Page 330 - Chemical Process Equipment - Selection and Design

294 MIXING AND AGITATION
TABLE 10.2. Agitation Results Corresponding to Specific Superficial Velocities
ft/sec Description ft/sec Description
Liquid Systems c. suspend all solids with the design settling velocity
0.1-0.2 low degree of agitation; a velocity of 0.2 ft/sec will completely off the bottom of the vessel
a. blend miscible liquids to uniformity when specific d. provide slurry uniformity to at least one-third of the
gravity differences are less than 0.1 liquid level
b. blend miscible liquids to uniformity if the ratio of e. be suitable for slurry drawoff at low exit nozzle
viscosities is less than 100 locations
c. establish liquid movement throughout the vessel 0.6-0.8 when uniform solids distribution must be approached; a
d. produce a flat but moving surface velocity of 0.6 ft/sec will
0.3-0.6 characteristic of most agitation used in chemical f. provide uniform distribution to within 95% of liquid
processing; a velocity of 0.6ft/sec will level
e. blend miscible liquids to uniformity if the specific g. be suitable for slurry drawoff up to 80% of liquid
gravity differences are less than 0.6 level
f. blend miscible liquids to uniformity if the ratio of 0.9-1 .o when the maximum feasible uniformity is needed. A
viscosities is less than 10,000 velocity of 0.9 ft/sec will
g. suspend trace solids (less than 2%) with settling h. provide slurry uniformity to 98% of the liquid level
rates of 2-4ft/min i. be suitable for slurry drawoff by means of overflow
h. produce surface rippling at low viscosities
0.7-1.0 high degree of agitation; a velocity of l.Oft/sec will Gas Dispersion
i. blend miscible liquids to uniformity if the specific 0.1-0.2 used when degree of dispersion is not critical to the
gravity differences are less than 1.0 process; a velocity of 0.2 ft/sec will
j. blend miscible liquids to uniformity if the ratio of a. provide nonflooded impeller conditions for coarse
viscosities is less than 100,000 dispersion
k. suspend trace solids (less than 2%) with settling b. be typical of situations that are not mass transfer
rates of 4-6 ft/min limited
I. produce surging surface at low viscosities 0.3-0.5 used where moderate degree of dispersion is needed; a
velocity of 0.5 ft/sec will
Solids Suspension c. drive fine bubbles completely to the wall of the
0.1-0.2 minimal solids suspension; a velocity of 0.1 ft/sec will vessel
a. produce motion of all solids with the design settling d. provide recirculation of dispersed bubbles back into
velocity the impeller
b. move fillets of solids on the tank bottom and 0.6-1.0 used where rapid mass transfer is needed; a velocity of
suspend them intermittently 1 .O ft/sec will
0.3-0.5 characteristic of most applications of solids suspension e. maximize interfacial area and recirculation of
and dissolution; a velocity of 0.3 ft/sec will dispersed bubbles through the impeller

[Chemineer, Co. Staff, Chem. Eng., 102-110 (26 April 1976); 144-150 (24 May 1976); 141-148 (19 July 1976)l.

Na &e Na
EXAMPLE 10.2 d/D d N [Eq. (111 [Eq.(2)1 (Fig. 10.7) Np P(HP)
Effects of the Ratios of Impeller and Tank Diameters
Power and rpm requirements will be investigated and compared 0.25 28.4 300 0.637 518 0.64 1.4 45.9
with the data of Table 10.3. The superficial velocity is 0.6ft/sec, 0.33 37.5 145 0.573 436 0.57 1.45 21.5
V = 5000 gals, Sp Gr = 1.0. Viscosities of 100 CP and 5000 CP will be 0.50 56.8 52 0.460 359 0.45 1.5 8.2
considered. With p = 100 cP, turbulence is fully developed.
With h/D = 1, D = h = 9.47 ft, Na
d/D d N 4 &, (Fig. 10.7) Np P
pumping rate Q = 0.6(~/4)(9.47)’= 42.23 cfs, 0.25 28.4 228 0.839 18,990 0.84 1.3 18.7
N, = l.037(10s)Q/Nd3 = 4.3793/Nd3 (1) 0.33 37.5 112 0.742 16,850 0.74 1.3 8.9
NRe = 10.7NdzS/p = 0.00214Nd2, p = 5000, (2) 0.50 56.8 40 0.597 13,800 0.60 1.3 3.2
P = NpN3dsS/l. 523( 1013), (3) Table 10.3 gives these combinations of HP/rpm as suitable: 25/125,
20/100, 10/56, 7.5137. The combination 10156 checks roughly the
N, from Figure 10.6. last entry at 5000cP. Table 10.3 also has data for viscosities of
For several choices of d/D, solve Eqs. (1) and (2) 25,000 cP, thus allowing for interpolation and possibly extra-
simultaneously with Figure 10.7. With p = 5000 cP; polation.