Azeotropic Distillation

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Ethanol Water Azeotrope Distillation

Azeotropic distillation is the process of separating all the components of an azeotropic mixture by the process of distillation. An azeotropic mixture consists of two or more liquids which cannot be separated through simple distillation since the vapours that are formed via boiling the azeotropic mixtures consist of the same proportions of the liquids as the mixture itself. Hence, the azeotropic distillation is a specialized distillation that involves using specific techniques in breaking the azeotropes.

One of the most common methods to break an azeotrope includes adding a material separation agent which has the capability for changing the molecular interactions between all the components of the azeotrope. Adding the material separation agent alters the activity of the coefficient of the compounds of the azeotropic mixture. It hence changes the relative volatility of the whole azeotropic mixture. Today, we will learn about azeotropic distillation, ethanol-water azeotrope distillation, and the difference between azeotropic distillation and extractive distillation.

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Material Separation Agent

Let us first learn about what is a material separation agent.

The addition of a material separation agent like benzene to a mixture of ethanol/water tends to change the molecular interactions and, in turn, eliminates the azeotrope. When added in the liquid phase, the new component tends to alter the activity coefficient of several compounds in several ways, hence altering the relative volatility of the mixture. Higher deviations from the Raoult's law tends to ease out achieving significant changes in the relative volatility along with the addition of the other component. 

In an azeotropic distillation, the volatility of the newly added component is the same as that of the mixture. A new azeotrope gets formed with one or more components depending on differences in polarity. If the material separation agent is opted for forming azeotropes having more than one component in the feed, it is called an entrainer. The added entrainer must be recovered by decantation, distillation, or another separation method and then returned to the top of the original column.

Distillation of Water/Ethanol

Let us now discuss the distillation of ethanol and water.

A historical example of an azeotropic distillation is its usage to dehydrate water and ethanol mixtures. To achieve this, a nearly azeotropic mixture gets delivered to the final column in which the azeotropic distillation tends to take place. Different entrainers are used in carrying out this process such as benzene, hexane, cyclohexane, pentane, acetone, isooctane, heptane, and diethyl ether. Out of all these compounds, benzene and cyclohexane have been into use the most widely. However, since benzene has been found to have carcinogenic properties, there was a decline in its usage. While this was the standard way to dehydrate ethanol previously, it lost its favour significantly because of the higher energy and capital cost that is associated with it. Another lesser toxic and favourable method than the usage of benzene for breaking the ethanol-water azeotrope is the usage of toluene.

Difference Between Extractive Distillation and Azeotropic Distillation

Now, let us look at the difference between extractive distillation and azeotropic distillation.

The main difference between extractive distillation and azeotropic distillation lies in the process followed during the separation of the mixture. In extractive distillation, a particular separation solvent has to be used in each mixture which should not form an azeotrope. Considering this, extractive distillation is considered a comparatively easier method when it is compared to the azeotropic distillation.

FAQ (Frequently Asked Questions)

1. How Can We Separate an Azeotropic Mixture by Distillation?

Ans: We can separate an azeotropic mixture by distillation by the following method.

Water and ethanol tend to form an azeotropic mixture which can be separated by the method of azeotropic distillation. For achieving this, several material separation agents like benzene, pentane, hexane, cyclohexane, acetone, and diethyl ether are generally used. 

However, benzene was the most common in use historically for the same purpose. However, since benzene has a carcinogenic nature it is known that there was a decline in its usage in the azeotropic distillation of the mixtures of ethanol and water. In modern practices, the azeotrope of ethanol and water is generally broken by using toluene. Other options suitable for the dehydration of the mixture of ethanol and water are isooctane, cyclohexane and heptane.

2. How to Break an Azeotrope?

Ans: For the azeotropes that boil low, the volatile compound is not entirely purified by the process of distillation. For obtaining the pure material, one should break the azeotrope. This process involves a separation method which does not depend on distillation. A more common approach includes the usage of molecular sieves. By treating 96% ethanol with the molecular sieve, it yields anhydrous alcohol. The sieve exhibits adsorbing the water from the given mixture. Also, the sieves can be regenerated subsequently through dehydration with the help of a vacuum oven.

3. Can You Separate Azeotropic Mixtures by Simple Distillation?

Ans: No, the azeotropic mixtures cannot be separated with the method of simple distillation.

An azeotropic mixture, or simply an azeotrope, is often called a constant boiling point mixture. It is a type of a mixture which contains two or more than two liquids that have proportions which cannot be changed or altered with the process of simple distillation. The reason you cannot separate azeotropes through the method of simple distillation is that when you boil azeotropes, the vapours tend to have the same proportions of its constituents to that of the unboiled mixtures.