Molecular sieve abstract
Using a catalyst comprising a silicoaluminophosphate molecular
sieve such as SAPO-11 and SAPO-41 and platinum or palladium, a
hydrogenation component, heavy oils are simultaneously subjected
to hydrocracking and isomerization reactions. The process selectively
produces middle distillates in high yields having good low temperature
fluid characteristics, especially reduced pour point and viscosity.
Molecular sieve claims
What is claimed is:
1. A process for selectively producing middle distillate hydrocarbons
by hydrocracking and isomerizing a hydrocarbonaceous feed wherein
at least 90% of said feed has a boiling point above about 600.degree.
F. comprising:
a. contacting under hydrocracking conditions wherein the pressure
is from about 200 psig to about 3000 psig and a conversion of greater
than about 50%, said hydrocarbonaceous feed with a catalyst comprising
a silicoaluminophosphate molecular sieve having a three-dimensional
microporous crystal framework structure of tetrahedral units whose
unit empirical formula on an anhydrous basis is:
wherein R represents at least one organic templating agent present
in the intracrystalline pore system; "m" represents the
moles of "R" present per mole of (Si.sub.x Al.sub.y P.sub.z)O.sub.2
and has a value from zero to about 0.3 "x", "y"
and "z" represent respectively, the mole fractions of
silicon, aluminum and phosphorus, said mole fractions being within
the compositional area bounded by points A, B, C, D and E on the
temary diagram which is FIG. 1 said silicoaluminophosphate selected
from the group consisting of SAPO-11 having an X-ray powder diffraction
pattern as set forth in Table I and SAPO-41 having an X-ray powdered
diffraction pattern as set forth in Table III and at least one hydrogenation
component selected from the group consisting of platinum and palladium;
(b) recovering a hydrocarbonaceous effluent wherein more than about
40% by volume of said effluent boils above about 300.degree. F.
and below about 725.degree. F. and has a pour point below 0.degree.
F.
2. The method of claim 1 wherein the silicoaluminophosphate is
SAPO-11.
3. The method of claims 1 and 2 wherein the hydrogenation component
is platinum.
4. The method of claims 1 and 2 wherein the hydrogenation component
is palladium.
5. The method of claim 1 wherein the hydrogenating component is
present in the range of 0.01% to 10% based on the weight of molecular
sieve.
6. The process of claim 1 wherein said process is conducted at
a temperature of from about 260.degree. C. to 482.degree. C., a
pressure of about 200 psig to about 3000 psig, a liquid hourly space
velocity of from about 0.1 hr.sup.-1 to about 20 hr.sup.-1 and
a hydrogen circulation rate of from 400 to 15000 SCF/bbl.
7. The method of claim 1 wherein said catalyst further comprises
an inorganic oxide matrix.
8. The process of claim 7 wherein said matrix is alumina.
9. The process of claim 1 wherein said catalyst further comprises
a nickel, cobalt, molybdenum, or tungsten component, or mixtures
thereof.
10. The process of claim 1 wherein said feed is a gas oil.
11. A process of claim 1 wherein said feed has a content of nitrogen-containing
impurities, calculated as nitrogen, which is below about 10 ppmw.
12. A process as in claim 1 wherein said hydrocarbon feed is selected
from the group consisting of petroleum distillates, solvent deasphalted
residua, and shale oils.
13. The process of claim 1 wherein greater than 50% by weight of
converted product boils above about 300.degree. F. and below 725.degree.
F.
14. A process of claim 1 wherein said catalyst is disposed downstream
of a reaction zone in which is hydrocarbon feed is contacted under
hydroprocessing conditions with an active hydrodenitrogenation catalyst.
15. A process of claim 14 wherein said hydrodenitrogenation catalyst
is disposed in a single reactor with said catalyst.
Molecular sieve description
BACKGROUND OF THE INVENTION
1. Field of the Invention
This invention pertains to a catalytic composition for use in a
hydrocracking process to maximize middle distillate yields for hydrocarbons
boiling above about 600.degree. F. In particular, this invention
concerns a hydrocracking and isomerization process to selectively
produce low pour point middle distillate hydrocarbons from a hydrocarbonaceous
feed boiling above about 600.degree. F. by contacting the feedstock
with a catalyst comprising a specific class of silicoaluminophosphate
molecular sieves and a hydrogenation component.
