Distillation is the process of purification of compounds based on their volatility. The different types of distillation include simple, fractional, steam, vacuum, and short path distillation.
Historical evidence suggests that the principle of distillation has been in use since ancient times. It is believed that ancient Arab chemists applied distillation for the first time to separate perfumes.
Distillation is a physical method of assorting mixtures depending upon the difference in the boiling point of the component substances. In simple words, 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 short, a highly volatile compound is separated from a less-volatile or non-volatile compound by using distillation.
Today, it is one of the most popular technique implemented for purification and separation of a mixture. There are several methods of distillation depending on the procedure and the instrument setup. Following are the common types.
~ In this process, the liquids are separated by evaporating and capturing them at various points in their cooling cycle, and then immediately channeled into a condenser.
~ Simple distillation is used for a mixture in which the boiling point of the components differ by at least 158°F (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 a process of repeating distillation on the collected liquid, in order to enhance the purity of the separated compounds.
~ This method is still used to obtain pure drinking water out of hard or contaminated water and also used to make homemade perfumes, liquor, and soaps.
~ Fractionating column is used to separate constituents whose volatility is nearly similar or differs by 77°F (25°C, at 1 atmosphere pressure) and which cannot be separated by simple distillation.
~ In the fractionating column, the plates are arranged, and the compounds with the least boiling point are collected at the top, while those with higher boiling point are present at the bottom.
~ Fractional distillation is used for the alcohol purification and gasoline purification in petroleum refining industries.
~ It is a more effective and easy to use process, compared to simple distillation method. It is, however, comparatively expensive than any other types of distillation.
~ Steam distillation is used for the purification of mixtures in which the components are temperature or heat sensitive; for example, organic compounds.
~ In this process, steam is introduced to the apparatus and the temperature of the compounds are depressed, by vaporizing them at 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 part, water and compound, which is then purified by using simple distillation.
~ The process requires some initial training and skill to operate the equipment. It also requires periodic maintenance.
~ Steam distillation is widely used for large-scale separation of essential oils, fats, waxes, and perfumes.
~ 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 392°F), which tend to decompose at their boiling temperature.
~ Vacuum distillation can be conducted without heating the mixture, as is usually followed in other distillation types.
~ For the separation of some aromatic compounds, vacuum distillation is used along with steam distillation. When vacuum distillation is combined with fractional distillation method, components of a mixture get separated very easily.
Short Path Distillation and Other Classifications
~ In short path distillation, the separated compounds are condensed immediately, without traveling to 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. Thermal sensitive compounds can also be separated by following short path distillation.
~ This method is mainly used for the separation of organic compounds with high molecular weight, especially in the pharmaceutical industries. It is also used for processing edible oils and fats.
~ Another method of classifying distillation is based on the amount of compound fed to the columns. There are two types of distillation columns namely, batch and continuous.
~ In the batch column method, the feed to the column is provided in batch-wise manner. The distillation process gets carried out when a batch of feed is introduced to the column.
~ In the continuous column method, the feed to the column is introduced continuously without any interruptions. A continuous operation is used commonly in the industrial applications.
Out of the five main types, simple and fractional distillation types are the two widely used methods that are applied even today in petroleum-refining industries, laboratories, and chemical factories.