Two-pressure process

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Principles of the two-pressure process (schematic)

The two-pressure process (also known as pressure swing rectification and pressure swing distillation ) is a process used in industrial chemistry to separate binary azeotropic mixtures by rectifying two rectification columns connected in series at significantly different pressures and temperatures.

The prerequisite for this is the significant change in the azeotropic composition of the mixture with the change in pressure and temperature.

The method is based on the fact that the respective phase equilibrium is on different sides of the azeotrope in the two columns. In the first column, which is mostly operated at a low pressure, one of the two components can be drawn off purely at the bottom of the column, while the azeotrope is drawn off at the top, but in a different composition than in the feed. This mixture is fed into the second column and is no longer an azeotropic mixture there, since it is operated there at higher pressure. In the second column, the second component can be drawn off at the bottom, while an azeotropic mixture is formed in the top, but again with a significantly different composition than in the feed of the second column. This azeotropic mixture is fed back into the first column.

The two components can each be withdrawn in the bottom (they are the high boilers ) because they have a higher boiling point than the azeotropic mixture (in these two cases the low boilers ).

use

The two-pressure process is only used relatively rarely, since the basic requirement of the high pressure dependency of the azeotropic composition is rarely given to a sufficient extent. One example is the separation of tetrahydrofuran and water .

The following data shows that the azeotropic point shifts from 94 mole percent THF at 22 kPa to 70 mole percent at 350 kPa.

T [K] P [kPa] x THF [mol / mol] x water [mol / mol] source
298.15 21,819 0.941 0.059
337.87 101.325 0.828 0.172
375.90 349,958 0.6975 0.3025

literature

  1. ↑ Dortmund database .
  2. ↑ Shnitko VA, Kogan VB, J.Appl.Chem.USSR, 41 (6), pp. 1236-1242, 1968.
  3. ^ Cigna R., Sebastiani E., Ann.Chim. (Rome), 54 (11), pp. 1048-1059, 1964.
  4. ↑ Gmehling J. , Boelts R., J. Chem. Eng. Data, 41 (2), pp. 202-209, 1996.