Methods of Separation - Fractional Distillation Method

Fractional distillation:

Fractional distillation is often used to separate mixtures of liquids that have similar boiling points. 

It is the same as simple distillation except for the fact that it requires a fractionating column additionally.

Application:

1. Purification of air

2. Purification and separation of many organic compounds.

The experiment of Separation of two miscible liquids by fractional distillation

Aim: To separate a mixture of two or more miscible liquids for which the difference in boiling points is less than 25 K

Requirements: Fractionating column, distillation flask, a mixture of acetone and water, thermometer, condenser, etc.

Principle: A simple fractionating column is a tube packed with glass beads. The beads provide a surface for the vapours to cool and condense repeatedly. Basically, fractional distillation involves repeated distillations and condensations and hence many cycles of this ensure a purer substance.

Procedure:

• Take the mixture in a distillation flask.

• Fit it with a thermometer and arrange the apparatus as shown

• Heat the mixture slowly keeping a close watch at the thermometer.

Observation: Pure liquid was obtained.

Inference/ Result: Pure component could be extracted from a mixture of components even with a little difference in their boiling points.

Experiment of obtaining a different type of gases from the air by fractional distillation.

Aim: Air is a homogeneous mixture and can be separated into its components by fractional distillation.

Principle: This liquid air is allowed to warm-up slowly in a fractional distillation column, where gases get separated at different heights depending upon their boiling points.

Steps:

• Under high pressure, the air is compressed and then cooled by reducing the temperature. This liquefies the air.
• The liquid air is then passed through the fractional distillation column.
• Inside the column, warmer air is present at the bottom and cool air at the top.
• Each gas starts to separate at different temperatures because of their different boiling points.
• 

• Considering the table above, oxygen has more boiling point so it will flow out of the flask through a tube in the bottom while nitrogen will move out from the top.

• Hence by this method, we can easily separate various components of air.