Twin screw granulation : a simpler re-derivation of quantifying fill level

The fill level is defined as the volume occupied by the powder and granules inside the twin-screw granulator in proportion to the maximum barrel channel void 'free' volume. In literature, the fill level is one of the key factors that determine the final granule properties as it relies on several factors such as the screw speed, screw element geometry, mass flow rate and barrel length. However, quantitative prediction of the fill level in twin-screw granulation (TSG) is still a developing area, which is required to enable effective development of process design space, to yield a product with desired quality attributes for all process scale levels (small to large equipment). In this study, a simple geometrical model is presented that predicts the barrel channel fill level in TSG. This model relates the volumetric flow rate to the forward volumetric conveying rate of the screws when they advance in the axial direction. Experimentation was conducted to validate the model by analytically measuring mass hold-up, the amount of material remaining in the barrel after steady state was reached, as the fill level is proportional to mass hold-up. Furthermore, the trends in the extent of granulation with the proposed model were investigated. Good agreement was found between the proposed fill level model and the mass hold-up for various screw elements, therefore the model provides a more practical measure of the fill level in TSG.