Vaporliquid Equilibrium Fundamentals

Distillation columns can be used to separate chemical components when there are differences in the concentrations of these components in the liquid and vapor phases. These concentration differences are analyzed and quantified using basic thermodynamic principles covering phase equilibrium. Vapor—liquid equilibrium (VLE) data and analysis are vital components of distillation design and operation.

Vapor Pressure

The liquid phase of any pure chemical component, species /, exerts a certain pressure at a given temperature. This pressure is called the pure component "vapor pressure" Pr It is a physical property of each component.

Vapor-pressure data are obtained by laboratory experiments where both liquid and vapor phases of a pure component are held in a container (see Figure 2.4). Pressure is measured at various temperatures. The temperature at which the pure component exerts a pressure of one atmosphere is called its "normal boiling point." Light components have low normal boiling points and heavy components have high normal boiling points.

* For a further discussion of flooding, see pages 424—430 of reference 8.

t This occurs at about 60 percent of design vapor rates for sieve trays and about 25 percent of design vapor rate for valve trays.

When the data are plotted on linear coordinates (see Figure 2.5), a nonlinear dependence of vapor pressure on temperature is obtained. Vapor-pressure data often can be described by the Antoine equation*:


Pj = vapor pressure of/th component in any pressure units

(commonly mm Hg, psia, atmospheres, kPa) T = absolute temperature (degrees Kelvin or Rankine)

Aj and Bj = constants over reasonable range of temperatures

Therefore, vapor-pressure data are usually plotted using coordinates of log pressure versus reciprocal of absolute temperature as illustrated in Figure 2.6. Note that the constants A, and B, must be determined for each pure component.

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