A procedure based on a rules of thumb used to provide a shortcut solution to a complex problem rather than an algorithm. In the case of separating a multicomponent mixture, the rules of thumb are based on a general understanding of the factors that determine distillation cost. This includes the capital costs that comprise the column itself, as well as reboiler and condenser costs, and operating costs mainly associated with the heat flux from the bottom to top of the column. If all separations are equally difficult the whole cost of the train would be minimized if the sum of all the flows from the bottoms and tops of columns were minimized. This is the case if each column produces approximately equal quantities of material from top and bottom. The fewer times that any component in large excess is distilled, the less cost in treating it will be involved. Any arrangement that sets about removing components in large excess early in the train are likely to be favoured. Highly corrosive components should also be removed early as they are likely to cause damage to equipment. For separations of unequal difficulty, it is clearly not favoured to treat a large quantity of material with a difficult separation. It is preferable to delay difficult separations to later in the train when smaller quantities are being dealt with. It is also beneficial not to separate components at all, if separation is not needed. The following heuristics are therefore applied:

1. 1 Remove highly corrosive components first.

2. 2 Favour equimolar splits.

3. 3 Split by widest difference in relative volatility first.

4. 4 Take a single pure component from the top of the column.

5. 5 Remove components in large excess first.

6. 6 Do not separate components that are acceptable as mixed products.

The set of heuristics, however, may not suggest the same decision in each case.