NOVELTY - Making (m1) a carbon membrane involves: heating polymer precursor, in a chamber to temperature at which pyrolysis byproducts are evolved; and flowing an inert gas through the chamber during the heating step, where the inert gas contains oxygen (less than 40 ppm).    USE - For making carbon membrane which is used in an asymmetric carbon molecule sieve hollow fiber module, in a gas separation apparatus for separating acid gas components from a natural gas stream, for optimizing the CO2/CH4 selectivity of a carbon membrane, for making a carbon membrane having a predetermined degree of CO2 permeability, and for making carbon membrane having a predetermined degree of CO2/CH4 selectivity (claimed).    ADVANTAGE - The oxygen concentration, impacts the overall separation performance of the membranes and allows the membrane performance to be modified to optimize selectivity and permeability. The carbon molecular sieve membranes have optimized gas separation performance properties, such properties being optimized by controlling the concentration of oxygen in the pyrolysis atmosphere.    DETAILED DESCRIPTION - INDEPENDENT CLAIMS are included for the following:    (1) an asymmetric carbon molecule sieve hollow fiber module comprising a sealable enclosure, the enclosure having; several membranes contained in sealable enclosure, at least one of the membrane is produced by process (m1); an inlet for introducing a feed stream comprising natural gas; an outlet for permitting egress of a permeate gas stream; and, another outlet for permitting egress of a retentate gas stream;    (2) separating (m2) acid gas components from a natural gas stream involving: introducing the natural gas stream into asymmetric carbon molecule sieve hollow fiber module at a pressure of at least 20 bar;    (3) reducing (m3) the concentration of acid gases in a natural gas stream involving: heating a membrane in a chamber to at least a temperature at which pyrolysis byproducts are evolved, where membrane is formed of asymmetric hollow polymer fibers; flowing an inert gas through the chamber, where the inert gas contains oxygen (less than 40 ppm); and flowing the natural gas stream through the membrane to produce a retentate stream having a reduced concentration of acid gases;    (4) optimizing (m4) the carbon dioxide (CO2)/methane (CH4) selectivity of a carbon membrane involving forming the carbon membrane by: providing a polymer precursor in the form of asymmetric hollow polymer fibers; heating the precursor in a chamber to at least a temperature at which pyrolysis byproducts are evolved; and, flowing an inert gas through the chamber during the heating step, where the inert gas contains oxygen (less than 10 ppm);    (5) a carbon membrane produced by the process (m4);    (6) making (m5) a carbon membrane having a predetermined degree of carbon dioxide (CO2) permeability involving: a) heating a polymer precursor in a chamber to at least a temperature at which pyrolysis byproducts are evolved; and, b) flowing a gas through the chamber during the heating step, where the concentration of oxygen in the gas is selected to produce a carbon membrane having the predetermined degree of CO2 permeability;    (7) making (m6) a carbon membrane having a predetermined degree of CO2/methane (CH4) selectivity involving: heating a polymer precursor in a chamber to at least a temperature at which pyrolysis byproducts are evolved; and, flowing a gas through the chamber during the heating step, where the concentration of oxygen in the gas is selected to produce a carbon membrane having the predetermined degree of CO2/CH4 selectivity;    (8) a gas separation apparatus, comprising one carbon membrane produced by pyrolyzing a polymer precursor in an atmosphere having a first oxygen concentration, and another carbon membrane produced by pyrolyzing a polymer precursor in an atmosphere having a second oxygen concentration, where the first and second oxygen concentrations are differ by at least 2 ppm oxygen to inert gas;    (9) making (m7) at least two carbon membranes having different predetermined degrees of CO2/CH4 permeability involving: providing a first polymer precursor; heating the first precursor in a first chamber to at least a temperature at which pyrolysis byproducts are evolved; flowing a first gas through the first chamber during the first heating step, the concentration of oxygen in the first gas being selected to produce a first carbon membrane having a first predetermined degree of CO2/CH4 permeability; providing a second polymer precursor; heating the second precursor in a second chamber to at least a temperature at which pyrolysis byproducts are evolved; and flowing a second gas through the second chamber during the second heating step, the concentration of oxygen in the second gas being selected to produce a second carbon membrane having a second predetermined degree of CO2/CH4 permeability, where the concentration of oxygen in the first gas and the second gas differ by at least 2 ppm;    (10) first and second carbon membranes made by the process (m7).