Practical powder blending: Blend segregation

Practical powder blending: Blend segregation
As appeared in April 2014 PBE
Copyright CSC Publishing
www.powderbulk.com
I
n this column, we’ll examine segregation, a major cause of blending failures, as we continue the
discussion of batch blending failure
modes in the last two “Mixing Mechanics” columns (April and September 2013). Segregation (also known
as demixing) commonly occurs during processing of powder blends that
contain ingredients with even slightly
different particle sizes. While segregation can occur in various situations,
in every case it leads to the same —
and usually undesirable — outcome:
As the powder blend flows through
the process, the blend becomes less
homogeneous and displays increasingly varied composition.
In the last 20 years, researchers who
study bulk solids behavior have proposed many mechanisms to explain
why blend segregation occurs in different situations. However, all known
blend segregation mechanisms share
a common cause: In a powder blend,
free-flowing particles or particle agglomerates that have different properties — especially size and density —
will follow different trajectories as the
blend flows. The different trajectories
separate the particles as they move
downstream, causing the composition, particle size and particle size distribution, and other properties of
different portions of the blend (and
thus, different units of the end product) to fluctuate substantially. This
phenomenon, known as differential
motion, helps explain why and how
segregation takes place.
Let’s explore the most common scenarios for blend segregation: inside
the blender, during blender discharging and filling of downstream receiving equipment, and during sampling.
Segregation inside the
blender
While a blender’s purpose is to increase blend homogeneity, segregation can and does happen inside the
blender. Different batch blender types
exhibit different forms of segregation.
Fernando J. Muzzio
Practical powder blending:
Blend segregation
Most tumbling blenders, such as the
V-blender (also known as a twin-shell
blender or Patterson-Kelley blender),
double-cone blender, and bin blender,
provide repetitive rotation around one
axis. As described in the September
2013 column, the tumbling blender
provides fast convective mixing in the
powder’s motion direction (typically,
around the blender’s rotation axis) but
very slow dispersive mixing across the
powder’s motion direction (typically,
axially — along the blender’s rotation
axis). As the powder tumbles around
during blending, particles of different
sizes tend to segregate along the
blender’s rotation axis. This phenomenon produces axial bands of different
particle sizes in the rotating blender.
In contrast, a convective blender, such
as a ribbon blender or plowshare
blender, typically has a rotating impeller or blade along the blender’s
axis and imparts a more complex
flow, with convection as the dominant
blending mechanism in all directions.
As a result, this blender tends to produce less intense segregation, except
in a dead spot (a zone where the impeller fails to agitate the blend), where
larger and denser particles tend to accumulate.
In both tumbling and convective
blenders, another segregation mechanism — fluidization segregation —
can occur. As the blender operates, the
powder becomes aerated and reaches
its minimum bulk density. When the
blender is stopped, the blend settles,
releasing some of the excess air and
dragging the blend’s finer particles
along with it. This yields a vertically
stratified blend, in which the upper
layers have a higher concentration of
finer particles.
Segregation during blender
discharging and filling of
downstream receiving
equipment
Segregation occurs even more commonly when the blend is discharged
from the blender into a downstream
drum, bin, hopper, or chute.
As the powder flows from the blender
into a receiving container, it typically
forms a heap. Particles rolling down
the heap’s slope often exhibit sifting
segregation, in which smaller particles fall vertically into the interstices
between larger particles and concentrate near the heap’s center. Larger
particles can’t fall straight down to the
same extent and instead roll downhill,
accumulating around the heap’s
perimeter. When the powder heap in
the container is then emptied into the
next process unit, the blend’s composition and particle size distribution
fluctuate. We regularly observe this
phenomenon in our daily lives, as in
the container of bleu cheese crumbles
shown in Figure 1, where the small
crumbles have settled down between
the large crumbles.
Fluidization segregation can also
occur, just as in the blending operation
itself, when the discharged blend is
loaded into a chute. As the powder fills
the chute, it must displace the air in the
chute. This creates a countercurrent
flow of powder and air. The air can
drag smaller, lighter particles vertically
upward, creating significant fluctuations in the blend’s composition.
Copyright CSC Publishing
In both cases, if the powder in the receiving equipment (whether a container or chute) is further processed
without reblending it, the end product’s composition often fluctuates.
