Abstract:
The present invention provides an electrically driven oxygen separation assembly and method of applying an electrical potential thereto in which one or more tubular membrane elements are provided having an anode layer, a cathode layer, an electrolyte layer and two current collector layers adjacent to and in contact with the anode layer and the cathode layer and situated on the inside and outside of the at least one tubular membrane element. The potential is applied to one of the two current collector layers at two central spaced locations of the at least one tubular membrane element and to the other of the two current collector layers at least at opposite end locations thereof. As a result the electric current flow through the tubular membrane element is divided into two parts flowing between the two central spaced locations and the opposite end locations.

Description:
FIELD OF THE INVENTION 
       [0001]    The present invention relates to an electrically driven oxygen separation assembly and method in which the oxygen is separated with the use of one or more tubular membrane elements of the assembly. More particularly the present invention relates to such an oxygen separation assembly and method in which the electrical potential is applied at opposed electrodes of the tubular membrane element or elements at two central spaced locations and at least at two end locations of the tubular membrane element outwardly spaced from the two central spaced locations. 
       BACKGROUND OF THE INVENTION 
       [0002]    Electrically driven oxygen separators are used to separate oxygen from oxygen containing feed, for example, air. Additionally, such devices are also used in purification application where it is desired to purify an oxygen containing feed by separating oxygen from the feed. Electrically driven oxygen separators can utilize tubular membrane elements having a layered structure containing an electrolyte layer capable of transporting oxygen ions when subjected to an elevated temperature, cathode and anode electrode layers located at opposite surfaces of the electrolyte layer and current collector layers to supply an electrical current to the cathode and anode electrode layers. 
         [0003]    When the tubular membrane elements are subjected to the elevated temperature, the oxygen contained in a feed will ionize on one surface of the electrolyte layer, adjacent the cathode electrode layer by gaining electrons from an applied electrical potential. Under the impetus of the applied electrical potential, the resulting oxygen ions will be transported through the electrolyte layer to the opposite side, adjacent the anode layer and recombine into elemental oxygen. 
         [0004]    The tubular membrane elements are housed in an electrically heated containment to heat the tubular membrane elements to an operational temperature at which oxygen ions will be transported. Additionally, such tubular membrane elements can be manifolded together such that the oxygen containing feed is passed into the heated containment and the separated oxygen is withdrawn from the tubular membrane elements through a manifold. In certain purification applications, the oxygen containing feed can be passed through the interior of the tubular membrane elements and the separated oxygen can be withdrawn from the containment. 
         [0005]    Typical materials that are used to form the electrolyte layer are yttrium stabilized zirconia and gadolinium doped ceria. The electrode layers can be made of mixtures of the electrolyte material and a conductive metal, a metal alloy or an electrically conductive perovskite. Current collectors in the art have been formed of conductive metals and metal alloys, such as silver as well as mixtures of such metals and metallic oxides. 
         [0006]    In order to apply the electrical potential to the tubular membrane elements, conductors can be attached to the current collector layers. Such conductors are attached at single locations to connect the tubular membrane elements in a series or parallel electrical connection. The problem with this is that the electrical current is unevenly distributed throughout the length of each of tubular elements resulting in hot spots developing at the connection of the conductors to the tubular membrane elements. Such hot spots can lead to failure of the tubular elements. Additionally, since the distribution of the electrical current is uneven, ionic conduction of the oxygen ions through the electrolyte layer is also uneven in that it occurs, to a large extent, at the connection of the conductors to the current collection layers. The effect of this is in order to achieve a target separation of oxygen, the tubular membrane elements are unnecessarily long or there are an excessive number of such elements. This not only increases fabrication costs, but also, the electricity costs involved in the heating of the tubular elements. 
         [0007]    A yet further problem is that the tubular membrane elements project through insulators and/or the heated containment that can also be insulated. Thus, at the projecting ends of the tubular membrane elements, a temperature is produced that is about 500° C. less than the temperature of the tubular elements within the heated containment that can be about 700° C. At these temperature transition zones it has been found that the electrolyte layer can undergo a chemical reduction in which the electrode chemically reduces into an electronic conductor leading to another point at which the tubular membrane elements will fail over time. 
