Extractive distillation:
Extractive distillation is a process used in chemical engineering to separate two or more components from a mixture. It involves the addition of a solvent to the mixture, which modifies the relative volatility of the components and allows for their separation. The solvent used in extractive distillation is chosen based on its ability to increase the relative volatility of the desired component.

Increased relative volatility:
The relative volatility of a component in a mixture is a measure of how easily it vaporizes compared to the other components. A higher relative volatility means that the component will vaporize more readily, making it easier to separate from the mixture. By adding a solvent to the mixture, the relative volatility of the desired component can be increased, facilitating its separation.

Role of solvent:
The solvent plays a crucial role in extractive distillation by altering the liquid-vapor equilibrium of the mixture. It interacts with the components of the mixture, forming new liquid and vapor phases with different compositions. This change in equilibrium allows for the separation of the desired component from the other components.

Selection of solvent:
The choice of solvent depends on several factors, including its selectivity for the desired component and its compatibility with the other components in the mixture. The solvent should have a high affinity for the desired component, enabling it to form a separate liquid phase with a higher concentration of the desired component. Additionally, the solvent should not react chemically with the components in the mixture.

Advantages of extractive distillation:
1. Increased separation efficiency: By using a solvent to increase the relative volatility, extractive distillation can achieve higher separation efficiencies compared to conventional distillation techniques.
2. Reduced energy consumption: Extractive distillation can often operate at lower temperatures and pressures, resulting in lower energy requirements.
3. Versatility: Extractive distillation can be used for a wide range of separation tasks, including the removal of close-boiling components, purification of azeotropic mixtures, and separation of heat-sensitive compounds.

In conclusion, the use of a solvent in extractive distillation is essential for increasing the relative volatility of the desired component and facilitating its separation from the mixture. This technique offers several advantages over conventional distillation methods, making it a valuable tool in the field of chemical engineering.