Agitation and Mixing
Many operations depend upon effective agitation and mixing of
components
Agitation: induced motion of a material
Mixing: random distribution of two initially separate phases
A single homogeneous material such as water in a tank can be
agitated but not mixed until another material is added to tank
Agitation and mixing
agitation is a means whereby mixing of phases can be accomplished
and by which mass and heat transfer can be enhanced between
phases or with external surfaces
Mixing is concerned with all combinations of phases of which the
most frequently occurring ones are
1. GASES WITH GASES
2. GASES INTO LIQUIDS: DISPERSION.
3. GASES WITH GRANULAR SOLIDS: FLUIDIZATION, PNEUMATIC
CONVEYING; DRYING
4. LIQUIDS INTO GASES: SPRAYING.
5. LIQUIDS WITH LIQUIDS: DISSOLUTION, EMULSIFICATION, DISPERSION
6. LIQUIDS WITH GRANULAR SOLIDS: SUSPENSION.
7. PASTES WITH EACH OTHER AND WITH SOLIDS.
8. SOLIDS WITH SOLIDS: MIXING OF POWDERS.
Agitation and Mixing
interaction of gases, liquids, and solids also may take place
Example: hydrogenation of liquids in the presence of a slurred solid
catalyst where the gas must be dispersed as bubbles and the solid
particles must be kept in suspension
Purpose of Agitation
Suspending solid particles in a liquid
Blending miscible liquids e.g. methanol-water
Dispersing a gas through a liquid in the form of small bubbles
Dispersing a second liquid, immiscible with first to form an emulsion or
suspension of fine drops
Promoting heat transfer between liquid and a coil or jacket.
Agitated Vessels
Round bottom to eliminate corners
where fluid cannot penetrate
Impeller is mounted on a shaft
Shaft driven by a motor
Baffles to reduce tangential motion of
fluid
Design of an Agitated Vessel
Flow patterns in Agitated Vessels
Depends upon
Type of impeller
Characteristics of the liquid (esp viscosity)
Size and proportions of the tank, baffles and the impeller
3 velocity components
Liquid velocity at any point has 3 components
1. Radial – perpendicular to the shaft of impeller
2. Longitudinal – parallel with shaft
3. Tangential or rotational – tangent to a circular path around the
shaft.
Radial and longitudinal comp are useful in mixing
When the shaft is vertical and centrally located; tangential
component is disadvantageous – creates a vortex
Impellers
2 types
1. Generate currents parallel with the axis
of impeller shaft Axial-flow impeller
2. Generate currents in a radial or
tangential direction Radial flow
impellers
Axial flow impellers impose basically
bulk motion, and are used in
homogenization processes
Radial flow impellers impose shear
stress to the fluid, and are used to mix
immiscible liquids
Axial is in left and Radial is in right side.
High Efficiency Impellers
High efficiency impellers are designed to
produce more uniform axial flow and better
mixing
It Reduces power requirements
In high efficiency impellers, blades are sometimes
folded to decrease the blade angle near tip
It is used to mix low to moderate viscosity liquids
but not for very viscous liquids or dispersing gases.
Impellers for highly viscous liquids
Helical ribbon impeller
Having diameter almost equal to inside dia
of tank
Promotes liquid motion all the way to the
tank wall with very viscous liquids
Anchor Impeller
Creates no vertical motion
Less effective than helical
Promotes better heat transfer
Anchor May have scrapers to remove liquid from
tank wall
Helical Ribbon
3 types are used based upon low-to-moderate viscosity liquids
1. Propellers
2. Turbines
3. High efficiency impellers
Propeller
Axial flow, high speed impeller is used for liquids of low viscosity
Its rotation forces liquid downward until deflected by the floor vessel
Propeller blades cut or shear the liquid
Produces a helix in the fluid
Propeller
One revolution will move the liquid longitudinally a fixed distance
depending upon angle of inclination of impeller blades
Ratio of distance to propeller diameter is called pitch of impeller.
(square pitch=1)
2 or more propellers may be used on a single shaft; directing liquid in
same direction
Turbines
Disk Turbine like straight blade turbine
creates zones of high shear rate
Dispersing a gas in a liquid
Pitched blade turbine is used when
good overall circulation is important
Straight blade turbine impeller
Straight blade force liquid radially and
tangentially with no vertical movement.
Current moves outward to vessel wall and
then either upward or downward
Also called paddles
Vortex formation and its disadvantages
If solid particles are present, they will
be thrown at the outside by
centrifugal force; and move
downward and to the centre of the
tank at bottom
Instead of mixing; (reverse)
concentration occurs
Relative velocity b/w blades and
liquid is reduced
Hence power that can be absorbed
by the liquid is limited
Prevention of Swirling (Vortex)
In small tanks, impeller can be mounted off
centre
In large tanks, agitator may be mounted in the
side of the tank with an angle with the radius
Installing baffles in large tanks (2,3 or 4)
Flow patterns in agitated
vessels
