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7.3.3 Fluidized bed process simulation
Fluid bed processors are used in the pharmaceutical industry for various
unit operations, such as mixing, drying, granulation, and coating. Solid
particles in fl uid bed processors are fl uidized, that is, suspended in air that
moves upwards through the processing chamber and counteracts the
gravitational forces acting on the particle bed. Agglomeration/coating is
achieved by spraying the binder/coating liquid on fl uidized particles. There
are different types of fl uid bed processors and, depending on nozzle
position spraying can be performed from the top, from the bottom, or into
the bed in a tangential direction (Fukumori and Ichikawa, 2006; Dixit and
Puthli, 2009). Drying can be achieved by introducing hot air into the
fl uidized bed. The main advantage of the fl uid bed processor is the ability
to perform different unit operations within the same piece of equipment,
reducing the costs, processing time, and mass losses, which would be due
to the transfer from one piece of equipment to another. However, there are
numerous parameters that can affect the product quality, including
apparatus design (direction of fl uid fl ow, distributor plate design, processing
chamber geometry, type and position of nozzle, etc.), process (fl uidizing air
fl ow rate, fl uidizing air temperature and humidity, atomizing air pressure,
liquid fl ow rate, etc.), and formulation of related parameters (binder/
coating material type and quantity, binder/coating solvent type, powder
particle density, size distribution, shape, surface roughness, etc.) (Summers
and Aulton, 2007; Dixit and Puthli, 2009). Therefore, process optimization
usually requires laborious and extensive experimental work and thorough
process understanding, which is the main obstacle for the wider use of fl uid
bed processors in the pharmaceutical industry. Application of numerical
modeling techniques, such as CFD, might improve process understanding
and reduce the experimental work.
One of the most important factors affecting the effi ciency of the fl uid
bed process is the air fl ow and its distribution within the processing
chamber. An air distributor plate controls the movement and distribution
of the air entering the chamber, and thus the movement of particles.
Therefore, the air distributor plate design is one of the most critical
equipment related parameters, and different types of air distributor plates
have been designed (Dixit and Puthli, 2009). Depypere et al. (2004) used
CFD to investigate the effects of the air distributor design and the
upstream air supply system on the airfl ow in a top-spray fl uid bed
processor. CFD simulations were verifi ed by experimental methods,
using air mass fl ow rate, pressure drop, and inner wall temperature
recordings. CFD modeling revealed that the lateral air inlet results in a
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