Page 221 - Distillation theory

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6.7 Distillation Complexes with Thermal Coupling Flows 195

equilibrium coefficients of components 1 ÷ 3 on the concentration of component 1
is shown in Fig. 6.11a, dependence of the concentration of component 1 in pseudo-
product point x on its concentration in reversible distillation trajectory tear-off

D
point x t , according to Eq. (6.13), is shown in Fig. 6.11b and the dependence of
rev,e
the relative rate of the entrainer E/D on this concentration x t , according to Eq.
rev,e
(6.16), is shown in Fig. 6.11c.
It is seen from Fig. 6.11a that K 3 = 1 in points x 1 = 0.35 and x 1 = 0.85 and that
K 3 = K 1 in point x 1 = 0.9. In accordance with this (see Eq. 6.13) in point x t =
rev,1
0.35 x =±∞, in point x t = 0.85 x =+∞, and in point x t = 0.9 x = 0
D,i rev,1 D,i rev,1 D,i
(Fig. 6.11b). Along with that, in the segment [0; 0.35] x < 0 (i.e., point x lies

D,i D
on the continuation of side 1-2 behind vertex 2) in the segment [0.35; 0.85] x D,i >
1 (i.e., point x lies on the continuation of side 1-2 behind vertex 1) and in the

D
segment [0.9; 1.0] 0 < x D,i < 1 (i.e., point x lies on side 1-2) (Fig. 6.8).

D
According to Fig. 6.11c, the parameter E/D →∞ at x t → 0 and (E/D) min =
rev,1
0.5 at x t = 0.75.
rev,1
As it becomes evident from this example, to determine (E/D) min at m m = 2
it is sufficient to scan the values of phase equilibrium coefficients of the com-
t
ponents in the intermediate section trajectory tear-off segment Reg and to use
e
Eq. (6.16).
Inamoregeneralcaseatm m >2insteadofthetrajectorytear-offsegment,there
is a tear-off region, in points of which it is necessary to determine the phase equi-
librium coefficients and then to determine the values of the parameter (E/D) min .
For example, for the split at Fig. 6.9c the parameter (E/D) min depends on composi-
tion of entrainer (i.e., on location of point x E at side 3-4). Therefore, to determine
(E/D) min it necessary to scan the phase equilibrium coefficients of the components
in all the points of face 1-3-4. As a result, the value (E/D) min and the optimal
composition of the entrainer x E (i.e., optimal correlation in it of concentrations of
components 3 and 4) will be found.
For the split in Fig. 6.9b, the composition of the top product x D , as a rule, is
fixed because it is conditioned by the correlation of components 1 and 2 in the
mixture under separation. Therefore, to determine (E/D) min it is sufficient to scan
t
the phase equilibrium coefficients of the components in tear-off points x rev for the
pseudoproduct points x on continuation of line x D − x E .

D

6.7. Distillation Complexes with Thermal Coupling Flows

6.7.1. Kinds of Distillation Complexes with Thermal Coupling Flows
Three kinds of distillation complexes with thermal coupling flows (with branching
of liquid and/or vapor flows) – columns with side stripping, columns with side
rectifiers and complexes with full thermal coupling flows, called Petlyuk column –
are used in industry at present.
For three-component mixtures, these kinds of complexes can be easily obtained
from three splits of three-component mixture in the first in motion column of the