Page 29 - Chemical Technology

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Chemical Technology • January 2013
solids handling
particles settle on the bottom of the tank for more than 2
seconds. All designs are to utilize a single HE-3 impeller
one-third the tank diameter off-bottom. The horsepower,
speed and impeller diameter combinations that satisfy
the process objective can be determined using solids'
suspension design procedures such as the article pre-
sented by Corpstein and others.20
The following designs shown in Table 1 (right) all satisfy
the off-bottom solids suspension process requirement.
Even though using small impellers operating at high
speeds reduces capital costs and power, the high tip
speeds of these designs lead to short wear lives. Using
an intermediate size impeller that minimizes tip speed
at the cost of higher capital and power costs maximizes
the wear life. As can be observed, the torque, and
consequently the capital cost, increase dramatically with
impeller to tank diameter ratios greater than 0,45 due to
changes in the flow pattern generated by the impeller.
Scale-down studies
Since there is often an erosion-corrosion synergistic
effect that cannot be predicted a priori with today’s
current data, any material selection should be studied
on a smaller scale, before making a final choice. Since
it is important to model the same hydraulic behaviour
over the blade, the authors recommend that the ratio of
mean particle diameter to impeller diameter not exceed
0,008. For example, a mean particle diameter of 1 mm
would suggest a small-scale impeller no less than 125
mm (4,9 in) diameter. It is also important to ensure that
fluid regimes haven’t changed. If the impeller operation
is turbulent on the large scale, it should also be turbulent
on the small scale.
An optimum agitator horsepower and speed selec-
tion can be made as described above for the full scale.
However, in order to determine the rate of erosion, scale-
down studies should be made. A geometric scale-down
for an equal level of solid suspension will result in a tip
speed that will always be lower on the smaller scale,
except when scaling down geometrically for very slow
settling solids (<0,1 m/min). Wear rate, as previously
demonstrated, is a strong function of velocity. Therefore
all scale-down tests should be made at equal tip speed.
As an example, if we examine the 55-inch diameter
impeller solution above for the above described problem
and scale this down to an 18-inch diameter tank we
would have the following comparison.
Conclusions and summary
The rate of erosion is dependent on the following major
static environmental factors: chemical environment;
hardness of the solid particles; density of the solid
particles; percent solids; the shape of the solids; the size
of the solids including whether or not tramp solids are
present; and type of impeller. The dynamic factors affect-
ing erosion rate are: fluid regime; impact velocity; impact
frequency and angle of impact. As there are no good
means currently of predicting erosion rate, small-scale
studies should be conducted emulating as much of the
total environment as possible. These small-scale studies
should be conducted using equal tip speed to mimic the
full-scale rate of erosion.
Erosion in most mixing processes is a fatigue process
generally accelerated by a liquid corrosive environment.
The fatigue process occurs on a micro or localized scale,
and, as with macro-scale fatigue, two stages of the ero-
sion process have been observed. There is an incuba-
tion period followed by the formation and growth of pits
involving the removal of the metal or material. One of
two routes is generally utilized in dealing with an erosion
application. The high velocity areas such as the blades
are either made from hard materials, or coated with hard
ceramic materials such as tungsten carbide or silicon
carbide. Alternatively, the blades are covered with some
type of elastomeric covering.
Impeller selection is important especially in turbulent
flow conditions. High efficiency impellers will gener-
ally erode at a slower rate because the backside of the
blades has minimized shedding vortices. In laminar flow,
from an erosion standpoint, most impellers behave simi-
larly due to a lack of vortices. Thus, the selection of the
impeller should be based primarily on what is required to
accomplish the desired process result.
A number of horsepower and speed selections that
satisfy the process requirement should be examined
to conduct an economic analysis. The lowest possible
speed may not be the most economical. It is best to first
design the most cost optimum agitator for the full scale.
Then in order to estimate the corrosion rate, scale down
on the basis of equal tip speed.
Table 1: Possible process design selections for example
Impeller
Diameter,
m
Impeller
Dia./Tank
Dia.
Impeller
Speed,
rpm
Impeller
Power,
kW
Impeller
Torque,
kNm
Impeller
Tip Speed,
m/min
Relative
Wear
Life *
Approx.
Relative
Cap. Cost
0.889 0.243 159 3.49 0.210 444 1.00 1.00
1.016 0.278 123 2.94 0.228 392 1.54 1.06
1.143 0.313 102 3.15 0.294 366 1.96 1.25
1.220 0.347 92 3.59 0.372 367 1.95 1.45
1.397 0.382 84 4.05 0.460 369 1.92 1.67
1.524 0.417 75 4.61 0.587 359 2.10 1.95
1.651 0.451 70 5.23 0.713 363 2.02 2.22
1.778 0.486 67 6.32 0.972 374 1.82 2.71
1.905 0.521 65 9.16 1.345 389 1.59 3.35
* Relative wear based on assumed velocity exponent of 3.5
Table 2: Comparison of equal suspension to equal tip speed
Condition
Impeller
Dia.,
cm
Impeller
Speed,
rpm
Impeller Tip
Speed,
m/min
Full Scale Design, 144 in. dia. Tank 139.7 84 369
Scale-down Design based on Equal Suspension 17.46 456 250
Scale-down Design Based on Equal Tip Speed 17.46 672 369
References are available from the editor at
chemtech@crown.co.za