On Powder
Flowability
JENIKE & JOHANSON, INC.
James K. Prescott* and Roger A. Barnum
The term powder flowability is
used loosely and has generally
been more closely associated to
the test method used to measure
it than the significance to the
process. To the formulator,
flowability is linked to the
product. To the engineer,
flowability relates to the process.
Relating powder flowability
results to actual behavior in the
production process is the true
reason flowability is measured.
This article connects typical
powder handling processes to
flow property measurements of
value to the formulator and the
process engineer.
James K. Prescott is
senior project engineer and
Roger A. Barnum is
project engineer at Jenike &
Johanson, Inc., One
Technology Drive, Westford,
MA 01886, tel. 978.392.0300,
fax 978.392.9980, e-mail
jkprescott@jenike.com
*To whom all correspondence
should be addressed.
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Pharmaceutical Technology
OCTOBER 2000
S
everal pharmaceutical processes,
including blending, transfer, storage, feeding, compaction, and fluidization, involve powder handling.
(The term powder is used predominantly
throughout this article, but these concepts
also apply to other bulk solids — fine and
coarse — such as dust, granulations, and
granules either as single substances or as
multicomponent blends.) The flow of
powder during manufacturing dictates
the quality of the product in terms of its
weight and content uniformity. Flow also
affects manufacturing efficiency, because
it can determine whether bins can be used
or hand scooping will be required, to what
extent product (if any) is scrapped at the
beginning or end of the run, and the allowable production rate of product (e.g.,
blend times and compression speeds).
During formulation development, the
flow of a blend may affect excipient selection and may dictate whether direct
compression is used or some form of
granulation is required. A full understanding of powder-flow behavior is essential when addressing segregation problems. In extreme cases, the success or
failure of a product has hinged on its flow
behavior during manufacturing.
Given the importance of powder flow,
the pharmaceutical industry still relies
suprisingly heavily on flow properties that
are poorly understood and applied. To be
sure, science currently has little to offer
about several aspects of powder flow, and
therefore prior experience is required.
However, much proven scientific understanding of bulk powder flow has not been
used fully by the pharmaceutical industry.
This article discusses powder flowability
in the context of various pharmaceutical
production processes. By connecting ap-
propriate test methods to the typical
pharmaceutical applications in which
interparticle motion occurs and powder
flowability is of concern, manufacturers
can decide which test methods best predict the flow behavior that will occur in a
given application. (References 1–10, 34,
and 36 describe some test methods, including the strengths and weaknesses of
particular testers.)
Flowability defined
A simple definition of powder flowability
is the ability of a powder to flow. By this
definition, flowability is sometimes
thought of as a one-dimensional characteristic of a powder, whereby powders can
be ranked on a sliding scale from freeflowing to nonflowing. Unfortunately, this
simplistic view lacks the science and understanding sufficient to address common
problems encountered by the formulator
and equipment designer.
Those who work with powder, whether
in the lab or in production, quickly recognize that powder flow is complex. Flow
behavior is multidimensional and does in
fact depend on many powder characteristics. For this reason, no one test could
ever quantify flowability. To address this
multivariable problem, some suggest that
all possible test values be considered; others propose that these values be factored
into a single flowability index.
Flowability can never be expressed as a
single value or index. In fact, flowability is
not an inherent material property at all.
Flowability is the result of the combination of material physical properties that
affect material flow and the equipment
used for handling, storing, or processing
the material. Equal consideration must be
given to both the material characteristics
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Figure 1: An example of a rathole.
and the equipment. The same powder may
flow well in one hopper but poorly in
another. Likewise, a given hopper may
handle one powder well but cause another
powder to hang up. Therefore, a more accurate definition of powder flowability is
the ability of powder to flow in a desired
manner in a specific piece of equipment.
With this definition, the term free flowing,
so commonly used, becomes meaningless
unless the specific equipment handling the
material is specified.
The specific bulk characteristics and
properties of a powder that affect flow and
that can in principle be measured are
known as flow properties. Examples of flow
properties include density (compressibility), cohesive strength, and wall friction.
These flow properties refer to the behavior of the bulk material and arise from the
collective forces acting on individual particles (e.g., van der Waals, electrostatic,
surface tension, interlocking, friction).
Rumpf and Podczeck provide more information about underlying particle properties that contribute to flow behavior
(11,12).
Flow property data themselves do not
refer to specific equipment that may
handle the powder and, therefore, should
not be confused with flowability. Flow
property data refer to the powder alone. To
be clear, the terms powder flow and powder
flow properties should not be used synonymously. Powder flow is an observation and
should refer to a description of how material will flow (or did flow) in a given piece
of equipment (e.g., “the powder flow
through the press hopper was steady, without surging”). Powder flow properties
should refer to test results of the powder
(e.g.,“at a consolidating pressure of 10 psf,
the unconfined yield strength is 2 psf ”).
When discussing or reporting flowability,
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OCTOBER 2000
one must include both the powder flow
properties and the handling equipment.
Flowability is a factor for several
processes in the pharmaceutical industry.
