Note : Les descriptions sont présentées dans la langue officielle dans laquelle elles ont été soumises.
CA 02012682 1999-11-23
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1. Field of the Invention
This invention relates to a process for the
preparation of cobalt catalysts wherein the metal is
impregnated and dispersed as a thin film, or coating, on the
peripheral or outside surface of a particulate carrier, or
support, particularly a titania carrier, or support.
2. Backaround of the Invention
Particulate catalysts are commonly employed in
chemical, petroleum and petrochemical processing for
chemically altering, or converting various gas and liquid
feeds to more desirable products. Such catalysts are formed by
dispersing a catalytically active metal, or metals, or the
compounds thereof upon particulate carriers, or supports. In
accordance with the conventional wisdom the catalytically
active metal, or metals is generally dispersed as uniformly as
possible throughout the particle, providing a uniformity of
the catalytically active sites from the center of a particle
outwardly.
Fischer-Tropsch synthesis, a process for the
production of hydrocarbons from carbon monoxide and hydrogen
(synthesis gas), has been commercially practiced in some parts
of the world. This process may gain wider acceptance, perhaps
in this country, if adequate improvements can be made in the
existing technology. The earlier Fischer-Tropsch catalysts
were constituted for the most part of non-noble metals
dispersed throughout a porous inorganic oxide support. The
Group VIII non-noble metals, iron, cobalt, and nickel have
been widely used in Fischer-Tropsch reactions, and these
metals have been promoted with various other metals, and
supported in various ways on various substrates, principally
alumina. Most commercial experience, however, has been based
on cobalt and iron catalysts. The first commercial Fischer-
Tropsch operation utilized a cobalt catalyst, though later
more active iron catalysts were also commercialized. The
cobalt and iron catalysts were formed by compositing the metal
CA 02012682 1999-11-23
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throughout an inorganic oxide support, The early cobalt
catalysts, however, were of generally low activity
necessitating the use of a multiple staged process, as well as
low syn gas throughout. The iron catalysts, on the other hand,
produce too much carbon dioxide from the synthesis gas with
too little of the carbon monoxide being converted to
hydrocarbons.
3. Obiects
It is, accordingly, the primary objective of the
present invention to provide further improvements in forming
these high productivity Fischer-Tropsch cobalt catalysts via a
process for impregnating by spraying a catalytically effective
amount of cobalt, or cobalt and an additional metal, or
metals, on the peripheral or outside surface of a particulate
inorganic oxide support, particularly a particulate titania or
titanium-containing support.
A further object is to provide a spray coating
process, as characterized in the above-stated primary
objective, wherein the cobalt, or cobalt and an additional
metal, or metals, is sprayed, impregnated and dispersed upon
the surface of said particulate support particles while the
latter are maintained in heated condition and in a fluidized
state.
A particular object of this invention is to provide
a process for the preparation of cobalt catalysts, and
promoted cobalt catalysts, useful in Fischer-Tropsch synthesis
via use of a process for impregnating by spray coating a
compound comprised of cobalt, or compounds comprised of cobalt
and a promoter metal, or metals, on the peripheral or outside
surface of a particulate inorganic oxide support, particularly
a particulate titania or titanium-containing support, while
the latter is maintained in heated condition and in a
fluidized state.
A further, and more specific object is to provide a
novel process for spray coating and impregnating cobalt, or
cobalt and an additional cobalt modifier or promoter metal, or
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metals, on a heated fluidized particulate titania or titania-
containing support, especially one wherein the titania or
titania component has a high rutile:anatase ratio.
