Fractional Distillation

Download transcript

The petrol we put in our cars, the bottles that hold our milk, and the lubricant on bike chains all originate from one liquid.

Crude oil, found deep in the Earth's crust, and brought to the surface by oil rigs.

How do we get all these useful products, from this thick brown liquid?

The answer lies in an ingenious process called fractional distillation.

Fractional Distillation

In normal distillation, a mixture of liquids is heated and the liquid with the lowest boiling point evaporates first.

This vapour is extracted, and then cooled to become a liquid in its pure form.

Fractional distillation is a special version of this process, separating a mixture of liquids into many different parts, or fractions.

It's ideal for separating crude oil, which is a soup of different hydrocarbon compounds – all with different boiling points.

The main reason they have different boiling points, is down to the number of carbon atoms in their molecules.

The longer the chain of carbon atoms, the higher the boiling point.

Methane = 1 carbon atom.
Boiling point -162°C

Decane = 10 carbon atoms.
Boiling point 174°C

The crude oil is heated to a vapour, and pumped into a tall tower called a fractionating column, which is hot at the bottom, and cool at the top.

Fractions with long chains of carbon atoms, and so high boiling points – condense at the bottom of the column.

Whereas those with short chains, and so lower boiling points – condense at the top.

The condensed liquid is extracted, and each fraction has different properties.

Petrol

Those with smaller molecules, like petrol, are easily ignited so they make good fuels.

Those with bigger molecules, like bitumen, are un-reactive and viscous.

Bitumen

They are used as lubricants, for waterproofing and road surfacing.

The fractions in between include kerosene, used as aircraft fuel, and naphtha, used to make chemicals like plastics.

A wealth of substances that we'd find it hard to live without, exist thanks to this simple yet clever process – fractional distillation.