Abstract:
A method of separating oxygen using a ceramic membrane unit having one or more ceramic membranes, preferably formed of a mixed conducting ceramic, for instance, a perovskite, capable of conducting both oxygen ions and electrons. Oxygen is separated within the ceramic membrane unit under impetus of compressing an incoming oxygen containing feed. The compressor used in the compression is powered by the work of expansion produced by expanding a process stream composed of at least a portion of the retentate produced in the ceramic membrane unit. Prior to expansion, the process stream is cooled to allow the use of less expensive materials in expanders used in the expansion. As a result, expansion of the process stream alone is insufficient to meet the power requirements involved in the compression. Interstage expansion with reheating is used to make up for the power deficit.

Description:
FIELD OF THE INVENTION 
     The present invention relates to a method of separating oxygen from an oxygen containing gas with the use of an ceramic membrane unit. More particularly, the present invention relates to such a method in which the oxygen containing gas is compressed by a compressor powered by the expansion of a cooled process stream made up at least in part by a retentate formed in the ceramic membrane unit. Even more particularly, the present invention relates to such a method in which the expansion of the process stream is carried out in stages with interstage heating. 
     BACKGROUND OF THE INVENTION 
     Oxygen transport membranes have demonstrated an ability to separate high-purity oxygen from an oxygen containing stream with a purity of at least about 99% and with an oxygen recovery of about 60%. Such oxygen transport membranes are formed from a ceramic that is capable of transporting oxygen ions when both heated to a suitable operational temperature and the opposite sides of the membrane are subjected to an oxygen partial pressure differential. The oxygen ions are formed by oxygen atoms in an oxygen containing feed gaining two electrons at one surface of the membrane. The oxygen is reconstituted at the opposite surface of the membrane by the loss of the electrons from the oxygen ions thus to complete the separation of the oxygen from the feed. Typically, multiple oxygen transport membranes are housed in a ceramic membrane unit that functions to separate oxygen from the oxygen containing feed to produce both an oxygen permeate from the separated oxygen and a retentate from the feed after the separation of oxygen therefrom. 
     Suitable oxygen transport membrane materials are known as either mixed conducting or ionic. Mixed conducting materials conduct both the oxygen ions and the electrons that are formed upon reconstitution of elemental oxygen from the oxygen ions. Ceramic mixed conducting materials include but are not limited to perovskites. Ionic materials conduct only oxygen ions and thus require an external electric circuit for the return of the electrons. Common materials used in an ionic membrane include, but are not limited to, Yttrium Stabilized Zirconia. 
     In order to compress the oxygen containing feed, for instance, air, the feed is compressed in a compressor that is powered at least in part by the work extracted from a turboexpander. Typically, a retentate stream, composed of a retentate formed upon separation of the oxygen within the ceramic membrane unit, is expanded in the turboexpander. An example of this is shown in U.S. Pat. No. 5,516,359 in which feed air is compressed and then heated in a direct-fired burner. The resultant heated feed gas is introduced into the ceramic membrane unit to separate oxygen from the feed. The retentate is heated by another direct fired burner prior to its introduction into the turboexpander. The turboexpander is used to power the compressor. 
     U.S. Pat. No. 5,643,354 discloses an integrated process in which oxygen is recovered from an oxygen-containing feed gas and subsequently is consumed in a coal gasifier. The hot oxygen product exiting the ceramic membrane unit is cooled through indirect heat exchange with water and an expander is used to recover the work needed to drive the feed gas compressor. 
     “Ion Transport Membrane Technology for Oxygen Separation and Syngas Production”, 134 Solid State Ionics, Dyer et al. pp 21-33 (2000), discloses a process in which a hot, low-pressure oxygen product gas, produced by a ceramic membrane unit, is cooled by indirect heat exchange with a cooling medium. After being cooled, the oxygen is compressed to a final delivery pressure. Fuel is used both to heat the feed gas to the desired inlet temperature of the oxygen transport membranes contained in the ceramic membrane unit and to heat the non permeate (i.e., retentate) to the desired inlet temperature to the expander. The work recovered from the expander is used to drive both the feed gas compressor and an oxygen product blower or compressor. 
     “Advanced Oxygen Separation Membranes”, Report No. TDA-GRI-90/0303, Wright et al., The Gas Research Institute, pp 33-61 (1990), illustrates various schemes for integrating ceramic membrane units with electrical generation systems. In one such integration, feed air is compressed and heated by combustion supported by oxygen contained in a retentate stream that is produced in a ceramic membrane unit. The retentate stream is then fed into a turboexpander that is used to drive a feed air compressor. 
     U.S. Pat. No. 5,753,007 discloses a process for oxygen recovery from an oxygen-containing feed gas by the use of a ceramic membrane unit in which the retentate stream is cooled and then expanded to recover useful work. In this patent, the degree of cooling is sufficiently high that the work can be extracted for the use of processes that are less energetic than those in which electrical power also is generated. The feed gas can be heated through indirect heat exchange with both the retentate and oxygen product streams. Additionally, the feed stream may be heated further by a combustor interposed prior to the ceramic membrane unit. 
