Abstract:
A method of operating an electrochemical cell stack is provided. The method includes feeding a reactant gas to the cell stack. The flow of the reactant gas is halted to at least one cell in the cell stack. The voltage applied to the at least one cell is increased. The flow of reactant gas to the at least one cell is initiated in response to the voltage increasing above a threshold for a predetermined amount of time.

Description:
CROSS REFERENCE TO RELATED APPLICATIONS 
       [0001]    The present application is a nonprovisional application of U.S. Provisional Application Ser. No. 61/829,478 filed on May 31, 2013 entitled “Hydrogen Recycling Apparatus and Method of Operation,” the contents of which are incorporated herein by reference. 
     
    
     BACKGROUND OF THE INVENTION 
       [0002]    The subject matter disclosed herein relates to an electrochemical system for separating hydrogen from contaminants in a gas stream substantially composed of hydrogen, and in particular to a system for separating hydrogen from a mixture containing carbon monoxide (“CO”) contaminant. 
         [0003]    Polymer electrode membrane (“PEM”) systems use an electrochemical process to separate hydrogen from other elements or compounds. One example of PEM system is an electrolysis cell stack that separates water into hydrogen and oxygen gas. Another type of PEM system may be used to separate hydrogen from a gas stream comprised of hydrogen and other compounds such as CO 2 , CO, N 2 , etc. These types of PEM systems may be applicable to applications that use hydrogen as a process fluid, such as a heat treating operation where the heat treating is performed in a hydrogen atmosphere. 
         [0004]    The PEM system is typically comprised of a plurality of cells arranged electrically in series. Each of the cells consists of a polymer membrane with an anode and a cathode electrode arranged on opposite sides. When the PEM system is used to process the exhaust gas stream in an application such the aforementioned heat treating operation, operation may be stalled when an appreciable concentrations carbon monoxide (“CO”) is present in the hydrogen is fed to an electrode in a PEM cell. The hydrogen is adsorbed on the catalyst and effectively stops the operation of the cell. When carbon monoxide (“CO”) is fed to the anode of an electrochemical hydrogen pump or compressor, thermodynamics predicts that a fraction of the CO will participate in an equilibrium reaction with water, which is available due to PEM hydration, and form CO 2 . Typically, in practice, this reaction is very slow and the electrode catalyst is rapidly de-activated by the CO. 
         [0005]    Some prior art systems have tried to address this issue by operating at very high temperatures to accelerate the oxidation of CO. This often requires alternative, high temperature membranes. Another approach that is often explored is raising the cell voltage. When the cell voltage is raised, oxidation of CO is driven at the anode by the external power source, consuming water and forming hydrogen (“H 2 ”) at the cathode. This approach works to a degree. Typically, the high voltage is applied momentarily in a pulse or wave function of some sort. However, anytime the voltage applied to the cell is elevated, the power consumption is elevated. 
         [0006]    Pulsing the voltage to an elevated voltage also creates other problems. As the voltage is driven higher in an electrochemical hydrogen compressor, although oxidation of CO at the anode is promoted, more hydrogen is oxidized. In other words, the pump rate is increased. A portion of the resultant electrical current will go to oxidize hydrogen while another portion will go to oxidization of CO. So if a hydrogen pump or compressor is operating and the voltage is pulsed higher, the presence of the hydrogen on the anode functions almost as a sacrificial material, preventing the oxidation of CO in favor of oxidation of hydrogen. 
         [0007]    Accordingly, while existing PEM systems are suitable for their intended purposes, the need for improvement remains, particularly in providing a PEM system where hydrogen is separated from CO at lower power consumption levels and/or lower operational temperatures. 
       BRIEF DESCRIPTION OF THE INVENTION 
       [0008]    According to one aspect of the invention, a method of operating an electrochemical cell stack is provided. The method includes feeding a reactant gas to the cell stack. The flow of the reactant gas is halted to at least one cell in the cell stack. The voltage applied to the at least one cell is increased. The flow of reactant gas to the at least one cell is initiated in response to the voltage increasing above a threshold or the passing of an appropriate period of time. These and other advantages and features will become more apparent from the following description taken in conjunction with the drawings. 
