Resources | Subject Notes | Chemistry
Petroleum is a complex mixture of hydrocarbons. Fractional distillation is a process used to separate these hydrocarbons into different fractions based on their boiling points. Each fraction has a range of boiling points, allowing for the isolation of useful products.
Petroleum is heated to a high temperature (around 400°C) in a furnace, converting it into a gas.
The hot vapour is then passed into the bottom of a fractionating column.
The column is cooler at the top and gradually gets warmer as you move down.
The different hydrocarbons in the vapour condense at different temperatures and rise up the column.
Liquids condense into liquids and are collected at different levels in the column.
Each fraction is collected in a separate container.
The different fractions obtained from fractional distillation and their typical boiling point ranges and uses are:
| Fraction | Boiling Point Range (°C) | Uses |
|---|---|---|
| Petroleum Gas | -150°C to -40°C | Fuel for heating and cooking |
| LPG (Liquefied Petroleum Gas) | -40°C to 40°C | Fuel for heating and cooking, in cylinders |
| Petrol (Gasoline) | 40°C to 100°C | Fuel for cars |
| Naphtha | 70°C to 200°C | Heating oil, solvent |
| Kerosene | 120°C to 250°C | Jet fuel, heating oil |
| Diesel Oil | 250°C to 300°C | Fuel for diesel engines (trucks, buses, trains) |
| Bitumen | 300°C to 400°C | Road surfacing (asphalt) |
The efficiency of fractional distillation depends on the difference in boiling points between the different fractions. A larger difference in boiling points leads to a cleaner separation. However, some overlap in boiling points means that some impurities may still be present in each fraction.
The efficiency of separation can be affected by:
The difference in boiling points of the components in the petroleum.
The rate of heating.
The temperature gradient within the fractionating column.