Molecular sieve abstract
An improved process for catalytically dewaxing a hydrocarbon oil
feedstock by contact of the feedstock with a catalyst is disclosed
where the catalyst comprises an intermediate pore size silicoaluminophosphate
molecular sieve and at least one Group VIII metal.
Molecular sieve claims
What is claimed is:
1. A process for dewaxing a hydrocarbon oil feedstock boiling above
350.degree. F. and containing straight chain and slightly branched
chain hydrocarbons comprising contacting said feedstock under mild
cracking and isomerization conditions including a pressure of from
about 15 psig to about 3000 psig with a catalyst comprising SAPO-11
and a Group VIII metal selected from the group consisting of platinum
and palladium.
2. The method of claim 1 wherein the metal is platinum.
3. The method of claim 1 wherein the Group VIII metal is present
in the range of 0.01% to 10% based on the weight of molecular sieve.
4. The process of claim 1 wherein said process is conducted at
a temperature of from about 200.degree. C. to 475.degree. C., a
pressure of about 15 psig to about 3000 psig, a liquid hourly space
velocity of from about 0.1 hr.sup.-1 to about 20 hr.sup.-1 a hydrogen
circulation rate of from 500 to about 30000 SCF/bbl.
5. The process of claim 1 wherein the feedstock is a middle distillate
oil.
6. The process of claim 1 wherein the feedstock is a lube oil.
7. The process of claim 1 wherein the feedstock contains less than
50 ppm of nitrogen.
8. The process of claim 1 wherein the feedstock contains less than
10 ppm of nitrogen.
9. A process of claim 1 wherein the feedstock is a hydroprocessed
feedstock.
10. The process of claim 4 or 1 wherein said pressure is between
about 200 psig and 3000 psig.
Molecular sieve description
BACKGROUND OF THE INVENTION
1. Field of the Invention
This invention is concerned with a process for catalytically dewaxing
a hydrocarbon oil. In particular, it is concerned with dewaxing
a hydrocarbon oil feedstock by contacting the feedstock with a catalyst
comprising an intermediate pore size silicoaluminophosphate molecular
sieve and at least one Group VIII metal.
2. Description of the Prior Art
Processes for dewaxing petroleum distillates are well known. Dewaxing
is required when highly paraffinic oils are to be used in products
which need to remain mobile at low temperatures, e.g., lubricating
oils, heating oils, jet fuels. The higher molecular weight straight
chain normal and slightly branched paraffins which are present in
oils of this kind, are waxes which are the cause of high pour points
and high cloud points in the oils. If adequately low pour points
are to be obtained, these waxes must be wholly or partly removed.
In the past, various solvent removal techniques were used, e.g.,
propane dewaxing, MEK dewaxing, but these techniques are costly
and time consuming. Catalytic dewaxing processes are more economical
and achieve this end by selectively cracking the longer chain n-paraffins,
to produce lower molecular weight products, some of which may be
removed by distillation.
Because of their selectivity, prior art dewaxing catalysts generally
comprise an aluminosilicate zeolite having a pore size which admits
the straight chain n-paraffins either alone or with only slightly
branched chain paraffins, but which excludes more highly branched
materials, cycloaliphatics and aromatics. Zeolites such as ZSM-5
ZSM-11 ZSM-12 ZSM-23 ZSM-35 and ZSM-38 have been proposed for
this purpose in dewaxing processes and their use is described in
U.S. Pat. Nos. 3700585; 3894938; 4176050; 4181598; 4222855;
4229282 and 4247388.
Since dewaxing processes of this kind function by means of cracking
reactions, a number of useful products become degraded to lower
molecular weight materials. For example, waxy paraffins may be cracked
down to butane, propane, ethane and methane and so may the lighter
n-paraffins which do not, in any event, contribute to the waxy nature
of the oil. Because these lighter products are generally of lower
value than the higher molecular weight materials, it would obviously
be desirable to limit the degree of cracking which takes place during
a catalytic dewaxing process.
A dewaxing catalyst has now been found which has superior selectivity
with respect to the nature of the products obtained in a dewaxing
process. By use of certain silicoaluminophosphate molecular sieve
catalysts in the dewaxing process, hydrocarbon oil feedstocks may
be effectively dewaxed wherein the products obtained are of higher
molecular weight than those obtained using the prior art aluminosilicate
zeolites. Also, and especially with respect to lubricating oil feedstocks,
the products obtained from the dewaxing process of this invention
have better viscosities and viscosity indexes at a given pour point
as compared to the above-described prior art process using aluminosilicate
zeolites.
