 to 3163.44 kPaa); and the operating temperature is
from 600° to
800°F (315.56° to 426.67°C), preferably operating
temperature of from 700° to 750°F
(371.11° to 398.89°C).
In a preferred embodiment, the reduced sulfur content of the recovered
gasoline stream
is from 5 to 10% of the initial sulfur content.
In another embodiment, the distillation of the olefinic gasoline stream is
less than SO%
and the olefin saturation of the product gasoline measured in terms of bromine
number is less
than SO~/o of the initial bromine number.
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The present invention has several advantages over gasoline desulfurization
processes
known in the prior art, including high desulfixrization with higher gasoline
yields and reduced
octane loss at almost any desulfurization level. The dual functional catalyst
used in the present
invention has the advantage of desulfiuizing and cracking the feedstock in one
vessel, in
contrast to prior art processes that use separate vessels for desulfiuizing
and cracking. The
Iower operating pressure of the present invention also provides the advantage
of increasing the
stability of the dual functional catalyst. Additionally, the present
irrvention provides the
advantage of lower hydrogen consumption by cracking olefins directly before
hydrogenation.
As new gasoline regulations permit lower amounts of sulfur, an increasing
amount of
o cracked gasoline has to be hydrofinished. This typically results in a severe
octane loss. The
process of the present invention provides high desulfurization and high
gasoline yields by using
a dual functional catalyst that both desulfurizes and cracks the olefinic
naphthas. In addition,
by adjusting the process parameters, the amount of olefins in the product
gasoline can be
tailored to meet different target specifications.
The present invention maintains octane and desulfiuizes olefinic naphtha by
olefin
cracking at low pressure using a dual functional catalyst that has a very
strong desulfurization
function as well as a selective cracking function. This produces a low sulfur,
high octane
product with geatly improved gasoline yields compared to other processes. The
dual catalyst
system performs separate desulfurization by Cobalt Molybdenum and paraffin
cracking by
2o zeolite. In a preferred embodiment, the process uses a CoMo/ZSM-5 catalyst.
At higher
pressures, this dual functional catalyst can be used for conventional
hydrotreating of olefinic
naphtha to paraffins and subsequent paraffin cracking to higher octaves. It
has been
discovered that at lower pressures, the dual functional catalyst has an
unexpected different
chemistry. At lower pressures, olefins are cracked directly to lighter
material before the
2s olefins are saturated over the metals. A significant advantage of the
process of the present
invention is that the desulfurization function has been added without
sacrificing cracking
activity. This results in minimal octane losses at almost any desulfiuization
level. Another
advantage of the dual functional catalyst is that both hydrotreating and
paraffin cracking can
be done in the same reactor vessel. In addition, the lower operating presssure
increases the
3o stability of the dual functional catalyst.
The dual fi~nction catalyst of the present invention employs metals to remove
heteroatoms, such as sulfur and nitrogen, while saturating the olefins. Once
saturated, the
zeolite portion of the catalyst selectively cracks the low octane paraffins
raising the octane at
the expense of gasoline yields loss.
3s The chemistry of the catalytic reaction changes at lower pressure favoring
olefin
cracking instead of olefin saturation from the metals. This has a very
positive impact on
product yields and properties. The most important result is that significant
octane loss from
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saturation does not occur and, therefore, the operating temperature can be
adjusted to achieve
high or mid level desulfurization without significant octane loss. At the
lower operating
pressure, hydrogen consumption is significantly less and the tolerance to
nitrogen poisons has
been found to be greater. In addition, the light gases in the gasoline product
become more
s olefinic.
The present invention can operate at surprisingly low temperatures
(400° to 750°F
versus 675° to 800°F) (204.44° to 398.89°C versus
357.22° to 426.67°C) for the typical
catalytic hydrodesulfurization process) and provides higher gasoline yields
than conventional
hydrodesulfurixation processes, especially at less than 95% desulfixrization.
