Aspen Plus 7.2 is employed for simulating these hybrid processes; note that membrane unit is an ACM module, which is exported from Aspen Custom Modeler 7.2. The built-in optimizer in Aspen Plus is used for optimization. There are some assumptions in the membrane unit simulation. Firstly, the temperature difference between inlet and permeate streams of the membrane unit was specified as fixed at 20◦C (Ahmad et al., 2012a,b); in reality, this value varies with the pressure difference between inlet and permeate streams, due to Joule-Thompson effect. Secondly, temperature and pressure of the retentate stream are assumed to be the same as those of the inlet gas; this assumption is acceptable and has been used by other researchers such as Peters et al. (2011) and Ahmad et al. (2012a,b). Lastly, the selectivity and the permeability data for CA membrane is assumed to be not changing with temperature and pressure.