Chemical Engineering July 2012 - 31

FIGURE 2. This double planetary/disperser
hybrid mixer is
equipped with a pair of high
speed dispersers on each of
two shafts, in addition to two
sets of helical planetary blades.
The addition of disperser blades to
the traditional double planetary mixer
enables it to handle applications that
include both high-viscosity and lowviscosity
stages
another and signal the need
for a change in agitation.
Example 2: Adding a high
shear agitator to accommoRecognizing
this, many forwardthinking
production engineers are
now testing periodically, and not just
when a plant expansion or the addition
of a new production line provides
an obvious opportunity to upgrade.
They test to stay current on new developments
in mixing technology, explore
opportunities to improve both current
and future production lines, and to
make sure their companies remain in
the passing lane of global competition.
Go with the flow
Many engineers who visit the test laboratory
of a mixer manufacturer are surprised
by the fact that subtle changes
in a mixer's configuration or operation
can yield an enormous improvement in
performance. Virtually all arrive with
at least an idea of the type of mixer
they want to use, and often their instincts
turn out to be correct - with
the simple addition of another agitator.
Example 1: Adding a low-shear agitator
to create a uniform pigment
dispersion.2 The production engineer
in this case had used a high speed
disperser for years to disperse a variety
of liquid pigment blends in a base
material. Operating in a batch with a
lotion-like consistency - a viscosity
of approximately 20,000 cP - the disperser
provided plenty of shear energy.
A 10 h.p. disperser in a 50-gal batch
required about 60 min to complete
the dispersion. Trials were arranged
to search for potential improvements
related to blade size and design, and
perhaps the use of multiple blades
mounted on a single shaft.
At this batch size and viscosity, an
8-in.-dia. high-speed disperser operating
with a tip speed of 5,000 ft/min
2. All of the test scenarios in this article are
drawn from actual trials in the Ross Test & Development
Center in Hauppauge, NY. However,
certain details were omitted or changed to safeguard
customer confidentiality or clarify the essential
message of the example.
creates only a mild vortex. Pigments
added to the light-colored base material
provide a vivid display of uniformity
- or in this case, slow progress
toward uniformity. Material near the
disperser was quickly dispersed and
assumed a uniform appearance. Meanwhile,
slow-moving swirls of color near
the vessel wall indicated limited flow
within the batch.
In actual production, the cycle time
for this application had been 60 minutes,
but most of that time was wasted.
The mixer dispersed the pigments immediately
once they contacted the
blade. The limiting factor was the flow
within the vessel, not the blade design.
We recognized that flow could be improved
by adding a low-shear agitator
that would complement the action of
the high shear agitator.
In a dual-shaft mixer, a slow-turning,
two-wing anchor agitator improves
flow by moving material from the vessel
wall toward the high shear agitator
(Figure 1). Teflon scrapers prevent a
layer from remaining on the wall and
bottom of the vessel. By improving
flow, the anchor essentially feeds material
to the disperser and accelerates
the dispersion process.
With the complementary action of
these two agitators, the batch reached
target uniformity in 15 min, a 75%
improvement compared to the cycle
time required by the disperser operating
alone.
An agitator for each stage
Mix cycles can often be accelerated by
identifying key inflection points during
the process and recognizing the need
to apply different forms of agitation
during different stages. Substantial
changes in viscosity, for example, generally
distinguish one mixing stage from
date the lowered viscosity of a
conductive coating. The double planetary
mixer has been around for more
than 50 years, and it is still a reliable
workhorse for high-viscosity mixing.
Since the dispersion of conductive carbon
is generally processed at viscosities
up to about 1 million cP during
the mix cycle, it is a typical application
for the double planetary mixer. In this
scenario, a manufacturer had already
used double planetary mixers to prepare
conductive coatings. He scheduled
a test to confirm the choice of a
new mixer for scale-up.
Replicating the process in the test
laboratory, conductive carbon powders
were added to a solvent base, along
with a variety of binder materials.
Planetary mixing required 20 min of
kneading at 1 million cP.
The next phase of the process was
far more time-consuming. Letting
the batch down from 1 million cP to
10,000 cP required 90 min, because
the solvent must be added slowly.
Dosing the solvent gradually allows
it to be incorporated without forming
clumps of the conductive paste, which
bob in the low-viscosity mix and resist
breaking down further.
The slow pace of the let-down stage
of this cycle made it an excellent target
for improvement. The key was to understand
that it was slow only because
the mixing action of the planetary
blades became steadily less effective
as viscosity fell. At viscosities below
200,000 cP, planetary blades generate
very little shear and are unable to
incorporate the low-viscosity diluent
into the paste.
The solution was to switch from a
traditional double planetary to a double
planetary/disperser hybrid mixer
(Figure 2). This mixer extends the versatility
of the double planetary mixer
by adding two disperser shafts, each of
which can be equipped with one or two
disperser blades. These high-speed
agitators orbit the vessel in tandem
with the planetary blades and apply
intense shear.
In this application, the high-shear
agitators were turned on for the
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