Page 307 - Distillation theory

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8.4 Multicomponent Azeotropic Mixtures: Presynthesis 281

distillation complexes). Therefore, to decrease the volume of calculations, vari-
ous heuristic rules can be used (e.g., to use “heat pump” only if [T reb − T con ] <
◦
20 C).

8.4. Multicomponent Azeotropic Mixtures: Presynthesis
In contrast to zeotropic mixtures, the main problem for azeotropic ones at syn-
thesis is that of determining possible splits in each potential separation column
(the problem of presynthesis). Initially, the analysis of possible splits of homoge-
neous azeotropic mixtures in simple columns came to nothing more than three-
component mixtures and the mode of infinite reflux. This analysis was based on
diagrams of residue curves. It was proved that the curvature of boundaries be-
tween distillation regions at infinite reflux can be used for separation (Balashov,
Grishunin, & Serafimov, 1970; Balashov, Grishumin, & Serafimov, 1984; Laroche
et al., 1992). This analysis is of great practical importance for the choice of en-
trainers with the purpose of separation of binary azeotropic mixtures.
The analysis of possible splits in the mode of infinite reflux with the purpose of
synthesis of separationflowsheets was extended to multicomponentmixtures (Pet-
lyuk, Kievskii, & Serafimov, 1977a; Petlyuk, Avetyan, & Inyaeva, 1977; Petlyuk,
1979; Petlyuk, Kievskii, & Serafimov, 1979; Baburina & Platonov, 1990; Safrit &
Westerberg, 1997; Rooks et al., 1998; Sargent, 1998), (Doherty & Malone,
2001).
The analysis of possible splits was realized for three-component mixtures
not only at infinite, but also at finite, reflux (Petlyuk et al., 1981; Petlyuk,
Vinogradova, & Serafimov, 1984; Wahnschaft et al., 1992; Kiva, Marchenko, &
Garber, 1993; Poellmann & Blass, 1994; Krolikowski et al., 1996; Davydyan et al.,
1997). In some works (Wahnschaft, Le Redulier, & Westerberg, 1993; Bauer &
Stichlmair, 1995), various strategies of synthesis are examined, but to determine
possible splits it is proposed to use simulation software, which, in the majority
of cases, requires too large volumes of calculations and is not systematic for the
reason of great number of parameters.
Development of trajectory tear-off theory at sharp separation and finite reflux
from the boundary elements of the concentration simplex (Petlyuk & Danilov,
2000a) created a general presynthesis method for multicomponent azeotropic
mixtures.
The theory of trajectory tear-off was extended to the section located between
the cross-section of entrainer input and that of the main-feeding input in columns
of sharp extractive distillation (Petlyuk & Danilov, 1999), which included it into
the general method of presynthesis.
Presynthesis on the basis of the theory of trajectory tear-off from the bound-
ary elements of concentration simplex takes into consideration the sharp splits
important for practical usage in columns with one and two feedings that are not
embraced by the methods of presynthesis on the basis of residue curve structure.
In a number of cases, this manages without expensive ways of separation, without
entrainers, with a minimum number of columns.