There is an additional complexity evident in figs. 7.5c and 7.5d. Considering the former we see that it is defined by two distinct types of behaviour on either sides of the point termed as the “azeotrope”. At this point the P-x1 and the P-y1 curves converge; i.e., the two phases are identical in composition at this point. On left side of the azeotrope  , while on the other side, The reverse situation holds for the system depicted in fig. 7.4d. Such systems are not uncommon in the process industry and always pose a difficulty in purifying a mixture to compositions higher than the azeotropic point. This is because during a distillation process when the mixture composition arrives at the azeotropic point the two phases become identical in composition, and the liquid composition does not alter further during evaporation.
The phase behaviour of the same systems as in fig. 7.5 is depicted as T-x-y diagrams in fig. 7.6. In particular we refer to the figs 7.6b and 7.6d. As expected the P-x curves appear inverted on a T-x diagram. The first kind of system (fig. 7.6b) is said to show a maximum boiling point azeotrope, while that depicted by fig. 7.5d shows a minimum boiling point azeotropic behaviour. |
