A Preliminary Study On The Mechanism Of Enhancing The Cryogenic Falling Film Flow And Mass Transfer By Surface Textures

The demand for widely used industrial gas is increasing with the development of modern society.Cryogenic distillation is the most effective method to produce high purity industrial gas products on a large scale.At present,cryogenic air separation equipment is developing on a large scale,and its energy consumption problem is becoming urgent.The cryogenic distillation column is the core of the air separation unit,the optimization of its internal structured packings has become one of the main methods to save energy.The optimization of the structured packing has gradually developed into the improvements of surface local structure.As one of the main directions of surface optimization,surface textures have been explored in the field of room-temperature fluid in-depth,but lack of specific research on cryogenic fluids.However,the difference of physical properties between cryogenic fluid and room-temperature one will lead to a great difference in flow characteristics between them,which will affect their mass transfer performance.As a result,to what degree and how the commercially used surface textures enhance the cryogenic mass transfer and whether the textures’ dimensions are the best fit for the cryogenic process are still unclear.In this paper,the influences of surface textures on cryogenic distillation were studied as follows:1.The characteristics of LOX and LN2 fluid flow and mass transfer on the textured surfaces were compared based on the computational fluid dynamics method,and the mechanisms were revealed.The method to enhance the cryogenic mass transfer by surface textures was also illustrated from the perspective of local turbulence.Based on the geometric size of Mellapak 250 Y packing,2D sinusoidal,triangular,and plane models were established to compare and analyze the flow and mass transfer characteristics under different liquid Reynolds numbers.Different from the plane,the flow and mass transfer distribution of the textured surface reach the peak in the trough region and the minimum in the peak region.Under simulated conditions,the sinusoidal surface and the triangular surface can enhance mass transfer by 50% and 25%,respectively.In addition,the enhancing mechanism of the textures on the cryogenic mass transfer process is by enhancing the local disturbance of the liquid film: the textures cause the flow to change its direction,increasing the flow intensity and the fluctuations.At the same time,vortices are generated at the gas-liquid interface in the trough,thus promoting the gas-liquid mixing and strengthening the mass transfer process.2.The enhancing characteristics of sinusoidal textures of different sizes were compared based on the computational fluid dynamics method.The dimension optimization was carried out from the perspective of mass transfer force and resistance,and the method of textures enhancing cryogenic mass transfer was further explained.The surface textures provide an additional force for the interphase mass transfer process,which can be quantitatively represented by the increase of the normal velocity at the gas-liquid interface and the influence range of vortices.Meanwhile,it also leads to the increase of resistance in the flow direction,thus thickening the liquid film and increasing the mass transfer resistance.To further enhance the mass transfer,it is necessary to reduce the mass transfer resistance based on sufficient external mass transfer force.The Mellapak 250 Y texture size was used as the reference to compare the enhancement of sinusoidal texture with different amplitude heights(0.4mm,0.2mm)and wavelength(3.5mm,4.0mm)on the cryogenic LOX-LN2 mass transfer.Under the simulated conditions,the mass transfer resistance of the texture with a period of 3.5mm is reduced by 22%,the average normal velocity is 66% of that of the reference,and the enhancement is further increased by about 3% compared with the reference when the vortex range is increased by 25%.3.The visualized experimental setup of cryogenic falling film flow and mass transfer process was designed and built,and the preliminary operation was carried out.The experimental setup is to realize the local falling film flow and mass transfer process on the packing surface and can be used to verify and revise the theoretical model and provide first-hand experimental data.The experimental setup can measure the performance of various packings under different working conditions.The whole experiment process is filmed by the visualization system,and the flow details can be obtained.In this paper,the preliminary experiment was carried out and the flow images of liquid nitrogen falling film were taken,which confirmed the effectiveness of the experimental system and provided a solid foundation for further exploration of the distillation process.