Study On The Hydrodynamics And Transport Processes During Centrifugal Short-path Distillation

Short-path distillation, as a kind of new advanced liquid-liquid separationtechnique, has been wildly used in some areas associated closely with our daily life,such as food, medication, petroleum, paper, cosmetics, and so on.Short-path distillation is a non-equilibrium distillation process under high-vacuum.Centrifugal short-path distiller is an effective one. It relies on a rotating conical disk.Consequently, driving forces are established to make the liquid on the disk to be avery thin film. This distiller type has the characteristics of thin liquid film, shortresidence time, simultaneous mass and heat transfer and high separation efficiency.However, it also has some shortages such as complex structure and high demand ofachievement and maintenance of high-vacuum. Published literatures mostly focus onmolecular distillation process so that there are relatively little basic theoreticalresearches of the short-path distillation. In this study, the hydrodynamics and transportprocesses of centrifugal short-path distillation has been investigated by combiningcomputational fluid dynamics (CFD) and experimental methods. The major work andconclusions are presented as follows:The development and difference between molecular distillation and short-pathdistillation are illustrated in terms of separation principle, distiller type and structure,application areas and development prospects etc.. Through analyzing and discussingthe existing theoretical researches, several drawbacks are pointed out and the majorwork in this paper is indicated as well.Under some reasonable hypotheses, the whole computational domain can beassumed to be a two-dimensional axis symmetric swirl model. A new CFD model,based on the volume of fluid (VOF), is built up to analyze the detailed flowcharacteristics such as the liquid film thickness, residence time, viscosity distributionand the resultant velocity distributions etc.. The liquid film thickness is comparedwith data in reported literatures. Experiments of residence time are carried out tovalidate the reliability of simulation results.To obtain information of transport processes, species transport model is also addedinto CFD simulation. A binary system with dibutyl phethalate–dibutyl sebacate(DBP-DBS), is used as an example for investigation. Under low operating pressure ranges, for the first time, transport processes in liquid phase, vapor phase andinterfacial region are taken into account simultaneously. The concentration andtemperature distributions in two phases, liquid overall mass transfer coefficient andoptimum separation parameters (operating pressure, evaporator temperature and feedflow rate) are analyzed and discussed. Under the same operating ranges andconditions, experiments are conducted. The absolute difference between simulationresults and experimental data does not exceed10%. Additionally, for comparisonpurpose, the Langmuir evaporation model is also studied and its calculation results are7~16%higher than experimental data. Comparisons between simulations andexperiments have verified that transfer mechanism introduced in this paper is muchmore accurate than the Langmuir evaporation rate theory to describe the transferprocesses.