Azeotropic Distillation

In chemistry, azeotropic distillation is any chemical process using which, a liquid mixture containing azeotropes, can be separated into its original components, by the addition of another substance (for instance an entrainer), to enable the distillation. Azeotropes have a number of uses, but are specifically used in the creation of stable non flammable compounds, made up of a combination of flammable and non flammable constituents. To begin with, it's necessary to first understand the several concepts that the definition deals with - azeotropes, distillation and entrainers for a start. The following is a detailed explanation.

Terms and Definitions

Distillation
This is a process by which a liquid mixture is separated, by boiling the liquid, collecting the vapors and then condensing them to attain the original components. Distillation functions on the premise that different substances have different boiling points, therefore each component of the liquid mixture will vaporize at a different temperature. Thus, each component can be separately captured, cooled and condensed. The simplest example among the various types of distillation is the separation of salt from sea water. When the solution is boiled, water in the solution evaporates at boiling point, leaving the salt behind. The resultant water vapor is collected and condensed, to water.

Azeotropes
An azeotrope is a mixture of two or more liquids that cannot be separated by distillation. It is also known as a constant boiling mixture. When an azeotrope is boiled, the resultant gaseous form or vapor has the same composition, or ratio of components as the liquid state, which is why separation isn't possible through distillation. To split up the azeotrope, a material separation agent or entrainer is added, which basically 'breaks' the azeotrope. Various parameters are used to classify azeotropes. Given below are two ways to classify them.

• Positive and Negative Azeotropes
A positive azeotrope is one in which the boiling point is lower than any of its components. An example is a water-ethanol mixture, composed of 95.63% ethanol and 4.37% water by weight. This azeotrope boils at 78.2 ºC which is lower than that of water (100 ºC) or ethanol (78.4 ºC). This azeotrope is also called a minimum boiling mixture, or a pressure maximum azeotrope. Conversely an azeotrope is negative, if its boiling point is higher than any of its components. An example of this, is hydrochloric acid 20.2% and 79.8% water (by weight). The boiling point of this azeotrope is 110 ºC, which is higher than that of water as well as hydrogen chloride. Negative azeotropes are also called maximum boiling mixtures or pressure minimum azeotropes.

• Number of Components
Most azeotropes are binary (made up of two constituents) or tenary (made up of three constituents), though they can be made of more than three constituents as well.

Entrainers
This is a substance that is added to an azeotrope to facilitate the separation. A specific entrainer has to be identified for the process of azeotropic distillation, as not all entrainers will work with a given azeotrope. A residual curve map is used to identify which entrainer is suitable. Entrainers are divided into different types, depending on the function they perform to enable the separation:

• Entrainers that react with one of the components of the azeotrope
• Entrainers that induce a liquid-phase separation
• Entrainers that ionically dissociate
• Entrainers that do not cause a liquid phase separation

There are different types of azeotropic distillation as well, like homogeneous and heterogeneous azeotropic distillation. The former is a separation process of azeotropes where the entrainer is completely miscible with the liquid. Depending on whether a single component or all the components of an azeotrope need to be separated, continuous distillation is carried out, until the desired constituent is achieved. In the latter, the entrainer will form an azeotrope with one or more of the components of the initial azeotropic mixture, enabling separation of the remaining constituent by a distillation sequence. Azeotropic distillation is an important process in chemical engineering. Understanding the terms used in the discussion of this process will give you the ability to grasp the intricacies of this topic.