         [0009]    According to another aspect of the invention, a system is provided. The system including a source of reactant gas. The reactant gas including hydrogen and carbon monoxide. An electrochemical cell having an anode and a cathode is provided. The anode having a catalyst configured to convert during operation at least a first portion of the reactant gas to carbon dioxide, the cathode having a catalyst configured to convert during operation at least a second portion of the reactant gas to hydrogen gas. A controller is operably coupled to the source of reactant gas, the controller configured to halt the flow of reactant gas to the electrochemical cell and reinitiate the flow of reactant gas in response to the voltage increasing above a threshold for a predetermined amount of time. 
     
    
     
       BRIEF DESCRIPTION OF THE DRAWING 
         [0010]    The subject matter, which is regarded as the invention, is particularly pointed out and distinctly claimed in the claims at the conclusion of the specification. The foregoing and other features, and advantages of the invention are apparent from the following detailed description taken in conjunction with the accompanying drawings in which: 
           [0011]      FIG. 1  is a schematic illustration of a heat treating process having a hydrogen recycling apparatus in accordance with an embodiment of the invention; 
           [0012]      FIG. 2  is a schematic illustration of the hydrogen recycling apparatus of  FIG. 1  in accordance with an embodiment of the invention; 
           [0013]      FIG. 3  is a schematic illustration of a reaction within a cell of the hydrogen recycling apparatus of  FIG. 2 ; 
           [0014]      FIG. 4  is a flow diagram of a process for separating hydrogen from contaminants in gas stream; 
           [0015]      FIG. 5  is a graphical illustration of the current density and voltage within a cell of the hydrogen recycling apparatus of  FIG. 2 ; and 
           [0016]      FIG. 6  is a flow diagram of another process for operating a fuel cell. 
       
    
    
       [0017]    The detailed description explains embodiments of the invention, together with advantages and features, by way of example with reference to the drawings. 
       DETAILED DESCRIPTION OF THE INVENTION 
       [0018]    Embodiments of the present invention provide advantages in the separation of hydrogen from a gas stream that includes contaminants, such as CO. Embodiments of the invention provide advantages in restoration of catalyst in an electrochemical cell to allow operation of the electrochemical cell at high CO levels. Still further embodiments of the invention provide advantages in allowing the operation of a fuel cell on a gas stream having CO. 
         [0019]    Referring now to  FIG. 1 , an exemplary process  20  is shown having an electrochemical hydrogen recycling device  22 . The process  20  typically includes a bulk hydrogen storage system  24  and a bulk nitrogen storage system  26 . The gas from storage systems  24 ,  26  are coupled to a manifold on a gas flow control panel  28 . The gas flow control panel  28  is configured to flow the desired volume and mixture of gas to a process device, such as furnace  30  for example. Where the process device is a furnace  30 , a material to be heat treated is fed from a first roll  32  into the furnace  30  where the material is processed in an atmosphere that is primarily hydrogen. In the exemplary embodiment, the atmosphere within the furnace  30  is approximately 99% hydrogen. The material exits the furnace  30  and is collected on a material roll  34 . 
         [0020]    A process waste gas  36  exits the furnace  30 . In a prior art system, this process waste, which contains about 100 ppm of CO with the remainder being hydrogen is vented to the atmosphere. In the exemplary embodiment, this process waste gas  36  is directed via conduits  38 ,  40  into the hydrogen recycling apparatus  22 . As will be discussed in more detail below, the hydrogen recycling apparatus separates the hydrogen from the CO and flows the separated hydrogen via conduit  42  back to the gas flow control panel  28 . The CO, some of which may now be converted to CO 2  is transferred via conduit  44  to the gas stream  36  where it is vented to atmosphere. 
