As mentioned earlier, some binary azeotrope mixtures lose azeotropic behaviour when the system pressure is changed. In this case, separation can be achieved without using an additional entrainer. For example, in the case of ethanol-water mixture (azeotropic composition 89.4 mole% at 101.325 kPa), an azeotrope does not form below a pressure of approximately 9.2 kPa.
Pressure-swing distillation is a method for separating a pressure-sensitive azeotrope that utiltizes 2 columns operated in sequence at 2 different pressures.
For example, consider the case of a minimum-boiling azeotrope with T-x-y curves at P2 as shown in the Figure below (left). As the pressure is reduced to P1, the azeotropic concentration moves toward a smaller percentage of A (xP2 to xP1). The set-up for a pressure-swing distillation is shown in the Figure below (right).
The total feed F1 to Column T-1 operating at the lower pressure P1 is the sum of the fresh feed F at mole fraction xF, and the recycled distillate D2 at mole fraction xD2 (close to azeotropic mole fraction xP2). The mole fraction of the total feed F1 is xF1 , and it is richer in A than the azeotropic mole fraction of xP1 at pressure P1. The bottoms leaving column T-1 is almost pure A. The distillate leaving T-1 is D1 , having the mole fraction xD1 . This mole fraction is richer in A than the azeotropic mole fraction of xP1, but less rich in A than the azeotrope mole fraction of xP2 .
Distillate D1 is sent to Column T-2 as feed F2 . Nearly pure B is obtained from bottom of T-2, and the distillate D2 is recycled to Column T-1.
The changes on the phase diagram is shown the Figure below.
[ see pp.612-613, "Separation Process Principles", J.D. Seader and E.J. Henley, John Wiley & Sons ]
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