Fractional Distillation

Fractional distillation is the process of separating crude oil into groups of hydrocarbons with similar numbers of carbon atoms. We call these groups of hydrocarbons “fractions”.

Short-chain hydrocarbons

Hydrocarbons with few carbon atoms are called 'short-chain hydrocarbons'.

Long-chain hydrocarbons

Hydrocarbons with lots of carbon atoms are called ‘long-chain hydrocarbons’.

Fractional Distillation Process

Different-sized hydrocarbons have different boiling points. Fractional distillation separates hydrocarbons using their different boiling points. The steps of the process are:

• Evaporation

  • Crude oil is heated until it evaporates.
  • Crude oil vapour is put into a fractionating column at the bottom and rises upwards.

• Condensation

  • The temperature is highest at the bottom of the column. Long-chain hydrocarbons condense at the bottom and are collected as liquids.
  • Short-chain hydrocarbons have lower boiling points. They pass up the column and condense at lower temperatures nearer the top.

• Collection

  • The fractions are collected. They are then processed to create end products:
  • Fuels (e.g. petrol, diesel) are a common end product.
  • The petrochemical industry can use some fractions as feedstock (material used in an industrial process) to make solvents, lubricants, detergents etc.