Azeotropic Distillation Process | Binary separation Technique

The azeotropic distillation process is a type of distillation process that is used in the chemical industry to separate liquid from the mixture and today we are going to learn what is an Azeotropic distillation, what are azeotropic mixtures along with examples. As an introduction, Distillation is a process of separating liquids from mixtures and is widely used in chemical industries and especially refineries and solvent recovery plants.

There are different types of distillation processes available to separate liquid components from the liquid mixture and are used based on the process requirement. those are discussed also. The distillation process works on the boiling point of the components present in the mixture and relative volatility. If the difference between the boiling point of the components is very less then the component present in the mixture can’t be separated using simple distillation. In that scenario, we need to use Azeotropic distillation.

Today we are going to get into the detail of the Azeotropic distillation process along with an example of Azeotropic distillation. so without wasting more time, lets dive into the definition followed by examples and related questions.


What is azeotropic distillation?

Azeotropic distillation also refers as a constant boiling point mixture is a process in which the substance present has a constant boiling point at a constant composition which makes it impossible to separate them using a simple distillation process. In order to perform separation of these types of mixture, we need to use a special distillation technique known as Azeotropic distillation.

These substances behave as if they are one component with one constant boiling point hence they are known as an azeotropic mixture. If we boil the binary azeotropic mixture, the vapors formed from the boiling mixture contain almost the same composition as the liquid itself which makes simple distillation not possible.

To perform the separation, we have to break this azeotrope, the most common practice is followed is the addition of an additional component referred to as entrainer. This foreign material know as entrainer when added to the mixture, it alters the relative volatility of the component and forms minimum or maximum boiling azeotrope with any one component. This process of distillation in which we add entrainer to break the azeotrope and create a new azeotrope either minimum or maximum boiling azeotrope is known as Azeotropic distillation.

Now we had understand what is azeotropic distillation, let us understand what are the different types of azetorpe.

What are Azeotropic mixtures and it types of azeotropes?

Azeotropic mixtures are the constant boiling mixture in which they boils at a constant boiling point and the composition of components in vapor are the same as the in the liquid. There are two types of azeotropes i.e. 1. Minimum boiling azeotrope and 2. Maximum boiling Azeotrope.

Minimum boiling azeotropes

Minimum boiling azeotropes are those which boil at a lower temperature than the boiling point of any one component of the mixture Maximum boiling azeotropes are those which boil at a higher temperature than the boiling point of each component in a pure state.

Example of Minimum boiling Azeotrope

In  95.63% ethanol and 4.37% water (by mass) mixture, the boiling point of ethanol is 78.4 °C and the boiling point of water is 100 °C. They form an azeotrope and the boiling point of the azeotrope is 78.2 °C which is lower than both the component, hence it is a minimum boiling azeotrope.

Maximum Boling Azeotrope

Maximum boiling azeotropes are those which boil at a higher temperature than the boiling point of each component in a pure state.

Example of Maximum Boling Azeotrope

In a mixture of hydrogen chloride, the boiling point of hydrogen chloride is -84 °C and boiling point of water is 100 °C and the boiling point of azeotrope is 110 °C. As the boiling point of the azeotrope is more than all both the component, it is called maximum boiling point.

Another term you had to learn in the definition of Azeotropic distillation is the foreign material that is added to the mixture referred to as entrainer. Let us understand entrainer in detail along with how to select entrainer.

What is Entrainer and Selection of Entrainer in azeotropic distillation?

In Azeotropic distillation, entrainer plays an important role as this makes the azeotropic distillation process happen. entrainer is a specific chemical that is being added to the mixuture which alters the relative volatility of the mixture and form a new azeotrope with either of the components. The entrainer selected must have the same volatility as the original azeotropic mixture to be separated. The selection criteria for entrainer are listed below.

