7 Things About high shear mixer You'll Kick Yourself for Not Knowing
Previously relegated to a relatively narrow niche of mixing applications, the high-shear rotor / stator mixer (HSM)
has become a staple of many applications in the chemical process industry (CPI). The ability to apply extreme
shear and shorten mixing cycles gives these mixers wide appeal for applications involving the formulation of
immiscible liquids into emulsions or the dispersion of agglomerated powders into a liquid medium. The
emergence of new variations on the original rotor / stator mixer concept has extended the utility of the HSM to
more diverse applications, particularly over the past decade. For example, traditional HSMs are commonly used
today for high-intensity mixing, dispersion, disintegration, emulsification, and homogenization in both top-entry
batch configurations and inline models.
Uses range from gum, ink, fumed silica, calcium carbonate, and active drugs dispersions to emulsions such as
beauty creams, lotions, and flavours. They are still widely misunderstood, however, despite the growing popularity
of HSMs in many industries. Scientists from industry and universities have focused primarily on exploring the
dynamics of modern low-shear mixing technologies, such as axial and radial-flow turbines. With only a few
notable exceptions, in terms of fundamental research, high-shear mixing has been largely overlooked to unlock its
mysteries and help users better predict mixing results, particularly during scale-up.
Since the body
of literature available for rotor / stator mixing predictive technology is extremely thin, the implementation of
HSMs is often approached empirically-with strong emphasis on application-specific testing and development by
individual manufacturers in the process industries. A few users have invested heavily in narrowly defined
applications such as those involving emulsion polymers and pigment dispersions with HSMs and achieved
remarkable performance. Others on their own have been less successful. Many prospective HSM users depend on
mixer manufacturers ' advice, which often hold a closely guarded secret to their proprietary design guidelines. The
result of this lack of available knowledge about high-shear mixing is the proliferation of misconceptions about the
proper application and use of HSMs. There are numerous misconceptions commonly held and errors in the
application are commonly made. Readers who can avoid these errors will save time and money in searching for
the best rotor / stator mixer and reduce their risk of choosing a configuration for a mixing system that looks fine
in the laboratory but does not perform properly on the floor of the plant.
Scaling up: Scaling up is a crucial phase for almost every project that impacts the business in a variety of ways,
from proper planning of floor layout and equipment installation, to operating procedures, to net operating and
capital cost effects on the bottom line. In laboratory tests, misjudging the time needed to achieve mixing
equilibrium in just a few seconds can ultimately cost your company millions of dollars, not to mention wasted time
and effort and increased wear and tear on the equipment during commercial production.
The laboratory tabletop HSM is usually the first step in investigating the specific advantages of rotor / stator
technology for a particular application. This common laboratory device is usually fitted with a variety of
interchangeable attachments that allow it to function in a variety of mixing modes-as a traditional HSM, as a
propeller mixer, and as a "saw tooth" disperser with high speed. This flexibility is important in the design of the
bench scale, as it enables the person involved in research and development to rapidly evaluate several different
However, as useful as the laboratory scale mixer may be, it is also the source of one of the most common and
costly errors in the scale from laboratory scale HSM to pilot scale and device output. If laboratory testing is carried
out routinely and with great care and precision, slight errors in over-processing on the benchtop can result in
enormous projection-scale errors. Such errors are especially common, as many engineers underestimate the
extraordinarily high performance-to-product-volume ratio of the laboratory mixer.
Until we move on, let's discuss another concept: combining effects of equilibrium. This is the point at which the
mixed product has acquired a target feature for practical purposes-such as a particular droplet or distribution of
particle size-that will not change significantly, no matter how long you continue to process the material. This is the
point at which we reach the particle size of the equilibrium when we work with dispersions. It's the average droplet
size of emulsions.
Whether we deal with emulsions or dispersions, this is quite certain: with a labscale mixer, we can achieve an
equilibrium far quicker than with a scaled-up pilot or production unit.
We can hit this mark in one tank turnover or several hundred tank turnovers depending on the application and the
rotor / stator model we use.
Consider this typical real-world scenario with a laboratory scale mixer test. Take a two-liter beaker and add the
following ingredients to prepare an emulsion: liquid phase— oil phase— air and oil-miscible surfactant. But
consider this: That little 1-3/8-in before pushing the start button and heading down the hall for another cup of
coffee. The rotor / stator generator will run at 100 liters per minute or more output on your mixer. With a 2-liter
batch in the beaker, it converts every 1.2 seconds into one full batch turnover. Assuming that 10 tank-turnovers
produce the desired emulsion in this application (a plausible number for many simple emulsions), this means that
in just 12.0 seconds you can achieve mixing balance!
This is the point where human nature takes over in the real world. You keep the tabletop batch running for five
minutes as you go for tea, and when you check the results you can find that your emulsion's droplet size
distribution is where you want it to be. It's a win! But what was actually going on? For five minutes you analyzed
the batch, rotated the batch more than 250 times, and found the correct endpoint. Yet the product did not change
once it hit its mixing balance in just 12 seconds-so the remaining four minutes and 48 seconds did not result in
any significant change in the mixed material. That's the margin by which the mixing balance was potentially
overshot. In a lab-scale example, over-processing by four minutes and 48 seconds may not seem like a big deal-
but consider the implications in terms of productivity, energy costs, manpower, and wear and tear when such a
mistake is propagated to a larger pilot or production-scale unit during scale.
Now, use the example above to speed up the scale-up requirements. Note that in 500-gallon batches you will
need to manufacture this material. If you think you'll need 250 tank turnovers to achieve your system goals
(instead of 10, which is really all you want), then you'll pick a top-entry, batch HSM emulsifier that will process
125,000 gallons over an appropriate period of time through its rotor / stator generator. We expect a 30-hp unit
with a 7-in.-dia, based on experience. The rotor pumps approximately 500 gal / min. Therefore, it will take 250
minutes (4 hours, 10 minutes) for our 250 tank turnovers (125,000 gallons). Both ventures have a capacity of
approximately two lots per8-hour period, or 10 per week of one shift. If we had better understood at the
laboratory level that the system target was accomplished in just 12 seconds (10 turnovers), we might have
estimated that the same production unit would complete the task in about 10 minutes. Such ventures require
about 240 lots a week-an improvement of 230 lots a week.