These include
● powder transfer through comparatively
large equipment such as providing consistent flow out of a blender, bin, drum
hopper, Y-branch/chute, press hopper,
or dust collector
● powder storage, which could for example
result in caking tendencies within a vial,
drum, bin, or bag after shipping or storage time
● separation of a small quantity of powder from the bulk — specifically just before the creation of individual doses such
as during tableting, encapsulation, and
vial filling, where feed consistency to and
through the equipment governs the uniformity of weight of the dose
● blending, in which the quality of the resulting blend depends on the type of
blender used and on the flow behavior
of powder during the blend cycle
● compaction processes (e.g., roller compaction and tablet compression)
● fluidization, whether for assisting flow
or for fluidized-bed processing such as
granulation and drying
● purely comparative, physical test methods to compare two powders (or two
lots or two suppliers). This test method
may be used as a quality control (QC)
check. In this case only, the flow property measurement may be intended
solely to distinguish between powders,
not necessarily providing any specific
insight into the behavior of a particular
powder within the equipment.
For each of these applications, various
types of handling equipment can be used.
The flow behavior of the same powder
between applications may be quite different. For example, a powder that flows
well through a bin may flow poorly at the
tablet press. Currently, no universal
mathematical model exists to predict
powder flow behavior in every situation,
nor is such a universal model for powder
flow anywhere in sight. A promising approach, but still currently far into the future for those in industry, is discrete element modeling. However, this approach
still relies on physical measurements of
individual particles and those surfaces
with which they interact, and it requires
precise numerical models and massive
computational power by current standards. Therefore, in working with the
technology available to industry today,
each of the previously listed processes requires appropriate flow property tests to
characterize and predict the actual flow
behavior within the equipment.
Powder transfer
When a raw or in-process material or
product is in bulk form, it must be transferred between pieces of equipment for
storage, transportation, or processing.
These transfers are usually driven by gravity and typically involve dropping product from a blender or portable container,
bin, silo, or drum (collectively referred to
as a bin in this article) by opening a valve.
Typically, these steps involve large quantities of powder with particles in contact
with one another, and these contacts may
be changing continuously. The size of the
equipment involved, particularly the outlets and transfer chutes (if used), also is
usually large (e.g., greater than several
inches in diameter).
Several problems can develop as material flows through the equipment. If the
powder has cohesive strength, an arch or
rathole may form. An arch is a stable obstruction that forms within the hopper
section (i.e., the converging portion of the
bin) usually near the bin outlet. Such an
arch supports the rest of the bin’s contents,
preventing discharge of the remaining
powder. A rathole is a stable pipe or vertical cavity that empties above the bin outlet (see Figure 1). Material is left stranded
in stagnant zones that usually remain in
place until an external force is applied to
dislodge it. Erratic flow is the result of an
obstruction alternating between an arch
and a rathole.
Other flow problems related to the state
of aeration or density of the powder can
occur during powder discharge. The discharge of sufficiently fine powders can create flooding: When a rathole collapses, the
falling particles entrain air and become
fluidized. If the solids-handling equipment cannot handle fluids, powder will
flood through the system uncontrollably.
Even if the powder is contained, its bulk
density can undergo dramatic variations
once fluidized, negatively affecting downstream equipment. On the other hand,
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Moving
Stagnant
Funnel flow
Mass flow
Figure 2: Examples of funnel flow and mass flow patterns.
Figure 3: Model test showing flow profiles in
a hopper.
flow-rate limitations also may occur when
fine powders are handled. The expansion
of voids during flow can create upward air
pressure gradients at the outlet of discharge equipment. During discharge, this
gradient acting against gravity reduces or
limits the discharge rate.
The occurrence of these flow problems
is strongly affected by the flow pattern of a
powder during discharge from a bin. Two
flow patterns can develop: funnel flow and
mass flow (see Figure 2). In funnel flow, an
active flow channel forms above the outlet
with nonflowing powder at the periphery.
This is a first in–last out flow sequence. As
the level of powder decreases, layers of nonflowing powder may or may not slide into
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Pharmaceutical Technology
OCTOBER 2000
the flowing channel, often resulting in the
formation of a stable rathole. In addition,
funnel flow can increase the extent to which
segregation affects the discharging powder.
In mass flow, all of the powder is in motion whenever any is withdrawn. Powder
from the center as well as from the periphery moves toward the outlet. Mass
flow provides a first in–first out flow sequence, eliminates stagnant powder, provides a steady discharge with a consistent
bulk density, and yields a flow that is uniform and well controlled. Mass flow also
reduces the extent to which some types of
segregation affect the powder. Although
all of the material is moving, velocity profiles may still exist within the hopper (see
Figure 3).
Requirements for achieving mass flow
include sizing the outlet large enough to
prevent an arch from forming and ensuring that the hopper walls are steep and
smooth enough to promote flow along
them. Several flow properties are relevant
to making such predictions. These properties are based on a continuum theory of
powder behavior — namely, that powder
behavior can be described as a gross phenomenon, neglecting the interaction of
individual particles. The application of
this theory using these properties has been
proven over the past 40 years in thousands
of installations handling the full spectrum
of powders used in industry (13).
Armed with information about the flow
properties of a powder, engineers can optimize the selection of transfer equipment.
These same properties can be used as a
basis for retrofitting existing equipment
to correct flow problems. Formulators can
use these properties during product development to predict flow behavior in existing equipment. Flow properties that are
generally of most interest are described in
the following paragraphs.
Cohesive strength. Consolidation of powder may create arching and ratholing
within transfer equipment. These behaviors are related to the cohesive strength of
the powder, which is a function of the applied consolidation pressure. To show the
significance of this property, one could
imagine squeezing a material such as wet
sand or snow in one’s hand. The material
may gain sufficient strength to retain its
shape once the hand is opened. In a lab,
cohesive strength can be measured accurately by a direct shear method. ASTM
standard D6128-97 describes the most
universally accepted method (2).
