1.1.4 Azeotropes

Icigure L.2 and 1.a are. phase diagrams for "normal" systems. In such ¡systems, as tne composition of the heavier component increase, so do 'the dew point and the bubble point.

iIf the components exhibit strong physical or chemical interaction, the j iphase diagrams may be different from those shown in Figure 1.3 or j1.4(a), and more like those shown in Figure 1.4(b) to 1.4(d). In such ; systems there is a critical composition (the point of intersection of | the equilibrium curve with the 45° diagonal) for which the vapor and ! liquid compositions are identical. Once this vapor and liquid j composition is reached, the components cannot be separated at the given pressure. Such mixtures are called azeotropes. I

A minimum boiling azeotrope (Figure 1.4(b)) boils at a temperature ]

flower than either of the pure components. When distilling a system | I made up of these components, the top product is the azeotrope. The ! : bottom product is the high boiling point component when the more ;

I volatile components are identical. Once this is present at low !

|concentrations. On the other hand, when the low boiling point ¡component is present at high concentrations, the bottom product is the ¡more volatile component.

!A maximum boiling azeotrope (Figure 1.4 (c)) boils at a temperature ¡higher than either of the pure components, and will therefore always !

ileave at the bottom of the.column.

¡If liquid phase separation occurs (Figure 1.4(d)), the boiling ¡temperature of the mixture as well as the vapor phase composition j jremain constant until one of the liquid phases disappears. Under such ! ¡conditions, a mixture of the two liquids will leave the top of the j column while either of the components will leave at the bottom, i

¡depending on the composition.

P, ccmston*
P, corsroir

figure 1.4- Phase diagrams for various types of binary systems.

CM. Van Winkle, "Distillation", 1967, by courtesy, McGraw-Hill Book Company.)

1.1.5 Effect of Temperature, Pressure and Composition on K-Values

For the purpose of this discussion, equation (1-7) is simplified by 'cTTittiag ¿ha ?">ypting correction, whicli is usually small at low ¡pressures, to give:

Combining equation (1-8) with the definition of relative volatility, the following is obtained:

where o o o

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