Abstract:
A joint assembly for joining a ceramic membrane of tubular form to a tube sheet used in supporting the ceramic membrane within a reactor. The ceramic membrane is received within a fixture connected to the tube sheet and a follower, extending into the fixture, exerts a force on a high temperature sealing element located between the fixture and the ceramic membrane to effect a seal and to hold the ceramic membrane in place. The force can be exerted directly on the sealing element or on the ceramic membrane itself.

Description:
RELATED APPLICATIONS  
       [0001]    This is a continuation in part of U.S. Ser. No. 09/534,998, now abandoned. 
     
    
     
       FIELD OF THE INVENTION  
         [0002]    The present invention relates to a joint assembly for joining a ceramic membrane of tubular form to a tube sheet used in supporting the ceramic membrane within a reactor. More particularly, the present invention relates to such a joint assembly in which the ceramic membrane is received and sealed within a fixture connected to the tube sheet.  
         BACKGROUND OF THE INVENTION  
         [0003]    Ceramic membranes are used to separate a gaseous component from a mixture, for instance, oxygen from air. Ceramic membranes are gas tight and function by allowing ions to selectively migrate through the membrane. The disassociation and ionization of the selected gas occurs at a membrane surface where electrons are picked up from near surface electronic states. The flux of the ions is charged compensated by a simultaneous flux of electronic charge carriers through the ceramic membrane. When the ions arrive at the opposite side of the membrane, the individual ions release their electrons and recombine to form gas molecules. The driving force for such transport can be a differential partial pressure of the selected gas applied across the membrane or an external source of electric power.  
           [0004]    Ceramic membranes can be employed in the form of tubes located within reactors. Critical to the success of the reactor is both the survival of such ceramic membranes and adequate sealing at a location where the tubular ceramic element is joined with the reactor, generally at a tube sheet. Since ceramic membranes exhibit ion conductivity at temperatures that are well above 500° C., generally in the range of between about 600° C. and about 1000° C. the joint assembly and seal between the ceramic membrane and the tube sheet are subjected to extreme environmental conditions.  
           [0005]    A major obstacle in developing a viable seal are the unique thermomechanical properties of ceramic materials, e.g. high thermal expansion and compositional dilation and the high operational temperatures of the membranes. Both factors prohibit the use of common fixed joining techniques such as glass sealing or brazing. Instead, joining techniques that do not rigidly affix the ceramic within the reactor are used such as non-bonding, compression type joint assembles. For instance, both U.S. Pat. No. 5,820,654 and U.S. Pat. No. 5,820,655 use either a sliding or fixed seal with a bellows at the juncture of the ceramic membrane and the tube sheet.  
           [0006]    The prior art has provided high temperature seals that are used to seal structural panels. For instance, U.S. Pat. No. 4,917,302 utilizes a stack of ceramic wafers located within a rectangular groove along the side of a movable engine panel. The engine panel is sealed to an adjacent side wall by the ceramic wafers which are held in position by a pressurized linear bellows that also fits within the groove. U.S. Pat. No. 5,082,293 shows a similar seal except that the sealing element instead of consisting of a stack of wafers is made up of multiple layers of a fiber wound about a core. The materials for such fibers can be alumina-boriasilicate or silicon-carbide. U.S. Pat. No. 5,301,595 discloses a rope seal having a core of ceramic fibers and a cover of stainless steel. The rope seal is designed to seat within a groove in one component and bear against a flat wall of another component. U.S. Pat. No. 4,394,023 shows a high temperature valve stem packing that incorporates graphite seal rings composed of coiled graphite tape held between metal packing adapter rings that bear against the graphite seal rings.  
           [0007]    All of the foregoing materials are attractive for sealing a ceramic membrane to a tube sheet due to their high temperature performance. As will be discussed, the present invention provides a joint assembly that is designed to advantageously utilize high temperature sealing materials, such as those identified above and to hold the ceramic membrane in place.  