         [0008]    As will be discussed, the present invention provides an oxygen separation assembly that utilizes one or more tubular membrane elements and a related method in which, among other advantages, the current is more evenly distributed along the length of the tubular membrane elements as compared with prior art. Further each of the tubular elements can be modified to resist failure in the temperature transition zone as discussed above. 
       SUMMARY OF THE INVENTION 
       [0009]    The present invention provides, in one aspect, an electrically driven oxygen separation assembly. In accordance with this aspect of the present invention, at least one tubular membrane element is provided having an anode layer, a cathode layer, an electrolyte layer located between the anode layer and the cathode layer and two current collector layers located adjacent to and in contact with the anode layer and the cathode layer and situated on the inside and outside of the at least one tubular membrane element. The two current collector layers allow an electrical current to be applied by a power source to the electrode layers to in turn induce oxygen ion transport through the electrolyte layer from the cathode layer to the anode layer. A set of conductors are connected to one of the two current collector layers at two central spaced locations of the at least one tubular membrane element and to the other of the two current collector layers at least at opposite end locations of the at least one tubular membrane element, outwardly spaced from the two central spaced locations, so that the power source is able to apply the electrical current through the set of conductors between the two central spaced locations and at least the two opposite end locations. As a result, the electrical current flowing through the at least one tubular membrane element is divided into two parts flowing between the two central spaced locations and the opposite end locations. 
         [0010]    The division of the electrical current allows the electrical current to be more evenly distributed throughout the tubular membrane element to prevent hot spots from developing and leading to failure of the tubular membrane element. Additionally, the even distribution of the electrical current allows more of the tubular membrane element to be used efficiently in separating the oxygen. 
         [0011]    The one of the two current collector layers can be situated on the outside of the at least one tubular membrane element with the cathode layer being adjacent to the one of the two current collector layers. Outer, opposed end sections of the at least one tubular membrane element can be retained within insulation members and the cathode layer and the one of the two current collector layers partially extend along a length dimension of the at least one tubular membrane element such that the outer, opposed end sections of the at least one tubular membrane element are devoid of the cathode layer and the one of the two current collector layers. It is to be noted here that since the outer, opposed end sections are retained within insulation members, there is a temperature transition zone within the end sections as discussed above. However, since there is no cathode layer and as will be discussed, also possibly no anode layer there is no electrical current being conducted in this region leading to a chemical reduction of the electrolyte and a possible failure thereof. In this regard, it is to be noted that the “two opposite end locations” do not have to be located at the physical ends of the at least one tubular membrane element and under circumstances in which there is no anode layer, such locations should be inwardly spaced from such physical ends so as to lie outside of the insulation members. 
         [0012]    A current distributor of elongated configuration can be located within the at least one tubular membrane element, extending between the ends of the at least one tubular membrane element and in contact with the other of the two current collectors at a plurality of points situated within the tubular membrane elements. The conductors connected to the opposite end locations of the tubular membrane elements are connected to opposite ends of the current distributor. The current distributor can be of helical configuration. 
         [0013]    The at least one tubular membrane element can be provided with opposed end seals, opposed, sealed electrical feed-throughs penetrating the opposed end seals and an outlet tube penetrating one of the opposed end seals to discharge oxygen. The conductors connected to the at least one tubular membrane element at the two opposite end locations pass through electrical feed-throughs and are connected to the current distributor. 
         [0014]    The at least one tubular membrane element can be a plurality of tubular membrane elements. The plurality of tubular membrane elements can be electrically connected in series by the set of the conductors with a first pair of the conductors connected to the two central spaced locations of a first of the tubular membrane elements, a second pair of the conductors connected to the opposite end locations of a second of the tubular membrane elements and remaining pairs of the conductors linking pairs of remaining tubular membrane elements at the two central spaced locations and at least the opposite end locations thereof so that the first pair of conductors and the second pair of conductors are able to be connected to an electrical power source. 
         [0015]    The one of the two current collectors can be situated on the outside of each of the tubular membrane elements adjacent the cathode layer and the other of the two current collectors can be situated on the inside of the tubular membrane elements adjacent the anode layer. 
