Abstract:
An assembly for separating molecular hydrogen from a volume of gas. The assembly includes a first conduit through which a gas at a first pressure flows, wherein the gas at least partially contains hydrogen. A second conduit intersects the first conduit. The second conduit is maintained at a pressure less than the first pressure of the first conduit. A hydrogen permeable membrane is disposed within the second conduit, wherein the membrane prevents the gas from flowing directly into the second conduit. Since the membrane is hydrogen permeable, a predetermined flow rate of hydrogen permeates through the membrane into the second conduit. The hydrogen permeable membrane contains a layer of hydrogen permeable material. The layer of hydrogen permeable material has a top surface and a bottom surface. A first metal mesh element is bonded to the top surface of the layer of hydrogen permeable material. Similarly, a second metal mesh element is bonded to the bottom surface of the layer of hydrogen permeable material, wherein the hydrogen permeable material is deformed into the second metal mesh. The mesh element supports the thin hydrogen permeable layer and prevents it from rupturing or collapsing.

Description:
REFERENCE TO DOCUMENT DISCLOSURE 
     The matter of this application corresponds to the matter contained in Disclosure Document 444763 filed Sept. 21, 1998, wherein this application assumes the priority date of that document. 
    
    
     BACKGROUND OF THE INVENTION 
     1. Field of the Invention 
     The present invention relates to devices and methods that are used to separate molecular hydrogen from a volume of gas. More particularly, the present invention is related to methods and devices that separate hydrogen from a volume of gas by exposing the gas to a hydrogen permeable material through which only atomic hydrogen can readily pass. 
     2. Description of the Prior Art 
     In industry there are many applications for the use of molecular hydrogen. However, in many common processes that produce hydrogen, the hydrogen gas produced is not pure. Rather, when hydrogen is produced, the resultant gas is often contaminated with water vapor, hydrocarbons and other contaminants. In many instances, however, it is desired to have ultra pure hydrogen. In the art, ultra pure hydrogen is commonly considered to be hydrogen having purity levels of at least 99.999%. In order to achieve such purity levels, hydrogen gas must be actively separated from its contaminants. 
     In the prior art, one of the most common ways to purify contaminated hydrogen gas is to pass the gas through a conduit made of a hydrogen permeable material, such as palladium or a palladium alloy. As the contaminated hydrogen gas passed through the conduit, atomic hydrogen would permeate through the walls of the conduit, thereby separating from the contaminants. In such prior art processes, the conduit is kept internally pressurized and is typically heated to several hundred degrees centigrade. Within the conduit, molecular hydrogen disassociates into atomic hydrogen on the surface of the conduit and the conduit absorbs the atomic hydrogen. The atomic hydrogen permeates through the conduit from a high pressure side of the conduit to a low pressure side of the conduit. Once at the low pressure side of the conduit, the atomic hydrogen recombines to form molecular hydrogen. The molecular hydrogen that passes through the walls of the conduit can then be collected for use. Such prior art systems are exemplified by U.S. Pat. No. 5,614,001 to Kosaka et al., entitled Hydrogen Separator, Hydrogen Separating Apparatus And Method For Manufacturing Hydrogen Separator. 
     Conduits made of palladium and palladium alloys are highly expensive. As such, it is highly desirable to use as little of the palladium as possible in manufacturing a hydrogen gas purification system. However, in the prior art, the conduits made from palladium and palladium alloys hold gas under pressure and at high temperatures. Accordingly, the walls of the conduit cannot be made too thin, else the conduit will either rupture or collapse depending on the pressure gradient across the wall of the conduit. 
     A typical prior art conduit made from palladium or a palladium alloy would have a wall thickness of approximately 89 μm. The thickness of the wall of the conduit is directly proportional to the amount of purified hydrogen that passes through that wall in a given period of time. As such, in order to make the conduit more efficient, a thinner wall is also desirable. However, as has already been stated, a conduit wall cannot be made so thin that it ruptures or collapses under the pressure of the gases being passed through that conduit. 
     To further complicate matters, conduits made from palladium and palladium alloys may become less efficient over time as the interior walls of the conduits become clogged with contaminants. In order to elongate the life of such conduits, many manufacturers attempt to clean the conduits by reverse pressurizing the conduits. In such a procedure, the exterior of the conduit is exposed to pressurized hydrogen. The hydrogen passes through the conduit wall and into the interior of the conduit. As the hydrogen passes into the interior of the conduit, the hydrogen may remove some of the contaminants that were deposited on the interior wall of the conduit. 