However, the
1o preferred operating temperature is typically below 700°F
(371.11°C). The octane
enhancement chemistry of the present invention is dominated by olefin
cracking, in contrast to
a conventional process, in which paraffin cracking is responsible for the
octane enhancement.
Under the low-pressure operating conditions of the present invention, the
catalyst has been
found to be more nitrogen tolerant and can be operated at higher liquid hourly
space velocities
(LHS~. This results in the production of more gasoline. In addition, the dual
functional
catalysts of the present invention, such as a CoMo promoted ZSM-5 catalyst,
operated at low
pressure do not require a high degree of denitrogenation for octane
enhancement.
FEEDSTOCK
The feed to the process comprises a sulfur-containing petroleum fi~action that
boils in
2o the gasoline boiling range, which can be regarded as extending from C6 to
500°F (260°C)
although lower end points below the 500°F (260°) end point are
more typical. Feeds of this
type include light naphthas typically having a boiling range of C4 to
330°F (166°C), full range
naphthas typically having a boiling range of CS to 420°F
(215.56°C), heavier naphtha fractions
boiling in the range of 260° to 420°F (126.67° to
215.56°C), or heavy gasoline fractions
boiling at, or at least within, the range of 330° to 500°F
(165.56° to 260°C), preferably from
330° to 420°F (166° to 215.56°C). While the mast
preferred feed appears at this time to be a
heavy gasoline produced by catalytic cracking; or a light or full range
gasoline boiling range
fraction, the best results are obtained when, as described below, the process
is operated with a
gasoline boiling range fraction which has a 95 percent point (determined
according to ASTM
so D 86) of at least 325°F (162.78°C) and preferably at least
350°F (176.67°C), for example, 95
percent points (T95) of at least 380°F (193.33°C) or at least
400°F (204.44°C). The process
can be applied to thermally cracked naphthas such as pyrolysis gasoline, coker
naphtha and
visbreaker naphtha as well as catalytically cracked naphthas such as thermofor
catalytic
cracking (TCC) or fluid catalytic cracking (FCC) naphtha since both types are
usually
characterized by the presence of olefinic unsaturation and the presence of
sulfur. From the
point of view of volume, however, the main application of the process is
likely to be with
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catalytically cracked naphthas, especially FCC naphthas and for this reason,
the process will be
described with particular reference to the use of catalyticatly cracked
naphthas.
The process can be operated with the entire gasoline fraction obtained from
the
catalytic cracking step or, alternatively, with part of it. Because the sulfur
tends to be
concentrated in the higher boiling fractions, it is preferable, particularly
when unit capacity is
limited or a high degee of sulfur removal is required, to separate the higher
boiling fractions
and process them through the steps of the present process without processing
the lower
boiling cut. The cut point between the treated and untreated fractions can
vary according to
the sulfur compounds present. A cut point in the range of from 100° to
300°F (37.78° to
l0 148.89°C) is preferred, and a cut point in the range of 200°
to 300°F (93.33° to 148.89°C) is
the most preferred. The exact cut point selected will depend on the sulfur
specification for the
gasoline product as well as on the type of sulfur compounds present; lower cut
points will
typically be necessary for lower product sulfur specifications. Sulfur which
is present in
components boiling below 180°F (82.22°C) is mostly in the form
of mercaptans, which can be
removed by extractive type processes. However, hydrotreating is appropriate
for the removal
of thiophene and other cyclic sulfur compounds present in higher boiling
components, that is,
component fractions boiling above 180°F (82.22°C). Treatment of
the lower boiling fraction
in an extractive type process coupled with hydrotreating of the higher boiling
component can
represent a preferred economic process option. Such a variant of the process
is described in
2o U.S. Serial No. 08/042,189 filed 30 March 1993 now U.S. Patent No.
5,360,532 and U.S.
Serial No. 07/001,681 filed 7 January 1993 now U.S. Patent No. 5,318,690.