  • It should enhance the relative volatility of the key component significantly.
  • Require solvent quantity to be added to the azeotropic mixture should not be excessive.
  • It should be soluble in the feed components.
  • It should be relatively inexpensive and easily available.
  • It should be stable at the operating condition of both the column.
  • it should be non-reactive with the feed components.
  • It should have low latent heat of vaporization.
  • it should be non-corrosive and non-toxic.
  • It should not form an immiscible liquid mixture at any point in the column.

Azeotropic distillation ethanol-water

As an example, we will take the example of heterogeneous azeotropic distillation of ethanol and water separation which can also be referred to as dehydration of 92.4 wt% ethanol using cyclo-hexane as a entrainer All the data of the component are described in below table.

ComponentBoiling Point
Ethanol78.25 °C
Water100 °C
Azeotrope78.09 °C
Cyclo-Hexane80.74 °C

As shown in the figure, we have two distillation columns C1 and C2 and one decanter. In Column C1, 89% mole fraction ethanol is feed where the cyclohexane is added as an entertainer (recycled from the decanter). This entrainer forms a Minimum boiling azeotrope with water. From the column C1, where we get pure ethanol from the bottom and from the top we get tertiary azeotrope. The vapor from the top of column C1 is condensed which is a heterogeneous mixture into the decanter and here get two different phases i.e. organic layer and the aqueous layer.

The organic layer is rich in entrainer and is recycled back to column C1 where the aqueous layer is feed to column C2 for further separation. In column C2, again separation is performed and we get pure water from the water and the top product containing ethanol-water and entrainer. The vapor from the top of column C2 is condensed and is feed back to column C1.

Azeotropic Distillation Ethanol-Water

FAQ (Azeotropic Distillation Process)

1. Why can’t Azeotropic Mixtures be Separated by Simple Distillation?

Ans – As discussed in the introduction, distillation is a separation process that occurs based on the difference of boiling point of the substance, and to use atmospheric distillation, the difference should be a minimum of 15°C. As the difference is very low, it can’t be separated by atmospheric distillation.

2. What are the two types of Azeotropes?

Ans – The two types of Azeotropes are minimum boiling azeotropes and maximum boiling azeotropes. This can be decided by the boiling point of the azeotrope.

3. How Azeotropes are classified and explained?

Ans – Azeotropes are classified based on the deviation from the Raoult’s law. The solutions which show a large positive deviation from the Raoultsnegative deviation from Raoult’s law form maximum boiling azeotropes at specific compositions

4. Where is azeotropic distillation used?

Azeotropic distillation is used in various industries such as polymer, chemical, Agrochemical industries and pharmaceutical industries.

Wrapping Up

This was a detailed article on what is the azeotropic definition and the Azeotropic distillation process. If you have any doubts regarding this topic then feel free to ask in the comment section, we will help you to clear your doubt. Also, give us suggestions on which topic we should cover in the next post. You can download our app Chemical Tweak to stay updated with out latest articles.

Avatar Of Ronak Prajapati

Me - Founder of Chemical Tweak - A Chemical Engineer by profession and a part-time self-trained technology blogger.

3 thoughts on “Azeotropic Distillation Process | Binary separation Technique”

  1. Way to go Ronak, I am closer to get my separation going in the lab. I need to separate H2O from i-Propanol and Propanone. I know they all form a ternary azeotrope but using Hexanes (as entrainer), it may be greatly simplified. I guess using the exptal setup shown above, one can easily achieve this separation.

  2. Hey!,

    Article was great and provide essential info about Azeotropic distillation, However there is one doubt that can we prefer pressure swing distillation for separation of binary mixture which alter relative volatility.

    • Hello,
      Thanks for appreciation,
      Now coming back to your question, Yes, when the boiling point of the mixture is very close, we need to opt for azeotropic distillation. The only problem with the pressure swing distillation is intensive equilibrium and azeotropic data is required. For Binary mixtures, if it possible to separate the compound by addition of additive then we prefer that as it is cost effective and to create pressure difference for pressure swing distillation, we need complex design, equipment and facilities.


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