Spherical Agglomeration: An Investigation of the Breakage Mechanism by Isolating the Breakage Rate Process

This thesis contributes to the field of spherical agglomeration by:
• developing a simple model system to isolate the breakage process in the spherical agglomeration and investigate the effect of shear rate and bridging liquid viscosity on the breakage rate, and
• developing a mathematical model to describe the breakage of spherical agglomeration.
Spherical agglomeration is a size enlargement process of crystals which happens in situ by adding immiscible bridging liquid. The advantages of this process include the potential to improve the flowability, solubility and compactibility of crystalline drugs. It can be used in the pharmaceutical industry for the production of tablets. As the mechanisms in the spherical agglomeration process are not fully understood yet, we proposed a three-step process mechanism for spherical agglomeration: wetting and nucleation; growth and consolidation; attrition and breakage. A few reports concerning the breakage of spherical agglomerates, but no specific study has been done.
This thesis developed a simple model system to represent the spherical agglomeration process and a method to measure the agglomerate size distribution. The model system consisted of polystyrene beads as the primary particles and kerosene and mixtures of kerosene – petroleum jelly as the bridging liquid. Meanwhile, the suitable method to measure the agglomerate size distribution in this study was wet sieving. Significantly fewer agglomerates breakage occurred during the process, and reproducible results could be achieved.
An investigation of the breakage mechanism was done by using the agglomerates produced from the model system developed. The effect of mixing was investigated in the breakage-only experimental setup, the contracting nozzle. The main finding was an increase in the shear rate led to a reduction in the size of the final agglomerate. The effect of bridging liquid viscosity on the breakage mechanism was also investigated in the contracting nozzle. It was found that the fraction of agglomerate breakage was increased as the viscosity of the bridging liquid decreased.
A further investigation of the shear rate and bridging liquid viscosity during the breakage process was conducted in an oscillating baffled reactor. It can be concluded that increasing shear rate and decreasing bridging liquid viscosity have decreased the agglomerate size. This study detected laminar and turbulent flow in the contracting nozzle, and only turbulent flow was detected inside the OBR. It was found that the agglomerates in the turbulent flow regime experienced high breakage; on the contrary, low breakage was found in the laminar flow regime.
Modelling of the breakage process in spherical agglomeration has been limited. In this study, population balances modelling was used to describe the spherical agglomeration process because it can incorporate the mechanisms such as breakage and attrition into the model. The population balance model was a good model to describe the breakage process in the spherical agglomeration for a bimodal distribution. A good approximation was found between the experimental data and the model with a bimodal distribution. However, a size distribution with trimodal distribution poorly fits the model. It could be because the model developed was only suitable for attrition and fragmentation. Breakage mode could be found through the model. The peaks in the size distribution could be the clue for the type of breakage mode in the process. Attrition and fragmentation were the types of breakage for the bimodal size distribution in this study.