Fractional Distillation
Chemistry: AQA A Level, topic 5.2.1
Fractional Distillation
Fractional distillation is the process of separating crude oil into groups of hydrocarbons with similar numbers of carbon atoms. We call these groups of hydrocarbons “fractions”.
Long-chain hydrocarbons
- Hydrocarbons with lots of carbon atoms are called ‘long-chain hydrocarbons’.
Short-chain hydrocarbons
- Hydrocarbons with few carbon atoms are called 'short-chain hydrocarbons'.
The Process of Fractional Distillation
Different-sized hydrocarbons have different boiling points. Fractional distillation separates hydrocarbons using their different boiling points. The steps of the process are:
Evaporation
- Crude oil is heated until it evaporates.
- Crude oil vapour is put into a fractionating column at the bottom and rises upwards.
Condensation
- The temperature is highest at the bottom of the column. Long-chain hydrocarbons condense at the bottom and are collected as liquids.
- Short-chain hydrocarbons have lower boiling points. They pass up the column and condense at lower temperatures nearer the top.
Collection
- The fractions are collected. They are then processed to create end products:
- Fuels (e.g. petrol, diesel) are a common end product.
- The petrochemical industry can use some fractions as feedstock (material used in an industrial process) to make solvents, lubricants, detergents etc.
Test yourself on Fractional Distillation
Two hydrocarbon molecules have very different numbers of carbon atoms. Would fractional distillation place these molecules in the same fraction or in different fractions?
What do we call the process of separating crude oil into fractions?
What is not an end product created from crude oil fractions?
What are these changes of state?
What are the steps in the process of fractional distillation?