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					Lecture # 6

SEDIMENTATION

Recap to
FILTARTION


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Recap to Filtration
  Discussion on the definition of filtration
  How filtration is carried out?
  Discussion on types of filtration
  Rate of filtration
  Filter media, its material of construction and
   properties
  Filter Aids
  Types of filters on the basis of their mechanism or
   technique
  Modes of operation
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Recap to Filtration

       Cake filtration
       Different types of cake filters
 1.     Filter Press
 2.     Shell & leaf filter
 3.     Rotary drum filter
 4.     Suspended batch centrifuge
 5.     Discontinuous vacuum filter
 6.     Horizontal belt filter
 7.     Continuous filtering centrifuges
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Recap to Filtration

       Clarifying filtration
       Discussion on definition
       Principles of clarification
       Liquid clarification
 1.     Cartridge filter
       Gas clarification
 1.     Pad filters
 2.     Bag filters
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Recap to Filtration
   Cross flow filtration
      Discussion on definition
      Differences b/w cross flow & conventional
       filtration
 Operation of cross flow filtration
 Performance of cross flow filtration
 Types of cross flow filtration
1. Reverse Osmosis
2. Microfiltration
3. Ultrafiltration
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4. Nanofiltration
           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, centrifugal
   acceleration.



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Mechanism behind Sedimentation

    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 buoyant force
3. The drag force



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 Lecture # 7

Mechanism behind sedimentation
Sedimentation Processes




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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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Why we study these terms in
     Sedimentation?



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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 heavies 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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