apparatus of fig2 is operated by supplying inlet air saturated with water and containing ambient quantities of carbon dioxide such as 300 to 600 ppm and hydrocarbons into the psa unit at the inlet 24 the stream may be flowed first through bed 10 via valve 16 with bed 10 being at a relatively high pressure as at the start of the on line period of bed 10 bed 12 has just come off line and is to be equilibrated with bed 14 which is at low pressure and has just been regenerated valves 16 and 34 are therefore open and all the others are presently closed to equilibrate beds 12 and 14 valves 54 and 58 are opened when the pressures are sufficiently equilibrated valve 54 is closed and valve 64 is opened to depressurise bed 12 via silencer 70 at the same time valve 68 is opened with valve 58 remaining open so as to repressurise bed 14 with product gas at a constant and relatively slow rate bed 12 is now regenerated by purging with dry co 2 free nitrogen by closing valve 64 and opening valves 48 and 30 when bed 12 is regenerated bed 10 comes off line with valves 16 and 34 closing regeneration of bed 12.