US20030157390A1 - Electrical current generation system - Google Patents

Abstract

An electrical generating system consists of a fuel cell, and an oxygen gas delivery. The fuel cell includes and 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 exchange between the anode and cathode channel. The oxygen gas delivery system is coupled to the cathode gas inlet and delivers oxygen gas to the cathode channel. The electrical current generating system also includes gas recirculation means couple to the cathode gas outlet for recirculating a portion of cathode exhaust gas exhausted from the cathode gas outlet to the cathode gas inlet.

Description

CROSS REFERENCE TO RELATED APPLICATIONS
• The present application is a continuation-in-part of prior international application No. PCT/CA99/00823, filed Sep. 14, 1999, which claimed priority from U.S. provisional application No. 60/100,091, filed Sep. 14, 1998, and Canadian patent application No. 2,274,240, filed Jun. 10, 1999, which applications are incorporated herein by reference.[0001]
FIELD
• The present invention relates to a fuel cell for the generation of electrical current. In particular, the present invention relates to a fuel cell-based electrical generation system which employs pressure swing adsorption for enhancing the efficiency of the fuel cell.[0002]
BACKGROUND
• Fuel cells provide an environmentally friendly source of electrical current. One form of fuel cell used for generating electrical power includes an anode for receiving hydrogen gas, a cathode for receiving oxygen gas, and an alkaline electrolyte. Another form of fuel cell 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. In both instances, oxygen gas which enters the cathode reacts with hydrogen ions which cross the electrolyte to generate a flow of electrons. Environmentally safe water vapor is also produced as a byproduct. However, several factors have limited the widespread use of fuel cells as power generation systems.[0003]
• In order to extract a continuous source of electrical power from a fuel cell, it is necessary to provide the fuel cell with a continuous source of oxygen and hydrogen gas. However, with atmospheric air as the direct source of oxygen to the cathode channel, performance of PEM fuel cells is severely impaired by the low partial pressure of oxygen and the concentration polarization of nitrogen, while alkaline fuel cells require a pretreatment purification system to remove carbon dioxide from the feed air. Further, as the average oxygen concentration in a cathode channel with atmospheric air feed is typically only about 15%, the size of the fuel cell must be undesirably large in order to provide sufficient power for industrial applications.[0004]
• In order to achieve a partial pressure of oxygen through the cathode channel sufficient for the attainment of competitive current densities from a PEM fuel cell system, particularly for vehicular propulsion, it is necessary to compress the air feed to at least 3 atmospheres before the air feed is introduced to the cathode channel. As will be appreciated, the power input necessary to sufficiently compress the air feed reduces the overall efficiency of the fuel cell system. It has been proposed to use polymeric membranes to enrich the oxygen, but such membranes actually reduce the oxygen partial pressure and the reduction in total pressure more than offsets the limited enrichment attainable.[0005]
• 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 of 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 and the alkaline fuel cell electrolyte in more than trace levels.[0006]
• In prior art PEM fuel cells operating with an autothermal or partial oxidation fuel processor, ambient air is used as the oxidant. This results in a large load of nitrogen having to be heated and then cooled through the fuel processor system. The substantial volume of nitrogen contributes to pressure losses throughout the fuel processor and anode channels, or alternatively to the cost and physical bulk penalties of making those passages larger.[0007]
• While water recovery from fuel cell exhaust is highly desirable for efficient fuel processor operation, the conventional fuel cell discharges its oxygen-depleted cathode exhaust gas to atmosphere, and thus requires an extra condenser to recover water which then must be vaporized in the fuel processor at a substantial energy cost. This condenser adds to the radiator cooling load which is already a problem for automotive fuel cell power plants in view of the large amount of low grade heat which must be rejected.[0008]
• 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.[0009]
• 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 (vacuum-PSA) separate gas fractions from a gas mixture by coordinating pressure cycling and flow reversals over an 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 adsorbent bed is elevated while the gas mixture is flowing through the adsorbent bed 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 adsorbent bed, while the more readily adsorbed component is concentrated adjacent the first end of the adsorbent bed. 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 bed, and a “heavy” product (a gas fraction enriched in the more strongly adsorbed component) is exhausted from the first end of the bed.[0010]
• However, the conventional system for implementing pressure swing adsorption or vacuum pressure swing adsorption uses two or more stationary adsorbent beds in parallel, with directional valving at each end of each adsorbent bed to connect the beds in alternating sequence to pressure sources and sinks. This system is often difficult and expensive to implement due to the complexity of the valving required.[0011]
• Further, the conventional PSA system makes inefficient use of applied energy, because feed gas pressurization is provided by a compressor whose delivery pressure is the highest pressure of the cycle. In PSA, energy expended in compressing the feed gas used for pressurization is then dissipated in throttling over valves over the instantaneous pressure difference between the adsorber and the high pressure supply. Similarly, in vacuum-PSA, where the lower pressure of the cycle is established by a vacuum pump exhausting gas at that pressure, energy is dissipated in throttling over valves during countercurrent blowdown of adsorbers whose pressure is being reduced. A further energy dissipation in both systems occurs in throttling of light reflux gas used for purge, equalization, cocurrent blowdown and product pressurization or backfill steps. These energy sinks reduce the overall efficiency of the fuel cell system.[0012]
• Additionally, conventional PSA systems can generally only operate at relatively low cycle frequencies, necessitating the use of large adsorbent inventories. The consequent large size and weight of such PSA systems renders them unsuitable for vehicular fuel cell applications. Thus, a conventional PSA unit for oxygen concentration would require an adsorbent bed volume of about 400 L, and an additional installed volume of about 100 L for pressure enclosures and PSA cycle control valves, in order to deliver a product flow containing about 200 L/min oxygen which would be sufficient for a 40 kW fuel cell.[0013]
• Accordingly, there remains a need for an efficient fuel cell-based electrical generation system which can produce sufficient power for industrial applications and which is suitable for vehicular applications. There also remains a need for compact, lightweight hydrogen and oxygen PSA systems that operate at higher cycle frequencies and are suitable for vehicular fuel cell-based applications.[0014]
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.[0015]
• The electrical current generating system, according to a first embodiment of the present invention, comprises a fuel cell, and an oxygen 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 exchange between the anode and cathode channel. The oxygen gas delivery system is coupled to the cathode gas inlet and delivers oxygen gas to the cathode channel.[0016]
• The electrical current generating system also includes gas recirculation means coupled to the cathode gas outlet for recirculating a portion of cathode exhaust gas (which is still enriched in oxygen relative to ambient air, and carries fuel cell exhaust water and fuel cell waste heat) from the cathode gas outlet to the cathode gas inlet.[0017]
• In some embodiments, at least a portion of the cathode exhaust gas is returned to the inlet of an autothermal or partial oxidation fuel processor (or reformer) for reacting a hydrocarbon fuel with oxygen and steam in order to generate raw hydrogen or syngas.[0018]
• In a preferred implementation of the first embodiment, the oxygen gas delivery system comprises an oxygen gas separation system for extracting enriched oxygen gas from air. Preferably, the oxygen gas separating system comprises an oxygen pressure swing adsorption system including 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 pressure swing adsorption system also includes 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 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 fee compartment, a light reflux exit compartment and a light reflux return compartment.[0019]
• 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 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.[0020]
• In another variation, restrictor orifices are disposed between the light reflux exit compartment and the light reflux return compartment for pressure letdown in replacement of the light reflux expander. The gas recirculating means comprises a compressor coupled to the cathode gas outlet for supplying oxygen gas to the cathode gas inlet, and a restrictive orifice disposed between the cathode gas outlet and a pressurization compartment for recycling a portion of the oxygen gas as feed gas to the pressure swing adsorption system. As a result, energy recovered from the cathode gas outlet can be used to help pressurize the cathode gas inlet through the PSA system.[0021]
• The electrical current generating system, according to a second embodiment of the present invention, 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 and an anode gas outlet, 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 exchange between the anode and cathode channel.[0022]
• 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 includes a hydrogen gas inlet for receiving a first hydrogen gas feed from the anode gas outlet, and a hydrogen gas outlet coupled to the anode gas inlet for delivering hydrogen gas received from the first hydrogen gas feed to the anode channel with increased purity.[0023]
• In a preferred implementation of the second 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 second hydrogen gas fee from hydrocarbon fuel, and a hydrogen pressure swing adsorption system coupled to the reactor for purifying hydrogen gas received from the first and second hydrogen gas feeds. 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.[0024]
• 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 shift reactor for removal of impurities, and then delivered as the second hydrogen gas feed to the hydrogen pressure swing adsorption system.[0025]
• 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 fee. 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 used to preheat air and/or fuel supplied to the autothermal reformer.[0026]
BRIEF DESCRIPTION OF THE DRAWINGS
• The preferred embodiment of the present invention will now be described, by way of example only, with reference to the drawings, in which:[0027]
• FIG. 1 is a sectional view of a rotary PSA module suitable for use with the present invention, showing the stator and rotor situated in the stator.[0028]
• FIG. 2 is a sectional view of the module of FIG. 1, with the stator deleted for clarity.[0029]
• FIG. 3 is a sectional view of the stator shown in FIG. 1, with the rotor deleted for clarity.[0030]
• FIG. 4 is an axial section of the module of FIG. 1.[0031]
• FIG. 5 shows a typical PSA cycle attainable with the PSA system shown in FIGS.[0032]1 to4.
• FIG. 6 shows one variation of the PSA cycle with heavy reflux, attainable with the PSA system shown in FIGS.[0033]1 to4.
• FIG. 7 shows a pressure swing adsorption apparatus for separating oxygen gas from air, suitable for use with the present invention, and depicting the rotary module shown in FIG. 1 and a compression machine coupled to the rotary module.[0034]
• FIG. 8 shows a pressure swing adsorption apparatus for purifying hydrogen gas, suitable for use with the present invention, and depicting the rotary module shown in FIG. 1 and a compression machine coupled to the rotary module.[0035]
• FIG. 9 shows an electrical current generating system, according to a first embodiment of the present invention, including an oxygen-separating PSA system for supplying enriched oxygen to the fuel cell cathode channel with energy recovery from light reflux expansion to boost the pressure of oxygen circulating in the fuel cell cathode loop.[0036]
• FIG. 10 shows a first variation of the electrical current generating system shown in FIG. 9, but with the PSA system including a countercurrent blowdown expander driving a free rotor exhaust vacuum pump for vacuum-PSA operation.[0037]
• FIG. 11 shows a second variation of the electrical current generating system shown in FIG. 9, with a portion of the oxygen enriched gas discharged from the fuel cell cathode being used for a pressurization step for the PSA system.[0038]
• FIG. 12 shows an electrical current generating system, according to a second embodiment of the present invention, including an oxygen-separating PSA system for supplying enriched oxygen to the fuel cell cathode channel, and a hydrogen-separating PSA system for supplying enriched hydrogen to the fuel cell anode channel, with the hydrogen-separating PSA system receiving feed gas from a stream reformer.[0039]
• FIG. 13 shows an electrical current generating system, according to a variation of the electrical current generating system shown in FIG. 12, but with the hydrogen-separating PSA system receiving feed gas from an autothermal reformer.[0040]
• FIG. 14 shows an electrical current generating system with carbon dioxide removal and oxygen enrichment for an alkaline fuel cell, and with an oxygen accumulator.[0041]
• FIG. 15 shows an axial section of a rotary PSA module with rotating adsorbers and stationary distributor valves.[0042]
• FIG. 16 shows a transverse section of the module of FIG. 15.[0043]
• FIG. 17 shows a transverse section of the module of FIG. 15.[0044]
• FIG. 18 shows a transverse section of the module of FIG. 15.[0045]
• FIG. 19 shows a transverse section of the module of FIG. 15.[0046]
• FIG. 20 shows a transverse section of the module of FIG. 15.[0047]
• FIG. 21 shows a transverse section of the module of FIG. 15.[0048]
• FIG. 22 shows an axial section of a rotary PSA module with stationary adsorbers and rotating distributor valves.[0049]
DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS
• To aid in understanding the present invention, a pressure swing adsorption process and associated apparatus, suitable for use with the present invention, will be described first, with reference to FIGS. 1 through 6. Thereafter, an oxygen-separating pressure swing adsorption system and a hydrogen-separating pressure swing adsorption system will be described with reference to FIGS. 7 and 8 respectively. Two embodiments of the invention, together with variations thereon, will then be described commencing with FIG. 9.[0050]
• FIGS. 1, 2,[0051]3 and4
• A[0052]rotary module10 which is suitable for use as part of the present invention is shown in FIGS. 1, 2,3 and4. The module includes arotor11 revolving aboutaxis12 in the direction shown byarrow13 withinstator14. However, it should be understood that the invention is not limited to PSA systems having rotary modules. Rather other arrangements may be employed without departing from the scope of the invention. For instance, if desired, the present invention may be employed with multiple stationary adsorbent beds in parallel, with directional valving at each of each adsorbent bed to connect the beds in alternating sequence to pressure sources and sinks. However, as will become apparent, therotary module10 is preferred since it provides highly desirable features of efficiency and compactness.
• Rotary PSA modules include those with rotating adsorber modules to provide the rotary valve function, and those with a fixed adsorber module cooperating with rotating valves, preferably coaxial to the adsorber module. In a particular embodiment, the rotary PSA module is a cylindrical axial flow adsorber module, with feed and product rotary valve faces at opposite ends of the adsorber module. The adsorber module may rotate or remain stationary, the later version including fluid transfer between the casing and the rotating valve rotors.[0053]
• In general, the[0054]rotary module10 may be configured for flow through the adsorber elements in the radial, axial or oblique conical directions relative to the rotor axis. For operation at high cycle frequency, radial flow has the advantage that the centripetal acceleration will lie parallel to the flow path for most favorable stabilization of buoyancy-driven free convection. Radial flow configurations may be preferred for large module capacities.
• Axial flow configurations may be preferred for smaller module capacities, and advantageously allow increased compactness of fuel cell systems that incorporate PSA systems, at least for smaller power ratings below about 300 kW. Compactness is an important consideration for enabling practicable use of PSA systems in automotive fuel cell power plants.[0055]
