U.S. patent application number 14/265617 was filed with the patent office on 2014-08-21 for apparatus for steam-methane reforming.
This patent application is currently assigned to CompactGTL Limited. The applicant listed for this patent is CompactGTL Limited. Invention is credited to Michael Joseph Bowe, Clive Derek Lee-Tuffnell, Jason Andrew Maude, John William Stairmand, Ian Frederick Zimmerman.
Application Number | 20140234168 14/265617 |
Document ID | / |
Family ID | 26246832 |
Filed Date | 2014-08-21 |
United States Patent
Application |
20140234168 |
Kind Code |
A1 |
Bowe; Michael Joseph ; et
al. |
August 21, 2014 |
APPARATUS FOR STEAM-METHANE REFORMING
Abstract
Apparatuses for use in plants for processing methane, the
apparatuses comprising a plurality of reaction modules each
including a plurality of Fischer-Tropsch reactors operable to
convert a gaseous mixture including carbon monoxide and hydrogen to
a liquid hydrocarbon. Each module may be disconnected and taken
away for servicing while allowing the plant to continue to operate.
In some of the apparatuses, each Fischer-Tropsch reactor comprises
a plurality of metal sheets arranged as a stack to define first and
second flow channels for flow of respective fluids, the channels
being arranged alternately to ensure good thermal contact between
the fluids in the channels.
Inventors: |
Bowe; Michael Joseph;
(Cleveland, GB) ; Lee-Tuffnell; Clive Derek;
(Cleveland, GB) ; Maude; Jason Andrew; (Cleveland,
GB) ; Stairmand; John William; (Cleveland, GB)
; Zimmerman; Ian Frederick; (Cleveland, GB) |
|
Applicant: |
Name |
City |
State |
Country |
Type |
CompactGTL Limited |
Cleveland |
|
GB |
|
|
Assignee: |
CompactGTL Limited
Cleveland
GB
|
Family ID: |
26246832 |
Appl. No.: |
14/265617 |
Filed: |
April 30, 2014 |
Related U.S. Patent Documents
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Application
Number |
Filing Date |
Patent Number |
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13235784 |
Sep 19, 2011 |
8753589 |
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14265617 |
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12081303 |
Apr 14, 2008 |
8021633 |
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13235784 |
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10497785 |
Jun 29, 2004 |
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12081303 |
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Current U.S.
Class: |
422/49 |
Current CPC
Class: |
B01J 2219/2482 20130101;
B01J 2219/32475 20130101; B01J 2219/2466 20130101; C01B 2203/0811
20130101; B01J 2219/2485 20130101; C01B 3/384 20130101; C01B
2203/1064 20130101; C01B 2203/82 20130101; B01J 19/2495 20130101;
B01J 2219/2497 20130101; B01J 2219/00135 20130101; C01B 2203/1035
20130101; C10G 2300/807 20130101; C10G 2300/42 20130101; C01B
2203/107 20130101; Y02P 20/52 20151101; B01J 2219/2493 20130101;
B01J 2219/2498 20130101; B01J 2219/2465 20130101; C01B 2203/062
20130101; C10G 2/32 20130101; B01J 2219/2453 20130101; B01J
2219/00117 20130101; C01B 2203/1052 20130101; C01B 2203/148
20130101; C01B 2203/142 20130101; B01J 2219/2479 20130101; C01B
2203/0883 20130101; B01J 12/007 20130101; C01B 2203/1241 20130101;
B01J 2219/3221 20130101; C01B 2203/0233 20130101; B01J 2219/2459
20130101; B01J 19/249 20130101; C01B 2203/84 20130101; B01J
2219/32408 20130101; B01J 2219/32466 20130101 |
Class at
Publication: |
422/49 |
International
Class: |
B01J 12/00 20060101
B01J012/00 |
Claims
1. An apparatus for use in a plant for processing methane, the
apparatus comprising a plurality of reaction modules each including
a plurality of Fischer-Tropsch reactors operable to convert a
gaseous mixture including carbon monoxide and hydrogen to a liquid
hydrocarbon, wherein each module may be disconnected and taken away
for servicing, whilst allowing the plant to continue to
operate.
2. An apparatus according to claim 1 wherein each Fischer-Tropsch
reactor comprises a plurality of metal sheets arranged as a stack
to define first and second flow channels for flow of respective
fluids, the channels being arranged alternately to ensure good
thermal contact between the fluids in the channels.
