The Major Types of Industrial Scrubbers

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For decades, industrial scrubbers have provided an effective means of purifying exhaust streams produced in manufacturing plants, power stations and other industrial facilities, and modern scrubbers can remove pollutants more effectively than ever before. By using an industrial scrubber that offers the right capabilities, you can keep harmful pollutants from entering the atmosphere.

In this scrubber selection guide, we’ll look at the three major types of industrial scrubbers to choose from: wet scrubbers, dry scrubbers and electrostatic precipitators (ESPs). Each scrubber type offers its own uses, advantages and potential drawbacks.

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What Are Industrial Scrubbers?

Industrial scrubbers are devices used to remove pollutants from industrial exhaust streams. These pollutants include particulates and gases that would be harmful to the environment and human health. The exhaust stream passes through the scrubber where it is stripped of harmful contaminants before being released into the environment.

The scrubber catches and contains the pollutants, so the exhaust stream that leaves the scrubber has been “scrubbed” of its impurities. A variety of industries use scrubbers to make their operations more hygienic and environmentally responsible.

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How Industrial Scrubbers Are Used to Protect the Environment

Humans were causing air pollution even in ancient societies, but with the Industrial Revolution, the problem became far more prominent. The industrial processes that propelled the world forward also came with some negative repercussions. It wasn’t until the latter half of the 20th century that the U.S. began to recognize the severity of the problem and take governmental action to reduce the number of toxins that were released into the air.

Scrubbers have been instrumental in helping industrial facilities dramatically reduce the contaminants they release into the environment. The result is cleaner air that’s healthier for the planet and for people. Some common contaminant types that scrubbers can remove from exhaust streams include:

  • Particulate matter: Some contaminants are solid particles that can pollute the air. For example, this includes the carbon particles that smoke contains, along with flammable and explosive dust. Particulates in the air can create a hazy appearance and can pose health risks. The smallest particles, which are less than 2.5 micrometers across, are the biggest threat to a person’s health since they can get into the lungs and the bloodstream.
  • Acid gases: Acid gases are one of the most common pollutants that necessitate a scrubber. They are harmful and often corrosive. Some of the biggest culprits include sulfur oxides, carbon dioxide, hydrogen sulfide, nitric oxide, nitrogen dioxide and hydrochloric acid. Carbon dioxide is responsible for about 81% of greenhouse gas emissions from human activities in the U.S. Acid gases like sulfur dioxide and nitrogen oxides contribute to acid rain, which is damaging to the environment.
  • Volatile organic compounds: Some scrubbers may also remove volatile organic compounds (VOCs) — human-made chemicals that can be detrimental to human health and the well-being of the environment. When VOCs enter the air uninhibited, photochemical reactions can lead to the formation of ozone, fine particulates and other gaseous pollutants, which help to create smog. Some VOCs can also disrupt ecosystems and contribute to acid rain.

It’s easy to see how allowing these pollutants to enter the atmosphere can cause both immediate and long-term problems for the environment. Three types of scrubbers were designed to solve this issue. Scrubbers can help industrial facilities comply with environmental protection regulations and make their processes more eco-friendly.

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Types of Industrial Scrubbers

The biggest distinction between the types of scrubbers is what they use to remove contaminants. The original scrubber type, the wet scrubber, uses liquid to wash exhaust streams. Dry scrubbers use dry media instead, and electrostatic precipitators use an electric charge to remove particles from the air. Learn more about the differences between wet and dry scrubbers in our other blog post.

Wet Scrubbers

The first industrial scrubbers available were wet scrubbers, and these scrubbers continue to be a popular option today. Wet scrubbers are metal or composite containers that contain liquid. As a gas stream passes through the container, contaminants bind with the water and remain in the container. The purified gas then leaves the scrubber.

All wet scrubbers use liquid to purify, but that liquid can differ depending on the application and the type of pollutants you need to remove. Water is an effective choice for removing many contaminants, but you could also use a wet scrubber with a different type of liquid. The chemical composition and electric charge of a liquid will determine which contaminants it’s able to pull from a gas stream.

