Page 290 - Academic Press Encyclopedia of Physical Science and Technology 3rd Chemical Engineering

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Encyclopedia of Physical Science and Technology EN006C-252 June 27, 2001 14:15

90 Fluid Mixing

energy spectra. In general, the energy dissipation per unit Qualitative and, hopefully, quantitative estimates of
volume around the impeller is approximately 100 times how the process result will be measured must be made
higher than in the rest of the tank. This results in an rms ve- in advance. The evaluations must allow one to establish
locity fluctuation ratio to the average velocity on the order the importance of the different steps in a process, such as
of 10:1 between the impeller zone and the rest of the tank. gas-liquid mass transfer, chemical reaction rate, or heat
Because there are thousands of specific processes each transfer.
year that involve mixing, there will be at least hundreds
of different situations requiring a somewhat different pilot It is seldom possible, either economically or
plant approach. Unfortunately, no set of rules states how time-wise, to study every potential mixing variable or
to carry out studies for any specific program, but here are to compare the performance of many impeller types.
a few guidelines that can help one carry out a pilot plant In many cases, a process needs a specific fluid regime
program. that is relatively independent of the impeller type used
to generate it. Because different impellers may require

For any given process, take a qualitative look at the different geometries to achieve an optimum process
possible role of fluid shear stresses. Try to consider combination, a random choice of only one diameter of
pathways related to fluid shear stress that may affect each of two or more impeller types may not tell what
the process. If there are none, then this extremely is appropriate for the fluid regime ultimately required.
complex phenomena can be dismissed and the process Often, a pilot plant will operate in the viscous region
design can be based on such things as uniformity, while the commercial unit will operate in the
circulation time, blend time, or velocity specifications. transition region, or alternatively, the pilot plant may
This is often the case in the blending of miscible fluids be in the transition region and the commercial unit in
and the suspension of solids. the turbulent region. Some experience is required to
If fluid shear stresses are likely to be involved in estimate the difference in performance to be expected
obtaining a process result, then one must qualitatively upon scaleup.
look at the scale at which the shear stresses influence In general, it is not necessary to model Z/T ratios
the result. If the particles, bubbles, droplets, or fluid between pilot and commercial units, where Z is the
clumps are on the order of 1000 µm or larger, the liquid level.
variables are macroscale and average velocities at a In order to make the pilot unit more like a commercial
point are the predominant variable. unit in macroscale characteristics, the pilot unit
impeller must be designed to lengthen the blend time
When macroscale variables are involved, every geo- and to increase the low maximum impeller zone shear
metric design variable can affect the role of shear stresses. rate. This will result in a greater range of shear rates
They can include such items as power, impeller speed, than is normally found in a pilot unit.
impeller diameter, impeller blade shape, impeller blade
width or height, thickness of the material used to make All of these conditions can be met using smaller D/T
the impeller, number of blades, impeller location, baffle ratios and narrower blade heights than are used normally
location, and number of impellers. in a pilot unit. If one uses the same impeller type in both
Microscale variables are involved when the particles, the pilot and commercial units, however, it may not be
droplets,baffles,orfluidclumpsareontheorderof100µm possible to come close to the long blend time that will
or less. In this case, the critical parameters usually are be obtained in the commercial unit. Radial flow impellers
power per unit volume, distribution of power per unit vol- can be excellent models in a pilot plant unit for axial flow
ume between the impeller and the rest of the tank, rms impellers in a commercial unit.
velocity fluctuation, energy spectra, dissipation length,
the smallest microscale eddy size for the particular power
level, and viscosity of the fluid. III. LIQUID–SOLID CONTACTING

The overall circulating pattern, including the Solids suspension involves producing the required distri-
circulation time and the deviation of the circulation bution of solids in the tank and is essentially a physical
times, can never be neglected. No matter what else a phenomenon. The criterion is normally a physical descrip-
mixer does, it must be able to circulate fluid tion of the degree of uniformity required in the suspension.
throughout an entire vessel appropriately. If it cannot, A key variable for solids suspension is the settling velocity
then that mixer is not suited for the tank being of the solids. This is usually measured by timing the fall
considered. velocity of individual solid particles in a defined depth of