     Due to the generally cylindrical shape of most prior art hydrogen purification conduits, the conduits are capable of withstanding a fairly high pressure gradient when the interior of the conduit is pressurized higher than the exterior of the conduit. However, when such conduits are cleaned and the external pressure of the conduit is raised higher than the interior pressure, a much lower pressure gradient must be used, else the conduit will implode. 
     In the prior art, improved conduit designs have been developed that attempt to minimize the amount of palladium used in a conduit, yet increase the strength of the conduit. One such prior art device is shown in U.S. Pat. No. 4,699,637 to Iniotakis, entitled Hydrogen permeation membrane. In the Iniotakis patent, a thin layer of palladium is reinforced between two layers of mesh. The laminar structure is then rolled into a conduit. Such a structure uses less palladium, however, the conduit is incapable of holding the same pressure gradient as solid palladium conduits. Accordingly, the increase in efficiency provided by the thinner palladium layer is partially offset by the decreased pressure limits, and thus gas flow rate, that are capable of being processed. 
     Another prior art approach to limiting the amount of palladium used is to create membranes that are placed over apertures, like a skin on a drum. A pressure gradient is then created on opposite sides of the membrane, thereby causing hydrogen to flow through the membrane. Such prior art systems are exemplified by U.S. Pat. No. 5,734,092 to Wang et al., entitled Planar Palladium Structure. A problem associated with such prior art systems is that the palladium or palladium alloy membrane is typically positioned in a level plane, wherein a pressure gradient exists from one side of the membrane to the other. Since the membrane is flat, it has little structural integrity when trying to resist the forces created by the pressure gradient. Accordingly, in order to prevent the membrane from rupturing, solid perforated substrates are used to reinforce the membrane. The solid perforated substrates, however, are complicated to manufacture, restrict the flow through the membrane, and reduce the efficiency of the overall system. 
     A need therefore exists in the art of hydrogen purification for a system and method that can handle high flow rates of gas, per unit area, and yet uses only a minimal amount of hydrogen permeable material. A need also exists for a hydrogen purification system capable of withstanding large pressure gradients in opposite directions. 
     SUMMARY OF THE INVENTION 
     The present invention is an assembly for separating molecular hydrogen from a volume of gas containing both hydrogen and other contaminants. The assembly includes a first conduit through which a gas at a first pressure flows, wherein the gas at least partially contains hydrogen. A second conduit intersects the first conduit. The second conduit is maintained at a pressure less than the first pressure of the first conduit. A hydrogen permeable membrane is disposed within the second conduit, wherein the membrane prevents the gas from flowing directly into the second conduit. Since the membrane is hydrogen permeable, a predetermined flow rate of hydrogen permeates through the membrane into the second conduit. 
     The hydrogen permeable membrane contains a layer of hydrogen permeable material. The layer of hydrogen permeable material has a top surface and a bottom surface. A first metal mesh element is bonded to the top surface of the layer of hydrogen permeable material. Similarly, a second metal mesh element is bonded to the bottom surface of the layer of hydrogen permeable material. The mesh element supports the thin hydrogen permeable layer and prevents it from rupturing as it creates a barrier in between the first conduit and the second conduit. Furthermore, the layer of hydrogen permeable material is deformed into the second metal mesh, thereby giving the hydrogen permeable material added structural integrity and a place to expand when atomic hydrogen is absorbed. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     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: 
     FIG. 1 is a cross-sectional view of one preferred embodiment of the present invention assembly; 
     FIG. 2 is a cross-sectional view of a hydrogen permeable membrane in accordance with the present invention; 
     FIG. 3 is a schematic of an exemplary method of manufacture for a hydrogen permeable membrane in accordance with the present invention; 
     FIG. 4 is an enlarged exploded view of the segment of the embodiment contained in FIG. 1 that contains the hydrogen permeable membrane; and 
     FIG. 5 is a partially cross-sectioned perspective view of a segment of the present invention containing an alternate embodiment of a mounting for a hydrogen permeable membrane. 
