Partial pressure swing adsorption system for providing hydrogen to a vehicle fuel cell Number:7,520,916 from the United States Patent and Trademark Office (PTO) owispatent

Title: Partial pressure swing adsorption system for providing hydrogen to a vehicle fuel cell

Abstract: A method of operating a fuel cell system includes providing a fuel inlet stream into a fuel cell stack, operating the fuel cell stack to generate electricity and a hydrogen containing fuel exhaust stream, separating at least a portion of hydrogen contained in the fuel exhaust stream using partial pressure swing adsorption, and providing the hydrogen separated from the fuel exhaust stream to a hydrogen storage vessel or to a hydrogen using device.

Patent Number: 7,520,916 Issued on 04/21/2009 to McElroy,   et al.

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Inventors:	McElroy; James F. (Suffield, CT), Finn; John E. (Mountain View, CA), LeVan; M. Douglas (Brentwood, TN), Venkataraman; Swaminathan (Cupertino, CA), Mitlitsky; Fred (Livermore, CA)
Assignee:	Bloom Energy Corporation (Sunnyvale, CA)
Appl. No.:	11/188,123
Filed:	July 25, 2005

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Current U.S. Class:	95/96 ; 429/13; 429/19; 95/119; 95/139; 96/121; 96/126
Current International Class:	B01D 53/02 (20060101)
Field of Search:	95/96,119,139 96/121,126 429/12,13,19

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References Cited [Referenced By]

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U.S. Patent Documents

3488266	January 1970	French
4041210	August 1977	Van Dine
4182795	January 1980	Baker et al.
4532192	July 1985	Baker et al.
4792502	December 1988	Trocciola et al.
4898792	February 1990	Singh et al.
4917971	April 1990	Farooque
4983471	January 1991	Reichner et al.
5034287	July 1991	Kunz
5047299	September 1991	Shockling
5084362	January 1992	Farooque
5143800	September 1992	George et al.
5169730	December 1992	Reichner et al.
5170124	December 1992	Blair et al.
5302470	April 1994	Okada et al.
5441821	August 1995	Merritt et al.
5498487	March 1996	Ruka et al.
5501914	March 1996	Satake et al.
5505824	April 1996	McElroy
5527631	June 1996	Singh et al.
5573867	November 1996	Zafred et al.
5601937	February 1997	Isenberg
5686196	November 1997	Singh et al.
5733675	March 1998	Dederer et al.
5741605	April 1998	Gillett et al.
5955039	September 1999	Dowdy
6013385	January 2000	DuBose
6051125	April 2000	Pham et al.
6106964	August 2000	Voss et al.
6280865	August 2001	Eisman et al.
6329090	December 2001	McElroy et al.
6403245	June 2002	Hunt
6436562	August 2002	DuBose
6451466	September 2002	Grasso et al.
6531243	March 2003	Thom
6623880	September 2003	Geisbrecht et al.
6821663	November 2004	McElroy et al.
6924053	August 2005	McElroy
2001/0049035	December 2001	Haltiner, Jr. et al.
2002/0015867	February 2002	Cargnelli et al.
2002/0028362	March 2002	Prediger et al.
2002/0058175	May 2002	Ruhl
2002/0106544	August 2002	Noetzel et al.
2003/0157386	August 2003	Gottmann
2003/0162067	August 2003	McElroy
2003/0196893	October 2003	McElroy
2003/0205641	November 2003	McElroy
2004/0005492	January 2004	Keefer et al.
2004/0081859	April 2004	McElroy et al.
2004/0191597	September 2004	MdElroy
2004/0191598	September 2004	Gottmann et al.
2004/0197612	October 2004	Keefer et al.
2004/0202914	October 2004	Sridhar et al.
2004/0224193	November 2004	Mitlitsky et al.
2005/0048334	March 2005	Sridhar et al.
2005/0164051	July 2005	Venkataraman et al.

Foreign Patent Documents

	WO 2004/013258		Feb., 2004		WO
	WO 2004/093214		Oct., 2004		WO
	WO 2004/093214		Oct., 2004		WO

