Note : Les descriptions sont présentées dans la langue officielle dans laquelle elles ont été soumises.
CA 02324699 2000-10-27
CARBON MONOXIDE REMOVAL FROM HYDROGEN FEED TO FUEL CELL
FIELD OF THE INVENTION
The present invention relates to a fuel cell-based electrical generation
system
which employs pressure swing adsorption for enhancing the efficiency and
durability
of the fuel cell.
BACKGROUND OF THE INVENTION
Fuel cells provide an environmentally friendly source of electrical current.
One form of fuel cell used for generating electrical power, particularly for
vehicle
propulsion and for smaller scale stationary power generation, includes an
anode
channel for receiving a flow of hydrogen gas, a cathode channel for receiving
a
flow of oxygen gas, and a polymer electrolyte membrane (PEM) which separates
the anode channel from the cathode channel. Oxygen gas which enters the
cathode
reacts with hydrogen ions which cross the electrolyte to generate a flow of
electrons. Environmentally safe water vapour is also produced as a byproduct.
External production, purification, dispensing and storage of hydrogen
(either as compressed gas or cryogenic liquid) requires costly infrastructure,
while
storage of hydrogen fuel on vehicles presents considerable technical and
economic
barriers. Accordingly, for stationary power generation, it is preferred to
generate
hydrogen from natural gas by steam reforming or partial oxidation followed by
water gas shift. For fuel cell vehicles using a liquid fuel, it is preferred
to
generate hydrogen from methanol by steam reforming or from gasoline by partial
oxidation or autothermal reforming, again followed by water gas shift.
However,
the resulting hydrogen contains carbon monoxide and carbon dioxide impurities
which cannot be tolerated respectively by the PEM fuel cell catalytic
electrodes in
more than trace levels.
CA 02324699 2000-10-27
-2-
The conventional method of removing residual carbon monoxide from the
hydrogen feed to PEM fuel cells has been catalytic selective oxidation, which
compromises efficiency as both the carbon monoxide and a fraction of the
hydrogen are consumed by low temperature oxidation, without any recovery of
the
heat of combustion. Palladium diffusion membranes can be used for hydrogen
purification, but have the disadvantages of delivery of the purified hydrogen
at low
pressure, and also the use of rare and costly materials.
Pressure swing adsorption systems (PSA) have the attractive features of
being able to provide continuous sources of oxygen and hydrogen gas, without
significant contaminant levels. PSA systems and vacuum pressure swing
adsorption systems (VPSA) separate gas fractions from a gas mixture by
coordinating pressure cycling and flow reversals over an adsorber or adsorbent
bed
which preferentially adsorbs a more readily adsorbed gas component relative to
a
less readily adsorbed gas component of the mixture. The total pressure of the
gas
mixture in the adsorber is elevated while the gas mixture is flowing through
the
adsorber from a first end to a second end thereof, and is reduced while the
gas
mixture is flowing through the adsorbent from the second end back to the first
end. As the PSA cycle is repeated, the less readily adsorbed component is
concentrated adjacent the second end of the adsorber, while the more readily
adsorbed component is concentrated adjacent the first end of the adsorber. As
a
result, a "light" product (a gas fraction depleted in the more readily
adsorbed
component and enriched in the less readily adsorbed component) is delivered
from
the second end of the adsorber, and a "heavy" product (a gas fraction enriched
in
the more strongly adsorbed component) is exhausted from the first end of the
adsorber.
However, the conventional system for implementing pressure swing
adsorption or vacuum pressure swing adsorption uses two or more stationary
adsorbers in parallel, with directional valuing at each end of each adsorber
to
connect the adsorbers in alternating sequence to pressure sources and sinks.
This
system is often cumbersome and expensive to implement due to the large size of
CA 02324699 2000-10-27
-3-
the adsorbers and the complexity of the valuing required. Further, the
conventional PSA system makes inefficient use of applied energy because of
irreversible gas expansion steps as adsorbers are cyclically pressurized and
depressurized within the PSA process. Conventional PSA systems could not be
applied to fuel cell power plants for vehicles, as such PSA systems are far
too
bulky and heavy because of their low cycle frequency and consequently large
adsorbent inventory.
SUMMARY OF THE INVENTION
According to the invention, there is provided a fuel cell based electrical
generation system which addresses the deficiencies of the prior art fuel cell
electrical generation systems, particularly as to purification of reformate
hydrogen.
The electrical current generating system comprises a fuel cell, an oxygen
gas delivery system, and a hydrogen gas delivery system. The fuel cell
includes
an anode channel having an anode gas inlet for receiving a supply of hydrogen
gas, a cathode channel having a cathode gas inlet and a cathode gas outlet,
and an
electrolyte in communication with the anode and cathode channel for
facilitating
ion transport between the anode and cathode channel. The oxygen gas delivery
system is coupled to the cathode gas inlet and delivers air or oxygen (e.g.
oxygen
enriched air) to the cathode channel.
The oxygen gas delivery system may simply be a air blower. However,
for superior performance it incorporates an oxygen pressure swing adsorption
system, including a rotary module having a stator and a rotor rotatable
relative to
the stator, for enriching oxygen gas from air. The rotor includes a number of
flow paths for receiving adsorbent material therein for preferentially
adsorbing a
first gas component in response to increasing pressure in the flow paths
relative to
a second gas component. The pressure swing adsorption system also may include
compression machinery coupled to the rotary module for facilitating gas flow
through the flow paths for separating the first gas component from the second
gas
CA 02324699 2000-10-27
-4-
component. The stator includes a first stator valve surface, a second stator
valve
surface, and plurality of function compartments opening into the stator valve
surfaces. The function compartments include a gas feed compartment, a light
reflux exit compartment and a light reflux return compartment.
In one variation, the compression machinery comprises a compressor for
delivering pressurized air to the gas feed compartment, and a light reflux
expander
coupled between the light reflux exit compartment and the light reflux return
compartment. The gas recirculating means comprises a compressor coupled to the
light reflux expander for supplying oxygen gas, exhausted from the cathode gas
outlet, under pressure to the cathode gas inlet. As a result, energy recovered
from
the pressure swing adsorption system can be applied to boost the pressure of
oxygen gas delivered to the cathode gas inlet.
The oxygen gas delivery system is coupled to the cathode gas inlet and
delivers oxygen gas to the cathode channel. The hydrogen gas delivery system
supplies purified hydrogen gas to the anode gas inlet, and may have provision
for
recirculating hydrogen gas from the anode gas exit back to the anode gas inlet
with
increased purity so as to avoid accumulation of impurities in the anode
channel.
