Oxygen production by staged mixed conductor membranes

Abstract: High-purity oxygen is recovered from air by a high-temperature ion transport membrane system comprising two or more stages in which each stage operates at a different feed side to permeate side pressure ratio. Operation of the system in multiple stages at controlled pressure ratios produces oxygen at a lower specific power consumption compared with single-stage operation.

Description: FIELD OF THE INVENTION The invention pertains to the recovery of oxygen by a high temperature ion transport membrane system, and in particular to the use of multiple membrane zones or stages in such a system. BACKGROUND OF THE INVENTION Oxygen is an economically important gas which is widely used in large-scale industrial applications. New uses for oxygen are emerging in advanced high-temperature processes for iron and steel manufacture, coal gasification, integrated gasification combined cycle power generation, and oxygen-enriched combustion. In these large-scale applications, the cost of oxygen produced by conventional cryogenic or noncryogenic technology is a major portion of the overall operating cost, and lower oxygen cost will encourage the commercialization of these emerging technologies. New oxygen separation processes which can be thermally integrated with these advanced high-temperature processes will reduce the energy consumed in oxygen production, which in turn will promote the technical and commercial development of such integrated systems. Oxygen can be recovered from air at high temperatures by inorganic oxide ceramic materials utilized in the form of selectively permeable nonporous ion transport membranes. An oxygen partial pressure differential or a voltage differential across the membrane causes oxygen ions to migrate through the membrane from the feed side to the permeate side where the ions recombine to form electrons and oxygen gas. An ion transport membrane of the pressure-driven type is defined herein as a mixed conductor membrane, in which the electrons simultaneously migrate through the membrane to preserve internal electrical neutrality. An ion transport membrane of the electrically-driven type is defined herein as a solid electrolyte membrane in which the electrons flow from the permeate side to the feed side of the membrane in an external circuit driven by a voltage differential. A comprehensive review of the characteristics and applications of ion transport membranes is given in report entitled "Advanced Oxygen Separation Membranes" by J. D. Wright and R. J. Copeland, Report No. TDA-GRI-90/0303 prepared for the Gas Research Institute, September 1990. The operation and heat integration of ion transport membranes with combustors, gas turbines, and power recovery systems are disclosed in U.S. Patents 4,545,787, 5,035,727, 5,118,395, and 5,174,866. An article entitled "Separation of Oxygen by Using Zirconia Solid Electrolyte Membranes" by D. J. Clark et al in Gas Separation and Purification 1992, Vol. 6, No. 4, pp. 201-205 discloses an integrated coal gasification-gas turbine cogeneration system with recovery of oxygen for use in the gasifier. Nonpermeate from the membrane is combusted with gas from the gasifier and passed to the gas turbine cogeneration system. Each of the high-temperature, nonporous, ion transport membrane systems summarized above is characterized by the use of a single membrane stage for oxygen recovery, and the use of multiple stages or zones is not disclosed. The separation of gas mixtures by nonporous polymeric membranes and by porous diffusion membranes is well-known in the art. Membrane separation systems utilizing these principles operate at ambient or near-ambient temperatures, and are characterized by permeate selectivities which are significantly lower than the extremely high oxygen selectivities achieved by ion transport membranes. As a result, permeate streams are gas mixtures enriched in certain components relative to the feed composition. Staged operation of nonporous polymeric membranes and porous diffusion membranes is disclosed in the art, wherein staged operation increases the purity and/or recovery of the desired product. EP-A 0 521 784 and US-A 4,894,068 disclose membrane processes for the production of high-purity nitrogen from air. EP-A 0 521 784 teaches the production of 98 % pure nitrogen from air using a polymide membrane. US-A 4,894,068 teaches the production of nitrogen containing less than 1,000 parts per million (ppm) of oxygen and uses a polysulfone polymer. In EP-A 0 521 784 the use of the permeate gas from a given stage to sweep the permeate side of the previous stage is a required feature of the process. US-A 4,894,068 discloses staged serious membrane separators for the production of nitrogon which operate in the same manner as the process of EP-A 0 521 784 except for the sweep feature. The membrane material of US-A 4,894,068 is a polymeric membrane, i.e. a