         SUMMARY OF THE INVENTION  
         [0008]    The present invention provides a joint assembly for joining a ceramic membrane of tubular configuration to a tube sheet. The joint assembly has a fixture connected to the tube sheet and including a passageway having a narrow end section, located at one end of the passageway, to receive the ceramic membrane. A sealing surface, defined by the passageway, is located at the one end thereof. The ceramic membrane extends through the narrow end section of the passageway so that an open end of the ceramic membrane is located within the passageway and an adjacent lateral surface of the membrane is surrounded by the sealing surface. At least one sealing element is located between the sealing surface and the lateral surface of the ceramic membrane. A follower is located within the passageway and bears against the at least one sealing element in a direction towards the narrow end section of the passageway. This action simultaneously drives the at least one sealing element against the sealing surface and the lateral surface of the ceramic membrane by compression of the at least one sealing element. As a result, a seal is effected between the fixture and the ceramic membrane and the ceramic membrane is held in place by frictional forces developed between the at least one sealing element and the ceramic membrane. The follower is provided with an inner passage in communication with the open end of the ceramic membrane to allow permeate or feed to flow through the follower.  
           [0009]    The sealing surface can be formed by a tapered section of said passageway, tapering towards the narrow end section. In such case, the passageway is also provided with an annular end surface connecting the tapered section of said passageway with the narrow end section. The follower has a fusto-conical end element configured to fit within said tapered section of said passageway and to bear against said at least one sealing element.  
           [0010]    In a further aspect of the present invention the fixture bears against the ceramic membrane to compress the at least one sealing element. In this aspect of the present invention, the fixture of the joint assembly includes a passageway having a narrow end section, located at one end of the passageway. A sealing surface surrounds and is located adjacent to the narrow end section. The ceramic membrane has an enlarged end portion at an open end thereof. The enlarged end portion is located within the passageway with the ceramic membrane extending from the narrow end section of the passageway. At least one sealing element is located between the sealing surface and the enlarged end portion of the ceramic membrane and a follower, located within the passageway, bears against the enlarged end portion of the ceramic membrane in a direction towards the narrow end section of the passageway. This action compresses the at least one sealing element between the sealing surface and the enlarged end portion of the ceramic membrane. As a result, a seal is effected between the fixture and the ceramic membrane and the ceramic membrane is held in place, against the sealing surface. The follower is provided with an inner passage in communication with the open end of the ceramic membrane to allow feed or permeate to flow through the follower.  
           [0011]    The enlarged end portion of the ceramic membrane can be formed by an outwardly flared portion of the ceramic membrane to produce an outwardly flared lateral surface thereof. In such embodiment of the invention, the sealing surface is formed by a tapered section of the passageway, tapering towards the narrow end section. The at least one sealing element is a cone seal gasket located between said outwardly flared lateral surface and said sealing surface. An annular butt seal gasket is located between the follower and the open end of said ceramic membrane in alignment with the inner passageway of the follower.  
           [0012]    In yet another alternative embodiment the sealing surface is an annular end surface of the passageway surrounding the narrow end section thereof. The enlarged end portion has an annular undersurface located opposite to the annular end surface and the at least one sealing element comprises a annular butt seal gasket located between said annular undersurface and said annular end surface. The follower also has an end section having a cavity configured to receive the enlarged end portion of said ceramic membrane. In such embodiment, the enlarged end portion of the ceramic membrane and the cavity can be of fusto-connical configuration. In such case, a cone-seal gasket is located between the enlarged end portion and the cavity. A butt seal gasket is located between the open end of the ceramic membrane and the follower, in alignment with the passage thereof.  
           [0013]    The at least one sealing element can be formed of a rope-like packing wound around the lateral surface of the ceramic membrane. Alternatively, the at least one sealing element can be formed of a paper or felt stuffing of the ceramic material. The ceramic material can be an aluminosilicate fiber or a zirconia fiber. Advantageously, the ceramic material can be infiltrated with a particulate material and preferably such particulate material can be a ceramic or a metal. The sealing element can generally be formed of a ceramic mineral such as vermiculite. The at least one sealing element can also be formed of a layer of a ceramic powder or a graphite packing.  
           [0014]    In any embodiment of the present invention, the fixture can be provided with an inlet port for introduction of a buffer gas into the fixture. 
