Granule breakage in twin screw granulation: Effect of material properties and screw element geometry

© 2017 Elsevier B.V.This study is the first to explicitly measure the influence of material dynamic yield strength (DYS) and screw element geometry on the breakage process in twin screw granulation. Granule breakage is the key mechanism for controlling granule size within the Twin Screw Granulator. Novel experiments which isolated breakage from other granulation rate processes were performed using conveying and distributive mixing element configurations and 2 and 3 mm cylindrical pellets of model materials (DYS from 0.5 to 137 kPa). Daughter size distributions and survivor pellet shape visualization was used to infer that the breakage mechanism in conveying elements (CE) is primarily edge chipping whereas in distributive mixing elements (DME), breakage is a combination of chipping and crushing. The maximum size of granule that could remain unbroken (3.49 mm for CE and 3.18 mm for DME) was determined by the largest available gap size in the element as measured by an analysis of the screw elements' open volume geometry. Below the maximum size, breakage probability varied inversely with granule strength up to 9 kPa. For granules stronger than 9 kPa DYS, breakage characteristics are independent of formulation properties and depend only on screw element geometry. This helps explain why twin screw granulation is more robust with respect to formulation changes compared to high shear wet granulation. Implications for using the results for both optimizing screw element design and calculating kinetic parameters for population balance modeling are discussed.