polysulfone polymer preferable coated with a silicone coating. The membrane preferable has a separation factor of oxygen with respect to nitrogen of greater than 4 which is similar to the oxygen to nitrogen selectivity of 4 to 8 as taught in EP-A 0 521 784. The successful development and commercialization of oxygen production by ion transport membranes will require flexible systems which maximize energy utilization and allow operation of system components at optimum conditions. The invention disclosed below and described in the claims which follow advances the art and provides improved methods for the production of oxygen by means of multiple ion transport membranes. SUMMARY OF THE INVENTION The invention is defined by claim 1. The subclaims define advantageous embodyments. BRIEF DESCRIPTION OF THE DRAWINGS Fig. 1 is a schematic flowsheet of a single-stage ion transport membrane system for recovering high-purity oxygen. Fig. 2 is a schematic flowsheet of a two-stage series ion transport membrane system for recovering high-purity oxygen according to the present invention. Fig. 3 is a schematic flowsheet of an alternative two-stage series ion transport membrane system for recovering high-purity oxygen according to the present invention. DETAILED DESCRIPTION OF THE INVENTION The present invention is an improved high-temperature process for recovering high purity oxygen from an oxygen-containing gas mixture, preferably air, by means of a multiple-stage ion transport membrane system. The stages are arranged in series configuration. A key feature of the invention is that each of the stages is operated such that the ratio of the feed side pressure to the permeate side pressure is different. In systems with multiple series stages, these ratios are controlled such that the ratio is higher in each succeeding stage. This results in a reduction of the specific power consumption compared with a single-stage system. Preferably, the ion transport membranes are pressure-driven mixed conductor membranes, although electrically-driven solid electrolyte membranes optionally may be utilized if desired. The definition of the term "stage" has various meanings in the field of gas separation in general and the membrane art in particular. The term has been used in the art to designate series membrane,modules as well as parallel membrane modules, including operating modes such as recycle, countercurrent operation, and the like. In the present specification, the term "stage" defines an individual ion transport membrane module which is divided by the membrane into a feed side and a permeate side. A system may comprise two or more stages arranged in series, and each stage is characterized by a different ratio of the feed pressure to the permeate pressure. Each stage exhibits an extremely high and theoretically infinite oxygen selectivity, which is characteristic of an ion transport membrane because oxygen is the sole diffusing gas. All stages therefore produce high-purity oxygen permeate streams which are essentially equivalent in oxygen purity. This is a key feature which distinguishes the present invention from previously disclosed multi-stage membrane systems. Such previously disclosed systems utilize solid polymeric membranes, porous diffusion membranes, or other types of membranes, all of which have modest selectivities compared with ion transport membranes. Such multi-stage membrane systems in series configuration are characterized by the fact that each stage produces a mixed permeate at a different composition. Fig. 1 illustrates a single-stage ion transport membrane system in which air or other oxygen-containing gas 101 is compressed to 1-35 bar (15-500 psia) in compressor 103 and heated in exchanger 105 by indirect heat exchange with hot gas streams defined later. Heated and compressed stream 107 is optionally further heated in combustor 109 and passed into the feed side of ion transport membrane zone 113 which is divided into a feed side and a permeate side by membrane 115. Combustor 109 can be a direct combustor in which fuel 111 is combusted with oxygen-containing stream 107 to form combustion products which flow as stream 112 directly to membrane zone 113. Alternatively, combustor 109 can be an indirect system, i.e. a process furnace, in which stream 107 is indirectly heated by combustion of fuel 111 with outside air (not shown), whereby the heated stream 112 passes without composition change to membrane zone 113. Stream 112 is typically heated to 500°-1200°C prior to membrane zone 113. High purity oxygen permeate 117 at greater than 98 vol% O 2 is withdrawn and optionally cooled in exchanger 105 against compressed feed 104. Cooled product 119 is compressed as necessary in compressor 121 to yield oxygen product 123. Alternatively, compressor 121 can be replaced by a vacuum blower to withdraw permeate 117 at a subatmospheric-pressure thus maximizing the oxygen partial pressure driving force across