3. An apparatus according to claim 2 wherein both the first flow
channels and the second flow channels are less than 5 mm deep in a
direction normal to the center plane of the sheets
4. The apparatus according to claim 1, wherein at least one of the
plurality of Fisher-Tropsch reactors includes a removable
catalyst.
5. The apparatus according to claim 1, wherein at least one of the
plurality of Fisher-Tropsch reactors includes a catalyst selected
from the group comprising iron, cobalt and fused magnetite.
6. The apparatus according to claim 5, wherein the catalyst is
supported on .gamma.-alumina.
7. The apparatus according to claim 6, wherein the .gamma.-alumina
has a surface area of 140-230 m2/g.
8. The apparatus according to claim 1, wherein at least one of the
plurality of Fisher-Tropsch reactors includes a catalyst comprising
about 35% by weight of cobalt.
9. The apparatus according to claim 5, wherein the catalyst is
provided with a promoter selected from the group comprising
ruthenium, platinum, gadolinium and rhenium.
10. The apparatus according to claim 1, wherein the reaction module
is small enough to be transported in an ISO structural frame.
11. The apparatus according to claim 1, wherein the plurality of
Fisher-Tropsch reactors are arranged such that the gaseous mixture
may flow in parallel through the plurality of Fisher-Tropsch
reactors.
Description
CROSS-REFERENCE TO RELATED APPLICATIONS
[0001] This is a continuation of prior pending U.S. application
Ser. No. 13/235,784, filed on Sep. 19, 2011 and currently allowed,
which is a continuation of prior pending application Ser. No.
12/081,303, filed on Apr. 14, 2008 and issued as U.S. Pat. No.
8,021,633, which is a continuation of application Ser. No.
10/497,785, filed on Jun. 29, 2004 and abandoned.
BRIEF DESCRIPTION OF THE FIGURES
[0002] FIG. 1 shows a flow diagram of a chemical process of the
invention.
[0003] FIG. 2 shows a plan view of a reactor suitable for
performing a step of the process shown in FIG. 1.
DETAILED DESCRIPTION
[0004] This invention relates to a chemical process, and to a plant
including catalytic reactors suitable for use in performing the
process.
[0005] A process is described in WO 01/51194 (Accentus plc) in
which methane is reacted with steam, to generate carbon monoxide
and hydrogen in a first catalytic reactor; the resulting gas
mixture is then used to perform Fisher-Tropsch synthesis in a
second catalytic reactor. The overall result is to convert methane
to hydrocarbons of higher molecular weight, which are usually
liquid under ambient conditions. The two stages of the process,
steam/methane reforming and Fisher-Tropsch synthesis, require
different catalysts, and catalytic reactors are described for each
stage. The catalytic reactors enable heat to be transferred to or
from the reacting gases, respectively, as the reactions are
respectively endothermic and exothermic; the heat required for
steam/methane reforming is provided by combustion of methane. A
potential problem with this process is that other reactions may
occur in the steam/methane reformer reactor, either to generate
carbon dioxide, or to generate coke. It is suggested that the
reformer may incorporate a platinum/rhodium catalyst, the reaction
being performed at 800.degree. C. The suggested process relies on a
steam/methane ratio that is close to 1:1, as the rhodium catalyst
is apparently resistant to coking. An improved way of performing
this process has now been found.
[0006] According to the present invention there is provided a
process for performing steam/methane reforming to generate carbon
monoxide and hydrogen, wherein the gas mixture is caused to flow
through a narrow flow channel between metal sheets separating the
flow channel from a source of heat, the flow channel containing a
fluid-permeable catalyst structure, the residence time in the
channel being less than 0.5 second, and both the average
temperature along the channel and the exit temperature of the
channel being in the range 750.degree. C. to 900.degree. C.,
wherein the steam is supplied at least in part by condensing water
vapor from combustion of a combustible gas, preferably comprising
methane.
[0007] The present invention also provides a plant for performing
steam/methane reforming that is particularly adapted for use on an
oil rig, a floating platform or a ship. Under such circumstances
space is limited, and the weight of the equipment must be
minimized.
[0008] Preferably the residence time is less than 0.1 s, but
preferably at least 0.02 s. It is presumed that such short reaction
times enable the process to operate under non-equilibrium
conditions, so that only those reactions that have comparatively
rapid kinetics will occur. It is also preferable that the ratio of
steam to methane should be in the range 1.2 to 2.0, more preferably
1.3 to 1.6, more preferably about 1.4 or 1.5. Under these
conditions the proportion of methane that undergoes reaction can
exceed 90%. Furthermore the selectivity in formation of carbon
monoxide rather than carbon dioxide can exceed 85% and even
90%.
