Note : Les descriptions sont présentées dans la langue officielle dans laquelle elles ont été soumises.
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SLURRY HYDROCARBON SYNTHESIS
PROCESS WITH INCREASED CATALYST LIFE
BACKGROUND OF THE DISCLOSURE
Field of the Invention
The invention relates to a hydrocal-bon synthesis process with
increased catalyst life. More particularly, the invention relates to a slurry
catalytic hydrocarbon synthesis process employing a supported cobalt metal
catalyst in which catalyst half life is increased by using a syngas feed
containing
less than fifty parts per billion of nitrogenous, catalyst deactivating
species.
Background of the Invention
Slurry hydrocarbon synthesis (HCS) processes are known. In a
slurry HCS process a synthesis gas (syngas) comprising a mixture of H2 and CO
is bubbled up as a third phase through a slurry in a reactor in which the
slurry
liquid comprises hydrocarbon products of the synthesis reaction and the
dispersed, suspended solids comprise a suitable Fischer-Tropsch type hydro-
carbon synthesis catalyst. Reactors which contain such a three phase slurry
are
sometimes referred to as "bubble columns", as is disclosed in U.S. Patent
5,348,982. Irrespective of whether the slurry reactor is operated as a
dispersed
or slumped bed, the mixing conditions in the slurry will typically be
somewhere
between the two theoretical conditions of plug flow and back mixed. It is also
known that Fischer-Tropsch type catalysts useful for forming hydrocarbons from
a syngas are rapidly, but reversibly deactivated by certain nitrogenous
species in
the syngas feed, particularly HCN and NH3. Syngas made from hydrocarbon
feedstocks which contain nitrogen (i.e., natural gas) or nitrogen containing
compounds (i.e., resids, coal, shale, coke, tar sands, etc.) invariably
contains
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HCN and NH3 which contaminate the reactive slurry and deactivate the catalyst.
Certain oxygenates and carbonaceous compounds which are formed in the slurry
as by-products of the HCS reaction are also believed to cause rapid
deactivation.
Deactivation of such catalysts by HCN and NHS may be reversed and catalytic
activity restored (rejuvenated) by contacting the deactivated catalyst with
hydrogen or a hydrogen containing gas (rejuvenating gas). Deactivation of such
catalysts by these species is reversible and catalytic activity is restored
(the
catalyst rejuvenated) by contacting the deactivated catalyst with hydrogen
either
continuously or intermittently as is disclosed, for example, in U.S. Patents
5,260,239; 5,268,344 and 5,283,216. While methods have been suggested for
reducing the HCN and NHS content of syngas down to about 0.1 ppm ( 100 ppb)
by catalytic hydrolysis (U.S. 4,769,224) and chemical scrubbing (U.S.
5,068,254), it has now been found that even as little as 100 vppb of a
combined
total of HCN and NH3 in the syngas will result in a catalyst half life of only
four
days for the case of a supported Co metal catalyst in an HCS slurry. It has
now
been found that reducing the level of the HCN and NHS catalyst poisons in the
syngas below SO ppb produces increased catalyst life and requires less
catalyst
rejuvenation. A method for achieving such low levels has also been found and
is
disclosed in copending US Patent Nos. 6,063,349, 5,968,465 and 6,107,353.
SUMMARY OF THE INVENTION
The present invention relates to a slurry hydrocarbon synthesis
(HCS) process employing a supported cobalt metal catalyst in which the short
term catalyst half life is at least 10 days, preferably at least 30 and more
prefer-
ably at least 40 or more days. By short term half life is meant that the
catalytic
activity caused by reversible deactivation of the catalyst is SO% that of
fresh
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catalyst and that this loss is substantially restored (the catalyst
rejuvenated) by
contacting the deactivated catalyst with a rejuvenating gas comprising H2.
Catalyst activity is defined in terms of the CO conversion to hydrocarbons.