2. Description of the Prior Art
Hydrocracking, used either in a one-step process or in a multistep
process coupled with hydrodenitrofication and/or hydrodesulfurization
steps, has been used extensively to upgrade poor-quality feeds and
to produce middle distillate materials. Over the years, much development
work has been done into finding improved cracking conditions and
catalysts. Tests have used catalysts containing only amorphous materials
and catalysts containing zeolites composited with amorphous materials.
Large pore size zeolites such as zeolites X and Y are presently
considered the most active hydrocracking catalysts. However, high
activity is not the only essential characteristic of midbarrel cracking
in catalysts. Midbarrel selectivity, namely, the percentage of total
conversion accounted for by products boiling within the midbarrel
range, from about 300.degree. F. to about 725.degree. F., is also
important. In fact, as noted in U.S. Pat. No. 3853742 Ward, Dec.
10 1974 many commercial midbarrel hydrocracking processes do not
use zeolitic catalysts due to their relatively low midbarrel selectivity.
Also, middle distillates conventionally serve as fuels such as
diesel oils, furnace oils, and the like. For convenience, in the
handling and in the use of these middle distillates, it is desirable
for the pour point to be as low as practical consistent with the
temperatures to which they may be exposed. Specifications for these
products often include a requirement that the pour point may not
exceed a certain maximum value. In some instances it is necessary
to subject these distillate fuels to additional processing whose
principle purpose is to reduce the pour point of the feed stream.
Pour point can also be lowered by lowering the distillate end point,
but this reduces yield.
As noted in U.S. Pat. No. 4486296 Oleck, Dec. 4 1984 that
while zeolite catalysts have been employed in hydrocracking processes
and although they may be effective in providing distillate yields
having one or more properties consistent with the intended use of
the distillate, these catalysts have come, in general, to suffer
the disadvantage of not providing product yields having good low
temperature fluidity characteristics, especially reduced pour point
and viscosity.
The prior art has utilized a separate dewaxing process in order
to reduce the pour point of the middle distillate using such selective
intermediate pore size zeolites as ZSM-5 (U.S. Pat. No. RE. 28398),
and ZSM-23 (European Patent Application No. 0092376).
Other methods in the art to produce middle distillates possessing
acceptable viscosity and pour point properties include processes
wherein the hydrocarbon feeds are concurrently or sequentially subjected
to hydrocracking and dewaxing in a continuous process using a large
pore size zeolite hydrocarbon cracking catalyst such as zeolite
X or zeolite Y and intermediate pore zeolite dewaxing zeolite catalyst,
such as ZSM-5 (U.S. Pat. No. 3758402).
These processes have two drawbacks. The first is that while the
pour point is reduced, the viscosity is increased, possibly above
acceptable limits. Secondly, the process operates by cracking wax
primarily to light products (e.g., C.sub.3 -C.sub.4) thereby significantly
reducing distillate yield. International Application No. WO86/03694
published July 3 1986 under the PCT discloses a hydrocracking process
to produce high octane gasoline using a catalyst comprising silicoaluminophosphates
alone or in combination with traditional hydrocracking catalysts
such as zeolite aluminosilicates.
As noted above, the prior art processes for preparing middle distillates
in high yields having appropriate viscosity and pour points involve
the use of multisteps and/or multicatalysts.
It has now been discovered that middle distillate products can
be selectively produced in a simplified process and over a single
catalyst, in high yields and having reduced pour points and viscosities
as compared to prior art processes.
It has now been found that heavy hydrocarbon oils may be simultaneously
hydrocracked and hydrodewaxed to produce a midbarrel liquid product
of improved yields and having satisfactory pour point and viscosity.
This desirable result is obtained by the use of a catalyst which
contains a SAPO-11 or SAPO-41 silicoaluminophosphate molecular sieve
component, and which includes a platinum or palladium hydrogenation
component to promote isomerization. Other conventional type metals,
for example, nickel, tungsten, cobalt, molybdenum or combinations
of these metals with platinum or palladium, e.g., Co-Mo, Ni-W, may
also be used.
SUMMARY OF THE INVENTION
In accordance with the present invention, there has been discovered
a process for selectively producing middle distillate hydrocarbons
by hydrocracking and isomerizing a hydrocarbonaceous feed wherein
at least 90% of said feed has a boiling point above about 600.degree.
F. comprising:
(a) contacting under hydrocracking conditions said hydrocarbonaceous
feed with a catalyst comprising a silicoaluminophosphate molecular
sieve selected from the group consisting of SAPO-11 and SAPO-41
and at least one hydrogenation component selected from the group
consisting of platinum and palladium; and
(b) recovering a hydrocarbonaceous effluent. With respect to the
effluent, preferably more than about 40% by volume boils above about
300.degree. F. and below about 725.degree. F. and has a pour point
below 0.degree. F.
