Abstract:
A module and an apparatus incorporating such module utilizing a plurality of tubular membrane elements, each configured to separate oxygen from an oxygen containing feed stream when an electric potential difference is applied to induce oxygen ion transport in an electrolyte thereof. The tubular membrane elements can be arranged in a bundle that is held in place by end insulating members. The insulating members can be positioned within opposed openings of end walls of a heated enclosure and can incorporate bores to allow an oxygen containing feed stream to flow past exposed ends of the tubular membrane elements for cooling the end seals of such elements. Further, first and second manifolds can be provided in a module in accordance with the present invention to collect separated oxygen from two separate portions of the tubular membrane elements.

Description:
FIELD OF THE INVENTION  
       [0001]    The present invention relates to an oxygen separation module and apparatus that incorporates a plurality of tubular membrane elements, each configured to separate oxygen from an oxygen containing feed stream when an electric potential difference is applied to produce oxygen ion transport through an electrolyte of the tubular membrane elements. More particularly, the present invention relates to such an oxygen separation module and apparatus in which manifolds to collect the separated oxygen are positioned at opposite ends of the tubular membrane elements and are connected to the tubular membrane elements such that a portion of the tubular membrane elements are connected to one of the manifolds and another portion of the tubular membrane elements are connected to the other of the manifolds. 
       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. The device can also be configured to separate H2O into H2 and O2 or CO2 into CO and O2. 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 or scandium 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]    The tubular membrane elements can be contained in one or more modules in which in each module, the tubular membrane elements are arranged in bundles and are held in place by end insulation members adjacent to the opposite ends of the tubular membrane elements. These modules can be positioned within insulated, heated enclosures to heat the tubular membrane elements to an operational temperature at which oxygen ion transport can occur. The insulated enclosure also has inlets and outlets within end walls of the enclosure to allow an oxygen containing feed stream to be passed into the enclosure and thereby to contact the tubular membrane elements. As a result of the oxygen separation, a retentate stream is formed that is discharged from the enclosure through the outlet. This type of electrically driven oxygen separation device is shown in U.S. Patent Appln. Ser. No. 2010/076280 A1. 
         [0007]    As can be appreciated, it is important that electrically driven oxygen separation devices reliably deliver the oxygen and at the lowest cost possible. With respect to reliability, a major problem with electrically driven oxygen separation devices, is that failure of the tubular membrane elements can occur. As a result, the oxygen containing feed stream will pass through the point of failure in a particular tubular membrane and little if any oxygen will be separated by the membrane that has the defect. Since, a major advantage of supplying oxygen from an electrically driven oxygen separation device is that the oxygen can be produced at ultra-high purity, the defective tubular membrane element will result in an unacceptable decrease in purity of the oxygen product. Therefore, as a result of such failure, the electrically driven oxygen separation device will have to be removed from service. Furthermore, such a device is most useful if the outlet of oxygen separation modules are connected to a storage tank and the oxygen is stored at pressure. In the case of a tube failure, the stored oxygen in the tank will discharge through the fractured ceramic tube. In order to reduce costs, the electrically driven oxygen separator has to be assembled in a cost efficient manner. In the patent application discussed above, the use of modules of such elements coupled with polymeric end seals go a long way toward reducing assembly costs. However, such ends seals represent another possible point of failure because they have only a limited ability to withstand the high temperatures that are necessary to induce the oxygen ion transport in the tubular membrane elements. 
         [0008]    As will be discussed, the present invention provides a module and an electrically driven oxygen separation device that, among other advantages is capable of operating upon failure of one or more tubular membrane elements and that is specifically designed to cool the end seals. 
       SUMMARY OF THE INVENTION 
       [0009]    The present invention provides, in one aspect, a module for an electrically driven oxygen separator that incorporates a plurality of tubular membrane elements. Each of the tubular membrane elements is configured to separate oxygen from an oxygen containing feed stream when an electric potential difference is applied to induce oxygen ion transport in an electrolyte thereof. A first manifold and a second manifold, configured to collect the oxygen, are spaced apart from one another with the tubular membrane elements situated between the first manifold and the second manifold. The first and second manifold are connected to the tubular membrane elements such that oxygen is received by the first manifold from a first portion of the tubular membrane elements and by the second manifold from a second portion of the tubular membrane elements. 
         [0010]    Therefore, upon failure of at least one of the tubular membrane elements in either the first portion or the second portion of the tubular membrane elements, oxygen is able to be collected from either the first portion or the second portion of the tubular membrane elements that do not include the at least one of the tubular membrane elements that has failed. Check valves can be provided to pneumatically isolate the failed tube and associated first or second tubular membrane elements. While the oxygen will of course be delivered at a lower rate after such a failure, unlike electrically driven oxygen separators of the prior art, the failure of one or more elements will not necessarily result in the electrically driven oxygen separator being withdrawn from service. 
         [0011]    End seals can be located at opposite ends of the tubular membrane elements. Each of the first manifold and the second manifold have a collection element to collect the oxygen produced by the tubular elements and first and second elongated elements connected at one end to the collection element and at the other end penetrating the end seals at the opposite ends of the tubular membrane elements. The first of the elongated elements are of tubular configuration to conduct the oxygen and the second of the elongated elements are configured to prevent flow of the oxygen to the collection element of each of the first manifold and the second manifold. The first of the elongated elements alternating with the second of the elongated elements such that as between two adjacent tubular membrane elements, the oxygen flows from one of the two adjacent tubular membrane elements to the collection element of the first manifold and from the other of the two adjacent tubular membrane elements to the collection element of the second manifold. The second of the elongated elements can be of solid configuration. 
         [0012]    The end seals can comprise plug-like members located within the tubular membrane element and formed by an elastomer to produce hermetic seals at the opposite ends of the tubular membrane elements and deposits of a ceramic adhesive located within the tubular membrane elements adjacent to the plug-like members and positioned to prevent outward movement of the plug-like members. The tubular membrane elements can be arranged in a bundle and are held in place by two opposed end insulation members located adjacent to the opposed ends of the tubular membrane elements. In such case, each of the first manifold and the second manifold has a spider-like configuration with the first and the second elongated elements radiating from the collection element of each of the first manifold and the second manifold. Further, each of the two opposed end insulation members has an inlet opening for passage of the oxygen containing feed stream. 
         [0013]    Each of the tubular membrane elements has an inner anode layer, an outer cathode layer and an electrolyte layer located between the anode layer and the cathode layer to form the electrolyte. Two current collector layers are located adjacent to and in contact with the anode layer and the cathode layer and situated on the inside and outside of the tubular membrane element to allow the electrical potential to be applied by a power source. The tubular membrane elements can be connected in series and contain current distributors of helical configuration in contact with one of the two current collector layers on the inside of the tubular membrane elements to conduct an electrical current applied by electrical conductors passing through the feed throughs. 
         [0014]    In another aspect, the present invention provides an electrically driven oxygen separation apparatus. Such apparatus is provided with an enclosure having two inlet regions, a heated interior region located between the inlet regions and having opposed end walls positioned adjacent to the inlet regions, opposed openings defined in the end walls, a sidewall connecting the end walls and heating elements positioned to heat the heated interior region. An outlet extends from the heated interior region and through the sidewall. A plurality of tubular membrane elements are provided that are each configured to separate oxygen from an oxygen containing feed stream when an electric potential difference is applied to induce oxygen ion transport in an electrolyte thereof. End seals are located at opposite ends of the tubular membrane elements. 
         [0015]    The tubular membrane elements are arranged in a bundle and held in place by two opposed end insulation members located adjacent to the opposed ends of the tubular membrane elements. The bundle is positioned within the enclosure and with the end insulation members situated within the openings of the end walls and the opposed ends of the tubular membrane elements and the end seals thereof projecting outwardly from the end insulation members and into the two inlet regions. At least one manifold is connected to the tubular membrane elements and configured to collect the oxygen produced by the tubular membrane elements. Inlets are located within the two end insulation members for passage of two oxygen containing feed streams from the inlet regions to the heated interior region and two blowers connected to the two inlet regions to circulate the oxygen containing feed stream into the inlet region and past the end seals to cool the end seals and then, through the inlets, thereby to contact the membrane elements inside the heated enclosure and to discharge a heated retentate from the heated enclosure that is formed by separation of the oxygen from the oxygen containing feed stream. The cooling of the end seals help to prevent failure of the electrically driven oxygen separator in the first instance. 
