Chemical Engineering January 2018 - 33

Facts At Your Fingertips
High-Shear Mixing
Department Editor: Scott Jenkins
H
igh-shear, high-speed mixing
equipment supplies the
mechanical energy necessary
to reduce the size of solid particles
and liquid droplets in emulsions and
dispersions. Emulsions are mixtures
of two normally immiscible liquids in
which tiny particles of one liquid are
suspended in another. Dispersions
refer to solid particles distributed uniformly
throughout a continuous medium.
High-shear mixing processes
are common across the chemical
process industries (CPI), including in
foods, cosmetics, pharmaceuticals,
greases and lubricants, specialty
chemicals, paints, inks and others.
This one-page reference provides
information on high-shear mixing operations
and equipment.
Rotor-stator mixers
Rotor-stator mixers are standard
workhorses used throughout the CPI
for preparing
fine
dispersions and
emulsions. The traditional design features
a four-blade rotor running at tip
speeds in the range of 3,000-4,000
ft/min within a close-tolerance fixed
stator. The mixer creates mechanical
and hydraulic shear by continuously
drawing product components into
the rotor and expelling them radially
through the openings in the stator.
Rotor-stator mixing devices are offered
in both batch and inline configurations.
Multi-stage designs are
also available. These typically consist
of 2-4 rows of teeth, and run at the
same maximum tip speeds as their
single-stage counterparts.
Particle size profile
Any high-shear, high-speed mixing
operation - whether the process
goal is powder dispersion, deagglomeration
or emulsification - generates
a Gaussian distribution of particle
size. The objective is usually to
produce the narrowest distribution
possible with equipment that meets
both process and business needs.
In some applications, quality control
is generally concerned with average
particle size, while others follow
strict standards pertaining to the larg20
15
Single-stage
rotor/stator
Multi-stage rotor/stator
Colloid mill
Ultra-high shear mixer
High pressure homogenizer
10
5
FIGURE 1. Various technologies can achieve different particle sizes (particle fineness) in emulsions
1
2
3
4
5
6
Tank turnovers
7
8
9
est diameter particles permitted in
a suspension. Particle size profile is
just one of many indicators of a good
product. Depending on the end use,
other properties like conductivity, stability,
viscosity, color, gloss and so on,
whether directly or indirectly related to
particle size, are also considered during
equipment testing and selection.
Particle size equilibrium
The majority of particle or droplet size
reduction occurs within the first few
passes through an inline mixer, or
the first few tank turnovers in a batch
mixer. This phenomenon is true for
almost any dispersion or emulsion.
Past this stage of sharp decrease in
particle size, the distribution hovers
at an equilibrium (see Figure). Additional
processing will gradually narrow
the distribution curve, but extending
the process for this purpose alone
is almost always unprofitable. If the
equilibrium particle or droplet size
achieved in a single-stage or multistage
rotor-stator is larger than desired,
manufacturers are often forced
to use more expensive, higher-energy
devices, such as high-pressure homogenizers
and colloid mills.
In a high-pressure homogenizer,
the fluid is subjected to very high
shear stress as it is forced through
a specially designed homogenization
nozzle at pressures of 150-200
MPa. However, high-pressure homogenizers,
colloid mills (which also
work on a rotor-stator principle) and
CHEMICAL ENGINEERING WWW.CHEMENGONLINE.COM
JANUARY 2018
10
similar equipment are commonly associated
with several drawbacks, including
higher capital cost and lower
throughput than mixing technologies,
along with frequent clogging,
labor-intensive cleaning and high
maintenance requirements.
Ultra-high shear
Among the recent developments in
mixing are more cost-effective rotorstator
designs capable of delivering
intense mixing along with rugged efficiency.
Ultra-high-shear mixers offer
the ability produce dispersions and
emulsions superior to those made in
multi-stage rotor-stators and colloid
mills. In certain applications, these can
replace high-pressure homogenizers,
delivering comparable size reduction
at significantly higher throughputs.
Ultra-high-shear mixers do not eliminate
the premixing process - the
feed must be a dispersion or emulsion
in liquid form. Simple to operate,
much like a regular inline rotor-stator
mixer, these behave like a centrifugal
pumping device. Materials are fed by
gravity or pumped to the mixer. With
the assistance of an auxiliary pump,
ultra-high-shear mixers can process
non-flowing, viscous materials like
gels, pastes and creams.
n
Editor's note: Material in this
month's column was contributed by
Christine Banaszek, Charles Ross
& Son Co. (Hauppauge, N.Y.; www.
mixers.com).
Facts At Your Fingertips
Sponsored by:
32a
Average particle size, microns

http://www.mixers.com http://WWW.CHEMENGONLINE.COM

Chemical Engineering January 2018

Table of Contents for the Digital Edition of Chemical Engineering January 2018