Page 295 - 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 95

FIGURE 24 Typical plot of K factor, ratio of horsepower with gas-on to horsepower with gas-off at constant speed,
as a function of superficial gas velocity with two different gas inlets.

direction. It has the characteristic of giving the same mass of these have been done with pure liquid components,
transfer as the radial flat blade turbine shown in Fig. 1a and do not apply to the complicated chemicals used in-
but does not drop off in power as much as the radial flow dustrially. Small amounts of surface active agents make
turbine does. Figure 25 shows the K factor which relates dramatic differences in emulsion characteristics, so it is
the power drawn when the gas is on to the power drawn in not usually possible to calculate in advance the mixer
the ungased liquid. This means that variable speed drives needed to provide the particular type of emulsion parti-
do not have to be used many times to keep the desired cle size. Therefore, test work is normally required where
power in the gased condition. conditions required in the pilot plant are evaluated for
scaleup.
Large tanks have a longer blend time and a much greater
V. LIQUID–LIQUID CONTACTING
variety of shear rates than small tanks, therefore, emulsion
characteristics on full scale are difficult to predict. Usu-
A. Emulsions
ally, the pilot plant work is aimed at trying to elaborate
There is a large class of processes where the final product the key role of maximum impeller zone shear rate, aver-
is an emulsion. It includes homogenized milk, shampoos, age impeller zone shear rate, and general circulation rate
polishing compounds, and some types of medical prepa- and velocity out in the main part of the tank. If these can be
rations. A key factor is the chemistry of the product, to even qualitatively determined, scaleup to full scale can be
ensure that the emulsion remains stable over a desired done with reliability. There are a variety of mixers used in
product shelf-life, when produced properly by the fluid these processes. For various types of emulsion polymer-
mixer. There are numerous correlations in the literature ization, it is typical to use axial flow impellers because the
on drop size in a two-phase liquid–liquid system, relative shear rate requirements do not demand the use of radial
to fluid properties and mixer variables. However, most flow impellers. Getting into the other end of the spectrum,
where extremely high shear mixers are needed, various
kinds of radial flow blades, usually with very narrow width
blades, allow speeds to go up to 1000 or 2000 rpm giving
very intense shear rates that are needed for many types of
emulsion processes.

B. Liquid Extraction
Figure 26 shows a typical curve relating the performance
of many kinds of mixing devices for liquid extraction.
The main advantage of using mixing or some type of
FIGURE 25 Radial flow turbine with blades. mechanical energy, compared to packed plate or spray