Cox Vapor Pressure Charts
The most important term in both K values and the relative volatility j equation is the vapor pressure term.
EFFECT OF TEMPERATURE This effect can be illustrated with the aid of j a Cox chart (Figure 1.5, Reference 8). This chart shows a steep increase of vapor pressure with temperature. For this reason, the K value for each component increases steeply with temperature.
The Cox chart also shows that in general, for members of a homologous series, vapor pressure of the less volatile component increases faster with temperature than the vapor pressure of the more volatile o components. This causes pr (equation l9a) to diminish as temperature is raised. As the critical point is approached, the ratio o pr approaches unity. Applying equation 19, relative volatility decreases as temperature is raised, until it approaches unity as the critical point is approached.
The above trend is valid for members of a homologous series. For components which are not members of a homologous series, the reverse trend may occur over a limited temperature range, causing relative volatility to increase as the equilibrium temperature is raised (equation 19). However, as temperature is raised further and approaches the critical point, relative volatility will eventually diminish, and will reach unity at the pseudocritical point of the mixture.
; (R.H. Perry, "Chemical Engineers Handbook", 1963, by courtesy, .McGrawHill Book Company.)
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