Page 186 - Fluid mechanics, heat transfer, and mass transfer

MIXING
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& The words mixing and agitation are often used center vertical axis, all turbulent flow will be
interchangeably. tangential.
. What are macro- and mesomixing? & Axial flow impellers generate flow parallel to the axis
& Mixing at nanometer level involves high shear stres- (or shaft), either upward or downward.
ses and turbulence. The energy dissipation rate de- & Radial flow impellers create flow perpendicular to the
termines whether mixing processes are at macro, axis or in a radial direction.
meso, or micro levels. The overall mixing process & Some impellers, such as pitched blade turbine, create
occurs within a flow field continuum that covers a a mixed flow pattern, that is, partially axial and
wide range of time- and length scales that are indic- partially radial.
ative of these mixing scales. . What are primary and secondary flows in a mixing
& For two miscible liquids, the initial large-scale dis-
process?
tribution by flow patterns that cause gross dispersion & The primary flow is directly induced by the rotation
is considered as macromixing.
of the impeller. The secondary (or entrained) flows
& The breakdown of large eddies into smaller ones via
are due to the flowing liquid dragging adjacent liquid
an eddy cascade is termed mesomixing. This is and entraining it.
followed by fluid engulfment in small eddies and & Secondary flows allow the entire contents of a vessel
subsequent laminar stretching of these eddies, with
to circulate even when a small impeller is inducing
molecular diffusion as the final mechanism to obtain
the primary flow. The secondary flow component is
uniform composition; this process is termed as mi-
smaller relative to primary flow for larger impellers.
cromixing. The length scale for this final diffusion
. What are the factors that influence efficiency of mixing
process is determined by the size of the smaller
eddies formed, which is referred to as Kolmogorov processes?
& Design of impeller, diameter of impeller, speed, and
length scale. These hydrodynamic processes are
baffles.
important to understand nanoscale phenomena.
. What are the variables that influence power require-
. Classify process criteria for mixing processes.
ments of an agitator?
& Physical Processing: suspension, dispersion, emul-
& Fluid circulation rate (pumping capacity) and fluid
sion, blending, pumping, and so on.
shear rate.
& Application Processing: liquid–solid, gas–liquid,
& The pumping capacity of an impeller is defined as the
immiscible liquids, miscible liquids, and so on.
volumetric flow rate normal to the impeller discharge
& Chemical Processing: dissolving, absorption, extrac-
area.
tion, reactions, heat transfer, and so on.
& The fluid shear rate is based on the velocity of the
. Give examples of common processes that require
fluid leaving the tips of the impeller.
agitation.
& A large impeller running at slow speed has a high
& Blending of two miscible or immiscible liquids, such
pumping capacity and a low fluid shear rate, while a
as hydrogen peroxide and water.
small impeller running at high speed has a high shear
& Dissolving solids in liquids, such as salt in water.
rate and a low pumping capacity. This means the
& Dispersing a gas in a liquid as fine bubbles, such as
power input may be distributed in different ways by
oxygen in a suspension of microorganisms for waste selecting the appropriate ratio of impeller to tank
treatment. size.
& Dispersion of droplets of one immiscible liquid in . What are the characteristic features of batch and con-
another in liquid–liquid extraction or in heteroge- tinuous mixing processes?
neous reaction processes. & The mixers commonly used in batch processes (often
& Suspension of fine solid particles, such as metallic
referred to as dynamic mixers) are freestanding
pigments in car paint. machines in which the ingredients are loaded, agi-
& Agitation of a fluid to eliminate temperature gradi- tated, and then discharged, typically into holding
ents arising from poor heat transfer or to increase heat bins, smaller containers, or packaging equipment.
transfer between the fluid and a heating element in & In general, batch mixing processes are energy inten-
the vessel. sive. Energy is imparted to an impeller via an electric
. What are the basic flow patterns in mixing processes? motor.
& Tangential, axial, and radial flows. For a cylindrical & The mixers associated with continuous processes are
vessel without baffles and any type of impeller on the generally integral parts of large production lines and