Page 62 - Distillation theory

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36 Basic Concepts of Distillation

1
a) b)
2
x E
3 x x B
x F+E
E

x F

1 3
13 x F
x D
2
x B
Figure 2.14. (a) A sequence with recycle for extractive distillation
(first column, extractive column; second column, column of entrainer
recovery); (b) the distillation trajectory of extractive column for sep-
aration of acetone(1)-water (2)-methanol(3) mixture (water-
entrainer). x F , initial feed; x F+E , total feed into first column.

that converts, for example, a binary mixture into a three-component mixture with
a quite definite structure type, a more easily separable one).
The heteroazeotropic distillation (Fig. 2.13a) is applicable only for the mixtures
that have a heterogeneous region in the liquid phase Reg L−L .
Figure 2.13b shows a structure of concentration space of mixture benzene
(entrainer)(1)-isopropyl alcohol(2)-water(3). The mixture has a ternary azeotrope
and three binary azeotropes.
Duetotheentrainerandthedecanter,itispossibletoseparatebinaryazeotropic
mixture x F into products x B (almost pure isopropyl alcohol) and x D (contaminated
water), which may be purified easily in the second column. Point x D lies not on
the distillation trajectory but on the liquid–liquid tie line (x D ≡ x L2 ).
Figure 2.14a shows a flowsheet of the column of extractive distillation and,
in Fig. 2.14b, an example of acetone(1)-water(entrainer)(2)-methanol(3) mixture
with section trajectories is shown. This mixture, which is impossible to separate
sharply into acetone (x D ) and methanol-water mixture (x B ) in the single-feed
column, may be separated into these products in the column with an extractive
section located between two feed inlets.

2.9. Is Process Opposite to Distillation Process Possible?
If there is a reversible distillation process, then there should be also an opposite
process, which may be called a process of the distilled flow mixing.
Figure 2.15b shows a McCabe-Thiele diagram with the operation lines of the
opposite process of distillation, and Fig. 2.15a illustrates a scheme of this process.
In the opposite process, a light component is introduced into the column bot-
tom, a heavy component is introduced into the column overhead, and a mixture
is removed from the column middle part. In the reboiler, the heat is fed at low
temperature T D and, from the condenser, the heat is removed at high temperature
T B . Thus, the opposite process acts as the heat pump, which transfers heat from