Distillation Principle and Types

Distillation: it is a separation process which is used to separate desired component from a liquid mixture by making use of differences in volatilities of different components of the mixture.

Working principle of distillation: heating a mixture at a specific temperature, i.e. boiling point of liquid, is vaporization takes place. The hot vapor is collected and condensate to separate the components of substances. Highly volatile substances are separated from less volatile compounds in distillation.

Principle of distillation:

  1. Condensation of vapor without allowing any liquid to return to the still i.e. without refluxing.
  2. Return of the part of condensate to the still under such condition that this returning liquid is brought into intimate contact with vapors on their way to their condenser. This method is also called distillation with reflux or rectification or fractional distillation.

Types of distillation: There are several types of distillation based upon procedure and instrumental set up.

1. Simple distillation: It is done for mixture in which boiling point of component differ by at least 70°C and also, for mixtures contaminated with non-volatile particle (solid or oil) that are nearly pure with less than 10 % contamination.


2. Double distillation: Repeated distillation of collected liquid in order to enhance the purity of separated compounds.



3. Steam distillation: It is used for purification of mixture in which the components are temperature or heat sensitive. Example; organic compounds like perfumes. In this system, steam is introduced by heating water which allows the compounds to boil at lower temperature. Temperature sensitive compounds are separated before decomposition. The resultant liquid has two phases which consist of water and compound. This can be further purified by separating funnel or simple distillation.

Fig: Steam distillation




4. Vacuum distillation: It is special method of separating compounds a pressure lower than standard atmospheric pressure. Under this condition, boiling point decreases. A saturation temperature is directly proportional to saturation pressure. Vacuum distillation is best suitable for separation of compounds with higher boiling points. It can also be conducted without heating the mixture as usually followed in other distillation process. Example, separation of some aromatic compounds. It is used along with steam distillation. Liquid boils when its vapor pressure is equal to atmospheric pressure. Boiling point of liquid may be lowered to desired temperature by reducing pressure on its surface.

Fig: Vacuum distillation

5. Short path distillation: Thermally sensitive compounds can be separated by following short path distillation. Here the separated compounds are condensed immediately without travelling the condenser. The condenser is fused in a vertical manner between heating flask and collecting flask. Pressure is maintained below atmospheric pressure. It is used for separation of organic compounds with higher molecular weight esp. in pharmaceutical industries.

6. Fractional distillation: When volatility of component is nearly similar or differ only by 25°C (at 1 atm. Pressure) they cannot be separated from simple distillation. This method is used where constituents are separated by a fractionating column where plates are arranged and compounds with the least boiling point are collected at the top while those with higher boiling point are present at bottom. Series of components are separated simultaneously one after another. Example; alcohol purification and gasoline purification in petroleum refining industries.


Fig: Fractional distillation



About Author

Name : Pratiksha Shrestha


Ms. Shrestha holds masters degree in food engineering and bioprocess technology from Asian Institute of Technology (AIT) Thailand. She is currently working for Government of Nepal at Department of Food Technology and Quality Control (DFTQC), Kathmandu. She is also a teaching faculty in College of Applied food and Dairy Technology (CAFODAT) affiliated to Purbanchal university, Nepal.