In a preferred embodiment, the oxygen gas separation system comprises an
oxygen pressure swing adsorption system, and the hydrogen gas separation
system
comprises a reactor for producing a first hydrogen gas feed from hydrocarbon
fuel, and a hydrogen pressure swing adsorption system coupled to the reactor
for
purifying hydrogen gas received from the first hydrogen gas feed. Hydrogen gas
from the anode exit may be recirculated to the hydrogen pressure swing
adsorption
system as a second hydrogen gas feed. Both pressure swing adsorption systems
include a rotary module having a stator and a rotor rotatable relative to the
stator.
The rotor includes a number of flow paths for receiving adsorbent material
therein
for preferentially adsorbing a first gas component in response to increasing
pressure in the flow paths relative to a second gas component. The function
compartments include a gas feed compartment and a heavy product compartment.
CA 02324699 2000-10-27
- 5 -
The feed gas to the hydrogen PSA system is reformate gas or syngas,
generated in alternative fuel processing methods known to the art by steam
reforming (e.g. of methanol or natural gas or light hydrocarbons), or by
autothermal reforming or partial oxidation (e.g. of natural gas, gasoline or
diesel
fuel). The CO content of methanol reformate (generated by relatively low
temperature steam reforming of methanol) is typically about 1 % . Other fuel
processors (e.g. steam methane reformers, and POX or autothermal reformers
operating on any feedstock) operate at much higher temperature, and preferably
include a lower temperature water gas shift reactor stage to reduce to CO
content
to about 1 % .
The reformate gas contains hydrogen plus the basic impurity components of
C02, CO and water vapour. If generated by air-blown POX or autothermal
reforming, the reformate gas will also contain a large inert fraction of
nitrogen and
argon. The fraction of inert atmospheric gases can be greatly reduced if an
oxygen PSA system is used to supply the POX or autothermal reformer, either
directly from the PSA, or as humid and still oxygen enriched air that has been
passed through the fuel cell cathode channel which was directly fed oxygen-
enriched air from the PSA.
In one variation, the oxygen pressure swing adsorption system includes a
compressor coupled to the gas feed compartment for delivering pressurized air
to
the gas feed compartment, and a vacuum pump coupled to the compressor for
extracting nitrogen product gas from the heavy product compartment. The
reactor
comprises a steam reformer, including a burner, for producing syngas, and a
water gas shift reactor coupled to the steam reformer for converting the
syngas to
the second hydrogen gas feed. The hydrogen pressure swing adsorption system
includes a vacuum pump for delivering fuel gas from the heavy product
compartment to the burner. The fuel gas is burned in the burner, and the heat
generated therefrom is used to supply the endothermic heat of reaction
necessary
for the steam reformer reaction. The resulting syngas is delivered to the
water gas
CA 02324699 2000-10-27
-6-
shift reactor for removal of impurities, and then delivered as the second
hydrogen
gas feed to the hydrogen pressure swing adsorption system.
In another variation, the invention includes a burner for burning fuel. The
reactor comprises an autothermal reformer for producing syngas, and a water
gas
shift reactor coupled to the autothermal reformer for converting the syngas to
the
second hydrogen gas feed. The compressor of the oxygen pressure swing
adsorption system delivers pressurized air to the burner, and the heavy
product gas
is delivered from the hydrogen pressure swing adsorption system as tail gas to
be
burned in the burner. The compression machine of the oxygen pressure swing
adsorption system also includes an expander coupled to the compressor for
driving
the compressor from hot gas of combustion emitted from the burner. Heat from
the burner may also be applied to preheat air and/or fuel supplied to the
autothermal reformer.
Independently of whether PSA is used for oxygen enrichment, the present
invention provides a hydrogen PSA apparatus for purifying the reformate. The
hydrogen PSA may be designed to deliver high purity hydrogen, or else may be
designed less stringently to achieve adequately high removal of noxious
components (harmful to the fuel cell) such as CO, H2S, halogens, methanol,
etc.
In the latter case, the hydrogen PSA would in its first pass only achieve
partial
removal of less harmful constituents (e.g., N2, Ar and C02). In that case,
anode
tail gas would then preferably be recycled to the feed end of the PSA inlet
for use
in a feed pressurization step, thus avoiding any need for mechanical
recompression. Even when high hydrogen purity is specified for the PSA, this
feature enables a small bleed from the end of the anode channel back to the
feed
pressurization step of the hydrogen PSA, as would be desirable for avoiding a
strict dead-headed configuration with the risk of accumulation in the anode
channel
of any contaminant slip due to equipment imperfections or operational
transient
upsets.
CA 02324699 2000-10-27
Operating temperature of the adsorbers in the hydrogen PSA unit of the
invention will preferably be elevated well above ambient, as the reformate gas
is
supplied at a temperature after water gas shift of typically about
200°C, while
operating temperatures of PEM fuel cells may extend from 80°C to about
100°C.
Alternatively, the adsorbers may be operated at a lower temperature if the
reformate is cooled, thus providing an opportunity for partial removal of
water and
any methanol vapour by condensation before admission to the hydrogen PSA unit.
Advantages of operation at moderately elevated temperature are ( 1 ) reformate
coolers and water condensers upstream of the hydrogen PSA can be avoided, (2)
PSA removal of water vapour and C02 may be more readily achieved at
moderately elevated temperature compared to ambient temperature, (3) CO can be
more selectively adsorbed than C02 over Cu(I) loaded adsorbents particularly
at
elevated temeprature, and (4) kinetics of CO sorption and desorption on CO-
selective sorbents will be greatly enhanced at higher temperature.
Consequently a
preferred operating temperature range for the adsorbers is from about
80°C to
about 200°C, and a more preferred operating range is from about
100°C to about
160°C .
The hydrogen PSA unit may be configured to support a temperature
gradient along the length of the flow channels, so that the temperature at the
first
end of the adsorbers is higher than the temperature at the second end of the
adsorbers.
Especially in the mode of design for low purity hydrogen with anode
recycle, the hydrogen PSA may use CO-selective adsorbents with CO-complexing
ions Cu(I) or Ag introduced by ion exchange or impregnation into a suitable
adsorbent carrier. Prior art CO-selective adsorbents have used a wide
diversity of
zeolites, alumina or activated carbon adsorbents as carriers. With CO-
selective
adsorbents, enhanced hydrogen recovery may be achieved while tolerating some
accumulation of non-CO impurities circulated through the fuel cell anode loop.