Higher cut
points will be preferred in order to minimize the amount of feed which is
passed to the
hydrotreater and the final selection of cut point together with other process
options such as
the extractive type desulfurization will, therefore, be made in accordance
with the product
2s specifications, feed constraints and other factors.
The sulfur content of these catalytically cracked fractions will depend on the
sulfur
content of the feed to the cracker as well as on the boiling range of the
selected fraction used
as the feed in the process. Lighter fractions, for example, will tend to have
lower sulfur
contents than the higher boiling fractions. As a practical matter, the sulfur
content will exceed
30 50 ppmw and, in most cases, the sulfur content will be in excess of 500
ppmw. For the
fractions which have 95 percent points over 380°F (193.33°C),
the sulfur content can exceed
1,000 ppmw and can be as high as 4,000 to 5,000 ppmw, or higher. The nitrogen
content is
not as characteristic of the feed as the sulfur content and is preferably not
greater than 20
ppmw, although higher nitrogen levels typically up to 70 ppmw can be found in
certain higher
35 boiling feeds with 95 percent points in excess of 380°F
(193.33°C). The nitrogen level will,
however, usually not be greater than 250 or 300 ppmw. As a result of the
cracking which
precedes the steps of the present process, the feed to the
hydrodesulfurization step will be
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olefinic, with an olefin content of at Ieast 5% by weight and more typically
in the range of 15
to 20 weight percent (wt.%), although higher olefin levels, for example 40
wt.%, or even
higher, can be encountered in specific charge stocks, such as gasoline
obtained from resid
catalytic cracking (RCC) processes.
PROCESS CONFIGLm ATTON
The present invention includes the use of a dual functional catalyst that has
a very
strong desulfurization function as well as a selective cracking function to
treat the
sulfur-containing, gasoline boiling range feed. The dual catalyst functions as
a conventional
hydrotreating catalyst to separate sulfur from the feed molecules and convert
it to hydrogen
to sulfde. The dual catalyst also contains an intermediate pore size zeolite
that promotes
catalytic cracking.
The catalyst used in the hydrodesulfurization step is suitably a conventional
desulfurization catalyst made up of a Group VI and/or a Group VIII metal on a
suitable
substrate. The Group VI metal is preferably molybdenum or tungsten and the
Group VIII
metal preferably nickel or cobalt. Combinations, such as NiMo, CoMo and
NiCoMo, are
typical with CoMo used in preferred embodime~tQ, nrt;e; metals which possess
hydrogenation
functionality are also usefl"1 in this service. The support for the catalyst
is conventionally a
porous sotid, u~,;aily alumina, or silica-alumina but other porous solids such
as magnesia,
titani_p or silica, either alone or mixed with alumina or silica-alumina can
also be used, as
2o convenient.
The particle size and the nature of the hydrotreating catalyst will usually be
determined
by the type of hydrotreating process which is being carried out, although in
most cases, a
down-flow, fixed bed process is preferred.
The hydrogenation reaction and the cracking reaction performed by the dual
functional
catalyst are complimentary because the hydrogenation reactions are exothermic,
and result in a
rise in temperature, while the cracking reaction is an endothermic reaction.
Therefore, the
hydrotreating conditions are adjusted not only to obtain the desired degree of
desulfurization
but also to produce the optimum temperature for promotion of the desired shape-
selective
cracking reactions.
3o The preferred catalysis for this invention contain zeolite-type crystals
and, molt
preferably, intermediate pore size zeolites. For purposes of this invention,
the term "zeolite" is
meant to represent the class of porotectosilicates, i.e., porous crystalline
silicates, that contain
silicon and oxygen atoms as the major components. Other components can be
present in
minor amounts, usually less than 14 mole.%, and preferably less than 4 mole.%.
These
components include alumina, gallium, iron, boron and the like, with aluminum
being preferred,
and used herein for illustrative purposes. The minor components can be present
separately or
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8
in mixtures in the catalyst. They can also be present intrinsically in the
structure of the
catalyst.
