Some of the heavier fractions that are produced in the fractional distillation of crude oil are not very useful.
The hydrocarbons in them are made up of large molecules.
They are thick liquids or solids with high boiling points.
They are difficult to vaporise and do not burn easily - so they are poor fuels.
Yet the main demand from crude oil is for fuels.
Fortunately these large hydrocarbon molecules can be broken down in a process called cracking. This process takes place at an oil refinery in a steel vessel called a cracker.
In the cracker, a heavy fraction distilled from crude oil is heated to vaporise the hydrocarbons.
The vapour is then either passed over a hot catalyst or mixed with steam and heated to a very high temperature. The hydrocarbons are cracked as thermal decomposition reactions take place The large molecules split apart to form smaller, more useful ones.
Question 3