A continuous countercurrent dryer is to be designed to dry 800 lb of wet porous solid per hour from 140 percent moisture to 20 percent, both on the dry basis. Air at 120°F dry-bulb and 70°F wet-bulb temperature is to be used. The exit humidity is to be 0.012. The average equilibrium moisture content is 5 percent of the dry weight. The total moisture content (dry basis) at the critical point is 40 percent. The stock may be assumed to remain at a temperature 3°F above that of the wet-bulb temperature of the air throughout the dryer. The heat-transfer coefficient is 12 Btu/ft²·h. The area exposed to the air is 1.1 ft² per pound of dry solids. How long must the solids remain in the dryer?