     An important consideration in the fabrication of any equipment that is used to separate oxygen is its cost. The cost of acquiring a turboexpander increases with its operating temperature due to the use of more exotic and/or more expensive materials. It therefore would be desirable from the standpoint of cost to be able to utilize a turboexpander at a lower temperature, for instance, preferably in a range of between about 300° C. and about 650° C. However, as the inlet temperature to the turboexpander decreases, there is less energy that can be extracted from a stream to be expanded and, therefore, less energy that is available to drive the feed air compressor. The energy able to be extracted from a stream sufficiently cooled to allow the use of turboexpanders designed to operate at low temperatures can be less than that required to operate the feed air compressor. 
     As will be discussed, the present invention provides a method of separating oxygen from an oxygen containing feed that is particularly applicable to the use of temperature limited turboexpander components and that can generate sufficient energy from the turboexpansion to drive the feed air compressor as well as other components. Other advantages will become apparent from the following discussion. 
     SUMMARY OF THE INVENTION 
     A method of separating oxygen from an oxygen containing gas is provided in which a feed stream containing the oxygen containing gas is compressed to produce a compressed feed stream. The compressed feed stream is heated. A ceramic membrane unit also is heated to an operational temperature. Oxygen is separated from the compressed feed stream within the ceramic membrane unit to produce both a retentate that contains residual components of the feed stream and an oxygen permeate formed by the separated oxygen. A process stream composed of at least a portion of the residual components of the retentate is cooled to a temperature below the operational temperature of the ceramic membrane unit. The process stream is expanded with the performance of work in an initial stage of expansion. An expansion stage, as described herein, is comprised of all system components that may be utilized to recover work from an inlet stream. Initial and subsequent stages of expansion are separated by a separate reheating step in which the expanded stream is reheated prior to entering the next stage of expansion. The process stream, after the initial stage of expansion, is reheated, then expanded with the performance of work in a subsequent stage of expansion. 
     The work of expansion produced by the initial stage of expansion is insufficient to meet the power requirements for the compression of the feed stream, and a sum of the work of expansion of the initial and subsequent expansion stages is at least sufficient to meet the power requirements for the compression of the feed stream. At least a part of a sum of the work of expansion of the initial and subsequent stages of expansion is applied to the compression of the feed stream. An oxygen product stream is extracted from the ceramic membrane unit that is composed of the oxygen permeate. 
     By having more than one stage of expansion with interstage reheating, sufficient energy can be recovered to power the compressor and, as will be discussed, additional accessories such as product and fuel compressors. At the same time, since the process stream is cooled, less expensive, temperature limited expanders can be utilized. Additionally, the method of the present invention also allows the temperature of the ceramic membranes of the ceramic membrane unit to be set independently of the temperature of the stream to be expanded for power recovery. This can be important when specific low operating temperatures are required for the longevity of the material used in the ceramic membrane. 
     In accordance with an additional aspect of the present invention, the residual components contained within the retentate include residual oxygen; and the process stream is formed by extracting a retentate stream from the ceramic membrane unit, introducing a fuel stream into the retentate stream, and combusting the fuel in the presence of the residual oxygen contained within the retentate stream. The compressed feed stream is heated through indirect heat exchange with the process stream, thereby cooling the process stream. The compressed feed stream, after having been heated, is introduced into the ceramic membrane unit to separate part of the oxygen contained within the compressed feed stream, thereby heating the ceramic membrane unit to the operational temperature. Thus, in this aspect of the present invention, the energy for heating the feed stream, and therefore the ceramic membrane unit, to an operational temperature is through indirect heat exchange with a process stream formed by combusting a fuel in the presence of residual oxygen in the retentate. 
     In another aspect of the present invention, the process stream is formed from a retentate stream composed of the retentate and extracted from the ceramic membrane unit. The compressed feed stream is heated through indirect heat exchange with the process stream, thereby cooling the process stream. A heated feed stream is formed by introducing a fuel stream into the compressed feed stream, after having been heated, and combusting the fuel in the presence of part of the oxygen contained within the compressed feed stream. The heated feed stream is introduced into the ceramic membrane unit to separate a remaining part of the oxygen contained within the compressed feed stream, thereby forming the oxygen permeate and heating the ceramic membrane unit to its operational temperature. 
     In yet another aspect of the present invention, the residual components include residual oxygen, and the compressed feed stream is heated through indirect heat exchange with a retentate stream extracted from the ceramic membrane unit. The compressed feed stream, after having been heated, is divided into first and second subsidiary streams. The process stream is formed by combining the retentate stream with the first subsidiary stream and a fuel stream and combusting the fuel stream in the presence of oxygen contained in said retentate stream and the first subsidiary stream. The second subsidiary stream is heated indirectly from the combustion of the fuel stream, thereby cooling said process stream. The second subsidiary stream is introduced into the ceramic membrane unit to separate the oxygen contained therein to form the oxygen permeate and to heat said ceramic membrane unit to its operational temperature. 
