Fractional Distillation & Petrochemicals (AQA GCSE Chemistry)

• Crude oil as a mixture is not a very useful substance but the different hydrocarbons that make up the mixture, called fractions, are enormously valuable, with each fraction having many different applications
• Each fraction consists of groups of hydrocarbons of similar chain lengths
• The fractions in crude oil (petroleum) are separated from each other in a process called fractional distillation
• The molecules in each fraction have similar properties and boiling points, which depend on the number of carbon atoms in the chain
• 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

Diagram showing the process of fractional distillation to separate crude oil in a fractionating column

• 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 petrochemical industry is hugely important for modern society and development
• The fuels that are used in most modern methods of transport (cars, trains, airplanes, etc.) are all based on oil products
• Polymers, lubricants, solvents, detergents and adhesives are all products that are obtained from crude oil
• The array of fractions in crude oil and the huge range of compounds we can produce from them all stem from carbon’s ability to form multiple strong covalent bonds with itself leading to a huge number of organic compounds
• The main fractions and their uses are described in the following table:

Petrochemicals Uses Table