Vapor pressure is a physical property of a pure chemical component. It is the pressure that a pure component exerts at a given temperature when both liquid and vapor phases are present. Laboratory vapor pressure data, usually generated by chemists, are available for most of the chemical components of importance in industry.
Vapor pressure depends only on temperature. It does not depend on composition because it is a pure component property. This dependence is normally a strong one with an exponential increase in vapor pressure with increasing temperature. Figure 1.1 gives two typical vapor pressure curves, one for benzene and one for toluene. The natural log of the vapor pressures of the two components are plotted against the reciprocal of the
Distillation Design and Control Using Aspen™ Simulation, By William L. Luyben Copyright © 2006 John Wiley & Sons, Inc.
absolute temperature. As temperature increases, we move to the left in the figure, which means a higher vapor pressure. In this particular figure, the vapor pressure PS of each component is given in units of millimeters of mercury (mmHg). The temperature is given in Kelvin units.
Looking at a vertical constant-temperature line shows that benzene has a higher vapor pressure than does toluene at a given temperature. Therefore benzene is the "lighter" component from the standpoint of volatility (not density). Looking at a constant-pressure horizontal line shows that benzene boils at a lower temperature than does toluene. Therefore benzene is the "lower boiling" component. Note that the vapor pressure lines for benzene and toluene are fairly parallel. This means that the ratio of the vapor pressures does not change much with temperature (or pressure). As discussed in a later section, this means that the ease or difficulty of the benzene/toluene separation (the energy required to make a specified separation) does not change much with the operating pressure of the column. Other chemical components can have temperature dependences that are quite different.
If we have a vessel containing a mixture of these two components with liquid and vapor phases present, the concentration of benzene in the vapor phase will be higher than that in the liquid phase. The reverse is true for the heavier, higher-boiling toluene. Therefore benzene and toluene can be separated in a distillation column into an overhead distillate stream that is fairly pure benzene and a bottoms stream that is fairly pure toluene.
Equations can be fitted to the experimental vapor pressure data for each component using two, three, or more parameters. For example, the two-parameter version is ln Pj = Cj + Dj/T
The Cj and Dj are constants for each pure chemical component. Their numerical values depend on the units used for vapor pressure [mmHg, kPa, psia (pounds per square inch absolute), atm, etc.] and on the units used for temperature (K or °R).
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