What is Distillation?

Distillation is a process/technique to separate more volatile components from less volatile components using the principle of relative volatility. In chemical engineering, distillation is one of the most common and important topics that is involved in almost every industry and process. 

Mechanism of Distillation

Distillation involves the difference in vapor pressure or boiling temperature of the components. Relative volatility helps us understand how easy or difficult the separation is going to be. It is known as the separation factor for distillation.

For a binary mixture to be separated (A and B), it should have two different boiling points. The one with the lower boiling point is called the more volatile component (A), and the one with the higher boiling point is called the less volatile component (B). The more volatile component has high vapor pressure and the less volatile component will have less vapor pressure. For example, in the separation of benzene and toluene, the boiling point of benzene is 80°C and toluene is around 110°C. Here, the more volatile component is benzene and toluene is the less volatile component.  The more volatile components are also lighter than the less volatile ones.

$Volatility of component A = \frac{Partial pressure of component A}{Mole fraction of component A in solution}$

$Relative volatility = \frac{Volatility of component A}{Volatility of component B}$

Vapor Pressure

The vapor pressure of a liquid component can help us understand how the separation of components is going to work during the distillation process.  When a liquid is heated, it changes its state from liquid to vapor. This is called vaporization. The change from a gaseous state to liquid is called condensation. The component with a lower boiling point exerts more vapor pressure than the component with a higher boiling point.

Types of Distillation

The distillation process can be done either in a batch or in continuous form.

Batch distillation is useful and preferred for the separation of small quantities by chemical engineers. The liquid feed which is to be separated is taken in a still pot and is heated until vapors are generated. Vapors exit through the top of the column and are known as distillate. This distillate is then cooled, known as condensate, and obtained as a product. The remaining liquid (residual) can be returned as reflux if required. This remaining mixture in the still pot is richer in the less volatile component which can be further treated.

Continuous distillation includes multi-stages of vapor-liquid contact in plates or trays. An ideal tray is one where the vapor and liquid mixtures are at equilibrium.

If the boiling point of a binary mixture is close, separation becomes difficult. This type of mixture, which cannot be separated by simple distillation, is separated by azeotropic distillation. In azeotropic distillation, azeotropic breakers or entertainers are used to break the azeotrope formed.

For example, water and acetic acid have close boiling points. To distill a mixture of water and acetic acid, the trainer used is ester to break the water-acetic acid azeotrope. Ester forms a minimum boiling point azeotrope either with water or acetic acid. This minimum boiling azeotrope escapes through the top of the distillation column, and acetic acid can be withdrawn from the bottom of the column as a product. The ether-water azeotrope is further treated.

Another form of distillation is flash distillation, which is the most common form.

Boiling Point

The boiling point needs to be analyzed for two cases - for constant pressure and varied temperature, and for constant temperature and varied pressure in a system.

Case 1: Constant Pressure and Varied Temperature

As boiling starts at constant pressure, the components (A and B) will be present in both the liquid and vapor phase, but their mole fractions will be different. The mole fraction of the more volatile component (A) will be greater in the vapor phase.

If we have to find the boiling points of the individual pure components, we can use Antoine’s method. As we increase or decrease pressure, the boiling point of the component also changes, as pressure and temperature are related to each other. As we decrease the pressure, the boiling point of a component also decreases and vice-versa.

For a binary mixture, the temperature at which the liquid first starts to boil is called the bubble point temperature and the point at which the vapor starts to condense is called the dew point temperature. Another term is the tie line, which gives us an idea about the amount of the more volatile component in the vapor phase and the liquid phase. It also gives us an idea about what percent of feed there is in liquid and the vapor.

Case 2: Constant Temperature and Varied Pressure

The constant temperature of binary components in a phase diagram is mainly used to analyze the solution behavior.

Common Mistakes

Students might confuse distillation with similar separation topics, such as absorption or stripping, which also require multi-components. Also, there are different processes of distillation, such as steam distillation, flash distillation, vacuum distillation, and a few more types in chemical engineering.

Context and Applications

Distillation is used by chemical engineers in industries and processes such as ethanol-water separation, pharmaceutical industries, petroleum industries, etc.

The concept holds relevance in various undergraduate and postgraduate courses, including in degree courses such as Bachelors of Science in Petroleum and Chemistry, Bachelors of Engineering in Chemistry, and Masters in Engineering in Petroleum and Oil.

Related Concepts

• Refining of petroleum products
• Pressure and temperature variations
• Raoult’s law

Practice Problems

1. What is the process involving heating of feed to form vapor and then cooling the vapor to obtain a pure liquid called?

1. Sublimation
2. Distillation
3. Crystallization
4. Evaporation

Answer: b

Explanation: The process involving heating of feed to form vapor and then cooling the vapor to obtain a pure liquid is called distillation.

2. What are the boiling points of benzene and toluene?

1. 80°C and 110°C
2. 110°C and 130°C
3. 100°C and 80°C
4. 80°C and 100°C

Answer: a

Explanation: The boiling points of benzene and toluene are 80°C and 110°C respectively.

3. What is the boiling point of aniline?

1. 400K
2. 550K
3. 457K
4. 100K

Answer: c

Explanation: The boiling point of aniline is 457K.

4. What is the separation factor in the distillation process known as?

1. Heat transfer
2. Relative volatility
3. Mass transfer
4. Raoult's coefficient

Answer: b

Explanation: The separation factor in the distillation process is known as relative volatility.

5. What is the vapor that leaves through the top of the distillation column called?

1. Distillate
2. Condensate
3. Mass transfer
4. Feed

Answer: a

Explanation: The part of the feed that vaporizes through the top of the column is called distillate and the cooled vapors are known as condensate.