Section 1 of 4What Crude Oil Is
Crude oil is a valuable raw material, but only once it has been separated into its components.
Finite resource
- Crude oil is non-renewable, because it formed over millions of years and is being used far more quickly than it can be replaced.
Mixture
- Crude oil is a complex mixture, mainly of hydrocarbons, and it also contains dissolved natural gas.
Hydrocarbons
- Compounds containing only carbon and hydrogen atoms, arranged as chains or rings of carbon atoms.
Fractions
- Groups of hydrocarbons with similar chain lengths, which have similar boiling points and uses.
Section 2 of 4Fractional Distillation
Fractional distillation relies on evaporation and condensation at different heights in a column.
1
Crude oil is heated until most of it vaporises, and the vapours rise into the fractionating column.
2
The column is very hot at the bottom and cooler at the top, so the temperature falls as you move up.
3
Hydrocarbons with high boiling points condense near the bottom and are tapped off as liquids.
4
Hydrocarbons with low boiling points rise further up and condense near the top, while the smallest molecules are collected as gases.
Section 3 of 4Uses of the Fractions
Uses of the Fractions
Top fractions
- The smallest hydrocarbons are collected as gases, such as the bottled gas used for cooking and heating.
Middle fractions
- Medium-length hydrocarbons are used to make petrol, kerosene and diesel fuels.
Bottom fractions
- Long-chain hydrocarbons are used for heavy fuel oil and lubricating oils.
Value of fractions
- Each fraction has its own supply and demand, which affects how valuable it is to the refinery.
Section 4 of 4Exam Checklist & Tips
10.1.1 – Key Points for the Exam
- Hydrocarbon A compound that contains only carbon and hydrogen atoms.
- Fraction A group of hydrocarbons with similar boiling points, produced by fractional distillation.
- Fractional distillation The separation of a mixture into fractions by heating it and condensing the vapours at different heights.
- Fractionating column A tall column that is hot at the bottom and cool at the top, used for fractional distillation.
- Finite resource A resource that cannot be replaced at the rate it is used, such as crude oil.
Common Mistakes
- Defining a hydrocarbon without the word only; it must contain carbon and hydrogen atoms only.
- Saying the fractionating column is hot at the top; it is hot at the bottom and cooler at the top.
- Describing crude oil as a single compound; it is a mixture of many hydrocarbons.
- Saying that fractions are separated by chemical reaction;
- fractional distillation is a physical process based on boiling points.