         [0016]    As set forth above, the at least one manifold can be a first manifold and a second manifold spaced apart from one another with the tubular membrane elements situated between the first manifold and the second manifold. The first manifold and the second manifold are connected to the tubular membrane elements such that oxygen is received by the first manifold from a first portion of the tubular membrane elements and by the second manifold from a second portion of the tubular membrane elements. Upon failure of at least one of the tubular membrane elements in either the first portion or the second portion of the tubular membrane elements, oxygen is able to be collected from either the first portion or the second portion of the tubular membrane elements that do not include the at least one of the tubular membrane elements that has failed. 
         [0017]    Each of the first manifold and the second manifold can be provided with a collection element to collect the oxygen produced by the tubular elements and first and second elongated elements connected at one end to the collection element and at the other end penetrating the end seals at the opposite ends of the tubular membrane elements. The first of the elongated elements are of tubular configuration to conduct the oxygen and the second of the elongated elements are configured to prevent flow of the oxygen to the collection element of each of the first manifold and the second manifold. The first of the elongated elements alternate with the second of the elongated elements such that as between two adjacent tubular membrane elements, the oxygen flows from one of the two adjacent tubular membrane elements to the collection element of the first manifold and from the other of the two adjacent tubular membrane elements to the collection element of the second manifold. 
         [0018]    The end seals can comprise plug-like members located within the tubular membrane element and formed by an elastomer to produce hermetic seals at the opposite ends of the tubular membrane elements and deposits of a ceramic adhesive located within the tubular membrane elements adjacent to the plug-like members and positioned to prevent outward movement of the plug-like members. As indicated above, the cooling of such end seals has proven to be critical for preventing failure of the electrically driven oxygen separation device. The second of the elongated elements can be of solid configuration. Each of the first manifold and the second manifold has a spider-like configuration with the first and the second elongated elements radiating from the collection element of each of the first manifold and the second manifold. 
         [0019]    Each of the membrane elements has an inner anode layer, an outer cathode layer and an electrolyte layer located between the anode layer and the cathode layer to form the electrolyte. Two current collector layers are located adjacent to and in contact with the anode layer and the cathode layer and situated on the inside and outside of the tubular membrane element to allow the electrical potential to be applied by a power source. The tubular membrane elements can be connected in series and the tubular membrane elements can contain current distributors of helical configuration in contact with one of the two current collector layers on the inside of the tubular membrane elements to conduct an electrical current applied by electrical conductors passing through the feed throughs. 
     
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         [0020]    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: 
           [0021]      FIG. 1  is a schematic sectional view of an electrically driven oxygen separation apparatus of the present invention; 
           [0022]      FIG. 2  is an elevational view of a module of the present invention; 
           [0023]      FIG. 3  is an enlarged, fragmentary perspective view of the module shown in  FIG. 2 ; 
           [0024]      FIG. 4  is a schematic, transverse cross-sectional view of a tubular membrane element used in a module of the present invention; and 
           [0025]      FIG. 5  is a schematic, sectional view of a tubular membrane element used in a module of the present invention. 
       
    
    
     DETAILED DESCRIPTION 
       [0026]    With reference to  FIG. 1 , an electrically driven oxygen separator  1  of the present invention is illustrated that has two modules  10  housed within an enclosure  12 . It is understood that there could be more or fewer modules  10  depending upon the application of an oxygen separation in accordance with the present invention. 
         [0027]    With reference to  FIG. 2 , each of the module  10  are formed by a bundle of tubular membrane elements that are divided into a first portion of the tubular membrane elements  14  and a second portion of the tubular membrane elements  16 . The first and second portions of the tubular membrane elements are held in position by end insulation members  18  and  20  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 oxygen that is separated by such first and second portions of the tubular membrane elements  14  and  16  are collected by first and second manifolds  22  and  24  that as illustrated are spaced apart from one another with the first and second portions of the tubular membrane elements  14  and  16  located between the first and second manifolds  22  and  24 . 