membrane 115. Nonpermeate stream 125 is optionally heated by direct or indirect combustor 129 and cooled against compressed feed 104 in exchanger 105 to yield cooled nonpermeate stream 133. The present invention is illustrated in one embodiment as shown in Fig. 2. Air or other oxygen-containing gas 201 is compressed to 1-35 bar (15-500 psia) in compressor 203 and heated in exchanger 205 by indirect heat exchange with hot gas streams defined later. Heated and compressed stream 207 is optionally further heated in combustor 209 and passed into the feed side of ion transport membrane zone 213 which is divided into a feed side and a permeate side by ion transport membrane 215. Combustor 209 can be a direct combustor in which fuel 211 is combusted with oxygen-containing stream 207 to form combustion products which flow as stream 212 directly to membrane zone 213. Alternatively, combustor 209 can be an indirect system, i.e. a process furnace, in which stream 207 is indirectly heated by combustion of fuel 211 with outside air (not shown), whereby the heated stream 212 passes without composition change to membrane zone 213. Stream 212 is typically heated to 500°-1200°C prior to membrane zone 213. High purity oxygen permeate 217 at greater than 98 vol% O 2 is withdrawn and cooled in exchanger 205 against compressed feed 204. Cooled product 219 is compressed with additional oxygen product 220 to the required product pressure in the second stage of two-stage compressor 221 to yield oxygen product 223. Nonpermeate stream 225 flows through the feed side of membrane zone 227 which is divided into a feed side and a permeate side by ion transport membrane 229. High purity oxygen permeate 231 at greater than 98 vol% O 2 is withdrawn and cooled in exchanger 205 against compressed feed 204. Cooled product 233 is compressed in the first stage of compressor 221 to yield intermediate pressure product 220, which is combined with product 219 and compressed to yield oxygen product 223. Nonpermeate stream 235 is cooled against compressed feed 204 in exchanger 205 to yield oxygen-depleted waste stream 237. Optionally, nonpermeate stream 235 can be heated in combustor 239 prior to exchanger 205. Combustor 239 can be a direct combustor in which fuel 241 is combusted with oxygen-containing stream nonpermeate stream 235 to form combustion products which flow as stream 243 directly to exchanger 205. Alternatively, combustor 239 can be an indirect system, i.e. a process furnace, in which stream 235 is indirectly heated by combustion of fuel 241 with outside air (not shown), whereby the heated stream 243 passes without composition change to exchanger 205. Ion transport membranes 215 and 229 are preferably solid mixed conductor ceramic membranes which selectively permeate oxygen in the form of oxygen ions driven by a negative oxygen partial pressure gradient in the direction of permeation. Preferred membrane materials of the mixed conductor type are described in U.S. Patent 5,240,480 . This patent discloses a membrane which comprises a porous layer with an average pore radius of less than 10 microns upon which is deposited a nonporous dense layer, in which both the porous substrate and the nonporous dense layer comprise multicomponent metallic oxides capable of conducting electrons and oxygen ions. This composite membrane operates at temperatures above 500°C and recovers high purity oxygen in the pressure-driven mode. Representative membranes are described in which the porous layer and/or the dense layer are formed from a multicomponent metallic oxide selected from the group consisting of La 0.2 Ba 0.8 Co 0.8 Fe 0.2 O 3-x , Pr 0.2 Ba 0.8 Co 0.8 Fe 0.2 O 3-x , and La 0.2 Ba 0.8 Co 0.6 Cu 0.2 Fe 0.2 O 3-x where x is between 0 and 1. A key feature of the invention is that membrane zones 213 and 227 operate at different ratios of the feed pressure to the permeate pressure. The feed side pressures are essentially equivalent, which means that the pressures differ only by the minimal pressure drop through the feed sides of the two zones and related piping. The permeate side pressures are different, however, and are set by the suction pressures in the two stages of compressor 221. The pressure ratio of feed to permeate is lower in the first stage than in the second stage. In this staged membrane operation a different oxygen partial pressure ratio is controlled in each stage which enables a better distribution of the oxygen partial pressure driving force through the system, which leads to a thermodynamically more reversible process compared with the single-stage system of Fig. 1. The staged system of Fig. 2 therefore operates with a lower power consumption compared with the single stage system of Fig. 1 for an equivalent final stage permeate pressure, final oxygen product pressure, and oxygen recovery. This is illustrated in more detail in the Examples which follow. The embodiment of Fig. 2 can be operated with alternative