Simulation Of Rayleigh Convection During Dichloromethane Absorption Mass Transfer Using Lattice Boltzmann Method

The emission of chlorinated volatile organic compounds,such as dichloromethane（DCM）,has put human health and the atmosphere at significant risk.The development of DCM waste gas purification-absorption technology aligns with the principles of the circular economy.However,previous reports on using absorption to treat DCM waste gas have neglected the interface mass transfer mechanism.It is possible that a density gradient caused by the concentration gradient of DCM near the liquid phase of the interface in the gas-liquid mass transfer process,which is opposite to the direction of gravity,may cause Rayleigh convection.Rayleigh convection significantly affects the gas-liquid mass transfer efficiency.Therefore,this paper focuses on the gas-liquid mass transfer process of DCM absorption,using a two-dimensional unsteady dual distribution lattice Boltzmann method（LBM）to simulate Rayleigh convection.The accuracy of the model is verified through Schlieren experiments and theoretical analysis.This paper aims to obtain information on the concentration and velocity distribution of the flow field of the gas-liquid mass transfer process.Based on this,the paper further studies the evolution of interface instability phenomena,the critical time for the onset of convection,the evolution of convection structure,and other critical data on interface mass transfer.Firstly,the DCM-water solution was selected as the research object and the LBM model was utilized to simulate the Rayleigh convection phenomenon caused by discrete or continuous DCM diffusion sources in this paper.The results show that the simulated single plume convection structure is consistent with the observed schlieren experiment.The process of convection structure from generation to fusion is closely related to the rapid increase in instantaneous mass transfer flux.Two mechanisms simultaneously exist between convective structures:induced fusion and mutual inhibition.Meanwhile,the simulation results of mass transfer data show that increasing the gas phase concentration can effectively shorten the critical time for the onset of convection and increase the instantaneous mass transfer flux(10^-6 kg·m^-2·s^-1～10^-5 kg·m^-2·s^-1).When the gas phase concentration increased from 50000 mg·m^-3 to 100000 mg·m^-3,the critical time for the onset of convection is shortened by 33.4 s～50.3 s for discrete diffusion sources,and the instantaneous mass transfer flux is increased by 1.99～6.66 times at different diffusion source numbers when the diffusion source number is 2.For continuous diffusion sources,the corresponding critical time for the onset of convection is shortened by 54.5 s,and the instantaneous mass transfer flux is increased by 1.60～2.20 times at different times.What’s more,when the gas phase concentration is fixed,the increase of the number of diffusion sources leads to the critical time for the onset of convection,shows a trend of first prolonging and then shortening,attributed to the combined effect of inducing fusion and mutual inhibition between the convection structures.Secondly,the random concentration perturbation LBM model is established using the interface concentration perturbation model parameters（perturbation probability P and perturbation amplitude CD）to explore the interface instability phenomenon and the critical parameters at the onset of Rayleigh convection.The results show that under the condition of gas phase concentration of 100000 mg·m^-3,5×10^-4≤P≤10^-3 and 0<C_D≤10^-9 kg·m^-3,the critical time for the onset of convection achieved by simulation（114.7 s～120.8 s）basically coincides with the theoretically predicted value（118.1 s）,and the maximum relative error does not exceed 2.92%.The model parameters play almost negligible roles in the enhancing Rayleigh convection on interface mass transfer.Finally,we have attempted to investigate the flow field characteristics before and after the onset of Rayleigh convection as well as its impact on mass transfer according to the random concentration perturbation LBM model(P=5×10^-4,C_D=10^-10 kg·m^-3).The results indicate that the evolution law of the simulated plume convection structure agreed with the observed schlieren experiment.The position of the plume convection structure accompanies multiple circulating flows,making the turbulent velocity of the flow field reach a large order of magnitude(10^-4 m·s^-1).The large order of magnitude is the key to the exchange and fusion of high and low concentration liquids between the interface and the liquid phase.Furthermore,the quantitative analysis results of the instantaneous mass transfer coefficient(10^-6 m·s^-1)and the instantaneous mass transfer enhancement factor reveal that the simulation results are generally correlated with the penetration theory before the onset of Rayleigh convection.While after the onset of convection,the simulation results are accorded well with the surface renewal theory when the characteristic scale is taken as the width(2.2×10^-3 m)of the largest plume convection structure obtained by simulation.The maximum instantaneous mass transfer enhancement factor at different gas phase concentrations can reach more than 5.8,and the enhancement effect of Rayleigh convection on gas-liquid mass transfer is significant.