Page 293 - Distillation theory

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8.2 Zeotropic Mixtures 267

minimum number of columns are compared, it is necessary to estimate both kinds
of expenditures – energy ones and capital ones. For this purpose, in a number of
works simplified methods of design calculation of simple columns and of distilla-
tioncomplexes(methodFenske–Underwood–Gilliland)andsimplifiedmodelsfor
estimation of the expenditures (Kafarov et al., 1975; Malone et al., 1985; Glinos &
Malone, 1988) were used.
Simplified methods of this type are good only for rough estimations because
empiricalratiosofGillilandandtheassumptionaboutconstancyofrelativevolatil-
ities of components can lead to essential mistakes.
At present, after the development of methods of automatic design calculation
(see Chapter 7), the application of simplified methods should be rejected and
rigorous methods should be used. To decrease the volume of calculations, some
stages can be excluded from these methods (those of iterations and of parameter
optimization).

8.2.3. Preferability Regions for Ternary Mixtures

Depending on the composition of the mixture under separation, this or that sep-
aration flowsheet is preferable. Many researchers investigated the location of
preferability regions in the concentration triangle.
The comparison of sequences with direct and indirect separation in the first
column, with three columns, with prefractionator and complex column, and of
Petlyuk column (Figs. 8.1 and 6.12c, d) was made in the works (Petlyuk & Platonov,
1965; Petlyuk et al., 1965). The summary flow rate of vapor V and expenditures of
energy of separation W were used for estimation of the expenditures.
The mixtures with volatilities α AC = 1.2, α BC = 1.1 and with compositions x FA =
x FC at different x FB were examined. It turned out for these conditions that, if esti-
mated by summary rate of vapor flow V, the flowsheet with three columns is more
preferable than the flowsheets with two columns at x FB > 0,3; the flowsheet with
prefractionator and complex column is more preferable at any x , and Petlyuk
FB
column is the most preferable at any x FB . In the case of estimation by energy
expenditures, W the flowsheet with three columns and the flowsheet with prefrac-
tionator and complex column are the most preferable ones. Petlyuk column yields
to these flowsheets because for it all heat is brought in at the highest temperature
and is withdrawn at the lowest.
The comparison of various distillation complexes and of ordinary flowsheets
is given in Tedder and Rudd (1978). Columns with side strippings and side recti-
fiers, Petlyuk columns, flowsheet with prefracionator, and also some other feasible
configurations of two columns were examined. It was shown, in particular, that
Petlyuk columns are preferable at big content of average volatile component.
In the monograph (Stichlmair & Fair, 1998), the preferability regions in con-
centration triangle for mixture 2.2-dimethylbutane(A)-2-dimethylpentane(B)-
hexane(C)(relative volatilities α AC = 1.887 and α BC = 1.329) were shown. The
comparison was made in accordance with summary vapor flow V.The flow-
sheets with a minimum number of simple columns, with three columns, and with