         [0028]    The first and second manifolds  22  and  24  are connected to the first and second portions of the tubular membrane elements  14  and  16  such that oxygen is received by the first manifold  22  from the first portion of the tubular membrane elements  14  and by the second manifold  24  from the second portion of the tubular membrane elements  16 . With additional reference to  FIG. 3 , the connection of the first manifold  22  is illustrated. Each of the first and second manifolds  22  and  24  are provided with first elongated elements  26  and second elongated elements  28  that radiate in a spider-like arrangement from a collection element  30  that actually collects the oxygen that is separated by the first and second portions of the tubular membrane elements  14  and  16 . As illustrated, the first portion of the tubular membrane elements  14  alternate with the second portion of the tubular membrane elements  16  and the elongated elements  26  alternate with the elongated elements  28 . The elongated elements penetrate the end seals  70  and  72  provided in opposite ends of both of the first and second portions of the tubular elements  14  and  16 . The first elongated elements  26  are hollow tubes and the second elongated elements  28  are of solid configuration, although such elongated elements  28  could be hollow tubes that are plugged. In any case, since the first elongated elements  26  are hollow tubes, the oxygen will flow from the first portion of the tubular membrane elements  14  to the collection element  30  while the oxygen will not flow from the second portion of the tubular membrane elements  16  to the collection elements  30 . At the opposite end of the module  10 , however, the second manifold  24 , that is identical to the first manifold  22 , is slightly rotated such that the first elongated elements  26  penetrate the end seals  72  of the second portion of the tubular membrane elements  16  and the second elongated elements  28  penetrate the end seals  70  of the first portion of the tubular membrane elements  16 . As a result, the oxygen produced by the second portion of the tubular membrane elements  16  is collected by the collection element of the second manifold  24 . Consequently, if one or more of the first portion of the tubular membrane elements  14  fail, oxygen will still able to be produced, albeit at a lower flow rate, from the second portion of the tubular membrane elements  16  that have not failed and vice-versa. 
         [0029]    As can be appreciated, it is possible to construct an embodiment of the present invention in which there is no such alternation of tubular membrane elements and elongated elements. For example the first portion of the tubular membrane elements  14  could be located on one side and the second portion of the tubular membrane elements  16  could be located on the other side of the module. In such case, the first elongated elements  14  would be located one side of the module  10  and the second elongated elements  16  would be located on the opposite side. Furthermore, embodiments of the present invention are also possible in which the tubular membrane elements are located in the same plane. As can be appreciated, the manifold in such case would have an elongated collection element with elongated elements extending therefrom at right angles to penetrate the end seals of the tubular membrane elements. In any embodiment, the tubular membrane elements are divided into portions such that one manifold will conduct the oxygen from one portion and the other manifold will conduct oxygen from the other portion. 
         [0030]    With additional reference to  FIG. 4 , each of the tubular membrane elements  14  is provided with a cathode layer  34 , an anode layer  36  and an electrolyte layer  38 . Two current collector layers  40  and  42  are located adjacent the anode layer  36  and the cathode layer  34 , respectively, to conduct an electrical current to the anode layer and the cathode layer. Tubular membrane elements  16  are identical to tubular membrane elements  14 . Although the present invention has application to any composite structure making up a tubular membrane element  14 , for exemplary purposes, the cathode layer  36  and the anode layer  34  can be between about 10 and about 50 microns thick and the electrolyte layer  38  can be between about 10 microns and about 1 mm. thick, with a preferred thickness of about 500 microns. The electrolyte layer  38  is gas impermeable and can be greater than about 95 percent dense and preferably greater than 99 percent dense. Each of the cathode layer  36  and the anode layer  34  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  38  can be 6 mol % scandium oxide, 1 mol % cerium oxide doped zirconium oxide. The current collector layers  40  and  42  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 S7000-02 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  44  contacts the current collector layer  40  and permeates through pores thereof to the cathode layer  36  which as indicated above is also porous. The oxygen ionizes as a result of an electrical potential applied to the cathode and anode layers  34  and  36  at current collector layers  40  and  42 . The resulting oxygen ions are transported through the electrolyte layer  38  under the driving force of applied potential and emerge at the side of the electrolyte layer  38  adjacent the anode layer  34  where electrons are gained to form elemental oxygen. The oxygen permeates through the pores of the anode layer  36  and the adjacent current collector  42  where the oxygen passes into the interior of the tubular membrane elements  14 . The same function, in the same manner would be obtained for tubular membrane elements  16 . 
