Distillation for azeotropic mixture(maximum boiling azeotrope,minimum boiling azeotrope)

Distillation for azeotropic mixture

Distillation is the most widely used separation technique in the chemical and petroleum industry. However, not all liquid mixture are amenable to ordinary fractional distillation.

When the components of the system have low relative volatilities (1.00 < a < 1.05), separation becomes difficult and expensive because a large number of trays are required and, usually, a high reflux ratio as well. Both equipment and utilities costs increase markedly and the operation can become uneconomical. If the system forms azeotropes, as in a benzene and cyclohexane system, a different problem arises, - the azeotropic composition limits the separation, and for a better separation this azeotrope must be bypassed in some way. At low to moderate pressure, with the assumption of ideal-gas model for the vapour phase, the vapour-liquid phase equilibrium (VLE) of many mixture can be adequately describe by the following Modified Raoult’s Law:  

 

When g_i = 1, the mixture is said to be ideal Equation 1 simplifies to Raoult’s Law. Non-ideal mixtures   (g_i ¹ 1) can exhibit either positive (g_i > 1) or negative deviations (g_i < 1) from Raoult’s Law. In many highly non-ideal mixtures these deviations become so large that the pressure-composition (P-x, y) and temperature-composition (T-x, y) phase diagrams exhibit a minimum or maximum azeotrope point. 

In the context of the T-x, y phase diagram, these points are called the minimum boiling azeotrope (where the boiling temperature of the azeotrope is less than that of the pure component) or maximum boiling azeotrope (the boiling temperature of the azeotrope is higher than that of the pure components). About 90% of the known azeotropes are of the minimum variety. At these minimum and maximum boiling azeotrope, the liquid phase and its equilibrium vapour phase have the same composition, i.e.,

x_i = y_i for i = 1, …, c (2)

Two main types of azeotropes exist, i.e. the homogeneous azeotrope, where a single liquid phase is in the equilibrium with a vapour phase; and the heterogeneous azeotropes, where the overall liquid composition which form two liquid phases, is identical to the vapour composition.