Page 63 - Separation process engineering

equilibrium data. In practice, the measurement is fairly difficult and a variety of special equilibrium stills
have been developed. Marsh (1978) and Van Ness and Abbott (1982, Section 6-7) briefly review
methods of determining equilibrium. With a static equilibrium cell, concentration measurements are not
required for binary systems. Concentrations can be calculated from pressure and temperature data, but the
calculation is complex.

If we obtained equilibrium measurements for a binary mixture of ethanol and water at 1 atm, we would
generate data similar to those shown in Table 2-1. The mole fractions in each phase must sum to 1.0. Thus
for this binary system,

(2-3)

Table 2-1. Vapor-liquid equilibrium data for ethanol and water at 1 atm y and x in mole fractions

where x is mole fraction in the liquid and y is mole fraction in the vapor. Very often only the composition
of the most volatile component (ethanol in this case) will be given. The mole fraction of the less volatile
component can be found from Eqs. (2-3). Equilibrium depends on pressure. (Data in Table 2-1 are
specified for a pressure of 1 atm.) Table 2-1 is only one source of equilibrium data for the ethanol-water
system, and over a dozen studies have explored this system (Wankat, 1988), and data are contained in the
more general sources listed in Table 2-2. The data in different references do not agree perfectly, and care
must be taken in choosing good data. We will refer back to this (and other) data quite often. If you have
difficulty finding it, either look in the index under ethanol data or water data, or look in Appendix D under
ethanol-water VLE.