Simulation Based Investigation On Gas- Liquid Transport Process In Structured Packed Bed

Compared with tray or random packings,structured packing has many advantages,such as high efficiency,high mass transfer efficiency and low amplification effect,so it has been widely used in gas-liquid mass transfer processes like distillation,absorption and others.The further study on the mass transfer and hydrodynamic characteristics of structured packing is beneficial for the optimal design of structured packing columns,optimization of mass transfer processes and the development of a new high efficient structured packing.The mass transfer performance of structured packing benefits from its internal uniform and regular surface structure.The structural parameters of these regular surfaces determine the mass transfer performance.However,the macroscopic mass transfer experiment or the empirical rule cannot meet the needs of understanding the influence of these structural parameters on the mass transfer performance of structured packing.The method of computational mass transfer has been proposed and could be used to simulate the influence of structure parameters on the mass transfer performance,and thus provided a new and powerful tool to accurately evaluate the mass transfer performance of structured packings.In this paper,the mass transfer process of structured packing was simulated by the method of computational mass transfer,so as to understand the mass transfer,diffusion and hydrodynamic characteristics of structured packing,in order to obtain a reliable method to accurately predict the mass transfer performance of structured packing.In this paper,the RNG k-?turbulence model was used to simulate the liquid distribution in inclined plate and a representative element unit?REU?of Mellapak 500Y structured packing.The volume of fluid?VOF?and the continuum surface force?CSF?models were used to solve the gas-liquid two-phase flow and to describe the effect of surface tension on fluid flow respectively.The simulation results of the inclined plate were compared with the experimental results to verify the reliability of the numerical method and the models.The results show that the simulation results of VOF-CSF mixing method are in good agreement with the experimental results,and the deviation is less than 15%.The simulation results of structured packing show that the smaller the contact angle is,the more uniform the liquid distribution is on the packing surface,and the viscosity has less influence on the liquid distribution.In order to study the mass transfer process of structured packing,the model for predicting gas-liquid phase mass transfer coefficient was established.The calculation model were applied to both the chemical absorption processes of CO₂ and H₂S respectively in NaOH aqueous solution and the physical absorption process of CO₂ in water in the REU of Mellepak 500Y structured packing.The interface of liquid volume fraction 0.2 was identified as the location of the effective mass transfer interface between gas and liquid phases.And then,the simulated mass transfer coefficient was compared with the empirical model,and the deviation is less than 10%.By comparing the simulation results of DNS and turbulence model,it was found that the turbulence model can be used,instead of the DNS,to simulate the mass transfer process with sufficient accuracy.From the DNS simulation results,it was found that the numerical values of turbulent diffusion coefficients in structured packing vary greatly in different directions,and exhibit obvious anisotropic characteristics.In this paper,pseudo single-phase simulation of the whole column was carried out for the random packing column and the structured packing absorber.During the simulation of structured packing absorber,mass transfer coefficient was determined by both empirical model and the calculation model established in this paper.The simulation results of concentration distribution of CO₂ were compared with the experimental results,and the deviation is less than 5%.The simulation results show that the increase of liquid load led to the increase in the efficiency of CO₂ absorption.The turbulent viscosity at the center of the tower was larger,and the closer to the tower wall,the smaller the turbulent viscosity.The gas phase turbulent diffusion coefficient increased with the increase of tower height.