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Distillation Trajectories in Infinite Complex
Columns and Complexes
6.1. Introduction
This chapter extends the geometric description of the distillation process to infinite
complex columns and complexes, and then on this basis to develop methods of
their calculation.
Here we understand by complex columns a countercurrent cascade without
branching of flows, without recycles and bypasses, which, in contrast to simple
columns, contains more than two sections. The complex column is a column with
several inputs and/or outputs of flows. The column of extractive distillation with
two inputs of flows – feed input and entrainer input – is an example of a complex
column.
We understand by distillation complex a countercurrent cascade with branch-
ing of flows, with recycles or bypasses of flows. Columns with side stripping or
side rectifier and columns with completely connected thermal flows (the so-called
“Petlyukcolumns”)areexamplesofdistillationcomplexeswithbranchingofflows.
A column of extractive distillation, together with a column of entrainer regenera-
tion, make an example of a complex with recycle of flows. Columns of this complex
work independently of each other; therefore, we do not examine it in this chapter,
and the questions of its usage in separation of azeotropic mixtures and questions
of determination of entrainer optimal flow rate are discussed in the following
chapters.
The fundamental difference between complex columns and complexes and
simple columns lies in the availability of intermediate sections (besides the top
and the bottom ones). The intermediate sections exchange vapor and liquid flows
with other sections or with the decanter.
Therefore, for the intermediate sections, the equations of material balance
should be transformed in such a way that the flow rate and composition of a
pseudoproduct – that is, the difference between the outgoing and the incoming
flows–shouldbesubstitutedtherefortheflowrateandcompositionoftheproduct.
Figure 6.1 shows an example of a closed contour for obtaining material balance
equations for the intermediate section.
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