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Moisture content after 50 s simulation in: (a) particle
phase; and (b) gas phase (reprinted from Karlsson
et al., 2009; with permission from John Wiley & Sons)
Figure 7.15
tube, due to the increasing amount of moisture in the gas phase and the
decreasing relative velocity between the phases. Moisture evaporation
was followed by a temperature decrease in both the particle and gas
phases. The simulated moisture content and temperature of the air were
in good agreement with experimental measurements. The infl uence of
spray rate, inlet air temperature, and moisture content on drying was
investigated. It was found that higher air temperature gave rise to faster
drying, with no regions with saturated air, while higher spray rate and
higher moisture content in the inlet air resulted in larger regions of the air
saturated with water. Rajniak et al. (2009) used CFD coupled with a
population balance model to analyze gas-solid fl ow and granule growth
within a Wurster fl uid bed processor. The authors concluded that further
work is required for development of more effective algorithms for
solution of the CFD-PB models. They found that simulations with the
CFD-PB model are computationally demanding and still not practical for
fi tting to experiments, but can provide useful information that can be
used for development of simplifi ed models.
Fries et al. (2011) coupled the Discrete-Element-Method and CFD
simulations to develop a model combining gas and particle dynamics
with a simple model of particle wetting. The infl uence of the apparatus
geometry (Wurster vs. top-spray fl uid bed granulator) and process/
equipment related parameters was also analyzed. Simulation results
revealed considerable differences in particle motion and air velocity
inside the investigated granulators (Figure 7.16). In the Wurster processor,
directed high velocity motion of the particles within the draft tube was
observed, while particle motion within the top-spray granulator was
random. The average air velocity was lower in the top-spray granulator
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