Production of oxygen by ion transport membranes with steam utilization

Abstract: Oxygen is recovered from a heated, compressed, oxygen-containing feed gas by selective permeation of oxygen through an ion transport membrane separation system and the hot, pressurized non-permeate gas is passed through an expansion turbine to recover power for compressing the feed gas and optionally generating electric power. Membrane performance and process heat recovery are enhanced by the addition of water to selected process streams. Steam raised by process heat is used to sweep the permeate side of the membrane to increase the oxygen permeation rate. The injection of partially or fully vaporized water into the expansion turbine inlet gas recovers process heat and increases mass flow to the turbine.

Description: FIELD OF THE INVENTION The invention pertains to the recovery of oxygen by high temperature ion transport membranes, and in particular to the introduction of water into processes using such membranes to improve oxygen recovery and heat utilization. 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, oxygen-enriched combustion, and in particular integrated gasification combined cycle power generation. 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 non-porous 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 such 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. In the recovery of oxygen from air at high temperatures typically 700°C to 1100°C using ion transport membranes, a significant amount of heat energy is available in the membrane permeate and non-permeate streams. The effective use of this energy in the overall operation of a mixed conductor membrane system is necessary if the system is to be competitive with conventional cryogenic technology for large scale oxygen production. Energy recovery and effective utilization thereof is possible by the integration of compressors, combustors, hot gas turbines, steam turbines, and heat exchangers with the mixed conductor membrane module. U.S. Patent 4,545,787 discloses the production of oxygen and net power in the integrated operation of a mixed conductor ceramic membrane. Air is compressed, heated, and passed through a membrane separator to produce an oxygen permeate and an oxygen-containing non-permeate stream. The non-permeate stream is combusted with a fuel and the hot combustion gases are expanded in a hot gas turbine. The turbine provides shaft power for the compressor and drives a generator for export of electricity, and turbine exhaust is optionally used to cogenerate steam or to preheat the compressed air membrane feed. Alternately, the membrane is placed downstream of the combustion step. U.S. Patent 5,035,727 describes the recovery of oxygen by a solid electrolyte membrane in conjunction with an externally-fired gas turbine in which compressed air is heated indirectly and passed through the membrane module. Non-permeate gas is expanded through a hot gas turbine, the turbine exhaust is heated by direct combustion, and the combustion products provide heat indirectly to the membrane feed. Steam is recovered from the waste heat after heat exchange with the membrane feed. U.S. Patent 5,118,395 describes the recovery of oxygen from gas turbine exhaust utilizing a solid electrolyte membrane with the coproduction of electric power and steam. A supplemental burner heats the turbine exhaust prior to the membrane, and steam is generated by the membrane non-permeate stream. Related U.S. Patent 5,174,866 discloses a similar system in which intermediate turbine exhaust is passed through the membrane and the membrane non-permeate stream is further expanded through another turbine stage. In both patents, turbine shaft power is used to drive the air compressor and an electric generator. The report by J. D. Wright and R. J. Copeland identified above discloses at p. 55 a gas turbine-driven ceramic membrane system in which air is compressed, heated indirectly in a fired heater, and passed through the membrane to yield oxygen and non-permeate gas. The non-permeate gas is combusted with natural gas in the fired heater and the combustion products are expanded through a hot gas turbine to drive the compressor and generate electric power. Heating of the air feed to the membrane and the combustion of fuel and non-permeate gas prior to the turbine thus are accomplished in a single integrated combustion chamber. U.S. Patent 5,245,110 (equivalent to PCT International Publication No. WO 93/06041) discloses the integration of a gas turbine with an oxygen-selective membrane system. The permeate side of the membrane is swept with air to yield an enriched air product containing about 35 vol% oxygen. The enriched air product is used in a hydrocarbon reformer or gasifier process, and tail gas from the reformer or gasifier is introduced into the gas turbine combustor to balance the flow of hot gas to the turbine. The nitrogen from the permeate and membrane sweep air replaces the mass lost when oxygen is consumed in the reformer or gasifier process, which maintains the turbine in a desired mass and thermal balance. 