What is vapor-liquid equilibrium (VLE) and why is it important?

Vapor-liquid equilibrium (VLE) is the state where liquid and vapor phases coexist with no net mass transfer - the rates of evaporation and condensation are equal. At VLE, temperature, pressure, and chemical potential (or fugacity) of each component are equal in both phases. VLE data is essential for designing distillation columns, flash drums, evaporators, and any separation involving phase change. VLE behavior is described by Raoult's law (ideal) or activity coefficient models (non-ideal).