Solutions to selected self-study problems

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Muthusamy, K., Wu, Z.H. and Mujumdar, A.S. Low-Rank Coal Drying Technologies—Current Statusand New Developments. Drying Technology 2009, 27(3), 403-415.Q 3. It is proposed to dry 20 tons of a dry specialty chemical powder in a circular fluid bed dryer. The wetfeed enters at 10% moisture content at 10 °C. The moisture content of the product is to be less than 1%.Average particle size is 400 µm. Specific of the dry material is 0.84 kJ/kg·K. Ambient air of specific heat of1.05 kJ/kg·K is heated to 150 °C by a heat exchanger with 60% efficiency. Calculate the total heat load,dryer dimensions and drying cost per ton of the product. (Please make necessary assumptions and use theproperties of air from literature). What are the other dryer types which can be used for this applicationand also provide advantages and limitations of these dryers over fluid bed dryer.Q3 Hints. Calculate the total heat needs to be supplied based in the tonnage and the moisture to be removedand other thermal properties of solids and water. Based on heat load calculate the diameter of the dryerusing the total gas flow at exit. Using the standard dimension rules decide the height of disengaging sectionetc. Calculate the blower power and other parameters.Q 4. A spry dryer is used to produce 500 kg/h of dried powder with 0.5% moisture content. The liquidfeed with a moisture content of 55 % (by wt) enters at 20 °C. The ambient air (10 °C) is heated to 200 °Cbefore it enters the dryer. Calculate the dryer dimensions to produce the necessary dried product. Suggesttype of atomizer which may be used for this application. Data:Specific heat of product: 1.25 kJ/kg·K; For water and air properties: please refer to relevant literatureQ 5. In a laboratory drying test of a granular material under certain drying conditions, it took 4.8 hours todry from 30% to 7.5% moisture content ( dry basis) while it took 4.75 hours to dry it from 24% to 5%moisture content. You are asked to determine the total drying time required to reduce the moisturecontent from 40% to 3% if the drying conditions are unchanged. The equilibrium moisture content for theconditions used for drying kinetic study is 1.4%; all moisture contents mentioned are on dry basis(Assume the ratio of dry mass to surface area of solid to be 18.825 kg/m2). The falling rate period islinear.Q5. Hints. From the first two conditions given (for 4.8 hours and 4.75 hours) calculate iteratively the value ofcritical moisture content and the drying rate in constant rate period. Using these values calculate the timerequired to dry the material from 40% to 3%
Q 6. A cake of crystalline solids is to be dried from 4.5 to 0.2 % moisture content (dry basis)by blowinghot air through the cake. The average diameter of the nonporous particles is 0.2 mm which are insoluble inwater hence the equilibrium moisture can be neglected. The cake is 2 cm thick and the apparent density ofis 2800 kg of dry solids/m 3 . The hot air enters the cake at 850 kg hr -1 m 2 at a dry bulb temperature of30 °C and 60% humidity. Determine the drying time.Please refer to the appropriate textbooks/handbooks for the correlations.Q 7. The surface moisture of crystals of NaCl of 0.5-mm average particle size is to be removed in acontinuous, direct-heat dryer without a significant change to the particle size. What types of dryers wouldbe suitable for this application?Q 7 Hints. Students should answer based on reading from the references/Lecture notes
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