For the two scenarios, TSMAHRD process is optimized in a similar way, except for the decision variables reduced from three to two (with P1stPermfixed at 233.6 kPa as in the reference case).Another difference is the compressor; whether a parallel compressor is required or not depends on the flow rate of the first permeate stream. If it is less than or the same as that in the reference case, the original compressor is sufficient; else, a parallel compressor is required. The optimizer in Aspen Plus gave optimal values to design variables as: SA= 5367 m2 and SB= 560 m2 for scenario one ;and SA= 8126 m2, SB= 1189 m2 for scenario two. For scenario one, the first stage membrane is fully used, while 30% of the second stage membrane is bypassed; flow rate of the first permeate is 427.6 kmol/h, less than that in the reference case (481.6 kmol/h),and so no additional compressor is required. For the second scenario, membrane area of 2758 m2 is added for the first stage membrane and 390 m2 is added for the second stage membrane. Also, flow rate of the first permeate doubles from 481.6 kmol/h to987.9 kmol/h, and so a parallel compressor is to be installed.