• As shown in FIG. 2, the[0056]rotor11 is of annular section, having concentrically toaxis12 an outercylindrical wall20 whose external surface isfirst valve surface21, and an innercylindrical wall22 whose internal surface issecond valve surface23. The rotor has (in the plane of the section defined byarrows15 and16 in FIG. 4) a total of “N” radialflow adsorber elements24. An adjacent pair ofadsorber elements25 and26 are separated bypartition27 which is structurally and sealingly joined toouter wall20 andinner wall22.Adjacent adsorber elements25 and26 are angularly spaced relative toaxis12 by an angle of 360°/N. Since the adsorber elements and the valve surfaces are thereby integrated into a single unit, and the adsorber elements are located in close proximity to the valve surfaces with minimal dead volume, therotary module10 is more compact and efficient than conventional PSA systems.
• [0057]Adsorber element24 has afirst end30 defined bysupport screen31 and asecond end32 defined bysupport screen33. The adsorber may be provided as granular adsorbent, whose packing voidage defines a flow path contacting the adsorbent between the first and second ends of the adsorber. However, as described in copending U.S. patent application Ser. No. 08/995,906, the description therein being incorporated herein by reference, preferably the adsorber element is provided as an array of laminated thin sheets extending between the first and second ends of the adsorber, the sheets having an adsorbent such as a zeolite supported on a reinforcement matrix, and with flow channels established by spacers between the sheets. The laminated sheet adsorber, with sheet thickness of approximately 150 microns and using type X zeolites, has greatly reduced mass transfer and pressure drop resistances compared to conventional granular adsorbers, so that satisfactory oxygen enrichment operation has been achieved with PSA cycle periods in the order of 1 second and as low as 0.4 second. Consequently, the adsorbent inventory is radically reduced compared to conventional PSA cycle periods of about 1 minute, with the size of the module being smaller by some two orders of magnitude compared to conventional PSA equipment of equivalent capacity. As a result, an exceptionally compact PSA module that can be used, rendering the invention particularly suitable for vehicular fuel cell power plants.
• First aperture or[0058]orifice34 provides flow communication fromfirst valve surface21 throughwall20 to thefirst end30 ofadsorber24. Second aperture ororifice35 provides flow communication fromsecond valve surface23 throughwall22 to thesecond end31 ofadsorber24. Support screens31 and33 respectively provideflow distribution32 betweenfirst aperture34 andfirst end30, and betweensecond aperture35 andsecond end32, ofadsorber element24.
• As shown in FIG. 3,[0059]stator14 is a pressure housing including an outer cylindrical shell orfirst valve stator40 outside theannular rotor11, and an inner cylindrical shell orsecond valve stator41 inside theannular rotor11.Outer shell40 carries axially extending strip seals (e.g.42 and43) sealingly engaged withfirst valve surface21, whileinner shell41 carries axially extending strip seals (e.g.44 and45) sealingly engaged withsecond valve surface23. Preferably, the azimuthal sealing width of the strip seals is greater than the diameters or azimuthal widths of the first andsecond apertures34 and35 opening through the first and second valve surfaces.
• A set of first compartments in the outer shell each open in an annular sector to the first valve surface, and each provide fluid communication between its angular sector of the first valve surface and a manifold external to the module. The angular sectors of the compartments are much wider than the angular separation of the adsorber elements. The first compartments are separated on the first sealing surface by the strip seals (e.g.[0060]42). Proceeding clockwise in FIG. 3, in the direction of rotor rotation, a firstfeed pressurization compartment46 communicates byconduit47 to firstfeed pressurization manifold48, which is maintained at a first intermediate feed pressure. Similarly, a secondfeed pressurization compartment50 communicates to secondfeed pressurization manifold51, which is maintained at a second intermediate feed pressure higher than the first intermediate feed pressure but less than the higher working pressure.
• For greater generality,[0061]module10 is shown with provision for sequential admission of two feed mixtures, the first feed gas having a lower concentration of the more readily adsorbed component relative to the second feed gas.First feed compartment52 communicates tofirst feed manifold53, which is maintained at substantially the higher working pressure. Likewise,second feed compartment54 communicates tosecond feed manifold55, which is maintained at substantially the higher working pressure. A firstcountercurrent blowdown compartment56 communicates to firstcountercurrent blowdown manifold57, which is maintained at a first countercurrent blowdown intermediate pressure. A secondcountercurrent blowdown compartment58 communicates to secondcountercurrent blowdown manifold59, which is maintained at a second countercurrent blowdown intermediate pressure above the lower working pressure. Aheavy product compartment60 communicates to heavyproduct exhaust manifold61 which is maintained at substantially the lower working pressure. It will be noted thatcompartment58 is bounded bystrip seals42 and43, and similarly all the compartments are bounded and mutually isolated by strip seals.
• A set of second compartments in the inner shell each open in an angular sector to the second valve surface, and each provide fluid communication between its angular sector of the second valve surface and a manifold external to the module. The second compartments are separated on the second sealing surface by the strip seals (e.g.[0062]44). Proceeding clockwise in FIG. 3, again in the direction of rotor rotation,light product compartment70 communicates tolight product manifold71, and receives high product gas at substantially the higher working pressure, less frictional pressure drops through the adsorbers and the first and second orifices. According to the angular extension ofcompartment70 relative tocompartment52 and54, the light product may be obtained only from adsorbers simultaneously receiving the first feed gas fromcompartment52, or from adsorbers receiving both the first and second feed gases.
• A first light[0063]reflux exit compartment72 communicates to first lightreflux exit manifold73, which is maintained at a first light reflux exit pressure, here substantially the higher working pressure less frictional pressure drops. A first cocurrent blowndown compartment74 (which is actually the second light reflux exit compartment), communicates to second lightreflux exit manifold75, which is maintained at a first cocurrent blowdown pressure less than the higher working pressure. A second cocurrent blowdown compartment or third lightreflux exit compartment76 communicates to third lightreflux exit manifold77, which is maintained at a second cocurrent blowdown pressure less than the first cocurrent blowdown pressure. A third cocurrent blowdown compartment or fourth lightreflux exit compartment78 communicates to fourth lightreflux exit manifold79, which is maintained at a third cocurrent blowdown pressure less than the second cocurrent blowdown pressure.
• A[0064]purge compartment80 communicates to a fourth lightreflux return manifold81, which supplies the fourth light reflux gas which has been expanded form the third cocurrent blowdown pressure to substantially the lower working pressure with an allowance for frictional pressure drops. The ordering of light reflux pressurization steps is inverted from the ordering or light reflux exit or cocurrent blowdown steps, so as to maintain a desirable “last out—first in” stratification of light reflux gas packets. Hence a first lightreflux pressurization compartment82 communicates to a third lightreflux return manifold83, which supplies the third light reflux gas which has been expanded from the second cocurrent blowdown pressure to a first light reflux pressurization pressure greater than the lower working pressure. A second lightreflux pressurization compartment84 communicates to a second lightreflux return manifold85, which supplies the second light reflux gas which has been expanded from the first cocurrent blowdown pressure to a second light reflux pressurization pressure greater than the first light reflux pressurization pressure. Finally, a third lightreflux pressurization compartment86 communicates to a first lightreflux return manifold87, which supplies the first light reflux gas which has been expanded from approximately the higher pressure to a third light reflux pressurization pressure greater than the second light reflux pressurization pressure, and in this example less than the first feed pressurization pressure.
• Additional details are shown in FIG. 4.[0065]Conduits88 connectfirst compartment60 tomanifold61, with multiple conduits providing for good axial flow distribution incompartment60. Similarly, conduits89 connectsecond compartment80 tomanifold81.Stator14 hasbase90 withbearings91 and92.Motor95 is coupled toshaft94 to driverotor11. The rotor could alternatively rotate as an annular drum, supported by rollers at several angular positions about its rim and also driven at its rim so that no shaft would be required. A rim drive could be provided by a ring gear attached to the rotor, or by a linear electromagnetic motor whose stator would engage an arc of the rim. Particularly for hydrogen separation applications, the rotor drive may be hermetically enclosed within the stator housing to eliminate hazards related to seal leakage. Outercircumferential seals96 seal the ends of outer strip seals42 and the edges offirst valve surface21, while innercircumferential seals97 seal the ends of inner strip seals44 and the edges ofsecond valve surface23.Rotor11 hasaccess plug98 betweenouter wall20 andinner wall22, which provides access for installation and removal of the adsorbent inadsorbers24.
• FIGS. 5 and 6[0066]
• FIG. 5 shows a typical PSA cycle which would be obtained using the foregoing gas separation system, while FIG. 6 shows a similar PSA cycle with heavy reflux recompression of a portion of the first product gas to provide a second feed gas to the process.[0067]
• In FIGS. 5 and 6, the[0068]vertical axis150 indicates the working pressure in the adsorbers and the pressure in the first and second compartments. Pressure drops due to flow within the adsorber elements are neglected. The higher and lower working pressures are respectively indicated bydotted lines151 and152. The lower working pressure may be nominally or approximately ambient atmospheric pressure, or may be a subatmospheric pressure established by vacuum pumping. The higher working pressure may typically be in the range of twice to four times the lower working pressure, based on the ratio of absolute pressures.
• The[0069]horizontal axis155 of FIGS. 5 and 6 indicates time, with the PSA cycle period defined by the time interval betweenpoints156 and157. Attimes156 and157, the working pressure in a particular adsorber ispressure158. Starting fromtime156, the cycle for a particular adsorber (e.g.24) begins as thefirst aperture34 of that adsorber is opened to the firstfeed pressurization compartment46, which is fed by first feed supply means160 at the firstintermediate feed pressure161. The pressure in that adsorber rises frompressure158 attime157 to the firstintermediate feed pressure161. Proceeding ahead, first aperture passes over a seal strip,first closing adsorber24 tocompartment46 and then opening it to secondfeed pressurization compartment50 which is feed by second feed supply means162 at the secondintermediate feed pressure163. The adsorber pressure rises to the second intermediate feed pressure.
• [0070]First aperture34 ofadsorber24 is opened next tofirst feed compartment52, which is maintained at substantially the higher pressure by a third feed supply means165. Once the adsorber pressure has risen to substantially the higher working pressure, its second aperture35 (which has been closed to all second compartments since time156) opens tolight product compartment70 and deliverslight product166.
• In the cycle of FIG. 6,[0071]first aperture34 ofadsorber24 is opened next tosecond feed compartment54, also maintained at substantially the higher pressure by a fourth feed supply means167. In general, the fourth feed supply means supplies a second feed gas, typically richer in the more readily adsorbed component than the first feed gas provided by the first, second and third feed supply means. In the specific cycle illustrated in FIG. 6, the fourth feed supply means167 is a “heavy reflux” compressor, recompressing a portion of the heavy product back into the apparatus. In the cycle illustrated in FIG. 5, there is no fourth feed supply means, andcompartment54 could be eliminated or consolidated withcompartment52 extended over a wider angular arc to the stator.
• While feed gas is still being supplied to the first end of[0072]adsorber24 from eithercompartment52 or54, the second end ofadsorber24 is closed tolight product compartment70 and opens to firstreflux exit compartment72 while delivering “light reflux” gas (enriched in the less readily adsorbed component, similar to second product gas) to first light reflux pressure let-down means (or expander)170. Thefirst aperture34 ofadsorber24 is then closed to all first compartments, while thesecond aperture35 is opened successively to (a) second lightreflux exit compartment74, dropping the adsorber pressure to the firstcocurrent blowdown pressure171 while delivering light reflux gas to second light reflux pressure letdown means172, (b) third lightreflux exit compartment76, dropping the adsorber pressure to the secondcocurrent blowdown pressure173 while delivering light reflux gas to third light reflux pressure letdown means174, and (c) fourth lightreflux exit compartment78, dropping the adsorber pressure to the thirdcocurrent blowdown pressure175 while delivering light reflux gas to fourth light reflux pressure letdown means176.Second aperture35 is then closed for an interval, until the light reflux return steps following the countercurrent blowdown steps.
• The light reflux pressure let-down means may be mechanical expanders or expansion stages for expansion energy recovery, or may be restrictor orifices or throttle valves for irreversible pressure let-down.[0073]
• Either when the second aperture is closed after the final light reflux exit step (as shown in FIGS. 5 and 6), or earlier while light reflux exit steps are still underway,[0074]first aperture34 is opened to firstcountercurrent blowdown compartment56, dropping the adsorber pressure to the first countercurrent blowdownintermediate pressure180 while releasing “heavy” gas (enriched in the more strongly adsorbed component) to first exhaust means181. Then,first aperture34 is opened to secondcountercurrent blowdown compartment58, dropping the adsorber pressure to the first countercurrent blowndownintermediate pressure182 while releasing heavy gas to second exhaust means183. Finally reaching the lower working pressure,first aperture34 is opened toheavy product compartment60, dropping the adsorber pressure to thelower pressure152 while releasing heavy gas to third exhaust means184. Once the adsorber pressure has substantially reached the lower pressure whilefirst aperture34 is open tocompartment60, thesecond aperture35 opens to purgecompartment80, which receives fourth light reflux gas from fourth light reflux pressure let-down means176 in order to displace more heavy gas intofirst product compartment60.
• In FIG. 5, the heavy gas from the first, second and third exhaust means is delivered as the[0075]heavy product185. In FIG. 6, this gas is partly released as theheavy product185, while the balance is redirected as “heavy reflux”187 to the heavy reflux compressor as fourth feed supply means167. Just as light reflux enables an approach to high purity of the less readily adsorbed (“light”) component in the light product, heavy reflux enables an approach to high purity of the more readily adsorbed (“heavy”) component in the heavy product so that high recovery of the less readily adsorbed (“light”) product can be achieved.
• The adsorber is then repressurized by light reflux gas after the first and second apertures close to[0076]compartments60 and80. In succession, while thefirst aperture34 remains closed at least initially, (a) thesecond aperture35 is opened to first lightreflux pressurization compartment82 to raise the adsorber pressure to the first lightreflux pressurization pressure190 while receiving third light reflux gas from the third reflux pressure letdown means174, (b) thesecond aperture35 is opened to second lightreflux pressurization compartment84 to raise the adsorber pressure to the second lightreflux pressurization pressure191 while receiving second light reflux gas from the second light reflux pressure letdown means172, and (c) thesecond aperture35 is opened to third lightreflux pressurization compartment86 to raise the adsorber pressure to the third lightreflux pressurization pressure192 while receiving first light reflux gas from the first light reflux pressure letdown means170. Unless feed pressurization has already been started while light reflux return for light reflux pressurization is still underway, the process (as based on FIGS. 5 and 6) begins feed pressurization for the cycle aftertime157 as soon as the third light reflux pressurization step has been concluded.
