# Vapor Liquid Equilibrium

Vapor liquid equilibrium stands for equilibrium condition between vapor and liquid of a pure compound or a mixture. Under vapor liquid equilibrium, a system is under dynamic equilibrium where rate of vaporization in the system is equal to the rate of condensation in the system.

Phase equilibrium for pure components and mixtures is defined by phase rule or Gibbs phase rule. As per phase rule (or Gibbs phase rule)

F = C – P + 2

where F is the number of degrees of freedom, C is the number of components and P is the number of phases in thermodynamic equilibrium with each other.

The number of degrees of freedom is the number of independent intensive variables, i.e. the largest number of thermodynamic parameters such as temperature or pressure that can be varied simultaneously and arbitrarily without determining one another.

For vapor liquid equilibrium of a pure chemical,

P = 2 and C = 1,

giving degree of freedom, F = 1 as per phase rule (or Gibbs phase rule)

This implies that a system comprising a pure chemical existing in vapor liquid equilibrium can be defined by either fixing the temperature of the system or the pressure of a system. Therefore, vapor liquid equilibrium of a pure chemical can be defined by a curve on pressure-temperature (P-T) plot.

Similarly, a binary system comprising of two chemicals existing in vapor liquid equilibrium will have a degree of freedom, F = 2 and this system can be defined by fixing any two independent intensive variables among temperature of the system, pressure of the system and concentration of either of the two chemicals in the system. Therefore, vapor liquid equilibrium of a binary system can be defined by a phase envelope on pressure-temperature (P-T) plot.

Phase envelope can also define vapor liquid equilibrium of multicomponent systems (C > 2) in case composition of the system is known as in this case if either of the pressure of the system or the temperature of the system is known then the total number of known independent intensive variables including known concentrations of C-1 chemicals in the system will become equal to degree of freedom of the system, F.

EQ-COMP is a vapour liquid equilibrium software which can calculate various vapour liquid equilibrium properties like bubble point, hydrocarbon dew point, water dew point, phase envelope, two phase composition, compressibility factors etc. for non polar or mildly polar hydrocarbons and inorganic gases and their binary and multicomponent mixtures.