Mixing of Solids - Chemical Engineering documents 2012
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Transcript Mixing of Solids - Chemical Engineering documents 2012
Mixing of Solids
By
Sidra Jabeen
Department of Chemical Engineering,
University of Engineering & Technology Lahore
Mixing
Intermingling of two or more separate components to form
more or less uniform product.
Some other terms are
Agitation
Blending
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Agitation
Agitation refers
to the induced
motion of the
material in a
circulatory
pattern in some
type
of
container.
Mixing
Mixing is the
random
distribution, into
and through one
another, of two or
more
initially
separate phases.
Blending
Blending
is
the
mixing
of
two
components to form a
homogenous mixture.
Sometimes, blending
is the term used for
mixing of liquids.
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Mixing of solids
Liquids
–
currents
are
generated
Liquid
Solid – no such currents
so mixing is done by
other means
–
well-mixed
product (homogeneous)
Solids
–
identifiable
phases
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Types of mixture
Perfect mixture / Solution
Segregated mixture / Suspension
Random mixture / Colloidal Dispersion
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Perfect mixture / Solution
Solutions are mixtures made by mixing a solute and a solvent.
The solute is the substance that dissolves. The solvent is the
substance that does the dissolving. Solutions are homogeneous
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Suspension / Segregated mixture
Suspensions are heterogeneous mixtures of a solid and a
liquid in which the solid does not dissolve. Particles of one
component have a greater probability of being found in greater
amount in one part of the mixture than the other.
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Random mixture / Colloidal Dispersion
Colloidal Dispersions are mixtures with characteristics part
way between a solution and a suspension. Colloidal
dispersions may appear homogeneous but are actually
heterogeneous.
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Types of mixers
Mixing is by slow speed agitation of the mass with an impeller
By tumbling
By Centrifuging smearing and impact
For cohesive solids – powerful and heavier machines
For free flowing solids – light in construction
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Mixers for non cohesive solids
Mixer for dry and free flowing solids are
Light in construction
Moderate power consumption
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Mixers for cohesive solids
Appropriate contact
Mixing elements cannot generate flow currents
High viscosity
Mixing is by combination of low speed
Shear
Smearing
Wiping
Folding
Stretching
compressing
Mixer have
high power consumption
Cooling is required
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Mixers for Cohesive solids
Different types of mixers are:
1.
Change-Can mixer
2.
Kneaders
3.
Dispersers
4.
Masticators
5.
Continuous kneaders
6.
Mixer extruders
7.
Mixing rolls
8.
Muller and pan mixers
9.
Pug mills
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1. Change-Can Mixers
For viscous liquids and light pastes as:
Food processing
Paint manufacturing
5 to 100 gal. in size
Change-Can mixer are:
1.
Pony mixer
2.
Beater mixer
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1. Pony mixers
Agitator consist of vertical blades held on rotating head
positioned near the wall of the can
Blades are slightly twisted
Agitator is mounted eccentrically
Can rest on a turn table driven in direction opposite to that of
agitator
All the material in the can is brought to the blade to be mixed
When mixing is completed –blades are cleaned and can is
replaced
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Change Can Mixer (Pony Mixer)
2. Beater mixers
Can or vessel is stationary
Agitator has a planetary motion
Visits all parts of vessel
Beaters are shaped to pass with close clearance over the side &
bottom of mixing vessel
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Change Can Mixer (Beater)
Kneaders, Dispersers and Masticators
Kneading – “squashing the mass flat, folding it over on itself
and squashing it once more”
Also tear the mass apart and shear it between a moving blade
& a stationary surface.
For deformable & plastic solids
Large energy requirements
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Kneaders, Dispersers and Masticators
Mixing is done by two heavy blades on horizontal shaft in a
short trough with a saddle shaped bottom
Blades
turn towards each other at top, drawing mass
downward over the point of the saddle, then shearing it
between the blades and wall
They turn at different speeds usually 1.5 : 1 or 2 : 1
Mixing time – 5 to 20 min or longer then trough is tilted for
discharging or can be unloaded through an opening
Cooling is provided commonly
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Banbury mixer
Heavy duty two-arm mixer
30 to 40 rpm
Solids enter from top and discharge from bottom
Applications:
Compound rubber and plastic solids
Masticate crude rubber
Devulcanize rubber scrap
Make water dispersion and rubber solution
Shorter time and small batches
Cooling is done by:
Water spraying on walls
Circulating through hollow agitator shaft
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Banbury Mixer
Banbury Mixer
Design of Blades
Sigma blade:
Used for general purpose kneading
Edges are serrated to give a shredding action
S-type Double-naben (fish-tail blade):
Effective with heavy plastic materials
develop high shear force
Z-type Disperser blade:
Heavier and develop high shearing forces
Disperse powders or liquids into rubbery masses.
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Kneader
(Sigma Blades)
Kneader
(Sigma Blades)
Double Naben Blades
Z-type Disperser blade
Continuous Kneaders
Continuous operation
deals light to fairly heavy materials (heavy, stiff, or gummy
materials)
consist of
Single horizontal shaft
Slow in motion
Rows of teeth on shaft arranged in spiral pattern
Stationary teeth on the wall of casing
Close clearance between dynamic and stationary teeth
Shaft turns and also reciprocates in the axial direction
Smearing action b/w teeth in axial or longitudinal direction as
well as radial shear
Several tons per hour
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Mixer Extruders
The discharge of continuous kneader is restricted by covering
it with an extrusion die
Pitched blades of the rotor build up considerable pressure in
the material
Material is cut and folded and subjected to additional shear
Contain one or two horizontal shafts, rotating but not
reciprocating , carrying blades set in a helical pattern.
Pressure is built by reducing the pitch of helix or by reducing
the diameter of chamber or both
Continuously mix, compound, and work thermoplastics, clays,
and other hard-to-mix materials.
Heating jacket is provided.
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Mixer Extruder
Mixing Rolls
Smooth metal rolls at variable speeds
For pastes & deformable solids to intense shear
3 to 5 horizontal rolls in vertical stack
Material moves from slower to faster ones
Batch roll mills has two rolls set in horizontal plane and
require long mixing time and attention
Used for dispersing solids in rubber and plastic materials
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Mixing Rolls