7.3.2.1 Vapor-Liquid Phase Equilibrium Diagrams VLE differs from SLE

in two ways. First, VLE is sensitive to both temperature and pressure, whereas

SLE is not affected much by pressure. Second, with VLE, all the components are

usually distributed in both vapor and liquid phases, whereas with SLE, it is

common that the solid phase is pure (single-component). To show the VLE

relationships graphically, we usually either (1) hold the pressure P constant and

plot the vapor (y) and liquid (x) mole fractions as a function of temperature T, or

(2) hold T constant and plot y, and x, as functions of P. Both T-x-y and P-x-y

diagrams are useful, but we will focus on T-x-y diagrams because it is more

common to design separation units to operate at constant P.

Figure 7.7 is a T-x-y diagram for ethanol (E) and water (W), with P= 1 atm.

(For the same data in table form, see Table B.13.) The normal boiling point

temperature T, 100°C for pure water is shown on the left-hand side (where ye

0.0 and yw

1.0) and 7,78.3°C for pure ethanol is shown on the right-hand

side (where yg=1.0).

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