(i) The superficial gas velocity at which fluidisation begins.
(ii) The air velocity at which the particles begin to leave the bed.
(iii) What is the depth of the bed when the air velocity is twice that required for fluidisation.
- Particle diameter 80 microns
- Particle density 1,650 kg / m3
- Bed voidage at rest 0.367
- Bed voidage at incipient fluidisation 1.05 epsilon
- Bed height at rest 1.6 m
Q - 2 : A batch centrifuge is used to centrifuge an aqueous slurry. Using the data below calculate :
(a) The volume of dry cake produced per batch.
(b) The filtration time per batch.
(c) Calculate the average hourly throughput if :
- Cycle time = (filtration time x 150 %) + 5 minutes for unloading
- Solids concentration in slurry, Cx = 0.35 kg solid / kg slurry
- Cake compressibility, a = 1.28 x 1,011 m / kg
- Cake density, rc = 2,200 kg / m3
- Centrifuge speed = 4,200 rpm
- Basket diameter = 720 mm
- Basket length = 1.8 m
- Maximum cake thickness = 120 mm
- Distance from liquid surface to axis of rotation = 240 mm
State any assumption made.
Q - 3 : (a) A centrifugal pump operates at a speed of 1,440 rpm with an impeller of inside diameter 200 mm and an outside diameter of 400 mm. The backward facing blades are at an angle of 30O at the inside of the impeller and 25O at the outer edge. The width of the flow passage across the impeller is 35 mm. Calculate the water flowrate, the pressure drop across the pump and the pump efficiency if the pump uses 150 kW. (b) A centrifugal pump is located 5 m above the surface of the water in an open tank from which it is being fed. The water is at 50OC and the friction head on the pump suction line amounts to 2 m. Calculate the net positive suction head.
Q - 4 : (a) For the homogeneous flow model, the pressure gradient in a pipe of diameter (di) inclined at an angle of (theta) to the horizontal is given by :
(b) Air and water flow at 0.08 kg / sec and 0.32 kg / sec respectively in a horizontal tube of inside diameter 25 mm. The mean pressure in 9.90bar (absolute) and the pressure drop across a length of 5 m in 0.215 bar. What is the value of the friction factor ?
Data :
At 9.90 bar VG = 8.28 x 10-2 m3 / kg
VL = 1.02 x 10-3 m3 / kg
Q - 5 : A standard (Rushton) configuration CSTR open to the atmosphere of volume 1.5 m3 is used to agitate water at 80 rpm such that impeller consumption is 200 W. If air is sparged to the vessel at a rate of 10 m3/hr while the impeller power output remains constant, what new reactor speed will be observed ?
Q - 6 : Write notes on the scale up of liquid mixing systems describing the various types of similarity which can exists. By what percentage should impeller diameter change if the rotational speed is decreased by one third in scaling up a Rushton configuration reactor system which requires constant power per unit volume.
