Influence of the dimensions of spheroniser plate protuberances on the production of pellets by extrusion-spheronisation

This study systematically investigated the influence of the dimensions of square-patterned pyramidal protuberances on a spheronisation plate on pellet yield, size and shape distributions, surface tensile strength and surface morphology was investigated using a 45 wt% microcrystalline cellulose/ water paste. Tests were conducted using four different extrudate diameters (1.0, 1.5, 2.0 and 2.5 mm) generated by screen extrusion and seven plate geometries, including a flat plate as a control, allowing the relative size of extrudate to protuberance feature to be studied. Geometrical analyses of the protuberance shapes provide some insight into the observed differences, with protuberance angle being significant in many cases. Sharper protuberances reduced yield (promoting breakage and attrition) but tended to give narrower size distributions and more uneven pellet surface. Pellet yield for 1 mm extrudates was also subject to losses caused by fragments passing through the 1.0 mm gap between the plate and spheroniser wall. Pellet tensile strength was noticeably greater for 1.0 mm diameter extrudates, which is attributed to the greater extensional strain imparted on the paste during the extrusion step. For some geometries there is an optimal ratio of extrudate to protuberance dimensions.