Abstract:
A hydrogen diffusion cell that is used to purify contaminated hydrogen gas. The hydrogen diffusion cell has at least one hydrogen diffusion structure that separates a first area from a second area. Normally, the pressure in the first area is kept higher than the pressure in the second area. This causes a pressure differential that causes hydrogen gas to permeate from the first area to the second area. However, an extreme pressure differential can occur when the second area is at its maximum pressure and the first area is inadvertently vented to ambient pressure. Under this extreme pressure differential hydrogen gas permeates from the second area back into the first area at a maximum reverse flow rate. A flow restrictor is provided that limits the flow of gas exiting the first area to a flow rate no greater than the maximum reverse flow rate.

Description:
BACKGROUND OF THE INVENTION  
         [0001]    1. Field of the Invention  
           [0002]    In general, the present invention relates to hydrogen diffusion cells. More particularly, the present invention relates to hydrogen diffusion cells that contain wound coils of palladium tubing.  
           [0003]    2. Description of the Prior Art  
           [0004]    In industry, there are many known techniques for separating hydrogen from more complex molecules in order to produce a supply of hydrogen gas. One such technique is electrolysis, wherein hydrogen gas is obtained from water. Regardless of how hydrogen gas is obtained, the collected hydrogen gas is typically contaminated with secondary gases, such as water vapor, hydrocarbons and the like. The types of contaminants in the collected hydrogen gas are dependent upon the technique used to generate the hydrogen gas.  
           [0005]    Although contaminated hydrogen gas is useful for certain applications, many other applications require the use of pure hydrogen. As such, the contaminated hydrogen gas must be purified. One technique used to purify hydrogen is to pass the hydrogen through a hydrogen diffusion cell. A typical hydrogen diffusion cell contains at least one coil of palladium tubing. The palladium tubing is heated and the contaminated hydrogen gas is directed through the palladium tubing. When heated, the palladium tubing is permeable to hydrogen gas but not to the contaminants that may be mixed with the hydrogen gas. As such, nearly pure hydrogen passes through the palladium tubing and is collected for use.  
           [0006]    To make the hydrogen gas permeate through the palladium tubing, a pressure differential is typically maintained between the pressure of the contaminated hydrogen gas within the palladium tubing and the pressure of the purified hydrogen gas surrounding the palladium tubing. During the operation of the hydrogen diffusion cell, this pressure differential is typically kept at about twenty pounds per square inch. The structure of the palladium tubing is adequate to operate within this pressure differential without rupturing or otherwise deforming, provided that the pressure within the tubing is greater than the pressure surrounding the tubing. However, on occasions, improper operation and maintenance practices may produce a reverse pressure differential within the hydrogen diffusion cell. During a period of a reverse pressure differential, the pressure surrounding the palladium tubing surpasses the pressure within the palladium tubing. Since, the palladium tubing is typically very thin, only a small reverse pressure differential can cause the palladium tube to collapse.  
           [0007]    Periods of reverse pressure differential typically occur during maintenance periods or when the hydrogen diffusion cell is first shut down. When the hydrogen diffusion cell is running properly, the pressure of the contaminated hydrogen gas within the palladium tubing and the pressure of the gas surrounding the palladium tubing are well controlled. However, when the hydrogen diffusion cell is shut off, an operator often vents the contaminated hydrogen gas from within the palladium tubing before venting the pressure of the purified hydrogen gas surrounding the palladium tubing. This results in a reverse pressure differential that can damage the palladium tubing.  
           [0008]    A need therefore exists for a system and method of preventing a hydrogen diffusion cell from experiencing reverse pressure to a degree that can cause damage to the palladium tubing within the hydrogen diffusion cell. This need is met by the present invention as it is described and claimed below.  