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. Membrane non-permeate is combusted with gas from the gasifier and passed to the gas turbine cogeneration system. A combined cycle power generation system is a highly efficient system which utilizes a gas turbine to drive an electric generator, wherein-heat is recovered from the turbine exhaust as steam which drives an additional electric generator. A description of typical combined cycle power generation systems is given in The Chemical Engineer , 28 January 1993, pp. 17-20. The compressor, combustor, and expansion turbine are carefully designed and integrated to maximize the efficiency of each component and thus the efficiency of the integrated system. Preferably these systems are operated at steady-state design conditions, since significant deviations from these conditions will adversely affect system efficiency. European Patent EP 0 739 649 (published 30 October 1996; Priority: 25 April 1994) discloses a method for the recovery of oxygen from an oxygen-containing gas comprising passing a compressed and heated oxygen-containing gas into a membrane separation zone to yield a hot oxygen permeate product and a hot oxygen-depleted non-permeate gas. The document further discloses to pass the non-permeate gas through an expansion turbine to generate shaft power and withdrawing exhaust gas therefrom. In addition the addition of water or steam to said non-permeate gas before the passage thereof through the expansion turbine is described. 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. In many situations, the recovery of oxygen and excess energy from an ion transport membrane system must be maximized in order to minimize operating costs. In other situations, low capital cost will be of primary importance and operating cost of less importance. The invention disclosed below and described in the following claims addresses these latter situations with a method for producing oxygen by an ion transport membrane system in which water is introduced into the system to improve oxygen recovery and heat utilization. SUMMARY OF THE INVENTION The present invention is a method for the recovery of oxygen from an oxygen-containing gas by compressing a stream of oxygen-containing gas and heating the entire compressed steam to yield a hot pressurized feed, and passing the hot pressurized feed into a membrane separation zone comprising one or more oxygen-selective ion transport membranes which divide the zone into a feed side and a permeate side. The hot pressurized feed flows through the feed side, oxygen permeate gas is produced in the permeate side, and a hot oxygen-depleted non-permeate gas is withdrawn form the feed side of the membrane zone. Water is introduced into the hot oxygen-depleted non-permeate gas, no heat being added to the gas either before or after the water addition. The resulting water-containing oxygen-depleted non-permeate gas is passed through an expansion turbine to generate shaft power. Exhaust gas is withdrawn from the expansion turbine. Preferably the oxygen-containing feed gas is air, and typically the oxygen permeate product is high purity oxygen containing at least 98 vol% oxygen. Water can be introduced as a liquid, a vapor, or a mixture of liquid and vapor. At least a portion of the water can be preheated by indirect heat exchange with the hot oxygen permeate product. Optionally at least a portion of the water is preheated by indirect heat exchange with the exhaust gas from the expansion turbine. The oxygen-containing gas is heated following compression by combusting the gas with a fuel. Shaft power of the expansion turbine optionally is used at least in part to drive an electric power generator. BRIEF DESCRIPTION OF THE DRAWINGS Fig. 1 is a schematic flowsheet of an embodiment of the present invention. DETAILED DESCRIPTION OF THE INVENTION The present invention utilizes a solid ceramic ion transport membrane integrated with compression, heating, and energy recovery steps to produce oxygen or an oxygen-steam mixture from an oxygen-containing gas, preferably air. The membrane operates by a mechanism in which 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 oxygen ions recombine to form oxygen gas and free electrons. 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. The term pressure-driven means that oxygen ions move through the membrane in the direction of decreasing partial pressure. 