[0009] The catalytic reactor preferably comprises a plurality of
metal sheets arranged to define first and second flow channels, the
channels being arranged alternately to ensure good thermal contact
between the fluids in them. Appropriate catalysts should be
provided in each channel, depending on the required reaction. To
ensure the required good thermal contact, both the first and the
second flow channels are preferably less than 5 mm deep in the
direction normal to the sheets. More preferably both the first and
the second flow channels are less than 3 mm deep. Corrugated or
dimpled foils, metal meshes, or corrugated or pleated metal felt
sheets may be used as the substrate of the catalyst structure
within the flow channels to enhance heat transfer. Since good heat
transfer is needed for achieving high CO selectivity in the
steam/methane reforming, a preferred structure comprises a metal
foil with a thin coating comprising the catalyst material.
[0010] As described in WO 01/51194, such a reactor may be used for
performing methane/steam reforming, the alternate channels
containing a methane/air mixture so that the exothermic oxidation
reaction provides the necessary heat for the endothermic reforming
reaction. For the oxidation reaction several different catalysts
may be used, for example palladium, platinum or copper on a ceramic
support; for example copper or platinum on an alumina support
stabilized with lanthanum, cerium or barium, or palladium on
zirconia, or more preferably platinum/palladium on gamma alumina
with a metal loading of about 10% by weight (relative to the
alumina). This catalyst composition is preferably in a coating of
thickness 20 to 200 .mu.m on a surface in the channel, preferably
on the foil. For the reforming reaction also several different
catalysts may be used, for example nickel, platinum, palladium,
ruthenium or rhodium, which may be used on ceramic coatings; the
preferred catalyst for the reforming reaction is platinum with
rhodium as a promoter, on alumina or stabilized alumina. Again the
catalyst metal is preferably about 10% by weight compared to the
alumina, and is provided as a 10-200 .mu.m coating, preferably 10
to 50 .mu.m. The oxidation reaction may be carried out at
substantially atmospheric pressure, while although the reforming
reaction may be carried out at elevated pressure, for example up to
2 MPa (20 atmospheres), operation at atmospheric pressure is
preferred, or possibly slightly elevated pressure for example in
the range 0 to 200 kPa above atmospheric pressure.
[0011] It will be appreciated that the materials of which the
reactor are made are subjected to a severely corrosive atmosphere
in use, for example the temperature may be as high as 900.degree.
C., although more typically around 800.degree. C. or 850.degree. C.
The reactor may be made of a metal such as an aluminum-bearing
ferritic steel, in particular of the type known as Fecralloy which
is iron with up to 20% chromium, 0.5-12% aluminum, and 0.1-3%
yttrium. For example it might comprise iron with 15% chromium, 4%
aluminum, and 0.3% yttrium. When this metal is heated in air it
forms an adherent oxide coating of alumina which protects the alloy
against further oxidation; this oxide layer also protects the alloy
against corrosion under conditions that prevail within for example
a methane oxidation reactor or a steam/methane reforming reactor.
Where this metal is used as a catalyst substrate, and is coated
with a ceramic layer into which a catalyst material is
incorporated, the alumina oxide layer on the metal is believed to
bind with the oxide coating, so ensuring the catalytic material
adheres to the metal substrate.
[0012] For some purposes the catalyst metal might instead be
deposited directly onto the adherent oxide coating of the metal
(without any ceramic layer).
[0013] The gases produced by the steam/methane reforming process
described above are preferably then subjected to Fischer-Tropsch
synthesis. This may be performed using a second such reactor, with
a different catalyst. Where excess hydrogen remains, after the
Fisher-Tropsch synthesis, this hydrogen is preferably separated
from the desired products, and fed back to the combustion flow
channels of the steam/methane reforming reactor. Combustion of a
mixture of methane and hydrogen with air in these channels has been
found to give more uniform temperature, and also enables the
combustion reaction to be initiated more readily when the reactor
is cold.
[0014] The invention will now be further and more particularly
described, by way of example only, and with reference to the
accompanying drawings in which:
[0015] FIG. 1 shows a flow diagram of a chemical process of the
invention; and
[0016] FIG. 2 shows a plan view of a reactor suitable for
performing a step of the process shown in FIG. 1.