By measuring the required shear force
for various vertical loads, one can develop
a relationship describing the cohesive
strength of the powder as a function of
the consolidating pressure (14). This relationship, known as a flow function, can
be analyzed to determine the minimum
outlet diameters for bins, press hoppers,
blender outlets, etc. to prevent arching and
ratholing (see Figure 4).
Other testing methods use the same
principles of consolidation and shearing
to determine the cohesive strength of a
bulk powder. Annular (ring) shear testers
produce a rotational, rather than a lateral,
displacement between cell halves containing material. Because of the unlimited
travel that can be achieved with this type
of test cell, the loading and shearing operations are more readily adapted to automation. Successful use of this test method
to demonstrate differences in cohesive
strength relating to handling characteristics has been discussed in the industry
(15–18).
Internal friction. Internal friction values
are important when characterizing the
flow properties of a powder. Such friction
is caused by solid particles flowing against
each other and is expressed as an angle of
internal friction. This angle can be measured using the cohesive strength tests described previously.
Wall friction. Used in a continuum
model, wall friction (particles sliding along
a surface) is expressed as the wall friction
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Wall friction angle, f9
Unconfined yield strength
sliding to the normal force
applied to the wall material
coupon. A plot of the meaCohesive
sured shear force as a funcmaterial
tion of the applied normal
Easy-flowing
load generates a relationship
material
known as the wall yield locus.
This flow property is a function of the powder handled
and the wall surface in contact with it. Variations in the
Major consolidating pressure
material or the wall surface
finish can have a dramatic
Figure 4: Example of flow functions.
effect on the resulting friction coefficient (19). Wall
friction can be used to deNormal force
Cover
termine the hopper angles
Bracket
required to achieve mass
Ring
flow (see Figure 6). Any
combination of w9 and uc
Bulk solid
that lie in the mass flow reShear
gion provides mass flow. The
force
grey bounding zone is an
Sample of wall material
uncertain region.
Bulk density. The bulk
Figure 5: Set up for wall-friction test.
density of a given powder is
not a single or dual value;
rather it varies as a function
uc
of the applied consolidating
408
pressure. Various methods
Funnel
can be used in industry to
flow
308
measure bulk density. These
methods incorporate vari208
Mass
ous-sized containers that are
flow
measured for volume after
108
being loosely filled with a
known mass of material and
08
then are measured again
08
108 208 308 408 508
after vibration or tapping.
Hopper angle from vertical (uc)
USP 24/NF 19 describes one
frequently
used approach
Figure 6: Example of a mass flow angle design for a conical
for
measuring
the bulk
hopper.
(loose) and tapped density
angle or the coefficient of sliding friction. (20). Although such methods can offer
As the coefficient of sliding friction in- some repeatability with respect to the concreases, the hopper or chute walls must be ditions under which measurements are
steeper for a powder to flow along them. taken, they don’t represent the actual comThe friction coefficient can be measured paction behavior a powder may undergo
by sliding a sample of powder in a test cell during bulk transfer operations.
across a stationary wall surface using a
In a more complete approach, one can
shear tester (2,14). Figure 5 shows one assess the degree to which a powder comarrangement of a test cell. In this case, a pacts as a function of the applied pressure
coupon of the wall material being evalu- (13,14). Results often are expressed as a
ated is held in place on the frame of the straight line on a log-log plot (see Figure
machine, with a cell of powder placed 7). In the literature discussing bulk solids,
above. The coefficient of sliding friction is the slope of this line typically is called comthe ratio of the shear force required for pressibility. The resulting data can be used
66
Pharmaceutical Technology
OCTOBER 2000
to accurately determine capacities for processing, storage, and transfer equipment
as well as to provide information for evaluating wall friction and feeder operation
requirements.
Permeability. The expansion of voids
during flow can create air pressure gradients within the powder bed, resulting in
flow-rate limitations when fine powders
are handled. The permeability of a powder (i.e., its ability to allow air to pass
through it) controls the discharge rate that
can be achieved. Sizing the outlet of a bin
or choosing the diameter of a transfer
chute should take into consideration the
desired powder flow rate.
Permeability is measured as a function
of bulk density (13). The method typically
used involves measuring the flow rate of
air at a specific pressure drop through a
sample of known density and height. Once
this relationship is determined (see Figure
8), it can be used to calculate critical powder discharge rates that will be achieved
for steady-flow conditions though various
orifice sizes. Higher rates may occur, but
they will do so in a nonsteady or erratic
manner, which can have undesirable side
effects such as poor weight control or
flooding. Permeability values can be used
to calculate the time required for fine powders to settle or deaerate in the equipment
and to design efficient drying or purging
systems (discussed later in this article).
Once these properties have been determined for a powder, they can be used to
analyze existing equipment to prevent or
solve handling problems (14,21). For instance, ratholing within a portable container or feed hopper could possibly be
prevented by selecting a wall surface finish that provides mass flow at the existing
hopper angle or by changing the hopper
design. An insert can be properly designed
to activate flow in a hopper that would
otherwise be too shallow to provide mass
flow. Arching can be eliminated by increasing the outlet size of a hopper to
greater than the minimum requirement
determined from the cohesive-strength
test. Problems achieving weight control
also can be eliminated by the use of mass
flow, which provides a uniform bulk density at the outlet that is nearly independent of the material level. If limiting discharge rates occur, an analysis of the
handling system can provide guidelines
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Bulk density, g/cm3
1
0.1
10
100
1000
Consolidating pressure (lb/ft2)
der as a function of the consolidating
pressure after storage at rest then is
developed (14).
