Azeotropic Distillation: Separating Liquid Mixtures

As an engineer or engineering student, you are always looking for ways to separate and purify liquid mixtures that are quick and easy.

But you may not know about azeotropic distillation.

Not only does this process separate mixtures, but it can also make pure components, which is hard to do with traditional distillation methods.

Azeotropic distillation is a complicated but interesting process that has become very important in the engineering and chemical industries.

In this article, I will explain how azeotropic distillation works, as well as its benefits, drawbacks, and common uses.

With this knowledge, you will be able to take your separation techniques to the next level.

Introduction to Azeotropic Distillation

Formal definition:

A process by which a liquid mixture is separated into pure components with the help of an additional substance or solvent.

Azeotropic distillation is a way to separate the parts of a mixture by making an azeotrope, which is a mixture of the parts that boils at the same temperature.

Distillation can not be used to separate this kind of mixture into its parts because all of the parts have the same amount of vapor and liquid.

Extractive distillation, on the other hand, is a similar way to separate mixtures, but instead of an entrainer, it uses a solvent to separate the mixtures.

Azeotropic Distillation

In azeotropic distillation, an entrainer is used to change how volatile the different parts of the mixture are, which makes it possible to separate them.

By combining one or more of the components with the entrainer, a new azeotrope is made.

This new azeotrope can be separated from the original mixture by fractional distillation.

The entrainer is usually a part of the mixture that has a low boiling point and a high relative volatility compared to the other parts.

If the mixture forms a minimum-boiling azeotrope, which is the azeotrope with the lowest boiling point, the entrainer will combine with the more volatile part of the original mixture to form an azeotrope.

In this case, the azeotrope with the lowest boiling point that forms when the entrainer is added evaporates first.

This is called the distillate.

If not, if an azeotrope with maximum boiling point forms with the entrainer, the more volatile part of the original mixture will evaporate first.

This makes it possible for fractional distillation to separate the parts of the original mixture.

Extractive Distillation

In extractive distillation, each mixture must use a different separation solvent, and the solvents should not tend to form an azeotrope.

The solvent changes the mixture's vapor-liquid equilibrium, which makes it possible to separate the parts.

Extractive distillation can be used to separate mixtures that tend to form azeotropes, which is not possible with azeotropic distillation.

Constant Boiling Mixtures

A mixture that boils at a constant rate, also called a "azeotropic" mixture, has effects on fractional distillation of less-than-ideal mixtures (azeotropes).

If Raoult's Law is broken in a positive way, it makes a vapor pressure curve with a maximum value that is not pure A or pure B.

If these mixtures have a lot of steam, their boiling points will be low.

The molecules move around easily, and fractional distillation alone is not enough to get pure ethanol because it will still have water and other impurities.

The Art of Complicated Separations: Exploring Azeotropic Distillation

Still hard to get and hard to understand? Let me change the point of view a bit:

Are you tired of the same old boring distillation processes that just separate mixtures without adding any extra flair? Do you want to make your separation techniques harder and more confusing than they need to be? Well, I have the answer for you! Azeotropic distillation is a process that takes simple separation and turns it into a maze of extra substances and solvents.

Why use just one thing to separate mixtures when you can use two or three? Who wants to be simple when you can be complicated? Azeotropic distillation: Sometimes, the best way to clean up your parts is to make the process as complicated as possible.

OK, that was just a joke disguised as a TV commercial. Now let's go back to the explanation.

The Azeotropic Distillation Process

Azeotropic distillation has a lot of advantages over other ways to separate things, like how easy it is to separate the entrainer and the product you want, how easy it is to use, how little energy it uses, and how cheap it is.

Types of Entrainers

For azeotropic distillation, there are three types of entrainers: homogeneous, heterogeneous, and extractive.

Homogeneous entrainers can mix with all of the feed's components, but heterogeneous entrainers form two liquid phases that can not mix.

Extractive entrainers do not make an azeotrope with any of the feed components.

Instead, they remove only the more polar or less polar components based on their polarity.

Homogeneous and Heterogeneous Azeotropic Distillation

There are two types of azeotropic distillation, called homogeneous and heterogeneous, that depend on how the entrainer interacts with the feed mixture.

Homogeneous azeotropic distillation is easier to use and more efficient than heterogeneous azeotropic distillation because the entrainer can mix with all of the components in the feed.

Heterogeneous azeotropic distillation, on the other hand, uses an entrainer that can not mix with any of the feed mixture's parts.

This can cause the column to fail with small changes in pressure or the decanter to shut down with small leaks.

So, it is more affected by how it is run than homogeneous azeotropic distillation.

Limitations of Azeotropic Distillation

Azeotropic distillation has some benefits, but it also has some problems.

Some solutions can not be separated as well by distillation if they have azeotropes in their vapor-liquid equilibrium states.

Also, the choice of azeotropic agent can have a big effect on the process and, if not handled properly, can pollute the environment.

Also, using azeotropic distillation still uses a lot of energy, and it may not always produce the level of purity that is needed for the target products.

Applications of Azeotropic Distillation

Dehydration of Ethanol

One of the most well-known ways that azeotropic distillation is used is to remove water from mixtures of ethanol and water.

The azeotropic mixture goes to the last column, which is where azeotropic distillation happens.

In the past, many different entrainers were used for this, but benzene was used the most until it was found to cause cancer.

Most of the time, cyclohexane is used to break the ethanol-water azeotrope in modern science.

This process is very important in the fuel ethanol industry, where water needs to be taken out of ethanol to keep engines from breaking down and to increase the amount of ethanol in fuel blends.

