Chemical Engineering August 2013 - 41

product properties. In any case, understanding
the limits of successful reaction,
and understanding the problems
that may arise outside the acceptable
range, will help engineers determine
both the methods and the range of
scaleup options.
For complicated chemical reactions,
power per volume is often the most
effective scaleup method, especially
where mixing affects the product distribution.
With slower reactions, uniform
blending commonly associated
with flow patterns and liquid velocity
may be more important for maintaining
batch uniformity. Reaction rate
and blending uniformity are both essential
for scaleup.
10. Scaleup uncertainties. A variety
of uncertainties arise during scaleup.
In simple cases, deciding which mixing
mechanism is critical to the process
and keeping the critical variable
constant is easy. Geometric similarity
and constant velocity (equal tip speed)
scaleup may be sufficient for batch
and continuous processes involving
basic blending. However, if multiple
reactions are involved, one reaction
may be limiting in the small scale
while another may be limiting in the
larger scale.
Some of these interacting hydrodynamic
effects also may appear in
solids-suspension applications, where
local turbulence may lift particles off
the bottom of the tank and local velocity
will move particles away from the
bottom and into the upper portion of
the tank. The result is the determination
of a scaleup exponent that falls
between equal power per volume, associated
with turbulence, and equal
tip speed, associated with velocity. The
effects may depend on the particle
size, settling rate and concentration.
Scale changes always involve
changes in the relative amount of surface
and volume effects. A scale- ratio
change in a length dimension, such
as tank diameter, results in a scaleratio-squared
change in area, tank
diameter squared, and a scale-ratiochange
cubed in volume, tank diameter
cubed. Thus, any increase in scale
will result in volume effects becoming
more significant when compared with
area effects.
A significant downside to this effect
involves any chemical reaction resulting
in a significant heat of reaction
(exothermic or endothermic). In the
small scale, heat transfer may not be
a problem, but with scaleup, the heattransfer
surface area decreases compared
with the heat of reaction, which
is a volume effect. Temperature control
becomes more difficult as the tank
size increases. For an exothermic reaction,
reduced heat removal will cause
a bulk temperature rise, which could
lead to a runaway reaction.
In most scaleup efforts, the certainty
of making the correct scaleup
decisions may never be resolved until
the large-scale process has been built
and is operating. Even at the large
scale, the operating limits of the process
may not be well defined. Several
aspects of scaleup may improve with
size, such as reduced impact of viscosity
as represented by increasing Reynolds
number, while other aspects may
become more difficult with increased
size, such as the reduction of the surface-to-volume
ratio.
The key to effective mixing scaleup
is always to carry out a thorough investigation
of the effects of mixing on
the small scale. That investigation involves
not just deciding what works,
but learning the limits and avoiding
situations that do not work. Simple
liquid blending may not need any experimental
testing because of a basic
understanding of mixing. However, innovative
applications involving complicated
chemical reactions or nonNewtonian
fluids may be much more
difficult to understand and scale up
successfully. The process of investigating
mixing effects on the small scale
will often be a critical tool in creating
a successful large-scale process. ■
Edited by Suzanne Shelley
Author
David S. Dickey (d.dickey@
mixtech.com) has run his own
consulting business called
MixTech,
Inc.
since 1998
(www.mixtech.com). He has
more than 35 years experience
designing mixing equipment
and solving process or
mechanical problems with
most types of fluid and powder
mixing equipment. He received
his B.S.Ch.E. from the
University of Illinois and completed his M.S. and
Ph.D. degrees in chemical engineering at Purdue
University. He is a fellow of the AIChE, and is
a past president of the North American Mixing
Forum (NAMF).
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Chemical Engineering August 2013

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