Simulation Of Transfer Process Of Vapor Flows In Molecular Distillation

Molecular distillation, also called short path distillation, is a kind of nonequilibrium distillation that is operated under high vacuum condition and is used to separate mixture by the difference of mean free path below the atmospheric boiling point of components. However, the application of molecular distillation is limited due to the insufficient theoretical research on the transfer process of molecular distiller, especially the transition flow in the vapor phase which implies that the transfer equations deduced by the continuity hypothesis won’t reflect the transfer phenomena in the vapor phase accurately. Therefore, to promote the application of the technology, it is necessary to simulate the transfer process in the molecular distiller.In this study, the transport phenomena of vapor phase in molecular distiller are considered. Based on the direct simulation Monte Carlo method, one- and two- dimensional models of vapor flows in the molecular distillation are established. Since the liquid composition varies along the liquid flow direction, the two- dimensional model makes it possible for a comparison between simulated and experimental results. Since most of the mixtures to be separated are polyatomic, and to reflect the reality, molecular rotation is introduced into the models and the general Larsen-Borgnakke method is applied to evaluate the exchange of the rotational and translational energy in a collision. The simulated results show that the introduction of molecular rotation results in a higher temperature field in the vapor space than the model only with molecular translation and a reduced distillation rate, which is closer to the experiment. In the operation process, it may be difficult to determine a proper condensing temperature in advance which can make sure every molecule approaching to the condensing surface is cooled down. Therefore, a reflecting boundary condition is included in the models. The boundary condition results in that the vapor temperature just above the condensing surface is higher than the condenser which is reasonable. Simulated results are tested to the experiment:the average relative error of distillation rate is 3.3% and the maximum relative error of the molar compositing of distillation is about 5%. The tests show that the models represent well the transport phenomena that occur in the vapor space, and it can be applied to design or check in the separation process.The influences of operating and equipment parameters on the separation process are also include in the study. The simulated results lead to the following conclusion:(1) The study of the evaporation and condensation of two parallel plate shows that with the increase of distance from evaporating surface, the macroscopic variables in the vapor phase such as number density, overall temperature and collision rate decrease whereas the macroscopic velocity and mean free path are inverse; With the increase of distance between evaporator and condenser, the evaporation efficiency decreases; With the increase of evaporating temperature or condensing temperature, number density, overall temperature and collision rate in the vapor phase increase whereas the mean free path and evaporation efficiency decrease. Furthermore, the rising evaporating surface leads to a greater macroscopic velocity while the rising condensing temperature is inverse; With the increase of volatile component in the liquid phase, number density, collision rate and macroscopic velocity in the vapor phase increase while the overall temperature, mean free path and evaporation efficiency decrease. With the increase of inert gas pressure, umber density, collision rate and overall temperature in the vapor phase increase whereas the macroscopic velocity, mean free path and evaporation efficiency decrease which is disadvantage to the separation process. (2) The study of the evaporation and condensation of two cylindrical surfaces shows that the distributions of number density, collision rate and overall temperature in the vapor phase of convex evaporating surface are lower than the concave one while the evaporation efficiency, the distributions of macroscopic velocity and mean free path are inverse. Furthermore, with the increase of ratio of condensing surface to evaporating surface, the increment of evaporation efficiency decreases.