This document discusses mechanisms and equipment for liquid mixing. It describes that mixing is done to produce a uniform mixture and requires both bulk flow and intensive mixing to break down inhomogeneities. The mechanisms of laminar and turbulent mixing are then covered. For laminar mixing, high viscosity liquids are elongated and thinned in regions of high shear near impellers. Turbulent mixing relies on eddy diffusion throughout the vessel. Common mixing equipment includes mechanical agitators with vessels, baffles and various impeller types selected based on liquid viscosity, as well as extruders and static mixers.

1. LIQUID MIXING

2. CONTENTS
LIQUID MIXING
LIQUID MIXING MECHANISMS
MIXING EQUIPMENTS
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3. MIXING
• Mixing is to be carried out in order to produce a uniform mixture.
• There must be bulk or convective flow so that there are no dead
(stagnant) zones. Secondly, there must be a zone of intensive or
high-shear mixing in which the inhomogeneities are broken down.
• Both these processes are energy-consuming and ultimately the
mechanical energy is dissipated as heat.
• Depending upon the fluid properties, primarily viscosity, the flow in
mixing vessels may be laminar or turbulent, with a substantial
transition zone in between the two.
• Frequently both flow types will occur simultaneously in different
parts of the vessel.
• Laminar and turbulent flow arise from different mechanisms, and it is
convenient to consider them separately.
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4. LIQUID MIXING
MECHANISMS
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5. LAMINAR MIXING
o Usually associated with high viscosity liquids which may be either Newtonian or non-
Newtonian.
o The inertial forces therefore tend to die out quickly, and the impeller of the mixer must
cover a significant proportion of the cross-section of the vessel to impart sufficient bulk
motion.
o Because the velocity gradients close to the rotating impeller are high, the fluid elements
in that region deform and stretch.
o Repeatedly elongate and become thinner each time the fluid elements pass through the
high shear zone.
o Extensional or elongational flow usually occurs simultaneously. This is a result of the
convergence of the streamlines and consequential increased velocity in the direction of
flow. As the volume remains constant, there must, be a thinning or flattening of the fluid
elements.
o Both of these mechanisms (shear and elongation), give rise to stresses in the liquid
which then effect a reduction in droplet size and an increase in interfacial area, by which
means the desired degree of homogeneity is obtained.
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8. TURBULANT MIXING
• For low viscosity liquids (less than 10 mN s/m2 ), the bulk
flow pattern in mixing vessels with rotating impellers is
turbulent.
• The inertia imparted to the liquid by the rotating impeller is
sufficient to cause the liquid to circulate throughout the
vessel and return to the impeller.
• Turbulent eddy diffusion takes place throughout the vessel
but is a maximum in the vicinity of the impeller.
• Eddy diffusion is inherently much faster than molecular
diffusion and, consequently, turbulent mixing occurs much
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9. MIXING EQUIPMENTS
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10. MIXING EQUIPMENTS
Mechanical
agitation
Extruders
Static mixers
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11. MECHANICAL AGITATION
• Most commonly used method of mixing liquids.
• Essentially there are three elements in such devices.
1.Vessels
2.Baffles
3.Impellers
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12. Vessels
• Vertically mounted cylindrical tanks up to 10 m in diameter.
• The base of the tanks may be flat, dished, or conical, or
specially contoured, depending upon factors such as ease
of emptying, or the need to suspend solids.
• May be mounted horizontally, for the batch mixing of
viscous pastes and doughs using ribbon impellers and
Zblade mixers.
• In such units, the working volume of pastes and doughs is
often relatively small, and the mixing blades are massive in
construction.
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13. Baffles
• Thin strips about one-tenth of the tank diameter in
width.
• Typically four equi-spaced baffles may be used.
• Prevent gross vortexing, which is detrimental to
mixing particularly in low viscosity systems.
• generally not required for high viscosity liquids.
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14. • Selected for a particular duty largely on
the basis of liquid viscosity.