Consider a blending process for pharmaceutical tablets where the powder
blend segregates in a hopper downstream from the blender: Every time
the hopper is filled and emptied, a
“concentration wave” is created in the
blend flowing to the tableting machine, causing significant composition fluctuations that will affect the
finished tablets’ product quality or
cause tableting problems.
Segregation during sampling
Another common form of segregation
results from using a sample thief (or
thief sampler) to extract a blend sample from the blender. This device has
an inner cylinder with sample cavities, each with a hole in the cylinder
wall, and an outer cylinder, also with
holes and a pointed tip. Before the
thief is inserted into the powder, the
inner cylinder is twisted so its holes
don’t align with the outer cylinder’s
holes; when the thief reaches the
proper location in the powder bed, the
inner cylinder is twisted again so that
the inner and outer cylinder holes
align, which allows the powder to
enter the sample cavities. The inner
cylinder is twisted again to close the
holes before the thief is removed.
One problem with using this device is
that as the thief is inserted into the powder blend, particles are dragged along
with it, which segregates the powder
Figure 1
Sifting segregation in container of
bleu cheese crumbles
along the thief’s path. A bigger problem
is that opening the thief’s cavities to
capture samples can cause differential
motion in the blend, so the resulting
samples don’t truly represent the
blend’s composition at the thief location. Thief sampling tends to undersample larger particles (greater than 600
microns) because the larger the particles, the lower their concentration in
each sample. In an extreme case, samples with 10 to 30 percent fewer large
particles than the blend actually contains can be consistently extracted from
an otherwise homogeneous blend.
Minimizing blend
segregation
Take the following steps to help minimize segregation problems in your
batch blends.
Correctly diagnose the cause. Start
by correctly diagnosing the problem.
Blend segregation is driven primarily
by particle size and density differences among ingredients in the blend,
and most segregation problems occur
in blends with substantial amounts of
free-flowing ingredients. To determine how likely your blend is to segregate, run tests of blend samples with
a segregation tester. (Several models
are commercially available; they can
also be accessed in independent or
university labs.) The instrument
quantifies the intensity of sifting and
fluidization segregation mechanisms
in each sample. If the results show
that your blend tends to segregate,
you may be able to reduce or minimize this tendency while maintaining
acceptable flow properties by adjusting the particle sizes of ingredients or
the blend’s cohesiveness.
Consider your process. Next, examine your process and determine
whether you can modify it to minimize the blend’s segregation tendencies. This can involve changes such as
eliminating hoppers or modifying
their cone angles to promote mass
flow, reducing the number of blend
discharges the process requires, or
adding another blending step just
prior to the blend’s final processing.
However, while such process modifications are often effective in reducing
segregation’s impact on product quality, they don’t remove the problem’s
root cause: the intrinsic tendency of
free-flowing blends containing ingredients with different particle sizes to
segregate.
Convert from batch to continuous
processing. If modifying your batch
process can’t minimize blend segregation, you can take a more radical step:
converting your batch process to a
continuous one. A properly designed
continuous process that achieves nearplug-flow material behavior after
blending will typically display minimal, if any, segregation. While this
less common approach to minimizing
segregation has additional benefits,
such as better process controllability
and a smaller equipment footprint, it
also requires a major change to your
existing process and is only effective if
you manufacture a relatively large
quantity of product each year.
Next time I’ll focus on a very different
source of blend homogeneity problems, also driven by ingredient properties: ingredient agglomeration. PBE
Fernando J. Muzzio is director of the
National Science Foundation’s Engineering Research Center on Structured
Organic Particulate Systems (http://er
cforsops.org/) and Professor II, chemical and biochemical engineering, Rutgers University, Piscataway, N.J. He
can be reached at 732-445-3357 (fj
muzzio@yahoo.com). He earned his
BS in chemical engineering at the University of Mar del Plata, Buenos Aires,
and his PhD in chemical engineering at
the University of Massachusetts,
Amherst. He has published more than
200 peer-reviewed papers on mixing
and blending, presented at numerous
conferences, and earned several
patents.
The author will answer your questions
in future issues. Direct questions to
him at fjmuzzio@yahoo.com or via
the Editor, Powder and Bulk
Engineering, 1155 Northland Drive,
St. Paul, MN 55120 (toneill@cscpub
.com).
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