Free-moisture content Drying rate 0.20 96°F 113°F 0.16 136°F 165 F 0.12 192°F 0.08 219 F- 0.04 248 F +277°F- 302°F 20 40 60 80 100 Relative humidity, percent Moisture content, Ib H,0/lb dry solids
Raw cotton having an initial total moisture content of 95% (dry basis) and a dry density of 43.7 lb/ft3 is to be dried batchwise to a final moisture content of 10% (dry basis) in a cross-circulation tray dryer. The trays, which are insulated on the bottom, are each 3 cm high, with an area of 1.5 m2 , and are completely filled. The heating medium, which is air at 160oF and 1 atm with a relative humidity of 30%, flows across the top surface of the tray at a mass velocity of 500 lb/h-ft2 . Equilibrium-moisture content isotherms for the cotton are given in Figure. Experiments have shown that under the given conditions, critical moisture content will be 0.4 lb water/lb bone-dry cotton, and the falling-rate drying period will be like that of Figure, based on freemoisture content. Determine: (a) amount of raw cotton in pounds (wet basis) that can be dried in one batch if the dryer contains 16 trays; (b) drying time for the constant-rate period; (c) drying time for the fallingrate period; (d) total drying time if the preheat period is 1 h.
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