Research On Ultrafine Particle Agglomerates In Gas-solid Flow Of Fliudized Bed

Superfine particle is a kind of Geldart C particle, which is hard to form nomalfliudization, and is demonsted to form agglomerate fliudization later. Because theunique propertise and great prospects for development of superfine particle, it’sfliudization characteristic is of increasing importance. In this paper, a kind of micro-SiO2particle was used to form agglomerate fluidization on superfine particlefluidzed bed. Two aspects, macroscope phenomenon and agglomerates’characteristic, were reseached seriously.Three stages, including un-fluidized stage, unstable fluidized stage, and stablefluidized stage, were detected. The expansion ratio rose up to8times higher withthe superficial velocity increased to7.93cm/s. Bed collapse of this kind of particlecan be divided into two stages, the hindered settling stage and the reductioncompaction stage, instead of three stages like type A particles. No sharp transitionbetween these two stages and compaction stage lasts a long time. This indicates thatthere is no bubble in the bed and particles are fluidized in the so-called agglomerateparticulate fluidization (APF) regime. Settling velocity of the hindered settlingstage almost kept a constant and it was equal to superficial velocity, because therewas a high bed-voidage in this stage.In this paper, we studied total voidage of bed, bed-voidage (voidage betweenagglomerates) and agglomerate-voidage (voidage within agglomerates). It suggestedthat when fluidization velocity was low, and bed had not expanded fully, gas volumein agglomerates cannot be neglected. With the aid of pressure sensors, multi-pointmeasurement of the absolute pressure was imlemented along the bed heightdirection with the superficial gas velocity changes. It’s found that the pressure dropof the bed bottom (about1/4height) occupied half of total pressure. And then, wecan find that, bed-voidage (voidage between agglomerates) of the bottom of the bedwas0.5-0.65with large gradient changes. Meanwhile, the upper section of the bedhas a high bed-voidage of0.9with little gradient changes. By contrast of result ofmulti-gas-velocity, bed-voidage of the bottom changed more. And we can concludethat, when the gas velocity was increased, the increases of bed height were mainlybecause of the expansion of the bottom of the fluidized bed.We studied the agglomerate from three aspects, size, roundness, and voidage. The size of agglomerates was recorded by2D PIV, and its distribution probability isalmost in subject to the Gaussian distribution, with average of200μm-300μm andmaxi-size of700μm-1500μm. It turned out that the calculated value by initialparticles were less than the measured value by three orders of magnitude, and themaximum agglomerate diameter was recommend to calculate the minimumfluidization velocity. Analysis on the agglomerate shape showed a betteragglomerate circularity Gaussian distribution, an average of approximately0.5,which were lower than nano-particle agglomerates. And there was no significantassoications between superficial velocity and the circularity values. Some largeagglomerates which had a higher mechanical strength with diameter between1mm-3mm can be obseved at the bottom of the fluidized bed. It’s density is slightlyhigher,about1.318times, than the bulk density. The microscopic void structure(mesopores and macropores with diameter15nm-1300nm) of agglomerates didn’tinfluenced by fluidization significantly. Microporous struceure didn’t exist inagglomerates. The changes of density and viodage of agglomerates were the resultof changes of some larger void structure (>1300nm). By the anslysis onsedimentation of agglomerates, we found that settling velocity distribute from0.0001m/s to0.1120m/s, with a average of0.05218m/s. Particle trcking method canearn more reasonable single particle settling velocity values. By analysis the singleparticle settling velocity, we discussed different agglomerates density values withdifferent drag coefficient. It more reasonable that the drag coefficient CD=6.5-1.0,and x=ρ_a/ρ_b ranged from1.15-1.77.