Extractive Distillation is defined as distillation in the presence of a miscible, high boiling, relatively non-volatile component, the solvent, that forms no azeotrope with the other components in the mixture.
Used for mixtures having a low value of relative volatility, nearing unity. Mixtures having a low relative volatility can not be separated by simple distillation because the volatility of both the components in the mixture is nearly the same causing them to evaporate at nearly the same temperature to a similar extent, whereby reducing the chances of separating either by condensation.
The method of extractive distillation uses a solvent, which is generally nonvolatile, has a high boiling point and is miscible with the mixture but doesn't form an azeotropic mixture. The solvent interacts differently with the components of the mixture whereby causing their relative volatilities to change. This causes the mixture to be separated through distillation. The component with the greater volatility separates out as the top product. The bottom product consists of a mixture of the solvent and the other component, which can be separated easily owing to the fact that the solvent doesn't form an azeotrope with it. The two can be separated by any of the methods available.
A very essential part of this type of distillation is the selection of the solvent, which plays 'the role' of separating the two components. Certain points to be kept in mind are that the solvent should alter the relative volatility significantly or the attempt might turn out to be futile. Should be economical (quantity to be used, its cost itself and the availabilty). Should be easily separable from the bottom product. Should not react chemically with the components/mixture; should not cause corrosion in the equipment either. A Classic example to be cited here could be the azeotropic mixture of benzene and cyclohexane, the solvent used here could be aniline or any other suitable alternative.