Distillation is one of the most critical mass transfer operations. Distillation is a method of separation of two liquids using thermal energy. Distillation separates the liquids with the difference in volatility (boiling point or vapor pressure).
So in this article, we are going to learn about distillation operation. We will start by learning what is distillation. Then we will see what are types of distillation operations. Also, I will explain different types of distillation with examples. Then we will learn about what is difference between evaporation and distillation. Then we will conclude this article.
So let's start by learning what distillation is.
What is Distillation?
Distillation Definition
Distillation is a mass transfer that is used to separate two miscible liquids with different boiling points by application of heat energy. Distillation is a gas-liquid contact mass transfer operation.
In gas-liquid contact mass transfer operation, two gas (vapor) and liquid phases come in contact with each other, The mass transfer takes place, and both phases leave.
The mass transfer may take place till the equilibrium is attained or may not be attained before phases leave.
As stated from distillation is used to separate two liquids that are miscible to each other and can not be separated from physical separation processes equipment such as decenter or centrifuges.
Distillation Process
In distillation, the feed is heated to the specified temperature. The more volatile component of feed vaporizes and forms a vapor phase.
The vapor generated on heating contains a higher fraction of more volatile components, which has a lower boiling point.
The liquid fraction that does not vaporize has a higher fraction of less volatile components due to its higher boiling point.
Vapors generated by heating in the reboiler are condensed in a condenser and taken out as distillate. Some amount of distillate is sent back to the column as reflux to maintain liquid flow on top of the distillation tower. The top product taken out is called the distillate, from the bottom of the column, taken out as bottoms or heavy cut.
Distillation can be operated as a batch operation as well as a continuous operation. On a laboratory scale, batch distillation is mostly used. But on an industrial scale, continuous distillation is most widely used.
Distillation Apparatus
At the lab scale, distillation is carried out in a glass still. Glass still is placed on a platform that has a heating provision. Still contains a liquid mixture which is needed to be separated. The top of the still is connected with the condenser. The condenser has a small tube inside in which vapors flow and the outer tube flows water which condenses the vapors. Condensed vapors are collected in a collector.
Industrial Distillation Equipments
In industries, Distillation is carried out in tray or packed columns. Tray columns are extensively used for distillation in industries. Packed columns are also used for specific applications.
Industrial distillation equipment contains three major components:
- Reboiler: Vaporizes the feed and generated vapor.
- Distillation column: Mass transfer between vapor and liquid takes place.
- Condenser: Condense vapor comes out from the top of the distillation column.
Types of Distillation
- Simple Distillation: Simple distillation is carried out between two liquids having significant relative volatility. Simple distillation is one of the most ancient techniques of separation of two liquids with a significant difference in their boiling point. Mostly simple distillation is carried out at atmospheric pressure. Simple distillation can be carried out at a laboratory scale by feeding a mixture of two liquids (Feed) to a flask and heating it to a specified temperature by laboratory heater. The top of the flask should be joined with a condenser tube that condenses the vapor that comes out from the top of the flask and condensed vapors are collected in another flask. An example of Simple distillation is the Production of Rose Water from a mixture of water and rose petals.
- Vacuum Distillation: When we want to distill out temperature-sensitive substances. Thermal sensitive materials are which degrades at a temperature lower than their boiling point. By reducing pressure from atmospheric pressure (By generating vacuum) boiling point of materials can be reduced. From this technique, (By reducing pressure) distillation of thermal sensitive materials can be carried out. Vacuum distillation is used to separate heavy vacuum gas oil and light vacuum gas oil from heavy bottoms.
- Fractional Distillation: Fractional distillation is also one type of distillation in which mass transfer happens in more than one stage. At every stage mass transfer between liquid and gas (vapor) takes place. First of all Feed liquid is vaporized in a reboiler. Vapour generated passed through all the stages and go to the top of the column by which it got condensed and some part of it send back to a column as reflux to maintain liquid flow in the column. An example of fractional distillation is to separate petroleum products from crude oils.
- Steam Distillation: Steam distillation is generally used for the distillation of liquids that are immiscible to water. Direct contact between steam and feed mixture is carried out and vaporization of the high volatile component takes place. The use of steam distillation is to separate some components from the hydrocarbon mixture.
- Azeotropic Distillation: When a liquid mixture forms azeotrope, Azeotropic distillation is carried out by adding a third component called the trainer. Azeotrope formation takes place when feed is vaporized and vapors generated have the same composition as feed. By adding a third component called entrainer which forms a lower boiling azeotrope with one of the components of the feed. An example is the separation of water and ethanol mixture by adding toluene.
- Extractive Distillation: When the relative volatility of two-component of feed is low, from simple distillation it is very difficult to separate these two components. By adding the third component called solvent which forms a higher boiling azeotrope with one of the components in the feed. An example of extractive distillation is a distillation of benzene and cyclohexane mixture by aniline solvent.
What is the difference between evaporation and distillation?
Vaporization is the process of the separation of more volatile solvents from non-volatile solutes. For example, When seawater is subjected to sublight, Water evaporates and salt being a non-volatile component remains unevaporated. Where distillation is the process of separation of two miscible liquids having different boiling points. For example, benzene and toluene are separated by distillation due to differences in their boiling points.
Conclusion
So, Distillation is a gas-liquid mass transfer operation that uses thermal energy to separate two miscible liquids having a difference in boiling points. There are various types of distillation processes such as simple distillation, batch distillation, fractionation, steam distillation, azeotropic, and extractive distillation.