Abstract: The modeling of two-phase flow was analyzed by using process simulation software PROII with the aim of predicting the distributions of pressure，temperature，evaporation rate as well as volumetric ratio of vapor to gas （G/L） in a vacuum transfer line. The model was developed based on the pressure drop model and the multi-stage flash model. The results indicated that the pressure drop，temperature decline， the variation of evaporation rate and G/L primarily occurred in the transition section and the junction site of the transfer line. In comparison with the high-velocity transfer line，the pressure and temperature decline was clearly higher in the case of the low-velocity transfer line. In the premise of the evaporation rate at the outlet of transfer line remaining approximately the same，the temperature at the inlet of transfer line was lower in the case of the low-velocity transfer line，which means more efficient energy transfer. Meanwhile，the low-velocity transfer line offered more moderate change of evaporation rate and G/L ratio，in favor of decreasing the amount of liquid entrainment at the feed section of a vacuum tower. Furthermore，the predictions were in good agreement with the industrial field data in the case of the low-velocity transfer line，which verified the reliability of the model.

Key words: vacuum distillation, transfer line, process simulation, two-phase flow, pressure drop, evaporation rate