Mechanically aided scrubbers are a form of
pollution control technology. This type of technology is a part of the group of
air pollution
Air pollution is the presence of substances in the Atmosphere of Earth, air that are harmful to humans, other living beings or the environment. Pollutants can be Gas, gases like Ground-level ozone, ozone or nitrogen oxides or small particles li ...
controls collectively referred to as
wet scrubbers.
In addition to using liquid sprays or the exhaust stream,
scrubbing systems can use
motors to supply energy. The motor drives a
rotor or
paddles 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 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
pollutants 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
wet scrubbers 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
pollutants 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. Achieving these 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. 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 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 engine ...
and
fouling. 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. 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 ratios. 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 (pL)
, 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, though can also remove gaseous pollutants from the exhaust stream.
References
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.
Pollution control technologies
Air pollution control systems
Scrubbers
Wet scrubbers
Liquid-phase and gas-phase contacting scrubbers