Direct sequence heuristic

Heuristic 2. Sequences that remove the lightest components alone one by one in column overheads should be favored. In other words, favor the direct sequence. Heuristic 3. A component composing a large fraction of the feed should be removed first. 

Direct Sequence Heuristic Prefer removing the most volatile species first. 

The marginal cost approach explains (at least partially) many of the commonly published heuristics used to select the better sequences. We have already seen how it explains the direct sequence heuristic, which is stated as follows ... 

Direct Sequence Heuristic All other aspects of the problem being equal, remove the most volatile species first. 

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.

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. 

Removing systematically the most plentiful component in intermediate mixtures will drastically reduce the cost of separation, both as investment and energy consumption. This heuristics typically leads to a direct sequence. 

The mixture is shared approximately in equal parts between HC1, VCM and EDC. Applying the heuristics in Chapter 3, the following direct sequence can be developed ... 

Repeated application of this heuristic gives what is called the direct sequence. For example, the direct sequence for our problem is A/BCDE, B/CDE, C/DE, and D/E. (The indirect sequence—the other one with a special name—is ABCD/E, ABC/D, AB/C, A/B.)... 

This heuristic favots the direct sequence because it removes the products one-by-one as distillates and therefore minimizes vapor Row in the column. Lockhart6 points out, however, that the direct sequence is not optimal when the least-volatile component is the primary constituent of the feed stream, in this case, the indirect sequence is preferred. 

When the number of products is three or four, designing and costing all possible sequences can best determine the most economical sequence. Often, however, unless the feed mixture has a wide distribution of component concentrations or a wide variation of relative volatilities for the possible separation points, the costs will not vary much and the sequence selection may be based on operation factors. In that case, the direct sequence is often the choice. Otherwise, a number of heuristics that have appeared in the literature, starting in 1947, have proved useful for reducing the number of sequences for detailed examination. The most useful of these heuristics are ... 

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. 

---