    
    
     DETAILED DESCRIPTION OF THE INVENTION 
     Referring to FIG. 1, there is shown a schematic of an exemplary embodiment of the present invention hydrogen purification apparatus  10 . The apparatus  10  contains at least one supply conduit  12  that is coupled to a source of contaminated hydrogen gas at a first pressure, P 1 . The supply conduit  12  is fabricated from stainless steel or an equivalent alloy that is capable of retaining the contaminated hydrogen gas  14  at the first pressure P 1  and at a predetermined operating temperature. A drain conduit  16  intersects each of the supply conduits  12 . The drain conduit  16  is used to receive purified hydrogen gas  18 , as will later be explained. The drain conduit  16  is maintained at a second pressure P 2 , which is less than that of the first pressure P 1  in the supply conduit  12 . According, a positive pressure gradient exists between the supply conduit  12  and the drain conduit  16 . 
     In FIG. 1, only one drain conduit  16  is shown intersecting the supply conduit  12 . Such an arrangement is merely exemplary and it should be understood that one or any plurality of drain conduits  16  may intersect the same supply conduit  12 . Obviously, by varying the number of drain conduits  16  used per supply conduit  12 , the flow rate for the overall assembly can be selectively controlled. 
     The drain conduit  16  is preferably made from the same material as is the supply conduit  12 . Accordingly, the rate of expansion is relatively the same between the supply conduit  12  and the drain conduit  16  across a range of temperatures. This reduces the stresses that exist in the joint between the supply conduit  12  and the drain conduit  16 . Furthermore, the interior of the drain conduit  16  is preferably micro-polished using a passivating process. Such a polished finish helps to prevent contaminates from the surface of the conduit from re-entering the purified hydrogen stream. The same polish can also be used on the interior of the supply conduit  12 , if desired. 
     The first end  20  of the drain conduit  16  terminates within the interior of the supply conduit  12 . A membrane  22  is suspended across the interior of the drain conduit  16  near the first end  20 . The membrane  22  is circular in shape to match the circular interior of the drain conduit  16 . The diameter D of the membrane  22  ranges from approximately ⅛ inch to ¾ inches. The diameter D of membrane  22  is selected depending upon the pressure gradient that exists between the supply conduit  12  and the drain conduit  16 . The membrane  22  is sealed against the interior of the drain conduit  16  in a manner that will later be explained. The membrane  22  itself is a hydrogen permeable assembly of a unique construction that will also be later described. As such, the membrane  22  only permits hydrogen gas to pass from the supply conduit  12  into the drain conduit  16 . Therefore, by collecting hydrogen gas  18  from the drain conduit  16 , purified hydrogen gas can be had. 
     Referring to FIG. 2, it can be seen that the membrane  22  is comprised of a layer  30  of hydrogen permeable material supported between two mesh elements  32 ,  34 . The hydrogen permeable material  30  can be a film of palladium, a palladium alloy such as PdAg, a palladium alloy layered with secondary materials such as tantalum, or some other combination of materials known in the art to be substantially hydrogen permeable. In the shown exemplary embodiment, the hydrogen permeable layer  30  of the membrane  22  is PdAg, having a thickness of only 2 μm. At this thickness, the hydrogen permeable layer  30  of the shown membrane  22  is forty four times thinner than the 89 μm wall thickness of many prior art hydrogen separation conduits. Accordingly, the hydrogen permeable layer  30  of the shown example membrane is approximately forty four times more efficient per unit area than the cited prior art example. Furthermore, the amount of PdAg used is approximately one forty fourth the amount used on the prior art example, per unit of hydrogen purified. The cost savings for material embodied by the present invention membrane is therefore clearly present. 
     The use of a 2 μm layer of hydrogen permeable material is merely exemplary and it should be understood that other thickness can be used in the present invention. The thickness of the hydrogen permeable layer  30  selected for use is dependent upon the material being used, the pressures within the apparatus and the flow rate of purified hydrogen desired. For example, a PdAg based hydrogen permeable layer  30  would preferably be rolled to a thickness of between 2 μm and 20 μm. A PdTaPd based hydrogen permeable layer would preferably be rolled to a thickness of between 2 μm and 50 μm. 
     The thin size of the hydrogen permeable layer  30  in the present invention would cause it to rupture if not reinforced. The reinforcement is provided by the mesh elements  32 ,  34  that are present along the top surface and the bottom surface of the hydrogen permeable layer  30  of the membrane  22 . The mesh elements  32 ,  34  can be of either a single or double weave. In a preferred embodiment, the mesh elements  32 ,  34  are stainless steel, having a mesh spacing of approximately 10 microns. However, other materials and other mesh sizes can be used as desired. The preferred mesh size, however, is preferably less than 50 microns. The mesh elements  32 ,  34  are also preferably chemically polished to remove any irregularities that may tear the hydrogen permeable layer  30  when attached to the hydrogen permeable layer  30 . 
