A Study On Modeling Of Vapor Liquid Two-phase Flow In The Transfer Line And Structure Optimization

The transfer line refers to a pipeline connection between the furnace and the vacuum column in the crude oil diatillation unit. A well-designed transfer line can increase the evaporation rate of the feed in the vacuum distillation tower, improve oil product quality and save energy for vacuum distillation unit. Different mathematical models have been established in this paper to improve fundamental understanding of gas-liquid two-phase flashing flow in the transfer line. Moreover, the distribution regularities of pressure, temperature and other parameters in the transfer lines with various structures have been investigated with the models on the basis of a large amount of filed data.Firstly, the one-dimensional, two-phase flow model is developed on the basis of pressure drop model and the multi-stage flash model. The axial pressure, temperature, velocity and evaporation rate are predicted by the model and it is found that variations of parameters principally occur in the transition section and the junction site. The analysis of the seven industrial transfer lines show that the pressure drops of Type A are bigger than others because the transition section merged to the low speed section directly, the pressure decreased gradually in the transfer lines of Type B, the pressure drop of which is the lowest; the pressure drop of Type C with two-stage enlargement structure decreased sharply twice.Secondly, the three-dimensional, compressible single flow model is developed by a commercial computational fluid dynamics (CFD) package. The results of the CFD simulation provided an insight into the phenomena occurring within the flow field in the pipe. It is also found that the secondary motion occurred at the transiton section and swirls existed at the junction section, which are the main reasons for the pressure drop of the two sections. The flow field analysis of the seven transfer lines shows that the bends, difference in elevation, diameter and junction structures have great influence on the pressure drop. Moreover, the suitable junction structure and two-stage enlargement is useful for decreasing the pressure drop.Thirdly, the three-dimensional multi-stage flash & VOF two-phase flow coupling model is developed to simulate the true situation in the transfer line. The results show that the pressure drop mainly occurred in the last segment of the transition section and the first segment of the junction. In addition, the flow regime shows a transition situation between annular flow and mist flow, and liquid are large droplets. The analysis and comparison of the three models developed in this thesis are also studied.Last, the structure optimization of the transition section, junction and low-speed section are studied by single flow model and a retrofitted structure is developed. The retrofitted structure is simulated by three-dimensional multi-stage flash & two-phase flow coupling model and it is found that the pressure drop of the new structure compared with the original one can be decreased by 36.8%. The decrease of pressure drop has a great influence on the vacuum distillation unit. On the one hand, the evaporation rate can be improved at a certain vacuum furnace outlet temperature. On the other hand, the outlet temperature of the vacuum furnace can be lowered at a certain transfer line outlet pressure, which can improve product quality and also has the advantage of energy conservation.