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Azeotropes or azeotropic mixtures have always been a topic of interest due to their unique properties and the inability to separate them completely using conventional distillation. A classic example of azeotropes occurs in winemaking wherein an Ethanol-water mixture forms an azeotropic mixture at 96% Ethanol by volume which prohibits its further purification by distillation. In this blog, let’s look at how this happens and how we can separate such azeotropic mixtures.

Before diving into azeotropes and azeotropic distillation, let’s have a quick look at the distillation process.

What are azeotropes or azeotropic mixtures?

Azeotropes are constant boiling point mixtures. Azeotropes are mixtures of two or more liquids whose composition cannot be altered or changed by simple distillation. This occurs because the vapour’s constituent ratios are identical to those of the unboiled mixture when an azeotrope is boiled. Azeotropes are also known as constant boiling point mixtures since distillation leaves their composition unaltered.

There is a distinctive boiling point for each azeotrope. An azeotrope’s boiling point is either lower or higher than the boiling points of any of its constituents. Depending on the boiling point deviation, we have two types of azeotropes as follows:

Maximum boiling azeotropes

Maximum boiling azeotropes are those mixtures that have a boiling point higher than any of their constituents. These azeotropes show a large negative deviation from Raoult’s Law. So we can call them negative azeotropes or pressure minimum azeotropes.

Hydrochloric acid at a concentration of 20.2% and 79.8% water (by mass) is an example of a negative azeotrope. Water and hydrogen chloride both boil at 100 °C and 84 °C, respectively, but the azeotrope boils at 110 °C, exceeding the boiling points of both of its ingredients. Any hydrochloric acid solution can boil at a maximum temperature of 110 °C. Other negative azeotropes include:

Nitric acid (68%)/water, which boils at 120.2 °C at 1 atm
Hydrofluoric acid (35.6%)/water, which boils at 111.35 °C
Water with perchloric acid (71.6%), 203 °C boiling point
Water and sulfuric acid (98.3%), boiling at 338 °C

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Minimum boiling azeotropes

Minimum boiling azeotropes are those mixtures that have a boiling point higher than any of their constituents. These azeotropes show a large positive deviation from Raoult’s Law. So we can call them positive azeotropes or pressure maximum azeotropes.

The mixture of 95.63% ethanol and 4.37% water (by mass), which boils at 78.2 °C, is a well-known example of a positive azeotrope. The azeotrope boils at 78.2 °C, which is lower than any of its components as ethanol boils at 78.4 °C and water boils at 100 °C.

Azeotropic Distillation

Since the vapours of azeotropes produced after boiling have the same composition as that of its liquid mixture, conventional distillation techniques can’t separate azeotropes. Hence we should add an additional component ie the entrainer, which can first break the existing azeotrope and make one of the components of the azeotrope more volatile than the other. In other words, azeotropic distillation is the process of converting a binary azeotrope into a ternary azeotrope by the addition of an entrainer.

An entrainer is a substance that we introduce to an azeotropic mixture to break it by changing the molecular interactions and creating a new azeotrope with a different composition and boiling point. The characteristics of the azeotropic mixture that undergoes separation determine the appropriate entrainer. The entrainer should be easily separable from the other components of the azeotropic mixture and form a new azeotrope with one of them.

The entrainer can change the activity coefficient of different compounds in different ways when added to the liquid phase, changing the relative volatility of a mixture. Greater deviations from Raoult’s law make it simpler to add another component and create considerable changes in relative volatility. In azeotropic distillation, the additional component has the same volatility as the mixture and one or more of the components combine to generate a new azeotrope due to polarity differences.

The most common types of entrainers in azeotropic distillation include:

Extractive entrainers
Azeotropic entrainers

Extractive Entrainers

Extractive entrainers are substances having a higher boiling point than the initial mixture and combining with one of the components in the azeotropic mixture to generate a new azeotrope. The mixture is heated after the addition of the extractive entrainer.

The extractive entrainer combines one of the original mixture’s components to create a new azeotrope as the mixture’s temperature rises. We can distill out the new azeotrope from the original mixture since it has a higher boiling point than the latter. A further distillation separates the entrainer from the isolated component.

Azeotropic Entrainers

Azeotropic entrainers are substances having a lower boiling point than the initial mixture and produces a new low boiling azeotrope. The most well-known example is the water – ethanol azeotrope when benzene or cyclohexane is added. The ternary azeotrope, which is 7% water, 17% ethanol, and 76% cyclohexane, boils at 62.1 °C with cyclohexane acting as the entrainer. The water/ethanol azeotrope is given just enough cyclohexane to engage all of the water in the ternary azeotrope. The azeotrope ( Benzene – water ) vaporises when the combination is then heated, leaving a residue that is almost entirely made up of ethanol.

