Page 291 - 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 91

the top and replacing it, allowing the slurry to flow into
the container. Neither of these methods gives the abso-
lute percentage of solids at the measurement point. In the
case of a tube, the withdrawal velocity of the tube can af-
fect the percent of solids that comes out of the discharged
slurry as well as the orientation of the tube relative to the
flow pattern in the tank. In the case of the sample con-
FIGURE 17 Effect of settling velocity to achieve a 60% suspen- tainer, its location, fill rate, and other variables can affect
sion of particle sizes when there is a mixture of particle sizes. the actual solids composition measured compared to the
actual. On the other hand, as long as measurement tech-
mother liquor. When there is a wide range of particle sizes, niques are consistent, a reliable effect of mixer variables
there may well be a wide range of settling velocities. can be determined, which is of value in predicting operat-
Much of the literature is based on experimental data ing conditions for full-scale units. One such test on a pilot
with similarly sized particles and observations of the speed plant scale yields data shown in Fig. 18, which shows the
required to keep particles in motion with at most 1 or 2 difference in axial and radial flow of solids suspension
sec of rest on the bottom of the tank. This is done by characteristics and indicates, as we mentioned previously,
visually observing solids in a transparent tank. This, of- that axial flow impellers require less horsepower for the
course, means that relatively small-scale experiments are same degree of solids suspension.
conducted and that this particular criterion cannot be used The use of the new type of fluidfoil impeller has reduced
for studies in large-sized tanks or in field tests. the power required for solids suspension to about one-half
◦
Sizing procedures to design a mixer for one closely to two-thirds of the values formerly used with 45 pitch
sized particle settling velocity are modified considerably blade turbines.
when there are other solids present. Figure 17 shows the In continuous flow, the only point in the tank that must
effect of settling velocity on power when there are other be equal to the feed composition for steady-state operation
solids present in the system. The slope is much less pro- is the drawoff point. Thus, if the drawoff point is at the
nounced than it is when a single particle size alone is being bottom, middle, or top of the tank, different average tank
suspended. compositions can result, even though the composition of
Much of the literature correlations for solids suspension the entrance and exit streams are the same. If the mixer
arebasedontheso-calledcriticalimpellerspeed.Attempts is large enough to provide complete uniformity of all the
to duplicate experiments between various investigators of- solids, including the coarse particles as well as the fine par-
ten yield deviations of ±30–50% from the critical speed ticles, then the drawoff point does not make any difference
shown by other investigators. Because power is propor- in the composition of the tank. However, if the mixer is de-
tional to speed cubed, power varies on the order of 2 to signed only to just suspend the solids to the drawoff point,
3 times, which is not sufficiently accurate for industrial then tank compositions vary widely, depending upon the
full-scale design. Therefore, many approximate, conser- drawoff conditions.
vative estimates have been made in the literature as gen- Many times a fillet can be left in a tank, which will
eral guidelines for choosing mixers for solids suspension. reduce the horsepower considerably for what will be
Table IV is one such guideline for solid particles of a required to completely clean out the last corners of a flat
closely sized nature. bottom tank. Depending upon the value of the solids in the
Thestudyofsolidssuspensioninquantitativetermsnor- process, they may either be left to form their own fillet,
mally involves a method of sampling. Typically, samples or the tank may be streamlined by using concrete or other
are withdrawn from the side of the mixing tank through materials to give a more streamlined shape.
openings or tubes inserted into the vessel wall. It may also When solids increase in percentage, the effect is to make
be done by submerging a container and quickly removing the process requirement more difficult, and a curve similar
to that in Fig. 19 results, until a point which often occurs
TABLE IV Motor Horsepower for Estimating around 40–50% by weight solids, at which there may be
Purposes for Solids Suspension a
a discontinuity. At this point, the viscosity of the slurry
Settling velocity 1 /min 2 /min 4 /min is becoming a parameter, which reduces the settling ve-

locity and, thus, minimizes its importance as a criterion
Off bottom 1 2 5
to one in which we are essentially blending and providing
Uniform 1.5 5 15
motion through a pseudo-plastic fluid. Then as the solids
a 15,000 gal tank; D /T = 0.33. C /D = ; axial percentage gets up toward 70 or 80% (and this point can be
1
2
flow turbine; 1–20% solids by weight. normalized by relating it to the percentage of the ultimate