Page 332 - Chemical Process Equipment - Selection and Design

296 MIXING AND AGlTATiON
TABLE 10.4. Suspension of Solids; Power and Impeller Speed (hp/rpm)
for Two Settling Velocities, as a Function of the Superficial
Velocity of the Liquid; Pitched Blade Turbine Impeller

Volume (gal)
10ft/min 25ft/min
ft/sec 1000 2000 5000 1000 2000 5000
0.1 11190 211 90 51125 21190 211 25 51125
l/lOO 3/84 11190 2/84 3/84
3/68 1/100 1.5184 3/68
2/45 1.5156 2/45
0.2 1/100 21125 7.51125 211 25 3/84 1511 55
1.5184 51100 1011 00
5/84 7.5168
3/56 5/45
0.3 21190 2/84 3/37 1.5184 51125 10184
1.5156 3/68
2/45
0.4 21155 511 55 7.5184 2 /84 7.51155 7.5145
1.51100 5/56 1.5156 51100
3/56
0.5 1.5184 3/84 1511 55 2/68 7.51125 15/84
211 25 1011 00 2/56 5/84 10156
7.5168 7.5137
5/45
0.6 21100 51125 10184 3/84 5/56 2511 25
1.5168 3/68 2011 00
3/56 15/68
2/45 10145
0.7 2/84 7.51155 15/84 7.51155 151155 301100
1.5156 7.51125 10156 51125 10/100 25/84
5/84 7.5/45 51100 7.5184 20168
7.5137 3/68 7.5168 15/56
0.8 3/84 7.5184 2511 25 7.51125 10184 6011 55
5/56 2011 00 5/84 4011 00
15/68 30168
10145 25/56
0.9 7.51155 151155 4011 55 10/125 15/84 751190
51125 10/100 3011 00 7.51100 601125
51100 7.5168 25/84 5011 00
3/68 20168 40184
1 .o 7.51125 2011 00 50/1 00 1511 55 30/155 751125
5/84 15/84 40184 1011 00 251125 751100
10184 30168 20/100 60184
25/56 50184
[Gates. Morton, and Fondy, Chem. Eng., 144-150 (24 May 1976)].

10.2-10.5 are one such process; the one developed by Oldshue of horsepower and ratio of impeller and vessel diameters will do the
(1983) will be examined shortly. required task. Example 10.3 employs this method, and makes a
Suspension of solids is maintained by upward movement of the comparison with the Chemineer method of Tables 10.2 and 10.3.
liquid. In principle, use of a draft tube and an axial flow impeller
will accomplish this flow pattern most readily. It turns out,
however, that such arrangements are suitable only for low solids 10.6. GAS DISPERSION
contents and moderate power levels. In order to be effective, the
cross section of the draft tube must be appreciably smaller than that Gases are dispersed in liquids usually to facilitate mass transfer
of the vessel, so that the solids concentration in the draft tube may between the phases or mass transfer to be followed by chemical
become impractically high. The usually practical arrangement for reaction. In some situations gases are dispersed adequately with
solids suspension employs a pitched blade turbine which gives both spargers or porous distributors, but the main concern here is with
axial and radial flow. the more intense effects achievable with impeller driven agitators.
For a given tank size, the ultimate design objective is the
relation between power input and impeller size at a specified SPARGERS
uniformity. The factors governing such information are the slurry
volume, the slurry level, and the required uniformity. The method Mixing of liquids and suspension of solids may be accomplished by
of Oldshue has corrections for these factors, as Fl, F,, and F3. bubbling with an inert gas introduced uniformly at the bottom of
When multiplied together, they make up the factor F4 which is the the tank. For mild agitation a superficial gas velocity of 1 ft/min is
ordinate of Figure 10.8(d) and which determines what combinations used, and for severe, one of about 4 ft/min.