Distillation is a mass transfer operation aimed at seperating a liquid solution into its components. The liquid solution is heated to force its components, which have different boiling points, into the gas phase.

A typical distillation system contains a distillation column where the separation of liquid components is carried out, a reboiler to provide the necessary vaporisation for the distillation process, a condenser and a reflux drum.

A distillation column contains a vertical shell and internals such as trays/plates or packings. The liquid solution that is to be processed is known as the feed and is introduced into the column on one or more trays.

As the liquid flows down, it comes in contact with rising vapor on the plates where mass transfer between the phases takes place in both directions.

The vapor and liquid contacting on a plate try to come equilibrium. The heat of condensation given by the condensing vapor is taken up by the liquid and part of it vaporizes. While this enriches the liquid in less volatile component further, the vapor is also enriched in the more volatile component further.

These partial vaporizations and partial condensations are repeated on each plate. As a result of these, the liquid solution flowing down from plate to plate continuously enriches in the less volatile component. By providing sufficient number of the plates (or packing height) in this region of the column, the liquid reaching bottom of the column can be made almost pure in the less volatile component. This liquid is withdrawn as bottom product.

The vapor leaving the column is directed to a condenser, where it condenses totally and part of this condensate (liquid) is returned back to the top of the column as reflux liquid to provide the liquid phase rich in the more volatile component on the plates above the feed plate. The remaining part of the condensate is withdrawn as top product or distillate.