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Unformatted text preview: MURDOCH
UNIVERSITY PERTH, WESTERN AUSTRALIA . .
Examinations  Semester 1, 2007 nit EXM224 PRINCIPLES OF UNIT OPERATIONS
ExamType  Internal & External 3 hours plus 10 minutes reading time INSTRUCTIONS Time Allowed 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 Universit_v_ NIL Provided by the Candidate
CLOSED BOOK EXAMINATION CALCULATOR PERMITTED (NO QWERTY KEYBOARD) Question 1 (a) (b) One of the products of a processing plant is fed at a ﬂow rate of 3 t/h to a
powder production unit P, followed by two screens S1 and S2. Seventy ﬁve
percent (75%) of the powder from P passes through S1 (ﬁrst screen), and the
rest (oversize material from S1) is returned to P. The undersize material from S2
(second screen) is 60% of the feed to 32. This fraction is fed to the next unit
operation. The oversize fraction from S2 is also returned to P. Carry out a mass balance to calculate:
(i) the mass ﬂow rate of material passing through each screen
(ii) the overall efﬁciency of the screening process. [6 marks]
[4 marks] Water enters a boiler at 18°C and 138 kPa through a pipe at an average velocity
of 1.52 m/s and a volume ﬂowrate of 4.32 m3/h. Exit steam at a height of 12.4 m
above the liquid inlet leaves at 138 kPa, and a velocity of 10.1 m/s at 149°C.
The values of enthalpy of the two streams at 18°C and 149°C (from steam tables) are 76.98 kJ/kg and 2771 kJ/kg respectively. Calculate the heat added
per kg steam at steady state. AE_dQ dW " 2dmi
F—E—F+Z(Hl+gZI+O.SVI)—‘a‘t’ i=1 [10 marks] Question 2 A ﬂuid (SG = 0.85) is being pumped from a sump at atmospheric pressure of 101 kPa
to a tank under a pressure of 160 kPa using a galvanized steel pipe of internal diameter
100 mm and a total length of 90 m at a volume ﬂow rate of 48 m3/h. The ﬂuid is at rest
in both tanks. The elevation of the ﬂuid level in the sump and the receiving tank is 2 m
and 12 m respectively. The efﬁciency of the pump is 81%. Friction factor of the pipe is
0.021. A list of ﬁttings and other relevant information is shown below. Number and type of ﬁttings Resistance per ﬁtting (K) 20 union (screwed) 0.05
1 lift valve (with strainer) 10
1 gradual enlargement (transition, 20 °) 0.4
1 globe valve (half open) 12.5
1 gate valve 0.2
3 elbow 0.9
1 sudden exit 1.0
V2 V2
—p—‘+zI +—‘+hA hL =p—2+z2 +—2
7 2g 7 2g
L V2
hL ( f D + 2 K) 2g
Calculate:
(i) the ﬂuid velocity [5 marks]
(ii) the total head loss [5 marks]
(iii) the total system head [5 marks]
(iv) power input [5 marks] Question 3 (a) Discuss the main ﬂow regimes encountered in settling slurries.
[5 marks] (b) A nonsettling slurry consisting of ﬁne phosphate particles exhibits Newtonian
behaviour. The concentration of solids in the slurry is 20% solid by volume.
The slurry is to be pumped through a 5 cm diameter pipe at a velocity of 2 m/s.
The slurry viscosity may be calculated by: “5 =1+2.5(:V+locv2
#0 You may use the Moody’s diagram given in Figure 1 (page 4) to obtain the
friction factors. SG of phosphate particles is 3.2. Determine: (i) The frictional head loss over a 20 m length of pipe. [10 marks]
(ii) Wall shear stress.
[3 marks]
(iii) The amount of solids delivered per hour.
[2 marks] Question 4 (a) Many metals can be dissolved by oxidation using dissolved oxygen by a general
reaction (for a divalent metal ion): 2M + 02 + 4H* = 2M2+ + 2H20 For highly reactive metals at elevated temperatures, the rate of dissolution is
controlled by the transfer of dissolved oxygen from the gas phase into the
aqueous phase. (i) Calculate the maximum rate of dissolution of M (in mol s'l) in a stirred
batch reactor open to the atmosphere at 25°C. The surface area exposed
to the atmosphere is 1m2 and the solubility of oxygen in equilibrium
with air at this temperature is 2 x 104 mol L". The mass transport
coefﬁcient for transfer of dissolved oxygen from the interface under
these conditions is 1x104cm s". [6 Marks] (ii) Sketch the graph of the concentration of M2+ in the reactor as a function
of time. [2 Marks] (iii) Suggest with reasons, two methods whereby the rate of dissolution of M
could be increased. [2 Marks] (b) Ion exchange resin has found numerous applications in the mineral processing
industry. The recovery of dilute concentrations of ions, such as gold, zinc,
copper and uranium oxide are common in hydrometallurgical processes. The kinetics of cation adsorption onto resin (AiS —> Air) has often been linked to
a second order reaction rate. For such system ﬁnd the outlet concentration of Ais
as a function of inlet concentration, volumetric flow rate and volume of the
vessel at steady state conditions, if:
(i) The ion exchange vessel is assumed to be a CSTR.
(ii) The ion exchange vessel is considered as a Plug Flow Reactor. [10 marks] Question 5 (a) Air at 206.8 kPa an average temperature of 477.6 K is being heated as it ﬂows
through a tube of 25.4 mm internal diameter. The heating medium is 488.7 K
steam condensing outside the tube. Calculate the heat ﬂux. Q = h A AT
Nu = h—D
k
D 0.33 0.14
Nu =1.86[Re.Pr——j (Laminar ﬂow)
L #W
0.14
Nu = 0.027 Reo'8 Pro‘33 (Turbulent ﬂow)
7%
k = 0.03984 W/(m.K)
Pr = 0.686
Re = 1.1 x 104
Bulk viscosity of air at 477.6 K = 2.60 x 10‘5 Pa.s
Viscosity of air at wall temperature 488.7 K = 2.64 x 10'5 Pas.
[12 marks]
(b) Grinding mills are generally very inefﬁcient, difﬁcult to control, and costly, in terms of both steel (grinding media) and power consumption. Researchers have
made a signiﬁcant effort to model the energy loss from grinding mills arising in
the form of heat transfer from load to outside atmosphere using pilot dry batch
mills. The load represents the grinding media (steel balls). (i) Calculate the rate of heat transfer from the load to atmosphere per square
meter of the mill surface at steady state. Overall heat transfer coefﬁcient
is 15 W/(m2.K). Load temperature and atmospheric temperature are 74
OC, and 20 0C respectively.
[3 marks]
(ii) Brieﬂy discuss the mechanism of heat transfer from the load to
atmosphere and the effect of feed material on the calculated value in part
(i).
[5 marks] Figure 1 (Moodyfs diagram) 3% a .23 a. 5:3 .52 “View
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