[0017] The invention relates to a chemical process for converting
methane to longer chain hydrocarbons. The first stage involves
steam/methane reforming, that is to say the reaction:
H.sub.2O+CH.sub.4.fwdarw.CO+3H.sub.2
[0018] This reaction is endothermic, and may be catalyzed by a
platinum/rhodium catalyst in a first gas flow channel. The heat
required to cause this reaction may be provided by combustion of
methane, that is to say:
CH.sub.4+2O.sub.2.fwdarw.CO.sub.2+2H.sub.2O
[0019] which is an exothermic reaction, and may be catalyzed by a
platinum/palladium catalyst in an adjacent second gas flow channel.
Both these reactions may take place at atmospheric pressure,
although alternatively the reforming reaction might take place at
an elevated pressure. The heat generated by the combustion reaction
would be conducted through the metal sheet separating the adjacent
channels.
[0020] The gas mixture produced by the steam/methane reforming can
then be used to perform a Fischer-Tropsch synthesis to generate a
longer chain hydrocarbon, that is to say:
nCO+2nH.sub.2.fwdarw.(CH.sub.2).sub.n+nH.sub.2O
[0021] which is an exothermic reaction, occurring at an elevated
temperature, typically between 200 and 350.degree. C., for example
230.degree. C., and an elevated pressure typically between 2 MPa
and 4 MPa, for example 2.5 MPa, in the presence of a catalyst such
as iron, cobalt or fused magnetite, with a promoter such as
potassium. The exact nature of the organic compounds formed by the
reaction depends on the temperature, the pressure, and the
catalyst, as well as the ratio of carbon monoxide to hydrogen. A
preferred catalyst is .gamma.-alumina (as a coating) of surface
area 140-230 m.sup.2/g, with about 35% by weight of cobalt with a
ruthenium, platinum, gadolinium or rhenium promoter and a basicity
promoter such as ThO.sub.2. The heat given out by this synthesis
reaction may be used to provide at least part of the heat required
by the steam/methane reforming reaction, for example a heat
transfer fluid may be used to transfer the heat from a reactor in
which the Fischer-Tropsch synthesis is occurring, also ensuring the
temperature in the Fischer-Tropsch reactor remains steady, the heat
being used to preheat at least one of the gas streams supplied to
the reforming reactor.
[0022] These reactions would be particularly advantageous if they
could be performed at sea, for example on a floating production
platform or an oil rig, as they would enable stranded gas resources
to be exploited. Stranded gas or associated gas reserves represent
an untapped source of fuel, but cannot readily be exploited because
they are often located remotely, and the gas flows may not be large
enough to justify construction of a pipeline or a plant to produce
liquefied natural gas. Currently such gas is usually flared or
re-injected. A plant for producing liquid hydrocarbon by
steam/methane reforming followed by Fischer-Tropsch synthesis on
land is known, but conventional plant for this purpose is much too
large to conveniently be accommodated on the deck of a floating
structure, and indeed some of the processes employed would be
vulnerable to wave-induced motion. However, by performing such
reactions using compact catalytic reactors, for example as
described in PCT/GB2002/004144, which are typically one-tenth the
volume of a conventional reactor for the same duty, it is feasible
to perform this process on a rig or a floating structure.
[0023] It will be appreciated from the equations given above that
steam must be provided to perform the steam/methane reforming
reaction, and indeed to ensure that the catalysts do not become
coated with coke it is, necessary to provide more moles of steam
than of methane. Ideally all the water provided to perform
steam/methane reforming could be recovered after the
Fischer-Tropsch synthesis, but in practice additional water is
required because of inefficiencies in each stage. For operation on
a rig or a floating structure at sea, it will be appreciated that
such water could in principle be provided by a distillation plant
fed with sea water, but boiling sea water generates salt, and tends
to lead to corrosion problems; it would be preferable if the steam
could be provided by the chemical process plant itself
[0024] Referring now to FIG. 1, the plant and overall chemical
process is shown as a flow diagram. The feed gas 4 consists
primarily of methane, with a small percentage (say 10%) of ethane
and propane. It may also contain compounds of sulphur that would be
detrimental to catalysts. It is first passed through a fluidic
vortex scrubber 5 in which it flows radially inward in
counter-current to droplets of a de-sulphurisation liquid. This may
for example comprise an aqueous solution of a chelated ferric salt
that reacts with sulphurous compounds and is thereby reduced to the
ferrous form. The liquid is recirculated by a pump 6 through a
fluidic vortex scrubber 7 in which it is contacted by air to
regenerate the ferric salt and to form a precipitate of sulphur,
and then through a filter 8 and a pump 9 back to the scrubber 5.
The feed gas 4 is hence de-sulphurised. The vortex scrubbers 5 and
7 are not vulnerable to wave-induced motion.
