Azeotrope rectification

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The azeotropic rectification , also known as the entrainer process, is a chemical-technical separation process for the extraction of pure substances from azeotropic mixtures.

principle

In the azeotrope rectification, an entrainer is added to the azeotropic mixture , which forms a ternary, low -boiling heteroazeotrope. The heteroazeotrope is used to drag out a component from a binary mixture. A heteroazeotrope is characterized in that a in the composition of the azeotrope miscibility gap in the liquid phase occurs. The liquid breaks down into two phases with different compositions. This property is used to recover the entrainer.

Example of absolute ethanol

Azeotropic rectification, absolute ethanol

In the case of ethanol - Dehydration four main process steps are performed:

  • Rectification of the ethanol / water mixture. This first step already provides pure water (boiling temperature 100 ° C) at the bottom of the column, but an azeotropic mixture of about 96 mol% ethanol and 4 mol% water (boiling temperature 78.15 ° C) at the top of the column (at normal pressure). A higher enrichment can no longer be achieved through rectification.
  • The azeotropic mixture is pumped into a second separating column, in which an entrainer is also added. In the past, benzene was mostly used as an entrainer, but today substances such as cyclohexane and toluene are preferred. The benzene forms a low-boiling ternary azeotrope with water and ethanol (boiling point approx. 65 ° C, composition approx. 22.8 mol% ethanol, 53.8 mol% benzene and 23.4 mol% water), which via the Can be withdrawn at the top of the column. With cyclohexane as an entrainer, the composition is about 7 mol% water, 17 mol% ethanol and 76 mol% cyclohexane at a boiling point of 63.4 ° C. All of the remaining water is thus evaporated overhead. Although ethanol is also drawn off overhead, pure ethanol (approx. 99.7 mol%, boiling temperature 78.3 ° C.) can be drawn off in the bottom, since a large excess of it has already been fed into this column.
Decomposition of the azeotropic mixture
  • The ternary mixture is condensed and breaks down into two phases, one rich in benzene and one poor in benzene. The two phases are separated in a phase separator (also known as a decanter ). The benzene-rich phase (> 80 mol%) is returned directly to the previous column.
  • The low-benzene and thus water-rich phase is split up in a further column. At the bottom of this regeneration column, a water-ethanol mixture can be drawn off and fed back to the first column. The ternary azeotrope arises at the top of this column and is fed back to the phase separator.

In this process, as in all rectification processes, a 100% pure product cannot be achieved, as this would only be possible with infinitely high separation columns. Therefore, on the one hand, benzene is no longer used because it is carcinogenic ; on the other hand, the ethanol produced here is further purified using other processes.

Entrainer selection

A suitable additive is mostly sought today through searches in fact databases containing experimental azeotropic points published in scientific literature . Alternatively, prediction methods are used to calculate the azeotropic points and also the miscibility gap. Larger deviations are generally to be expected here, which are therefore verified with measurements before large-scale application.

See also

Web links

literature

  1. ↑ Manfred Baerns, Arno Behr, Axel Brehm, Jürgen Gmehling, Kai-Olaf Hinrichsen, Hanns Hofmann, Ulfert Onken, Regina Palkovits, Albert Renken: Technische Chemie . John Wiley & Sons, 2014, ISBN 978-3-527-67409-1 , pp. 323 ( limited preview in Google Book search).
  2. ^ Klaus Schwetlick: Organikum . Wiley, 2009, ISBN 978-3-527-32292-3 , pp. 56 ( limited preview in Google Book search).
  3. ↑ Swietoslawski W., Karpinski B., CRHebd.Seanc.Acad.Sci.Paris, 198, pp. 2166-2167, 1934.