Types of Distillation

Distillation is a physical method of assorting mixtures depending upon the difference in the boiling point of the component substances. The working principle of distillation is to heat a mixture at a specific temperature, collect the hot vapors and condense to separate the component substance. In simpler terms, a highly volatile compound is separated from a less-volatile or non-volatile compound by using distillation. As per evidences, the principle of distillation has been used since ancient times. It is believed that the ancient Arab chemists applied distillation for the first time to separate perfumes. Today, it is one of the most popular technique implemented for purification and separation of a mixture.

The Different Types of Distillation

There are several methods of distillation depending on the procedure and the instrument setup. Each distillation type is used for purification of compounds having different properties. Following are the common types.

Simple Distillation
Simple distillation is practiced for a mixture in which the boiling point of the components differ by at least 70° C. It is also followed for the mixtures contaminated with nonvolatile particles (solid or oil) and those that are nearly pure with less than 10 percent contamination. Double distillation is the process of repeating distillation on the collected liquid in order to enhance the purity of the separated compounds.

Fractional Distillation
Those mixtures, in which the volatility of the components is nearly similar or differs by 25° C (at 1 atmosphere pressure), cannot be separated by simple distillation. In such cases, fractional distillation is used whereby the constituents are separated by a fractionating column. In the fractionating column, the plates are arranged and the compound with the least boiling point are collected at the top while those with higher boiling point are present at the bottom. A series of compounds are separated simultaneously one after another. Fractional distillation is used for the alcohol purification and gasoline purification in petroleum refining industries.

Steam Distillation
Steam distillation is used for the purification of mixtures, in which the components are temperature or heat sensitive; for example, organic compounds. In the instrument setup, steam is introduced by heating water, which allows the compounds to boil at a lower temperature. This way, the temperature sensitive compounds are separated before decomposition. The vapors are collected and condensed in the same way as other distillation types. The resultant liquid consists of two phases, water and compound, which is then purified by using simple distillation. Steam distillation is practiced for the large-scale separation of essential oils and perfumes.

Vacuum Distillation
Vacuum distillation is a special method of separating compounds at pressure lower than the standard atmospheric pressure. Under this condition, the compounds boil below their normal boiling temperature. Hence, vacuum distillation is best suited for separation of compounds with higher boiling points (more than 200°C), which tend to decompose at their boiling temperature. Vacuum distillation can be conducted without heating the mixture, as usually followed in other distillation types. For the separation of some aromatic compounds, vacuum distillation is used along with steam distillation.

Short Path Distillation
Thermal sensitive compounds can also be separated by following short path distillation. In this technique, the separated compounds are condensed immediately without traveling the condenser. The condenser is configured in a vertical manner between the heating flask and the collecting flask. Similar to vacuum type, the pressure is maintained below the atmospheric pressure. Short path distillation is used for the separation of organic compounds with high molecular weight, especially in the pharmaceutical industries.

Another method of classifying distillation is based on the column type used in the process. There are two types of distillation columns namely, batch and continuous. In the former case, the feed is provided in the batch wise manner; whereas, in the latter type, the feed to the column is introduced continuously. A continuous operation is used commonly in the industrial applications.