Scrubbers have played a very important role in industrial ventilation for many decades. These are the only devices which can remove particulate matter as well as gaseous impurities. Wet scrubbing of industrial gases has improved over many years to accommodate fine cleaning requirements of today’s industry. Any activity like burning metal forming, chemical reaction or material handling will generate pollutants like particulate matter, unwarranted gases and these pollutants needs to be removed from the gas stream and cleaned gases are to be left in the atmosphere to ensure pollution free environment.
Scrubbers can be grouped into 2 types. Type 1 is for particulate collection, where equipment like wet cyclones, cyclonic scrubbers and venturi scrubbers are involved, and type 2 is for gaseous absorption / removal, where equipment like static packed bed, dynamic packed bed, baffle plate scrubbers or impingement plate scrubbers are involved.
In its simplest form, a venturi scrubber consists of a restriction in a gas duct into which water is introduced. The scrubbing occurs at the restriction where high gas velocities atomize the water into fine droplets. After scrubbing, the gas and water are separated in a cyclonic separator, the water gravitating to a pumping tank for re-use and the clean gas leaving from the top of the separator.
Impingement Plate Scrubber
The gas enters the lower section of the scrubber and rises through a zone of spray, usually water, created by a group of low pressure sprays. These sprays humidify the gas and strip it of coarser dust particles. The gas is now prepared for intensified cleaning action during its passage through the impingement baffle plate stage, which is flooded with water or other scrubbing liquid during the cleaning process. The plate consists of two parts: A perforated sheet having circular boles & Impingement baffles above the sheet, a baffle being located directly above the sheet, a baffle being located directly above each perforation. When gas passes through these holes perforation, high velocity jet is created and each jet strikes against a baffle. Since the baffle is located at the vena contracta of the jet, the gas stream strikes the baffle at the moment of its greatest velocity giving the highest target efficiency for the pressure drop through the perforation.
Advantages of Cyclones
- High temperature gases will be cooled and washed in one equipment.
- Corrosive gases can be neutralized simultaneously with the collection of particulate matter.
- Soluable material may be recovered and pumped in slurry form for additional treatment, if necessary.
- Possibility of fire or explosive hazard is eliminated.
- When gases are near due point condition, wet collectors avoid the problem or building up sticky deposits on dry surface. This is an important factor to be considered before going in for a dry system with a pre-cooler. If the dew point is very high, one will have to insulate the dry system.
Some of the typical applications for wet collectors are
- Gas cleaning for blast furnaces, boiler flue gas.
- Black liquor recovery for pulp and paper.
- Refusal incineration scrubbing.
- Dolomite lime kiln exhaust.
- Zinc oxide fume collection.
- Aluminium smelting furnace exhaust ventilation.
- Bagasse fired and oil fired stream/power generation.
Apart from these, non-metallic scrubbers are used for acid scrubbing, SO x scrubbing and NOx scrubbing.