methods of oxygen final product withdrawal. In one alternative, compressor 221 is replaced by a two-stage vacuum blower in order to increase the overall oxygen partial driving force across membranes 215 and 229; final oxygen product 223 is produced at atmospheric pressure and can be compressed further if necessary. In another alternative, two different oxygen product streams are produced at different total pressures. This can be accomplished for example by withdrawing stream 219 at atmospheric pressure and withdrawing stream 231 at subatmospheric pressure by means of a vacuum blower; other configurations are possible as long as different permeate pressures are maintained in membrane zones 213 and 227. While Fig. 2 illustrates the use of two stages or zones in series, any number of stages can be used. The use of two and three stages is illustrated in the Examples which follow. An alternative embodiment of the series membranes of Fig. 2 is given in Fig. 3. In this alternative, the permeate sides of the membranes operate at essentially the same pressure, and the feed side pressures are different. Air or other oxygen-containing gas 401 is compressed to 15-500 psia in compressor 403 and heated in exchanger 405 by indirect heat exchange with hot gas streams defined later. Heated and compressed stream 407 is optionally further heated in combustor 409 and passed into the feed side of ion transport membrane zone 413 which is divided into a feed side and a permeate side by membrane 415. Combustor 409 is preferably an indirectly fired system, i.e. a process furnace, in which stream 407 is indirectly heated by combustion of fuel 411 with outside air (not shown), whereby the heated stream 412 passes without composition change to membrane zone 413. Stream 412 is typically heated to 500°-1200°C prior to membrane zone 413. High purity oxygen permeate 417 at greater than 98 vol% O 2 is withdrawn from membrane zone 413. Nonpermeate stream 419 is cooled in exchanger 405 against compressed feed 404, cooled stream 421 is compressed in compressor 423, and compressed stream 425 is heated against hot process steams in exchanger 405. Heated compressed stream 427 is optionally further heated in indirect combustor 409 and passes as stream 429 into the feed side of membrane zone 431 which is divided into a feed side and a permeate side by ion transport membrane 433. Preferably, ion transport membranes 415 and 433, are mixed conductor membranes as described above. High purity oxygen permeate 435 typically at greater than 98 vol% O 2 is withdrawn, combined with product steam 417, and combined stream 437 is cooled in exchanger 405 against compressed feed 404. Cooled product 439 optionally is compressed in compressor 441 to yield oxygen product 443. Nonpermeate stream 445 is cooled against compressed feed 404 in exchanger 405 to yield oxygen-depleted waste stream 447. Optionally, nonpermeate stream 445 can be heated in combustor 449 prior to exchanger 405. Combustor 449 can be a direct combustor in which fuel 451 is combusted with oxygen-containing stream nonpermeate stream 445 to form combustion products which flow as stream 453 directly to exchanger 405. Alternatively, combustor 449 can be an indirect system, i.e. a process furnace, in which stream 445 is indirectly heated by combustion of fuel 451 with outside air (not shown), whereby the heated stream 453 passes without composition change to exchanger 405. In the embodiment of Fig. 3, membrane zones 413 and 431 operate at different ratios of the feed side pressure to the permeate side pressure, a key feature of the invention as discussed earlier. This is achieved by operation with essentially equal membrane zone permeate pressures and different feed pressures, in contrast with the embodiment of Fig. 2 in which the membrane zone feed pressures are essentially equal and the permeate pressures are different. In Fig. 3 the pressure ratio in the first stage is lower than in the second stage. Staged membrane operation thus enables a better distribution of the oxygen partial pressure driving force through the system, which leads to a thermodynamically more reversible process compared with the single-stage system of Fig. 1. The staged system of Fig. 3, as in the alternative embodiment of Fig. 2, operates with a lower power consumption compared with the single stage system for an equivalent final stage permeate pressure, final oxygen product pressure, and oxygen recovery. In each of the embodiments of the invention, a pressurized oxygen-containing nonpermeate stream is produced (e.g. streams 237 of Fig. 2, 447 of Fig. 3). Power can be recovered from any these streams directly by means of an expansion turbine, or any of the streams can be combusted in a gas turbine combustor and the hot combustion gas expanded to generate power. The ion transport membranes utilized in the embodiments described above are typically solid ceramic assemblies in the form of tubes, sheets, or