CA 02324699 2000-10-27
_g_
Numerous copper based CO-selective adsorbents have been disclosed by
Rabo et al (U.S. Patent No.4,019,879), Hirai (U.S. Patent No.4,587,114),
Nishida
et al (U.S. Patent No. 4,743,276), Tajima et al (U.S. Patent No. 4,783,433),
Tsuji et al (U.S. Patent No. 4,914,076), Xie et al (U.S. Patent No.
4,917,711),
Golden et al (U.S. Patent Nos. 5,126,310; 5,258,571; and 5,531,809), and Hable
et al (U.S. Patent No. 6,060,032). Use of some such CO-selective adsorbents in
pressure swing adsorption processes for removal or concentration of CO has
been
commercially established at industrial scale.
Use of some such adsorbents for removing CO from reformate for PEM
fuel cells has been investigated by researchers at the Argonne National
Laboratory,
as reported in the 1998 annual report of the Fuel Cells for Transportation
Program
of the U.S. Department of Energy, Office of Advanced Transportation
Technologies. Bellows (U.S. Patent No. 5,604,047) discloses use of selected
noble metals, and the carbides and nitrides of certain metals, as carbon
monoxide
adsorbents in a steam displacement purge cycle for removal of CO from
reformate
feed to fuel cells.
Potential problems with CO-selective adsorbents used to purify hydrogen
from reformate include (1) compatibility with water vapour that may deactivate
the
adsorbent or cause leaching of impregnated constituents, (2) over-reduction by
hydrogen causing the CO-complexing ion to reduce to inert metallic form, and
(3)
relatively slow kinetics of CO-complexing as compared to physical adsorption.
In the present invention, the active adsorbent preferably including a CO-
selective component is supported on thin adsorbent sheets which are layered
and
spaced apart by spacers defining flow channels, so as to provide a high
surface
area parallel passage support with minimal mass transfer resistance and flow
channel pressure drop. With crystalline adsorbents such as zeolites, and
amorphous adsorbents such as alumina gel or silica gel, the adsorbent sheet is
formed by coating or in-situ synthesis of the adsorbent on a reinforcement
sheet of
inert material, e.g. a wire mesh, a metal foil, a glass or mineral fiber
paper, or a
CA 02324699 2000-10-27
-9-
woven or nonwoven fabric. Active carbon adsorbent may also be coated onto a
reinforcement sheet of inert material, but adosrbent sheets of active carbon
may
also be provided as self-supporting carbon fiber paper or cloth. Adsorbers of
the
layered adsorbent sheet material may be formed by stacking flat or curved
sheets;
or by forming a spiral roll, with the flow channels between the sheets
extending
from the first end of the adsorber to the second end thereof; to fill the
volume of
the adsorber housing of the desired shape. Typical thickness of the adsorbent
sheet may be in the range of about 100 to about 200 microns, while flow
channel
spacing between the sheets may be in the range of about 50 to about 200
microns.
In the present invention, the adsorbent material contacting the flow
channels between the first and second ends of the adsorbers may in general be
selected to be different in distinct zones of the flow channels, so that the
adsorbers would have a succession of zones (e.g. a first zone, a second zone,
a
third zone, a perhaps additional zones) with distinct adsorbents proceeding
along
the flow channels from the first end to the second end.
In a first variant configured to deliver high purity hydrogen, the adsorbent
in a first zone of the adsorbers adjacent the first end will be a dessicant to
achieve
bulk removal of water vapour in that first zone, the adsorbent in a second
zone in
the central portion of the adsorbers will be selected to achieve bulk removal
of
C02 and some removal of CO, and the adsorbent in a third zone of the adsorbers
will be selected to achieve final removal of CO and substantial removal of any
nitrogen and argon. A suitable dessicant for the first zone is alumina gel. A
suitable adsorbent for the second zone is 13X zeolite, or SA, or active
charcoal.
Suitable adsorbents for the third zone may be a strongly carbon monoxide and
nitrogen selective adsorbent selected from the group including but not limited
to
Na-LSX, Ca-LSX, Li-LSX, Li- exchanged chabazite, Ca- exchanged chabazite,
Sr- exchanged chabazite. The zeolite adsorbents of this group are
characterized
by strong hydrophilicity, corresponding to selectivity for polar molecules.
This
first variant relying on physical adsorption will operate most effectively at
relatively lower temperatures, unlikely to exceed much more than about
100°C
CA 02324699 2000-10-27
-10-
although certain adsorbents such as Ca- or Sr-exchanged chabazite would remain
adequately effective for CO and N2 removal at temperatures to about
150°C.
In a second similar variant also configured to deliver high purity hydrogen,
the adsorbent in the second or third zone may be a more strongly carbon
monoxide selective adsorbent such as a Cu(I)-exchanged zeolite. The zeolite
may
for example be an X or a Y zeolite, mordenite, or chabazite. For stability
against
over-reduction while contacting nearly pure hydrogen, the exchangeable ions of
the zeolite may be a mixture of Cu(I) and other ions such as Na, Li, Ca, Sr,
other transition group metals or lanthanide group metals. The mixed ions may
also or alternatively include Ag as a minor component for enhanced CO-
selectivity.
In a third variant configured to deliver at least partially purified hydrogen
with CO nearly completely removed, the adsorbent in a first zone of the
adsorbers
adjacent the first end will be a dessicant to achieve bulk removal of water
vapour
in that first zone, the adsorbent in a second zone in the central portion of
the
adsorbers will be selected to achieve bulk removal of C02 and some removal of
CO, and the adsorbent in a third zone of the adsorbers will be selected to
achieve
final removal of CO and partial removal of any nitrogen and argon. A suitable
dessicant for the first zone is alumina gel. A suitable adsorbent for the
second
zone is alumina gel impregnated with Cu(I), or active carbon impregnated with
Cu(I). Suitable adsorbents for the third zone may be similar to those used in
the
second zone, or may be a CO- and nitrogen selective adsorbent as in the first
or
second variants above.
In a fourth variant configured to deliver at least partially purified hydrogen
with CO nearly completely removed, the adsorbent in some or all zones of the
adsorbers will be a moderately hydrophobic adsorbent selected from the group
including but not limited to active carbon and Y-zeolite, and preferably
containing
Cu(I) for enhanced CO- selectivity in a zone adjacent the second end of the
adsorbers.
CA 02324699 2000-10-27
-11-
In a fifth variant configured to deliver at least partially purified hydrogen
with CO nearly completely removed, the adsorbent in some or all zones of the
adsorbers will be a strongly hydrophobic adsorbent selected from the group
including but not limited to silicalite and dealuminified Y-zeolite. The
hydrophobic adsorbent may preferably contain Cu(I) for enhanced CO-
selectivity.
