How the column works

Crude oil is heated so that most of it vaporises. The vapour enters a fractionating column, which is hottest near the bottom and cooler near the top. As vapours rise, they cool and condense at different heights. A fraction is a group of hydrocarbons with similar boiling points. Shorter-chain hydrocarbons usually have lower boiling points and are collected higher in the column. Longer-chain hydrocarbons usually have higher boiling points and condense lower down. Refinery gases are collected at the top of the column, petrol in the upper section, diesel in the lower section, and bitumen residue remains at the bottom. Petrol is used as fuel for many cars, and bitumen is used for road surfacing or roofing. The fractions are mixtures, not single pure substances.