Patent Publication Number: US-8114193-B2

Title: Ion transport membrane module and vessel system

Description:
CROSS-REFERENCE TO RELATED APPLICATIONS 
     This application is a divisional of application Ser. No. 11/699,145 now U.S. Pat. No. 7,335,247, filed on Jan. 29, 2007, which is a divisional of application Ser. No. 10/635,695 now U.S. Pat. No. 7,179,323, filed on Aug. 6, 2003, the specifications and claims which are incorporated by reference and made a part of this Application. 
    
    
     STATEMENT REGARDING FEDERALLY SPONSORED RESEARCH OR DEVELOPMENT 
     This invention was made with Government support under Cooperative Agreement No. DE-FC26-97FT96052 between Air Products and Chemicals, Inc. and the U.S. Department of Energy. The Government has certain rights to this invention. 
    
    
     BACKGROUND OF THE INVENTION 
     The permeation of oxygen ions through ceramic ion transport membranes is the basis for a variety of gas separation devices and oxidation reactor systems operating at high temperatures in which permeated oxygen is recovered on the permeate side as a high purity oxygen product or is reacted on the permeate side with oxidizable compounds to form oxidized or partially oxidized products. The practical application of these gas separation devices and oxidation reactor systems requires membrane assemblies having large surface areas, means to contact feed gas with the feed sides of the membranes, and means to withdraw product gas from the permeate sides of the membranes. These membrane assemblies may comprise a large number of individual membranes arranged and assembled into modules having appropriate gas flow piping to introduce feed gas into the modules and withdraw product gas from the modules. 
     Ion transport membranes may be fabricated in either planar or tubular configurations. In the planar configuration, multiple flat ceramic plates are fabricated and assembled into stacks or modules having piping means to pass feed gas over the planar membranes and to withdraw product gas from the permeate side of the planar membranes. In tubular configurations, multiple ceramic tubes may be arranged in bayonet or shell-and-tube configurations with appropriate tube sheet assemblies to isolate the feed and permeate sides of the multiple tubes. 
     The individual membranes used in planar or tubular module configurations typically comprise very thin layers of active membrane material supported on material having large pores or channels that allow gas flow to and from the surfaces of the active membrane layers. The ceramic membrane material and the components of the membrane modules can be subjected to significant mechanical stresses during normal steady-state operation and especially during unsteady-state startup, shutdown, and upset conditions. These stresses may be caused by thermal expansion and contraction of the ceramic material and by dimensional variance caused by chemical composition or crystal structure changes due to changes in the oxygen stoichiometry of the membrane material. These modules may operate with significant pressure differentials across the membrane and membrane seals, and stresses caused by these pressure differentials must be taken into account in membrane module design. In addition, the relative importance of these phenomena may differ depending on whether the modules are operated in gas separation or oxidation service. The potential operating problems caused by these phenomena may have a significant negative impact on the purity of recovered products and on membrane operating life. 
     There is a need in the field of high temperature ceramic membrane reactor systems for new membrane module and vessel designs that address and overcome these potential operating problems. Such designs should include features to allow efficient operation, long membrane life, minimum capital cost, and the ability to specify membrane systems over a wide range of production rates. Embodiments of the invention disclosed herein address these design problems and include improved module and vessel designs for both oxygen production and oxidation systems. 
     BRIEF SUMMARY OF THE INVENTION 
     An embodiment of the invention relates to an ion transport membrane system comprising a pressure vessel having an interior, an exterior, an inlet, and an outlet; a plurality of planar ion transport membrane modules disposed in the interior of the pressure vessel and arranged in series, each membrane module comprising mixed metal oxide ceramic material and having an interior region and an exterior region, wherein any inlet and any outlet of the pressure vessel are in flow communication with exterior regions of the membrane modules; and one or more gas manifolds in flow communication with interior regions of the membrane modules and with the exterior of the pressure vessel. 
     Each planar membrane module typically comprises a plurality of wafers having planar parallel surfaces; the pressure vessel may be cylindrical and may have an axis that is parallel to some or all of the planar parallel surfaces of the wafers. 
     The system may further comprise a flow containment duct disposed in the interior of the pressure vessel, wherein the flow containment duct surrounds the plurality of planar ion transport membrane modules and is in flow communication with any inlet and any outlet of the pressure vessel. The one or more gas manifolds may comprise an inlet manifold and an outlet manifold; the interior region of any planar membrane module may be in flow communication with the inlet manifold via a secondary inlet manifold and may be in flow communication with the outlet manifold via a primary outlet manifold; and within the flow containment duct, the secondary inlet manifold and the primary outlet manifold of any planar membrane module may be combined to form a nested manifold. The flow containment duct may comprise an oxidation-resistant metal alloy containing iron and one or more elements selected from the group consisting of nickel and chromium. 
     The one or more gas manifolds may be disposed in the interior of the pressure vessel or exterior to the pressure vessel. The one or more gas manifolds may be insulated internally, externally, or internally and externally. 
     At least two of the planar ion transport membrane modules define a module axis, wherein the pressure vessel may be cylindrical and may have an axis that is parallel to the module axis. At least two of the planar ion transport membrane modules may define a module axis, wherein the pressure vessel may be cylindrical and may have an axis that is perpendicular to the module axis. 
     The system may further comprise insulation disposed in the interior of the pressure vessel. The insulation may be disposed in a region between an interior surface of the pressure vessel and the membrane modules, wherein the insulation forms a cavity that surrounds the membrane modules and the cavity is in flow communication with any inlet and any outlet of the pressure vessel. Alternatively, the insulation may be in contact with the interior surface of the pressure vessel. In another alternative, the insulation may not be not in contact with the interior surface of the pressure vessel. In yet another alternative, the system may further comprise a flow containment duct disposed in the interior of the pressure vessel, wherein the planar ion transport membrane modules are disposed within the duct, and wherein the insulation is disposed between an interior surface of the pressure vessel and an exterior surface of the duct. 
     The insulation may (a) be in contact with the interior surface of the pressure vessel and is not in contact with the exterior surface of the duct; (b) be in contact with the interior surface of the pressure vessel and is in contact with the exterior surface of the duct; (c) not be in contact with the interior surface of the pressure vessel and not be in contact with the exterior surface of the duct; or (d) not be in contact with the interior surface of the pressure vessel and be in contact with the exterior surface of the duct. 
     The system may further comprise a flow containment duct disposed in the interior of the pressure vessel and in flow communication with the inlet and outlet of the pressure vessel, wherein the planar ion transport membrane modules are disposed within the duct, wherein the insulation is disposed between an interior surface of the duct and the membrane modules, and wherein the insulation forms a cavity that surrounds the membrane modules and is in flow communication with any inlet and any outlet of the pressure vessel. The system may further comprise insulation around the exterior of the pressure vessel. 
     The one or more gas manifolds may comprise metal and the ion transport modules may comprise ceramic, wherein connections between the one or more gas manifolds and the modules may include ceramic-to-metal seals, and wherein the ceramic-to-metal seals may be surrounded by the insulation. 
     The insulation may comprise one or more materials selected from the group consisting of fibrous alumina, fibrous alumina silicate, porous alumina, porous alumina silicate. The insulation may comprise one or more materials selected from the group consisting of magnesium oxide, calcium oxide, copper oxide, calcium carbonate, sodium carbonate, strontium carbonate, zinc oxide, strontium oxide, and alkaline-earth-containing perovskites. 
     The system may further comprise a guard bed disposed between any inlet of the pressure vessel and a first membrane module. This guard bed may contain one or more materials selected from the group consisting of magnesium oxide, calcium oxide, copper oxide, calcium carbonate, sodium carbonate, strontium carbonate, zinc oxide, strontium oxide, and alkaline-earth-containing perovskites. 
     The ion transport membrane system may further comprise
         (a) one or more additional pressure vessels, each having an interior, an exterior, an inlet, and an outlet;   (b) a plurality of planar ion transport membrane modules disposed in the interior of each of the one or more pressure vessels and arranged in series, each membrane module comprising mixed metal oxide ceramic material and having an interior region and an exterior region, wherein any inlet and any outlet of the pressure vessel are in flow communication with exterior regions of the membrane modules; and   (c) one or more gas manifolds in flow communication with interior regions of the membrane modules and with the exterior of the pressure vessel;
 
wherein at least two of the pressure vessels are arranged in series such that the outlet of one pressure vessel is in flow communication with the inlet of another pressure vessel.
       

     Alternatively, the ion transport membrane system may further comprise
         (a) one or more additional pressure vessels, each having an interior, an exterior, an inlet, and an outlet;   (b) a plurality of planar ion transport membrane modules disposed in the interior of each of the one or more pressure vessels and arranged in series, each membrane module comprising mixed metal oxide ceramic material and having an interior region and an exterior region, wherein any inlet and any outlet of the pressure vessel are in flow communication with exterior regions of the membrane modules; and   (c) one or more gas manifolds in flow communication with the interior regions of the membrane modules and with the exterior of the pressure vessel;
 
wherein at least two of the pressure vessels are arranged in parallel such that any inlet of one pressure vessel and any inlet of another pressure vessel are in flow communication with a common feed conduit.
       