monolithic honeycomb. Any solid ceramic membrane material which selectively permeates oxygen can be utilized in the present invention. The active ceramic material preferably is selected from materials of the mixed conductor type as described in U.S. Patent 5,240,480. This patent discloses a membrane which comprises a porous layer with an average pore radius of less than 10 microns upon which is deposited a nonporous dense layer, in which both the porous substrate and the nonporous dense layer comprise multicomponent metallic oxides. capable of conducting electrons and oxygen ions. This composite membrane operates at temperatures above 500°C and recovers high purity oxygen by the mechanism discussed earlier. Representative membranes are described in U.S. Patent 5,240,480 in which the porous layer and/or the dense layer are formed from a multicomponent metallic oxide selected from the group consisting of La 0.2 Ba 0.8 Co 0.8 Fe 0.2 O 3-x , Pr 0.2 Ba 0.8 Co 0.8 Fe 0.2 O 3-x , and La 0.2 Ba 0.8 Co 0.6 Cu 0.2 Fe 0.2 O 3-x where x is between 0 and 1. Ion transport membranes of the electrically-driven solid electrolyte type optionally may be employed in the membrane separation zones. Preferred membranes of the solid electrolyte type can be fabricated by depositing a thin layer of multicomponent oxide on a porous substrate as described in U.S. Patent 5,160,618. A preferred membrane comprises yttria-stabilized zirconia which has been deposited into the micropores of a lanthanum-doped alumina surface layer of a porous alumina substrate having an average pore diameter of less than about 50 nanometers and on the surface layer of the alumina substrate wherein the thickness of the yttria-stabilized zirconia is 0.5 microns or less. The yttria-stabilized zirconia layer is deposited at a temperature of 700-1100°C at a pressure of 1 to 760 torr over a time period ranging from 1 to 120 minutes by the following method. At least two metal halides, for example yttrium chloride and zirconium chloride, are vaporized on one side of the substrate described above and an oxidizing gas such as a mixture of oxygen and water is contacted with the other side of the substrate. The two gas mixtures diffuse and react within the pores of the porous surface layer to deposit the corresponding metal oxides therein, forming the membrane capable of separating an oxygen-containing gas mixture by the mechanism described earlier. This thin coating of active membrane material can be deposited on tubes, sheets, or monolithic honeycomb before or after assembly in a membrane module. EXAMPLE 1 The single stage and two stage mixed conductor membrane systems of Figs. 1 and 2 were simulated by means of heat and material balances, and the specific power requirements for the feed compressor and vacuum blower were calculated. The same simulation was also carried out for a three stage system (not shown) in which a third stage in series is added to the system of Fig. 2, and a three-stage vacuum blower is used. In all cases the systems operate with the following common parameters: membrane temperature, 850°C; oxygen production rate, 20 tons/day; oxygen recovery from feed, 11.6%; compressor discharge pressure, 2 bar 29.7 psia; membrane zone feed side pressure, 1.7 bar (24.7 psia) (pressure drop through the membrane zones was assumed negligible); unmixed flow in the feed side; permeate withdrawal by vacuum blower delivering oxygen product at 1 bar (14.7 psia); air pressure drop through main exchangers 105 and 205, 0,34 bar 5 psi; and oxygen pressure drop through main exchangers 105 and 205, 0,1 bar 75 torr (1.45 psi). The adiabatic efficiencies of the feed air compressor and the vacuum blower are 78% and 57% respectively. The feed and permeate oxygen partial pressures for the one, two, and three stage cases are summarized in Table 1. Feed and Permeate Oxygen Partial Pressures for Example 1 (torr) 1 torr = 1.33mbar Stage 1 Stage 2 Stage 3 Single Stage System: feed in 268 - - nonpermeate out 135 - - permeate 120 - - Two Stage System: feed in 268 200 - nonpermeate out 200 135 - permeate 180 120 - Three Stage System: feed in 268 220 180 nonpermeate out 220 180 135 permeate 200 160 120 In these three cases, the final stage oxygen permeate is withdrawn at 120 torr. For the multiple stage systems, it is seen that stage 1 permeate is withdrawn at a progressively higher pressure as the number of stages increases. The multiple stage systems operate at successively increasing ratios of the feed side to the permeate side oxygen partial pressure. The calculated specific power and relative power for the three cases are summarized in Table 2. The calculated specific power is the total for the air feed compressor and the oxygen vacuum blower. Comparison of Specific Power for Cases of Example 1 Total Stages kW hr/ton O 2 Relative Power One 330 1.0 Two 312 0.95 Three 307 0.93 The utilization of stages yields a better distribution of the overall oxygen partial