Solutions to selected self-study problems

Self-Study problemsIndustrial Transfer Processes (ME5202)(2011-2012)Please ignore the questions inredYou may refer to internet sources, textbooks and handbooks for data, empirical correlation etc. Several ofthe problems are open-ended and can have multiple correct responses.Q 1. It is required to design a dryer to dry sand with a throughput of 10 t/h.1. Recommend possible dryer types for this purpose. Discuss possible fuels that may be used tosupply the heat necessary for drying. Compare relative advantages and limitations of each fueltype both in terms of cost and emission of greenhouse gases.2. It is proposed to use a continuous fluid bed for this application. Estimate the size and operatingconditions you would recommend for drying sand from 10% db to 1% db. Obtain necessary datafor design calculations from the open literature. What is the minimum operating velocity and whatshould be the range of operating velocity for the fluid bed?3. Discuss briefly possible ways of improving the energy efficiency of such a bed dryer.Q1 (1) Hints - Students need to find this themselvesQ1 (2) Hints - Based on the sand properties from literature and assumed drying conditions including the bedheight, calculate the total heat needed to be supplied to dry 10 t/h of sand from 10% to 1%. This muchquantity of heat needs to be supplied using the fluidizing gas (air in this case). from heat balance calculatethe quantity of air to be used. For selected sand properties calculate the minimum fluidization velocity andfind the actual velocity used which will give the diameter of the continuous fluid bed dryer. Refer to Van'tLand 1991 for more detailed calculations.Q1 (3) Hints - Students can refer to handbook of Industrial Drying for more detailsQ 2. It is desired to dry lignite from 60% moisture dry basis to 25% dry basis . Production capacity neededis 100t/h of feed material.1. Examine possible alternative drying strategies and discuss them in order of your preference. Statebriefly reasons for your choice. List all key selection criteria you have considered. Refer to heopen literature for properties and drying kinetics data you may need. You may select appropriatephysical and thermo physical property data for lignite. Provide the references used in yourestimation.2. Estimate the improvement in calorific value obtained by drying. Based on current market valueof lignite what is the economic advantage of drying such high rank coal (per ton of dry coal basis).Q 2 Hints. Students should answer based on reading from the references belowReference:Jangam, S.V., Karthikeyan, M., Mujumdar, A.S. A critical assessment of industrial coal drying technologies:Role of energy, emissions, risk and sustainability, Drying Technology 2011, available online.

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

Q09. An insoluble granular material wet with water is being dried in a pan 0.457 × 0.457 m and 25.4 mmdeep. The material is 25.4 mm deep in the metal pan, which has a metal bottom of thickness z M = 0.610mm having a thermal conductivity k M = 43.3 W/m.K. The thermal conductivity of the solid can be assumedas k S=0.865 W/m.K. Heat transfer is by convection from an air stream following parallel to the top dryingsurface and the bottom metal surface at a velocity of 6.1 m/s and having a temperature of 65.6 °C andhumidity H = 0.01 kg H 2O / kg dry air. The top surface also receives direct radiation from steam heatedpipes whose surface temperature T R = 93.3 °C. The emissivity of the solid is ε = 0.92. Estimate the rate ofdrying for the constant-rate period.State all assumptions you make.Examine the following1. Effect of changing air velocity to 8 m/s , other parameters being held constant2. Effect of reducing humidity to 0.005 kg/kg, other parameters held constant3. Thickness of the bed reduced to 15 mm, other parameters held constant4. If instead of drying air flowing parallel to the drying surface, what if we use an array of impinging jets?Comment on this design suggestion.