ANS:A - added to alter the relative volatility of the mixture.

In extractive distillation, the solvent is added to alter the relative volatility of the mixture. Here's how it works:

1. Altering Relative Volatility: Extractive distillation is a variation of conventional distillation used to separate close-boiling or azeotropic mixtures. It involves adding a third component, called a solvent or an entrainer, to the feed mixture. This solvent interacts preferentially with one of the components in the feed mixture, altering the relative volatility between the components.
2. Enhancing Separation: By adding the solvent, the relative volatility between the components in the feed mixture changes, making it easier to achieve separation. The solvent forms a separate liquid phase with one of the components, effectively breaking the azeotrope or enhancing the differences in volatility between the components.
3. Separation Process: During the distillation process, the components in the feed mixture are vaporized and rise through the column. As they encounter the solvent, they may preferentially partition into the solvent phase, depending on their solubility and affinity for the solvent. This leads to improved separation efficiency compared to conventional distillation.
4. Solvent Characteristics: While the solvent can vary in its properties depending on the specific application, it is typically chosen based on its ability to alter the relative volatility of the components in the feed mixture. The solvent should have a high affinity for one of the components and should be immiscible or partially miscible with the other components.

1. Added to alter the relative volatility of the mixture:
   • In extractive distillation, the addition of a third component, known as a solvent or an entrainer, is a fundamental aspect of the process. This solvent is carefully chosen based on its ability to modify the relative volatility of the components in the feed mixture.
   • The primary objective of adding the solvent is to improve the separation of components that have similar boiling points or form azeotropic mixtures. These mixtures are challenging to separate by conventional distillation methods due to their close boiling points or the formation of constant boiling mixtures.
   • The solvent interacts preferentially with one of the components in the feed mixture, altering the vapor-liquid equilibrium conditions in the distillation column. This change in equilibrium conditions leads to an increase in the relative volatility between the components, making it easier to achieve separation.
   • The choice of solvent depends on various factors, including its selectivity towards one of the components, its compatibility with the feed mixture, and its ease of recovery from the distillation process.
2. Of high volatility:
   • While the volatility of the solvent can vary depending on the specific application and the components being separated, it is generally advantageous for the solvent to have a relatively high volatility.
   • A solvent with high volatility can vaporize easily at the operating conditions of the distillation column, ensuring effective contact with the components in the feed mixture.
   • The high volatility of the solvent facilitates its distribution throughout the column, allowing it to interact efficiently with the vapor and liquid phases and enhance mass transfer and separation efficiency.
   • However, the selection of the solvent's volatility should be balanced with other considerations, such as its solubility properties and its impact on the overall process economics.
3. Present in overhead stream:
   • In extractive distillation, the solvent is typically introduced into the distillation column at a point where it can interact with the components as they vaporize and rise through the column.
   • As the vaporized components ascend the column, they come into contact with the solvent, which may form a separate liquid phase with one of the components.
   • The solvent, along with the separated component, is carried overhead in the distillation column's vapor stream.
   • After leaving the column, the overhead stream, which contains the solvent and the separated component, undergoes further processing to separate the solvent from the component for subsequent recovery and reuse.
4. Of high viscosity to give high tray efficiency:
   • While high tray efficiency is desirable in distillation processes for achieving effective separation of components, using a solvent with high viscosity is not typically advantageous.
   • High viscosity can impede mass transfer and mixing in the distillation column, potentially reducing separation efficiency rather than enhancing it.
   • Solvents with high viscosity may lead to increased resistance to flow and higher pressure drops across the column, which can affect the overall performance and energy consumption of the distillation system.
   • Therefore, when selecting a solvent for extractive distillation, it is generally preferable to choose one with a viscosity that allows for efficient mass transfer and mixing while minimizing the negative impact on process performance.