Conditions for the Occurrence of Azeotropic Behavior

At the azeotropic point, the mole fractions of all components in the liquid phase are identical with the mole fractions in the vapor phase for homogeneous systems. 

This leads to the fact that all K-factors and all relative volatilities show a value of 1 at the azeotropic point and that the system cannot be separated by ordinary distillation. A reliable knowledge of all azeotropic points for the system to be separated is of essential importance for the synthesis and design of separation processes. 

For a binary system, the following relations are valid for homogeneous systems at the azeotropic point using the simplified Eq. (5.18) of Approach B  

Using an equation of state (Approach A) the following relation is obtained for the azeotropic point  

From Eq. (5.46), it can be seen that azeotropic behavior can easily occur in a binary system if the vapor pressures of the two components are very similar, since in this case already very small deviations from Raoult s law are sufficient to fulfill the equation and to create an azeotropic point either with positive or negative deviation from Raoult s law. If the vapor pressures are identical (e.g., at the Bancroft point), the binary system shows the azeotropic behavior. 

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From Figure 5.47, it can be concluded that the occurrence of azeotropic points can be calculated if besides the activity coefficients at infinite dilution the ratio of the vapor pressures is known. Azeotropic behavior occurs if the following condition for positive resp. negative deviation from Raoult s law is fulfilled (see Figure 5.47) ... 

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