Page 271 - Chemical Process Equipment - Selection and Design

238 DRYERS AND COOLING TOWERS

EXAMPLE 9.3 With moisture content of the stock as a parameter, the humidity of
Drying with Changing Humidity of‘ Ai in a Tunnel Dryer the air is calculated by moisture balance from
A granular material deposited on trays or a belt is moved through a
tunnel dryer countercurrently to air that is maintained at 170°F with H, = 0.0125 + (1400/7790)(W - 0.1). (4)
steam-heated tubes. The stock enters at 1400 Ib dry/hr with
W = 1.161b/lb and leaves with 0.1 lb/lb. The air enters at 5% The corresponding relative humidities and wet bulb temperatures
relative humidity (H, = 0.0125 lb/lb) and leaves at 60% relative and corresponding humidities H, are read off a psychrometric chart.
humidity at 170°F (H, = 0.203 Ib/lb). The air rate found by moisture The equilibrium moisture is found from the relative humidity by
balance is 7790 lb dry/hr: Eq. (2). The various corrections to the rate are applied in Eq. (3).
The results are tabulated, and the time is found by integration of
7790 Ib/hr the rate data over the range 0.1 < W < 1.16.
7 STM r 170 F, Ha =
170 F, H, = 0.203 , 0.0125
4 W 4 W Hg Y RH W, Rate l/Rate
+ b 1.16 0.203 0.210 0.239 4.184
1 .oo 0.174 0.182 0.273 3.663
0.9 0.1 56 0.165 0.303 3.257
0.8 0.138 0.148 0.341 2.933
0.7 0.120 0.130 0.341 2.933
Drying tests reported by Walker, Lewis, McAdams, and Gilliland, 0.58 0.099 0.110 0.335 0.044 0.356 2.809
Principles of Chemical Engineering, McGraw-Hill, New York, 0.50 0.084 0.096 0.29 0.040 0.333 3.003
(1937, p. 671) may be represented by the rate equation 0.4 0.066 0.080 0.24 0.035 0.308 3.247
0.3 0.048 0.061 0.18 0.028 0.213 4.695
0.045
-loo-= dW { 0.28 (lb/lb)/hr, 0.58 < W < 1.16, (1) 0.2 0.030 0.0315 0.119 0.021 0.162 6.173
0.011
0.050
0.1
0.0125
9.804
0.102
d9 0.28(W - We)/(0.58 - We), We < W <0.58.
The air was at 95°F and 7% relative humidity, corresponding to a The drying time is
humidity driving force of H, - H, = 0.0082. Equilibrium moisture
content as a function of the fraction relative humidity (RH), and 0.10 dw
assumed independent of temperature, is represented by 9 = = 4.21 hr, by trapezoidal rule.
We = 0.0036 + 0.1539(RH) - 0.097(FW)2. (2)
The length of tunnel needed depends on the space needed to
The critical moisture content is assumed indpendent of the drying ensure proper circulation of air through the granular bed. If the bed
rate. Accordingly, under the proposed operating conditions, the moves through the dryer at 10 ft/hr, the length of the dryer must be
rate of drying will be at least 42 ft.
0.28(H, - H,) 0.58 < W < 1.16, W -
0.0082 ’ c-m \ f\ m m m -
W
\
\
-loo-= 0.28(Hs - H,)(W - We) We < W < 0.58. (3) I I I m Stock
0.0082(0.58 - 0.014) ’ --c 00 W

have led to the development of a considerable variety of equipment. Fluidized bed dryers, for example, are operated as batch or
The most elaborate classification of dryers is that of Kroll (1978) continuous, for pharmaceuticals or asphalt, at rates of hundreds or
which assigns one of 10 letters for the kind of solid and one of seven many thousands of pounds per hour.
numbers for the kind of operation. As modified by Keey (1972), it An important characteristic of a dryer is the residence time
comprises 39 main classes and a total of 70 with subclasses. Less distribution of solids in it. Dryers in which the particles do not move
comprehensive but perhaps more practical classifications are shown relatively to each other provide uniform time distribution. In spray,
in Table 9.1. They take into account the method of operation, the pneumatic conveying, fluidized bed, and other equipment in which
physical form of the stock, special features, scale of production, and the particles tumble about, a substantial variation in residence time
drying time. develops. Accordingly, some particles may overdry and some
In a later section, the characteristics and performances of the remain wet. Figure 9.5 shows some data. Spray and pneumatic
most widely used equipment will be described in some detail. Many conveyors have wide time distributions; rotary and fluidized bed
types are shown in Figure 9.4. Here some comparisons are made. units have narrower but far from uniform ones. Differences in
Evaporation rates and thermal efficiencies are compared in Table particle size also lead to nonuniform drying. In pneumatic con-
9.2, while similar and other data appear in Table 9.3. The wide veying dryers particularly, it is common practice to recycle a
spreads of these numbers reflect the diversity of individual designs portion of the product continuously to ensure adequate overall
of the same general kind of equipment, differences in moisture drying. In other cases recycling may be performed to improve the
contents, and differences in drying properties of various materials. handling characteristics when the feed material is very wet.