Preferably oxygen is displaced by a co current flow of cold nitrogen, following which krypton and xenon are collected by thermal regeneration and counter current purging with hot nitrogen 20, thus avoiding potentially explosive concentrations of {HEAD ~ hydrocarbons} and oxygen in {TAIL ~ the desorbed gas} 22 
Preferably oxygen is displaced by a co current flow of cold nitrogen , following which krypton and xenon are collected by thermal regeneration and counter current purging with hot nitrogen 20 , thus avoiding potentially explosive concentrations of { HEAD ~ hydrocarbons } and oxygen in { TAIL ~ the desorbed gas } 22
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