pressure driving force through the system, which leads to a thermodynamically more reversible process compared with the single stage system. As seen in Table 2, this results in a decrease in the specific power as the number of stages increases; the two, stage system operates with a lower power consumption than the single stage system, and the three stage system operates with a lower power consumption than the one and two stage systems. EXAMPLE 2 The single stage and two stage mixed conductor membrane systems of Figs. 1 and 2 were simulated by means of heat and material balances in a manner similar to that of Example 1. In Example 2, the single stage process of Fig. 1 and the two stage process of Fig. 2 are used to produce two different oxygen product streams of equal flow rates at 10.3 and 1 bar (150 and 14.7 psia) respectively. In the process of Fig. 1, cooled oxygen product 119 is withdrawn at atmospheric pressure, one half is taken as a first product, and the other half is compressed to 10.3 bar 150 psia by compressor 121 as a second product. In the process of Fig. 2, cooled oxygen product 233 is withdrawn at atmospheric pressure; vacuum blower 221 is not used. Cooled oxygen product 219 is compressed to 10.3 bar 150 psia (not shown). The two systems operate with the following common parameters: membrane temperature, 850°C; total oxygen production rate, 20 tons/day; oxygen recovery from feed, 11.6%; feed compressor discharge pressure, 12.4 bar (179.7 psia); membrane zone feed side pressure, 12.05 bar (174.7 psia) (pressure drop through the membrane zones was assumed negligible); air pressure drop through main exchangers 105 and 205, 0,34 bar (5 psi); and oxygen pressure drop through main exchangers 105 and 205, 0,13 bar (1.9 psi). No credit is taken for power recovery from pressurized nonpermeate streams 133 or 237. The adiabatic efficiency for oxygen product compression in both systems is 78%. The feed and permeate oxygen partial pressures for the one and two stage cases are summarized in Table 3. Feed and Permeate Oxygen Partial Pressures for Example 2 (torr) 1 torr = 1.33 mbar Stage 1 Stage 2 Single Stage System: feed 1897 - nonpermeate 964 - permeate 860 - Two Stage System: feed 1897 1430 nonpermeate 1430 964 permeate 1367 860 The specific power and relative specific power for oxygen product compression were calculated and are given in Table 4. Comparison of Specific Power for Cases of Example 2 Total Stages Product Compression Power, kW hr/ton Relative Power One 36.9 1.0 Two 26.7 0.73 It is seen that staging of the membrane system results in a significant decrease in the compression power required to produce two product streams at different pressures. The utilization of a different pressure ratio in each stage of the two stage system yields a better distribution of the overall oxygen partial pressure driving force through the system, which leads to a thermodynamically more reversible process compared with the single stage system. As seen in Table 4, this results in a decrease in the specific product compression power as the number of stages increases; the two stage system operates with a lower power consumption than the single stage system. The two stage system is especially adapted to the efficient production of two oxygen product streams at different pressures. The staged operation of high temperature mixed conductor membranes thus offers significant power reduction compared with the operation of a single stage system at comparable process conditions. The staging of mixed conductor membrane systems is fundamentally different from staged systems which use polymeric or porous diffusion membranes because the permeate from each stage of a mixed conductor membrane system is essentially a pure component while the permeate streams from stages of these other membrane systems are mixtures enriched in certain components relative to the membrane feed. Staging in these other membrane systems is used exclusively to increase product purity and recovery; conversely, staging in ion transport membrane systems has no effect on product purity and therefore would not be used to increase product purity. The reduction in power requirements by staging in the present invention is therefore unexpected in view of the teachings in the art regarding these other types of staged membrane systems. The essential characteristics of the present invention are described completely in the foregoing disclosure. One skilled in the art can understand the invention and make various modifications thereto without departing from the scope of the claims which follow. Examples for such modifications are: The temperature in each of said membrane separation zones is between 500 and 1200°C. The heating of the resulting compressed gas mixture is accomplished at least in part by indirect heat exchange with said first and second high-purity oxygen permeate streams or with said second oxygen-containing non-permeate stream.

