Wet Scrubbers are air pollution control devices for removing particles and gases from industrial exhaust stream. It is operated by introducing the dirty gas stream with a scrubbing liquid as water. Then gases are collected in the scrubbing liquid. Wet scrubbers are usually the most appropriate air pollution-control device for collecting both, particulate and gas in a single system alone.
DESIGN OF WET SCRUBBER:
The design of wet scrubbers depend on the industrial process conditions and the nature of the air pollutants involved in device. Inlet gas features and dust properties (if present) are of more importance. Scrubbers can also be designed to collect particulate matter and gaseous pollutants. The versatility of wet scrubber allows it to be built in numerous configurations. And all are designed to provide good contact between the liquid and polluted gas streams.
FACTS ABOUT WET SCRUBBER:
The typical wet scrubber system consists of a scrubbing vessel, a duct-work and fan system, a mist eliminator, a pumping, a spent scrubbing liquid treatment and an exhaust stack. Modern control technique are used to monitor these systems and make necessary adjustments.
These are common in various industrial applications which includes pollutant reduction at petroleum refineries, the large scale chemical processes, acid manufacturing plants and steel making steel.
OPERATION OF WET SCRUBBER:
It relays on the liquid spray to remove dust particles from gas streams. It is also used for the fume absorption hence it is very useful in fume extraction system. It removes all air pollutants by inertia of diffusional impaction and reactions with absorbent or reagent slurry or absorption into a liquid solvent. The inlet gas stream enters to the bottom of the tower and moves upward while liquid is sprayed downward from one or more level.
They are also used for removal of noxious and corrosive contaminants components from the flue gases generated by many process in process plans. The gas is then washed with suitable liquid using spray nozzles. The suitable packed bed provides large contact surface for the gases and liquid to mix and hence traps the pollutants.
Since wet scrubbers vary in complexity and its method of operation so we divide it in following categories. Scrubbers for particle collection are categorized by the gas-side pressure drop of the systems.
Scrubbers can be classified by pressure drop as follows:
1. Low-energy scrubbers have pressure drops of less than 12.7 cm of water.
2. Medium-energy scrubbers have pressure drops between 12.7 and 38.1 cm of water.
3. High-energy scrubber have pressure drops greater than 38.1 cm of water.
But most of the scrubber operates over a wide range of pressure drops.
>> Gives high collection efficiency of solid & gaseous pollutant.
>> Provides Maximum pressure drop.
>> Fumes are also be absorbed
>> Maintenance cost is low
>> Complete wetting of the product
>> Emission of minimum product through exhaust
>> Ability to handle high temperature
1) Small space requirements: Scrubbers reduce the temp & volume of the unsaturated exhaust streams. So vessel sizes, fans and ducts downstream are smaller than those of other control devices. It results in lower capital costs and more flexibility.
2) No secondary dust sources: Once particulate matter is collected, it cannot escape from hoppers or during transport.
3) Handles high-temperature, high-humidity gas streams: There is no temperature limit or condensation problem can occur in bag-houses.
4) Minimal fire and explosion hazards: It is mainly because various dry dusts are flammable. By using water eliminates the probability of explosions.
1) Corrosion Problems: Generally water and dissolved pollutants can form highly corrosive acid solution. So proper construction materials are very important and wet-dry interface areas can also result in corrosion.
2) High power requirements: The high collection efficiencies for particulate matter are attainable only at high pressure drops which results in high operating costs.
3) Water-disposal problems: The settling ponds or sludge clarifier is needed to meet waste-water regulation meter.
4) Difficult product recovery: De-watering and drying of scrubber sludge makes recovery of dust very expensive and difficult for reuse.