Page 290 - Distillation theory

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264 Synthesis of Separation Flowsheets

calculation (i.e., calculation without participation of the designer) of simple dis-
tillation columns and distillation complexes with branching of flows, a program of
calculation of each sequence, a program of estimation of expenditures for each
sequence, a program of automatic identification of each feasible sequence, and a
program of systematic sorting of these sequences and their selection. The program
system should also include limitations on complexity of the distillation complexes
used in synthesis that can be installed in accordance with the user’s desire (limi-
tation on the number of sections or on the number of columns entering into the
complex).
Only the development of the methods of automatic design calculation of sim-
ple columns and distillation complexes with branching of flows described in the
previous chapters and the increase of performance of computers makes real the
creation of such a software product. Traditional methods of design calculation
based on calculation investigations with the help of simulation software are of no
use when solving a synthesis task.
For azeotropic mixtures, the main difficulty of the solution of the task of syn-
thesis consists not in the multiplicity of feasible sequences of columns and com-
plexes but in the necessity for the determination of feasible splits in each potential
column or in the complex. The questions of synthesis of separation flowsheets
for azeotropic mixtures were investigated in a great number of works. But these
works mainly concern three-component mixtures and splits at infinite reflux. In
a small number of works, mixtures with a larger number of components are
considered; however, in these works, the discussion is limited to the identifica-
tion of splits at infinite reflux and linear boundaries between distillation regions
∞
Reg . Yet, it is important to identify all feasible splits, not only the splits fea-
sible in simple columns at infinite reflux and at linear boundaries between dis-
tillation regions. It is important, in particular, to identify the splits feasible in
simple columns at finite reflux and curvilinear boundaries between distillation
regions and also the splits feasible only in three-section columns of extractive
distillation.
ThetheoryoftrajectorybundlesdescribedinChapters5and6ensuresthepossi-
bility of identification of all feasible splits of multicomponent azeotropic mixtures.
The software for synthesis of separation units for multicomponent azeotropic mix-
tures should include, besides the module of identification of feasible splits, a mod-
ule of preliminary selection of these splits (i.e., choice of the most “interesting”
splits, a module of determination of necessary recycle flow rates, a module of
choice of entrainers, and also modules entering into the system of synthesis for
zeotropic mixtures).
Besides general questions of synthesis for zeotropic and azeotropic mixtures,
we also discuss a particular but very important from the practical point of view task
of conceptual designing of units of petroleum refining. The peculiarity of these
units is the limitation of separability for the reason of thermolability. Therefore,
the most important criterion of efficiency of these units is the degree of recovery
of the most valuable (“light”) oil products.