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Method of operating an oxygen-permeable mixed conducting membrane having an oxidant feed side and a permeate side, which method comprises controlling the differential strain between the oxidant feed side and the permeate side by varying either or both of the oxygen partial pressure and the total gas

Method of operating an oxygen-permeable mixed conducting membrane having an oxidant feed side and a permeate side, which method comprises controlling the differential strain between the oxidant feed side and the permeate side by varying either or both of the oxygen partial pressure and the total gas pressure on either or both of the oxidant feed side and the permeate side of the membrane while changing the temperature of the membrane from a first temperature to a second temperature.

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The invention claimed is: 1. A method of operating an oxygen-permeable mixed conducting membrane having an oxidant feed side and a permeate side, which method comprises controlling the differential strain between the oxidant feed side and the permeate side by varying the oxygen partial pressure on

The invention claimed is: 1. A method of operating an oxygen-permeable mixed conducting membrane having an oxidant feed side and a permeate side, which method comprises controlling the differential strain between the oxidant feed side and the permeate side by varying the oxygen partial pressure on either or both of the oxidant feed side and the permeate side of the membrane while the temperature of the membrane changes from a first temperature to a second temperature. 2. The method of claim 1 wherein the first temperature is greater than the second temperature. 3. The method of claim 1 wherein the first temperature is less than the second temperature. 4. The method of claim 1 wherein the oxygen partial pressure is controlled on either or both of the oxidant feed side and the permeate side of the membrane by varying either or both of the oxygen mole fraction and the total gas pressure on either or both of the oxidant feed side and the permeate side of the membrane, respectively. 5. The method of claim 1 wherein the oxygen partial pressure on the permeate side of the membrane is controlled by (a) contacting the permeate side of the membrane with a gaseous mixture comprising one or more reducing gases selected from CO, H2, and CH4 and one or more oxygen-containing gases selected from CO2 and H2O; and (b) varying the composition of the gaseous mixture and optionally the total gas pressure on the permeate side of the membrane. 6. The method of claim 5 wherein the mixed conducting metal oxide material has the general stoichiometric composition (Ln1-xAx)w(B1-yB'y)O 3-δ, wherein Ln represents one or more elements selected from La, the D block lanthanides of the IUPAC periodic table, and Y; wherein A represents one or more elements selected from Mg, Ca, Sr and Ba; wherein B and B' each represent one or more elements selected from Sc, Ti, V, Mn, Fe, Co, Ni, Cu, Cr, Al, Zr, Mg, and Ga; wherein 0≦x≦1, 0≦y≦1, and 0.95xCa1-x)wFeO3-δ wherein 1.0>x>0.5, 1.1≧w≧1.0, and δ is a number which renders the composition charge neutral. 8. The method of claim 4 wherein the mixed conducting metal oxide material has the general stoichiometric composition (LaxSr1-x)wCoO3-δ wherein 1.0>x>0.1, 1.05≧w>0.95, and δ is a number which renders the composition charge neutral. 9. The method of claim 8 wherein the mixed conducting metal oxide material has the general stoichiometric composition (La0.4Sr0.6)wCoO3-δ wherein 1.05≧w>0.95 and δ is a number which renders the composition charge neutral. 10. A method of operating an oxygen-permeable mixed conducting membrane having an oxidant feed side and a permeate side, wherein the method comprises (a) heating the membrane to a first temperature and contacting the feed side with a dioxygen-containing gas; (b) determining the oxygen partial pressure on the feed and permeate sides of the membrane; (c) determining the differential strain between the feed side and the permeate side of the membrane at the first temperature; and (d) maintaining the differential strain between the feed and permeate sides during cooling at a value substantially equal to the differential strain determined in (c) or at a value or values within a selected range of values while cooling the membrane. 