Abstract:
Methods and systems for carbon monoxide clean-up are provided. The methods and systems utilize water gas shift reactors having water gas shift catalysts and hydride heat exchangers having metal hydrides. The methods and systems allow hydrogen from a reactant stream to be stored in the metal hydride during carbon-monoxide clean-up and subsequently released into the reactant stream. It is emphasized that this abstract is provided to comply with the rules requiring an abstract which will allow a searcher or other reader to quickly ascertain the subject matter of the technical disclosure. It is submitted with the understanding that is will not be used to interpret or limit the scope or meaning of the claims. 37 CFR 1.72(b).

Description:
BACKGROUND OF THE INVENTION  
         [0001]    The present invention relates to integrated systems and methods for carbon monoxide clean up. More particularly, the present invention relates to carbon monoxide clean-up systems having water gas shift reactors and heat exchangers having metal hydrides therein and methods of removing carbon monoxide from a reactant stream employing such systems.  
           [0002]    Hydrogen fuel cells have become an increasingly attractive source of power for a variety of applications. However, the storage, transportation, and delivery of hydrogen presents a number of difficulties. Thus, hydrogen fuel cell systems may be equipped with reforming systems for producing hydrogen from an alternate fuel source such as a hydrocarbon fuel. However, these reforming systems often require extensive carbon monoxide removal subsystems because hydrogen fuel cells are generally not tolerant of carbon monoxide. The subsystems add to the cost, complexity, and size of reforming systems.  
           [0003]    Thus, there remains a need in the art for carbon monoxide clean-up systems that are more cost effective, less complex, and smaller.  
         SUMMARY OF THE INVENTION  
         [0004]    This need is met by the present invention that provides carbon monoxide clean-up systems and methods of using the same. Additionally, fuel cell systems and vehicles are provided.  
           [0005]    In accordance with an embodiment of the present invention, a device comprising a carbon monoxide clean-up system is provided. The carbon monoxide clean-up system comprises a first water gas shift reactor having a first water gas shift catalyst; a first hydride heat exchanger having a first metal hydride disposed therein; and a second water gas shift reactor having a second water gas shift catalyst. The first hydride heat exchanger is in communication with the first water gas shift reactor, and the first hydride heat exchanger is disposed such that a reactant stream may pass through the first water gas shift reactor prior to passing through the first heat exchanger. The second water gas shift reactor is in communication with the first heat exchanger, and the second water gas shift reactor is disposed such that the reactant stream may pass through the second water gas shift reactor after passing through the first heat exchanger. The first hydride heat exchanger may be disposed so that the reactant stream may again pass through the first hydride heat exchanger after passing through the second water gas shift reactor.  
           [0006]    In accordance with an embodiment of the present invention, a method of removing carbon monoxide from a reactant stream is provided. The method comprises providing a carbon monoxide clean-up system and passing a reactant stream comprising carbon monoxide and hydrogen through the carbon monoxide clean-up system. The carbon monoxide clean-up system comprises: a first water gas shift reactor having at one least first water gas shift catalyst; a first hydride heat exchanger having a first metal hydride disposed therein; and a second water gas shift reactor having a second water gas shift catalyst. The first hydride heat exchanger is in communication with the first water gas shift reactor, and the second water gas shift reactor is in communication with the first heat exchanger.  
           [0007]    The reactant stream may be passed through the first water gas shift reactor, and the first water gas shift reactor is configured such that the concentration of carbon monoxide in the reactant stream is reduced upon passage of the reactant stream through the first water gas shift reactor. The reactant stream may passed through the first hydride heat exchanger subsequent to passing the reactant stream through the first water gas shift reactor, and the first hydride heat exchanger is configured such that the first metal hydride is hydrided with the hydrogen and the concentration of hydrogen in the reactant stream is reduced upon passage of the reactant stream through the first hydride heat exchanger. The reactant stream may passed through the second water gas shift reactor subsequent to passing the reactant stream through the first hydride heat exchanger, and the second water gas shift reactor is configured such that the concentration of carbon monoxide in the reactant stream is further reduced upon passage of the reactant stream through the second water gas shift reactor. The reactant stream is passed through the first hydride heat exchanger subsequent to passing the reactant stream through the second water gas shift reactor, and the first hydride heat exchanger is configured such that the first metal hydride is dehydrided and the concentration of hydrogen in the reactant stream is increased upon passage of the reactant stream through the first hydride heat exchanger.  
