Principle:-                                                          

The separation of components from a liquid mixture through distillation      depends on the differences in boiling points of the individual components. A liquid is said to ‘boil’ when its vapour pressure equals the surrounding pressure. Liquids with high vapour pressure boil at lower temperatures and are termed as ‘volatile’ liquids. So, distillation simply occurs because of the differences in the volatility of the various components of a liquid mixture.

Apparatus:- A simple distillation apparatus consists of three parts; a flask equipped with a thermo meter and one outlet tube for the passage of vapour, a condenser and a vessel in which the condensed vapour is collected.

Method:- The liquid mixture is placed in the flask and heated. The substance to vapourize first is the one that has the lowest boiling point. The vapour then passes through the condenser where, on being cooled, it reverts to its liquid form or simply ‘condenses’ and gets collected in a receiving vessel. The product so obtained is known as the ‘distillate’. Substances having a higher boiling point remain in the flask and constitute the ‘residue’.

Distillation Columns: There are many types of distillation columns, each designed to perform specific types of separation. Based on operation, the distillation columns are classified into batch and continuous columns. In batch operation, the distillation process is carried out batch wise, while in continuous column process, there is a continuous feed stream.

Simple Distillation: It is effective for separating a volatile liquid from a nonvolatile substance or when separating two liquids that after in boiling point by about 50 degrees or more. Besides, the distillation process is commonly used for removing microorganisms, dissolved solids, organic compounds, heavy metals, and radionuclide from water. Although most impurities of water are removed through distillation, some volatile organic contaminants, certain pesticides and volatile solvents that boil at temperatures very close to water remain in the distillate.

Fractional Distillation: In this process, when the substance with the lowest boiling point has been removed, the temperature is raised and the distillation process is repeated for separating out the substance having the next lowest boiling point. A more efficient method of fractional distillation involves placing a vertical tube called a fractionating column between the flask and the condenser. This column is filled special material on which the vapours can repeatedly condense and reevaporate as they move towards the top, thus effectively distilling the vapours many times. So, the more volatile liquids tend to reevaporate and keep moving upwards and accumulate at the top. The column can be tapped at various levels to remove different fractions. Fractional distillation is commonly used in refining petroleum, where some of the fractions include gasoline, benzene, kerosene, fuel oils, lubricating oils, and paraffin.