Heuristic Rules of Synthesis

The easiest and the oldest way of synthesis is to use a number of heuristic rules, such as first isolate the lightest product (direct separation), choose such a boundary of first division that the amounts of the top and bottom products should be the closest ones (the principle of dichotomy) (Harbert, 1957), choose such a boundary of first division that the separation should be the easiest one (Thompson King, 1972), and the most difficult separation should be the last one. These heuristics obtained from the experience of calculation are supplemented by such evident heuristics as follows obnoxious and corrosion-active components should be removed at the beginning and it is desirable to obtain end products as distillates (Hendry, Rudd, Seader, 1973). 

The first group of heuristic rules can be substantiated if some assumptions about the mixture under separation are accepted. Such substantiation was made in the works (Modi Westerberg, 1992) and (Westerberg Wahnschafft, 1996) using the Underwood method for calculation of summary vapor flow in the sequence of column in the minimum reflux mode. 

The heuristic way of synthesis is very simple and almost does not require calculations however, its application can lead to nonoptinaal sequences of separation and, therefore, to substantial excessive expenditures. Heuristic rules can contradict each other. For example, it is really not too hard to find a mixture for which the first and the second rule recommend different boundaries of division. Such an example is given in the work (Malone et al., 1985) for the mixture with volatiUties of the components = 2.59 and age = 1-2 and composition (0.05 0.05 0.90). 

According to the first rule, it is direct separation that should be in the first column (A B,C) (B C) according to the second rule, there must be indirect separation (A,B C) (A B). Calculation check-up proves that summary vapor flow in the two columns is 60% larger at direct separation than at indirect separation in the first column. 

to correctly choose the best sequence of columns, the estimation of expenditures on separation is necessary. 

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