Lec#4 MS - Sedimentation

8/3/2019 Lec#4 MS - Sedimentation

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MECHANICAL SEPARATION

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` Definition of filtration` How filtration is carried out?

` Rate of filtration

` Filter media

` Material of construction and properties

` Filter Aids

` Types of filters on the basis of their

mechanism or technique` Modes of operation

` Cake filtration

` Filter Press

` Shell & leaf filter

` Rotary drum filter

`

Suspended batch centrifuge` Discontinuous vacuum filter

` Horizontal belt filter

` Continuous filtering centrifuges

` Clarifying filtration

` Principles of clarification` Liquid clarification

` Cartridge filter

` Gas clarification

` Pad filters

` Bag filters

` 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

`

Reverse Osmosis` Microfiltration

` Ultrafiltration

` Nanofiltration

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Sedimentation is the tendency for particles in suspension or molecules insolution 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.

MECHANISM

` Mechanics of fluid motion

` Terminal velocity

` Drag coefficient & Reynolds number

` Motion of spherical particles

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Three forces acting on a particle moving through the fluid1. The External Force

2. The Buoyant Force

3. The Drag Force

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Mechanism behind Sedimentation

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TERMINALVELOCITY

` 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.

DRAG CO-EFFICIENT

` 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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FREE SETTLING

` When the particle is at sufficient distance from the boundaries of thecontainer 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 becolliding, the process is called hindered settling.

TYPES OF SEDIMENTATION PROCESSES

` Gravity Settling Processes

` Centrifugal Settling Processes

Q. Why we need to apply the centrifugal force?

` Because of Brownian Motion

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` 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.

TYPES [GRAVITYSETTLERS]

Classifier A device that separates the solids into two fractions.

Clarifier A settler that removes virtually all the particles from a liquid.

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` Separate particles on the basis of size, in situation in which the density ofthe fine particles is the same as that of the larger ones.

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` 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.

GRAVITY CLARIFIER:

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` Process by which clumps of solids in water aggregate through biological orchemical 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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` Particles finer than 0.1 m (10-7m) in water remain continuously in motiondue 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 VanderWall's forces. These larger and

heavier particles are called FLOCS.

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Flocculants, or flocculating agents, are chemicals that promote flocculation

by causing colloids and other suspended particles in liquids to aggregate,

forming a FLOC.

USES:

` Flocculants are used in water treatment processes to improve the

sedimentation or filterability of small particles.

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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`

To increase the settling rate, the force of gravity acting on the particle may bereplaced by a much stronger centrifugal force.

CYCLONES

` Cyclones, is a Centrifugal Settling Device, used for the separation of solidsfrom gases.

` Cyclones operate to collect relatively large size particulate matter from agaseous stream through the use of centrifugal forces.

Working

` Dust laden gas is made to rotate in a decreasing diameter pathway forcingsolids 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.

Hydro-Cyclones cyclones used for separating solids from liquids.

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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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`

A hydro cyclone is a device to classify,separate or sort particles in a liquidsuspension based on the densities of theparticles.

A hydro cyclone may be used to separate

` solids from liquids or` separate liquids of different density.

` A hydro cyclone will normally have acylindrical section at the top where liquid isbeing fed tangentially, and a conical base. Theangle, and hence length of the conical section,plays a role in determining operatingcharacteristics.

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`

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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Lec#4 MS - Sedimentation

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