Molecular Sieves

A common approach involves the use of molecular sieves. Treatment of 96% ethanol with molecular sieves gives anhydrous alcohol, the sieves having adsorbed water from the mixture. The sieves can be subsequently regenerated by dehydration using a vacuum oven.

Shall we wrap up?

Conclusion

In this blog, we had a short discussion on azeotropes, their formation, properties and the methods of separating them. Azeotropic distillation, pressure swing distillation and molecular sieves are some of the existing methods available. In case of any doubts, please feel free to ask in the comments section. Happy Learning!

Steam distillation is a separation process in which we separate a mixture of immiscible components by introducing steam and subsequently condensing the vapours. In this blog, I will walk you through steam distillation and its principles. First, let us understand the instances in which we opt for Steam distillation over other separation processes.

What is Steam Distillation?

In the typical distillation process, we usually have a mixture of components that are miscible with one another. The vapour pressure that the combination exerts on heating depends on the components that make up the mixture.

To start boiling, the vapour pressure of the mixture should become equal to the atmospheric pressure or the pressure to which it is subjected to. Hence we must heat the system of the liquid mixture to a temperature where the system can create enough vapour to equalise the operating pressure or the atmospheric pressure.

The temperature that must be attained depends on the operating pressure; if it is less than one atmospheric pressure, the temperature that is to be attained is relatively lower; if it is greater than one atmospheric pressure, the temperature to be attained is relatively higher.

In some circumstances, it might not be possible to perform this. Some of those instances are as follows:

When separating materials with very high boiling points, we have to supply more heat to raise the temperature of the mixture. As a result, the procedure uses more energy and is more expensive.
If the mixture contains any thermally unstable components, raising the temperature too high could cause the components to decompose and have an impact on their qualities.
The process becomes energy-intensive if we have a binary combination in which one component boils at a high temperature while the other is non-volatile in nature.
We can easily handle these situations using the method of steam distillation.

Steam Distillation Principle

In the previous blog, we saw Raoult’s law which states that the partial pressure of each component in a miscible ideal mixture is equal to the product of its vapour pressure and mole fraction.

P_a= Xa * P^v

Hence it is clear that the liquid components can’t exert their actual vapour pressure but a corrected vapour pressure (or what we call the partial pressure) which is always less than its pure component vapour pressure ( since mole fraction is always less than 1 )

But, in the case of liquid mixtures in which the components are non-miscible, they can exert their entire vapour pressure as its partial pressure. That is, the total pressure becomes equal to the sum of the individual vapour pressures for immiscible liquid mixtures. Their combined vapour pressures can easily reach the external pressure before the vapour pressure of either of the individual components cross it. Hence the boiling point of the mixture would be lesser than the boiling point of either of the components.

Now, let us assume that water is one of the components in the immiscible mixture. Then we can bring that mixture to a boil at under 100 ⁰C in one atmosphere ( Boiling Point of water at 1 ATM = 100 ⁰C ) if we keep the pressure constant at 1 ATM. In other words, we can lower the operating pressure needed to boil the mixture by introducing steam.

The main concept behind steam distillation is that we use steam to help create the pressure needed to balance the operating pressure. We must be careful to only employ components that are immiscible with water while using steam.

Steam Distillation Process

Consider a binary mixture where component A is a high-boiling component and component B is a non-volatile component. Let’s say A is insoluble in water. We feed the mixture into the column. Using a steam coil, we raise the feed mixture’s temperature. A sparger forces the steam through another steam line. Steam enters the column through the feed mixture and adds to the vapour pressure. When it reaches the working pressure, it causes the creation of vapours of A at a significantly lower temperature. The non-volatile component is eliminated as residue but remains in the feed. Steam and Component A is routed via a condenser where they are easily separated after condensation.

Steam Distillation Advantages

We frequently use steam distillation since it has various advantages over other extraction methods. They are as follows:

the process produces organic compounds devoid of solvents;
Additional separation procedures are not necessary;
It has a huge processing capacity on an industrial scale;
Inexpensive equipment

Shall we wrap up?

Conclusion

In this blog, we saw the process of steam distillation, its advantages and its applications.

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Category Archives: chemical engineering

Azeotropes – Definition, Types, Properties and Methods of Separation