     The mesh elements  32 ,  34  have a mesh spacing that is preferably in the order of 2 to 20 times wider than the hydrogen permeable layer  30  is thick. This provides the room needed by the hydrogen permeable layer  30  to deform into the mesh element. In the embodiment of FIG. 2, both of the mesh elements  32 ,  34  are shown to be made of metal threads that are generally equivalent in size. Such an embodiment is merely exemplary and it should be understood that the mesh elements on either side of the hydrogen permeable membrane can be different sizes. 
     Additionally, the shown embodiment only shows one mesh element on either side of the hydrogen permeable membrane. It should be understood that multiple layers of mesh can be added to either side of the hydrogen permeable membrane  30  and that the different mesh elements can be of different sizes. 
     The mesh elements  32 ,  34  can be attached to the hydrogen permeable layer  30  of the membrane using either a brazing method or a tacking method. Both methods will be later explained. As such, the mesh elements  32 ,  34  attach to the hydrogen permeable layer  30 . However, as can be seen from FIG. 2, the hydrogen permeable layer  30  does not lay flat between the mesh elements  32 ,  34 . 
     Rather, the hydrogen permeable layer  30  is buckled in between the points of attachment to the mesh elements  32 ,  34 . The pattern of the buckling in the hydrogen permeable layer  30  makes the hydrogen permeable layer  30  bow into concave regions  37  in between points of attachment. The concave regions  37  serve three functions. First, the concave regions  37  added a degree of structural integrity to the hydrogen permeable layer  30  in between the points of attachment with the mesh elements  32 ,  34 . The structural integrity added by the concave regions  37  enables the hydrogen permeable layer  30  to withstand a higher pressure gradient of gas than would a flat hydrogen permeable layer of the same material. 
     The second advantage of the concave regions  37  is that it provides for the controlled expansion of the hydrogen permeable layer  30 . As the hydrogen permeable layer  30  absorbs hydrogen, the hydrogen permeable layer  30  expands. If the hydrogen permeable layer were flat, the expansion of that layer could cause stress points that would subtract from the structural integrity of the hydrogen permeable layer. By adding concave regions  37  to the hydrogen permeable layer  30 , the expansion of the hydrogen permeable layer  30  causes the radius of curvature associated with the hydrogen permeable layer  30  to change. Accordingly, the areas in which the hydrogen permeable layer  30  expands is predicted and managed in a way that the expansion does not detract from the structural integrity of the layer. 
     The third advantage of the concave regions  37  is that the concave shape of these regions  37  do increase the exposed surface area of the hydrogen permeable material per unit area. Since more of the hydrogen permeable material is exposed, the resultant flow rate for the membrane is increased. 
     The embodiment of FIG. 2, shows a membrane  22  assembled using a brazing method. In the brazing method the hydrogen permeable layer  30  is placed between the two mesh elements  32 ,  34 . A fine coat of brazing powder is present on each of the mesh elements  32 ,  34 . A suitable brazing powder would be gold or a gold alloy having a powder particle size in the order of one micron. The assembly is then placed in a vacuum furnace, wherein the assembly is compressed and heated until the brazing powder melts between mesh elements  32 ,  34  and the hydrogen permeable layer  30 , thereby bonding these elements together at brazing points  36 , which correspond to points of contact between the mesh elements  32 ,  34  and the hydrogen permeable layer  30 . 
     To create the concave regions  37  in the hydrogen permeable layer  30 , the membrane  22  is exposed to a large pressure gradient. In this procedure, the membrane is heated, one side of the membrane  22  is exposed to high pressured gas. The pressure gradient from one side of the membrane  22  to the other causes the hydrogen permeable layer  30  to buckle away from the high pressure gas, thereby producing the concave regions  37  previously described. 
     Referring to FIG. 3, an alternate method of manufacturing the present invention membrane is shown. In this method, the hydrogen permeable layer  30  is provided, as is shown in step  1 . The hydrogen permeable layer  30  placed between the two micro-polished mesh elements  32 ,  34 , as is shown in step  2 . The mesh elements  32 ,  34  can be temporarily attached to the hydrogen permeable layer  30  with an adhesive, if desired. The assembly is then compressed between ceramic plates  38 , or some other compression device, as is shown in step  3 . The entire assembly is placed in a vacuum furnace  40 , as is shown in step  4 . There are many different devices available in the art for asserting a compression force on an object in a vacuum furnace. Any such prior art device can be adapted for use with the present invention. 