         [0021]    Turning now to  FIGS. 2-3 , the hydrogen recycling apparatus  22  is shown. The apparatus  22  includes an electrochemical cell stack  46 . The cell stack  46  is comprised of a plurality of cells  48  arranged in series or in parallel. Each of the cells  48  includes a polymer electrode membrane assembly (“MEA”)  50 . The electrode membrane assembly  50  includes a proton exchange membrane  52  and electrodes (anode  54  and cathode  56 ) disposed at opposing sides of proton exchange membrane  52 . As shown, both anode  54  and cathode  56  are positioned in ionic communication with proton exchange membrane  52  within the active areas of the cell. The active areas are defined as being the areas of a cell at which hydrogen is dissociated from a gas stream to produce hydrogen gas on the cathode and CO 2  on the anode. 
         [0022]    Proton exchange membrane  52  can comprise electrolytes that are preferably solids under the operating conditions of the electrochemical cell. Useful materials from which the membrane  52  can be fabricated include proton conducting ionomers and ion exchange resins. Ion-exchange resins useful as proton conducting materials include hydrocarbon and fluorocarbon-type resins. Fluorocarbon-type resins typically exhibit excellent resistance to oxidation by halogen, strong acids, and bases. One family of fluorocarbon-type resins having sulfonic acid group functionality is NAFION® resins (commercially available from E. I. du Pont de Nemours and Company, Wilmington, Del.). 
         [0023]    Anode  54  and cathode  56  are fabricated from catalytic materials suitable for performing the needed electrochemical reaction (e.g., the dissociation of hydrogen gas). Suitable catalytic materials for anode  54  and cathode  56  includes, but is not limited to, platinum, palladium, rhodium, carbon, gold, tantalum, tungsten, ruthenium, iridium, osmium, alloys thereof, and the like, as well as combinations of the foregoing materials. Anode  54  and cathode  56  are positioned adjacent to, and preferably in contact with, their respective proton exchange membrane  52  and are defined by structures comprising discrete catalytic particles adsorbed onto a porous substrate. Adhesion of the catalytic particles onto the substrate may be by any method including, but not limited to, spraying, dipping, painting, imbibing, vapor depositing, combinations of the foregoing methods, and the like. Alternately, the catalytic particles may be deposited directly onto opposing sides of proton exchange membranes  52  or onto the support members. 
         [0024]    It should be appreciated that each cell  48  may include further components as is known in the art. These additional components may include gas diffusion layers, flow fields, current collectors and the like. 
         [0025]    The process waste gas  36  is routed through conduits  38 ,  40  into a conduit  58  having a control valve  60 . The valve  60  controls the flow of process waste gas  36  to the electrochemical cell stack  46  and hence the flow to cells  48 . The electrochemical cell stack  46  includes a first output  62  and a second output  64 . The first output  62  is fluidly coupled to the anode side of each cell  48  and connects the electrochemical cell stack  46  to the conduit  44  which vents the balance of the waste gas (e.g. CO 2 ) to the atmosphere. The second output  64  is fluidly coupled to the cathode side of each cell  48  and transfers the separated hydrogen gas to conduit  42  and back to the gas flow control panel  28 . It should be appreciated that the apparatus  22  may include additional components, such as phase separator  66  and control valves for example, as is known in the art and a means of maintaining membrane hydration as known in the art. 
         [0026]    The transfer or feeding of a gas stream with high levels of CO to an electrochemical cell  46  will rapidly result in a degradation of the electrochemical cell  50  due to de-activation of the catalyst by the CO. Referring now to  FIG. 4 , a method  70  is shown for restoring catalyst by electro-oxidizing the CO. The process  70  starts in block  72  where the waste gas stream or reactant of H 2 +CO is fed to the electrochemical cell stack  46 . The waste gas stream is processed for a predetermined amount of time and then process  70  proceeds to block  74  where the flow of waste stream gas is halted. When the waste gas stream is halted to the cells  48 , the catalyst can be restored by removing the sacrificial hydrogen. When the valve  60  leading the cell anode is closed, the cell  48  will go to the maximum voltage set on the power supply (not shown) coupled to the electrochemical cell stack. Since the electrode is starved of H 2  and the applied potential is raised simultaneously, causing the electro-oxidizing the CO on the electrode catalyst. As a result, the catalyst is restored. 
