Separation Train Synthesis

Beginning with the need to separate a C-component mixture into several products, altenia-live sequences of two-product distillation towers are considered in this section. Although the synthesis strategies are not as well defined for highly nonideal and azeotropic mixtures, several steps are well recognized and are described next. It should be mentioned that these strategies continue to be developed, and variations are not uncommon. 

Identify the azeotropes. Initially, it is very helpful to obtain estimates of the temperature, pressure, and composition of the binary, ternary. azeotropes associated with the C-component mixture. For all of the ternary submixtures, these can be determined, as described above, by preparing residue curve or distillation-line maps. When it is necessary to estimate the quaternary and higher-component azeotropes, as well as the binary and ternary azeotropes, the methods of Fidkowski et al. (1993) and Eckert and Kubicek (1997) are recommended. When the C-component mixture is the effluent from a chemical reactor, it may be helpful to include the reacting chemicals, that is, to locate any azeotropes involving these chemicals as well as the existence of reactive azeotropes. This information may show the potential for using reactive distillation operations as a vehicle for crossing distillation boundaries that complicate the recovery of nearly pure species. 

In view of the above, many factors need to be considered in selecting an entrainer, factors that can have a significant impact on the resulting separation train. Two of the more important guidelines are the following  

The effects of these and other guidelines must be considered as each separator is designed and as the separation sequence evolves. More recently, Peterson and Partin (1997) showed that temperature sequences involving the boiling points of the pure species and the azeotrope temperatures can be used to effectively categorize many kinds of residue curve maps. This classification simplifies the search for an entrainer that has a desirable residue curve map, for example, that does not involve a distillation boundary. 

As illustrated throughout this section, process simulators have extensive facilities for preparing phase-equilibrium diagrams T-x-y, P-x-y, x-y. ), and residue curve maps and binodal curves for ternary systems. In addition, related but independent packages have been developed for the synthesis and evaluation of distillation trains involving azeotropic mixtures. These include SPLIT by Aspen Technology, Inc., and DISTIL by Hyprotech (now Aspen Technology, Inc., which contains MAYFLOWER developed by M.F. Doherty and M.F. Malone at the University of Massachusetts). 

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Process synthesis is introduced mostly using heuristics in Part One (Chapters 3 and 5), whereas Part Two provides more detailed algorithmic methods for chemical reactor network synthesis, separation train synthesis, the synthesis of reactor-separator-recycle networks, heat and power integration, mass integration, and the optimal design and sequencing of batch processes. 

From exhaustive application of alternative simple distillation operators to all possible separations for this four-component system, or from application of ranked-list-based separations synthesis methods, it is easily shown that there are five different separation train structures for this four-component problem. Each can be generated systematically or since this pattern of solutions is already well known, each can be written down immediately or design heuristics can be used to generate one or more of the structures expected to be most suitable. After each structure is synthesized, its performance can be analyzed and evaluated with a flowsheet simulator. 

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). 

Figure 7.12 Synthesis problem and separation train for Example 7.2 (a) paraffin separation problem (b) sequence developed from heuristics.

Sections 7.4 and 7.5 deal primarily with the synthesis of separation trains for liquid-mixture feeds. The primary separation techniques are ordinary and enhanced distillation. If the feed consists of a vapor mixture in equilibrium with a liquid mixture, the same techniques and synthesis procedures can often be employed. However, if the feed is a gas mixture and a wide gap in volatility exists between two groups of chemicals in the mixture, it is often preferable, as discussed in Section 7.1, to partially condense the mixture, separate the phases, and send the liquid and gas phases to separate separation systems as discussed by Dou (1988) and shown in Figure 7.44. Note that if a liquid phase is produced in the gas separation system, it is routed to the liquid separation system and vice versa. 

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