Page 287 - 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

Fluid Mixing 87

FIGURE 14 The velocity spectra in the axial direction for an axial impeller A200.

today is specifying mixers for industrial processes based where ν is the dynamic viscosity and e is the power per
on meeting criteria of this type. This is largely because unit volume.
processes are so complex that it is not possible to define
the process requirements in terms of these fluid mechanics II. SCALEUP RELATIONSHIPS
parameters. If the process results could be defined in terms
of these parameters, sufficient information probably exists
Scaleup involves determining the controlling factors in a
to permit the calculation of an approximate mixer design.
process, the role that mixing plays, and the application of
It is important to continue studying fluid mechanics pa-
a suitable scaleup technique. In this section, the general
rameters in both mixing and pipeline reactors to establish
scaleup relationships will be presented, and the particular
what is required by different processes in fundamental
types of processes involved will be covered. Section X
terms.
will cover pilot planting, how runs are made to determine
Recently, one of the most practical results of these stud-
the controlling factor, and how to choose a suitable design
ies has been the ability to design pilot plant experiments
relationship for that situation.
(and, in many cases, plant-scale experiments) that can es-
Table III is a key for understanding scaleup relation-
tablish the sensitivity of process to macroscale mixing
ships. In the first column are listed many design vari-
variables (as a function of power, pumping capacity, im-
ables involved in mixing processes. These include power,
peller diameter, impeller tip speeds, and macroscale shear
power per unit volume, speed, impeller diameter, impeller
rates) in contrast to microscale mixing variables (which
are relative to power per unit volume, rms velocity fluctu-
ations, and some estimation of the size of the microscale TABLE III Properties of a Fluid Mixer on Scaleup
eddies). Pilot scale
Another useful and interesting concept is the size of the Property (80 Liters) Plant scale (17.280 liters)
eddies, L, at which the power of an impeller is eventually
dissipated. This concept utilizes the principles of isotropic P 1.0 216 7776 36 0.16
P/Vol 1.0 1.0 36 0.16 .0007
turbulence developed by Komolgoroff [1]. The calcula-
N 1.0 0.3 1.0 0.16 .03
tions assume some reasonable approach to the degree of
D 1.0 6.0 6.0 6.0 6.0
isotropic turbulence, and the estimates do give some idea
Q 1.0 65 216 36 6.0
as to how far down in the microscale size the power per
Q/Vol 1.0 0.3 1.0 0.16 .03
unit volume can effectively reach
ND 1.0 1.8 6.0 1.0 0.16
2
ND ρ 1.0 10.8 36 5.8 1.0
3
L = (ν /e) 1/4 µ