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 l-9a) to diminish as temperature is raised. As the critical point is approached, the ratio o pr approaches unity. Applying equation 1-9, 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 1-9). However, as temperature is raised further and approaches the critical point, relative volatility will eventually diminish, and will reach unity at the pseudo-critical point of the mixture.

Cox Chart Vapor Pressure
Fio. 1-5 Cox chart of vapor praam of norma] parsfia hydrocarbon. (£■«• and Lae«r, rarioiw." otan/rrd Univmuy Prmi. Stanford. Calif.. 1839.)

; (R.H. Perry, "Chemical Engineers Handbook", 1963, by courtesy, .McGraw-Hill Book Company.)

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  • ursula
    What is a cox chart pressure?
    8 years ago

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