This new time-dependent flow function
can be analyzed to determine the minimum outlet diameters for converging
geometries to prevent arching and ratholing. A comparison with the continuousflow test results will show what strength
the powder has gained and what changes
in equipment geometry are needed to overcome these gains. Conversely, the tests may
show that, after a given time period and
set of external influences, the material can
no longer be expected to flow under gravity alone. Such important distinctions provide valuable insight into the storage
requirements for a powder and whether
environmental controls or special handling
are needed to avoid potential problems.
Figure 7: Example of a bulk density versus consolidating pressure plot.
Separating a small quantity of
powder from the bulk
for changing capacity, discharge method,
or outlet size.
Analysis of the discharge flow pattern
can provide insight into other types of
problems. Of particular interest is segregation, which can be very complex in
terms of its cause and compounding factors. Segregation that has led to side-toside variations in material concentration
often can be minimized using mass flow,
which recombines these different regions
at the outlet (22).
Finally, if new equipment is being considered, potential problems can be prevented. Using flow property data, engineers can accordingly select handling
equipment.
In pharmaceutical processes involving
powder handling, at some point a small
quantity of powder must be separated
from a larger blend or mass of powder,
e.g., when producing tablets or filling
vials. Solid dosage forms weighing less
than a gram often are created from a batch
perhaps larger than 1000 kg. Ideally, each
of these dosage forms is identical in
weight and composition. For most systems — in particular for automated, highspeed systems — consistent powder flow
at this point in the process is essential to
provide consistent weight of individual
doses. Because of poor powder flow, a
slow-speed process that works well may
not work at all when rates are increased.
Therefore, at higher speeds, a steady feed
rate and a consistent bulk density of the
powder are required.
In many of these operations, particles
are forced into a die cavity or vial (opening). If the powder has sufficient fluidity,
gravity alone can quickly fill the opening.
A combination of these two methods also
can be used. With equipment that creates
an individual dose, the openings are usually small, typically less than 0.25 in.
Manufacturers often use mechanical assistance in the form of paddle feeders,
wipers, doctor blades, or agitator arms.
These devices force the powder into the
cavity and/or aerate the powder so it behaves more like a liquid and less like a
Powder storage
During processing, immediate transfer and
consumption of a powder are not always
possible or desired because of the need for
analysis, equipment availability, or transporting between facilities and companies.
In such cases, bulk material is stored in a
container (e.g., a package, drum, or vessel).
As in the case of flowing material, particles
are in contact with each other. However,
these contacts do not change continuously.
Prolonged contact, in conjunction with
moisture and pressure from the weight of
material above, may alter the bonds between particles as well as the particles
themselves. Moisture migration, aging, re68
Pharmaceutical Technology
OCTOBER 2000
crystallization, or reactions that absorb or
give off heat may lead to dramatic gains
in cohesive strength, agglomeration of
smaller particles into larger ones, or caking. External forces such as vibration induced during transportation or storage
methods (stacking of containers that deform) exacerbate these effects. Expansion
and contraction resulting from temperature changes also can significantly contribute to increased consolidation. Each
of these factors can create a problem by
itself, but combining the factors compounds problems further. As a result of
these conditions, an increase of cohesive
strength can make discharge from the container extremely difficult or impossible
because of the problems caused by arching or ratholing. If stronger bonds lead to
significant caking, the material may not
be usable at all.
The potential for these time-dependent effects to occur can be investigated
by measuring the gain in cohesive
strength of the powder after time at rest.
The direct-shear tester is a method in
which a vertical load can be applied and
held for a specified time (2). External influences such as variable humidity or
temperature can be applied during these
tests. After the appropriate time has
passed, a horizontal shearing force is applied and measured. The correlation between the cohesive strength of the pow-
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Permeability K0, ft/s
0.01
0.001
0.0001
0.1
1
3
Bulk density, g/cm
Figure 8: Example of a permeability versus bulk density plot.
powder. Complicating the flow behavior
— and incidentally, complicating the
analysis — is the typical presence of significant ambient vibration that is transmitted to the powder and, depending on
how and where it is applied, could assist
or hinder flow.
Another variable that must be considered is the effect of triboelectrification, or
building of a static charge, created by the
fast flow of powder. This phenomenon is
transient and can be difficult to reproduce,
identify, and quantify. Yet triboelectrification can have a pronounced effect on
the interparticle forces that affect flow.
In these processes, particle velocities are
quite high, and there is significant interparticle motion; that is, contacts between
particles are short lived. In fact, as the powder becomes aerated, particle-to-particle
contacts may not be sufficient to transmit
solids pressure, and the assumption that
the powder can be treated as a contact bed
fails. Because of this behavior, and because
of the small openings and low solids pressures involved, techniques used for hopper design cannot be used to predict flow.
Although many operations could be
considered, tableting is quite common
and illustrates the principles involved. In
tableting, the ability of the powder to flow
into the small die is key to producing consistent tablet weights. This tendency is
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OCTOBER 2000
governed by the ability of particles to separate from one another (low particle-toparticle cohesion) and the state of aeration of the powder. In turn, the state of
aeration is governed by other flow properties of the powder such as permeability
and compressibility, as well as the equipment that handles the powder (e.g., presshopper shape) and handling history (e.g.,
storage time before compression and flow
rate through the equipment). However,
even if one knows these parameters one
cannot predict flow behavior during separation of the dose.