         SUMMARY OF THE INVENTION  
         [0009]    The present invention is a hydrogen diffusion cell that is used to purify contaminated hydrogen gas. The hydrogen diffusion cell has at least one hydrogen diffusion structure that separates a first area from a second area. Once at an operating temperature, hydrogen gas can diffuse through the hydrogen diffusion structure at a diffusion rate that is dependent upon the pressure differential between the first area and the second area. Normally, the pressure in the first area is kept higher than the pressure in the second area. This causes a pressure differential that causes hydrogen gas to permeate from the first area to the second area. However, an extreme pressure differential can occur when the second area is at its maximum pressure and the first area is inadvertently vented to ambient pressure. Under this extreme pressure differential hydrogen gas permeates from the second area back into the first area at a maximum reverse flow rate.  
           [0010]    A flow restrictor is provided that limits the flow of gas exiting the first area. The flow restrictor is calibrated to have a flow rate no greater than the maximum reverse flow rate. Accordingly, should the first area ever be inadvertently vented to ambient pressure, gas cannot leave the first area at a rate grater than hydrogen gas can permeate back into the first area from the second area. The restrictor therefore prevents the occurrence of reverse pressure differentials. 
       
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS  
       [0011]    For a better understanding of the present invention, reference is made to the following description of exemplary embodiments thereof, considered in conjunction with the accompanying drawings, in which:  
         [0012]    [0012]FIG. 1 is an partially exploded perspective view of the front of a hydrogen diffusion cell in accordance with the present invention; and  
         [0013]    [0013]FIG. 2 is a partially exploded perspective view of the rear of a hydrogen diffusion cell in accordance with the present invention.  
     
    
     DETAILED DESCRIPTION OF THE INVENTION  
       [0014]    Referring to FIG. 1, a first exemplary embodiment of a hydrogen diffusion cell  10  is shown in accordance with the present invention. The diffusion cell  10  contains a supply tube  12 , a drain tube  14  and an output tube  15 . The supply tube  12  supplies unpurified hydrogen gas to the hydrogen diffusion cell  10 . The drain tube  14  removes the unused, unpurified hydrogen gas from the hydrogen diffusion cell  10 . The output tube  15  removes purified hydrogen gas from the hydrogen diffusion cell  10 . The supply tube  12 , drain tube  14 , and output tube  15  are all made of stainless steel or another inert high strength alloy. The supply tube  12 , drain tube  14  and-output tube  15  all pass through an end cap plate  16 . The supply tube  12 , drain tube  14  and output tube  15  are welded to the end cap plate  16  at the points where they pass through the end cap plate  16 . To prevent stresses caused by expansion and contraction, the end cap plate  16  is preferably made of the same material, as is the supply tube  12 , drain tube  14  and output tube  15 .  
         [0015]    On the supply tube  12  is located a clustered set of brazing flanges  20 . Each brazing flange  20  is a short segment of tubing that is welded to the supply tube  12 . The short segment of tubing is made of the same material as is the supply tube  12 . Within each clustered set of brazing flanges  20 , each brazing flange  20  is a different distance from the end cap plate  16 . Furthermore, each brazing flange  20  in the clustered set radially extends from the supply tube  12  at an angle different from that of any of the other brazing flanges  20  in that same clustered set.  
         [0016]    In the embodiment shown in FIG. 1, there is only one clustered set of brazing flanges  20  on the supply tube  12  and that clustered set contains two brazing flanges  20 . Such an embodiment is merely exemplary. It should be understood that multiple clustered sets of brazing flanges  20  can be present on the supply tube  12  and any plurality of brazing flanges  20  can be contained within each clustered set.  
         [0017]    The drain tube  14  also contains clustered sets of brazing flanges  22 . The brazing flanges  22  are of the same construction as those on the supply tube  12 . The number of clustered sets of brazing flanges  22  on the drain tube  14  corresponds in number to the number of clustered sets of brazing flanges  20  present on the supply tube  12 . Similarly, the number of brazing flanges  22  contained within each clustered set on the drain tube  14  corresponds in number to the number of brazing flanges  20  in each clustered set on the supply tube  12 .  
