Greta Jimenez
Scrubbers are devices used to control air pollution by removing suspended particles and gases from industrial exhaust or flue gas streams. They are commonly used to minimize environmental impact and reduce air contamination. Wet scrubbers are the most widely recognized type, using a solvent to trap particles and pollutants from the air, while dry scrubbers do not utilize a liquid and instead use a reaction center where the sorbent or dry reaction material absorbs contaminants. Wet scrubbers are considered more effective at removing pollutants than dry scrubbers, with the ability to remove solids, mists, and gases simultaneously. They are also used for heat recovery from hot gases and can remove organic and inorganic pollutants, including those that cause illness, deplete the ozone layer, or disrupt ecosystems.
| Characteristics | Values |
|---|---|
| Type | Wet scrubber, dry scrubber, semi-dry scrubber |
| Use | Removing pollutants from industrial exhaust or flue gas streams |
| Pollutants Removed | Particulate matter, gases, VOCs, acid gases, water-soluble inorganic contaminants, toxic gases, corrosive gases, odours |
| Efficiency | Wet scrubbers >90%, Venturi scrubbers >98% for particles >0.5 μm, Dry scrubbers 80+% acid gas removal |
| Energy Usage | Low, medium, high |
| Operating Problems | Corrosion, plugged nozzles, poor gas-liquid contact, inadequate liquid flow, liquid re-entrainment |
| Indicators of Performance | Pressure differential, liquid flow rate, scrubber liquid outlet concentration |
| Liquid Used | Water, dimethylaniline, amines, sodium hydroxide, calcium hydroxide, sodium carbonate |
| Other Components | Cyclone, spray tower, packed bed, electrostatic precipitator |
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What You'll Learn
Venturi scrubbers
The design of a Venturi scrubber consists of three sections: a converging section, a throat section, and a diverging section. The inlet gas stream enters the converging section, where the velocity of the gas increases as the area decreases. Liquid is introduced at the throat or the entrance to the converging section, where it mixes turbulently with the gas, producing a large number of tiny droplets. This process facilitates the collection of particles in the liquid droplets and the absorption of gases.
While Venturi scrubbers are effective for particulate removal, they have limitations in terms of gaseous pollutant removal due to the short contact time between the liquid and gas phases. To maximize gas absorption, Venturi scrubbers are designed to operate at lower gas velocities and higher liquid-to-gas ratios. Ejector Venturi scrubbers, which derive energy from a high-pressure liquid spray, are often used when higher gas solubility is required, although they still face limitations in gas removal due to short contact times.
Overall, Venturi scrubbers play a crucial role in air pollution control, particularly in industrial settings, by effectively removing particulate matter and, to a lesser extent, gaseous pollutants from exhaust systems. Their versatility and high collection efficiencies make them a popular choice for a wide range of applications.
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Wet scrubbers for gaseous control
Wet scrubbers are used to remove pollutants from exhaust gas streams. The process involves using a liquid, usually water, to absorb the pollutant. Wet scrubbers are often referred to as absorbers and have removal efficiencies of over 90%, depending on the pollutant being absorbed.
Wet scrubbers are commonly used to remove acid gases, water-soluble inorganic contaminants, and volatile organic compounds. They are also effective at removing water-soluble toxic and corrosive gases like hydrochloric acid (HCl) and ammonia (NH3) from process exhaust gas. The choice of absorbent liquid depends on the solubility of the gas and should be relatively inexpensive. While water is the most common solvent, other liquids such as dimethylaniline or amines may be used for gases with low water solubility.
The primary indicators of wet scrubber performance are pressure differential, liquid flow rate, and scrubber liquid outlet concentration. The performance of a wet scrubber is influenced by the amount of soluble gas in the gas stream and the concentration of the solute gas in the liquid film in contact with the gas. Wet scrubbers can be designed with a large surface area to maximize contact between the liquid and gas stream, improving removal efficiency.
Wet scrubbers have several advantages, including low capital and installation costs and simplicity of design. However, one significant drawback is the potential transfer of air pollution to water pollution. Additionally, the absorption equilibrium may not always be sufficient to meet clean air regulations, and the use of modified solvents can increase costs.
Wet scrubbers are susceptible to operating problems, especially if the pressure differential, liquid flow, and gas flow are not carefully controlled. Common issues include low gas and liquid flow rates, condensation, poor liquid distribution, and pollutant re-entrainment.
