Page 57 - Distillation theory

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2.6 Minimum Reflux Mode: Fractionation Classes 31

D, x D
a)

F, x
F

B, x
B
D, x
b) D
Figure 2.8. A location of zones of constant concentration (pinches) in
columns for distillation of three-component mixtures under minimum
F, x F reflux: (a) R < R 1 lim (first class of fractionation), (b) R 1 lim < R < R 2 lim
(second class of fractionation), (c) R > R 2 lim (third class of fractiona-
tion); pinches are shaded.
B, x B

D, x D
c)

F, x F

B, x B

2.6.2. Distillation of Three-Component Mixtures
For mixtures with n ≥ 3 side by side with the first and third classes of fractionation,
an intermediate class – the second class – exists.
Let’s consider the change of compositions of three-component ideal mixture
products in the concentration triangle (Fig. 2.7) under the same conditions as
before for the binary one. With the increase of R in the first fractionation class,
points x D and x B are moving in opposite directions and transferred along the
straight line passing through the “vapor–liquid” feed tie-line x F → y F (Fig. 2.7a).
The zones of constant concentrations of the column are in the feed cross-section
(Fig. 2.8a).
In the case of R 1 = R 3 (boundary mode of the first fractionation class), point
lim
x D reaches side 1-2. At this time, the trajectory of distillation of the rectifying
section (Fig. 2.7a) is situated along side 1-2 from point x D up to the tear off point
t
x , and later it comes inside the concentration triangle up to point x F. Under these
r
conditions, the trajectory of the stripping section is located completely inside the
concentration triangle. The zones of constant concentrations of the column are
given in Fig. 2.8b.