4. Statement of the Invention
These objects and others are achieved in accordance
with this invention embodying a process for the production of
a catalyst wherein cobalt, or cobalt and an additional metal,
or metals, catalytically effective in a Fischer-Tropsch
synthesis reaction is coated as a catalytically active layer,
or film, upon the peripheral or outside surface of a
particulate porous inorganic oxide support by contacting and
impregnating with a liquid dispersion, suitably a suspension
or solution, of a compound of said metal, or metals, the
support particles in a fluidized bed while the particles are
maintained at temperature above about 50°C, preferably at
temperature ranging from about 50°C to about 100°C, and more
preferably from about 70°C to about 90°C. Preferred inorganic
oxide supports are alumina, silica, silica-alumina and
titania, and of these a titania or titania-containing support
is most preferred; especially a rutile titania support, or
support wherein the titania or titania component has a weight
ratio of rutile:anatase of at least about 3:2. The
catalytically active surface layer, or film, ranges in average
thickness from about 20 microns to about 250 microns,
preferably from about 40 microns to about 150 microns, with
the loading of the cobalt expressed as the weight metallic
cobalt per packed bulk volume of catalyst ranging from about
0.01 grams (g) per cubic centimeter (cc) to about 0.15 g/cc,
preferably from about 0.03 g/cc to about 0.09 g/cc catalyst.
Metals such as rhenium, zirconium, hafnium, cerium,
thorium and uranium, or the compounds thereof, can be added to
cobalt to increase the activity and regenerability of the
catalyst. Thus, the thin catalytically active layers, or
films, formed on the surface of the support particles,
especially the titania or titania containing support
particles, can include in addition to a catalytically active
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amount of cobalt any one or more of rhenium, zirconium,
hafnium, cerium, thorium and uranium, or admixtures of these
with each other or with other metals or compounds thereof.
Preferred thin catalytically active layers, or films,
supported on a support, notably a titania or a titania-
containing support, thus include cobalt-rhenium, cobalt-
zirconium, cobalt-hafnium, cobalt-cerium, cobalt-thorium and
cobalt-uranium, with or without the additional presence of
other metals or compounds thereof.
The support particles are fluidized in a bed and
sprayed with a suspension, or solution containing a compound
of cobalt, or a compound or compounds of cobalt and an
additional metal or metals, catalytically effective in a
Fischer-Tropsch synthesis reaction. The temperature of the bed
during spraying should be above 50°C, preferably from about
50°C to about 100°C, and more preferably from about 70°C
to
about 90°C. Suitably, the support particles are fluidized with
a gas, preferably air, and the fluidized bed of support
particles is sprayed with the suspension or solution via one
or a plurality of nozzles. A key and novel feature of the
process is that a very high particle drying capacity is
generated during spraying by operating within a specified
range of conditions of temperature and flow rate of the air,
solution concentration of the metal, or metals, compound, or
compounds, and flow rate of the suspension, or solution from
the nozzle, or nozzles, into the bed. In forming a metal, or
metals, film, or coating, of the required uniform thickness on
the particulate particles, e.g., as in coating a cobalt metal
film on a particulate titania support, the temperature of the
bed during spraying, or spraying temperature, must be
maintained at above about 50°C, and preferably from about 50°C
to about 100°C, and more preferably from about 70°C to about
90°C, and the ratio of the solution feed rate to the
fluidization gas, or air rate must be maintained below about
0.6 grams (g) solution per cubic feet (ft3) of air, preferably
from about 0.1 g/ft3 to about 0.6 g/ft3, and more preferably
from about 0.3 g/ft3 to about 0.5 g/ft3. These parameters taken
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together provide high metals recovery, control the drying
capacity, and generate catalysts with a uniform metal, or
metals, film, or coating ranging in average thickness from
about 20 microns to about 250 microns, preferably from about
40 to about 150 microns, with the loading of the metal, or
metals expressed as the weight metallic metal per packed bulk
volume of catalyst ranging from about 0.01 g per cubic
centimeter (cc) to about 0.15 g/cc, preferably from about 0.03
g/cc to about 0.09 g/cc catalyst. If the temperature of the
fluidizing gas is on the higher side, then solution: gas ratios
on the higher side of the expressed ranges are preferred; and
conversely, if the temperature of the fluidizing gas is on the
lower side, then solution: gas ratios on the lower side, of
the expressed ranges are preferred. The level of a metal, or
metals, loading in the solution and the porosity of the
support to some degree also effect the thickness of the metal,
or metals, film, or coating, at a given set of spraying condi-
tions. With cobalt, e.g., when operating within the expressed
preferred range of conditions, the thickness of the cobalt
film will range from about 100 microns to about 250 microns
for a volumetric cobalt loading of about 6 g/100 cc.
