Relative Volatility

One of the most useful ways to represent VLE data is by employing "relative volatility," which is the ratio of the y/x values [vapor mole fraction over (divided by) liquid mole fraction] of two components. For example, the relative volatility of component L with respect to component H is defined in the following equation:

yH/Xh

The larger the relative volatility, the easier the separation.

Relative volatilities can be applied to both binary and multicomponent systems. In the binary case, the relative volatility a between the light and heavy components can be used to give a simple relationship between the composition of the liquid phase (x is the mole fraction of the light component in the liquid phase) and the composition of the vapor phase ( y is the mole fraction of the light component in the vapor phase):

1J. Gmehling et al., Vapor-Liquid Equilibrium Data Collection, DECHEMA, Frankfurt/Main, 1993.

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