CHEE3004 - University of Queensland Assignment 2012

CHEE3004 Homework Assignment 7 2012 A porous solid is dried in a batch dryer under constant drying conditions. Seven hours re required to reduce the moisture content from 35 to the critical value of 20 percent. The equilibrium moisture content is 4 percent. All moisture contents are on a dry basis. Assuming that the rate of drying is proportional to the free-moisture content, how long should it take to dry a sample from 35 to 25 percent under the same conditions? A continuous dryer is to be designed to dry 800 lb of wet porous solid per hour from 140 percent moisture to 20 percent, both on a 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 co-efficient is 12 Btu/ft2.h.?F. The area exposed to the air is 1.1 ft2 per pound of solids. How long must the solids remain in the dryer? A wet filter cake in a pan 1 ft x 1 ft and 1 in. thick is dried on the top surface with air at a wet bulb temperature of 80 ?F and a dry bulb of 120 ?F flowing parallel to the surface at a velocity of 2.5 ft/s. The dry density of the cake is 120 lbm/ft3 and the critical free moisture content is 0.09 lb H20 /lb dry solid. How long will it take to dry the material from a free moisture content of 0.20 lb H20/lb dry material to the critical moisture content? A batch of wet solid was dried on a tray dryer using constant drying conditions and a thickness of material on the tray of 25.4 mm. Only the top surface was exposed. The drying rate during the constant rate period was R = 2.05 kg water/kg dry solid. The ratio Ls/A used was 24.4 kg dry solid/ m2 exposed surface. The initial free moisture was X1 = 0.55 and the critical moisture content Xc = 0.22 kg free moisture/ kg dry solid. Calculate the time to dry a batch of this material from X1 = 0.45 to X2 = 0.30 using the same drying conditions but a thickness of 50.8 mm with drying from the top and bottom surfaces. This is due on Friday the 21st of September by 17:00 through the faculty chute as we do not have a tute on Friday (and Monday is the vac).