How Do Wet Scrubbers Work to Remove Air Pollution?


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Minimizing environmental impact is a crucial priority for many industries, for legal, ethical and practical reasons. While air pollutants can be harmful, their effective removal is possible with a wet air scrubber. Wet scrubbers offer a contained, efficient solution for eliminating unwanted air contaminants, making wet scrubbers a great option for air pollution control. They can remove organic and inorganic pollutants, including those that cause illness, deplete ozone or disrupt ecosystems.

Understanding how wet scrubbers work will help you choose the best system for your industry-specific needs. A modern packed-bed air scrubber with low pressure drops and high mass-transfer efficiencies will benefit your operation, the environment and, in turn, all living things within a wide radius of your industrial setting.

Read on to learn how wet scrubbers work, what pollutants they remove, what wet scrubbers are used for and more.

What Are Wet Scrubbers and How Do They Work?

Industrial wet scrubbers are instruments that use liquid solvents to remove pollution from air or liquid. Wet scrubbers work by absorbing pollutants with water or a water-based solvent. In the wet scrubber, the contaminated gas enters at the bottom and passes upward through the packed bed and downward-flowing solvent sprays. The pollutants are collected in the solvent droplets, and before the gas leaves the scrubber, it passes through a mist eliminator to capture any droplets.

In general, wet scrubbers achieve a higher level of pollutant removal than dry scrubbers. They’re vital for industrial manufacturing or wastewater treatment facilities, as they keep pollutants from contaminating outside air. Any industries that produce air pollutants might use wet air scrubbers of various designs and construction. Both wet and dry remove gas pollutants and related odors from industrial exhaust streams.

How Wet Scrubbers Work to Remove Air Pollution

Key Factors That Determine a Wet Scrubber’s Effectiveness

The solvent’s composition affects its ability to absorb contaminants. The critical factor is the solvent’s electric charge. A positive, negative or neutral charge will allow the solvent to bind with different inorganic pollutants. Some of the most common wet scrubber solvents include:

  • Water, or H2O.
  • Sodium hydroxide, or NaOH.
  • Calcium hydroxide, or Ca(OH)2. 
  • Sodium carbonate, or Na2CO3.

As the solvent absorbs contaminants, a new solvent is necessary to purge the pollutants and replace the evaporated liquid. Solvent flooding can be either continuous or periodic. A wet scrubber’s effectiveness depends on how much gas makes contact with the solvent, which requires a stream of energy. Low-energy scrubbers can remove large particles, while high-energy scrubbers can remove the tiniest particles.

All wet scrubbers, whether high or low energy, have a mist eliminator component. The mist eliminator collects and removes the contaminated droplets from the solvent. An effective mist eliminator is a fundamental component of any wet scrubber. Before discharging or reusing scrubbing liquid, it’s essential to treat it to ensure total pollutant removal.

Why are wet scrubbers necessary?

Why Are Wet Scrubbers Necessary?

Over the past several decades, rising concerns about greenhouse gases and air quality have brought air purifying techniques to the forefront of environmental conversations. Due to mounting evidence of climate change and the impact air pollution can have on the population, reducing air contamination while still continuing to develop transportation and manufacturing processes is an essential goal.

Currently, nearly all United States facilities on federal or private sites are subject to the U.S. Clean Air Act (CAA). This law stipulates that, for a list of nearly 200 pollutants, the Maximum Available Control Technology (MACT) must be used to reduce contamination before waste products can be discarded. In addition to general fines and punishments for not following these laws, the CAA also carries criminal penalties for knowingly violating CAA regulations, improperly handling particularly hazardous waste, tampering with monitoring devices, negligence and many other illegal activities.

What Are Wet Scrubbers Used For?

What Are Wet Scrubbers Used For?

Wet scrubbers are effective for removing a wide range of different air pollutants. When determining whether to use a wet scrubber, facility managers should consider which air pollutants their processes generate and the potential dangers those contaminants pose.

What Pollutants Can a Wet Scrubber Control?

A wet scrubber can control specific airborne pollutants. Industries that produce such pollutants should consider using wet scrubbers to minimize environmental impact and human safety risks. They can help reduce any of the following air pollutants.

1. Inorganic Fumes, Vapors and Gases

Many industries use wet scrubbers to control airborne fumes, vapors and gases. These inorganic pollutants can cause environmental disruption and adverse health effects if they enter the atmosphere in large quantities. The inorganic gases a wet scrubber can remove from the air include some of the most harmful air pollutants, as designated by the Environmental Protection Agency. Some of the most common such pollutants include:

  • Chromic acid.
  • Hydrogen sulfide.
  • Ammonia.
  • Chlorides.
  • Fluorides.
  • Sulfur dioxide.

