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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 3035 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
nonsettling 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 roastleach—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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