Fractional Distillation of Crude Oil (Edexcel GCSE Chemistry: Combined Science)

• 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 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

Uses of the different fractions obtained from petroleum (crude oil)

• 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 a huge number of organic compounds
• The main fractions and their uses are described in the following table:

Uses of Crude Oil Fractions

Properties of the main fractions of crude oil

• Some properties of hydrocarbons depend on the size of their molecules, including boiling point, viscosity and flammability
• These properties influence how hydrocarbons are used as fuels

Number of Carbon & Hydrogen Atoms

• • 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

Decane is an alkane, C₁₀H₂₂ and is a component in some fuels

Boiling point

• As the molecules get larger, the intermolecular forces of attraction between the molecules becomes greater as there is more surface area contact between them
• This means that more heat is needed to separate the molecules, hence with increasing molecular size there is an increase in boiling point

Viscosity

• Viscosity refers to the ease of flow of a liquid
• High viscosity liquids are thick and flow less easily
• Viscosity also increases with increasing chain length
  • This is also due to the increased intermolecular forces of attraction as molecular size increases
  • Increased viscosity means that higher alkanes are useful as lubricants in machinery as they are less likely to burn and function to reduce friction between moving parts

Trends in the Main Fractions

Ease of Ignition

• Molecular size again influences the ease of ignition or flammability of hydrocarbons
• Smaller hydrocarbon molecules are more flammable and are easier to ignite than larger molecules
• This makes them very useful as fuels, releasing large amounts of energy when they burn