Wet vs. Dry Industrial Scrubbers for Air Pollution

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Wet and dry scrubbers both work to reduce air pollution by treating exhaust gases. However, there are differences between the mechanisms each type of scrubber uses and thepollutants they best neutralize. Choosing which type of scrubber to invest in for your air pollution control requires knowing how these systems work and their pros and cons.

Wet Scrubbers

There are a variety of wet scrubbers that remove pollutants from exhaust gases. These different types of wet scrubbers all work by wetting the gas to separate contaminants. Wet scrubbers are highly effective at acid and contaminant removal.

What Are Wet Scrubbers?

Wet scrubber types differ in how they expose the exhaust gas to the liquid. These variations of wet scrubbers include the following types:

  • Packed bed scrubbers
  • Venturi scrubbers
  • Spray towers
  • Cyclone spray chambers
  • Orifice scrubbers

For all wet scrubbers, the proportion of exhaust gas to liquid is vital, as it relates to the surface area contact of the gas with the liquid. The pressure and speed of the incoming exhaust gas also influence how well the scrubber works. The scrubbing liquid also determines what the unit removes from the exhaust gas. While water is common as a liquid, caustic scrubbing liquid may be an option for facilities that must remove acids from the exhaust gas.

Various scrubber types have different uses and levels of performance. Therefore, to understand how wet scrubbers work, you need to learn about the specific operation of each type.

Packed Bed Scrubbers

Packed bed scrubbers, also known as packed towers, send gas through beds of tower packing. As the gas moves upward through the beds, the scrubbing liquid sprays down on top. In these types of scrubbers, the type of packing makes a significant difference in optimizing the surface contact between gas and liquid. The gas continues to move up through a mist eliminator section of the tower, where the liquid containing the pollutants collects, allowing the cleaned gas to exit.

Tower packing comes in a variety of types, including different materials and shapes. The best type to use depends on the composition of the gas and the design of the tower. Ideal tower packing will maximize the gas to liquid contact and prevent excessive pressure drops.

When choosing between wet scrubbers for removing gaseous pollutants, packed beds stand out as an excellent option.

Venturi Scrubbers

Venturi scrubbers use an hourglass-shaped chamber through which exhaust gases pass at high pressure. Scrubbing liquid enters the gas stream at a lower pressure. The high pressure of the gas turns the scrubbing liquid into a fine mist, which traps gaseous and particulate matter into droplets. Unfortunately, this type of scrubber requires a lot of power to accommodate the high pressure drops.

A variation of the Venturi scrubber is the jet model. This type delivers the scrubbing liquid at high pressure into the narrowest part of the scrubber. While this method overcomes the extreme pressure drops, it does not operate as efficiently as a standard Venturi scrubber.

For a lower-cost method that removes both particulate and gaseous materials, a Venturi scrubber may be the best option.

Spray Towers

Spray towers have a simple method of scrubbing pollutants from gas. The exhaust enters a chamber fitted with multiple spray nozzles that create a mist of the scrubbing liquid. Droplet size must receive careful control to ensure maximum capture of pollutants and removal from the gas. The cleaned gas escapes through the top of the chamber. After treating the gas, liquid recirculates through a treatment system before returning to the tower.

Thanks to their simplistic operation, spray towers have lower power consumption requirements. However, these towers do not operate as efficiently to remove particulate matter from the gas. Plus, facilities need systems to treat the liquid before reusing it. Lastly, spray towers have heavy maintenance requirements to keep the spray nozzles clear to produce the required droplet size for operation.

Facilities that require low power options for gaseous pollutant control can use this type of wet scrubbing tower.

Cyclone Spray Chambers

A cyclone spray chamber takes sprayers and integrates them with high speeds that create a cyclonic movement of the gases. Gas enters a chamber at high speed into the side of the chamber. The fast-moving gases naturally move in a cyclone fashion around the round spray chamber. A centrally located spray manifold contains multiple spray nozzles to coat the gas with the scrubbing liquid. As the gas rises, it passes straightening vanes at the top of the chamber to break up the cyclonic movement before it exits the top.

The high-speed movement of the gas encourages the gas to collect more droplets. Consequently, the nozzles can use smaller spray sizes, reducing the liquid required and enhancing efficiency. To achieve faster speeds and overcome the high pressure drops, though, cyclone spray chambers use much more energy than simpler spray towers do.

However, cyclone spray chambers overcome some of the shortcomings of less efficient spray towers. First, these chambers have better particulate removal capabilities than spray towers. Additionally, they need less liquid to operate.