         [0016]    The tubular membrane elements can be arranged in a bundle and held in a radial array by opposed insulation members located at outer, opposite end sections of the tubular membrane elements. The tubular membrane elements can be provided with opposed end seals, opposed, sealed electrical feed-throughs penetrating the opposed end seals and outlet tubes penetrating the opposed end seals at one end of the bundle to discharge the oxygen. The conductors connected to the tubular membrane elements at the two opposite end locations pass through electrical feed-throughs and are in electrical contact with the other of the two current collectors. A manifold is connected to the outlet tube and has a common outlet to discharge the oxygen that is discharged from the outlet tube. The cathode layer and the one of the two current collector layers can partially extend along a length dimension of the tubular membrane elements such that the outer, opposed end sections of the tubular membrane elements are devoid of the cathode layer and the one of the two current collector layers. As indicated above, a current distributor can be employed with the conductors connected to the opposite end locations of the tubular membrane elements being connected to opposite ends of the current distributor. The current distributor can be of helical configuration. 
         [0017]    In another aspect, the present invention provides a method of applying an electric potential in an electrically driven oxygen separation assembly. In accordance with this aspect of the present invention the electric potential is applied to at least one tubular membrane element having an anode layer, a cathode layer, an electrolyte layer formed of the electrolyte material and located between the anode layer and the cathode layer and two current collector layers located adjacent to and in contact with the anode layer and the cathode layer and situated on the inside and outside of the at least one tubular membrane element. The electric potential is applied to one of the two current collector layers at two central spaced locations of the at least one tubular membrane element and to the other of the two current collector layers at least at opposite end locations of the at least one tubular membrane element, outwardly spaced from the two central spaced locations, so that an electrical current flowing through the at least one tubular membrane element, induced by the applied electric potential, is divided into two parts flowing between the two central spaced locations and the opposite end locations. 
         [0018]    The one of the two current collector layers is located on the outside of the tubular membrane element. The cathode is located adjacent the one of the two current collector layers and the oxygen containing feed contacts the outside of the tubular membrane element. The oxygen is collected on the inside of the tubular membrane element and is withdrawn from the inside of the tubular membrane element. As indicated above, the cathode layer and the one of the two current collector layers can partially extend along a length dimension of the tubular membrane element such that outer, opposed end sections of the tubular membrane element are devoid of the cathode layer and the one of the two current collector layers located adjacent to the at least one of the cathode layer. The current can be applied to the other of the current collectors at a plurality of points situated within the tubular membrane element between the end locations thereof. 
     
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         [0019]    While the specification concludes with claims that distinctly point out the subject matter that Applicants regard as their invention, it is believed that the invention will be understood when taken in connection with the accompanying drawings in which: 
           [0020]      FIG. 1  is a schematic sectional view of bundles of tubular membrane elements of an electrically driven oxygen separation assembly in accordance with the present invention illustrated within a heated containment and with the electrical connections to such elements not shown; 
           [0021]      FIG. 2  is a perspective view of bundled tubular membrane elements utilized in  FIG. 1 ; 
           [0022]      FIG. 3  is a schematic sectional view of a tubular composite membrane utilized in the tubular membrane elements illustrated in  FIGS. 1 and 2 ; 
           [0023]      FIG. 4  is a fragmentary, schematic sectional view of an oxygen separation assembly utilized in  FIG. 1  illustrating the electrical connection thereof to a power source; 
           [0024]      FIG. 5  is a schematic, sectional view of the electrical connection of composite membrane elements utilized in an oxygen separation assembly shown in  FIG. 1 ; and 
           [0025]      FIG. 6  is a graphical representation of the temperature profile along the length of a tubular membrane element of an oxygen separation assembly of the present invention compared with a tubular membrane element of an oxygen separation assembly of the prior art. 
       
    
    
     DETAILED DESCRIPTION 
       [0026]    With reference to  FIG. 1 , an oxygen separator  1  is illustrated that has oxygen separation assemblies  10  housed within a heated containment  12 . Oxygen separation assemblies  10  are each formed by tubular membrane elements  14  that are held in a bundle-like position by end insulation members  16  and  18  that are fabricated from high purity alumina fiber. The tubular membrane elements for exemplary purposes can have an outer diameter of about 6.35 mm., a total wall thickness of about 0.5 mm. and a length of about 55 cm. The end insulation members  16  and  18  are retained within opposite openings  20 ,  22  and  24  and  26  defined in insulated end walls  28  and  30  of heated containment  12 . Heated containment  12  can be of cylindrical configuration having an insulated sidewall  32  connecting the end walls  28  and  30 . A heated insulation layer  34  is coaxially positioned within insulated sidewall  32  and contains heating elements to heat the tubular membrane elements  14  to an operational temperature at which oxygen ion transport will occur when an electrical potential is applied to such elements. 