           [0008]    The carbon monoxide clean-up system may further comprise: a second hydride heat exchanger having a second metal hydride disposed therein, wherein the second metal hydride heat exchanger is in communication with the second water gas shift reactor; and a third water gas shift reactor having a third water gas shift catalyst, wherein the third water gas shift reactor is in communication with the second hydride heat exchanger.  
           [0009]    The reactant stream is passed through the first water gas shift reactor, and the first water gas shift reactor is configured such that the concentration of carbon monoxide in the reactant stream is reduced upon passage of the reactant stream through the first water gas shift reactor. The reactant stream is passed through the first hydride heat exchanger subsequent to passing the reactant stream through the first water gas shift reactor, and the first hydride heat exchanger is configured such that the first metal hydride is hydrided with the hydrogen and the concentration of hydrogen in the reactant stream is reduced upon passage of the reactant stream through the first hydride heat exchanger. The reactant stream is passed through the second water gas shift reactor subsequent to passing the reactant stream through the first hydride heat exchanger, and the second water gas shift reactor is configured such that the concentration of carbon monoxide in the reactant stream is further reduced upon passage of the reactant stream through the second water gas shift reactor.  
           [0010]    The reactant stream is passed through the second hydride heat exchanger subsequent to passing the reactant stream through the second water gas shift reactor, and the second hydride heat exchanger is configured such that the second metal hydride is hydrided with the hydrogen and the concentration of hydrogen in the reactant stream is reduced upon passage of the reactant stream through the second hydride heat exchanger. The reactant stream is passed through the third water gas shift reactor subsequent to passing the reactant stream through the second hydride heat exchanger, and the third water gas shift reactor is configured such that the concentration of carbon monoxide in the reactant stream is further reduced upon passage of the reactant stream through the third water gas shift reactor. The reactant stream is passed through the second hydride heat exchanger subsequent to passing the reactant stream through the third water gas shift reactor, and the second hydride heat exchanger is configured such that the first metal hydride is dehydrided and the concentration of hydrogen in the reactant stream is increased upon passage of the reactant stream through the second hydride heat exchanger. The reactant stream is passed through the first hydride heat exchanger subsequent to passing the reactant stream through the second hydride heat exchanger, and the first hydride heat exchanger is configured such that the first metal hydride is dehydrided and the concentration of hydrogen in the reactant stream is increased upon passage of the reactant stream through the first hydride heat exchanger. 
       
    
    
     BRIEF DESCRIPTION OF THE SEVERAL VIEWS OF THE DRAWINGS  
       [0011]    The following detailed description of the preferred embodiments of the present invention can be best understood when read in conjunction with the following drawings, where like structure is indicated with like reference numerals and in which:  
         [0012]    [0012]FIG. 1 is a schematic illustration of a carbon monoxide clean-up system in accordance with the present invention.  
         [0013]    [0013]FIG. 2 is a schematic illustration of another carbon monoxide clean-up system in accordance with the present invention.  
         [0014]    [0014]FIG. 3 is a schematic illustration of yet another carbon monoxide clean-up system in accordance with the present invention.  
         [0015]    [0015]FIG. 4 is schematic illustration of a fuel cell system in accordance with the present invention.  
         [0016]    [0016]FIG. 5 is a schematic illustration of a vehicle having a fuel processing system and an electrochemical reaction cell in accordance with the present invention. 
