Azeotropes – Introduction, Types, Example and Applications

An overview of the Azeotrope

An azeotrope (/əˈziɒtrəp/, az-i-ˈɒt-rəp/) is a mixture of two or more liquids that forms a single-phase liquid with no detectable change in composition over time. This contrasts with azeotropes in which the mixture changes composition over time.

Azeotropes are formed when two liquids are mixed in a ratio such that the resulting mixture has the same vapor pressure as the two individual liquids. The vapor pressure of a mixture is the sum of the vapor pressures of the individual liquids. When the vapor pressure of the two liquids is the same, the two liquids are said to form an azeotrope.

The azeotropic composition is said to be “unstable” because a small change in the ratio of the two liquids will cause a significant change in the vapor pressure of the mixture. As a result, the azeotropic composition is not permanent and will eventually change over time.

What are Azeotropes?

An azeotrope is a mixture of two or more liquids that are incapable of being separated by distillation. Azeotropes are formed when the vapour pressure of the individual liquids are equal and when the liquids are completely miscible. Azeotropes have a distinctive boiling point, which is lower than the boiling point of any of the individual liquids.

Types of Azeotropes

There are two main types of azeotropes: homogeneous azeotropes and heterogeneous azeotropes.

1. Homogeneous Azeotropes: A homogeneous azeotrope is a type of azeotrope where the components of the mixture are completely miscible in each other in all proportions. The boiling point of a homogeneous azeotrope is constant and independent of pressure, making it useful for separation and purification processes.

Homogeneous azeotropes are further classified into two types: minimum boiling azeotropes and maximum boiling azeotropes.

• Minimum Boiling Azeotropes: Minimum boiling azeotropes are formed when the components of the mixture have a negative deviation from Raoult’s law, which means the vapor pressure of the mixture is lower than the vapor pressure predicted by Raoult’s law. These azeotropes have a boiling point lower than either of the components and are usually found in systems where the components have a significant difference in boiling points.
• Maximum Boiling Azeotropes: Maximum boiling azeotropes are formed when the components of the mixture have a positive deviation from Raoult’s law, which means the vapor pressure of the mixture is higher than the vapor pressure predicted by Raoult’s law. These azeotropes have a boiling point higher than either of the components and are usually found in systems where the components have a similar boiling point.

2. Heterogeneous Azeotropes: A heterogeneous azeotrope is a type of azeotrope where the components of the mixture are not completely miscible in each other. These azeotropes have a boiling point range rather than a single boiling point, making them less useful for separation and purification processes.

Heterogeneous azeotropes are further classified into two types: minimum boiling heterogeneous azeotropes and maximum boiling heterogeneous azeotropes.

• Minimum Boiling Heterogeneous Azeotropes: Minimum boiling heterogeneous azeotropes are formed when the components of the mixture are partially miscible and have a boiling point lower than either of the components.
• Maximum Boiling Heterogeneous Azeotropes: Maximum boiling heterogeneous azeotropes are formed when the components of the mixture are partially miscible and have a boiling point higher than either of the components.

Examples of Azeotropes :

Azeotropes, which are mixtures of liquids that have constant boiling points and compositions, have several applications in various industries. Here are some common applications of azeotropes:

• Distillation and Fractionation: Azeotropes find extensive use in distillation and fractionation processes. Since azeotropic mixtures have constant boiling points, they can be separated from other components by distillation without decomposition. This property is especially valuable in industries such as petroleum refining, where azeotropic distillation is employed to separate and purify crude oil fractions.
• Solvent Recovery: Azeotropes play a vital role in solvent recovery processes. In some cases, azeotropic mixtures can form with solvents, making it difficult to separate them from the desired product. However, techniques like extractive distillation or pressure swing distillation can be employed to break the azeotrope and recover the pure solvent.
• Industrial Chemistry: Azeotropes are utilized in various chemical processes. For instance, azeotropic distillation is used in the production of anhydrous ethanol, where an azeotropic mixture of ethanol and water is distilled to obtain nearly pure ethanol. Azeotropic mixtures also aid in the synthesis and purification of chemicals by facilitating the separation of reactants, by-products, or impurities.
• Air Conditioning and Refrigeration: Certain azeotropic mixtures have suitable properties for use as refrigerants or working fluids in air conditioning and refrigeration systems. Azeotropic refrigerants, such as R-502 (a mixture of chlorodifluoromethane and chloropentafluoroethane), offer specific advantages like azeotropic behavior and desirable temperature-pressure characteristics for efficient heat transfer.
• Polymerization and Polymer Processing: Azeotropic mixtures can be utilized in polymerization reactions and polymer processing. For example, azeotropes are employed as reaction media or solvents to control the reaction conditions, enhance reaction rates, or modify the properties of the resulting polymers. Additionally, azeotropic mixtures can act as effective solvents for dissolving and processing polymers due to their unique boiling points and solvating abilities.
• Analytical Chemistry: Azeotropic mixtures are employed in analytical chemistry techniques, such as extraction and sample preparation methods. Azeotropic solvents can enhance the efficiency of extractions by modifying the solvent properties and facilitating the separation of desired compounds from complex mixtures.
• Energy Generation: In certain energy generation systems, azeotropic mixtures can be utilized as working fluids in heat transfer applications or as heat exchange media. Azeotropic mixtures can offer improved thermodynamic properties and stability, resulting in enhanced energy efficiency and performance.

These are just a few examples highlighting the diverse applications of azeotropes in various industries and scientific fields. The unique properties of azeotropic mixtures make them valuable for processes ranging from separation and purification to chemical synthesis and energy generation.