Agglomeration and flow transitions

In agglomeration using liquid binders, powders are agitated in the presence of liquid to form structured products but the detailed flow fields during processing have not been investigated. Systematic experiments have been performed in a mechanical mixer (diameter 250mm) with ploughshare impellers to examine the effect of processing variables on the velocity fields and torque during agglomeration. Liquid to powder ratio, impeller rotation speed and initial powder volume fill were varied and agglomerate properties were measured. Coincident transitions of the mechanism of agglomeration and type of flow were induced by varying the impeller speed. At around 4Hz the mode of the agglomerate size distribution increased rapidly together with an increase in tap bulk density, a weakening of the agglomerates, and a change in the relationship between torque and time. Tracers were used to show that there was an associated change in the size enlargement mechanism from crushing and layering to coalescence. Detailed velocity information, obtained using a positron camera, showed that at around 4Hz the particle angular velocity increased rapidly while mean axial velocity and the axial dispersion coefficient were greatest. Some particles left the bed as the transition from gravity-dominated to centrifuging flow occurred. For all conditions around 90% of the agglomerates were moving at less than 30% of the impeller tip speed and it is likely that the impeller Froude number (Fr) is not the appropriate criterion to designate the transition from gravity-dominated to centrifuging flow. For instance at 4Hz (Fr=8), around 5% of the agglomerates and none of the sand had a particle Froude number (Frp) greater than 1. At 6Hz (Fr=18), 20% of the agglomerates and 5% of the sand had Frp>l. Hence the particle Froude number is a better criterion to determine the onset of centrifuging flow. Taken together, the agglomerate properties and velocity distributions were consistent with the current binary collision model for agglomeration. In order to further improve knowledge of agglomeration it is essential to conduct integrated studies of agglomerate properties and flow fields using well-defined systems together with physically-based approaches to modelling.EThOS - Electronic Theses Online ServiceGBUnited Kingdo