Page 268 - Chemical Process Equipment - Selection and Design

9.2. RATE OF DRYING 235

init101 moisture content UMCI

nstant rate period

Time
(a)

0
Relative, humidity %
(b)
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UOISTURE C5NTEWT.LES./LB.OF DRY SAND
7. SATURATION
(C) (d)
Figure 9.3. (a) Classic drying curve of moisture content against time; a heat-up period in which no drying occurs also is usually present
(Proctor and Schwartz, Inc.; Schweitzer, p. 4.144). (h) Equilibrium moisture content as a function of relative humidity; many other data are
tabulated in Chemical Engineers Handbook (McGraw-Hill, New York, 1984, 20.12). (These data are from National Academy ~f Science,
copyright 1926.) (c) Rate of drying as a function of % saturation at low (subscript 1) and high (subscript 2) drying rates: (A) glass spheres,
60 pm, bed 51 mm deep; (B) silica flour, 23.5 pm? 51 rnm deep; (C) silica flour, 7.5 pm, 51 mm bed; (D) silica flour, 2.5 p, 45 mm deep (data
of Newitt et al., Trans. Inst. Chem. Eng. 27, 1 (1949). (d) Moisture content, time and drying rates in the drying of a tray of sand with
superheated steam; surface 2.35 sqft, weight 27.125 lb. The scatter in the rate data is due to the rough numerical differentiation (Wenzel,
Ph.D. thesis, Umuersity of Michigan, 1949). (e) Temperature and drying rate in the drying of sand in a tray by blowing air across it. Dry bulb
76.1°C, wet bulb 36.0"C (Ceaglske and Hougen, Trans. AIChE 33, 283 (1937). (f) Drying rates of slabs of paper pulp of several thicknesses
[after McCready and McCabe, Trans. AIChE 29, 131 (1933)l. (8) Drying of asbestos pulp with air of various humidities [McCready and
McCabe, Trans. .AIChE 29, 131 (1933)l. (h) Effect of temperature difference on the coefficient K of the falling rate equation -dW/dO = KW
[Sherwood and Comings, Trans. AIChE 27, 118 (1932)l. (i) Effect of air velocity on drying of clay slabs. The data are represented by
--
R = ~.OU'.~~(H,,, H). The dashed line is for evaporation in a wetted wall tower (Walker, Lewis, McAdams, and GiZZiZand, Principles of
Chemical Engineering, McGraw-Hill, New York, 1937).