It has long been recognized that liquid flow pattern especially liquid circulation

or reverse flow

is of great importance in determining tray efficiency.Research work concerning tray efficiency calculation is reviewed and a mathematical model which combines computational fluid dynamic model with mass transfer equation to predict the details of the mass transfer process on a distillation tray is proposed. By solving the proposed model

the velocity distribution of liquid phase and concentration profiles of vapor and liquid phases can be calculated simultaneously. Complete mixing and partial mixing of vapor phase is investigated in simulating the details of mass transfer process. The result reveals that the tray efficiency calculated in the assumption of complete mixing of vapor phase is greater than the real value. The computed results verify the qualitative assumption that the vapor leaving the liquid circulation region has very little change in concentration. The quantitative results of tray efficiency vs. the size of circulation region is given.