Mechanically aided scrubbers are a form of

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. 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 type ...

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 ...

s. In addition to using liquid sprays or the exhaust stream, scrubbing systems can use

motor An engine or motor is a machine designed to convert one or more forms of energy into mechanical energy. Available energy sources include potential energy (e.g. energy of the Earth's gravitational field as exploited in hydroelectric power ge ...

s to supply energy. The motor drives a rotor or

paddle A paddle is a handheld tool with an elongated handle and a flat, widened distal end (i.e. the ''blade''), used as a lever to apply force onto the bladed end. It most commonly describes a completely handheld tool used to propel a human-powered wa ...

s which, in turn, generate water droplets for gas and particle collection. Systems designed in this manner have the advantage of requiring less space than other scrubbers, but their overall power requirements tend to be higher than other scrubbers of equivalent efficiency. Significant power losses occur in driving the rotor. Therefore, not all the power used is expended for gas–liquid contact.

Types

There are fewer mechanically aided scrubber designs available than liquid- and gas-phase contacting collector designs. Two are more common: ''

centrifugal fan A centrifugal fan is a mechanical device for moving air or other gases in a direction at an angle to the incoming fluid. Centrifugal fans often contain a ducted housing to direct outgoing air in a specific direction or across a heat sink; such ...

scrubbers'' and ''mechanically induced spray scrubbers''. A ''centrifugal-fan scrubber'' can serve as both an air mover and a collection device. Figure 1 shows such a system, where water is sprayed onto the fan blades concurrently with the moving exhaust gas. Some gaseous

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 and particles are initially removed as they pass over the liquid sprays.
The liquid droplets then impact on the blades to create smaller droplets for additional collection targets. Collection can also take place on the liquid film that forms on the fan blades. The rotating blades force the liquid and collected particles off the blades. The liquid droplets separate from the gas stream because of their centrifugal motion. Centrifugal-fan collectors are the most compact of the

s since the fan and collector comprise a combined unit. No internal pressure loss occurs across the scrubber, but a power loss equivalent to a pressure drop of 10.2 to 15.2 cm (4 to 6 in) of water occurs because the blower efficiency is low. Another mechanically aided scrubber, the ''induced-spray'', consists of a whirling rotor submerged in a pool of liquid. The whirling rotor produces a fine droplet spray. By moving the process gas through the spray, particles and gaseous

s can subsequently be collected. Figure 2 shows an induced-spray scrubber that uses a vertical-spray rotor.

Particle collection

Mechanically aided scrubbers are capable of high collection efficiencies for particles with diameters of 1 μm or greater. However, achieving these high efficiencies usually requires a greater energy input than those of other scrubbers operating at similar efficiencies. In mechanically aided scrubbers, the majority of particle collection occurs in the liquid droplets formed by the rotating blades or rotor.

Gas collection

Mechanically aided scrubbers are generally not used for gas

absorption Absorption may refer to: Chemistry and biology *Absorption (biology), digestion ** Absorption (small intestine) *Absorption (chemistry), diffusion of particles of gas or liquid into liquid or solid materials * Absorption (skin), a route by which ...

. The contact time between the gas and liquid phases is very short, limiting absorption. For gas removal, several other scrubbing systems provide much better removal per unit of energy consumed.

Maintenance problems

As with almost any device, the addition of

moving parts Machines include both fixed and moving parts. The moving parts have controlled and constrained motions. Moving parts are machine components excluding any moving fluids, such as fuel, coolant or hydraulic fluid. Moving parts also do not includ ...

leads to an increase in potential maintenance problems. Mechanically aided scrubbers have higher maintenance costs than other wet collector systems. The moving parts are particularly susceptible to

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 engin ...

and

fouling Fouling is the accumulation of unwanted material on solid surfaces. The fouling materials can consist of either living organisms (biofouling) or a non-living substance (inorganic or organic). Fouling is usually distinguished from other surf ...

. In addition, rotating parts are subject to vibration-induced fatigue or wear, causing them to become unbalanced. Corrosion-resistant materials for these scrubbers are very expensive; therefore, these devices are not used in applications where corrosion or sticky materials could cause problems.

Summary

Mechanically aided scrubbers have been used to control exhaust streams containing

particulate matter 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 ...

. They have the advantage of being smaller than most other scrubbing systems, since the fan is incorporated into the scrubber. In addition, they operate with low

liquid-to-gas ratio An important parameter in wet scrubbing systems is the rate of liquid flow. It is common in wet scrubber terminology to express the liquid flow as a function of the gas flow rate that is being treated. This is commonly called the liquid-to-gas ratio ...

s. Their disadvantages include their generally high maintenance requirements, low absorption efficiency, and high operating costs.
The performance characteristics of mechanically aided scrubbers are given in Table 1.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:4" , - ! colspan="6" style="height:50px; align:center" , Table 1. Operating characteristics of mechanically aided scrubbers , - , align="center" width="12%" , Pollutant , align="center" , Pressure drop (Δp) , align="center" , Liquid-to-gas ratio (L/G) , align="center" , Liquid-inlet pressure (p_L) , align="center" , Particle diameter , align="center" , Applications , - , align="center" width="12%" rowspan="2" , Particles , align="center" rowspan="2" , 10-20 cm of water (4.0-8.0 in of water) , align="center" , 0.07-0.2 L/m³ (centrifugal)
0.5-1.5 gal/1,000 ft³ (centrifugal) , align="center" rowspan="2" , 20-60 psig (centrifugal) , align="center" rowspan="2" , 1 μm , align="center" rowspan="2" , Mining operations
Food product industries
Chemical industry
Foundries and steel mills , - , align="center" , 0.5-0.7 L/m³ (spray rotor)
4-5 gal/1,000 ft³ (spray rotor) Note: These devices are used mainly for particle collection; however, they can also remove gaseous pollutants that are present in the exhaust stream.

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 Scrubbers Wet scrubbers Liquid-phase and gas-phase contacting scrubbers