     The system may further comprise an additional plurality of planar ion transport membrane modules disposed in the interior of the pressure vessel and arranged in series, wherein the plurality of planar ion transport membrane modules and the additional plurality of planar ion transport membrane modules lie on parallel axes. 
     A further embodiment if the invention is an ion transport membrane system comprising
         (a) a pressure vessel having an interior, an exterior, an inlet, and an outlet;   (b) a plurality of planar ion transport membrane modules disposed in the interior of the pressure vessel and arranged in a series of banks of modules, each bank containing two or more modules in parallel, each membrane module comprising mixed metal oxide ceramic material and having an interior region and an exterior region, wherein any inlet and any outlet of the pressure vessel are in flow communication with exterior regions of the membrane modules; and   (c) one or more gas manifolds in flow communication with interior regions of the membrane modules and with the exterior of the pressure vessel.       

     An alternative embodiment of the invention is an ion transport membrane system comprising
         (a) a pressure vessel having an interior, an exterior, an inlet, and an outlet;   (b) a plurality of ion transport membrane modules disposed in the interior of the pressure vessel and arranged in series, each membrane module comprising mixed metal oxide ceramic material and having an interior region and an exterior region, wherein any inlet and any outlet of the pressure vessel are in flow communication with exterior regions of the membrane modules; and   (c) one or more gas manifolds disposed in the interior of the pressure vessel and in flow communication with the interior regions of the membrane modules and with the exterior of the pressure vessel.       

     Another alternative embodiment of the invention is an ion transport membrane system comprising
         (a) a pressure vessel having an interior, an exterior, an inlet, and an outlet;   (b) a membrane stack or module assembly disposed in the interior of the pressure vessel, the assembly having a plurality of planar wafers comprising mixed metal oxide ceramic material, each wafer having an interior region and an exterior region, and a plurality of hollow ceramic spacers, wherein the stack or module assembly is formed by alternating wafers and spacers such that the interiors of the wafers are in flow communication via the hollow spacers, the wafers are oriented parallel to one another, and the alternating spacers and wafers are oriented coaxially to form the stack or module such that the wafers are perpendicular to the stack or module axis;   (c) a gas manifold shroud assembly disposed around the membrane stack or module assembly within the interior of the pressure vessel, wherein the shroud assembly separates the stack or module into at least a first wafer zone and a second wafer zone, places any inlet of the pressure vessel in flow communication with exterior regions of the wafers in the first wafer zone, and places exterior regions of the wafers in the first wafer zone in series flow communication with exterior regions of the wafers of the second wafer zone.       

     This ion transport membrane system may further comprise a plurality of additional wafer zones formed by the gas manifold shroud assembly, wherein the shroud assembly places the additional wafer zones in series flow communication with one another, and wherein one of the additional wafer zones is in flow communication with any outlet of the pressure vessel. 
     Another embodiment of the invention includes a method for the recovery of oxygen from an oxygen-containing gas comprising
         (a) providing an ion transport membrane separator system comprising
           (1) a pressure vessel having an interior, an exterior, an inlet, and an outlet;   (2) a plurality of planar ion transport membrane modules disposed in the interior of the pressure vessel and arranged in series, each membrane module comprising mixed metal oxide ceramic material and having an interior region and an exterior region, wherein any inlet and any outlet of the pressure vessel are in flow communication with exterior regions of the membrane modules; and   (3) one or more gas manifolds in flow communication with the interior regions of the membrane modules and with the exterior of the pressure vessel;   
           (b) providing a heated, pressurized oxygen-containing feed gas stream, introducing the feed gas stream via any pressure vessel inlet to the exterior regions of the membrane modules, and contacting the feed gas stream with the mixed metal oxide ceramic material;   (c) permeating oxygen ions through the mixed metal oxide ceramic material, recovering high purity oxygen gas product in the interior regions of the membrane modules, and withdrawing the high purity oxygen gas product from the interior regions of the membrane modules through the gas manifolds to the exterior of the pressure vessel; and   (d) withdrawing an oxygen-depleted oxygen-containing gas from any pressure vessel outlet.       

     The pressure of the oxygen-containing feed gas typically is greater than the pressure of the high purity oxygen gas product. The ion transport membrane separator system further may comprise a flow containment duct that has an interior and an exterior and is disposed in the interior of the pressure vessel, and wherein the flow containment duct surrounds the plurality of planar ion transport membrane modules and is in flow communication with any inlet and any outlet of the pressure vessel such that the oxygen-containing feed gas passes through the interior of the flow containment duct. 
     The pressure differential between the interior and the exterior of the flow containment duct at any point between the inlet and outlet of the pressure vessel may be maintained at a value equal to or greater than zero, wherein the pressure in the interior of the duct may be equal to or greater than the pressure in the pressure vessel exterior to the duct. 
     Another embodiment of the invention relates to an oxidation process comprising
         (a) providing an ion transport membrane reactor system comprising
           (1) a pressure vessel having an interior, an exterior, an inlet, and an outlet;   (2) a plurality of planar ion transport membrane modules disposed in the interior of the pressure vessel and arranged in series, each membrane module comprising mixed metal oxide ceramic material and having an interior region and an exterior region, wherein any inlet and any outlet of the pressure vessel are in flow communication with exterior regions of the membrane modules; and   (3) one or more gas manifolds in flow communication with interior regions of the membrane modules and with the exterior of the pressure vessel;   
           (b) providing a heated, pressurized reactant feed gas stream, introducing the reactant feed gas stream via any pressure vessel inlet to the exterior regions of the membrane modules;   (c) providing an oxygen-containing oxidant gas to the interior regions of the membrane modules, permeating oxygen ions through the mixed metal oxide ceramic material, reacting oxygen with components in the reactant feed gas stream in the exterior regions of the membrane modules to form oxidation products therein, and withdrawing the oxidation products from the exterior regions of the membrane modules through any outlet to the exterior of the pressure vessel to provide an oxidation product stream; and   (d) withdrawing oxygen-depleted oxygen-containing gas from the interior regions of the membrane modules via the one or more manifolds to the exterior of the pressure vessel.       

     In this embodiment, the pressure of the pressurized reactant feed gas stream typically is greater than the pressure of the oxygen-containing oxidant gas. The ion transport membrane reactor system may further comprise a flow containment duct that has an interior and an exterior and is disposed in the interior of the pressure vessel, wherein the flow containment duct surrounds the plurality of planar ion transport membrane modules and is in flow communication with the inlet and the outlet of the pressure vessel such that the pressurized reactant feed gas stream passes through the interior of the flow containment duct. 
     The pressure differential between the interior and the exterior of the flow containment duct at any point between the inlet and outlet of the pressure vessel may be maintained at a value equal to or greater than zero, and the pressure in the interior of the duct may be equal to or greater than the pressure in the pressure vessel exterior to the duct. 
     The pressurized reactant feed gas stream may comprise one or more hydrocarbons having one or more carbon atoms, one of which may comprise methane. The oxidation product stream may comprise hydrogen and carbon oxides. 
     Another embodiment of the invention relates to an ion transport membrane reactor system comprising
         (a) a pressure vessel having an interior, an exterior, an inlet, and an outlet;   (b) a plurality of ion transport membrane modules disposed in the interior of the pressure vessel, wherein a first plurality of modules are arranged in series; and   (c) catalyst disposed between any two membrane modules in the first plurality of modules.
 
The reactor system may further comprise a second plurality of modules arranged in series, wherein the first plurality of modules is arranged in parallel with the second plurality of modules.
       

     In this reactor system, the catalyst may be disposed between any modules that are arranged in parallel, between any modules that are arranged in series, or between any modules that are arranged in parallel and between any modules that are arranged in series. The catalyst may comprise one or more metals or compounds containing metals selected from the group consisting of nickel, cobalt, platinum, gold, palladium, rhodium, ruthenium, and iron. The catalyst may be placed between a number of modules in series and the activity of the catalyst may vary at different locations between the modules in series. 
    
