Influence of measurement cell size on predicted attrition by the Distinct Element Method

Abstract

During agitated drying and mixing processes, particle beds are exposed to shear deformation. This leads to particle attrition, the extent of which is dependent on the prevailing stresses and strains in the bed. The distributions of shear stresses and strain rates within the bed are highly non-uniform, requiring attention to localised conditions. Therefore a narrow angular sector of the bed is divided radially and vertically into a number of measurement cells, within which the stresses and strain rates are calculated throughout one rotation by the Distinct Element Method. These are then used in an empirical relationship of material breakage to predict the extent of attrition due to agitation. Here we investigate the influence of the measurement cell size on the estimated stresses and strain rates, and the subsequent effect on the predicted attrition. The measurement cell size is altered by varying the measurement sector size and the number of radial and vertical divisions within it. The median particle size is also varied to establish its influence on the predicted attrition. An increase in the average number of particles in a given cell, by varying the particle size or measurement cell dimensions, leads to a reduction in the estimated stresses and strain rates, and therefore a reduction in the predicted attrition. Comparison of the predicted attrition with the experimental breakage in the agitated vessel shows that the prediction method is accurate when the cell dimensions are comparable to the width of a naturally occurring shear band.