         [0018]    A plurality of concentric coils  24 ,  26  are provided. The concentric coils  24 ,  26  are made from palladium or a palladium alloy. The process used to make the coils is the subject of co-pending U.S. patent application Ser. No. 09/702,637, entitled METHOD AND APPARATUS FOR WINDING THIN WALLED TUBING, the disclosure of which is incorporated into this specification by reference.  
         [0019]    The number of brazing flanges  20 ,  22  in each clustered set corresponds in number to the number of coils  24 ,  26 . One end of each coil  24 ,  26  extends into a brazing flange  20  on the supply tube  12 . The opposite end of each coil  24 ,  26  extends into a brazing flange  22  on the drain tube  14 . The concentric coils  24 ,  26  have different diameters so that they can fit one inside another. Furthermore, each coil has a slightly different length so that the ends of the coils align properly with the different brazing flanges  20 ,  22  on the supply tube  12  and the drain tube  14 , respectively.  
         [0020]    In the embodiment of FIG. 1, there are two coils  24 ,  26 . As such, there are two brazing flanges  20  on the supply tube  12  and two brazing flanges  22  on the drain tube  14 . It will be understood that more than two concentric coils can be used. In any case, the number of supply brazing flanges  20  and drain brazing flanges  22  matches the number of coils used. Furthermore, in FIG. 1, the palladium coils have a length only slightly smaller than that of the cylindrical casing  28 . It will be understood that multiple palladium coils can be linearly aligned within the cylindrical casing, wherein each of the palladium coils is much shorter than the cylindrical casing. Hydrogen diffusion cells having multiple coils that are linearly aligned are disclosed in co-pending U.S. patent application Ser. No. 09/702,637 that was previously incorporated into this application by reference.  
         [0021]    In FIG. 1, the coils  24 ,  26  have a nearly constant radius of curvature from one end to the other. As such, the coils  24 ,  26  do not contain any natural stress concentration points that may prematurely crack as the coils  24 ,  26  expand and contract. To further increase the reliability of the hydrogen diffusion cell  10 , the brazing flanges  20  on the supply tube  12  and the brazing flanges  22  on the drain tube  14  are treated. The brazing flanges  20 ,  22  are chemically polished prior to brazing. Such a preparation procedure produces high quality brazing connections that are much less likely to fail than brazing connections with untreated brazing flanges.  
         [0022]    The output tube  15  extends down the center of the hydrogen diffusion cell  10 . The coils  24 ,  26  surround the output tube  15 . As such, the output tube  15  extends down the center of the concentrically disposed coils  24 ,  26 . The length of the output tube  15  is at least as long as the length of the coils  24 ,  25 . As such, the output tube is present along the entire length of the coils  24 ,  26 .  
         [0023]    The output tube  15  is perforated along its length. The perforation enables purified hydrogen gas to pass into the output tube  15 . The holes  29  used to perforate the output tube  15  can have a constant diameter. However, in a preferred embodiment, the holes  29  increase in diameter along the length of the output tube  15 , as the output tube  15  extends away from the end cap plate  16 . In this manner, the draw of hydrogen gas into the output tube  15  through the various holes  29  remains relatively constant along the entire length of the output tube  15 .  
         [0024]    Once the coils  24 ,  26  placed around the output tube  15  and are attached to both the supply tube  12  and the drain tube  14 , the coils  24 ,  26  are covered with a cylindrical casing  28 . The cylindrical casing  28  is welded closed at the end cap plate  16 , thereby completing the assembly.  
         [0025]    To utilize the hydrogen diffusion cell  10 , the cell  10  is heated. Once at the proper temperature, contaminated hydrogen gas is fed into the supply tube  12 . The contaminated hydrogen gas fills the coils  24 ,  26 . Purified hydrogen gas permeates through the coils  24 ,  26  and is collected in the cylindrical casing  28 . The purified hydrogen gas is drawn into the output tube  15 . The remainder of the contaminated hydrogen gas is drained through the drain tube  14  for reprocessing.  