       
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS  
       [0015]    While the specification concludes with claims distinctly pointing out the subject matter that Applicants regard as their invention, it is believed that the invention will be better understood when taken in connection with the accompanying drawings in which:  
         [0016]    [0016]FIG. 1 is a schematic view of a joint assembly in accordance with the present invention;  
         [0017]    [0017]FIG. 2 is an enlarged fragmentary view of an embodiment of the joint assembly shown in FIG. 1;  
         [0018]    [0018]FIG. 3 is an enlarged fragmentary view of an alternative embodiment of the joint assembly shown in FIG. 1;  
         [0019]    [0019]FIG. 4 is a schematic view of an alternative embodiment of a joint assembly in accordance with the present invention.  
         [0020]    [0020]FIG. 5 is an enlarged fragmentary view of an alternative embodiment of sealing elements used in the embodiment shown in FIG. 2; and  
         [0021]    [0021]FIG. 6 is an enlarged fragmentary view of an alternative embodiment of sealing elements used in the embodiment shown in FIG. 2. 
     
    
     DETAILED DESCRIPTION  
       [0022]    With reference to FIG. 1, a joint assembly  1  is illustrated for sealing in open end of a ceramic membrane  2  of tubular configuration to a tube sheet  3 . Joint assembly  1  includes a fixture  10  connected to tube sheet  3  and a follower  12  having a hex-like head  13 .  
         [0023]    Follower  12  and its hex-like head  13  are provided with internal bores  14  and  15 , respectively, that form an inner passage to allow a permeate or feed stream to flow through follower  12  and therefore joint assembly  1 . Follower  12  fits within the fixture  10  with a threaded engagement  17  to exert pressure against a sealing element  16 . In this regard, hex-like head  13  of follower  12  facilitates the threading of follower  12  into fixture  10 .  
         [0024]    It is to be noted that fixture  10  and follower  12  can be fabricated from HAYNES 230 alloy. HAYNES 214 and INCOLOY 800 are other possible materials.  
         [0025]    With additional reference to FIG. 2, fixture  10  is provided with a passageway  18  having a narrow end section  20  located at one end of passageway  18 . A sealing surface  22  is formed by an inner tapered section of a passageway  18  that tapers towards narrow end section  20 . Narrow end section  20  and inner tapered sealing surface  22  are connected by an annular end surface  23 . When an open end  24  of ceramic membrane  2  is received within narrow end section  20 , an adjacent lateral surface  26  of ceramic membrane  2  is surrounded by sealing surface  22 .  
         [0026]    Sealing element  16  can be in the form of four coils of a rope-type packing is positioned between sealing surface  22  and lateral surface  26  of ceramic membrane  2  and against annular end surface  23 .  
         [0027]    The rope-type packing of sealing element  16  can be formed of an aluminosilicate fiber or a zirconia fiber. In this regard, sealing element  16  can be a rope-like material having a diameter of about 1.59 mm and formed of plied and twisted filaments of alumina-boriasilicate. Such filaments are sold commercially as NEXTEL 312 ceramic fibers as a product of 3M Ceramics Materials Department, 3M Center, St. Paul, Minn., 55144, United States. The particular rope-like material, described above, can be obtained in finished form from Coltronics Corp., Brooklyn, N.Y., United States, as item# CT301.  
         [0028]    In addition to rope-like packings, sealing element  16  can be formed of a paper or felt stuffing of the ceramic material. Ceramic minerals are also possible such as vermiculite.  
         [0029]    The sealing efficiency of any of the ceramic materials mentioned above can be enhanced by infiltrating the fibers with a particulate such as a refractory ceramic (e.g. Al 2 O 3 , ZrO 2 , MgO and etc.), or a powder of ceramic material used in fabricating ceramic membrane  2 , or possibly a metal such as gold. The particulate material can be applied by dipping or spraying a slurry.  
         [0030]    Follower  12  is provided with a fusto-conical end element  30  that fits within the tapered section of passageway  18 . End element  30  bears against sealing element  16  in a direction taken towards narrow end section  20  by action of threaded engagement  17 . This action drives sealing element  16  against sealing surface  22 , lateral surface  26  of ceramic membrane  2 , and annular end surface  23 , thereby to compress sealing element  16 . As a result, a seal is effectuated between fixture  10  and ceramic membrane  2  and therefore also tube sheet  3  due to the connection of fixture  10  and tube sheet  3 . At the same time a strong frictional engagement is produced to hold ceramic membrane  2  in place.  