2. Volatile Organic Compounds

A wet scrubber can also remove volatile organic compounds from the air. VOCs are human-made chemicals produced during specific manufacturing processes, especially those used to make paint, pharmaceuticals and refrigerants. They become trapped in specific solids or liquids and release into the air over time. Some VOC chemicals can cause severe short and long-term health effects. The manufacturing of the following materials release VOCs:

  • Paints.
  • Paint strippers.
  • Varnishes
  • Fuels.
  • Industrial cleaning supplies.
  • Pest control solutions.
  • Building materials.
  • Adhesives.

A wet scrubber can help minimize industrial VOC content, creating safer and cleaner air.

3. Particulate Matter

Other air pollutants include particulate matter, which encompasses a wide range of solid particles and liquid droplets. While some are large enough to see with the naked eye, others are minuscule. PMs can be as large as 10 micrometers and as tiny as 2.5 micrometers or smaller. For perspective, a strand of human hair is generally about 50 to 70 micrometers in diameter, which is 30 times larger than the tiniest PM particle. PMs can include any of the following, given a size of 10 micrometers or smaller:

  • Dust.
  • Dirt.
  • Soot.
  • Smoke.
  • Pollen.

Since PMs are so diverse, their sources and hazard levels vary. Some PMs can cause health issues if inhaled — if small enough, they can even enter the bloodstream. In large quantities, PMs can cause haziness and reduced visibility. Because of these negative effects, regulating PM quantity is a crucial aspect of air pollution mitigation. Wet scrubbers can help remove PMs from the air.

4. Hazardous Air Pollutants

Hazardous air pollutants, also called toxic air pollutants, have proven or suspected grave health implications. They may cause cancer, birth defects and other concerns. Some examples of HAPs include the following.

  • Benzene: Benzene is a pollutant in gasoline. It’s colorless or has a light yellow color, as well as a sweet odor. It has high flammability and evaporates rapidly. Breathing in or ingesting benzene causes internal cells to malfunction. It can impair bone marrow’s ability to produce enough red blood cells, causing anemia. Changing blood levels can also weaken the immune system.
  • Perchloroethylene: Some dry cleaning facilities release this pollutant into the air. In metalworking facilities, perchloroethylene cleans and degreases metal. It’s colorless and non-flammable, with a sweet odor. High-level exposure can lead to nervous system failures, resulting in unconsciousness or death.
  • Methylene chloride: Methylene chloride, also called dichloromethane, is a solvent and paint stripper. Many industries use this chemical. Like perchloroethylene, it’s colorless and sweet-smelling. Exposure may cause cancer, suffocation or cognitive impairment.

Because HAPs have proven dangerous, preventing them from entering the atmosphere is crucial for public health and safety. Wet scrubbers help keep the environment safer by removing these chemicals from the air.

Where Are Wet Scrubbers Used?

Where Are Wet Scrubbers Used?

Wet scrubbers are necessary in a wide range of industries since many industrial facilities produce various air pollutants. Some environments in which a wet scrubber may be essential include:

  • Food and agriculture.
  • Electroplating.
  • Acid plants.
  • Fertilizer plants.
  • Steel mills.
  • Asphalt plants.

How Targeted Pollutants Affect Scrubber Design

A wet scrubber’s design depends on the targeted pollutants. Because air contaminants vary in many ways, including size, shape and electric charge, their capture also differs.

Packed Bed Height

The packed bed height size can vary based on the targeted pollutant and efficiency goals. This factor impacts how long the gas stream is in contact with the solvent liquid, which results in more effective particle absorption. Depending on the application in question, a packed-bed wet scrubber’s efficiency may be between 50 and 95%. These scrubber types work well against inorganic fumes, gases and odors.

Scrubbing Liquid Type

Another variable factor is the solvent’s composition. Acidic pollutants necessitate a caustic scrubbing liquid, while caustic pollutants necessitate an acidic scrubbing liquid. The solvent’s electric charge impacts which contaminants it attracts and binds to. Depending on the contaminants a facility produces, different solvents will prove most effective.

Liquid Flow Rate

Liquid flow rate, also called liquid-to-gas ratio, is the proportion of solvent liquid to gas treated. A higher liquid flow rate has improved efficiency since the volume of droplets is greater in a given cross-section. More solvent means more pollutants captured, at least to a point. If a facility produces relatively high air contaminants, a wet scrubber with a high liquid flow rate may be necessary.

Target Gas Velocity

The target gas velocity is a significant factor. The speed at which the contaminated gas moves through the wet scrubber system impacts pollutant collection efficiency. A higher level of inertia leads to more collisions and impaction, which result in captured pollutants. Different velocities are necessary depending on the particles in question. A greater velocity calls for stronger force and, therefore, more energy, so wet scrubbers with high velocities can be more expensive to purchase and operate.