         [0026]    The wall thickness of the tubing used to make the coils  24 ,  26  is thin to provide for rapid permeation of hydrogen gas through the walls of the coils  24 ,  26 . However, since the walls of the palladium coils  24 ,  26  are thin, the palladium tubing is easily crushed or otherwise damaged by forces externally applied to the palladium tubing. As has been previously mentioned, such external forces occur when the nydrogen diffusion cell  10  experiences a reverse pressure differential and the pressure within the cylindrical casing  28  surpasses the pressure within the palladium coils  24 ,  26 .  
         [0027]    Referring to FIG. 2, it can be seen that within the drain tube  14  of the hydrogen diffusion cell  10  is disposed flow restrictor assembly. The flow restrictor assembly consists of a tube  32  that has a very small and precisely manufactured internal conduit  33 . The flow restrictor tube  32  has two ends. One end of the flow restrictor tube  32  is left open. The opposite end of the flow restrictor tube  32  is coupled to a gland plate  34  that is part of a seal assembly. The flow restrictor tube  32  extends through the gland plate  34  and is welded to the center of the gland plate  34 . As such, the internal conduit  33  of the flow restrictor tube  32  remains open and exposed on the forward face of the gland plate  34 .  
         [0028]    The drain tube  14  extends through the end cap  16  of the hydrogen diffusion cell  10 . The drain tube  14  terminates with a threaded termination  36  that receives the gland plate  34  at the end of the flow restrictor tube  32 . The flow restrictor tube  32  passes into the interior of the drain tube  14  until the gland plate  34  at the end of the flow restrictor tube  32  is received within the threaded termination  36 . A seal nut  38  is then used to seal the gland plate  34  into place, thereby completing the seal assembly and completely sealing the interior of the drain tube  34 , other than through the conduit  33  of the flow restrictor tube  32  that remains open on the face of the gland plate  34 . As such, it will be understood that any gas that is drawn through the drain tube  14  out of the hydrogen diffusion cell  10  must pass through the flow restrictor tube  32 .  
         [0029]    Contaminated hydrogen gas is drawn out of the hydrogen diffusion cell  10  through the drain tube  14 . This means that the contaminated hydrogen gas is drawn through the flow restrictor tube  32  as is passes out of the drain tube  14 . The purpose of the flow restrictor tube  32  is to restrict the flow of contaminated gas from within the palladium coils  24 ,  26  (FIG. 1) so that the pressure within the palladium coils cannot be accidentally allowed to dip below the pressure surrounding the palladium coils within the cylindrical casing  28 .  
         [0030]    When the hydrogen diffusion cell  10  is in operation, the area within the cylindrical casing  28  that surrounds the coils  24 ,  26  (FIG. 1) is maintained within a predetermined range of operating pressures. Under normal operating conditions, the range of pressures maintained within the cylindrical casing  28  are less than the range of pressures maintained within the coils  24 ,  26  (FIG. 1). As such, there is always a positive pressure differential between the interior of the coils and the space surrounding the coils. This caused hydrogen gas to permeate out through the coils into the cylindrical casing  28 . However, should the interior of the coils be inadvertently vented to ambient pressure, a reverse pressure differential occurs. The maximum reverse pressure differential occurs when the gas pressure within the cylindrical casing  28  is at the top of its operating pressure range and the interior of the coils are inadvertently vented to ambient pressure.  
         [0031]    For a given range of operating temperature and pressure differentials, the palladium coils in every hydrogen diffusion cell have a maximum hydrogen diffusion rate at which hydrogen gas can diffuse through the palladium coils. In addition to operating temperature and pressure differentials, the diffusion flow rate is a function of the diameter of the palladium tubing, the thickness of the palladium tubing, the composition of the palladium tubing, and the length of the palladium tubing present in the coils. The range of operating pressures maintained in the cylindrical casing  28 , surrounding the coils  24 ,  26  (FIG. 1) are known. Likewise, the range of operating temperatures for the hydrogen diffusion cell  10  are known and the physical characteristics of the coils  24 ,  26  (FIG. 1) are known. Assuming that the pressure in the coils  24 ,  26  (FIG. 1) were suddenly vented to ambient pressure when the hydrogen diffusion cell  10  was operating at its maximum operating temperature and the pressure within the cylindrical casing  28 . A reverse pressure differential would occur, wherein hydrogen gas would diffuse from the cylindrical casing  28  surrounding the coils  24 ,  26  (FIG. 1) back into the coils  24 ,  26  (FIG. 1). Since the operating temperature is at its maximum and the reverse pressure differential is at its maximum, the reverse diffusion rate would also be at its maximum.  
