Fractional Distillation of Crude Oil (Oxford AQA International A Level (IAL) Chemistry): Revision Note

Fractional Distillation of Crude Oil

• Crude oil, or petroleum, is currently the world's main source of organic chemicals

• It is a mixture of mostly alkanes

  • Alkanes are saturated hydrocarbons, which means that:

    • They contain only single covalent carbon-carbon bonds

    • They are compounds made from only hydrogen and carbon

  • They can be straight chain or branched chain

• As a mixture, it isn’t a very useful substance

  • However, the different hydrocarbons that make up the mixture are useful

• The mixture of alkanes from crude oil is separated into fractions, based on boiling points

  • Each fraction has different uses and consists of groups of hydrocarbons of similar chain lengths

  • Hydrocarbons of similar chain length have similar boiling points

• The fractions are separated from each other in a process called fractional distillation

• The size and length of each hydrocarbon molecule determines the fraction an alkane will be separated into

• The size of each molecule is directly related to how many carbon and hydrogen atoms the molecule contains

The fractional distillation of crude oil

• Fractional distillation is carried out in a fractionating column which has a temperature gradient 

  • It is very hot at the bottom of the column and cooler at the top

• During the process of fractional distillation:

  • The crude oil is heated and vapourises

  • The vapours of hydrocarbons enter the column which has a temperature gradient

  • The vapours of hydrocarbons with high boiling points condense at the bottom of the column

  • The vapours of hydrocarbons with lower boiling points rise up the column and condense at the top

• Crude oil contains small amounts of other compounds

  • Some of these compounds may contain sulfur, and when burned, sulfur dioxide can be produced

    • This is one of the causes of acid rain