Agitation and Mixing of Liquids

Many processing operations depend for their success

on the effective agitation and mixing of fluids.
Agitation. It refers to the induced motion of a
material in a specified way, usually in a circulatory
pattern inside some sort of container.
Mixing. It is the random distribution in to and
through one another, of two or more initially separate
phases.
e.g. tank full of cold water can be agitated but can not
be mixed until some other material is added to it.
Term Mixing is applied to a variety of operations,
differing widely in the degree of homogeneity of the
mixed material.
e.g. two gases and sand, gravel, cement and water
tumbled in a rotating drum.
Types of Mixing
Single-phase liquid mixing. In many instances, two or
more miscible liquids must be mixed to give a product
of a desired specification, such as, for example, in the
blending of petroleum products of different
viscosities. This is the simplest type of mixing as it
involves neither heat nor mass transfer, nor indeed a
chemical reaction.
Mixing of immiscible liquids
When two immiscible liquids are stirred together, one
phase becomes dispersed as tiny droplets in the second
liquid which forms a continuous phase.

Gas-liquid mixing
Numerous processing operations involving chemical
reactions, such as wastewater treatment, oxidation of
hydrocarbons, and so on, require good contacting
between a gas and a liquid.
Gas-Liquid Mixing
Liquid-solids mixing
Mechanical agitation may be used to suspend
particles in a liquid in order to promote mass transfer
or a chemical reaction. The liquids involved in such
applications are usually of low viscosity, and the
particles will settle down when agitation ceases.
Gas-liquid-solids mixing
In some applications such as catalytic hydrogenation
of vegetable oils, slurry reactors, evaporative
crystallization, and so on, the success and efficiency of
the process is directly influenced by the extent of
mixing between the three phases.
Solids-solids mixing
Mixing together of particulate solids, sometimes
referred to as blending, is a very much dependent, on
the character of the particles — density, size, size
distribution, shape and surface properties and also on
the differences of these properties in the
components. Mixing of sand, cement and aggregate
to form concrete and of the ingredients in gunpowder
preparation are examples of the mixing of solids.
Agitation and Mixing
Purpose of Agitation
•Suspending solid particles,
•Blending miscible liquids e.g. water and methyl
alcohol,
•Dispersing a gas through the liquid in the form of
small bubbles,
•Dispersing a second liquid, immiscible with the first,
to form an emulsion or a suspension of fine drops,
•Promoting heat transfer between the liquid and a coil
or jacket.