The silicoaluminophosphate molecular sieves used in the process
of this invention are described in U.S. Pat. No. 4440871 which
is incorporated totally herein by reference.
A preferred embodiment is a process using a catalyst comprising
SAPO-11 and a platinum or palladium component.
Among other factors, the present invention is based on my discovery
that using a silicoaluminophosphate catalyst such as SAPO-11 and
SAPO-41 containing a metal such as platinum and/or palladium, in
a hydrocracking and isomerization reaction of hydrocarbonaceous
feeds boiling above 600.degree. F., results in unexpectedly high
yields of middle distillate having excellent pour point characteristics.
In the process of the invention, the hydrocarbon feedstock is heated
with the catalyst under conversion conditions which are appropriate
for hydrocracking. During the conversion, the aromatics and naphthenes
which are present in the feedstock undergo hydrocracking reactions
such as dealkylation, ring opening, and cracking, followed by hydrogenation.
The long-chain paraffins, which are present in the feedstock, undergo
mild cracking reactions to yield non-waxy products of higher molecular
weight than compared to products obtained using the prior art dewaxing
zeolitic catalysts such as ZSM-5 and at the same time, a measure
of isomerization takes place so that not only is the pour point
reduced by reason of the cracking reactions described above, but
in addition the n-paraffins become isomerized to isoparaffins to
form liquid-range materials which contribute to low viscosity, low
pour point products.
The process enables heavy feedstocks, such as gas oils, boiling
above 600.degree. F. to be more selectively converted to middle
distillate range products having improved pour points in contrast
to prior processes using large pore catalysts, such as zeolite Y,
and the consumption of hydrogen will be reduced even though the
product will conform to the desired specifications for pour point
and viscosity.
In contrast to dewaxing processes using shape selective catalysts
such as zeolite ZSM-5 the yields will be improved and the viscosity
kept acceptably low. The latter is ensured by the bulk conversion
involving not only the cracking of low viscosity paraffins but high
viscosity components (e.g., multi-ring naphthenes) as well. Thus,
the present process is capable of effecting a bulk conversion together
with a simultaneous dewaxing. It is also possible to operate at
partial conversion, thus effecting economies in hydrogen consumption
while still meeting pour point and viscosity requirements.
Overall, the process also achieves enhanced selectivity for the
production of middle distillate range materials wherein greater
than 50% by weight of the converted products boil in the middle
distillate range having improved pour points; and the yield of gas
and products boiling below the distillate range is reduced.
BRIEF DESCRIPTION OF THE DRAWINGS
FIG. 1 is a ternary diagram showing the compositional parameters
of the silicoaluminophosphates of U.S. Pat. No. 4440871 in terms
of mole fractions of silicon, aluminum, and phosphorus.
FIG. 2 is a ternary diagram showing the preferred compositional
parameters of the silicoaluminophosphates in terms of mole fractions
of silicon, aluminum, and phosphorus.
FIG. 3 is a graph showing a comparison for a crystalline silicoaluminophosphate
catalyst used in the process of this invention and a sulfided cogelled
nickel-tungsten-silica-alumina catalyst with respect to yields.
FIG. 4 is a graph showing a comparison for a crystalline silicoaluminophosphate
catalyst used in the process of this invention and a ZSM-5 catalyst
with respect to yields.
DETAILED DESCRIPTION
Feedstocks
The feedstock for the process of the invention comprises a heavy
hydrocarbon oil such as a gas oil, coker tower bottoms fraction,
reduced crude, vacuum tower bottoms, deasphalted vacuum resids,
FCC tower bottoms, or cycle oils. Oils derived from coal, shale,
or tar sands may also be treated in this way. Oils of this kind
generally boil above 600.degree. F. (316.degree. C.) although the
process is also useful with oils which have initial boiling points
as low as 436.degree. F. (260.degree. C.). Preferably at least 90%
of the feed will boil above 600.degree. F. (316.degree. C.) and
most preferably at least about 90% of the feed will boil between
700.degree. F. (371.degree. C.) and about 1200.degree. F. (649.degree.
C.). These heavy oils comprise high molecular weight long-chain
paraffins and high molecular weight ring compounds with a large
proportion of fused ring compounds. During the processing, both
the fused ring aromatics and naphthenes and paraffinic compounds
are cracked by the SAPO-containing catalyst to middle distillate
range products. A substantial fraction of the paraffinic components
of the initial feedstock also undergo conversion to isoparaffins.
