Fractional Distillation

Fractional distillation is a type of distillation that involves the separation of a mixture into its constituent substances based on the differences in their boiling point (volatility). It is practiced for those mixtures, in which the boiling point of the components differ by less than 25ºC at one atmospheric pressure. The component with the least boiling point is separated first, while the component with the highest boiling point is drawn off at the last. If the boiling point of the constituents differ by more than 25ºC, then simple distillation procedure is implemented for separation.

Working of Fractional Distillation

In the process of fractional distillation, a specific apparatus is used for boiling the mixture and collecting a series of fractions of the components. The laboratory setup for fractional distillation includes a round-bottom boiling flask, condenser, adapter (that connects boiling flask and condenser), fractionating column (within the adapter) and a collection flask. A thermometer is fixed to the adapter for monitoring the temperature.

The fractionating column is usually composed of plates arranged in a horizontal manner, which enhances the separation by allowing condensation, evaporation and re-condensation of the separated components. The vapor of the component substance after passing the fractionating column is passed to the condenser, which is fixed with a water inlet and water outlet pipe. In the water inlet pipe, cold water is constantly supplied to cool down or condense the separated vapor. Eventually, warm water after being circulated through the condenser is removed via the outlet pipe. The component undergoes condensation in the condenser, which is then collected in the collection flask as a purified liquid.

The components with lower boiling point tends to collect at the top portion of the fractionating column, while those with high boiling point settle at the bottom of the column. Though fractional distillation provides better separation of constituents, it requires more energy and longer time than the usual simple distillation. The more the number of plates, the longer is the distillation. For these reasons, fractional distillation is implemented for purifying complex mixtures with many components.

Uses of Fractional Distillation

Crude oil that contains other impurities are separated by fractional distillation. In fact, fractional distillation is the most common procedure followed for separation of mixtures in petroleum refining industries and chemical industries. In petroleum refining, gasoline is separated first followed by kerosene, heating oil and lubricating oil. In industries, large fractionation towers (vertical structures) are used for distillation process. Under controlled conditions of feed (complex mixture), temperature and condenser, product is being collected continuously. This technology implemented in industries is referred to as continuous fractional distillation.

Fractional distillation is also practiced in large scale alcoholic fermentation in order to purify alcoholic beverages. Beer and wine produced after fermentation of grains and grapes contain less than 15 percent alcohol. It is due to the fact that a high concentration of alcohol kills the yeast that are responsible for proceeding fermentation. In such a condition, fractional distillation is practiced to purify alcohol and make stronger alcoholic drinks. In addition, fractional distillation is also used for solvent recycling, extraction of essential oils and purification of fragrances in perfume industries.