Separation flowsheet synthesis columns

Further, we call triangles 1-2-13 and 3-2-13 product simplexes Regsmp- This notion has great significance for separation flowsheets synthesis, because for a feed point xp located inside the product simplex one can get all the components and azeotropes that are vertexes of this simplex in a sequence of (n - 1) columns. 

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. 

This chapter is the central one of the book all previous chapters being introductory ones to it, and all posterior chapters arising from this one. Distillation process in inhnite column at finite refiux is the most similar to the real process in finite columns. The difference in results of finite and infinite column distillation can be made as small as one wants by increasing the number of plates. Therefore, the main practical questions of distillation unit creation are those of separation flowsheet synthesis and of optimal design parameters determination (i.e., the questions of conceptual design) that can be solved only on the basis of theory of distillation in infinite columns at finite reflux. 

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. 

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. 

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. 

While synthesizing separation flowsheets, it is necessary to consider the pos-sibUity of thermodynamic improvement and thermal integration. Therefore, for each sequence, identified in the process of synthesis, it is necessary to realize possible thermodynanric improvements and thermal integration of the columns. The estimation of expenditures on separation is made taking into consideration these modifications, if these expenditures are smaller than for the sequence without modifications under consideration. This estimation of expenditures is used while comparing the sequence under consideration to all other possible sequences. 

In the given example mixture 2,3,4, that is bottom product of the first column, is zeotropic therefore, its separation in the second and third columns presents no difficulty. Figure 8.23 shows nine preferable separation flowsheets for this example, obtained by means of synthesis. All these flowsheets separate four-component mixtures using a minimum number (three) of columns without additional entrain-ers, but with application of autoentrainer. Component 2 or 4 or mixtures 2,4 or 3,4 are used as an autoentrainer. Therefore, each of nine separation flowsheets contains one column with one or two feedings where recycle flow of autoentrainer is brought in. 

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. 

Very little process synthesis literature has dealt successfully with the problem of selection between alternative methods of separation. Indeed, most process synthesis literature has dealt with the selection of the flowsheet sequences Tor a single sepamtion method with only simple, sharp separations between components of ndjacent selectivity, without recycle. Furthermore, most of this literature has used only distillation in illustrative epplications. In practice, much use is made of recycle, nonsharp eplits. and complex columns (in distillation) and, of coorse. distillation is not always the separation merited of choice. 

The synthesis of the liquid separation system for the HDA process has been discussed in Chapter 7. The flowsheet consists of three columns C-1 (stabiliser), C-2 (production), and (C-3) recycle. For the same operation point discussed before Table 17.8 presents some characteristics of the three columns. Note that a cooler (duty 730 kW) should be inserted before the column C-2 to bring the feed at its bubble point. The design is valid for a benzene purity of 99.8 %, as well as for a loss of toluene in Heavies of about 0.4%, imposed by the limitation of reboiler temperature in the recycle column below 200 °C. 

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. 

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