11. The method of claim 10 wherein the differential strain is maintained by varying the oxygen partial pressure on either or both of the oxidant feed side and the permeate side of the membrane. 12. The method of claim 11 wherein the oxygen partial pressure on either or both of the oxidant feed side and the permeate side of the membrane is controlled by varying either or both of the oxygen mole fraction and the total pressure on either or both of the oxidant feed side and the permeate side of the membrane, respectively. 13. The method of claim 10 wherein the oxygen partial pressure on the permeate side of the membrane is controlled by (a) contacting the permeate side of the membrane with a gaseous mixture comprising one or more reducing gases selected from CO, H2, and CH4 and one or more oxygen-containing gases selected from CO2 and H2O; and (b) varying the composition of the gaseous mixture and optionally the total gas pressure on the permeate side of the membrane. 14. The method of claim 10 wherein the mixed conducting metal oxide material has the general stoichiometric composition (Ln1-xAx)w(B1-yB'y)O 3-δ, wherein Ln represents one or more elements selected from La, the D block lanthanides of the IUPAC periodic table, and Y; wherein A represents one or more elements selected from Mg, Ca, Sr and Ba; wherein B and B' each represent one or more elements selected from Sc, Ti, V, Mn, Fe, Co, Ni, Cu, Cr, Al, Zr, Mg, and Ga; wherein 0≧x≧1, 0≧y≧1, and 0.95xCa1-x)wFeO3-δ wherein 1.0>x>0.5, 1.1≧w≧1.0, and δ is a number which renders the composition charge neutral. 16. The method of claim 14 wherein the mixed conducting metal oxide material has the general stoichiometric composition (LaxSr1-x)wCoO3-δ wherein 1.0>x>0.1, 1.05≧w≧0.95, and δ is a number which renders the composition charge neutral. 17. The method of claim 16 wherein the mixed conducting metal oxide material has the general stoichiometric composition (La0.4Sr0.6)wCoO3-δ wherein 1.05≧w>0.95 and δ is a number which renders the composition charge neutral. 18. A method of operating a mixed conducting membrane oxygen recovery system, the method comprising (a) providing at least one membrane module comprising a membrane made of mixed conducting metal oxide material, wherein the membrane module has an oxidant feed side and a permeate side; (b) heating the membrane and membrane module to a first temperature, contacting the oxidant feed side with an oxygen-containing gas, and withdrawing an oxygen-enriched gas from the permeate side; and (c) controlling the differential strain between the oxidant feed side and the permeate side by varying the oxygen partial pressure on either or both of the oxidant feed side and the permeate side of the membrane module while cooling the membrane and the membrane module. 19. The method of claim 18 wherein the mixed conducting metal oxide material has the general stoichiometric composition (Ln1-xAx)w(B1-yB'y)O 3-δ, wherein Ln represents one or more elements selected from La, the D block lanthanides of the IUPAC periodic table, and Y; wherein A represents one or more elements selected from Mg, Ca, Sr and Ba; wherein B and B' each represent one or more elements selected from Sc, Ti, V, Mn, Fe, Co, Ni, Cu, Cr, Al, Zr and Ga; wherein 0≦x≦1, 0≦y≦1, and 0.95xSr1-x)wCoO3-δ wherein 1.0>x>0.1, 1.05≧w>0.95, and δ is a number which renders the composition charge neutral. 21. The method of claim 20 wherein the mixed conducting metal oxide material has the general stoichiometric composition (La0.4Sr0.6)wCoO3-δ wherein 1.05≧w>0.95 and δ is a number which renders the composition charge neutral. 22. The method of claim 18 wherein the oxygen partial pressure on either or both of the oxidant feed side and the permeate side of the membrane is controlled by varying either or both of the oxygen mole fraction and the total pressure on either or both of the oxidant feed side and the permeate side of the membrane, respectively. 23. A method of operating a mixed conducting membrane hydrocarbon oxidation system, which method comprises (a) providing at least one membrane module comprising a membrane made of mixed conducting metal oxide material, wherein the membrane module has an oxidant feed side and a permeate side; (b) heating the membrane and membrane module to a first temperature, introducing an oxygen-containing gas into the oxidant feed side of the membrane module, introducing a hydrocarbon-containing gas into the permeate side of the membrane module, and withdrawing a hydrocarbon oxidation product from the permeate side of the membrane module; and (c) controlling the differential strain between the oxidant feed side and the permeate side by varying the oxygen partial pressure on either or both of the oxidant feed side and the permeate side of the membrane while cooling the membrane and membrane module. 