Entrainer Selection

In principle, any substance can be an entraîner but not every substance can "break" the azeotrope and/or improve relative volatility of the close boiling point mixture. Separation factor (i.e. relative volatility) is an important physical quantity in azeotropic distillation. It is desirable to assess the influence that the entraîner will have on the vapor-liquid equilibrium whenever any column is designed and simulated [31, 32],

In some cases the entraîner and the two components of i and j being separated can produce three-phase (vapor-liquid-liquid) equilibrium. Two liquid phases may be in equilibrium with a vapor phase. For the three-phase equilibrium the solubilities of components i and j in the upper liquid phase are denoted respectively by x' and x', the solubilities of components A and B in the lower liquid phase respectively by x" and x'j , and the corresponding activity coefficients by y in the upper liquid phase and Y in the lower liquid phase, respectively. The relative volatility for components i and j is related to the overall composition X, according to ^ _ rf.Pft (x':' -X,)Yl+{Xl-x',)yJ

a" r^jPj&j (*," - x, )r, + (x, - x; )Yl where û: and 6. are correction factor for high pressure. At low or even middle pressure, it can be approximately regarded as 6, = 0. « 1.

Over the two-liquid phase region Eq. (1) gives relative volatilities for three-phase equilibrium. In the composition range where only an upper liquid phase and a vapor phase exist, Eq. (1) reduces to

When only a lower liquid phase is in equilibrium with a vapor phase, Eq. (1) becomes rip,0,

Similar relations have already been obtained in chapter 1 (Eqs. (63) and (64)), where for the

\VIL-BH)(P-P, low pressure, 0i =0j ~ 1 ; and for the middle pressure, 0i = exp

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