VARIABLES INDICATING THE COST OF VAPOR-LIQUID-EQUILIBRIUM SEPARATION PROCESSES

In process synthesis, cost-effective process flowsheets must be genera ted and unpromising flowsheet alternatives must be discarded. Currentl y, there are no simple economic criteria for comparing alternatives to distillation columns for vapour-liquid equilibrium (VLE) separation p rocesses. This work introduces a pair of 'capacity variables' which ma y serve as cost indicators for processes involving vaporization and co ndensation. These variables are defined for an equilibrium stage and e xtended to describe a separation process comprised of many such stages . These definitions are applied to a range of VLE separation processes , and consistent results are obtained for all the processes considered . The capacity variables for the process are found to be related to th e total internal vapour and liquid production requirements, which are related to both capital and energy costs of the process. The overall c apacity variables are calculated for a simple ideal binary distillatio n column. The capacity variables are seen to follow closely trends in cost. The optimum redux ratio predicted by the capacity variables is a round 1.5 times the minimum reflux ratio, which is close to the accept ed heuristic rule of 1.03-1.3 times the minimum reflux ratio. The capa city variables are thus shown to be potentially useful variables for a ssessing costs in process synthesis exercises. Copyright (C) 1996 Else vier Science Ltd