Wet scrubbers use fairly simplistic yet effective technology. Engineers found that maximizing the surface area of the liquid in the wet scrubber allows for more effective purification. They also determined that agitating the solution in the scrubber helps by keeping the gas and scrubber liquid more thoroughly mixed.

Wet scrubbers offer some valuable advantages, as well as some potential disadvantages, compared to other scrubber options.

Advantages of Wet Scrubbers

Some notable advantages of wet scrubbers include their:

  • Versatile applications: Wet scrubbers can remove a host of contaminants, including both gases and particulate matter. Wet scrubbers are also a good choice for handling high-temperature, high-humidity gas streams, unlike ESPs and some other types of filtration systems. Wetting contaminants also allows these scrubbers to safely handle flammable and combustible materials.
  • High collection efficiency: Wet scrubbers also have exceptionally high rates of collection efficiency. In other words, they’re able to effectively remove contaminants almost in their entirety, leaving very little the final exhaust stream. Most modern wet scrubbing systems can remove up to 95% of contaminants.
  • Compact size: Wet scrubbers are also smaller than some other scrubber system types. This makes them more convenient to retrofit into an existing industrial facility. You can find the most convenient placement for your wet scrubber without interfering with other machinery. Smaller-sized machines can also mean lower initial costs.

Disadvantages of Wet Scrubbers

As with any technology, wet scrubbers also come with some potential disadvantages, including:

  • Disposal complications: Wet scrubbers produce a concentrated slurry from the contaminants they trap, and this effluent stream must be disposed of properly. Nonhazardous slurries aren’t too complicated to dispose of, but toxic effluent must be treated and disposed of according to hazardous waste disposal guidelines.
  • Operating costs: Stationary wet scrubber systems have demanding horsepower and pressure-drop requirements. This can mean higher operating costs. They may also require more maintenance than some other systems since solids can collect around the wet-dry interface, and acidic slurries can be highly corrosive.

Dry Scrubbers

Dry scrubbers have become an increasingly common option alongside wet scrubbers. They offer some advantages over wet scrubbers but also come with some limitations compared to wet scrubbers. As the name suggests, dry scrubbers do not use a liquid scrubber to purify. Instead, they use dry sorbents. This is the main difference between wet and dry industrial scrubbers. Otherwise, these devices share several similarities. Like a wet scrubber, a dry scrubber can remove both harmful gases and particles from an exhaust stream.

Dry scrubbers are mostly used to remove acidic components from gas exhausts that result from combustion processes. In these applications, the dry sorbent is an alkaline slurry. When the gas interacts with the alkaline substance, it undergoes absorption, effectively neutralizing the pH of the gas. The impurities turn into solid salts, which the particulate control device removes from the system. There are two basic methods dry scrubbers can use:

  • Dry sorbent injection: Some dry scrubbers inject a sorbent that the gas directly mixes with as it passes through. This injection and resulting reaction can happen at various locations in the process. Wherever it occurs, the system absorbs the acidic components from the gas and creates solid salts. These dry scrubbers are known as dry sorbent injectors (DSIs).
  • Spray dryer absorbent method: Other dry scrubbers spray the alkaline slurry into the gas stream as it passes through. The heat of the exhaust gas causes water droplets to evaporate, so the purified gas that leaves the system is non-saturated. These scrubbers are known as spray dryer absorbers (SDAs), spray driers or semi-dry scrubbers.

Dry scrubbers are less efficient than wet scrubbers at removing toxins, but they are still suitable in some applications and can be more convenient.