In a sixth variant configured to deliver at least partially purified hydrogen
with CO nearly completely removed, the adsorbent in the first or second zone
of
the adsorbers will include a component catalytically active at the operating
temperature of that zone for the water gas shift reaction. The catalytically
active
component may be any known water gas shift catalyst, e.g. Cu-Zn0 based
catalysts. Preferably, the catalytically active component may be metal
carbonyl
complexes of a transition group metal or a mixture of transition group metals
(e.g.
Cu, Ag, Ni, Pd, Pt, Rh, Ru, Fe, Mo, etc.) inserted into the zeolite cages of
e.g.
an X or Y zeolite. A portion of the carbon monoxide sorbed onto the
catalytically
active component may then react with water vapour by the water gas shift
reaction
to generate carbon dioxide and additional hydrogen. It is known [J.J.
Verdonck,
P.A. Jacobs, J.B. Uytterhoeven, "Catalysis by a Ruthenium Complex
Heterogenized in Faujasite-type Zeolites: the Water Gas-shift Reaction",
J.C.S.
Chem. Comm., pp. 181-182, 1979] that ruthenium complexes stabilized within X
or Y zeolites provide greater water-gas shift catalytic activity than
conventional
copper based catalysts.
In a seventh variant configured to deliver at least partially purified
hydrogen with CO nearly completely removed, the adsorbent in the first zone of
the adsorbers is an adsorbent selective at the elevated operating temperature
of the
first zone for carbon dioxide in preference to water vapour. Suitable such
adsorbents known in the art include alumina impregnated with potassium
carbonate, and hydrotalcite promoted with potassium carbonate. The adsorbent
in
the second zone of the adsorbers will include a component catalytically active
at
the operating temperature of that zone for the water gas shift reaction. As in
the
sixth variant above, the catalytically active component in the second zone may
be
CA 02324699 2000-10-27
-12-
a known water gas shift catalyst, or may be a transition group metal dispersed
in
zeolite cages and reversibly forming a metal carbonyl complex at the operating
temperature of the second zone. The second or preferably third zone of the
adsorbers contains adsorbent with some useful working capacity for carbon
monoxide and other impurity components at the operating temperature of that
zone. Because carbon dioxide is strongly adsorbed in the first zone, the
concentration of carbon dioxide in the second zone is maintained at a reduced
level
by the PSA process, while water vapour concentration remains relatively high
in
the second zone. Hence, in this seventh variant the water gas shift reaction
equilibrium is continually shifted by the PSA process which continually
removes
both hydrogen and carbon dioxide from the catalytically active second zone
while
preventing passage of carbon monoxide into the hydrogen product passing the
third
zone, so that essentially all carbon monoxide is consumed to generate carbon
dioxide and additional hydrogen. This is an example of a PSA reactor or
"sorption enhanced reactor", driving the water gas shift reaction
substantially to
completion while achieving adequate purification of the hydrogen.
Industrial H2 PSA is normally conducted at considerably elevated pressures
( > 10 tiara) to achieve simultaneous high purity and high recovery ( -- 80 % -
85 % ). Fuel cell systems operating with pressurized methanol reformers or in
integration with gas turbine cycles may operate at relatively high pressures.
However, most PEM fuel cell systems operate at ambient to about 3 tiara
pressure.
As feed pressure and the overall working pressure ratio of the PSA are
reduced,
productivity and recovery of a simple cycle deteriorate. Under given pressure
conditions, use of CO-selective adsorbents should significantly improve
recovery
at specified product CO concentration, if hydrogen purity with respect to
other
impurities such as nitrogen and carbon dioxide can be relaxed.
At very low feed pressures (e.g. 2 - 3 tiara), the H2 PSA would need
supplemental compression to achieve high recovery. We may consider vacuum
pumping to widen the working pressure ratio, or alternatively "heavy reflux"
which is recompression and recycle to the PSA feed of a fraction of its
exhaust
CA 02324699 2000-10-27
-13-
stream at full pressure. Vacuum and heavy reflux options may be combined in
PSA systems for reformate purification. We have successfully used the heavy
reflux option, without CO-selective adsorbents or any vacuum pumping, in a lab
bench PSA device to achieve -,- 95 % recovery from synthetic methanol
reformate
at ~ 3 tiara feed pressure.
To get heavy reflux in a very low pressure PSA, the vacuum pump may be
configured so that part of its flow is reinjected into the PSA feed. Extremely
high hydrogen recovery can then be obtained (even at a fairly low overall
pressure
ratio) just by pumping enough heavy reflux. The vacuum level can be traded
against the mass flow of heavy reflux.
A fuel cell may be a standalone power plant, or else it may be integrated
with some type of combustion engine. In the case of a standalone fuel cell,
all
mechanical power for air handling compression and any oxygen and/or hydrogen
PSA units must be provided as electrical power by the appropriately sized fuel
cell
stack. In this case, tight constraints apply to the recovery level that must
be
achieved by the H2 PSA at specified purity. In the absence of any useful
export
use for high grade heat, an efficient heat balance requires that the heating
value of
combustible waste gases (H2, CO and unreacted fuel) be matched to the heat
demand of the fuel processor. For a fuel cell with steam reforming (e.g.
methanol
or natural gas), nominal hydrogen recovery by the H2 PSA has to be about 75 %
to 80% as the PSA tail gas is burned to heat the reformer; while for a POX or
autothermal reformer, hydrogen recovery by the PSA needs to be extremely high
(at least 90 % to 95 % ) as such reformers can only use a limited amount of
external
combustion heat from burning PSA tail gas or fuel cell anode tail gas, e.g.
for
preheating feed oxygen/air and fuel reactants to the reformer.
BRIEF DESCRIPTION OF THE DRAWINGS
Fig. 1 shows an axial section of a rotary PSA module.
CA 02324699 2000-10-27
-14-
Figs. 2 through SB show transverse sections of the module of Fig. 1.
Fig. 6 is a simplified schematic of a PEM fuel cell power plant with a
steam reforming fuel processor, a PSA unit for reformate hydrogen purification
by
at least removal of CO, and a VPSA unit for oxygen enrichment.
DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS
Figs. 1 - 5
Fig. 1 shows a rotary PSA module 1, which includes a number "N" of
adsorbers 3 in adsorber housing body 4. Each adsorber has a first end 5 and a
second
end 6, with a flow path therebetween contacting a nitrogen-selective
adsorbent. The
adsorbers are deployed in an axisymmetric array about axis 7 of the adsorber
housing
body. The housing body 4 is in relative rotary motion about axis 7 with first
and
second functional bodies 8 and 9, being engaged across a first valve face 10
with the
first functional body 8 to which feed gas mixture is supplied and from which
the
heavy product is withdrawn, and across a second valve face 11 with the second
functional body 9 from which the light product is withdrawn.