• The pressure variation waveform in each adsorber would be a rectangular staircase if there were no throttling in the first and second valves. Such throttling is needed to smooth pressure and flow transients. In order to provide balanced performance, preferably all of the adsorber elements and the apertures are closely identical to each other.[0077]
• The rate of pressure change in each pressurization or blowdown step will be restricted by throttling in ports (or in clearance or labyrinth sealing gaps) of the first and second valve means, or by throttling in the apertures at first and second ends of the adsorbers, resulting in the typical pressure waveform depicted in FIGS. 5 and 6. Alternatively, the apertures may be opened slowly by the seal strips, to provide flow restriction throttling between the apertures and the seal strips, which may have narrow tapered clearance channels so that the apertures are only opened to full flow gradually. Excessively rapid rates of pressure change would subject the adsorber to mechanical stress, while also causing flow transients which would tend to increase axial dispersion of the concentration wavefront in the adsorber. Pulsations of flow and pressure are minimized by having a plurality of adsorbers simultaneously transiting each step of the cycle, and by providing enough volume in the function compartments and associated manifolds so that they act effectively as surge adsorbers between the compression machinery and the first and second valve means.[0078]
• It will be evident that the cycle could be generalized in many variations by having more or fewer intermediate stages in each major step of feed pressurization, countercurrent blowdown exhaust, or light reflux. If desired, combined feed and product pressurization steps (or combined cocurrent and countercurrent blowdown steps) at intermediate pressures may be performed from both first and second valves simultaneously. The pressure at which feed pressurization begins, may differ from the pressure at which countercurrent blowdown begins. Furthermore, in air separation or air purification applications, a stage of feed pressurization (typically the first stage) could be performed by equalization with atmosphere as an intermediate pressure of the cycle. Similarly, a stage of countercurrent blowdown could be performed by equalization with atmosphere as an intermediate pressure of the cycle.[0079]
• FIG. 7[0080]
• FIG. 7 is a simplified schematic of a PSA system for separating oxygen from air using nitrogen-selective zeolite adsorbents. The light product is concentrated oxygen, while the heavy product is nitrogen-enriched air usually vented as waste. The cycle[0081]lower pressure152 is illustrated as nominally atmospheric pressure, although avacuum pressure152 could be used as will be illustrated in FIG. 8. Feed air is introduced throughfilter intake200 to afeed compressor201. The feed compressor includes compressorfirst stage202,intercooler203, compressorsecond stage204,second intercooler205, compressorthird stage206,third intercooler207, and compressorfourth stage208. Thefeed compressor201 as described may be a four stage axial compressor withmotor209 as prime mover coupled byshaft210. The intercoolers are optional. With reference to FIG. 5, the feed compressor first and second stages are the first feed supply means160, delivering feed gas at the firstintermediate feed pressure161 viaconduit212 andwater condensate separator213 to firstfeed pressurization manifold48. Feed compressorthird stage206 is the second feed supply means162, delivering feed gas at the secondintermediate feed pressure163 viaconduit214 andwater condensate separator215 to secondfeed pressurization manifold51. Feed compressorfourth stage208 is the third feed supply means165, delivering feed gas at thehigher pressure151 viaconduit216 andwater condensate separator217 to feedmanifold53. Light product oxygen flow is delivered fromlight product manifold71 byconduit218, maintained at substantially the higher pressure less frictional pressure drops.
• The PSA system of FIG. 7 includes energy recovery expanders, including light reflux expander[0082]220 (here including four stages) and countercurrent blowdown expander221 (here including two stages), coupled to feedcompressor201 byshaft222. The expander stages may be provided for example as radial inflow turbine stages, as full admission axial turbine stages with separate wheels, or as partial admission impulse turbine stages combined in a single wheel.
• Light reflux gas from first light[0083]reflux exit manifold73 flows at the higher pressure viaconduit224 andheater225 to first light pressure letdown means170 which here is first lightreflux expander stage226, and then flows at the third lightreflux pressurization pressure192 byconduit227 to the first lightreflux return manifold87. Light reflux gas from second lightreflux exit manifold75 flows at the firstcocurrent blowdown pressure171 viaconduit228 andheater225 to second light reflux pressure letdown means172, here thesecond expander stage230, and then flows at the second lightreflux pressurization pressure191 byconduit231 to the second lightreflux return manifold85. Light reflux gas from third lightreflux exit manifold77 flows at the secondcocurrent blowdown pressure173 viaconduit232 andheater225 to third light refluxpressure pressurization pressure190 byconduit235 to the third lightreflux return manifold83. Finally, light reflux gas from fourth lightreflux exit manifold79 flows at the thirdcocurrent blowdown pressure175 viaconduit236 andheater225 to fourth light reflux pressure letdown means176, here the fourth lightreflux expander stage238, and then flows at substantially thelower pressure152 byconduit239 to the fourth lightreflux return manifold81.
• Heavy countercurrent blowdown gas from first[0084]countercurrent blowdown manifold57 flows at first countercurrent blowdownintermediate pressure180 byconduit240 toheater241 and thence tofirst stage242 of thecountercurrent blowdown expander221 as first exhaust means181, and is discharged from the expander to exhaust manifold243 at substantially thelower pressure152. Countercurrent blowdown gas from secondcountercurrent blowdown manifold59 flows at second countercurrent blowdownintermediate pressure182 byconduit244 toheater241 and thence tosecond stage245 of thecountercurrent blowdown expander221 as second exhaust means183, and is discharged from the expander to exhaust manifold243 at substantially thelower pressure152. Finally, heavy gas from heavyproduct exhaust manifold61 flows byconduit246 as third exhaust means184 toexhaust manifold243 delivering theheavy product gas185 to be vented at substantially thelower pressure152.
• [0085]Optional heaters225 and241 raise the temperatures ofgases entering expanders220 and221, thus augmenting the recovery of expansion energy and increasing the power transmitted byshaft222 fromexpanders220 and221 to feedcompressor201, and reducing the power required fromprime mover209. Whileheaters225 and241 are means to provide heat to the expanders,intercoolers203,205 and207 are means to remove heat from the feed compressor and serve to reduce the required power of the higher compressor stages. Theintercoolers203,205,207 are optional features.
• If[0086]light reflux heater249 operates at a sufficiently high temperature so that the exit temperature of the light reflux expansion stages is higher than the temperature at which feed gas is delivered to the feed manifolds byconduits212,214 and216, the temperature of the second ends35 of theadsorbers24 may be higher than the temperature of their first ends34. Hence, the adsorbers have a thermal gradient along the flow path, with higher temperature at their second end relative to the first end. This is an extension of the principle of :thermally coupled pressure swing adsorption” (TCPSA), introduced by Keefer in U.S. Pat. No. 4,702,903.Adsorber rotor11 then acts as a thermal rotary regenerator, as in regenerative gas turbine engines having acompressor201 and anexpander220. Heat provided to the PSA process byheater225 assists powering the process according to a regenerative thermodynamic power cycle, similar to advanced regenerative gas turbine engines approximately realizing the Ericsson thermodynamic cycle with intercooling on the compression side and interstage heating on the expansion side. In the instance of PSA applied to oxygen separation from air, the total light reflux flow is much less than the feed flow because of the strong bulk adsorption of nitrogen. Accordingly the power recoverable from the expanders is much less than the power required by the compressor, but will still contribute significantly to enhanced efficiency of oxygen production.
• If high energy efficiency is not of highest importance, the light reflux expander stages and the countercurrent blowdown expander stages may be replaced by restrictor orifices or throttle valves for pressure letdown. The schematic of FIG. 7 shows a single shaft supporting the compressor stages, the countercurrent blowdown or exhaust expander stages, and the light reflux stages, as well as coupling the compressor to the prime mover. However, it should be understood that separate shafts and even separate prime movers may be used for the distinct compression and expansion stages within the scope of the present invention.[0087]
• FIG. 8[0088]
• FIG. 8 shows a vacuum-PSA system, also with heavy product reflux as could be used to achieve high recovery in hydrogen purification for fuel cell power plant. The raw hydrogen may be provided in certain stationary applications from chemical process or petroleum refinery offgases. However, in most fuel cell applications, the raw hydrogen gas feed will be provided by processing of a hydrocarbon or carbonaceous fuel, e.g. by steam reforming of natural gas or methanol, or by autothermal reforming or partial oxidation of liquid fuels. Such hydrogen feed gases typically contain 30% to 75% hydrogen. Using typical adsorbents such as zeolites, carbon dioxide, carbon monoxide, nitrogen, ammonia, and hydrogen sulfide or other trace impurities will be much more readily adsorbed than hydrogen, so the purified hydrogen will be the light product delivered at the higher working pressure which may be only slightly less than the feed supply pressure, while the impurities will be concentrated as the heavy product and will be exhausted from the PSA process as “PSA tail gas” at the lower working pressure. This tail gas will be used as fuel gas for the fuel processing reactions to generate hydrogen, or else for a combustion turbine to power PSA compression machinery for the fuel cell power plant.[0089]
• The PSA system of FIG. 8 has[0090]infeed conduit300 to introduce the feed gas at substantially the higher pressure tofirst feed manifold53. In this example, all but the final pressurization steps are achieved with light reflux gas, with the final feed pressurization step being achieved throughmanifold55.
• The PSA system includes a[0091]multistage vacuum pump301 driven byprime mover209 throughshaft210, and optionally bylight reflux expander220 throughshaft309. Thevacuum pump301 includes afirst stage302 drawing heavy gas byconduit246 from firstproduct exhaust manifold61, and compressing this gas throughintercooler303 tosecond stage304. Vacuum pumpsecond stage304 draws heavy gas from secondcountercurrent blowdown manifold59 throughconduit244, and delivers this gas byintercooler305 tothird stage306 which also draws heavy gas from firstcountercurrent blowdown manifold57 throughconduit240. Thevacuum pump stage306 compresses the heavy gas to a pressure sufficiently above ambient pressure for a portion of this gas (heavy product gas or PSA tail gas) to be delivered for use as fuel has in heavyproduct delivery conduit307. The remaining heavy gas proceeds fromvacuum pump301 toheavy reflux compressor308 which attains substantially the higher working pressure of the PSA cycle.
• The compressed heavy gas is conveyed from compressor[0092]fourth stage308 byconduit310 tocondensate separator311. If desired (as for combustion in an expansion turbine as in the embodiment of FIG. 13), the entire heavy product stream could be compressed throughcompressor308, so that the heavy product fuel gas may be delivered at the highest working pressure by alternative heavyproduct delivery conduit312 which is externally maintained at substantially the higher pressure less frictional pressure drops. Condensed vapors (such as water) are removed throughconduit313 at substantially the same pressure as the heavy product inconduit312. The remaining heavy gas flow, after removal of the first product, gas, flows byconduit314 to thesecond feed manifold55 as heavy reflux to the adsorbers following the feed step for each adsorber. The heavy reflux gas is a second feed gas, of higher concentration in the more readily adsorbed component or fraction than the first feed gas.
• FIGS. 9 and 10[0093]
• Turning now to FIGS. 9 and 10, fuel cell-based electrical current generating systems, according to a first embodiment of the present invention, are shown using a rotary PSA system similar to that shown in FIG. 7 as the basic building block. However, it should be understood that the invention is not limited to electrical current generating systems having rotary PSA modules. Rather other arrangements may be employed without departing from the scope of the invention.[0094]
• In FIG. 9, the PSA system separates oxygen from air, using nitrogen-selective zeolite adsorbents, as previously described. The light product is concentrated oxygen, while the heavy product is nitrogen-enriched air usually vented as waste. The cycle[0095]lower pressure152 is nominally atmospheric pressure, unless an optional vacuum pump is provided as in FIG. 8. Feed air is introduced throughfilter intake200 to afeed compressor201. The feed compressor includes compressorfirst stage202, compressorsecond stage204, compressorthird stage206, and compressorfourth stage208. Thefeed compressor201 as described may be a four stage axial compressor withmotor209 as prime mover coupled byshaft210. The compressor stages may be in series as shown, or alternatively in parallel. Intercoolers between compressor stages are optional. The feed compressor first and second stages deliver feed gas at the firstintermediate feed pressure161 viaconduit212 andwater condensate separator213 to firstfeed pressurization manifold48. Feed compressorthird stage206 delivers feed gas at the secondintermediate feed pressure163 viaconduit214 andwater condensate separator215 to secondfeed pressurization manifold51. Feed compressorfourth stage208 delivers feed gas at thehigher pressure151 viaconduit216 andwater condensate separator217 to feedmanifold53. Light product oxygen flow is delivered fromlight product manifold71 byconduit218, maintained at substantially the higher pressure less frictional pressure drops.
• The apparatus of FIG. 9 includes energy recovery expanders, including light reflux expander[0096]220 (here including four stages) and countercurrent blowdown expander221 (here including two stages).Expander221 is coupled to feedcompressor201 byshaft222. The expander stages may be provided for example as radial inflow turbine stages, as full admission axial turbine stages with separate wheels, or as partial admission turbine stages combined in a single wheel. If high energy efficiency were not of highest importance, the light reflux expander stages and/or the countercurrent blowdown expander stages could be replaced by restrictor orifices or throttle valves for pressure letdown.
• Light reflux gas from light[0097]reflux exit manifold73 flows at the higher pressure viaconduit224 andheater225 to first lightreflux expander stage226, and then flows at the third lightreflux pressurization pressure192 byconduit227 to the first lightreflux return manifold87. Light reflux gas from second lightreflux exit manifold75 flows at the firstcocurrent blowdown pressure171 viaconduit228 andheater225 to thesecond expander stage230, and then flows at the second lightreflux pressurization pressure191 byconduit231 to the second lightreflux return manifold85. Light reflux gas from third lightreflux exit manifold77 flows at the secondcocurrent blowdown pressure173 viaconduit232 andheater225 to thethird expander stage234, and then flows at the first lightreflux pressurization pressure190 byconduit235 to the third lightreflux return manifold83. Finally, light reflux gas from fourth lightreflux exit manifold79 flows at the thirdcocurrent blowdown pressure175 viaconduit236 andheater225 to fourth lightreflux expander stage238, and then flows at substantially thelower pressure152 byconduit239 to the fourth lightreflux return manifold81.
• Heavy countercurrent blowdown gas from first[0098]countercurrent blowdown manifold57 flows at first countercurrent blowdownintermediate pressure180 byconduit240 toheater241 and thence tofirst stage242 of thecountercurrent blowdown expander221, and is discharged from the expander to exhaust manifold243 at substantially thelower pressure152.
• [0099]Optional heaters225 and241 raise the temperature ofgases entering expanders220 and221, thus augmenting the recovery of expansion energy and increasing the power transmitted byshaft222 fromexpanders220 and221 to feedcompressor201, and reducing the power required fromprime mover209.
