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Mass Transfer 2
CPB 20103
CPB 20103 MASS TRANSFER 2 TUTORIAL 2 DRYING OPERATIONS 1. Air of 20oC and I bar with relative humidity of 80% is heated to 50 oC. The heated air flows along a flat tray filled with completely wetted material. The heat transfer coefficient is 35 W/m2.K and the latent heat of vaporization is 2.45 x 106 J/kg. Calculate the constant drying rate. [R = 3.43 x 10-4 kg/m2.s] 2. A wet solid with a moisture content of 0.50 kg H 2O/kg dry solid occupies 0.1 m2 on a flat tray. A laboratory experiment shows that the critical moisture content is 0.2 kg H 2O/kg dry solid and the constant drying rate is 0.36 g/m2.s. Calculate the time required to reduce the moisture content of the wet solid on the tray containing 2 kg of dry solid from 0.40 to 0.20 kg H2O/kg dry solid. [3.09hr] 3. A porous solid is dried in a batch dryer under constant drying conditions. 5 hours are required to reduce the moisture content from 35 to 10 percent. The critical moisture content was found to be 20 percent and the equilibrium moisture 4 percent. All moisture contents are on the dry basis. Assuming that the rate of drying during the falling-rate period is proportional to the free-moisture content, calculate the time required to dry a sample of the same solid from 35 to 5 percent under the same drying conditions. [t = 9.67hr] 4. A granular solid is to be dried in a tunnel dryer from 50% to 10% moisture under identical conditions in 3.0 x 10 4 s. The critical and equilibrium moisture contents are 12% and 3%, respectively. Calculate the time required to dry the solid from 50% moisture to 5% moisture if all moisture contents quoted are on a dry basis. [t = 7.37hr] 5. A granular insoluble solid material wet with water is being dried in the constant rate period in a pan 0.61 m x 0.61 m and the depth of material is 25.4 mm. The sides and bottom are insulated. Air flows perpendicular to the top surface at a velocity of 3.05 m/s and has a dry bulb temperature of 60oC and wet bulb temperature of 29.4 oC. The pan
FG/T2/July2011
Mass Transfer 2
CPB 20103
contains 11.34 kg of dry solid having a free moisture content of 0.35 kg H2O/kg dry solid and the material is to be dried in the constant rate period to 0.22 kg H2O/kg dry solid. Calculate the drying rate and the time in hours needed. [R = 1.642 kg/h.m2, t = 2.41h]
6. A slab with a wet weight of 5 kg originally contains 50% moisture (wet basis). The slab is 600 by 900 by 75 mm thick. The equilibrium moisture content is 5% of the total weight when in contact with air of 20 oC and 20% humidity. The drying rate is given in Table 1 for contact with air of the above quality at a definite velocity. Drying is from one face. Determine the time required to dry the slab to 10% moisture content (wet basis). Table 1 Wet – slab weight, kg 9.1 7.2 5.3 4.2 3.3 2.9 2.7
Drying rate, kg/m2.h 4.9 4.9 4.4 3.9 3.4 2.0 1.0
7. A wet filter cake in a pan 1 ft x 1 ft square and 1 inch thick is dried on the top surface with air at wet bulb temperature of 80 oF and a dry bulb of 120oF flowing parallel to the surface at a velocity of 2.5 ft/s. The dry density of the cake is 150 lbm/ft 3 and the critical free moisture content is 0.09 lb H2O/lb dry solid. How long will it take to dry the material from a free moisture content of 0.20 lb H2O/lb dry sold to the critical moisture content? [t = 17.03h] 8. Slabs of filter cake with a bone-dry density of 1600 kg/m 3 are to be dried from an initial free moisture content of 90% (dry basis) to a final free moisture content of 5% (dry basis) batchwise in trays that are 1 m long by 0.5 m wide with a depth of 3 cm. Drying will take place only from the top surface. The drying air conditions are 1 atm, 60 oC and a 39oC wet bulb temperature. Air flows perpendicular to the top surface at velocity of 3.5 m/s. Experiments under these drying conditions show a critical free moisture content of 45% (dry basis) with assuming that the drying rate is a linear with free moisture content through origin. Determine:
FG/T2/July2011
Mass Transfer 2
CPB 20103
a) b)
The drying time for the constant rate period in hours. The drying time for the falling rate period in hours.
9. A wet solid is to be dried in a pan 0.61 m by 0.61 m and the depth of material is 25.4 mm under constant rate period. The sides and bottom are insulated. Air flows parallel to the top surface at a velocity of 3.05 m/s and has a dry bulb temperature of 60 oC and wet bulb temperature of 29.4 oC. The pan contains 11.34 kg of dry solid having a moisture content of 40% and the material is to be dried in the constant rate period to the critical moisture content of 20%. The equilibrium moisture was found to be 4%. All moisture contents are on the dry basis. a) Calculate the drying rate and the time required in hours. [R = 1.599 kg/h.m2, t = 3.82hr] b)
Calculate the drying rate and the time needed in hours if the depth of material is reduced to 12.7 mm. [R = 1.599 kg/h.m2, t = 1.91hr]
10.A wet solid is dried with air at 60oC and a humidity of 0.010 kg H2O/kg dry air. The air flows parallel to the flat, wet surface of the solid at a velocity of 5 m/s. Under these conditions the heat transfer coefficient is given by h = 14.3( v )0.8, where is the density of the mist air in kg moist air per unit volume moist air. The surface area available for heat exchange with air is 0.25m 2 and the latent heat of vaporization is 2.45 x 106 J/kg. Calculate the total amount of water evaporating per unit time. [R = 1.71 x 10-4 kg/s]
FG/T2/July2011