Note: Descriptions are shown in the official language in which they were submitted.
l~~O 92/21609 ~ ~ ~ ~ ~ ~ ~ PCTI LJS92l04?~3
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CATALYST/HEAT-TRANSFER I~iEDIUM FOR SYNGAS GENERATION
FIELD OF THE INVENTION
This invention relates to a fluid-bed or spouted-bed process
for preparing synthesis gas, carbon monoxide and hydrogen, from lower
alkanes, preferably methane, in the presence of both relatively inert
solids acting primarily as heat carriers and small amounts, relative .
to the heat carrying solids, of a catalytic material.
More particularly this invention relates to a process for
' reacting a lower alkane, e.g., methane, with oxygen in the presence of
other gas phase components, preferably steam at elevated temperatures
and pressures, and in the presence of both fluidized, relatively inert
solids and catalytic solids.
In fluid-bed processes the entire solids inventory of both
catalytic and inert solids is in a state of fluidization, while in
spouted-bed processes only that portion of the bed through which the
gases are injected are in a fluidized state.
BACKGROUND OF THE INVENTION
The production of synthesis gas by either partial oxidation
or steam-reforming is well known and there are extensive literature
references to these processes. The processes may be used separately
~ or they may be combined. Thus, the steam-reforming reaction is highly
endothermic and is described as:
CH4 + H20 -~ CO + 3H2 (1)
while the partial oxidation reaction is highly exothermic and is
described by:
CHI, + 02 -~ CO + H2 + H20
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The combined reaction employing a 2/I CHt,/02 feed ratio is
described as:
2CH~ + 02 - 2C0 + 4H2
In addition to these reactions, the mildly exothermic water
gas shift reaction also occurs:
CO + H20 ~ H2 + C02
The representation of the combined process shows that the
ratio of produced hydrogen to carbon monoxide is 2/1; the approximate
stoichiometric hydrogen/carbon monoxide ratio for producing higher
hydrocarbons by a hydrocarbon synthesis process, such as the Fischer-
Tropsch process over a catalyst with little or no water gas shift
activity.
A number of patents illustrate these processes, and U.S.
Patent No. 4,888,131 contains an extensive, hut not exhaustive listing
thereof.
Fluid bed processes are well known for the advantages they
provide in heat and mass transfer characteristics. Such processes
allow for substantially isothermal reactor conditions, and are usually
effective in eliminating temperature runaways or hot spats; however,
with 02 injection while camplete elimination of hot spots is impos
e
Bible although the fluid bed does tend to minimize the intensity
thereof. They are not, however, without their disadvantages: cata-
lyst strength or attrition resistance is important for maintaining the
integrity of the catalyst and minimizing the formation of fine parti-
ales that may be lost from the system, especially those particles not
recoverable by use of cyclones and deposited in down atream equipment
causing fouling or reverse reactions as temperature is decreased;
erosivity, or the tendency to erode equipment must be contained, since
attrition resistance is often an inverse function of erosivity.
VSO 92121609 PCTli1S92/04753
2~~~~~2~ v
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Additianally, the relatively high temperatures, e.g., above
about 1650°F, found in reforming reactions where oxygen is present can
cause agglomeration of the catalyst particles leading to lower cata-
lytic efficiency (e.g., lower conversion), larger particles that are
more difficult to fluidize, greater wear on equipment due to greater
momentum and impact forces, and clogging of lines. For example, a
common catalytic material, nickel, even when deposited in small
amounts on a suitable carrier, e.g., Iess than about 5 wtX nickel on
a support, tends to soften at reaction temperatures (due to its
reactivity with the support phase with concomitant formation of
reactive/lower melting mono- and polymetalic oxide phases), which
become sticky, and generally lead to particle agglomeration. Particle
agglomeration, in fact, tends to i,nerease as the amount of nickel
present in the catalyst bed increases or as the Ni containing phase is
subjected to multiple oxidizing and reducing cycles as it is trans-
ported through the fluid bed. The behavior of Idi/A1203 in H2 and
steam rich environments has been reported, E. Ruckenstein et al, J
Catal skis 100 1-16 (1986). Thus, maintaining the amount of nickel at
rather low levels in the catalyst bed minimizes particle agglomera-
tion. 0n the other hand sufficient nickel is required for providing
economical feed conversions to synthesis gas, i.e., within about 200°F
approach to equilibrium, thereby minimizing the level of CN4 exiting
the syngas generation zone.
