In this research a 3-D two phase CFD model in Eulerian-Eulerian framework was developed to study sieve trays efficiency, hydraulics and mass transfer. To estimate the efficiency of the sieve trays, two trays with similar geometry but unequal hole diameters were used while the effects of the holes and bubbles diameters were observed. The results were compared with the available experimental data for distillation of methanol and n-propanol mixture. Mass transfer coefficients were estimated by the Higbie and surface renewal stretch (SRS) mass transfer models. It has been found that the liquid flow pattern on the tray which had smaller hole diameter is closer to the plug flow pattern and tends to have a larger mass transfer rate. In addition, while comparing the SRS model theory with the Higbie model, it has been found that using the SRS model reduces the sensitivity of the simulation results to the bubble diameters. (c) 2012 Published by Elsevier Ltd.