• In the instance of PSA applied to oxygen separation from air, the total light reflux flow is much less than the feed flow because of the strong bulk adsorption of nitrogen. Accordingly the power recoverable from the expanders is much less than the power required by the compressor, but will still contribute significantly to enhanced efficiency of oxygen production. By operating the adsorbers at moderately elevated temperature (e.g. 40° to 60° C.) and using strongly nitrogen-selective adsorbents such as Ca—X, Li—X or lithium chabazite zeolites, the PSA oxygen generation system can operate with favorable performance and efficiency. Calcium or strontium exchanged chabazite may be used at higher temperatures, even in excess of 100° C., reflecting the extraordinary capacity of these adsorbents for nitrogen, their nitrogen uptake being too close to saturation at lower temperatures near ambient for satisfactory operation.[0100]
• While higher temperature of the adsorbent will reduce nitrogen uptake and selectivity for each zeolite adsorbent, the isotherms will be more linear and humidity rejection will be easier. Working with adsorbents such as Ca—X and Li—X, recent conventional practice has been to operate ambient temperature PSA at subatmospheric lower pressures in so-called “vacuum swing adsorption” (VSA), so that the highly selective adsorbents operate well below saturation in nitrogen uptake, and have a large working capacity in a relatively linear isotherm range. At higher temperatures, saturation in nitrogen uptake is shifted to more elevated pressures, so that optimum PSA cycle higher and lower pressures are also shifted upward.[0101]
• The enriched oxygen product gas is delivered by[0102]conduit218,non-return valve250, and conduit251 to the inlet ofoxygen product compressor252 which boosts the pressure of product oxygen delivered byconduit253.Compressor252 may be a single stage centrifugal compressor, driven directly through shaft254 bylight reflux expander220 or alternatively by a motor.Light reflux expander220 may be the sole power source tocompressor252, in whichcase expander220 andcompressor252 together constitute a free rotor turbo-booster255. Since the working fluid in bothexpander220 andcompressor252 is enriched oxygen, the free rotor turbo-booster embodiment has the important safety feature of not requiring a shaft seal to an external motor. Preferably, energy recovered from light reflux expansion is used to raise the delivery pressure of the light product, here oxygen.
• The compressed enriched oxygen is delivered to a[0103]fuel cell260, byconduit253 tocathode inlet261 of fuelcell cathode channel262.Fuel cell260 may be of the polymer electrolyte membrane (PEM), with theelectrolyte265 separatingcathode channel262 fromanode channel266. Hydrogen fuel is supplied toanode inlet267 ofanode channel266 byhydrogen infeed conduit268.
• The enriched oxygen passes through[0104]cathode channel262 tocathode exit270, as a fraction of the oxygen reacts with hydrogen ions crossing the membrane to generate electrical power and reacting to form byproduct water. The cathode exit gas leaving the cathode channel inconduit280 from cathode exit270 (in this preferred embodiment) is still significantly enriched in oxygen relative to ambient air concentration of approximately 21%. A minor portion of this gas is purged as cathode purge gas fromconduit280 bypurge valve285 and purgeexhaust286, and the balance of the cathode exit gas is retained as cathode recycle gas. The cathode recycle gas is conveyed byconduit281 towater condensate separator282 where excess liquid water is removed from the cathode exit gas, which remains saturated in water vapor. The humid cathode recycle gas is then blended with incoming enriched oxygen form the PSA system byconduit283 connecting to conduit251.
• [0105]Conduits251,253,280,281 and283 thus form a cathode loop withcathode channel262,compressor252 andwater condensate separator282.Heat exchanger225 may cool the oxygen-enriched gas to be compressed bycompressor252, by removing waste heat from the fuel cell cathode loop to heat light reflux gas before expansion inexpander220. Enough of the cathode exit gas is purged bypurge valve285 to avoid excessive build-up of argon and nitrogen impurities in the cathode loop. In a practicable example, the product oxygen concentration inconduit218 may be 90% oxygen, with equal amounts of argon and nitrogen impurities. With a small purge flow, oxygen concentrations atcathode inlet261 and atcathode exit270 may be respectively 60% and 50%.
• As discussed above, a PEM fuel cell operating with atmospheric air as oxidant may typically require air compression to at least 3 atmospheres in order to achieve a sufficiently high oxygen partial pressure over the cathode for competitive current density in the fuel cell stack. Oxygen concentration at the cathode inlet would be 21%, and at the cathode exit typically only about 10% oxygen. The present invention can achieve much higher average oxygen concentration over the fuel cell cathode channel, e.g. 55% compared to approximately 15%. Hence, the operating pressure may be reduced to about 1.5 atmospheres while still retaining a substantial enhancement of oxygen partial pressure over the cathode. With higher oxygen partial pressure over the cathode, fuel cell stack power density and efficiency can be enhanced, as is particularly crucial in automotive power plant applications. Mechanical compression power required by the apparatus of the present invention (using high performance adsorbents such as Li—X) will be less than that required for the air compressor of a PEM fuel cell system operating at 3 atmospheres air supply pressure, further enhancing overall power plant efficiency.[0106]
• An important benefit in this example apparatus is that the oxygen enriched gas entering[0107]cathode inlet261 is humidified by blending with the much larger stream of saturated cathode recycle gas. Another benefit is that energy recovery from the PSA unit can be applied to boost pressure and drive recycle circulation in the cathode loop, while fuel cell waste heat can be applied toheat exchangers225 and241 to enhance expansion energy recovery in the PSA unit. Yet another benefit is that suitable cathode channel circulation flow velocities to assure satisfactory water removal from PEM fuel cells are readily achieved.
• While recycle of cathode gas has benefits as discussed above, it will be understood that the invention may also be practiced without any such recycle feature, so that the cathode gas from[0108]cathode exit270 may alternatively be either discharged to atmosphere or else removed to another use such as assisting combustion within a fuel processor.
• Another variation is to operate the oxygen PSA unit to deliver oxygen at a relatively high concentration (e.g. in the range of 60% to 95%, or more preferably 70% to 90% oxygen concentration), while bypassing a fraction of the compressed air feed from[0109]conduit216 past the PSA module to blend directly with product oxygen inconduit70,conduit283 orconduit253. In this approach, the blended bypass air and PSA oxygen product (plus any recycle cathode gas from cathode exit port270) may have a mixed oxygen concentration in the range of e.g. 30% to 50% so that a substantial benefit of partial oxygen enrichment over the fuel cell cathode is provided, while the size and power consumption of the PSA unit is reduced.
• Turning to FIG. 10, an oxygen-separating PSA-based fuel cell system is shown, similar to the fuel cell system in FIG. 9, but with a countercurrent blowdown expander driving a free rotor exhaust vacuum pump. Thus, in FIG. 9 dashed[0110]line290 represents an optional feedair bypass conduit290 with aflow control valve291, communicating between compressedair feed conduit216 andconduit283 in the cathode recycle loop. In the embodiment of FIG. 10,shaft222 coupling thecountercurrent blowdown expander221 to feedcompressor201 has been removed. Instead,vacuum pump301 is used to depress the low pressure of the cycle below atmospheric pressure, drawing nitrogen-enriched waste gas from heavyproduct exhaust compartment61 viaconduit246 andoptional heater302.Pump301 is powered bycountercurrent blowdown expander304 expanding countercurrent blowdown gas from firstcountercurrent blowdown manifold57 viaconduit240 andoptional heater241.Vacuum pump301 andexpander304 are coupled byshaft305, and together constitute a free rotorvacuum pump assembly306. Such a free rotor vacuum pump offers attractive advantages of efficiency and capital cost. Alternatively, a motor could be coupled to an extension ofshaft305.
• The countercurrent blowdown gas from second[0111]countercurrent blowdown manifold59 exits that manifold at a pressure which is substantially atmospheric or slightly greater according to the amount of throttling restriction associated withconduit244.
• FIG. 11[0112]
• FIG. 11 shows a fuel cell-based electrical current generating system, similar to the electrical current generating system of FIG. 9, but without light reflux energy recovery, and with a portion of oxygen enriched gas discharged from the fuel cell cathode being used for a pressurization step. The illustrative four stages of light reflux pressure letdown are achieved irreversibly over[0113]adjustable orifices350,351,352 and353, which respectively connectconduits224 and227,228 and231,232 and235, and236 and239.Orifices350,351,352 and353 are actuated throughlinkage354 by actuator(s)355. Adjustment of the orifices is desirable to enable turndown of the PSA apparatus to operation at reduced cycle frequency and reduced flow rates when the fuel cell power plant is operated at part load.
• The fuel cell has a cathode recycle loop defined (in the loop flow direction) by[0114]water condensate separator360,conduit361 conveying enriched oxygen tocathode channel inlet261,cathode channel262,conduit362 conveying cathode exhaust gas fromcathode channel exit270 tocathode recycle conduit365 includingcathode recycle blower363 to pressurize the cathode recycle gas for admission tocondensate separator360.Separator360 removes fuel cell water exhaust condensate from the cathode recycle loop, while also humidifying the dry concentrated oxygen admitted from the PSA system conduit fromconduit218.
• A portion of the cathode exhaust is removed from the[0115]conduit362 byconduit371, branching fromcathode recycle conduit365. This portion of the cathode exhaust gas is recycled to the feed end of the PSA (or alternatively vacuum-PSA) apparatus, and is conveyed byconduit371 to water condensate separator373 and thence tofirst pressurization manifold48 communicating to thefirst valve face21. A throttle valve373 may be provided inconduit371 to provide a pressure letdown as required from the pressure atcathode exit270 tofirst pressurization manifold48.
• Recycling a portion of the cathode exhaust gas to the PSA unit feed has several advantages, including (1) reducing the volume of feed gas to be compressed, (2) eliminating the requirement to purge any cathode exhaust gas from the cathode loop, and (3) recovering some waste energy from the fuel cell cathode loop by using this gas to help pressurize the fuel cell from the feed end. This oxygen rich gas must be admitted to the feed end of the PSA unit, because it is saturated with water vapor which would deactivate the adsorbent if admitted directly to the second valve face at the product end. By introducing it to the feed end of the beds after the low pressure step and before any direct pressurization with feed air, a favorable concentration profile is established since this gas is richer in oxygen than feed air, but also contains a greater load of impurities than the product oxygen-enriched gas.[0116]
• Because argon is concentrated with oxygen by the PSA unit, argon will be concentrated both within the cathode loop and in the PSA enriched oxygen product in this embodiment. If no cathode purge is provided, argon can only exit the system through the exhaust of the PSA unit. Since the PSA unit typically achieves about 60% recovery of oxygen and argon when ordinary air is used as the only feed for pressurization to the first valve face, about 40% of argon admitted with feed gas may be exhausted in each cycle. The fractional elimination of recycle argon introduced with initial feed pressurization steps will be lower, since the main feed is introduced subsequently to push the recycle argon deeper into the absorbers. Hence, a small amount of purge from the cathode loop may be desirable. Cathode exhaust gas recycle to the PSA unit feed may also be blended directly with feed air introduced at the same or lower pressure as the[0117]cathode channel exit270.
• FIG. 12[0118]
• FIG. 12 shows a fuel-cell based electrical[0119]current generating system400, according to a second embodiment of the present invention, comprising afuel cell402, an oxygen-generatingPSA system404, and a hydrogengas production system406. The fuel cell comprises ananode channel408 including ananode gas inlet410 and ananode gas outlet412, acathode channel414 including acathode gas inlet416 and acathode gas outlet418, and aPEM420 in communication with theanode channel408 and thecathode channel414 for facilitating ion exchange between theanode channel408 and thecathode channel414.
• The oxygen-[0120]PSA system404 extracts oxygen gas from feed air, and comprises arotary module10, and acompressor422 for delivering pressurized feed air to the feed compartments424 of therotary module10. Preferably, the oxygen-PSA system404 includes a vacuum pump426 (or alternatively countercurrent blowdown expander) coupled to thecompressor422 for withdrawing nitrogen-enriched gas as heavy product gas from the blowdown compartments428 of therotary module10. The oxygen-PSA system404 also includes a light productgas function compartment430 coupled to thecathode gas inlet416 for delivering oxygen-enriched gas to thecathode channel414. Cathode recycle may be provided as in the embodiments of FIGS.9-11.
• The hydrogen[0121]gas production system406 comprises a hydrogen-generatingPSA system432, and a fuel processor reactor434 coupled to the hydrogen-PSA system432 for supplying a first hydrogen gas feed to the hydrogen-PSA system432. The hydrogen-PSA system432 comprises arotary module10 including a firstfeed gas compartment436 for receiving a first hydrogen gas feed from the reactor434, apressurization compartment438 for receiving a hydrogen gas feed from theanode gas outlet412, a light product compartment440 for delivering hydrogen gas to theanode gas inlet410, and a blowdown compartment441 for delivering tail gas as heavy product gas to the reactor434. Preferably the hydrogen-PSA system432 includes a vacuum pump442 (or alternately a countercurrent blowdown expander) provided between the blowdown compartment441 and the reactor434 for extracting the tail gas from the blowdown compartment441.
• According to the purity level of the hydrogen gas recycled from the[0122]anode gas exit412,pressurization compartment438 may cooperate with either the first or second valve of the rotary module, the latter being preferred if the purity of this stream is relatively high. The hydrogen-PSA system432 may also include aheavy reflux compressor443 delivering heavy reflux gas to a secondfeed gas compartment444 to improve the fractional recovery of hydrogen gas. The calorific fuel gas requirements of the hydrogengas production system406 will determine the correct recovery of hydrogen gas.
• The reactor[0123]434 comprises a steam reformer445, including a burner446 and catalyst tubes (not shown), and a watergas shift reactor448. The burner446 includes afirst burner inlet450 for receiving the tail gas from theblowdown compartment442, and a second burner inlet452 for receiving air or humid oxygen-enriched gas from thecathode channel414. Thesteam reformer444 is supplied through a fuel inlet454 with a hydrocarbon fuel, such as methane gas, plus water at a feed pressure which is the working pressure of the fuel cell plus an allowance for pressure drops through thesystem406. The fuel is preheated and steam is generated by heat exchanger455, recovering heat from the flue gas of burner446. The methane fuel gas and steam mixture is them passed through the catalyst tubes, while the tail gas and the oxygen-enriched gas are burned in the burner446 to elevate the temperature of the methane fuel gas mixture to the temperature necessary (typically 800° C.) for conducting endothermic steam reforming reactions of the methane fuel gas mixture:
• CH₄+H₂O−CO+3H₂
• CH₄+2H₂O−CO₂+4H₂
• The resulting syngas (approximately 70% H[0124]₂, with equal amounts of CO and CO₂as major impurities, and unreacted CH₄and N₂as minor impurities) is cooled to about 250° C., and then passed to the watergas shift reactor448 for reacting most of the CO with steam to produce more H₂and CO₂:
• CO+H₂O−CO₂+H₂
• The resulting gas reactants are then conveyed to the[0125]first feed compartment436 of the hydrogen-PSA system432 for hydrogen purification, with the heavy product tail gas being returned to the steam reformer434 form theblowdown compartment442 for combustion in the burner446.
• In one variation, the reactor[0126]434 comprises a partial oxidation reactor, and instead of the methane gas mixture being steam reformed, the methane gas mixture is reacted in the partial oxidation reactor with a portion of the humid oxygen-enriched gas received from thecathode channel414, through an optional conduit456, for partial oxidation of the methane gas:
• CH₄+½O₂−CO+2H₂
• The resulting syngas is again cooled to about 250° C., and then passed to the water[0127]gas shift reactor448 for reacting most of the CO with steam to produce more H₂and CO₂:
• CO+H₂O−CO₂+H₂