Processes similar to fluid-bed steam-reforming processes for
the preparation of synthesis gas are also illustrated by US patent
s
4,758,375 and European patent publication 0163 385 Bl relating to
spouted-bed technology and the use of inert materials in the bed.
An.object of this im~ention, therefore, is taking advantage
of fluid-bed or spouted-bed processes for the production of synthesis
gas from lower alkanes, e.g., C1-C4, feeds while substantially elimi-
nating particle growth at elevated temperatures. Another abject of
this invention is approaching a minimum nickel concentration in the
reactor while contirniing to provide economic conversion levels.
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CA 02109592 2000-05-24
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SUMMARY OF THE INVENTION
These objects and other objects of this invention are
met by conducting a fluid-bed or spouted-bed, steam-
reforming, partial-oxidation process involving a lower
alkane feed, e.g., methane, at elevated temperatures in the
presence of non-catalytic or essentially non-catalytic, heat
carrying solids, and periodically injecting sufficient
catalytic material to maintain conversion levels of less
than or equal to a 250°F approach to equilibrium. The
invention thus minimizes the total amount of catalytic
material in the fluid-bed reaction zone at any point in
time, thereby minimizing any agglomeration or sintering
effects due to the presence of catalytic material, and
allows for conducting the reaction at higher temperatures
approaching the heat limit of the non-catalytic solids.
Higher operating temperatures, in turn, provide better
conversion of feed to synthesis gas.
According to one aspect of the present invention there
is provided a process for converting a feed comprising
methane to a product comprising hydrogen and carbon monoxide
which comprises: (a) reacting the feed in a fluidized
reaction zone at elevated temperatures with oxygen in the
presence of a first, essentially non-catalytic heat carrying
solids; (b) periodically adding to the reaction zone a
second solid comprising a nickel containing steam reforming
catalyst; and (c) maintaining a methane leak in the product
equivalent to no more than a 250°F approach to equilibrium.
According to a further aspect of the present invention
there is provided a process for converting a feed comprising
methane to a product comprising hydrogen and carbon monoxide
which comprises: (a) reacting the feed in a fluidized
reaction zone at a temperature above about 1650°F in the
presence of oxygen, steam, and carbon monoxide, and also in
the presence of a first, essentially non-catalytic heat
CA 02109592 2000-05-24
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carrying solids; (b) periodically adding to the reaction
zone a second solid comprised of a nickel containing steam
reforming catalyst and obtaining an active nickel loading in
the bed of at least about 0.02 wt.%; and (c) maintaining a
methane leak in the product equivalent to no more than a
250°F approach to equilibrium.
According to another aspect of the present invention
there is provided in a steam reforming-partial oxidation
process for converting, in a fluidized bed or spouted bed
reaction zone, a feed gas comprising methane to a product
comprising unconverted methane, hydrogen and carbon monoxide
at elevated temperatures, in the presence of oxygen, non-
catalytic heat carrying solids and catalytic solids, the
catalytic solids deactivating during the reaction, the
improvement comprising periodically adding to the reaction
zone sufficient nickel containing steam reforming catalyst
such that the unconverted methane in the product is
maintained at no more than a 250°F approach to equilibrium.
DETAILED DESCRIPTION OF THE INVENTION
The steam-reforming, partial=oxidation process
contemplated herein operates in a reactor wherein at least
one part of the solids inventory is in a fluidized state and
wherein heat carrying or heat transfer materials are
suspended in a flowing fluid at average bed temperatures
above about 1650°F, preferably at or above about 1700°F.