The process is of particular utility with highly paraffinic feeds
because, with feeds of this kind, the greatest improvement in pour
point may be obtained. However, most feeds will contain a certain
content of polycyclic compounds.
As discussed hereinafter, the feedstocks may have been subjected
to a hydrofining and/or hydrogenation treatment, which may have
been accompanied by some hydrocracking, before being supplied to
the present process.
Silicoaluminophosphate Catalysts Compositions (SAPOs)
As mentioned above, the process combines elements of hydrocracking
and isomerization. The catalyst used in the process has an acidic
component, and a platinum or palladium hydrogenation component.
The acidic component comprises a SAPO-11 and SAPO-41 silicoaluminophosphates,
which are described in U.S. Pat. No. 4440871 and reference is
made to this patent for details of these molecular sieves and their
preparation, which patent is incorporated totally herein by reference.
The SAPO-11 and SAPO-41 silicoaluminophosphate molecular sieves
(SAPOs) suitable for use in the instant process comprise a molecular
framework of corner-sharing [SiO.sub.2 ] tetrahedra, [AlO.sub.2
] tetrahedra and [PO.sub.2 ] tetrahedra, [i.e., (Si.sub.x Al.sub.y
P)O.sub.2 tetrahedral units], and which functions when combined
with a platinum or palladium hydrogenation component to convert
at effective process conditions the aforementioned feedstock to
middle distillate products.
The unit empirical formula for a SAPO may be given on an "as-synthesized"
basis or may be given after an "as-synthesized" SAPO composition
has been subjected to some post treatment process, e.g., calcined.
The term "as-synthesized" herein shall be used to refer
to the SAPO composition(s) formed as a result of the hydrothermal
crystallization but before the SAPO composition has been subjected
to post treatment to remove any volatile components present therein.
The actual value of "m" for a post-treated SAPO will depend
on several factors (including: the particular SAPO, template, severity
of the post-treatment in terms of its ability to remove the template
from the SAPO, the proposed application of the SAPO composition,
and etc.) and the value for "m" can be within the range
of values as defined for the as-synthesized SAPO compositions although
such is generally less than the as-synthesized SAPO unless such
post-treatment process adds template to the SAPO so treated. A SAPO
composition which is in the calcined or other post-treated form
generally has an empirical formula represented by Formula (1), except
that the value of "m" is generally less than about 0.02.
Under sufficiently severe post-treatment conditions, e.g., roasting
in air at high temperature for long periods (over 1 hr.), the value
of "m" may be zero (0) or, in any event, the template,
R, is undetectable by normal analytical procedures.
The above silicoaluminophosphates are generally synthesized by
hydrothermal crystallization from a reaction mixture comprising
reactive sources of silicon, aluminum and phosphorus, and one or
more organic templating agents. Optionally, alkali metal(s) may
be present in the reaction mixture. The reaction mixture is placed
in a sealed pressure vessel, preferably lined with an inert plastic
material, such as polytetrafluoroethylene, and heated, preferably
under autogenous pressure at a temperature of at least about 100.degree.
C., and preferably between 100.degree. C. and 250.degree. C., until
crystals of the silicoaluminophosphate product are obtained, usually
for a period of from 2 hours to 2 weeks. While not essential to
the synthesis of SAPO compositions, it has been found that in general
stirring or other moderate agitation of the reaction mixture and/or
seeding the reaction mixture with seed crystals of either the SAPO
to be produced, or a topologically similar composition, facilitates
the crystallization procedure. The product is recovered by any convenient
method such as centrifugation or filtration.
After crystallization the SAPO may be isolated and washed with
water and dried in air. As a result of the hydrothermal crystallization,
the as-synthesized SAPO contains within its intracrystalline pore
system at least one form of the template employed in its formation.
Generally, the template is a molecular species, but it is possible,
steric considerations permitting, that at least some of the template
is present as a charge-balancing cation. Generally the template
is too large to move freely through the intracrystalline pore system
of the formed SAPO and may be removed by a post-treatment process,
such as by calcining the SAPO at temperatures of between about 200.degree.
C. and to about 700.degree. C. so as to thermally degrade the template
or by employing some other post-treatment process for removal of
at least part of the template from the SAPO. In some instances the
pores of the SAPO are sufficiently large to permit transport of
the template, and, accordingly, complete or partial removal thereof
can be accomplished by conventional desorption procedures such as
carried out in the case of zeolites.
