Consider the partially miscible binary mixture composed of -butanol and water at 101.325 °C. The nonrandom two-liquid (NRTL) model developed by Renon and Prausnitz [1, 2] is adequate for determining liquid-liquid equilibrium (LLE) data. Since this mixture presents a heteroazeotrope with a boiling point of 93.49 °C, we consider only temperatures below this threshold value. The magenta-shaded region indicates where the validity of our simulation fails. Indeed, for °C we have, in addition to the two liquid phases, a third vapor phase.