In general, the concentration of the metal compound,
or compounds, in the suspension or solution should be greater
than about 5 wt.%, preferably 10 wt. o. Preferably a solution
is employed, and the solution can be saturated or
supersaturated with the metal compound, or metal compounds.
The optimum concentration is probably best determined
dependent upon the specific compound, or compounds employed.
Low concentrations will require longer spraying time, and
conversely high concentrations will decrease the spraying
time. Balance between the specific solution concentration and
period required for spraying for obtaining the required metal,
or metals, loading on film thickness is required.
Several types of fluidizing bed apparatus are found
in the literature, but the specific type and size of fluid bed
device per se do not appear to have an effect on the depth of
impregnation of the metal, or metals, below the peripheral
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surface of the particulate support. Known devices include,
e.g., fluid bed granulator/dryers equipped with nozzles
entering either above or below the bed, rotary granulators,
and Wurster columns. Such devices are available at Glatt Air
Techniques, Inc. of Ramsey, New Jersey, viz. a one-liter top
sprayer (Versa-Glatt model) and a 20 liter top sprayer, Model
GPCG-5, and at the Coating Place, Inc. of Verona, Wisconsin,
viz. a 20 liter bottom sprayer. Less breakage of the particles
appears to occur with the top entering nozzle. Typically, in
operating these units, the flow rate of the air is adjusted to
that which is required to properly fluidize the support
particles, and the rate of metals solution addition is then
adjusted in relation to the flow rate of the air.
A cobalt-titania catalyst is particularly preferred.
The cobalt-titania catalyst is one wherein the cobalt, or the
cobalt and a promoter, is dispersed as a thin catalytically
active film upon titania, or a titania-containing carrier, or
support, in which the titania has a rutile:anatase weight
ratio of at least about 3:2, as determined by ASTM D 3720-78:
Standard Test Method for Ratio of Anatase to Rutile In
Titanium Dioxide Pigments By Use of X-Ray Diffraction.
Generally, the catalyst is one wherein the titania has a
rutile:anatase ratio ranging at least about 3:2 to about
100:1, or greater, and more preferably from about 4:1 to about
100:1, or greater. Where any one of rhenium, zirconium,
hafnium, cerium, thorium, or uranium metals, respectively, is
added to the cobalt an a promoter to form the thin
catalytically active film, the metal in added to the cobalt in
concentration sufficient to provide a weight ratio of
cobalt: metal promoter ranging from about 30:1 to about 2:1,
preferably from about 20:1 to about 5:1. Rhenium and hafnium
are the preferred promoter metals, rhenium being more
effective in promoting improved activity maintenance on an
absolute basis, with hafnium being more effective on a cost-
effectiveness basis. These catalyst compositions, it has been
found, produce at high productivity, with low methane
selectivity, a product which is predominately Clo+ linear
. CA 02012682 1999-11-23
paraffins and olefins, with very little oxygenates. These
catalysts also provide high activity, high selectivity and
high activity maintenance in the conversion of carbon monoxide
and hydrogen to distillate fuels.
After the spraying is completed, the impregnated
metal compounds must be decomposed to the corresponding metal
oxides. Preferably, the catalyst is contacted with oxygen,
air, or other oxygen-containing gas at temperature sufficient
to oxidize the metal, or metals component, e.g., cobalt to
convert the cobalt to Co;jO4. Temperatures ranging above about
150°C, and preferably above about 200°C are satisfactory to
convert the metals, notably cobalt, to the oxide. Temperatures
above about 500°C are to be avoided. Suitably, the
decomposition of the metal compound, or compounds in achieved
at temperatures ranging from about 150°C to about 300°C. The
decomposition step can be conducted in the fluidized bed
device or in an external oven, or calcination device.
Cobalt catalyts produced by the process of this
invention have proven especially useful for the preparation of
liquid hydrocarbons from synthesis gas at high productivities,
with low methane formation. They contain essentially all of
the active metal, notably cobalt, on the outer peripheral
surface of the support particles, while the metal is
substantially excluded from the inner surface of the
particles. The high metal, or metals, loading in a thin rim,
or shell, on the outer surface of the particles maximizes
reaction of the hydrogen and carbon monoxide at the surface of
the catalytic particle. This avoids the normal diffusion
limitation of most prior art catalysts, the catalyst behaving
more ideally, approaching the behavior of a powdered catalyst
which in not diffusion limited. However, unlike in the use of
powdered catalysts the flow of the reactants through the
catalyst bed, because of the larger particle size of the
catalyst, is virtually unimpeded. The production of this type
of catalyst on a large scale, while very difficult if at all
feasible by known techniques, is readily accomplished via the
process of this invention.
