Computational fluid dynamics: applications in pharmaceutical technology - Computer-Aided Applications in Pharmaceutical Technology - page 248

base of the vessel revealed that there is no dead zone of mixing between
the regions above and below the paddle (at the level of the paddle), as
previously assumed. The authors also observed that the time needed for
complete mixing may largely differ, depending on the paddle rotational
speed applied (Figure 7.10). They also simulated the fl uid fl ow around a
cylindrical compact positioned at the base of the vessel. It was found that
fl uid fl ow above the planar surface of a compact undergoes solid body
rotation. Fluid fl ow next to the curved surface was more complex, with
high shear rates for a region within approximately 3 mm from the base of
the compact, associated with a higher dissolution rate in this region.
D'Arcy et al. (2005) investigated the infl uence of different locations of
the cylindrical compacts of benzoic acid within the vessel on dissolution
rate and variability in dissolution results. CFD was used to examine the
relationship between variability in dissolution rate and variation in local
hydrodynamics. Cylindrical compacts (diameter 13 mm) were fi xed to one
of three positions: central (in the centre of the vessel base); position 1 (next
to the central position); and position 2 (next to the position 1). Dissolution
was investigated from top planar surface, from curved side surface, and

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Path-lines of fl uid fl ow tracked with time for 5 seconds
from an initial plane 0.5 mm above the base of the
USP paddle dissolution vessel at 25, 50, 100, and
150 rpm (reprinted from McCarthy et al., 2004; with
permission from Springer)

Figure 7.10