SEMESTER 1, 2016
Examination for the Degree of B.E. (Chemical/Pharmaceutical)
Examination for the Degree of M.Eng. (Chemical)
105527
105994
CHEMENG 3035 Multi-Phase Fluid and Particle Mechanics
CHEMENG 7050 Multi-Phase Fluid and Particle Mechanics PG
Official Reading Time:
Writing Time:
Total Duration:
10 mins
180 mins
190 mins
Part
Questions
Time
Marks
A
B
Answer all 1 question
Answer ANY 3 out of 4 questions
60 mins
120 mins
60 marks
120 marks
180 Total
Instructions
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Part A is a Closed Book examination – Calculators are not permitted.
Part B is an Open Book examination during which reference material may be
consulted, and calculators may be used.
Part A must be handed in before commencing Part B.
Part B may be attempted early provided that Part A has been handed in.
Answer Parts A and B in separate books.
Begin each question on a new page.
The marks for each question are indicated.
Write your name and Student ID number on all loose diagrams/papers.
Examination materials must not be removed from the examination room.
Materials
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
Course notes and text books are permitted for Part B.
Calculators are permitted for Part B only.
The use of a dictionary is permitted.
Graph paper.
Attachments:
1. Drag coefficient – Reynolds number chart for non-spherical particles
2. Eckert correlation and Packing factor data
3. Graph papers
DO NOT COMMENCE WRITING UNTIL INSTRUCTED TO DO SO
Examination 2016
CHEMENG 3035 (105527)
CHEMENG 7050 (105994)
PART A. CLOSED BOOK
Question 1
(60 marks)
Give brief answers to the following questions. Marks for individual questions are as indicated.
a.
Consider a ring-shaped particle having outside diameter D, thickness a, and
length L. Define, and derive expressions for, the following properties of the
particle in terms of its dimensions (D, a, L):
b.
A.
Volume-equivalent diameter (dV)
B.
Surface-volume diameter (dSV)
C.
Sphericity ()
[6]
Name and define the most appropriate mean particle size to describe the
particles in the following situations:
c.
A.
Aerosols
B.
Ink jet printing
C.
Finely ground coffee
[3]
Describe two techniques for particle size measurements. Explain why particle
size values obtained from these different techniques may not necessarily be the
same. What considerations should be made when selecting a particular
technique for particle size measurement?
d.
[4]
What is log-normal size distribution? Explain how mean particle sizes and
standard deviation can be obtained from this mode of size distribution.
e.
Under
what
condition
does
hindered
settling
occur?
[3]
Describe
the
characteristics of this type of settling behaviour, and explain how the hindered
settling velocity can be determined experimentally and analytically.
f.
[4]
What are the main variables that affect the velocity of a gas bubble rising in a
liquid? Describe how the terminal velocity of the bubble changes with increasing
size. Also, explain how the bubble velocity can be determined given its size. [4]
g.
Define two characteristic dimensions that can be used to describe a porous
medium formed by solid particles, and show that these dimensions are
equivalent.
[4]
Question 1 continues next page
Multi-Phase Fluid & Particle Mechanics
Page 2 of 11
Examination 2016
CHEMENG 3035 (105527)
CHEMENG 7050 (105994)
Question 1 (continued)
h.
Consider a porous medium initially filled by a liquid. A gas is then injected under
pressure to displace the liquid from the medium.
A.
Define relative permeability and saturation, and describe, with the aid of
diagrams, the flow characteristics of each fluid phase in terms of relative
permeability as a function of saturation.
B.
Explain how the frictional pressure drop across the porous medium can be
determined from the individual phase pressure drops.
i.
[3]
[2]
Describe the four groups of powder according to the Geldart classification. What
would the likely mode of fluidisation be when each of these powders is fluidised
by air?
j.
[4]
What are the two limiting fluid velocities in a fluidised bed? How can these
velocities be determined for beds of particles having diverse sizes?
k.
[4]
Under what conditions would a bed of particles be aggregatively fluidised?
Describe the various stages of bubbling fluidisation as the fluid velocity is
increased.
l.
[4]
What are the four possible concentration zones found in type II settling slurry?
Clearly explain how the movements of the interfaces associated with these
zones can be predicted from batch solids flux data.
m.
[5]
The general filtration equation is usually given in the following form
R   dV
 C 
P   S2 V  m 
A  dt
 A
A.
Explain how the parameters  and Rm for compressible materials can be
determined from constant-rate filtration experiments.
B.
[5]
Under what circumstance is “constant-rate followed by constantpressure filtration” practised? Develop an expression for the filtrate
volume as a function filtration time for this hybrid mode of filtration
operation. Define all terms used.
[5]
END OF PART A
Multi-Phase Fluid & Particle Mechanics
Page 3 of 11
Examination 2016
CHEMENG 3035 (105527)
CHEMENG 7050 (105994)
PART B. OPEN BOOK
ANSWER ANY THREE (3) QUESTIONS
Question 2
(40 marks)
A fluidised bed is to be used for mixing three solid powder ingredients of a
pharmaceutical formulation in the production of tablets by pressure granulation.
Three different types of powder, with characteristics shown in the Table below, are
fed to the mixer at a total quantity of 200 kg per load. The bed is fluidised by air at
atmospheric pressure and 20oC.
Powder
Quantity
(kg)
Volume
Diameter
Density
Sphericity
(kg/m3)
 (-)
dV (m)
a.
E
100
200
960
0.81
Z
60
150
1020
0.81
Y
40
125
850
0.81
Calculate the number volume-mean diameter (dVn), the number surface-volume
mean diameter (dSVn), and the average density of the powder mixture.
b.
