Binary Phase Diagram: Vapor liquid equilibrium behavior of binary systems can be represented by a temperature composition diagram at a given constant pressure. The bubble- and dew point- curves converge at the two ends, which represent the saturation points of the two components. Equilibrium composition of any mixture in two-phase region can be found by drawing a horizontal line. Certain non-ideal systems deviate so much from these as to form maxima or minima at an intermediate composition rather than at on end or the other of the diagram. Such a composition is called an "azeotropic composition". Phase equilibrium is important in design of distillation columns. Such design is commonly based on use of a xy diagram, a plot of equilibrium vapor composition y versus liquid composition x for a given binary system at a given pressure.

Example 6.4: Calculate the vapor-liquid equilibrium composition of an ideal binary mixture of benzene and toluene at 760 mm Hg. Given the following information:

Boiling point of benzene, t_b = 80.1 °C

Boiling point of toluene, t_t = 130.13 °C

Solution: Using the vapor pressure data from previous examples, we can establish the following equilibrium values.

t/°C	Α	y	x
85	4.66	0.956	0.824
90	4.53	0.903	0.672
95	4.42	0.839	0.542
100	4.30	0.765	0.43
105	4.20	0.678	0.334
110	4.10	0.577	0.25
115	4.005	0.461	0.176
120	3.915	0.328	0.111
125	3.831	0.176	0.053
130	3.75	0.005	0.001