Abstract: An apparatus for purifying feed air by adsorption of water vapor and carbon dioxide therefrom includes a purification vessel in which are defined vertically arrange chambers and another purification vessel in which are defined vertically arranged chambers. None of the chambers is in a series flow relationship with any of the others. Each chamber contains a bed of one or more adsorbents which is able to adsorb water vapor and carbon dioxide from the feed air and which forms a reversible part of a gas flow path extending between locations external of the vessels. Valves are associated with the vessel and are operable to permit during a first adsorption period of time simultaneous flows of air to to purified through at least two of the chambers and during a second regeneration period of time simultaneous reverse flows of regeneration gas through the same chambers.