• The resulting gas reactants are then conveyed to the[0128]first feed compartment436 of the hydrogen-PSA system432 for hydrogen purification, with the heavy product tail gas being purged from the hydrogen-PSA system432.
• In another variation, the reactor[0129]434 comprises as autothermal reformer and a watergas shift reactor448, and instead of the methane gas mixture being endothermically steam reformed or exothermically partially oxidized, the methane gas mixture is reacted in the autothermal reformer by a thermally balanced combination of those reactions, followed by reaction in the watergas shift reactor448. Since the hydrogen-PSA heavy product tail gas will always have some fuel value even in the limit of very high heavy reflux, a burner446 would be provided for efficiently preheating air and or fuel feeds to any autothermal reactor. Unless the fuel processing reactions include an endothermic reforming component as an energy-efficient sink for tail gas fuel combustion, another economic use (as in embodiment of FIG. 13) should be provided if the net fuel processing reactions are highly endothermic, as in the case of simple partial oxidation.
• Oxygen enrichment autothermal or partial oxidation fuel processor advantageously reduces the heating load of reactants entering the reaction chamber, and also reduces the cooling load of hydrogen-rich product gases delivered to the hydrogen PSA unit and the fuel cell anode, owing to the depletion of nitrogen from those gas streams. A further advantage of the present invention is the ability of the hydrogen PSA unit to remove ammonia in addition to carbon monoxide and hydrogen sulfide, which contaminants are all extremely detrimental to PEM fuel cell performance and life expectancy. Ammonia may be formed in fuel processors where hydrocarbons are catalytically reformed to hydrogen in the presence of any atmospheric nitrogen. The oxygen PSA reduces this problem by front-end removal of nitrogen, while the hydrogen PSA removes any residual traces of ammonia.[0130]
• FIG. 13[0131]
• It would be apparent that a deficiency of the electrical[0132]current generating system400 relates to the necessity of driving thecompressor422 and thevacuum pumps426,444 with a portion of the electrical power generated by the fuel cell. FIG. 13 shows a fuel cell based electricalcurrent generating system500, which addresses this deficiency.
• The electrical[0133]current generating system500 is substantially similar to the electricalcurrent generating system400, comprising thefuel cell402, an oxygen-generatingPSA system504, and a hydrogen gas production system506. The oxygen-PSA system504 extracts oxygen gas from feed air, and comprises arotary module10, acompressor522 for delivering pressurized feed air to the feed compartments524 of therotary module10, acombustion expander523 coupled to thecompressor522, a starter motor (not shown) coupled to thecompressor522, and a light productgas function compartment530 coupled to thecathode gas inlet416 for delivering oxygen-enriched gas to thecathode channel414. The oxygen-PSA system504 may also have a countercurrent blowdown or heavy product exhaust compartment531 cooperating with a vacuum pump and/or expander, as illustrated in previous embodiments.
• The hydrogen gas production system[0134]506 comprises a hydrogen-generatingPSA system532, and areactor534 coupled to the hydrogen-PSA system for supplying a first hydrogen gas feed to the hydrogen-PSA system532. The hydrogen-PSA system532 comprises arotary module10 including a firstfeed gas compartment536 for receiving a first hydrogen gas feed from thesteam reformer534, a pressurization compartment538 (communicating with either the first or second valve) for receiving a second hydrogen gas feed from theanode gas outlet412, a light product compartment540 for delivering hydrogen gas to theanode gas inlet410, and a blowdown compartment541 for delivering tail gas as heavy product fuel gas to thereactor534. As in previous embodiments, the blowdown compartment541 may cooperate with an exhaust vacuum pump and/or expander (not shown) for extracting the tail gas from the blowdown compartment541.
• The[0135]reactor534 comprises anautothermal reformer544, a burner546, and a watergas shift reactor548. The burner546 includes heater tubes549, a first burner inlet550 for receiving the tail gas from theblowdown compartment542, and asecond burner inlet552 for receiving compressed air from thecompressor522 second stage. As will be apparent from FIG. 13, thecompressor522 second stage compresses a portion of the feed air which is not delivered to the oxygen-generatingPSA system504.
• The[0136]expander523 and thecompressor522 together comprise a gas turbine, and expands combustion product gas emanating from the burner546 so as to increase the pressure of feed air to the feed compartments524. As will be appreciated, the thermal energy of combustion of the hydrogen-PSA tail gas is used to drive the fuel cell accessory gas purification and compression machinery. As shown in FIG. 13, additional feed gas compression energy may be obtained from the exothermic heat of reaction of the watergas shift reactor548 through preheat exchangers555.
• The[0137]autothermal reformer544 is supplied through afuel inlet554 with a hydrocarbon fuel gas, such as methane gas and, in the example shown, is reacted with oxygen-enriched gas received under pressure from thecathode channel414 throughbooster blower556. Cathode recycle may not be justified, or at least may be reduced, if the oxygen-enriched gas delivered from the cathode exit can be used advantageously for fuel processing (to reduce nitrogen load and enhance combustion). The resultant syngas is then cooled, and then passed to the watergas shift reactor548 for reacting most of the CO with steam to produce more H₂and CO₂. The resulting gas reactants are then conveyed to thefirst feed compartment536 of the hydrogen-PSA system532 for hydrogen purification.
• In some embodiments, at least a portion of the cathode exhaust gas (which is still enriched in oxygen relative to ambient air, and carries fuel cell exhaust water and fuel cell waste heat) is returned to the inlet of an autothermal or partial oxidation fuel processor (or reformer) for reacting a hydrocarbon fuel with oxygen and steam in order to generate raw hydrogen or syngas. The oxygen reacts autothermally with a portion of the fuel to produce carbon monoxide and heat which furthers reaction of remaining fuel with steam to generate hydrogen. Excess steam helps prevent any coking in the reformer or fuel processor, and subsequently reacts at lower temperature with the carbon monoxide in a water gas shift reactor to generate more hydrogen mixed with waste carbon dioxide. Residual carbon monoxide, the carbon dioxide and any other impurities can then be removed by a hydrogen PSA unit according to the invention.[0138]
• Delivery of still enriched oxygen gas from the fuel cell cathode to the inlet of the fuel processor (1) reduces the inert load of nitrogen entrained with atmospheric air as oxidant, (2) enhances circulation velocities within the fuel cell cathode channel for effective water removal, (3) directly recovers exhaust water from the cathode for the fuel processor in direct accordance with water demand for fuel processing, (4) delivers that water largely in vapor form to avoid costly condensation and revaporization steps, (5) delivers some fuel cell waste heat usefully to the fuel processor inlet, and (3) enhances overall system efficiency through desirable thermal integration.[0139]
• FIG. 14[0140]
• Embodiment[0141]600 illustrates further aspects of the invention. For alkaline fuel cells, the crucial problem is removal of CO₂from both feed oxidant and hydrogen streams. The oxygen-PSA and hydrogen-PSA systems of this invention as described above will remove CO₂very effectively, since CO₂is much more strongly adsorbed than other permanent gas impurities. Oxygen enrichment is beneficial for all types of fuel cells in increasing voltage efficiency, although not usually justified except at high current densities. Alkaline fuel cells can use an under-sized oxygen-PSA for very effective carbon dioxide removal along with modest oxygen enrichment, or may use the same PSA device with an adsorbent lacking nitrogen/oxygen selectivity (e.g. activated carbon, or high silica zeolites) for carbon dioxide clean-up without oxygen enrichment. The rotary PSA module and compression machinery of this invention for entirely suitable for this role.
• Alkaline fuel cells operating on ambient air feed typically operate near atmospheric pressure, at about 70° C. Under such conditions, the water vapor saturated cathode exhaust stream of nitrogen-enriched air serves to remove fuel cell product water while maintaining electrolyte water balance. Operation of alkaline fuel cells at higher temperature may be desirable for high efficiency with less costly electrocatalyst materials, or else for thermal integration to a methanol reformer using fuel cell waste heat to vaporize reactants and even drive the endothermic reaction. But with increasing stack exhaust temperatures, operation with ambient air composition may rapidly become impracticable. At higher temperatures, the nitrogen rich cathode exhaust simply carries too much water vapor out of the system, unless the total pressure is uneconomically raised or else a condenser for water recovery is included.[0142]
• With oxygen enrichment, the volume of the cathode exhaust can be adjusted to achieve water balance for any alkaline fuel cell. Reasonable low stack working pressures become practicable, e.g. about 3 atmospheres for a cathode exit temperature of 120° C. If oxygen enrichment is carried out to the full capability of oxygen-PSA, e.g. approaching 95% oxygen purity, the cathode exhaust stream becomes dry steam with a modest concentration of permanent gases. This steam product may be useful for diverse applications, including fuel processing of hydrocarbon feedstocks to generate hydrogen.[0143]
• Embodiment[0144]600 shows an oxygen-PSA (also performing CO₂removal) as shown in FIG. 12. The hydrogen side of the system is simplified in this example to show only the anode gas inlet of pure hydrogen. Oxygen at more than 90% purity is supplied to thecathode gas inlet416, while concentrated water vapor is delivered fromcathode gas exit418 and conveyed directly tosteam expander610.Expander610 discharges to vacuumcondenser612, from which liquid condensate is removed bypump614, while the permanent gas overheads are withdrawn through conduit byvacuum pump426 of the oxygen-PSA.Expander610 may assistmotor616 to drive the compression machinery of the oxygen-PSA, thus improving overall efficiency of the fuel power plant by approximately 2 to 3%.
• A final aspect of the invention (for any type of fuel cell) is the optional provision of light product gas accumulators for the PSA units, and particularly for the oxygen-PSA as illustrated in FIG. 14.[0145]Oxygen product accumulator660 includes anoxygen storage vessel661 charged from thelight product compartment430 throughnon-return valve662, at substantially the upper pressure of the PSA process or optionally at an elevated pressure generated by a smallaccumulator charging compressor663. A peakingoxygen delivery valve665 and abackflush valve666 are provided on either side ofnon-return valve667 so as to enable oxygen delivery from the storage vessel respectively forward to the fuel cell cathode inlet or backward to the oxygen-PSA unit.
• The oxygen storage vessel is charged during normal operation, particularly during intervals of stand-by or idling when the oxygen-PSA attains highest oxygen purity. The optional charging compressor may be operated when the plant is idling, or (in vehicle applications) as an energy load application of regenerative braking. The peaking[0146]oxygen delivery valve665 is opened during intervals of peak power demand, so as to increase supply of concentrated oxygen to the cathode when most needed. If the oxygen accumulator is large enough, the oxygen-PSA compressor422 andvacuum pump426 could be idled during brief intervals of peak power demand, so as to release the power normally consumed by internal accessories to meet external demand. Then, the size of the fuel cell stack (in a power plant required to meet occasional specified peak power levels) can be reduced for important cost savings.
• When the fuel cell power plant is shut down, the oxygen-[0147]PSA compressor422 is stopped first to drop the internal pressure for an initial blowdown of all absorbers. Then,backflush valve666 is opened to release a purging flow of oxygen to displace adsorbed nitrogen and some adsorbed water vapor from the absorbers over a short time interval. The absorbers are then left precharged with dry oxygen at atmospheric pressure, thus enabling fast response of the oxygen-PSA for the next plant start-up.
• FIGS.[0148]15-21
• FIG. 15 shows a[0149]rotary PSA module701 configured for axial flow and particularly suitable for smaller scale oxygen generation and hydrogen purification.Module701 includes a number “N” ofadsorbers703 inadsorber housing body704. Each adsorber has afirst end705 and asecond end706, with a flow path therebetween contacting a nitrogen-selective adsorbent. The adsorbers are deployed in an axisymmetric array aboutaxis707 of the adsorber housing body. Thehousing body704 is in relative rotary motion aboutaxis707 with first and secondfunctional bodies708 and709, being engaged across afirst valve face710 with the firstfunctional body708 to which feed air is supplied and from which nitrogen-enriched air is withdrawn as the heavy product, and across asecond valve face711 with the secondfunctional body709 from which oxygen-enriched air is withdrawn as the light product.
• In preferred embodiments as particularly depicted in FIGS.[0150]15-21, theadsorber housing704 rotates and shall henceforth be referred to as theadsorber rotor704, while the first and second functional bodies are stationary and together constitute astator assembly712 of the module. The first functional body shall henceforth be referred to as thefirst valve stator708, and the second functional body shall henceforth be referred to as thesecond valve stator709.
• In the embodiment shown in FIGS.[0151]15-21, the flow path through the adsorbers is parallel toaxis707, so that the flow direction is axial, while the first and second valve faces are shown as flat annular discs normal toaxis707. 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 onaxis707. The steps of the process 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.[0152]15-21 are cross-sections ofmodule701 in the planes defined by arrows712-713,714-715, and716-717.Arrow720 in each section shows the direction of rotation of therotor704.
• FIG. 16 shows section[0153]712-713 across FIG. 15, which crosses the adsorber rotor. Here, “N”=72. Theadsorbers703 are mounted betweenouter wall721 andinner wall722 ofadsorber rotor704. Each adsorber comprises a rectangularflat pack703 ofadsorbent sheets723, withspacers724 between the sheets to define flow channels here in the axial direction.Separators725 are provided between the adsorbers to fill void space and prevent leakage between the adsorbers. Other preferred configurations for the adsorber module may be provided by forming the adsorbent sheets and intervening spacers in trapezoidal packs or spiral rolls constituting each adsorber.
• Alternatively the entire adsorber rotor may be formed as a spiral roll of one adsorbent sheet or a plurality of adsorbent sheets, the spiral roll formed concentric with the[0154]axis707 and with spacers between adjacent layers of the spirally rolled adsorbent sheet(s), and with at least some of the spacers at narrow angular intervals extending along the entire length between the first and second ends as barriers to transverse flow, so as to partition the spiral roll into many channels, each of which serves as a distinct adsorber. The first and second ends of the spiral roll would directly coincide with the first and second valve faces respectively.
• 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 air separation to produce enriched oxygen, typical adsorbents are X, A or chabazite type zeolites, typically exchanged with lithium, calcium, strontium, magnesium and/or other cations, and with optimized silicon/aluminium 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.[0155]
• 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 aluminium foils. 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.[0156]
• 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 at PSA cycle frequencies in the range of 30 to 150 cycles per minute.[0157]
• FIG. 17 shows the porting of[0158]rotor704 in the first and second valve faces respectively in the planes defined by arrows714-715, and716-717. Anadsorber port730 provides fluid communication directly from the first or second end of each adsorber to respectively the first or second valve face.
• FIG. 18 shows the first[0159]stator valve face800 of thefirst stator708 in thefirst valve face710, in the plane defined by arrows714-715. Fluid connections are shown to afeed compressor801 inducting feed air frominlet filter802, and to anexhauster803 delivering nitrogen-enriched second product to a secondproduct delivery conduit804.Compressor801 andexhauster803 are shown coupled to adrive motor805.
