Chemical Engineering April 2022 - 43

Critical measurements
S-L ratio
TABLE 2 . AVALANCHE TEST DATA FOR THREE PEARLESCENT COSMETIC PIGMENTS
Gold
0.32
Bronze
0.30
Avalanche energy, kJ/kg
Break energy, kJ/kg
Hurst index
Rest angle, deg
Surface fractal
26.8 ± 26.7
82.0 ± 17.9
0.296
45.2
4.49
relevantly differentiate materials, is
increasingly common [1, 2]. This approach
is highly effective in identifying
which powder testers will provide
a return on investment by delivering
unique and valuable information.
Case study 1
This example involves characterizing
the processability of innovative pharmaceutical
excipients. A relatively
recent innovation in the field of pharmaceutical
excipients is the introduction
of microcrystalline cellulose
(MCC) in the form of spherical pellets,
such as Cellets. Coating these
inert, neutral, starter cores produces
optimized particles for combinatory
and controlled-release tablets. A
Wurster fluid-bed coater is a popular
choice for this process (Figure 1).
In an experimental study, the properties
of Cellets of different sizes were
investigated using a range of methods.
Ref. 3 contains full experimental
details. Here, the focus is specifically
on the measurements carried
out to investigate flowability, which
included tapped-density and flowthrough-an-orifice
testing (JV1000
and Flowability Tester Model BEP2,
respectively, both Copley Scientific,
U.K.), avalanche testing (GranuDrum,
GranuTools sprl, Belgium)
and flow-energy measurement (FT4
Powder Rheometer). Data were
gathered for Avicel PH102 (MCC
powder) for comparative purposes.
The aim was to determine a) whether
the materials could be differentiated
and b) whether test conditions and
results could be linked to likely bulksolids
behavior in different regions of
the coater.
Particle-size data for the Cellets
samples are shown in Table 1,
along with the results from tappeddensity
and flow-through-an-orifice
tests. Tapped-density data are presented
in the form of Hausner Ratio
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43
White
0.25
20.0 ± 15.7
79.5 ± 8.2
0.525
45.4
2.92
35.7 ± 35.5
104.1 ± 24.0
0.803
46.1
5.14
(HR), the ratio of tapped to poured
powder density, and provide no differentiation
of the Cellets samples.
Differences are observed, however,
in the Avicel PH102, which has an
HR of 1.21, indicating a classification
of " fair " flowability, relative to the " excellent "
flow of the Cellets samples
(HR in the range 1.09 to 1.11). Flowthrough-an-orifice
data only provide
limited differentiation of the Cellets,
particularly when standard deviation
values are taken into account, and
there is no clear trend - the poorer
flowability of the Avicel PH102 is only
identified by a " no flow " result.
The avalanche tester reports two
parameters: dynamic angle of repose;
and cohesive Index
(Figure 2). Dynamic-angleof-repose
data exhibit significant
variability and, as with
the preceding techniques,
fail to differentiate the Cellets
samples, though Avicel
PH102 is again classified
as behaving differently. Cohesive
index provides some
differentiation, but not between the
Cellets 200 and 350 samples. Cohesive
index is a measure of the extent
to which individual flow-angle measurements
deviate from the average
over the course of the test. The test
is essentially a measure of variance,
with higher values associated with
more cohesive powders.
The avalanching data show varying
trends with respect to drum
rotation speed. For example, dynamic
angle of repose increases
with rotation speed for all the Cellets
samples, but is relatively consistent
for the Avicel PH102, particularly
at higher rotation speeds. With respect
to cohesive index, the Cellets
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