Heterogeneous Azeotropic Distillation

Adding an entrainer that makes a separate phase is part of a subset of industrial azeotropic distillation methods.

Similar to extractive distillation, this process of adding an entrainer that makes a new phase is called entrainment.

A common way to use this method is to mix benzene with water and ethanol to make a new heterogeneous azeotrope with a lower boiling point that can be separated in the usual way.

This method is especially useful for getting rid of impurities in organic solvents, cleaning essential oils, and getting back hydrocarbons.

Other uses

Azeotropic distillation has a number of uses in industry besides dehydration of ethanol and heterogeneous azeotropic distillation, such as:

• Isomers, which have almost the same physical and chemical properties, can be separated using azeotropic distillation.
• Azeotropic distillation is used in the pharmaceutical, chemical, and polymer industries to get solvents back from waste streams.
• Acid gases are taken out of natural gas streams using a process called azeotropic distillation.
• Separation of essential oils: Azeotropic distillation is used to separate and clean essential oils from plant matter.
• Removal of water from solvents: Azeotropic distillation is used in the chemical and pharmaceutical industries to remove water from solvents.

Challenges in Azeotropic Distillation

Azeotropic distillation is a good way to separate azeotropic mixtures, but it comes with some challenges that must be dealt with to get the best results.

Entrainer Selection:

In the synthesis and conceptual design of AD processes, the choice of an entrainer is very important because it determines the order of separation.

The entrainer must form a strong azeotrope with one of the mixture's components and be easy to separate from the product you want.

The choice of entrainer affects the product's quality and purity, and if it is not used properly, it can also pollute the environment.

Excess Entrainer's Effects:

Adding an excess amount of entrainer can cause an increase in energy consumption, as the additional entrainer must be separated from the desired product.

This can raise the cost of making the product and may also lower its quality.

In some cases, an excess of entrainer may lead to the formation of a third azeotrope, which can complicate the separation process.

Insufficient Entrainer:

If you use too little entrainer, the product might not be completely separated or might not be as pure.

So, in azeotropic distillation, the amount of entrainer should be optimized so that the desired level of separation and purity can be reached with the least amount of energy and cost.

How Azeotropes Are Made:

When an entrainer changes the relative volatility of an azeotropic mixture, it forms either a maximum boiling or minimum boiling azeotrope.

Adding an entrainer can further enhance the purity of an azeotropic component.

But if you choose too much or too little entrainer, it can change how well the separation process works.

After the Azeotropic Point:

Distilling an azeotropic mixture beyond its azeotropic point is possible, but this process utilizes alternative cost-intensive separation techniques like pressure swing distillation.

Instead of trying to distill past the azeotropic point, it is important to find the right amount of entrainer to get a good separation.

Azeotropic Distillation in Practice

Entrainers in Azeotropic Distillation

Because it can form a low-boiling azeotrope with water, benzene was often used as an entrainer in azeotrope distillation.

Toluene is better than benzene, though, because benzene can cause cancer.

An entrainer works well if it can combine with one of the components in the feed mixture to make a new azeotrope.

This changes the relative volatility of the components and makes it possible to separate them.

The entrainer should be easy to separate from the product you want, and you can get it back by distilling, decanting, or some other method.

In azeotropic distillation, the effect of too much entrainer depends on the system being used.

When too much entrainer is added, the amount of energy used goes up because the extra entrainer has to be separated from the desired product.

This can raise the cost of making the product and may also lower its quality.

Sometimes, too much entrainer can cause a third azeotrope to form, which can make the process of separating more difficult.

On the other hand, if you use too little entrainer, the product might not be completely separated or might not be as pure.

So, the right amount of entrainer should be used to get the level of separation and purity needed with the least amount of energy and cost.

Deep eutectic solvents (DES) are becoming more popular as entrainers in azeotropic distillation because they are better for the environment than traditional solvents.

Azeotropic mixtures like benzene-cyclohexane and acetonitrile-water are easy to separate with DES.

DES can also be used in a technique called "extractive distillation," which is similar to distillation and involves adding a solvent to get one of the feed mixture's parts out.

Azeotropic Distillation in Comparison to Steam Distillation

With steam distillation, heat-sensitive materials like natural aromatic compounds are separated from organic materials.

Azeotropic distillation, on the other hand, is used to separate the parts of an azeotropic mixture.

Mixtures that are azeotropic are hard to separate with traditional distillation because their boiling points stay the same and the vapours are the same as the liquid mixture.

Since steam distillation is used to separate materials that are sensitive to heat, it is hard to say what the azeotropic temperature of water and toluene is.

Toluene and water, on the other hand, form an azeotrope at 84.1°C that is made up of 22.85% water and 77.15wt% toluene.

Since it acts like a single compound, a mixture of toluene and water with this make-up cannot be separated further by simple distillation.

You can separate this mixture by using azeotropic distillation with an entrainer or extractive distillation with a solvent.

Video: Azeotrope Displacement and Separation

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Use cases

Conclusion

In conclusion, azeotropic distillation is a useful way to separate complicated mixtures into their pure parts.

It is a powerful tool for engineers and scientists because of its unique ability to make pure components quickly.

But, like any separation method, it has its limits and needs careful thought about the composition of the mixture, the choice of entrainer, and the process conditions to work well.

As you continue to look into the possibilities of azeotropic distillation, keep in mind that this process is not only a technical challenge, but also an opportunity for creative problem-solving and new ideas.

If you think about azeotropic distillation in the right way, it can lead to new engineering discoveries and ideas.

So keep an open mind and try new things. There are a lot of options.