Isobaric Vapor-Liquid Equilibrium Diagram for a Constant Relative Volatility Mixture

One of the simplest equations relating vapor pressure of a pure component to temperature is given by:

where is in Kelvin and is in bars.

The above equation is named August equation, after the German physicist Ernst Ferdinand August (1795-1870). Thus, vs. is a straight line. As shown in the snapshots, if , then the two straight lines (i.e., vapor pressures for two components, and ) will be parallel.

Relative volatility assuming ideal behavior is given by: . If the constants and are equal then the relative volatility is independent of temperature and we have where , , and are the constants that appear in the August equation for components and .

Vapor-liquid equilibrium data can be easily computed for constant relative volatility binary mixtures. Indeed, the following relationships can be derived:

where and are the mole fraction of the vapor and liquid phases in equilibrium.

One also has analytical expressions for the bubble temperature, ,and for the dew temperature, , which are as follows:

and

where is the total pressure, , , and are the constants that appear in the August equation for components and and α is the relative volatility. One snapshot shows the isobaric vapor-liquid equilibrium diagram for a particular constant relative volatility mixture at.