Thus, if under a given set of HCS conditions fresh catalyst produces a CO
conversion of 80 mole %, the catalyst half life is realized when the
conversion
drops to 40%, as a result of contact with the reversibly deactivating
nitrogenous
species in the synthesis gas (syngas) feed. By reversibly deactivating
nitrogenous species is meant HCN, NH3 and mixture thereof. It is also an
embodiment of the process of the invention that the catalyst will have a long
term half life of at least 100 days and preferably at least 200 days. It has
been
found that there is also an unrejuvenable catalyst activity loss which occurs
over
time, which cannot be restored by contacting the catalyst with 1-12, but which
can
be restored by regeneration. The unrejuvenable, but regenerable loss in
catalyst
activity drops continuously, so that eventually the catalyst has a CO
conversion
activity at its long team half life only half or 50% of fresh catalyst and
this long
term activity Loss cannot be restored (the catalytic activity cannot be
rejuvenated)
by contacting the deactivated catalyst with H2 or a HZ containing rejuvenating
gas. Thus, by long term half life is meant the time it takes the catalyst to
have
only half the activity of fresh catalyst and that this activity loss, while
reversible,
is not restored by a rejuvenation process in which the deactivated catalyst is
contacted with H2 or an H2 containing gas. Instead, the catalyst has to be
separated fram the slung and regenerated by processes that include oxidation
or
burning, rereduction of the catalytic metals) and, optionally, passivation in
CO
and/or syngas. Thus, long term loss of catalyst activity in the context of the
invention is regenerable, but not rejuvenable with H2. Further, regenerable
activity loss is different from irreversible catalyst activity loss due to,
for
example, sulfur poisoning, which requires catalyst replacement. The relatively
long short term and Long term catalyst life in the practice of the invention
is
achieved by using a syngas feed in which the total level of the catalyst
deactivat-
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ing nitrogenous species HCN, NH3 and mixture thereof is less than 50 vppb
(volume parts per billion), preferably less than 20 vppb, and still more
preferably
less than 10 vppb. A slurry HCS catalyst useful in the practice of the
invention
comprises a cataIytically active cobalt component dispersed and supported on a
particulate inorganic refractory oxide carrier or support, and preferably.as a
thin
catalytically active surface layer, ranging in thickness from about 5-200
microns.
It is also preferred the catalyst have a productivity of at least 150 hr-~ at
200°C,
preferably at least 500 hr-~ and more preferably at least 1000 hr-1. By produc-
tivity is meant the standard volume of CO converted per volume of catalyst per
hour. In a further embodiment, the catalyst employed in the process of the
invention will have a methane selectivity of less than I O mole % and
preferably
less than 5 mole %. This means that less than i 0% of the CO converted is
converted to methane. In one embodiment the catalyst comprises catalytically
effective amounts of Co and one or more of Re, Ru, Fe, Ni, Th, Zr, Hf, U, Mg
and La on a suitable inorganic support material, and preferably one which
comprises one or more refractory metal oxides. Preferred supports for Co
containing catalysts comprise titanic and titanic-silica composites,
particularly
when employing a slurry HCS process in which higher molecular weight,
primarily paraffinic liquid hydrocarbon products are desired. Useful catalysts
and their preparation are known and illustrative, but nonlimiting examples may
be found, for example, in U.S. Patents 4,568,663; 4,663,305; 4,542, I22;
4,621,072 and 5,545,674, with those disclosed in U.S. 5,545,674 being
particularly preferred.
An HCS slurry process of the invention comprises reacting a
syngas which contains HCN, HN3 or mixture thereof in the presence of a solid,
particulate HCS catalyst in a slurry which comprises the catalyst and gas
bubbles
in a hydrocarbon slurry liquid, at reaction conditions effective to produce
hydro-
carbons from the syngas, wherein the total amount of HCN, HN3 or mixture
~ ~ ... . . .
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thereof in the syngas is less than 50 vppb, preferably less than 20 vppb and
more
preferably less that 10 vppb to achieve a short term catalyst half life
greater than
days, preferably greater than 30 days and more preferably greater than 40
days, and with a long term catalyst half life greater than 100 and preferably
greater than 200 days. Those skilled in the art will appreciate the unusually
large
difference in catalyst half life resulting from a relatively small difference
in
concentration of the HCN and NH3 catalyst deactivating species in the syngas
feed. It was not known that the relatively small differences and extremely low
concentrations of HCN and NHS in the syngas feed would make such a large
difference in catalyst half life. The increased catalyst half life reduces
catalyst
rejuvenation requirements and concomitant hydrogen consumption, while main-
taining good productivity and selectivity to liquid hydrocarbon products. The
process of the invention has been demonstrated with a slurry HCS process in
which the syngas is bubbled up through a three phase HCS slurry comprising the
particulate catalyst and gas bubbles in a hydrocarbon slurry liquid, and in
which
the catalyst comprised a catalytically active cobalt component dispersed and
supported on a particulate inorganic refractory oxide carrier or support, as a
thin
catalytically active surface layer which met the above requirements for
productivity and methane make. This catalyst was of the type disclosed and
claimed in the '674 patent referred to above.
