Heuristics for Sequencing

The procedure known as the list-processing method allows the designer to identify feasible sequences for separating ideal or slightly nonideal zeotropic mixtures by means of simple distillation columns. Table 3.13 presents a list of more specific heuristics for sequencing, supplementary to those in Table 3.7. The first one shows 

Perform difficult separations the last, but before separations of azeotropes. 

Remove firstly the lightest component one by one as overhead products. 

Remove components in order of decreasing percentage of the feed. 

This operation will reduce the cost of the next separation. 

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It is interesting to reflect on the heuristics for sequencing of distillation columns in Chapter 11. Heuristics 2, 3 and 4 from Section 11.1 tend to minimize the flowrate of nonkey components. Heuristic 1 relates to special circumstances when there is a particularly difficult separation8. 

Simple distillation Relative volatility a Use heuristics for sequencing. Not feasible if a < 1.1... 

Discussion. The use of heuristics for sequencing multicomponent distillation systems, as well as for separation processes in general, must be used with some discretion on the pan of the design engineer. For certain separation problems, some heuristics will be more pertinent than others and for this reason the systematic application of heuristics is difficult. Since heuristics also usually apply to ample, sharp separations. more complex schemes that may be advantageous often are ignored. For example, Fetlyuk ct al.7 and Stupin and Lockhart showed that more complex separation schemes can be an improvement to the... 

Table 7.23 presents generic heuristics for sequencing the separation of liquid mixtures. More rules, specific for the separations of zeotropic mixtures by distillation, will be shown later. The separation for azeotropic mixtures is treated in Chapter 9, although the emphasis is on the solvent selection and not on the number of splits. 

Probably the most common method used for sequence selection for simple distillation columns is heuristic. Many heuristics have been proposed, but they can be summarized by the following four ... 

TL Bailey, M Grihskov. The megaprior heuristic for discovering protein sequence patterns. Intelligent Systems in Molecular Biology 4 15-24, 1996. 

Apply the heuristics for the sequencing of simple distillation columns to the problem in Table 11.9. Is the sequence shown in Figure 11.26 likely to be a good or bad sequence based on the heuristics ... 

Table 11.10 presents some heuristics for using complex distillation columns to separate a ternary mixture into its pure component products. On the basis of these heuristics and those for simple columns, suggest two sequences containing complex columns that can be used to separate the mixture described in Table 11.9 into relatively pure products. 

The requirements for short-term planning, especially with regard to campaignhandling as well as the need to consider sequence-dependent setup and finite resource capacities on most resources lead to the selection of the PP/DS optimizer as the most suitable planning method. In addition to the optimizer, PP/DS offers numerous heuristics for automating production planning and detailed scheduling tasks. 

Indirect reduction, 14 501 Indirect sequence heuristic, for simple distillation, 22 299... 

Bailey, T. L., and Gribskov, M. (1996). The megaprior heuristic for discovering protein sequence patterns. In Proceedings of the Fourth International Conference on Intelligent Systems for Molecular Biology, pp. 15-24. AAAI Press, Menlo Park. 

Heuristics are reliable, well-established rules for reducing the number of potential alternative sequences with minimum effort, and often lead to near-optimal separation system designs. Most of the heuristics for distillation sequencing were originally formulated from parametric studies. A number of Heuristics have been suggested, some of which contradict each other (5—8). Heuristic methods have also been extended to sequencing nonsharp distillation separations and to combinations of distillation, mixing, and stream bypass operations (9—11). 

Table 3.1 shows general heuristics for split generation applicable to all managers. The removal of toxic, hazardous and corrosive materials has the highest priority. Next, troublesome impurities should be dealt with. Matching directly the products by the shortest sequence of splits is optimal in most cases. When no choice is obvious, dividing the feed as equal as possible is often the best strategy. More specific heuristics will be formulated later in this chapter. 

Table 3.11 General heuristics for separation sequencing of liquid mixtures.

Table 3.13 Heuristics for separation sequencing of zeotropic mixture.

Heuristics far Multicomponent Distillation Sequences. The origin of meny heuristics can be traced to multicomponent distillation problems and, with the possible exception of heat exchenge network synthesis, most process synthesis study has been in this area. This is not surprising since distillation is the backbone of the chemical industry. Heuristics for multicomponent distillation are summarized in Table 4.2-2 and discussed below. 

Table 7.12 General sequencing heuristics for the Gas Split Manager...

Simple distillation Relative volatility oc Use heuristics for split sequencing. Not feasible if a< LI. Check thermal stability of components. 

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