The SAPOs are preferably formed from a reaction mixture having
a mole fraction of alkali metal cation which is sufficiently low
that it does not interfere with the formation of the SAPO composition.
Although the SAPO compositions will form if alkali metal cation
are present, such reaction mixtures are not generally preferred.
A reaction mixture, expressed in terms of molar oxide ratios, having
the following bulk composition is preferred:
wherein "R" is a template; "a" has a value
great enough to constitute an effective concentration of "R"
and is within the range of from greater than zero (0) to about 3;
"b" has a value of from zero to 500; "x", "y"
and "z" represent the mole fractions, respectively of
silicon, aluminum and phosphorus wherein x, y and z each have a
value of at least 0.01. The reaction mixture is preferably formed
by combining at least a portion of the reactive aluminum and phosphorus
sources in the substantial absence of the silicon source and thereafter
combining the resulting reaction mixture comprising the aluminum
and phosphorus sources with the silicon source. When the SAPOs are
synthesized by this method the value of "m" in Formula
(1) is generally above about 0.02.
Though the presence of alkali metal cations are not preferred,
when they are present in the reaction mixture it is preferred to
first admix at least a portion of each of the aluminum and phosphorus
sources in the substantial absence of the silicon source. This procedure
avoids adding the phosphorus source to a highly basic reaction mixture
containing the silicon and aluminum source.
The reaction mixture from which these SAPOs are formed contain
one or more organic templating agents (templates) which can be most
any of those heretofore proposed for use in the synthesis of aluminosilicates.
The template preferably contains at least one element of Group VA
of the Periodic Table, particularly nitrogen, phosphorus, arsenic
and/or antimony, more preferably nitrogen or phosphorus and most
preferably nitrogen. The template contains at least one alkyl, aryl,
araalkyl, or alkylaryl group. The template preferably contains from
1 to 8 carbon atoms, although more than eight carbon atoms may be
present in the template. Nitrogen-containing templates are preferred,
including amines and quaternary ammonium compounds, the latter being
represented generally by the formula R'.sub.4 N+ wherein each R'
is an alkyl, aryl, alkylaryl, or araalkyl group; wherein R' preferably
contains from 1 to 8 carbon atoms or higher when R' is alkyl and
greater than 6 carbon atoms when R' is otherwise, as herein before
discussed. Polymeric quaternary ammonium salts such as [(C.sub.14
H.sub.32 N.sub.2)(OH).sub.2 ].sub.x wherein "x" has a
value of at least 2 may also be employed. The mono-, di- and tri-amines,
including mixed amines, may also be employed as templates either
alone or in combination with a quaternary ammonium compound or another
template.
Representative templates, phosphorus, aluminum and silicon sources
as well as detailed process conditions are more fully described
in U.S. Pat. No. 4440871 which is incorporated totally herein
by reference.
When used in the present process, the SAPO-11 and SAPO-41 silicoaluminophosphate
molecular sieves are employed in admixture with at least one platinum
or palladium hydrogenating component. Non-noble metals, such as
tungsten, vanadium, molybdenum, nickel, cobalt, chromium, and manganese,
may optionally be added to the catalyst. Combinations of these metals
with platinum or palladium, such as cobalt-molybdenum, cobalt-nickel,
nickel-tungsten or cobalt-nickel-tungsten, are also useful with
many feedstocks. The amount of any one metal ranges from about 0.01%
to 10% and preferably 0.1 to 5% by weight of the molecular sieve.
The techniques of introducing catalytically active metals to a molecular
sieve are disclosed in the literature, and preexisting metal incorporation
techniques and treatment of the molecular sieve to form an active
catalyst are suitable, e.g., ion exchange, impregnation or by occlusion
during sieve preparation. See, for example, U.S. Pat. Nos. 3236761;
3226339; 3236762; 3620960; 3373109; 4202996; and 4440871
which patents are incorporated totally herein by reference.
The hydrogenation metal utilized in the process of this invention
can mean one or more of the metals in its elemental state or in
some form such as the sulfide or oxide and mixtures thereof. As
is customary in the art of catalysis, when referring to the active
metal or metals it is intended to encompass the existence of such
metal in the elementary state or in some form such as the oxide
or sulfide as mentioned above, and regardless of the state in which
the metallic component actually exists the concentrations are computed
as if they existed in the elemental state.
The physical form of the silicoaluminophosphate catalyst depends
on the type of catalytic reactor being employed and may be in the
form of a granule or powder, and is desirably compacted into a more
readily usable form (e.g., larger agglomerates), usually with a
silica or alumina binder for fluidized bed reaction, or pills, prills,
spheres, extrudates, or other shapes of controlled size to accord
adequate catalyst-reactant contact. The catalyst may e employed
either as a fluidized catalyst, or in a fixed or moving bed, and
in one or more reaction stages.
