Comparative study of the fluid dynamics of bottom spray fluid bed coaters

Abstract

Fluid dynamics of pellets processed in bottom spray traditional Wurster coating and swirl accelerated air (precision) coating were compared with the intent to understand and facilitate improvements in the coating processes. Fluid dynamics was described by pellet mass flow rate (MFR) obtained using a pellet collection system and images captured using high speed photography. Pellet flow within the partition column was found to be denser and slower in Wurster coating than in precision coating, suggesting a higher tendency of agglomeration during the coating process. The influence of partition gap and load on the MFR indicated that the mechanism of transport of pellets into the coating zone in precision coating depended on a strong suction, whereas in Wurster coating, pellets were transported by a combination of peripheral fluidization, gravity, and weak suction pressure. In precision coating, MFR was found to increase uniformly with air flow rate and atomizing pressure, whereas MFR in Wurster coating did not correlate as well with air flow rate and atomizing pressure. This demonstration showed that transport in precision coating was air dominated. In conclusion, fluid dynamics in precision coating was found to be air dominated and dependent on pressure differential, thus it is more responsive to changes in operational variables than Wurster coating.

Classification of Annular Bed Flow Patterns and Investigation on Their Influence on the Bottom Spray Fluid Bed Coating Process

“…Hence, the differences in coat uniformity observed within the scope of this study could not be attributed to particle recirculation within the partition column. For particle mass flow rate, AAI-20 was reported to induce the highest particle mass flow rate followed by AAI-24 and AAI-30 due to "Venturi" effect (16). According to the validated model developed by Turton et al, higher mass flow rate within the partition column reduced cycle-time, and particles underwent more coating cycles through the spray zone, thus achieving more rapid attainment of uniform coat layers (2).…”

“…2-b-iii). This flow pattern was due to gravity and "Venturi" effect (16), which drew particles into the up-bed region via the partition column inlet. Under this flow pattern, freshly coated particles moved in pulsating rhythm smoothly and orderly via the staging annular bed for another coating cycle.…”

“…Hence, the influences of confounding factors, i.e., the possible influences of particle recirculation within the partition column and mass flow rates, on coat uniformity have to be scrutinized and ruled out first before establishing the relationship between annular bed flow patterns and particle coat uniformity. The relationship between AAIs and particle mass flow rates has been studied (16). However, the influence of AAIs on particle recirculation probability is still unknown and needs to be determined experimentally.…”

A Feasibility Study on Pellet Coating Using a High-Speed Quasi-continuous Coater

“…Pellet coating is traditionally carried out using the Wurster coater, a bottom-spray fluid bed coater (1,3). However, the Wurster coater has been associated with problems of pellet agglomeration during the coating process.…”

“…However, the Wurster coater has been associated with problems of pellet agglomeration during the coating process. This was partly attributed to the dense flow of pellets within the central partition column, where the close proximity of pellets with respect to one another often results in extensive agglomeration (3,4). Unpredictable airflow patterns in the partition gap, defined as the gap between the base of the central partition column and the air distribution plate, further exacerbated the problem of agglomeration (5).…”