Refining 3-Hydroxypropionitrile By Molecular Distillation And Simulation Of Its Wiped Film Flow Using CFD Technique

Application of molecular distillation can be traced back to the beginning of last century efficiently separating unusual mixture under high vacuum and liquid state. With the development of high vacuum technique, application and research on molecular distillation are carried out more and more deeply and widely. Especially up to late 20th century when people gradually become enjoying natural material, significance of molecular distillation has been revealed as an effective natural material purification method. At the same time, theoretical research on molecular distillation has also been carried out largely. However, among all kinds of existing mathematical models none has really and correctly described the molecular distilling process. Moreover, computational fluid dynamics (CFD) simulation belongs to an innovative method of studying molecular distilling process. CFD simulations with software were mainly accomplished to study flow field of evaporating film in a wiped molecular distillatory, and an experiment of applying wiped molecular distillation was also completed to observe phenomenon of the thin liquid film flow and obtain qualitative conclusions. According to these conclusions and phenomenon, simulating results were evaluated and verified. What's more in this dissertation, study on liquid thin-film was concentrated on film thickness, velocity distribution and flow state in the film. Taking advantage of CFD simulating software, Fluent, and based on three hypotheses to simplify modeling and computation process, mathematic theoretic models were built in two dimensions and pure fluid dynamics simulations were executed of evaporating liquid thin-film in the wiped molecular distillation under atmospheric temperature and pressure. RNG and Realizable k-εturbulent models were compared with simulation results in rotating reference frame to estimate their effectiveness in this study. At the same time, volume of fluid (VOF) was also built to track interface between liquid and gas as well as enhanced-wall function method to further deal with near-wall flow. By comparison, it is found that little difference exists between the above two turbulent models. And the RNG turbulent model was then applied to further study the liquid film. With respectively changing the wiper speed and feed flow, velocity distribution in the thin-film was simulated as well as turbulence distribution and wave interface between liquid and gas. Simulating results show that laminar, turbulent and wavy flow states are simultaneously subsisting in the evaporating liquid thin-film, and turbulence appears in the arched wave before wipers. With increase of wiper speed or decrease of feeding flow, the undulation becomes more enhanced as well as its turbulent intensity and dissipation. Away from wipers, the liquid film flow belongs to laminar state and is less affected by wiper rotation and feeding speed. Furthermore, an experiment on purifying 3-hydroxypoinitrile from a nearly pure medium product by wiped molecular distillation was carried out to obtain the flow phenomenon and separation degree with change of wiper speed and feed flow, that is, under different thin-film flow state. Comparison of simulating and experiment results, another conclusion can be induced that with increase of wiper speed or decrease of feeding flow, better separation degree can be attained because surface area of the evaporating liquid film is enlarged and then its mass and heat transfer process are enhanced.