         [0032]    The flow rate of gas through the flow restrictor tube  32  is a function of the diameter of the conduit  33  within the flow restrictor tube  32  and the length of the flow restrictor tube  32 . The flow rate selected for the flow restrictor tube  32  is equal to or less than the maximum reverse diffusion flow rate, as defined in the previous paragraph. Accordingly, should any line connected to the drain tube  14  of the hydrogen diffusion cell  10  be accidentally vented during the operation of the hydrogen diffusion cell  10 , the contaminated hydrogen could only exit the drain tube  14  at the flow rate allowed by the flow restrictor tube  32 . As the pressure in the palladium coils  24 ,  26  (FIG. 1) drops below the pressure within the cylindrical housing  28 , hydrogen gas would diffuse back into the interior of the palladium coils  24 ,  26  (FIG. 1) from space within the cylindrical housing  28 . Since hydrogen gas can diffuse back into the coils  24 ,  26  (FIG. 1) at a rate equal to or less than the rate that gas flow through the flow restrictor tube  32 , equilibrium is immediately reached. Thus, the pressure within the palladium coils  24 ,  26  (FIG. 1) will equalize with the pressure surrounding the palladium coils until both pressures match atmospheric pressure. The presence of the flow restrictor tube  32  therefore prevents gas from being drawn out of the palladium coils  24 ,  26  (FIG. 1) faster than gas can permeate back into the palladium coils. The problems associated with creating a reverse pressure differential are therefore eliminated.  
         [0033]    The flow restriction tube  32  is positioned within the drain tube  14  of the hydrogen diffusion cell  10 . Consequently, the flow restrictor tube  32  is present with the structure of the hydrogen diffusion cell  10  as the hydrogen diffusion cell  10  operates. The hydrogen diffusion cell  10  has an operational temperature of at least 400 degrees Celsius. As a result, the flow restrictor tube  32  is also maintained this operating temperature. By maintaining the flow restrictor tube  32  at the operational temperature of the hydrogen diffusion cell  10 , the flow restrictor tube  32  is kept well above the condensation temperature of any water vapor. Accordingly, if water vapor is contained within the contaminated hydrogen gas that is drawn through the flow restrictor tube  32 , the water vapor does not condense and obstruct the small internal conduit  33  of the flow restrictor tube  32 .  
         [0034]    In the embodiment of FIG. 2, the flow restrictor tube  32  is shown as a curved tube that has a generally U-shaped configuration. Such a configuration is merely exemplary and any length or shape of tubing can be used. If the flow restrictor tube  32  has a very small internal conduit  33 , for example a 0.0007 inch diameter, only a small length of this tubing may be needed and no curves on the flow restriction tube would be required. However, the smaller the internal diameter of a flow restrictor tube  32 , the more likely it is that a speck of contamination would block flow restrictor tube  32 . Consequently, a flow restrictor tube  32  with an internal diameter of at least 0.001 is recommended. With such a diameter tube, a long length of tube  32  may be required to obtain the desired restricted flow rate. In such a scenario, the length of flow restrictor tube  32  can be bent so that the tube will fit within the confines of the drain tube  14  in the hydrogen diffusion cell  10 .  
         [0035]    There are many variations to the present invention device that can be made. For instance, the length and diameter of the flow restrictor tube can be changed. Furthermore, flow restrictor other than lengths of tubing can also be used, provided such flow restrictors fit within the confines of the drain tube. It will therefore be understood that a person skilled in the art can make numerous alterations and modifications to the shown embodiments utilizing functionally equivalent components to those shown and described. All such modifications are intended to be included within the scope of the present invention as defined by the appended claims.