The dual functional catalyst promotes cracking reactions that convert low
octane
para~ns into higher octane products, both by the selective cracking of heavy
para~ns to
lighter paraffins and by cracking low octane n-paraffins, in both cases with
the generation of
olefins. Ring-opening reactions can also take place, leading to the production
of further
quantities of high octane gasoline boiling range components. The dual
functional catalyst also
improves product octane by dehydrocyclization/aromatization of paraffins to
alkylbenzenes.
The extent of the desulfurization will depend on the feed sulfur content and,
of course,
to on the product sulfur specification with the reaction parameters selected
accordingly. It is not
necessary to go to very iow nitrogen levels but low nitrogen levels can
improve the activity of
the cracking catalyst. Normally, the denitrogenation which accompanies the
desulfurization
will result in an acceptable organic nitrogen content. However, if it is
necessary to increase
the denitrogenation in order to obtain a desired level of activity in the
cracking step, the
i5 operating conditions for the hydrogenation reaction can be adjusted
accordingly.
The operating conditions are selected to produce a controlled degree of
cracking.
Typically, the temperature of the reactor will be 300° to 800°F
(148.89° to 426.67°C),
preferably 400° to 750°F {204.44° to 398.89°C).
The reactor pressure will typically operate
at from 100 to 600 psig (790.86 to 4238.35 kPaa), preferably 200 to 400 psig
(1581.71 to
20 3163.44 kPaa) with comparable space velocities, typically from 0.1 to 10
LHSV (hc''), and
preferably from 0.5 to 5 LHSV (hr''). The present catalyst combination of
molybdenum on
ZSM-5 has been found to be effective at low pressures below 200 psig (1480.36
kPaa) and
even below 150 psig (1034.25 kPaa). Hydrogen to hydrocarbon ratios typically
of 100 to
5,000 scf/bbl (17.8 to 890 n.l.l.'' ), preferably 500 to 2,500 scf/bbl (89 to
445 n.l.l.'' ) are
25 selected to minimize catalyst aging.
Consistent with the objective of restoring lost octane while retaining overall
product
volume, the conversion to products boiling below the gasoline boiling range
(Cs-) during the
cracking is held to a minimum and distillation of the gasoline feed stream is
maintained below
50%. However, because the cracking of the heavier portions of the feed can
lead to the
3o production of products still within the gasoline range, the conversion to
Cs- products is at a
low level, in fact, a net increase in the volume of Cs+ material can occur
during this stage of
the process.
The acidic component of the dual functional catalyst is an intermediate pore
size
zeolite. Zeolites of this type are characterized by a crystalline structure
having rings of
35 ten-membered rings of oxygen atoms through which molecules obtain access to
the
intracrystalline pore volume. These zeolites have a Constraint Index from 2 to
12, as defined
in U.S. Patent No. 4,016,218, to which reference is made for a description of
the method of
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9
determining Constraint Index and examples of the Constraint Indices for a
number of zeolites.
ZeoIites of this class are well-known intermediate ore size aluminosilicate
zeolites; typical
members of this class are the zeolites having the structures of ZSM-5 (U. S.
Patent Nos.
3,702,886 and Re 29,948); ZSM-11 (U.S. Patent No. 3,709,979); ZSM-12 (U.S.
Patent No.
s 3,832,449); ZSM-22 (U.S. Patent No. 4,656,477); ZSM-23 (U.S. Patent No.
4,076,842);
ZSM-35 (U.S. Patent No. 4,016,245); ZSM-48 (U.S. Patent No. 4,397,827); ZSM s7
(U.S.
Patent No. 4,046,685); ZSM-s8 (U.S. Patent No. 4,417,780); M-41S (U.S. Patent
No.
5,098,684) and MCM-22 (U.S. Patent Nos. 4,954,325 and 4,962,256). ZSM-S is the
preferred zeolite for use in the present process. The aluminosilicate forms of
these zeolites
to provide the requisite degree of acidic functionality and for this reason
are the preferred
compositional forms of the zeolites. Other isostructural forms of the
intermediate pore size
zeolites containing other metals instead of aluminum such as gallium, boron or
iron can also be
used.
