Presented by:
Nirmit Dave

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AGITATION AND MIXING
Agitation:-
Agitation refers to the induced motion of a material in a specified way, usually in circular pattern inside some sort of container.
Mixing:-
Mixing is the random distribution, into and through one another , of two or more initially separate phases.
AGITATION AND MIXING
Purposes of agitation:-
1.Suspending solid particles.
2.Blending miscible liquids.
3.Dispersion of liquid, which is immiscible with other liquids, to form
emulsion or a suspension of fine drops.
4.Dispersion of gas through a liquid in the form of small bubble.
5.Promoting heat transfer between the liquids and a coil or jacket.
AGITATION AND MIXING
Agitated Vessels:-
AGITATION AND MIXING
Agitated vessels:-
- In agitation mostly cylindrical vessel with a vertical axis is used.
- The tank bottom is rounded, not flat, to eliminate sharp corners or regions into which fluid currents would not penetrate.
- Liquid depth is approximately equal to the diameter of the tank.
- An impeller is mounted on shaft, that is, a shaft is supported from above.
- The shaft is driven by a motor, sometimes, directly connected to the shaft but more often connected to it through a speed-reducing gearbox.
- Accessories such as inlet and outlet lines, coils, jackets, and wells for thermometers or other temperature-measuring devices are usually included.
- Baffles are usually included to reduce tangential motion.
AGITATION AND MIXING
Impellers:-
1.Axial flow impellers:-
Generate currents parallel with the axial of the impeller shaft.
2.Radial flow impellers:-
Generate currents in a radial or tangential direction .
- For low to moderate viscosity liquids propellers, turbines, and high efficiency impellers are used.
- For very viscous liquids, the most widely used impellers are helical impellers and anchor agitators.
AGITATION AND MIXING
Various type of impellers:-
AGITATION AND MIXING
Propellers:-
- It is an axial flow, high speed impeller for liquids of low viscosity. Small propellers speed is 1150 to 1750 rpm; larger one speed is 400 to 800 rpm.
- The direction of rotation is usually chosen to force the liquid downward, and the flow currents leaving the impeller continue until deflected by the floor of vessel.
- As shown in figure the standard three-blade marine propellers with square pitch are most common; four blade, toothed, and other designs are sometimes employed for special purposes.
AGITATION AND MIXING
Turbines:-
- The simple straight blade turbine pushes the liquid radically and tangentially with almost no vertical motion at the impeller.
- The currents it generates travel outward to the vessel wall and then flow either upward or downward. Such impellers are sometimes called paddles
- It can rotate at the speed of 20 to 150 rpm.
- A pitched blade turbine is used when good overall circulation is important, because it provides some axial flow in addition to the radial flow.
AGITATION AND MIXING
Flow pattern:-
The way a liquid moves in an agitated vessel depends on
- Type of impeller
- Characteristics of the liquid, specially its viscosity
- Size and proportions of the tank
- Baffles
- Impeller
The liquid velocity at any point in the tank has three components
1. Radial component acts in a direction perpendicular to the shaft of impeller.
2. Longitudinal component acts in a direction parallel with the shaft.
3. Tangential component or rotational acts in a direction tangent to a circular path around the shaft.
AGITATION AND MIXING
- When the shaft is vertical and centrally located in the tank, the tangential component is generally disadvantageous. The tangential flow follows a circular path around the shaft and create a vortex in the liquid.
- If solid particles are present, circulatory currents tend to throw the outside by centrifugal force; from there they move downward and to the center of the tank at the bottom. Instead of mixing, its reverse concentration occurs.
- If the swirling is strong, the flow pattern in the tank is virtually the same regardless of the design of the impeller, and gas from above the liquid is drawn down into the charge.
AGITATION AND MIXING
Prevention of swirling:-
Circulatory flow and swirling can be prevented by three methods.
1. In small tanks, the impeller can be mounted off center. The shaft is moved away from the centerline of the tank, then tilted in a plane perpendicular to the direction of the move.
2. In larger tanks, the agitator may be mounted in the side of the tank,
with the shaft in a horizontal plane but at an angle with a radius.
3. In large tanks with vertical agitators, the preferable method of reducing swirling is to install baffles, which impede rotational flow without interfering with radial or longitudinal flow.
- Except in very large tanks, four baffles are sufficient to prevent swirling and vortex formation.
- For viscous liquids even narrower baffles are generally used, and baffles are not needed at all when µ>10 Pa•s.
- Baffles are also not needed with side-entering, inclined, or off-center propeller.
AGITATION AND MIXING
Flow number:-
- Volumetric flow rate in the agitator is proportional to the cube of impeller diameter.
q∞nDa³
- The ratio of these two quantities is called the flow number NQ, which is defined by
NQ=q/nDa³
- For marine propellers NQ=0.50
- For a four blade 45º turbine NQ=0.87
- For a disk turbine NQ=1.30
- For HE-3 high efficiency impeller NQ=0.47
AGITATION AND MIXING
Velocity patterns and velocity gradient:-
- As the fluid leaves the impeller blades, the radial component of the fluid velocity, at the centerline of the impeller is about 0.6 times the tip speed.
- The radial velocity decrease with vertical distance from the centerline.
- As the jet travels away from the impeller, it slows down because of the increased area for flow and because more liquid is entrained.
- Along the centerline of the impeller, the velocity drops more or less linearly with radial distance.
- The total volumetric flow increases with radius because of further entrainment and then drops near the vessel wall because the flow has started to divided into the upward and downward circulation loops.
- The velocity gradients in an agitated vessel vary widely from one point to another in the fluid.
AGITATION AND MIXING
- Increasing the impeller speed increases the tip velocity and the circulation rate. It does not increase the fluid velocity at a given location in the same proportion, for a fast-moving jet entrains much more material from the bulk of the liquid than a slower-moving jet does, and the jet velocity drops very quickly with increasing, distance from the impeller.
- The velocity near the wall is very much less because near the wall shear rate is much lower and the apparent viscosity may be much higher.
- To avoid forming stagnant regions in the vessel an axial flow impeller or helical impeller is required.