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6
MIXING
6.1 Mixing 163 & The efficiency of mixing depends on the efficient use
6.2 Mixing Equipment 169 of energy to generate flow of the components.
6.2.1 Baffled Vessels 169 & Mixing reduces concentration and temperature
6.2.2 Impellers 172
gradients in a mixture, thus exerting a favorable
6.2.3 Turbine and Propeller Mixers 173
effect on the overall rates of mass and heat transfer.
6.2.4 Paddle Mixers 176
& Important aspects in the design of a mixer include the
6.2.5 Draft Tubes 177
6.2.6 Other Types of Mixers 177 following:
6.2.6.1 Static Mixers 183 ➢ Provision of adequate input energy (for an appro-
6.2.7 Solids Mixing 186 priate time).
6.2.8 Mixer Seals 194
➢ Design of the mechanism for introducing the
energy.
➢ Properties of the components.
. How is mixing achieved? What are its objectives? What
are its applications?
& Mixing processes use mechanical means to forceful-
6.1 MIXING
ly circulate materials of separate phases to yield
. What is mixing? What are its objectives and a random distribution of the components.
characteristics? & Mixing objectives can encompass blending, gas
& Mixing is a unit operation in which a relatively induction, homogenization, particle reduction, emul-
uniform mixture is obtained from two or more com- sification, and so on. A homogeneous mixture is
ponents. In some cases, mixingis requiredto promote one in which the separate molecules being combined
mass transfer or chemical reaction. are interspersed. A heterogeneous mixture contains
a random dispersion of materials having distinguish-
& Typically, the objectives include creating a suspen-
sion of solid particles, blending miscible liquids, able phases.
dispersing gases through liquids, blending or & Mixing applications involve contacting and interfac-
dispersing immiscible liquids in each other, and ing of a variety of substances, including gas–gas,
promoting heat transfer. liquid–liquid, solid–solid, gas–liquid, solid–liquid,
& The degree of uniformity achievable varies widely. It and gas–paste.
is easy to achieve virtually complete homogeneity . What is the difference between mixing and agitation?
when mixing miscible liquids or mixing soluble & Mixing refers to any operation used to change a
solids into a liquid, but it can be difficult to achieve nonuniform system into a uniform one (the random
a homogeneous result when mixing two solids, distribution of two or more initially separated
mixing two highly viscous liquids, or mixing items phases).
with widely varying densities, especially if the & Agitation implies forcing a fluid by mechanical
amount of one component is very small compared means to flow in a circulatory or other pattern inside
to the amounts of the others. a vessel.
Fluid Mechanics, Heat Transfer, and Mass Transfer: Chemical Engineering Practice, By K. S. N. Raju
Copyright Ó 2011 John Wiley & Sons, Inc.
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