[0025] Alternatively the sulphur-contaminated natural gas may be
reacted with hydrogen, at a temperature of 200-500.degree. C., over
a hydro-desulphurisation catalyst, to convert mercaptans to
H.sub.2S. The gas can then be passed through a bed of an adsorbent
(such as ZnO so the H.sub.2S reacts to give H.sub.2O and ZnS). Some
adsorbents can be regenerated in situ, producing SO.sub.2.
[0026] The de-sulphurised feed gas 10 is passed through a heat
exchanger 11 so it is at about 400.degree. C. and is then supplied
via a fluidic vortex mixer 12 to a first catalytic reactor 14; in
the mixer 12 the feed gas is mixed with a stream of steam that is
also at about 400.degree. C., the streams entering the mixer 12
through tangential inlets and following a spiral path to an axial
outlet so they become thoroughly mixed. Both streams may be at
atmospheric pressure, or for example at a pressure of say 100 kPa
above atmospheric. The flows are preferably such that the steam;
methane molar ratio (at the steam/methane reforming stage) is
between 1.4 and 1.6, preferably 1.5. The first part of the reactor
14 is a pre-reformer 15 with a nickel methanation catalyst at
400.degree. C., in which the higher alkanes react with the steam to
form methane (and carbon monoxide); extra steam is required to
ensure the desired steam/methane ratio is achieved after this
pre-forming stage (this pre-reformer 15 would not be required if
the feed gas 4 contained substantially no higher alkanes). An
alternative catalyst for the pre-reformer is platinum/rhodium. The
second part of the reactor 14 is a reformer 16 with a
platinum/rhodium catalyst, in which the methane and steam react to
form carbon monoxide and hydrogen. This reaction may be performed
at around 850.degree. C., as described below.
[0027] The heat for the endothermic reactions may be provided by
combustion of methane over a palladium or platinum catalyst within
adjacent gas flow channels 17 of the reactor 14. The catalyst may
incorporate a metal hexaaluminate (such as magnesium hexaaluminate)
or more preferably .gamma.-alumina, with 5-20% (say 10%) by weight
palladium/platinum catalyst. The methane/oxygen mixture may be
supplied to the channels 17 in stages along their length, to ensure
combustion occurs throughout their length. The exhaust gases from
the combustion channels 17 are passed through a sea water-cooled
heat exchanger 19 to cause at least part of the water vapor to
condense, the remaining gases being released to the atmosphere as
exhaust while the liquid water is fed through the duct 24 (see
below).
[0028] The hot mixture of carbon monoxide and hydrogen emerging
from the reformer 16 is then quenched by passing through a heat
exchanger 18 to provide the hot steam supplied to the vortex mixer
12, and then through the heat exchanger 11 in which it loses heat
to the feed gas. The mixture is then further cooled to about
100.degree. C. by passing through a heat exchanger 20 cooled by
water. Any water vapor that condenses is separated from the gas
stream into duct 25. The gases are then compressed through a
compressor 22 to a pressure in the range 1.0 MPa to 2.5 MPa (10 to
25 atm.).
[0029] The stream of high pressure carbon monoxide and hydrogen is
then supplied to a catalytic reactor 26 in which the gases react,
undergoing Fischer-Tropsch synthesis to form a paraffin or similar
compound. This reaction is exothermic, preferably taking place at
about 230.degree. C., and the heat generated may be used to preheat
the steam supplied to the heat exchanger 18, using a heat exchange
fluid such as helium circulated between heat exchange channels in
the reactor 26 and a steam generator 28. During this synthesis the
volume of the gases decreases. The resulting gases are then passed
into a condenser 30 in which they exchange heat with water
initially at 25.degree. C. The higher alkanes (say C5 and above)
condense as a liquid, as does the water, this mixture of liquids
being passed to a gravity separator 31; the separated higher
alkanes can then be removed as the desired product.
[0030] The water from the separator 31 is returned via the heat
exchangers 28 and 18 to the mixer 12. The water from the ducts 24
and 25 is also combined with this water stream. The water in the
separator 31 may also contain alcohols (which may be formed in the
Fischer-Tropsch reactor 26), so the water may first be
steam-stripped to remove such soluble organic compounds before it
is returned to the mixer 12. If water that contains alcohols is
returned to the mixer 12, the alcohols will be reformed to produce
CO, CO.sub.2 and H.sub.2.
