Fractional Distillation & Petrochemicals (AQA GCSE Chemistry: Combined Science)

What is fractional distillation?

• 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 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 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 fractional distillation coloumn

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

How does fractional distillation work?

• 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 

Uses of petrochemicals

• 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:

Crude oil fractions

Fraction	Use
Liquified petroleum gas	Domestic heating & cooking
Petrol	Fuel for cars
Kerosene	Jet fuel
Diesel	Diesel engines
Heavy fuel oil	Ships and power stations