    
     
       BRIEF DESCRIPTION OF SEVERAL VIEWS OF THE DRAWINGS 
         FIG. 1  is a schematic front view of a membrane wafer stack or module for use in oxygen recovery or in oxidation processes according to embodiments of the present invention. 
         FIG. 2A  is a side view of the membrane wafer stack or module of  FIG. 1  for use in oxidation processes. 
         FIG. 2B  is a side view of the membrane wafer stack or module of  FIG. 1  for use in oxygen recovery. 
         FIG. 3A  is a sectional view of a membrane wafer of  FIGS. 1 ,  2 A, and  2 B. 
         FIG. 3B  is another sectional view of the membrane wafer of  FIGS. 1 ,  2 A, and  2 B. 
         FIG. 3C  is a sectional view of an alternative membrane wafer of  FIGS. 1 ,  2 A, and  2 B. 
         FIG. 3D  is another sectional view of the alternative membrane wafer of  FIGS. 1 ,  2 A, and  2 B. 
         FIG. 4A  is a schematic side view of the interior of a membrane separator vessel for use in oxygen recovery. 
         FIG. 4B  is a cross sectional view of  FIG. 4A . 
         FIG. 5  is a schematic side view of the interior of a membrane reactor vessel for use in oxidation processes. 
         FIG. 6  is a cross sectional view of  FIG. 5 . 
         FIG. 7  is an embodiment of  FIG. 4B  showing the placement of insulation material. 
         FIG. 8  is a second embodiment of  FIG. 4B  showing an alternative placement of thermal insulation material. 
         FIG. 9  is a third embodiment of  FIG. 4B  showing an alternative placement of thermal insulation material. 
         FIG. 10  is a fourth embodiment of  FIG. 4B  showing an alternative placement of thermal insulation material. 
         FIG. 11  is a fifth embodiment of  FIG. 4B  showing an alternative placement of thermal insulation material. 
         FIG. 12  is a sixth embodiment of  FIG. 4B  showing an alternative placement of thermal insulation material. 
         FIG. 13  is a seventh embodiment of  FIG. 4B  showing the placement of insulation thermal material. 
         FIG. 14  is a schematic side view of the interior of an alternative membrane vessel and module arrangement for use in oxygen recovery or in oxidation processes. 
         FIG. 15  is a sectional plan view of a flow containment duct in  FIG. 4A  having coaxial parallel membrane modules. 
         FIG. 16  is a sectional plan view of a flow containment duct with offset banks of parallel membrane modules. 
     
    
    
