Fractional distillation is a type of distillation which involves the separation of miscible liquids. The process involves repeated distillations and condensations and the mixture is usually separated into component parts. The separation happens when the mixture is heated at a certain temperature where fractions of the mixture start to vaporize.
The basic principle of this type of distillation is that different liquids boil and evaporate at different temperatures. So when the mixture is heated, the substance with lower boiling point starts to boil first and convert into vapours.
Few fractional distillation apparatuses are required for the process. It includes distilling flask, condenser, receiver, fractionating column, thermometer and heat source.
After setting up the apparatus, a mixture of two miscible liquids A and B is taken where A has more volatility than substance B. The solution is added into the distilling flask while the fractionating column is connected at the tip of the flask. Heat is applied which increases the temperature slowly. The mixture then starts to boil and vapours start rising in the flask. The vapours are from the volatile component A. The vapours then start moving through the fractionating column into the condenser where it is cooled down to form a liquid which is collected in the receiver.
Throughout the process, vaporization and condensation take place repeatedly until the two mixtures are separated completely.
Fractional distillation is one of the popular separation techniques used in several industries. While the principle behind the process remains the same, the distillation is carried out on a larger scale. Usually, huge vertical cylindrical columns are known as “distillation columns” or “distillation or fractionation towers” are used. These industrial towers use reflux which ensures complete separation of the mixtures.
A common example of fractional distillation in industries is the separation of various components of crude oil. Crude oil normally contains substances such as paraffin wax, gasoline, diesel, naphtha, lubricating oil and kerosene. The distillation process helps in separating these components effectively.
Crude oil is added in the chamber and is heated with high-pressure steam. The mixture starts boiling and vapour is formed. At this point, various substances enter into the vapour phase. The vapour rises up in the fractional distillation column which consists of several plates. The plates have holes that allow the vapour to pass through it. The temperature is usually kept low at the top of the fractionating column.
Here, components with the highest boiling point will condense in the lower part of the column while substances with a low boiling point will condense at the top. The condensed vapours or liquid fractions are then removed from the sides of the column. The collected liquid fractions can further be passed through condensers to cool them even more.