Other References

"Type BPL Granular Carbon", Calgon Product Bulletin, Calgon Corporation, Activated Carbon Division, 2 pgs. cited by other . Levan, M. Douglas et al., "Fixed-Bed Adsorption of Gases: Effect of Velocity Variations on Transition Types", AIChE Journal, vol. 34, No. 6, Jun. 1988, pp. 996-1005. cited by other . Sward, Brian K. et al., "Simple Flow-Through Apparatus for Measurement of Mass Transfer Rates in Adsorbent Particles by Frequency Response", Fundementals of Adsorption, K. Kaneko et al., eds., vol. 7, 2002, pp. 29-36. Published by IK International of Japan. cited by other . Walton, Krista S. et al., "A Novel Adsorption Cycle for CO.sub.2 Recovery: Experimental and Theoretical Investigations of a Temperature Swing Compression Process", Revised Manuscript, submitted in 1995 to Separation Science & Technology, 30 pgs. cited by other . Yong, Zou et al., "Adsorption of carbon dioxide at high temperature--a review", Separation and Purification Technology, vol. 26, 2002, pp. 195-205. cited by other . U.S. Appl. No. 11/124,120, filed May 9, 2005, Valensa et al. cited by other . U.S. Appl. No. 11/188,118, filed Jul. 25, 2005, LeVan et al. cited by other . U.S. Appl. No. 11/188,120, filed Jul. 25, 2005, LeVan et al. cited by other . Berlier, Karl et al., "Adsorption of CO.sub.2 on Microporous Materials. 1. On Activated Carbon and Silica Gel", J. Chem. Eng. Data, 1997, vol. 42, pp. 533-537. cited by other . EG & G, Parsons, Inc., SAIC. Fuel Cell Handbook. 5.sup.th Edition. USDOE. Oct. 2000. 9-1-9-4; 9-12-9-14. cited by other . LeVan, M. Douglas et al., "Adsorption and Ion Exchange", Perry's Chemical Engineers' Handbook (7.sup.th Edition), 1997, 66 pgs. cited by other . Manchado, M. Cabrejas et al., "Adsorption of H.sub.2, O.sub.2, CO, and CO.sub.2 on a .gamma.-Alumina: Volumetric and Calorimetric Studies", Langmuir, vol. 10, 1994, pp. 685-691. cited by other . Olivier, Marie-Georges et al., "Adsorption of Light Hyrdocarbons and Carbon Dioxide on Silica Gel", J. Chem. Eng. Data, 1997, vol. 42, pp. 230-233. cited by other . Qi, Nan et al., "Adsorption equilibrium modeling for water on activated carbons", Carbon, vol. 43, 2005, pp. 2258-2263. cited by other . Rudisill, Edgar N. et al., "Coadsorption of Hydrocarbons and Water on BPL Activated Carbon", Ind. Eng. Chem. Res., 1992, vol. 31, pp. 1122-1130. cited by other . Sward, Brian K. et al., "Frequency Response Method for Measuring Mass Transfer Rates in Adsorbents via Pressure Perturbation", Adsorption, vol. 9, 2003, pp. 37-54. cited by other . Yang, Ralph T., "Adsorbents: Fundamentals and Applications", 2003, 4 pgs. cited by other . Yong, Zou et al., "Adsorption of Carbon Dioxide on Basic Alumina at High Temperatures", J. Chem. Eng. Data, 2000, vol. 45, pp. 1093-1095. cited by other.

Primary Examiner: Smith; Duane
Assistant Examiner: Hawkins; Karla
Attorney, Agent or Firm: Foley & Lardner LLP

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Claims

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What is claimed is:

1. A method of operating a fuel cell system, comprising: providing a fuel inlet stream into a fuel cell stack; operating the fuel cell stack to generate electricity and a hydrogen containing fuel exhaust stream; separating at least a portion of hydrogen contained in the fuel exhaust stream using partial pressure swing adsorption; and providing the hydrogen separated from the fuel exhaust stream to a hydrogen storage vessel or to a hydrogen using device; wherein the step of separating comprises: (a) a first feed/purge step comprising: providing a feed gas inlet stream comprising at least a portion of the fuel exhaust stream into a first adsorbent bed; collecting a feed gas outlet stream comprising at least one separated component of the feed gas at a first output; providing a purge gas inlet stream into a second adsorbent bed; and collecting a purge gas outlet stream at a second output; (b) a first flush step, conducted after the first feed/purge step, the first flush step comprising: providing the purge gas inlet stream into the first adsorbent bed; collecting the purge gas outlet stream, which comprises at least one component of the feed gas that was trapped in a void volume of the first adsorbent bed, at the first output; providing the feed gas inlet stream into the second adsorbent bed; and collecting the feed gas outlet stream, which comprises a portion of the purge gas that was trapped in a void volume of the second bed, at the second output; (c) a second feed/purge step, conducted after the first flush step, the second feed/purge step comprising: providing the feed gas inlet stream into the second adsorbent bed; collecting the feed gas outlet stream comprising at least one separated component of the feed gas at the first output; providing the purge gas inlet stream into the first adsorbent bed; and collecting the purge gas outlet stream at the second output; and (d) a second flush step, conducted after the second feed/purge step, the second flush step comprising: providing the purge gas inlet stream into the second adsorbent bed; collecting the purge gas outlet stream, which comprises at least one component of the feed gas that was trapped in a void volume of the second adsorbent bed, at the first output; providing the feed gas inlet stream into the first adsorbent bed; and collecting a feed gas outlet stream, which comprises a portion of the purge gas that was trapped in a void volume of the first bed, at the second output.

2. The method of claim 1, wherein: the fuel inlet stream comprises a hydrocarbon fuel inlet stream; the fuel cell stack comprises a solid oxide fuel cell stack; the fuel exhaust stream comprises hydrogen, carbon monoxide, water vapor and carbon dioxide; and the step of separating comprises adsorbing at least a majority of the carbon dioxide and a portion of the water vapor in the fuel exhaust stream in at least one adsorbent bed while allowing at least a majority of the hydrogen and carbon monoxide in the fuel exhaust stream to be passed through the at least one adsorbent bed.

3. The method of claim 2, the step of separating comprises: providing unpressurized fuel exhaust stream into a first adsorbent bed to adsorb at least a majority of the carbon dioxide and a portion of the water vapor in the fuel exhaust stream in the first adsorbent bed until the first adsorbent bed is saturated while regenerating a second adsorbent bed by providing air having a relative humidity of 50% or less through the second adsorbent bed to desorb adsorbed carbon dioxide and water vapor; and providing unpressurized fuel exhaust stream into the second adsorbent bed to adsorb at least a majority of the carbon dioxide and a portion of the water vapor in the fuel exhaust stream in the second adsorbent bed until the second adsorbent bed is saturated while regenerating the first adsorbent bed by providing air having a relative humidity of 50% or less through the first adsorbent bed to desorb adsorbed carbon dioxide and water vapor.

