In order to study the mechanisms of fines ejection at the bed surface. and the mixing of fines, two sets of experiments were carried out.
Cracking catalyst, mean size 58 um , was continuously injected into a bed containing sand (450 um). The axial and radial fines concentration in the bed and the elutriation rate were measured. If diffusional mixing process is operative, then for continuous injection of fines to the bottom of the bed, the fines concentration would be higher at the bottom and lower at the top. However, experiment showed an inverse concentration gradient of fines. An expression is developed to predict this profile by assuming that the fines migrate upwards in the bubble wake and downwards by eddy diffusion. The radial concentration profile is zero, Size analysis of the fines in the bed indicated the existence of fines stratification according to size.
Single bubble injection experiments were carried out by injecting a single bubble into a bed fluidised at Umfc. The quantity of fines ejected at the bed surface per bubble was determined. The experimental results are compared with the predictions from the bubble wake and the bubble roof ejection mechanisms. Another fines ejection or migration mechanism is suggested - the ejection of fines dispersed &thin the bubble void. This mechanism has not been previously suggested. There is reasonably good agreement between theory and experimental results.