Reaction pressures may vary widely, for example, from about
atmospheric pressure to about 100 atmospheres. 4~Ihere the
product synthesis gas will be used in hydrocarbon synthesis
reactions, the pressure may be chosen so that intermediate
recompression of the synthesis gas can be avoided and the
synthesis gas will flow directly, after some product
separation and finds recovery, to the hydrocarbon synthesis
reactor, e.g., at pressures of about 10-50 atmospheres,
preferably 10-40 atmospheres, more preferably 10-40
CA 02109592 2000-05-24
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atmospheres. By virtue of this invention the high end of the
temperature range is no longer limited by catalyst
disintegration or catalyst agglomeration and the temperature
may range to within about 50°F of the softening point of the
heat carrying, essentially non-catalytic materials. (By non-
catalytic we mean that the steam-reforming or partial
oxidation
CA 02109592 1998-04-09
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process is either not catalyzed or only poorly catalyzed by the heat
carrying materials. Thus, the heat carrying solids are inert or
substantially inert for this steam-reforming or partial-oxidation
reaction). More preferably, however, average bed temperatures may range
from about 1650°F to 2000°F, still more preferably from about
1700°F to
about 1800°F at the preferred operating pressure of 20-40 atm. As the
preferred pressure decreases to about 10-20 atm, the preferred operating
temperature will be decreased accordingly to maintain desired methane
conversion.
The feed material to be reformed is any reformable alkane,
usually a lower alkane, e.g., C1-C4, preferably comprising methane or
natural gas which contains a high concentration of methane, e.g.,
greater than about 70% methane,-preferably greater than 80% methane,
more preferably, greater than 90% methane based on the total carbon
content of the feed. Such feed gases will likely contain up to about 10%
ethane, up to about 3% propane and trace amounts of C4-Ce. Condensate and
known contaminants, such as hydrogen sulfide, in the gas should be
removed, e.g., by well known processes. Typical feeds may also contain
some COz and nitrogen as well as some CO, H2, olefins and oxygenated
products from recycle operations, e.g., from Fischer-Tropsch processes.
The heat carrying solids may be any fluidizable material that
maintains its integrity at reaction conditions. These materials may be
Group II metal oxides, rare earth oxides, alpha alumina, modified alpha
aluminas, or alpha alumina containing oxides.
The heat carrying materials are generally attrition resistant
at reaction conditions and have a mean particle diameter ranging
from about 20 to 150 microns, preferably 30-150 microns, more
preferably 30-120 microns. Alumina materials, especially fused tabular
alumina, described in US Patent Nos. 4,888,131 and 4,952,389 are
particularly applicable for heat carrying materials. Generally,
these materials are at least about 98% alpha alumina with
substantially no silica. Silica tends to volatilize to Si(OH)4 at
W~ 92/2609 PCT/US92/04753
-s-
reaction temperatures, impairing the integrity of the particle.
Silica content is, therefore, less than about 1 wtX, preferably less
than about 0.5 wtX. Preferred materials are alpha aluminas, tabular
or fused, and rare earth stabilized alpha aluminas, e.g., containing
about 0.1 to 1.0 wtX rare earth.
Materials useful as heat carrying solids generally have
rather low surface areas, e.g., less than about 2 m2/gm, usually less .
than about 1 m2/gm.
The heat carrying materials are substantially inert or
non-catalytic with respect to the steam-reforming reaction. Conse-
quently, even though not preferred, some of these materials may be
comprised of spent or deactivated catalyst. The reaction itself may
lead to deactivation of the catalyst, particularly nickel containing
catalysts, which may then be used as heat carrying solids. The
deactivation rate of an individual catalyst will be a function of its
chemical and physical properties as well as the synthesis gas genera-
tion operating conditions. As a given catalyst is being used, it will
be possible to determine the precise deactivation rate, and from that
the rate at which fresh catalyst will need to be added.
The catalyst used herein may be any conventional steam-
reforming catalyst, or autothermal or combined reforming catalyst.
Such catalysts can be described as being selected from the group
. consisting of uranium, Group VII metals, and Group VIII noble and
non-noble metals. The metals are generally supported on inorganic
refractory oxides similar to the heat carrying materials already
described. Preferred catalyst setals are the Group VIII metals,
particularly nickel. In the case of nickel, any nickel containing
material is useful, e.g., nickel supported on alpha alumina, nickel
aluminate materials, nickel oxide, and preferably a supported nickel
containing material.
The catalyst may have a similar particle size distribution as
that found in the heart carrying material or it may have a somewhat
larger particle size, e.g.,.from 70-250 microns or larger. The larger
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WO 92!21609 PCT/U~92/04753
2~~9~~2
_ 7 _
particles may be more sintering resistant. Even though more sintering
prone, more finely divided catalyst, < 70p, may be desired because
. their fluid dynamic properties make them more accessible to the gas
phase reactants.
