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 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 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: 0.9810-3 Pas 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