Within the realm of tablet presses, the
design and operation of the feed frames
used to meter powder onto the die table
can vary. Between press manufacturers,
and even within a line of presses, there are
various paddle diameters, heights, speeds,
shapes, as well as various numbers of paddles and blades. This means that a powder
can have differences in flow between equipment, simply because of the variations with
the feed frame itself. For a given design,
control of the feed rate onto the die table
is critical. If the rate is too slow, the dies
are starved. If it’s too fast, then the powder can deaerate and/or densify in an inconsistent manner before fill. Both of these
situations lead to erratic tablet weights.
Again, flowability is a function of the
material being handled and the equipment
used to handle the material. As Shangraw
states, “There are two basic approaches to
increasing die feeding efficiency: a) to force
material into the die cavity; b) to improve
the flow properties of material directly
above the die cavity so that the material
will naturally flow downward. The latter
approach appears to be more realistic and
serves as the basis for most tablet machine
modifications for improvement of die fill”
(23). Note that as is often the case, there
is confusion with terminology in these
statements. Flow properties cannot be
modified without some chemical or physical change to the material (e.g., moisture
or particle size). Flowability, on the other
hand, can be improved by machine modifications, which may serve to aerate the
material to allow it to “naturally flow” and
improve die filling.
Because of the complexity of powder
flow through a feed frame, onto a table,
and into dies, no first principles have been
developed to describe or model the flow
behavior. The continuum model, which
works well for powder flowing in a bin, is
less descriptive of this process and fails to
always predict the true flow behavior.
Without a model or theory of how powder flows within a feed frame, it is impossible to apply any bench-scale test
results to predict true performance. Empirical modeling is therefore required. Because of the complexity of the problem,
nothing less than an actual full-scale test
can describe the true flow behavior. Such
tests can be inconvenient, expensive, or
impossible.
Unfortunately in most cases,“The most
efficient means of measuring the effectiveness of a glidant in a powder blend” (or
other behavior) “is to compress the blend
and determine weight variation” (21). Fullscale tests have their drawbacks, especially
when the next new material or press is presented. To paraphrase Lippens, another
problem with empirical approaches is the
possibility of a never-ending series of articles in which authors prove that their (new)
empirical equation based on their own data
predicts behavior better than an already
published empirical relation (24). This situation is a result of the nature of the multivariable problem of combining different
powders, test methods, and applications.
Instead of theoretical models or detailed, complete, proven, empirical modwww.phar maportal.com
Figure 9: Examples of (a) free-flowing and (b) weakly cohesive powder blends (courtesy of F.J.
Muzzio and T. Shinbrut, Rutgers University).
els to predict flow through a press feed
frame, the industry relies on surrogate tests
as indicators. Traditional tests are angle of
repose, Carr indices (3), Hausner ratio (the
ratio of tapped to loose bulk density)
minimum orifice diameter, and flow rate
through a funnel. Shear cell and avalanche
tests also are used, though less often. None
of these small-scale tests simulate the state
of aeration of the powder before the feed
frame nor the effects of the feed frame itself, both of which have a significant effect on the arching tendency and the maximum flow rate of the powder. Most
important, none of these tests give a physical parameter directly linked to the flow
at the press. Instead, it is hoped that flowtest result trends will correlate to tablet
weight relative standard deviation (RSD),
as may have been the case with a prior
product or application.
Arguably, tests for minimum orifice diameter and flow rate through a funnel seem
more applicable for predicting the maximum rate of compression because these
measurements most closely simulate the
need to fill a small die as fast as possible.
However, rarely have published papers cited
a strong correlation between any of these
flow properties and weight variations during the creation of a dose. Nyquist discovered a correlation by relating shear cell data
to tablet weight RSD and frequency of
tablet machine adjustments (17).
Because none of these flow property test
results can reliably predict flow behavior,
the easiest test method often is used without regard to the significance of the results. This is analogous to the anecdote of
the person searching for a lost wallet under
the streetlight because the light is better,
although the wallet was lost well away
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OCTOBER 2000
from the streetlight in the dark. Unfortunately, no one yet has shed light on how
to properly address powder flow on the
press using first principles that can be
quantified with a bench-scale tester.
Capsule filling is another example of
how flow properties can be used as a predictor. Unlike a tableting operation, a
capsule-filling operation requires the
powder to have sufficient strength to
form a plug and remain in the dosage
tube until the plug is ejected into the capsule shell. (These criteria can be determined based on wall friction and cohesive strength tests.) At the same time, the
remaining bed of powder must be sufficiently free flowing to collapse to fill in
the void that was created. Again, empirical approaches are usually taken in which
a correlation of flow behavior to some
standard flow property test is desired.
Other applications in which a unit dose
is created and powder flow is a concern
— outside of traditional tablets and capsules — include inhalers and electrostatic
deposition of active drug onto tablets.
These and other novel processes require
new tools to investigate the effect of flowability during the creation of the unit dose.
Flow of powder during blending
Blending is accomplished by a combination of three primary mechanisms: shear,
convection, and diffusion (or dispersion).
