Distillation sequence heuristics

Four columns are needed to produce the desired products. Considering the Sharp Distillation Sequencing heuristics, heuristic (/) does not apply, as there is more than one product in this mixture. Fatty acids are moderately corrosive, but none is particularly more so than the others, so heuristic (2) does not apply. The most volatile product, the caproic and capryflc mixture, is a small (10 mol %) fraction of the feed, so heuristic (3) does not apply. The least volatile product, the oleic—stearic acids, is 27% of the feed, but is not nearly as large as the capric—lauric acid product, so heuristic (4) does not apply. The spht between lauric and myristic acids is closest to equimolar (55 45) and is easy. Therefore, by heuristic (5) it should be performed first. The boiling point list implies that the distillate of the first column contains caproic, capryflc, capric, and lauric acids. This stream requires only one further separation, which by heuristic (/) is between the caproic—capryflc acids and capric—lauric acids. 

The most volatile product (myristic acid) is a small fraction of the feed, whereas the least volatile product (oleic—stearic acids) is most of the feed, and the palmitic—oleic acid split has a good relative volatility. The palmitic—oleic acid split therefore is selected by heuristic (4) for the third column. This would also be the separation suggested by heuristic (5). After splitting myristic and palmitic acid, the final distillation sequence is pictured in Figure 1. Detailed simulations of the separation flow sheet confirm that the capital cost of this design is about 7% less than the straightforward direct sequence. 

Fig. 1. Fatty acid distillation sequence (a) sequence generated by ranked heuristics and (b) more expensive direct sequence.

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

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). 

The six sequencing heuristics are formulated to reduce the separation load on downstream columns, favoring easier separations early and difficult separations in the absence of nonkey components. If only two products are to be derived from a mixture and all of the components in one product are more volatile than all of the components in the other product, then the next split should divide the mixture into the two products. The presence of hazardous or corrosive materials can gready increase costs, and such components should be removed as early as possible. The most plentiful product in a mixture should be removed (if it can be) with one separation and if the relative volatility is favorable. Direct sequences, ie, removing a light product as distillate, generally are favored over indirect sequences, ie, removing a heavy product as bottoms. If no product dominates the feed composition, then separations that yield approximately equimolar splits are favored. Only if no other heuristic applies should the easiest separation be performed next. 

Heuristic methods have been widely used for synthesis of distillation sequences to avoid lengthy calculations. Many heuristics are intuitive, e.g., Remove corrosive components first, Remove most plentiful... 

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. 

The following stream at 100°F and 20 psia is to be separated into the four indicated products. Determine the best distillation sequence by the heuristics. 

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. 

Considerable research has been carried out to facilitate the selection of desirable configurations. A few simple heuristics are generally employed to guide the selection of a few sequences, which may be subjected to detailed analysis (King, 1980 Doherty and Malone, 2001). Consideration of the energy required for separation is very important in these deliberations. (An elementary introduction to the energy required for separation in distillation is provided in Sections 10.1.3, 10.1.4.2, 10.2.2.1 and 10.2.2.2.) We list a few simple sequencing heuristics below the rest... 

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