Chemical Engineering November 2021 - 24

all breakage will depend on specific
material properties and operating
conditions. Since typical processes
have multiple unit operations, the
impact of attrition on all downstream
unit operations, and upstream equipment
influenced by recycle loops,
must be considered.
FIGURE 3. Different attrition mechanisms for solid particles show different effects in analyses of particlesize
distribution
amination of the particles before and
after the handling event can lead to a
deeper understanding of the attrition
mechanism (Figure 3). Such analysis
is necessary to delineate the underlying
mechanisms, identify test methods
to quantify them, and design and
select suitable equipment and operating
conditions to mitigate them.
Fragmentation/fracture can occur
instantaneously due to the presence
of internal or external flaws
under subjected stress conditions
and loading rates. Multiple repeated
impacts (cyclic loading) can cause
defects, dislocations or failure planes
within the particle, which can subsequently
fail due to fatigue. Fragmentation
leads to smaller particles of different
sizes and shapes that may not
resemble the original particle. These
smaller fragments may have sharp
corners which are then more prone
to surface abrasion and chipping.
Fines generated by breakage of
asperities due to high compressive
stresses, low repetitive tangential
stress, and high shear stress
at lateral cracks or sharp edges
are collectively considered surface
abrasion. The overall particle-size
distribution largely remains unchanged,
except for the formation of
a new fines fraction and an increase
in particle " roundness " (reduction of
edges and asperities).
These two mechanisms mainly
occur simultaneously in a system.
The relative extent of each mechanism
depends on material properties
and process conditions. The
threshold energy, stress or velocity
required to cause fragmentation/
fracture is higher than abrasion.
As shown in Figure 4, abrasion will
occur without fragmentation at low
impact velocities, whereas fragmentation
will be the dominant mechanism
at higher velocities.
Mechanisms at bulk-solids level.
Treating the behavior of particles
as an ensemble (or bulk) is a practical
approach toward developing
a mechanistic view of the process.
These mechanisms are quantifiable
and more amenable to modeling
with continuum modeling using average
properties. The primary attrition
mechanisms at the bulk level are
summarized in Table 1.
TABLE 2. RELATIONSHIP BETWEEN BULK- AND
PARTICLE-LEVEL ATTRITION MECHANISMS
Particle-level mechanism
Bulk-level
mechanism
Bulk Sheer
Contact sliding /
friction
Dilated flow
Impact
Crushing
Trapping, pinching,
chopping
Vibrations
Viscous shearing
24
Fragmentation /
fracture
Yes
Somewhat
Somewhat
Yes
Yes
Yes
Somewhat
Somewhat
Surface abrasion /
chipping
Yes
Yes
Yes
While the bulk-solids-level mechanisms
are well understood for various
unit operations, the particle degradation
or breakage still occurs at
the particle level. Therefore, a fundamental
understanding of attrition
behavior still requires developing
the linkage between
bulk-level and particle-level
mechanisms. A general
guidance for the relationship
between bulk- and particlelevel
mechanisms is shown
in Table 2.
Somewhat
Somewhat chipping
No
Yes
Yes
Mechanisms at unit operation
(equipment) level.
Table 3 is an attempt to
summarize the contributing
mechanisms for particle
breakage for key unit operations.
The relative impact of
these mechanisms on overAttrition
testers
An excellent survey of attrition tests
can be found in the attrition guide by
BMHB [1] and in Bemrose and Bridgewater
[2]. Various standard tests have
been documented in ASTM/BS/ISO
standards (they are listed in the online
version of this article). The list of
standards is indicative of the fact that
various industries have developed
relevant tests to quantify attrition and
rank materials for their propensity for
breakage (friability).
These tests can be classified as
single-particle tests or multi-particle
tests (bulk materials). The singleparticle
tests are suitable for coarser
particles (larger than ~300 µm) and
measure the integrity of particles
through various impact and crushing
mechanisms (ASTM D4179).
These tests replicate the fragmentation
mechanism. For instance, these
tests can be useful to compare the
strength of coarse particles (such as
catalyst pellets) to understand breakage
or attrition in packed beds.
Multi-particle or bulk tests are commonly
used to compare materials or
estimate the extent of fines generation
in the process. Commonly used attrition
testers [1,2] are shown in Table 4.
These testers are typically used for
either (i) comparing different materials
and their behavior (process function)
in a given unit operation (same
mechanism, different products) or
(ii) comparing different processes
to compare attrition rates (different
mechanisms at play).
It has been demonstrated in the literature
that the relative rankings depend
on the specific tester and the
testing conditions. Therefore, careful
selection of tester and the operating
conditions that represent the process
of interest is critical for making
useful evaluations and comparisons.
Each tester replicates a set of
attrition mechanisms and relevant
process functions under controlled
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