Page 215 - Fluid mechanics, heat transfer, and mass transfer

MIXING EQUIPMENT 193

. Summarize selection of solids mixing equipment for
different applications.
& Table 6.11 gives a summary of mixing action and
applications for different types of mixers.
. What is the minimum agitator speed to keep solids
under suspension in a vessel? What other considerations
influence the settling process?
& The most common method for estimating the
minimum suspension condition is the following
Zwietering correlation for the minimum agitator
speed:

a 0:2 0:1
N min ¼CðD T =D A Þ d m ðgDrÞ 0:45 W 0:13 =r 0:55 D 0:85 ;
p l s l A
ð6:13Þ
FIGURE 6.21 Viscosity ranges for different types of mixers.
where D A is the impeller diameter (m), D T is the
tank diameter (m), d p is the particle diameter (m),
W s is the ratio of weight of solids to weight of
work well and quickly with all sorts of mixtures, 3
liquid (%), r l is the liquid density (kg/m ), Dr is
including particles of dissimilar size, shape, and
the density difference between solids and liquid
specific gravity. And unlike mechanical blending, 3
(kg/m ), m l is the liquid viscosity (Pa s), g is the
sonic devices are more efficient with dense powders 2
gravitational constant (m/s ), and C and a depend
than with fluffy materials.
on the characteristics of the agitator. For propeller,
& The technology has its limitations, the most obvious
turbine, and flat blade agitators, C and a are 1.4
being a capacity limit of 200 L. and 1.5, respectively.
& Sonic blending also does not work well with moist-
& Dished tank bottoms and baffles help prevent set-
ened blends, probably because of sonic damping by tling in cylindrical tanks. Baffles impart a high
water.
vertical component of velocity and help eliminate
. Summarize, graphically, selection of mixers based on dead zones. If baffles are undesirable due to slime
viscosity ranges. buildup or other reasons, the agitator should be
& Figure 6.21 gives a graphical representation of offset and mounted at an angle to reduce swirl and

viscosity ranges of materials for different mixing increase vertical motion (10–15 from vertical þ
equipment. offset).

TABLE 6.11 Types of Mixers and Their Mixing Action and Applications
Type of Mixer Mixing Action Applications
Rotating: cone, double Tumbling action Blending dry, free-flowing powders, granules, and crystals. For
cone, drum example, pharmaceuticals, food, and chemicals
Air blast fluidization Air blast lifts and mixes particles Dry powders and granules. For example, milk powder, detergents,
and chemicals
Horizontal trough mixer Rotating element produces contraflow Dry and moist powders. For example, tablet granulation, food,
with ribbon blades, movement of materials chemicals, and pigments
paddles, or beaters
Z-blade mixers Shearing and kneading by the specially Mixing heavy pastes, creams, and doughs. For example, bakeries,
shaped blades rubber doughs, and plastic dispersions
Pan mixers Vertical, rotating paddles, often with Mixing, whipping, and kneading of materials ranging from low-
planetary motion viscosity pastes to stiff doughs. For example, food, pharma-
ceuticals, chemicals, printing inks, and ceramics
Cylinder mixers, Shearing and kneading action Compounding of rubbers and plastics. For example, rubbers,
single and double plastics, and pigment dispersions
Jones R L. Mixing equipment for powders and pastes. Chemical Engineering (London) 1985;419:41.