is the superficial contact time of the gas in the adsorbent bed this parameter is defined as where l is the bed length and v o is the superficial velocity of the feed gas through the bed based on the empty bed volume the superficial contact time may be defined for all adsorbent in the bed including a pretreatment layer or alternatively may be defined for the pretreatment layer only a minimum superficial contact time is required to select an adsorbent for contaminant removal the adsorbent beds in the embodiments of the present invention may utilize one or more layers of pretreatment adsorbent to remove impurities such as water and co 2 and one or more layers of adsorbent to effect separation of the main constituents i e oxygen and nitrogen in the feed gas the feed gas may be air or any other oxygen containing gas during the operation of the adsorption cycle cooling air may be passed over the outer surfaces of the adsorbent columns to promote heat transfer from the columns the following examples illustrate embodiments of the present invention but do not limit embodiments of the invention to any of the specific details described therein a large single bed pvsa system with a bed diameter of 30 inches 2 r 0 052 cm u2212 1 and a bed depth of 4 feet 7 of which is a pretreatment layer of nax zeolite and the remainder of which is a main adsorbent layer of.