A number of methods have been found to achieve the low
concentration of the HCN to NH3 in the syngas useful in the practice of the
invention. These include catalytic hydrolysis of the HCN to NH3, followed by
scrubbing with water to dissolve out the NH3 and, optionally, the use of guard
beds containing one or more solid adsorbents, preferably acidic, to adsorb any
HCN and NH3 that may break through. This process is disclosed in
US Patent 6,107,353 referred to above. Another method comprises
cryogenic separation of nitrogen from natural gas used as a syngas
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feed, so that not enough nitrogen is present in the natural gas to produce the
catalyst deactivating species in the syngas generating unit. In this process
too,
solid adsorbent beds will be placed between the syngas generation and the HCS
reactor(s), in the event of a nitrogen break through upstream of the syngas
generating unit and result in increasing the concentration of the catalyst
deactivating species in the syngas. In a more specific embodiment of a slurry
HCS process, the invention comprises reacting a synthesis gas comprising a
mixture of H2 and CO and containing HCN, NH3 or mixture thereof, in the
presence of a hydrocarbon synthesis catalyst in a slurry comprising said
catalyst
and gas bubbles in a hydrocarbon slurry liquid, under reaction conditions
effective to form hydrocarbons from said syngas, said catalyst comprising a
catalytically active cobalt component dispersed and supported on a particulate
inorganic refractory oxide ca~Tier or suppout, as a thin catalytically active
surface
layer, said catalyst having a productivity of at least 150 hr-1 and less than
5 mole
methane make from said synthesis gas, and wherein the amount of said HCN,
NH3 or mixture thereof present in said gas is less than 50 vppb so as to
achieve a
short term catalyst half life of at least 10 days. The hydrocarbon slurry
liquid
comprises hydrocarbon products of the HCS reaction which are liquid at the
reaction conditions and a portion is continuously or intermittently withdrawn
from the slurry HCS reactor as long as the hydrocarbons are being produced.
The hydrocarbon liquid withdrawn from the reactor comprises CS+, primarily
paxaffinic hydrocarbons and is typically upgraded into more valuable products
by one or more conversion operations, or sold neat. As the HCS reaction
progresses, the catalyst loses activity due to the presence of the HCN, NH3 or
mixture thereof in the syngas and must be either continuously or
intermittently
rejuvenated by bubbling H2 or an H2 containing gas up through the slurry in
which it contacts the catalyst and at least partially, and preferably
substantially
completely, restores the catalytic activity, as is disclosed in the prior art
referred
_.
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to above and more preferably after all or at least a portion of the CO has
been
removed from the slurry.
DETAILED DESCRIPTION
In a Fischer-Tropsch slurry HCS process, a syngas comprising a
mixture of H2 and CO is bubbled up into a reactive slurry in which it is
catalytically converted into hydrocarbons and preferably liquid hydrocarbons.
The mole ratio of the hydrogen to the carbon monoxide may broadly range from
about 0.5 to 4, but which is more typically within the range of from about 0.7
to
2.75 and preferably from about 0.7 to 2.5. The stoichiometric mole ratio for a
Fischer-Tropsch HCS reaction is 2.0, but there are many reasons for using
other
than a stoichiometric ratio as those skilled in the al-t know and a discussion
of
which is beyond the scope of the present invention. In a slurry HCS process
the
mole rario of the H2 to CO is typically about 2.1/1. Slurry HCS process condi-
tions vary somewhat depending on the catalyst and desired products. Typical
conditions effective to form hydrocarbons comprising mostly CS+ paraffins, and
preferably C,o+ paraffins (e.g., CS+-C2oo), in a slwry HCS process employing a
catalyst comprising a supported cobalt component include, for example,
temperatures, pressures and hourly gas space velocities in the range of from
about 320-600oF, 80-600 psi and 100-40,000 V/hrN, expressed as standard
volumes of the gaseous CO and H2 mixture (0°C, 1 atm) per hour per
volume of
catalyst, respectively. Slurry catalyst rejuvenation conditions of temperature
and
pressure are similar to those for hydrocarbon synthesis and are disclosed in
the
prior art. The syngas may be formed by various means, including contacting a
hot carbonaceous material such as coke or coal, with steam, or from a feed
comprising methane. A feed comprising methane is preferred for convenience,
cleanliness and because it does not leave large quantities of ash to be
handled
and disposed of. The methane containing gas feed is obtained from natural gas
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_g_
or by burning coal, tar, liquid hydrocarbons and the like and is fed into a
syngas
generator. The production of syngas from methane by either partial oxidation,
steam reforming or a combination thereof is well known as is disclosed, for
example, in U.S. Patent 4,888, I3 I . In many cases it is preferred to
catalytically
partially oxidize and steam reform the methane in a fluid bed syngas
generating