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Dry scrubbers
Scrubbers are used to remove harmful materials from industrial exhaust gases before they are released into the environment. Wet scrubbers use a liquid medium, typically water, to remove pollutants from exhaust gases.
The basic operations of a dry scrubber involve a few key steps. Firstly, the polluted gas stream, which typically contains various pollutants, including particulate matter, acid gases, and possibly volatile organic compounds (VOCs), is taken into the dry scrubber system. Next comes the optional step of gas cooling, where the hot gas stream is potentially cooled before sorbent introduction to optimize scrubbing. The third step is sorbent injection, where a sorbent material is injected into the gas stream. The sorbent is either a dry reagent or a slurry mixture that absorbs the acid gases, which then react to form solid salts. The final step is filtering out the pollutant-containing powder through a fabric filter or with an electrostatic precipitator.
One disadvantage of dry scrubbers is that they cannot remove all contaminants from the gas. Another is the residual waste powder that is produced, which must be disposed of as it is a hazardous material. This waste must be handled by specialists because of its chemical makeup.
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Packed-bed scrubbers
Scrubbers are used to remove pollutants from gases. Wet scrubbers are commonly used for this purpose, and they work by bringing the gas stream into contact with a scrubbing solution, which is usually water. Pollutants are removed from the gas stream and transferred to the liquid, which can then be disposed of. Wet scrubbers are used to remove dust and inorganic contaminants, as well as toxic and corrosive gases like hydrochloric acid (HCl) and ammonia (NH3). They can also be used for heat recovery from hot gases.
The Monroe Vertical Packed Tower Scrubber is a commonly used packed-bed scrubber with a vertical counter-flow design. It provides efficient scrubbing of vapors and is used in most industries. This scrubber is not suitable when solid particulate matter is present in the airstream. The Monroe Horizontal Packed Bed Scrubber is another option, used when there is limited headroom or when a low profile is required for rooftop applications.
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Scrubber performance indicators
Wet scrubbers are used to remove liquid or solid particles from a gas stream by transferring them to a liquid. The liquid used is most commonly water, but other liquids can be used depending on the solubility of the gas and the solubility of the solute gas in the liquid film in contact with the gas. Wet scrubbers are used to clean air, fuel gas, or other gases of various pollutants and dust particles.
Wet scrubbers are susceptible to several operating problems, especially if the pressure differential, liquid flow, and gas flow are not kept relatively constant. The primary indicators of wet scrubber performance are:
- Pressure differential
- Liquid flow rate
- Scrubber liquid outlet concentration
The less significant indicators of gaseous pollutant control efficiency for wet scrubbers are:
- Gas flow rate
- Neutralizing chemical feed rate
- Scrubber outlet gas temperature
Parameters that can be monitored as an alternative to scrubber liquid outlet concentration include scrubber liquid pH, scrubber liquid specific gravity, and scrubber makeup/blowdown rates.
Other factors that can influence the efficiency and performance of a wet scrubber include:
- The type of packing material used must meet the requirements of the scrubber. For instance, stainless steel materials work well in high-temperature environments but not in the presence of hydrochloric acid.
- Freshwater makeup needs to meet the scrubber system’s fluid needs.
- Biological growth can occur in systems with high pH values, especially in the warm, wet environment inside the wet scrubber’s beds. To prevent the growth of algae and bacteria inside the scrubber, consider regularly washing the interior of the unit with acid or using systems that include chlorination or bromination.
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Frequently asked questions
Wet scrubbers are used to remove organic and inorganic pollutants from industrial exhaust streams. They are also used to remove acid gases, water-soluble inorganic contaminants, and volatile organic compounds.
Wet scrubbers use a liquid, usually water, to remove pollutants from an exhaust stream. The liquid is used to absorb the pollutant, with the gas exiting the scrubber cleaner than the gas that entered.
There are several types of wet scrubbers, including spray tower scrubbers, venturi scrubbers, packed-bed scrubbers, condensation scrubbers, impingement plate scrubbers, mechanically-aided scrubbers, and orifice scrubbers.
Wet scrubbers offer a contained and efficient solution for eliminating unwanted air contaminants, making them a great option for air pollution control. They can also handle explosive and flammable gases safely.
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