Fractional Distillation
Fractional distillation is the process used to separate crude oil into different, useful fractions. Each fraction contains hydrocarbons with similar boiling points.
What is crude oil?
- Crude oil is a fossil fuel made from ancient marine organisms
- It is a mixture of hydrocarbons, mainly alkanes
- Crude oil itself is not useful – it must be separated into parts by fractional distillation
What is fractional distillation?
- A method that separates a mixture based on different boiling points
- Used to separate hydrocarbons in crude oil
- Each fraction contains hydrocarbons with a similar number of carbon atoms
How it works
- Crude oil is heated until it evaporates
- Vapour enters the fractionating column, which is hot at the bottom and cooler at the top
- Hydrocarbons condense at different heights depending on their boiling points
- The fractions are collected separately
Fractions and their uses
Fraction | Carbon chain length | Boiling point | Use |
Refinery gases | Very short | Very low | Bottled gas (heating) |
Gasoline (petrol) | Short | Low | Fuel for cars |
Kerosene | Medium | Medium | Jet fuel |
Diesel | Longer | Higher | Fuel for cars, lorries |
Fuel oil | Long | Very high | Ships, power stations |
Bitumen | Very long | Highest | Roads and roofing |
Trends in the fractions
As you go down the column (longer chains):
- Boiling point increases
- Viscosity increases (gets thicker)
- Flammability decreases
- Darker in colour
Questions
- What property is used to separate hydrocarbons in fractional distillation?
- Which part of the column is hottest?
- Which fraction is used in jet fuel?
- What happens to flammability as chain length increases?
- What is bitumen used for?
Summary
- Crude oil is separated by fractional distillation
- The column has a temperature gradient (hot at bottom, cool at top)
- Hydrocarbons condense when they reach their boiling point
- Each fraction has different chain lengths and uses
- Shorter chains = lower boiling point, more flammable, lighter