U.S. patent application number 10/421048 was filed with the patent office on 2003-11-06 for fuel processing system.
Invention is credited to Bradley, Mark, Holland, Robert, O' Connor, Kevin, Peppley, Brant, Schubak, Gary.
Application Number | 20030204993 10/421048 |
Document ID | / |
Family ID | 24480888 |
Filed Date | 2003-11-06 |
United States Patent
Application |
20030204993 |
Kind Code |
A1 |
Holland, Robert ; et
al. |
November 6, 2003 |
Fuel processing system
Abstract
A method and apparatus for processing a hydrocarbon fuel
comprises: a primary fuel processing reactor for converting a feed
stream to a first reformate stream comprising hydrogen; a first
hydrogen separator located downstream of the primary fuel
processing reactor and fluidly connected thereto for receiving the
first reformate stream, the first separator comprising a first
membrane for separating the first reformate stream into a first
hydrogen-rich stream and a first retentate stream; and a secondary
fuel processing reactor fluidly connected to the first separator
for receiving and converting the first retentate stream to a second
reformate stream comprising hydrogen. A fuel cell power generation
system includes the present apparatus and a fuel cell stack fluidly
connected thereto for receiving hydrogen-rich streams
therefrom.
Inventors: |
Holland, Robert; (Richmond,
CA) ; Schubak, Gary; (Vancouver, CA) ;
Bradley, Mark; (North Vancouver, CA) ; O' Connor,
Kevin; (Maple Ridge, CA) ; Peppley, Brant;
(Kingston, CA) |
Correspondence
Address: |
Robert W. Fieseler
McAndrews, Held & Malloy, Ltd.
34th Floor
500 West Madison Street
Chicago
IL
60661
US
|
Family ID: |
24480888 |
Appl. No.: |
10/421048 |
Filed: |
April 23, 2003 |
Related U.S. Patent Documents
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Application
Number |
Filing Date |
Patent Number |
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10421048 |
Apr 23, 2003 |
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09619204 |
Jul 19, 2000 |
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6572837 |
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Current U.S.
Class: |
48/127.9 ;
422/198; 422/600; 429/420; 429/425; 429/454; 429/492; 48/197R;
48/198.1; 48/198.3; 48/198.7; 48/61; 48/62R; 48/63 |
Current CPC
Class: |
C01B 2203/0233 20130101;
C01B 2203/0283 20130101; C01B 3/382 20130101; C01B 2203/0244
20130101; C01B 2203/0261 20130101; C01B 2203/0405 20130101; C01B
3/16 20130101; C01B 2203/143 20130101; C01B 2203/142 20130101; C01B
2203/1205 20130101; C01B 3/501 20130101; C01B 2203/0288 20130101;
C01B 2203/146 20130101; C01B 2203/047 20130101 |
Class at
Publication: |
48/127.9 ; 48/61;
48/62.00R; 48/63; 48/197.00R; 48/198.1; 48/198.3; 48/198.7;
422/190; 422/193; 422/194; 422/198; 429/17; 429/19; 429/20 |
International
Class: |
B01J 008/04 |
Claims
What is claimed is:
1. A fuel processing system comprising: (a) a primary fuel
processing reactor for converting a feed stream to a first
reformate stream comprising hydrogen; (b) a first hydrogen
separator located downstream of said primary fuel processing
reactor and fluidly connected thereto for receiving said first
reformate stream, said first separator comprising a first membrane
for separating said first reformate stream into a first
hydrogen-rich stream and a first retentate stream; and (c) a
secondary fuel processing reactor fluidly connected to said first
separator for receiving and converting said first retentate stream
to a second reformate stream comprising hydrogen.
2. The fuel processing system of claim 1 wherein said first
hydrogen separator is fluidly connected to said secondary fuel
processing reactor for receiving said second reformate stream, and
said second reformate stream is introduced into said first
reformate stream via a compressor.
3. The fuel processing system of claim 1 wherein said first
hydrogen separator is fluidly connected to said secondary fuel
processing reactor for receiving said second reformate stream, and
said second reformate stream is introduced into said first
reformate stream via an ejector.
4. The fuel processing system of claim 1, further comprising: (d) a
second hydrogen separator located downstream of said secondary fuel
processing reactor and fluidly connected thereto for receiving said
second reformate stream, said second separator comprising a second
membrane for separating said second reformate stream into a second
hydrogen-rich stream and a second retentate stream.
5. The fuel processing system of claim 1 wherein said primary fuel
processing reactor is selected from the group consisting of steam
reformers, partial oxidation reformers, catalytic partial oxidation
reformers, autothermal reformers, and plasma reformers.
6. The fuel processing system of claim 5 wherein said secondary
fuel processing reactor is selected from the group consisting of
steam reformers, partial oxidation reformers, catalytic partial
oxidation reformers, autothermal reformers, plasma reformers, and
shift reactors.
7. The fuel processing system of claim 1 wherein said feed stream
comprises synthesis gas or a reformate stream from a
high-temperature reformer, and wherein said primary and secondary
fuel processing reactors are shift reactors.