4. The method of claim 2, wherein the step of providing the hydrogen comprises providing the hydrogen separated from the fuel exhaust stream to the hydrogen storage vessel.

5. The method of claim 2, wherein the step of providing the hydrogen comprises providing the hydrogen separated from the fuel exhaust stream to the hydrogen using device.

6. The method of claim 2, wherein: the hydrogen using device comprises a PEM fuel cell system located in a vehicle; and the step of providing the hydrogen comprises providing the hydrogen to the hydrogen storage vessel and then providing the hydrogen from the hydrogen storage vessel to the PEM fuel cell system, or providing the hydrogen to the PEM fuel cell system without providing the hydrogen to the hydrogen storage vessel.

7. The method of claim 6, further comprising scrubbing carbon monoxide from a hydrogen containing stream before the step of providing hydrogen to the PEM fuel cell system and after the step of separating the hydrogen from the fuel exhaust stream by partial pressure swing adsorption.

8. The method of claim 7, wherein the step of scrubbing carbon monoxide comprises scrubbing carbon monoxide using pressure swing adsorption or a Sabatier reaction.

9. The method of claim 2, further comprising: dividing the hydrogen separated from the fuel exhaust stream into a first stream and a second stream; providing the first stream into the hydrocarbon fuel inlet stream; and providing the second stream to the hydrogen storage vessel or the hydrogen using device.

10. The method of claim 9, further comprising alternating between providing the hydrogen separated from the fuel exhaust stream into the hydrocarbon fuel inlet stream and providing the hydrogen separated from the fuel exhaust to the hydrogen storage vessel.

11. The method of claim 1, further comprising: humidifying the fuel inlet stream using water vapor contained in the fuel exhaust stream by using a humidifier; after the step of humidifying, condensing and removing at least a part of the water vapor in the fuel exhaust stream; performing the step of separating after the step of condensing and removing; and providing all hydrogen separated from the fuel exhaust stream to a hydrogen storage vessel or to a hydrogen using device without recycling hydrogen separated from the fuel exhaust stream into the fuel inlet stream and without recycling a portion of the fuel exhaust stream into the fuel inlet stream.

12. The method of claim 1, further comprising: separating the fuel exhaust stream into at least two streams; recycling a first fuel exhaust stream into the fuel inlet stream; separating at least a portion of hydrogen and carbon monoxide contained in a second fuel exhaust stream using the partial pressure swing adsorption; providing a first portion of the separated hydrogen to the hydrogen using device or to the hydrogen storage vessel; and providing a second portion of the separated hydrogen to the fuel inlet stream.

13. A fuel cell system, comprising: a fuel cell stack; a partial pressure swing adsorption unit comprising a plurality of adsorbent beds; a first conduit which operatively connects a fuel exhaust outlet of the fuel cell stack to a first inlet of the partial pressure swing adsorption unit; a second conduit which operatively connects a purge gas source to a second inlet of the partial pressure swing adsorption unit; and a third conduit which operatively connects an outlet of the partial pressure swing adsorption unit to a hydrogen using device or to a hydrogen storage vessel; the plurality of adsorbent beds comprise a first adsorbent bed and a second adsorbent bed; in operation, the first adsorbent bed performs the following functions: (a) receives the feed gas inlet stream comprising at least a portion of the fuel cell stack fuel exhaust stream from the first conduit and provides at least one separated component of the feed gas to the third conduit in a first feed/purge step; (b) receives the purge gas inlet stream from the second conduit and provides a purge gas outlet stream, which comprises at least one component of the feed gas that was trapped in a void volume of the first bed to the third conduit in a first flush step, conducted after the first feed/purge step; (c) receives a purge gas inlet stream from the second conduit and provides a purge gas outlet stream to an output different from the third conduit in a second feed/purge step, conducted after the first flush step; and (d) receives the feed gas inlet stream from the first conduit and provides a feed gas outlet stream, which comprises a portion of the purge gas that was trapped in a void volume of the first bed, to at an output different from the third conduit in a second flush step, conducted after the second feed/purge step; and in operation, the second bed performs the following functions: (a) receives a purge gas inlet stream from the second conduit and provides a purge gas outlet stream to at an output different from the third conduit in a first feed/purge step; (b) receives the feed gas inlet stream from the first conduit and provides the feed gas outlet stream, which comprises a portion of the purge gas that was trapped in a void volume of the second bed, to an output different from the third conduit in a first flush step, conducted after the first feed/purge step; (c) receives the feed gas inlet stream from the first conduit and provides the feed gas outlet stream comprising at least one separated component of the feed gas to the third conduit in a second feed/purge step, conducted after the first flush step; and (d) receives the purge gas inlet stream from the second conduit and provides the purge gas outlet stream, which comprises at least one component of the feed gas that was trapped in a void volume of the second bed to the first conduit in a second flush step, conducted after the second feed/purge step.

14. The system of claim 13, wherein: the fuel cell stack comprises a solid oxide fuel cell stack; the plurality of adsorbent beds comprise a material which preferentially adsorbs carbon dioxide and water vapor to hydrogen and carbon monoxide; and the system lacks a compressor which in operation compresses the fuel cell stack fuel exhaust stream to be provided into the partial pressure swing adsorption unit.

15. The system of claim 13, wherein the third conduit operatively connects an outlet of the partial pressure swing adsorption unit to the hydrogen storage vessel.

16. The system of claim 13, wherein the third conduit operatively connects an outlet of the partial pressure swing adsorption unit to the hydrogen using device.

