DIstillatoion

The fundamental basis of distillation is the physical equilibrium between the liquid and vapor phases of a system. Equilibrium is the condition reached after an infinite time of contact between the phases. In practice, liquid-vapor systems normally reach a state close to equilibrium in a comparatively short time of contact. At equilibrium, the composition in the vapor phase is usually different from that in the liquid. (If this is not the case, then an azeotrope is present, as discussed later.) The relationship of equilibrium concentrations of a component between phases is described by the equilibrium ratio . Other terms used for this ratio can include distribution coefficient, equilibrium constant, K-constant , or simply volatility.


Relative volatility is a useful tool to judge the feasibility and ease of a distillation separation. In general, the larger the relative volatility between two key components, the easier and less costly will be the separation of those keys. If the relative volatility between the keys is unity, then their separation by ordinary distillation is impossible. Such a situation exists with azeotropes. The relationship between relative volatility and difficulty of separation may be illustrated by application of a simple relationship


By Rajendra