• [0160]Arrow720 indicates the direction of rotation by the adsorber rotor. In the annular valve face betweencircumferential seals805 and806, the open area of firststator valve face800 ported to the feed and exhaust compartments is indicated by clear angular segments811-816 corresponding to the first functional ports communicating directly to functional compartments identified by the same reference numerals811-816. The substantially closed area ofvalve face800 between functional compartments is indicated by hatchedsectors818 and819 which are slippers with zero clearance, or preferably a narrow clearance to reduce friction and wear without excessive leakage. Typicalclosed sector818 provides a transition for an adsorber, between being open tocompartment814 and open tocompartment815. 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.819) 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 air to feed pressurization compartments[0161]811 and812, and to feedproduction compartment813.Compartments811 and812 have successively increasing working pressures, whilecompartment813 is at the higher working pressure of the PSA cycle.Compressor801 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 ofcompartments811 and812, and then the final pressurization and production throughcompartment813. 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 acompartment811 to813. Alternatively,compressor801 may deliver all the feed air to the higher pressure, with throttling of some of that air to supply feed pressurization compartments811 and812 at their respective intermediate pressures.
• Similar,[0162]exhauster803 exhausts nitrogen-enriched heavy product gas from countercurrent blowdown compartments814 and815 at the successively decreasing working pressures of those compartments, and finally fromexhaust compartment816 which is at the lower pressure of the cycle. Similarly tocompressor801,exhauster803 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. 18, the lower pressure is ambient pressure, so[0163]exhaust compartment816 exhaust directly to heavyproduct delivery conduit804.Exhauster803 thus provides pressure letdown with energy recovery to assistmotor805 from the countercurrent blowdown compartments814 and815. For simplicity,exhauster803 may be replaced by throttling orifices as countercurrent blowdown pressure letdown means fromcompartments814 and815.
• In some preferred embodiments, the lower pressure of the PSA cycle is subatmospheric.[0164]Exhauster803 is then provided as a vacuum pump, as shown in FIG. 19. 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. 19, the early countercurrent blowdown stream fromcompartment814 is released at ambient pressure directly to heavyproduct delivery conduit804. If for simplicity a single stage vacuum pump were used, the countercurrent blowdown stream fromcompartment815 would be throttled down to the lower pressure over an orifice to join the stream fromcompartment816 at the inlet of the vacuum pump.
• FIGS. 20 and 21 shows the second stator valve face, at section[0165]716-717 of FIG. 15. Open ports of the valve face are second valve function ports communicating directly to a lightproduct delivery compartment821; a number of light reflux exit compartments822,823,824 and825; and the same number of light reflux return compartments826,827,828 and829 within the second stator. The second valve function ports are in the annular ring defined bycircumferential seals831 and832. 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[0166]light reflux expander840 is shown in FIGS. 15 and 20 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 returncompartments822 and829,823 and828,824 and827, and825 and826 as illustrated. Thelight reflux expander840 may power a lightproduct booster compressor845 bydrive shaft846, which delivers the oxygen enriched light product tooxygen delivery conduit847 and compressed to a delivery pressure above the higher pressure of the PSA cycle. Illustrating the option of light reflux pressure letdown with energy recovery, a split streamlight reflux expander840 is provided to provide pressure let-down of four light reflux stages with energy recovery. The light reflux expander serves as pressure let-down means for each of four light reflux stages, respectively between light reflux exit and returncompartments822 and829,823 and828,824 and827, and825 and826 as illustrated.
• [0167]Light reflux expander840 is coupled to a light productpressure booster compressor845 bydrive shaft846.Compressor845 receives the light product fromconduit725, and delivers light product (compressed to a delivery pressure above the higher pressure of the PSA cycle) todelivery conduit250. Since the light reflux and light product are both enriched oxygen streams of approximately the same purity,expander840 andlight product compressor845 may be hermetically enclosed in a single housing which may conveniently be integrated with the second stator as shown in FIG. 15. This configuration of a “turbocompressor” oxygen booster without a separate drive motor is advantageous, as a useful pressure boost of the product oxygen 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. 21 shows the simpler alternative of using a[0168]throttle orifice850 as the pressure letdown means for each of the light reflux stages.
• Turning back to FIG. 15, compressed feed air is supplied to[0169]compartment813 as indicated byarrow825, while nitrogen enriched heavy product is exhausted from compartment817 as indicated byarrow826. The rotor is supported by bearing860 withshaft seal861 onrotor drive shaft862 in thefirst stator708, which is integrally assembled with the first and second valve stators. The adsorber rotor is driven bymotor863 as rotor drive means.
• As leakage across outer[0170]circumferential seal831 on thesecond valve face711 may compromise enriched oxygen purity, and more importantly may allow ingress of atmospheric humidity into the second ends of the adsorbers which could deactivate the nitrogen-selective adsorbent, abuffer seal870 is provided to provide more positive sealing of abuffer chamber871 betweenseals831 and871. A desiccant873 (e.g. alumina, silica gel, sodium hydroxide, potassium hydroxide, magnesium perchlorate, barium oxide, phosphorous pentoxide, calcium chloride, calcium sulfate, calcium oxide, or magnesium oxide) may be included inbuffer chamber871 to provide more moisture protection. 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 exhauster803), buffer chamber is filled with dry enriched oxygen product at a buffer pressure positively above ambient pressure. Hence, minor leakage of dry oxygen outward may take place, but humid air may not leak into the buffer chamber. In order to further minimize leakage and to reduce seal frictional torque,buffer seal871 seals on a sealingface872 at a much smaller diameter than the diameter ofcircumferential seal831.Buffer seal870 seals between arotor extension875 ofadsorber rotor704 and the sealingface872 on thesecond valve stator709, withrotor extension875 enveloping the rear portion ofsecond valve stator709 to formbuffer chamber871. Astator housing member880 is provided as structural connection between first valve stator8 andsecond valve stator709.
• FIG. 22[0171]
• FIG. 22 shows an axial section of a rotary PSA embodiment, in which the adsorbers and[0172]adsorber body housing904 are stationary, while the first and second distributor valve bodies arefirst valve rotor908 andsecond valve rotor909 respectively engaged withfirst valve face910 andsecond valve face911. Thefirst valve rotor908 is driven bymotor1063 throughshaft1062, while the second valve rotor9 is driven by connectingshaft1110. As thearrows720 in FIGS.16-21 indicate the direction of relative motion of theadsorber housing904 relative to the distributor valve bodies, the rotation ofvalve rotors908 and909 for the stationary adsorber embodiment of FIG. 22 may be in the opposite direction toarrows720 when referred to thestationary housing904. Thefirst valve rotor908 is installed withinfirst valve housing1112, andsecond valve rotor909 is installed withinsecond valve housing214.Housings1112 and1114 are assembled withadsorber housing904 to form a complete pressure containment enclosure for thePSA module901.
• Alternative valve configurations and pressure balancing devices for the first and second rotating distributor valves for embodiments with stationary adsorbers are disclosed by Keefer et al in U.S. Pat. No. 6,063,161, the disclosure of which is incorporated herein.[0173]
• For all externally connected flow functions where feed is provided to the first valve body, exhaust is withdrawn from the first valve body, or gas is withdrawn from or returned to the second valve body, fluid transfer chambers are provided for each such flow function, in order to establish fluid communication from the housing to the rotor for that function. Each fluid transfer chamber is an annular cavity in the valve rotor or its housing, providing fluid communication at all angular positions of the rotor between a functional compartment in the rotor and the corresponding external conduit connected to the housing. Rotary seals must be provided for each such fluid transfer chamber to prevent leakage. A particular advantage of rotary adsorber embodiments is the elimination of such fluid transfer chambers and associated seals, since in those embodiments the first and second valve bodies are stationary and may be connected directly to external flow functions.[0174]
• A[0175]feed transfer chamber1120 communicates betweenfeed conduit1081 andfunctional compartment1013 in thefirst valve rotor908, and anexhaust transfer chamber1122 communicates to exhaustconduit1082 and functional compartment1016 in thefirst valve rotor908.Transfer chambers1120 and1122 are separated byrotary seal1124. This configuration is suitable for a single stage feed compressor1001 and a single stage exhauster1003. Narrow clearance gaps infirst valve face910 may extend fromcompartment1013 over the annular sectors corresponding tocompartments812 and811 in FIGS. 18 and 19, and from compartment1016 over the annular sectors corresponding tocompartments815 and814 in FIGS. 18 and 19, so as to provide throttling for gentle pressurization and depressurization of each adsorber being opened tocompartments1013 or1016. While additional transfer chambers could be provided for additional compression or exhaust stages, it will be appreciated that this complication could be avoided with rotary adsorber embodiments.
• A[0176]product transfer chamber1126 communicates betweenproduct delivery conduit1047 andfunctional compartment1021 in thesecond valve rotor909.Rotary seal1127 is provided fortransfer chamber1126. Additional transfer chambers could be provided in pairs for exit and return of each light reflux stage. This may be necessary if alight reflux expander840 is provided as shown in FIG. 20. However, it will be much simpler in the stationary adsorber embodiments to avoid fluid transfer between thesecond valve rotor909 and itshousing1114 for light reflux stages. For stationary adsorber embodiments, it will thus be preferred to usethrottle orifices850 for pressure let-down of each light reflux stage as shown in FIG. 21, with theseorifices850 installed withinrotor909 so that no fluid transfer betweenrotor909 andhousing1114 is required for light reflux. Onesuch orifice1050 is shown in FIG. 22, with the connection to lightreflux exit compartment1025 being out of the sectional plane and accordingly not shown in the view of FIG. 22.
• Axial Flow Oxygen Enrichment PSA Unit[0177]
• The specific productivity and the yield of an axial flow rotary oxygen enrichment PSA module similar to[0178]module701 of FIG. 15 was measured with air feed at 30° C. The module had a total volume of 18 L (not including the compressor, the vacuum pump, and the rotor drive motor), and a contained adsorber volume of 8.2 L. Specific productivity is defined as normal liters of contained product oxygen delivered per hour per liter of adsorber volume, and yield is defined as fractional recovery of oxygen contained in the product from oxygen contained in the feed air.
• Operating in a vacuum-PSA mode at a cycle frequency of 100 cycles/minute with feed air compressed to 1.5 bars absolute and vacuum exhaust at 0.5 bars absolute, oxygen product at 70% purity was obtained with specific productivity of 1500 NL/L-hour, at a yield of 47%.[0179]
• Operating in a vacuum-PSA mode at a cycle frequency of 100 cycles/minute with feed air compressed to 1.5 bars absolute and vacuum exhaust at 0.5 bars absolute, oxygen product at 70% was obtained with specific productivity improved to 1650 NL/L-hour and yield improved to 50.5%.[0180]
• Operating in a vacuum-PSA mode at a cycle frequency of 100 cycles/minute with feed air compressed to 1.7 bars absolute and vacuum exhaust at 0.32 bars absolute, oxygen product at 80% purity was obtained with a specific productivity of 2500 NL/L-hour, at a yield of 56%.[0181]
• Operating in a positive pressure PSA mode at a cycle frequency of 100 cycles/minute with feed air compressed to 3 bars absolute and exhaust at atmospheric pressure, oxygen product at 80% purity was obtained with a specific productivity of 1320 NL/L-hour, at a yield of 25.5%.[0182]
• Operating in a positive pressure PSA mode at a cycle frequency of 100 cycles/minute with feed air compressed to 3 bars absolute and exhaust at atmospheric pressure, oxygen product at 70% purity was obtained with a specific productivity of 1750 NL/L-hour, at a yield of 33%.[0183]
• For comparison, conventional oxygen industrial vacuum-PSA and PSA systems using granular adsorbent operate at cycle frequencies of only about 1 cycle/minute, achieving specific productivities of about 30 NL/L-hour. Specific productivities in the range of 130 to 170 NL/L-hour have been reported for a prior art rotary PSA device (See Vigor et al., U.S. Pat. No. 5,658,370). Keller et al. (U.S. Pat. No. 4,354,859) achieved specific productivities for oxygen in the range of 210 to 270 NL/L-hour by operating at cycle frequencies of 45 to 50 cycles/minute with a granular adsorber of 40 to 80 mesh zeolite.[0184]
• The high specific productivities realized with the axial flow rotary PSA system of the disclosure are thus substantially better than prior art systems, by one to two full orders of magnitude. High productivities from the disclosed compact PSA units are decisively important for automobile fuel cell applications, as prior art devices are far too bulky and heavy to be considered for such applications.[0185]
• It is to be noted that the extreme compactness of the PSA devices provided for fuel cell systems by the present invention is enabled by operating at high cycle frequency (greater than 50 cycles per minute, and more preferably greater than 100 cycles per minute) with the dual enabling aspects of the rotary PSA mechanism and the adsorbent modules formed of thin adsorbent sheets with narrow channels spaced therebetween. At a specific productivity of 1500 NL/L-hour, the above mentioned PSA unit delivered about 200 NL/min of contained oxygen from a module volume of only 16 L. This oxygen flow rate would suffice for a 40 kW fuel cell, or for an even larger fuel cell if additional oxygen is provided by blending bypass air with the PSA product oxygen.[0186]
• Hydrogen Purification PSA[0187]
• Hydrogen purification PSA is particularly advantageous because it successfully removes impurities to a level that are not incompatible with PEM fuel cells. Such impurities include many compounds that are inherent to fossil fuel reforming or hydrogen recovery from chemical plant off-gases. For example, several particularly problematic compounds, such as ammonia, carbon monoxide, hydrogen sulfide, methanol vapor, and chlorine are removed effectively by PSA.[0188]
• Methanol is a preferred feedstock for PEM fuel cells in stationary and automotive applications. Steam reforming of methanol generates raw hydrogen or syngas containing hydrogen, carbon dioxide and significant levels of carbon monoxide and unreacted methanol. The hydrogen PSA of the present disclosure has been tested on synthetic methanol syngas containing approximately 1% carbon monoxide, methanol vapor, and water vapor along with carbon dioxide as the bulk impurity. Each of the impurities was reduced below a concentration of 50 ppm.[0189]
• Autothermal reforming, steam reforming or partial oxidation of other hydrocarbons (e.g. natural gas, gasoline, or diesel fuel) invariably produces raw hydrogen or syngas containing carbon dioxide and frequently nitrogen as bulk impurities, with carbon monoxide and frequently hydrogen sulfide as potentially harmful contaminants. All of these impurities can be adequately removed by the hydrogen PSA unit of the present disclosure.[0190]
• The ability of the hydrogen PSA unit to remove severely harmful components such as carbon monoxide, methanol vapor and hydrogen sulfide is most important for PEM fuel cells, to extend their operating life, improve their reliability, and potentially also reduce their cost by enabling lower noble metal catalyst loadings on the fuel cell electrodes.[0191]
• The ability of the hydrogen PSA unit to remove bulk impurities such as carbon dioxide and nitrogen allows the fuel cell to operate with a high partial pressure of hydrogen and with a much smaller cathode purge flow, thus improving electrochemical energy conversion efficiency while also improving hydrogen utilization within the fuel cell anode channel.[0192]
• The foregoing description is intended to be illustrative of the preferred embodiments of the present invention. Those of ordinary skill may envisage certain additions, deletions and/or modifications to the described embodiments which, although not specifically described or referred to herein, do not depart from the spirit or scope of the present invention as defined by the appended claims.[0193]