24. The method of claim 23 wherein the hydrocarbon-containing gas comprises methane and the hydrocarbon oxidation product comprises hydrogen and carbon monoxide. 25. The method of claim 23 wherein the differential strain is maintained by varying either or both of the oxygen mole fraction and the total gas pressure on either or both of the oxidant feed side and the permeate side of the membrane. 26. The method of claim 25 wherein the oxygen partial pressure on the oxidant feed side is controlled by varying the oxygen mole fraction on the oxidant feed side. 27. The method of claim 25 wherein the oxygen partial pressure on the permeate side of the membrane is controlled by (a) contacting the permeate side of the membrane with a gaseous mixture comprising one or more reducing gases selected from CO, H2, and CH4 and one or more oxygen-containing gases selected from CO2 and H2O; and (b) varying the composition of the gaseous mixture and optionally the total gas pressure on the permeate side of the membrane. 28. The method of claim 23 wherein the mixed conducting metal oxide material has the general stoichiometric composition (Ln1-xAx)w(B1-yB'y)O 3-δ, wherein Ln represents one or more elements selected from La, the D block lanthanides of the IUPAC periodic table, and Y; wherein A represents one or more elements selected from Mg, Ca, Sr and Ba; wherein B and B' each represent one or more elements selected from Sc, Ti, V, Mn, Fe, Co, Ni, Cu, Cr, Al, Zr, Mg, and Ga; wherein 0≦x≦1, 0≦y≦1, and 0.95xCa1-x)wFeO3-δ wherein 1.0>x>0.5, 1.1≧w≧1.0, and δ is a number which renders the composition charge neutral. 30. A mixed conducting membrane system comprising (a) a membrane module having a membrane comprising mixed conducting metal oxide material, wherein the membrane module has an oxidant feed side and a permeate side; (b) means for heating and cooling the oxidant feed side and the permeate side of the membrane module; (c) means for changing the oxygen partial pressure on either or both of the oxidant feed side and the permeate side of the membrane module while heating and cooling the oxidant feed side and the permeate side of the membrane module. 31. The system of claim 30 wherein the means for controlling the oxygen partial pressure on either or both of the oxidant feed side and the permeate side of the membrane module comprises either or both of (1) means for varying the oxygen mole fraction and (2) means for varying the total gas pressure. 32. The system of claim 31 wherein the means for controlling the oxygen partial pressure on the permeate side of the membrane module comprises (1) means for contacting the permeate side of the membrane with a gaseous mixture comprising one or more reducing gases selected from CO, H2, and CH4 and one or more oxygen-containing gases selected from CO2 and H2O; and (2) means for varying the composition of the gaseous mixture and optionally the total gas pressure on the permeate side of the membrane. 33. The system of claim 30 wherein the mixed conducting metal oxide material has the general stoichiometric composition (Ln1-xAx)w(B1-yB'y)O 3-δ, wherein Ln represents one or more elements selected from La, the D block lanthanides of the IUPAC periodic table, and Y; wherein A represents one or more elements selected from Mg, Ca, Sr and Ba; wherein B and B' each represent one or more elements selected from Sc, Ti, V, Mn, Fe, Co, Ni, Cu, Cr, Al, Zr, Mg, and Ga; wherein 0≦x≦1, 0≦y≦1, and 0.95xCa1-x)wFeO3-δ wherein 1.0>x>0.5, 1.1≧w≧1.0, and δ is a number which renders the composition charge neutral. 35. The system of claim 33 wherein the mixed conducting metal oxide material has the general stoichiometric composition (LaxSr1-x)wCoO3-δ wherein 1.0>x>0.1, 1.05≧w>0.95, and δ is a number which renders the composition charge neutral. 36. The system of claim 35 wherein the mixed conducting metal oxide material has the general stoichiometric composition (La0.4Sr0.6)wCoO3-δ wherein 1.05≧w>0.95 and δ is a number which renders the composition charge neutral.