     
    
     DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS  
       [0017]    Referring to FIG. 1, a carbon monoxide clean-up system  10  in accordance with an embodiment of the present invention is illustrated. The carbon monoxide clean-up system comprises a first water gas shift reactor  12 , a first hydride heat exchanger  14 , and a second water gas shift reactor  16 . The first hydride heat exchanger  14  is in communication with the first water gas shift reactor  12  and the second water gas shift reactor  16 . A reactant stream  18  comprising carbon monoxide and hydrogen is provided to the carbon monoxide clean-up system  10 , and the reactant stream passes through the carbon monoxide clean-up system  10 .  
         [0018]    The first water gas shift reactor  12  is any suitable water gas shift reactor having a first water gas shift catalyst. For example, the first water gas shift reactor  12  may be a monolith containing a first water gas shift catalyst. It will be understood that the first water gas shift reactor  12  may have more than one water gas shift catalyst. The reactant stream  18  passes through the first water gas shift reactor  12 , and a standard water gas shift reaction takes place on the water gas shift catalyst. The water gas shift reaction is generally: 
         CO+H 2 O→H 2 +CO 2   
         [0019]    Thus, the first water gas shift reactor  12  is configured such that the concentration of carbon monoxide in the reactant stream is reduced.  
         [0020]    The first hydride heat exchanger  14  is any suitable heat exchanger of any suitable configuration. For example, the first hydride heat exchanger  14  may be a tube type, coil type, or plate type heat exchanger. The first hydride heat exchanger  14  has a first metal hydride disposed therein. For purposes of defining and describing the present invention, “metal hydride” shall be understood as referring to an intermetallic compound capable of absorbing hydrogen. The first metal hydride may be any suitable metal hydride, and the first hydride heat exchanger  14  may be coated or filled with the metal hydride in any suitable manner. It will be understood that the first hydride heat exchanger  14  may alternatively have more than one metal hydride disposed therein.  
         [0021]    The reactant stream  18  is passed through the first hydride heat exchanger  14  subsequent to passing through the first water gas shift reactor  12 . The first metal hydride is selected such that hydrogen in the reactant stream is absorbed by the metal hydride at the temperature of the reactant stream  18  and at the partial pressure of hydrogen in the reactant stream  18 , after passing through the first water gas shift reactor  12 . Thus, the metal hydride becomes hydrided and the concentration of hydrogen in the reactant stream  18  is reduced. Generally, the metal hydride becomes fully hydrided for the given temperature and partial pressure of hydrogen of the reactant stream  18 .  
         [0022]    The metal hydride may be selected such that it absorbs hydrogen at a temperature between about 200° C. to about 350° C. More generally, the metal hydride is selected such that is absorbs hydrogen at a temperature between about 250° C. to about 300° C. The reactant stream  18  may be cooled after passing through the first water gas shift reactor  12  and prior to passing through the first hydride heat exchanger  14  because the water gas shift reaction is generally exothermic. Thus, the reactant stream  18  may be cooled to a suitable temperature at which the metal hydride may absorb hydrogen. The reactant stream  18  may be cooled in any suitable manner. For example, the reactant stream  18  may be cooled using a separate heat exchanger  17 .  
         [0023]    Suitable metal hydrides include those containing Ti, Mg, and Pd. For example, the metal hydride may be selected from metal hydrides of the Mg 2 Ni family. The metal hydrides may be doped with platinum group metals (PGM). Such PGM doped metal hydrides are described in U.S. Pat. No. 6,165,643, the disclosure of which is incorporated by reference herein. PGM doped metal hydrides may enhance the kinetics of the hydration/dehydration in the first hydride heat exchanger  14 , and, thus, the size of the first hydride heat exchanger  14  may be reduced.  
         [0024]    The second water gas shift reactor  16  is any suitable water gas shift reactor having a second water gas shift catalyst. It will be understood that that second water gas shift reactor  16  may have more than one water gas shift catalyst. The reactant stream  18  is passed through the second water gas shift reactor  16  subsequent to passing through the first hydride heat exchanger  14 . A standard water gas shift reaction takes place on the second water gas shift catalyst, and the concentration of carbon monoxide in the reactant stream  18  is further reduced.  
