When designing distillation columns, one of the key steps is to calculate the number of equilibrium stages needed. This can be achieved for binary separations via a number of methods, one of these is the Ponchon-Savarit method. In this method, the enthalpy, $h$, curves for the vapour and liquid phases are used with knowledge of the equilibrium tie lines and operating points calculated from mass balances on the column.
The video below gives a quick summary of the derivation of the key lines needed for the Ponchon-Savarit method.
The graph below shows an example Ponchon-Savarit graph. The key design parameters can be varied to help understand how they affect the design.