Simulation Of Transfer Process Of Vapors In Molecular Distillation Based On DSMC Method

Molecular distillation, also named short-path distillation, is a kind of non-equilibrium distillation that is operated under high vacuum and far below the atmospheric boiling point of components to separate mixtures by the differences of mean free path. The application of molecular distillation technology is limited due to the insufficient theoretical research on the transfer process of vapors in molecular distillation. Since the transfer equations deduced by the continuity hypothesis don't reflect the transfer phenomena of transition flow in the vapor space accurately, it's necessary to find an appropriate method to simulate the transfer process of vapors in molecular distillation.The direct simulation Monte Carlo(DSMC) method is a mature and reliable numerical simulation method for rarefied gas non-equilibrium flow, and it has been widely used in the subject of dilute gas flow dynamics. Based on the molecular gas dynamics, DSMC method simulates the molecular movement and collisions between gas molecules described by the Boltzmann equation from the microscopic aspect.In this dissertation, the vapor flow model is established to investigate the transfer process of dilute vapors in molecular distillation by the DSMC method. The variable hard sphere(VHS) model that is more consistent with the real gas molecules in collision mechanism is employed. In addition, since most of the mixtures to be separated are diatomic or polyatomic molecules, the molecular rotational energy is included as well as molecular translational energy. And the Larsen-Borgnakke model is adopted to simulate the energy exchange between molecular rotation and translation in the inelastic collision process. Compared to the experimental data, the evaporation efficiency calculated by the DSMC simulation is much accurate. It shows that the new model established in this paper represents the real vapors transfer phenomena very well, and therefore the new model can be applied to guide the enlarge design and calculation of the molecular distillatory.In this study, the influences of operating parameters and structure parameters on the vapor flow characteristics in the space of parallel plane and coaxial cylinder are discussed in details. The distributions of vapor flow characteristics such as number density, velocity, temperature, collision frequency and mean free path are provided. In addition, the effects of operating parameters and structure parameters on the evaporation efficiency in different molecular distillation devices are also simulated and analyzed. The results indicate that the evaporation efficiency decreases when the evaporator temperature, condenser temperature or the distillation gap increases. Especially, the evaporator temperature influences the evaporation efficiency significantly. The evaporation efficiency decreases obviously as the inert gas pressure stays high. In addition, with the same operating condition in the coaxial cylinder molecular distillatory, the distribution tendency of vapor flow characteristics in the vapor spaces of convex evaporating surface is consistent with that of concave evaporating surface. However, the change speed of concave evaporating surface tends to be gentler. Coaxial cylinder molecular distillatory with the convex evaporating surface is more efficient than molecular distillatory with the concave evaporating surface.