This document discusses agitation and mixing of fluids. It begins by introducing agitation as a means of mixing phases to enhance mass and heat transfer. It then defines agitation and mixing. The main purposes of agitation are to suspend solids, blend liquids, disperse gas in liquid, form emulsions or suspensions, and promote heat and mass transfer. Agitation methods include mechanical, hydraulic, pneumatic and pipeline mixing. The document also discusses the basic components of a stirred tank including the vessel, baffles, impellers and motor. It provides details on impeller types, size, location and flow patterns for effective mixing.
2. INTRODUCTION
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. In its most general sense, the
process of mixing is concerned with all
combinations of phases like Gas, Liquid, solid. It is
the heart of the chemical industry. Here, we are
going to discuss about agitation and mixing
concerned with fluids only.
3. AGITATION
It refers to the induced motion of a “homogenous”
material in a specified way.
MIXING
It is the random distribution, into and through one
another, of two or more initially separate phases.
4. PURPOSES OF AGITATION
• Suspending solid particles
• Blending miscible liquids
• Dispersing a gas through the liquid
• Dispersing a second liquid to form an emulsion or
suspension
• Promoting heat transfer
• Enhancement of mass transfer between dispersed
phases.
9. A BASIC STIRRED TANK DESIGN
THE VESSEL
BAFFLES
IMPELLER
MOTOR
10. THE VESSEL
A dished bottom requires
less power than a flat
one. Economic and
manufacturing
considerations, however,
often dictate higher ratios
of depth to diameter.
11. BAFFLES
Baffles are needed to
prevent vortexing and
rotation of the liquid mass
as a whole. A baffle width
one-twelfth the tank
diameter, w = Dt/12; a
length extending from
one half the impeller
diameter, d/2, from the
tangent line at the bottom
to the liquid level, but
sometimes terminated
just above the level of the
eye of the uppermost
impeller.
12. VORTEX
If solid particles present
within tank; it tends to
throw the particles to the
outside by centrifugal
force. Power absorbed by
liquid is limited. At high
impeller speeds, the
vortex may be so deep
that it reaches the
impeller.
14. IMPELLERS
An impeller is a rotating
component of
a centrifugal pump which
transfers energy from
the motor that drives the
pump to the fluid being
pumped by accelerating
the fluid outwards from
the center of rotation. The
velocity
achieved by the impeller
transfers into pressure
when the outward
movement of the fluid is
confined by the pump
casing. Impellers are
usually short cylinders
with an open inlet (called
an eye) to accept
incoming fluid, vanes to
push the fluid radially,
and a splined, keyed, or
threaded bore to accept a
drive-shaft.
15. IMPELLERS
Impellers in agitated tanks
are used to mix fluids or
slurry in the tank. This
can be used to combine
materials in the form of
solids, liquids and gas.
Mixing the fluids in a tank
is very important if there
are gradients in
conditions such as
temperature or
concentration.
16. IMPELLER TYPES
There are two types of
impellers, depending on
the flow regime created
(see figure):
Axial flow impeller
Radial flow impeller
17. IMPELLER TYPES
Radial flow impellers impose essentially shear stress to
the fluid, and are used, for example, to mix immiscible
liquids or in general when there is a
deformable interface to break. Another application of
radial flow impellers are the mixing of very viscous
fluids.
Axial flow impellers impose essentially bulk motion, and
are used on homogenization processes, in which
increased fluid volumetric flow rate is important.
18. IMPELLER TYPES
Impellers can be further
classified principally into
three sub-types
• Propellers
• Paddles
• Turbines
19. IMPELLER SIZE
This depends on the kind of impeller and operating
conditions described by the Reynolds, Froude, and
Power numbers as well as individual characteristics
whose effects have been correlated. For the popular
turbine impeller, the ratio of diameters of impeller and
vessel falls in the range, d/Dt=0.3-0.6, the lower values
at high rpm, in gas dispersion, for example.
IMPELLER SPEED
With commercially available motors and speed reducers,
standard speeds are 37, 45, 56, 68, 84, 100, 125, 155,
190, and 320 rpm.
20. IMPELLER LOCATION
Expert opinions differ somewhat on this factor. As a first
approximation, the impeller can be placed at 1/6 the
liquid level off the bottom. In some cases there is
provision for changing the position of the impeller on the
shaft. For off-bottom suspension of solids, an impeller
location of 1/3 the impeller diameter off the bottom may
be satisfactory. Criteria developed by Dickey (1984) are
based on the viscosity of the liquid and the ratio of the
liquid depth to the tank diameter, h / Q .
22. A GOOD MIXING SHOULD ACHIEVE THE
FOLLOWING:
1. Minimum power
requirement.
2. Efficient mixing in
optimum time.
3. Best possible economy.
4. Minimum maintenance,
durable and trouble free
operation.
