Page 336 - Chemical Process Equipment - Selection and Design

300 MIXING AND AGITATION

.
a"
a

Q x 102
NDI
X = ( P / V)( d/0)'.85

(3 Figure 10.12. Relation between power input, P/V HP/1000 gal,
superficial liquid velocity u,ft/sec, ratio of impeller and tank
diameters, d/D, and superficial gas velocity us ft/sec. [Hicks and
Gates, Chem. Eng., 141-148 (19 July 1976)l.
% have its own feed nozzle, as in Figure 10.13(b), but usually the
*

0.0 streams may be combined externally near the blender and then
a One manufacturer gives these power ratings:
given the works, as in Figure 10.13(a).

Tanksize (gal)
Motor HP 1 5 10 30
2
3
1
0.5
0.1
Another ties in the line and motor sizes:
Line size, (in.) 1-4 6-8 10-12
0. a Motor HP 0.5 1 2
But above viscosities of 1OcP a body one size larger than the line
size is recommended.
Other devices utilize the energy of the flowing fluid to do the
mixing. They are inserts to the pipeline that force continual changes
0.3
0.02 0.04 0.06 0.08 of direction and mixing. Loading a section of piping with tower
(+) packing is an example but special assemblies of greater convenience
have been developed, some of which are shown in Figure 10.14. In
(b) each case manufacturer's literature recommends the sizes and
pressure drops needed for particular services.
Figure 10.11. Power consumption. (a) Ratio of power consumptions The Kenics mixer, Figure 10.14(a), for example, consists of a
of aerated and unaerated liquids. Q is the volumetric rate of the succession of helical elements twisted alternately in opposite
gas: (0) glycol; ( X ) ethanol; (V) water. [After Calderbank, Trans. directions. In laminar flow for instance, the flow is split in two at
Inst. Chem. Eng. 36, 443 (1958)l. (b) Ratio of power consumptions each element so that after n elements the number of striations
of aerated and unaerated liquids at low values of Q/Nd3. Six-bladed becomes 2". The effect of this geometrical progression is illustrated
disk turbine: (0) water; (a) methanol (10%); (A) ethylene glycol in Figure 10.14(b) and points out how effective the mixing becomes
(8%); (A) glycerol (40%); P, = gassed power input; P = ungassed after only a few elements. The Reynolds number in a corresponding
power input; Q = gas flow rate; N = agitator speed; d = agitator- empty pipe is the major discriminant for the size of mixer, one
impeller diameter. [Luong and Volesky, AIChE J. 25, 893 (1979)l.
manufacturer's recommendations being
4, Number of Elements
10.7. IN-LINE BLENDERS AND MIXERS Less than 10 24
10-2000 12-18
When long residence time is not needed for chemical reaction or More than 2000 6
other purposes, small highly powered tank mixers may be suitable,
with energy inputs measured in HP/gal rather than HP/1000 gal. Besides liquid blending applications, static mixers have been
They bring together several streams continuously for a short contact used for mixing gases, pH control, dispersion of gases into liquids,
time (at most a second or two) and may be used whenever the and dispersion of dyes and solids in viscous liquids. They have the
effluent remains naturally blended for a sufficiently long time, that advantages of small size, ease of operation, and relatively low cost.
is, when a true solution is formed or a stable emulsion-like mixture. The strong mixing effect enhances the rate of heat transfer from
When it is essential that the mixing be immediate each stream will viscous streams. Complete heat exchangers are built with such