What is Equilibrium and Vapour Liquid Equilibrium (VLE)?

Equilibrium is a static condition in which no changes occur in the macroscopic properties of a system with time.
In engineering practice, only macroscopic properties are taken into consideration, microscopic property level of equilibrium is generally ignored.
Because at the microscopic level if a system is not in equilibrium it will not introduce significant error in calculation.
Example: consider an isolated system containing liquid and vapour phases in equilibrium. Near the interphase of vapour and liquid molecules of high velocities overcome the surface force and passes into other phases (liquid to vapour and vapour to liquid). And the average rate of passage of molecules is the same in both directions. And the net interphase mass transfer of molecules is zero.
Common coexisting phases in industrial practice are vapour and liquid. That’s why our main concern is vapour liquid equilibrium, although liquid/liquid, vapour/solid, and liquid/solid systems are also found.
Vapour Liquid Equilibrium is the state of coexistence of liquid and vapour phases.
When the phases (vapour and liquid) are not in equilibrium and they come in contact mass transfer occurs between them and the composition of the phases changes until equilibrium is achieved.
This phenomenon is used in processes such as distillation, absorption, and extraction to carry out the separation.

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