Effect of granulation scale-up on the strength of granules

Abstract

Granulation is commonly used to enlarge particle size to impart desirable characteristics and functionality to the granules. In this work, the effect of operating scale of the granulator on the physical properties of granules is analysed. Three scaling up rules of constant tip speed, constant shear stress and constant Froude number have been evaluated using 1, 5 and 50 l Cyclomix high shear mixer granulators, calcium carbonate powder and Polyethylene glycol (PEG) as binder. The strength of granules produced in different granulator scales is analysed by side crushing test. The results indicate that the condition of constant tip speed for scale-up produces granules with relatively similar strength. When the condition of constant shear stress is used to scale-up the granulator, the granules produced in 5 and 50 l have relatively similar strength, but 1 l produces much weaker granules than those of 5 and 50 l. Work has also been carried out to analyse the flow field of granules in the granulator to provide a better insight into the velocity and stress profiles as a function of equipment scale. Positron Emission Particle Tracking (PEPT) analysis shows that under constant shear stress condition macroscopic flow field of 1 l and 5 l granulators are different, a feature which could affect the final structure of the agglomerates. The PEPT results are used to describe the velocity gradient in the Distinct Element Method (DEM) simulation of the agglomerate deformation under bulk shearing. DEM modelling of microscopic interactions in agglomerate behaviour within a shearing bed shows that flow conditions of 1 l granulator make the agglomerate with a higher elongation factor and lower packing fraction, indicating that the agglomerate would be weaker. This feature has also been observed experimentally.