Good day Sir/Mam
Non-ideal mixtures of components forming an azeotrope are very difficult to separate to a purity beyond their azeotropic composition by using ordinary distillation. In such cases, pressure swing distillation (PSD) is a very common practice in chemical industries to separate such mixtures.
The principle behind PSD is to use two columns operated at sequence in two different pressures. So the pre-requisite for the process is that the composition of azeotrope should change significantly with change in pressure. Therefore assigning the opeerating pressure depends on the component system and the pressure at which their azeotropic composition changes significantly.
In DWSIM, the distillation column pressure can be entered under the Column Configuration >> Stages where one can enter the pressure for every stages individually or one can also enter the top & bottom pressure and extrapolate to get the pressure of stages.