In preferred embodiments as particularly depicted in Figs. 1 - 5, the adsorber
housing 4 rotates and shall henceforth be referred to as the adsorber rotor 4,
while the
first and second functional bodies are stationary and together constitute a
stator
assembly 12 of the module. The first functional body shall henceforth be
referred to
as the first valve stator 8, and the second functional body shall henceforth
be referred
to as the second valve stator 9.
In the embodiment shown in Figs. 1 - 5, the flow path through the adsorbers
is parallel to axis 7, so that the flow direction is axial, while the first
and second valve
faces are shown as flat annular discs normal to axis 7. However, more
generally the
flow direction in the adsorbers may be axial or radial, and the first and
second valve
faces may be any figure of revolution centred on axis 7. The steps of the
process
CA 02324699 2000-10-27
-15-
and the functional compartments to be defined will be in the same angular
relationship
regardless of a radial or axial flow direction in the adsorbers.
Figs. 2 - 5 are cross sections of module 1 in the planes defined by arrows 12
- 13, 14 - 15, and 16 - 17. Arrow 20 in each section shows the direction of
rotation
of the rotor 4. Fig. 2 shows section 12 - 13 across Fig.l, which crosses the
adsorber
rotor. In this example, "N" = 72. The adsorbers 3 are mounted between outer
wall
21 and inner wall 22 of adsorber wheel 208. Each adsorber comprises a
rectangular
flat pack 3 of adsorbent sheets 23, with spacers 24 between the sheets to
define flow
channels here in the axial direction. Separators 25 are provided between the
adsorbers
to fill void space and prevent leakage between the adsorbers. In other
configurations,
the adsorbent sheets may be formed in curved packs or spiral rolls.
Satisfactory adsorbent sheets have been made by coating a slurry of zeolite
crystals with binder constituents onto the reinforcement material, with
successful
examples including nonwoven fibreglass scrims, woven metal fabrics, and
expanded
aluminum foils. The adsorbent sheets comprise a reinforcement material, in
preferred
embodiments glass fibre, metal foil or wire mesh, to which the adsorbent
material is
attached with a suitable binder. For applications such as hydrogen
purification, some
or all of the adsorbent material may be provided as carbon fibers, in woven or
nonwoven form to serve as its own reinforcement material. Spacers are provided
by
printing or embossing the adsorbent sheet with a raised pattern, or by placing
a
fabricated spacer between adjacent pairs of adsorbent sheets. Alternative
satisfactory
spacers have been provided as woven metal screens, non-woven fibreglass
scrims, and
metal foils with etched flow channels in a photolithographic pattern.
Typical experimental sheet thicknesses have been 150 microns, with spacer
heights in the range of 100 to 150 microns, and adsorber flow channel length
approximately 20 cm. Using X type zeolites, excellent performance has been
achieved
in oxygen separation from air and hydrogen purification from reformate at PSA
cycle
frequencies in the range of 30 to 150 cycles per minute.
CA 02324699 2000-10-27
-16-
As shown in Fig. 1, the adsorbers 3 comprise a plurality of distinct zones
between the first end 5 and the second end 6 of the flow channels, here shown
as three
zones respectively a first zone 26 adjacent the first end 5, a second zone 27
in the
middle of the adsorbers, and a third zone 28 adjacent the second end 6. The
first
zone typically contains an adsorbent or dessicant selected for removing very
strongly
adsorbed components of the feed gas mixture, such as water or methanol vapour,
and
some carbon dioxide. The second zone contains an adsorbent typically selected
for
bulk separation of impurities at relatively high concentration, and the third
zone
contains an adsorbent typically selected for polishing removal of impurities
at
relatively low concentration.
In embodiments with three zones, the first zone may be the first 10% to
20% of the flow channel length from the first end, the second zone may be the
next roughly 40% to 50% of the channel length, and the third zone the
remainder.
In embodiments with only two adsorber zones, the first zone may be the first
10%
to 30% of the flow channel length from the first end, and the second zone the
remainder. The zones may be formed by coating the different adsorbents onto
the
adsorbent support sheet material in bands of the same width as the flow
channel
length of the corresponding zone. The adsorbent material composition may
change abruptly at the zone boundary, or may be blended smoothly across the
boundary. Particularly in the first zone of the adsorber, the adsorbent must
be
compatible with significant concentrations of water vapour.
For air separation to produce enriched oxygen, alumina gel may be used in the
first zone to remove water vapour, while typical adsorbents in the second and
third
zones are X, A or chabazite type zeolites, typically exchanged with lithium,
calcium,
strontium, magnesium and/or other cations, and with optimized silicon/aluminum
ratios as well known in the art. The zeolite crystals are bound with silica,
clay and
other binders, or self bound, within the adsorbent sheet matrix.
In a first variant configured to deliver high purity hydrogen, the adsorbent
in a first zone of the adsorbers adjacent the first end will be a dessicant to
achieve
CA 02324699 2000-10-27
-17-
bulk removal of water vapour in that first zone, the adsorbent in a second
zone in
the central portion of the adsorbers will be selected to achieve bulk removal
of
C02 and some removal of CO, and the adsorbent in a third zone of the adsorbers
will be selected to achieve final removal of CO and substantial removal of any
nitrogen and argon. A suitable dessicant for the first zone is alumina gel. A
Illustrative suitable adsorbents for the second zone are 13X zeolite, or SA,
or active
charcoal. Suitable adsorbents for the third zone may be a strongly carbon
monoxide and nitrogen selective adsorbent selected from the group including
but
not limited to Na-LSX, Ca-LSX, Li-LSX, Li- exchanged chabazite, Ca- exchanged
chabazite, Sr- exchanged chabazite. The zeolite adsorbents of this group are
characterized by strong hydrophilicity, corresponding to selectivity for polar
molecules. This first variant relying on physical adsorption will operate most
effectively at relatively lower temperatures, unlikely to exceed much more
than
about 100°C although certain adsorbents such as Ca- or Sr-exchanged
chabazite
remain adequately effective for CO and N2 removal at temperatures up to about
150°C.