     The drawings of  FIGS. 1-16  are not necessarily to scale. 
     DETAILED DESCRIPTION OF THE INVENTION 
     Embodiments of the present invention are directed toward the design and operation of ion transport membrane systems that utilize multiple membrane modules operating in series for use in either oxygen recovery or oxidation processes. It has been found that when oxygen transport across a membrane results in an exothermic reaction, for example in the production of synthesis gas from methane, the degree of reactant conversion across an individual membrane must be limited to prevent an excessive temperature gradient across the membrane. It also has been found that when a membrane is transporting oxygen into a lower-pressure permeate stream, the amount of oxygen extraction across an individual membrane must be limited to prevent an excessive oxygen vacancy gradient in the membrane material between the leading edge and trailing edge of the membrane. Excessive temperature or oxygen vacancy gradients may cause excessive stresses in the membranes that could limit the membrane life quite drastically. 
     The present invention addresses these problems by orienting multiple membrane modules or banks of modules in series so that the amount of oxygen extracted across membranes in each module is sufficiently low to prevent an excessive oxygen vacancy gradient in the membrane material. The amount of oxygen extracted across each individual module may be limited by appropriate module sizing, and the total desired degree of oxygen extraction may be achieved by operating a selected plurality of modules in series. When oxygen transport across a membrane results in an exothermic reaction, the degree of reactant conversion across individual membranes in each module should be sufficiently low to prevent an excessive temperature gradient across the membrane in the flow direction. The degree of conversion across each individual module may be limited by appropriate module sizing, and the total desired conversion may be achieved by operating a plurality of selected modules in series. 
     The gas flowing over the outside of the membranes in each membrane module preferably is at a higher pressure than the gas on the inside of the membranes in the interior of the module as described below. In order to minimize gas-phase mass transport resistance, the higher pressure gas should be directed across the outer surface of the membranes at high velocity and as uniformly as possible. 
     Because of the unique operating conditions of an ion transport membrane system, the system design may include a pressure vessel, an optional gas flow containment device or duct disposed within the vessel and surrounding the series membrane modules, and thermal insulation within the vessel to allow the vessel wall to operate at a lower temperature than the membrane modules. The proper physical positioning of each of these components as described below improves the prospects for fabrication, installation, and long-term operability of the system. In addition, other internal design features are disclosed that may contribute to the long-term reliability of the overall ion transport membrane system. 
     The following definitions apply to terms used in the description of the embodiments of the invention presented herein. 
     An ion transport membrane module is an assembly of a plurality of membrane structures which has a gas inflow region and a gas outflow region disposed such that gas flows across the external surfaces of the membrane structures. Gas flowing from the inflow region to the outflow region of a membrane module changes in composition as it passes across the surfaces of the membrane structures in the module. Each membrane structure has an oxygen-containing gas feed side and a permeate side separated by an active membrane layer or region that allows oxygen ions to permeate therethrough. Each membrane structure also has an interior region and an exterior region. In one embodiment, in which the membrane module is operated as an oxygen separation device, the oxygen-containing gas feed side may be adjacent to the exterior region of the membrane structure and the permeate side may be adjacent to the interior region of the membrane structure. 
     In an alternative embodiment, in which the membrane module is operated as an oxidation reaction device, the oxygen-containing gas feed side may be adjacent to the interior region of the membrane structure and the permeate side may be adjacent to the exterior region of the membrane structure. In this alternative embodiment, a reactant feed gas flows through the exterior region of the membrane structure and reacts with the permeated oxygen. Thus in this embodiment the permeate side is also the reactant gas side of the membrane structure. 
     A membrane structure may have a tubular configuration in which an oxygen-containing gas flows in contact with one side of the tube (i.e., in either the interior region or the exterior region of the tube) and oxygen ions permeate through active membrane material in or on the tube walls to the other side of the tube. The oxygen-containing gas may flow inside or outside of the tube in a direction generally parallel to the tube axis, or conversely may flow over the outer side of the tube in a direction which is not parallel to the tube axis. A module comprises multiple tubes arranged in bayonet or shell-and-tube configurations with appropriate tube sheet assemblies to isolate the feed and permeate sides of the multiple tubes. 
     Alternatively, the membrane structure may have a planar configuration in which a wafer having a center or interior region and an exterior region is formed by two parallel planar members sealed about at least a portion of the peripheral edges thereof. Oxygen ions permeate through active membrane material that may be placed on either or both surfaces of a planar member. Gas can flow through the center or interior region of the wafer, and the wafer has one or more gas flow openings to allow gas to enter and/or exit the interior region of the wafer. Thus oxygen ions may permeate from the exterior region into the interior region, or conversely may permeate from the interior region to the exterior region. 
     Components of a membrane module include an active membrane layer that transports or permeates oxygen ions and may also transport electrons, structural components that support the active membrane layer, and structural components to direct gas flow to and from the membrane surfaces. The active membrane layer typically comprises mixed metal oxide ceramic material and also may comprise one or more elemental metals. The structural components of the membrane module may be made of any appropriate material such as, for example, mixed metal oxide ceramic materials, and also may comprise one or more elemental metals. Any of the active membrane layer and structural components may be made of the same material. 
     Single modules may be arranged in series, which means that a number of modules are disposed along a single axis. Typically, gas which has passed across the surfaces of the membrane structures in a first module flows from the outflow region of that module, after which some or all of this gas enters the inflow region of a second module and thereafter flows across the surfaces of the membrane structures in the second module. The axis of a series of single modules may be parallel or nearly parallel to the overall flow direction or axis of the gas passing over the modules in series. 
     Modules may be arranged in banks of two or more parallel modules wherein a bank of parallel modules lies on an axis that is not parallel to, and may be generally orthogonal to, the overall flow direction or axis of the gas passing over the modules. Multiple banks of modules may be arranged in series, which means by definition that banks of modules are disposed such that at least a portion of gas which has passed across the surfaces of the membrane structures in a first bank of modules flows across the surfaces of the membrane structures in a second bank of modules. 
     Any number of single modules or banks of modules may be arranged in series. In one embodiment, the modules in a series of single modules or in a series of banks of modules may lie on a common axis or common axes in which the number of axes equals one or equals the number of modules in each bank. In another embodiment described below, successive modules or banks of modules in a series of modules or banks of modules may be offset in an alternating fashion such that the modules lie on at least two axes or on a number of axes greater than the number of modules in a bank, respectively. Both of these embodiments are included in the definition of modules in series as used herein. 
     Preferably, the gas in contact with the outer surfaces in the exterior regions of the membrane modules is at a higher pressure than the gas within the interior regions of the membrane modules. 
     A flow containment duct is defined as a conduit or closed channel surrounding a plurality of series membrane modules which directs flowing gas over modules in series. 
     A manifold is an assembly of pipes or conduits which directs gas to enter and/or exit the interior regions of the membrane modules. Two manifolds may be combined by installing a first or inner conduit within a second or outer conduit wherein the first conduit provides a first manifold and the annulus between the conduits provides a second manifold. The conduits may be concentric or coaxial, wherein these two terms have the same meaning. Alternatively, the conduits may not be concentric or coaxial but may have separate parallel or nonparallel axes. This configuration of inner and outer conduits to provide a combined manifold function is defined herein as a nested manifold. 
     Flow communication means that components of membrane modules and vessel systems are oriented relative to one another such that gas can flow readily from one component to another component. 
     A wafer is a membrane structure having a center or interior region and an exterior region wherein the wafer is formed by two parallel planar members sealed about at least a portion of the peripheral edges thereof. Active membrane material may be placed on either or both surfaces of a planar member. Gas can flow through the center or interior region of the wafer, i.e., all parts of the interior region are in flow communication, and the wafer has one or more gas flow openings to allow gas to enter and/or exit the interior region of the wafer. The interior region of the wafer may include porous and/or channeled material that allows gas flow through the interior region and mechanically supports the parallel planar members. The active membrane material transports or permeates oxygen ions but is impervious to the flow of any gas. 
     Oxygen is the generic term for forms of oxygen comprising the element having an atomic number of 8. The generic term oxygen includes oxygen ions as well as gaseous oxygen (O 2  or dioxygen). An oxygen-containing gas may include, but is not limited to, air or gas mixtures comprising one or more components selected from the group consisting of oxygen, nitrogen, water, carbon monoxide, and carbon dioxide. 
     A reactant gas or reactant feed gas is a gas comprising at least one component which reacts with oxygen to form an oxidation product. A reactant gas may contain one or more hydrocarbons, wherein a hydrocarbon is a compound comprising primarily or exclusively hydrogen and carbon atoms. A hydrocarbon also may contain other atoms, such as, for example, oxygen. 
     Synthesis gas is a gas mixture containing at least hydrogen and carbon oxides. 
     An ion transport membrane is an active layer of ceramic membrane material comprising mixed metal oxides capable of transporting or permeating oxygen ions at elevated temperatures. The ion transport membrane also may transport electrons as well as oxygen ions, and this type of ion transport membrane typically is described as a mixed conductor membrane. The ion transport membrane also may include one or more elemental metals thereby forming a composite membrane. 
     An ion transport membrane system is a generic term for an array of multiple ion transport membrane modules used for oxygen recovery or for oxidation reactions. An ion transport membrane separation system is an ion transport membrane system used for separating and recovering oxygen from an oxygen-containing gas. An ion transport membrane reactor system is an ion transport membrane system used for oxidation reactions. 
     The series membrane modules in the embodiments of the present invention may be fabricated in either tubular or planar configurations as described above. Planar configurations are preferred for many applications, and various configurations of planar membrane modules are possible. Planar membrane module configurations are described, for example, in copending U.S. patent application Ser. No. 10/394,620 filed on Mar. 21, 2003, which application is incorporated herein by reference. 