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 mechanically perfect membrane of either type operating without gas leakage is infinitely selective for oxygen; in practical applications a high-purity oxygen product containing at least 98 vol% oxygen is achievable. The key feature of invention as described herein is the selective introduction of water into the system to improve energy efficiency, reduce equipment requirements, and improve the performance of the ion transport membrane. Referring to Fig. 1, oxygen-containing gas 1, preferably air, is compressed in compressor 101 to a pressure between 3.45 and 34.5 bar (50 and 500 psia), preferably 5.5-20.7 bar (80 to 300 psia). Compressor 101 is a centrifugal, axial, screw, or reciprocating compressor, optionally multistaged, and optionally intercooled. When operating without intercooling in an adiabatic mode, compressed feed 3 will be at a temperature of 182-593°C (360 to 1,100°F); when operated with intercooling in an isothermal mode, compressed feed 3 will be at 65.6-149°C (150 to 300°F). Compressed feed 3 passes into direct-fired combustor 105, which is preferably gas-fired and utilizes fuel gas 9 which is natural gas, synthesis gas comprising hydrogen and carbon monoxide, refinery fuel gas containing mixed hydrocarbons, or another combustible gas mixture. Combustor 105 may be of the type known and used in the gas turbine art, or any other type of combustor, provided that gaseous fuel can be combusted with excess air to yield an acceptable feed for membrane separation zone 107 described below. Combustor 105 is operated with sufficient excess air such that hot combustion stream 11 contains about 10-20 vol% oxygen at a temperature of 427-1093°C (800 to 2,000°F), preferably 538-871°C (1,000 to 1,600°F). Stream 11 passes through the feed side of membrane separation zone 107 comprising membrane 108, preferably a mixed conductor membrane, wherein oxygen diffuses through the membrane driven by an oxygen partial pressure differential in the range of 0.14-5.5 bar (2 to 80 psi), and hot high purity oxygen stream 13 typically containing at least 98 vol% oxygen is withdrawn therefrom at 0.14-2.1 bar (2 to 30 psia). Hot non-permeate stream 15 is withdrawn at near feed pressure and contains 6 to 18 vol% oxygen. Membrane 108 operates in the temperature range of 427-1093°C (800 to 2,000°F), preferably 538-871°C (1,000 to 1,600°F). Membrane separation zone 107 typically is sized and operated such that up to about 90% of the oxygen in membrane feed 11 is recovered as product 13. Alternatively, ion transport membrane 108 can be of the solid electrolyte type as described earlier which is driven by a voltage differential across the membrane in which electrons are conducted through an external circuit with porous electrodes attached to the surfaces of the membrane. In this mode of operation the oxygen permeate product can be recovered at or above the feed pressure. Ion transport membrane 108 is typically a solid ceramic assembly in the form of tubes, sheets, or a monolithic honeycomb. The membrane divides membrane separation zone 107 into a feed side and a permeate side. Typical compositions of the active membrane material are given in representative articles by Y. Teraoka et al in Chemistry Letters, 1985, pp.1743-1746 and by H. Iwahara et al in Advances in Ceramics, Vol. 24: Science and Technology of Zirconia III, pp. 907-914, or in the article by J. D. Wright and R. J. Copeland earlier cited. Any solid ceramic membrane material which selectively permeates oxygen in the form of oxygen ions, of either the mixed conductor or solid electrolyte type described above, can be utilized in the present invention. Preferred membranes 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 by the mechanism discussed earlier. 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. 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, which is incorporated herein by reference. 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 micrometers or less. The yttria-stabilized zirconia layer is deposited at a temperature of 700-1100°C at a pressure of 0.133-101.3 kPa (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. Referring again to Fig. 1, hot non-permeate 15, now containing 6 to 18 vol% oxygen, flows into water introduction zone 109. Water 17, which can be a liquid, vapor, or a mixture of liquid and vapor, is introduced into non-permeate stream 15 in water introduction zone 109. In this specification, the term water generically defines the compound H 2 O in any phase or coexisting phases. If water 17 is vapor (i.e. steam), water introduction zone 109 is a simple gas mixing zone. If water 17 contains liquid, water introduction zone 109 is a direct-contact vaporizer designed to eliminate liquid droplets which could impinge on downstream equipment before completely vaporizing. The combined stream 21 in which all water is present as vapor typically at 1000 to 1400°F is passed through hot gas expansion turbine 111 to generate shaft power, and exhaust 23 is withdrawn therefrom at 93-427°C (200 to 800°F) and a pressure up to 1.38 bar (20 psia). Expansion turbine 111 is of the type typically used for hot gas mixtures containing water vapor, for example such as turbines manufactured by Dresser-Rand Corporation for use in the nitric acid industry. Preferably, turbine 111 can be mechanically linked with compressor 101 by shaft 113 whereby the required compression power is supplied by turbine 111. Optionally, a portion of the shaft