11. Agitation & Mixing [EngineeringDuniya.com]

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Agitation & Mixing of fluids
In process industries many operations are dependent on effective agitation and mixing of fluids.
Agitation refers to forcing a fluid by mechanical means to flow in a circulatory or other pattern inside a vessel.
Mixing usually implies the blending of two or more separate phases, such as a fluid and a powdered solid, or two fluids, and causing them to be randomly distributed through one another.
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Purposes of Agitation
Blending of two miscible liquids, such as ethanol and water.
Dissolving solids in liquids, such as salt in water.
Dispersing a gas in a liquid as fine bubbles, such as oxygen from air in a suspension of microorganisms for fermentation or for the activated sludge process in waste water treatment.
Suspending of fine solid particles in a liquid, such as in the catalytic hydrogenation of a liquid where solid catalyst particles and hydrogen bubbles are dispersed in the liquid.
Agitation of the fluid to increase heat transfer between the fluid and a coil or jacket in the vessel wall.
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Equipment for agitation
Generally, liquids are agitated in a cylindrical vessel which can be closed or open to the air.
The height of liquid is approximately equal to the tank dia.
An impeller mounted on a shaft is driven by an electric motor.
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Measurements of turbine
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Various types of agitators
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Power needed in Agitated vessels
In the design of an agitated vessel, an important factor is the power required to drive the impeller.
The presence or absence of turbulence can be correlated with the impeller Reynolds number NRe,i, defined as,
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Power number vs. Reynolds number
One of important dimensionless number used in Agitation is Power number, NP
There are standard graphs to calculate power required for agitation with respect to the type of impeller.
From the graph of NP vs. NRe,i we can calculate the power required for agitation
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Power correlation for different impellers
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A flat blade turbine agitator with disk having six blades is installed in a tank. The tank dia Dt is 1.83m, the turbine dia Da is 0.61m. The tank contains four baffles. The turbine is operated at 90rpm and the liquid in the tank has a viscosity of 10cP and a density of 929kg/m3.
Calculate the required kW of the mixer.
For the same conditions, except for the solution having a viscosity of 100 000cP, cal the required kW
NRei = 5.185x104
NP=5---P=1.324kW
NRei = 5.185
NP=14 P=3.71kW
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It is desired to agitate a liquid having a viscosity of 1.5x10-3 Pa.s and a density of 969 kg/m3 in a tank having a dia of 0.91m. The agitator will be a six-blade open turbine having a dia of 0.305m operating at 180rpm. The tank has four vertical baffles each with a width of 0.076m. Calculate the required kW
NP=2.5
Power = 0.172kW
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A fermentation broth with viscosity 10-2 Pa-s and density 1000kg/m3 is agitated in a 50m3 baffled tank using a marine propeller (refer curve 5) 1.3 m in dia. Calculate the power required for a stirred speed of 4 rps.
P = 83kW
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Effect of viscosity on power requirement
A cylindrical bioreactor of dia 3m has four baffles. A Rushton turbine (refer Curve 1) mounted in the reactor has a dia one third the tank dia and is operated at a speed of 90rpm. The density of the fluid is approximately 1 g/cm3. The reactor is used to culture an anaerobic organism that doesn’t require gas sparging. The broth can be assumed Newtonian. As the cells grow, the viscosity of the broth increases.
Compare power requirements when the viscosity is:
Approximately that of water;
100 times greater than water; and
104 times greater than water
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Transportation of fluids
Pump classifications: 
NPSH
Characteristics and constructional details of centrifugal pumps 
Cavitation
Positive displacement pumps: 
Piston pumps 
Plunger pumps 
Diaphragm pump
Rotary pumps 
Gear pumps 
Lobe pumps 
Screw pumps
Airlift pump 
Jet pump 
Selection of pumps 
Fans, blowers, and compressors
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3RD SESSIONAL TEST
Includes both Problems and theory……
FLUIDIZED BED
SETTLING VELOCITY
AGITATION & MIXING
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