Objective function in TSMAHRD is the same as that in OSMAHRD; however, there are more decision variables, which are: first stage membrane area (SAm2); second stage membrane area (SBm2); pressure of first permeate stream (P1stPermkPa); and outlet pressure of the compressor after first stage membrane (PComOutletkPa, equal to the inlet pressure of second stage membrane). Their corresponding lower and upper bounds are specified as: