Crude Oil & Fractional Distillation (Edexcel IGCSE Chemistry: Double Science)

• Crude oil is a mixture of hydrocarbons
• As a mixture it 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 molecules in each fraction have similar properties and boiling points, which depend on the number of carbon atoms in the chain
• The fractions in petroleum 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

Fractional distillation of crude oil

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 

Properties of the main fractions of crude oil

Viscosity

• This refers to the ease of flow of a liquid
• High viscosity liquids are thick and flow less easily
• If the number of carbon atoms increases, the attraction between the hydrocarbon molecules also increases which results in the liquid becoming more viscous with the increasing length of the hydrocarbon chain
• The liquid flows less easily with increasing molecular mass

Colour

• As carbon chain length increases the colour of the liquid gets darker as it gets thicker and more viscous

Melting point/boiling point

• As the molecules get larger, the intermolecular attraction becomes greater
• So more heat is needed to separate the molecules
• With increasing molecular size there is an increase in boiling point

Volatility

• Volatility refers to the tendency of a substance to vaporise
• With increasing molecular size hydrocarbon liquids become less volatile
• This is because the attraction between the molecules increases with increasing molecular size

Trend in boiling point of the main fractions

Fraction	Number of carbon atoms	Boiling point	Boiling point & viscosity increase going down
Refinery gas	1-4	Below 25	↓
Gasoline / petrol	4-12	40-100
Naptha	7-14	90-150
Kerosene / paraffin	12-16	150-240
Diesel / gas oil	14-18	220-300
Fuel oil	19-25	250-320
Lubricating oil	20-40	300-350
Bitumen	More than 70	More than 350

What are the fractions used for? 

• 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

Fraction	Use
Liquified petroleum gas	Domestic heating & cooking
Petrol	Fuel for cars (gasoline)
Kerosene	Jet fuel (paraffin)
Diesel	Diesel engines (gas oil)
Heavy fuel oil	Ships & power stations
Bitumen	Surfacing roads and roofs