Chemical Engineering October 2011 - 76

Solids Processing
of relevant measurement techniques
plays an important role. Matching
powder properties with the demands
of the process is essential to robust,
trouble-free operation. In contrast, a
poor match will give rise to the problems
that are routinely encountered by
solids processors: variable flowrates,
stoppages/blockages, sub-optimal finished
product quality and excessively
high manual-input requirements.
Measuring powders
Over the decades, in an effort to produce
tools that aid understanding,
powder scientists and engineers have
developed many different characterization
techniques. Bulk, shear and
dynamic properties are all now commonly
used as input to the design and
operating strategies, with universal
powder testers incorporating all three
types of measurement in a single instrument.
Important bulk properties
include density, compressibility and
permeability, while shear properties
define both the ability of a consolidated
powder to transition from a stationary
into a dynamic state, and the
ease with which it will move against
the surface of the process equipment.
Dynamic properties relate directly to
flowability, since they are measures
of a powder in motion, and quantify
cohesion as well as sensitivity to
such influencing factors as aeration,
consolidation, flowrate, moisture and
electrostatic charge.
Core dynamic parameters, such as
basic flowability energy (BFE) and
specific energy (SE), quantify how easily
a conditioned powder moves when
it is subjected to compacting motion,
and when disturbed in an unconfined
way, respectively. By characterizing
samples in a compacted, aerated or
even fluidized state, the effect of air on
the baseline BFE measurement, and
hence on flow behavior, can be readily
quantified. The effect of other variables
such as flowrate and moisture content
is equally easy to evaluate.
Conditioning the sample prior to
measurement is an important preparation
step, involving closely prescribed,
gentle displacement of the
powder. Excess air is released from
an overly aerated bed, while a consolidated
sample is broken up. The goal
Figure 2. This
figure shows the
extrusion pressure
as a function of
piston displacement
for five different
powders (at
piston velocity of 10
mm/s). As the piston
pushes down on the
powder extrusion,
pressure rises to
an imposed limit at
which point extrusion
is stopped
100
20
40
60
80
10
20
30
40
50
Piston displacement, mm
Figure 3. This
figure shows the
influence of piston
type and extrusion
speed on the extrusion
performance of
limestone powder.
Faster extrusion
speeds are more efficient,
but the type
of piston used has
little impact
10
20
30
40
50
60
70
5
10
Piston velocity, mm/s
in either case is to achieve a homogeneous,
loosely packed, slightly aerated
powder bed. This conditioning step
ensures high reproducibility, which
supports the sensitivity of dynamic
characterization. Techniques that fail
to employ such a preparation step
are likely to provide results that are
a function of the way the powder was
handled and loaded by the operator, as
much as they reflect true differences
between test samples.
The following case study highlights
the type of information that can be
generated using a universal powder
tester, focusing on the contrast between
forced and unconfined flow behavior.
As the results illustrate, powders
that flow freely when unconfined
can " lock up " and exert high resistance
to forced flow, while more cohesive materials,
which flow poorly under gravity,
extrude when forced to flow.
Case study
Investigating the extrusion behavior
of different powders. Physical
and flow properties (dynamic, bulk
68 ChemiCal engineering www.Che.Com oCtober 2011
and shear) were measured for five different
materials: limestone (CRM116),
three grades of lactose (spray dried,
coarsely milled and finely milled)
and calcium sulphite hemihydrate.
A universal powder tester was used,
employing standard methodologies
for all flow measurements [1, 2,
3]. Fitting the instrument with an
extrusion device also enabled the extrusion
behavior of the powders to be
studied (Figure 1).
During extrusion tests, a piston
forces a conditioned sample of known
volume through a 25-mm bore cylinder
and out of an 18-mm diameter orifice.
During the test, the compressing
piston moves down at constant speed
until a force limit is reached, at which
point the amount of extruded powder
is weighed to determine an extrusion
percentage: the mass of extruded material
compared to the initial mass
of powder in the cylinder. Compression
tests were carried out using
a vented piston, which allows air
to escape through the piston face, and
the results were compared to data
15
20
60
70
Spray dried
lactose
Coarsely milled
lactose
Finely milled
lactose
Calcium sulphite
hemihydrate
Limestone
Solid piston
Vented piston
Extrusion pressure, kPa
Extrusion pressure, kPa
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Chemical Engineering October 2011

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