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exam paper 2008

# exam paper 2008 - MURDOCH U N IV E R s I T Y Examlnatlons...

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Unformatted text preview: MURDOCH . . _ U N IV E R s I T Y Examlnatlons Semester 1, 2008 v PERTH, WESTERN AUSTRALIA ExamType : Internal & External Reading time : 10 minutes (Note taking allowed onto Exam Paper ONLY) Exam Duration : 3 hours INSTRUCTIONS This paper contains 3 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 Universiﬂ ANSWER BOOKLET Provided by the Candidate CLOSED BOOK EXAM - CALCULATOR PERMITTED (NO QWERTY KEYBOARD) Question 1 (20 marks) (a) Figure 1 shows a continuous atmospheric gravity separator used to separate two immiscible liquids A (SG = 0.81) and B (SG = 0.98). The depth of the two layers is to be maintained as shown in Figure 1. The total depth of the two immiscible liquids is ﬁxed by the overﬂow line for the light liquid A. The heavy liquid B discharges through an overﬂow leg with an adjustable height y maintained above the bottom of the vessel as shown in Fig. 1. The vessel and the overﬂow lines are vented to the atmosphere (Figure 1). Calculate the value of y in meters. Figure 1 Vent ' y Heavy Liqu I I 3 Heavy liquid Light liquid overﬂow overﬂow [6 marks] (b) Calculate the volume ﬂow rate of water in the pipe system shown in Figure 2 if the manometer deﬂection, h, is 250 mm. Figure 2 Direction of water ﬂow ‘—“‘* 60 mm 30 mm diamete . diameter I I Mercury (sg = 13.54) Water Bernoulli Equation : (p/y) + z +(V2/2g) = Constant g = 9.81 m 5'2 [14 marks] 1 Question 2 (20 marks] i) Brieﬂy discuss the ﬂow regimes encountered when the velocity of ﬂow is progressively increased over a settled bed of solids in a pipe. Also specify which regime gives the lowest pressure gradient. [4 marks] ii) With the aid of a shear stress versus velocity gradient plot, show how the following non-settling slurries behave: a) Newtonian‘slurry b) Bingham plastic slurry c) Pseudo plastic slurry [3 x 2 marks] iii) Determine the pressure gradient required to pump a non—settling slurry having 40% solids by mass of silica sand through a horizontal 5 cm diameter pipe at an average velocity of 1.5 m/s. [10 marks] Assume: a) The friction factor for smooth pipes carrying water may be calculated using the Blasius equation: f = 0.316 Re'o'25 b) The slurry viscosity is given by: f;— = 1+ 2.5Cv +1ocj- where C" = volume fraction of solids in the slurry and H0 = viscosity of ﬂuid. c) Viscosity of water = 0.001 Nsm‘2 . .,, be) Question 3 (20 marks] (a) An air stream from atmosphere (21% Oz and 79% N2) at 32 °C is to be used in a dryer. For this purpose air is ﬁrst preheated to 65 °C in a steam heater. Average heat capacity of air is 29.2 kJ/(kmol K). The air ﬂow is 500 kmol/h. The steam enters the heater saturated at 149 °C (AH = 2746.5 kJ/kg), is condensed and cooled, and leaves as a liquid at 138 °C (AH = 578.5 kJ/kg). Calculate the steam ﬂow rate in t/h. Assume a 12% heat loss in this process. [10 marks] (b) At the point of maximum efﬁciency a centrifugal pump running at 900 rpm delivers 38 US of water with a head of 20 m. The power input is 9.2 kW and efﬁciency 81%. Calculate capacity, head, power, and efﬁciency at a pump speed of 1800 rpm. 1/2 1/3 Afﬁnity laws: 92— : Ill = .121 = [fl—2] = [i] Q1 n1 D1 hl ‘PI [10 marks] Question 4 [20 marks[ The reductive dissolution of manganese dioxide particles (assume spherical particles of diameter 100 um and density 3 g/cm3 ) by ferrous ions occurs by the overall reaction Mn02(s) + 2Fe2+ + 4H“ = Mn2+ + 2Fe3+ + 2H20 The mass transfer coefﬁcient for transport of ferrous ions to the surface of the suspended manganese dioxide particles is 1 x 10'3 cm 5']. i) Calculate the maximUm possible initial rate of reaction (in mol Mn cm'2 s'1 and in g Mn (kg Mn02)'1 min'l) at a ferrous ion concentration of 0.02 mol drn'3 assuming that the reaction is mass transport controlled. [8 marks] ii) How would this calculated rate change with time? Why? Sketch the expected curve of fraction of MnOz dissolved as a function of time. [4 marks] iii) Assuming that the above calculated rate is constant for small extents of reaction, estimate the fraction of MnOz dissolved in an ideal CSTR reactor which has a feed slurry containing 0.01mol drn'3 ferrous ions. The residence time (1:) in the reactor is 10 min. [8 marks] Note: The surface area/unit volume of a sphere is 6/d where d is the diameter. The CSTR equation is C = C0 + Rate x r. In this case use C in mol/unit area of solid and note that the rate is a negative quantity. Question 5 [20 marks] (a) A roast-leach—electrowin process produces zinc metal from a sulﬁde concentrate which contains 80% ZnS, 10% SiOz (by mass), and moisture. The ﬂuid bed roasting of the concentrate is carried out at a feed rate of 5 t/h using 10% excess air (21% 02, 79% N2) to ensure the complete conversion of ZnS to ZnO. Calculate the mass ﬂow rate of air to the roaster. (molar masses: ZnS = 97.4, 02 = 32, N2 = 28, air = 28.84). [8 marks] (b) The cylindrical ﬂuidized bed roaster used in part (a) of this question is of 2 m internal diameter and 5 m height made up of steel of 5 mm thickness and conductivity 45 W/(m K). The cylindrical surface and ends have ﬁrebrick linings of 0.25 m thickness and a mean heat conductivity of 1.5 W/(m K). The inner temperature of the lining is 500°C. The outer temperature of the steel shell is 50°C. Calculate the heat losses through the reactor walls. Clearly state the assumptions involved in the calculation. Fourier equation: Q = AT ; Logarithmic mean area = Am, = M _x_ ln(A2 / A1) A k ‘ [12 marks] End of Question Paper ...
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exam paper 2008 - MURDOCH U N IV E R s I T Y Examlnatlons...

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