ANS:B - extractive distillation

Extractive distillation is a variant of the conventional distillation process used to separate close-boiling or azeotropic mixtures, or to enhance the separation of components that have similar volatilities. It involves the addition of a third component, known as an entrainer or solvent, to the mixture being distilled. The entrainer forms a separate liquid phase with one or more of the components in the mixture, thereby altering the relative volatility of the components and facilitating their separation. Here's how extractive distillation works:

1. Introduction of Entrainer: A third component, called an entrainer or solvent, is added to the feed mixture before entering the distillation column. The entrainer is chosen based on its ability to form azeotropes or to preferentially interact with one or more components in the mixture.
2. Formation of Liquid Phases: Inside the distillation column, the entrainer forms a separate liquid phase with one or more of the components from the feed mixture. This results in the formation of two or more liquid phases within the column: the original feed mixture and the entrainer-rich liquid phase.
3. Altered Relative Volatility: By forming a separate liquid phase, the entrainer alters the relative volatility of the components in the mixture. This change in relative volatility helps to overcome the limitations of conventional distillation, allowing for better separation of components with similar boiling points or azeotropic behavior.
4. Separation: As the mixture is heated and vaporized in the distillation column, the components are carried upwards in the form of vapor. The entrainer-rich liquid phase flows downwards through the column. As the vapor and liquid phases move counter-currently, they undergo multiple stages of vapor-liquid equilibrium, leading to the separation of the components based on their differences in volatility.
5. Recovery of Components: The overhead vapor product from the distillation column contains the more volatile components, while the bottom product contains the less volatile components and the entrainer. The entrainer can be separated from the bottom product and recycled back to the column, if desired.

Extractive distillation is commonly used in various industries, including petrochemicals, pharmaceuticals, and chemicals, to separate mixtures that are difficult to separate by conventional distillation methods. It offers advantages such as improved separation efficiency, reduced energy consumption, and the ability to handle a wider range of feed compositions and operating conditions compared to traditional distillation techniques.