Azeotropic distillation

The azeotropic rectification, also called entrainer process is a chemical-technical separation method for obtaining pure substances from azeotropic mixtures.

Principle

The azeotropic mixture in the azeotropic rectification, an entrainer is added that forms a low-boiling ternary heteroazeotrope. The heteroazeotrope is used auszuschleppen a component of a binary mixture. A heteroazeotrope is characterized in that in the composition of the azeotrope has a miscibility gap appears in the liquid phase. The liquid decomposes into two phases with different composition. This property is used to recover the entraining agent.

Example ethanol dehydration

In the case of ethanol dehydration four main process steps are carried out:

• Rectification of the ethanol / water mixture. This first step is already supplying pure water ( boiling point 100 ° C) at the bottom of the column, however, an azeotropic mixture of about 90 mole % ethanol and 10 mole - % of water ( boiling point about 78 ° C) at the top of the column (at atmospheric pressure ). A higher enrichment can no longer be reached via a rectification.

• The azeotropic mixture is pumped into a second separation column in which an entraining agent is additionally added. As entrainer benzene was used earlier for the most part, but today substances such as cyclohexane and toluene are preferred. Benzene forms with water and ethanol, a low-boiling ternary azeotrope (boiling point about 65 ° C, composition approximately 22.8 mol% ethanol, 53.8 mol% benzene and 23.4 mol% water) over top of the column can be removed. Using cyclohexane as an entrainer, the composition is about 7 mole% water, 17 mol% ethanol, and 76 mol% of cyclohexane at a boiling point of 63.4 ° C. Overhead is evaporated so that all the residual water. Although this overhead also ethanol is deducted may be deducted in the bottom of pure ethanol (about 99.7 mol%, the boiling temperature about 78 ° C ), since it has already been fed in large excess in this column.

• The ternary mixture is condensed and decomposes into two phases, one rich and one poor in benzene. In a phase separator (also referred to as a decanter ), the two phases are separated. The benzene- rich phase (> 80 mol% ) of the previous column is fed directly back.

• The benzene poor and thus water-rich phase is split in a further column. At the bottom of this regeneration column, a water - ethanol mixture can be removed, which is fed to the first column again. At the top of this column, the ternary azeotrope formed that once the phase separator is fed.

In this process, as in all rectification not one hundred percent pure product can be obtained, since this would only be possible with an infinitely high separation columns. Therefore, on the one hand no more benzene is used because it is carcinogenic, on the other hand, the ethanol produced here is also cleaned or with other procedures.

Entrainer selection

A suitable additive is usually included today by searching in factual databases, published in the scientific literature experimental azeotropic points searched. Alternatively, prediction methods are used to calculate the azeotropic points and also the miscibility gap. Here ia are expected larger deviations, which are therefore verified before large-scale application with measurements.