Page 211 - Fluid mechanics, heat transfer, and mass transfer

MIXING EQUIPMENT 189

➢ This process leads to highly flexible production & Tube mixers utilize open pipes within a bin. The
and has been popular in the pharmaceutical, food, pipes have notches in them to allow pellet or granular
and powdered metal industries. material to partially flow in and out of the tubes over
➢ The greatest benefit of in-built mixing technology the height of the bin, or for reintroduction into a lower
is the elimination of a transfer step from a mixer to portion of the bin, such as in a mixing chamber.
a container, by which segregation through various & These mixers can handle much larger volumes of
mechanisms can result. material than tumbler or convective mixers, since no
➢ No cleaning between batches is required and the free board space is required and their technology can
mix is stored in a sealed container until use. be applied to storage bins or silos.
➢ Optimum in-bin tumbler mixers incorporate mass & Another type of hopper mixer, called a planetary or
flow technology, where all of the material is in conical screw mixer, is commonly used for cohesive
motion whenever any is discharged, to ensure that powder mixing.
the mix does not segregate during container & The planetary screw is composed of a screw con-
discharge. veyor inside a conical hopper. The screw is located
. What are convection mixers? What are their operational so that one end is near the apex of the cone and the
characteristics? other end is near the top of the hopper, with the tip
of the flights near the wall of the hopper. The screw
& Convection Mixers: These mixers use a fixed
rotates while revolving around the walls of the
U-shaped or cylindrical shell with an internal rotat-
hopper, pulling material up from the bottom.
ing element (impeller) such as a ribbon, paddle, or
& It has the ability to handle a wide range of materials,
plow. Due to the action of the impeller, the particles
from free flowing to cohesive.
are moved rapidly from one location to another
within the bulk of the mixture. & Potential disadvantages include sifting segregation
within the mixer, possible segregation during mix
➢ The mixing action can range from relatively gentle
discharge, and a dead region at the bottom of the cone
to aggressive, depending on the agitator design
during mixing.
and speed and the use of intensifiers, which are
known as choppers. & These mixers are commonly jacketed for heating or
& These mixers work well with cohesive materials, cooling of a material during the mixing cycle.
which normally need substantially longer mixing & The Nauta-type mixer can also be fitted with
times in tumbler mixers. a vertically oriented ribbon mixer, although there
are limitations on its capacity, given the high level of
& They require less headroom, allowing liquid addition
operating torque and horsepower.
and heating or cooling and potential for continuous
. What are the characteristics of fluid bed mixers?
operation instead of only batch mixing as with tum-
bler mixers. & Effective for materials that will fluidize and those
with similar settling characteristics.
& These mixers are less likely to experience mix seg-
regation during discharge, since the impellers typi- & These mixers use high flow rates of gas to fully
cally operate during this process. fluidize powders in order to rapidly mix components.
The gas can also be used to process (e.g., heat or cool)
& Total capacity for these mixers generally ranges from
3
3
28 to 28 10 L (1–1000 ft ), and working capacity the mix.
is normally 50–70% of the total capacity. & Particulates should be fine, free-flowing powders that
have a narrow size distribution and are close in
. What are hopper mixers?
particle density.
& Hopper mixers are usually cone-in-cone to tube-type
& Fluidized mixers combine fluidization and convec-
units, where particles flow under the influence of
tive features, yielding rapid mixing times with a high
gravity in a contact bed fashion without moving
degree of blend uniformity.
parts, making them attractive for highly abrasive
bulk materials, given their wear potential. & Added jets of air that produce spouting are said to be
effective in decreasing the time required to achieve
➢ With the former unit, the inner cone produces a
good mixing.
pronounced faster flow through the inner hopper
compared to the outer annulus section, thereby & Very rapid: 1–2 min compared to around 15 min for a
allowing moderate mixing of material. tumbler mixer.
& These hopperstypically require two to fourpasseswith & This type of mixer consists of twin counterrotating
a recirculation system to achieve proper uniformity. paddled agitators that mechanically fluidize the