Transcript SEDIMENTATION 11/7/2015 Definition Sedimentation is the tendency for particles in suspension or molecules in solution to settle out of the fluid in which they.

SEDIMENTATION
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Definition
Sedimentation is the tendency for particles in
suspension or molecules in solution to settle out of
the fluid in which they are entrained, and come to
rest against a wall. This is due to their motion
through the fluid in response to the forces acting on
them: these forces can be due to gravity or
centrifugal acceleration.
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Mechanism behind Sedimentation
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Mechanics of fluid motion
Terminal velocity
Drag coefficient & Reynolds number
Motion of spherical particles
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Mechanics of fluid motion
Three forces acting on a particle moving
through the fluid
1. The external force
2. The buoyancy force
3. The drag force
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Terminal velocity
The velocity at which the driving forces
(external forces) are cancelled out by the
resistive forces (drag forces). Once an object
has reached terminal velocity, the object is
not accelerating (a=0), therefore it is not
speeding up or slowing down. It is a constant
velocity unless the driving forces or the
resistive forces change.
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Drag coefficient & Reynolds
number
 Drag Coefficient is a dimensionless quantity
which is used to quantify the resistance of an
object in a fluid environment
 Reynolds number gives the condition in
which one type of flow changes to other type
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Settling
 Free Settling
When the particle is at sufficient distance from the boundaries
of the container or from other particles so that, its fall is not
affected by them.
 Hindered Settling
If the motion of the particles is impeded by other particles
which will happen when the particles are near each other,
though they may not be colliding, the process is called
hindered settling.
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Types of Sedimentation processes
 Gravity settling processes
 Centrifugal settling processes
Why we need to apply the centrifugal force?
Because of
Brownian Motion
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Gravity Settling Processes
Particles heavier than the suspending fluid
may be removed from a gas or a liquid in a
large settling tank, in which the fluid velocity
is low and the particles have ample time to
settle out.
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Types of gravity settlers
 A settler that removes virtually all the
particles from a liquid is known as a clarifier.
 A device that separates the solids into two
fractions is called as a classifier.
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1. Gravity Classifier
Separate particles on the basis of size, in
situation in which the density of the fine
particles is the same as that of the larger ones.
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Classifier
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2. Clarifier
Clarifiers are used for gravity separation
under hindered settling conditions to convert
a dilute slurry of fine particles into a clarified
liquid and a concentrated suspension.
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Gravity Clarifier
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Flocculation
Process by which clumps of solids in water aggregate through
biological or chemical action so they can be separated from
water
OR
The process of forming floc particles when a chemical
coagulant or flocculent such as alum or ferric chloride is
added to the water
OR
The gathering together of fine particles in water by gentle
mixing after the addition of coagulant chemicals to form
larger particles
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Flocs
Particles finer than 0.1 µm (10-7m) in water remain
continuously in motion due to electrostatic charge
(often negative) which causes them to repel each
other. Once their electrostatic charge is neutralized
by the use of coagulant chemical, the finer particles
start to collide and agglomerate (combine together)
under the influence of Vander wall's forces. These
larger and heavier particles are called flocs.
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Flocculants
Flocculants, or flocculating agents, are chemicals that
promote flocculation by causing colloids and other
suspended particles in liquids to aggregate, forming a floc.
Flocculants are used in water treatment processes to improve
the sedimentation or filterability of small particles. For
example, a flocculent may be used in swimming pool or
drinking water filtration to aid removal of microscopic
particles which would otherwise cause the water to be turbid
(cloudy) and which would be difficult or impossible to
remove by filtration alone.
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Centrifugal settling processes
To increase the settling rate, the force of
gravity acting on the particle may be replaced
by a much stronger centrifugal force.
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Centrifugal settling devices
 For the separation of solids from gases,
cyclones are used.
 Cyclones used for separating solids from
liquids are called hydro cyclones.
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Cyclones
 Cyclones operate to collect relatively large size
particulate matter from a gaseous stream through
the use of centrifugal forces. Dust laden gas is made
to rotate in a decreasing diameter pathway forcing
solids to the outer edge of the gas stream for
deposition into the bottom of the cyclone.
Efficiencies is 90% in particle sizes of 10 microns
or greater are possible.
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Advantages & Disadvantages
Advantages
 Reduces internal access needs
 Operation at elevated temperatures possible
 Few moving parts, few mechanical / electrical ignition
sources
 Optimal flow rate difficult to adjust
Disadvantages
 Prone to internal erosion / corrosion
 Low efficiency for small diameter material
 Hopper recirculation / flow distribution problems
 High energy costs for volumetric flow requirements
 Dew point agglomeration, bridging, and plugging
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Hydrocyclone
 A hydro cyclone is a device to classify, separate or
sort particles in a liquid suspension based on the
densities of the particles. A hydro cyclone may be
used to separate solids from liquids or to separate
liquids of different density.
 A hydro cyclone will normally have a cylindrical
section at the top where liquid is being fed
tangentially, and a conical base. The angle, and
hence length of the conical section, plays a role in
determining operating characteristics.
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Advantages
► Excellent abrasion resistance
► Excellent corrosion resistance
► Excellent chemical resistance /acids, bases, oils/
► Excellent temperature resistance
► Very good resistance to hydrolysis and
micro- organisms
► Very good pressure resistance
► High resilience
► Successful application by sorting aggressive and
high temperature gases and gas mixture
► Successful application in flue gas desulphurization
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