         [0025]    Once the reactant stream  18  has passed through the second water gas shift reactor  16 , it is again passed through the first hydride heat exchanger  14 . Because the reactant stream  18  is depleted of hydrogen prior to reentering the first hydride heat exchanger  14 , the partial pressure of hydrogen in the reactant stream  18  has been reduced. Thus, the first metal hydride is dehydrided when the reactant stream  18  passes through the first hydride heat exchanger  14  after passing through the second water gas shift reactor  16 , and the concentration of hydrogen in reactant stream  18  is increased. Additionally, the reactant stream  18  may be heated prior to passing through the first hydride heat exchanger  14  after exiting the second water gas shift reactor  16  in order to increase the rate at which the metal hydride is dehydrided. For example, the reactant stream  18  may be heated to a temperature between about 250° C. to about 450° C. More generally, the reactant stream  18  may be heated to a temperature between about 250° C. to about 350° C. The reactant stream  18  may be heated in any suitable manner. For example, the reactant stream  18  may be heated using a heat exchanger  19 .  
         [0026]    The first and second water gas shift catalysts may be any suitable water gas shift catalysts. Generally, the first and second water gas shift catalysts are selected such that an operational temperature of the first and second water gas shift catalysts and a hydridable temperature of the first metal hydride are approximately equal. For purposes of defining and describing the present invention “operational temperature” shall be understood as referring to a temperature range at which a given water gas shift catalyst may operate. For purposes of defining and describing the present invention “hydridable temperature” shall be understood as referring to a temperature range in which a given metal hydride may be hydrided. For example, the first metal hydride may absorb hydrogen in a temperature range of about 200° C. to about 350° C. Thus, standard low temperature water gas shift catalysts, such as Cu—Al—Zn, may be used as the first and second water gas shift catalyst. Other water gas shift catalysts include Pd/Fe formulations and Pt/Ce formulations.  
         [0027]    Once the reactant stream  18  has passed through the first hydride heat exchanger  14  after exiting the second water gas shift reactor, the reactant stream  18  generally has a greatly reduced concentration of carbon monoxide. For example, in a reactant stream  18  which entered the carbon monoxide clean-up system  10  with about 6% carbon monoxide, the reactant stream  18  may have a concentration of less than about 70 ppm carbon monoxide on exiting the carbon monoxide clean-up system  10 . It will be understood that other carbon monoxide concentrations in the reactant stream  18  entering and exiting the carbon monoxide clean-up system are possible and are a matter of system design. It will be further understood that the carbon monoxide clean-up system  10  may produce a reactant stream  18  that may be used to feed a hydrogen fuel cell stack without the need for a partial oxidation or methanation unit, which may consume hydrogen. Additionally, the reactant stream  18  may be heated or cooled as needed prior to being provided to a fuel cell stack or the like.  
         [0028]    Referring to FIG. 2, a carbon monoxide clean-up system  10  in accordance with the present invention is illustrated. The carbon monoxide clean-up system  10  comprises a first water gas shift reactor  12   a  having a first water gas shift catalyst, a first hydride heat exchanger  14  having a first metal hydride, and a second water gas shift reactor having a second water gas shift catalyst  16  as discussed above. The first water gas shift reactor  12   a  is configured such that the reactant stream  18  is cooled while passing through the first water gas shift reactor  12   a.  Thus, the first water gas shift reactor  12   a  is cooled. The first water gas shift reactor  12   a  may be cooled in any suitable manner. For example, the first water gas shift reactor  12   a  may be proximate to a heat exchanger  21 .  
