Page 303 - Chemical Process Equipment - Selection and Design

9.9. FLUIDIZED BED DRYERS 267

EXAMPLE 9.&(contanuedj 39
decremented vabue of W,, assume a value of T,, and proceed. The 48
solution is tabulated.
58
w T T' @(set)
0.035 60 450 0
0.0325 73.04 378.2 0.0402 68
0.03 75.66 352.2 0.0581 78
0.025 77.41 315.3 0.0872 88
0.02 77.23 286.7 0.1133 98
0.015 76.28 261.3 0.1 396
0.01 75.15 236.4 0.1 687 198
0.005 74.67 208.4 0.2067
0.003 75.55 192.4 0.2317 119
0.001 79.00 165.0 0.2841
128
When going directly from 0.035 to 0.001,
139
T2 = 80.28,
T; = 144.04, 148
0 = 0.3279 sec. 159
The calculation could be repeated with a smaller air rate in order to 169
reduce its exit temperature to nearer 220"C, thus improving thermal 178
efficiency.
In the vessel with diameter = 0.6 an, the air velocities are 138
25.0 m/sec at 450°C inlet 198
5.15 m/sec at 165°C outlet
288
20.1 m/sec average. 219
228
The vessel height that will provide the needed residence time is
H = U,B = 20.1(0.2841) = 5.70 rn.
239
Very fine particles with zero slip velocity will have the same
holdup time as the air. The coarsest with settling velocity of
10 m/sec will have a net forward velocity of
Us = 20.1 - 10 = 10.1 m/sec,
which corresponds to a holdup time of
B = 5.7/10.1= 0.56 sec,
which is desirable since they dry more slowly.
After the assumption of a,, other quantities are evaluated in
the order shown in this program.

9 8 ! Exanir~ 1 E? 9 . 8 . Pneuma t: i c c12r-f
we::' i n9 dryer
26 ! Findins the exit solids te
MP T2 by trial, then all de^
enden t quant i ? i e5

9.14(a). This process is faster and much less labor-intensive than installed). The relatively large power requirements of fluidized bed
tray drying and has largely replaced tray drying in the pharma- dryers are counterbalanced by their greater mechanical simplicity
ceutical industry which deals with small production rates. Drying and lower floor space requirements.
rates of 2-10Ib/(hr)(cuft) are reported in this table, with drying Air rates in Table 9.15 range from 13 to 793 SCFM/sqft, which
times of a fraction of an hour to several hours. In the continuous is hardly a guide to the selection of an air rate for a particular case.
operations of Table 9.15, the residence times are at most a few minutes. A gas velocity twice the minimum fluidization velocity may be taken
Thermal efficiency of fluidized bed dryers is superior to that of as a safe prescription. None of the published correlations of
many other types, generally less than twicle the latent heat of the minimum fluidizing velocity is of high accuracy. The equation of
water evaporated being required as heat input. Power requirements Leva (Fluidization, McGraw-Hill, New York, 1959) appears to be
are a major cost factor. The easily dried materials of Table 9.15(a) as good as any of the later ones. It is
show evaporation rates of 58-103 lb/(hr)(HP installed) but the
more difficult materials of Table 9.15(d) show only 5-18 lb/(hr) (HP (9.20)