         [0031]    Good sealing results were obtained for a ceramic membrane having an outer diameter of about 1.27 cm. and with a sealing element  16  as has been specifically described above and illustrated in FIG. 1. Such results were obtained with sealing surface  22  having a length of about 1.9 cm.(as measured along the side of fixture  10 ) and a taper of about 3 degrees. Annular end surface  23  had a width, as measured along a radius thereof, of about 1.02 mm. When joint assembly  1  was completely assembled, sealing element  16  was compressed to a length (as measured along the side of fixture  10 ) of about 9.53 mm.  
         [0032]    [0032]FIG. 3 illustrates a joint assembly  1 ′ that is used to join a ceramic membrane  2 ′ to a tube sheet such as that designated by reference numeral  3 . Joint assembly  1 ′ includes a fixture  32  that is connected to the tube sheet  3 . Fixture  32  is provided with a passageway  33  having a tapered section to form a sealing surface  34  surrounding and located adjacent to a narrow end section  36  of passageway  33 . As illustrated, ceramic membrane  2 ′ projects from narrow end section  36  of passageway  33 . Ceramic membrane  2 ′ is provided with an enlarged end portion  40  located at an open end  38  thereof, that is outwardly flared to produce an outwardly flared lateral surface  41 . When ceramic membrane  2 ′ is positioned within narrow end section  36 , lateral surface  41  of ceramic membrane  2 ′ is surrounded by sealing surface  34 .  
         [0033]    A sealing element  42  in the form of a cone seal gasket fits between sealing surface  34  and lateral surface  41  of ceramic membrane  2 ′. An optional, annular butt seal gasket  45  is positioned between a follower  44  extended into passageway  33  of fixture  32  and in alignment an internal bore  46  of a passageway for the flow of a permeate through fixture  33 . Sealing element  42  and annular butt seal gasket  45  can be fabricated from a ceramic fiber paper (formed from the same materials set forth above). Although not illustrated, sealing element  42  and annular butt seal gasket  45  could be replaced by a wound rope-like seal as illustrated for sealing element  16 .  
         [0034]    Although not illustrated, follower  44  and fixture  32  can be provided with a threaded engagement in the same manner as threaded engagement  17  of fixture  10  and follower  12 . Follower  44  is provided with a cylindrical end element  47  to bear against annular butt seal gasket  45  when follower  44  is driven within fixture  32  by such threaded engagement. This acts to provide a seal between bore  46  and open end  38  of ceramic membrane  2 ′. Furthermore, such action in turn forces lateral surface  41  of ceramic membrane  2 ′ against sealing element  42  and sealing surface  34 . As a result, sealing element  42  is compressed to seal ceramic membrane  2 ′ within fixture  32  and therefore also with respect to the tube sheet. At the same time, ceramic membrane  2 ′ is held in place due to the force exerted by follower  44 .  
         [0035]    It is to be noted that in many applications, a seal produced by a joint assembly in accordance with the present invention will produce some leakage of a permeate such as oxygen. In order to prevent leakage, fixture  32  can optionally be provided with an enlarged bore  46  adjacent to inner tapered sealing surface  34  and an inlet port  48  in communication with bore  34  to allow introduction of an inert buffer gas to prevent leakage from the sealing arrangement described above. Fixture  10  could be provided with a similar arrangement.  
         [0036]    With reference to FIG. 4, is an alternative embodiment illustrating a joint assembly  1 ″ that is used to join a ceramic membrane  2 ″ to tube sheet  3 . Joint assembly  1 ″ is provided with a fixture  50  connected to tube sheet  3 . A ceramic membrane  2 ″ is fabrication with an enlarged end portion  52  located at an open end  53  thereof. Enlarged end portion  52  fits within fixture  50  with the remainder of ceramic membrane  2 ″ projecting from fixture  50 . A follower  54  by a threaded connection (not shown but as in other embodiments) bears against enlarged end portion  52 .  
