6.4 Pressure swing distillation

Pressure affects vapor-liquid equilibrium, and in the systems that form azeotrope it will affect the composition of the azeotrope.

The ethanol-water system has an azeotrope at 0.8943 mole fraction ethanol at 1 atm pressure [see also principles of this section].

If the pressure is reduced from 760 torr to 100 torr, it is observed (see fig.1) that the composition of the azeotrope goes more and more to 1 to completely disappear at pressure below 70mmHg [not visible in fig.1].

At this pressure, the separation could be done in a ordinary distillation column. Unfortunately the costs to get the disappearance of the azeotrope are normally very high and in any case the separation would require a very high and large distillation column. Therefore a distillation process operating at this conditions is not convenient.

However even if not completely disappearing, the azeotrope composition is affected in any case by the pressure [fig.2].

If the shift in composition (D₁-D₂) due to the change of the pressure (P₁-P₂) is large enough, one can think about a process where two distillation columns operate at the two different pressures P₁ and P₂.

Tipically, P₁ is the atm pressure, while P₂ is a higher pressure.

Fig.3 shows the process configuration for a system like the one in fig.2: