Page 189 - Fluid mechanics, heat transfer, and mass transfer

MIXING 167

. Define and explain agitation intensity number. TABLE 6.3 Mixing Intensity Versus Impeller Tip Speed
& Tank turnover rate has been used as a criterion for Tip Speed
agitation intensity.
& Agitation intensity is directly proportional to the rate Intensity N l (m/min) (ft/min)
at which tank contents are turned over. Low 5.3–6.8 150–200 500–650
& Agitation intensity is assumed to vary linearly with Medium 6.8–8.4 200–245 650–800
tank fluid velocities. Very high 8.4–11.6 245–335 800–1100
& Range of agitation intensity (scale) is defined by bulk
fluid velocities, V b , ranging from 1.8 to 18 m/min & For settling velocities above 4.5 cm/s, intense pro-
(6–60 ft/min). peller agitation is needed.
& A 1–10 scale is established for the above range. . Define impeller blend number.
& Agitation intensity number & Impeller blend number is used to predict the blend
time in a mixing system:
N l ¼ V b =ð6ft=minÞ: ð6:8Þ
2:3
N B ¼ nuðD=TÞ ; ð6:9Þ
V b ¼ q/A, where q is the capacity at volumetric flow
rate and A is the cross-sectional area of the vessel. where n is the rotational shaft speed (rpm) and u is the
. Give examples of applications of flow-sensitive mixing blend time (s).
systems in process industry, indicating the ranges of . Define impeller force number.
agitation scales for each of these applications. & Impeller force number, N F , correlates the axial force,
& Table 6.2 classifies mixing in terms of mixing inten- F ax , or the thrust generated by an impeller. F ax is
sity along with applications in industry. used in the correlation to predict cavern dimensions
. How is mixing intensity related to impeller tip speeds? and is also important for mechanical design
considerations.
& Table 6.3 gives the relationship between mixing
2
4
N F ¼ F ax =rn D : ð6:10Þ
intensity and impeller tip speed.
. Illustrate how solids settling velocities and intensity of & Cavern is used to describe the well-mixed, turbulent
agitation are related. region around the impeller. Equations are developed
& For settling velocities around 0.9 cm/s, solids sus- to predict cavern diameters.
pension can be accomplished with turbine or propel- . Give an equation for blend time to achieve the desired
ler impellers. concentration?
& Grenville gives the blend time to achieve within 5%
of the desired concentration as
TABLE 6.2 Examples of Mixing Processes Based on Mixing
2 1=3
ðblend timeÞ 95% ¼ 5:4ðT=DÞ =N p ð6:11Þ
Intensity Scales N:
Agitation
& This correlation is claimed to be valid for turbulent
Applications
Intensity, N i
conditions with N Re > 10,000 and a liquid depth
Mild, 1 Noncritical blending operations; blending equal to the vessel diameter, T. D is the impeller
to prevent concentration surges; storage diameter, N p is the power number, and N is rpm.
or holding tanks; feed tanks; equalization
basins; water treatment; flocculation; and . While mixing low to moderate viscosity fluids, how
so on (surface barely in motion) turbulence and vortex formation are to be handled?
Moderate, 2–3 Make up tanks; reaction tanks; blend tanks; & Turbulence should be induced to entrain slow
pigment suspension (paint); maintaining moving parts within faster moving parts. Turbulence
suspension (surface in strong motion)
is highest near the impeller and liquid should be
Vigorous, 3–8 Critical mixing operations; most heat
circulated through this region as much as possible.
transfer; pH control; reactors; blend tank
& Avortex should be avoided because adjoining layers
for adhesives
Violent, 6–10 Special critical applications; high shear of circulating liquids travel at a similar speed and
requirements; critical heat transfer; entrainment does not take place—the liquids simply
emulsion polymerization; monomer rotate around in the mixer.
emulsions in water with stabilizers; bulk . What are the superficial fluid velocity ranges in a mixer
polymerization; polymer in molten form for mild and intense mixing?
or solution in monomer; reactors; surface
& Mild agitation results from superficial fluid velocities
boiling; splashing; vortexing
of 0.03–0.06 m/s (0.10–0.20 ft/s).