Isovolatility Lines and Equilibrium Vectors
Consider a ternary mixture of chloroform, acetone, and methanol (components 1, 2 and 3, respectively) at atm. Vapor-liquid equilibrium data for this non-ideal mixture are computed using the modified Raoult's law and the Wilson model. This mixture has three binary azeotropes (represented by , , and ) and a ternary azeotrope labeled . The Demonstration plots the isovolatility lines (i.e., the locus of the mixtures for which the relative volatility of components and , , is equal to 1), shown in blue, for the three possible binary combinations. The ternary azeotrope appears naturally as the intersection of these three lines (as seen in the last snapshot, all relative volatilities are equal to 1 at this point). Each line ends at the corresponding binary azeotrope. Equilibrium vectors (or tie lines) are shown with red arrows. It is clear that as one approaches an azeotrope, the size of the arrow becomes small because an azeotrope satisfies = where is the liquid mole fraction and is the vapor mole fraction of a component of this ternary mixture. All lines generated by the equilibrium vectors intersect at one of the vertices (i.e., they will meet at vertex for binary with and ). Finally, the Demonstration shows the values of the relative volatilities for all possible binary mixtures at any location on the ternary diagram.