     The use of the indefinite articles “a” and “an” means one or more when applied to any feature of the present invention described in the specification and claims. The use of “a” and “an” does not limit the meaning to a single feature unless such a limit is specifically stated. 
     An exemplary planar membrane module is illustrated in  FIG. 1 , which is a schematic front view of a membrane wafer stack or module for use in oxygen recovery or in oxidation processes according to embodiments of the present invention. The stack or module in this example comprises a plurality of planar wafers  1  separated by hollow spacers  3  and having an optional cap  5 . The wafers and spacers are placed and joined in alternating fashion as shown and form stack or module axis  7 . The wafers may be any shape in plan view, but square or rectangular shapes are generally preferred. The dimension of any side of a square or rectangular wafer may be between 2 and 45 cm. The number of wafers in a stack may range up to 1000. 
     The exterior region of the stack or module is that region surrounding the outer surfaces of the wafers and spacers. As described in detail below, wafers  1  have interior regions which are placed in flow communication with the interiors of spacers  3  wherein gas-tight seals are formed between the wafers and spacers. Opening  9  in bottom hollow spacer  11  allows gas to enter and/or exit the interior region of the stack or module wherein the interior region of the module is formed by the interior regions of the wafers and the openings in the hollow spacers. Thus opening  9  is in flow communication with the interior region of the module. 
     A side view of the module of  FIG. 1  is shown in  FIG. 2A , which illustrates an exemplary configuration for use in oxidation processes. In this example, spacers  201  between wafers  200  each have two separate sets of openings  203  and  205 . Openings  203  in spacers  201 , and additional openings in spacers disposed above and below spacers  201 , form an internal manifold that is in flow communication with the interior regions of the wafers by way of appropriately placed openings (not shown) through the layers of the wafers at the left ends of the wafers. These openings through the layers of the wafers also place the internal openings  203  of spacers  201  and the internal openings in spacers above and below spacers  201  in flow communication with each other. Likewise, openings  205  in spacers  201 , and additional openings in spacers disposed above and below spacers  201 , form an internal manifold that is in flow communication with the interior regions of the wafers by way of appropriately placed openings (not shown) through the layers of the wafers at the right ends of the wafers. These openings through the layers of the wafers also place the internal openings  205  of spacers  201  and the internal openings in spacers above and below spacers  201  in flow communication with each other. 
     In this example configuration, gas stream  207  flows upward through the internal manifold formed by openings  203  and openings above them, and then flows horizontally through the interior regions of the wafers. Gas from the interior regions of the wafers then flows downward through the interior manifold formed by openings  205  and openings above them, and exits the module as gas stream  209 . A second gas  211  at the gas inflow region of the module flows through the exterior region of the module on either side of spacers  201  and in contact with the outer surfaces of wafers  200 . Gas  213 , after contacting the outer surfaces of wafers  200 , flows through the gas outflow region of the module. The module may operate in a typical temperature range of 600 to 1100° C. 
     The module of  FIG. 2A  may be used as part of an oxidation reactor system wherein representative gas  211  is a reactant gas and representative gas  207  is an oxidant or oxygen-containing gas. The oxygen-containing gas  207  flows through the internal manifold via openings  203  and through the interior regions of the wafers, oxygen permeates the active membrane material in the planar members of the wafers, and oxygen-depleted gas  209  flows from the module via openings  205 . Permeated oxygen reacts with reactant components in reactant gas or reactant feed gas  211  as the gas flows over the outer surfaces of the wafers and forms oxidation products. Exit gas  213  from the module contains the oxidation products and unreacted components. In one example embodiment, reactant gas  211  comprises methane or a methane-containing feed gas and exit gas  213  is a mixture of unreacted methane, hydrogen, carbon oxides, and water, oxygen-containing gas  207  is air, and oxygen-depleted gas  209  is enriched in nitrogen and depleted in oxygen relative to gas  207 . Typically, the pressure of gases  211  and  213  is higher than the pressure of the gas in the interior region of the module. 
     An alternative side view of the module of  FIG. 1  is shown in  FIG. 2B , which illustrates an exemplary configuration for use in processes for the recovery of high purity oxygen from an oxygen-containing gas. In this example, spacers  215  between wafers  217  have openings  219  wherein openings  219  and additional openings in spacers disposed below spacers  215  form an internal manifold that is in flow communication with the interior regions of the wafers. Opening  221  thus places the interior region of the module in flow communication with a product gas conduit (not shown). Oxygen-containing gas  223 , for example air, at the gas inflow region of the module flows through the exterior region of the module on either side of spacers  215  and in contact with the outer surfaces of wafers  217 . After contacting the outer surfaces of wafers  217 , oxygen-depleted gas  225  flows through the gas outflow region of the module. The module may operate in a typical temperature range of 600° C. to 1100° C. 
     As the oxygen-containing gas flows through the exterior region of the module and the gas contacts the outer surfaces of the wafers, oxygen permeates the active membrane material in the planar members of the wafers and high purity oxygen gas collects in the interior region of the module. High purity oxygen product gas  227  flows from opening  221 . Typically, the pressure of oxygen-containing gases  223  and  225  is higher than the pressure of the high purity oxygen in the interior region of the module. 
     One possible exemplary configuration of the interior regions of the wafers in  FIGS. 1 ,  2 A, and  2 B is illustrated in the sectional views of  FIGS. 3A and 3B . Referring to  FIG. 3A , which represents section  2 - 2  of  FIG. 1 , the wafer has outer support layers  301  and  303  of porous ceramic material that allows gas flow through the pores. Dense active membrane layers of  305  and  307  are in contact with outer support layers  301  and  303  and are supported by supporting ribs  321  and  329  which are part of flow channel layers  315  and  317 . These ribs are in turn supported by slotted support layer  309  that has openings or slots  313  for gas flow. Open channels  319  and  325  are in flow communication via openings or slots  313 . Optionally, support layers  301  and  303  may not be required when the module of  FIG. 2B  is used for recovering oxygen from an oxygen-containing gas. 
     The term “dense” refers to a ceramic material through which, when sintered or fired, a gas cannot flow. Gas cannot flow through dense ceramic membranes made of mixed-conducting multi-component metal oxide material as long as the membranes are intact and have no cracks, holes, or imperfections which allow gas leaks. Oxygen ions can permeate dense ceramic membranes made of mixed-conducting multi-component metal oxide material at elevated temperatures, typically greater than 600° C. 
       FIG. 3B , which represents section  4 - 4  of  FIGS. 2A and 2B , illustrates a wafer section rotated 90 degrees from the section of  FIG. 3A . This section shows identical views of outer support layers  301  and  303  and of dense active membrane material layers  305  and  307 . This section also shows alternate views of slotted support layer  309  and flow channel layers  315  and  317 . Open channels  331  are formed between alternating supporting ribs  333  and allow gas flow through the interior region of the wafer. The interior region of the wafer is therefore defined as the combined open volume within flow channel layer  315 , flow channel layer  317 , and slotted support layer  309 . 
     The dense active membrane layers  305  and  307  preferably comprise a mixed metal oxide ceramic material containing at least one mixed-conducting multi-component metal oxide compound having the general formula (La x Ca 1-x ) y FeO 3-δ  wherein 1.0&gt;x&gt;0.5, 1.1≧y&gt;1.0, and δ is a number which renders the composition of matter charge neutral. Any appropriate material can be used for porous support layers  301  and  303 , and this material may be, for example, a ceramic material having the same composition as that of active membrane layers  305  and  307 . Preferably, porous support layers  301  and  303  are mixed-conducting multi-component metal oxide material. Any appropriate material can be used for the structural members of slotted support layer  309  and flow channel layers  315  and  317 , and this material may be, for example, a ceramic material having the same composition as that of active membrane layers  305  and  307 . The material of channeled support layer preferably is a dense ceramic material. In one embodiment, active membrane layers  305  and  307 , porous support layers  301  and  303 , slotted support layer  309 , and flow channel layers  315  and  317  all may be fabricated of material having the same composition. 
     Dense active membrane layers  305  and  307  optionally may include one or more oxygen reduction catalysts on the oxidant side. The catalyst or catalysts may comprise metals selected from or compounds containing metals selected from the group consisting of platinum, palladium, ruthenium, gold, silver, bismuth, barium, vanadium, molybdenum, cerium, praseodymium, cobalt, rhodium and manganese. 
     Porous support layers  301  and  303  optionally may include one or more catalysts to promote hydrocarbon oxidation, reforming, and/or other reactions that occur in the porous layer. The catalyst or catalysts may be disposed on either or both surfaces of porous support layers  301  and  303 , or alternatively may be dispersed throughout the layer. The one or more catalysts may comprise metals selected from or compounds containing metals selected from the group consisting of platinum, palladium, rhodium, ruthenium, iridium, gold, nickel, cobalt, copper, potassium and mixtures thereof. If desired for structural and/or process reasons, an additional porous layer may be disposed between active membrane layers  305  and  307  and the adjacent flow channel layers  315  and  317  respectively. 
     Another possible configuration of the interior regions of the wafers for the oxygen recovery application in  FIGS. 1 ,  2 A, and  2 B are illustrated in the sectional views of  FIGS. 3C and 3D . Referring to  FIG. 3C , which represents section  2 - 2  of  FIG. 1 , the wafer has outer dense layers  351  and  353 . Porous ceramic layers of  355  and  357  are in contact with outer dense layers  351  and  353 . Porous ceramic layer  355  is supported by supporting ribs  371 , which are part of flow channel layer  365 . Porous ceramic layer  355  is in contact with flow channels  366 , which are part of flow channel layer  365 . Porous ceramic layer  357  is in contact with flow channels  368 , which are part of flow channel layer  367 . 
     Ribs  371  are supported in turn by flow channel layer  358  that has openings or slots  363  for gas flow. Flow channel layer  367  is supported by ribs  373  of flow channel layer  359 , and bridges  379  form the ends of flow channels  368 . Bridges  372  form the ends of the flow channels  363  and flow channels  368  are in flow communication with flow channels  374  of flow channel layer  359 . Open channels  374  and  363  are in flow communication. 
       FIG. 3D , which represents section  4 - 4  of  FIGS. 2A and 2B , illustrates a section of the wafers rotated 90 degrees from the section of  FIG. 3C . This section shows identical views of outer dense layers  351  and  353  and of porous ceramic layers  355  and  357 . Porous ceramic layer  355  is supported by flow channel layer  365 . Porous ceramic layer  355  is in contact with flow channels  366 , which are part of flow channel layer  365 . Porous ceramic layer  357  is supported by ribs  378  of flow channel layer  367 . Porous layer  357  is in flow communication with flow channels  368 , which are part of flow channel layer  367 . 