     It is to be noted that the operational temperature can be in a range from between about 600° C. and about 1200° C. The temperature below said operational temperature to which said process stream is cooled is preferably in a range of between about 300° C. and about 650° C. Further, after said initial stage of expansion, the process stream is reheated preferably to a reheated temperature in a range of between about 350° C. and about 650° C. 
     In any of the foregoing aspects of the present invention, the expansion exhaust stream can be reheated through indirect heat exchange with the oxygen product stream. Additionally, when the sum of the work of expansion is in excess of that required to compress the feed stream, the sum of said work of expansion is applied additionally to compression of the fuel and oxygen product streams. The expansion exhaust stream is reheated preferably through indirect heat exchange with the oxygen product stream. The ceramic membrane unit can be purged with either a reactive or non-reactive purge stream to increase the separation of the oxygen. 
     The use of a reactive purge also can be used in connection with a method of the present invention that is designed to produce a nitrogen product. In such a method, a retentate stream is extracted from the ceramic membrane unit and introduced into a further ceramic membrane unit. A reactive purge stream is introduced into the further ceramic membrane unit to separate further oxygen from the retentate stream and thereby to produce a further retentate and further oxygen permeate. In accordance with such a method, the process stream is formed from said further retentate. The compressed feed stream is heated through indirect heat exchange with the process stream and/or a further oxygen permeate stream, composed of the further oxygen permeate. The compressed feed stream, after having been heated, is introduced into the ceramic membrane unit to separate the oxygen therefrom and to heat the ceramic membrane unit to its operational temperature. A nitrogen product stream is formed at least in part from a subsequent expansion exhaust stream that is produced from the subsequent expansion. The expansion exhaust stream can be reheated through indirect heat exchange with both the product stream and the further oxygen permeate stream after its having exchanged heat with the compressed feed stream. 
     In the aforementioned aspect of the present invention, when the sum of said work of expansion is in excess of that required to compress the feed stream, the subsequent expansion exhaust stream can be divided into first and second nitrogen containing streams. The first nitrogen containing stream is used to form the nitrogen product stream, and the reactive purge stream is formed from a fuel stream and said second nitrogen containing stream. The reactive purge and oxygen product streams are compressed, and the sum of said work of expansion is applied additionally to compression of the reactive purge and oxygen product streams. 
     In accordance with another method of the present invention that can be used to produce a nitrogen product, the compressed feed stream is heated through indirect heat exchange with the process stream. A heated feed stream is formed by introducing a fuel stream into the compressed feed stream, after having been heated, and the fuel is combusted in the presence of part of the oxygen contained within the compressed feed stream. The heated feed stream is introduced into the ceramic membrane unit to separate a remaining part of the oxygen contained within the compressed feed stream and to heat the ceramic membrane unit to its operational temperature. A retentate stream is extracted from the ceramic membrane unit and is introduced into a further ceramic membrane unit. A reactive purge stream is introduced into the further ceramic membrane unit to separate further oxygen from the retentate stream and thereby to produce a further retentate and further oxygen permeate. The process stream is formed from the further retentate, and a nitrogen product stream is formed at least in part from a subsequent expansion exhaust stream produced from the subsequent expansion. The process stream can be reheated through indirect heat exchange with the product stream and the further oxygen permeate stream. 
     With respect to the foregoing aspect of the present invention, the sum of said work of expansion is in excess of that required to compress the feed stream. A first nitrogen containing stream, composed of part of the process stream, forms the nitrogen product stream. The reactive purge stream is formed from a further fuel stream and a second nitrogen containing stream composed of a further part of the process stream. The fuel and oxygen product streams are compressed and the sum of the work of expansion is additionally applied to compression of said fuel and oxygen product streams. 
     A method of the present invention can be adapted for use of a low pressure burner. In accordance with such adaptation, the process stream is formed from a retentate stream composed of the retentate and extracted from the ceramic membrane unit. The compressed feed stream is heated through indirect heat exchange with heat generated through combustion of a fuel and through indirect heat exchange with the process stream, thereby cooling said process stream. The process stream is reheated through indirect heat exchange with a flue gas stream composed of flue gas produced from the combustion of the fuel and the oxygen product stream. The compressed feed stream can be heated through further indirect heat exchange with the heat generated through combustion of the fuel, so that the compressed feed stream initially undergoes the indirect heat exchange with the process stream and subsequently undergoes the indirect heat exchange with heat generated through combustion of the fuel, prior to the further indirect heat exchange. The ceramic membrane unit optionally may be subjected to a purge with steam to increase the oxygen separation. The process stream and an auxiliary air stream can be preheated through indirect heat exchange with the heat generated through the combustion of the fuel. The streams can then be used to support the combustion of the fuel. 
     It is to be noted, that the term, “ceramic membrane unit” as used herein and in the claims means any type of reactor that can be used to separate oxygen from an oxygen containing stream and that utilizes oxygen transport membranes, either mixed conducting or ionic, for such separation. 
    
