Page 203 - Fluid mechanics, heat transfer, and mass transfer

MIXING EQUIPMENT 181

mounted in the aeration basin. These are used in & The general principle is that mixing performance
wastewater treatment and increasing oxygen supply depends on direct contact between the mixing ele-
to reduce eutrophication of stationary ponds, bior- ment and the material.
eactors, and so on. & Flow inside the material is laminar, not turbulent.
. What are planetary or pan mixers? What are their & The material must be brought to the mixing elements
characteristic features and applications? or the mixing elements must travel to all parts of the
& Planetary mixers are used for a variety of liquid and vessel.
solid mixing applications, from simple mixtures to & Mixing occurs by kneading the material against the
sophisticated reactions, involving high temperature, vessel wall or against other material, folding un-
vacuum, or internal pressure. This type of mixer is mixed food or other material into the mixed part,
employed in batch operations. and shearing to stretch the material.
& During the mix cycle, two rectangular shaped . Name mixers used for pastes and viscous materials.
stirrer blades revolve around the tank on a central & Anchor impeller: The anchor impeller design is best
axis. Each blade revolves on its own axis simulta-
suited for mixing of high-viscosity fluids. The design
neously, at approximately the speed of the central
produces radial flow at low speeds. These types of
rotation.
impellers often incorporate wipers that remove ma-
& With each revolution on its own axis, the mixing
terial from the vessel walls during mixing, which
elements move in a planetary path, visiting all parts enhances heat transfer.
of the mixing pan, normally with only small clear- & Change-can mixers: The container is a separate unit
ance from the pan wall.
easily placed in or removed from the frame of the
& This movement provides homogeneity of the mate-
mixer.
rial being mixed and does not depend on the flow & Helical blade mixers.
characteristics of the mix. Instead, the stirrers cover
& Double arm kneading mixers: Consist of two
every point within the mix tank.
counterrotating blades in a rectangular trough
& Double planetary mixers have no packing glands or
curved at the bottom to form two longitudinal
bearings in the product zone, and hence, cleaning
half cylinders and a saddle section. Mixing action
between batches is minimized.
is a combination of bulk movement, smearing,
& In a rotating pan mixer, the mixing vessel is mounted stretching, folding, dividing, and recombining as the
on a rotating turntable. The mixing elements rotate in material is pulled and squeezed against blades. Dif-
a fixed position near the pan wall. ferent blade designs are used for specific applica-
& Various designs of mixing elements may be used. tions. Sigma blade is most widely used for mixing
& The mixer is equipped with a hydraulic lift that foods, epoxy resins, adhesives, and other plastic
permits the stirrer blades to be lowered and raised mass.
in and out of the mixing tank. & Figure 6.12 illustrates common designs of double
& The mixer is capable of handling low-viscosity fluids arm kneader mixers.
to very high-viscosity pastes and dough-like materi- & Screw mixers and extruders: Extensively used in
als. Typical capacities of these units range from 38 to plastics industry for uniform mixing of various ad-
more than 1200 L (10 to more than 300 gallons). ditives, stabilizers, fillers, and so on.
. What is the principle of operation of continuous paste & Mixing rolls.
mixers? & Muller mixer.
& A common principle is to force material through & Banbury mixer: Used for mixing rubbers and plas-
obstructions such as perforated plates, meshes, grids, tics. It is a high-intensity mixer with power input up
and so on, by means of screw conveyors. The material to 6000 kW/m . Speeds are up to 40 rpm.
3
is kneaded and sheared between the screws and the . What are the features and good points of twin shaft
walls of a trough and further acted on mechanically ribbon and paddle mixers?
by being forced through or past obstructions.
& These are most commonly used continuous mixers,
. What are the objectives involved in mixing of pastes and
being highly flexible and capable of high output rates.
high-viscosity materials?
& Twin overlapping screws rotating down at the outer
& Uniform mixing may not be the only objective; for
walls bring the material into a compression triangle
example, mechanical action is required for dough
in the center of the casing. This confined region
development.
allows relatively high shear loads to develop at high