Chemical Engineering August 2013 - 34

Cover Story
Feature Report
Ten Things You May Not Know
About Liquid Mixing Scaleup
Close attention to these
aspects of scaleup can ensure
greater success
David S. Dickey
MixTech, Inc.
T
he scaleup of liquid mixing systems
does not have to be a mystery.
The principles are quite well
understood, although knowing
which principles apply is not always
obvious. Follow this guidance to improve
operation.
1. Liquid mixing scaleup. Liquid
mixing is the most common use of
scaleup methods in chemical engineering.
The concept of scaleup is to
take small-scale results and predict
results in larger-scale equipment
(Figure 1). The term " liquid mixing, "
more broadly called agitation, can be
extended to include most types of mechanically
induced fluid motion that is
carried out for the purpose of increasing
the uniformity of concentration,
composition or temperature.
In this article, the term mixing applies
to all types of mechanically induced
fluid motion, including blending,
suspension and dispersion. Fluid
mixing also includes the blending of
viscous fluids, the suspension of solids
in liquids, the dispersion of gases in
liquids, the dispersion of immiscible
liquids and other efforts to combine
materials that behave as fluids.
The reason that scaleup efforts are
so prevalent for mixing processes is
that the possible combinations of tank
geometry, impeller types and process
applications related to
combining
fluids are almost limitless. To understand
how fluids behave when they
are mixed or agitated, you must understand
the fluid properties, the tank
size and shape, the impeller type and
the rotational speed of the impeller. In
basic terms, three factors determine
the size of any mixer required: 1) How
much fluid must be agitated; 2) How
difficult the fluid is to move; 3) How
intense the agitation needs to be for the
application.
The answer to the question of " how
much " can be quantified by either
volume or mass. However, because
fluid density is an important factor in
turbulent power, all combinations of
volume, mass, and density should be
known. The " how difficult " question
is measured by the properties of the
fluid. Viscosity describes the difficulty
for the motion of liquids, and may be
applied to high concentrations of solids
in slurries or pastes.
The settling rate of solids for a solids
suspension or the volumetric flowrate
of a gas for gas dispersion are also difficult
factors for multi-phase systems.
Small-scale testing is often the best
- and sometimes the only - way to
decide " how intense " mixing needs to
be. Properly designed and evaluated
testing will include all three factors.
Deciding " how much " fluid needs to
be mixed may also involve knowledge
about the shape of the tank. Tall, slender
tanks and short, fat tanks may
present different mixing problems for
the same quantity of fluid. Tank features,
such as the existence of baffles
32 CHEMICAL ENGINEERING WWW.CHE.COM AUGUST 2013
FIGURE 1.
Scaleup applies
to any kind of deScale-up
velopment
or testing
conducted in
smaller equipment,
and using those
results to design
and operate successfully
in larger
equipment. While
absolute size does
not matter, the
small-scale results
must be relevant
to the large-scale
equipment
and feed locations, may also influence
mixing intensity. Testing is usually
done with a fluid that has the same
properties as expected in the full-scale
application, although a test fluid with
a different viscosity may be chosen
if the flow pattern will change with
scale. For instance, the flow pattern
in larger tanks, with a higher impeller
Reynolds number, is less influenced by
viscosity than in smaller tanks.
Scaleup efforts will never be successful
unless the real objective and
critical variables have been identified
correctly. For instance, scaleup for
product quality may be different from
scaleup for yield quantity. In some
cases, a high-quality product may result
in less quantity, or the opposite.
For example, the investigator should
ask if a minimum requirement exists
for either quality or quantity, and how
much can be achieved for the other objective
within the limiting constraint?
The process of testing should explore
the alternatives so that engineers
can choose a successful direction for
scaleup and truly understand how
certain adjustments might be made
on the large-scale system to compensate
for potential uncertainties.
Testing to identify a range of possible
conditions that will be successful provides
valuable information about how
production-scale adjustments can be
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Chemical Engineering August 2013

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