What is Fractional Distillation?
The simple distillation process cannot be used for separating components of a mixture that have very close boiling points. Fractional distillation is used to separate miscible liquids if the difference in their boiling points is less than 25∘ C.
While the basic apparatus remains the same as in the distillation process, there is an additional fractionating column. The column has glass beads which provide vapours with the surface to condense again and again.
Fractional distillation
Let us understand how fractional distillation works with an example.
When a mixture of two liquids A and B having boiling points of 90∘ C and 100∘ C, respectively, is heated, both liquids start vaporising at almost the same time. But due to the beads in the fractionating column, the vapours condense and the liquid keeps moving back into the flask.
In the process, liquid A, which has the lower boiling point starts rising up the column. At 90∘ C the condensed liquid that pours out is the purest form of A. And at 100∘ C, liquid B pours out in its purest form.
Between the two temperatures, the liquid is still a mixture. It will have to undergo another round of fractional distillation.
When liquids with very close boiling points mix, they have to undergo multiple rounds of fractional distillation to be separated.
Application of fractional distillation
Crude oil is separated in huge refineries with the help of fractional distillation. But the process is slightly different. The fractionating column has multiple openings. When the different components heat up differently, they reach only a particular point in the tube. Depending on these points, the different components are taken out. The ones which have a low boiling point reach the topmost opening while the ones with the highest boiling point remain at the base.
Application of fractional distillation in refining crude oil