Synthesis of separation sequences

The synthesis of separation sequences for non-ideal mixtures is handled nowadays by means of Residue Curve Maps. Major issues are feasibility and entrainer selection. However, there are still important unsolved problems (Chapter 9). 

Azeotropic distillation. Azeotropes could exist in the initial mixture or created by means of an entrainer. The synthesis of separation sequences involving azeotropes is a complicated matter, but systematic methods based on Residue Curve Maps are available. These methods will be developed in larger extent in Chapter 9. 

This chapter presents two applications of MINLP methods in the area of separations. Section 9.1 provides an overall introduction to the synthesis of separation systems. Section 9.2 focuses on sharp heat-integrated distillation sequencing. Section 9.3 presents an application of nonsharp separation synthesis. 

S. H. Cheng and Y. A. Liu. Studies in chemical process design and synthesis 8. A simple heuristic method for systematic synthesis of initial sequences for sloppy multicomponent separations. I ECRes., 27 2304, 1988. 

There is no doubt that, in many instances, relatively smtple approaches are still anfficient to accomplish particular control objectives. However, for example, in the case of separation sequences that involve complex flow arrangements and thermal onapling, more advanced process control strategies will be necessary. As process synthesis methodologies improve, control strategy evaluation and process design and... 

Simple Heaiiatic Method for Systematic Synthesis of Initial Sequences for Multioompoannt Separations, AIChE J., 29, 926 (1983). 

The following subchapters will present the methodology in more detail. However, some more complex subjects will be developed in separate chapters, particularly the energy integration and the synthesis of distillation sequences for non-ideal mixtures. 

The Hierarchical Approach developed in this chapter incorporates a knowledge-based procedure for the synthesis of separations. This consists of dividing the separation section in three subsystems gas vapour, liquid and solid separations. Each subsystem is further managed by selectors, which makes use of unit operations. Split sequencing is based mainly on heuristics, although may include algorithmic or optimisation techniques. This chapter describe in more detail the synthesis of distillation trains for zeotropic distillations, the non-ideal case being left for the Chapter 9. 

A sequence may be simple as in Fig. 14.1 or complex as in Fig. 14.2. It is simple if each separator performs a relatively sharp split between two key components and if neither products nor energy is recycled between separators. In this chapter, methods for the synthesis of simple sequences containing simple separators are presented. 

Most chemical processes are dominated by the need to separate multicomponent chemical mixtures. In general, a number of separation steps must be employed, where each step separates between two components of the feed to that step. During process design, separation methods must be selected and sequenced for these steps. This chapter discusses some of the techniques for the synthesis of separation trains. More detailed treatments are given by Douglas (1995), Bamicki and Siirola (1997), and Doherty and Malone (2001). 

In the previous section we developed heuristics for synthesis of distillation sequences for almost ideal systems unfortunately, many of these heuristics do not apply to nonideal systems. Instead, we must use a different set of operational suggestions and the tools developed in section 8.3. distillation and residue curves. The purpose of the operational suggestions is to first develop a feasible separation scheme and then work to improve it. 

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. 

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. 

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