Distillation is a technique whereby liquids are separated on the basis of differences in their boiling points. It is the mainstay of the petroleum industry, and is carried out on a very large scale in so-called FRACTIONATING TOWERS, illustrated above.
Crude oil is heated in a furnace, causing most of the components to vapourise and enter the tower. The vapours rise up the tower, causing a temperature gradient to be established, with the hottest regions near the bottom, and the coolest near the top. As the vapours rise, they condense partially, the liquid being collected in trays, from which the liquids are either recycled or drawn off as fractions.
Gases rich in gasoline vapour leave the tower at its top, from where they pass into a condenser, which condense the gasoline and passes it into the gasoline stream, known as "straight-run gasoline". The uncondensed gases are led off and collected.