5. Compactness.
23. AGITATOR DESIGN
Factors affecting the designing of the agitator:
1. Type of vessel
2. Circulation pattern.
3. Location of the agitator
4. Shape and size of the vessel
5. Diameter and width of the agitator
6. Method of baffling
7. Power required
8. Shaft overhang
9. Type of stuffing box or seal, bearing, drive system etc.
24. MIXING FLOW PATTERNS:
(i) Axial flow.
1. Impeller makes an angle
of less than 90o with the
plane of rotation thus
resultant flow pattern
towards the base of the
tank (i.e. marine
impellers).
2. More energy efficient
than radial flow mixing.
3. More effective at lifting
solids from the base of
the tank.
25. MIXING FLOW PATTERNS:
(ii) Radial flow.
1. Impellers are parallel to
the axis of the drive
shaft.
2. The currents travel
outward to the vessel
wall & then either
upward or downward.
3. Higher energy is
required compared to
axial flow impellers.
26. MIXING TIME
The 'mixing time' is the time
measured from the
instant of addition until
the vessel contents have
reached a specified
degree of uniformity
when the system is said
to be 'mixed'. Standard
measures of homogeneity
such as the striation
thickness in laminar flow or
the coefficient of variation in
turbulent flow can be used
to answer this question
quantitatively.
27. MIXING OF LIQUIDS
Since natural diffusion in liquids is relatively slow, liquid
mixing is most commonly accomplished by rotating an
agitator in the liquid confined in a tank. It is possible to
waste much of this input of mechanical energy if the
wrong kind of agitator is used.
In general, agitators can be classified into the
following two groups.
1. Agitators with a small blade area which rotate at high
speeds. These include turbines and marine type
propellers.
2. Agitators with a large blade area which rotate at low
speeds. include anchors, paddles and helical screws.
28. SMALL BLADE HIGH SPEED AGITATORS
Small blade high speed agitators are used to mix low
to medium viscosity liquids. Two of the most
common types are the six-blade flat blade turbine
and the marine type propeller. Flat blade turbines
used to mix liquids in baffled tanks produce radial
flow patterns primarily perpendicular to the vessel
wall. In contrast marine type propellers used to mix
liquids in baffled tanks produce axial flow patterns
primarily parallel to the vessel wall
29. LARGE BLADE LOW SPEED AGITATORS
Large blade low speed agitators include anchors,
gates, paddles, helical ribbons and helical screws.
They are used to mix relatively high viscosity liquids
and depend on a large blade area to produce liquid
movement throughout a tank. Since they are low
shear agitators they are useful for mixing shear
thickening liquids.
30. SCALE UP OF LIQUID-LIQUID MIXING
Scale-up of agitated immiscible liquid–liquid systems can
be a challenge that should not be taken lightly. The
problems arise from incomplete or inaccurate process
information and few quantitative tools to deal with
complex technology. A successful scale-up does not
mean that identical results are obtained at two different
scales, but rather, that the scale-up results are
predictable and acceptable. Problem correction at large
scale is costly, time consuming, and sometimes not
possible. The scale-up of certain liquid–liquid processes
can be straightforward. Dilute dispersions are the
easiest processes to scale up.
31. SCALE UP OF LIQUID-LIQUID MIXING
The first step is to understand the goals of the
process and to acquire accurate data for all
components, including physical, chemical, and
interfacial properties as well as reaction kinetics.
This also includes the influence of minor impurities.
Differences in the quality of raw materials need to
be considered.
32. CFD MODEL
Examination of the flow
patterns in the proposed full
scale vessel using CFD can
help visualize potential
problems related to design.
Once the CFD model has
been developed and
validated, design and
operating parameters can
be compared to determine
design sensitivities. One
observation seems to hold
universally—better results
are always obtained in small
equipment.
33. INDUSTRIAL APPLICATION OF
AGITATION
• blending of two miscible liquids as ethyl alcohol 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 treatment
• liquid-liquid dispersion, such as dispersion of
pigment in solvents
• suspending of fine solid particles in a liquid, as in
catalytic hydrogenation of a liquid
• agitation of the fluid to increase heat transfer
between the fluid and a coil or jacket in the vessel
wall.
34. CONCLUSION
Agitation is the heart of bioprocess engineering. The
success of a bioprocessing depends on this process.
This process is not only used for mixing but also used
for multiple purposes like heat transfer, mass transfer
and so on. It may sound like simple but difficulties in
here can affect the whole process. That’s why McCabe
rightly quoted “Many processing operations depend for
their success on the effective agitation & mixing of
fluids”.
Often we seem to use mixing and agitation as synonyms of each other, but in Bioprocess Engineering they have major difference.
Difference between agitation and mixing....
Agitation is the process of keeping a mixture that has been mixed in the proper mixed state required for the 'end' product. Mixing refers to the actual stirring of different liquids and/or materials to blend them together into an end product or mixture. Once this mixture is 'mixed' it may require agitation to keep the mixture in the proper 'mixed' state.
