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.