Page 214 - Fluid mechanics, heat transfer, and mass transfer

MIXING
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Differences in these properties can also cause un- ➢ Segregating mixers have mainly diffusive me-
mixing or segregation during mixing or mechanical chanisms, encouraging the movement of indi-
jiggling of the mixture. vidual particles, making segregation more sig-
nificant. Non-impeller mixers tend to be of this
& Experience shows that materials with a size greater
type.
than 75 mm will segregate readily during mechanical
jiggling of the mixture, but those below 10 mm will ➢ Less segregating mixers have mainly convective
not segregate appreciably. mixing mechanisms. These are typically impeller
types in which blades, screws, ploughs, and so on
& Means of overcoming segregation and poor mixing
sweep groups of particles through the mixing
include the following:
zone.
➢ Comminution to smaller sizes.
. Illustrate the working of a solids static mixer for solids
➢ Use of powders with a narrow size distribution.
mixing.
➢ Use of the same volume-average diameter for all
& Figure 6.20 illustrates a static mixer for solids
components.
mixing.
➢ Granulation.
& A solids static mixer employs gravity as the
➢ Coating processes.
driving force for promoting the flow and mixing
➢ Controlled continuous mixing.
process. It is characterized by multiple element
. What is the criterion for selection of mixers for solids? separations and recollation of the flow stream. As
& Selection of mixers must take into account any the mechanics of the process is limited by the free-
tendency toward segregation. flowing behavior of the bulk material, the opera-
tion is generally gently repetitive, and hence more
& This may be evaluated from a heap test, in which a
suitable for handling delicate products than a
well-mixed material is poured through a funnel to
mechanical mixer that normally exerts high local
form a heap.
and individual forces on the constituent particles
& If the composition of samples taken from the
and bulk regions.
outside varies significantly from compositions of
. What is acoustic/sonic mixing?
samples taken from the center of the heap, the
material is likely to segregate during mixing or & Sonic waves cause powders to fluidize, turn over, and
later processing. mix rapidly. Sonic mixers use acoustic fluidization.
Acoustic fluidization creates bubbling through the
. How are solids mixers classified with respect to
powder and a high-intensity mixing zone. Mixing
segregation?
chamber shape helps to focus sonic energy into
& Mixers can be classed into two groups with respect to
mixing regions. Sonic mixing/blending is said to
segregation:

FIGURE 6.20 Static mixer.