17. The system of claim 13, further comprising a selector valve having an inlet operatively connected to an outlet of the partial pressure swing adsorption unit, a first outlet operatively connected to the hydrogen storage vessel or to the hydrogen using device, and a second outlet operatively connected to a fuel inlet of the fuel cell stack.

18. The system of claim 17, further comprising a carbon monoxide scrubbing device having an inlet operatively connected to an outlet of the partial pressure swing adsorption unit and an outlet operatively connected to a PEM fuel cell system located in a vehicle, wherein in operation, the carbon monoxide scrubbing device scrubs carbon monoxide being provided with the hydrogen from the partial pressure swing adsorption unit and provides the hydrogen either directly or indirectly to the PEM fuel cell system.

19. The system of claim 18, wherein the carbon monoxide scrubbing device comprises a pressure swing adsorption unit or a Sabatier reactor.

20. The system of claim 13, further comprising: a condenser and water separator having an inlet which is operatively connected to the fuel cell stack fuel exhaust outlet and an outlet which is operatively connected to an inlet of the partial pressure swing adsorption unit; and a fuel humidifier having a first inlet operatively connected to a hydrocarbon fuel inlet conduit, a second inlet operatively connected to the fuel cell stack fuel exhaust outlet, a first outlet operatively connected to the fuel cell stack fuel inlet, and a second outlet operatively connected to the condenser and water separator, wherein in operation, the fuel humidifier humidifies a fuel inlet stream using water vapor contained in a fuel cell stack fuel exhaust stream.

21. The system of claim 13, further comprising: a condenser and water separator having an inlet which is operatively connected to the fuel cell stack fuel exhaust outlet and an outlet which is operatively connected to an inlet of the partial pressure swing adsorption unit; and a multi-way valve having an inlet operatively connected to the fuel cell stack fuel exhaust outlet, a first outlet operatively connected to the condenser and water separator, and a second outlet operatively connected to the fuel cell stack fuel inlet conduit.

22. A fuel cell system, comprising: a fuel cell stack; and a separation means for separating at least a portion of hydrogen contained in a fuel cell stack fuel exhaust stream using partial pressure swing adsorption and for providing the hydrogen separated from the fuel exhaust stream to a hydrogen storage vessel or to a hydrogen using device; wherein the separation means comprises: a first means for providing a feed gas inlet stream comprising at least a portion of the fuel cell stack fuel exhaust stream; a second means for providing a purge gas inlet stream; a third means for collecting at least one separated component of the feed gas; a fourth means for: (a) receiving the feed gas inlet stream from the first means and for providing at least one separated component of the feed gas to the third means in a first feed/purge step; (b) receiving the purge gas inlet stream from the second means and for providing a purge gas outlet stream, which comprises at least one component of the feed gas that was trapped in a void volume of the fourth means to the third means in a first flush step, conducted after the first feed/purge step; (c) receiving a purge gas inlet stream from the second means and for providing a purge gas outlet stream to an output different from the third means in a second feed/purge step, conducted after the first flush step; and (d) receiving the feed gas inlet stream from the first means and for providing a feed gas outlet stream, which comprises a portion of the purge gas that was trapped in a void volume of the fourth means, to at an output different from the third means, in a second flush step, conducted after the second feed/purge step; a fifth means for: (a) receiving a purge gas inlet stream from the second means and for providing a purge gas outlet stream to at an output different from the third means in a first feed/purge step; (b) receiving the feed gas inlet stream from the first means and for providing the feed gas outlet stream, which comprises a portion of the purge gas that was trapped in a void volume of the fifth means, to an output different from the third means in a first flush step, conducted after the first feed/purge step; (c) receiving the feed gas inlet stream from the first means and for providing the feed gas outlet stream comprising at least one separated component of the feed gas to the third means in a second feed/purge step, conducted after the first flush step; and (d) receiving the purge gas inlet stream from the second means and for providing the purge gas outlet stream, which comprises at least one component of the feed gas that was trapped in a void volume of the fifth means to the first means in a second flush step, conducted after the second feed/purge step; and a sixth means for dividing the hydrogen separated from the fuel exhaust stream into a first stream and a second stream, for providing the first stream into the hydrocarbon fuel inlet stream, and for providing the second stream to the hydrogen storage vessel or the hydrogen using device.

23. The system of claim 22, wherein the separation means is a means for separating at least the portion of hydrogen contained in the fuel cell stack fuel exhaust stream using partial pressure swing adsorption and for providing the hydrogen separated from the fuel exhaust stream to a PEM fuel cell system located in a vehicle either by providing the hydrogen to the hydrogen storage vessel, which then provides the hydrogen to the PEM fuel cell system, or by providing the hydrogen to the PEM fuel cell system without providing the hydrogen to the hydrogen storage vessel.

24. The system of claim 22, further comprising a scrubbing means for scrubbing carbon monoxide from a gas stream provided from the separation means to the hydrogen storage vessel or to the hydrogen using device.

25. The system of claim 22, wherein: the fuel inlet stream comprises a hydrocarbon fuel inlet stream; the fuel cell stack comprises a solid oxide fuel cell stack; the fuel exhaust stream comprises hydrogen, carbon monoxide, water vapor and carbon dioxide; and the separation means is a means for adsorbing at least a majority of the carbon dioxide and a portion of the water vapor in the fuel exhaust stream while allowing at least a majority of the hydrogen and carbon monoxide in the fuel exhaust stream to be passed through.

