Steam distillation

A distillation method in which steam is blown into a substance with a high boiling point, and the substance is distilled at a temperature lower than the boiling point. Substances that decompose during normal distillation or have boiling points that are too high can be distilled under reduced pressure, but if steam is blown in to reduce the partial pressure of the high boiling point components, the sum of the vapor pressure of the high boiling point components and water becomes the total pressure, so distillation can be performed at atmospheric pressure and at a temperature below the normal boiling point. This is called steam distillation. If the overhead product of the distillation apparatus is an oil that is insoluble in water, it will form two liquid phases with water, so the water can be easily separated.

Steam distillation is often more expensive than vacuum distillation, and is therefore often used for small batch distillations where reduced pressure is inconvenient or where steam is blown directly into the boiling liquid in the still.

[Toyohiko Hayakawa]

Source: Shogakukan Encyclopedia Nipponica About Encyclopedia Nipponica Information | Legend

沸点の高い物質に水蒸気を吹き込んで、その沸点よりも低い温度で留出させる蒸留法。普通の蒸留では分解してしまう物質や、沸点が高すぎる物質の蒸留は、減圧蒸留を用いればよいが、高沸点成分の分圧を減少させるため水蒸気を吹き込んで蒸留すれば、高沸点成分と水の蒸気圧の和が全圧となるため、大気圧下で通常の沸点以下の温度で蒸留ができる。これを水蒸気蒸留という。蒸留装置の塔頂製品が水に不溶な油類ならば、水と2液相を形成するので水を簡単に分離できる。

　水蒸気蒸留は多くの場合、減圧蒸留より高価につくため、減圧が不便または小規模な回分蒸留でよく用いられる。この場合は、蒸留缶の沸騰液に直接水蒸気を吹き込んで行う回分蒸留である。

［早川豊彦］

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