Physical Chemistry , 1st ed.

(Darren Dugan) #1
Fractional distillations are among the most important and energy-demanding
processes, especially in the petrochemical industry.
Phase diagrams can also be plotted in terms of temperature—usually the
boiling point (BP) of the liquid—versus composition. However, unlike the
pressure-composition phase diagram, there is no simple straight-line equation
to express one of the lines, so in temperature-composition phase diagrams
both bubble point and dew point lines are curved. An example is shown in
Figure 7.10, which corresponds to Figure 7.5. Notice that the higher compo-
nent of vapor pressure, component 2, has the lower boiling point for the pure

7.3 Two Components: Liquid/Liquid Systems 177

Figure 7.9 Some fractional distillation apparatus. (a) A laboratory scale fractional distillation
apparatus. (b) A microscale fractional distillation setup. Microscale equipment uses small
amounts, so it is appropriate when only small amounts of material are available. (c) Fractional
distillation on an industrial scale is a common process. This shows the hardware for large-scale
distillations.

T(BP 2 )

T(BP 1 )

Temperature

0.5
x 1 , y 1

0.0 1.0

T(BP) vs. x 1 :
the bubble point line

T(BP) vs. y 1 :
the dew point line
Figure 7.10 Temperature-composition
phase diagrams are more common than
pressure-composition diagrams. Notice,
however, that neither line is straight, and
that the lines indicating the boiling process
and the condensation process are switched
from the pressure-composition diagrams.
Compare to Figure 7.5.

© William Wright/Fundamental Photographs

Receiving
flask

Condenser

Thermometer

Sample
flask

Fractional
distillation
column Water

(a)


(b)

(c)
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