power of turbine 111 via shaft 115 is used to produce electricity in generator 117. Optionally, turbine exhaust 23 is utilized to preheat boiler feed water 25 and/or 27 in heat exchange zone 112 to obtain preheated water 29 and/or 31 as liquid, vapor, or a mixture of liquid and vapor. The term boiler feed water as used in this specification defines water which has been treated by means known in the art for use in generating steam for steam turbines and other applications requiring high purity steam. Boiler feed water is typically produced by demineralization and ion exchange of ordinary utility water from wells or surface sources. Preheated water 29 optionally provides at least a portion of water 17 which is used as earlier described. Optionally and preferably, oxygen permeate 45 is further cooled against coolant stream 47 in heat exchange zone 127 and withdrawn through oxygen blower 129 to provide final oxygen product 49 at or above atmospheric pressure. In this preferred option, the permeate side of membrane separation zone 107 operates at a pressure in the range of 2 to 30 psia which increases the oxygen partial pressure driving force across membrane 108 and in turn reduces the membrane area required for a given oxygen production rate. Blower 129 optionally is driven via shaft 114 using a portion of the work recovered in expansion turbine 111. Preheating of water 17 prior to introduction into the non-permeate stream can be accomplished by several alternative methods. A preferred method is to preheat boiler feed water 39 against hot high purity oxygen stream 13 in heat exchange zone 123. Heated water 41 provides at least a portion of water 17 for introduction into the non-permeate stream prior to expansion turbine 111. Optionally, preheated water 29 is combined with preheated water 41 to provide preheated water 17. In another optional mode, preheated water 31 is combined with preheated water 41 and the combined stream is further heated against hot oxygen permeate 13 in heat exchange zone 125 to yield heated water 43 for use as earlier described. The essential feature of the invention illustrated in the embodiment of Fig. 1 is the addition of water 17 to hot non-permeate stream 15 and the introduction of the resulting water vapor-containing stream into expansion turbine 111. This feature allows the cooling of expansion turbine 111 inlet stream 21 which in turn allows the use of a less complex turbine which operates at lower temperatures than a typical gas turbine. In the present invention, expansion turbine 111 operates at 871-816°C (700 to 1500°F), preferably 538-760°C (1000 to 1400°F); a conventional gas turbine by comparison operates at 871-1538°C (1600 to 2800°F). EXAMPLE 1 A heat and material balance was carried out to illustrate the invention of Fig. 1. Air 1 is compressed in compressor 101, which is a four-stage intercooled compressor having an isentropic efficiency of 82% per stage. The compressor is driven by expansion turbine 111 which has an isentropic efficiency of 83%. In this operating mode, the inlet operating temperature of the turbine is limited to 677°C (1250°F). In this mode, heat exchange zones 103, 112, 125, and 127 are not used. Oxygen permeate 13 is cooled in heat exchange zone 123 against boiler feed water 39 to yield water 43 which provides water 17 which is vaporized in vaporization zone 109. Oxygen 45 is withdrawn at a pressure of 0.47 bar (6.8 psia) by vacuum blower 129 which delivers oxygen product 49 at 1.14 bar (16.6 psia). The process is operated to yield 1016.05 kg (1.0 ton)/day of oxygen 49 from an air feed 3 of 9652.5 kg (9.5 ton)/day for an oxygen recovery of 45%. Membrane separation zone 107 is operated at 1562°F (850°C) and an inlet pressure of 14.8 bar (214.7 psia). Electric generator 117 is driven by expansion turbine 111 to generate 55.8 mJ (15.5 kW) of electric power. A stream summary for the heat and material balance is given in Table 1. The present invention thus provides a simple high temperature method to produce oxygen or an oxygen-steam mixture by means of an ion transport membrane separation system in which steam is used to improve process heat recovery at low capital cost. The addition of internally-heated water to the non-permeate gas prior to the expansion turbine recovers thermal energy from the process, allows independent control of the expansion turbine inlet, and provides operating flexibility by controlling mass flow to the expansion turbine. The embodiments of the invention exhibit the additional feature that many of the heat exchange zones are essentially simple steam boilers; the use of high temperature gas-gas heat exchangers is minimized or is not required. Direct injection of water converts the heat contained in the process streams into recoverable pressure energy simply and at low capital cost, and this energy is readily recovered in a single low-cost expansion turbine. This turbine operates at lower temperatures than conventional gas turbines, which can reduce maintenance requirements.