8. The fuel processing system of claim 1 wherein said primary fuel
processing reactor is a steam reformer.
9. The fuel processing system of claim 8 wherein said secondary
fuel processing reactor is a steam reformer or a shift reactor.
10. The fuel processing system of claim 1 wherein said first
membrane is selected from the group consisting of palladium
membranes, palladium alloy membranes, platinum membranes, platinum
alloy membranes, titanium alloy membranes, ceramic membranes,
zeolite molecular sieve membranes, carbon molecular sieve
membranes, inorganic poly-acid membranes, and composite membranes
thereof.
11. The fuel processing system of claim 10 wherein said first
membrane comprises a palladium membrane or palladium alloy
membrane.
12. The fuel processing system of claim 11 wherein said first
membrane is supported.
13. The fuel processing system of claim 1 wherein said first
hydrogen separator is independently selected from the group
consisting of plate-and-frame, spiral wound, and hollow fiber
modules.
14. The fuel processing system of claim 4 wherein said first and
second membranes are independently selected from the group
consisting of palladium membranes, palladium alloy membranes,
platinum membranes, platinum alloy membranes, titanium alloy
membranes, ceramic membranes, zeolite molecular sieve membranes,
carbon molecular sieve membranes, inorganic poly-acid membranes,
and composite membranes thereof, and wherein said first and second
membranes can be of the same type or different.
15. The fuel processing system of claim 14 wherein said first and
second membranes comprise palladium membranes or palladium alloy
membranes.
16. The fuel processing system of claim 15 wherein said first and
second membranes are supported.
17. The fuel processing system of claim 4 wherein said first and
second hydrogen separators are independently selected from the
group consisting of plate-and-frame, spiral wound, and hollow fiber
modules, and wherein said first and second hydrogen separators can
be of the same type or different.
18. The fuel processing system of claim 5, further comprising a
fuel supply for supplying said feed stream to said primary
reformer.
19. The fuel processing system of claim 18 wherein said feed stream
comprises a fuel selected from the group consisting of gasoline,
diesel, natural gas, ethane, butane, light distillates, dimethyl
ether, methanol, ethanol, propane, naphtha, kerosene, and
combinations thereof.
20. The fuel processing system of claim 19 wherein said fuel is
methanol.
21. The fuel processing system of claim 1, further comprising a
water supply for supplying water vapor to at least one of said
primary and secondary fuel processing reactors.
22. The fuel processing system of claim 1, further comprising a
heating device for heating said second reformate stream to a
temperature within a predetermined temperature range.
23. The fuel processing system of claim 4, further comprising a
heating device for heating said second reformate stream to a
temperature within a predetermined temperature range, said heating
device being located upstream of said secondary hydrogen
separator.
24. The fuel processing system of claim 1, further comprising an
oxidant supply for supplying oxidant to at least one of said
primary and secondary fuel processing reactors.
25. A fuel cell power generation system comprising: a fuel
processing system according to claim 1, and a fuel cell stack
comprising at least one fuel cell fluidly connected to receive said
first hydrogen-rich stream from said fuel processing system.
26. The fuel cell power generation system of claim 25 wherein said
at least one fuel cell is a solid polymer electrolyte fuel
cell.
27. A fuel cell power generation system comprising: a fuel
processing system according to claim 4, and a fuel cell stack
comprising at least one fuel cell fluidly connected to receive said
first and second hydrogen-rich streams from said fuel processing
system.
28. The fuel cell power generation system of claim 27 wherein said
at least one fuel cell is a solid polymer electrolyte fuel cell.
Description
CROSS-REFERENCE TO RELATED APPLICATION(S)
[0001] This application is a continuation-in-part of U.S. patent
application Ser. No. 09/619,204 filed Jul. 19, 2000, now U.S. Pat.
No. 6, ______, ______ issued ______, entitled "Fuel Processing
System". The '204 application hereby incorporated by reference in
its entirety.
FIELD OF THE INVENTION
[0002] The present invention relates to methods and apparatus for
producing substantially pure hydrogen from a hydrocarbon fuel, and
in particular, to an apparatus comprising a plurality of fuel
processing reactors and hydrogen separation membrane units
connected in series.
BACKGROUND OF THE INVENTION
[0003] The search for alternative power sources has focused
attention on the use of electrochemical fuel cells to generate
electrical power. Unlike conventional fossil fuel power sources,
fuel cells are capable of generating electrical power from a fuel
stream and an oxidant stream without producing substantial amounts
of undesirable by-products, such as sulfides, nitrogen oxides and
carbon monoxide. However, the commercial viability of fuel cell
systems will benefit from the ability to efficiently and cleanly
convert conventional hydrocarbon fuel sources, such as, for
example, gasoline, diesel, natural gas, ethane, butane, light
distillates, dimethyl ether, methanol, ethanol, propane, naphtha,
kerosene, and combinations thereof, to a hydrogen-rich gas stream
with increased reliability and decreased cost. The conversion of
such fuel sources to a hydrogen-rich gas stream is also important
for other industrial processes, as well. Several technologies are
available for converting such fuels to hydrogen-rich gas
streams.
