Attrition and breakage are particularly important rate process in the high shear granulation; however, they are still poorly understood and only limited studies have been reported in the literature. The wet granulation process possesses a high/rapid shear flow; however, the flow patterns can be complex, and there are no reliable methods to experimentally measure shear stresses in these granulators. In this study, both experimental and modelling techniques were used to investigate deformation of granular materials. Experimentally, an annular shear cell was used due to its relatively simple shear profiles. A computational model was also developed to gain information on the particle scale to help understand pellet deformation in the annular shear cell. The objectives of this work are: 1) to study the effect of material properties and operating parameters on wet granular pellet deformation in an annular shear cell; 2) to attempt to predict pellet deformation through DEM modelling and theoretical failure theory and 3) to correlate the experiments with prediction.
Granular pellets (composed of either lactose or ballotini glass beads with different viscosity of silicone oils) surrounded by background glass beads were chosen as the model materials. Young’s modulus, plastic and elastic stiffness and yield stresses of the pellets were experimentally measured from two types of compression tests and used as the input materials in the DEM simulations. Annular shear cell experiments were tested for lactose 10cSt and ballotini 10cSt pellets (composed of lactose or ballotini glass beads with 0.01 Pa.s silicone oil) and sheared under shearing conditions; the effect of shear rate, normal pressure and size of background beads were investigated. The pellet deformation was evaluated based on two measures of deformation, degree of deformation through pellet elongation (%) and deformation fraction (%). The experimental results were compared to the Stokes deformation number, 〖St〗_def and the range of critical Stokes deformation number, 〖St〗_def^* were determined.
A “DEM unit shear cell”, representing a small section of the annular shear cell, was developed using DEM simulations. A model of a DEM pellet was created by agglomerating multi-sphere particles in a cylindrical geometry, surrounded by background beads and sheared in between a stationary bottom plate and moving top plate. A linear elastic-plastic without adhesion was chosen as the contact force model. Deviatoric stress was computed to predict the deformation stress of the pellet. For analysis of von Mises failure theory, the computed von Mises stress was compared to yield stresses of the pellets at different compression speeds between 1 and 125.7 mm/s and the intersection times between the von Mises stress and yield stresses were determined to represent the predicted yielding time of the pellet. A correlation was made between the predicted yielding time and the experimental data of lactose 10cSt and beads 10cSt pellets, e.g., pellet elongation (%) and deformation fraction (%) for operating parameters; shear rate, normal pressure and size of background beads.
Experimental pellet deformation in the annular shear cell showed increasing pellet elongation (%) and deformation fraction (%) for lactose 10cSt and beads 10cSt pellets can be obtained with increasing normal pressure, higher shear rate, larger background beads and longer shearing times. The Stokes deformation number, 〖St〗_def has potential to be used as a predictor for some of the experiments results by changing the shear rate.
Results from the DEM simulations showed that the deviatoric and von Mises stresses increased with shearing time for all conditions studied. It was found that lactose 10cSt pellets had higher deviatoric stress with increasing shear rate, increasing normal pressure and decreasing size of background beads. Deviatoric stress of ballotini 10cSt pellets was increased with increasing normal pressure and decreasing size of background beads. Only small differences in the deviatoric stress of the ballotini pellets were observed for all the shear rates used. The von Mises failure theory predicted that lactose 10cSt pellets had decreasing yielding times with increasing shear rate, increasing normal pressure and decreasing size of background beads. The von Mises failure theory predicted that ballotini 10cSt pellets had decreasing yielding time with increasing normal pressure and decreasing size of background beads.
The results of pellet deformation prediction through DEM simulations and the experiments were indirectly correlated. The correlation was made between the predicted yielding time to the elongation and deformation faction measured experimentally in the annual shear cell. It would be expected that higher predicted yielding time would result in lower experimentally measured deformation. There were some conditions where this expectation was met, overall, the model developed is a semi-prediction for the experimentally observed deformation.
In conclusion, a new method for investigating the pellet deformation is presented, using a combination of experiment, modelling and theoretical tools.
