Fractional distillation of crude oil
Crude oil despite being one of the most valuable natural resources is a useless, smelly and thick
black "liquid" when it comes out of the ground. The reason for this is that it is a mixture. It is a
mixture of compounds called hydrocarbons.
Hydrocarbons are compounds containing only the elements
hydrogen and carbon. There are thousands of different hydrocarbons
all mixed together in crude oil.
Some of the hydrocarbon molecules are small, some are medium sized and some are large. At an oil
refinery the mixture is separated into different fractions of hydrocarbons which contain similar sized molecules, this is easily
done since similar sized molecules have similar boiling points.
The image below shows what happens to the crude oil
at the oil refinery and explain how it is separated into its various fractions.
At the oil refinery:
- The crude oil passes through a hot pipe in a furnace, this vapourises most of the hydrocarbon molecules in the oil.
- The oil then enters the fractionating column. This is a tall narrow cylinder with a temperature gradient,
it gets cooler the higher you go up the column.
- The hot vapours rise inside the fractionating column. When a hot vapour (gas) meets a surface cooler
than its boiling point then it will condense. This is why for example if you boil a kettle too close to
a cool window condensation collects on the window. The steam from the kettle which maybe around 1300C,
hits the window which is cooler than the boiling point of water, so the steam condenses. The same process
will happen inside the fractionating column. Hot vapours will rise until they meet a bubble cap cooler
than their boiling point, the vapours will then condense and the liquids formed will be run off and
- Any fraction or part of the oil that was not vapourised falls to the bottom of the column and is collected. This will be the fractions containing bitumen (tar) and greases.
- The larger or longer the hydrocarbon molecule the higher
will be its boiling point. The fractionating column will basically separate out molecules by their boiling points into separate fractions. Each fraction will
contain many different hydrocarbon molecules each with a similar boiling point.
As you can see lots of very valuable substance such as fuels, petrol, diesel, kerosene for modern
transport, oil to lubricate engines and machinery as well as feedstock molecules in the naphtha
fraction which may end being used in medicines, cosmetics, solvents, detergents, plastic as well
as many other everyday products.
- Each fraction collected will contain a mixture of many different molecules. These molecules will be of a similar size with similar boiling points.
- Crude oil is a mixture of many different sized hydrocarbon molecules.
- Crude oil is separated out into different fractions at an oil refinery. Here the crude oil is heated until most
of it vapourises. The vapourises then enter a tall fractionating column where they condense at different heights inside the column.
- The fractionating column has a temperature gradient, it is hot at the bottom
and cooler at the top.
- Small hydrocarbon molecules have low boiling points, large hydrocarbon molecules have high boiling points. Inside the
fractionating column large hydrocarbon molecules condense at the bottom and as you rise up the column the smaller molecules condense higher up.