Chemical Engineering March 2017 - 75

difficulties can result if degradation
causes a large percentage of fines
to be produced, particularly if the filtration
equipment is not capable of
handling the fines satisfactorily. Filter
cloths and screens will rapidly block
if they have to cope with unexpectedly
high flowrates of fine powder.
The net result is that there is usually
an increase in pressure drop across
the filter, and this could be a significant
proportion of the total pressure
available in a low-pressure system.
Flow problems. In many systems
there is a need to store the conveyed
material in a hopper or silo. Flow functions
can be determined for bulk particulate
materials, from which hopper
wall angles and opening sizes can be
evaluated, to ensure that the material
flows reliably at the rate required. A
change in particle-size distribution of
a material, as a result of conveying
operations, however, can result in a
significant change in flow properties.
Thus, a hopper designed for a material
in the " as-received " condition
may be totally unsuitable for the material
after it has been conveyed. As
a result, it may be necessary to fit an
expensive flow aid to the hopper to
solve the problem.
Potential explosion problems.
Many solid materials, when they
occur in a dust cloud, can ignite and
cause an explosion. Dust clouds
are clearly quite impossible to avoid
within a pneumatic conveying system,
and so this poses a hazard with
regard to the safe operation of such
systems. Of those materials that are
potentially explosive, research has
shown that it is only the fraction of
the material with particle sizes less
than about 200 μm that poses the
risk of explosion. Degradation and
attrition caused by pneumatic conveying,
however, can result in the
generation of a considerable number
of fines, particularly if the material
is friable. Even if the material did
not present a problem with respect
to explosions in the " as-received "
condition, the situation could be very
different after the material has been
conveyed [4].
The influence of velocity. The results
of a program of tests [1] carried
out with 5-mm spherical aluminumoxide
particles impacted at 90 deg
against a steel target are presented
in Figure 8. In this plot, the experimental
data have been included to
150
100
Material: Low-density
polyethylene (LDPE)
Pipeline:
Bore: 100 mm
Material: aluminum
Surface: sandblasted
50
Conveying conditions
Solids loading ratio: 8.0
Material temperature: 50oC
20
30
40
Conveying line exit air velocity, m/s
FIGURE 12. Low-density polyethylene (LDPE) pellets can degrade by forming " streamers "
show how the relationship was derived
and to show the limits of scatter
in the results.
Figure 8 shows that there is a very
rapid transition in particle
velocity
from zero breakage to total degradation.
Below a particle velocity of
about 9 m/s, only elastic deformation
occurs and there is essentially
zero particle degradation. Above
a particle velocity of about 25 m/s,
however, the stress induced by the
impact is always sufficient to damage
every particle. It is interesting to
note that within the transition region,
the number of unbroken particles at
any given velocity is very consistent,
and that a smooth transition is obtained
from one extreme to the other
over this range of velocity.
The influence of surface material.
With erosive wear of surface
materials, it has been found that
the resilience of the surface material
can have a significant influence
on erosive wear, and that rubber
and polymers can offer better wear
resistance than metals having a very
high hardness value in certain cases.
Since the mechanisms of erosion
and degradation have many similarities,
it is quite possible that resilient
materials could offer very good resistance
to particle degradation.
The results of tests carried out on
four different target materials with
the 5-mm spherical aluminum-oxide
particles are presented in Figure 9.
In each case, the targets were 5-mm
thick and they were impacted by the
aluminum oxide particles at 90 deg.
This shows very clearly that target
material can have a very marked effect
on degradation.
Although there is little difference
in the maximum value of particle
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MARCH 2017
velocity at which no degradation occurs,
varying from 12 m/s for steel
to about 17 m/s for Plexiglas (poly
methyl methacrylate) and aluminum,
very significant differences exist in
the transition region between no
degradation and total degradation.
In the case of the steel and glass targets,
the transition is very rapid. For
the aluminum and Plexiglas, however,
the transition is very slow, and
so a high-velocity impact against
these materials would only result in
limited damage occurring.
The influence of particle impact
angle. Particle impact angle is the
same as that used in the above
erosive-wear work (Figure 4). Impact
angle has been shown to be a
major variable with regard to the erosive
wear of surface materials, and
hence is an important consideration
in terms of material selection and the
specification of components such as
pipeline bends. In relation to particle
degradation, it is equally important,
for as the impact angle reduces,
so the normal component of velocity
decreases. This will have a direct
bearing on the deceleration force on
the particles. The results of a comprehensive
program of tests carried
with the 5-mm aluminium-oxide particles
aimed at investigating the influence
of particle-impact angle are
presented in Figure 10 [1].
Figure 10 shows that there is little
change in the response to degradation
until the impact angle is below
about 50 deg. There is then a very
marked difference in performance,
with only small incremental changes
in impact angle. With a decrease in
particle-impact angle, it would appear
that there is little change in the
particle velocity at which the onset of
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50
60
Streamer generation, g/m.t.
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Chemical Engineering March 2017

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