Vapor–liquid equilibrium

Vapor–liquid equilibrium (VLE) is a condition where a liquid and its vapor are in equilibrium with each other, a situation where the rate of evaporation equals the rate of condensation on a molecular level such that there is no net vapor-liquid interphase mass transfer. VLE is a fundamental principle in chemical engineering and thermodynamics, playing a critical role in processes such as distillation, absorption, and extraction.

Overview

Vapor–liquid equilibrium involves two phases: the liquid phase and the vapor phase. The equilibrium is governed by the Raoult's Law for ideal mixtures and the Henry's Law for dilute solutions, which describe how the vapor pressure of the liquid phase and the concentration of the vapor phase relate at a given temperature. The equilibrium condition is described by the phase rule, which provides a way to calculate the degrees of freedom in a multiphase system.

Phase Diagrams

Phase diagrams are graphical representations that show the conditions of temperature and pressure under which the phases of a substance exist. In the context of VLE, a phase diagram typically includes a bubble point curve and a dew point curve. The area between these curves represents the range of conditions under which both liquid and vapor phases coexist in equilibrium.

Mathematical Modeling

The mathematical modeling of vapor–liquid equilibrium involves the use of equations such as the Clausius-Clapeyron Equation to describe the phase transition between liquid and vapor. Additionally, activity coefficients, which account for non-ideal behavior, are used in conjunction with Raoult's and Henry's laws to predict the composition of the liquid and vapor phases.

Applications

Vapor–liquid equilibrium data and concepts are essential in the design and operation of equipment for distillation, absorption, and other separation processes. In distillation, for example, the VLE data for the mixture being separated determines the number of theoretical stages required to achieve the desired separation.

See Also

• Chemical equilibrium
• Le Chatelier's Principle
• Thermodynamics
• Separation process