Page 270 - Chemical Process Equipment - Selection and Design

9.3. CLASSIFICATION AND GENERAL CHARACTERISTICS OF DRYERS 237

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EXAMPLE 92 dW 0.03292, 0.04 < W < 0.168,
Drying Time over Constant and Falling Rate Periods with dB 0.823W, W<O.O4.
Constant Gas Conditions
The data of Figure 9.3(d) were obtained on a sample that contained Accordingly, the drying time is
27.125 lb dry sand and had an exposed drying surface of 2.35 sqft.
Take the case of a sample that initially contained 0.1681b
1
moistnre/lb dry material and is to be dried to W = (4.005 Ib/lb. In o=-- w-wc +~ln(z)
these units, the constant rate shown on the graph is transformed to 0.03292 0.823
0.168-0.04 1
- 0.03292 +-In 0.823 (~~~5)
-
1 dW
-_-=___ 0.38 (Wb) /@r)(sqft),
2.35 d6 27.125 = 6.42 hr.
which applies down to the critical moisture content W, = 0.04 lb/lb. This checks the reading off the plot of the original data on Figure
The rate behavior over the whole moisture range is 9.3(d).

the evaporate assumes the wet bulb temperature of the air. section cannot be applied readily. The sizing of such equipment is
Constant rate zones are shown in (d) and (e), and (e) reports that essentially a scale-up of pilot plant tests in similar equipment. Some
temperatures aire truly constant in such a zone. manufacturers make such test equipment available. The tests may
The moisture content at which the drying rate begins to decline establish the residence time and the terminal conditions of the gas
is called critical. Some of the variables on which the transition point and solid. Dusting behavior and possible need for recycling of gas
depends are indicated in Figures 9.3(c) and (g). The shape of the or of dried material are among the other factors that may be noted.
falling rate curve sometimes may be approximated by a straight Such pilot plant data are cited for the rotary dryer of Example
line, with equation 9.6. For the pneumatic conveying dryer of Example 9.8, the tests
establish heat and mass transfer coefficients which can be used to
calculate residence time under full scale operation.
(9.14)
Scale-up factors as small as 2 may be required in critical cases,
but factors of 5 or more often are practicable, particularly when the
where We is the equilibrium moisture content. When W, is zero as it tests are analyzed by experienced persons. The minimum
often is of nonporous granular materials, the straight line goes dimensions of a test rotary dryer are 1 ft dia by 6 ft long. A common
through the origin. (d) and (h) illustrate this kind of behavior. The criterion is that the product of diameter and rpm be in the range
drying time is found by integration of the rate plots or equations. 25-35. A laboratory pneumatic conveying dryer is described by
The process is illustrated in Example 9.2 for straight line behavior. Nonhebel and Moss (1971). The veseel is 8 cm dia by about 1.5 m
Other cases require numerical integration. Each of the examples of long. Feed rate suggested is 100g/min and the air velocity about
Figure 9.3 corresponds to a particular substantially constant gas lm/sec. They suggest that 6-12 passes of the solid through this
condition. This is true of shallow bed drying without recirculation of equipment may be needed to obtain the requisite dryness because
humid gas, but in other kinds of drying equipment the variation of of limitations in its length.
the rate with time and position in the equipment, as well as with the The smallest pilot spray dryer supplied by Bowen Engineering
moisture content, must be taken into account. Co. is 30 in. dia by 2.5-6.0 ft high. Atomization is with 15 SCFM of
An approximation that may be justifiable is that the critical air at 1OOpsig. Air rate is 250 actual cfm at 150-1000°F.
moisture content is roughly independent of the drying conditions Evaporation rates of 15-80 lb/hr are attained, and particles of
and that the falling rate curve is linear. Then the rate equations may product range from 5 to 40 ym.
be written A pilot continuous multitray dryer is available from the
Wyssmont Co. It is 4 ft dia by 5 ft high with 9 trays and can handle
w, < w < w,, 25-200 lb/hr of feed.
1 dW- Batch fluidized bed dryers are made in quite small sizes, of the
A do we < w < w,. (9.15) order of 100 lb/hr of feed as the data of Table 9.14(a) show, and are
suitable for pilot plant work.
Examples 9.3 and 9.4 apply these relations to a countercurrent
dryer in which the humidity driving force and the equilibrium 9.3. CLASSlFlCATlON AND GENERAL CHARACTERISTICS OF
moisture content vary throughout the equipment. DRYERS
LABORATORY AND PILOT PLANT TESTING Removal of water from solids is most often accomplished by
contacting them with air of low humidity and elevated temperature.
The techniques, of measuring drying of stationary products, as on Less common, although locally important, drying processes apply
trays, are relatively straightforward. Details may be found in the heat radiatively or dielectrically; in these operations as in freeze
references made with the data of Figure 9.3. Mass transfer drying, the role of any gas supply is that of entrainer of the
resistances were eliminated by Wenzel through use of superheated humidity.
steam as the drying medium. The nature, size, and shape of the solids, the scale of the
In some practical kinds of dryers, the flow patterns of gas and operation, the method of transporting the stock and contacting it
solid are so complex that the kind of rate equation discussed in this with gas, the heating mode, etc. are some of the many factors that