A modified duplex PSA has been proposed, which could yield sharp separation and has the potential for process intensification. The performances for the original and the modified duplex PSA have been evaluated for two types of systems: the CO2-N-2-13X zeolite with single component adsorption and the N-2-O-2-5A zeolite with competing adsorption. Part 1 of the companion papers presents the simulation studies on the separation of the CO2-N-2 mixture. The purities of both products were in excess of 99 mol % with the modified duplex PSA against 78 mol 96 CO2 and 95 mol % N-2 with the original duplex PSA for the same conditions. The bed sizes and energy requirements for the modified duplex PSA were one-half of those for the original duplex PSA. Part 2 presents studies on the separation of the N-2-O-2 mixture.