Page 291 - Distillation theory

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8.2 Zeotropic Mixtures 265

8.2. Zeotropic Mixtures

8.2.1. Heuristic Rules of Synthesis
Theeasiestandtheoldestwayofsynthesisistouseanumberofheuristicrules,such
as “first isolate the lightest product” (direct separation), “choose such a boundary
of first division that the amounts of the top and bottom products should be the clos-
est ones” (the principle of dichotomy) (Harbert, 1957), “choose such a boundary
of first division that the separation should be the easiest one” (Thompson & King,
1972), and “the most difficult separation should be the last one.” These heuristics
obtained from the experience of calculation are supplemented by such evident
heuristics as follows: “obnoxious and corrosion-active components should be re-
moved at the beginning” and “it is desirable to obtain end products as distillates”
(Hendry, Rudd, & Seader, 1973).
The first group of heuristic rules can be substantiated if some assumptions about
the mixture under separation are accepted. Such substantiation was made in the
works (Modi & Westerberg, 1992) and (Westerberg & Wahnschafft, 1996) using
the Underwood method for calculation of summary vapor flow in the sequence
of column in the minimum reflux mode.
The heuristic way of synthesis is very simple and almost does not require calcu-
lations; however, its application can lead to nonoptimal sequences of separation
and, therefore, to substantial excessive expenditures. Heuristic rules can contra-
dict each other. For example, it is really not too hard to find a mixture for which
the first and the second rule recommend different boundaries of division. Such an
example is given in the work (Malone et al., 1985) for the mixture with volatilities
of the components α = 2.59 and α = 1.2 and composition (0.05; 0.05; 0.90).
AB BC
According to the first rule, it is direct separation that should be in the first
column (A : B,C) → (B : C); according to the second rule, there must be indi-
rect separation (A,B : C) → (A : B). Calculation check-up proves that summary
vapor flow in the two columns is 60% larger at direct separation than at indirect
separation in the first column.
Thus, to correctly choose the best sequence of columns, the estimation of ex-
penditures on separation is necessary.

8.2.2. Estimation of the Expenditures on Separation

At the stage of synthesis, it is not necessary to determine all expenditures on sep-
aration for each sequence of columns being compared. The number of alternative
sequences being great, an aspiration to simplify as much as possible the calcula-
tion of these expenditures, not committing errors in estimation of preferability of
these sequences, arises.
For estimation of the expenditures, it was proposed in many works to use the
summary vapor flow V in all columns of the sequence under consideration, cal-
culated according to the Underwood method for the minimum reflux mode. Such
approach presupposes that the mixture is a close-to-ideal one and that expensive