Three corn. membrane modules were used for sepn. of CO2/N-2 mixts. (10, 40 and 70 vol. % CO2 in N-2, gas flow rate 0.05 kmol/h and feed pressure 1.2-7.5 bar). The modified polyimide-based membrane gave the highest efficiency of CO2 recovery. The process was also simulated by using a math. model verified on exptl. data. The model can be used for both process design and optimization.