The optimizer in Aspen Plus gave optimal values: SA= 5352 m2,SB= 811 m2, P1stPerm= 229.6 kPa and PComOutlet= 5496 kPa. Note that optimal value of PComOutletis near its upper bound; hence, the compressor after second stage membrane increases pressure from 5496 kPa to 5516 kPa, and its power consumption is merely 1.29 kW. Given these results, the latter compressor can be avoided in TSMAHRD by setting PComOutlet= 5516 kPa. The new optimization problem has only three decision variables, which are: SA, SB and P1stPerm. Their respective lower and upper bounds are given in Eqs. (8)–(10). The optimizer in Aspen Plus gave optimal values of these design variables as: SA= 5368 m2, SB= 799 m2and P1stPerm= 233.6 kPa. Note that the permeate stream compressed from 233.6 kPa to 5516 kPa by a two-stage compressor with inter-stage cooler. The pressure ratio of each stage is specified following the heuristic in Seider et al. (2010).