MIXED-INTEGER OPTIMIZATION OF COMBINED SOLVENT-EXTRACTION AND DISTILLATION PROCESSES

Liquid solvent extraction with a distillative purification of its prod ucts is frequently used to separate close boiling or azeotropic mixtur es. In this paper a methodology for the synthesis of superstructures f or the mixed integer optimization of such processes is presented. A ra ther specific and small superstructure is generated in order to keep t he rigorous process model solvable. The highly nonideal behavior of pa rtially immiscible mixtures makes a rigorous modeling using excess Gib bs energy (g(E)) models for phase equilibrium calculations necessary. For the synthesis of the superstructure only knowledge of the phase eq uilibrium (i.e. vapor-liquid and liquid-liquid equilibrium) is necessa ry. Two strategies make the combination of a mixed integer optimizatio n and a rigorous simulation of the process possible: Firstly, a rather specific superstructure for a liquid solvent extraction process is ge nerated. Secondly, the difficult liquid-liquid phase equilibrium calcu lation is separated from the process optimization task. The capabiliti es of the model are shown with an example process consisting of a liqu id solvent extraction column, two distillation columns, a decanter and several heat exchangers. It is possible to perform simultaneously an optimization of the continuous operation variables as well as in some structural variables like the overall number of equilibrium stages, th e feed locations or the existance or non-existance of complete columns .