Distillation separates two or more liquid components in a mixture using the principle of relative volatility or boiling points. The greater the difference in relative volatility the greater the nonlinearity, and the easier it is to separate the mixture using distillation. The process involves the production of vapor by boiling the liquid mixture in a still and removal of the vapor from the still by condensation. Due to differences in relative volatility or boiling points, the vapor is rich in light components and the liquid is rich in heavy components.

Often a part of the condensate is returned (reflux) back to the still and is mixed with the outgoing vapor. This allows further transfer of the lighter components to the vapor phase from the liquid phase, and transfer of heavier components to the liquid phase from the vapor phase. Consequently, the vapor stream becomes richer in light components, and the liquid stream becomes richer in heavy components. Different types of devices called plates, trays or packing are used to bring the vapor and liquid phases into intimate contact to enhance the mass transfer. Depending on the relative volatility and the separation task (i.e. purity of the separated components), more trays (or more packing materials) are stacked one above the other in a cylindrical shell to form a column

The distillation column process can be carried out in continuous, batch or in semi-batch (or semi-continuous) mode. Large-scale industrial distillation applications include both batch and continuous fractional, vacuum, azeotropic, extractive, and steam distillation. The most widely used industrial applications of continuous, steady-state fractional distillation are in petroleum refineries, petrochemical and chemical plants, and natural gas processing plants.