    
     BRIEF DESCRIPTIONS OF THE DRAWINGS 
     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: 
     FIG. 1 is a schematic, process flow diagram of an apparatus used in carrying out a method in accordance with the present invention; 
     FIG. 2 is a schematic, process flow diagram of alternative embodiment of the method illustrated by FIG. 1; 
     FIG. 3 is a schematic, process flow diagram of an apparatus used in carrying out an alternative embodiment of a method in accordance with the present invention; 
     FIG. 4 is a schematic, process flow diagram of alternative embodiment of the method illustrated by FIG. 3; 
     FIG. 5 is a schematic, process flow diagram of an apparatus used in carrying out an alternative embodiment of a method in accordance with the present invention that utilizes a burner of the type shown in FIG. 4; 
     FIG. 6 is a schematic, process flow diagram of an alternative embodiment of the method illustrated by FIG. 3 that additionally incorporates a further ceramic membrane unit to separate further oxygen from the feed and thereby allow the production of a nitrogen product; 
     FIG. 7 is a schematic, process flow diagram of an alternative embodiment of the method illustrated by FIG. 6 that does not use a direct fired burner; 
     FIG. 8 is a schematic, process flow diagram of an alternative embodiment of the method illustrated by FIG. 1 that uses a steam purge in the ceramic membrane unit; 
     FIG. 9 is a schematic, process flow diagram of an apparatus used in carrying out a method in accordance with the present invention that uses a low pressure burner; 
     FIG. 10 is a schematic, process flow diagram of an apparatus used in carrying out a method in accordance with the present invention that utilizes a low pressure burner and steam purge within the ceramic membrane unit; and 
     FIG. 11 is an alternative embodiment of the method illustrated by FIG. 10 that does not use a steam purge in the ceramic membrane unit. 
     The same reference numbers were carried through the drawings for elements that had the same function and/or design in order to avoid needless repetition in the description of such elements. 
    