The SAPO-11 and SAPO-41 molecular sieves may be composited with
other materials resistant to the temperatures and other conditions
employed in the process. Such matrix materials include active and
inactive materials and synthetic or naturally occurring zeolites
as well as inorganic materials such as clays, silica and metal oxides.
The latter may be either naturally occurring or in the form of gelatinous
precipitates, sols or gels including mixtures of silica and metal
oxides. Inactive materials suitably serve as diluents to control
the amount of conversion in the dewaxing process so that products
can be obtained economically without employing other means for controlling
the rate of reaction. The silicoaluminophosphate may be incorporated
into naturally occurring clays, e.g., bentonite and kaolin. These
materials, i.e., clays, oxides, etc., function, in part, as binders
for the catalyst. It is desirable to provide a catalyst having good
crush strength, because in petroleum refining the catalyst is often
subjected to rough handling. This tends to break the catalyst down
into powder-like materials which cause problems in processing.
Naturally occurring clays which can be composited with the catalyst
include the montmorillonite and kaolin families, 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.
Fibrous clays such as halloysite, sepiolite and attapulgite can
also be used as supports. 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, the catalysts may be composited
with porous inorganic oxide matrix materials and mixtures of matrix
materials such as silica, alumina, titania, magnesia, silica-alumina,
silica-magnesia, silica-zirconia, silica-thoria, silica-beryllia,
silica-titania, titania-zirconia as well as ternary compositions
such as silica-alumina-thoria, silica-alumina-titania, silica-alumina-magnesia
and silica-magnesia-zirconia. The matrix can be in the form of a
cogel.
In a preferred embodiment, the final catalyst will be a composite
and includes the silicoaluminophosphate, a platinum or palladium
hydrogenating metal component and the inorganic oxide matrix. The
preferred silicoaluminophosphate is SAPO-11 the preferred metal
component is platinum, and the preferred support is alumina. A wide
variety of procedures can be used to combine the molecular sieve
with the refractory oxide. For example, the molecular sieve can
be mulled with a hydrogel of the oxide followed by partial drying
if required and extruding or pelletizing to form particles of the
desired shape. Alternatively, the refractory oxide can be precipitated
in the presence of the molecular sieve. This is accomplished by
increasing the pH of the solution of a refractory oxide precursor
such as sodium aluminate or sodium silicate. The combination can
then be partially dried as desired, tableted, pelleted, extruded,
or formed by other means and then calcined, e.g., at a temperature
above 600.degree. F. (316.degree. C.), usually above 800.degree.
F. (427.degree. C.). Processes which produce larger pore size supports
are preferred to those producing smaller pore size supports when
cogelling.
Process Conditions
Although the catalyst used in this method exhibits excellent stability,
activity and midbarrel selectivity, reaction conditions must nevertheless
be correlated to provide the desired conversion rates while minimizing
conversion to less desired lower-boiling products. The conditions
required to meet these objectives will depend on catalyst activity
and selectivity and feedstock characteristics such as boiling range,
as well as organonitrogen and aromatic content and structure. They
will also depend on the most judicious compromise of overall activity,
i.e., conversion per pass and selectivity. For example, these systems
can be operated at relatively high conversion rates on the order
of 70 80 or even 90% conversion per pass. However, higher conversion
rates generally result in lower selectivity. Thus, a compromise
must be drawn between conversion and selectivity. The balancing
of reaction conditions to achieve the desired objectives is part
of the ordinary skill of the art.
Reaction temperatures generally exceed about 500.degree. F. (260.degree.
C.) and are usually above about 600.degree. F. (316.degree. C.),
preferably between 600.degree. F. (316.degree. C.) and 900.degree.
F. (482.degree. C.). Hydrogen addition rates should be at last about
400 and are usually between about 1000 and about 15000 standard
cubic feet per barrel. Reaction pressures exceed 200 psig (13.7
bar) and are usually within the range of about 500 to about 3000
psig (32.4 to 207 bar). Liquid hourly space velocities are less
than about 15 preferably between about 0.2 and about 10.