Claims: A process for recovering oxygen from an oxygen-containing gas mixture, comprising the steps of: (a) compressing (103) said oxygen-containing gas mixture; (b) heating (105) the resulting compressed gas mixture of step (a); (c) passing the resulting compressed and heated mixture of step (b) into a first membrane separation zone (213) comprising an oxygen-selective ion transport membrane (215) which separates said zone (213) into a feed side and a permeate side, and withdrawing from said zone (213) a first high-purity oxygen permeate stream at greater than 98 vol.-% oxygen and a first oxygen-containing non-permeate stream; and (d) passing said first oxygen-containing non-permeate stream into a second membrane separation zone (227) comprising an oxygen-selective ion transport membrane (229) which separates said zone (227) into a feed side and a permeate side, and withdrawing from said zone (227) a second high-purity oxygen permeate stream at greater than 98 vol.-% oxygen and a second oxygen-containing non-permeate stream; wherein each of said membrane separation zones (213, 227) operates at a different ratio of feed side pressure to permeate side pressure the ratio in the second zone being higher than the ratio in the first zone, and wherein said first and second high-purity oxygen permeate streams are withdrawn in total from said first and second membrane separation zones (213, 227) without the introduction of any portion of said permeate streams into said first and second membrane separation zones (213, 227). The process of Claim 1, wherein said ion transport membranes (215, 229) are mixed conductor membranes. The process of Claims 1 or 2, wherein said oxygen-containing gas mixture is air. The process of Claims 1 to 3, wherein said heating of the resulting compressed gas mixture of step (a) is accomplished at least in part by passing said compressed gas mixture through a direct or indirect combustor (209) prior to said first membrane separation zone (213). The process of Claims 2 to 4, wherein optionally said membrane separation zones (413, 431) operate at essentially equivalent permeate side pressures and different feed side pressures, which further comprises cooling said first oxygen-containing non-permeate stream, compressing the resulting cooled stream, and heating the resulting compressed stream prior to step (d), and wherein said heating of the resulting compressed stream is accomplished at least in part by passing said stream through an indirect combustor (409). The process of Claims 1 to 5, which further comprises heating said second oxygen-containing non-permeate stream in a direct or indirect combustor (239, 449) prior to said indirect heat exchange (205, 405) with the compressed gas mixture of step (a). The process of Claims 5 and 6, wherein said heating of the resulting compressed gas mixture of step (a) is accomplished at least in part by passing said compressed gas mixture through an indirect combustor (409) prior to said first membrane separation zone (413). The process of claim 1, wherein multiple, in particular three, membrane separation zones are used.

Inventors: SRINIVASAN RAJAGOPALAN SREE, THOROGOOD ROBERT MICHAEL, YEE TERRENCE FU, SRINIVASAN, RAJAGOPALAN SREE, THOROGOOD, ROBERT MICHAEL, YEE, TERRENCE FU
Applicants: AIR PROD & CHEM, AIR PRODUCTS AND CHEMICALS, INC.
Application year: 1995
Main classification: B01D53
IPC codes: C01B13, B01D53