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

Fractional Distillation

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

• As a mixture, it isn’t a very useful substance, but the different hydrocarbons that make up the mixture, called fractions, are useful, with each fraction having different applications

• It is a mixture of mostly alkanes, unbranched and branched chain

  • The mixture is separated into fractions, based on boiling points

• Each fraction 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 in which fraction it will be separated into

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

• Most fractions contain mainly alkanes, which are compounds of carbon and hydrogen with only single bonds between them

The Process of Fractional Distillation of Crude Oil

• Fractional distillation is carried out in a fractionating column which is very hot at the bottom and cool at the top

• Crude oil enters the fractionating column and is heated so vapours rise

• Vapours of hydrocarbons with very high boiling points will immediately condense into liquid at the higher temperatures lower down and are tapped off at the bottom of the column

• Vapours of hydrocarbons with low boiling points will rise up the column and condense at the top to be tapped off

• The different fractions condense at different heights according to their boiling points and are tapped off as liquids

• The fractions containing smaller hydrocarbons are collected at the top of the fractionating column as gases

• The fractions containing bigger hydrocarbons are collected at the lower sections of the fractionating column

• The 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