Mechanically aided scrubber
Encyclopedia
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
Air pollution is the introduction of chemicals, particulate matter, or biological materials that cause harm or discomfort to humans or other living organisms, or cause damage to the natural environment or built environment, into the atmosphere....

 controls collectively referred to as wet scrubber
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 liquid, by forcing it through a pool of liquid, or by...

s.

In addition to using liquid sprays or the exhaust stream, scrubbing systems
Scrubber
Scrubber systems are a diverse group of air pollution control devices that can be used to remove some particulates and/or gases from industrial exhaust streams. Traditionally, the term "scrubber" has referred to pollution control devices that use liquid to wash unwanted pollutants from a gas stream...

 can use motor
Electric motor
An electric motor converts electrical energy into mechanical energy.Most electric motors operate through the interaction of magnetic fields and current-carrying conductors to generate force...

s to supply energy. The motor drives a rotor or paddle
Paddle
A paddle is a tool used for pushing against liquids, either as a form of propulsion in a boat or as an implement for mixing.-Materials and designs:...

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
Pressure drop
Pressure drop is a term used to describe the decrease in pressure from one point in a pipe or tube to another point downstream. "Pressure drop" is the result of frictional forces on the fluid as it flows through the tube. The frictional forces are caused by a resistance to flow. The main...

 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
Centrifugal fan
A centrifugal fan is a mechanical device for moving air or other gases. It has a fan wheel composed of a number of fan blades, or ribs, mounted around a hub. As shown in Figure 1, the hub turns on a driveshaft that passes through the fan housing...

 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 cocurrently with the moving exhaust gas. Some gaseous pollutant
Pollutant
A pollutant is a waste material that pollutes air, water or soil, and is the cause of pollution.Three factors determine the severity of a pollutant: its chemical nature, its concentration and its persistence. Some pollutants are biodegradable and therefore will not persist in the environment in the...

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
Centrifuge
A centrifuge is a piece of equipment, generally driven by an electric motor , that puts an object in rotation around a fixed axis, applying a force perpendicular to the axis...

.

Centrifugal-fan collectors are the most compact of the wet scrubber
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 liquid, by forcing it through a pool of liquid, or by...

s since the fan and collector comprise a combined unit. No internal pressure loss
Pressure drop
Pressure drop is a term used to describe the decrease in pressure from one point in a pipe or tube to another point downstream. "Pressure drop" is the result of frictional forces on the fluid as it flows through the tube. The frictional forces are caused by a resistance to flow. The main...

 occurs across the scrubber, but a power loss equivalent to a pressure drop
Pressure drop
Pressure drop is a term used to describe the decrease in pressure from one point in a pipe or tube to another point downstream. "Pressure drop" is the result of frictional forces on the fluid as it flows through the tube. The frictional forces are caused by a resistance to flow. The main...

 of 10.2 to 15.2 cm (4 to 6 in) of water occurs because the blower
Blower
Blower may refer to:* USS Blower , a submarine of the United States Navy* a ducted centrifugal fan, especially when used in a heating, ventilating, and air-conditioning system* a supercharger on an internal-combustion engine...

 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 pollutant
Pollutant
A pollutant is a waste material that pollutes air, water or soil, and is the cause of pollution.Three factors determine the severity of a pollutant: its chemical nature, its concentration and its persistence. Some pollutants are biodegradable and therefore will not persist in the environment in the...

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. 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
Moving parts
The moving parts of a machine are those parts of it that move. Machines comprise both moving and fixed parts. The moving parts have controlled and constrained motions....

 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
Corrosion is the disintegration of an engineered material into its constituent atoms due to chemical reactions with its surroundings. In the most common use of the word, this means electrochemical oxidation of metals in reaction with an oxidant such as oxygen...

 and fouling
Fouling
Fouling refers to the accumulation of unwanted material on solid surfaces, most often in an aquatic environment. The fouling material can consist of either living organisms or a non-living substance...

. 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 ratio
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...

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.
Table 1. Operating characteristics of mechanically aided scrubbers
Pollutant Pressure drop (Δp) Liquid-to-gas ratio (L/G) Liquid-inlet pressure (pL) Particle diameter Applications
Particles 10-20 cm of water (4.0-8.0 in of water) 0.07-0.2 l/m3 (centrifugal)
0.5-1.5 gal/1,000 ft3 (centrifugal)
20-60 psig (centrifugal) 1 μm Mining operations
Food product industries
Chemical industry
Foundries and steel mills
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.
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