Steam distillation is a specific kind of distillation that is used for separating two or more temperature sensitive materials, such as: aromatic compounds.
Steam distillation is used to remove any volatile compound from a mixture by passing the steam through heated mixture and then condensing the volatiles along with the steam.
A Persian chemist, Ibn Sina invented the process of steam distillation in early 11th century in order to extract the essential oils from their sources. Its invention led to a kind of revolution in the industry of perfumery that continuously needs to extract the pure fragrances from their respective sources. The advent of steam distillation paved the way for several scientific developments including the extraction of several chemicals and compounds in their pure form. All of these were impossible before its invention.
On heating, the two mutually immiscible liquids exert their own vapor pressures in their mixture that changes with temperature. Their independent vapor pressures act as if the other vapor pressure is absent. As a result, the total vapor pressure of the mixture increases and the mixture starts boiling on the point where total vapor pressure exceeds the surrounding atmospheric pressure. Therefore, an organic compound can be purified at a much lower temperature than that required for its decomposition, provided the organic substance is insoluble in water. For example: despite having a boiling point of 156 °C, bromobenzene can be distilled at 95 °C by mixing it with water.
The primary uses of steam distillation are to purify the commercially useful volatile compounds and to remove any undesirable flavor from the fats and the oils. It is also used to synthetically produce some complex organic compounds, such as: orange oil and eucalyptus oil. It is known with another name (i.e. Steam Stripping) in the petrochemical plants and petroleum refineries, where it is heavily used to purify the different components of petroleum products. It is also used in consumer food production and aerosolized condiments production.
A question may arise with the use of steam distillation is that why it is necessary to use it to extract the fragrances from its source (e.g. flowers, buds, leaves, etc.), when the fragrance can be easily extracted by simply pressing the source material. This is because the simple pressure technique would yield very low quantity of fragrance. But using steam distillation, one can easily extract almost 100% of the fragrance from its source material.
Some substances are too sensitive for the heat to be used under steam distillation. Steam distilling a temperature sensitive substance can even lead to explosion. Hence in such a case, it is recommended to use the steam distillation in combination with vacuum distillation. Moreover, the apparatus used for steam distillation should be of highest quality, as the apparatus has to withstand a lot of pressure during the distillation process.