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.

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