In a second similar variant also configured to deliver high purity hydrogen,
the adsorbent in the second or third zone may be a more strongly carbon
monoxide
selective adsorbent such as a Cu(I)-exchanged zeolite. The zeolite may for
example be an X or a Y zeolite, mordenite, or chabazite. For stability against
over-reduction while contacting nearly pure hydrogen, the exchangeable ions of
the
zeolite may be a mixture of Cu(I) and other ions such as Na, Li, Ca, Sr, other
transition group metals or lanthanide group metals. The mixed ions may also or
alternatively include Ag(I) as a minor component for enhanced CO-selectivity.
In a third variant configured to deliver at least partially purified hydrogen
with CO nearly completely removed, the adsorbent in a first zone of the
adsorbers
adjacent the first end will be a dessicant to achieve bulk removal of water
vapour
in that first zone, the adsorbent in a second zone in the central portion of
the
adsorbers will be selected to achieve bulk removal of C02 and some removal of
CO, and the adsorbent in a third zone of the adsorbers will be selected to
achieve
CA 02324699 2000-10-27
-18-
final removal of CO and partial removal of any nitrogen and argon. A suitable
dessicant for the first zone is alumina gel. A suitable adsorbent for the
second
zone is alumina gel impregnated with Cu(I), or active carbon impregnated with
Cu(I). Suitable adsorbents for the third zone may be similar to those used in
the
second zone, or may be a CO- and nitrogen selective adsorbent as in the first
or
second variants above.
In a fourth variant configured to deliver at least partially purified hydrogen
with CO nearly completely removed, the adsorbent in some or all zones of the
adsorbers will be a moderately hydrophobic adsorbent selected from the group
including but not limited to active carbon and Y-zeolite, and preferably
containing
Cu(I) for enhanced CO- selectivity in a zone adjacent the second end of the
adsorbers.
In a fifth variant configured to deliver at least partially purified hydrogen
with CO nearly completely removed, the adsorbent in some or all zones of the
adsorbers will be a strongly hydrophobic adsorbent selected from the group
including but not limited to silicalite and dealuminified Y-zeolite. The
hydrophobic adsorbent may preferably contain Cu(I) for enhanced CO-
selectivity.
In a sixth variant configured to deliver at least partially purified hydrogen
with CO nearly completely removed, the adsorbent in the first or second zone
of
the adsorbers will include a component catalytically active at the operating
temperature of that zone for the water gas shift reaction. The catalytically
active
component may be any known water gas shift catalyst, e.g. Cu-Zn0 based
catalysts. Preferably, the catalytically active component may be metal
carbonyl
complexes of a transition group metal or a mixture of transition group metals
(e.g.
Cu, Ag, Ni, Pd, Pt, Rh, Ru, Fe, Mo, etc.) inserted into the zeolite cages of
e.g. an
X or Y zeolite. A portion of the carbon monoxide sorbed onto the catalytically
active component may then react with water vapour by the water gas shift
reaction
to generate carbon dioxide and additional hydrogen.
CA 02324699 2000-10-27
-19-
In a seventh variant configured to deliver at least partially purified
hydrogen
with CO nearly completely removed, the adsorbent in the first zone of the
adsorbers is an adsorbent selective at the elevated operating temperature of
the first
zone for carbon dioxide in preference to water vapour. Suitable such
adsorbents
known in the art include alumina impregnated with potassium carbonate, and
hydrotalcite promoted with potassium carbonate. The adsorbent in the second
zone of the adsorbers will include a component catalytically active at the
operating
temperature of that zone for the water gas shift reaction. As in the sixth
variant
above, the catalytically active component in the second zone may be a known
water
gas shift catalyst, or may be a transition group metal dispersed in zeolite
cages and
reversibly forming a metal carbonyl complex at the operating temperature of
the
second zone. The second or preferably third zone of the adsorbers contains
adsorbent with some useful working capacity for carbon monoxide and other
impurity components at the operating temperature of that zone. Because carbon
dioxide is strongly adsorbed in the first zone, the concentration of carbon
dioxide
in the second zone is maintained at a reduced level by the PSA process, while
water vapour concentration remains relatively high in the second zone. Hence,
in
this seventh variant the water gas shift reaction equilibrium is continually
shifted by
the PSA process which continually removes both hydrogen and carbon dioxide
from the catalytically active second zone while preventing passage of carbon
monoxide into the hydrogen product passing the third zone, so that essentially
all
carbon monoxide is consumed to generate carbon dioxide and additional
hydrogen.
The water gas shift reaction is thus driven substantially to completion, while
achieving adequate purification of the hydrogen.
Fig. 3 shows the porting of rotor 4 in the first and second valve faces
respectively in the planes defined by arrows 14 - 15, and 16 - 17. An adsorber
port
provides fluid communication directly from the first or second end of each
adsorber
to respectively the first or second valve face.
Figs. 4A and 4B show the first stator valve face 100 of the first stator 8 in
the
first valve face 10, in the plane defined by arrows 14 - 15. Fluid connections
are
CA 02324699 2000-10-27
-20-
shown to a feed compressor 101 inducting feed gas from inlet filter 102, and
to an
exhauster 103 delivering second product to a second product delivery conduit
104.
Compressor 101 and exhauster 103 are shown coupled to a drive motor 105.
Arrow 20 indicates the direction of rotation by the adsorber rotor. In the
annular valve face between circumferential seals 106 and 107, the open area of
first
stator valve face 100 ported to the feed and exhaust compartments is indicated
by clear
angular segments 111 - 116 corresponding to the first functional ports
communicating
directly to functional compartments identified by the same reference numerals
111 -
116. The substantially closed area of valve face 100 between functional
compartments
is indicated by hatched sectors 118 and 119 which are slippers with zero
clearance, or
preferably a narrow clearance to reduce friction and wear without excessive
leakage.
Typical closed sector 118 provides a transition for an adsorber, between being
open
to compartment 114 and open to compartment 1 I5. Gradual opening is provided
by
a tapering clearance channel between the slipper and the sealing face, so as
to achieve
gentle pressure equalization of an adsorber being opened to a new compartment.
Much wider closed sectors (e.g. 119) are provided to substantially close flow
to or
from one end of the adsorbers when pressurization or blowdown is being
performed
from the other end.
The feed compressor provides feed gas to feed pressurization compartments
111 and 112, and to feed production compartment 113. Compartments 111 and 112
have succcessively increasing working pressures, while compartment 113 is at
the
higher working pressure of the PSA cycle. Compressor 101 may thus be a
multistage
or split stream compressor system delivering the appropriate volume of feed
flow to
each compartment so as to achieve the pressurization of adsorbers through the
intermediate pressure levels of compartments 111 and 112, and then the final
pressurization and production through compartment 113. A split stream
compressor
system may be provided in series as a multistage compressor with interstage
delivery
ports; or as a plurality of compressors or compression cylinders in parallel,
each
delivering feed air to the working pressure of a compartment 111 to 113.