CA 02012682 1999-11-23
_g_
In conducting synthesis gas reactions the total
pressure upon the CO and H2 reaction mixture is generally
maintained above about 80 prig, and preferably above about 140
psig. It is generally desirable to employ carbon monoxide, and
hydrogen, in molar ratio of H2:C0 above about 0.5:1 and
preferably equal to or above about 1.7:1 to increase the
concentration of Clo+ hydrocarbons in the product. Suitably,
the H2:C0 molar ratio ranges from about 0.5:1 to about 4:1, and
preferably the carbon monoxide and hydrogen are employed in
molar ratio H2:C0 ranging from about 1.7:1 to about 2.5:1. In
general, the reaction is carried out at gas hourly space
velocities ranging from about 100 V/Hr/V to about 5000 V/Hr/V,
preferably from about 300 V/Hr/V to about 1500 V/Hr/V,
measured as standard volumes of the gaseous mixture of carbon
monoxide and hydrogen (O°C, 1 Atm.) per hour per volume of
catalyst. The reaction is conducted at temperatures ranging
from about 160°C to about 290°C, preferably from about
190°C to
about 260°C. Pressures preferably range from about 80 prig to
about 600 psig, more preferably from about 140 prig to about
400 psig. The product generally and preferably contains 60
percent, or greater, and more preferably 75 percent, or
greater, Clo+ liquid hydrocarbons which boil above 160°C
(320°F) .
The invention will be more fully understood by
reference to the following examples and demonstrations which
present comparative data illustrating its more salient
features. All parts are in terms of weight units except as
otherwise specified.
EXAMPLES 1-10
A series of supports, for use in preparing
catalysts, were obtained from reliable commercial sources or
prepared from particulate titania-alumina
(96.50 Ti/3.5o A12O,3), silica, and alumina both in extrudate
and spherical form. The identity of the supports, their
physical size and form, and certain physical characteristics
-- viz. surface area (S. A.)
CA 02012682 1999-11-23
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in square meters per gram (B.E.T.), pore volume (PV) as
measured by mercury intrusion, and porosity are given in Table
1.
TABLE 1
Support Composition Form SA, PV, Porosity
Number m~q cc
I 96.50 Ti02/ 0.8 mm 15 0.27 0.54
3.5% A1203 Extrudates
II 96.5% Ti02/ 0.8 mm 51 0.42 0.64
3.5% A1203 Extrudates
III 96.50 Ti02/ 0.8 mm 3 0.21 0.49
3 . A1203 Extrudates
5
o
IV Si02 2.4 mm 244 1.23 0.74
Spheres
V A1203 0.8 mm 288 0.43 0.56
Extrudates
The catalysts made from these supports are described in Table
2, the support employed in the catalyst preparation being
identified by the support number related back to Table 1.
Aqueous solutions of cobalt nitrate (11-13 wt.% Co)
and perrhenic acid (1-1.3 wt.% Re) were used in the
preparation of a series of 15 catalysts. These catalysts were
individually prepared in fluid bed sprayers, the Glatt Air
Techniques, Inc. 1-liter and 20-liter top sprayers,
respectively, and 20-liter bottom sprayer supplied by Coating
Place, Inc. The catalysts were calcined as designated in Table
2. Table 2 also lists the weight of the charge to the fluid
bed prayer, the solution rate in g/minute, the fluidizing air
rate in ft3/minute (CFM), solution: air ratio in g/ft3, the
average bed temperature during spraying, the method of
calcination, the wt.% Co-Re on the finished catalyst and the
average depth from the outer surface, or thickness of the Co-
Re on the outer surface of the support particles in microns,
viz. RIM thickness, as measured by an electron probe analyzer
CA 02012682 1999-11-23
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(produced by JEOL Company, Model No. JXA-50A). The thickness
of the Co-Re surface layer, or RIM, is a measure of the
success, or lack of success, of a given catalyst preparation.
