Difference Between Azeotropic and Extractive Distillation

What is Azeotropic Distillation?

Azeotropic distillation is a separation technique used to separate the components of a mixture by forming an azeotrope. Azeotropes are component mixtures with a constant boiling point. This type of mixture cannot be separated into components by simple distillation since all the components have the same boiling point. When an azeotropic mixture is boiled, the proportion of components in the liquid and its vapor phase is equal.

In azeotropic distillation method, a new component (known as the entrainer) is added to the azeotropic mixture in order to form a new azeotrope that boils at a lower temperature than the existing azeotrope. Then the system has two immiscible liquid phases with different boiling points (heterogeneous).

Azeotropic distillation employs various techniques to break or modify the azeotropic behavior, allowing for separation of the components. These techniques include:

  • Addition of Entrainer: An entrainer, which is a third component not present in the original mixture, is added to the system. The entrainer forms a new azeotrope with one of the components of the original mixture, altering the vapor-liquid equilibrium and facilitating separation.
  • Pressure Swing: By changing the pressure conditions during distillation, it’s possible to alter the boiling points of the components, thereby breaking the azeotrope. This is achieved by operating at pressures where the relative volatilities of the components are significantly different from those at atmospheric pressure.
  • Extractive Distillation: In this method, a selective solvent is added to the mixture. The solvent forms a new liquid phase with one of the components, creating a different vapor-liquid equilibrium system. This alters the relative volatility of the components and allows for their separation.

What is Extractive Distillation?

Extractive distillation is a sophisticated and highly efficient separation process utilized in the chemical industry to separate components of a mixture with similar boiling points that cannot be effectively separated using conventional distillation methods. This method relies on the introduction of a solvent or extractive agent to alter the vapor-liquid equilibrium and enhance separation.

The added component, known as the solvent, forms a separate liquid phase with one of the components and alters the relative volatility of the components. This results in the formation of an azeotrope, which is a mixture that boils at a constant temperature and has a composition different from the original mixture.

The mixture is then heated, and distillation is carried out. As the vapor rises through the distillation column, it contains a different composition of components due to the presence of the solvent. The components are then separated based on their differing affinities for the solvent and their vapor pressures.

After distillation, the solvent-rich fraction is separated from the desired components. The solvent is then recovered from this fraction, often through additional distillation or other separation techniques, and recycled back into the process.

Extractive distillation is useful in the extraction of essential oils, flavors, and fragrances from natural sources. It helps to concentrate and purify the desired aromatic compounds. In the production of specialty chemicals, extractive distillation separates and purifies high-value compounds. It is particularly useful when dealing with compounds that are sensitive to high temperatures.

Azeotropic vs Extractive Distillation

BasisAzeotropic DistillationExtractive Distillation
ObjectiveSeparate azeotropic mixturesSeparate non-azeotropic mixtures
Additional ComponentNo additional component addedRequires addition of an extractive agent or solvent
Boiling PointsComponents have very similar boiling pointsComponents have significantly different boiling points
Separation MechanismRelies on formation of temporary azeotropesModifies relative volatilities by adding a solvent
Solvent RecoveryNot applicable, no solvent usedSolvent recovery required from distillate
Energy RequirementsRequires less energyMay require more energy due to solvent regeneration
ApplicationsCommonly used for close-boiling mixtures like ethanol-waterUsed for mixtures with larger boiling point differences
ComplexityGenerally simpler setupMore complex setup due to addition of solvent

Key Takeaways

Azeotropic distillation

  • It aims to separate azeotropic mixtures, which are mixtures of liquids that boil at a constant temperature and composition. The goal is to break the azeotrope and obtain pure components.
  • No additional component is added to the system.
  • It is used when the components of the mixture have very similar boiling points.
  • It relies on the formation of temporary azeotropes, which are then separated using different methods such as pressure swing or entrainer addition.
  • It does not involve the use of a separate solvent, so there’s no solvent recovery step required.
  • It requires less energy compared to extractive distillation since it doesn’t involve the addition of a solvent.
  • It is commonly used in the petroleum and chemical industries for separating close-boiling mixtures like ethanol-water or benzene-toluene.
  • Its setups are generally simpler compared to extractive distillation systems.

Extractive Distillation

  • It is used to separate mixtures that form non-azeotropic systems. It involves adding an additional component (extractive agent or solvent) to the mixture to modify the relative volatilities of the components, thereby facilitating separation.
  • It requires the addition of an extractive agent or solvent to alter the vapor-liquid equilibrium and enhance the separation.
  • It is often employed when the components have significantly different boiling points.
  • It alters the relative volatility of components by introducing a third component (the solvent or extractive agent) that forms different azeotropes with the original components, facilitating their separation.
  • It involves the use of a solvent, which must be recovered from the distillate and recycled, adding an additional step to the process.
  • It may require more energy due to the additional heat needed for solvent regeneration.
  • It finds applications in separating mixtures with components having larger boiling point differences, such as separating close-boiling isomers or purifying products from complex mixtures.
  • Extractive distillation systems are more complex due to the addition of a solvent and the need for additional equipment for solvent recovery.