unit (FBSG) as is disclosed, for example, in U.S. Patents 4,888,131 and
5,160,456. Irrespective of the source of the methane, nitrogen or nitrogen
containing compounds are present in the methane containing gas fed into the
syngas generator, some of which are converted into NH3 and HCN during the
syngas formation. These will deactivate a Fischer-Tropsch HCS catalyst,
particularly those comprising Co as the catalytic metal. As the prior art
teaches,
deactivation by these species is reversible and the catalyst can be
rejuvenated by
contacting it with hydrogen. This restoration of the catalytic activity of a
reversibly deactivated catalyst is referred to as catalyst rejuvenation and is
disclosed, for example, in the 5,260,239; 5,268,344 and 5,283,216 patents
referred to above. It has also been found that both the short term and long
term
catalyst half life of a Co containing slurry HCS catalyst are unacceptably
short
unless the combined amount of the HCN and NH3 present in the syngas being
fed into an HCS reactor is less than 50 vppb, preferably less than 20 vppb and
more preferably less than 10 vppb, so that the short term or H2 rejuvenable
catalyst half life will be at least 10 days, preferably at least 30 days and
more
preferably at least 40 days and for the long term catalyst half life to be at
least
100 days and preferably at least 200 days. As mentioned above, with a Co metal
containing HCS catalyst of the type disclosed and claimed in U.S. Patent
5,545,674 in a reactive HCS slurry, 100 vppb of a combined total of HCN and
NH3 present in the syngas results in the catalyst having a half life of only 4
days.
By half life is meant that the overall activity of the catalyst body is
reduced by
50% in 4 days. An activity level of 50% is totally unacceptable. It means that
the productivity of the catalyst (and, concomitantly the reactor), measured in
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terms of CO conversion, is only 50% of what it should be in 4 days. A produc-
tivity level of at least 90% is desired. This means that in cyclic or
batchwise
rejuvenation, the reactor is taken off line for one-quarter of each day to
maintain
the activity level at no less than about 90%, during which time the catalyst
in the
reactor is rejuvenated with hydrogen. As a practical matter, the reactor, is
off
line more than one-quarter of each day, due to the time it takes to purge out
the
syngas, pass in the hydrogen or hydrogen containing catalyst rejuvenating gas
and then restart the HCS reaction. This results in a continuous average 25%
loss
of hydrocarbon production from the reactor, even with rejuvenation. Further,
as
the catalyst deactivates at otherwise constant conditions, the conversion
level
drops resulting in a decrease in liquid hydrocarbon make and a small increase
in
methane make. Alternatively, conversion can be held relatively constant
despite
the catalyst deactivation, by increasing the reactor temperature, but this
results in
a relatively large increase in methane make and consequent decrease in liquid
product make. At a combined HCN and NHS level of about 20 vppb in the
syngas, the catalyst half life is 20 days. This means that about every fourth
day
the catalyst has to be rejuvenated, using the same amount of time and
hydrogena-
tion for the rejuvenation as for the case above, yielding an average
production
loss of only about 6%. At about 40-50 vppb, it is about 15%. At a combined
level of about 10-12 vppb, the catalyst half life is about 40 days and the
catalyst
has to be rejuvenated for one-quarter of a day only every 8 days, yielding a
productivity loss of only about 3%. The catalyst half life is about 30 days
when
the combined amounts of HCN and NH3 is about 13-17 vppb. In the case of a
slurry HCS process, the catalyst in the slurry can be either continuously
rejuvenated with the reactor remaining on-line using the methods disclosed in
U.S. Patents 5,260,239 and 5,268,344. Nevertheless, the case of a catalyst
half
life of only 4 days will still consume five times more hydrogen rejuvenation
gas
than if the half life were 20 days, and ten times the amount required for a 40
day
half life.
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While prior art methods have included catalytic hydrolysis and
chemical scrubbing for reducing the synthesis gas HCN content to 0.01 vol% or
100 vppb, even 100 vppb of HCN in the syngas is an unacceptably high level.
Further, HCN removal by alkaline scrubbing and with alkaline ferrous sulphate
solutions is hampered by the presence of other acidic materials in the syngas,
particularly C02. Washing with water which contains chemicals is further
disadvantaged by process complexity, costly chemical consumption, and waste
disposal demands. Also, while NH3 is water soluble, HCN is not soluble enough
in water to be able to remove it down to the low levels of less than 50 vppb,
preferably less than 20 vppb and more preferably less than 10 vppb required to
achieve reasonable levels of catalyst half life. Chemical scrubbing processes
are
not selective enough to remove the HCN down to these levels. Some prior art
catalytic conversion processes have employed relatively low activity catalysts
which require excessive catalyst volume and/or high processing temperatures.
