Drying

DRYING: 1.

The initial moisture content of a food product is 77% (wet basis), and the critical moisture content is 30%(wet basis). If the constant drying rate is 0.1 kg H2O / m2.s, compute the time required for the product to begin the falling rate drying period. The product has a cube shape with 5- cm sides, and the initial product density is 950 kg/m3. 23.4 s b. 34.6 s c. 48.5 s d. 53.2

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A cabinet dryer is being used to dry a feed product from 68% moisture content (wet basis) to 5.5% moisture content (wet basis). The drying air enters the system at 54 degrees C and 10% RH. The product temperature is 25degreesC throughout drying. Compute the quantity of air required for drying on the basis of 1 kg of product solid. a. 224.65 kg dry air/kg solids b. 268.4 kg dry air/kg solids c. 302.27 kg dry air/kg solids d. 412.36 kg dry air/kg solids

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A concentrated dairy product is being spray dried to a final moisture content of 4% using 120 degrees Heated air with 7% RH. The density of the concentrated liquid is 1000 kg/m3 and the density of the dry product is 1250 kg/m3. Separate measurements have indicated that the critical moisture content for the product is 0.45 kg water/kg product and the particle diameter at the critical moisture content is 45 um. The latent heat of vaporization during the constant rate period of the drying is 2130 kJ/kg and it is 2375 kJ/kg during the falling rate period. Estimate the drying time for the product based on the largest droplet diameter of 120um. 2.31 s b. 3.25 s c. 4.08 s d. 4.65 s

A tunnel dryer is being designed for drying apple halves from an initial moisture content of 79% (wet basis) to a final moisture content of 5%(wet basis). An experimental drying curve for the product indicates that the critical moisture content is 25% (wet basis) and the time for constant rate drying is 5 minutes. Based on information provided, estimate the total drying time for the product. Assume 1 kg of product is the basis. 4.25 min b. 5.81 min c. 6.54 min d. 6.89 min

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fluidizeD -bed dryer is being used to dry diced carrots. The product enters the dryer with 80% moisture content (wet basis) at 25*C. The air used for drying enters the dryer at 120 *C after being heated from ambient air with 60% RH at 20*C. Estimate the production rate when air is entering the dryer at 700 kg dry air/hr and product leaving the dryer is 10% moisture content( wet basis). Assume product leaves the dryer at the wet bulb temperature of air and the specific heat of product solid is 2.0 kJ/kg .*C. Air leaves the dryer 10*C above the product temperature. a. 11.25 kg solids/hr b. 16.12 kg solids/Hr b. c. 18.69 kg solids/hr d. 20.15 kg solids/hr

7. A tray is to be used for drying a solid material . Each tray is 3 ft. wide by 4 ft. long and the wet material has depth of 2 in. The density of the wet material is 50 lb/ft3. The wet material contains 3 lb of water per pound of dry solid. How many trays are necessary to obtain 3,000 lb of a product containing 1 lb of water per pound of dry solids. a. 55 trays b. 60 trays c. 50 tray d. 65 trays

6. A granular material of density 5000 kg/m3 is to be dried in a fluidised bed dryer using directly heated air at 811 K. The particle size is 0.5 mm and 1.26 kg/s of water is to be removed from 12.6 kg/s of solid feed at 293 K. What diameter of bed should be specified? a. 3.20 m b. 4.60 m c. 5.30 m d. 5.80 m

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9. A 100 kg batch of granular solids containing 30 per cent moisture is to be dried in a tray drier to 15.5 per cent of moisture by passing a current of air at 350 K tangentially across its surface at a velocity of 1.8 m/s. If the constant rate of drying under these conditions is 0.0007 kg/s m2 and the critical moisture content is 15 per cent, calculate the approximate drying time. Assume the drying surface to be 0.03 m2/kg dry mass. a. 2.25 h b. 3.25 h c. 4.25 h d. 5.25 h

8. A porous solid is dried in a batch dryer under constant drying conditions. Five hours were required to reduce the moisture content from 30 to 12 lb water/lb dry solid. The critical moisture content is 18 lb water/lb dry solid and the equilibrium moisture is 5 lb water / lb dry solid. If the drying rate during the FRP is a straight line through the origin, calculate the time required to dry the solid from 30 to 8 lb water/lb dry solid. a. 4.75 h b. 7.74 h c. 10.15 h d. 15.2 h

10. An adiabatic tunnel dryer handles 100kg of banana chips per batch reducing the moisture content from 50%and 12% wet basis. Drying takes10 hours to

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complete by flowing air at C and 5% R.H. The air leaves the dryer saturated. Assuming that the rate of drying constant, the volume of air blown in is a. 7840m3/hr b. 222m3/hr c. 250 m3/hr d. 7000ft3/hr

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