Claims: A method for the recovery of oxygen from an oxygen-containing gas which comprises: a. compressing a stream of oxygen-containing gas and heating the entire compressed stream to yield a hot pressurized feed; b. passing said hot pressurized feed into a membrane separation zone comprising one or more oxygen-selective ion transport membranes which divide said zone into a feed side and a permeate side, wherein said hot pressurized feed flows through said feed side and oxygen permeate gas is produced in said permeate side, and withdrawing from said zone a hot oxygen-depleted non-permeate gas from said feed side and a hot oxygen permeate product from said permeate side; c. adding water to said hot oxygen-depleted non-permeate gas, without any addition of heat to said hot oxygen-depleted non-permeate gas, to form a combined stream in which all water is present as a vapor; and d. passing the combined stream, without any addition of heat thereto, through an expansion turbine to generate shaft power and withdrawing exhaust gas therefrom. The method of claim 1 wherein said oxygen-containing gas is air. The method of Claim 1 wherein said oxygen permeate product is high purity oxygen containing at least 98 vol% oxygen. The method of Claim 1 wherein said water is added as a liquid, a vapor, or a mixture of liquid and vapor. The method of Claim 4 wherein at least a portion of said water is preheated by indirect heat exchange with said hot oxygen permeate product. The method of Claim 4 wherein at least a portion of said water is preheated by indirect heat exchange with said exhaust gas from said expansion turbine. The method of Claim 1 wherein said oxygen-containing gas is heated following compression by combusting said gas with a fuel in a direct-fired combustor to produce a hot, oxygen-containing combustion product which provides said hot pressurized feed. The method of Claim 7 wherein at least a portion of said water is heated by indirect heat exchange with said hot oxygen-containing combustion product within said direct-fired combustor. The method of Claim 1 wherein said oxygen-containing gas is heated following compression by indirect heat exchange with a hot combustion product which is obtained by combusting said hot oxygen-depleted non-permeate gas with a fuel in a direct-fired combustor. The method of Claim 9 wherein at least a portion of said water is heated by indirect heat exchange with said hot oxygen-containing combustion product within said direct-fired combustor. The method of Claim 9 wherein said water is injected into said hot combustion product prior to said indirect heat exchange. The method of Claim 1 wherein the shaft power of said expansion turbine is used at least in part to drive an electric power generator.

Inventors: AGRAWAL RAKESH, BRENGEL DAVID DOUGLAS, FOSTER EDWARD PAUL, KANG DOOHEE, RUSSEK STEVEN LEE, AGRAWAL, RAKESH, BRENGEL, DAVID DOUGLAS, FOSTER, EDWARD PAUL, KANG, DOOHEE, RUSSEK, STEVEN LEE
Applicants: AIR PROD & CHEM, AIR PRODUCTS AND CHEMICALS, INC.
Application year: 1996
Main classification: B01D53
IPC codes: B01D53, C01B13