Chemical Engineering September 2015 - 63

Figure 2. The flow cup test is relatively easy to
setup and perform, and the data are used to calculate
the Carr index, Equations (1), and Hausner
ration, Equation (2)
and then becomes lodged in place.
This type of structure is referred to as
a " rathole " shown in Figure 1c. The
rathole may extend from top to bottom
of the bin and may change in
diameter of opening as a function of
powder depth.
Processors prefer mass flow for
obvious reasons. Cohesive materials
will generally exhibit core flow
in plant equipment as originally designed.
The hopper wall angle and
its material of construction have a direct
impact on flow behavior. Therefore
the challenge is to manage the
problem with the equipment that
exists, which means modifying the
formulation, or redesigning the bin
equipment, if practical.
Traditional tests for flowability
As mentioned earlier, there are three
common methods for predicting
flow: flow cup, angle of repose and
the tap test.
Flow cup. The most popular testing
method is the flow cup, which is
quick and easy to use. The cup is
basically an open cylinder with a removable
disc that is inserted into the
bottom (Figure 2). A family of discs,
each with a different hole diameter in
the middle, is provided with the cup.
Once the disc is in place, the cup is
filled with powder and the operator
observes whether the material discharges
through the hole. Processors
may know from experience
what difficulties they are likely to face
depending on the hole diameter that
is needed to allow the powder to discharge
from the cup. In a practical
sense, this instrument is used as a
" go " or " no-go " indicator for powder
processing on a regular basis.
Angle of repose. This is a simple
test method that observes powder in
a pile and measures the angle of the
pile relative to horizontal. Note that
both the angle-of-repose method
and the flow-cup test work with powders
that are loosely consolidated.
They do not attempt to evaluate the
powder as it settles, which is what
happens when powder is placed in a
containment vessel of any kind. This
phenomenon, called " consolidation, "
is an important distinction to keep in
mind because it has direct impact on
how flow behavior can change.
Tap test. The tap test takes a cylinder
of powder and shakes it to determine
how much settling will occur.
The change in volume of the powder
from start to finish is a measurement
of the powder's tendency to consolidate.
The " loose fill " density, ρpoured,
of the powder at the start of the test
is calculated by dividing the cylinder
volume into the weight of the
sample. The " tap density, " ρtapped,
is calculated by dividing the reduced
volume of powder at the end of the
test into the sample weight. The two
density values are compared to one
another, giving an indicator for the
consolidation that can take place
over time when the powder settles.
Two standard calculations that are
typically used by industry to evaluate
tap test data are called Carr Index
(Carr%) and Hausner Ratio (HR), as
defined in Equations (1) and (2):
(1)
(2)
Shear cell test for flowability
Shear cells measure the inter-particle
friction of powder materials. This
type of test has direct application
to predicting flow behavior in gravity
discharge for powders stored in
vessels of any kind. Shear cells were
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63
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Chemical Engineering September 2015

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