Chemical Engineering October 2012 - 50

Solids Processing
convective process. Shear in a highspeed
mixer will have a different end
result in that it will be much more
effective in breaking up agglomerates
of fine powders and distributing
small-particle-size material with
high surface activity.
Figure 1 is a cross section view
through a bin showing the displacement
of material as it flows. The pattern
shows how the faster velocity in
the center displaces material relative
to the slower material at the walls.
This displacement pattern can be
described as shear deformation, but
the blending that occurs is caused by
convection of collections of material
from the central part of the bin that
reach the outlet sooner than material
near the wall. If material withdrawn
from the bin is circulated back to the
top, the discreet fill pattern that was
placed into the bin will be smeared
and homogenized, and some degree of
blending will be achieved after several
recirculations [1].
In a high-speed agitated blender,
shear occurs with significantly higher
intensity near the agitator but affects
a relatively small volume of material.
The high-intensity shear can break
agglomerates, plate fine particles onto
the surface of larger particles and redistribute
particles with much higher
intensity, causing better diffusion of
small particles that may adhere to
themselves or other particles. Ideally,
the convective component of blending
in this type of blender brings all of the
material in the blender into the high
shear zones.
Blend quality
In casual conversation, terms such as
" uniform blend " , " homogenous mixture " ,
and " well blended " are often
used without quantifying what they
mean. In the heavily scrutinized pharmaceutical
industry, regulations and
guidelines dictate specific and often
rigorous methodology for quantifying
the uniformity of a mixture. Food and
other consumer products must also
meet trade regulations for delivering
the stated quantity of product and
composition of ingredients. Quantifying
the structure of a blend requires
defining not only the composition (for
instance, the proportion of each component
and allowable
variation) but also the
quantity of material
that is to be measured.
The compositional requirement
of a blend
is usually well known
because it is either the
entire objective of the
process to produce a
particular mixture, such
as a pharmaceutical
tablet or box of cereal,
or it is an intermediate
mixture that has well
defined requirements. Determining
the amount of material to measure is
sometimes less clear, and may be different
depending on who is answering
the question. In pharmaceutical applications
the answer is clear in that
it is almost always based on the unit
dose of the blend that makes up the
final tablet.
In other applications the sample
FIGURE 2. Sifting segregation occurs when a mixture
containing a range of particle sizes is allowed to form
a pile. Smaller particles sift between larger particles as
each layer of material slides along the pile surface
size subjected to scrutiny is not as
clear cut. Take, for example, a box
of cereal. In one specific example a
manufacturer produced a cereal with
a cereal particle and a marshmallow
candy particle. The packaging line
filled boxes from a series of conveyors
that fed a mixture of the two components
into a gravimetric packaging
machine that dispensed the required
weight of the blend into each package.
The two components of the mixture
were of similar size and density and
the only blending that occurred was
in the handling system along several
conveyors and in a surge hopper. Each
component was metered onto the conveyor
system at the correct ratio, but
there was not a discreet blending process.
The manufacturing plants fine
tuned their systems to get an acceptable
mixture into each box.
In this case, each box was always
within weight tolerance for the stated
package quantity, but there was no
direct control of the ratio of the two
components in any box. In an effort to
improve quality, a new packaging line
was installed that handled each component
individually up to the packaging
head where each component was
weighed in a series of weigh hoppers,
and then each box was filled from
several hoppers of each component,
42 CHEMICAL ENGINEERING WWW.CHE.COM OCTOBER 2012
selected by the packaging system
controller, to control both the proportion
of each component and the total
weight in a box. This would seem an
ideal solution since it would guarantee
both accurate package weight and
composition. From the producers point
of view, they had improved quality by
insuring that every package met their
quality standard.
However, to everyone's surprise,
consumer complaints from packages
produced at this plant went up. What
the plant had gained in compositional
accuracy they had sacrificed in uniformity.
Consumers found that individual
bowls of cereal poured from the new
packages had much wider variations of
the two components than boxes filled
from the old filling line. The new filling
line had given up control of blending,
even though the blending in the
old system was very " low tech " . The
new system allowed only about one
second from the time the ingredients
were separate until they were combined
in the package. There was very
little opportunity to do any blending,
and the result was that some boxes
were well blended while others were
segregated. From the manufacturers
point of view, they had " improved quality "
and were taking responsibility for
what they could control to the highest
degree of precision, but they were not
controlling the sample size of interest
to consumers.
Random blend. The simplified case
of blending two different ingredients
that do not have significant bonding
or other interactions is often referred
to as a random blend. In a random
blend, all individual particles are free
to move relative to each other, and
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Chemical Engineering October 2012

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