     The vacuum furnace  40  is evacuated to a low pressure at a temperature in the range of 1900° F. After a predetermined period of time at this temperature, the vacuum furnace  40  is cooled and the assembly is blown down with a non-reactive gas, such as argon or nitrogen. This procedure tacks the hydrogen permeable layer  30  to the mesh elements  32 ,  34  without the need for a brazing powder. Lastly, in step  5 , the membrane can be placed in a high pressure chamber  41 . The membrane is supported on one side of the membrane is pressurized with a non-reactive gas such as argon. The pressure P of the non-reactive gas is used to buckle the hydrogen permeable layer  30 , into the configuration previously described. The pressure of the non-reactive gas used to deform the hydrogen permeable material is dependent upon the thickness of the hydrogen permeable material, the composition of the hydrogen permeable material, temperature, and the mesh size of the mesh elements  32 ,  34  supporting the hydrogen permeable layer  30 . 
     FIG. 4 is an enlarged and exploded view of the first end  20  of the drain conduit  16  that was previously shown in FIG.  1 . Referring to FIG. 4, it can be seen that a depression is formed in the first end  20  of the drain conduit  16 . The depression creates a ridge  42  on the interior of the drain conduit  16  near the first end. The membrane  22  is advanced into the first end  20  of the drain conduit  16  until the peripheral edge of the membrane  22  rests upon the ridge  42 . 
     Once the membrane  22  is placed on the ridge  42 , the peripheral edge of the membrane  22  does not automatically create a gas impervious seal with the interior of the drain conduit  16 . Accordingly, brazing material  44  is placed along the peripheral edge of the membrane  22 . The brazing material  44  can be a gold-nickel alloy, a silver alloy or any appropriate brazing composition. An annular collar element  46  is then placed atop the brazing material  44  and the assembly is heated above the melting point of the brazing material  44 . As the brazing material  44  melts, it bonds both the membrane  22  and the annular collar  46  to the interior of drain conduit  16 . The brazing material  44  also creates a gas impermeable seal around the periphery of the membrane  22 . Consequently, any gas flowing from the supply conduit  12  to the drain conduit  16  must permeate through the membrane  22 . 
     Referring to FIG. 5, an alternate embodiment of the first end of the drain conduit is shown. In this embodiment, a cradle structure  50  is disposed at the first end of the drain conduit  16 . The cradle structure  50  has a disk shaped bottom surface  52  and a cylindrical wall  54  that extends upwardly from the periphery of the bottom surface  52 . A plurality of apertures  56  are disposed in the bottom surface  52  of the cradle structure  50 , wherein the apertures  56  lead through to the drain conduit  16 . 
     A membrane  22  rests upon the bottom surface  52  of the cradle structure  50  within the cylindrical wall  54 . A cap element  58  is positioned over the membrane  22 , wherein the cap element  58  also fits within the cylindrical wall  54  of the cradle structure  50 . A plurality of apertures  60  are also formed through the cap element  58 . The cap element  58  has a chamfered lower edge  62  and a recess  64  for retaining brazing material. During manufacture, the cap element  58  is used to press the membrane  22  against the bottom surface  52  of the cradle structure  50 . The assembly is heated until the brazing material melts and adheres the cap element  58  to the cylindrical wall  54  of the cradle structure  50 . The brazing material also adheres the membrane  22  to the cradle structure  50 , thereby creating a gas impermeable seal around the periphery of the membrane  22 . 
     Although the membrane  22  is pressed against the bottom surface of the cradle structure  50  and the bottom surface of the cap element  58 , the membrane  22  does not seal against these surfaces. Rather, since the top surface and the bottom surface of the membrane  22  are mesh elements, gas is free to flow in between the cap element  58  and the membrane  22  as well as between the cradle structure  50  and the membrane  22 . Accordingly, the apertures  60  in the cap element  58  expose the entire top surface of the membrane  22  to contaminated hydrogen gas from the supply conduit  12 . Similarly, the apertures  56  in the cradle structure  50  draw molecular hydrogen from the full bottom surface of the membrane  22 . 
     It will be understood that the various figures described above illustrate only exemplary embodiments of the present invention. A person skilled in the art can therefore 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.