Estimation of the Expenditures on Separation

At the stage of synthesis, it is not necessary to determine aU expenditures on separation for each sequence of columns being compared. The number of alternative sequences being great, an aspiration to simphfy as much as possible the calculation of these expenditures, not committing errors in estimation of preferability of these sequences, arises. 

For estimation of the expenditures, it was proposed in many works to use the summary vapor flow V in all columns of the sequence under consideration, calculated according to the Underwood method for the minimum reflux mode. Such approach presupposes that the mixture is a close-to-ideal one and that expensive 

Synthesis experiments (Kafarov et al., 1975 Ghnos Malone, 1984) proved that energy expenditures reflected by summary vapor flow V constitute the main share of yearly expenditures, and capital costs do not differ greatly for alternative sequences. Therefore, the flowsheet with smaller summary vapor flow V, as a rule, requires smaller total expenditures on separation. This allows, in the first approximation, to use the summary vapor flow V for estimation of preferability of the sequences at the above-mentioned hmitations. 

If expensive utilities of heat or cold are used in the columns, for example, at low-temperature separation, then the above-described way of expenditures estimation is not good. In this case, the temperature at which heat should be brought in in the reboilers and withdrawn in the condensers acquires great importance. The total energy expenditures on separation can be estimated by the value of energy of separation that depends on the amount of heat brought in and withdrawn, and on the temperatures at which this heat is brought in or withdrawn (see Chapter 4)  

For a column with one reboiler and one condenser in the case of liquid feeding, instead of Eq. (8.1), we have  

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