UNIT III CONTROL OF PARTICULATE CONTAMINANTS

Control Of Particulate Contaminants
Factors affecting Selection of Control Equipment – Gas
Particle Interaction – Working principle, Design and
performance equations of Gravity Separators, Centrifugal
separators Fabric filters, Particulate Scrubbers, Electrostatic
Precipitators – Operational Considerations.

Some of the particulate control equipment
• Gravity settling
• Centrifugal impaction
• Inertial impaction
• Direct interception
• Diffusion
• Electrostatic perception

Scrubber types
• Dry scrubber
• Wet gas scrubber
– Plate scrubber
– Packed-bed scrubber
– Spray scrubber
– Venturi scrubber
– Cyclone scrubber
– Baffle scrubber
– Impingement-entrainment scrubber
– Fluidized-bed scrubber

Advantages
• Simultaneously remove particulates and gaseous pollutants.
• Hot gases can be cooled down.
• Corrosive gases can be recovered and neutralize.
Disadvantages
• Lot of waste waters produced.
• Poses freezing problem in cold countries.
• Maintenance cost is high when corrosive materials are collected.

Factors affecting selection of control
equipment
• Environmental
• Engineering (gas stream characteristics,
contaminant characteristics)
• Economic (capital cost, operating cost,
durability)

• Gas particle interaction
• Gravity Separators
• Centrifugal separators
• Fabric filters
• Particulate Scrubbers
• Electrostatic Precipitators

Gas Particle Interaction
• Colorless gas that dissolves in water vapour to form
acid, and interact with other gases and particles in the
air.
• Contribute to the formation of smog, acid rain, water
quality deterioration, global warming, and visibility
impairment.
• Ozone (O3) gaseous pollutant when it is formed in the
troposphere.
• Aerosol particles offer reactive surfaces on which
important chemical reactions occur.

The gas which is responsible for air pollution is
• These six pollutants are
– carbon monoxide
– lead
– nitrogen oxides
– ground-level ozone
– particle pollution (often referred to as particulate matter),
and
– sulfur oxides.

Gravity settling chamber
• Gravity settling chambers are the oldest and simplest means of removing large
diameter suspended particles from a gas.
• Gravity settling chamber is used to remove large and abrasive particles greater
than 50 µ from a gas stream.
• This is a simple particulate collection device using the principle of gravity to
settle the particulate matter in a gas stream passing through its long chamber.
• The design of gravitational settling chamber is based on Stokes law
• Where vt = terminal settling velocity, m/s

Advantages
• Low initial cost
• Easy to design
• Low pressure drop
• Low maintenance cost
• Dry and continuous disposal of solid particulates.
Disadvantages
• Require large space
• Less collection efficiency
• Only larger size particles can be collected

Gravity Separators
• Gravity settling chambers are the oldest and simplest means of
removing suspended particles from a gas
• In principle, pollutants are removed by reducing the velocity of the gas
stream sufficiently to allow particles to settle out under the influence of
gravity
• The simplest chamber is merely a horizontal duct in which large
particles settle out on the floor.
• High-efficiency settling chambers are often fitted with baffles or
deflectors to change the gas flow direction.

They are called precleaners
• This type of technology is a part of the group of air
pollution controls collectively referred to as
“precleaners”, because they are oftentimes used to
reduce the inlet loading of particulate matter (PM)
to downstream collection devices by removing
larger, abrasive particles.

• Settling chambers are also referred to as gravity
settling chambers, gravity collectors, expansion
chambers, and outfall chambers.
• Multiple-tray settling chambers are also referred to
as Howard settling chambers.

It is a long duct fitted with hoppers

The length of the chamber determines the amount of
time the particles remain at the reduced rate.

Collection efficiency
• The collection efficiency of settling
chambers varies as a function of particle
size and settling chamber design. Settling
chambers are most effective for large and/or
dense particles

Advantages
• Low capital cost
• Very low energy cost
• No moving parts, therefore, few maintenance requirements and low
operating costs
• Excellent reliability
• Low pressure drop through device
• Device not subject to abrasion due to low gas velocity
• Provide incidental cooling of gas stream; Temperature and pressure
limitations are only dependent on the materials of construction
• Dry collection and disposal.

Disadvantages
• Relatively low PM collection efficiencies, particularly for
PM less than 50 micrometer in size
• Unable to handle sticky or tacky materials
• Large physical size
• Trays in multiple-tray settling chambers may warp during
high-temperature operations.

Centrifugal separator
• Centrifugal separation means separating substances with different
specific gravities by means of centrifugal force. Like water and oil,
substances are separated through natural sedimentation if they have
different specific gravities.
• Cyclone separators or simply cyclones are separation devices (dry
scrubbers) that use the principle of inertia to remove particulate matter
from flue gases. Cyclone separators is one of many air pollution control
devices known as precleaners since they generally remove larger pieces
of particulate matter.

Advantages of centrifugal separators from Bernoulli System
• Separation occurs without any interruption to the flow.
• Very little liquid is lost by purging.
• No need for any consumable material.
• Zero maintenance.
• Separation efficiency.
• Specific gravity table.

UNIT III CONTROL OF PARTICULATE CONTAMINANTS

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UNIT III CONTROL OF PARTICULATE CONTAMINANTS