Page 223 - Distillation theory

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6.7 Distillation Complexes with Thermal Coupling Flows 197

6.7.2. Petlyuk Columns

Columns with completely thermal coupling flows were patented in different
modifications by a number of authors: sequence with prefractionator (Brugma,
1942; Fig. 6.12d); column with dividing wall (Wright, 1949; Fig. 6.12f); and col-
umn with dividing wall for separation of four-component mixtures (Cahn et al.,
1962).
Later, these columns were independently rediscovered (Petlyuk, Platonov, &
Slavinskii, 1965; Platonov, Petlyuk, & Zhvanetskiy, 1970) on the basis of theoret-
ical analysis of thermodynamically reversible distillation because this distillation
complex by its configuration coincides with the sequence of thermodynamically
reversible distillation of three-component mixture (see Chapter 4), but in con-
trast to this sequence it contains regular adiabatic columns. The peculiarities of
Petlyuk columns for multicomponent mixtures are (1) total number of sections is
n(n − 1) instead of 2(n − 1) in regular separation sequences; (2) it is sufficient to
have one reboiler and one condenser; (3) the lightest and the heaviest components
are the key components in each two-section constituent of the complex; and (4)
n components of a set purity are products.
The modifications of Petlyuk column in Fig. 6.12c,e,f are thermodynamically
equivalent. The modification in Fig. 6.12e has the advantage that vapor flow from
the second three-section column is selected in the top and some middle cross-
section, and is directed to the first three-section column (i.e., there are no vapor
flows of different directions passing from one column into the other). This allows
to keep the pressure in the second column at the level slightly above that of the first
one. Therefore, valves allowing to regulate splitting of vapor flows can be installed
at one or both vapor flows. This modification was introduced in the works (Kaibel,
1987; Smith & Linnhoff, 1988; Agrawal & Fidkowski, 1998).
Petlyuk columns decrease energy expenditures for separation of three-
component mixtures, on the average, by 30% due to their thermodynamical ad-
vantages: (1) in the preliminary column, the composition of flows in feed cross-
section is close to feed composition (i.e., thermodynamic losses at mixing of flows
are nearly absent); (2) these losses at the mixing of flows at the ends of the columns
are nearly absent; (3) absence of reboiler or condenser at output of component
2 decreases energy expenditures due to the fact that liquid and vapor flows are
used twice in the sections located above and below output of component 2; and
(4) thermodynamic losses for the reason of repeated mixing of flows in the second
column at regular separation sequence are absent (the concentration of compo-
nent 2 at the end of the first column at direct split along distillation trajectory
decreases, which requires additional expenditures of energy in the second column
for obtaining pure component 2).
An interesting new application of Petlyuk columns is mentioned in the work
(Agrawal, Woodward, & Modi, 1997) – in order to remove microadmixtures and
obtain products of very high purity at air separation.
The modification in Fig. 6.12f (column with a dividing wall) allows, besides that,
to decrease capital expenses, on the average, by 30%.