Distillation

Introduction to Distillation

Distillation glassware

Setting up a distillation

Mini-distillation

Steam distillation

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Introduction to Distillation

Distillation is a technique for separating liquids from one another, using the differences in boiling point between the individual components. Liquids are vaporised through heating either at atmospheric pressure or at reduced pressure (under vacuum) and the vapours are condensed and collected. Components are separated on the basis of boiling point. The wider the separation in boiling point, the easier the distillation is to achieve. Distillations may be used to separate and isolate a number of compounds (which is very common on an industrial scale, for example the fractional distillation of crude oil), or to purify a single compound from impurities present.

Type of distillation

There are a variety of different distillation methods, each which are used for a specific purpose/application depending on the properties of the distilates involved. The choice of a distillation method involves considering the composition of the mixture and the thermal characteristics (e.g Boiling points, vapour pressures and decomposition temperatures) of the components.

Distillation type

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When this method is emplyed

Example use case

Simple distillation

Used to distil one major component, separating it from small quantities of other impurities. This is often used on compounds that have relativley low boiling points (50-200 ℃) at atmospheric pressure.

Often used to purify starting materials or products.

Vacuum distillation

The use of vacuum may enable compounds with higher boiling points to be distilled, as the reduced pressure lowers the boiling point. Vacuum distillation isn't strictly a type of distillation (in principle be used in any distillation setup), but the term is most commonly referring to its use with simple and short path distillation setups. A Pressure-Temperature Nomograph is often consulted to predict boiling points based on the measured vacuum pressure.

Regularly used for the purification of starting materials, particularly aromatic compounds, with boiling points at atmospheric pressure which are >200 °C.

Short path distillation

Short path distilation is a type of vacuum distillation that is used when trying to distill compounds that a thermally unstable. They employ the use of very low pressure and short condensor columns to limit the amount of time the compound is at high heat and reduce sample loss

#Mainly used to distil very small volumes.

Kugelrohr is a type of short path distillation technique which has no column at all but instead a cooled middle collection trap is used

Fractional distillation

Fractional distillation is used to separate multiple components which have differing boiling points. Fractional distillations require long path lengths to provide the temperature gradients nescessary to provide effective separation.

Extensively used in process chemistry on large scale, for example in the seperation of components in crude oil, with very limited ## laboratory use for small scale preparations.

Steam distillation

Steam distillation involves the distillation of organic compounds using steam. Compounds are carried over in the distillation at temperatures below their boiling point, providing there is sufficient vapour pressure for a compound at the temperature of the steam.

Often used to distil thermally unstable compounds, particularly in natural product extraction. For example, limonene (BP 176 °C) can be extracted from citrus fruits using steam distillation.

Cryogenic distillation

Cryogenic distillation is a type of fractional distillation that is done at temperatures far below 0 ℃, where a mixture of gasses is cooled down and liquified and then are allowed to sperate them by their boiling points as they warm up.

An example of where this is used is in natural gas processing to remove carbon monoxide and hydrogen sulfide