The zeolite catalyst possesses suiflcient acidic functionality to bring about
the desired
~s reactions to restore the octane lost in the hydrotreating reaction. The
catalyst should have
sufficient acid activity to have cracking activity that is sufficient to
convert the appropriate
portion of the feed, suitably with an alpha value of at least 10, usually in
the range of 20 to
800, and preferably at least s0 to 200 (values measured prior to addition of
the metal
component). The alpha value is one measure of the acid activity of a catalyst;
it is a measure
20 of the ability of the catalyst to crack normal hexane under prescribed
conditions. This test has
been widely published and is conventionally used in the petroleum cracking
art, and compares
the cracking activity of a catalyst under study with the cracking activity,
under the same
operating and feed conditions, of an amorphous silica-alumina catalyst, which
has been
arbitrarily designated to have an alpha activity of 1. The alpha value is an
approximate
2s indication of the catalytic cracking activity of the catalyst compared to a
standard catalyst.
The alpha test gives the relative rate constant (rate of normal hexane
conversion per volume of
catalyst per unit time) of the test catalyst relative to the standard catalyst
which is taken as an
alpha of 1 (Rate Constant = 0.016 sec.''). The alpha test is described in U.S.
Patent No.
3,354,078 and in J. Catalysis, 4, 527 (1965); 6, 278 (1966); and 61,395
(1980), to which
3o reference is made for a description of the test. The experimental
conditions of the test used to
determine the alpha values referred to in this specification include a
constant temperature of
538°C and a variable flow rate as described in detail in J. Catalysis,
61,395 (1980).
The zeolite component of the dual functional catalyst will usually be
composited with a
binder or substrate because the particle sizes of the pure zeolite are too
small and lead to an
3s excessive pressure drop in a catalyst bed. This binder or substrate, which
is preferably used in
this service, is suitably any refractory binder material. Examples of these
materials are well
known and typically include silica, silica-alumina, silica-zirconia, silica-
titania, alumina.
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The dual functional catalyst also contains Group VIB and Group VII metals,
such as
cobalt and molybdenum, components which improve catalyst desulfurization
activity, stability
as well as for improving product quality as described above. Typically, the
cobalt and
molybdenum will be in the oxide or the sulfide form; it can readily be
converted from the oxide
form to the sulfide by conventional pre-sulfiding techniques. A molybdenum
contern of 1 to
10 wt.%, conventionally 5 to 10 wt.%, (as metal) is suitable although higher
metal loadings
typically up to 15 wt% can be used. A cobalt content of 0.5 to 5 wt.% {as
metal),
conventionally 3 to 4 wt.%, is suitable.
The molybdenum component can be incorporated into the dual functional catalyst
by
l0 conventions) procedures such as impregnation into an extrodate or by
mulling with the zeolite
and the binder. When the molybdenum is added in the form of an anionic complex
such as
molybdate, impregnation or addition to the muller will be appropriate methods.
The particle size and the nature of the catalyst will usually be determined by
the type of
conversion process which is being carried out with operation in a down-flow,
fixed bed
is process being typical and preferred.
The conditions of operation and the catalysts should be selected based on the
characteristics of the feed so that the gasoline product octane is not
substantially lower than
the octane of the feed gasoline boiling range material; that is, not lower by
more than 1 to 10
octane numbers and usually, not more than 1 to 3 octane numbers, depending on
the nature of
2o the feed. It is preferred also that the volume of the product should not be
substantially less
than that of the feed although yields as low as 80% can be achieved with
certain feeds under
particular conditions. In some cases, the volumetric yieid and/or octane of
the gasoline boiling
range product can be higher than those of the feed, as noted above and in
favorable cases, the
octane barrels (that is the octane number of the product times the volume of
product) of the
25 product will be higher than the octane barrels of the feed.