The amount of catalyst in the bed is that sufficient to bring
the activity to within 250°F of equilibrium, preferably to within
100°F of equilibrium, more preferably to within 50°F of
equilibrium
when the overall reaction is being carried out at relatively high
pressures, e.g., 20-40 atm where achieving high levels of CHI conver-
sion would otherwise be difficult.
The steam-reforming reaction is equilibrium limited. That
is, at any particular reaction temperature an equilibrium conversion
can be calculated based on the partial pressure of the gaseous compo-
nents of the system and the relative rate constants of the forward and
reverse reactions. This calculation is easily established and carried
out by one skil-~ed in the art. However, calculating the equilibrium
conversion at any particular temperature is not a part of this inven-
tion. The only importance it plays is that the activity of the
catalyst is such that the conversion of feed, e.g., methane, to
Synthesl.S gas is such that it is equal or greater to that which would
be obtained within 250°F approach to the equilibrium conversion for a
particular temperature. Thus the approach to equilibrium is simply
another way of measuring effective activity of the catalyst-heat
,transfer solids mixture.
In order to minimize the ability of a catalyst, e.g., nickel
containing catalyst, to cause particle agglomeration, the amount of
nickel containing particles in the reaction bed should be minimized
while maintaining sufficient nickel for providing adequate catalyst
activity. Consequently, the nickel loading on a supported particle
should be reasonably high, for example, 1 to 20 wtX, bearing in mind
that the support usually has a low surface area, with little porosity
and can hold relatively low amounts of catalytic metal. The nickel
loading in the bed which is constituted of heat carrying, relatively
inert particles and supported nickel, catalytic particles should be at
CVO 92/21609 PCT/US92/0~753
2~.~9~~~
_g_
least about 0.01 wtX based on total bed solids, preferably at least
about 0.02 wtX, more preferably about 0.02 wtX to about 3.0 wtX, and
most preferably about 0.2 to 1.~ wtX. This loading is for nickel
acting as a catalyst for the steam reforming reaction, i.e., active
nickel, since there may be some totally or substantially deactivated
nickel, i.e., spent nickel, in the~reaction zone acting as a heat
carrying solid. Since the nickel loading on the catalytic particle
may vary widely, as stated above, the amount of nickel containing
catalytic solids can be easily calculated based on the total bed
weight the weight of nickel in the total bed, and the nickel loading
on the nic'~el containing solids.
The catalyst containing material may be added continuously to
the fluid-bed or may be added at regular intervals. Significant
increases or decreases in the weight of the fluidized material should
be avoided so as not to disturb the fluidizing characteristics of the
bed. Thus, the rate and timing of the addition of catalyst should be,
generally, balanced by the normal losses from any fluid-bed system,
that is, fines or materials o~ less than about 20 microns mean
diameter which cannot be trapped by cyclones for return to the bed,
and other materials, such as spent catalyst that is removed from time
to time from the fluid-bed.
Regardless of whether active catalytic material is continu-
ously fed to the fluid-bad or spouted-bed or injected at regular or
,irregular intervals, the rate and timing of catalyst addition is such
that conversion of feed, e.g., methane, and feed leak or methane leak,
i.e., the volume X of unconverted feed or methane in the product
gases, is within 250°F of equilibrium. One skilled in the art can
easily picture a plot of activity (ordinate) v. time (abscissa) where
the activity line is relatively horizontal (constant addition of fresh
or active catalyst) ar effects a saw tooth-type curve (periodic
addition of fresh or active catalyst where activity decreases with
time and then jumps with each injection of catalyst).
The unconverted feed or methane leaving the fluid-bed as
product gas is usually less than about 10X, preferably less than about .
i~V~ 92/216U9 PCT/U~92/~753
_ 2:~0~~~~
8X, more preferably less than about 5X based on the total level of the
hydrocarbon or methane being fed to the reactor.
The fluidized reaction zone may contain a fluid bed of
particles or a spouted bed of particles. The design and engineering
of fluid-bed or spouted-bed reactors for the conversion of methane or
lower alkanes to synthesis gas is easily accomplished with relation-
shigs and techniques well known in the art, see, e.g., 0. Levenspiel
and K. Dunii, Fluidization Engineering, Wiley, New York (1969) and
references therein and see K. Methane and N. Epsteain, Spouted Beds,
Academic Press, New York, 1974 and references therein. Fluid bed
processes are preferred.
