Fractional distillation is the primary method refiners use to separate hydrocarbon mixtures such as crude oil into its components.

The main examples of fractional distillation in a refinery are the atmospheric distillation and vacuum distillation units, and the various fractionators built into conversion units.

How it works

Fractional distillation uses the different boiling points for the components in a hydrocarbon mixture to achieve separation. Typically, the mixture is heated until most of the volume evaporates (boils off). The vapors are passed upwards through a series of trays and gradually lowered in temperature. Different components (called fractions or cuts) will condense back into liquid on different trays based on their boiling points, and are then drawn off. The lighter, lower-boiling point fractions will condense higher up, and the heavier, higher boiling point fractions will condense lower down. In practice, the lightest fraction is withdrawn out the top as a vapor, and the heaviest fraction remains in a liquid state the whole time and is drawn out at the bottom.

For complex mixtures such as crude oil, the separated fractions still contain a wide mix of different components with boiling points in a range rather than at a single temperature.