Page 207 - Distillation theory

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6.4 Trajectories of Intermediate Sections of Extractive Distillation Columns 181

the height of the column, which requires the calculation of the minimum reflux
mode at different successions. For the column with two feeds, one has to begin
the calculations with the regular succession (that means bringing in the feed with
lower bubble temperature into the higher cross-section of the column). If it turns
out that the energy consumption at separation is smaller than in the column with
one mixed feed, then one can leave the inverse succession unexamined. Otherwise,
one has to carry out the calculations for the inverse succession.

6.4. Trajectories of Intermediate Sections of Extractive
Distillation Columns
Columns of extractive distillation represent an important particular case of
columns with two feeds. The peculiarity consists of the fact that the entrainer is
their top feed and the mixture being separated is their bottom feed. The entrainer
can consist of one or more components, included or not included in the mixture
under separation. In the first case, the separation process is called autoextractive
distillation; in the second case, it is called just extractive distillation.
Columnsofextractiveandautoextractivedistillationarewidelyusedinindustry
(Benedict&Rubin,1945;Drew,1979;Happe,Cornell,&Eastman,1946;Hoffman,
1964; Kogan, 1971). The separation process of binary mixtures with azeotrope,
having minimum bubble temperature, with the help of heavy entrainer brought
into the column higher than the main feed was discovered empirically and started
to be used in the 1940s in connection with military needs – in particular, for
extraction of butadiene and toluene.
The feasibility of azeotropic mixture separation and bringing in light entrainer
in vapor phase into the cross-section lower than the main feed was shown along
with regular extractive distillation (Kiva et al., 1983).
Theoretical analysis of the separation of azeotropic mixtures with the help of
extractive distillation was carried out in the works (Levy & Doherty, 1986; Knight
& Doherty, 1989; Knapp & Doherty, 1990; Knapp & Doherty, 1992; Wahnschafft
& Westerberg, 1993; Knapp & Doherty, 1994; Wahnschafft, Kohler, & Westerberg,
1994; Bauer & Stichlmair, 1995; Rooks, Malone, & Doherty, 1996; Stichlmair &
Fair, 1998; Doherty & Malone, 2001). Characteristic peculiarities of the process
of extractive distillation of binary azeotropic mixtures were investigated in these
works. More general conception of the processes of extractive and autoextractive
distillation on the basis of the theory of intermediate section trajectory tear-off
from boundary elements of concentration simplex was introduced in the works
(Petlyuk, 1984; Petlyuk & Danilov, 1999). Trajectory bundles of intermediate sec-
tion for multicomponent mixtures were examined in the latter work.

6.4.1. Sharp Extractive Distillation of Three-Component Mixtures
In contrast to columns with two feeds examined in the previous section for which
composition and amount of each feed flow were set, only the main feed (i.e., the
mixture under separation F) is set in columns of extractive distillation. The amount