Chemical Engineering August 2013 - 39

Another scaleup consideration is
how much of a change can be done
successfully. For instance, doubling a
length dimension increases the volume
by a factor of eight. Thus, an increase
of ten times the length scale
results in 1,000 times the volume.
For complicated scaleup applications
or those with limited small-scale
data, a two-, three- or four-fold increase
in a linear dimension may create
significant uncertainties. In other
cases, laboratory data - even for a
chemical reaction - may allow for
scale changes involving a 10- or even
20-fold increase in the length dimensions
and the corresponding increase
in volume. Many large processes undergo
limited experimental testing, in
very small scales, yet mixing requirements
can be met with large increases
in size.
Geometric similarity is not an essential
for scaleup, provided the process
requirements are well understood and
the critical mixing variables are held
constant. A large-scale mixed tank,
especially a pressure vessel, may be
much taller in relation to its diameter
compared with the small-scale tank
that was used for development testing
and geometry modifications. For storage
tanks, small-scale testing is rarely
done, but large storage tanks may be
shorter in relationship to diameter.
8. Viscous mixing. Applications involving
viscous mixing often experience
many problems during scaleup,
so small-scale testing is especially important.
Rarely does a single viscosity
value adequately describe the properties
of a viscous fluid. High-viscosity
fluids are usually non-Newtonian,
which means that the apparent viscosity
includes effects of not only
temperature but also shear rate, time
dependence, and possibly yield stress.
Making things more complicated, the
properties that complicate viscosity
are often those properties that are
sought to enhance the quality of the
end products.
Mixing viscous materials often involves
changing from turbine-style
impellers to large-diameter,
closeclearance
impellers. Even with turbine-style
impellers for moderately
high viscosities, more impellers and
larger-diameter impellers are needed
just to move all of the fluid in a tank.
As a result, testing at a small scale can
be useful in determining which type,
size, or number of impellers works
best for moving the fluid.
Because flow is often laminar,
stretching and folding mechanisms
must replace turbulence and random
motion for mixing. These slower processes
increase blend time and make
complete motion more difficult. For all
of the experience in mixer design with
low-viscosity liquids, limited experience
and design guidance is available
for mixing high-viscosity fluids using
close-clearance impellers.
However, a bright side to scaleup exists
with viscous blending. Most viscous
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Chemical Engineering August 2013

Table of Contents for the Digital Edition of Chemical Engineering August 2013