Calculate the minimum air velocity required to fluidise the bed.
c.
What is the allowable maximum air velocity for this fluidised bed solids mixer?
d.
In order to ensure complete mixing, it is required to fluidise the solids to a depth
equal to two (2) times the bed diameter. For a fluidising air velocity of 0.1 m/s,
calculate the diameter of the fluidised bed and the air flow rate required for the
operation. Assume particulate fluidisation.
Refer Attachment 1 for drag coefficient (CD) versus particle Reynolds number (Rep)
for non-spherical particles.
Multi-Phase Fluid & Particle Mechanics
Page 4 of 11
Examination 2016
CHEMENG 3035 (105527)
CHEMENG 7050 (105994)
Question 3
(40 marks)
You are planning to use an existing 1.2-m diameter tower, packed with 3.6 m of 50mm ceramic Raschig rings, as a counter-current gas scrubber to treat process air
using water as the absorption liquid. The air is to be fed at 1.1 atm and 300C and the
desired degree of gas absorption can be achieved with a water-to-air mass flow rate
ratio of 5.
a.
You found that flooding just occurred in the packed tower when operating under
the condition stated above. What is the corresponding air flow rate [in m3/h]?
b.
One option you plan to try is replace the existing packing with new plastic Pall
rings having the same nominal size (50 mm). Would this solve the flooding
problem? What would the resulting pressure drop across the packed tower be
with the new packing?
c.
How much higher flow rates could be operated before flooding occurs in the
tower packed with the 50-mm Pall rings?
Data:
Air:
Molecular mass:
Water:
29
Gas constant (R): 0.082 atm.m3/kgmol.K
Average Density:
990 kg/m3
Average Viscosity: 0.9810-3 Pas
See Attachment 2 for Packing factor (Fp) data and Eckert correlation.
Multi-Phase Fluid & Particle Mechanics
Page 5 of 11
Examination 2016
CHEMENG 3035 (105527)
CHEMENG 7050 (105994)
Question 4
(40 marks)
A slurry containing 20 wt% solids (SG = 1.8) in water at 25oC is to be filtered in a
plate-and-frame filter press. The slurry and filter medium are first tested in a constantpressure laboratory filter that has an area of 0.05 m2, at a fixed filtration pressure
drop of 300 kPa. It is found that 1.0 L of filtrate is collected after 10 seconds, and 4.0
L is collected after 75 second of filtration. The plate and frame filter has 20 frames,
with 0.873 m2 of filter medium per frame, and operates at a constant filtration rate
of 120 L/min of filtrate. The filter is operated until the pressure drop reaches 300 kPa,
at which time it is shut down for washing and cleaning. The filter cake is washed at
the same rate as the filtration rate using a volume of wash water equal to one-quarter
of the total filtrate volume collected. Dismantling, cleaning and refitting of the filter
takes 20 min. Assuming incompressible cake, determine:
a.
The specific cake resistance , and the medium resistance Rm.
b.
Volume of filtrate collected and thickness of filter cake produced per cycle.
c.
The filtration cycle time.
Multi-Phase Fluid & Particle Mechanics
Page 6 of 11
Examination 2016
CHEMENG 3035 (105527)
CHEMENG 7050 (105994)
Question 5
(40 marks)
The settling data in the Table below was obtained from batch settling experiments
with slurries containing fine mineral solids (SG = 2.3) in water.
a.
Solids volume fraction
Settling Velocity
c (v/v)
vp (mm/s)
0.068
0.729
0.102
0.549
0.136
0.418
0.170
0.306
0.204
0.224
0.272
0.109
0.340
0.047
0.408
0.016
0.442
0.009
Use the graph paper provided, construct a curve showing the relationship
between solids flux and solids concentration.
b.
A continuous thickener is required to process 280 m3/h of the feed slurry
containing 20 wt% solids. The underflow velocity is to be fixed at 0.6 m/h.
Determine the minimum cross sectional area required for the thickener, and the
highest solids concentration that can be achieved in the underflow.
c.
The thickener to be used has a cross sectional area of 120 m2. Assess the
performance
of
the
thickening
operation
by
determining
the
solids
concentrations in the underflow, in the overflow, and in the thickening zone
below the feed line.
d.
During operation under the same flow conditions as above, the feed
concentration may increase by 20% of the design value. Estimate how this
change impacts on the thickener performance in terms of solids concentrations
of the exit streams, and in the thickening zone below the feed line.
END OF PART B
END OF THE EXAMINATION PAPER
Multi-Phase Fluid & Particle Mechanics
Page 7 of 11
Examination 2016
CHEMENG 3035 (105527)
CHEMENG 7050 (105994)
ATTACHMENT 1 – Drag Coefficient – Reynolds Number chart for non-spheres
Multi-Phase Fluid & Particle Mechanics
Page 8 of 11
Examination 2016
CHEMENG 3035 (105527)
CHEMENG 7050 (105994)
ATTACHMENT 2: ECKERT CORRELATION
G 2FP L0.2 w

g G  L  L
Table: Packing Factor (Fp) for Some Packing Materials
Packing Type
Nominal size, mm
Packing factor Fp, m-1
Raschig rings,
ceramic
25
50
75
25
38
50
90
510
215
120
170
105
82
52
Pall rings,
polypropylene
Multi-Phase Fluid & Particle Mechanics
Page 9 of 11
Examination 2016
Multi-Phase Fluid & Particle Mechanics
CHEMENG 3035 (105527)
CHEMENG 7050 (105994)
Page 10 of 11
Examination 2016
Multi-Phase Fluid & Particle Mechanics
CHEMENG 3035 (105527)
CHEMENG 7050 (105994)
Page 11 of 11