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The present invention relates to an asymmetric hollow-fiber gas separation membrane made of a polyimide having a specific repeating unit, an improved tensile elongation at break of 15% or more as a hollow-fiber membrane itself, an oxygen gas permeation rate (P′O2) of 4.0×10−5 cm3(

The present invention relates to an asymmetric hollow-fiber gas separation membrane made of a polyimide having a specific repeating unit, an improved tensile elongation at break of 15% or more as a hollow-fiber membrane itself, an oxygen gas permeation rate (P′O2) of 4.0×10−5 cm3(STP)/cm2·sec·cmHg or more and a gas ratio of permeation rate of oxygen to nitrogen (P′O2/P′N2) of 4 or more that are measured at 50° C., a gas separation method and a gas separation membrane module using the asymmetric hollow-fiber gas separation membrane. In addition, the present invention relates to an asymmetric hollow-fiber gas separation membrane obtained by heat-treating the asymmetric hollow-fiber gas separation membrane at a maximum temperature of from 350 to 450° C. The asymmetric hollow-fiber gas separation membrane has sufficient mechanical strength even after the heat-treatment at a maximum temperature of from 350 to 450° C.

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What is claimed is: 1. An asymmetric hollow-fiber gas separation membrane comprising a polyimide, and having an improved tensile elongation at break of 15% or more as a hollow-fiber membrane itself, the polyimide comprising a repeating unit substantially represented by the following general formula

What is claimed is: 1. An asymmetric hollow-fiber gas separation membrane comprising a polyimide, and having an improved tensile elongation at break of 15% or more as a hollow-fiber membrane itself, the polyimide comprising a repeating unit substantially represented by the following general formula (1): wherein, 20 to 80 mol % of A is a tetravalent unit based on the biphenyl structure represented by the formula (2), 20 to 80 mol % of A is a tetravalent unit based on the diphenylhexafluoropropane represented by the formula (3), 0 to 30 mol % of A is a tetravalent unit based on the phenyl structure represented by the formula (4), 30 to 70 mol % of R is a divalent unit represented by the formula (5) and/or formula (6), wherein, in formula (5), each of R1 and R2 is a hydrogen atom or organic group, and n is any number of 0, 1 and 2, and in formula (6), each of R1 and R2 is a hydrogen atom or organic group, and X is —CH2— or —CO—, and 30 to 70 mol % of R is a divalent unit based on the biphenyl structure represented by the formula (7), wherein, in formula (7), Y is a chlorine or bromine atom, and n is any number of 1 to 3. 2. The asymmetric hollow-fiber gas separation membrane according to claim 1, having an oxygen gas permeation rate P′O2 of 4.0×10−5 cm3(STP)/cm2·sec·cmHg or more and a gas ratio of permeation rate of oxygen to nitrogen P′O2/P′N2 of 4 or more that are measured at 50° C. 3. A gas separation method comprising: contacting a mixed gas containing oxygen gas and nitrogen gas to a gas supply side of the asymmetric hollow-fiber gas separation membrane according to claim 1, selectively permeating the oxygen gas to a gas permeation side of the asymmetric hollow-fiber gas separation membrane, and separating and recovering an oxygen-enriched mixed gas and a nitrogen-enriched mixed gas from the mixed gas. 4. The gas separation method according to claim 3, wherein a bore-side of the asymmetric hollow-fiber gas separation membrane is selected as the gas supply side and the outside of the asymmetric hollow-fiber gas separation membrane is selected as the gas permeation side. 5. A hollow-fiber gas separation membrane module comprising: a hollow-fiber element essentially comprising a hollow-fiber bundle formed by binding a number of the asymmetric hollow-fiber gas separation membranes according to claim 1, the hollow-fiber element accommodated in a vessel in such a manner that a bore-side space of the asymmetric hollow fiber gas separation membrane is isolated from an outside room of the membrane, and a tube plate embedding and fixing at least one end of the hollow-fiber bundle while each end is opened, the vessel having an inlet for mixed gas, an outlet for non-permeated gas, and an outlet for permeated gas. 6. An asymmetric hollow-fiber gas separation membrane obtained by heating the asymmetric hollow-fiber gas separation membrane according to claim 1 at a maximum temperature in the range of 350° C. to 450° C. 7. A gas separation method comprising: contacting a mixed gas enriched with an organic compound vapor to a gas supply side of the asymmetric hollow-fiber gas separation membrane according to claim 6, permeating selectively the organic compound vapor to a gas permeation side of the asymmetric hollow-fiber gas separation membrane, and separating and recovering the mixed gas enriched with an organic compound vapor. 8. A hollow-fiber gas separation membrane module comprising: a hollow-fiber element essentially comprising a hollow-fiber bundle formed by binding a number of the asymmetric hollow-fiber gas separation membranes according to claim 6, the hollow-fiber element accommodated in a vessel in such a manner that a bore-side space of the asymmetric hollow-fiber gas separation membranes is isolated from an outside room of the membrane, and a tube plate embedding and fixing at least one end of the hollow-fiber bundle while each end is opened, the vessel having an inlet for mixed gas, an outlet for non-permeated gas, and an outlet for permeated gas. 9. A gas separation method comprising: contacting a mixed gas containing oxygen gas and nitrogen gas to a gas supply side of the asymmetric hollow-fiber gas separation membrane according to claim 2, selectively permeating the oxygen gas to a gas permeation side of the asymmetric hollow-fiber gas separation membrane, and separating and recovering an oxygen-enriched mixed gas and a nitrogen-enriched mixed gas from the mixed gas. 10. The gas separation method according to claim 9, wherein a bore-side of the asymmetric hollow-fiber gas separation membrane is selected as the gas supply side and the outside of the asymmetric hollow-fiber gas separation membrane is selected as the gas permeation side. 11. A hollow-fiber gas separation membrane module comprising: a hollow-fiber element essentially comprising a hollow-fiber bundle formed by binding a number of the asymmetric hollow-fiber gas separation membranes according to claim 2, the hollow-fiber element accommodated in a vessel in such a manner that a bore-side space of the asymmetric hollow-fiber gas separation membranes is isolated from an outside room of the membranes, and a tube plate embedding and fixing at least one end of the hollow-fiber bundle while each end is opened, the vessel having an inlet for mixed gas, an outlet for non-permeated gas, and an outlet for permeated gas. 12. An asymmetric hollow-fiber gas separation membrane obtained by heating the asymmetric hollow-fiber gas separation membrane according to claim 2 at a maximum temperature in the range of 350° C. to 450° C. 13. A gas separation method comprising: contacting a mixed gas containing an organic compound vapor to the gas supply side of the asymmetric hollow-fiber gas separation membrane according to claim 12, permeating selectively the organic compound vapor to a gas permeation side of the asymmetric hollow-fiber gas separation membrane, and separating and recovering the mixed gas enriched with an organic compound vapor. 14. A hollow-fiber gas separation membrane module comprising: a hollow-fiber element essentially comprising a hollow-fiber bundle formed by binding a number of the asymmetric hollow-fiber gas separation membranes according to claim 12, the hollow-fiber element accommodated in a vessel in such a manner that a bore-side space of the asymmetric hollow-fiber gas separation membranes is isolated from the outside room thereof, and a tube plate embedding and fixing at least one end of the hollow-fiber bundle while each end is opened, the vessel having an inlet for mixed gas, an outlet for non-permeated gas, and an outlet for permeated gas.