Separation flowsheet synthesis azeotropic mixture

For azeotropic mixtures, the main difficulty of the solution of the task of synthesis consists not in the multiplicity of feasible sequences of columns and complexes 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 identification of splits at infinite reflux and linear boundaries between distillation regions Reg° . Yet, it is important to identify all feasible splits, not only the spUts feasible in simple columns at infinite reflux and at linear boundaries between distillation regions. It is important, in particular, to identify the spUts 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. 

In industry, it is necessary to deal with very complicated mixtures for which structural matrices can serve as an instrument of separation flowsheets synthesis. In Wahnschafft (1997), the example of plant for separation of coal tar in South Africa (20 components, more than 200 azeotropes) consisting of 40 columns is given. 

The geometric distillation theory also allowed the development of the general methods of separation flowsheets synthesis for azeotropic mixtures and design calculation of simple and complex distillation columns, which is examined in the chapters to follow. 

This is especially important for the solution of the task of separation flowsheet synthesis of multicomponent azeotropic mixtures (see Chapter 8) because this, in many cases, uses autoextractive distillation (i.e., to exclude the application of entrainers). 

After identification of several preferable sequences, choosing among the optimum sequences, taking into consideration possible thermodinamic improvements and thermal integration of columns, arises. This task is similar to the synthesis of separation flowsheets of zeotropic mixtures (see Section 8.3), and it should be solved by the same methods (i.e., by means of comparative estimation of expenditures on separation). The methods of design calculation, described in Chapters 5 7 for the modes of minimum reflux and reflux bigger than minimum, have to be used for this purpose. In contrast to zeotropic mixtures, the set of alternative preferable sequences for azeotropic mixtures that sharply decreases the volume of necessary calculation is much smaller. 

Chapter 11 Acrylonitrile by Ammoxidation of Propene illustrates the synthesis of a flowsheet in which a difficult separation problem dominates. In addition, large energy consumption of both low- and high-temperature utilities is required. Various separation methods are involved from simple flash and gas absorption to extractive distillation for splitting azeotropic mixtures. The problem is tackled by an accurate thermodynamic analysis. Important energy saving can be detected. 

Petlyuk, F. B., Danilov, R. Yu. (2000). Synthesis of Separation Flowsheets for Multicomponent Azeotropic Mixtures on the Basis of the Distillation Theory. Synthesis Finding Optimal Separation Flowsheets. Theor. Found. Chem. Eng., 34,444-56. 

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