Modelling And Simulation Of Gas-solid Mass Transfer In Turbulent Fluidized Beds

The turbulent fluidized bed(TFB)reactors have been widely used in the chemical and petrochemical industries owing to its advantages such as vigorous gas-solid contact,well heat and mass transfer and high yield.Many studies have been conducted on the TFB hydrodynamics,however,there were few reports on the investigation of the gas-solid mass transfer behavior in TFBs,especially by the CFD simulation.In this work,according to the heterogeneous gas-solid flow,the meso-structure in the TFB was proposed to be divided into the quasi discrete void phase and the quasi discrete cluster phase,where six parameters of component concentrations were employed to describe mass transfer behavior,which are Cc,Cd,Csc,Csd,Cf and Csf.A structure-based mass transfer model was proposed according to the mass conservation and the CFD simulation was employed to validate the reliability of the model and to investigate the mass transfer in the turbulent regime.With the flow structure in TFBs divided into the quasi homogeneous dilute and dense phases,a structure-based mass transfer model was proposed according to the mass conservation among different phases.The equation for the average mass transfer coefficient can be obtained by summing up the mass transfer rates in each phase,which reflects the effect of the meso-structure on the mass transfer.The one-and two-dimensional transport equations of gas components were obtained according to the mass conservation,and the above parameters of the component concentrations can then be solved accordingly.The structure-based model was applied to simulate the mass transfer behavior by the commercial software Fluent and the experimental data of the methane combustion and the ozone decomposition from literature were employed to validate the model.It showed that the simulated results are in good agreements with the experimental data.The numerical simulation also demonstrates that the low component concentration tends to be closely related to the high catalyst holdup at high reaction rates.However,there is lack of direct connection between the reactant concentration and the catalyst holdup due to the flow and the diffusion.With the increase in inlet gas velocity,the ozone concentration increases because of the increase in amount of ozone entering and decrease in residence time,whereas more ozone decomposes because of the enhanced mass transfer.In addition,the mass exchange between the cluster phase and the void phase is the control step for the ozone decomposition process.Further investigations were carried out to test the effect of gas velocity and the correlation for interphase mass transfer coefficient on the simulation results with the ozone decomposition as the model reaction.The simulation results indicate that the mass transfer model with the Foka correlation,based on the cluster phase,achieves best predictions,while other correlations give large errors.The effects of the average cluster diameter and the solid holdup on the component concentration were also analyzed in this paper.