         [0027]    During operation of the oxygen separator  1 , an oxygen containing feed stream  36  is introduced into the interior of heated containment  12  by way of an inlet  37  to contact the outside of the tubular membrane elements  14 . By means of a potential applied to the tubular membrane elements  14 , the oxygen is converted to oxygen ions that are transported to the interior of such elements  14 . The separated oxygen is then discharged through manifold arrangements  38  having a spider-like arrangement of tubes  39  connected to a compression fitting  40  having bores (not illustrated) to receive oxygen streams from the tubes  39  and to discharge an oxygen stream  42  from the compression fittings  40 . Although not illustrated, the compression fittings  40  could be connected to a common discharge pipe or other manifold to collect and discharge the separated oxygen. The oxygen depleted retentate is discharged as a retentate stream  44  from an outlet  46  of the heated containment  12 . 
         [0028]    With additional reference to  FIG. 2 , it can be seen that each of the end insulation members  16  and  18  is provided with slots  48  to hold the tubular membrane elements  14  in place. In the particular illustrated embodiment, each of the bundles consists of six of such tubular membrane elements  14 . Each of the tubular membrane elements  14  are provided with end seals that are formed by end caps  50  located at opposite ends thereof. Electrical feed-throughs  52  and  54  penetrate the end caps  50 . Additionally, outlet tubes  56  penetrate the end caps  50  at one end of the tubular membrane elements  14 . 
         [0029]    It is understood that the discussion of oxygen separator  1  is for illustrative purposes only and is not intended to be limiting on the application of the invention or the scope of the appended claims. In this regard, the present invention has application to an oxygen separator having a single tubular membrane element  14  or such a tubular membrane element  14  utilized for purposes other than in the production of oxygen. For example, the invention has applicability to a purifier that is used to remove oxygen from an oxygen containing feed stream and as such, the feed stream could be fed to the interior of tubular membrane elements. 
         [0030]    With reference to  FIG. 3 , each tubular membrane element  14  is provided with a cathode layer  58 , an anode layer  60  and an electrolyte layer  62 . Two current collector layers  64  and  66  are located adjacent the anode layer  58  and the cathode layer  60 , respectively, to conduct an electrical current to the anode layer and the cathode layer. Although the present invention has application to any composite structure making up a tubular membrane element  14 , for exemplary purposes, the cathode layer  58  and the anode layer  60  can be between about 10 and about 50 microns thick and the electrolyte layer  62  can be between about 100 microns and about 1 mm. thick, with a preferred thickness of about 500 microns. The electrolyte layer  62  is gas impermeable and can be greater than about 95 percent dense and preferably greater than 99 percent dense. Each of the cathode layer  58  and the anode layer  60  can have a porosity of between about 30 percent and about 50 percent and can be formed from (La 0.8 Sr 0.2 ) 0.98 MnO 3-δ . The electrolyte layer  62  can be 6 mol % scandium doped zirconia. The current collector layers  64  and  66  can each be between about 50 and about 150 microns thick, have a porosity of between about 30 percent and about 50 percent and can be formed from a powder of silver particles having surface deposits of zirconium oxide. Such a powder can be produced by methods well known in the art, for example by wash-coating or mechanical alloying. For exemplary purposes, a silver powder, designated as FERRO S11000-02 powder, can be obtained from Ferro Corporation, Electronic Material Systems, 3900 South Clinton Avenue, South Plainfield, N.J. 07080 USA. The size of particles contained in such powder is between about 3 and about 10 microns in diameter and the particles have a low specific surface are of about 0.2 m 2 /gram. Zirconia surface deposits can be formed on such powder such that the zirconia accounts for about 0.25 percent of the weight of the coated particle. 
