Distillation is the oldest and most common refinery process. It is based on the well-known principle that liquids with different boiling points can be separated by evaporation / condensation.

Crude oil distillation involves feeding crude oil at the boiling point (400°C) into a large fractionation column. The oil is split up into a series of product fractions, depending on desired boiling point ranges. Evaporated oil on its way up through the column encounters condensed liquid on its way down. They get in contact and exchange components at a number of sieve trays in the column. This contact causes light fluid molecules to evaporate and heavy gas molecules to condense. The further up the column we get, the lighter the fluid found at the sieve tray is. Part of the crude oil cannot be evaporated, and is extracted as heavy oil at the bottom of the column.

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