As the pressure is reduced to P1, the azeotropic concentration moves toward a smaller ). The set-up for a pressure-swing distillation is shown in Fig. The total feed F1 to Column T1 operating at the lower pressure P1 is the sum of the fresh feed F at mole fraction x F , and the recycled distillate D2 at mole fraction x D2 (close to azeotropic mole fraction x P2 ). The mole fraction of the total feed F1 is x F1 , and it is richer in A than the azeotropic mole fraction of x P1 at pressure P 1 . The bottoms leaving column T1 is almost pure A. The distillate leaving T1 is D 1 , having the mole fraction xD 1 . 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 x P2 .