Solid flow rate prediction in silo discharge of aerated cohesive powders

Aerated discharge of six cohesive powders differing in particle size, density and flow properties is presented. These samples were characterized by means of fluidization experiments and rheology shear tests carried out in a ring shear tester. In the silo discharge experiments the measurements of the discharge rates and the mass of residual solids as a function of the aeration rate were carried out. In addition, photographic techniques highlighted the aggregative behavior of these powders. Solids aggregates, in fact, were visible within the aerated beds of solids during the fluidization experiments and in the streams of the discharging solids. Experimental data on powder flow properties and on fluidization were analyzed in order to evaluate aggregate diameters. These values compared fairly well with the aggregate diameters observed and allowed evaluation of the voidage values external to the aggregates. These results were used in a modified form of the De Jong and Hoelen equation to predict the solid discharge rate with reasonable accuracy. A modified version of this procedure has been proposed to evaluate the aggregate density and diameter starting from the vibrated density of the powder. This makes preliminary powder fluidization experiments unnecessary in the estimation of the solid discharge rates. (C) 2007 American Institute of Chemical Engineers.