Polymer engineers have developed a new dense polymer membrane for H2/CH4 separation. The permeate composition resulted in 80% H2 and a residue of 42% H2 with a H2 composition of 50% H2 in the feed. The feed and permeate absolute pressures were 100 and 15 lbf/in2 . The permeate flow was 20% of the feed flow. What is the membrane selectivity? ( = 2 4/4 2 )