The optimizer in Aspen Plus gave optimal membrane area of2568 m2. As can be seem from Table 4, amortized capital cost (CACC)for the optimal solution is 213,164 $/year, which represents only3.3% of EAOC, due to the simple structure and low cost of added one-stage membrane. On the other hand, cost of methane loss is1,639,117 $/year, representing 25.5% of EAOC; note that 1 kg of ethane, propane and butane loss is calculated as 1.9 kg, 2.8 kg and3.6 kg of methane loss, respectively, because of their higher value.