Cyclonic spray scrubbers are an air

pollution control Pollution is the introduction of contaminants into the natural environment that cause adverse change. Pollution can take the form of any substance (solid, liquid, or gas) or energy (such as radioactivity, heat, sound, or light). Pollutants, the ...

technology. They use the features of both the dry cyclone and the spray chamber to remove pollutants from gas streams. Generally, the inlet gas enters the chamber tangentially, swirls through the chamber in a corkscrew motion, and exits. At the same time, liquid is sprayed inside the chamber. As the gas swirls around the chamber,

pollutant A pollutant or novel entity is a substance or energy introduced into the environment that has undesired effects, or adversely affects the usefulness of a resource. These can be both naturally forming (i.e. minerals or extracted compounds like o ...

s are removed when they impact on liquid droplets, are thrown to the walls, and washed back down and out. Cyclonic scrubbers are generally low- to medium-energy devices, with

pressure drop Pressure drop is defined as the difference in total pressure between two points of a fluid carrying network. A pressure drop occurs when frictional forces, caused by the resistance to flow, act on a fluid as it flows through the tube. The main de ...

s of 4 to 25 cm (1.5 to 10 in) of water. Commercially available designs include the irrigated cyclone scrubber and the cyclonic spray scrubber. In the ''irrigated cyclone'' (Figure 1), the inlet gas enters near the top of the scrubber into the water sprays. The gas is forced to swirl downward, then change directions, and return upward in a tighter spiral. The liquid droplets produced capture the pollutants, are eventually thrown to the side walls, and carried out of the collector. The "cleaned" gas leaves through the top of the chamber. The ''cyclonic spray'' scrubber (Figure 2) forces the inlet gas up through the chamber from a bottom tangential entry. Liquid sprayed from nozzles on a center post (manifold) is directed toward the chamber walls and through the swirling gas. As in the ''irrigated cyclone'', liquid captures the pollutant, is forced to the walls, and washes out. The "cleaned" gas continues upward, exiting through the straightening vanes at the top of the chamber. This type of technology is a part of the group of

air pollution Air pollution is the contamination of air due to the presence of substances in the atmosphere that are harmful to the health of humans and other living beings, or cause damage to the climate or to materials. There are many different typ ...

controls collectively referred to as

wet scrubber The term wet scrubber describes a variety of devices that remove pollutants from a furnace flue gas or from other gas streams. In a wet scrubber, the polluted gas stream is brought into contact with the scrubbing liquid, by spraying it with the ...

Particulate collection

Cyclonic spray scrubbers are more efficient than

spray tower A spray tower (or spray column or spray chamber) is gas-liquid contactor used to achieve mass and heat transfer between a continuous gas phase (that can contain dispersed solid particles) and a dispersed liquid phase. It consists of an empty c ...

s, but not as efficient as

venturi scrubber A venturi scrubber is designed to effectively use the energy from a high-velocity inlet gas stream to atomize the liquid being used to scrub the gas stream. This type of technology is a part of the group of air pollution controls collectively refe ...

s, in removing

particulate Particulates – also known as atmospheric aerosol particles, atmospheric particulate matter, particulate matter (PM) or suspended particulate matter (SPM) – are microscopic particles of solid or liquid matter suspended in the air. The te ...

from the inlet gas stream. Particulates larger than 5 μm are generally collected by impaction with 90% efficiency. In a simple

, the velocity of the particulates in the gas stream is low: 0.6 to 1.5 m/s (2 to 5 ft/s). By introducing the inlet gas tangentially into the spray chamber, the cyclonic scrubber increases gas velocities (thus, particulate velocities) to approximately 60 to 180 m/s (200 to 600 ft/s). The velocity of the liquid spray is approximately the same in both devices. This higher particulate-to-liquid relative velocity increases particulate collection efficiency for this device over that of the spray chamber. Gas velocities of 60 to 180 m/s are equivalent to those encountered in a

. However, cyclonic spray scrubbers are not as efficient as venturi scrubbers because they are not capable of producing the same degree of useful turbulence.

Gas collection

High gas velocities through these devices reduce the gas-liquid contact time, thus reducing absorption efficiency. Cyclonic spray scrubbers are capable of effectively removing some gases; however, they are rarely chosen when gaseous pollutant removal is the only concern.

Maintenance problems

The main maintenance problems with cyclonic scrubbers are nozzle plugging and

corrosion Corrosion is a natural process that converts a refined metal into a more chemically stable oxide. It is the gradual deterioration of materials (usually a metal) by chemical or electrochemical reaction with their environment. Corrosion engi ...

or erosion of the side walls of the cyclone body. Nozzles have a tendency to plug from particulates that are in the recycled liquid and/or particulates that are in the gas stream. The best solution is to install the nozzles so that they are easily accessible for cleaning or removal. Due to high gas velocities, erosion of the side walls of the cyclone can also be a problem. Abrasion-resistant materials may be used to protect the cyclone body, especially at the inlet.

Summary

The

s across cyclonic scrubbers are usually 4 to 25 cm (1.5 to 10 in) of water; therefore, they are low- to medium-energy devices and are most often used to control large-sized particulates. Relatively simple devices, they resist plugging because of their open construction. They also have the additional advantage of acting as

entrainment Entrainment may refer to: * Air entrainment, the intentional creation of tiny air bubbles in concrete * Brainwave entrainment, the practice of entraining one's brainwaves to a desired frequency * Entrainment (biomusicology), the synchronization o ...

separators because of their shape. The liquid droplets are forced to the sides of the cyclone and removed prior to exiting the vessel. Their biggest disadvantages are that they are not capable of removing submicrometer particulates and they do not efficiently absorb most pollutant gases.
Table 1 lists typical operating characteristics of cyclonic scrubbers.US EPA Air Pollution Training Institute
developed in collaboration with North Carolina State University, College of Engineering (NCSU) {, class="wikitable" style="width:100%; align:center; cellpadding:2; cellspacing:1" , - ! colspan="6" style="height:50px; align:center" , Table 1. Operating characteristics of spray towers , - , align="center" width="17%" , Pollutant , align="center" , Pressure drop (Δp) , align="center" , Liquid-to-gas ratio (L/G) , align="center" , Liquid-inlet pressure (p_L) , align="center" , Removal efficiency , align="center" , Applications , - , align="center" width="17%" , Gases , align="center" rowspan="2" , 4–25 cm of water
(1.5-10 in of water) , align="center" rowspan="2" , 0.3-1.3 L/m³ (2-10 gal/1,000 ft³) , align="center" rowspan="2" , 280–2,800 kPa (40-400 psig) , align="center" , Only effective for very soluble gases , align="center" rowspan="2" , Mining operations
Drying operations
Food processing
Foundries , - , align="center" width="17%" , Particulates , align="center" , 2-3 μm diameter

Bibliography

* Bethea, R. M. 1978. Air Pollution Control Technology. New York: Van Nostrand Reinhold. * McIlvaine Company. 1974. The Wet Scrubber Handbook. Northbrook, IL: McIlvaine Company. * Richards, J. R. 1995. Control of Particulate Emissions (APTI Course 413). U.S. Environmental Protection Agency. * Richards, J. R. 1995. Control of Gaseous Emissions. (APTI Course 415). U.S. Environmental Protection Agency. * U.S. Environmental Protection Agency. 1969. Control Techniques for Particulate Air Pollutants. AP-51.

References

Pollution control technologies Air pollution control systems Wet scrubbers Liquid-phase and gas-phase contacting scrubbers