Study On Rarefied Gas Flowing In Molecular Distillation And Purification Of Natural Products

Molecular distillation (MD), which also defined as short path distillation (SPD), is a special separation technology without boiling, non-equilibrium and continual distillation process at high vacuum condition. Since MD came out in 1930s, enormous developments of MD has been acquired with its lots of virtues, especially tremendous excess in separating thermal sensitive natural product. However, lack of deep studies on MD made the molecular distillatory in industry restrained. Therefore, combined with mass and heat transfer in liquid film, it is important for optimizing operation condition and enlarging equipment to study rarefied gas flowing in distillatory. In this dissertation, the space in distillatory is divided into three sections, including evaporating kinetic layer, steady flowing layer and condensing kinetic layer. In fact, rarefied gas flowing is produced by temperature difference between evaporation surface and condensation surface. The flowing process, which can be simply regarded as one dimension flowing, is studied on basis of energy conservation law and multi-composition moment equation of two parallel planes. Many properties of rarefied gas can be established by four-moment equation, including macroscopic and microscopic properties, mass and heat transfer equations. What's more, these equations are solved with Delphi program. The mathematical models can predict the profile of rarefied gas, such as numerical density, kinetic temperature and macroscopic velocity, and the profile of distillate, too. So the flowing model make it possible to apply experimental distillatory to industrial equipment. The influence of operating and structure parameters on mass transfer efficiency and separation efficiency is studied, including composition in liquid film, partial pressure of inert gas, evaporation and condensation temperature, and distance between evaporating and condensing surfaces. It can be concluded from simulation that macroscopic temperature in steady flowing layer falls with velocity growing when evaporating and condensing temperature are constant, and that mass transfer efficiency and separation efficiency are decreasing with evaporating temperature increasing. However, the mass transfer efficiency is sensitive for condensing temperature. Besides, the effect of distance of two surfaces on mass transfer efficiency and separation efficiency is quite week whether taking inert partial pressure into consideration or not. The technological conditions of concentrating effective compositions from seal oil ethyl ester by molecular distillation are studied in this dissertation. In distillation process, the influence of operating parameters on total content and yield of effective compositions is investigated, such as distillation temperature, operating pressure, feed rate and operating stage. The fractionation temperatures of each composition in seal oil ethyl ester are discussed, and economic value on this item is evaluated. To meet the crystallization separation process for isomer, molecular distillation is applied to a preliminary separation of para-acetaminophenylacetate ethyl for the first time. The effect of above experimental parameters on relative content and separation efficiency of two components has been studied. In addition, several methods of improving product yield are presented in molecular distillation process. The optimal conditions could be included via repeated experiments, and reduces the crystallization load of isomer. The study of molecular distillation provides theoretical value for its new application yield.