[0004] Steam reformers convert hydrocarbons to reformate gas
streams that contain hydrogen. Hydrocarbon feedstock and steam are
reacted in reactors filled with catalyst (typically nickel-,
copper- or noble metal-based), and hydrogen, carbon dioxide
(CO.sub.2) , and carbon monoxide (CO) are produced. For example,
the following principal reactions occur in the steam reforming of
methane (and natural gas): 1 CH 4 + H 2 O CO + 3 H 2 CO + H 2 O CO
2 + H 2 CH 4 + 2 H 2 O CO 2 + 4 H 2 ( I )
[0005] The overall reaction (I) is highly endothermic, and is
normally carried out at elevated catalyst temperatures in the range
from about 650.degree. C. to about 875.degree. C. Such elevated
temperatures are typically generated by the heat of combustion from
a burner incorporated in the fuel processing reactor. Steam
reforming is adversely affected by sulfur and/or other contaminants
in the feedstock. Accordingly, fuel feed purification may be
required prior to steam reforming.
[0006] Partial oxidation systems are based on substoichiometric
combustion to achieve the temperatures necessary to reform the
hydrocarbon feedstock. Feedstock and oxidant (oxygen or air, for
example) are reacted to form hydrogen and CO. Taking methane as an
example, the process is based mainly on the exothermic partial
oxidation of the hydrocarbon.
2CH.sub.4+O.sub.2 2CO+4H.sub.2 (II)
[0007] Other reactions may also occur, including endothermic
cracking and/or pyrolysis, and endothermic reforming with carbon
dioxide. Combustion of the feedstock, according to the following
reaction, is minimized:
CH.sub.4+2O.sub.2 CO.sub.2+2H.sub.2O (III)
[0008] Partial oxidation is generally performed at high
temperatures (1200-1650.degree. C.). The heat required to drive the
reactions is typically supplied by oxidizing a fraction of the
fuel.
[0009] Catalytic partial oxidation systems employ catalysts to
accelerate the reforming reactions at lower temperatures. The
desirable result can be soot-free operation, since soot is a common
problem with non-catalytic partial oxidation approaches, and
improved conversion efficiencies from smaller and lighter
equipment. However, common catalysts are susceptible to coking by
feedstocks that are high in aromatic content at the low
steam-to-carbon ratios typically employed.
[0010] Autothermal reforming is an approach that combines catalytic
partial oxidation and steam reforming. A significant advantage of
autothermal reforming technology is that the exothermic combustion
reaction (II or III) is used to drive the endothermic reforming
reaction (I).
[0011] More recently, a plasma reformer process has been developed
that employs an electric arc to generate very high temperatures for
reforming the fuel. The high temperature conditions avoid the need
for catalysts.
[0012] In addition to the fuel processing step, other processing
steps are generally performed to reduce the sulfur and/or CO
content of the fuel gas to meet fuel cell requirements. Absorbent
beds may be utilized to remove sulfur-containing compounds from the
fuel gas, for example. A water gas shift reactor ("shift reactor")
is often employed to reduce the CO concentration in the fuel gas in
order to avoid poisoning of the catalyst employed in the fuel cells
and to produce additional hydrogen fuel. In the shift reactor, CO
is combined with water in the presence of a catalyst to yield
carbon dioxide and hydrogen according to the following
reaction:
CO+H.sub.2O CO.sub.2+H.sub.2 (IV)
[0013] In many instances, the reformate stream exiting the shift
reactor is often passed through a selective oxidizer, to further
reduce the concentration of CO present in the stream.
[0014] With respect to reliability and cost, conventional reformers
have some disadvantages with respect to fuel cell use. For example,
in vehicular applications in particular, conventional reformers
tend to be quite large, which impacts material costs and
undesirably increases the size and weight of the fuel cell power
generation system, as a whole. Several approaches have been used in
an effort to reduce the size and weight of hydrocarbon reforming
systems without undesirable loss of performance.
[0015] For example, a conventional reformer can be followed by a
hydrogen separation unit. A hydrogen separation unit employs a
hydrogen-permeable membrane material to separate essentially pure
hydrogen from the reformed fuel gas ("reformate"). Typically, these
membranes are made of palladium or palladium alloy films supported
by porous ceramic substrates, but may be made of other materials
with high selectivity and high permeability for hydrogen. The
reformer is typically operated at a relatively high pressure (for
example, 20 to 35 barg) to provide a good hydrogen partial pressure
on the high-pressure side of the palladium membrane. However, the
amount of feedstock that can be reformed is limited by
pressure-related equilibrium considerations. For example, the
conversion of methanol to hydrogen is limited to about 92% at 35
barg. As well, the palladium membrane is limited as to the amount
of hydrogen it can produce by such factors as temperature, the
hydrogen partial pressure across the membrane, and equilibrium
considerations. A typical reformer/palladium system produces in the
range of 75% fuel efficiency at optimum conditions.