The extent to which each of these mechanisms occurs is a function of many variables. One set of variables relates to the
equipment itself such as blender type and
speed of operation. Another set of critical
variables involves the flow properties of
the powders being blended. For instance,
with the diffusion mechanism, particles
migrate (or diffuse) through a dilated or
expanded bed of powder. The ability of
the bed to dilate and the ability of particles
to migrate depend highly upon the cohesive strength of the powder. Powders with
little cohesive strength dilate more readily. Therefore, shorter blend times can be
achieved if the major component of the
blend is more free flowing. On the other
hand, if convection is considered, a moderately cohesive powder may blend faster
because chaotic patterns are introduced
(see Figure 9). Ultimately, though, freeflowing blends may segregate easily upon
subsequent handling, particularly if the
different components of the blend do not
adhere to one another. A better blend may
be achieved if the minor component is
somewhat cohesive or has a tendency to
adhere to the major component of the
blend. (This latter instance is referred to
as an ordered or adhesive blend). In addition, a better blend may be obtained and
maintained if the blend as a whole is
slightly cohesive compared with a blend
that is free flowing (25).
As with other applications, powder flow
behavior in a blender is a function of the
equipment used and the material properties — they cannot be separated. As with
flow through a press feed frame, flow during blending is complex, and currently no
first principles exist that adequately describe
blending behavior or blendability. Ideally,
one would conduct a bench scale test on
each component to be blended. These results would then describe the appropriate
blender to use, the proper order of addition
of each component, and the appropriate
speed of operation to provide a homogenous blend in the shortest time. The scientific community appears to be a long way
from developing such first principles or
even complete numerical descriptors of
blending behavior. Therefore, although a
cohesive-strength test may be a useful indicator of how a blender may perform, ultimately, blender selection is an empirical,
try-it-and-see, experienced-based approach.
Clearly, given the importance of a uniform
blend, further work is needed in this area.
Unfortunately for the formulator, the
flow properties of a blend of materials cannot be determined based on the flow properties of the individual components that
make up a blend. A blend consisting of an
excipient with 1% active ingredient does
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not have 99% of the flow behavior of the
excipient summed with 1% of the flow behavior of the active. As an example of the
complexity of the problem, consider colloidal silicon dioxide: By itself, the material is very difficult to handle in bulk form
because of its poor flow properties. However, when added in small amounts to
other powders, it is a glidant that improves
flow behavior.
Compaction processes
Dry granulation processes such as roller
compaction, pelletization, and slugging
are commonly used to improve flow properties or to produce a more uniform blend.
Flow properties used in the analysis of
bulk powder flow also are used to analyze
compaction processes.
A compaction process involves three
basic steps: the application of consolidating stress to the powder, the removal of
the stress, then ejection of the compact
(26). When stress is applied to the powder, particles initially rearrange themselves
to produce a higher density. Ultimately,
however, the particles deform and transition from elastic deformation to plastic
deformation or brittle fracture. As this
happens, bonding takes place between particles. The removal of air from the powder also occurs during the consolidation
step. As stress is applied and the bulk density is increased, the powder flows within
the compact.
The strength of the resulting compact
will depend on the maximum solids pressure (particle-to-particle) generated during
the consolidation step. This maximum
solids pressure is equal to the total pressure
applied (e.g., by the rolls or punch) minus
the interstitial air pressure that develops
within the compact. It is important to note
that the total applied pressure is easily and
commonly measured; however, the solids
pressure alone — which is virtually impossible to measure — provides bonding
strength. The distribution between the solids
pressure and the interstitial air pressure is
a function of the powder flow properties.
The last two steps of compaction — removal of stress and ejection of the compact — involve elastic recovery, air expansion and escape, flaw development, and
possibly fracture initiation such as capping
and lamination. Ideally, little, if any, interparticle motion takes place within the com74
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OCTOBER 2000
pact at this point, though flow properties
can once again be used to ensure that flow
(compact failure) does not occur.
Powder flow behavior in a roll press is
a good example of how powder flow properties can be used to predict compaction
behavior. (Miller provides a review of roll
compaction processes [27].) As stated previously, the resulting strength of the compact will depend on the generated solids
pressure (i.e., the total applied pressure less
the interstitial air pressure that develops).
The total pressure developed within the
rolls is a function of the feed (initial) pressure, the compressibility of the powder, the
geometry of the rolls (e.g., the diameter
and gap), and the nip angle. The nip angle
demarcates the region where powder is
slipping against the rotating rolls from the
region where powder is grabbed by the
rolls and compacted. The nip angle primarily is a function of the internal friction
of the powder (determined by shear-cell
measurements), the compressibility of the
powder (determined by a uniaxial compression test), and the friction between the
powder and the roll surface (determined
by wall-friction tests). These are the same
test methods used for determining flow in
a bin, except that they are conducted at
much higher pressures, representative of
the pressure in the rolls. In addition, these
known flow properties and the geometry
of the rolls can be used to calculate the
peak pressure applied to the powder (28).
Because the total applied pressure is a
strong function of the initial solids pressure at the top of the rolls, consistent feed
of partially deaerated powder into the rolls
is crucial to produce a uniform compact.
If the powder is sufficiently free flowing
and dense, gravity-driven flow into the
rolls may be possible, in which case the
design approach for the feed hopper is as
described by Jenike (14). If the powder is
poor flowing so that compaction is required to improve flow (as is often the
case), then gravity-driven feed into the
rolls may be impossible, necessitating the
use of a screw as a force feeder. This screw
forces powder into the rolls, thereby providing precompression.
