Extractive distillation

This is a distillation method in which a substance that is less volatile (has a higher boiling point) than any of the raw materials and significantly changes the relative volatility of the two raw materials is added as a solvent when the vapor pressures of the liquid mixture are close and difficult to separate by normal distillation, or when the two components to be separated form an azeotrope. The raw materials have a low affinity for the solvent and are taken out from the top of the tower, while the components with a high affinity are selectively extracted into the solvent and taken out from the bottom of the tower together with the solvent. The mixed liquid from the bottom of the tower is separated from the solvent in another distillation tower, and the solvent is recycled. For example, benzene and cyclohexane form an azeotrope, but when phenol is added and distillation is performed, cyclohexane is taken out from the top of the tower and a mixed liquid of benzene and phenol is taken out from the bottom of the tower, and the phenol is separated in another distillation tower and recycled.

The solvent for extractive distillation should be one that changes the relative volatility of the two components to be separated most favorably and is easy to recover later, but it should also be easy to obtain and chemically stable, etc. Once the solvent is decided, it is necessary to calculate the amount to be used, which stage near the top of the column to continuously supply it to, and the recovery column and recycle, so it is complicated, but there are established calculation methods.

[Toyohiko Hayakawa]

Source: Shogakukan Encyclopedia Nipponica About Encyclopedia Nipponica Information | Legend

蒸気圧が接近して普通の蒸留では分離しにくい液体混合物や、分離すべき二成分が共沸混合物を形成するとき、原料中のどの成分よりも揮発度が小さく（沸点が高く）原料二成分の相対揮発度を大きく変化させる物質を溶剤として加えて行う蒸留をいう。原料中の溶剤との親和力の小さい成分は塔頂から取り出され、親和力の大きい成分は溶剤に選択的に抽出されて、溶剤とともに塔底から取り出される。塔底からの混合液は別の蒸留塔で溶剤を分離し、溶剤は循環使用する。たとえば、ベンゼンとシクロヘキサンは共沸混合物をつくるが、フェノールを添加して蒸留を行うと、塔頂からシクロヘキサンが、塔底からベンゼンとフェノールの混合液が取り出され、フェノールは別の蒸留塔で分離し循環使用される。

　抽出蒸留の溶剤は、分離すべき二成分系の相対揮発度をもっとも大きく有利に変化させ、しかも後の回収が容易なものがよいが、そのほかにも入手が容易で化学的に安定であるなど総合的に優れたものがよい。溶剤が決まったら量をどのくらいにするか、塔頂近くのどの段に連続的に供給するか、さらに回収塔とリサイクルの計算を行うので複雑であるが計算法は確立されている。

［早川豊彦］

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