[0016] One approach to increasing the fuel efficiency of such a
system is to recycle the retentate from the palladium unit back to
the reformer and recover the unreformed fuel and/or hydrogen.
However, such recycling systems suffer from several disadvantages.
First, they generally include a compressor for the recycling loop,
which introduces a parasitic load into the system and also
increases its size, cost, and complexity. Second, recycling the
retentate effectively dilutes the feedstock. The introduction of
diluent gases increases the mass flows and thereby increases the
size and cost of the reformer if hydrogen production capacity is to
be maintained. Third, the retentate will be enriched in CO (and
possibly hydrogen), as compared to the feedstock. As a result, the
equilibrium conditions in the mixed feedstock/retentate will be
less favorable with respect to hydrogen formation, by LeChatelier's
principle. Accordingly, such recycling systems are less than
optimal.
[0017] A similar approach employs staged hydrogen separation units
downstream of the fuel processing reactor. The amount of hydrogen
recoverable in a first palladium membrane unit, for example, is
limited by the hydrogen partial pressure in the reformate stream
and the hydrogen partial pressure differential across the membrane.
A second palladium membrane unit is employed to recover some of the
hydrogen in the retentate from the first unit. While equilibrium
conditions in the second unit favor further hydrogen recovery, the
hydrogen partial pressure of the retentate stream may be less than
favorable.
[0018] Another approach is to incorporate a reformer and a hydrogen
separation membrane together into a "membrane reactor". Such an
integral arrangement enhances both the reforming and separating
functions. Hydrogen formed in the reforming reaction can be
continually removed by the separation membrane, thereby creating
equilibrium conditions in the reformer favoring hydrogen formation.
The formation of hydrogen on one side of the membrane also assists
to maintain a hydrogen partial pressure favoring separation.
However, this approach has rarely been successful in practice. It
greatly increases the complexity of design and also greatly
increases the complexity of maintenance of the unit.
[0019] Accordingly, it would be desirable to have a hydrocarbon
fuel reforming system of relatively simple design, capable of high
hydrogen recovery rates, and of adequate reliability, size, weight
and cost for use in various industrial applications, including fuel
cell applications. Embodiments of the present system address one or
more of these concerns.
SUMMARY OF THE INVENTION
[0020] A fuel processing system is provided comprising:
[0021] (a) a primary fuel processing reactor for converting a feed
stream to a first reformate stream comprising hydrogen;
[0022] (b) a first hydrogen separator located downstream of the
primary fuel processing reactor and fluidly connected thereto for
receiving the first reformate stream, the first separator
comprising a first membrane for separating the first reformate
stream into a first hydrogen-rich stream and a first retentate
stream; and
[0023] (c) a secondary fuel processing reactor fluidly connected to
the first separator for receiving and converting the first
retentate stream to a second reformate stream comprising
hydrogen.
[0024] In a preferred embodiment of the present fuel processing
system, the first hydrogen separator is fluidly connected to the
secondary fuel processing reactor for receiving the second
reformate stream in addition to or in combination with the first
reformate stream. The second reformate stream may be introduced
into the first reformate stream via a compressor or an ejector.
[0025] Alternatively, the fuel processing system may further
comprise a second hydrogen separator located downstream of the
secondary fuel processing reactor and fluidly connected thereto for
receiving the second reformate stream, the second separator
comprising a second membrane for separating the second reformate
stream into a second hydrogen-rich stream and a second retentate
stream.
[0026] In the present fuel processing system, the primary fuel
processing reactor may comprise a steam reformer, partial oxidation
reformer, catalytic partial oxidation reformer, autothermal
reformer, or a plasma reformer, for example. The secondary fuel
processing reactor may comprise any of the foregoing or may
comprise a shift reactor. If the feed stream comprises synthesis
gas or a reformate stream from a high-temperature reformer, both
the primary and secondary fuel processing reactors may comprise
shift reactors. In a preferred embodiment, the primary fuel
processing reactor is a steam reformer and the secondary reformer
is a steam reformer or a shift reactor.
[0027] The present fuel processing system may further comprise a
fuel supply for supplying fuel to the primary fuel processing
reactor, an oxidant supply for supplying oxidant to at least one of
the primary and secondary fuel processing reactors, and/or a water
supply for supplying water vapor to at least one of the primary and
secondary fuel processing reactors. The fuel processing system may
also further comprise a heating device for heating the second
reformate stream to a temperature within a predetermined
temperature range.
[0028] The first and second membranes found in the first and second
hydrogen separators, respectively, may be independently selected
from the group consisting of palladium membranes, palladium alloy
membranes, platinum membranes, platinum alloy membranes, titanium
alloy membranes, ceramic membranes, zeolite molecular sieve
membranes, carbon molecular sieve membranes, inorganic poly-acid
membranes, and composite membranes thereof. They may be supported,
and may be constructed as plate-and-frame, spiral wound, or hollow
fiber modules, if desired. The membranes of the first and second
hydrogen separators may be the same or different.