     Ribs  378  are supported in turn by flow channel layer  359  that has openings or slots  374  for gas flow. Flow channel layer  365  is supported by ribs  375  of flow channel layer  358 . Bridges  371  form the ends of flow channels  366 . Bridges  376  form the ends of the flow channels  374  and flow channels  366  are in flow communication with flow channels  363  of flow channel layer  358 . Open channels  374  and  363  are in flow communication. 
     The interior region of the wafer therefore is defined as the combined open volume within flow channel layer  365 , flow channel layer  367 , flow channel layer  358  and flow channel layer  359 . The flow channels in layers  365  and  358  may be orthogonal to each other, as may be the flow channels in layers  367  and  359 . Alternatively, flow channels  358  and  359  may be replaced by a single flow channel layer that comprises flow channels that radiate from the center of the wafer and are in flow communication with a central port in the center of the wafer. 
     Exemplary compositions for the dense active membrane are described in U.S. Pat. No. 6,056,807, which is incorporated herein by reference. Dense active membrane layers  351  and  353  preferably comprise a mixed metal oxide ceramic material containing at least one mixed-conducting multi-component metal oxide compound having the general formula (La x Sr 1-x )Co y O 3-δ  wherein 1.0&lt;x&lt;0.4, 1.02≧y&gt;1.0, and δ is a number which renders the composition of matter charge neutral. Any appropriate ceramic material can be used for porous support layers  355  and  357 , and may be, for example, material of the same composition as that of active membrane layers  351  and  353 . Preferably, porous support layers  355  and  357  are mixed-conducting multi-component metal oxide material. Any appropriate material can be used for the structural members of flow channel layers  365 ,  367 ,  358  and  359 , and this material may be, for example, a ceramic material having the same composition as that of active membrane layers  351  and  353 . The material of channeled flow layers preferably is a dense ceramic material. 
     In one embodiment, active membrane layers  351  and  353 , porous support layer  355  and  357 , and channeled flow layers  358 ,  359 ,  365  and  367  all may be fabricated of material having the same composition. 
     Optionally, a porous layer may be applied on the outside surface of dense layers  351  and  353 . Other exemplary configurations for the interior regions of the wafers for the oxygen generation application are given in U.S. Pat. No. 5,681,373, which is incorporated herein by reference. 
     Embodiments of the present invention utilize multiple membrane modules arranged in series as defined above. The series modules in turn may be installed in one or more vessels with appropriate gas flow containment ducts, conduits, and/or manifolds to direct gas streams to and from the modules. One of these embodiments is illustrated in  FIG. 4A , which is a schematic side view of the interior of an exemplary membrane separator vessel for use in recovering high purity oxygen from an oxygen-containing gas. Membrane modules  401 ,  403 ,  405 ,  407 , and  409  are installed in series in optional flow containment duct  411  within pressure vessel  413 . These membrane modules may be, for example, similar to the module described above with reference to  FIGS. 1 and 2B . Optional flow containment duct  411  has inlet  415  to direct inlet gas stream  417  through the duct to contact the outer surfaces of the wafers in modules  401  to  409 . The inlet gas stream is a pressurized oxygen-containing oxidant gas, for example air, that has been heated by any appropriate method (not shown) to a temperature of 600° C. to 1100° C. The pressure of the gas within duct  411  may be in the range of 0.2 to 8 MPa. The flow containment duct preferably comprises an oxidation-resistant metal alloy containing iron and one or more elements selected from the group consisting of nickel and chromium. Commercially-available alloys that may be used for flow containment ducts include Haynes® 230, Incolloy 800H, Haynes® 214, and Inconel® 693 alloys. 
     The gas pressure in the interior of flow containment duct  411  preferably is greater than the gas pressure in the interior of pressure vessel  413  between the inner wall of the vessel and the outer wall of flow containment duct  411 . The pressure differential between the interior and the exterior of duct  411  at any point between the inlet and outlet of pressure vessel  413  preferably is maintained at a value equal to or greater than zero, wherein the pressure in the interior of the duct is equal to or greater than the pressure in the pressure vessel exterior to the duct. This may be accomplished, for example, by purging the space outside the duct with a gas at lower pressure than the process gas inside the duct; allowing flow communication between the space outside the duct and the process gas in the duct at process gas outlet  421 ; introducing a purge gas into the space outside the duct, or withdrawing the purge gas through a purge gas outlet while using pressure controllers on a purge gas outlet to maintain a lower pressure in the space outside the duct than inside the duct. 
     As the oxygen-containing gas passes in series over the surfaces of the wafers in membrane modules  401  to  409 , oxygen permeates the dense active membrane layers and collects in the interior regions of the modules. Oxygen-depleted gas stream  419  exits the duct and pressure vessel via outlet  421 . High purity oxygen permeate product from the interior regions of the modules flows via primary manifolds  423 ,  425 ,  427 ,  429 , and  431 , secondary manifolds  433 ,  435 ,  437 ,  439 , and  441 , and main manifold  445 , and exits the system as high purity gas product stream  447 . At least two of membrane modules  401  to  409  define a module axis which may be parallel to or coincident with the axis of pressure vessel  413  or with the axis of flow containment duct  411 . 
     While the exemplary membrane separator vessel described above has a single inlet for feed gas to the membrane modules, a single flow containment duct, and a single outlet from the membrane modules, other embodiments are possible in which multiple inlets, multiple flow containment ducts, and/or multiple outlets may be used. For example, a pressure vessel may have two (or more) flow containment ducts, each having one or more inlets and one or more outlets. Generically, when a separator vessel is described as having an inlet and an outlet, this means that it has one or more inlets and one or more outlets. Generically, when a separator vessel is described as having a flow containment duct, this means that it has one or more flow containment ducts. 
     Another view of the exemplary membrane separator vessel of  FIG. 4A  is given by section  6 - 6  as shown in  FIG. 4B . In this embodiment, a bank of three membrane modules  401   a ,  401   b , and  401   c  is installed in parallel in duct  411  and has three primary manifolds  423   a ,  423   b , and  423   c  that are connected to secondary manifold  433 . Secondary manifold  433  is connected in turn to main manifold  445 . Alternatively, one membrane module, two parallel membrane modules, or more than three parallel membrane modules may be used in each bank. 
     While secondary manifolds  433 ,  435 ,  437 ,  439 , and  441 , and main manifold  445  are located in the interior of pressure vessel  413  in the embodiments of  FIGS. 4A and 4B , these manifolds may be located outside of the pressure vessel in an alternative embodiment. Primary manifolds  423 ,  425 ,  427 ,  429 , and  431  would pass through the wall of pressure vessel  413  in this alternative embodiment. 
     In an alternative embodiment, planar membrane modules  401  through  409  may be replaced by tubular membrane modules placed in series relationship relative to the longitudinal flow of gas through optional duct  411 . These modules may utilize multiple single tubes or may utilize bayonet-type tubes, and the modules may be oriented such that gas flows across the tubes in crossflow or contacts the tubes in parallel flow. In this alternative embodiment, all manifolds are located inside the pressure vessel as shown in  FIGS. 4A and 4B . 
     Another embodiment of the invention is illustrated in  FIG. 5 , which is a schematic side view of the interior of an exemplary membrane reactor vessel for use in oxidation processes. Membrane modules  501 ,  503 ,  505 ,  507 , and  509  are installed in series in flow containment duct  511  within pressure vessel  513 . These membrane modules may be, for example, similar to the module described above with reference to  FIGS. 1 and 2A . Optional flow containment duct  511  has inlet  515  to direct inlet gas stream  517  through the duct to contact the outer surfaces of the wafers in modules  501  to  509 . The inlet gas stream is a reactant feed gas containing one or more components which react with oxygen at elevated temperatures wherein the inlet reactant feed gas is heated by any appropriate method (not shown) to a temperature of 600° C. to 1100° C. The pressure of the gas within duct  511  may be in the range of 0.2 to 8 MPa. An example of a reactant feed gas is a mixture of steam and natural gas wherein the natural gas comprises mostly methane with smaller amounts of light hydrocarbons. The mixture may be prereformed at a temperature below about 800° C. to yield a reactant feed gas containing steam, methane, and carbon oxides. Other oxidizable reactant feed gases may include, for example, various mixtures of hydrogen, carbon monoxide, steam, methanol, ethanol, and light hydrocarbons. 
     The gas pressure in the interior of flow containment duct  511  preferably is greater than the gas pressure in the interior of pressure vessel  513  between the inner wall of the vessel and the outer wall of flow containment duct  511 . The pressure differential between the interior and the exterior of duct  511  at any point between the inlet and outlet of pressure vessel  513  preferably is maintained at a value equal to or greater than zero, wherein the pressure in the interior of the duct is equal to or greater than the pressure in the pressure vessel exterior to the duct. This may be accomplished, for example, by purging the space outside the duct with a gas at lower pressure than the process gas inside the duct; allowing flow communication between the space outside the duct and the process gas in the duct at the process gas outlet,  559 ; introducing a purge gas into the space outside the duct, and withdrawing the purge gas through a purge gas outlet while using pressure controllers on a purge gas outlet to maintain a lower pressure in the space outside the duct than inside the duct. 
     The interior regions of membrane modules  501  to  509  are in flow communication with two manifold systems, one to introduce an oxygen-containing oxidant gas into the modules and the other to withdraw oxygen-depleted oxidant gas from the modules. The first of these manifold systems comprises main inlet manifold  519 , primary inlet manifolds  521 ,  523 ,  525 ,  527 , and  529 , and secondary inlet manifolds  531 ,  533 ,  535 ,  537 , and  539 . The second of these manifold systems comprises main outlet manifold  541  and primary outlet manifolds  543 ,  545 ,  547 ,  549 , and  551 . 
     In an alternative configuration (not shown) to the configuration of  FIG. 5 , secondary inlet manifolds  531 ,  533 ,  535 ,  537 , and  539  may be combined with primary outlet manifolds  543 ,  545 ,  547 ,  549 , and  551 , respectively, when located within flow containment duct  511 . Two manifolds may be combined by installing a first or inner conduit within a second or outer conduit wherein the first conduit provides a first manifold and the annulus between the conduits provides a second manifold. The conduits may be concentric or coaxial; alternatively, the conduits may not be concentric or coaxial and may have separate parallel or nonparallel axes. This configuration of inner and outer conduits to provide a combined manifold function is defined herein as a nested manifold. 
     In this alternative configuration, gas  553  would flow through the central conduit and gas  555  would flow through the annulus of each set of these nested manifolds. The nested manifolds would transition to separate manifolds exterior to flow containment duct  511 , i.e., would transition to secondary inlet manifolds  531 ,  533 ,  535 , and  539  and primary outlet manifolds  543 ,  545 ,  547 ,  549 , and  551  as shown in  FIG. 5 . Optionally, primary outlet manifolds  543 ,  545 ,  547 ,  549 , and  551  may be nested within secondary inlet manifolds  531 ,  533 ,  535 ,  537 , and  539 , respectively, within flow containment duct  511 . In this option, gas  555  would flow through the central conduit and gas  553  would flow through the annulus of each set of these nested manifolds. In generic terms, therefore, the secondary inlet manifolds and the primary outlet manifolds may be nested when located within flow containment duct  511 , and either a secondary inlet manifold or a primary outlet manifold may be provided by the annulus 
