Chemistry Concepts

http://www.bbc.co.uk/bitesize/intermediate2/chemistry/carbon_compounds/fuels/revision/6/

Fractional distillation

Fractional distillation is different from distillation in that it separates a mixture into a number of different parts, called fractions. A tall column is fitted above the mixture, with several condensers coming off at different heights. The column is hot at the bottom and cool at the top. Substances with high boiling points condense at the (hot) bottom and substances with lower boiling points condense on the way to the (cold) top.

The crude oil is evaporated and its vapours condense at different temperatures in the fractionating column. Each fraction contains hydrocarbon molecules with a similar number of carbon atoms.

Oil fractions

The diagram below summarises the main fractions from crude oil and their uses, and the trends in properties. Note that the gases leave at the top of the column, the liquids condense in the middle and the solids stay at the bottom.

The Fractioning Column

As you go up the fractionating column, the hydrocarbons have:

• lower boiling points
• lower viscosity (they flow more easily)
• higher flammability (they ignite more easily)

This means that in general hydrocarbons with small molecules make better fuels than hydrocarbons with large molecules.