• Propellers, turbines, paddles, anchors,
helical ribbons and screws are usually
mounted on a central vertical shaft in a
cylindrical tank.
• It is necessary to move
from a propeller to a turbine and then, in
order, to a paddle, to
an anchor and then to a helical
ribbon and finally to a screw as
the viscosity.
• Propellers, turbines and paddles are
generally used with relatively low
viscosity systems and operate at
high rotational speeds.
• For large vessels, more than one impeller
are mounted on the same shaft.
(a) Three-bladed propeller (b) Six-bladed disc
turbine (Rushton turbine) (c) Simple paddle
(d) Anchor impeller (e) Helical ribbon
Impellers
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15. • Propellers are frequently of the
three-bladed marine type and
may be arranged as angled side-
entry units.
• The anchor and ribbon are
arranged with a close clearance
at the vessel wall.
• Helical ribbons or interrupted
ribbons are often used in
horizontally mounted cylindrical
vessels.
• Kneaders, Z- and sigma-blade,
and Banbury mixers are used
for the mixing of high-viscosity
liquids, pastes, rubbers, doughs.
(g) Z-blade mixer
(h) Banbury mixer
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16. AGITATOR
SELECTION
GUIDE
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17. AGITATOR
ARRANGEMENT
S AND FLOW
PATTERNS
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18. EXTRUDERS
• Mixing duties in the plastics industry are often carried out in
either single or twin screw extruders.
• The feed to such units usually contains the base polymer in
either granular or powder form, together with additives such
as stabilisers, pigments and plasticisers.
• During processing in the extruder the polymer is melted and
the additives mixed.
• Extrudate is delivered at high pressure and at a controlled
rate from the extruder for shaping by means of either a die
or a mould.
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19. • Single-screw extruder:
Shearing which occurs in the
helical channel between the barrel and
the screw is is not intense, and therefore
this device does not give good mixing.
• Twin-screw extruder:
May be co or counter-rotatory.
There are regions where the rotors are in
close proximity thereby generating
extremely high shear stresses. Twin-
screw units can yield a product of better
mixture quality than a single-screw
machine.
Single-screw extruder
Co-rotating twin-
screw extruder
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20. STATIC MIXERS
• Static devices which promote turbulent mixing in
pipelines provide an inexpensive way of continuously
mixing fluids.
• Sometimes called "in-line" or "motionless" mixers.
• pumped through a pipe containing a series of specially
shaped stationary blades.
• Can be used with liquids of a wide range of viscosities
in either the laminar or turbulent regimes.
• The flow patterns within the mixer are complex.
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21. Series of stationary helical blades mounted
in a circular pipe is used to divide and twist
the flowing streams.
Mixing proceeds by a distributive process
similar to the cutting and folding
mechanism.
The division and rotation of the fluid at each
element causes rapid radical mixing.
Important in eliminating any radial gradients
of composition, velocity and possibly
temperature that might exist in the material.
Twisted-blade type static
mixer elements
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22. Sulzer type
SMX static
mixer
• Mixing element consists of a
lattice of intermeshing and
interconnecting bars
contained in a pipe 80 mm
diameter. It is recommended
for viscous materials in
laminar flow. The mixer
shown is used in food
processing, for example
mixing fresh cheese with
whipped cream.
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23. • A simple mixing tee,
followed by a length of
pipe is suitable for
mixing low viscosity
fluids (50 mN s/m2)
providing the flow is
turbulent, and the
densities and flow-rates
of the fluids are similar.
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24. • With injection mixers in
which the one fluid is
introduced into the
flowing stream of the
other through a
concentric pipe or an
annular array of jets,
mixing will take place by
entrainment and
turbulent diffusion. Such
devices should be used
where one flow is much
lower than the other.
Annular
Injection
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25. THANK
YOU
• REFERENCE
Coulson and Richardson's
Chemical Engineering Design Volume 1 and 2
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