Fiberglass reinforced plastic

Materials of Construction

Wet scrubbers can consist of a wide range of materials, including metals and plastics. Choosing the right material can minimize cost concerns such as rusting and corrosion. Some possible wet scrubber materials include the following.

  • Fiberglass-reinforced plastic: FRP is an economic, lightweight material. It works well in both alkaline and acidic environments. It’s best suited for low- to medium-temperature environments and inorganic pollutants. FRP makes a good liner material for steel corrosion protection, thanks to its high corrosion resistance.
  • Carbon steel: Carbon steel is a popular choice for wet scrubbers. It works best for dust collection or basic pollutants, as opposed to acidic ones.
  • Cast iron: Cast iron is an affordable choice for a wet scrubber material. A cast-iron wet scrubber may incorporate stainless steel or FRP lining for better corrosion resistance.
  • Cast-iron alloys: A cast-iron alloy containing nickel has stronger corrosion resistance. It’s a good material when polluted gas is high temperature or contains abrasive particles.
  • Stainless steel: Stainless steel, a temperature-resistant material, is ideal for high-temperature environments. However, it’s best to use a different material if hydrochloric acids are present.
  • Polyvinyl chloride: PVC has significant limitations, as it cannot handle high temperatures. However, in a low-temperature environment, it is effective. You may find PVC in specific elements of your wet scrubber, such as the packed bed or the spray nozzle.
  • Furfuryl alcohol resin: Furfuryl alcohol resin works well against organic pollutants. It has strong acid and alkali resistance.

Choosing construction materials depends on the application, targeted pollutants and other priorities. Some materials have less initial expense than others. For instance, cast iron is the least expensive material in terms of its initial cost. However, a corrosion-resistant material may last longer, justifying a higher price. In addition, internal lining materials can vary. Some choices include the following.

  • Carbon graphite: Carbon graphite works well for smaller, simpler wet scrubbers.
  • Teflon: Teflon is suitable for extreme-temperature environments and other severe conditions.
  • Kynar: A Kynar-lined FRP wet scrubber is an excellent choice for targeted chlorinated hydrocarbons.

Advantages of Wet Scrubbers

A packed-bed wet scrubber is advantageous for many reasons. Efficient wet scrubbers use the best construction materials and designs to create a cost-effective and successful system.

Relatively Low Pressure Drop

Efficiency reduces cost and extends a wet scrubber’s life, and a low pressure drop is a crucial efficiency factor. The pressure drop is the change in pressure as the gas travels through the wet scrubber. The most efficient types of wet scrubbers minimize pressure drops.

High Mass-Transfer Efficiencies

The more material a wet scrubber can move, the more pollutants it will eliminate from the air. A high mass-transfer efficiency achieves greater pollutant scrubbing. Several adjustments help achieve peak mass transfer efficiency.

Packing Height or Type Adjustments

The wet scrubber’s packing height and type are adjustable. Various heights and types work best against different pollutants, improving mass transfer and removing more particles from the air.

Relatively Low Capital Cost

Compared to other pollution-minimizing solutions, packed-bed wet scrubbers have a low capital cost, both for initial setup and lifetime maintenance. If you need a cost-effective contaminant removal system, a packed-bed wet scrubber is the best choice.

Small Space Requirements

Packed-bed wet scrubbers take up a relatively small space since one wet scrubber can remove many different types of pollutants. considering how many pollutants they can remove. Many facilities only need one wet scrubber system for total pollutant removal. It simultaneously minimizes cost and infrastructure concerns, resulting in a simplified solution.

Collects Particulate Matter as Well as Gases

While wet scrubbers remove toxic gases from the air, they also target particulate matter. PMs include allergens and illness-causing soot and smoke. A wet scrubber’s ability to remove such contaminants results in a cleaner, safer environment for miles beyond the pollutant-producing facility.

Contact MACH Engineering for More Information

Contact MACH Engineering for More Information

The efficient removal of industrial air pollutants helps keep the environment stable and safe. Packed-bed wet scrubbers remove particulate contaminants and harmful gases before they have the chance to cause adverse environmental impacts. By using efficient wet air scrubbers, professionals in a wide range of industries can help protect human, animal and plant life.

If you’re interested in learning more about wet air scrubbers, contact MACH Engineering. At MACH Engineering, we design state-of-the-art odor and emission control systems for specific industry applications. We use multiple construction materials, sizes and other elements to create the most effective possible solutions. Learn more about our custom equipment design services at MACH Engineering.

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