Agitated Vessels.
 Impellers and Propellers
An impeller is a rotor inside a tube or conduit used to
increase (or decrease in case of turbines) the pressure
and flow of a fluid.
Impellers in agitated tanks. They are used to mix
fluids or slurry in the tank. Mixing the fluids in a tank
is very important if there are gradients in conditions
such as temperature or concentration.
There are two types of impellers, depending on the
flow regime created.
•Axial flow impeller
•Radial flow impeller.
Axial flow impeller. Those that generates currents
parallel with the axis of the impeller shaft.
Radial flow impeller. Those that generates current in
radial or tangential direction.
Helical Impeller
Generally recognized as the best all around high
viscosity, laminar flow impeller. The helical impeller is
also good for heat transfer and blending of liquids and
solids from the surface. Generally used for
applications where high viscosity liquids are involved.
Axial, radial and helical impellers
Impellers can be further classified principally into
three sub-types (for low to moderate viscosity liquids)
•Propellers
•Paddles
•Turbines
A paddle is a tool used for pushing against liquids,
either as a form of propulsion in a boat or as an
implement for mixing.
Propellers
A propeller is an axial flow, high speed impeller for
liquids of low viscosity. The direction of liquid is
usually chosen to force the liquid downward, and the
flow current leaving the impeller continues until
deflected by floor of vessel. The highly turbulent
swirling column of the liquid leaving the impeller
entrains the stagnant liquid as it moves along and
propellers blades vigorously shear a liquid. Because of
persistent of flow currents, they are effective in large
vessels. Rarely exceeds 18 inch in diameter.
Diameter is the total outer circumference of the
propeller
Pitch is "the distance a propeller would move in one
revolution if it were moving through a soft solid, like a
screw through wood." For example, a 21-pitch
propeller would move forward 21 inches in one
revolution.
Or
It is the displacement a propeller makes in a
complete spin of 360° degrees.
 Flow Patterns in Agitated Vessels
The way the liquid moves in an agitated vessel
depends on many things:
•The type of impeller,
•The characteristics of liquid especially its viscosity,
•The size and proportions of tank, baffles, and
impeller.
Liquid velocity at any point in the tank has three
components.
First Velocity Component. It is radial and acts in a
direction perpendicular to the shaft of impeller.
Second velocity Component. It is axial and acts in the
direction parallel with the shaft.
Third Velocity Component. It is tangential or
rotational and acts in direction tangent to the circular
path around the shaft.
In the usual case of a vertical shaft, the radial and
tangential components are in a horizontal plane and
the axial component is vertical.
The radial and axial component are useful and provide
necessary for the mixing action. When the shaft is
vertically and centrally located in the tank, the
tangential component is generally disadvantageous.
Tangential flow follows a circular path around the
shaft and creates a swirl in the liquid.
If solid particles are present, circulatory currents tend
to throw the particles to the outside by centrifugal
force; from there they move downward and to the
centre of the tank at the bottom. Instead of mixing,
concentration occurs.
Since, in circulatory flow, the liquid flows with the
direction of motion of impeller blades, the relative
velocity between the blades and the liquid is reduced,
and the power that can be absorbed by the liquid is
limited. In an unbaffled vessel, swirl is introduced in
all types of impellers.
 Prevention of Swirling
•In small vessels, impellers can be mounted off centre. The
shaft is moved away from the centerline of the tank.
Baffles
•In large tank with the vertical agitator, the preferable
method of reducing swirling is to install baffles, which
impedes rotational flow without interfering with radial or
axial flow. A simple and effective baffling is attained by
installing vertical strips perpendicular to the wall of the
tank. Four baffles are enough to prevent swirling except in
large tanks. Baffles are fitted to the wall of vessels. These
take the form of thin strips about one-tenth of the tank
diameter in width.
A small clearance is left between the wall and the
baffle to facilitate fluid motion in the wall region.
Once the swirling is stopped, the specific flow pattern
in the vessel depends on the type of impeller.
 Draft Tubes
The return flow to an impeller of any type approaches
the impeller from all directions, because it is not under
the control of solid surface. The flow to and from a
propeller, e.g. is essentially similar to the flow of air to
and from fan operating in a room.
In most applications, there is no limitation, but when
the direction and velocity of flow to the suction of the
impeller are to be controlled, draft tubes are used.
These devices are used in the manufacturing of certain
emulsion, or where solid particles that tend to float on
the surface of the tank are to be dispersed in the liquid.
Draft tubes for propellers are mounted around the
impeller, and those for turbines are mounted
immediately above the impeller. Draft tubes add to
the fluid friction in the system; and for given power
input , they reduce the rate of flow, so they are not
used unless they are required.
 Flow Number
Flat blade Turbine impeller
• u2 is the velocity of blade
tip.
• Tangential/peripheral
component Vu2
• Radial component Vr2
• V2 is total liquid velocity.
 Static Mixer
For gases or low viscosity liquids can be mixed by
passing them through a length of open pipe or a pipe
containing orifice plates or segmented baffles.
Pipe length may be as short as 5 to 10 pipe diameter,
but 50 to 100 pipe diameter is recommended.
More difficult mixing task is accomplished by static
mixers. It consists of series of metal inserts placed in
pipe.
 Helical Element Mixer
Major type of static mixer, mainly used for viscous
liquids and pastes.
Each element, 1 to 5 pipe diameters in length, divides
the stream in to two, gives it a 180˚ twist and delivers
it to the next element, which is set at an angle of 90˚
to the trailing edge of the first element.
The second element divides the already divided
stream and twists it 180˚ in the opposite direction.
Successive elements further subdivides the stream
until mixing process can be finished by molecular
diffusion.
Recommended no. of helical elements is 6 for Re =
100 to 1,000, 12 for Re = 10 to 100, and 18 for Re <
10.
More elements are needed for very viscous liquids
because of lower molecular diffusivity.
Pressure drop per unit length is about 6 times that in
the empty pipe when Re < 10, but increases to about
50 to 100 times that in the empty pipe when Re =
2,000.
Striate . A thin narrow channel
 Suspension of Solid Particles
Particles of solids are suspended in liquids for many
purposes,
• To produce a homogenous mixture for feeding to
processing units.
• To dissolve solids
• To catalyze a chemical reaction

It is somewhat like fluidization of solids with liquids.

But to keep solids in suspension in a tank requires higher
average fluid velocities that would be needed in
fluidization
 Degree of Suspension
When solids are suspended in an agitated tank,
there are several ways to define the conditions
of suspension.
Nearly complete suspension with filleting.
• Few % of fillets of solids at the outer periphery
of the bottom or at any other places in the
tanks
• Suitable for processing units, not for
crystallization or chemical reaction.
 Degree of Suspension(cont…..)
Complete Particle Motion
• All the particles are either are suspended or
are moving along the tank bottom.
• Particles moving on the bottom have a much
lower mass transfer coefficient than
suspended particles affecting the performance
of unit.
 Degree of Suspension(cont…..)
Complete Suspension or complete off - bottom
suspension
All the particles are suspended off the tank
bottom or do not stay on the bottom more than
1 or 2s.
At this point, there will be concentration
gradient in the suspension and there may be a
region of clear liquid near the top of the tank.
 Degree of Suspension(cont…..)
Uniform Suspension
At stirrer speeds considerably above those
needed for complete suspension, there is no
longer any clear liquid near the top of the tank
and suspension appears uniform.