Advantages of Dry Scrubbers

Dry scrubbers have gained popularity due to some of the advantages they offer, including:

  • Simpler waste disposal: Compared to wet scrubbers, dry scrubbers offer simpler waste disposal. Rather than having to handle wet sludge, you only need to dispose of the dry particles collected. Heavier particles fall down to the bottom of the system and are trapped in a particle screen, and other contaminants are absorbed by the sorbent, which can easily be removed from the system.
  • Lower maintenance: These systems tend to require less maintenance than wet scrubbers, keeping operations simpler with less downtime.
  • Compact size: An advantage that wet and dry scrubbers have in common, compared to ESPs, is that they’re both relatively compact. Because dry scrubbers have a fairly small footprint, you can easily find the ideal spot to install them in your industrial facility.

Disadvantages of Dry Scrubbers

Dry scrubbers also come with some downsides, including their:

  • Limited efficiency: Like wet scrubbers, dry scrubbers can trap both gaseous and particulate contaminants. However, dry scrubbers are known to be less efficient than wet scrubbers, so they may allow more contaminants to escape into the environment.
  • Need for reactants: Dry scrubbers require special reactants to operate. This means you need to keep these reactants on-hand at all times, adding to your storage needs. These reactants can also be relatively costly, contributing to higher ongoing operating costs.

Electrostatic Precipitators

Electrostatic precipitators are unique compared to wet and dry scrubbers in that they are filterless devices. Rather than using some sort of material to absorb harmful pollutants, electrostatic precipitators use an electric field to attract contaminants and trap them inside the machine. The electric field either gives the particles a positive or negative charge. The particles are then attracted to plates in the device with the opposite charge.

Electrostatic precipitators are used to remove particles like dust and smoke. Wet scrubbers can also use an electric charge to attract contaminants, but with electrostatic precipitators, that is the sole method used for purification.

Electrostatic precipitators are classified into two types depending on the way they dispose of the contaminants they collect:

  • Dry ESPs: Most ESPs vibrate the collector plates when needed to loosen the collected particles. This process is known as rapping and keeps the contaminants dry during disposal.
  • Wet ESPs: Wet ESPs wash the collector plates with water. Typically, this method is used when the particles collected are sticky and pose low resistivity.

ESP systems can rotate the surface plates as needed to ensure there’s always enough charged surface area in the system to attract contaminants coming in.

Advantages of Electrostatic Precipitators

Electrostatic precipitators offer some benefits over alternative technologies, including their:

  • High collection efficiency: When it comes to particulate matter, ESPs offer impressive collection efficiencies that can exceed 99%. Though they are limited to particulate matter, ESPs are able to thoroughly collect a variety of contaminants, including corrosive, wet and high-temperature materials.
  • Large capacity: ESPs can also handle extremely large gas flow rates and heavy dust loads with low pressure losses. This makes ESPs a good choice in facilities with high-volume exhaust streams that only need to be purified of particulate matter.
  • Low operating costs: ESPs tend to be fairly low-maintenance. Add to this that ESPs don’t require any filtration media. The result is a system that is relatively inexpensive to operate. This also means you don’t need to store any extra components on-site to operate your ESP.

Disadvantages of Electrostatic Precipitators

There are also a few reasons why a company may not choose an ESP to filter their exhaust streams. The ESP’s disadvantages include its:

  • Large footprint: ESPs take up a large area, which makes it difficult to retrofit them into existing facilities. Once installed, moving the ESP to a new location or reconfiguring it can be a challenge.
  • Inability to filter gases: ESPs can be an effective choice for filtering particulate matter like dust or soot, but these systems cannot be used to remove gaseous pollutants. This greatly limits the useful applications of ESPs since many industrial operations produce both gaseous and solid pollutants that must be filtered.

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Trust MACH Engineering for Your Wet Scrubber Needs

At MACH Engineering, we can provide the solutions you need to outfit your facility with an industrial wet scrubber that makes your processes environmentally sustainable. These solutions include complete wet scrubber systems, short-term rentals, scrubber tower internals and design services. We pride ourselves on our ability to customize a scrubber system that is optimized for your requirements. Contact us today to learn more about how MACH Engineering can help with your waste purification needs.


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