Non-hydrocarbon

Compounds in this group contain nitrogen, sulphur and oxygen (NSO’s).

Free nitrogen gas may be generated during the formation of petroleum.

Sulphur organic compounds are often foul smelling. The best known is H2S.

Oxygen compounds include alcohols, ethers and organic acids.

Fractional Distillation

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.

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.

Where do Product come out of a Still?

A distillation tower splits crude oil into separate fractions.

Each fraction is a mix of hydrocarbons. Each fraction has its own range of boiling points and comes off at a different level in the tower.

In reality, a single tower could not cover the full range of temperatures needed to split up the heavier fractions.

The table shows the names and uses of the fractions that come from the distillation process. It also shows the ranges of hydrocarbons in each fraction.