[0029] The feed stream may comprise a fuel selected from the group
consisting of gasoline, diesel, natural gas, ethane, butane, light
distillates, dimethyl ether, methanol, ethanol, propane, naphtha,
kerosene, and combinations thereof.
[0030] A fuel cell power generation system is also provided. In one
embodiment the fuel cell power generation system comprises:
[0031] (a) a primary fuel processing reactor for converting a feed
stream to a first reformate stream comprising hydrogen;
[0032] (b) a first hydrogen separator located downstream of the
primary fuel processing reactor and fluidly connected thereto for
receiving the first reformate stream, the first separator
comprising a first membrane for separating the first reformate
stream into a first hydrogen-rich stream and a first retentate
stream;
[0033] (c) a secondary fuel processing reactor fluidly connected to
the first separator for receiving and converting the first
retentate stream to a second reformate stream comprising hydrogen;
and
[0034] a fuel cell stack comprising at least one fuel cell fluidly
connected to receive the first hydrogen-rich stream from the fuel
processing system.
[0035] Another embodiment of the present fuel cell power generation
system further comprises a second hydrogen separator located
downstream of the secondary fuel processing reactor and fluidly
connected thereto for receiving the second reformate stream, the
second separator comprising a second membrane for separating the
second reformate stream into a second hydrogen-rich stream and a
second retentate stream, and the fuel cell stack is connected to
receive both first and second hydrogen-rich streams from the fuel
processing system. In either embodiment, the at least one fuel cell
may be a solid polymer electrolyte fuel cell.
[0036] A fuel processing method is also provided, comprising the
sequential steps:
[0037] (a) supplying a feed stream to a primary fuel processing
reactor;
[0038] (b) processing the feed stream in the primary fuel
processing reactor to produce a first reformate stream comprising
hydrogen;
[0039] (c) supplying the first reformate stream to a hydrogen
separator and separating the first reformate stream therein into a
first hydrogen-rich stream and a first retentate stream;
[0040] (d) supplying the first retentate stream to a secondary fuel
processing reactor and processing the first retentate stream
therein to produce a second reformate stream comprising hydrogen;
and
[0041] (e) supplying the second reformate stream to a hydrogen
separator and separating the second reformate stream therein into a
second hydrogen-rich stream and a second retentate stream.
[0042] In this method, the first and second reformate streams may
be supplied to the same hydrogen separator, or the first reformate
stream may be supplied to a first hydrogen separator in step (c),
and the second reformate stream may be supplied to a second
hydrogen separator in step (e).
[0043] In the present method, the primary fuel processing reactor
may comprise a steam reformer, partial oxidation reformer,
catalytic partial oxidation reformer, autothermal reformer, or
plasma reformer, for example. The secondary fuel processing reactor
may comprise any of the foregoing or may comprise a shift reactor.
Where the feed stream comprises synthesis gas or a reformate stream
from a high-temperature reformer, both the primary and secondary
fuel processing reactors may be shift reactors. In a preferred
embodiment, the primary fuel processing reactor is a steam reformer
and the secondary reformer is a steam reformer or a shift
reactor.
[0044] The first and second membranes found in the first and second
hydrogen separators may be independently selected from the group
consisting of palladium membranes, palladium alloy membranes,
platinum membranes, platinum alloy membranes, titanium alloy
membranes, ceramic membranes, zeolite molecular sieve membranes,
carbon molecular sieve membranes, inorganic poly-acid membranes,
and composite membranes thereof. They may be supported, and may be
constructed as plate-and-frame, spiral wound, or hollow fiber
modules, if desired. The first and second membranes may be the same
or different from each other.
[0045] The feed stream may comprise a fuel selected from the group
consisting of gasoline, diesel, natural gas, ethane, butane, light
distillates, dimethyl ether, methanol, ethanol, propane, naphtha,
kerosene, and combinations thereof.
[0046] The present method may further comprise supplying water
vapor, oxidant, or both, to the primary fuel processing reactor,
the secondary fuel processing reactor, or both, as desired. In
addition, the method may further comprise heating the second
reformate stream to a temperature within a predetermined
temperature range upstream of the hydrogen separator.
[0047] Although these embodiments of the apparatus and methods are
described herein as comprising two fuel processing reactors and one
or two hydrogen separators, additional reactors and separators may
be included. For example, a third fuel processing reactor may be
located downstream of the second hydrogen separator and fluidly
connected thereto for receiving and converting the second retentate
stream to a third reformate stream comprising hydrogen.
BRIEF DESCRIPTION OF THE DRAWINGS
[0048] FIG. 1 is a schematic block diagram of a preferred
embodiment of the present method and apparatus.
[0049] FIG. 2 is a schematic block diagram of another embodiment of
the present method and apparatus.
DETAILED DESCRIPTION OF PREFERRED EMBODIMENT(S)
[0050] As used in this description and in the appended claims,
hydrocarbon fuel means gaseous or liquid fuels comprising aliphatic
hydrocarbons and oxygenated derivatives thereof, and may further
comprise aromatic hydrocarbons and oxygenated derivatives thereof.