EXAMPLES
A full range feedstock was processed using the dual function catalyst system
of the
present invention at two different pressures (550 and 350 psig) (3893.6 kPaa
to 2514.60
kPaa). The feedstock properties are as follows:
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TABLE 1
PROPERTIES OF FEEDSTOC K
API Gravity 48.0
Motor Octane (M+O) 78,8
Research Octane (R+p) 90.0
Road Octane 84.4
Bromine Number 40.61
Sulfur, ppm 2802
Nitrogen, ppm 62
to
Distillation fD861. C
IBP 54
50 140
EP 220
EXAMPLE 1
In this example, the full range feedstock shown in Table 1 was desulfurized
under high
pressure conditions. initially, the feedstock was contacted with a commercial
cobalt
molybdenum catalyst to saturate diolefln at a low temperature. The diolefin-
removed
2o feedstock was desulfurized at a space velocity of 2.0 LHSV (liquid hourly
space velocity);
with 2,500 scf/bbl (445 n.l.l.'1 ) hydrogen circulation and 550 psig (3893.6
kPaa) total pressure
over a CoMo ZSM-5 at various temperatures. The results are summarized in Table
2.
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TABLE 2
AIGH PRESSURE BASE
ZSM-5 temperature 500F 600F 650F 700F 725F 750F
260C 316C 343C 371C 385C 410C
Octaves R+O 76.8 75.8 76 85 87.8 90.7
M+O 71.1 71.8 72 79.3 80.6 81.9
API 50.5 56.3 55.7 55.3 55.1 53.1
Sulfur, ppm 49 250 101 123 52 154
Nitrogen, ppm 8 15 <5 <5 <5 <5
Cs+CZ Nil 0.01 0.04 0.29 0.63 1.48
C3 0.02 0.04 0.36 4.61 5.51 10.59
C3s Nil Nil 0.01 0.02 0.02 0.06
ICe 0.02 0.25 0.32 3.47 3.66 6.16
NC4 0.11 1.24 1.02 2. 84 3 .22 5.3
7
C4- Nil Nil Nil 0.01 0.01 0.08
Cs+ 100.01 99.57 99.36 89.67 87.86 77.67
H2 consumption, 407 719 713 625 634 869
Scf/bbl (72.45 { I ( 126.91( 111.25( 112.85( 154.68
27.98 -i -1 -i -i
n.l.l. -1 n.l.l. n.l.l. n.l.l. n.l.l.
n.l.l.
EXAMPLE 2
In this example, the full range feedstock shown in Table 1 was desulfurized at
low
pressure conditions using the present invention. The diolefin-removed
feedstock was
desulfurized at 3.0 LHSV, 2,500 scflbbl (445 n.l.l.'~ ) hydrogen circulation,
300 psig (2169.85
kPaa) and cascaded over CoMo ZSM-5 at 3.0 LHSV at various temperatures. The
results are
l0 summarized in Table 3.
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TABLE 3
LOW PRESSURE DIOLEFIN SATURATION
ZSM-5 temperature 550F 600F 650F 700F 725F 750F
288C 316C 343C 371C 385C 410C
Octaves R+O 89.6 86.8 86 87.8 87.5 88
M+O 79 78 77.5 80 79.4 79.6
API 50.5 49.9 48.1 47.1 48.6 49.1
Sulfur, ppm 593 192 169 99 131 101
Nitrogen, ppm 44 42 38 30 14 10
Cl+C2 0.01 0.02 0.05 0.06 0.15 0.23
0.14 0.17 0.73 2.01 3.0 3.8
0.03 0.11 0.38 0.3 0.26 0.24
IC4 0.13 0.08 0.60 1.71 2.3I 2.71
nC4 0.20 0.13 0.48 1.23 I.84 2.29
Ce- 0.23 0.40 1.05 0.79 0.56 0.52
Cs+ 99.45 99.30 96.79 93.82 92.02 90.24
Hi consumption, 201 222 165 125 199 145
scf/bbl (36.78 (39.51 (29.37 (22.25 (35.42 (25.81
-i .i _i -i _1 _i
n.l.l. n.l.l. n.l.l. n.l.l. n.l.I. n.l.l.
