Chemical Engineering April 2022 - 42

ment use?
Sensitivity.
similar
Repeatability
What
is the ability to differentiate
among
samples?
and
reproducibility
have
an impact on sensitivity,
but some
techniques are inherently
more sensitive
than others.
Relevance.
How
well does the powder-property
inforFIGURE
4. The graph shows data from a sensory analysis of the hardness
and writing performance of cosmetic formulations containing different pigments,
as discussed in Case Study 2
density and particle properties in
isolation do not adequately describe
performance - and they rarely do
- what other techniques should be
considered? Few other bulk powder
properties appear consistently on
product specifications, and design
methodologies for powder processing
equipment are extremely limited.
Hopper design is the exception, and
points to the value of shear-cell testing.
However, even those adept at
shear-cell analysis are increasingly
aware of its limitations. This raises
the question of how to best identify a
powder testing regime that can provide
the information needed to compete
effectively.
The evolution of powder testing
Commercial powder testers vary
considerably on the basis of three
key characteristics:
Ease of use/productivity. How easy
is it to generate high-quality data?
How much time is required for the
measurement? What level of training
and expertise is required for instruAvicel
PH-102
Cellets 90
Cellets 100
Cellets 200
Cellets 350
42
D50, um
115
94
163
270
424
mation generated
with a given analytical
device correlate
with the aspects of
powder behavior
or process performance
that are of
greatest interest?
An optimal analytical solution will
generate sufficiently sensitive and
relevant data with the minimal level
of manual input.
Current practice and trends in the
application of powder testing are
driven not only by the need for better,
more informative data, but also
by growing appreciation of the economic
returns that modern instrumentation
can deliver. An enhanced
ability to efficiently handle the large
amounts of data that modern testers
can generate has been instrumental
in revealing this value.
Traditional powder testing techniques
- as exemplified by angle of
repose, flow through an orifice and
tapped density methods - generate
a single descriptor of flowability.
If this is not informative with respect
to the behavior of interest, then the
tester has no value. If it is informative,
then the tester potentially has value,
though poor repeatability and sensitivity
can be a crucial limitation when
it comes to differentiating similar
TABLE 1. DATA FOR EXCIPIENTS: PARTICLE SIZE, TOPPED DENSITY AND MASS FLOWRATE
Excipients
Particle size distribution (PSD)
Span
1.85
0.44
0.27
0.34
0.22
Hausner ratio
1.21
1.09
1.09
1.11
1.10
No flow
1.76
2.06
1.89
1.83
Mass flowrate, g/s
Av
powders. Either way, there are limited
amounts of data to interrogate.
Alternative options for powder
testing include avalanche or rotatingdrum
testers, which capture images
of the interface of a powder sample
as it tumbles within a drum rotating at
different speeds. In this environment,
powders exhibit a periodic flow pattern,
initially moving with the rotating
drum before reaching a certain
height at which the surface breaks
and the powder falls, or avalanches,
and flow establishes. This cyclical
behavior is repeated over and over
as the drum rotates. Mathematical
analyses of the captured images
generate a number of parameters,
but it is important to recognize that
they are all derived from the application
of a single technique, as with traditional
methods. Furthermore, avalanche
testing offers a limited scope
to modify the test conditions to simulate
a specific process. Drum speed
is the primary test variable.
In contrast, dynamic powder testers
that measure flow energy values
quantify powder behavior under a
range of different stress and flow regimes
of direct relevance to powder
processes. Flow energy is generated
from measurements of the forces
acting on the blade of the tester as
it rotates through a powder sample,
which can be in a consolidated, conditioned,
aerated or fluidized state.
Testers with this capability also offer
shear and bulk property measurement,
combining three distinctly different
testing techniques to generate
multiple parameters that provide
multi-faceted characterization for a
given powder.
With the limitations of single-numSD
-
0.09
0.01
0.01
0.01
ber
analysis now widely recognized,
and with advances in data analysis
software making it much easier to
efficiently interrogate large datasets,
the potential of modern testers is becoming
increasingly
apparent, and easier
to exploit. Applying a
broad range of analytical
techniques to
generate an array of
parameters, which
are then refined on
the basis of their ability
to sensitively and
CHEMICAL ENGINEERING WWW.CHEMENGONLINE.COM
APRIL 2022
http://WWW.CHEMENGONLINE.COM
Chemical Engineering April 2022

Table of Contents for the Digital Edition of Chemical Engineering April 2022
