Raw cotton having an initial total moisture content of 95% (dry basis) and a dry density 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 160°F 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 18.25. Experiments have shown that under the given conditions, the critical-moisture content hill be 0.4 lb water/lb bone-dry cotton and that the falling-rate drying period will be like that of Figure 18.3la,based on the free-moisture content. Determine:

(a) The amount of raw cotton in pounds (wet basis) that can be dried in one batch if the dryer contains 16 trays.

(b) The drying time for the constant-rate period.

(c) The drying time for the falling-rate period.

(d) The total drying time if the preheat period is 1 h.

0.20 96F 113F 0.16 136F 165F 0.12 192F 219F- 0.08 0.04 248F 277F- 302F 60 80 100 20 40 Relative humidity, percent Figure 18.25 Effect of temperature on the equilibrium-moisture content of raw cotton at 1 atm. Moisture content, Ib H2O/lb dry solids