         [0032]    It is to be noted, that although the cathode layer is located on the outside of the tubular membrane elements  14  and  16 , it is possible to reverse the layers so that the anode layer were located on the outside of the tubular membrane elements  14  and  16  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 continued reference to  FIG. 5 , it can be seen that the outer, opposite end sections of each of the tubular membrane elements  14  are located within end insulation members  18  and  20 . It is to be noted that the following discussion would have equal applicability to tubular membrane elements  16 . As a result, there is essentially no oxygen transport taking place at such locations. As illustrated, the ends of each of the tubular membrane elements  14  are devoid of both the cathode layer  36  and its associated current collector  40  and the anode layer  34  and its associated current collector  42  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 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 end insulation members. 
         [0034]    Tubular membrane elements  14  and  16  incorporate end seals  70  and  72  formed at the opposite ends thereof. Each of the end seals  70  and  72  are formed by plug-like members  74  and  76  that are each fabricated from an elastomer to effect a hermetic seal at the ends of the tubular membrane elements  14  and  16 . A suitable elastomer is a VITON® fluoroelastomer obtained through Dupont Performance Elastomers of Willmington, Del., United States of America. 
         [0035]    During operation of tubular membrane elements  14  and  16  oxygen will accumulate and will tend to force the plug-like members  74  and  76  in an outward direction and from the ends of tubular membrane elements  14  and  16 . In order to retain the plug-like members  74  and  76  within the end of tubular membrane elements  14  and  16 , deposits of a ceramic adhesive  78  and  80  are introduced into the ends of tubular membrane elements  14  and  16  at a location adjacent to plug-like member  74  and plug-like member  76 , respectively. A suitable ceramic adhesive can be a RESBOND™ 940 fast setting adhesive manufactured by Cotronics Corporation of Brooklyn, N.Y., United States of America. It is to be noted that other suitable means to retain plug-like member  74  and  76  could be employed such as mechanical keys located adjacent to plug-like member  74  that penetrate opposed transverse bores defined at the ends of tubular membrane elements  14  and  16  or sleeves cemented in place within the ends of tubular membrane elements  14  and  16 . 
         [0036]    As illustrated, an elongated element  28  penetrates the deposit  78  and the plug-like member  74  along with an electrical feed through  82  and an elongated elements  26  penetrates deposit  80  and plug-like member  76 . In this regard an axial bore  84  and  86  are defined within plug-like member  74  for penetration of electrical feed through  82  and the second elongated element  28 . An axial bore  88  is provided within plug-like member  76  for penetration of the elongated element  26 . 
         [0037]    In order to install plug-like members  74  and  76  within the end of tubular membrane elements  14  and  16 , the same is fabricated with a larger outer diameter than the inner diameter of tubular membrane elements  14  and  16  and then cooled with liquid nitrogen. The percentage difference in diameters can be about  10  percent. Thereafter, plug-like members  74  and  76  are installed in the ends of tubular membrane elements  14  and  16  and as such members warm to ambient temperature, the same expands to produce a hermetic seal within the ends of tubular membrane element  14  and  16 . Additionally, each of the bores  84 ,  86  and  88  are all sized smaller than the associated electrical feed through  82  and the elongated elements  28  and  26 . After installation and warming of the plug-like members  74  and  76 , the electrical feed through  84  and the elongated elements  28  and  26  are forced through the smaller bores to create hermetic seals. Thereafter, the ends are filled with the deposits of ceramic adhesive  78  and  80  to complete the end seals. As could be appreciated, other types of end seals are known in the art such as ceramic end caps and ceramic deposits within the tubes. 
         [0038]    The potential is applied to each of the tubular membrane elements  14  and  16  by means of a connection to the current collector layer  42  adjacent of the cathode layer  34  by means of a conductor  90  that is looped around the current collector layer  42  by a loop  92  that is held in place by silver paste  94 . Connection is established to current collector layer  40  adjacent the anode layer  36  by means of a conductor  90  that is attached to a current distributor  98  of helical configuration. Conductor  90  passes through the electrical feed through  82 . 
