Page 64 - Distillation theory

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38 Basic Concepts of Distillation

Moreover, the application of specific separation schemes and stripping columns,
in particular, is of primary importance.

2.11. The Problem of Designing Distillation Units

The main object of the distillation theory is the development of new methods of
designing the units for separation of mixtures into required products at minimum
costs. Two basic interconnected stages of the conceptual design of distillation units
are (1) synthesis of optimum flowsheet; and (2) determination of the best reflux
number, quantity of trays in the column sections, and optimum recycling flows.
The synthesis of optimum sequences for the multicomponent azeotropic mix-
ture is the issue of the distillation theory. Geometric theory of distillation over-
comes the principal part of this problem – the determination of possible splits
for each potential distillation column that may be included into the synthesized
sequence. The best feasible sequences selection is carried out on the basis of the
criteria of a minimum number of columns, as well as minimum liquid and vapor
flows, under the minimum reflux mode.
The analysis of the minimum reflux mode is used at the stage of sequence
selection, as well as at the stage of determination of optimum reflux ratios and the
quantity of column trays. The geometric theory of distillation makes it possible to
develop the general methods of calculation of minimum and more reflux mode.
For the mixtures characterized by infinite separability, the necessity for deter-
mination of possible splits no longer arises because they are known a priori.
Therefore, the problem of how to make the best sequence selection from the
many probable ones still exists.
The maximum recovery of the most valuable components from the feed (e.g.,
the separation of light oil products from the crude oil) is considered to be the
criterion of primary importance when choosing the best sequence used for ther-
molabile mixtures processing.
The second stage, including the selection of the best reflux numbers and the
quantity of column section trays, will be the important one. The geometric distilla-
tion theory makes it possible to determine the feasible compositions that are to be
in trays above and below the feed cross-section, then make the design calculations
of the trajectory of sections and determine the best ratio of section tray numbers.
The new algorithms allow for an increase in the design quality; and apart from
that, they make it possible to lower the separation costs and to practically exclude
the human participation in the process of calculation.

2.12. Questions

1. What is the “theoretical tray”?
2. What types of equations are used to calculate the distillation?
3. What are the distinctions in the representation of minimum, and more finite and
infinite reflux modes in the diagram of McCabe and Thiele?