[0031] Any lower alkanes or methane, and remaining hydrogen, pass
through the condenser 30 and are supplied to a refrigerated
condenser 32 in which they are cooled to about 5.degree. C. The
gases that remain, consisting primarily of hydrogen with carbon
dioxide, methane and ethane, are passed through a pressure-reducing
turbine 33 and fed via a duct 34 into a storage vessel 35, and
hence through a valve 36 into the combustion channel of the first
catalytic reactor 14. The condensed vapors, consisting primarily of
propane, butane and water, are passed to a gravity separator 37,
from which the water is combined with the recycled water from the
separator 31, while the alkanes are recycled to the feed line 10 so
as to be fed into the pre-reformer 15. As indicated by the broken
line, electricity generated by the turbine 33 may be used to help
drive the compressor 22.
[0032] When used in this fashion the overall result of the
processes is that methane is converted to higher molecular weight
hydrocarbons which are typically liquids at ambient temperatures.
The processes may be used at an oil or gas well to convert methane
gas into a liquid hydrocarbon which is easier to transport.
[0033] From the steam/methane reforming reaction given above one
would expect that the appropriate ratio between steam and methane
would be 1 to 1. However, at that ratio there is a significant risk
of coking, and a risk that a significant proportion of the methane
will not undergo the reaction. Increasing the proportion of steam
increases the proportion of methane that reacts, and decreases the
risk of coking, although if the proportion of steam is too high
then there is an increased likelihood of carbon dioxide formation.
It has been found that operating with a steam/methane ratio of
between 1.3 and 1.6, preferably 1.4 or 1.5, combined with short
residence times that are preferably no more than 100 ms, gives both
high selectivity for carbon monoxide formation and also a very high
proportion of methane undergoing reaction. The flow rates through
the reformer 16 are preferably such that the residence time is in
the range 20 to 100 ms, more preferably about 50 ms. The average
temperature along each channel in the reformer 16 is above
750.degree. C., preferably between 800.degree. C. and 900.degree.
C.
[0034] Such a short residence time enables the reactor 16 to
operate under what appears to be a non-equilibrium condition. The
competing reaction between carbon monoxide and steam to form the
unwanted products carbon dioxide and hydrogen has slower kinetics
than the steam/methane reforming reaction to form carbon monoxide
and hydrogen; and in the reforming reaction the reverse process has
slower kinetics than the forward reaction. The short residence time
allows insufficient time for the slower reactions to reach
equilibrium. Under these circumstances the proportion of methane
undergoing reaction may exceed 90%, and the selectivity for carbon
monoxide can exceed 90%.
[0035] Experimental measurements have been made, passing a
preheated steam/methane mixture (ratio 1.5) through a multichannel
reactor similar to that described below with reference to FIG. 2,
with a residence time of 50 ms. The temperature was measured near
the inlet and near the exit from a channel, and at other
intermediate positions to enable a mean value to be calculated.
Results have been obtained as in the Table:
TABLE-US-00001 Inlet Temp Exit Temp Mean Temp CO Selectivity CH4
Conversion (.degree. C.) (.degree. C.) (.degree. C.) (%) (%) 726
853 810 92.5 93.8 731 866 815 93.0 94.2
[0036] Selectivity for CO production is enhanced by operating with
an exit temperature above 800.degree. C., more preferably above
850.degree. C. The performance of the reactor can also be improved
by a pre-treatment, heating the reactor to about 850.degree. C. in
the presence of hydrogen, as this improves subsequent catalyst
activity.
[0037] As indicated above, the ideal hydrogen to carbon monoxide
stoichiometric ratio to feed to the Fischer-Tropsch synthesis
reactor would be about 2 moles hydrogen to 1 mole carbon monoxide.
This ratio cannot readily be obtained by steam/methane reforming:
as discussed above, at a steam/methane ratio of 1.0 the resulting
gas mixture has a hydrogen to carbon monoxide ratio 3 to 1, and at
the elevated steam/methane ratios that must be adopted to avoid
coking the hydrogen to carbon monoxide ratio is above 3, and may be
as high as 4 to 1. Consequently, after the Fischer-Tropsch
synthesis reaction has occurred there will be an excess of hydrogen
that remains Feeding this gas into the combustion channel of the
reactor 14 has been found to give a more uniform temperature
distribution, and also enables the combustion reaction to be
initiated more readily when the reactor is cold (as catalytic
combustion can then occur at a temperature as low as 15 or
20.degree. C.). The overall thermal efficiency of the process is
improved, the amount of methane fed directly to the combustion
channels is decreased, and the emission of carbon dioxide to the
environment is also reduced.