    
     DETAILED DESCRIPTION 
     With reference to FIG. 1 an apparatus is illustrated for separating oxygen from an oxygen containing gas contained within a feed stream  12 , typically air. Feed stream  12  is compressed by a feed air compressor  14  which can utilize interstage cooling between stages to produce a stream  16  composed of condensed water. The resultant compressed feed stream  18  is heated within a feed heater  20  and then is introduced into a ceramic membrane unit  22 . Ceramic membrane unit  22  contains one or more oxygen transport membranes. These ceramic membranes may be of any configuration, including but not limited to planar or tubular. Preferably, the oxygen transport membranes are fabricated from a mixed conducting ceramic. Compressed feed stream  18  heats the oxygen transport membranes contained within ceramic membrane unit  22  to their operational temperature and also produces a pressure differential across the oxygen transport membranes. This separates a portion of the oxygen from compressed feed stream  18  to produce an oxygen permeate and a retentate containing residual components of compressed feed stream  18  that have not separated. For instance, in the illustrated case, the residual components would include nitrogen, argon, other potential residual feed air components, and any oxygen that has not separated from compressed feed stream  18 . 
     A retentate stream  24  composed of the retentate is introduced into a direct fired burner  26  to support combustion of a fuel, for instance, natural gas. A fuel stream  28 , containing the fuel, is compressed to the pressure of retentate stream  24  by means of a compressor  30  to produce a compressed fuel stream  32 . Compressed fuel stream  32  is introduced thereupon into direct fired burner  26 . The combustion of the fuel produces a process stream  34  that is composed of at least a portion of the residual components of the retentate, for instance, nitrogen and also combustion products produced from the combustion of the fuel. 
     Process stream  34  is introduced into feed heater  20  to heat compressed feed stream  18  to the operational temperature of the oxygen transport membranes contained within ceramic membrane unit  22 . The resultant indirect heat exchange cools process stream  34  to a temperature below such operational temperature. As mentioned above, process stream  34  can be cooled sufficiently to enable it to be used with turboexpanders fabricated from less expensive, low temperature materials. 
     Process stream  34  is introduced into an initial stage of expansion provided by a turboexpander  38  that is coupled to feed gas compressor  14 . In this regard the term “coupled” does not necessarily mean a mechanical coupling. Commonly, turboexpander  38  is coupled to a generator which can be used to produce electricity, which in turn is used to drive feed gas compressor  14 . The electricity is not necessarily applied directly to feed gas compressor  14 . For instance, the electricity can be returned to the power grid from which electricity is distributed to feed gas compressor  14 . 
     The work of expansion produced by turboexpander  38  does not generate sufficient power to run feed gas compressor  14 . In order to gain the requisite energy, process stream  34 , which after expansion is designated as exhaust stream  40 , is reheated within an expander interstage heater  42  and then introduced into a second stage of expansion that is provided by a turboexpander  44 . The resultant turboexpander exhaust stream  46  is discharged from turboexpander  44 . The reheating adds the requisite energy to run feed air compressor  14  as well as other accessories. 
     Process stream  34  is reheated by an oxygen product stream  48  that is composed of the oxygen permeate produced within ceramic membrane unit  22 . Oxygen product stream  48  is introduced into expander interstage heater  42  in indirect heat exchange with process stream  34  (exhaust stream  40 ). Oxygen product stream  48  optionally may be cooled thereafter in an aftercooler  50  and compressed by a product compressor  52  to produce an oxygen product at pressure. The amount of energy produced by the initial and subsequent stages of expansion produced by turboexpanders  38  and  44  is sufficient to run not only the feed air compressor  14 , but also compressor  30  for compressing the fuel stream  28  and product compressor  52  for compressing oxygen product stream  48 . 
     With reference to FIG. 2, interstage reheating of process stream  34  is effectuated in part by expander interstage heater  42  and a burner  26 ′ which is provided with a heat exchange pass  27  to produce a further reheating of process stream  34  through combustion of fuel within burner  26 ′. The advantage of this embodiment of the present invention is that it allows for lower operational temperatures of ceramic membrane unit  22  without compromising the operation of any of the stages of expansion. 
     With reference to FIG. 3, a slightly different method is used to heat the ceramic membrane unit  22  to its operational temperature and to generate the process stream to be expanded (designated by reference number  34 ′). 
