Study On Flow And Mass Transfer Characteristics Of Cryogenic Oxygen-nitrogen On The Structured Packing Surface

Cryogenic air separation is the industrial gas production technology with the largest processing capacity and the highest product purity.Structured packing is one of the core internals in the cryogenic distillation column,but the development of structured packing of cryogenic distillation relies heavily on the results of hydraulic experiments and common fluids at room temperature due to the high cost and difficulty of conducting cryogenic distillation experiments.The physical properties of cryogenic fluids are significantly different from those of common fluids,lower viscosity,surface tension and contact angle leading to different liquid spreading and interphase mass transfer.It is necessary to reserve safety factor on the basis of hydraulic experiment at room temperature when applying structured packing in cryogenic distillation,introducing large redundancy and deviation.In this paper,the falling film flow and interphase mass transfer in cryogenic distillation process are studied theoretically and experimentally,investigating the effect of the main structure and surface micro-texture of the packing.The main research contents are as follows:（1）The visual images and flow parameters of liquid nitrogen（LN₂）on the surface of corrugated structured packing were obtained for the first time in the cryogenic falling film visualization experimental system,and the wetting and the resistance characteristics in the cryogenic structured packing were revealed through experiment result and numerical simulation.The flow characteristics of cryogenic fluid in the packing units were studied through CFD simulation.It is obtained that LN₂ can ensure the film flow in the packing units,and the interfacial area of LN₂ almost does not change with the equivalent contact angle.Therefore,the benefit of improving the wettability of cryogenic fluid by optimizing the surface micro-texture is very low.The dry pressure drop results of hydraulic experiment are suitable for cryogenic distillation,but their wet pressure drop are significantly different.A set of visualization experiment system for falling film flow and mass transfer of cryogenic fluid was designed and built,and the falling film flow experiment of LN₂ on the surface of single piece corrugated structured packing was carried out.LN₂ has extremely high wettability at a small liquid load,much higher than water under the same conditions.The liquid flow angle and the inlet wetting coverage of LN₂ are also higher than those of water.（2）The gas-liquid countercurrent mass transfer model in the structured packing was established,and the effect of the packing corrugation angle on the pressure drop resistance and the interphase mass transfer of cryogenic oxygen and nitrogen was clarified,which provided ideas for the optimization of packing type selection and structure improvement in cryogenic distillation.The effects of corrugation angle on flow parameters,mass transfer rate and pressure drop in cryogenic distillation were studied by CFD method.The reduction ratio of mass transfer rate of 60°packing is much lower than the pressure drop compared to the commonly used 45°packing.Although the increase of corrugation angle will sacrifice part of the mass transfer efficiency,it can greatly reduce the pressure drop loss.The mass transfer and pressure drop of packing with different corrugation angles were evaluated comprehensively using trade-off index,and it is concluded that 60°packing has the best overall performance.Therefore,it is proposed a segmented arrangement in cryogenic distillation,with 60°and 45°packing used in the middle and both ends of the packed column,which serve to reduce energy consumption and ensure product purity,respectively.Moreover,the 60°packing with higher specific surface area can also be used in the cryogenic distillation column to balance the mass transfer and pressure drop performance.In addition,the mass transfer loss of 50°packing is only8.8%compared with 45°packing,but the pressure drop can be reduced by 41.1%,which has good application potential.（3）Through visualization and mass transfer experiments,the effect of micro-texture structure on the film fluctuation enhancement and mass transfer efficiency of cryogenic fluid is revealed,the optimization direction of micro-texture of structured packing in cryogenic distillation is proposed,and the experimental correlation of cryogenic oxygen-nitrogen interphase mass transfer on the micro-texture surface is developed.The visualization experiments were carried out,and it is found that the visualization experiments revealed that LN₂ exhibited different forms of film fluctuation on the micro-textured plate and the flat plate,both of which can strengthen the interphase mass transfer.Based on the sinusoidal micro-textured test plate with wavelength of 2.8 mm and amplitude of 0.3 mm,the oxygen-nitrogen mass transfer performance of five micro-textured plates was tested and compared with the flat plate under the same conditions.When the wavelength is larger than 3.4 mm or the amplitude is lower than 0.3 mm,the mass transfer efficiency of the flat plate is better than that of the micro-textured plate.Increasing the amplitude and decreasing the wavelength can improve the mass transfer efficiency of the micro-textured plate.The gas-phase mass transfer coefficient can be improved by 47.4%and 11.5%compared with the flat plate when the amplitude is increased to 0.4 mm and the wavelength is reduced to 2.2 mm,respectively.The experimental results clarify the optimization direction of micro-texture on the surface of cryogenic distillation packing.An experimental correlation for mass transfer in cryogenic oxygen and nitrogen systems is also proposed.