Mechanical Operations Chemical Engineering Unit V Sainath K Page 1 AGITATION AND MIXING Many processing operations depend for their success on the effective agitation and mixing of fluids. Agitation and mixing are not synonymous. Agitation refers to the induced motion of a material in a specified way, usually in a circulatory pattern inside some sort of container. Mixing is the random distribution, into and through one another, of two or more initially separate phases. Example, a single homogeneous material, such as tankful of cold water, can be agitated, but it cannot be mixed until some other material (such as a quantity of hot water or some powdered solid) is added to it. PURPOSE OF AGITATION Liquids are agitated for a number of purposes, depending on the objectives of the processing step. These purposes include 1. Suspending solid particles 2. Blending miscible liquids, for example, methyl alcohol and water 3. Dispersing a gas through the liquid in the form of small bubbles 4. Dispersing a second liquid, immiscible with the first, to form an emulsion or suspension of fine drops 5. Promoting heat transfer between the liquid and a coil or jacket Often one agitator serves several purposes at the same time, as in the catalytic hydrogenation of a liquid. In a hydrogenation vessel, the hydrogen gas is dispersed through the liquid in which solid particles of catalyst are suspended, with the heat of reaction simultaneously removed by a cooling coil or jacket. AGITATED VESSELS Liquids are most often agitated in some kind of tank or vessel, usually cylindrical in form and with a vertical axis. The top of the vessel may be open to the air, more usually it is closed. The proportions of the tank vary widely, depending on the nature of the agitation problem. A standardized design such as that shown in Fig. 1 is applicable in many situations. The tank bottom is rounded, not flat, to eliminate sharp corners or regions into which fluid currents would not penetrate. The liquid depth is approximately equal to the diameter of the tank. An impeller is mounted on an overhung shaft, that is, a shaft 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.
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Mechanical Operations Chemical Engineering Unit V
Sainath K Page 1
AGITATION AND MIXING
Many processing operations depend for their success on the effective agitation and mixing of
fluids. Agitation and mixing are not synonymous.
Agitation refers to the induced motion of a material in a specified way, usually in a
circulatory pattern inside some sort of container.
Mixing is the random distribution, into and through one another, of two or more
initially separate phases.
Example, a single homogeneous material, such as tankful of cold water, can be
agitated, but it cannot be mixed until some other material (such as a quantity of hot water or
some powdered solid) is added to it.
PURPOSE OF AGITATION Liquids are agitated for a number of purposes, depending on the objectives of the processing
step. These purposes include
1. Suspending solid particles
2. Blending miscible liquids, for example, methyl alcohol and water
3. Dispersing a gas through the liquid in the form of small bubbles
4. Dispersing a second liquid, immiscible with the first, to form an emulsion or suspension
of fine drops
5. Promoting heat transfer between the liquid and a coil or jacket
Often one agitator serves several purposes at the same time, as in the catalytic
hydrogenation of a liquid. In a hydrogenation vessel, the hydrogen gas is dispersed through
the liquid in which solid particles of catalyst are suspended, with the heat of reaction
simultaneously removed by a cooling coil or jacket.
AGITATED VESSELS Liquids are most often agitated in some kind of tank or vessel, usually cylindrical in form and
with a vertical axis. The top of the vessel may be open to the air, more usually it is closed.
The proportions of the tank vary widely, depending on the nature of the agitation problem. A
standardized design such as that shown in Fig. 1 is applicable in many situations.
The tank bottom is rounded, not flat, to eliminate sharp corners or regions into which
fluid currents would not penetrate. The liquid depth is approximately equal to the diameter of
the tank.
An impeller is mounted on an overhung shaft, that is, a shaft 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.
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Figure 1 Typical agitation process vessel.
Accessories such as inlet and outlet lines, coils, jackets, and wells for thermometers or other
temperature-measuring devices are usually included.
The impeller causes the liquid to circulate through the vessel and eventually return to the
impeller. Baffles are often included to reduce tangential motion.
IMPELLERS Impellers agitators are divided into two classes. Those that generate currents parallel with the
axis of the impeller shaft are called axial-flow impellers; those that generate currents in a
radial or tangential direction are called radial-flow impellers.
The three main types of impellers for low-to moderate-viscosity liquids are propellers,
turbines, and paddles (high-efficiency impellers). For very viscous liquids, the most widely
used impellers are helical impellers and anchor agitators.
Propellers and pitched blade turbine are axial flow impellers; while paddle and flat
blade, disk flat blade turbines are radial flow mixers.
PROPELLER A propeller is an axial-flow, high-speed impeller for low viscosity liquids. It may be mounted
centrally, off-centre or at an angle to the tank. It is simple and portable.
Small propellers turn at either 1500 or 1750 rpm and larger one turn at 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 the vessel. The highly
turbulent swirling column of liquid leaving the impeller entrains stagnant liquid as it moves
along. The propeller blades vigorously cut or shear the liquid. Because of the persistence of
the flow currents, propeller agitators are effective in very large vessels.
Inlet
Discharge / outlet valve
Thermowell
Baffle
ImpellerVessel
Liquid
surface
Shaft
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Propellers rarely exceed 18 inch in diameter regardless of the size of the vessel. In a
deep tank two or more propellers may be mounted on the same shaft, usually directing the
liquid in the same direction.
Figure 2 Three-blade marine propeller.
Turbines A simple straight-blade turbine, as shown in Fig. 3 (a) and (b), pushes the liquid radially 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. In process vessels they
typically turn at 20 to 150 rpm.
Turbines are very effective over a wide range of viscosities up to 104 cP. Turbines
impellers drive the liquid radially against the wall, where the stream divides into two
portions. One of the portions flows downward to the bottom and then returns to the centre of
impeller from below while other flows upward towards the surface and finally returns to the
impeller from above. Turbines are especially effective in developing radial currents, but with
a baffled vessel they also induce vertical flows. To avoid vortexing and swirling with