         [0029]    The reactant stream  18  passes through the first water gas shift reactor  12   a , and the concentration of carbon monoxide is reduced. When the first water gas shift reactor  12   a  is cooled, the concentration of carbon monoxide in the reactant stream  18  is reduced more than the concentration of carbon monoxide in a reactant stream  18  is reduced when it passes through an uncooled water gas shift reactor. The reactant stream  18  then passes through the first hydride heat exchanger  14 , the second water gas shift reactor  16 , and again through the first hydride heat exchanger  14  as discussed above. After passing through the carbon monoxide clean-up system  10 , the overall carbon monoxide concentration in the reactant stream  18  is lower than in a similar system with an uncooled first water gas shift reactor. For example, the concentration of carbon monoxide in a reactant stream  18  may be reduced from about 6% to about 53 ppm after passing through the carbon monoxide clean-up system  10  illustrated in FIG. 2.  
         [0030]    Referring to FIG. 3, a carbon monoxide clean-up system  10  in accordance with an embodiment of present invention is illustrated. The carbon monoxide clean-up system  10  comprises a first water gas shift reactor  12 , a first hydride heat exchanger  14 , a second water gas shift reactor  16 , a second hydride heat exchanger  20 , and a third water gas shift reactor  22 . A reactant stream  18  comprising carbon monoxide and hydrogen is provided to the carbon monoxide clean-up system  10  and passes therethrough.  
         [0031]    The first, second, and third water gas shift reactors  12 ,  16 ,  22  are any suitable water gas shift reactors having first, second, or third water gas shift catalysts, respectively. The first, second, and third water gas shift catalysts may be the same catalyst or different catalysts. Additionally, the first, second, and third water gas shift reactors  12 ,  16 ,  22  may each have more than one water gas shift catalyst. The first and second hydride heat exchangers  14 ,  20  are any suitable heat exchangers having a first or second metal hydride disposed therein, respectively. The first and second metal hydrides may be the same or different. Additionally, the first and second hydride heat exchangers  14 ,  20  may each have more than one metal hydride disposed therein. The first, second, and third water gas shift catalysts are generally selected such that an operational temperature of the first, second, or third water gas shift catalysts and a hydridable temperature of the first or second metal hydrides are approximately equal.  
         [0032]    The reactant stream  18  passes through the first water gas shift reactor  12 , and the concentration of carbon monoxide in the reactant stream  18  is reduced as described herein. The reactant stream  18  then passes through the first hydride heat exchanger  14 , and hydrogen is absorbed by the first metal hydride. As described herein, the first metal hydride is selected such that hydrogen is absorbed at the temperature and partial pressure of hydrogen of the reactant stream  18  after the reactant stream  18  passes through the first water gas shift reactor  14 . The reactant stream  18  may be cooled after passing through the first water gas shift reactor  12  and prior to passing through the first hydride heat exchanger  14 . For example, the reactant stream  18  may be cooled by a heat exchanger  17 .  
         [0033]    The reactant stream  18  passes through the second water gas shift reactor  16  subsequent to passing through the first hydride heat exchanger  14 , and the concentration of carbon monoxide in the reactant stream  18  is reduced as discussed herein. The reactant stream  18  is then passed through the second hydride heat exchanger  20  such that the concentration of hydrogen in the reactant stream  18  is reduced as the second metal hydride becomes hydrided. The second metal hydride is selected such that hydrogen is absorbed at the temperature and partial pressure of hydrogen of the reactant stream  18  after the reactant stream  18  passes through the second water gas shift reactor  14 . The reactant stream  18  may be cooled after passing through the second water gas shift reactor  16  and prior to passing through the second hydride heat exchanger  16 . For example, the reactant stream  18  may be cooled by a heat exchanger  25 .  
         [0034]    The reactant stream  18  passes through the third water gas shift reactor  22  after passing through the second hydride heat exchanger  20 , and the concentration of carbon monoxide in the reactant stream  18  is reduced as described herein. The reactant stream  18  is then again passed through the second hydride heat exchanger  20 , and the second metal hydride in the second hydride heat exchanger  20  becomes dehydrided such that the concentration of hydrogen in the reactant stream is increased. A heat exchanger  23  may heat the reactant stream  18  prior to entering the second hydride heat exchanger  20  after exiting the third water gas shift reactor  22  in order to increase the rate at which the second metal hydride is dehydrided.  
