exam paper 2004 - MURDOCH U N I V E R S I T Y PERTH WESTERN...

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Unformatted text preview: MURDOCH U N I V E R S I T Y , , . PERTH, WESTERN AUSTRALIA FlrSt Semester Examinatlons 2004 Unit EXM224 PRINCIPLES OF UNIT OPERATIONS IN MINERAL PROCESSIN: Internal & External Time Allowed 3 hours plus 10 minutes reading time INSTRUCTIONS This paper contains 4 pages and 5 questions. Spend 30-35 minutes on each question. All questions are of equal value with respect to marks. Attempt all questions. EXAMINATION AIDS ALLOWED Provided by the University Provided by the Candidate Closed Book Examination ‘Calculator Permitted (no Qwerty Keyboard) Drawing Equipment Question 1. (a) Figure 1 shows the material flow in the extraction and refining unit operations of a metallurgical plant. Carry out a material balance to calculate the flow rates W, E, R and P. (10 marks) Figure 1 0.05% metal (waste) W (t/h) 10% metal 40% metal 96% metal 10 (V11) Extraction E (Uh) Refining P (Uh) (feed) (product) 20% metal (recycle) (b) An air stream at 32°C is to be preheated to 75°C in a steam heater. The air flow is 750 kmol/h. The steam enters the heater saturated at 145°C is condensed and cooled, and leaves as a liquid at 135°C. Calculate, using the data in Tables 1 and 2, the flow rate of steam input (kg/h) in this operation. (10 marks) Table 1 Mean molar heat capacities CP kJ/(kmol K) of gases at 101.325 kPa T (°C) N2 02 Air H20 25 29.14 29.38 29.19 33.59 100 29.19 29.66 29.29 33.85 200 29.29 30.07 29.40 34.24 Table 2 Enthalpy kJ/kg of saturated steam and water T (”C) Liquid Saturated Vapor 25 104.89 2547.2 50 209.33 2592.1 75 313.93 2635.3 100 419.04 2676.1 125 524.99 2713.5 130 546.31 2720.5 135 567.69 2727.3 140 589.13 2733.9 145 610.63 2740.3 150 632.21 2746.5 Question 2. A solvent must be delivered to point B in Figure 2 with a pressure 500 kPa. A pump is located at point A 15 m below point B. The two points are connected by a 60 m plastic pipe having an inside diameter of 50 mm. Other relevant information and equations are shown in relation to Figure 2. (a) Write the Bernoulli’s equation incorporating hA and h which represent the energy added to the fluid and head loss due to friction respectively.(3 marks) (b) Calculate hA assuming that h is negligibly small. (7 marks) (c) Calculate the power delivered by the pump. (10 marks) Figure 2 pB = 500 kPa plastic pipe L = 60 m D = 50 mm v = 0.1 m/s ply + v2/2g + z = constant (Bernulli’s equation) Power = hA 7 Q Y 2 P g specific gravity of solvent = 0.88 Question 3. A slurry having 50% solids by mass of 1mm sand particles is to be pumped through a horizontal 10 cm diameter pipe, at an average velocity of flow of 3 m/s. Calculate: ’ (a) The solids concentration of the slurry in % solids by volume. (2 marks) (b) The amount of solids transported in one hour in t/h. (3 marks) (c) The head loss per unit length of pipe. (15 marks) Assume: i) The friction factor for smooth pipes carrying water may be calculated using the Blasius equation: ‘ f= 0.316 Re'0'25 ii) The fractional excess pressure drop ¢ is given by: ¢ = 82.0,.(JCD .Fr l” v2 where Fr = Froude no = ————-— g D (s - 1) iii) The drag coefficient CD of 1mm sand particles when settling in still water is 0.87. iv) hf = f. (L/D). (v2/2g) Question 4. Copper is electrodeposited on the walls of a cylinder through which a dilute solution containing 1.0 g/l of copper is flowed at a rate of 1.0 litre/min. The cylinder is a pipe with internal diameter 10 cm and length 50 cm. The mass transfer correlation for laminar flow through a short pipe is given by Sh = 1.62 Re“3sc“3(d/L)“3 = den) where Re = dU/v, Sc = v/D and d is the pipe diameter, L its length, D = 5 x 10'6 cmzs‘l is the diffusion coefficient of copper ions, U is the linear flow-rate, v = 0.01 cmzs'l is the kinematic viscosity and kL is the mass transfer coefficient. a) Calculate the mass transfer coefficient. (8 marks) b) Assuming that the rate of deposition is controlled by mass transport of copper ions to the surface of the pipe, calculate the concentration of copper ions in the solution leaving the pipe. (9 marks) (For a tubular reactor, C = C0 exp(—kt) where 1: is the residence time.) 0) Suggest two methods for increasing the rate of removal of copper in this reactor.(3 marks) Question 5 (a) The pressure drop (AP) of a fluid across a packed bed (column) of solid particles is given by the following equation. AI:_150V,L¢(1—a)2 +1.75m2 (1-5) L ¢2D2 £3 ¢D£3 V—W'—J¥_—~v—_J first term second term (i) Briefly explain the significance of the terms involved in this equation. (ii) In a laboratory experiment, you were asked to measure the pressure drop for different values of v using a column packed with solid particles by the laboratory technician. Explain how you would use the results to comment on the nature of the fluid flow (laminar or turbulent). (iii) In one of the experiments you had to unpack the column and repack using the same sample of solid particles. How would this affect your results? (8 marks) (b) Figure 3 shows a short heat recuperator where the two flows are separated by a temperature resistant material of 10 mm thickness with a heat conductivity of k = 1-2 W / (m K). The heat transfer coefficient on the hot side is h(hot) = 55 W /(m2 K), on the cold side meow) = 20 W / (m2 K). Calculate the following. (i) the overall heat transfer coefficient, (ii) the heat flow per square meter if the temperature of the hot flow is 600°C, of the cold flow 400°C. Cl = h (AT) A (convection) q = k (AT/x)A (conduction) (12 marks) Figure 3 i Cooled gas Cold air _L__________________.______[_____" “"i :C j“:::::: :: ::':L‘—»‘ Hot air End of Question Paper ...
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