Distillation is defined as the process of separating the components from their liquid mixture using various techniques. While there are many methods of distillation including conventional, fractional, steam, etc., azeotropic distillation and extractive distillation are two methods that are often confused with each other.
Azeotropic Distillation vs Extractive Distillation
The main difference between azeotropic distillation and extractive distillation is in the process that each one follows to separate the components of a mixture. In azeotropic distillation, an azeotrope needs to be formed in the process whereas, extractive distillation restricts the formation of azeotropes.
Azeotropic Distillation is defined as a technique of separating the components of an azeotropic mixture with the help of a specialized distillation process. In an azeotropic mixture, there is more than one type of liquids that cannot be separated using simple distillation.
Extractive Distillation is defined as a distillation process of separating two components from a mixture with the addition of a high-boiling solvent. The solvent’s boiling point should be much higher than the components of the mixture so that new azeotropes cannot be formed.
Comparison Table Between Azeotropic and Extractive Distillation
| Parameters of Comparison | Azeotropic Distillation | Extractive Distillation |
| Azeotrope | Depends on the formation of the azeotrope. | Strictly resists the formation of the azeotrope. |
| Application | It is used while experimenting in labs. | It is used for manufacturing and production. |
| Product | It is obtained at the column’s top. | It is obtained at the column’s bottom. |
| Process | It is a complicated process. | It is a simpler process. |
| Time | It is more time-consuming. | It takes less time. |
What is Azeotropic Distillation?
Azeotropic Distillation is defined as a technique of separating the components of an azeotropic mixture with the help of a specialized distillation process. In an azeotropic mixture, there is more than one type of liquids that cannot be separated using simple distillation. This is because boiling liquids in the azeotropic mixture contain the same proportions.
In distillation, the components of a mixture can be separated due to differences in their volatilities whereas, in an azeotropic mixture, the volatilities of components are also similar. To overcome this hurdle, an additional volatile component is added to the mixture, known as an entrainer. As a result, the volatility of one of the liquids noticeably changes, thus separating it from the other.
Azeotropic mixtures are mainly of two types including minimum boiling azeotropes, which boil at relatively low temperatures, and maximum boiling azeotropes, which boil at relatively high temperatures.
Most commonly known azeotropic mixtures are composed of water and ethanol substances. To separate the two, additional solvents such as benzene, hexane, cyclohexane, etc., are required. In most cases, benzene is used for azeotropic distillation. The addition of an entrainer changes the molecular interactions to eliminate the azeotrope, and hence, it alters the relative volatility of the mixture.
What is Extractive Distillation?
Extractive Distillation is defined as a distillation process of separating two components from a mixture with the addition of a high-boiling solvent. The solvent’s boiling point should be much higher than the components of the mixture so that new azeotropes cannot be formed. It makes sure that the solvent is not vaporized at the boiling point of components.
Extractive distillation is most commonly used to separate toluene from iso-octane. The separation between these two is difficult by conventional distillation as their boiling points are nearly similar. In such a case, phenol, which has a much higher boiling point than the components, is added to the mixture.
This results in the production of phenol-toluene mixture that leaves at the bottom, while iso-octane is recovered as an overhead product. The phenol-toluene mixture is then separated into a different column using a simple distillation process. Phenol is the most used solvent in this process as it has a higher boiling point and prevents foaming.
Unlike azeotropic distillation, this process strictly restricts the formation of new azeotropes and separates the components solely with vaporization. Since azeotropes are absent in this process, this process is much simpler than azeotropic distillation. Extractive distillation is highly practical and is more widely used than other distillation processes.
Main Differences Between Azeotropic and Extractive Distillation
- Azeotrope formation takes place in azeotropic distillation whereas it does not take place in extractive distillation.
- In azeotropic distillation, the solvent doesn’t need to have a high-boiling point whereas, in extractive distillation, the solvent needs to have a very high-boiling point.
- Azeotropic distillation is a more complicated process when compared to extractive distillation, which is simple.
- Azeotropic distillation separates the component from the vapor phase whereas extractive distillation separates the component from a matrix of substances.
- The product obtained from azeotropic distillation is from the top of the column whereas the product obtained from extractive distillation is from the bottom of the column.
Conclusion
Azeotropic and extractive distillation are two of the most widely used distillation techniques in chemistry. While the two are similar in many areas, they carry significant differences in processing. While azeotropic distillation requires the production of an azeotrope, extractive distillation strictly avoids it.
Even though the two are very successful techniques, extractive distillation is preferred over azeotropic distillation in most cases. Extractive distillation doesn’t require the production of an azeotrope, and hence, it is much quicker and simpler than other methods. Furthermore, no large quantities of solvent need to be vaporized in order to separate the components.
On the other hand, in some cases, azeotropic distillation is preferred over extractive distillation. When working in labs, the end product needs to be as pure as it can be. In extractive distillation, the product can never be highly pure as the solvent obtained at the bottom of the solvent-recovery column most likely contains impurities. As the product obtained is at the top of the column in azeotropic distillation, it is highly pure.
The preference between these two distillation methods solely depends on the requirement. For most lab experiments, azeotropic distillation is preferred as it is highly pure. On the other hand, for manufacturing and production, extractive distillation is highly preferred.
References
- https://pubs.acs.org/doi/pdf/10.1021/ie50418a002
- https://www.tandfonline.com/doi/abs/10.1081/spm-120026627