Synthesis of Separation Flowsheets

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 summary expenditures on separation (energy expenditures and capital costs, taking into consideration their payback period). 

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 mixture 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 program system that would include as main modules programs of automatic design 

Only the development of the methods of automatic design calculation of simple 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 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. 

---

A method similar to that of product simplex and considering azeotropes as pseudocomponents was proposed for synthesis of separation flowsheets in Sargent (1994). 

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. 

The analysis of possible splits in the mode of infinite reflux with the purpose of synthesis of separation flowsheets was extended to multicomponent mixtures (Pet-lyuk, Kievskii, Serafimov, 1977a Petlyuk, Avetyan, Inyaeva, 1977 Petlyuk, 1979 Petlyuk, Kievskii, Serafimov, 1979 Baburina Platonov, 1990 Safrit Westerberg, 1997 Rooks et al., 1998 Sargent, 1998), (Doherty Malone, 2001). 

In the discussed examples, synthesis of separation flowsheets is carried out automatically on the basis of formal rules. In more comphcated cases, in the process of synthesis there can be no other variants other than variants of obtaining azeotropes as some products. These cases are discussed in the following section. 

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. 

---