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Description

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BACKGROUND OF THE INVENTION

The present invention relates generally to the field of fuel cell systems and more particularly to fuel cell systems with anode exhaust fuel recovery by partial pressure swing adsorption.

SUMMARY OF THE INVENTION

Fuel cells are electrochemical devices which can convert energy stored in fuels to electrical energy with high efficiencies. High temperature fuel cells include solid oxide and molten carbonate fuel cells. These fuel cells may operate using hydrogen and/or hydrocarbon fuels. There are classes of fuel cells, such as the solid oxide regenerative fuel cells, that also allow reversed operation, such that oxidized fuel can be reduced back to unoxidized fuel using electrical energy as an input.

BRIEF DESCRIPTION OF THE DRAWINGS

FIGS. 1, 2A, 2B, 2C, 2D, 3, and 4 are schematic diagrams of the partial pressure swing adsorption systems of the embodiments of the invention.

FIGS. 5 and 6 are schematic diagrams of fuel cell systems of the embodiments of the invention which incorporate the partial pressure swing adsorption systems.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

The embodiments of the invention provide a system and method in which partial pressure swing adsorption (i.e., concentration swing adsorption) is used to separate hydrogen from a high temperature fuel cell stack fuel exhaust stream and to provide the separated hydrogen as fuel to a hydrogen storage vessel or to a hydrogen using device, such as a low temperature fuel cell stack used to power a vehicle. If desired, at least a portion of the separated hydrogen may also be recycled into the fuel inlet stream of the high temperature fuel cell stack. Preferably, the high temperature fuel cell stack comprises a solid oxide fuel cell stack which operates on hydrocarbon fuel and the low temperature fuel cell stack comprises a PEM fuel cell stack which operates on hydrogen fuel. The first four embodiments described below are directed to various partial pressure swing adsorption gas separation methods and devices which may be used to separate hydrogen from the fuel exhaust stream, while the fifth and sixth embodiments are directed to the fuel cell systems which use partial pressure swing adsorption methods and devices for hydrogen separation.

The first embodiment of the invention provides a four-step partial pressure swing adsorption (i.e., concentration swing adsorption) cycle for gas separation, such as for recovering fuel from the fuel (i.e., anode side) exhaust of a solid oxide fuel cell stack. Two beds packed with an adsorbent material, such as activated carbon, are used to adsorb carbon dioxide and water (i.e., water vapor) from the fuel exhaust, allowing hydrogen and carbon monoxide to pass through the beds. The beds are regenerated, preferably countercurrently, with air dried to modest relative humidities, such as about 30% to about 50% relative humidity. For example, dry air for regeneration may be developed in a temperature swing adsorption cycle using silica gel or activated alumina. Flush steps are used to recover additional hydrogen and to prevent air from contaminating the recovered fuel. The duration of the adsorption and regeneration (i.e., feeding and purging) steps is preferably at least 5 times longer, such as 10-50 times longer than the duration of the flush steps.

Thus, a reliable, energy-efficient cycle for optimum gas separation is provided. For example, the cycle is a high efficiency cycle for maximum recovery of hydrogen and maximum rejection of carbon dioxide and air, based on a partial pressure swing adsorption (also referred to herein as concentration swing adsorption) with countercurrent purge and cocurrent flush steps. Since the beds are preferably regenerated with air, the sweeping of air left in the bed at the end of regeneration back into the fuel cell stack is not desirable. Furthermore, at the start of a regeneration step, the bed taken off stream contains hydrogen in the gas phase. Recovery of this hydrogen is desirable. The flush steps are used to remove the air left in the bed at the end of regeneration to prevent providing this air back into the fuel cell stack, and to provide the hydrogen remaining in the bed at the start of a regeneration step into the fuel inlet of the fuel cell stack.

While the system and method of the first embodiment will be described and illustrated with respect to an adsorption system which separates carbon dioxide from the hydrogen in a solid oxide fuel stack fuel exhaust stream, it should be noted that the system and method of the first embodiment may be used to separate any multicomponent gas stream that is not part of a fuel cell system or that is part of a fuel cell system other than a solid oxide fuel cell system, such as a molten carbonate fuel cell system for example. Thus, the system and method of the first embodiment should not be considered limited to separation of hydrogen from carbon dioxide. The adsorbent material in the adsorbent beds may be selected based on the gases being separated.

FIG. 1 illustrates a gas separation apparatus 1 of the first embodiment. The apparatus 1 contains a first feed gas inlet conduit 3, which in operation provides a feed gas inlet stream. If the apparatus 1 is used to separate hydrogen from a fuel cell stack fuel exhaust stream, then conduit 3 is operatively connected to the fuel cell stack anode exhaust. As used herein, when two elements are "operatively connected," this means that the elements are directly or indirectly connected to allow direct or indirect fluid flow from one element to the other. The apparatus 1 also contains a second purge gas inlet conduit 5, which in operation provides a purge gas inlet stream.

The apparatus contains a third feed gas collection conduit 7, which in operation collects at least one separated component of the feed gas. If the apparatus 1 is used to separate hydrogen from a fuel cell stack fuel exhaust stream and to recycle the hydrogen into the fuel inlet of the fuel cell stack, then conduit 7 is operatively connected to the fuel inlet of the fuel cell stack (i.e., either directly into the stack fuel inlet or to a fuel inlet conduit which is operatively connected to the stack fuel inlet). The apparatus also contains a fourth purge gas collection conduit 9, which in operation collects the feed gas outlet stream during the flush steps and collects the purge gas outlet stream during feed/purge steps.

