Gravitational Settling Chambers o5f6o

Gravitational Settling Chambers Simple gravity separation is the basis of one of the oldest collection devices—the settling chamber. A settling chamber is a long, enlarged section in the exhaust system. As the cross-sectional area increases, there is a corresponding decrease in the exhaust gas velocity, which permits coarser particles to fall from the air suspension. Settling chambers offer the advantages of modest construction, simple operation, dry collection, and small pressure drops. Structural space requirements are a significant disadvantage and are generally the limiting factor in use of settling chambers as primary collection methods. The design of a settling chamber is based on the settling velocity of the particles to be removed. Generally, chambers will have flow-through velocities less than 3 m/s and will effectively remove particles greater than 50 µm in size. Dry collectors, such as scrappers and screw conveyors, are used to collect the settled particles.

Gravimetric Efficiency : It is defined as the ration of the weight of the material collected or retained by the collector to the total amount entering the collector. G E (%)  100 x

Wc We

Where, GE is Gravimetric Efficiency, Wc is weight of material collected and We is the weight of material entering the collector.

Wet Scrubbers : Using a variety of methods, wet scrubbers are collection devices that wet particles in order to remove them from the gas stream. They utilize inertial impaction and/or Brownian diffusion as the particle collection mechanism, and droplets, sheets, and jets as the

liquid collection mechanism. Wet scrubbers typically use water as the cleaning liquid. Water usage and wastewater disposal requirements are important factors in the evaluation of a scrubber alternative, and both are influenced by the quantity of particles collected, the size distribution of the particles, and the presence of dissolved contaminants in the wastewater. Many applications recirculate the scrubber liquid, which reduces consumption demand but will lead to higher particulate and dissolved solids concentrations, requiring treatment and disposal.

Advantages and Disadvantages of Wet Scrubbers Advantages

Disadvantages

Absorbs gas phase emissions suggests pollution Compact size Efficient through wide loading range temperature gases Insensitive to moisture content

Condensate plume

Low capital cost handling required Low operating and maintenance cost Re-entrainment rare Versatility for hazardous emissions

Corrosion Inefficient with highRequires high power input Waste scrubber liquid

Fabric Filters One of the most efficient devices for removal of particulates is the fabric filter collector. Fabric filters have the capability of maintaining collection efficiencies above 99% for particle size down to 0.3 µm. The basic features of a fabric filter unit consist of woven or felted fabric, usually in the form of tubes (bags) that are suspended in a housing structure (baghouse). The emission stream is distributed by means of specially designed entry and exit plenum chambers, providing equal gas flow through the filtration medium. The particle collection mechanisms for fabric filters include inertial impaction, Brownian diffusion, interception, gravity settling, and electrostatic attraction. The particles are collected in dry form either on a dust cake ed by the fabric (most efficient) or on the fabric itself. The process occurs with a relatively low pressure drop requirement. Periodically, most of the dust cake is removed for disposal; the residual dust serves as an initial filtration aid while the new dust cake develops.

1As particles adhere to the fabric, the removal efficiency increases, but so does the pressure drop. The pressure drop is the sum of the pressure drop owing to the fabric and the drop owing to the caked or adhered particles, and is expressed as  x f xp     K f K p 

P   

2where

P





x K

= = = = =

overall pressure drop in meters of water, superficial gas velocity through the fabric (in m/s), gas viscosity (in P), thickness of filter (/) and particle layer (p) (in m), and filter (f) and particle layer (p) permeability.

Electrostatic Precipitators Electrostatic Precipitation Systems. The classification of electrostatic precipitators may be as dry or wet systems and/or as single- or two-stage systems. Dry Systems. The dry precipitator with plate-type collection electrodes is the predominant type of system in industrial applications and is the process most discussed in this section. Wet Systems. The primary difference in wet and dry systems is the method by which collected particles are cleaned from the collection electrode. Three common wet precipitator configurations are the plate type with horizontal gas flow, the concentric plate type with vertical gas flow, and the conventional pipe type with reversing vertical flow. Electrostatic precipitators use electrical energy to charge and collect particles. They have high removal efficiencies; in addition, they are effective for a variety of source categories and emission gas characteristics.

3The collection efficiency of an electrostatic precipitator may be estimated by an empirical equation  - A d    Q 

R  1 - exp 

Where 4A = total area of the collecting surface of the collection electrodes (m2), Q = flow rate of gas through the pipe (m3/s), d

= drift velocity 5( velocity of the particles toward the collecting electrode) = 0.5 d (m/s) D = diameter of the particle (  m) Advantages and Disadvantages of Electrostatic Precipitation Advantages: 1. Highly effective collection with efficiencies exceeding 99% in some cases. 2. Low power requirements and associated low operating power costs. 3. Capable of removing very small particles, even those not removable by other treatment schemes. 4. Dry dust collection often used, which may be useful for product and by-product recovery. 5. Pressure drops are small.

6. Temperature changes are small with dry systems; most impacted in spray-wet systems. 7. Low maintenance requirements due to few or no moving parts. 8. System tolerant to high temperatures, with no special provisions below 535°C (1000°F). 9. Collection efficiency and system capacity can be easily expanded with larger or additional units. 10. Some pollutants, such as acid and tar mists, are effectively collected by other schemes. Disadvantages: 1. 2. 3. 4. 5. 6. 7.

Capital investment is high. Space requirements are relatively large. Very low or high resistivity particles are difficult to remove. Relatively constant operating conditions are needed for efficient collection. Gaseous wastes are not affected. Safety is a concern due to high voltage. Wet systems produce sludges that may require dewatering before disposal.