Reformate means the gas stream comprising hydrogen produced from a
hydrocarbon fuel by a fuel processing reactor, including but not
limited to steam reformers, partial oxidation reformers, catalytic
partial oxidation reformers, autothermal reformers, plasma
reformers, and shift reactors. Oxidant means substantially pure
oxygen, or a fluid stream comprising oxygen, including air.
Synthesis gas means a gas mixture comprising carbon monoxide and
hydrogen such as that may be used as a feedstock for making
hydrocarbon compounds. As used herein, when two components are
fluidly connected to one another, there may be other components in
between them, and the other components may effect the fluid
connection but not eliminate it altogether.
[0051] The primary fuel processing reactor may, but need not,
convert most or substantially all of the feed stream to a reformate
stream. The primary fuel processing reactor will generally convert
more (or a greater percentage) of the feed stream than will the
secondary fuel processing reactor, although in certain embodiments,
the secondary fuel processing reactor will convert a greater volume
or percentage to hydrogen. Similarly, the primary hydrogen
separator may, but need not; separate volume or percentage of
hydrogen from reformate than does a secondary hydrogen separator,
if present.
[0052] The present method and apparatus employ fuel processing
reactors and hydrogen separation units in stages in order to
produce hydrogen from a fuel. Generally, a first-stage fuel
processing reactor converts the fuel into a reformate stream, which
is provided to a first-stage hydrogen separation device. The
reformate stream is separated into a hydrogen-rich stream
("permeate") and a hydrogen-depleted stream ("retentate").
Substantially pure hydrogen will generally permeate through the
membrane of the hydrogen separator, while the hydrogen-depleted
stream generally does not permeate the membrane and remains on the
original side of the membrane. The first-stage hydrogen separation
device typically removes about 40-60% of the hydrogen from the
reformate stream. The hydrogen-depleted retentate is then sent to a
second-stage fuel processing reactor where it is reformed into
additional hydrogen. Lowering the hydrogen concentration in the
first-stage retentate may shift the equilibrium of the reforming
reaction in the second-stage fuel processing reactor in favor of
further hydrogen production, thus increasing the overall fuel
conversion efficiency. This, in turn, may increase the driving
force and favor the recovery of additional hydrogen from a
second-stage hydrogen separation device. More than two stages may
be employed to further increase the recovery of hydrogen from the
fuel.
[0053] FIG. 1 is a schematic block diagram of a preferred
embodiment of the present method and apparatus. In fuel processing
system 100, a vaporized feed stream from feed source 102 is fed to
primary fuel processing reactor 104. Primary fuel processing
reactor 104 converts the feed stream to a first hydrogen-rich
reformate stream. The reformate stream is then fed to hydrogen
separation device 106, where it is separated into a hydrogen-rich
stream and a retentate stream by membrane 108. A portion of the
retentate stream is then fed to secondary fuel processing reactor
110, where it is converted to a second reformate stream comprising
hydrogen. The reformate stream from secondary fuel processing
reactor 110 is pressurized in compressor 112 and then introduced
into the first reformate stream supplied to hydrogen separation
device 106. Alternatively, an ejector could be used instead of
compressor 112 to pressurize the second reformate stream prior to
introduction into hydrogen separation device 106. The pressurized
reformate stream from secondary fuel processing reactor 110 could
also be introduced into hydrogen separation device 106 separately
from the reformate stream from fuel processing reactor 104. In any
case, hydrogen separation device 106 separates the reformate stream
of fuel processing reactors 104, 110 into a hydrogen-rich stream
that preferably comprises substantially pure hydrogen. A portion of
the retentate stream is typically diverted and used to fuel a
(catalytic) burner (not shown) for fuel processing reactor 104,
fuel processing reactor 110, or both. The hydrogen-rich stream from
hydrogen separation device 106 is supplied to unit 114. Unit 114
may comprise a storage tank or downstream equipment such as, for
example, a fuel cell stack for generating electricity from the
hydrogen-rich stream as part of a fuel cell power generation
system. For example, the method and apparatus may employ a fuel
cell stack such as that disclosed in U.S. Pat. No. 5,484,666, which
is incorporated by reference herein.
[0054] FIG. 2 is a schematic block diagram of another embodiment of
the present method and apparatus. In fuel processing system 200, a
vaporized feed stream from feed source 202 is fed to primary fuel
processing reactor 204. Primary fuel processing reactor 204
converts the feed stream to a first reformate stream comprising
hydrogen. The reformate stream is then fed to first hydrogen
separation device 206, where it is separated into a hydrogen-rich
stream, preferably a substantially pure hydrogen stream, and a
retentate stream by membrane 208. The retentate stream is then fed
to secondary fuel processing reactor 210, where it is converted to
a second reformate stream comprising hydrogen. The reformate stream
from secondary fuel processing reactor 210 is then fed to second
hydrogen separation device 212, where it is separated into a
hydrogen-rich stream, preferably a substantially pure hydrogen
stream, and a retentate stream by membrane 214. The retentate
stream from hydrogen separation device 212 may be exhausted:
alternatively, all or a portion of it may be used to fuel a burner
(not shown) for fuel processing reactor 204, fuel processing
reactor 210, or both. The hydrogen-rich streams from hydrogen
separation units 206, 212 are supplied to unit 216. Unit 216 may
comprise a storage tank or downstream equipment such as, for
example, a fuel cell stack for generating electricity from the
hydrogen-rich stream as part of a fuel cell power generation
system.
