Section 3.7.1.1: Note on Relative Volatility

A convenient measure for use particularly in approximate vapour-liquid equilibrium calculations is the relative volatility of one species with respect to an arbitrarily chosen reference or key component.

Relative volatility $\alpha_{i,r}$ of species i to a reference component r is defined as:

$\begin{displaymath}\alpha_{i,r} = \frac{K_i(T,P)}{ K_r(T,P)} \end{displaymath}$

For an ideal system:

$\begin{displaymath}K_i = \frac{P^*_i(T)}{ P} \end{displaymath}$

Hence:

$\begin{displaymath}\alpha_{i,r} = \frac{P^*_i(T)}{ P^*_r(T)} \end{displaymath}$

Although relative volatility is thus a function of temperature T, much of its usefulness results from it being a rather weak function of temperature, particularly for nearly ideal mixtures. This is because the slope of vapour pressure versus curves tend to be similar for similar components.

Thus over a reasonably limited range of temperatures relative volatility may be estimated at a suitable `average' or reference temperature and then regarded as a constant.

For an n component mixture there are of course n relative volatilities. However if the reference component is taken as on e of those in the mixture, which would be the usual practice, then the volatility of this component w.r.t. itself will be one.

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