Claims (77)

We claim:

1. An electrical current generating system comprising:

a fuel cell including an anode channel including an anode gas inlet for receiving a supply of hydrogen gas, a cathode channel including a cathode gas inlet and a cathode gas outlet, and an electrolyte in communication with the anode and cathode channel for facilitating ion exchange between the anode and cathode channel;

an oxygen gas delivery system coupled to the cathode gas inlet for delivering a gaseous stream enriched in oxygen gas to the cathode channel, the oxygen gas delivery system including a rotary pressure swing adsorption system for enriching oxygen in a gaseous feed.

2. The system according toclaim 1 and further comprising a first gas recirculation means coupled to the cathode gas outlet for recirculating a first portion of cathode exhaust gas exhausted from the cathode channel to the cathode gas inlet.

3. The current generating system according toclaim 1 where the pressure swing adsorption system includes a first feed gas inlet for receiving air feed as a first gas feed, and a gas outlet coupled to the cathode gas inlet.

4. The current generating system according toclaim 3 where the oxygen gas delivery system includes a gas inlet for receiving a first portion of cathode gas exhausted from the cathode channel and a gas outlet for delivering the gaseous stream enrich in oxygen gas to the cathode channel.

5. The current generating system according toclaim 4 where the oxygen gas delivery system includes a first gas recirculation means coupled to the cathode gas outlet for recirculating the first portion of cathode gas from the cathode channel to the cathode gas inlet.

6. The current generating system according toclaim 5, wherein the first gas recirculating means comprises a compressor for supplying the first cathode exhaust gas portion under pressure to the cathode gas inlet.

7. The current generating system according toclaim 6, wherein the first gas recirculation means includes a condensate separator coupled between the cathode gas outlet and the compressor for removing moisture from the first cathode exhaust gas portion.

8. The current generating system according toclaim 6, wherein the FIRST gas recirculating means directs the first cathode exhaust gas portion as feed gas to the gas separation system.

9. The current generating system according toclaim 4, wherein the gas separation system includes a second feed gas inlet, and the current generating system includes second gas recirculating means coupled to the cathode gas outlet for recirculating a second portion of the cathode exhaust gas to the second feed gas inlet.

10. The current generating system according toclaim 9 wherein the recirculating means comprises a restrictive orifice for delivering the second cathode exhaust gas portion to the gas separation system at a pressure less than a pressure of the air feed.

11. The current generating system according toclaim 9 where the pressure swing adsorption system comprises a rotary module including a stator and a rotor rotatable relative to the stator, the rotor including a plurality 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, and 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 component.

12. The current generating system according toclaim 11 wherein the plurality of flow paths are aligned with the axis of the rotor.

13. The current generating system according toclaim 11, wherein the stator includes a first stator valve surface, a second stator valve surface, a plurality of first function compartments opening into the first stator valve surface, and a plurality of second function compartments opening into the second stator valve surface, and the rotor includes a first rotor valve surface in communication with the second stator valve surface, and a plurality of apertures provided in the rotor valve surfaces and in communication with respective ends of the flow paths and the function compartments.

14. The current generating system according toclaim 13 where the compression machinery is coupled to a portion of the function compartments for maintaining the portion of function compartments at a plurality of discrete respective pressure levels between an upper pressure and a lower pressure for maintaining uniform gas flow through the portion of function compartments.

15. The current generating system according toclaim 13, wherein the function compartments include a light reflux exit compartment and a light reflux return compartment, the compression machinery comprises a light reflux expander coupled between the light reflux exit and return compartments, and the first gas recirculation means comprises a compressor coupled to the light reflux expander for supplying the first cathode exhaust gas portion under pressure to the cathode gas inlet

16. The current generating system according toclaim 15 wherein the rotary pressure swing adsorption system includes a heater disposed between the light reflux exit compartment and the light reflux expander for enhancing recovery of energy from light reflux gas exhausted from the light reflux exit compartment.

17. The current generating system according toclaim 15 where the function compartments include a gas feed compartment and a countercurrent blowdown compartment, and the compression machinery comprises a compressor coupled to the first feed gas inlet for delivering compressed air to the gas feed compartment, and an expander coupled to the compressor for exhausting heavy product gas enriched in the first gas component from the countercurrent blowdown compartment.

18. The current generating system according toclaim 15 where the function compartments include a countercurrent blowdown compartment and a heavy product compartment, and the compression machinery comprises an expander coupled to the countercurrent blowdown compartment.

19. The current generating system according toclaim 18 further comprising a vacuum pump coupled to the expander for extracting heavy product gas enriched in the first gas component at subatmospheric pressure from the heavy product compartment.

20. The current generating system according toclaim 13 where the function compartments include a gas feed compartment, and the gas recirculating means directs the first cathode exhaust gas portion as feed gas to the gas feed compartment.

21. The current generating system according toclaim 13 where the function compartments include a gas feed compartment, and the current generating system includes second gas recirculating means coupled to the cathode gas outlet for recirculating a second portion of the cathode exhaust gas to the gas feed compartment.

22. The current generating system according toclaim 21 where the second gas recirculating means comprises a restrictor orifice.

23. The current generating system according toclaim 11 where the adsorbent material is one of CA-X, Li—X, lithium chabazite zeolite, calcium-exchanged chabazite and strontium-exchanged chabazite.

24. The current generation system according toclaim 1 where the rotary pressure swing adsorption system enriches oxygen and removes carbon dioxide from an air feed.

25. The current generation system according toclaim 24 further comprising a hydrogen gas delivery system coupled to the anode gas inlet.

26. The current generation system according toclaim 25 where the hydrogen gas delivery system enriches hydrogen gas in a gaseous feed.

27. The current generation system according toclaim 26 further comprising an oxygen accumulator interposed between the oxygen gas delivery system and the cathode gas inlet

28. The current generation system according toclaim 27 in which the electrolyte is alkaline and is maintained at a working temperature greater that about 100° C., the oxygen gas delivery system is operated to supply oxygen of about 90% purity to the cathode gas inlet, so that the product water of the fuel cell is delivered as concentrated dry steam from the cathode gas outlet; the system including a steam expander to expand the steam from the cathode gas outlet to a vacuum condenser, a condensate pump to exhaust liquid from the condenser, and a vacuum pump cooperating with the oxygen pressure swing adsorption system and exhausting permanent gas overheads from the vacuum condenser.