     In this embodiment, the compressed feed stream  18  is heated in part through indirect heat exchange within feed heater  20 , and then is heated further within direct fired burner  26  to produce a heated feed stream  18 ′. Heated feed stream  18 ′ is introduced into ceramic membrane unit  22 . The process stream  34 ′ contains such components as residual oxygen that has not been consumed in the combustion of the fuel, combustion products from such combustion, and nitrogen. 
     With reference to FIG. 4, an alternative embodiment is illustrated that is similar to the embodiment of FIG.  3 . However, direct fired burner  26 ′ of FIG. 2 is utilized to split the interstage heating duty of process stream  34 ′ between expander interstage heater  27  and direct fired burner  26 ′. 
     FIG. 5 illustrates an embodiment of the present invention that combines several aspects of the aforementioned embodiments. A burner  54  is utilized that is provided with a heat exchange pass  56  (similar in design to heat exchange pass  27 ) and an additional inlet for an auxiliary air stream. Compressed feed stream  18 , after having been heated within feed heater  20 , is divided into first and second subsidiary streams  18   a  and  18   b , respectively. Retentate stream  24  is passed through feed heater  20  to heat compressed feed stream  18 . Subsequently, both the cooled retentate stream and the first subsidiary stream  18   a  are introduced into burner  54 . The resultant process stream  34 ″ contains nitrogen, any oxygen that has not been consumed in the combustion of fuel  28 , and the combustion products obtained from the combustion of fuel  28 . Second subsidiary stream  18   b  is passed through heat exchange pass  56  of burner  54 . Second subsidiary stream  18   b  is heated within heat exchange pass  56  to form a stream  18 ″ that is introduced into ceramic membrane unit  22  for oxygen separation. 
     With reference to FIG. 6, an embodiment of the present invention is illustrated that is designed to co-produce oxygen and nitrogen. In some aspects, it is similar to the embodiments shown in FIGS. 2 and 3 except that process stream  34 ′″ has much less oxygen due to further processing that will be described. 
     Retentate stream  24  is introduced into a further ceramic membrane unit  58  that functions as a de-oxo unit to remove any further residual oxygen contained within retentate stream  24 , thereby producing process stream  34 ′″ which is a retentate stream of ceramic membrane unit  58 . Process stream  34 ′″, after its initial and subsequent expansions, produces an exhaust stream  46  which is divided into first and second nitrogen containing streams  60  and  62 , respectively. First nitrogen containing stream  60  is introduced into a purifier  64  to produce a nitrogen product stream  66 . The impurities are rejected as a stream  68 . The purifier can be any unit, conventional or otherwise, that is designed to remove any of the residual components that may be present in the nitrogen product stream  60 . The second nitrogen containing stream  62  is combined with a fuel stream  70  and introduced into ceramic membrane unit  58  for combustion supported by permeated oxygen. This combustion consumes oxygen to lower the oxygen partial pressure, thereby increasing the degree of separation. The increased degree of separation can be expressed as either a purer process stream  34 ′″ or a lower compression requirement for feed air compressor  14 . Additionally, the combustion also heats ceramic membrane unit  58  to its operational temperature. 
     The resultant heated exhaust stream  72 , containing combustion products formed from the combustion in ceramic membrane unit  58 , then can be introduced into an expander interstage heater  42 ′ that is provided with a pass for heated exhaust stream  72  to help reheat process stream  34 ′″ in the form of turbine exhaust  40 . Heated exhaust stream  72 , after passage through expander interstage heater  42 , then subsequently may be cooled further in an aftercooler  74 . 
     FIG. 7 illustrates an alternative embodiment of a process flow diagram shown in FIG.  6 . The embodiment illustrated in this figure does not use a direct fired burner. Instead, fuel stream  26  is diluted with second nitrogen containing stream  62 , and then compressed within fuel compressor  30 . The resultant compressed fuel stream  32 ′ is introduced into oxygen transport membrane unit  58  for purposes that are identical to those illustrated in FIG. 6 for the introduction of compressed fuel stream  32 . The resultant heated exhaust stream  72 ′ then is passed through a feed heater  20 ′ having an extra pass for such stream, thereby to help heat compressed feed stream  18 . The heated exhaust stream  72 ′ then is passed through expander interstage heater  42 ′ to help the interstage reheating of process stream  34 ′″. 
