Effect Of The Plate Structure And The Surface Tension On The Falling Film Mass Transfer Process

Falling film flow is widely used in various process industry areas due to its unique advantages, such as simple structure, low power consumption and high heat and mass transfer coefficients under low flow rates and small temperature difference. The purpose of our research is to explore the separation effect of mass transfer process in the falling film evaporation. By improving the structure of falling film plates and the surface tension of the liquid, the mass transfer process of gas-liquid two-phase system is enhanced. Furthermore, the application prospect is introduced.In this paper, a new set of apparatus is designed and manufactured to investigate the different flow patterns of falling film. In order to observe the flow patterns of falling film, the liquid is heated by electric panels while falling down the flat plate and baffled plate. The flow pattern of heated film on flat plate reduces while the width of the film shrinks obviously. On the other hand, the fins of the baffled plate can redistribute the falling film, which enhances the turbulence and intensify the renewal of free surface of the liquid film. Thus, the flow pattern of heated film reduces little and the width of the film does not shrink much. This kind of construction sets a good example for the mass transfer enhancement of the falling film evaporation.In order to investigate the mass transfer efficiency of ethanol-water solution under different flow and heating conditions, the control variable method was used in the experiment. Under flow plates with different structures, various flow rates and heating powers, the separation performances of the falling film evaporation process were compared. With the increase of the flow rates and the decrease of the heating power, the mass fraction of light component in top distillate becomes higher. Meanwhile, the baffled plate obtained the highest mass fraction while the flat plate obtained the lowest of the three kinds of plates. The results proves that it is feasible to enhance the mass transfer efficiency of the falling film evaporation process by improving the structure of the falling film plate, optimizing the flow pattern and increasing the degree of turbulence.In order to investigate the influence of surface tension on the effect of falling film evaporation, a certain amount of surfactant was added to the ethanol-water solution. The surface tension of this solution under heating condition was measured. By contrasting mass fraction of light component in top distillate with and without surfactant, a better separation performance was obtained through the ethanol-water solution with surfactant. Since the surface tension of the solution can be reduced by surfactants, the surface wave is restrained to some extent and the effective area for mass transfer is increased. As a result, the mass transfer efficiency of the falling film evaporation process is obviously improved.At last, some measures to enhance falling film evaporation are given: improve the structures of the falling film plates, add suitable surfactant to reduce the surface tension of the solution, optimize the flow rate of the falling film and the heating power.