     Heated, pressurized oxygen-containing oxidant gas  553 , for example, air that has been heated by any appropriate method (not shown) to a temperature of 600 to 1100° C., enters main inlet manifold  519  and flows via primary inlet manifolds  521 ,  523 ,  525 ,  527 , and  529  and secondary inlet manifolds  531 ,  533 ,  535 ,  537 , and  539  to the inlets of membrane modules  501 ,  503 ,  505 ,  507 , and  509 . Oxygen from the oxidant gas in the interior regions of the membrane modules permeates the dense active membrane layers in the wafers of modules  501  to  509  and the permeated oxygen reacts with the reactive components in the exterior regions of the membrane modules. Oxygen-depleted oxidant gas exits the outlets of the interior regions of the membrane modules via primary outlet manifolds  543 ,  545 ,  547 ,  549 , and  551  and main outlet manifold  541 , and the final oxygen-depleted oxidant gas is withdrawn as gas stream  555 . Outlet gas stream  557 , which contains reaction products and unreacted feed components, is withdrawn from the reactor system via outlet  559 . 
     While the exemplary reactor vessel described above has a single inlet for reactant feed gas to the membrane modules, a single flow containment duct, and a single outlet from the membrane modules, other embodiments are possible in which multiple inlets, multiple flow containment ducts, and/or multiple outlets may be used. For example, a pressure vessel may have two or more flow containment ducts, each having one or more inlets and one or more outlets. Generically, when a reactor vessel is described as having an inlet and an outlet, this means that it has one or more inlets and one or more outlets. Generically, when a reactor vessel is described as having a flow containment duct, this means that it has one or more flow containment ducts. 
     Another view of the exemplary membrane reactor vessel of  FIG. 5  is given in section  8 - 8  as shown in  FIG. 6 . In this embodiment, a bank of three membrane modules  503   a ,  503   b , and  503   c  are installed in parallel in duct  511 . Oxidant gas flows through main inlet manifold  519 , primary inlet manifold  523 , and secondary inlet manifolds  533   a ,  533   b , and  533   c  to the inlets of membrane modules  503   a ,  503   b , and  503   c . Oxygen-depleted oxidant gas exits membrane modules  503   a ,  503   b , and  503   c  via primary outlet manifolds  545   a ,  545   b , and  545   c  (located behind secondary inlet manifolds  533   a ,  533   b , and  533   c ), secondary outlet manifold  561 , and main outlet manifolds  541   a  and  541   b . While three parallel membrane modules are shown in the embodiment of  FIG. 6 , one membrane module, two parallel membrane modules, or more than three parallel membrane modules may be used as desired. 
     A guard bed (not shown) may be installed in inlet  415  of pressure vessel  413  and/or in inlet  515  to pressure vessel  513  to remove trace contaminants from inlet stream  417  and/or  517 . Alternatively, the guard bed may be installed in the interior of the pressure vessel between the inlet and the first membrane module. The contaminants may include, for example, sulfur-, chromium- and/or silicon-containing gaseous species. The guard bed may contain one or more materials selected from the group consisting of magnesium oxide, calcium oxide, copper oxide, calcium carbonate, sodium carbonate, strontium carbonate, strontium oxide, zinc oxide, and alkaline-earth-containing perovskites. These materials react with and remove the contaminants from the inlet stream of reactant gas or oxygen-containing gas. 
     Additional pressure vessels may be installed in series with pressure vessel  413  such that the outlet gas from one vessel feeds the other vessel. Additional pressure vessels may be placed in parallel wherein a plurality of pressure vessels operate in parallel and in series. Likewise, additional pressure vessels may be installed in series with pressure vessel  513  such that the outlet gas from one vessel feeds the other vessel. Additional pressure vessels may be placed in parallel wherein a plurality of pressure vessels operate in parallel and in series. Guard beds may be placed between any series pressure vessels as desired. 
     In the embodiments described above, it is desirable to use internal insulation to maintain the walls of pressure vessel  413  and  513  at temperatures lower than the temperatures of the respective membrane modules  401  to  409  and  501  to  509 . This may be accomplished by various insulation alternatives in  FIGS. 7 through 13 , which illustrate insulation configurations for the embodiment of  FIGS. 4A and 4B  used for the recovery of oxygen from an oxygen-containing gas. Similar insulation configurations (not shown) may be used for the oxidation reactor embodiment of  FIGS. 5 and 6 . 
     The first of these alternatives is shown in  FIG. 7  wherein insulation  701  is disposed within and may be in contact with the interior walls of pressure vessel  703 . In this embodiment, a flow containment duct is not used; instead, cavity  705  is formed by the insulation itself and this cavity serves to direct gas flow over the exterior regions of the membrane modules. The insulation may be in contact with primary manifolds  423   a ,  423   b , and  423   c , secondary manifold  433 , and main manifold  445 . 
     A second insulation configuration is shown in  FIG. 8  wherein insulation  801  is disposed adjacent to and may be in contact with the inner wall of pressure vessel  413 . In this embodiment, flow containment duct  411  is used and preferably is not in contact with insulation  801 . The insulation preferably is not in contact with primary manifolds  423   a ,  423   b , and  423   c , secondary manifold  433 , and main manifold  445 . 
     A third insulation configuration is shown in  FIG. 9  wherein insulation  901  completely fills the interior region of the pressure vessel between the inner walls of the vessel and the exterior surfaces of flow containment duct  411 , primary manifolds  423   a ,  423   b , and  423   c , secondary manifold  433 , and main manifold  445 . The insulation may be in contact with the inner vessel walls and the exterior surfaces of flow containment duct  411 , primary manifolds  423   a ,  423   b , and  423   c , secondary manifold  433 , and main manifold  445 . 
     Another alternative insulation configuration is shown in  FIG. 10  wherein insulation  1001  forms a cavity  1003  around the membrane modules and this cavity serves to direct gas flow over the exterior regions of the modules. Insulation  1001  may be in contact with primary manifolds  423   a ,  423   b , and  423   c , and typically is not in contact with the inner walls of pressure vessel  413 . 
       FIG. 11  shows another alternative insulation configuration in which insulation  1101  surrounds flow containment duct  411 , which in turn surrounds the membrane modules as described above. Insulation  1101  may be in contact with primary manifolds  423   a ,  423   b , and  423   c , and typically is not in contact with the inner walls of pressure vessel  413  and the outer surface of flow containment duct  411 . 
     Another insulation configuration is shown in  FIG. 12  wherein insulation  1201  surrounds flow containment duct  411 , which in turn surrounds the membrane modules as described above. Insulation  1201  may be in contact with primary manifolds  423   a ,  423   b , and  423   c , typically is in contact with the outer surface of flow containment duct  411 , and typically is not in contact with the inner walls of pressure vessel  413 . 
     A final insulation configuration is shown in  FIG. 13  wherein insulation  1303  is placed within and typically in contact with the inner walls of flow containment duct  411 , wherein the insulation forms a cavity  1305  around the membrane modules and this cavity serves to direct gas flow over the exterior regions of the modules. Insulation  1303  may be in contact with primary manifolds  423   a ,  423   b , and  423   c.    
     In any of the embodiments described above of  FIGS. 7-13 , a metal-to-ceramic seal typically is used in primary manifolds  423   a ,  423   b , and  423   c  to transition from metal manifolds to the ceramic modules. Likewise, in the oxidation reactor embodiment of  FIG. 6  and corresponding insulation embodiments similar to those of  FIGS. 7-13 , a metal-to-ceramic seal typically is used in primary manifolds  533   a ,  533   b , and  533   c  to transition from metal manifolds to the ceramic modules. In the embodiments of  FIGS. 10-13  (and similar embodiments for the oxidation reactor), these seals preferably are located within insulation  1001 ,  1101 ,  1201 , and  1303  (in contact with manifolds  423   a ,  423   b , and  423   c  but not with manifold  433 ) to obtain desired seal operating temperatures. 
     In any of the embodiments of  FIGS. 7-13 , additional insulation (not shown) may be placed around the external surface of the pressure vessel, for example to protect operating personnel from a potentially hot vessel surface. This additional insulation also may serve to ensure that the vessel interior is above the dew point of any gas within the vessel. In any of the embodiments of  FIGS. 10-13 , additional insulation (not shown) may be placed adjacent to the inner surface of the pressure vessel. In any of the embodiments of  FIGS. 4A ,  4 B, and  5 - 13 , any of the manifolds may be insulated internally and/or externally (not shown). This insulation would serve to improve the thermal expansion uniformity of flow containment duct  411  and the manifolds. 
     The insulation used in the embodiments of  FIGS. 7-13  may contain alumina, alumino-silicate, silica, calcium silicate, or other conventional insulation materials suitable for use at elevated temperatures. The insulation may comprise, for example, one or more materials selected from the group consisting of fibrous alumina, fibrous alumino-silicate, porous alumina, and porous alumino-silicate. In the embodiments of  FIGS. 7 ,  10 , and  13 , wherein the insulation itself forms a cavity around the membrane modules, the interior walls of the cavity may be coated or covered with a material which prevents volatile components from the insulation from contacting the membrane modules. For example, the cavity may be lined with a foil made of a metal such as Haynes 214 to prevent Si-containing vapor species, which may be generated from insulation materials, and/or Cr-containing vapor species, which may be generated from metal piping materials, from contacting the membrane modules. 
     The insulation may include one or more additional materials selected from the group consisting of magnesium oxide, calcium oxide, copper oxide, calcium carbonate, strontium carbonate, sodium carbonate, zinc oxide, strontium oxide, and alkaline-earth-containing perovskites wherein these materials may be applied to the surface of the insulation and/or dispersed throughout the insulation. These additional materials may be used in place of or in addition to the guard bed or beds described above. These materials react with and remove contaminants that may be present in the inlet stream of reactant gas; these contaminants may include, for example, sulfur-, chromium-, silicon-, or oxygen-containing containing gaseous species. 
     An alternative embodiment for placing groups of wafers in a series flow configuration is shown in  FIG. 14 . In this embodiment, a tall stack is formed of wafers and spacers as described above and the stack is installed in pressure vessel  1401 . Inlet line  1403  and outlet line  1405  are connected to gas manifold shroud assembly  1407  which directs the flow of inlet gas  1408  to flow in alternating directions across groups of wafers and through outlet line  1405  as outlet stream  1409 . In this embodiment, the stack is divided by the shroud assembly into first wafer zone  1411 , second wafer zone  1413 , and third wafer zone  1415 . Inlet gas  1408  thus flows in series across wafer zones  1411 ,  1413 , and  1415  and exits via outlet line  1405 . While three wafer zones are shown here for illustration purposes, any number of wafer zones can be used as required. 