         [0035]    Once the reactant stream  18  is again passed through the second hydride heat exchanger  20 , it passes through the first hydride heat exchanger  14 . The metal hydride in the first hydride heat exchanger  14  is dehydrided as described herein, and the concentration of hydrogen in the reactant stream  18  is further increased. The carbon monoxide clean-up system  10  illustrated in FIG. 3 may provide an extremely low concentration of carbon monoxide in the reactant stream  18  after the reactant stream  18  passes therethrough. For example, for a reactant stream  18  having a 6.0% carbon monoxide concentration upon entering the carbon monoxide clean-up system  10 , the concentration of carbon monoxide in the reactant stream  18  upon exiting the carbon monoxide clean-up system  10  may be less than about 50 ppm.  
         [0036]    Referring to FIG. 4, an exemplary fuel cell system including a carbon monoxide clean-up system  10  is illustrated. The fuel cell system comprises a fuel processing system  11  with a primary reactor  7  and a carbon monoxide clean-up system  10 . The fuel processing system  11  provides the fuel cell stack  30  with a source of hydrogen. In the primary reactor  10 , a reactant mixture  13  that may contain a hydrocarbon fuel stream and an oxygen-containing stream is flowed into the primary reactor  7 . The oxygen-containing stream may comprise air, steam, and combinations thereof. The reactant mixture  13  may be formed by mixing a hydrocarbon fuel with a preheated air and steam input stream before flowing the reactant mixture into the primary reactor. After the reactant mixture  13  is flowed into the primary reactor  7 , the reactant mixture  13  passes over at least one reaction zone having at least one reforming catalyst and product gas stream  18  containing hydrogen and carbon monoxide is produced catalytically. The primary reactor  7  is generally an autothermal reactor in which hydrogen is produced by combined catalytic partial oxidation and steam reforming reactions, but may alternatively comprise any suitable reactor configuration.  
         [0037]    In one embodiment, the product gas stream  18  exiting the primary reactor  7  may comprise hydrogen, carbon dioxide, carbon monoxide, and trace compounds, and water in the form of steam. The product gas stream then passes through a carbon monoxide clean-up system  10  as described herein, and the concentration of carbon monoxide in the product gas stream  18  is reduced.  
         [0038]    The product gas stream  18  exiting the carbon monoxide clean-up system is then fed into a fuel cell stack  30 . As used herein, the term fuel cell stack refers to one or more fuel cells to form an electrochemical energy converter. As is illustrated schematically in FIG. 1, the electrochemical energy converter may have an anode side  34  and a cathode side  32  separated by diffusion barrier layer  35 . The carbon monoxide purged product stream  24 ′ is fed into the anode side  34  of the fuel cell stack  30 . An oxidant stream  36  is fed into the cathode side  32 . The hydrogen from the carbon monoxide purged product stream  24 ′ and the oxygen from the oxidant stream  36  react in the fuel cell stack  30  to produce electricity for powering a load  38 . A variety of alternative fuel cell designs are contemplated be present invention including designs that include a plurality of anodes  34 , a plurality of cathodes  32 , or any configuration where hydrogen is utilized in the production of electricity.  
         [0039]    Referring to FIG. 5, the device of the present invention may be a vehicle  48  and the vehicle may have a vehicle body  50  and an electrochemical catalytic reaction cell comprising a fuel cell  30 . The fuel cell  13  may be configured to at least partially provide the vehicle body with motive power. The vehicle  48  may also have a fuel processing system  30  to supply the fuel cell  13  with hydrogen, and the fuel processing system may include a carbon monoxide clean-up system as illustrated in FIG. 4. It will be understood by those having skill in the art that fuel cell  13  and fuel processing system  30  are shown schematically and may be used or placed in any suitable manner within the vehicle body  50 .  
         [0040]    It will be obvious to those skilled in the art that various changes may be made without departing from the scope of the invention, which is not to be considered limited to what is described in the specification.