The steam-reforming partial-oxidation reaction is carried out
in the presence steam and oxygen. The alkane feed to steam molar
ratio is at least about 1 preferably about 1 to 3, more preferably 1.5
to 2.5. The oxygen to alkane feed malar ratio is about 0.2 to 1.0,
preferably 0.4 to 0.6. The 02 is added to provide the sensible heat
for reactants and to maintain the overall reaction temperature at a
desired level. When oxygen is employed, the alkane feed and oxygen
should be separately diluted with steam andfor C02 and preheated
before injection into the fluidized bed reaction zone. The ratio of
steam to C02 is chosen so as to achieve the desired H2/CO product
ratio.
Example
A refractory lined three foot diameter fluid-bed reactor was
charged with about 14 klb of 55-65 micron average diameter fused
alumina and 50 lb. of an 8 wtX Ni catalyst supported on 0.3 wtX
Ia-A1203 support, 40-100 micron average diameter. The bed of heat
carrying solids and Ni catalyst contained ca. 0.025-0.03 wtX nickel.
The system Was initially brought to temperature via in situ combustion
of methane until the desired operating temperature was approached.
Natural gas plus steam and carbon dioxide were introduced at
the bottom of the, reactor through an 8 inch o.d. center post with four
dY~ 92/21609 Pd_'T6US92/04753
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1.04 inch i.d. tubes angled 30 degrees downward from horizontal.
These tubes were symetrically disposed around the outer circumference
of the post. This-~eed was simultaneously introduced through four
similar tubes positioned symmetrically around the vessel wall.
Oxygen together with nitrogen, carbon dioxide, other inert
diluent gases or mixtures thereof were introduced through eight
nozzles evenly spaced around the reactor wall at a level about 3 feet
above the methane feed zone. Each nozzle terminates in three 0.277
inch i.d. tubes; the center tube is aimed at the vertical center line
of the reactor at an angle 30 degrees below horizontal. The outer
tubes are angled 30 degrees to either side of the center tube and are
on a horizontal axis.
The system was operated for about ten days after which time
an additional 325 lb. batch of the Ni reforming catalyst was added to
the bed to give an overall Ni loading level of 0.2-0.3 wtX. Solids
that were eluded from the bed were recycled so as to maintain a nearly
constant inventory during this operating period.
Total feed rates (moles/hr) to the reactor were: 150
methane, 108 steam, 71 oxygen, 48 carbon dioxide and less than 10 of
nitrogen. The system was operated at an average bed temperature of
1700°F at a pressure of 360 psia over a 15 day period during which
nearly stable activity, i.e., little deactivation, was observed and
total solids attrition as measured by the formation of particles with
less than 38 microns average diameter and agglomeration as measured by
the formation of particles with over 90 microns average diameter were
minimal.
On-line analysis showed 80-95X overall conversion with the
outlet stream containing hydrogen and carbon monoxide with an H2/CO
molar ratio of ca. 1.9-2.0, steam, carbon dioxide and nitxogen. This
stream was shown to have less than 5X volume of methane present at the
end of the 15 day operating period.
Pt'T/ US92/04753
W~ 92f21609
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This example demonstrates the utility of a fluid-bed com-
prised of non-catalytic heat carrying solids with low levels of a
' nickel containing steam reforming catalyst to produce synthesis gas
from methane with high overall efficiency. .
In the example, the amount of nickel added to the bed was
mare than sufficient to maintain the conversion to well within a
"1Q0°F approach to equilibrium. Consequently, the rats of deactiva- '
Lion, e.g., the activity half life, is rather relatively long for this
amount of nickel. However, with increasing time of operation the
activity will continue to fall, and another injection of nickel
containing solids will be required to maintain a desired conversion
and overall approach to equilibrium.
~If the amount of nickel added to the system was less than
shown in the example, the rate of deactivation would increase faster
with time and another nickel addition would become necessary in a
shorter time period. Thus, the periodicity of nickel addition is a
function of the amount of nickel added to the bed, and the deactiva-
tion rate with additional nickel being required to maintain the
desired activity and overall approach to equilibrium.