     The embodiment of  FIG. 14  may be used as an oxygen recovery device or as an oxidation reactor device. When used as an oxygen recovery device, the stack is formed of wafers and spacers as earlier described with reference to  FIGS. 1 and 2B . In an oxygen recovery process, inlet gas  1408  is a heated, pressurized oxygen-containing gas (for example, air), outlet stream  1409  is an oxygen-depleted oxygen-containing gas, and stream  1417  flowing through outlet line  1419  is a high purity oxygen product stream typically at a lower pressure than the pressurized oxygen-containing gas. When used as an oxidation reactor system, the stack is formed of wafers and spacers as earlier described with reference to  FIGS. 1 and 2A . In an oxidation process, inlet gas  1408  is a heated, pressurized reactant gas and outlet gas  1409  is a mixture of oxidation reaction products and unreacted reactant gas components. Stream  1417  is an oxygen-depleted oxygen-containing gas stream typically at a lower pressure than the pressurized reactant gas. Fresh oxygen-containing oxidant gas (for example, air) flows into the stack through an internal stack manifold as described with reference to  FIG. 2A ; the inlet to this manifold is not seen in  FIG. 14  because it lies behind outlet line  1419 . 
     The embodiment of  FIG. 14  can be operated with multiple pressure vessels in series and/or in parallel as desired. Multiple modules may be installed in a single pressure vessel if desired. 
     The series membrane modules may be arranged in banks of parallel modules as earlier described with reference to  FIGS. 4A ,  4 B,  5 , and  6 . This is illustrated in  FIG. 15 , which is a sectional plan view (not to scale) of flow containment duct  511  and the membrane modules within the duct. In this exemplary embodiment, five banks of three parallel modules are arranged such that each individual set of series modules lies on a common module axis, i.e., modules  501   a ,  503   a ,  505   a ,  507   a , and  509   a  lie on the same axis, modules  501   b ,  503   b ,  505   b ,  507   b , and  509   b  lie on the same axis, and modules  501   c ,  503   c ,  505   c ,  507   c , and  509   c  lie on the same axis. Thus in this example there are three axes, equal to the number of modules in each bank. Each bank comprises a plurality of modules in parallel; for example, modules  501   a ,  501   b , and  501   c  constitute one bank of modules in parallel. A plurality of modules also may be arranged in series; for example, modules  501   c ,  503   c ,  505   c ,  507   c , and  509   c  constitute modules in series. The definition of series modules also can include banks of modules; for example, the bank of modules  501   a ,  501   b , and  501   c  is in series with the bank of modules  503   a ,  503   b , and  503   c . The module configuration in  FIG. 15  thus includes modules in series and modules in parallel. 
     As a practical matter, it may be desirable to promote substantial radial mixing (i.e., gas flow in directions deviating from the axis of a series of modules) of gas between successive banks of modules to minimize the deleterious effect of gas bypassing around the membrane modules. The module configuration in  FIG. 15  thus may be best described as including modules in parallel and banks of parallel modules operating in series. As in the design of many gas flow distribution systems, the degree of radial mixing can be maximized by proper selection of the axial and radial spacing between internal elements (i.e. membrane modules) and/or the use of flow baffles to promote gas mixing. 
     Inlet gas stream  1501  in inlet  1503  flows in series over each bank of radially-oriented (i.e., parallel) modules. With proper selection of the axial and radial spacing between modules, a small amount of gas may bypass modules  501   a ,  501   b , and  501   c , but eventually will contact downstream modules as it mixes or diffuses in a radial direction. Exit gas stream  1505  flows through outlet  1507 . The gas flow over each successive bank of modules defines the series arrangement of this embodiment wherein all or nearly all of the gas from one bank of parallel modules contacts the next bank of parallel modules in the series of modules. Any desired number of modules may be used in parallel radially and any desired number of banks of parallel modules may be used in series axially. 
     In an alternative embodiment of the invention related to  FIGS. 4A and 4B  or to  FIGS. 5 and 6 , banks of parallel membrane modules may be oriented in a staggered or offset series arrangement such that a first bank of three modules is followed in series by an offset second bank of three modules which in turn is followed in series by an offset third bank of three modules, and so forth. This is illustrated in  FIG. 16  wherein a first bank of three modules  502   a ,  502   b , and  502   c  is followed in series by a second bank of three modules  504   a ,  504   b , and  504   c  offset in a direction perpendicular to the axis of flow containment duct  511 . A third bank of three modules  506   a ,  506   b , and  506   c  is offset with respect to the second bank but the modules are coaxial with the modules in the first bank. This offset relationship may continue in similar fashion through the fourth bank of modules  508   a ,  508   b , and  508   c  and the fifth bank of modules  510   a ,  510   b , and  510   c . Each bank may comprise a plurality of modules in parallel; for example, modules  502   a ,  502   b , and  502   c  constitute one bank of modules in parallel. A plurality of modules also may be arranged in series; for example, modules  502   c ,  504   c ,  506   c ,  508   c , and  510   c  may constitute modules in series. The definition of series modules also can include banks of modules; for example, the bank of modules  502   a ,  502   b , and  502   c  is in series with the bank of modules  504   a ,  504   b , and  504   c . The module configuration in  FIG. 16  thus includes modules in series and modules in parallel. 
     The modules in  FIG. 16  lie on six axes, i.e., modules  502   c ,  506   c , and  510   c  lie on one axis, modules  504   c  and  508   c  lie on another axis, and so forth. These axes may be parallel to the overall flow direction of gas over the modules. In this embodiment, the number of axes is greater than the number of modules in each bank of modules. 
     In the embodiment of  FIG. 16 , inlet gas stream  1601  enters through inlet  1603  and flows over modules  502   a ,  502   b , and  502   c  in the first bank. A portion of this gas may bypass module  502   a  but, in the absence of significant radial mixing, will at least contact offset module  504   a . Gas that flows between modules  502   a ,  502   b , and  502   c , will at least contact the next series of offset modules  504   b  and  504   c . Portions of the gas that flows from module  502   a  in the first bank will contact at least two modules ( 504   a  and  504   b ) in the second bank. In this way, such an offset arrangement prevents gas from bypassing straight through a gap between rows of modules on a common axis. Instead, gas bypassing any module in a bank of modules will impinge directly on a module in the next bank of modules. In the absence of significant radial mixing, at least a portion of the gas from one or more of the modules in a bank will contact one or more of the modules in the next bank, and this defines the series arrangement of modules in this embodiment. 
     The definition of modules arranged in series according to the present invention thus includes both embodiments described above with reference to  FIGS. 15 and 16 . In these embodiments, the axes of banks of modules and the axes of series modules may be generally orthogonal, and the axes of series modules may be generally parallel to the overall direction of gas flow through the vessel. Alternative embodiments are possible wherein the axes of banks of modules are not generally orthogonal to the axes of the series modules and/or wherein the axes of series modules are not generally parallel to the overall direction of gas flow through the vessel. In these alternative embodiments, the banks of modules lie at acute angles to the overall direction of gas flow through the vessel. These alternative embodiments are included in the definition of modules arranged in series according to the present invention. 
     The series reactor system described above may be used in oxidation service to produce synthesis gas from a hydrocarbon-containing feed gas such as natural gas. In this application, reforming catalyst may be disposed between any series modules, any parallel modules, any series and parallel modules, and/or following the final modules in a vessel. The reforming catalyst promotes the endothermic reactions of water and/or carbon dioxide with hydrocarbons, especially methane, to generate hydrogen and carbon monoxide. The catalyst may be used to complement or balance the exothermic oxidation reactions that occur between permeated oxygen and reactants adjacent to the surfaces of the active membrane material in the modules. By appropriate use of reforming catalyst at strategic locations between the modules in a multiple-module series reactor system, the temperature profiles across the reactor and the product gas composition may be controlled to achieve optimum reactor operation. 
     An embodiment of the present invention is illustrated by the exemplary placement of appropriate catalyst between the modules of a multiple-module series oxidation reactor system. For example, referring to  FIG. 15 , catalyst  501   d ,  501   e , and  501   f  may be placed in series fashion in the space between any modules in the first bank of modules  501   a ,  501   b , and  501   c  and the second bank of modules  503   a ,  503   b , and  503   c . Alternatively, catalyst  501   d ,  501   e , and  501   f  may extend continuously between the inner walls of flow containment duct  511 . Likewise, catalyst may be placed between any or all of the second and third banks of modules, the third and fourth bank of modules, the fourth and fifth banks of modules, or following the fifth bank (not shown). Similarly, catalyst may be placed in series fashion between any or all of the offset banks of modules in the embodiment of  FIG. 16 . For example, referring to  FIG. 16 , catalyst  502   d ,  502   e , and  502   f  may be placed in series fashion in the space between the first and second banks of modules. Alternatively, catalyst  502   d ,  502   e , and  502   f  may extend continuously between the inner walls of flow containment duct  511 . In general, catalyst may be placed in series fashion between or downstream of any or all of the series banks of modules in  FIGS. 15 and 16 . 
     Additionally or alternatively, catalyst may be placed between the modules in a bank of parallel modules to promote reforming reactions in the gas passing between the modules. For example, in  FIG. 15  catalyst  505   d  and  505   e  may be placed between modules  505   a  and  505   b  and between  505   b  and  505   c . Alternatively, catalyst  505   d , and  505   e  may extend continuously in the axial direction between the first through fifth banks of modules. For example, in  FIG. 16  catalyst  506   d  and  506   e  may be placed between modules  406   a  and  506   b  and between  506   b  and  506   c . In general, catalyst may be placed in parallel fashion between any or all of the parallel modules in  FIGS. 15 and 16 . 
     In the broadest application of this concept, therefore, catalyst may be placed in the space between any two adjacent modules in the embodiments of  FIGS. 15 and 16 , or in any other embodiments with both of series and parallel module placement. In addition, when pressure vessel  513  is operated in series with another similar pressure vessel, catalyst may be placed between the vessels such that the effluent gas from one pressure vessel passes through the catalyst before passing into the second pressure vessel. 
     The catalyst may be varied in type and/or amount depending on the axial or radial location among the modules in the pressure vessel. In one alternative, for example, the catalyst activity may be varied in the axial direction for optimal control of the module temperatures through the reactor. For example, catalyst sections near the inlet of the reactor may comprise catalyst which is active at lower temperature (i.e. a high Ni loading), whereas in higher temperature regions of the reactor the optimal catalyst composition may involve lesser activity and greater thermal stability (i.e. a low Ni loading). In this way, optimal catalyst activity can be achieved at every axial location in the reactor, while maintaining thermal stability of the catalyst. Other catalyst arrangements are possible and fall within the scope of embodiments of the claimed invention. 
     The catalyst for use in this embodiment may include one or more metals or compounds containing metals selected from the group consisting of nickel, cobalt, platinum, gold, palladium, rhodium, ruthenium, and iron. The catalyst may be supported on alumina or other oxide supports and may include additions such as lanthanum or potassium. The catalyst may be placed between modules by any known means including, for example, using monoliths or using granular catalysts in appropriate catalyst holders that fit in the spaces between the modules.