[0038] Referring now to FIG. 2, a reactor 40 (suitable for example
for steam/methane reforming as reactor 14) comprises a stack of
Fecralloy steel plates 41, each plate being generally rectangular,
450 mm long and 150 mm wide and 3 mm thick, these dimensions being
given only by way of example. On the upper surface of each such
plate 41 are rectangular grooves 42 of depth 2 mm separated by
lands 43 (eight such grooves being shown), but there are three
different arrangements of the grooves 42. In the plate 41 shown in
the drawing the grooves 42 extend diagonally at an angle of
45.degree. to the longitudinal axis of the plate 41, from top left
to bottom right as shown. In a second type of plate 41 the grooves
42a (as indicated by broken lines) follow a mirror image pattern,
extending diagonally at 45.degree. from bottom left to top right as
shown. In a third type of plate 41 the grooves 42b (as indicated by
chain dotted lines) extend parallel to the longitudinal axis.
[0039] The plates 41 are assembled in a stack, with each of the
third type of plate 41 (with the longitudinal grooves 42b) being
between a plate with diagonal grooves 42 and a plate with mirror
image diagonal grooves 42a, and after assembling many plates 41 the
stack is completed with a blank rectangular plate. The plates 41
are compressed together and diffusion bonded, so they are sealed to
each other. Corrugated Fecralloy alloy foils 44 (only one is shown)
50 .mu.m thick coated with a ceramic coating of thickness 15 .mu.m
containing a catalyst material, of appropriate shapes and with
corrugations 2 mm high, can be slid into each such groove 42, 42a
and 42b. The corrugations extend parallel to the flow direction in
each case.
[0040] Header chambers 46 are welded to the stack along each side,
each header 46 defining three compartments by virtue of two fins 47
that are also welded to the stack. The fins 47 are one third of the
way along the length of the stack from each end, and coincide with
a land 43 (or a portion of the plates with no groove) in each plate
41 with diagonal grooves 42 or 42a. Gas flow headers 48 in the form
of rectangular caps are then welded onto the stack at each end,
communicating with the longitudinal grooves 41b. In a modification
(not shown), in place of each three-compartment header 46 there
might instead be three adjacent header chambers, each being a
rectangular cap like the headers 48.
[0041] In use of the reactor 40 for steam/methane reforming, a
steam/methane mixture is supplied to the header 48 at one end (the
right hand end as shown), and the resulting mixture of hydrogen and
carbon monoxide emerges through the header 48 at the other end.
Methane/air mixture is supplied to the compartments of both headers
46 at the other end (the left hand end as shown), and so exhaust
gas from the combustion process emerges through the compartments of
both headers 46 at the right hand end as shown. The flow path for
the mixture supplied to the top-left header compartment (as shown)
is through the diagonal grooves 42 into the bottom-middle header
compartment, and then to flow through the diagonal grooves 42a in
other plates in the stack into the top-right header compartment.
Hence the gas flows are at least partially counter-current, so that
the hottest region in the combustion channels, which is near the
inlet to those channels, is closest to the outlet for the
steam/methane reforming reaction.
[0042] The headers 46 and 48 each comprise a simple rectangular cap
sealed around its periphery to the outside of the stack so as to
cover part of one face of the stack. They may be welded onto the
outside of the stack. Alternatively, if neither of the gas flows
are at elevated pressures, it may be adequate to clamp the header
chambers 46, 48 onto the outside of the stack. In either case it
will be appreciated that after a period of use, if the catalyst in
either or both of the channels has become spent, then the headers
46 and 48 may be removed or cut off and the corresponding
catalyst-carrying foils 44 removed and replaced. The headers 46, 48
can then be re-attached.
[0043] It will be understood that the type and thickness of ceramic
on the corrugated foils 44 in the flow channels may be different in
successive plates 41 in the stack, and that the catalyst materials
may differ also. For example the ceramic might comprise alumina in
one of the gas flow channels, and zirconia in the other gas flow
channels. The reactor 40 formed from the plates 41 might also be
suitable for performing Fischer-Tropsch synthesis. Because the
plates 41 forming the stack are bonded together the gas flow
channels are gas tight (apart from communication with headers 46 or
48), and the dimensions of the plates 41 and grooves 42 are such
that pressures in the alternate gas flow channels may be
considerably different. Furthermore the pitch or pattern of the
corrugated foils 44 may vary along a reactor channel 42 to adjust
catalytic activity, and hence provide for control over the
temperatures or reaction rates at different points in the reactor
40. The corrugated foils 44 may also be shaped, for example with
perforations, to promote mixing of the fluid within the channels
42. Furthermore parts of the foils 44 may be devoid of
catalyst.
