Fractional distillation

Miscible liquids are ones that can mix together – like water and ethanol. Immiscible liquids are ones that can’t – like oil and water.

Fractional distillation separates miscible liquids that have different boiling points. It is useful for separating ethanol from a mixture of ethanol and water, and for separating crude oil into different products such as petrol, diesel and kerosene.

When the mixture is heated:

  • the liquids boil at their boiling point.
  • their vapours rise through a column which is hot at the bottom and cooler at the top.
  • the vapour condense when they reach a part of the column that is below the temperature of their boiling point.
  • the liquids which condense in the column drip back into the flask.
  • any gas that makes it to the top of the fractioning column enters the condenser where it is changed to a liquid.
  • the liquid runs into a collecting vessel.
  • distillates collected over a narrow temperature range are known as fractions.

Fractional distillation process to separate ethanol from water

In the first stage of ethanol distillation, water and ethanol solution is heated.

Heat the water and ethanol solution.