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Azeotropic Mixtures-Types, Examples, and Separation

October 25, 2023
written by Adeel Abbas

An azeotropic mixture is a combination of two or more liquids that maintain a constant composition when boiled or condensed. It means they cannot be fully separated by distillation alone. These mixtures have unique properties and are often used in industrial processes.

What Is an Azeotropic Mixture?

An azeotropic mixture, often referred to as an azeotrope, is a special type of liquid mixture that exhibits a unique behavior during distillation. In an azeotrope, the composition of the vapor phase is the same as the composition of the liquid phase.

This means that during distillation, the vapor and liquid phases maintain a constant composition, and the mixture cannot be further separated by simple fractional distillation.

Azeotrope Mixture Types

Azeotropes come in two primary types:

  1. Minimum Boiling Azeotrope: In a minimum boiling azeotrope, the azeotropic mixture has a boiling point lower than that of any of its individual components. As the mixture is heated, it vaporizes into a vapor phase with the same composition as the liquid phase. Common examples include the ethanol-water azeotrope (approximately 95.6% ethanol and 4.4% water) and the hydrochloric acid-water azeotrope (approximately 20.2% hydrochloric acid and 79.8% water).
  2. Maximum Boiling Azeotrope: In a maximum boiling azeotrope, the azeotropic mixture has a boiling point higher than that of any individual component. As it is heated, it forms a vapor phase with an identical composition to the liquid phase. A well-known example is the nitric acid-water azeotrope, containing approximately 68% nitric acid and 32% water.

Azeotrope Examples

Azeotropes can be found in various chemical and industrial processes. Some examples include:

  • Ethanol-Water Azeotrope: This is one of the most commonly encountered azeotropes. It occurs in the distillation of alcoholic beverages, where the azeotrope’s composition of approximately 95.6% ethanol and 4.4% water is responsible for limiting the concentration of ethanol that can be achieved by distillation.
  • Hydrochloric Acid-Water Azeotrope: Found in the chemical industry, this azeotrope is used to produce hydrochloric acid with a specific concentration.
  • Sulfuric Acid-Water Azeotrope: This azeotrope, with approximately 98.3% sulfuric acid and 1.7% water, is used in various chemical processes, including the dehydration of alcohols.
  • Acetic Acid-Water Azeotrope: With a composition of approximately 27.7% acetic acid and 72.3% water, this azeotrope is crucial in the production of acetic acid.

Azeotrope Separation

Separating components of an azeotropic mixture is challenging due to the constant composition of the vapor and liquid phases. Various techniques are employed to break the azeotrope:

  1. Azeotropic Distillation: Azeotropic distillation, or extractive distillation, involves adding a third component, called an entrainer or an azeotrope-breaking agent, to the mixture. The entrainer forms a new azeotrope with one of the components, facilitating their separation.
  2. Pressure Swing Distillation: By changing the pressure during distillation, it is possible to alter the composition of the azeotrope. This can help in separating the components.
  3. Pervaporation: Pervaporation is a membrane separation technique that exploits differences in the permeability of components through a selective membrane to separate the azeotropic mixture.

Azeotropic Distillation

Azeotropic distillation, as mentioned earlier, is a widely employed method to separate azeotropic mixtures. It involves the addition of an entrainer to the mixture to create a new azeotrope that can be separated more easily. This technique is used extensively in the chemical, petrochemical, and pharmaceutical industries to purify and separate components from challenging azeotropic mixtures.

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