Page 327 - Chemical Process Equipment - Selection and Design

10.3. CHARACTERIZATION OF MIXING QUALITY 2

1
a b c d
Figure 10.4. Agitator flow patterns. (a) Axial or radial impellers without baffles produce vortexes. (b) Offcenter location reduces the vortex.
(c) Axial impeller with baffles. (d) Radial impeller with baffles.

NFr = 7.454(1W4)N2d, Froude number, (10.5) 100
d = impeller diameter (in.),
D =vessel diameter (in.),
N = rpm of impeller shaft, 10
P = horsepower input,
Q = volumetric pumping rate (cuftfsec),
S = specific gravity,
tb = blend time (Hnin), 1
p = viscosity (cP).
The Froude number is pertinent when gravitational effects are 0.10
significant, as in vortex formation; in baffled tanks its influence is
hardly detectable. The power, flow, and blend time numbers change
with Reynolds numbers in the low range, but tend to level off above 0.01
NR, = 10,000 or so at values characteristic of the kind of impeller. 1 10 100 1000 10000 100000 1000000
Sometimes impellers are characterized by their limiting N,, as an REYNOLDS NUMBER
N, = 1.37 of a turbine, for instance. The dependencies on Reynolds (a)
number are shown on Figures 10.5 and 10.6 for power, in Figure Figure 10.5. Power number, N, = Pg,/N3D5p, against Reynolds
10.3 for flow and in Figure 10.7 for blend time. number, NRe = ND2p/p, for several kinds of impellers: (a) helical
Rough rules for mixing quality can be based on correlations of shape (Oldshue, 1983); (b) anchor shape (Oldshue, 1983); (c)
power input and pumping rate when the agitation system is several shapes: (1) propeller, pitch equalling diameter, without
otherwise properly designed with a suitable impeller (predominantly baffles; (2) propeller, s = d, four baffles; (3) propeller, s =2d,
either axial oir radial depending on the process) in a correct without baffles; (4) propeller, s =2d, four baffles; (5) turbine
location, with appropriate bafffing and the correct shape of vessel. impeller, six straight blades, without baffles; (6) turbine impeller,
The power input per unit volume or the superficial linear velocity six blades, four baffles; (7) turbine impeller, six curved blades, four
can be used as measures of mixing intensity. For continuous flow baffles; (8) arrowhead turbine, four baffles; (9) turbine impeller,
inclined curved blades, four baffles; (10) two-blade paddle, four
reactors, for instance, a rule of thumb is that the contents of the baffles; (11) turbine impeller, six blades, four baffles; (12) turbine
vessel should be turned over in 5-10% of the residence time. impeller with stator ring; (13) paddle without bafHes (data of Miller
Specifications of superficial linear velocities for different kinds of and Mann); (14) paddle without baffles (data of White and
operations are stated later in this chapter. For baffled turbine Summerford). All baffles are of width 0.1D [after Rushton, Costich,
agitation of reactors, power inputs and impeller tip speeds such as and Everett, Chem. Eng. Prog. 46(9), 467 (1950)].