     In a modification of the process shown in FIG. 7, a heat exchange pass may be included inside the ceramic membrane unit  58 . Prior to entering the ceramic membrane unit  22 , the heated compressed feed stream  18  is heated further by indirect heat exchange with the heated exhaust stream  72 ′ that is formed inside ceramic membrane unit  58 . 
     In yet another possible modification, part of exhaust stream  40 , prior to being reheated, could be taken as a product and then purified. The remainder would be heated in an expander interstage heater of similar design to expander interstage heater  42 , having two passes. The remainder of exhaust stream  40 , after having been reheated, would be combined with heated exhaust stream  72 ′ and such resultant combined stream would be introduced into turboexpander  44 . This process is useful in situations in which the required delivery pressure for a nitrogen product gas is equal to the pressure of the nitrogen product stream that exits the initial expansion stage, thereby eliminating the need to recompress the product nitrogen. Furthermore, by combining the heated exhaust stream  72 ′ with the waste nitrogen stream  35 ′, aftercooler  74  is eliminated. 
     FIG. 8 represents the embodiment based on FIG. 1 in which a steam purge is used in order to aid oxygen separation within an oxygen transport membrane unit  22 . The steam purge produces a permeate containing moisture and, therefore, a moisture containing oxygen product stream  48 ′. After passage through expander interstage heater  42 , water is condensed from moisture containing oxygen product stream  48 ′ in a condenser  78 . Condenser  78  contains a heat exchange pass  79  to heat an incoming water stream into a stream  80  made up of steam. The heating cools moisture containing oxygen product stream  48 ′ to condense water which is illustrated by a stream  82 . Stream  80  is heated further within a heater  84  and introduced into the oxygen transport membrane unit  22 . The steam sweeps the oxygen permeate from the oxygen transport membranes contained within ceramic membrane unit  22  to lower the oxygen pressure, thereby increasing the degree of oxygen separation. 
     With reference to FIG. 9, a low pressure burner is utilized in which an auxiliary air stream  92  is introduced into low pressure burner  90  by way of a blower  94  together with fuel stream  28 . Low pressure burner  90  has a pass  96  for heating compressed feed stream  18  which is subsequently heated further in feed heater  20 , and then yet heated further within a pass  98  of low pressure burner  90 . The resultant heated compressed feed stream  18  then is introduced into ceramic membrane unit  22 . Combustion taking place within low pressure burner  90  produces a heated flue gas stream  100  which is introduced into an expander interstage heater  42 ″ that is provided with a pass for such purpose. Heated flue gas stream  100  is thereafter cooled by an aftercooler  102 . Alternatively, stream  92  could be any oxygen-containing gas, such as exhaust stream  46 , which still may contain oxygen and have useful thermal energy that can be used to improve the overall thermal efficiency of the process. 
     With reference to FIG. 10 a low pressure burner  90 ′ is illustrated that is used in conjunction with a steam purge for ceramic membrane unit  22 . In this embodiment low pressure burner  90 ′ is provided with a heat exchange pass  96  in which the compressed feed stream  18 , after having been heated within feed heater  20 , serves to heat further compressed feed stream  18  prior to its introduction into ceramic membrane unit  22 . Heat exchange passes  106  and  108  are also provided to preheat exhaust stream  46  and auxiliary air stream  92  for introduction into low pressure burner  90 ′ to help support combustion of the fuel. 
     Heated flue gas stream  100  after having been cooled within expander interstage heater  42 ″ then is passed through a heat exchanger  110  to superheat a stream  112  made up of steam. Stream  112  is produced within a condenser  114  in which moisture is removed from moisture containing oxygen product steam  48 ′ to produce oxygen product stream  48 . The resultant superheated steam then is introduced into ceramic membrane unit  22 . Heated flue gas stream  100  is further cooled within an aftercooler  116 . Alternatively, stream  92  could be any oxygen-containing gas, such as exhaust stream  46 , which still may contain oxygen and have useful thermal energy that can be used to improve the overall thermal efficiency of the process. 
     With reference to FIG. 11, an embodiment of the present invention is illustrated that is based upon the use of low pressure burner  90 ′ illustrated in FIG.  10 . The difference between the two embodiments is that a steam purge is not used. Alternatively, stream  92  could be any oxygen-containing gas, such as exhaust stream  46 , which still may contain oxygen and have useful thermal energy that can be used to improve the overall thermal efficiency of the process. 
     The following table illustrates an exemplary process conducted with the process flow diagram illustrated for FIG.  1 . Table 2 illustrates the power required and recovered in the various illustrated components. 
     
