Page 60 - Distillation theory

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34 Basic Concepts of Distillation

Figure 2.11. A location of pinches
(shaded) in column for adiabatic distil-
lation at minimum reflux and reversible
distillation for equal product composi-
tion: (a) first class of fractionation (R <
1
R ) and reversible distillation, (b) sec-
lim
2
ond class of fractionation (R = R )
lim
and partially reversible distillation.

from the equilibrium conditions between y j−1 and x j and conditions of the mate-
rial balance for columns’ ends).
Hence, it appears that the maximum possible locations of points y D and x B are
the points of the straight-line intersection, which passes through the feed tie-line
with sides 1-2 and 2-3.
For the adiabatic column in the first class of fractionation, the product compo-
sitions coincide with the product compositions for the reversible distillation (Fig.
2.11a).
Generally speaking, for the first and second fractionation classes under the
minimum reflux mode, the points of compositions in the zones of constant con-
centrations (i.e., stationary points of the trajectory bundles) should be arranged
at the trajectories of reversible distillation built for the product points. It follows
from the conditions of the material balance and the phase equilibrium in the zones
of constant concentrations. Figure 2.11b illustrates the partially reversible process
(it is reversible in the column parts that are from the constant concentration
zones for the minimum reflux mode up to the column ends).
In the case of the reflux ratio alteration and conservation of the product compo-
sition, the stationary points of trajectory bundle are traveling along the reversible
distillation trajectories built for a given product, so the trajectories may be called
lines of stationarity. Thus, the analysis of the reversible distillation trajectory ar-
rangement in the concentration simplex is decisive in general geometric theory of
distillation.
The analysis of temperature alteration, as well as the vapor and liquid flows
along the trajectory of the reversible distillation, is the basis of the methods
for nonadiabatic distillation unit design; the background for developing the new