Modeling And Validation For Fluid Flow During The Distillation Process In A Wiped-film Distillatory

Application of molecular distillation can be traced back to earlier last century. With the development of application on molecular distillation carried out more and more deeply and widely,theoretical research on molecular distillation has also been carried out largely. Wiped-film molecular evaporator is a kind of high efficient and widely used molecular distillation equipment with complexity of fluid flow and heat transfer resulting from mechanical motion of wipers. It is important to predict the separation efficiency and optimize the equipment design and operation by studying the mass and heat transfer and developing a comprehensive mathematic model for wiped-film molecular distillation process.The movement of rotation wipers forms both falling liquid film on the evaporator wall and bow wave in front of the wipers. Liquid fluid flow and heat transfer in liquid film and bow wave in a wiped film molecular distillatory was indirectly studied by using a kind of computational fluid dynamics (CFD) software, Fluent 6.2.Three assumptions were introduced in order to simplify modeling processes. In rotating coordinate system, the volume of fluid (VOF) multiphase model and the Eurian model were applied to simulate different parts of the sdudy for the limitation of each model. The RNG k-εturbulent model treating near-wall flow was properly used in this simulation and a near-wall modeling method, enhanced wall treatment, was used to consider the effect of the surface of evaporating cylinder.A set of experimental equipment were designed to testify part of the simulation results by concerning the Residence Time Distribution (RTD) curves of the evaporating cylinder flow from the tracer concentration. Other simulating results were also compared with the published data and were discussed in several aspects accordingly.It was concluded that the liquid film on the cylindrical evaporator wall distributed well for the periodic wiping which would strengthen the mass and heat transfer inside of the liquid film and reduce the difference in temperature and concentration between the evaporator wall and the surface of liquid film leading to high separation efficiency. The simulation results correspond with experiments and a theory analysis in a wiped film evaporator similar to this proposed distillatory. The analyses of simulating result revealed that liquid flow and temperature were fluctuant in front and behind wipers and reversed flow appeared nearby the wipers which proved that wipers plays an crucial role in wiped-film molecular distillation. The effects of feed flow rate and rotational velocity were evaluated too, and it was found that wiper movements enhance mass transfer and heat transfer by reducing film thickness and renewing film surface periodically. All these analyses provide scientific evidences for enhancing the separation efficiency in the distillation process and optimize the operation.