Fractional distillation involves separation of a mixture into its constituents, based on the differences in their boiling point (volatility). It is practiced for those mixtures, whose boiling point of the components differ by less than 25º C as compared to one atmospheric pressure. The component with the least boiling point is separated first, while the one with the highest boiling point is separated last. If the boiling point of the constituents differ by more than 25º C, then the simple distillation procedure is implemented for separation.
A specific apparatus is used for boiling the mixture and collecting a series of fractions of the constituent substances. The laboratory setup includes a round-bottom boiling flask, condenser, adapter (that connects the 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 that are 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 former one, cold water is constantly supplied to cool down or condense the separated vapor. Eventually, warm water that is being circulated through the condenser, is removed via the latter one. The component undergoes condensation in the condenser, which is then collected in the collection flask as a purified liquid.
The constituents with lower boiling point tend to collect at the top portion of the fractionating column, while those with a higher boiling point settle at the bottom. Though this type of distillation provides better separation of constituents, it requires more energy and a longer time than the usual simple distillation. The more the number of plates, the longer is the process. For these reasons, fractional distillation is implemented for purifying complex mixtures with many components.
Crude oil that contains other impurities are separated by fractional distillation. In fact, it 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 are used for distillation processes. Under controlled conditions of feed (complex mixture), temperature, and condenser, the product is being collected continuously. This technology that is implemented in industries is referred to as continuous fractional distillation.
This type of distillation is also practiced on a large-scale for 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, it is also used for solvent recycling, extraction of essential oils, and purification of fragrances in perfume industries.