Page 289 - Distillation theory

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8

Synthesis of Separation Flowsheets

8.1. Introduction
Synthesis of a separation flowsheet consists of determining the best sequence of
distillation columns and complexes that will ensure the obtaining of the set of
products of a set quality from the initial mixture.
Distillation columns and complexes entering into this sequence differ by splits
and kinds of complexes. The best sequence is characterized by the smallest sum-
mary expenditures on separation (energy expenditures and capital costs, taking
into consideration their payback period).
A large number of various research works were dedicated to the task of synthe-
sis. However, these works present nothing more than examination of particular
examples of mixtures, and the results of these works prove the complicity of the
task of synthesis. The methods used in these works are too laborious to be widely
adopted in separation units designing. Therefore, while designing, the separation
flowsheet is chosen, as a rule, on the basis of analogy with existing units or some too
simple heuristic rules, or comparison of a small number of alternative sequences
of columns is carried out. Very often, it so happens that the most “interesting”
separation flowsheets remain unexamined. It concerns, first of all, flowsheets that
imply the use of distillation complexes and separation flowsheets for azeotropic
mixtures. Such practice leads to great excessive expenditures on separation.
For zeotropic mixtures, the main difficulty of the solution of synthesis task
consists of the large number of alternative sequences that have to be calculated
and compared with each other in terms of expenditures. This number greatly
increases when the number of the products into which the mixture should be
separated increases. The best sequence (or several sequences with close values
of expenditures) depends on the concentrations of the components in the mix-
ture under separation and on the field of phase equilibrium coefficients of the
components in the concentration simplex. To ensure the solubility of the task of
synthesis for multicomponent zeotropic mixtures, it is necessary to create a pro-
gram system that would include as main modules programs of automatic design

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