Page 81 - Separation process engineering

Comparing Eqs. (2-32c) and (2-27), the Raoult’s law K value is

(2-33)

This is handy, since extensive tables of vapor pressures are available (e.g., Boublik et al., 1984; Dean,
1985; Green and Perry, 2008). Vapor pressure is often correlated in terms of the Antoine equation

(2-34)

where A, B, and C are constants for each pure compound. These constants are tabulated in various data
sources (Boublik et al., 1984; Yaws et al., 2005). The equations based on Raoult’s law should be used
with great care, since deviations from Raoult’s law are extremely common.

Nonidealities in the liquid phase can be taken into account with a liquid-phase activity coefficient, γ .
i
Then Eq. (2-33) becomes

(2-35)

The activity coefficient depends on temperature, pressure, and concentration. Excellent correlation
procedures for activity coefficients such as the Margules, Van Laar, Wilson, NRTL, and UNIQUAC
methods have been developed (Poling et al., 2001; Prausnitz et al., 1999; Sandler, 2006; Tester and
Modell, 1997; Van Ness and Abbott, 1982; Walas, 1985). The coefficients for these equations for a wide
variety of mixtures have been tabulated along with the experimental data (see Table 2-2). When the binary
data are not available, one can use infinite dilution coefficients (Table 2-2; Carlson, 1996; Lazzaroni et
al., 2005; Schad, 1998) or the UNIFAC group contribution method (Fredenslund et al., 1977; Prausnitz et
al., 1980) to predict the missing data. Many distillation simulators use Eqs. (2-34), (2-35), and an
appropriate activity coefficient equation. Although a detailed description of these methods is beyond the
scope of this book, a guide to choosing VLE correlations for use in computer simulations is presented in
Table 2-4. For final designs, you must be confident in the VLE correlation used. Check the predictions
with experimental data such as VLE data, flash distillation results, or distillation column results.

Table 2-4. Approximate guides for selection of K-value methods.