Fractional distillation

An easy way to separate a mixture of miscible liquids is by a process called simple distillation.

In this process, a liquid mixture is separated on the basis of the boiling points of the constituents.

The liquid with the lower boiling point is distilled out first, followed by the liquid with the higher boiling point.

This process is successful when the liquids have a significant difference in their boiling points.

This ensures that the second component retains its liquid state even when the first liquid vaporises.

But if the boiling points of liquids have a very small difference, the vapour obtained will be a mixture.

In such a case, the mixture will not be separated properly. Thus simple distillation will not work.

To separate liquids with smaller difference in boiling points, we need a different process.

The process is called fractional distillation.

Fractional distillation follows the same principle as simple distillation.

The difference lies in the experimental set up.

Fractional distillation involves an additional equipment called fractionating column.

A simple fractionating column is a glass tube packed with glass beads.

Let us understand how fractional distillation works with a mixture of water and ethanol.

In this process, the mixture of ethanol and water is heated in a round bottom flask.

When the temperature of the mixture reaches 78⁰C, ethanol vapourises and reaches the first section of the column.

For a while, the temperature of the first section remains below 78⁰C. So, the vapours condense down.

On further heating, more ethanol vapours reach the first section of the column.

Ethanol vapours continue to condense until the temperature of this section is 78⁰C.

As the temperature of the first section increases, the vapours move up to the next section.

In the second section too, the vapours continue to condense until the temperature reaches 78⁰C.

This process continues through different sections of the column until the temperature of the topmost region is .

Once the top of the column reaches , ethanol moves through the condenser. The liquid is then collected in the collecting flask.

When the temperature of the topmost part goes beyond the collecting flask should be removed.

As ethanol vapour moves up the column, the temperature of the mixture in the round bottom flask also starts rising.

So, when the temperature of the mixture rises to , water starts boiling along with ethanol.

Since the boiling points of water and ethanol are different,

they condense at different sections of the column, depending on the temperature of that section.

This ensures separation of ethanol and water with considerable efficiency.

Fractional distillation has many applications.

It is used to separate crude oil. Crude oil contains products like diesel,petrol and paraffin wax.

Fractional distillation is also used to separate different components of air.

The first step is to liquify air. This is done by compressing and cooling.

Liquid air is then heated and allowed to move up through the fractionating column.

Depending on the boiling points, components of air are separated at different heights of the column.

Nitrogen with the lower boiling point is distilled from the top and oxygen with the higher boiling point is distilled from the bottom.

Revision

Simple distillation works when there is a large difference between between the boiling points of the liquids.

Fractional distillation is used to separate miscible liquids with a difference in boiling point less than 25 ⁰C.

In fractional distillation, vapours of both liquids pass through the fractionating column.

The liquids condense at different sections of the fractionating column, based on the temperature of the column.

Fractional distillation can be used to separate crude oil and components of air.

The End