A dryer is to be used for drying a wet porous solid. The solid material is 1.5 m square and 3 cm thick. The dry density of the material is 1200kg/m3 . Drying is done from both sides with air at a wet-bulb temperature 30oCand a dry bulb temperature 75oC. The air leaving the dryer has a temperature 40oC. Under the conditions of drying, the equilibrium moisture content is 5% and the critical moisture is 10%, dry basis. Assume that the heat transfer by radiation or by conduction is negligible and the convection heat transfer coefficient for the process is 60W/m2 . oC. The stock may be assumed to remain at the wet-bulb temperature of air throughout the dryer. The drying rate is proportional to the moisture content during the falling rate period.

(a) How long would it take to dry this material from an initial moisture content of 20% percent (dry basis) to a final moisture content of 7% percent? (b) What will be the final moisture content if the material is kept in the dryer for 48 hours? (c) If you want to dry the material faster, what operating condition should be changed? Give reason(s) for your answer