Fractional distillation

Crude oil or ‘petroleum’ consists of a complex mixture of numerous hydrocarbons formed by the decay of plant and animal matter over time.

Fractional distillation is the process used to separate the hydrocarbon mixture into various lighter and heavier fractions, based upon differences in boiling points.

The boiling point and melting point of the alkane series

Number of Carbon atoms

As can be seen by the pink line in the graph the larger the alkane molecule the higher the boiling point. This relationship between molecular size and boiling point is used to physically separate the raw mixture of hydrocarbons in a fractionating tower.

Fractional distillation

The crude oil is heated in a boiler and then cooled in a fractionating tower. As you move up the tower it becomes progressively cooler and the fractions condense out in order of decreasing boiling point. At the very top of the tower are the lighter C1 to C4 gaseous fractions and at the base of the tower are the heavier residues like bitumen which is used to make roads.

Fractional distillation allows raw petroleum to be separated into many useful fractions such as lubricating oils, diesel, kerosene, petrol etc which society can use.

The high demand of octane as a fuel is partially overcome by the catalytic cracking of the larger less useful carbon molecules. Larger petroleum molecules are broken down into more useful products like octane which is used as a fuel in the combustion engine and ethylene or ethene which is used in the manufacture of plastics.