Progress in the understanding of bulk solids attrition in dilute phase pneumatic conveying

To investigate attrition processes in pneumatic conveying, it is distinguished between process parameters, determining the stress conditions (SC) the particles are subjected to during conveying (process function), and material properties, being responsible for the individual reaction of different materials to these stress conditions (material function). For dilute phase conveying, the process function was determined for a pipe bend preceded by a straight pipe by employing computational fluid dynamics (CFD). The influence of the main parameters as obtained by dimensional analysis, namely, Stokes parameter, Froude number, Reynolds number and the ratio of bend radius to pipe diameter, was investigated and discussed. The results showed that the impact conditions were different from what is commonly expected. The impact angles in a pipe bend (rB/D=5; D=80 mm) were determined to lie between 5° and 35°. Consequently, the tangential impact velocity components are considerably higher than the normal ones. The experiments carried out to determine the material function revealed that the dominating attrition mechanisms differ between polypropylene (PP) particles on one hand and polymethylmethacrylates (PMMA) and polystyrenes (PS) on the other. By applying dynamic mechanical analysis (DMA), the glass transition temperature of the polymers was identified to be a key factor in the determination of the prevailing attrition mechanism. Based on these findings, a qualitative three-level model of the attrition process, involving stress mode, material-specific attrition mechanisms and basic (microscopic) attrition mechanisms, was developed.