The impact of channel fill level on internal forces during continuous twin screw wet granulation

The forces experienced by the particles inside a twin screw granulator (TSG) are one of the most difficult parameters to measure quantitatively. However, it is possible to perform accurately this measurement through the use of dye containing calibrated microencapsulated sensors (CAMES) whose rupture is directly dependant on their experienced shear stress. The current study measures the extent of local stresses in the transformation from powder to granules at different channel fills during TSG processing. Channel fill has shown good potential as a design tool, however, its validity for predicting particle size distributions has yet to be demonstrated in an 11-mm TSG. The results of this study showed that the particles within the twin screw granulator experienced stresses in the range of 350-1000 kPa and this value was not linear with the specific mechanical energy applied by the granulator. It was observed that the majority of these stresses were produced by material transport processes rather than the granulation in itself. In addition it was determined that the torque required by the TSG increases exponentially after a certain channel fill a feature that requires to be considered in order to design safer, predictable and reliable granulation workspaces.