The overall conversion rate is primarily controlled by reaction
temperature and liquid hourly space velocity. However, selectivity
is generally inversely proportional to reaction temperature. It
is not as severely affected by reduced space velocities at otherwise
constant conversion. Conversely, selectivity is usually improved
at higher pressures and hydrogen addition rates. Thus, the most
desirable conditions for the conversion of a specific feed to a
predetermined product can be best obtained by converting the feed
at several different temperatures, pressures, space velocities and
hydrogen addition rates, correlating the effect of each of these
variables and selecting the best compromise of overall conversion
and selectivity.
The conditions should be chosen so that the overall conversion
rate will correspond to the production of at least about 40%, and
preferably at least about 50% of products boiling below about 725.degree.
F. (385.degree. C.) per pass and preferably below about 725.degree.
F. and above about 300.degree. F. Midbarrel selectivity should be
such that at least about 40%, preferably at least about 50% of the
product is in the middle distillate range and preferably below about
725.degree. F. and above about 300.degree. F. The process can maintain
conversion levels in excess of about 50% per pass at selectivities
in excess of 60% to middle distillate products boiling between 300.degree.
F. (149.degree. C.) and 725.degree. F. (385.degree. C.). The pour
point of the middle distillate effluent obtained by the process
will be below about 0.degree. F. and preferably below -20.degree.
F.
The process can be operated as a single-stage hydroprocessing zone.
It can also be the second stage of a two-stage hydrocracking scheme
in which the first stage removes nitrogen and sulfur from the feedstock
before contact with the middle distillate-producing catalyst. The
catalyst can also be used in the first stage of a multistep hydrocracking
scheme. In operation as the first stage, the middle distillate-producing
zone also denitrifies and desulfurizes the feedstock; in addition,
it allows the second stage using the same catalyst or a conventional
hydrocracking catalyst to operate more efficiently so that more
middle distillates are produced overall than in other process configurations.
Nitrogen Content of Feedstocks
While the process herein can be practiced with utility when the
feed contains organic nitrogen (nitrogencontaining impurities),
for example as much as several thousand parts per million by weight
of organic nitrogen, it is preferred that the organic nitrogen content
of the feed be less than 50 more preferably less than 10 ppmw.
Especially good results, in terms of activity and length of catalyst
cycle (period between successive regenerations or start-up and first
regeneration), are experienced when the feed contains less than
10 ppmw of organic nitrogen. This is surprising in view of the art
(see, for example, U.S. Pat. No. 3894938).
Sulfur Content of Feedstocks
The presence of organic sulfur (sulfur-containing impurities) in
the feedstock does not appear to deleteriously affect the desired
hydrocracking of the feed, for example, in terms of activity and
catalyst life. In fact, hydrodesulfurizing of the feed of organic
sulfur is in large part a significant concurrent reaction. However,
the resulting product, in general, will contain at least some thiols
and/or thioethers as a result of inter-reaction of hydrogen sulfide
and olefinic hydrocarbons in the effluent product stream. Thus,
it may be desirable in some instances that the feed prior to use
in the process herein be hydrofined or hydrotreated for at least
a substantial removal of both organic sulfur- and nitrogen-containing
compounds.
Upstream hydrodenitrogenation can be performed in the reactor with
the molecular sieve-containing catalyst or preferably in a separate
reactor. When a separate hydrodenitrogenation reactor is used, it
may be desirable to remove, e.g., flash, light gaseous products
such as NH.sub.3 upstream of the reactor containing the molecular
sieve-containing catalyst. If the hydrotreating is performed in
the same reactor, the molecular sieve-containing catalyst is disposed
in one or more layers downstream of an active hydrodenitrogenation
catalyst. The single reactor should preferably be operated under
hydrotreating conditions sufficient to reduce the organic nitrogen
of the feed to 10 ppmw or less before the feed encounters the molecular
sieve-containing layer. The volume of hydrodenitrogenation catalyst
relative to molecular sieve-containing catalyst can vary over a
wide range, such as from about 0.1 to 1 to 20 to 1 preferably at
least 0.2 to 1 and more preferably at least 0.5 to 1. The ratio
depends upon such parameters as: (a) the organic nitrogen content
of the feedstock; (b) the hydrodenitrogenation and hydrocracking
activities of the upstream hydrotreating catalyst; and (c) the degree
of overall hydrocracking desired.
The upstream hydrotreating catalysts can be any of the conventional
catalysts having hydrodenitrogenation and hydrocracking activity.
