Heuristics for Determining Favorable Sequences

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  

Remove thermally unstable, corrosive, or chemically reactive components early in the sequence. 

Remove final products one by one as distillates (the direct sequence). 

Sequence separation points to remove, early in the sequence, those components of greatest molar percentage in the feed. 

Sequence separation points in the order of decreasing relative volatility so that the most difficult splits are made in the absence of the other components. 

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