[0044] It will be appreciated that the plates forming the stack may
be of a different size (typically in the range 400-1200 mm long,
150-600 mm wide), and that the diagonal grooves 42 and 42a may have
a different orientation (typically between 30.degree. and
90.degree.), for example the plates might be 800 mm by 400 mm, and
the grooves be at about 56.degree. to the longitudinal axis (if
there are three header compartments along each side) or at about
63.degree. (if there are four header compartments). In every case
the headers ensure the fluid in those sets of channels follows a
serpentine path along the reactor.
[0045] In a modification to the reactor 40, the foils 44 are again
of Fecralloy material, but the catalyst material is deposited
directly onto the oxide layer of the Fecralloy.
[0046] Particularly where the reactor 40 is to be used for
Fischer-Tropsch synthesis, the gas flow channels 42 for that
reaction may decrease in width, and possibly also depth, along
their length, so as to vary the fluid flow conditions, and the heat
or mass transfer coefficients. During the synthesis reaction the
gas volume decreases, and by appropriate tapering of the channels
42 the gas velocity may be maintained as the reaction proceeds.
Furthermore the pitch or pattern of the corrugated foils 44 may
vary along a reactor channel 42 to adjust catalytic activity, and
hence provide for control over the temperatures or reaction rates
at different points in the reactor 40.
[0047] When a reactor such as the reactor 40 is used for reactions
between gases that generate gaseous products then the orientation
of the channels is not of concern. However if a product may be a
liquid, it may be preferable to arrange the reactor 40 so that the
flow paths for this reaction slope downwardly, to ensure that any
liquid that is formed will drain out of the channels 42. With the
gas flowing along the corrugations in the foils 44, any liquid
tends to be entrained, so minimizing liquid build-up on the surface
of the catalyst.
[0048] It will be appreciated that the although the heat for the
steam/methane reforming reaction may be provided by catalytic
combustion in adjacent channels (as described above), as an
alternative the combustion may take place in an external burner
(such as a laminar flow burner), the very hot exhaust gases at
about 900 or 1000.degree. C. being passed through the second gas
flow channels of the reactor 14 of FIG. 1 in counter-current to the
methane flow; this can enable the reacting gases in the reformer 16
to reach a final temperature of as much as 900.degree. C. In this
case it is not essential to provide the foils in the combustion
channels with ceramic coating or catalyst, but such foils would
nevertheless enhance heat transfer between the second gas flow
channel carrying the hot exhaust gas and the reactants in the
pre-reformer and reformer channels, by transferring heat to the
separating plates 41. In the combustion channels of the catalytic
reactor 14, if catalytic combustion is used to generate the heat
(as indicated), the combustion catalyst may itself be coated with a
thin porous inert ceramic layer, so as to restrict the contact of
the gas mixture with the catalyst and so restrict the reaction rate
particularly at the start of the channel.
[0049] Particularly where hydrogen is unavailable, it may be
desirable to provide electrical heating by passing an electric
current directly through the plates forming the reactor. This may
be used initially to raise the temperature for example of the
reforming reactor 14 to say 400.degree. C. before supplying gases,
to ensure catalytic combustion occurs. Such electrical heating may
also be used during operation to adjust the reactor temperature.
Electrical heating may also be used in the vicinity of the outlet
from the reactor 14 to ensure that a temperature of say 900.degree.
C. is reached by the gases undergoing the reforming reaction.
[0050] As mentioned above the reactor may differ in size or shape
from that shown in FIG. 2. A single such plate might instead for
example be 1.0 m by 0.5 m. The stack forming a reactor might be 0.8
m thick. Several such reactors may be combined into a reaction
module, for example ten such reactors might form a module provided
with pipes interconnected so the gas flows are in parallel through
all the reactors in the module. Such a module may be small enough
to be transported in an ISO structural frame, and yet have
sufficient capacity to produce synthesis gas equivalent to 1000
barrels per day of synthetic oil.
[0051] A practical plant may need to include several such modules,
all being operated with gas flows in parallel, although it may not
be necessary to replicate the other features (e.g. heat exchangers
18, 11, 20 etc. and separators 31 and 35) of the plant. Thus there
might be say six or ten modules made up of the reactors 14, and the
same number of modules of Fischer-Tropsch reactors 26. This has the
benefit that if the catalysts in one module need to be replaced,
that module may be disconnected and taken away for servicing, while
allowing the plant to continue to operate at only slightly reduced
capacity.
* * * * *