What is meant by fractional distillation

The process in which a volatile liquid is first vaporised at its boiling point and the vapours are condensed to get back the liquid in pure form is called distillation.

If the difference in boiling points of the liquids in the mixture is less than 250C the method of fractional distillation is used to separate them. 

Fractional distillation method is same as simple distillation except that in the apparatus a fractionating column is fitted in between the distillation flask and the condenser.

It makes the appratus more efficient . A fractionating column is a wide tube packed with glass beads, which provides a larger surface area of cooling. Vapours of liquid having higher boiling point get condensed on the surface of these beads, while the vapours of lower boiling point liquid pass on upwards. Length of the column also increases the efficiency of the process.

Example:  In an oil refinery, the different fraction of petroleum products are separated by fractional distillation of crude petroleum.