See, for example, U.S. Pat. No. 3401125 incorporated herein by
reference. In general, such hydrotreating catalysts are porous composites
or inorganic matrix oxides such as alumina, silica, magnesia, etc.,
which contain one or more hydrogenation components such as transition
elements, particularly elements of Group VIB or Group VIII of the
Periodic Table of the Elements. Handbook of Chemistry and Physics,
45th Ed., Chemical Rubber Company. The Group VIB and/or Group VIII
or other transition elements can be present as metals, oxides, or
sulfides. The hydrotreating catalyst can also contain promoters
such as phosphorus, titanium and other materials known in the art,
present as metals, oxides or sulfides. The upstream hydrotreating
catalyst need not contain a silicoaluminophosphate component. Typical
upstream hydrogenation catalysts suitable for use herein contain
10 to 30 wt. % amorphous silica, 20 to 40 wt. % amorphous alumina,
15 to 30 wt. % Group VIB metal oxide, such as WO.sub.3 5 to 15
wt. % Group VIII metal oxide, such as NiO and 2 to 15 wt. % of a
promoter oxide, such as TiO.sub.2. The hydrotreating catalyst should
have an average pore size in the range of about 30 to 200 Angstroms
and a surface area of at least about 150 square meters per gram.
The improved process of this invention will now be illustrated
by examples which are not to be construed as limiting the invention
as described in this specification including the attached claims.
EXAMPLES
Example 1
A. Comparative Example
A hydrodenitrified vacuum gas oil (Table V) was hydrocracked over
a sulfided cogelled nickel-tungsten-silica-alumina catalyst containing
7.7 wt. % Ni and 19.4 wt. % W. The conditions were a catalyst temperature
of 670.degree. F., a reactor pressure of 2200 psig, a liquid hourly
space velocity (LHSV) of 1.3 and a once-through hydrogen rate of
8 MSCF/bbl. The conversion below 700.degree. F. was 56 wt. %, where
percent conversion is defined as ##EQU1## The liquid product was
distilled into fractions boiling in the following ranges: C.sub.5
-230.degree. F., 230.degree.-284.degree. F., 284.degree.-482.degree.
F., 482.degree.-698.degree. F., and 698.degree. F.+. The yields
of the 698.degree. F.- fractions are shown in FIG. 3 which shows
a diesel (482.degree.-698.degree. F.) yield of 36 wt. %. The inspections
of the diesel cut are given in Table VI below, showing a pour point
of +5.degree. F.
B. SAPO-11 was prepared as described below and identified as such
by X-ray diffraction analysis. More specifically, 115.6 g of 85%
H.sub.3 PO.sub.4 were added to 59 g of H.sub.2 O. To this were slowly
added 204.2 g of aluminum isoproxide ([(CH.sub.3).sub.2 CHO].sub.3
Al) and mixed until homogeneous. 8 g of H.sub.2 O were added to
60.2 g of Ludox AS-30 (30% silica aqueous sol) and the mixture slowly
added to the above with mixing until homogeneous. 45.6 g of di-n-propylamine
were then slowly added with mixing, again until homogeneous. Synthesis
was carried out in a Teflon bottle in an autoclave at 150.degree.
C. for 5 days.
The anhydrous molar composition of the calcined sieve was
The sieve was bound with 35% catapal alumina and made into 1/10-inch
extrudate. The extrudate was dried in air for 4 hours at 250.degree.
F., then calcined 2 hours at 450.degree. F. and 2 hours at 1000.degree.
F. The extrudate was then impregnated by the pore-fill method with
0.5 wt. % Pd using an aqueous solution of Pd(NH.sub.3).sub.4 (NO.sub.3).sub.2.
The catalyst was dried for 2 hours at 250.degree. F., then calcined
in air for two hours at 450.degree. F. and two hours at 900.degree.
F. It was then crushed to 24-42 mesh and used to hydrocrack the
feed of the above example at 750.degree. F., 2200 psig, 1.0 LHSV,
and 8M SCF/bbl once-through H.sub.2 to give 44 wt. % conversion
below 700.degree. F. Product yields are compared to those for the
Comparative Example catalyst in FIG. 3 showing the 482.degree.-698.degree.
F. diesel yield to be 7 wt. % higher. The inspections of the diesel
cut are given in Table VI below showing a pour point of -40.degree.
F.
C. The catalyst of Example B was also run at 750.degree. F., 1.3
LHSV, 2200 psig, and 8M SCF/bbl once-through H.sub.2 to give 47
wt. % conversion below 725.degree. F. The diesel end point was extended
from 698.degree. F. to 725.degree. F., thereby increasing diesel
yield another 11 wt. %. Despite the higher end point, the pour point
was still exceedingly low (-50.degree. F.). The inspections of the
diesel cut are given in Table VI below. |