Difference between extractive and azeotropic distillation

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Azeotropic rectification

If the separation factors for the separation of a mixture are close to one, separation by means of simple rectification is not possible or only at high investment and operating costs, i.e. columns with many trays and a very high reflux ratio.

The introduction of a further component can facilitate the separation here, even if the additional component makes a further separation step necessary, which consists in separating this component.

In addition to the possibility of feeding in an additional, high-boiling component that "extracts" one of the two components to be separated, there is also the possibility of using an additional component that forms an azeotrope (temperature minimum azeotrope) with one of the two components to be separated. forms. This then leaves the column at the head.

This azeotrope does not necessarily have to represent a new separation problem, since it occurs in a three-component mixture. If it is present alone, i.e. without the third component separated off in the bottom of the column, it either does not exist or it can be easily separated. Rectification is not used for separation, but other separation processes such as extraction or, in the case of a heteroazeotrope, a simple phase separation of two liquid phases that have formed.

When selecting the components for carrying out an azeotropic rectification, components are selected whose boiling points are in a similar range to that of the components to be separated in order to keep the additional energy consumption required as low as possible. Furthermore, these additional components must form an azeotrope with one of the two components, which can then be easily separated, possibly also using a different separation process.

The schematic structure of such a separation process is shown in the following figure. The mixture to be separated and the additional component are added to column I. The high boiler B is taken off at the bottom of the column, while the azeotrope from A + Z is taken off at the top of the column. This azeotrope is separated in separation process II. The recovered additional component Z is returned to the first column. Any losses must be compensated.

Typical separation problems that can be solved with the help of azeotropic rectification:
  • Ethanol-water additional components: benzene, cyclohexane, pentane, toluene, etc.
  • Propanol-water additional component: benzene
  • Aliphatic aromatic mixtures with a narrow boiling range. Additional components: acetone, butanone-2
  • Acetic acid-water additional component: butyl acetate
  • Pyridine-water additional components: benzene, toluene

Specialization: The separation of benzene and cyclohexane with acetone as an additional component

Consolidation / illustration: Ethanol absolute with benzene with animation of the process