This Demonstration plots the residue curve of a nonideal ternary mixture composed of benzene butanol and propanol Deviation from ideal behavior in the liquid phase is taken into account using the Wilson model in order to compute the activity coefficients The total pressure is 101325 kPa This mixture presents two azeotropes 1 between benzene and propanol marked in the ternary diagram by 77 mole fraction of benzene; 2 between benzene and butanol marked in the ternary diagram by 8862 mole fraction of benzene This ternary mixture has two distillation regions the yellow and green shaded areas with a distillation boundary connecting the two binary azeotropes the straight red segment in the diagram Benzene and butanol are stable nodes; propanol and are two saddle points and finally is an unstable node You can change the position of the locator to plot the residue curve passing at the locator position