Experimental Study On The Flow Characteristics Of Bubbles And Agglomerate In Gas-solid Fluidized Bed

Gas-solid fluidized bed is a kind of high efficiency reactor which is widely used in industrial production.It has the advantages of simple structure,high efficiency of heat and mass transfer and high efficiency of catalyst utilization.Previous studies have been carried out on dilute phase fluidized bed,but few studies have been carried out on dense phase fluidized bed because of the high particle concentration and high refractive index.Dense phase gas-solid fluidized bed is difficult to measure and describe due to its multi-scale,nonlinear and non-equilibrium characteristics.The research on dense phase gas-solid fluidized bed is mainly focused on Geldart A particles,but there is little research on Geldart B particles and admixture particles which are also widely used in industry.Experiments were carried out in a set of 300 mm large cold model gas-solid fluidized bed.Fiber optic probes were used to measure the solid holdup-time series signals of eight materials including FCC catalyst(Geldart A),sand(Geldart B),FCC catalyst mixed with silicon powder(Geldart C),and sand mixed with silicon powder.Agglomerate was identified according to the value of solid holdup,the pulsation frequency and the pulsation amplitude.The third-order statistical moment method was improved and the bubble phase threshold was calculated by means of traversing method.The Matlab program was written to realize the decoupling of solid holdup-time series signals in batches,and to calculate the flow characteristics of bubbles and agglomerate either.The flow characteristics of FCC catalysts at different axial heights of bubbles and agglomerate have just a little difference.The bubbles are constantly colliding,breaking and coalescence in the process of rising,and tend to converge to the middle of the bed.The flow characteristics of bubbles and agglomerate are relatively stable within the range of dimensionless radius less than 0.75,the flow characteristics of agglomerate are mainly changed near the wall,and the radial distribution is more uniform after the operation gas velocity increases.Compared with FCC catalyst,the flow characteristics of bubbles and agglomerate about sand are significantly different.the mean solid holdup of bubbles can reach 0.323 to 0.632.The occurrence frequency and mean rising speed of bubbles do not change along the radial position.But there is no agglomerate exists in the sand.After adding the silicon powder,the mean solid holdup of bubbles and agglomerate decrease about mixed FCC catalyst.The size of bubbles decreases,but the occurrence frequency increases,and the bubble flow characteristics are more uniform along the radial distribution.Due to the strong surface effect of silicon powder,the size and frequency of agglomerate increase slightly with the increase of silicon powder concentration.After adding the silicon powder,the flow characteristics of mixed sand particles changed significantly.The mean solid holdup,frequency of solid holdup signal pulsation and amplitude of the bubbles are closer to the emulsion phase,which could not be distinguished from the emulsion signal gradually with the increase of the concentration of silicon powder.The mean chord length of agglomerate increases significantly: the mean chord length of agglomerate near the wall at the concentration of20w% silicon powder can reach up to 21.739 mm,which will seriously affect the mass transfer,heat transfer,momentum transfer and chemical reaction efficiency of catalyst and gaseous reactant in industrial production.