Numerical Simulation Of Microscopic Characters In Vacuum Membrane Distillation Process

Membrane distillation(MD) is an emerging separation method which combines distillation process and membrane process. The low grade or low calorific energy sources can be used as the process driving force is one of MD's main advantages. And MD technology is the only membrane process which can separate and gain crystallized products directly. The hollow fiber vacuum membrane distillation(VMD) technology has the merits of high flux, flexible operation styles, high packing density of membranes and high energy efficiency when compared to other styles of MD(direct contact MD, air gap MD, sweep gas MD). The widely application of VMD in seawater desalination, industrial water treatment, chemical industrial, food and pharmaceutical and other industrial fields makes it significant to carry out deep research and comprehensive understanding of hollow fiber VMD process.The simulation software FLUENT was engaged in this paper to study the hollow fiber VMD process. A 3-D calculation model was established, and the user defined function(UDF) was engaged to realize the heat and mass transfer process of phase change of water and vapor on membrane surface. The indirect verification experiments were carried and the results showed that the simulation flux matches well with the experiment values, which indicate that the model built in our study can simulate the VMD process scientifically and subjectively. On this basis, a comprehensive analysis of the micro-scaled distribution of the flow parameters, mass and heat transfer parameters, and also the influence of operation conditions such as bulk velocity, temperature and vacuum pressure on these micro-scaled parameters were studied. We found that the distribution of heat and mass transfer parameters are closely related to the position of the membrane surface, the main contribution of permeate flux comes from the regions on the left and right sides of hollow fiber membrane which have higher velocity and bigger heat transfer coefficient. The thick boundary layers exist at back and forth sides are no good for improving the mass and heat transfer process of the whole fiber surface. The vapor-water mixing phenomenon was founded on the fiber surface, and the vapor generated by phase change was forced by the bulk velocity to flow to the back side of hollow fiber. The velocity can affect the distribution of temperature, evaporation rate, heat transfer coefficient on the fiber surface, and the improvement of velocity can strengthen the membrane distillation process very well. The improvement of vacuum pressure lead to an increase of membrane flux but a decrease of heat transfer coefficient on hot side fiber surface. These researches enriched the fundamental theories of MD process and provided a reference for enhancing the MD process.In addition, the influence of pore structure(porosity and tortuosity) on membrane flux of VMD process was discussed based on the simulation model, which may provide directional reference for the preparation of high efficiency membrane for MD.