the components of a gas mixture are separated by pressure swing adsorption in a plurality of adsorption vessels . in the first step of the half - cycle adsorption takes place in a first bed while the second bed undergoes countercurrent desorption . at the end of the first step the first bed is vented countercurrently and the first and second beds undergo , as a first bed equalization step , outlet - to - outlet equalization or outlet to both inlet and outlet equalization . the vent step may precede or be concurrent with the first bed equalization step . in a second equalization step the beds simultaneously undergo inlet - to - inlet and outlet - to - outlet equalization . the second bed is then further pressurized with nonadsorbed product gas . the half cycle is then repeated but with the first bed being substituted for the second bed and vice versa .