Thus, if the apparatus 1 is used to separate hydrogen from a fuel cell stack fuel exhaust stream, then the first conduit 3 comprises a hydrogen, carbon dioxide, carbon monoxide and water vapor inlet conduit, the second conduit 5 comprises a dry air inlet conduit, the third conduit 7 comprises a hydrogen and carbon monoxide removal and recycling conduit and the fourth conduit 9 comprises a carbon dioxide and water vapor removal conduit.

The apparatus 1 also contains at least two adsorbent beds 11, 13. The beds may contain any suitable adsorbent material which adsorbs at least a majority, such as at least 80 to 95% of one or more desired components of the feed gas, and which allows a majority of one or more other components to pass through. For example, the bed material may comprise zeolite, activated carbon, silica gel or activated alumina adsorbent material. Activated carbon is preferred for separating hydrogen and carbon monoxide from water vapor and carbon dioxide in a fuel cell stack fuel exhaust stream. Zeolites adsorb carbon dioxide as well. However, they adsorb water very strongly, and a very dry gas should be used for regeneration, which is difficult to obtain. Thus, zeolite beds can preferably, but not necessarily, be used to separate a gas stream which does not contain water vapor because an apparatus which uses zeolite beds to separate a water vapor containing gas may experience a slow degradation of performance.

The apparatus 1 also comprises a plurality of valves which direct the gas flow. For example, the apparatus may contain three four-way valves with "double-LL" flow paths: a feed valve 15, a regeneration valve 17 and a product valve 19. The feed valve 15 is connected to the first conduit 3, to the two beds 11, 13 and to the regeneration valve 17 by conduit 21. The regeneration valve 17 is connected to the second and fourth conduits 5 and 9, respectively, to the feed valve 15 by conduit 21 and to the product valve 19 by conduit 23. The product valve 19 is connected to the third conduit 7, to the two beds 11, 13 and to the regeneration valve 17 by conduit 23. The four-way valves may be used to redirect two flows at a time. Such valves are available in a wide range of sizes, for example, from A-T Controls, Inc., Cincinnati, Ohio, USA. If desired, each 4-way valve may be replaced by two 3-way valves or four 2-way valves, or by an entirely different flow distribution system involving a manifold.

Thus, the valves 15, 17, 19 are preferably operated such that the purge gas inlet stream is provided into the beds 11, 13 countercurrently with the feed gas inlet stream during the purge steps and cocurrently with the feed gas inlet stream during the flush steps. In other words, the first conduit 3 is operatively connected to the first and the second beds 11, 13 to provide the feed gas inlet stream into the first and the second beds in a first direction. The second conduit 5 is operatively connected to the first and the second beds 11, 13 through valves 17, 19 such that the purge gas inlet stream is provided into each of the first and the second beds 11, 13 in a different direction from the first direction (such as in the opposite direction) during the first and the second feed/purge steps, and the purge gas inlet stream is provided into the first and the second beds in the first direction (i.e., the same direction and the feed gas inlet stream) during the first and the second flush steps.

FIGS. 2A-2D illustrate the steps in the operation cycle of system 1. FIG. 2A shows the apparatus 1 during a first feed/purge step in which the first bed 11 is fed with a feed gas inlet stream, such as the fuel stack fuel exhaust stream, while the second bed 13 is fed with a purge gas, such as dried air, to regenerate the second bed 13.

The feed gas inlet stream is provided from conduit 3 through valve 15 into the first adsorbent bed 11. For a feed gas which contains hydrogen, carbon monoxide, carbon dioxide and water vapor, the majority of the hydrogen and carbon monoxide, such as at least 80-95% passes through the first bed 11, while a majority of the carbon dioxide, such as at least 80-95%, and much of the water vapor are adsorbed in the first bed. The feed gas outlet stream comprising at least one separated component of the feed gas, such as hydrogen and carbon monoxide, passes through valve 19 and is collected at a first output, such as the third conduit 7.

The purge gas inlet stream, such as dried air, is provided from the second conduit 5 through valve 17, conduit 23 and valve 19 into a second adsorbent bed 13. The purge gas outlet stream passes through conduit 21 and valves 15 and 17, and is collected at a second output, such as the fourth conduit 9.

In the first feed/purge step, the valve positions are such that valve 15 directs the feed to the first bed 11 and valve 19 directs the hydrogen product away to conduit 7. Valve 17 is positioned to sweep dry air counter currently through the second bed to remove carbon dioxide that was previously adsorbed. Some of the water in the feed gas steam is adsorbed on the adsorbent material, such as activated carbon, at the inlet of the first bed 11 and will be removed from the bed 11 when it is regenerated in a subsequent step. Carbon monoxide will be passed through the first bed 11 as the carbon dioxide wave advances.

FIG. 2B illustrates the apparatus 1 in a first flush step which is conducted after the first feed/purge step. In this step, the feed valve 15 and the regeneration valve 17 switch flow directions from the prior step, while the product valve 19 does not.

The purge gas inlet stream is provided from conduit 5 through valves 17 and 15 and conduit 21 into the first adsorbent bed 11. Preferably, this purge gas inlet stream is provided into the first bed 11 in the same direction as the feed gas stream in the previous step. The purge gas outlet stream, which comprises at least one component of the feed gas, such as hydrogen, that was trapped in a void volume of the first adsorbent bed, is collected at the first output, such as conduit 7.

