PURPOSE: To obtian a separation system for nonideal multicomponent mixtures by dividing components in a nonideal multicomponent liquid mixture into the prescribed groups of ternary systems and getting a total reflux elemental separation diagram for each system to confirm separation ability, etc.

CONSTITUTION: For the combination of components, that of those with about ≤2°C difference in boiling pt., from each other, that of those forming azeotrope, and that of those consisting of isomers or compounds having a similar functional group are selected. A total reflux elemental separation diagram is obtained by 'ASOG and UNIFAC', a system for giving the vapor-liquid equilibrium relationship of nonideal three components. For example, total reflux distillation curves for the methanil-ethanol- water system are obtained by ASOG, changing the feed composition and the number of stages. Each diagram represents an elemental separation diagram indicating the ratio of components when they are in vapor-liquid equilibrium in total reflux distrillation for each number of stages. Namely, ethanol and water are hard to separate because of having an azeotropic point at 351.32°K. It is concluded, however, that methanol and water can be separated by distillation. Thereby the behavior of each component on distillation is confirmed in advance and a separation system is synthesized.