s
The low-pressure data shows that motor octane (M+O) did not drop offwith
higher
temperat«re, while research octane (R+O) dropped only mildly compared to the
high-pressure
data. The synergism between the metals and the ZSM-5 allowed this to work. The
advantage
is higher gasoline yields and lower hydrogen consumption at equivalent
desulfurization: The
low pressure operation can tolerate a higher residual nitrogen content to
enhance product
octane. In comparison, the octane enhancement for the high pressure operation
is
accompanied by a very high degree of denitrogenation. This is because the ZSM-
5 is
preferentially cracking olefins prior to saturation. A better illustration is
shown below in
Example 3 for a lighter feed.
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EXAMPLE 3
In this example, the FCC gasoline feedstock shown in Table 4 was desulfurized
using
the low pressure process of the present invention. Initially, the feedstock
was contacted with a
commercial cobalt molybdenum catalyst for diolefin saturation at low
temperature. The
s diolefin-removed feedstock was desulfurized at 3.0 LHSV, and 2,500 scf/bbl
(445 n.l.l.'' )
hydrogen circulation; with 300 psig (2372.58 kPaa) total pressure over CoMo
ZSM-S at
various temperatures. The catalyst used was a standard hydrogen ZSM-5,
commonly used for
catalytic dewaxing or cracking, impregnated with 3.0 wt.% cobalt and 8.8 wt.%
molybdenum.
The results of the test are shown below in Table 5.
to The bromine numbers of the desulfuriaed gasoline products were measured to
determine the change in composition. The bromine number is a method of
calculating the
contents of an olefin. The number of grams of bromine absorbed by 100 grams of
gasoline
indicates the percentage of double bonds present. Thus, when the type and
molecular weight
is known, the contents of the olefin can be calculated.
is TABLE 4
C5/C6 FCC Gasoline
API Gravity 74.5
Motor Octane 79.8
Research Octane 94.7
2o Road Octane 87.3
Bromine Number 81.4
Sulfur, ppm 487
Nitrogen, ppm 15
25 Distillation ~(I~8_6y. °C
IBP 29
50 57
EP 101
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TABLE 5
LOW PRESSURE SATURATION WITH A LIGHT FEEDSTOCK
ZSM-5 temperature400F 500F 600F 700F 725F 770F
204 260C 316C 371C 385C 410C
Octaves R+O 93.5 93.4 89.2 83.2 86.2 88.8
M+O 79.1 79.4 78.8 78.3 80.8 81.8
API 73.8 73.4 73.7 71.1 68.8 65.8
Sulfur, ppm 360 239 62 34 36 25
Nitrogen, ppm 9 <5 <5 <5 <5 <S
Cl+C2 0.00 0.00 0.01 1.16 2.47 3.32
C3 0.00 0.02 0.14 5.79 9.87 12.40
C3' 0.00 0.00 0.09 0.66 0.60 0.54
IC4 O.I9 0.13 0.26 6.80 7.36 8.35
nC4 0.32 0.25 0.78 6.22 5.85 6.91
C,' 1.61 1.19 1.27 1.92 0.9 0.77
Cs~' 97.84 98.35 97.68 77.68 73.71 68.45
H2 consumption, -20 -28 116 416 354 344
scf/bbl (-3.56 (-4.984(20.65 (74.05 (63.01 (61.23
-i _i .i -i .i _i
n.l.l. n.l.l. n.l.l. n.l.l. n.l.l. n.l.l.
Bromine number 80.7 92.7 72.7 19.4 9.3 6.0
5
At less than 700°F (3? 1 °C), the bromine numbers indicate that
considerable olefins
have not been saturated while the olefins in the light gases indicate olefins
have been cracked
from heavier olefins.
