Fractional distillation

Oil refineries

Distillation column

A pipestill at Fawley

The crude oil is a mixture of lots of compounds - mainly hydrocarbons. Fractional distillation splits the crude oil into simpler mixtures called fractions. The different fractions are taken out of the still at different levels.

This happens in a distillation tower (which we shorten to still).

The crude oil is heated in a furnace to about 370°C and is pumped into the bottom of a distillation tower. Most of the hydrocarbons are gaseous, though the very thick ones are still a liquid even at this temperature.

A distillation tower is often called a still. The atmospheric still is the tall steel tower in the middle.

The crude oil is heated in the furnace to the right.

The different fractions come out at different levels. You can see the pipes that take them away.

The residue from the atmospheric still passes to the vacuum still on the left. Here it is split into more fractions.

The tower is like a giant heat exchanger - it removes heat from the gases as they rise up it. The temperature falls to 20°C by the time the vapours reach the top.

The vapours condense as they rise up the tower. The heavier ones (with higher boiling points) condense first. The thinner, runny ones get further up the tower before they condense. And the gases pass out of the top.

The distillation tower (or still) splits the crude oil into its separate fractions.

This is how it works. The hot mixture is pumped into the bottom. The tower acts as a heat exchanger, removing heat from the vapours as they rise. Some of them condense back into liquids and fall back down the column.

So the temperature gradually decreases as you go up the column. Different groups of hydrocarbons condense at different heights. The heaviest at the bottom, the lightest at he top.

Graphic and cutaway of still.

Bubble traps force the hot vapour and cooler liquids to mix. They also collect the condensed fractions and run them out of the tower.