Alternatively, compressor 101 may deliver all the feed gas to the higher
pressure, with
CA 02324699 2000-10-27
-21-
throttling of some of that gas to and 112 at their respective intermediate
pressures.
Similarly, exhauster 103 exhausts heavy product gas from countercurrent
blowdown compartments 114 and 115 at the successively decreasing working
pressures
of those compartments, and finally from exhaust compartment 116 which is at
the
lower pressure of the cycle. Similarly to compressor 101, exhauster 103 may be
provided as a multistage or split stream machine, with stages in series or in
parallel
to accept each flow at the appropriate intermediate pressure descending to the
lower
pressure.
In the example embodiment of Fig. 4A, the lower pressure is ambient pressure,
so exhaust compartment 116 communicates directly to heavy product delivery
conduit
104. Exhauster 103 thus is an expander which provides pressure letdown with
energy
recovery to assist motor 105 from the countercurrrent blowdown compartments
114
and 115. For simplicity, exhauster 103 may be replaced by throttling orifices
as
countercurrent blowdown pressure letdown means from compartments 114 and 115.
In some preferred embodiments, the lower pressure of the PSA cycle is
subatmospheric. Exhauster 103 is then provided as a vacuum pump, as shown in
Fig.
4B. Again, the vacuum pump may be multistage or split stream, with separate
stages
in series or in parallel, to accept countercurrent blowdown streams exiting
their
compartments at working pressures greater than the lower pressure which is the
deepest vacuum pressure. In Fig. 4B, the early countercurrent blowdown stream
from
compartment 114 is released at ambient pressure directly to heavy product
delivery
conduit 104. If for simplicity a single stage vacuum pump were used, the
countercurrent blowdown stream from compartment 115 would be throttled down to
the lower pressure over an orifice to join the stream from compartment 116 at
the inlet
of the vacuum pump.
If the feed gas is provided at an elevated pressure at least equal to the
higher
pressure of the PSA cycle, as may conveniently be the case of a hydrogen PSA
operating with e.g. methanol reformate feed, compressor 101 would be
eliminated.
CA 02324699 2000-10-27
-22-
To reduce energy losses from irreversible throttling over orifices to supply
feed
pressurization compartments e.g. 111, the number of feed pressurization stages
may
be reduced, sot that adsorber repressurization is largely achieved by product
pressurization, by backfill from light reflux steps. Alternatively, compressor
101 may
be replaced in part by an expander which expands feed gas to a feed
pressurization
compartment e.g. 111 from the feed supply pressure of the higher pressure to
the
intermediate pressure of that compartment, so as to recover energy for driving
a
vacuum pump 103 which reduces the lower pressure below ambient pressure so as
to
enhance the PSA process performance.
Figs. SA and SB shows the second stator valve face, at section 16 - 17 of Fig.
1. Open ports of the valve face are second valve function ports communicating
directly to a light product delivery compartment 121; a number of light reflux
exit
compartments 122, 123, 124 and 125; and the same number of light reflux return
compartments 126, 127, 128 and 129 within the second stator. The second valve
function ports are in the annular ring defined by circumferential seals 131
and 132.
Each pair of light reflux exit and return compartments provides a stage of
light reflux
pressure letdown, respectively for the PSA process functions of supply to
backfill, full
or partial pressure equalization, and cocurrent blowdown to purge.
Illustrating the option of light reflux pressure letdown with energy recovery,
a split stream light reflux expander 140 is shown in Figs. 1 and SA to provide
pressure
let-down of four light reflux stages with energy recovery. The light reflux
expander
provides pressure let-down for each of four light reflux stages, respectively
between
light reflux exit and return compartments 122 and 129, 123 and 128, 124 and
127, and
125 and 126 as illustrated. The light reflux expander 140 may power a light
product
booster compressor 145 by drive shaft 146, which delivers the oxygen enriched
light
product to oxygen delivery conduit 147 and compressed to a delivery pressure
above
the higher pressure of the PSA cycle.
Since the light reflux and light product have approximately the same purity,
expander 140 and light product compressor 145 may be hermetically enclosed in
a
CA 02324699 2000-10-27
- 23 -
single housing which may conveniently be integrated with the second stator as
shown
in Fig. 1. This configuration of a "turbocompressor" light product booster
without a
separate drive motor is advantageous, as a useful pressure boost of the light
product
can be achieved without an external motor and corresponding shaft seals, and
can also
be very compact when designed to operate at very high shaft speeds.
Fig. 5B shows the simpler alternative of using a throttle orifice 150 as the
pressure letdown means for each of the light reflux stages.
Turning back to Fig. 1, compressed feed gas is supplied to compartment 113
as indicated by arrow 125, while heavy product is exhausted from compartment
117
as indicated by arrow 126. The rotor is supported by bearing 160 with shaft
seal 161
on rotor drive shaft 162 in the first stator 8, which is integrally assembled
with the
first and second valve stators. The adsorber rotor is driven by motor 163 as
rotor drive
means.
As leakage across outer circumferential seal 131 on the second valve face 11
may compromise light product purity, and more importantly may allow ingress of
humidity into the second ends of the adsorbers which could deactivate the
nitrogen-
selective or CO-selective adsorbent, a buffer seal 170 is provided to provide
more
positive sealing of a buffer chamber 171 between seals 131 and 171. Even
though
the working pressure in some zones of the second valve face may be
subatmospheric
(in the case that a vacuum pump is used as exhauster 103), buffer chamber is
filled
with dry light product gas at a buffer pressure positively above ambient
pressure.
Hence, minor leakage of light product outward may take place, but humid feed
gas
may not leak into the buffer chamber. In order to further minimize leakage and
to
reduce seal frictional torque, buffer seal 171 seals on a sealing face 172 at
a much
smaller diameter than the diameter of circumferential seal 131. Buffer seal
170 seals
between a rotor extension 175 of adsorber rotor 4 and the sealing face 172 on
the
second valve stator 9, with rotor extension 175 enveloping the rear portion of
second
valve stator 9 to form buffer chamber 171. A stator housing member 180 is
provided
as structural connection between first valve stator 8 and second valve stator
9.
CA 02324699 2000-10-27
-24-
In the following figures of this disclosure, simplified diagrams will
represent
the PSA apparatus as described above. These highly simplified diagrams will
indicate
just a single feed conduit 181 to, and a single heavy product conduit 182
from, the
first valve face 10; and the light product delivery conduit 147 and a single
representative light reflux stage 184 with pressure let-down means
communicating to
the second valve face 11. Reference numerals pertaining to PSA units as
described
above will be unprimed for an oxygen enrichment PSA or VPSA unit, and primed
for
a hydrogen purification PSA or VPSA unit.