SUMMARY OF THE INVENTION
In accordance with the present invention, there has been discovered
a process of catalytically dewaxing a hydrocarbon oil feedstock
boiling above 350.degree. F. and containing straight chain and slightly
branched chain hydrocarbons, the improvement which comprises contacting
said oil feedstock with a catalyst comprising an intermediate pore
size silicoaluminophosphate molecular sieve, and at least one Group
VIII metal.
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.
It has been found that the present process provides selective conversion
of waxy n-paraffins to non-waxy paraffins. During processing the
waxy paraffins undergo mild cracking reactions to yield non-waxy
products of higher molecular weight than compared to products obtained
using the prior art zeolitic catalyst. 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 iso-paraffins to
form liquid range materials which contribute to a low viscosity,
low pour point product.
Because of the selectivity of the catalyst used in the process
of this invention, the gas yield is reduced, thereby preserving
the economic value of the feedstock.
Hydrogen consumption using the process of this invention is less
compared to prior art processes using conventional dewaxing catalysts
because isomerization does not consume hydrogen and cracking to
liquid range products consumes less hydrogen than cracking to gas.
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 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 ZSM-5 catalyst
with respect to lube yield at a given pour point in a dewaxing process.
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 viscosity index at a given pour point in a dewaxing
process.
FIG. 5 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 viscosity at a given pour point in a dewaxing process.
DESCRIPTION OF SPECIFIC EMBODIMENTS
Feedstock
The present process may be used to dewax a variety of feedstocks
ranging from relatively light distillate fractions up to high boiling
stocks such as whole crude petroleum, reduced crudes, vacuum tower
residua, cycle oils, synthetic crudes (e.g., shale oils, tar sand
oil, etc.), gas oils, vacuum gas oils, foots oils, and other heavy
oils. The feedstock will normally be a C.sub.10 + feedstock generally
boiling above about 350.degree. F. since lighter oils will usually
be free of significant quantities of waxy components. However, the
process is particularly useful with waxy distillate stocks such
as middle distillate stocks including gas oils, kerosenes, and jet
fuels, lubricating oil stocks, heating oils and other distillate
fractions whose pour point and viscosity need to be maintained within
certain specification limits. Lubricating oil stocks will generally
boil above 230.degree. C. (450.degree. F.), more usually above 315.degree.
C. (600.degree. F.). Hydroprocessed stocks are a convenient source
of stocks of this kind and also of other distillate fractions since
they normally contain significant amounts of waxy n-paraffins. The
feedstock of the present process will normally be a C.sub. 10 +
feedstock containing paraffins, olefins, naphthenes, aromatics and
heterocyclic compounds and with a substantial proportion of higher
molecular weight n-paraffins and slightly branched paraffins which
contribute to the waxy nature of the feedstock. During the processing,
the n-paraffins and the slightly branched paraffins undergo some
cracking or hydrocracking to form liquid range materials which contribute
to a low viscosity product. The degree of cracking which occurs
is, however, limited so that the gas yield is reduced, thereby preserving
the economic value of the feedstock.
Typical feedstocks include light gas oils, heavy gas oils and reduced
crudes boiling above 350.degree. F.
While the process herein can be practiced with utility when the
feed contains organic nitrogen (nitrogen-containing impurities),
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.
SILICOALUMINOPHOSPHATE CATALYSTS COMPOSITIONS (SAPOs)
Silicoaluminophosphate molecular sives (SAPOs) suitable for use
in the instant process comprise any molecular sieve having a silicoaluminophosphate
molecular framework which has an intermediate pore size and which
comprises 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 to convert
at effective process conditions the aforementioned feedstock to
dewaxed products and includes those silicoaluminophosphate molecular
sieves described in U.S. Pat. No. 4440871 which is incorporated
herein by reference.
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 assynthesized 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 hereinbefore
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.
By "intermediate pore size", as used herein, is meant
an effective pore aperture in the range of about 5.3 to 6.5 Angstroms
when the molecular sieve is in the calcined form. Molecular sieves
having pore apertures in this range tend to have unique molecular
sieving characteristics. Unlike small pore zeolites such as erionite
and chabazite, they will allow hydrocarbons having some branching
into the molecular sieve void spaces. Unlike larger pore zeolites
such as the faujasites and mordenites, they can differentiate between
n-alkanes and slightly branched alkanes on the one hand and larger
branched alkanes having, for example, quaternary carbon atoms.