29. Electrical current generated by the electrical current generating system according toclaim 1.

30. The current generating system according toclaim 8 where the rotary pressure swing adsorption system comprises a rotary module for implementing a pressure swing adsorption process having an operating pressure cycling between an upper pressure and a lower pressure, for extracting a first gas fraction and a second gas fraction from a gas mixture including the first and second fractions, the rotary module comprising:

a stator including a first stator valve surface, a second stator valve surface a plurality of first function compartments opening into the first stator valve surface, and a plurality of second function compartments opening into the second stator valve surface; and

a rotor rotatably coupled to the stator and including a first rotor valve surface in communication with the first stator valve surface, a second rotor valve surface in communication with the second stator valve surface, a plurality of flow paths for receiving adsorbent material therein, each said flow path including a pair of opposite ends, and a plurality of apertures provided in the rotor valve surfaces and in communication with the flow path ends and the functions ports for cyclically exposing each said flow path to a plurality of discrete pressure levels between the upper and lower pressure for maintaining uniform gas floe through the first and second function compartments, the function compartments comprising first and second gas geed compartments opening into the firs stator valve surface for delivering the gas mixture to the flow paths for sequentially exposing the flow paths to the second feed gas prior to the firs feed gas.

31. The current generation system according toclaim 30 where the second feed gas is enriched in oxygen relative to the first feed gas.

32. An electrical current generating system comprising:

a fuel cell including an anode channel including an anode gas inlet and an anode gas outlet, a cathode channel including a cathode gas inlet and a cathode gas outlet, and an electrolyte in communication with the anode and cathode channel for facilitating exchange between the anode and cathode channel;

an oxygen gas delivery system coupled to the cathode gas inlet for delivering oxygen gas to the cathode channel; and

a hydrogen gas delivery system coupled to the anode gas inlet for delivering a gaseous stream enriched in hydrogen gas to the anode channel, including a first rotary pressure swing adsorption system for enriching hydrogen in a gaseous feed

33. The electrical current generation system according toclaim 32 where the first rotary pressure sing adsorption system includes a first gas feed gas inlet for receiving a first gas feed comprising hydrogen gas and a gas outlet coupled to the anode gas inlet.

34. The electrical current generation system according toclaim 33 where the hydrogen gas delivery system includes a gas inlet for receiving a second gas feed from the anode gas outlet and a gas outlet for delivering the gaseous stream enriched in hydrogen gas to the anode channel.

35. The electrical current generation system according toclaim 34 where the second gas feed is passed through the first rotary pressure swing adsorption system.

36. The electrical current generation system according toclaim 35 where the first rotary pressure swing adsorption system includes a second feed gas inlet for receiving the second gas feed.

37. The electrical current generating system according toclaim 32 where the oxygen gas delivery system comprises an oxygen gas separation system for extracting oxygen gas from air, the gas separation system including a first feed gas inlet for receiving an air feed, and an oxygen gas outlet coupled to the cathode gas inlet for supplying enriched oxygen gas to the cathode channel from the air feed.

38. The electrical current generation system according toclaim 36 where the oxygen gas delivery system comprises a second rotary pressure swing adsorption system for extracting oxygen gas from the air, the second rotary pressure swing adsorption system including a first feed gas inlet for receiving an air feed an a gas outlet coupled to the cathode gas inlet for supplying a gaseous stream enriched in oxygen gas to the cathode channel.

39. The electrical current generating system according toclaim 36 where the hydrogen gas delivery system comprises a reactor for producing a second gas feed from hydrocarbon fuel, and wherein the first rotary pressure swing adsorption system is coupled to the reactor for receiving the first and second gas feeds.

40. The electrical current generating system according toclaim 39 wherein the hydrogen gas delivery system comprises a reactor for producing a second hydrogen gas feed from hydrocarbon fuel, and wherein the first rotary pressure swing adsorption system is coupled to the reactor and receives the first and second gas feeds.

41. The electrical current generating system according toclaim 40 where the first rotary pressure swing adsorption system receives the first and second gas feeds, and produces a gaseous stream enriched in hydrogen therefrom.

42. The electrical current generating system according toclaim 41 where the first rotary pressure swing adsorption system hydrogen includes a first feed gas inlet for receiving the first gas feed, and a second feed gas inlet for receiving the second gas feed.

43. The electrical current generating system according toclaim 42 where the first gas feed is provided at a pressure level different from a pressure level of the second gas feed.

44. The electrical current generating system according toclaim 43 where the reactor comprises a steam reformer, and a water gas shift reactor coupled to the steam reformer for producing the second gas feed.

45. The electrical current generating system according toclaim 44 where the steam reformer includes a burner including a first burner inlet coupled to the cathode gas outlet for receiving humid oxygen-enriched gas, and a second burner inlet for receiving heavy product from the first rotary pressure swing adsorption system for burning within the burner for providing endothermic heat of reaction for steam reforming the hydrocarbon fuel.

46. The electrical current generating system according toclaim 39 wherein the reactor comprises an autothermal reformer, and a water gas shift reactor coupled to the steam reformer for producing the second gas feed.

47. The electrical current generating system according toclaim 46 where the oxygen gas delivery system comprises a second rotary pressure swing adsorption system for extracting oxygen gas from air, the second rotary pressure swing adsorption system including a first feed gas inlet for receiving an air feed, and a gas outlet coupled to the cathode gas inlet for supplying a gaseous stream enriched in oxygen gas to the cathode channel, and the reactor comprises a burner including a first burner inlet for receiving air, and a second burner inlet for receiving a gaseous stream enriched in hydrogen gas from the first rotary pressure swing adsorption system for burning the received hydrogen gas within the burner for recovery of heat energy for pressurizing the air feed.

48. The electrical current generating system according toclaim 47 where at least one of the pressure swing adsorption systems comprises a rotary module including a stator and a rotor rotatable relative to the stator, the rotor including a plurality 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, and 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 component.

49. The current generating system according toclaim 48 where the stator includes a first stator valve surface, a second stator valve surface, a plurality of first function compartments opening into the first stator valve surface, and the rotor includes a first rotor valve surface in communication with the first stator valve surface, a second rotor valve surface in communication with the second stator valve surface, and a plurality of apertures provided in the rotor valve surfaces and in communication with respective ends of the flow paths and the function compartments.

50. The current generating system according toclaim 49 wherein the compression machinery is coupled to a portion of the function compartments for maintaining the portion of function compartments at a plurality of discrete respective pressure levels between an upper pressure and a lower pressure for maintaining uniform gas flow through the portion of function compartments.

51. The electrical generating system according toclaim 33 further comprising a gas recirculation means coupled to the cathode gas outlet for recirculating a portion of cathode exhaust gas exhausted from the cathode channel to the cathode gas inlet.

52. The electrical generating system according toclaim 39 further comprising a gas recirculation means coupled to the cathode gas outlet for recirculating a portion of cathode exhaust gas exhausted from the cathode channel to the reactor for producing hydrogen from hydrocarbon fuel.

53. Electrical current generated by the electrical current generating system ofclaim 33.

54. A method of generating an electric potential, comprising:

providing a fuel cell including an anode channel including an anode gas inlet, a cathode channel including a cathode gas inlet and a cathode gas outlet, and an electrolyte in communication with the anode and cathode channel for facilitating ion exchange between the anode and cathode channel;

supplying a gaseous stream enriched in oxygen gas to the cathode gas inlet, where the supplying step comprises supplying a first gas feed to a rotary pressure swing adsorption apparatus to produce a product gas stream enriched in oxygen gas; and

delivering the product gas stream to the cathode gas inlet.

55. The method according toclaim 54 and further comprising recirculating a portion of cathode gas exhausted from the cathode gas outlet to the cathode gas inlet.

56. The method according toclaim 55 where recirculating comprises delivering the exhaust gas portion to the rotary pressure swing adsorption apparatus as a second gas geed.

57. The method according toclaim 56 where the recirculating further comprises purging a remainder of the exhausted cathode gas.

58. The method according toclaim 57 where recirculating comprises delivering the exhaust gas portion at increased pressure to the cathode gas inlet.

59. The method according toclaim 58 where recirculating further comprises recovering waste heat from the fuel cell for enhancing recovery of expansion energy from the pressure swing adsorption apparatus.

60. The method according toclaim 54 where supplying hydrogen gas comprises recirculating a portion of cathode gas exhausted from the cathode gas outlet to the inlet of a fuel processor for reacting a hydrocarbon fuel to generate hydrogen.

61. A method for generating electrical potential, comprising:

providing a fuel cell including an anode channel including an anode gas inlet, a cathode channel including a cathode gas inlet and a cathode gas outlet, and an electrolyte in communication with the anode and cathode channel for facilitating ton exchange between the anode and cathode channel;

supplying a first gaseous stream enriched in hydrogen gas to the anode gas inlet, where supplying comprises supplying a first gas feed to a rotary hydrogen pressure swing adsorption apparatus to produce a first product gas stream enriched in hydrogen gas;

delivering the first product gas stream to the anode gas inlet; and

supplying a second gaseous stream enriched in oxygen gas to the cathode gas inlet.

62. The method according toclaim 61 and further comprising recirculating a portion of anode gas exhausted from the anode gas outlet to the anode gas inlet.

63. The method according toclaim 62 where recirculating comprises delivering the exhaust gas portion to the rotary hydrogen pressure swing adsorption apparatus as a second gas feed.

64. The method according toclaim 63 where supplying a second gaseous stream comprises supplying an air feed to a rotary oxygen pressure swing adsorption apparatus to produce a second product gas stream enriched in oxygen gas, and delivering the second product gas stream to the cathode gas inlet.

65. The method according to claim64where supplying hydrogen gas comprises supplying a hydrocarbon fuel to a reformer, reacting the fuel with oxygen-enriched gas from the cathode gas outlet, delivering a hydrogen gas feed from the reformer as a first gas feed to a hydrogen pressure swing adsorption apparatus, and delivering hydrogen-enriched gas extracted from the first gas feed as light product gas to the anode gas inlet.

66. The method according toclaim 65 where the reformer comprises a steam reformer including a combustor, and reacting comprises delivering the fuel to the combustor, and providing heat energy to the combustor by burning tail gas extracted from the rotary hydrogen pressure swing adsorption apparatus as heavy product gas with the oxygen-enriched gas from the cathode gas outlet in the combustor.

67. The method according toclaim 65 where the reformer comprises a steam reformer including a combustor, and reacting comprises delivering the fuel to the combustor, and providing heat energy to the combustor by burning tail gas extracted from the rotary hydrogen pressure swing adsorption apparatus as heavy product gas with a portion of the air feed in the combustor, and recovering heat of combustion from the combustor for delivering air under pressure to the rotary oxygen pressure swing adsorption apparatus.

68. The method according toclaim 65 where the reformer comprises an autothermal reformer including a water gas shift reactor, and the step of supplying a pressurized air feed comprises the steps of delivering air to a combustor, burning tail gas extracted from the hydrogen pressure swing adsorption apparatus as heavy product gas with the delivered air in the combustor, and recovering heat of combustion from the combustor for delivering air under pressure to the oxygen pressure swing adsorption apparatus.

69. An electrical current generating system comprising:

a fuel cell including an anode channel including an anode gas inlet and an anode gas outlet, a cathode channel including a cathode gas inlet and a cathode gas outlet, and an electrolyte in communication with the anode and cathode channel for facilitating ion exchange between the anode and cathode channel;

an oxygen gas delivery system coupled to the cathode gas inlet for delivering enriched oxygen gas to the cathode channel, the oxygen gas delivery system including a pressure swing adsorption system to enrich oxygen and to remove carbon dioxide from atmospheric air feed; and

a hydrogen gas delivery system coupled to the anode gas inlet for delivering purified hydrogen gas to the cathode channel.

70. The current generating system according toclaim 69 further including an oxygen accumulator interposed between the oxygen gas delivery system and the cathode gas inlet. The method according toclaim 65 where the reformer comprises a steam reformer including a combustor, and reacting comprises delivering the fuel to the combustor, and providing heat energy to the combustor by burning tail gas extracted from the hydrogen pressure swing adsorption apparatus as heavy product gas with the oxygen-enriched gas from the cathode gas outlet in the combustor.

71. The electrical current generation system according toclaim 1 where the rotary pressure swing adsorption system comprises a plurality of adsorbers having first and second ends communicating respectively to a first valve face and a second valve face, the plurality of adsorbers being housed in an adsorber housing body cooperating with first and second valve bodies respectively sealingly engaged with the adsorber housing body at the first and second valve faces.

72. The electrical current generation system according toclaim 71 where the adsorber housing body rotates relative to the first and second valve bodies.

73. The electrical current generation system according toclaim 71 where the first and second valve bodies rotate relative to the adsorber housing.

74. The electrical current generation system according toclaim 73 where the rotary pressure swing adsorption system further comprises transfer chambers providing fluid communication between the first valve body and respectively a feed air inlet conduit and an exhaust discharge conduit.

75. The electrical current generation system according toclaim 73 where the rotary pressure swing adsorption system further comprises a transfer chamber providing fluid communication between the second valve body and the product oxygen conduit to the cathode inlet.

76. The electrical current generation system according toclaim 1 further where the pressure swing adsorption system generates a gaseous stream enriched in oxygen, the gaseous stream enriched in oxygen having a first oxygen purity and being fluidly coupled to the cathode gas inlet, the system further comprising a compressed air inlet for mixing compressed air with the gaseous stream enriched in oxygen upstream of the cathode gas inlet, thereby forming a cathode feed gas stream having a second oxygen purity lower than the first oxygen purity.

77. The electrical current generation system where the first oxygen purity is in the range of from about 70% to about 90% and the second oxygen purity is in the range of from about 30% to about 40%.

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