         [0031]    During operation of the oxygen separator  1 , the oxygen contained in oxygen containing feed stream  36  contacts the current collector layer  64  and permeates through pores thereof to the cathode layer  58  which as indicated above is also porous. The oxygen ionizes as a result of an electrical potential applied to the cathode and anode layers  58  and  60  at current collector layers  64  and  66 . The resulting oxygen ions are transported through the electrolyte layer  62  under the driving force of applied potential and emerge at the side of the electrolyte layer  62  adjacent the anode layer  60  where electrons are gained to form elemental oxygen. The oxygen permeates through the pores of the anode layer  60  and the adjacent current collector  66  where the oxygen passes into the interior of the tubular membrane element  14 . 
         [0032]    It is to be noted, that although the cathode layer is located on the outside of the tubular membrane elements  14 , it is possible to reverse the layers so that the anode layer were located on the outside of the tubular membrane elements  14  and the cathode layer were located on the inside. Such an embodiment would be used where the device were used as a purifier. In such case the oxygen containing feed would flow on the inside of the tubular membrane elements  14 . 
         [0033]    With additional reference to  FIGS. 4 and 5 , the electrical potential, generated by a power source  70 , can be applied to the tubular membrane elements  14  by means of a set of conductors that are formed from wires, preferably silver. A first pair of the conductors  72  and  74  is connected to the two central spaced locations  76  of a first of the tubular membrane elements  14  at the current collector layer  64  and to the negative pole of the power source  70 . A second pair of the conductors  78  and  80  connect the anode layer  60  of a last of the tubular membrane elements  14  to the positive pole of the electrical power source by means of a silver wire  79  that joins conductor  78  and  80  and a wire  81  that is connected to the positive pole of electrical power source  70 . The second pair of conductors  78  and  80  is in electrical contact with current collector layer  66  adjacent the anode layer  60 , preferably at several points of contact, by means of a connection to opposite ends of a current distributor  82 , more clearly shown in  FIG. 4 , that can be of helical configuration and thus formed from a length of silver wire that is spirally wound into the helical configuration. Remaining pairs of conductors formed by insulated wires  84 ,  86  and  88 ,  90  link pairs of remaining tubular membrane elements  14  at the two central spaced locations  76  and to the ends of current distributors  80  employed within such tubular membrane elements  14 . The resulting electrical connection is a series electrical connection. However, a parallel electrical connection is also possible. Further, as indicated above, only a single of the tubular membrane elements  14  might be used in a particular device to which the present invention is employed and therefore, such embodiment would only utilize the first and second pairs of conductors  72 ,  74 ,  78  and  80 . 
         [0034]    With specific reference to  FIG. 5 , it is to be noted that for purposes of illustration, the cathode layer  58  and its associated current collector  64  are shown as a single element as well as the anode layer  60  and its associated current collector layer  66 . As shown in  FIG. 5 , the two spaced central locations  76  are formed by looping wires  86  and  90 , around the tubular membrane element  14  and holding the looped wires  92  in place by deposits of silver paste  94 . Wires  96  and  98  then pass through bores  96  and  98  provided within insulation members  16  and  18 , respectively. Although not illustrated, the wires  96  and  98  can be wrapped around the outside of the tubular membrane element  14  before being passed through the bores  96  and  98  to prevent them from sagging into other tubes. It is to be noted that the ends of each of the tubular membrane elements  14  are sealed by end caps  50  that are held in place by deposits  100  and the electrical feed-throughs  52  and  54  and the outlets  56  are all held in place by deposits  102 . It is to be noted that the end caps  50  can be formed by pressed or injected molded zirconia and the deposits  100  and  102  can be formed from a glass sealing material system, either a lead boro-silicate system or a barium alumino-silicate system. It is to be that there are other possible ways to form the end seals. For example, the glass sealing material itself or a mixture of such material with an oxide could be placed in the ends of the tubes. Such material could then be fired and cooled to solidification. The wires  84  and  88  pass through electrical feed-throughs  52  and  54  which are in turn sealed by deposits  104  of a braze material, preferably 50 percent Ag, Cu, Zn, Sn, Ni composition. 