The feed gas inlet stream is provided from conduit 3 through valve 15 into the second adsorbent bed 13. The feed gas outlet stream, which comprises a portion of the purge gas, such as air, that was trapped in a void volume of the second bed 13, passes through valves 19 and 17 and conduit 23 and is collected at an output different from the first output, such as at conduit 9.

Thus, in the first flush step, hydrogen trapped in the void volume of the first bed 11 is swept to product by the entering air and desorbing carbon dioxide. Air trapped in the void volume of the second bed 13 is purged from the bed 13 by the entering feed gas. This step improves the overall efficiency of the process by continuing to recover hydrogen that is trapped from the prior feed step and preventing air from the prior purge step from contaminating the hydrogen containing product after the next valve switch. This flush step is short, such as less than 1/5 of the time of the prior feed/purge step, such as 1/10 to 1/50 of the time of the prior step. For example, for an about 90 second feed/purge step, the flush step may be about 4 seconds.

FIG. 2C shows the apparatus 1 during a second feed/purge step which is conducted after the first flush step. In this step, the second bed 13 is fed with a feed gas stream, such as the fuel stack fuel exhaust stream, while the first bed 11 is fed with a purge gas, such as dried air, to regenerate the first bed 11. Thus, in this step, the flow paths in valves 17 and 19 switch. This step is generally the same as the first feed/purge step, but with the beds reversed.

The feed gas inlet stream is provided from conduit 3 through valve 15 into the second adsorbent bed 13. Preferably the feed gas inlet stream is provided into the second bed 13 in the opposite (i.e., countercurrent) direction from the direction in which the purge gas inlet stream is provided into the second bed 13 in the first purge step. The feed gas outlet stream, which comprises at least one separated component of the feed gas, such as hydrogen and carbon monoxide, is collected at the first output, such as in the third conduit 7. The purge gas inlet stream is provided from conduit 5 through valves 17 and 19 and conduit 23 into the first adsorbent bed 11. Preferably the purge gas inlet stream is provided into the first bed 11 in the opposite (i.e., countercurrent) direction from the direction in which the feed gas inlet stream is provided into the first bed 11 in the first feed step. The purge gas outlet stream is collected from the first bed 11 at an output different from the first output, such as at the fourth conduit 9.

FIG. 2D illustrates the apparatus 1 in a second flush step which is conducted after the second feed/purge step. In this step, the feed valve 15 and the regeneration valve 17 switch flow directions from the prior step, while the product valve 19 does not. This step is similar to the first flush steps, but with the beds reversed.

The purge gas inlet stream is provided from conduit 5 through valves 17 and 15 and conduit 21 into the second adsorbent bed 13. Preferably, this steam is provided into the bed 13 in the same direction as the feed gas inlet stream in the prior two steps. The purge gas outlet stream, which comprises at least one component of the feed gas, such as hydrogen, that was trapped in a void volume of the second adsorbent bed 13, is collected at the first output, such as the third conduit 7.

The feed gas inlet stream is provided from conduit 3 through valve 15 into the first adsorbent bed 11. The feed gas outlet stream, which comprises a portion of the purge gas, such as air, that was trapped in a void volume of the first bed 11, is collected at an output different from the first output, such as at the fourth conduit 9. Then the first feed/purge step shown in FIG. 2A is repeated. In general, the four steps described above are repeated a plurality of times in the same order.

It should be noted the feed gas inlet stream is preferably provided in each of the first 11 and the second 13 adsorbent beds in the same direction in the steps described above. In the first and the second flush steps, the purge gas inlet stream is provided into each of the first and the second adsorbent beds in the same direction as the feed gas inlet stream direction. In contrast, in the first and the second feed/purge steps, the purge gas inlet stream is provided into each of the first and the second adsorbent beds in a different direction, such as the opposite direction, from the feed gas inlet stream direction.

The countercurrent purge gas inlet stream flow is advantageous because it is believed that it will reduce the amount of carbon dioxide in the hydrogen product stream compared to a co-current flow during the purge steps. Some water will adsorb near the inlet of the carbon bed during the feed step. During the purge or regeneration step, the bed is purged counter currently with dried air. Because activated carbon is used for adsorption of carbon dioxide and activated carbon does not adsorb water appreciably at moderately low relative humidities, in order to prevent accumulation of water in the bed, the regeneration purge only needs to be dried to a relative humidity of roughly 30 to 50%. During the feed step, carbon monoxide will be pushed into the product (with the hydrogen) by using the beds efficiently for carbon dioxide removal (i.e., by advancing the carbon dioxide wave reasonably far into the beds). The countercurrent regeneration step will reduce the level of carbon dioxide in the hydrogen stream in comparison to a cocurrent regeneration step. The dual flush step will maximize both hydrogen recovery and air rejection from the hydrogen product.

As noted above, in the partial pressure swing adsorption method, the feed gas inlet stream is not pressurized prior to being provided into the first and the second adsorbent beds. Furthermore, the above four steps are preferably conducted without external heating of the adsorbent beds.

In operation, the first bed 11 performs the following functions. It receives the feed gas inlet stream from the first conduit 3 and provides at least one separated component of the feed gas to the third conduit 7 in a first feed/purge step. It receives the purge gas inlet stream from the second conduit 5 and provides a purge gas outlet stream, which comprises at least one component of the feed gas that was trapped in a void volume of the first bed to the third conduit 7 in a first flush step. It receives a purge gas inlet stream from the second conduit 5 and provides a purge gas outlet stream to an output different from the third conduit 7, such as the fourth conduit 9, in a second feed/purge step. It also receives the feed gas inlet stream from the first conduit 3 and provides a feed gas outlet stream, which comprises a portion of the purge gas that was trapped in a void volume of the first bed, to at an output different from the third conduit 7, such as the fourth conduit 9, in a second flush step.