[0055] By supplying hydrogen-depleted retentate to the secondary
fuel processing reactor, the equilibrium of the reforming reaction
is shifted in favor of the production of more hydrogen. Thus,
reaction conditions in the secondary reactor may be more favorable
to hydrogen production relative to a fuel processing reactor where
recycled retentate is mixed with fuel. More favorable reaction
conditions, in turn, may result in greater efficiency compared to
such fuel processing systems. Similarly, the reformate streams may
have higher hydrogen partial pressures relative to systems
employing retentate recycling or separation units in series, and
the hydrogen separation devices may operate at higher efficiency as
a result. The present method and apparatus is also simpler than
palladium membrane reactors and may be less costly to produce and
maintain.
[0056] Any suitable hydrocarbon fuel can be used as the feed
stream. Suitable such fuels include gasoline, diesel, natural gas,
ethane, butane, light distillates, dimethyl ether, methanol,
ethanol, propane, naphtha, kerosene, and combinations thereof, for
example, and may also include synthesis gas or the reformate from a
high-temperature reformer.
[0057] The present method and apparatus may employ any fuel
processing reactor capable of converting a hydrocarbon fuel stream
to a reformate stream comprising hydrogen. For example, steam
reformers, partial oxidation reformers, catalytic partial oxidation
reformers, autothermal reformers (including electrochemical
autothermal reformers (EATR)), plasma reformers, and shift reactors
can be used. The fuel processing reactors can operate at high or
low temperature, pressure, or both, depending on system
characteristics.
[0058] Where the present method and apparatus are part of a
stand-alone fuel processing system, the primary fuel processing
reactor is a reformer such as, for example, a steam reformer,
partial oxidation reformer, catalytic partial oxidation reformer,
autothermal reformer, or plasma reformer. The other fuel processing
reactor(s) may alternatively comprise a shift reactor. Subject to
the foregoing, the fuel processing reactors may be of the same type
or different. The same is true where the present method and
apparatus is part of a larger system. For example, the present
method and apparatus may be used for processing synthesis gas into
substantially pure hydrogen, or as part of a fuel processing system
downstream of a high-temperature reformer (such as in natural gas
reforming). In such circumstances, the fuel processing reactors may
be shift reactors or other similar devices. For fuel cell power
generation applications, a preferred fuel processing system
comprises two steam reformers or a primary steam reformer and a
secondary shift reactor.
[0059] The hydrogen separation membrane may comprise: a metal
membrane (for example, palladium, palladium alloy, or titanium
alloy membrane); a polymeric material (for example, porous or
microporous polyaramides, polyimides, polyketones, polysulfones,
siloxane- and silane-based polymers, and cellulose acetate-based
polymers); ceramic membranes (for example, porous silica membranes,
porous or dense metal oxide membranes); zeolite molecular sieves;
carbon molecular sieves; and inorganic poly-acids (for example,
poly-antimonic acids and polyphosphates); and composite membranes
thereof. The membranes may be supported or unsupported. They may be
flat films or films of various other shapes, such as cylinders, for
example. The membranes may comprise modules such as, for example,
plate-and-frame, spiral wound, or hollow fiber modules. In the
preferred embodiment, the hydrogen separation membranes of the
first and second hydrogen separation devices may be of the same
type or different. Palladium and palladium alloy membranes are more
preferred.
[0060] As will be understood by persons skilled in the art, the
present method and apparatus may further comprise other
steps/components depending on the particular system configuration
employed. Factors to be considered include the hydrocarbon fuel,
fuel processing reactor design, and hydrogen separation membrane
operating conditions. For example, if the fuel used is other than
an alcohol or ether, an upstream pre-treatment step (for example,
desulfurization) will probably be necessary prior to any catalytic
fuel processing reactor step to remove any catalyst poisons present
in the fuel. In addition, it may also be desirable to include an
upstream pre-reforming step prior to a first reforming step where
higher molecular weight fuels are employed.
[0061] Depending on the choice of fuel processing reactors, the
present method and apparatus may further comprise a water source
for supplying steam, an oxidant, or both, to the fuel processing
reactors depending on processing or reactor requirements. For
example, steam reformers require a source of steam, partial
oxidation reformers require an oxidant source, and autothermal
reactors typically require both. In fuel cell power generation
applications, oxidant may be supplied from the stack oxidant supply
or from the cathode exhaust, for example. Similarly, water may be
obtained from a stack water supply, or reaction product water may
be used. Depending on the relative operating temperatures of the
fuel processing reactors, heat exchange elements may also be
included, if desired. This may increase efficiency of the system
where, for example, the heat from an exothermic fuel processing
reactor could be supplied to an endothermic fuel processing
reactor.
