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How Does Extractive Distillation Work?

Published in Chemical Separation 5 mins read

Extractive distillation is a specialized chemical engineering technique used to separate liquid mixtures that are difficult or impossible to separate by conventional distillation methods. It specifically addresses challenges like azeotropes (mixtures with a constant boiling point) or components with very similar boiling points.

At its core, extractive distillation is the use of a third component to separate two close-boiling components in which one of the original components in the mixture is extracted by the third component and retained in the liquid phase to facilitate separation by distillation. This "third component," known as the extractive solvent or entrainer, plays a crucial role in altering the relative volatilities of the original mixture components, making their separation feasible.

The Core Principle

Traditional distillation relies on differences in boiling points to separate components. However, when components form azeotropes or have very close boiling points, their volatilities become too similar, making separation inefficient or impossible. This is where extractive distillation steps in:

  • Altering Volatility: The extractive solvent is added to the mixture. This solvent is carefully chosen to interact more strongly with one of the components (let's call it Component B) than the other (Component A).
  • Selective Interaction: By forming stronger intermolecular bonds with Component B, the solvent effectively increases the boiling point of Component B, making it less volatile and "extracting" it into the liquid phase.
  • Enhanced Separation: Component A, whose volatility is less affected by the solvent, becomes comparatively more volatile. This difference in volatility is what enables a clean separation in the distillation column.

The Role of the Extractive Solvent

The choice of extractive solvent is critical for the success of the process. An ideal solvent possesses several key characteristics:

  • High Boiling Point: It should boil at a much higher temperature than the components being separated, ensuring it remains in the liquid phase throughout the main separation column and can be easily separated later.
  • Selective Solvency: It must selectively dissolve or interact strongly with only one of the components in the original mixture.
  • Non-reactive: It should not chemically react with the components of the mixture.
  • Easily Separable: It must be easily separable from the extracted component in a subsequent step (e.g., by simple distillation or decantation).
  • Cost-Effective: Low cost, readily available, and ideally non-toxic and environmentally friendly.

The Process Steps

Extractive distillation typically involves a two-column system:

1. Main Extractive Distillation Column

This is the primary separation unit where the magic happens.

  • Feed Introduction: The original binary mixture (e.g., A + B) and the extractive solvent are continuously fed into this column. The solvent is often introduced near the top of the column.
  • Selective Interaction & Vaporization: As the mixture flows down the column, the solvent interacts with Component B, holding it in the liquid phase. Component A, being more volatile in the presence of the solvent, vaporizes and rises to the top.
  • Top Product: The vapor at the top of the column is primarily Component A, now purified. This vapor is condensed and collected.
  • Bottom Product: The liquid at the bottom of the column consists of the extractive solvent mixed with Component B.

2. Solvent Recovery Column

The bottom product from the main extractive column (solvent + Component B) is then sent to a second distillation column.

  • Separation: In this column, Component B is separated from the solvent. Since the solvent has a much higher boiling point, Component B typically vaporizes and is collected as the top product.
  • Solvent Recycle: The purified extractive solvent, remaining as the bottom product, is then cooled and recycled back to the main extractive distillation column, making the process economical.

Here's a simplified flow for the main column:

Feed Stream Role of Solvent (Example) Top Product (Vapor) Bottom Product (Liquid)
Component A + B Selectively binds with Component B Purified Component A Solvent + Component B
(e.g., Ethanol + Water) (e.g., Ethylene Glycol with Water) Anhydrous Ethanol Ethylene Glycol + Water

When is Extractive Distillation Used?

Extractive distillation is a powerful solution for challenging separations in various industries:

  • Breaking Azeotropes: This is perhaps its most common application, allowing the separation of mixtures like ethanol and water, which form an azeotrope that cannot be fully separated by simple distillation.
  • Close-Boiling Point Separations: For components with very similar boiling points (e.g., isomers), where the relative volatility is too low for efficient conventional distillation.
  • High Purity Requirements: When extremely high purity is needed for one or both components.

Examples of Applications

  • Ethanol-Water Separation: To produce anhydrous (water-free) ethanol, solvents like ethylene glycol or glycerol are used. These solvents interact strongly with water, allowing ethanol to distill off at a higher purity.
  • Aromatics Production: In the petrochemical industry, it's used to separate valuable aromatic compounds (like benzene, toluene, xylene) from non-aromatic hydrocarbons. Solvents such as sulfolane or N-methyl-2-pyrrolidone (NMP) are commonly employed.
  • Acetone-Methanol Separation: Another common azeotropic separation.

By introducing a carefully selected third component, extractive distillation ingeniously modifies the vapor-liquid equilibrium of a mixture, enabling efficient and precise separations that would otherwise be impossible.