Chemical Engineering May 2016 - 48

actually tend to degrade when exposed to
excessive shear. In other words, ultra-highshear
mixers offer a " sweet spot " of intensity
necessary for particle- or droplet-size reduction
beyond what regular single- and multistage
rotor-stator mixers can deliver.
At the same time, they are not exact drop-in
replacements for high-pressure homogenizers,
but offer an alternative strategy for certain
applications, including pharmaceutical,
cosmetic and food emulsions, fine pigment
dispersions, electronic inks and specialty
coatings, to name a few. Manufacturers reap
the advantages of lower equipment cost,
significantly higher production capacity,
faster changeover and easier maintenance.
Sanitary models, which can be cleaned
and sanitized in place, also make ultra-high
shear mixers a very practical option for sensitive
applications.
Case history snapshot 3. A pharmaceutical
company uses several ultra-high-shear mixers
to produce a topical emulsion made up
of a slightly thickened aqueous phase and
a smaller-quantity oil phase. The aqueous
phase is charged into a recirculation vessel
and the mixer is started up. once the mixer
has ramped up to the operating speed, the
oil phase is added to the tank at a controlled
rate, as the emulsion recirculates with the
aid of a pump. Within minutes, a tight distribution
with submicron median droplet size
is achieved.
4. Vertical blender/dryers. When drying
to very low moisture levels, conventional
ovens and tray dryers are often left to run
continuously for many hours, perhaps even
overnight. vacuum ovens and tray dryers
offer better drying rates compared to their
atmospheric counterparts, but agitated vacuum
dryers can be even more efficient. In
fact, combining mixing and drying in a single
simultaneous operation may offer substantial
improvements to production rate and
energy consumption.
Recognizing the potential savings from
lower operating costs and faster drying,
more and more manufacturers throughout
the CPI are upgrading to agitated vacuum
dryers. one of the most reliable systems
available is the vertical blender/dryer, which
features a slow-turning auger screw and a
conical vessel (Figure 6). The motion of the
auger screw gently lifts materials upward
and reintroduces them to the center of the
batch. As it orbits around the vessel, the
screw also nudges product near the top
to cascade slowly back down. The combination
of thorough mixing, deep vacuum
FIGURE 6. The agitator movement
and material flow pattern
shown here for a vertical
blender-dryer provides favorable
results for many
applications. Product is
heated through contact with
the jacketed sidewalls. The
vacuum-rated cover may
also be jacketed to avoid
condensation
and low horsepower makes the vertical
blender an extremely efficient dryer. With
a very low footprint requirement, the vertical
blender/dryer is especially practical for
large batch volumes up to 500 ft3 or even
larger, as long as the plant has enough
ceiling height.
As opposed to ovens and tray dryers
where product is stationary and relies on
convection for heat transfer, the materials
inside a vertical blender are heated via
conduction. The layer of product in direct
contact with the heated sidewalls is constantly
renewed, and under vacuum conditions,
this continuous but gentle turnover of
materials accelerates drying without affecting
bulk density or generating an excessive
amount of fines.
Another great benefit of the vertical blender
is that it can accommodate a wide range of
feed forms, from free-flowing powders and
pellets to wet granules and paste-like materials.
The low-impact blending mechanism
also makes it well-suited for delicate, abrasive
and high-purity applications.
one particular innovation that is an essential
feature of the modern vertical blender is
the fully top-supported screw agitator. WithChemiCal
engineering www.Chemengonline.Com may 2016
47
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Chemical Engineering May 2016

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