The total applied pressure is the sum of
the solids pressure (particle-to-particle)
and the interstitial air pressure. Thus, if
the rolls were operated infinitely slowly,
then the interstitial air pressure that de-
veloped would dissipate, leaving the solids
pressure equal to the total applied pressure. However, at typical compression
rates, not all air is dissipated, and interstitial air pressure develops. The ability for
this air to escape is governed by the permeability of the powder. As the compression rate increases or as the permeability
decreases, the interstitial air pressure increases. Because this interstitial air pressure is subtracted from the total applied
pressure, a higher total applied force is
needed at high roll speeds or with the use
of fine powders to achieve the same solids
pressure present at slow speeds.
However, applying a higher total pressure does not mean that the effect of entrained air can be neglected. The applied
pressure must be removed. If the interstitial air pressure is higher than the strength
of the compact, then the compact could
rupture upon removal of the load (29). The
permeability of the compact can be used
to determine the ability of the entrained
air to escape after the load is removed.
Air pressures are not the sole mechanisms that result in the spontaneous failure of a compact. The elastic recovery of
a compact also will induce stresses that
can contribute to the potential failure of
the compact. This elasticity can be measured using uniaxial compression tests.
Unlike air depressurization, true elastic
recovery does not highly depend on compression rate.
Air entrainment in the powder also can
create compaction problems. As powder
is compressed, air will flow up — counter
to the downward flow of powder into the
rolls. This upward airflow acts to retard
flow of powder, limiting the feed rate. The
air also has the potential to create erratic
flow by fluidizing a portion of the powder in the rolls. This results in striped
sheets as these pockets enter the rolls.
Ultimately, the acceptable rate of compaction depends highly on the permeability of the powder. Methods that accelerate
the rate of deaeration (e.g., inducing a vacuum within the feed screw) can therefore
improve the compaction process (27).
Tablet compaction is not much different from roll compaction. In tablet compaction, as with roll compaction, the solids
pressure is equal to the applied pressure less
the interstitial air pressure that develops.
The density, and hence strength, distribuwww.phar maportal.com
tion within a tablet is a function of the
shape of the tablet, the wall friction, and
internal friction of the powder. Capping or
lamination occurs when elastic recovery
and interstitial air pressures combine to
produce stresses that cause the failure (flow)
of the tablet. A yield locus (determined by
shear tests) can be used in the analysis (in
addition to other tools such as Heckel plots
and Hiestand’s tableting indices (26,32).
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Pharmaceutical Technology
Ramachandruni and Hoag demonstrated
a correlation between wall-friction tests (as
measured in an annular shear cell) and the
loading of lubricant relative to the tabletejection force (18). Other researchers have
found similar relationships (35).
Fluidization
Fluidization involves airflow counter to the
force of gravity through a bed of powder.
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In a fluidized state, particles are readily separated from each other. Fluidized handling
is of interest for several reasons: Some granulation processes rely on a fluidized bed for
agitation and mixing to generate uniform
particles. Fluidized feed systems can be used
in applications in which high material feed
rates or very fine powders are involved. The
discharging features of such systems may
include a permeable membrane or air injection points on a hopper surface. Purging or bed-drying processes also may use
such features, although the airflow rates are
typically lower because complete fluidization may not be required and, in fact, may
need to be avoided depending on the
process requirements. Pneumatic conveying, in which an expanded bed of powder
is transported via injected air between
points, can be considered an extreme case
of fluidized handling.
The ability of air to separate particles
depends on the flow properties of the
powder. If a bed is allowed to expand, its
bulk density decreases as the airflow
through the powder increases. Permeability of the powder is described as the
ability of the air to move through a stationary bed, which in turn is a function of
the bulk density. As the bed dilates and
reaches a minimum density, particles separate and move relative to one another. At
this point, defined as the minimum fluidization velocity, pressure drop across the
bed remains relatively constant as air velocity increases. From this information,
the airflow requirements for a fluidizing
process can be determined, and the supply system can be sized accordingly. This
information also can be used when complete fluidization must be avoided.
The ability of the particles to separate
from one another is based on particle-toparticle bonding or cohesion. Cohesive
powders may not fluidize easily and instead may simply form large flow channels allowing air to channel past stagnant
zones. Geldart developed correlations describing the ease with which materials can
be fluidized (30). These relationships describe the dependency of mean particle
diameter and particle density on general
fluidization trends. However, these are
purely empirical correlations for relating
material behavior trends to two particle
properties. Cohesiveness is neglected. For
example, addition of moisture to a dry
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powder increases its cohesiveness and reduces its ability to fluidize. However, because its particle size and density remain
the same, Geldart’s chart cannot distinguish this behavior. In general, relying on
past correlations may provide some guidance, but each new product will have a
new correlation (24). Once fluidized, powder flow could in principle be modeled as
a non-Newtonian fluid (31).
The settling or deaeration of powder
may be of as much a concern as the requirements for fluidizing it. Settling times
are influenced by the same properties that
affect fluidizing potential. These properties include the permeability of the bulk
powder as well as the mean size and density of the particles. Quicker settling times
often are desired to avoid flooding and
segregation. Flooding can result when the
retained air pressure within a bed acts as
a driving force if it is discharged from
below, as in a bin or hopper. Segregation
by air entrainment can result when the air
trapped within a material (e.g., during the
filling of a bin from above) escapes upward and carries with it finer particles that
are then deposited on the top surface.