         [0035]    As mentioned above, the two spaced central locations  76  of tubular membrane elements  14  provide for the electrical current induced in tubular membrane elements  14  to be distributed between the ends of such elements and the two spaced central locations  76  so that the current more uniformly distributed along the length of the tubular membrane elements  14 . As a result, more oxygen ion transport takes place in each of the tubular membrane elements  14  than had the potential been applied at solely two end locations of each of the tubular membrane elements  14  as in the prior art. Additionally, the temperature distribution is more uniform than in the prior art. 
         [0036]    It is to be noted that some advantage, though a lesser advantage than when the current distributor  82  is utilized, can be obtained by connecting the wires  84  and  90  at end locations of each of the tubular membrane elements  14  that are outwardly spaced from the two central locations  76 . For reasons that will be discussed, such end locations are preferably inside the tubes at regions thereof that are not surrounded by the end insulators  16  and  18 . A further point is that if the tubular membrane elements  14  were used for purification applications, the two spaced locations might be placed within such elements. Alternatively, in any embodiment of the present invention, the two spaced locations could be positioned adjacent to the anode layer  60 . 
         [0037]    In an example of typical operating conditions at a nominal operational temperature of 700° C., each of the tubular membrane elements is supplied with 1.1 volts, DC by a power supply rated to at least 6.6 volts. The resulting total current that flows through the entire circuit which includes the oxygen ion current through the electrolyte of the tubular membrane elements  14  which is about 22.5 amperes. Associated with this current is an oxygen flow of about 0.83 liters per tube or roughly 0.5 liters for the six tube bundle and out of the outlet  38  of the manifold  40 . 
         [0038]    Approximately half of the electrical current, about 11.25 amperes flows through the series circuit created between one end of each of the tubular membrane elements  14  to one of the two spaced central locations  76  and the other half flows through the series circuit created at the other half of the tubular membrane elements  14  between the other of the two central locations and the other opposite end thereof. In this manner the current is distributed relatively uniformly across the length of the tubular membrane elements  14 . This uniform current distribution is important because as each of the tubular membrane elements  14  heats as a result of the power dissipated during operation. With reference to  FIG. 6 , the temperature of a tubular membrane element was plotted where the electrical potential at the cathode was applied solely at the ends of the tube, close to the end caps  50  (the data presented in circles) and where the electrical potential at the cathode was applied at the central locations  76  (the data presented as squares). As is evident from the graph, the temperature rise and therefore the current distribution along the length of the tube are better managed by locating the conductors contacting the cathode at the center of the tubes. 
         [0039]    With continued reference to  FIG. 5 , it can be seen that the outer, opposite end sections of each of the tubular membrane elements are located within insulators  18  that in turn are located within insulated end wall  28  and  30  of heated containment  12 . As a result, there is essentially no oxygen transport taking place at such locations. At the same time, as indicated above, the temperature of each of the elements is increasing by about 500° C. As illustrated, the ends of each of the tubular membrane elements  14  are devoid of both the cathode layer  58  and its associated current collector  64  so that current does not flow within the tubular membrane elements  14  at such locations. It has been found that where the tubular membrane elements are designed with electrical current flow within such insulated end section, the ceramic will tend to undergo a chemical reduction reaction at such end sections with a consequent potential of a failure of the elements. It is to be noted, however, that advantageously, the anode layer  60  and its associated current collector layer  66  can also be dispensed with at such locations to ensure no current flow at the insulated ends of the tubular membrane elements. It is to be noted that embodiments of the present invention are possible in which the anode and cathode layers and their associated current collector layers extend to the physical ends of the tubular membrane elements  14  even when covered with an insulating member. 
         [0040]    As indicated above, embodiments of the present invention are possible without the current distributors  82 . In such case, anode layer  62  and its associated current collector  66  could end at the insulator members  16  and  18  and the wires  84  and  88  would be connected inside the tubular membrane elements  14  inwardly of the ends thereof and of the end insulator members  16  and  18 . As such, the end locations at which the potential would be applied would be inwardly spaced from the physical ends of the tubular membrane elements. 
         [0041]    Although the present invention has been described with reference to a preferred embodiment, as will occur to those skilled in the art, numerous changes, additions and omission may be made without departing from the spirit and scope of the present invention as set forth in the appended claims.