Agitated filter dryers (AFDs) are commonly used in many industries, however the mechanisms driving undesired agglomeration remain poorly understood. Undesired agglomeration can have several consequences including out of specification product, additional downstream processing and increased cycle times and cost.
This thesis proposes a novel mechanism consisting of three rate processes governing agglomeration during agitated drying: the formation of loosely bound agglomerates, consolidation and coalescence, and solidification of liquid bridges.
The influence of key material and process parameters such as agitation speed, agitation time, moisture content, temperature and fill level on the extent of agglomeration in an AFD were investigated. Higher agitation speeds resulted in a dynamic equilibrium between agglomerate growth and breakage. At lower speeds, breakage dominated behaviour shifted to agglomeration dominated over time. Moisture content played a critical role, with significant agglomeration observed even at low moisture contents. Increased fill levels promoted agglomeration due to reduced mixing efficiency, though this could be offset by using higher agitation speeds.
Different agglomerate growth dynamics of salicylic acid were identified by modifying primary particle size. The resulting agglomerate growth and breakage dynamics were categorised using a regime map, where agglomerate growth is described as a function of maximum pore saturation and Stokes deformation number. This is the first regime map of its kind developed specifically to describe agglomerate growth and breakage dynamics observed in an AFD.
The scale up of agglomeration behaviour using constant tip speed was investigated. In a larger AFD, scaling with constant tip speed while maintaining the geometric similarity captured agglomeration dynamics qualitatively but not quantitatively, likely due to the different shear profile in a larger AFD. Agglomeration dynamics observed in the conical dryer varied considerably, which was expected due to the very different mixing dynamics.
Overall, this work provides a mechanistic framework for understanding undesired agglomeration in AFDs.
