Chemical Engineering August 2013 - 40

Cover Story
mixing scaleup can be handled by equal
tip speed. The power requirements for
nearly all of the blending processes
depend on Reynolds number, which is
affected more strongly by a primary
length dimension, such as tank or impeller
diameter, than velocity. So, if tip
speed, which translates to fluid velocity,
is held constant during scaleup, the
Reynolds number increases in direct
proportion to the increase in the length
dimensions of the tank.
As the Reynolds number increases
with tank size, the effect of viscosity
in the laminar and transitional
ranges is reduced. This change means
easier fluid movement and potentially
greater mixing intensity. For the same
Reynolds number,
the large-scale
mixer could even be considered to be
operating as if the fluid had a lower
viscosity. The reduced effects of viscosity
should make mixing easier with
scaleup.
9. Chemical reactions. Most new
chemical reactions are developed at
the laboratory scale. Commonly available
laboratory mixers may be as
simple as magnetic stir bars in glass
beakers, or flasks on a shaker table.
These mixer types are not effective
in large-scale processes because the
rotational flow patterns do not create
sufficient vertical and horizontal motion
for good mixing. The only reason
that the swirling motion in laboratory
glassware is adequate is that in such
small containers the ingredients mix
quickly regardless of the flow pattern.
Scaleup of chemical reactions often
follows two different paths - an easy
one and a more difficult one. The easy
path occurs when the new reaction
is similar to one previously put into
production, where the reaction steps
and any exothermic heat generation
have already been successfully scaled.
Other reactions with different kinetics,
stoichiometry
and heat release
may require extensive pilot testing
before going into production.
While work in a pilot plant may provide
additional information about the
reaction, a pilot reactor should be designed
to test and observe the effects
that mixing has on the chemical reaction.
Pilot testing needs to investigate
the positive and negative effects that
mixing has on the reaction, in terms
of yield, quality and even the overall
success of the process. Mixing is necessary
to bring reactants together and
to distribute the reactants and product
uniformly in the tank. Heat addition
or removal may also be part of the
chemical process.
Information about the process must
also establish critical mixing variables
that are required to ensure the success
of the reaction. Insufficient mixing
may not provide adequate uniformity
or blending that is rapid enough
for the reaction to produce the desired
products. Excessive mixing may introduce
air, or may negatively impact
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Chemical Engineering August 2013

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