         [0037]    Fixture  50  includes a passageway  56  having a narrow end section  58 , and an annular end surface  60  surrounding and adjacent to the narrow section  58  of passageway  56 . Enlarged end portion  52  has a fusto-conical configuration to provide a cone-shaped lateral surface  62  and an annular undersurface  64  that is located opposite to the annular end surface  60  when ceramic membrane  2 ″ is received within fixture  50 . Follower  54  has an end section  66  to bear against enlarged end portion  52 . End section  66  has a cavity  68  also of fusto-conical configuration to receive enlarged end portion  52  of the ceramic membrane  2 ″. Cavity  68  is in communication with one end of a bore  70  serving as an inner passage for permeate to flow through follower  54 .  
         [0038]    A sealing element  72  in the form of annular butt seal gasket is located between annular undersurface  64  and annular end surface  60  to seal ceramic membrane  2 ″ within fixture  50  when follower  54  is driven towards narrow end section  56  and therefore annular end surface  60 . A cone seal gasket  74  can optionally be provided to seal enlarged end portion  52  within cavity  68 . Furthermore, an annular butt seal gasket  76 , located between open end  53  of ceramic membrane  2 ″ and follower  50  and in alignment with bore  70 , can optionally be provided to seal bore  70  to open end  53  of ceramic membrane  2 ″.  
         [0039]    With reference to FIG. 5, in place of a sealing element  16  for joint assembly  1 , a sealing element  16 ′ can be provided that consists of two coils  80  and  82  of a rope-type packing and a layer of a mineral or graphite packing  84 . With reference to FIG. 6 a sealing element  16 ″ can be provided in place of sealing element  16  that consists of two coils  86  and  88  of a rope-type packing sandwiching a mineral or graphite packing  90 .  
         [0040]    Other types of sealing elements are possible. However, any sealing element used to effectuate a seal in the extreme environment contemplated by the present invention must be designed to retain its shape within a temperature range of between about 700° C. and about 1200° C. It is important that the seal be able to withstand an oxygen partial pressure within a pressure range of between about 10 −18  atmospheres and about 3 atmosphere. Further, any such sealing element should additionally be able to withstand an absolute pressure within the pressure range of between about 1 atmospheres and about 70 atmospheres.  
         [0041]    Other types of fixtures in accordance with the present invention are possible. For instance, fixture  10  could be modified by continuing passageway  18  to narrow end section  20  with a constant diameter. As a result, the fixture would not have a tapered sealing surface  22  nor would follower  12  have a fusto-conical end element  30 . Through action of a threaded engagement between the follower and the fixture, a sealing element such as a rope seal could be compressed against the base of such modified bore to cause an outward deformation of the sealing element. The outward deformation surfaces of the sealing element would bear against both fixture and ceramic membrane to effect a seal with a frictional engagement to hold the membrane in place.  
         [0042]    A modification to joint assembly  1 ″ is also possible though use of a fixture having a bore of constant diameter, as has been discussed above. In such modification, enlarged end portion  52  would be given a cylindrical configuration and rope-like packing elements might be provided both between the lateral surface of the enlarged end portion and the passageway of the fixture and between the undersurface of such enlarged end portion and the annular end surface of the passageway. A cylindrical cavity at the end of the follower would at once receive the enlarged end portion, bear against packing elements located between the enlarged end portion and the surface of the passageway and also bear against the end section of the ceramic membrane to compress the packing element located between its undersurface and the annular end surface of the passageway. Thus, such embodiment would have elements of either joint assemblies  1  and  1 ″. The claims therefore are meant to cover such an embodiment.  
         [0043]    A further possible modification to the illustrated preferred embodiments as has been discussed with reference to either joint assembly  1 , joint assembly  1 ′ or joint assembly  1 ′″ is to use locking pins in place of a threaded engagement such that designated by reference numeral  17 .  
         [0044]    While the invention has been described with reference to a preferred embodiment, as will occur to those skilled in the art, numerous changes, additions and omissions can be made without departing from the spirit and the scope of the present invention.