Page 190 - Fluid mechanics, heat transfer, and mass transfer

MIXING
168
& Intense agitation results from velocities of TABLE 6.4 Agitation Achievable for Different Fluid
0.21–0.30 m/s (0.70–1.0 ft/s). Velocities
. Describe agitation achievable in relation to fluid veloc- Agitation Results Corresponding to Specific Superficial Velocities
ities for different fluid systems.
& Table 6.4 givesagitation achievable for different fluid Superficial
velocities. Velocity (cm/s) Description
. Give approximate ranges of power requirements for Liquid systems
different applications for baffled agitated tanks. 3–6
& Table 6.5 gives power requirements for baffled ves-
sels for different applications.
. “In a mixer, high pumping capacity is obtained by using
large diameter impellers at slow speeds compared to
higher shear rates obtained by using smaller impellers 9–18
and higher speeds.” True/False?
& True. The pumping capacity of mixing impellers
3
is proportional to ND , as indicated by the relation-
ship
3
Q / ND ; ð6:12Þ
where Q is the pumping capacity, N is the impeller 21–30
speed, and D is the impeller diameter.
. “Power requirements for mixing a gas with a liquid are
more than those for mixing the liquid alone.” True/
False?
& False. Power requirements for gas–liquid systems
can be 25–50% less, depending on gas/liquid ratios, Solids suspension
than those for liquid systems alone. 3–6
. What are the ranges of relative power requirements for
mixing a gas with a liquid and the liquid alone?
& Power to mix a gas and a liquid can be 25–50% less
than the power to mix the liquid alone.
9–15
. What are the disadvantages of vortex formation in
mixing operations?
& Once vortex reaches impeller, severe air entrainment
may occur.
& In addition to air entrainment, swirling mass of liquid
may generate an oscillating surge in the tank, which,
coupled with the deep vortex, may create a large Gas dispersions
fluctuating force acting on the shaft. 3–6
. What are the undesirable effects of vortex shedding?
& Severe vibration.
& Mechanical failure of cylindrical elements such as
suspended piping, transmission lines, heat exchanger 9–15
tubes, columns, stacks, and so on.
. How are vortices broken/eliminated?
& By the use of baffled tanks and vortex breakers. For
axial flow impellers, the effect of full baffling can be
18–30
achieved in an unbaffled vessel with an off-center and
angled impeller shaft location. Off-center angled
shaft eliminates vortexing and swirling to a large
extent.
Low degree of agitation; a velocity of 6 cm/s
will blend miscible liquids to uniformity
when Dr < 0.1; blend miscible liquids to
uniformity if m 1 /m 2 < 100; establish liq-
uid movement throughout the vessel;
produce a flat but moving surface
Characteristic of most agitations used in
chemical processing; a velocity of 18 cm/
s will blend miscible liquids to uniformity
if Dr < 0.6; blend miscible liquids to
uniformity if m 1 /m 2 < 10,000; suspend
trace solids (<2%) with settling rates of
0.60–1.2 m/min; produce surface rip-
pling at low viscosities
High degree of agitation; a velocity of
30 cm/s velocity will blend miscible li-
quids to uniformity if Dr < 1.0; blend
miscible liquids to uniformity if m 1 /m 2
< 100,000; suspend trace solids (<2%)
with settling rates of 1.2–1.8 m/min;
produce surging surface at low m
Minimal solids suspension; a velocity of
3 cm/s will produce motion of all solids
with the design settling velocity; move
fillets of solids on the tank bottom and
suspend them intermittently
Characteristic of most applications of solids
suspension and dissolution; a velocity of
9 cm/s will suspend all solids with the
design settling velocity completely off
the bottom of the vessel; provide slurry
uniformity to at least one-third of the
liquid level; be suitable for slurry draw-
off at low exit nozzle locations
Usedwhendegree of dispersion is not critical
to the process; a velocity of 6 cm/s will
provide nonflooded impeller conditions
for coarse dispersion; be typical of situa-
tions that are not mass transfer limited
Used where moderate degree of dispersion
is needed; a velocity of 15 m/s will drive
fine bubbles completely to the wall of the
vessel; provide recirculation of dispersed
bubbles back into the impeller
Used where rapid mass transfer is needed; a
velocity of 30 cm/s will maximize inter-
facial area and recirculation of dispersed
bubbles through the impeller