Abstract: The pinch zone in a distillation column is the stages where the liquid or vapor composition varies very little. It can be classified into upper, middle and lower pinch zones for ternary distillation according to the different locations of its occurence. In batch distillation with an infinite number of stages, different pinch zones may appear under the same instantaneous bottom composition when the column is operated at different reflux ratios. In this paper, the development of pinch zone for ternary systems in constant reflux operation is discussed.The method of determining minimum reflux ratio for constant reflux operation in multicomponent batch distillation is presented on the basis of Rayleigh formula, which makes the assumption of constant relative volatility unnecessary and can compute the distillate composition accurately. Thus, the method has overcome the shortcomings of Underwood formula, and provides a sound basis for short-cut design of multicomponent batch distillation column.