         [0027]    To control the amount of time the cells  48  are without the waste gas stream, the process  70  proceeds to query block  76  where it is determined if the cell voltages are equal to a desired voltage (e.g. the maximum voltage set of the power supply). If query block  76  returns a negative, the process  70  loops back to block  74 . If query block  76  returns a positive, the process  70  proceeds to block  78  where the flow of process waste gas resumes. The process  70  then proceeds to block  80  where the process waste gas flows to the electrochemical cell stack  46  for a predetermined amount of time. The longer that the valve  60  is closed, the more oxidation occurs. In the exemplary embodiment, the flow of process waste gas is halted approximately every seven minutes. In other embodiments, the voltage limit could be constrained by the applied voltage from the power supply, and the voltage could be left at this high voltage by leaving the valve closed for a longer period of time as shown in  FIG. 5 , when the valve is closed at time t 1 , the voltage gradually increases, while the current decreases. It should be appreciated that the process  70  provides advantages in that, since the current is decreasing, the power consumed by the electrochemical cell stack  46  also decreases while the catalyst is restored. When the flow of process waste gas is resumed, the voltage once again returns to its desired operating level. 
         [0028]    It should be appreciated that in other embodiments, the amount of time the flow of process waste gas is halted and/or the length of time during which it is allowed to continue may be a function of time only or a combination of time and voltage. It is contemplated that in some embodiments, the voltage and or time may be changed based on operating conditions or parameters. 
         [0029]    Referring now to  FIG. 6 , a method  82  is shown of operating a fuel cell in a manner that drives oxidation at the anode catalyst by inducing a reversal of the polarity of the cell voltage. In this embodiment, it is contemplated that each cell of the fuel cell may include a valve to allow the flow of fuel to be halted on a cell by cell basis. The method  82  starts by flowing the hydrogen containing gas stream in block  84 . The method  82  then proceeds after a predetermined amount of time to block  86  where the flow of gas is halted to a single cell. Since the other cells in the fuel cell are still receiving gas, the polarity of the isolated cell is reversed in block  88  driving oxidation at the anode in the isolated cell. The process  82  then proceeds to query block  90  where it is determined if the voltage in the isolated cell is at the desired voltage. In the exemplary embodiment, the desired voltage is the maximum voltage of the cell. 
         [0030]    If the query block returns a negative, the process  82  loops back to block  88 . If the query block  90  returns a positive, the process  82  proceeds to block  92  where the flow of the gas is reinitiated to the cell. The process  82  then proceeds to block  94  where the cell to be isolated is incremented to the next cell and the process  82  loops back to block  86  where the gas flow to the next cell is halted. The process continues until all of the cells have been isolated and oxidized. After all of the cells in the fuel cell have been oxidized, the fuel cell may operate for a predetermined amount of time, or in one embodiment may cycle back to the first cell that was isolated and the process repeats. 
         [0031]    It should be appreciated that the methods described herein may also be used in other electrochemical systems for restoring catalysts that are contaminated with other constituents of a process gas such as hydrocarbons or sulfur for example. 
         [0032]    While the invention has been described in detail in connection with only a limited number of embodiments, it should be readily understood that the invention is not limited to such disclosed embodiments. Rather, the invention can be modified to incorporate any number of variations, alterations, substitutions or equivalent arrangements not heretofore described, but which are commensurate with the spirit and scope of the invention. Additionally, while various embodiments of the invention have been described, it is to be understood that aspects of the invention may include only some of the described embodiments. Accordingly, the invention is not to be seen as limited by the foregoing description, but is only limited by the scope of the appended claims.