Fractional distillation
When a mixture holds several components that all vaporise, one condenser is not enough. The separation has to be done at controlled temperatures, one fraction at a time.
Why the simple apparatus will not do
If the solution or mixture to be separated holds several volatile components, simple distillation — or the apparatus used in simple distillation — cannot separate them. This distillation has to be done under controlled conditions, and special apparatus has to be used.
Two components, worked through
Suppose a component A in a mixture has a boiling point of 80 °C and a component B has a boiling point of 40 °C. On heating, this solution of A and B begins to boil at a little above 40 °C. The vapour formed then holds more of component B. If that vapour is collected near 40 °C and condensed, then as B leaves the solution the percentage of A in the mixture rises, and so the temperature at which the mixture boils rises too. Collecting and condensing the vapour at each appropriate temperature in this way separates the components. For this to work at all the boiling points must differ appreciably — that is, the volatilities must be appreciably different from each other.
Fractional distillation
Separating several components by distillation in this way, controlling the cooling conditions, is called fractional distillation.
The industry built on it
Crude oil is a mixture of a great many hydrocarbon components. At a crude oil refinery a distillation column is used to control the cooling conditions. The temperature at the various levels of that distillation column is maintained at various values, and the distillation happens separately at each of those places. Components with low boiling points (petroleum gas) are separated from the upper part of the column. Components with high boiling points (tar) collect at the bottom of the column.
Figure to be added
| Fraction | Boiling point | What it is used for |
|---|---|---|
| Petroleum gas | −25 °C | Gas cylinders, chemicals |
| Gasoline (petrol) | 40 – 75 °C | Petrol |
| Naphtha | 75 – 150 °C | Chemicals |
| Kerosene | 150 – 240 °C | Jet fuel |
| Diesel | 220 – 250 °C | Diesel |
| Lubricating oil | 250 – 300 °C | Lubricants |
| Fuel oil | 250 °C | Fuel for ships |
| Tar | Highest | Surfacing roads |
Fractional distillation of air
Fractional distillation is used to separate the components of atmospheric air as well. When atmospheric air is cooled to about −200 °C under pressure it becomes a liquid. That liquid is one holding several gaseous components. When it is heated again each component vaporises at its own boiling point. In this way nitrogen leaves at −196 °C, oxygen at −183 °C and carbon dioxide at −78.5 °C.