Abstract: Based on the two-dimensional, fixed-number mixing-pool model proposed previously by the author, a new mathematical model, designated as three-dimensional nonequilibrium mixing-pool model, for simulating an industrial distillation process is presented in this paper. The basic mathematical description of the model is given and the calculated result for a propylene-propane frac-tionation tower shows good agreement with the plant data. The proposed model is believed to have broad prospects in applying to large diameter industrial distillation columns.