       
         
               
               
               
               
               
             
               
               
               
               
               
             
           
               
                 TABLE 1 
               
               
                   
               
               
                 Stream 
                 Location 
                 Pressure 
                   
                 Temperature 
               
               
                   
               
             
             
               
                   
               
             
          
           
               
                 18 
                 After compressor 14 
                 200-500 psia 
                   
                   
               
               
                 18 
                 After feed heater 20 
                   
                   
                 600° C.-1200° C. 
               
               
                 48 
                 After ceramic membrane unit 22 
                 14-20 psia 
                   
                 600° C.-1200° C. 
               
               
                 24 
                 After oxygen transport 
                 180-500 psia 
                   
                 600° C.-1200° C. 
               
               
                   
                 membrane unit 22 
               
               
                 48 
                 After expander interstage 
                   
                   
                 120° C.-520° C.  
               
               
                   
                 heater 42 
               
               
                 48 
                 After aftercooler 50 
                   
                   
                 50° C. 
               
               
                 48 
                 After compressor 52 
                 30-100 psia 
               
               
                 32 
                   
                 180-500 psia 
               
               
                 34 
                 Prior to feed heater 20 
                 175-500 psia 
                   
                 800° C.-1600° C. 
               
               
                 34 
                 After cooling in feed heater 20 
                   
                   
                 350° C.-650° C.  
               
               
                 40 
                 After turboexpander 38, but 
                 35-120 psia 
                   
                 180° C.-500° C.  
               
               
                   
                 before expander interstage 
               
               
                   
                 heater 42 
               
               
                 40 
                 After expander interstage 
                   
                   
                 350° C.-650° C.  
               
               
                   
                 heater 42, but prior to 
               
               
                   
                 turboexpander 44 
               
               
                 46 
                   
                 14-15 psia 
               
               
                   
               
             
          
         
       
     
     
       
         
               
               
             
               
               
             
           
               
                 TABLE 2 
               
               
                   
               
               
                 Turbomachinery 
                 Power Required/Recovered (kW) 
               
               
                   
               
             
             
               
                   
               
             
          
           
               
                 Feed Gas Compressor 
                 3728.7 
               
               
                 Oxygen Product Compressor 
                 196.5 
               
               
                 Fuel Compressor 
                 49.03 
               
               
                 First Expander Stage 
                 1864.35 
               
               
                 Second Expander Stage 
                 1864.35 
               
               
                   
               
             
          
         
       
     
     While the present invention has been described with reference to preferred embodiments, as will occur to those skilled in the art, numerous changes, additions, and omissions may be made without departing from the present invention.