Fig. 6
Fig. 6 shows a fuel cell power plant 200, according to the present invention,
comprising a fuel cell 202, a steam reforming fuel processor 204, a hydrogen
purification PSA system 205, and an oxygen enrichment VPSA system 206. The
fuel cell comprises an anode channel 208 including an anode gas inlet 210 and
an
anode gas outlet 212, a cathode channel 214 including a cathode gas inlet 216
and
a cathode gas outlet 218, and a PEM electrolyte membrane 220 cooperating with
the anode channel 208 and the cathode channel 214 for facilitating ion
exchange
between the anode channel 208 and the cathode channel 214.
The oxygen VPSA system 206 extracts oxygen gas from feed air, and
comprises a PSA rotary module 1 and a compressor 101 for delivering
pressurized
feed air to the feed compartments of the rotary module 1. The oxygen VPSA
system 206 includes a vacuum pump 103 coupled to the compressor 101 for
withdrawing nitrogen enriched gas as heavy product gas from the blowdown and
exhaust compartments of the rotary module l, and discharging the nitrogen
enriched gas from conduit 225. The adsorbers 3 of rotary module 1 have a first
zone 26 loaded with a suitable dessicant such as alumina gel for removal of
water
vapour, and a second zone 27 loaded with a nitrogen-selective zeolite. Dry
oxygen enriched air as the light product gas of VPSA module 1 is delivered by
conduit 147 to water management chamber 230 for humidification, and thence by
conduit 231 to cathode inlet 216. A portion of the oxygen reacts with hydrogen
ions when electric current is generated, to form water in the cathode. The
cathode
CA 02324699 2000-10-27
- 25 -
exhaust gas now containing a reduced amount of oxygen (but still typically
oxygen-
enriched well above ambient air composition) is withdrawn from cathode exit
218
by conduit 232. A portion of the cathode exhaust gas is removed from conduit
232
by conduit 233 and flow control valve 234, and may either be vented to
atmosphere for purging nitrogen and argon accumulations, or else returned to
the
first valve face 10 of PSA module 1 as a feed pressurization stream at an
intermediate pressure below the higher pressure of the PSA cycle. The
remaining
cathode exhaust gas is supplied to suction port 240 of an ejector 242 which
serves
as cathode gas recirculation means. The ejector receives enriched oxygen from
conduit 147 through nozzle 244 which drives recirculation of cathode exhaust
gas
from suction port 240, and delivers the combined oxygen enriched gas stream to
water management chamber 230 where excess water is condensed. The excess
water is either exhausted through valve 250, or else is delivered as water
reactant to
fuel processor 204 by water pump 252 and conduit 254.
A hydrocarbon fuel is supplied to the fuel processor 204 by a feed pump or
compressor 260, is combined with water from conduit 254, and is vaporized and
preheated in heat exchanger 262. The preheated stream of fuel and steam is
then
admitted to steam reforming catalytic chamber 264, which is heated by burner
266
whose flue gas heats the heat exchanger 262. In the example that the fuel is
methane, the following steam reforming reactions take place:
CH4 + HZO ~ CO + 3H2
CH4 + 2Hz0 -~ COZ + 4H2
The resulting reformate or "syngas" (dry composition approximately 70%
Hz with roughly equal amounts of CO and COz as major impurities, and unreacted
CH4 and Nz as minor impurities) is cooled to about 250?C, and then passed to
the
water gas shift reaction zone 268 for reacting most of the CO with steam to
produce more HZ and CO,:
CO + HZO -~ COZ + H,
CA 02324699 2000-10-27
-26-
The hydrogen rich reformate still contains about 1 % to 2% CO after water
gas shift, along with substantial amounts of carbon dioxide and water vapour.
For
high performance and longevity of a PEM fuel cell, it is necessary that CO
concentration be reduced well below 100 ppm and preferably below 10 ppm.
Consequently, the impure reformate is admitted by conduit 270 to the higher
pressure feed port of hydrogen PSA unit 205, including rotary PSA module 1'.
The
adsorbers 3' of rotary module 1' have a first zone 26' loaded with a suitable
dessicant such as alumina gel for removal of water vapour, a second zone 27'
loaded with an adsorbent selective for CO removal and at least partial bulk
removal
of COZ, and a third zone 28' loaded with an adsorbent suitable for polishing
removal of CO and at least partial removal of other impurities such as NZ. The
invention provides numerous combinations and variations of suitable adsorbents
for
the three zones of the hydrogen PSA adsorbers, as already recited above.
Purified hydrogen light product from the hydrogen PSA module 1' is
delivered by conduit 147' to an ejector 242' which is recirculation means for
partial recirculation of hydrogen rich anode gas through fuel cell anode
channel
208. The hydrogen rich gas from ejector 242' is delivered to anode inlet 210,
passed through anode channel 208, and then exhausted from anode exit 212 in
part
back to the suction inlet of ejector 242'. Recirculation of anode gas through
the
ejector 242' is optional, so this ejector may be omitted. The remaining
portion of
the anode exhaust gas (or all of it in the case that ejector 242' is omitted)
is
conveyed by conduit 280 back to a feed pressurization port in the first valve
surface 10' of hydrogen PSA module 1', so as to retain hydrogen within the
system
while using the hydrogen PSA unit to reject impurities from the anode gas
loop. A
larger fraction of anode gas is recycled in this manner back to the PSA unit
when
adsorbent and PSA process combinations are selected that remove CO almost
completely while allowing some passage of other impurities such as N2 and
perhaps some C02. Conversely, only a small amount of anode exhaust gas is
recycled back to the PSA to prevent inadvertent impurity accumulations, when
the
adsorbents and PSA cycle are designed to achieve high purity hydrogen with
nearly
compete removal of CO and other impurities as well.
CA 02324699 2000-10-27
-27-
Exhaust second product gas from the hydrogen PSA module is exhausted
from valve face 10' by conduit 285 to burner 266.
It will be understood by those skilled in the art that the hydrogen PSA unit
of this invention, with the above specified combinations and variations of
adsorbents in the sequential zones of the adsorbers, may be applied in
conjunction
with alternative fuel processors, including partial oxidation or autothermal
reactors
for processing of heavy as well as light hydrocarbon fuels to generate
hydrogen
rich reformate, from which CO and other impurities must be removed.
