Separex Corp., through its subsidiary, Spectrum Separations, Inc., initiated work on membrane gas separation in 1979. Since that time, Spectrum has developed a cellulose acetate membrane and membrane element that exhibit greater selectivity and higher permeation rates for gas separations of commerical interest than previously found. These elements, when inserted into pipes, become part of a modular system capable of processing a wide range of feed flow rates. Up to 6 elements are connected in series in a single, 22-ft (6.7-m) long pipe or tube. A rubber U-cup attached to the element serves to seal the element with the inner diameter of the pressure tube, thereby forcing the feed gas to flow through the element. The pressure tubes usually contain 6 elements each and are mounted in racks on a skid. Commercial-size elements are typically 8 in. (203 mm) in diam by 42 in. (1070 mm) long and contain from 150 to 275 sq ft (14 to 26 sq m) of membrane area.