Effect of off-bottom clearance on properties of pellets produced by melt pelletization

This study examines the influence of off-bottom clearance on size and size distribution of pellets produced during melt pelletization at different postmelt impeller speeds and binder concentrations using lactose and polyethylene glycol. Melt pelletization of lactose powder 450 M in an 8-liter highshear mixer, the floor of which was made of polytetrafluoroethylene (PTFE), was carried out with polyethylene glycol 3000 as the meltable binder. Erosion of the PTFE floor of the mixer occurred with time of use and this caused a change in the off-bottom clearance. The findings showed that with a wider clearance, the size distribution of pellets was wider and pellets were much larger. These changes were the results of changes in the mixing intensity and impact frequency of the mass in relation to the eddies formed within the off-bottom clearance. The changes were not associated with the reduction in the circulating material load by entrapment into the off-bottom clearance. In the melt pelletization process, the quality of product was higher if the clearance was kept to a minimum. The off-bottom clearance was best measured at the beginning of each pelletization run because the PTFE floor of the mixer was prone to erosion.