Some aspects of the operation of a semi-continuous, middle-vessel distillation column that separates a ternary ideal mixture are analyzed. In general, middle-vessel distillation columns have a large vessel between the recti. cation and stripping sections and are usually operated in a batch or continuous fashion. Recently, Phimister and Seider (2000) presented a novel process that operates in a semi-continuous manner, showing that the process can separate a ternary ideal mixture into three nearly pure species, with the intermediate-boiling species concentrating in the middle vessel and the light and heavy species concentrating in the distillate and bottom products, respectively. By means of numerical simulations of middle-vessel continuous and middle-vessel batch processes, in this work we arrive in a natural manner at the middle-vessel semi-continuous process. Due to the fact the operation of semi-continuous middle-vessel distillation requires the use of controlled policies, a dual composition control configuration was designed. The control schemes used in this work are based on stable inverse-dynamics feedback control law by assuming complete knowledge of the process dynamics. As a result, a new operating policy based on refills of the middle vessel is proposed and discussed.