B

Azeotropic distillation column Solvent recovery column

Fig. 2. The double column process for heterogeneous azeotropic distillation.

Fig. 3. Binary T-x phase diagram at a fixed pressure for homogeneous azeotrope; (a) azeotropic boiling point above but not far from the upper critical solution temperature; adapted from the reference: S. Widagdo and W.D. Seider [29]; (b) azeotropic boiling point below the upper critical solution temperature (usual behavior).

Fig. 3. Binary T-x phase diagram at a fixed pressure for homogeneous azeotrope; (a) azeotropic boiling point above but not far from the upper critical solution temperature; adapted from the reference: S. Widagdo and W.D. Seider [29]; (b) azeotropic boiling point below the upper critical solution temperature (usual behavior).

For the binary azeotrope to be separated, the typical binary T-x phase diagrams at a fixed pressure are illustrated in Fig. 3 for homogeneous azeotrope and in Fig. 4 for heterogeneous azeotrope. At an azeotropic point, the relative volatility of binary azeotrope is unity because the overall liquid phase composition x( =yt. In Fig. 3 (a) the L - L two phases region may not exist; or even if exist, the operating temperature in the distillation column is over the highest temperature of L-L two phases region; In Fig. 3 (b) the azeotropic boiling point is below the upper critical solution temperature (usual behavior). In Fig. 4 two vapor-liquid envelops (Li - V and L2 - V) overlap with liquid-liquid envelop (Li - L2) at one line which is also the tie line of liquid-liquid equilibria. Provided overlapping only at one point (i.e. azeotropic point) which is also the critical point of liquid-liquid equilibria, then heterogeneous azeotrope becomes homogeneous azeotrope because in this case x, = x2 = xazeo.

Homogeneous Azeotrope

Fig. 4. Binary T-x phase diagram at a fixed pressure for heterogeneous azeotrope; adapted from the reference: S. Widagdo and W.D. Seider [29].

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