[yi y2 2 yNcl5 T f P Xi X2 2 xNci [y T f P x

Here we have introduced the mole fractions x and y in the liquid and vapour phases respectively, and we trivially have £1 xi = 1 and £ 1 yi = 1.

In ideal mixtures, the VLE can be derived from Raoult's law which states that the partial pressure p, of a component (i) in the vapour phase is proportional to the vapour pressure (po) of the pure component (which is a function of temperature only: po = po(T)) and the liquid mole fraction (x,):

Here po(T) depends on temperature so the K-values will actually be constant only close to the column ends where the temperature is relatively constant. On the other hand, the ratio po(T)/po(T) is much less dependent on temperature, which makes the relative volatility very attractive for computations. For ideal mixtures, a geometric average of the relative volatilities for the highest and lowest temperature in the column usually gives sufficient accuracy in the computations: « = y«ij,top ' ^.bottom.

We usually select a common reference component r (usually the least volatile or heavy component), and define:

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