Metal distribution was not found to be uniform across this
depth; the concentration often decreasing with increased
distance from the peripheral surface of the particles. The
depth within a particle, and measurements made from particle
to particle however generally showed good uniformity;
particularly as regards Catalyst Preparation Nos. 1 through
10, inclusively, which were successful in accordance with this
invention. In addition to average RIM thickness, metals
recovery is also an important criteria for success, metals
recovery exceeding over 90 wt.o except as regards Example 12.
CA 02012682 1999-11-23
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CA 02012682 1999-11-23
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CA 02012682 1999-11-23
-12-
The following conclusions can be drawn from the
preparations described in Table 2, to wit:
Preparations nos. 1-4a illustrate the strong
relationship of spraying temperature to RIM thickness, with
other conditions being relatively constant. The higher the
temperature, over the range from 50-100°C, the thinner the
RIM. Note that preparation nos. 1-3 were sprayed at a low
solution: air ratios.
Preparations 4a and 4b show that RIM thickness is
dependent to an extent on the final calcination step. The
molten nitrate salt migrates further into the support during
the heat-up, before it decomposes to the oxide. The migration
of the metals toward the center of the particles can be
minimized by quick heat-up, and the sprayer provides the best
way to do this. An alternative approach in to use low
solution: air ratio during spraying. The extra drying this
affords helps minimize the tendency of the RIM to migrate
later during the calcination. The solution: air ratio of 0.56
is at the high end of the acceptable range. The RIM increased
significantly when a small portion was withdrawn from the
sprayer immediately after spraying, and then calcined in a
tube unit.
Preparation no. 5, compared to preparation no. 2,
shows that RIM thickness is a function of cobalt loading. The
less cobalt sprayed onto the support, tho thinner the RIM at
constant spray conditions.
Preparation no. 6 shows that exceptionally thin RIMS
can be made at high metal loadings. In this case preparation
no. 5 was simply returned after calcining to the sprayer for
another impregnation. A very thin RIM resulted compared to a
single impregnation, preparation no. 2.
Preparations 7 and 8 show that RIM thickness is a
function of support porosity. The RIM is thinner (preparation
no. 7 vs preparation no. 4a) when the support has a higher
pore volume. The RIM in much thicker (preparation no. 8 vs
preparation nos. 4a, 7) when low pore volume is present.
Preparations 9 and 10 demonstrate success with
silica and alumina supports.
CA 02012682 1999-11-23
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silica and alumina supports.
Preparations 11 and 12 show the adverse effects of
operating outside the preferred temperature range. Preparation
no. 11 shows that spraying below 50°C does not give a RIM, in
spite of using a low solution: air ratio and depositing only
2.8o cobalt. Preparation no. 12 shows that spraying at 110°C
gives a thin RIM but only 600 of the metal sprayed ended up on
the catalyst. The rest of the cobalt and expensive rhenium
were carried out overhead as fines. "Spray drying" of the
solution and decomposition of the nitrate appear to have
occurred before the solution reached the support.
Preparations 13 and 14 show that good RIMS are not
obtained when high solution: air ratios are used.
Further, to demonstrate the utility of these RIM
catalysts, a series Of Fischer-Tropsch runs were made with the
catalysts which had been successfully prepared by the fluid
bed technique. All were successful, and the data obtained
agreed well with the predicted performance based on the cobalt
loading and RIM thickness of each catalyst preparation. The
results obtained with catalyst preparation 4a can be
considered illustrative. Thus, preparation no. 4a was used to
convert synthesis gas to heavy hydrocarbons. A portion of the
catalyst was reduced at 450°C for 1 hour and then reacted with
a feed containing 65% H2/31o CO/4o Ne at 200°C, 280 prig,
GHSV=1000. CO conversion was 79% after 20 hours operation.
Product selectivity, in terms of moles of CO converted to a
product per moles of CO reacted, was: 6.Oo CHq, 0.4o CO2, and
93.60 C2+.
It is apparent that various modifications and
changes can be made without departing the spirit and scope of
the present invention.
Having described the invention, what is claimed is