In operation, the second bed 13 performs the following functions. It receives a purge gas inlet stream from the second conduit 5 and provides a purge gas outlet stream to at an output different from the third conduit 7, such as the fourth conduit 9, in a first feed/purge step. It receives the feed gas inlet stream from the first conduit 3 and provides the feed gas outlet stream, which comprises a portion of the purge gas that was trapped in a void volume of the second bed 13, to an output different from the third conduit 7, such as the fourth conduit 9, in a first flush step. It receives the feed gas inlet stream from the first conduit 3 and provides the feed gas outlet stream comprising at least one separated component of the feed gas to the third conduit 7 in a second feed/purge step. It also receives the purge gas inlet stream from the second conduit 5 and provides the purge gas outlet stream, which comprises at least one component of the feed gas that was trapped in a void volume of the second bed 13 to the third conduit 7 in a second flush step.

Thus, at least a majority of the carbon dioxide and water vapor in the feed gas inlet stream is adsorbed by the first 11 and the second 13 adsorbent beds during the first and the second feed/purge steps, respectively. The adsorbed carbon dioxide and water vapor is removed from the first and the second adsorbent beds by the purge gas inlet stream during the second and the first feed/purge steps, respectively. The removed carbon dioxide and water vapor are collected with the purge gas outlet stream at the second output during the second and the first feed/purge steps.

It is noted that the regeneration (i.e., purging) of the bed will be accompanied by a cooling of the bed as CO.sub.2 desorbs. It is believed that this will shift adsorption equilibrium to lower partial pressures for CO.sub.2 and will slow regeneration. This and the expanding velocity front during regeneration may be taken into account in setting the purge gas (i.e., dry air) flow rate. For example, the inlet air volumetric flowrate for regeneration may be greater than, such as 1.5 times greater than, the outlet flowrate of hydrogen and carbon monoxide. It is believed that allowing for desorption of carbon dioxide during regeneration, the outlet flowrate for regeneration will exceed the inlet flowrate of the feed.

The apparatus 1 may have the following non-limiting features. The adsorbent bed material preferably comprises activated carbon for hydrogen separation from the fuel cell stack fuel exhaust. For example, Calgon BPL activated carbon, 6.times.16 or 4.times.10 mesh may be used. The beds 11, 13 may be cylindrical beds 2-12 inches in diameter and 1-6 feed long, such as 6 inches in diameter and 3 feet long, for example, depending on the size of the fuel cell stack and on the flow rate of the gases. The duration of the feed/purge steps may be more than 1 minute while the duration of the flush steps may be a few seconds. For example, the feed/purge duration may be 1 to 3 minutes, such as 1.5 minutes, while the flush duration may be 3-5 seconds, such as 4 seconds.

The method of the first embodiment is designed to provide a high hydrogen recovery (with flush steps), high carbon dioxide separation (with flush and countercurrent regeneration steps), high degree of air rejection (with flush steps), regeneration using a purge gas having a relatively low dryness, such as air having 30-50% relative humidity, low energy requirements, high robustness (i.e., easily tunable and adaptable to changes in operating conditions), simple operation with few moving parts, high scalability, and low to moderate capital cost.

The dry air for the purge steps may be obtained by any suitable method. For example, the dry air can easily be achieved using temperature swing adsorption cycle with water vapor absorbing beds, such as silica gel or activated alumina beds. Silica gel has a somewhat higher capacity for water than alumina. However, it will fracture if very dry and contacted with a water mist. If this is likely, a protective layer of a non-decrepitating silica gel can be used, or activated alumina can be used.

The temperature swing adsorption cycle uses two beds (i.e., beds other than beds 11, 13 shown in FIG. 1). One bed is used in the adsorption mode while the other is being regenerated (heated and cooled). The steps in the cycle are as follows.

In a first adsorption step, a working capacity of 10 mol H.sub.2O/kg of silica gel can be used. Considering the worst case, the air would be saturated with water at 30.degree. C. The partial pressure of water in air saturated at 30.degree. C. is 0.042 bar. For example, to produce a dry air flow rate of 144 slpm from this wet air, 0.28 mol/min of water must be removed. At the designated working capacity, silica gel is consumed at a rate of 0.028 kg/min. A bed containing 2 kg of silica gel can remain on stream for 72 minutes. Given a specific gravity of silica gel of 0.72 (corresponding to a bulk density of 45 lb/ft.sup.3), the bed will dry 4300 bed volumes of feed during this time (with 12,000 temperature corrected liters of wet feed dried by a bed 2.8 liters in volume). The dried air is provided through conduit 5 into the apparatus 1.

In a second heating step, the bed is heated counter currently with a warm feed (e.g., 80.degree. C. or other suitable moderately warm or hot temperature). The bed is heated after about 1000 bed volumes have been passed into it. Somewhat more energy will be required to heat metal parts also.

In a third cooling step, the bed is cooled cocurrently (same direction as adsorption) with the wet air feed. It will take about 800 bed volumes to cool the bed. This will deposit water at the bed inlet and use up some of the capacity for adsorption, reducing it to about 3500 bed volumes. While the first bed is undergoing the adsorption step, the second bed is undergoing heating or cooling steps. While the second bed is undergoing the adsorption step, the firs