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3.3 T Reactors o-Phase Agitated w 95

These losses are from the loss of usable energy in the form of work through the motor ,
shaft gearing, and bearings, and could be as high as 20–30%. The agitation intensity could
e,
v
be characterized as mild, vigorous, and intensi depending on the power deliered per v
unit volume of liquid. For w the approximate values are presented in T , ater able 3.2.
The discharge rate has been measured for seeral types of impellers, and discharge co- v
efficients have been calculated (Perry and Green, 1999). For turbines, N Q ranges from 0.7
to 2.9, depending on the impeller geometrical characteristics. F or a standard flat blade tur-
-blade 45° turbine and a baf- bine (Ruhston) in a baffled vessel, this v or a four alue is 1.3. F
essel,
essel,
fled v this value is 0.87, and for marine propellers and a baffled v it is 0.5
(McCabe et al ., 1993). More specif for the typical case of six-bladed turbines
ically
,
(Ruhston), the following approximation can be used (Perry and Green, 1999):
D T
N Q 0.93 (3.109)
D a
where D T is the tank diameter in m. Power number N p ranges from 0.35 to 7, depending
on the impeller type, its geometrical characteristics, and v ery essel design. This number is v
important since it is needed for the evaluation of power consumption, which in turn, is used
for the determination of mass transfer coefThe geometry of the agitation system icients. f
plays a critical role. The typical geometrical ratios are presented in T able 3.3.
In Figure 3.23, the various dimensions are presented.
Baffles are frequently arranged with a clearance , which may be half the baffle width,
between the baffle and the tank wall to preent accumulation of solids behind the baf v fles
(Treybal, 1980).
The height of the impeller above vessel floor is frequently called “clearance,” and is the
distance between the vessel bottom and the impeller centerline . On the other hand, C b is
the distance from the bottom to the bottom of the impeller. The relationship between these
two parameters is

W
Z C (3.110)
a b
2
In Table 3.4, the power number is presented for se eral cases. v
The values of N P are approximate. At low Reynolds numbers, about 300, the po wer
number curves for baffled and unbaffled tanks are identical (McCabe et al ., 1993). F or
higher N Re , the po fled tanks is lower than the values for baffled tanks. wer number for unbaf

Table 3.2

Agitation intensity versus poolume for w wer/v ater
Agitation Power/volume (kW/m 3 )
Mild 0.09–0.17
Vigorous 0.34–0.51
Intensive 0.68–1.69