tank 11 through valve 8 during the operation of the process each of the adsorption vessels a b and c is generally as shown in fig2 and 3 the vessel indicated by the reference numeral 20 is generally cylindrical in shape but has dished ends 22 and 24 a gas port 26 is formed in dished end 22 and a similar gas port 28 in the dished end 24 the gas ports 26 and 28 are coaxial with the longitudinal axis of the vessel 20 a support grid 30 is located at the bottom of the vessel 20 there is a similar retaining grid 32 located at the top of the vessel 20 both the grids 30 and 32 are perforate to enable gas to flow freely between the ports 26 and 28 the grid 30 supports a bed 34 of adsorbent particles the bed 34 comprises a lower layer 36 of an adsorbent such as a synthetic zeolite 5a or natural zeolite that is able selectively to adsorb nitrogen and carbon dioxide preferentially to oxygen on top of the layer 36 is a layer 38 of adsorbent or desiccant particles for example of alumina which are able to adsorb water from the incoming air the depth of the layer 36 is preferably at least twice the depth of the layer 38 in those steps of the process described with reference to fig1 in which air is fed to the vessel 20 through the port 26 water vapour is first adsorbed from the air in the layer 38 and then other constituents of the air are adsorbed by the layer 36 in preference to oxygen to enable a product gas stream rich in.