Other processes have employed sulfided catalysts which will leak out sulfur
and
irreversibly deactivate an HCS catalyst downstream. Processes which rely
primarily or solely on adsorption to remove the HCN and NH3 require imprac-
ticably large quantities of adsorbent to achieve useful operating times to
reduce
the combined HCN and NHS concentration to the desired levels. The methods
disclosed in the copending patent applications referred to above are preferred
for
achieving the low levels of HCN and NH3 required for acceptable catalyst half
life.
In a slurry HCS process according to the practice of the invention,
liquid and gaseous hydrocal-bon products are formed by contacting a syngas
comprising a mixture of H2 and CO with a Fischer-Tropsch type of HCS
catalyst, under shifting or non-shifting conditions and preferably under non-
shifting conditions in which little or no water gas shift reaction occurs,
~ i ,
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particularly when the catalytic metal comprises Co, Ru or mixture thereof.
Suitable Fischer-Tropsch reaction types of catalyst comprise, for example, one
or
more Group VIII catalytic metals such as Fe, Ni, Co, Ru and Re. Particularly
preferred in the practice of the invention is a catalyst in which the
catalytic metal
comprises a catalytically active cobalt component dispersed and supported on a
particulate inorganic refractory oxide carrier or support, with the total
thickness
of the catalytically active layer in the range of from about 5-200 microns.
For
support particles of a diameter greater than these values, the metal will be
impregnated as a thin surface layer no thicker than this range. For support
particles smaller than the upper limit of this range, the catalytic metal may
be
either uniformly impregnated throughout the particles or deposited as a
thinner)
surface layer. Paraffinic, CS+ hydrocarbon products are preferred and
preferably
more than 50% of the CS,. hydrocarbons will be paraffins. Preferably the
catalyst
will have a productivity in excess of 150 hl' at 200°C and exhibit a
methane
selectivity of less than 10%. More specifically and as set forth above, the
catalyst comprises catalytically effective amounts of Co and one or more of
Re,
Ru, Fe, Ni, Th, Zr, Hf, U, Mg and La on a suitable inorganic support material,
and preferably one which comprises one or more refractory metal oxides.
Preferred supports for Co containing catalysts comprise titania and titania-
silica
composites, particularly when employing a slung HCS process in which higher
molecular weight, primarily CS+ paraffinic liquid hydrocarbon products are
desired. Useful catalysts and their preparation are known and illustrative,
but
nonlimiting examples may be found, for example, in U.S. Patents 4,568,663;
4,663,305; 4,542,122; 4,621,072 and 5,545,674, with those disclosed in U.S.
5,545,674 being particularly preferred.
The hydrocarbons produced by an HCS process according to the
invention are typically upgraded to more valuable products, by subjecting all
or a
portion of the CS+ hydrocarbons to fractionation and/or conversion. By conver-
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sion is meant one or more operations in which the molecular structure of at
least
a portion of the hydrocarbon is changed and includes both noncatalytic process-
ing (e.g., steam cracking), and catalytic processing (e.g., catalytic
cracking) in
which a fraction is contacted with a suitable catalyst. If hydrogen is present
as a
reactant, such process steps are typically referred to as hydroconversion and
include, for example, hydroisomerization, hydrocracking, hydrodewaxing,
hydrorefining and the more severe hydrorefining referred to as hydrotreating,
all
conducted at conditions well known in the literature for hydroconversion of
hydrocarbon feeds, including hydrocarbon feeds rich in paraffins.
Illustrative,
but nonlimiting examples of more valuable products formed by conversion
include one or more of a synthetic crude oil, liquid fuel, olefins, solvents,
lubricating, industrial or medicinal oil, waxy hydrocarbons, nitrogen and
oxygen
containing compounds, and the Like. Liquid fuel includes one or more of motor
gasoline, diesel fuel, jet fuel, and kerosene, while lubricating oil includes,
for
example, automotive, jet, turbine and metal working oils. Industrial oil
includes
well drilling fluids, agricultural oils, heat transfer fluids and the like.
It is understood that various other embodiments and modifications
in the practice of the invention will be apparent to, and can be readily made
by,
those skilled in the art without departing from the scope and spirit of the
invention described above. Accordingly, it is not intended that the scope of
the
claims appended hereto be limited to the exact description set forth above,
but
rather that the claims be construed as encompassing all of the features of
patent-
able novelty which reside in the present invention, including all the features
and
embodiments which would be treated as equivalents thereof by those skilled in
the art to which the invention pertains.
