Study of hydrodynamic behaviour in a conical fluidized bed dryer using pressure fluctuation analysis and X-ray densitometry

Fluidized bed dryers (FBDs) are used in the pharmaceutical industry to remove excess moisture from granule prior to tablet formation. Currently, the hydrodynamics associated with FBDs are not fully understood and consequently a number of product quality and control issues still exist. To improve the understanding of FBDs, the hydrodynamics of drying and the influence of important fluidized bed design parameters, such as distributor design and vessel geometry, were studied using pressure fluctuation analysis and x-ray densitometry.;Distributor design studies using dry and wet granule in a conical fluidized bed suggest that the punched plate design limits bubble coalescence when compared to the perforated plate and Dutch weave mesh designs. Furthermore, the Dutch weave results in extensive segregation, which is undesirable from a fluidization perspective. Local drying hydrodynamic measurements using x-ray densitometry found that the punched and perforated plates generate a centralized bubbling core region during drying with a defluidized bed periphery in a cylindrical FBD. This fluidized core region grows as drying proceeds until the defluidized region disappears. Under the same operating conditions, a porous plate distributor creates extensive channelling and defluidization across the entire bed cross-section during the constant rate period of drying. These poor fluidization characteristics result from the porous plate introducing the gas into the bed as a fine dispersion.;Lastly, the hydrodynamics associated with the conical vessel geometry improves the circulation and mixing patterns in FBDs. This is especially the case in the entry region of the conical bed where the high inlet gas velocity prevents defluidization around the periphery of the bed. The straight walled geometry of the cylindrical bed resulted in defluidization in this area. As a result, the hydrodynamics associated with bubbling differ between the geometries over the course of drying.;As granule moisture content is reduced from its initial to final state, the velocity required to fully fluidize the granule decreases and the bed voidage increases. The changes in these fluidization properties are attributed to the decrease in the interparticle force load created by a reduction in liquid bridging as moisture is removed. During constant velocity drying in a conical FBD, these fluidization properties result in a bubbling fluidization state, which evolves into a bubble coalescing regime as drying proceeds. This behaviour was identifiable using pressure fluctuation time-series analysis techniques. This demonstrates the viability of using pressure fluctuation analysis as a monitoring tool for fluidized bed drying processes.