Deaeration behavior within a bin (e.g.,
peak air pressure) can be calculated given
the permeability and compressibility of
the powder as well as the in-feed rate and
bin geometry. These parameters also can
be used to determine the potential for
flooding and segregation.
Flow properties as comparative,
physical test methods
In many instances, QC checks must be
performed on powders to determine if certain attributes of the powder fall within a
predefined range. These attributes include
chemical composition, particle size, color,
moisture, and, often, flow properties. QC
checks can be made by a supplier before
shipping raw materials, by a user of incoming raw materials, or by technicians
for process control.
The applicability of the flow property
test highly depends on what the user is
trying to capture. For example, if one is
concerned that a certain batch of material may arch when transferred into a bin,
a shear test may be the most comprehensive QC test to conduct. Recognizing, however, that these QC checks may hold up
shipment or pace further processing of the
powder, faster test methods often are desired. One option is abbreviated shear-cell
testing.
Other QC checks include angle of repose, compressibility, Carr indices, the Johanson indicizer, flow funnels, minimum
orifice diameter, dynamic angle of repose,
or the Jenike & Johanson QC tester (10).
Any flow property test could in principle
be used as a QC test, and often the fastest,
most convenient test is selected. This
choice is acceptable, provided the user is
aware of the test’s limitations. Because test
methods such as angle of repose and flow
funnels do not isolate attributes of the
powder, the suitability of a test and the acceptance limits must be empirical. Applying the results of such tests relies on either extensive testing or experience and
judgment. In principle, several batches of
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Pharmaceutical Technology
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acceptable product must be made (defining the acceptable limits on quality) along
with several unacceptable batches. Determining good and bad without flow properties tests is the hard part. The QC flow
property test method must properly identify each case.
Often, different test methods will produce different results, which is no surprise
given the different physical mechanisms
involved with each test. Numerous studies have shown that for a group of materials, different test methods rank these materials differently with respect to flow (7,
33,36,37, and unpublished data from
Jenike & Johanson, Inc.). With different
rankings by each QC test, how does one
apply the results? Again, the application
and equipment must be considered, and
the test method most closely simulating
the flow behavior in the actual process
should be selected.
Example
One pharmaceutical company had been
producing tablets from a granulation. The
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Pharmaceutical Technology
OCTOBER 2000
product and process had been validated and
were in production. Increased demand for
this product then required increased production, which meant that either the press
speed would have to be increased or another
press would have to be purchased. The formulation required a specific particle size for
proper compaction (tablet formation) and
dissolution. Unfortunately, its particle-size
distribution resulted in a material which
was described by the company as poorly
flowing.
The press was operating at less than half
its capacity. Increased speed resulted in erratic tablet weights, sometimes forcing the
press to shut down. Compounding these
inefficiencies was the need to have an operator poke the material within the bin to
maintain flow to the press. Occasionally,
material within the press hopper would
arch, again forcing a press shutdown.
The system consisted of a single bin,
which held one batch. The bin comprised
a square straight-sided section, followed
by a pyramidal hopper with 458 side walls
converging to a 9-in. outlet. This bin was
Circle 59 or eINFO 59
placed above a single-sided press. Material discharged directly from the bin into
the press hopper, through a paddle feeder,
then into the dies.
The granulation within the bin discharged in a funnel flow pattern and as a
result, formed a stable flow channel or
rathole. If the operator were too aggressive in clearing the rathole, the collapsing
flow channel caused a surge of aerated material to enter the press hopper, affecting
the uniformity of flow into the press. In
this case, the flow problem resulted from
the bulk material flowing to the press as
opposed to flow within the paddle feeder.
Wall-friction tests were conducted on
the granulation against the bin wall surface. Test results confirmed that funnel
flow would be expected to occur. These
results also were used as the basis for a redesign of the bin to provide mass flow.
For this material, the maximum hopper
angle for mass flow was determined to be
288 (from vertical) for this surface. A new
bin with a 258 cone angle (from vertical),
and an identical outlet size and total
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height was designed and installed. The
existing press hopper, incidentally, was
sufficiently steep and smooth to provide
mass flow. With the new bin, mass flow
eliminated ratholing and hence the need
for the operator to poke the material. Further, the steady flow to the press hopper
allowed uniform feed onto the dies. Without modifications to the press or the formulation, the press speed was nearly dou-
bled and consistent tablet weights were
achieved. Powder flow was improved
without changing the powder flow properties. In this case, all other comparative
measures of flowability such as angle of
repose, compressibility, or flow funnel
times would not have provided any insight into the true cause of the problem
nor how to solve it.
Conclusion
Too often, the measurement of powder
flowability does not take into consideration the specific equipment used to handle
the powder. Only with an understanding
of the root causes of the problems that can
occur or are occurring can test methods be
selected to diagnose and then avoid these
situations.
References
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— Powder Metal Technologies and Applications, W.B. Eisen et al., Eds. (ASM International, Materials Park, OH, 1998), pp.
287–301.
2. American Society for Testing and Materials,
“Standard Shear Testing Method for Bulk
Solids Using the Jenike Shear Cell,” ASTM
Standard D6128-97 (1998).
3. American Society for Testing and Materials,
“Standard Test Method for Bulk Solids Characterization by Carr Indices,” ASTM Standard D6393-99 (1999).
4. G.E. Amidon,“Physical Test Methods for Powder Flow Characterization of Pharmaceutical
Materials: A Review of Methods,” Pharma.
Forum 25 (3), 8298–8305 (May–June 1999).
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