[0062] The choice of hydrogen separation device, and in particular,
the hydrogen separation membrane, also influences systems design.
The suitability of a particular membrane may depend on such factors
as, for example, process feed compositions, process feed pressures,
process temperatures and/or temperature cycles, and pressure
differentials across the membrane. For example, palladium and
palladium alloy membranes operate more efficiently at higher
temperatures. Where an endothermic fuel processing reactor, such as
a steam reformer, for example, is employed, it may be desirable to
include a heating device upstream of the hydrogen separation device
to heat the incoming reformate stream to within the optimal
operating temperature range of the membrane. Suitable such heating
devices include burners, electrical heaters, and oil bath heaters,
for example. As another example, hydrogen separation membranes also
have an optimal pressure range for hydrogen separation. The present
method and apparatus may further include means for pressurizing the
reformate streams supplied to the hydrogen separation device(s), if
desired. Preferably, the system pressure chosen will correspond to
the optimal operating pressure of the hydrogen separation membranes
employed, at least within the hydrogen separation units
themselves.
[0063] While the foregoing factors influencing system design have
been discussed separately, those skilled in the art will recognize
that they are inter-dependent and should be considered in relation
to each other. In particular, system pressure and temperature
parameters should be chosen to optimize the fuel processing reactor
reactions and hydrogen separation efficiencies.
[0064] The present method and apparatus is intended to permit
increased recovery of hydrogen relative to prior art systems
employing a single fuel processing reactor. At the same time, they
embody a simple design that may be simpler to maintain and repair
than current palladium membrane reactors. Further, they may also
provide for the ability to separately optimize fuel processing
reactors and hydrogen separation devices for each stage. The
following example is for purposes of illustration and is not
intended to limit the claims.
EXAMPLE 1
[0065] A fuel processing system was assembled using a Ballard Power
Systems prototype design 50 kW steam reformer, having a modified
tube and shell heat exchanger with commercial low-temperature shift
catalyst inside tubes (9 L of BASF R3-12 catalyst), as the primary
reformer, and first and second palladium membrane hydrogen
separation units. The hydrogen separation units were also
prototypes comprising planar palladium separation membranes in a
plate-and-frame construction, and were rated at 25 kW each.
[0066] The system was used to reform methanol. The primary steam
reformer was operated at 34.5-36.2 barg inlet pressure, steam to
carbon ratio of 1.4 to 1, and a 275-285.degree. C. reformer outlet
temperature. Reformate was passed through the first and second
palladium separation units in series. The separation units were
operated at about 34.5 barg inlet pressure and 285-300.degree. C.
inlet temperature. Retentate outlet pressure was about 33.8-35.2
barg and the permeate outlet pressure was 2.8-3.4 barg. The
retentate from the first palladium separation unit is fed to the
inlet of the second palladium separation unit, and the retentate
from the second unit is supplied as fuel to the primary reformer
burner--additional methanol is also used to supplement the
retentate stream supplied to the burner. Data set forth in Table 1
was compiled from this system.
[0067] The fuel processing system was then modified according to
the present method and apparatus by installing a secondary steam
reformer between the two palladium separation units. The secondary
steam reformer is another Ballard Power Systems prototype design,
with a modified tubular reactor and oil-jacket heating, and
containing commercial low-temperature shift catalyst inside the
reactor (2 L of BASF R3-12 catalyst). Otherwise, operating
conditions of both systems were the same.
[0068] Both systems were operated at 40% of their nominal reformer
pump settings and the hydrogen output of both systems was measured
using a Teledyne-Hastings 200-series mass flow meter calibrated to
500 SLPM of hydrogen. The flow rate of hydrogen produced by each
system was measured, as was the flow rate of air supplied to the
burner of the primary reformer. In addition, the output of the
primary burner methanol pump was measured as a percentage of
maximum output. The results are compiled in Table 1.
1 TABLE 1 Reformer Burner Burner Pump Pump Oxygen Secondary Output
(% H.sub.2 Flow Output Flow Reformer Max) (SLPM) (% Max) (SLPM)
Absent 40 250 9.4 90 Present 40 280 12.4 95 Percent 0% 12% 31% 6%
Increase
[0069] The data indicates that for the system employing the present
method and apparatus the hydrogen flow, and therefore the system
output, increased by 12% relative to the first system described
above. The data also indicates that about 30% more burner methanol
was required, partly to compensate for the lower fuel flow in the
retentate. However, this increase corresponds to an increase in
total methanol flow of about 6%. The burner oxygen flow increase is
of a similar magnitude. Thus, the data indicates that the present
method and apparatus may result in a significant increase in
hydrogen production without the need for costly and complicated
palladium membrane reactors.
[0070] While particular elements, embodiments and applications of
the present invention have been shown and described, it will be
understood, of course, that the invention is not limited thereto
since modifications may be made by those skilled in the art,
particularly in light of the foregoing teachings. It is therefore
contemplated that the appended claims cover such modifications as
incorporate those features that come within the scope of the
invention.
* * * * *