Fractional Distillation Of Crude Oil (AQA GCSE Combined Science: Synergy: Physical Sciences): Revision Note

Fractional distillation

What is fractional distillation?

• Crude oil as a mixture is not a very useful substance

• But, but the different hydrocarbons that make up the mixture, called fractions, are enormously valuable

  • Each fraction has 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

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 hydrocarbon fractions

• Some properties of hydrocarbons depend on the size of their molecules

• These properties include:

  • Boiling point

  • Viscosity (the ability of the substance to flow)

  • Flammability (how easily the substance burns)

• These properties influence how hydrocarbons are used as fuels

Boiling point

• The bigger the hydrocarbon, the higher its boiling point

• This is because the size of the intermolecular forces increases requiring more energy to overcome them

Graph to show the trend in boiling points of hydrocarbons

A graph of the boiling points of the first eight alkanes showing a gradually increasing trend

Viscosity

• Viscosity refers to the ease of flow of a liquid

  • The opposite of high viscosity is to say a liquid is runny

  • 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

• Longer chain hydrocarbons would not be as useful as fuels for cars as they would be too thick and clog the engine

• Increased viscosity means that longer chained alkanes are useful as lubricants in machinery as they are less likely to burn and function to reduce friction between moving parts

Flammability

• 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

Crude oil fractions

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: