Page 337 - Chemical Process Equipment - Selection and Design

10.8. MIXING OF POWDERS AND PASTES 301

EXAMPLE 110.4 From Table 10.2, a liquid velocity of 0.6-0.7ft/sec will give
HP and rpin Requirements of an Aerated Agitated Tank moderate to high dispersion. Table 10.5 gives possible HP/rpm
A tank contains 5000 gal of liquid with sp gr = 1.0 and viscosity combination of 301125, somewhat less than the value found here.
1OOcF that is aerated and agitated. The ratio of impeller to tank
diameters is d/13 = 9.4. Two sets of conditions are to be examined. b. With liquid circulation velocity specified,
a. The air rate is 972SCFM or 872ACFM at an average uL = 0.5 ft/sec.
submergence of 4 ft. The corresponding superficial gas velocity is
0.206 ft/sec or 0.063 m/sec. A mass transfer coefficient Use Figure 10.12:
kLa = 0.2/sec is required; Dickey's equation (10.6) applies. Find
the power and rpm needed. Y = iou,(d/D)1.2 = 10(0.5)(0.4)1.2 = 1.67,
b. The air rale is 296ACFM, 0.07ft/sec, 0.0213m/sec. The X=O.8,
required intensity of mixing corresponds to a liquid superficial P,/V = 0.8/(0.4)1.85 = 4.36 HP/1000 gal
velocity of 0.5 ftlsec. Find the power, rotation speed, and mass
transfer coefficients for sulfite oxidation and for fermentation.
(this does exceed the minimum of 1.6 from Figure 10.11),
a. d = 0.4(9.47) = 3.79 ft, 45.46 in., P' = 5(4.36) = 21.8,
kLa = 0.064(P6/V)0 'u: = 0.2, '
P,/V = [0.i!/0.064(0.206)o 2]1'0.7 = 8.00 HP, _- 296/(3.79)3N = 5.437/N,
Nd3 -
P, = 5(8.0) = 40.0 HP/5000 gal, NRe = 222N (part a),
Q/Nd3 = 8'72/(379)3N= 16.02/N,
NRe = 10.75Nd2S/p = 10.75(45.46)2N/100 = 222N N=- 78~442P (part a).
N
Equation (10.21,
Solve by trial, using Figure 10.10(a) and curve 2 of Figure 10.6.
Np = 1.523 10'3)P/N3d5S = 78,442P/N3.
N QIN~ p,/p &e N~ P FDg
Curve 2 of Figure (10.6) applies. PJP from Figure 10.10(a). Solve 100 0.0544 0.5194 22,200 4 51 26.5
by trial. 94 0.0576 0.5130 4 42.35 21.7 -2.8

QJNd GJP NR. Np P 5 The closest reading from Table 10.5 is HP/rpm = 25/100 which is a
400 0.160 0.324 22,200 4 51 16.5 good check.
150 0.107 0.422 33,300 4 172 72.6 For sulfite oxidation, at u, = 0.07 ft/sec,
127 0.1261 0.3866 28,194 4 104.5 40.4-40.0
PJV = 4.36 HP/1000 gal, from Figure 10.9,
The Bast entry of P, checks the required value 40.0. Find the
corresponding superficial liquid velocity with Figure 10.12: k6a = 0.07 lb mol/(cuft)/(hr)(atm).

X = (P/V)(d/D)1.85 = 8.04(0.4)1.s5 = 1.48, For fermentation, Eq. 10.6 gives
at uG = 0.206 ftlsec, Y = 2.0, kLa = 0.064(4.36)0.7(0.07)0.2
lb mol/(cuft)(sec)
= 0.105
:. uL = 2/11D(0.4)1-2 = 0.60 ft/sec lb mol/cuft '

mixing inserts in the tubes and are then claimed to have 3-5 times projects somewhat out of direct experience and where design by
normal capability in some cases. analogy may not suffice, testing in pilot plant equipment is a service
provided by many equipment suppliers.
10.8. MIXING OF POWDERS AND PASTES A few examples of mixers and blenders for powders and pastes
are illustrated in Figure 10.15. For descriptions of available
Industries such a5 foods, cosmetics, pharmaceuticals, plastics, equipment-their construction, capacity, performance, power
rubbers, and also some others have to do with mixing of high consumption, etc.-the primary sources are catalogs of manufac-
viscosity liquids or pastes, of powders together and of powders with turers and contact with their offices. Classified lists of manu-
pastes. Much of this kind of work is in batch mode. The processes facturers, and some of their catalog information, appear in the
are so diverse arnd the criteria for uniformity of the final product are Chemical Engineering Catalog (Reinhold, New York, annually)
SO imprecise that the nonspecialist can do little in the way of and in the Chemical Engineering Equipment Buyers Guide
equipment design, or in checking on the recommendations of (McGraw-Hill, New York, annually). Brief descriptions of some
equipment manufacturers. Direct experience is the main guide to types of equipment are in Perry's Chemical Engineers Handbook
selection of the best kind of equipment, predicting how well and (McGraw-Hill, New York, 1984 and earlier editions). Well-classified
quickly it will perform, and what power consumption will be. For descriptions, with figures, of paste mixers are in Ullmann (1972,