A radial U-flow adsorption unit for purifying air by removal of at least one contaminant selected from the group consisting of water, carbon dioxide, hydrocarbons and NO x , said unit comprising: an outer tubular side wall having a longitudinal axis and comprising a top end and a bottom end opposite said top end, said top and bottom ends being closed by top and bottom end walls respectively, said outer tubular side wall having an inner diameter, A; an elongated annular bed comprising at least one selectively adsorbent material , said annular bed being located co-axially within said outer tubular side wall thereby defining a first annular space between said side wall and said annular bed , said annular bed defining a central channel in fluid communication with said first annular space through said annular bed ; at least one baffle for reducing gas flow maldistribution in said first annular space ; an impermeable annular skirt supporting said annular bed on said bottom end wall , said annular skirt being located co-axially within said outer tubular side wall thereby defining a second annular space between said side wall and said annular skirt , said second annular space being in fluid communication with said first annular space , said annular skirt having an inner diameter, C; at least one gas inlet located in said outer tubular side wall at said bottom end and in fluid communication with said first annular space through said second annular space ; and a gas outlet located in said bottom wall of said unit and in fluid communication with said central channel , wherein the ratio of the inner diameter of the outer tubular side wall, A, to the inner diameter of the annular skirt, C, is from 3:1 to 11:10. an outer tubular side wall having a longitudinal axis and comprising a top end and a bottom end opposite said top end, said top and bottom ends being closed by top and bottom end walls respectively, said outer tubular side wall having an inner diameter, A; an elongated annular bed comprising at least one selectively adsorbent material , said annular bed being located co-axially within said outer tubular side wall thereby defining a first annular space between said side wall and said annular bed , said annular bed defining a central channel in fluid communication with said first annular space through said annular bed ; at least one baffle for reducing gas flow maldistribution in said first annular space ; an impermeable annular skirt supporting said annular bed on said bottom end wall , said annular skirt being located co-axially within said outer tubular side wall thereby defining a second annular space between said side wall and said annular skirt , said second annular space being in fluid communication with said first annular space , said annular skirt having an inner diameter, C; at least one gas inlet located in said outer tubular side wall at said bottom end and in fluid communication with said first annular space through said second annular space ; and a gas outlet located in said bottom wall of said unit and in fluid communication with said central channel , wherein the ratio of the inner diameter of the outer tubular side wall, A, to the inner diameter of the annular skirt, C, is from 3:1 to 11:10. <EOS>