         [0039]    Although the tubular membrane elements  14  and  16  could be connected in parallel, preferably a series connection is established in which the current collector  40  of each of the tubular membrane elements  14  and  16  is connected to the current collector  42  of the next in series of the tubular membrane elements  14  and  16 . Therefore, the current collector  40  of each particular first tubular membrane element  14  is connected to the current collector  42  of the second tubular membrane element  16  located directly adjacent thereto and the current collector  42  of the second tubular membrane element  16  is connected to the current collector elements  40  of the next, adjacent first tubular membrane element. Thus, as can best be seen in  FIG. 3 , the conductor  90  of each of the first tubular membrane elements  14  is connected to the end of the electrical feed through  82  of each of the adjacent second tubular membrane elements  16  and the conductor  90  passes through the second insulating member  20  for connection to such adjacent first tubular element  14  at loop  92  thereof. Since the first tubular membrane elements  16  and the second tubular membrane element  14  are reversed, at the first insulating member  18 , the conductor  90  connects to the electrical feed through  82  of each of the first tubular membrane elements  14 , passes through the first insulating member  18  and then is connected to the second tubular membrane elements  16  via the loop  92  thereof. This being said in case of two adjacent first and second tubular membrane elements  14  and  16 , such connection between the elements as aforesaid is not established and instead, power cords  100  and  102  are connected to the electrical feed through  82  of the second tubular membrane element  16  and the current collector layer  42  of the first tubular membrane element  14  so that the electrical potential can be applied to the first and second tubular membrane elements  14  and  16 . 
         [0040]    With reference again to  FIG. 1 , the enclosure  12  has two opposite end walls  104  and  106  provided within opposite openings  108  and  110  within which the insulating members  18  and  20  are lodged with the ends of the first and second tubular membrane elements  14  and  16  exposed. The opposite end walls  104  and  106  are connected by a sidewall  112  thereby define a heated enclosure  114  that is heated by heating elements  116  embedded within the sidewall  112 . Attached to the end walls  104  and  106  are inlet regions  120  and  122  defined by the interior of cowlings  124  and  126 , respectively. Attached to the cowlings  124  and  126  are blowers  128  and  130 , respectively, that direct feed air streams  44  and  44  to the inlet regions  120  and  122 . With brief reference to  FIG. 3 , the insulating member  20  is provided with an opening in the form of an axial bore  136  that allows part of the feed air stream  44  to flow past the ends of the tubular membrane elements  14 ,  16  and thereby cool the ends and the deposits of elastomer that form the end seals before passing into the heated enclosure  114  and contact the first and second tubular membrane elements  14  and  16 . Although not illustrated, insulating member  18  is provided with a like opening to allow at least a portion of the feed air stream  44  to flow past the exposed ends of the first and second tubular membrane elements  14  and  16  and into the heated enclosure  114  for the same purpose. The separation of the oxygen from the feed air streams  44  and  44  form a retentate that is discharged from the heated enclosure  114 , through an exhaust  136  as a retentate stream  138 . 
         [0041]    As can be appreciated, embodiments of the present invention are possible in which in place of the axial bores or other openings within insulating members  18  and  20 , openings could be situated within the end walls  104  and  106 . The ends of the first and second tubular membrane elements  14  and  16  would not be cooled to the same extent as in the illustrated embodiment. Also, the openings in the insulating members, such as the illustrated insulating members  18  and  20  could be used in connection with an embodiment that did not have the first and second manifolds  22  and  24  of the present invention; or in other words, a single manifold collecting oxygen from all tubular membrane elements used in such embodiment. 
         [0042]    With reference again to  FIG. 2 , oxygen product streams  140  and  142  are withdrawn from the first tubular elements  14  and the second tubular elements  16  by lines  144  and  146  connected to the collection elements  30  of second and first manifolds  24  and  22 , respectively. Although not illustrated, the lines would pass through the cowlings  124  and  126  and then to a collection tank that would collect the oxygen product at pressure. As mentioned above, a central advantage of having the separate portions of the tubular membrane elements  14  and  16  is to prevent failure of the oxygen separation device  1  upon failure of a tubular membrane element. Moreover, where oxygen separation device  1  is used to supply oxygen to a tank under pressure, if a tubular membrane element failed, then product would be lost from the tank. In order to prevent this, check valves  148  and  150  are provided to isolate the first tubular membrane elements  14  from the second tubular membrane elements  16 , respectively, and thereby to prevent the loss of pressurized product oxygen upon failure of a tubular membrane element of either of the two portions. 
         [0043]    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.