The effective pore size of the molecular sieves can be measured
using standard adsorption techniques and hydrocarbonaceous compounds
of known minimum kinetic diameters. See Breck, Zeolite Molecular
Sieves, 1974 (especially Chapter 8); Anderson et al., J. Catalysis
58 114 (1979); and U.S. Pat. No. 4440871 all of which are incorporated
herein by reference.
Intermediate pore size molecular sieves will typically admit molecules
having kinetic diameters of 5.3 to 6.5 Angstroms with little hindrance.
Examples of such compounds (and their kinetic diameters in Angstroms)
are: n-hexane (4.3), 3-methylpentane (5.5), benzene (5.85), and
toluene (5.8). Compounds having kinetic diameters of about 6 to
6.5 Angstroms can be admitted into the pores, depending on the particular
sieve, but do not penetrate as quickly and in some cases are effectively
excluded. Compounds having kinetic diameters in the range of 6 to
6.5 Angstroms include: cyclohexane (6.0), 23-dimethylbutane (6.1),
and m-xylene (6.1). Generally, compounds having kinetic diameters
of greater than about 6.5 Angstroms do not penetrate the pore apertures
and thus are not absorbed into the interior of the molecular sieve
lattice. Examples of such larger compounds include: o-xylene (6.8),
135-trimethylbenzene (7.5), and tributylamine (8.1).
The preferred effective pore size range is from about 5.5 to about
6.2 Angstroms.
In performing adsorption measurements to determine pore size, standard
techniques are used. It is convenient to consider a particular molecule
as excluded if it does not reach at least 95% of its equilibrium
adsorption value on the molecular sieve in less than about 10 minutes
(p/po=0.5; 25.degree. C.).
The preferred intermediate pore size silicoaluminophosphate molecular
sieves which are useful in the process of this invention include
SAPO-11 SAPO-31 and SAPO-41 and are described in U.S. Pat. No.
4440871 which is incorporated herein by reference.
Process Conditions
The catalytic dewaxing step of this invention may be conducted
by contacting the feed to be dewaxed with a fixed stationary bed
of catalyst, with a fixed fluidized bed, or with a transport bed,
as desired. A simple and therefore preferred configuration is a
trickle-bed operation in which the feed is allowed to trickle through
a stationary fixed bed, preferably in the presence of hydrogen.
The catalytic dewaxing conditions are dependent in large measure
on the feed used and upon the desired pour point. Generally, the
temperature will be between about 200.degree. C. and about 475.degree.
C., preferably between about 250.degree. C. and about 450.degree.
C. The pressure is typically between about 15 psig and about 3000
psig, preferably between about 200 psig and 3000 psig. The liquid
hourly space velocity (LHSV) preferably will be form 0.1 to 20
preferably between about 0.2 and about 10.
Hydrogen is preferably present in the reaction zone during the
catalytic dewaxing process. The hydrogen to feed ratios is typically
between about 500 and about 30000 SCF/bbl (standard cubic feet
per barrel), preferably about 1000 to about 20000 SCF/bbl. Generally,
hydrogen will be separated from the product and recycled to the
reaction zone.
The silicoaluminophosphate molecular sieve catalysts can be manufactured
into a wide variety of physical forms. Generally speaking, the molecular
sieves can be in the form of a powder, a granule, or a molded product,
such as extrudate having particle size sufficient to pass through
a 2-mesh (Tyler) screen and be retained on a 40-mesh (Tyler) screen.
In cases where the catalyst is molded, such as by extrusion with
a binder, the silico-aluminophosphate can be extruded before drying,
or, dried or partially dried and then extruded.
The molecular sieve can be composited with other materials resistant
to the temperatures and other conditions employed in the dewaxing
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 silicoaluminophosphates 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 silicoaluminophosphate
include the montmorillonite and kaolin families, which families
include the sub-bentonites, 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 silicoaluminophosphates
can be composited with porous 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.
The silicoaluminophosphate catalysts used in the process of this
invention can also be composited with other zeolites such as synthetic
and natural faujasites, (e.g., X and Y) erionites, and mordenites.
They can also be composited with purely synthetic zeolites such
as those of the ZSM series. The combination of zeolites can also
be composited in a porous inorganic matrix.
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.
|