Page 200 - Fluid mechanics, heat transfer, and mass transfer

MIXING
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& Modern rotor–stator mixers involve numerous (a) Low-viscosity liquids (high Reynolds number
concentric rows of intermeshing teeth and other applications).
proprietary designs that achieve the tight tolerances (b) High-viscosity liquids/slurries/suspensions (low
needed to impart high mechanical and hydraulic Reynolds number applications).
shear.
& (a) Relatively small, high-speed impellers, for
& As the liquids or solid slurries are drawn from the
example, propellers, turbines, and paddles.
vessel into the rotor inlet, they are accelerated and & (b) Larger, lower speed impellers, for example,
expelled either radially or axially at high velocity,
anchor, helical ribbons, and so on.
depending on the design. The materials are subject to
. What are the essential differences between impellers
intense shear as they are forced through the stator
used for low-viscosity liquids and high-viscosity li-
slots and the narrow gap between the ends of the rotor
quids, with respect to their constructional features?
blades and the stator, through the perforations in the
stator. & Low-viscosity liquids:
& As fluid enters the center of the stator, pumping vanes ➢ Diameters of impellers are about one-fourth to half
on the rotor, which spin at 55 m/s, accelerate the of the vessel diameter.
product through grooves in the respective parts, butin ➢ Impellers are placed at about one-third of the
opposite directions, resulting in opposed flow colli- liquid height above the base.
sion that imparts tremendous shear forces upon the ➢ Baffles are used to prevent vortex formation.
product. ➢ Baffle width is about 1/10th of the vessel diameter.
& This provides much higher shear rates than the
& High-viscosity liquids:
normal mixers.
➢ Heavy duty impellers with diameters just less than
& In many cases, fluids are forced to decelerate from
vessel diameter. Small clearance between impeller
300 m/s or more to zero across the extremely narrow
and vessel wall.
gaps.
➢ No baffles.
. What are the applications of rotor–stator mixers?
➢ Use of helical coils for heat transfer is difficult and
& Homogenization, dispersion, emulsification, grind-
expensive.
ing, dissolving, chemical reaction, cell disruption,
➢ Jacketed vessels are used for such heat transfer
coagulation (due to shear), and so on.
applications.
& Used in the manufacture of latexes, adhesives, per-
. What type(s) of impeller(s) is recommended for solid
sonal care and cleaning products, dispersion of che-
suspensions involving low and high settling velocities?
micals, agricultural formulations, and so on.
& For settling velocities around 0.9 cm/s, solids sus-
& Used in the fluid viscosity ranges up to 150 Pa s
pension can be accomplished with turbine or propel-
(150,000 cP). Above these values, extruders are used.
ler impellers.
. What are high shear mixers? What are their
& For settling velocities above 4.5 cm/s, intense pro-
applications?
peller agitation is needed.
& High shear mixers are basically stator–rotor devices
. What is a fluid foil impeller? What is its advantage/
with a wide variety of designs. The rotor is equipped
application?
with blades or teeth on the inner and outer surfaces.
The stator is patterned with slots, holes, and other & Fluid foil impeller uses curvature of the blades
perforations. instead of usual turbine blades.
& High shear mixing is an energy-intensive process that & It has improved blending and solids suspension
combines immiscible liquids to produce emulsions characteristics.
and dispersions through the controlled formation and . How is mixing in a reactor increased?
integration of droplets. & Adding/improving baffles.
& These are also used to improve deagglomeration and & Installing a higher rpm motor on the mixer.
improve blending of solid ingredients into liquids.
& Using a different mixer blade design.
& Applications include manufacture of mayonnaise,
& Using multiple impellers.
toothpaste, lubricants, paints, salad dressing, phar-
& Pumped recirculation: addition and/or increasing the
maceuticals, and a wide variety of other products.
quantity.
. Name different types of impellers used in mixing
& Use of in-line static mixers for mixing entering
equipment for the following applications:
streams.