Page 289 - Chemical Process Equipment - Selection and Design

256 DRYERS AND COOLING TOWERS

12 ft/sec. The diameter and length will be found. Since the inlet and
EXAMPLE 9.5 outlet conditions are specified and the moisture transfer is known,
W 1000 0.6 Ib/hr Ib/lb r
Scale-up of a Rotary Dryer
the heat balance can be made. The heat capacity of the solid is 0.24:
Tests on a laboratory unit come up with the stated conditions for
drying a pelleted material at the rate of 1000 lb dry/hr:
moisture evap = 1000(0.6-0.05) = 550 lb/hr
air rate = 550/(0.0428 - 0.013) = 18,4561b/hr
160 F
H = 0.0428
Off a psychrometric chart, the sp vol of the air is 15.9 cuft/(lb dry).
H = 0.013
The diameter is
W = 0.05 Ib/lb
Stock
110 F
l ' '
D=(
70
;:OF
F
=
3600(12)(1- 0.075)~/4
The length is 18,456(15.9) ) = 3.06 ft, say 3.0 ft.
The residence time is 20min. The speed is 3-4rpm. On the
average, 7.5% of the cross section is occupied by solid. Because of
dusting problems, the linear velocity of the air is limited to
EXAMPLE 9.6 With the allowable air rate of 750 lb/hr sqft, the diameter of
Design Details of a Countercurrent Rotary Dryer the dryer is
Pilot plants indicate that a residence time of 3hr is needed to
accomplish a drying with the conditions indicated on the sketch.
For reasons of entrainment, the air rate is limited to 7501bs D = v/11,633/750n/4 = 4.44 ft, say 4.5 ft.
dry/(hr)(sqft cross section). Properties of the solid are 501b/
cuft and 0.22Btu/(lb)("F). Symbols on the sketch are A = dry air, Say the solid occupies 8% of the cross section. With a solids
S = dry solid, W =water: density of 50 lb/cuft, the dryer volume,
136 F 150 psig steam V = 3(1000/50)/0.08 = 750 cuft,
A Ib/hr
H = 0.008
and the length is
Stock
s = 1000 W = 1 Ib/hr L = 750/(4.5)'~/4 = 47.2 ft.
w = 334
60 F
The standard number of flights is 2-4 times the diameter, or
In terms of the dry air rate, A lb/hr, the average moist heat capacity number = (2-4)4.5 = 9-18, say 12.
is
The product of rpm and diameter is 25-35
C = 0.24 + 0.45[0.008 + $(333/A)] = 0.2436 + 74.93/A.
.'. rpm = (25-35)/4.5 = 5.5-7.8, say 6.7.
In the dryer, the enthalpy change of the moist air equals the sum of
the enthalpy changes of the moisture and of the solid. Add 7% for The stm heater duty is
heat losses. With steam table data.
Q, = 11,633(0.2436)(290 - 60) = 651,733 Btu/hr,
(0.2436 + 74.93/A)A(290 - 136) = 1.07[333(1120.3) + l(228) 150 psig stm,
+ 1000(0.22)(260 - 60) - 334(28)] stm = 651,733/857 = 760.5 lb/hr.
= 1.07(407,936) = 43,6491,
:. A = 11,633 lb/hr. Evaporation efficiency is
q = 333/760.5 = 0.438 lb water/lb stm.
The exit humidity is
H = 0.008 + 333/11,633 = 0.03661b/lb, The efficiency of the dryer itself is
which corresponds to an exit dewpoint of 96"F, an acceptable value. vd = 407,936/651,733 = 0.626 Btu/Btu.