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Orifice Scrubbers

Orifice scrubbers operate based on a principle similar to that of the Venturi models. Both force gas to atomize the liquid at high speeds. In orifice scrubbers, the gas passes over a pool of scrubbing liquid, collecting droplets that it turns into mist along the way. As the gas laden with this liquid moves through the chamber, it reaches several baffles. When the gas strikes these baffles, it knocks the pollutant-containing liquid out, permitting the cleaned gas to pass through.

Impingement scrubbers are a subset of this type of device. Instead of passing the gas over a pool of liquid, they pass the gas first through a perforated tray covered or sprayed with liquid. The rest of the process is the same as other orifice scrubbers.

Compared with other models, orifice scrubbers have very low liquid recirculation. These models may be best for facilities that require low power inputs and have high efficiency.

Common Wet Scrubbers Air Pollution Applications

Common uses of wet scrubbers are for facilities that process propane and natural gas. Additionally, any type of facility that requires the removal of particulate matter and treatment of gaseous pollutants in the most thorough manner possible should use wet scrubbers.

Advantages of Wet Scrubbers

Wet scrubbers have several advantages for their use — most notably, their ability to process out more pollutants compared to dry scrubbers. Plus, they reduce fire hazards by keeping flammable pollutants wet inside the tower. These scrubbers are also the only means of pollution control that remove both particulate matter and gases from the exhaust in the same process. Different types of wet scrubbers will perform at varying levels when processing out both particulate and gaseous materials. For instance, spray towers don’t operate as well at removing particulate matter as Venturi scrubbers.

Disadvantages of Wet Scrubbers

The most significant downside to wet scrubbers is the need to dispose of the liquid waste from them. Due to pollutant contamination of this liquid, it requires treatment before disposal. Additionally, wet scrubbers can experience corrosion and require regular maintenance to keep them operating at peak efficiency.

Dry Scrubbers

Dry scrubbers offer an alternative to wet scrubbers, which require an additional wastewater disposal system. The cost-effectiveness of these scrubbers makes them popular for use in many facilities that need treatment of polluting exhaust gases.

What Are Dry Scrubbers?

Dry scrubbers do not use wet products to treat the exhaust gas. Instead, these systems use a dry reagent called a sorbent to either neutralize or separate the acids from the gas. As in wet scrubber systems, dry scrubbers need to maximize surface area contact between the sorbent and the gas to remove as much acid from the exhaust as possible. Filters in the system filter out particulate matter the sorbent cannot impact. After the sorbent passes through the gas, it becomes a hazardous material that requires special disposal.

While wet scrubbers perform better at removing more pollutants, the extra weight and disposal of the wastewater from them increases their operating costs. Dry scrubbers provide a lower-maintenance alternative thanks to their lighter-weight powder waste.

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How Do Dry Scrubbers Work?

Dry scrubbers have a three-step process to remove acids from exhaust gases. By taking out these pollutants, dry scrubbers can reduce the incidence of acid rain and lower harmful air pollution levels.

The first step requires cooling the exhaust gases to make treating the pollutants easier. Typically, an evaporative cooling unit cools and dilutes the exhaust gas during this stage.

The next stage exposes the gas to a combination of alkaline reagents in the sorbent mixture. Usually, sorbents are either hydrated lime, sodium sesquicarbonate or sodium bicarbonate, or a mixture of them. This process can occur in one of two ways, depending on the structure of the dry scrubber.

One way of adding sorbent is with a spray dryer absorbent. This process sprays the alkaline sorbent over the gas in a fine mist. The alkaline sorbent counters the acids in the pollution, neutralizing them. Spray dryer absorbent can remove up to 99% of hydrochloride and 95% sulfur dioxide from the gas.

A second method of mixing the gas with the sorbent is through a dry sorbent injection (DSI) process. In this method, the gas combines directly into the sorbent. The effectiveness of this process depends on the filtering method. However, when combined with fabric filtering, DSI can remove up to 90% of hydrochloride from the exhaust.

The third step filters out the pollutant-containing powder through a fabric filter or with an electrostatic precipitator. The former is highly effective and does not require power for operation. The latter can effectively remove particulate matter, too — however, it may use up to 4% of a power facility’s output to maintain its electrostatic charge. Due to the difficulty of cleaning the waste powder to remove pollutants, most plants discard it.

Common Dry Scrubbers Air Pollution Applications

Industrial applications of dry scrubbers include the removal of acids from their exhaust gases. Typically, facilities that lack the space or funds for a separate wastewater treatment system to handle water waste from wet scrubbers benefit most from using dry scrubbers. Therefore, these types of scrubbers find use in most types of plants, including energy production, manufacturing and waste treatment facilities.

Advantages of Dry Scrubbers

Dry scrubbers work well to remove sulfur dioxide and hydrochloride. They have lower use costs because the waste they produce does not require additional treatment and weighs less than the wastewater from wet scrubbers. For existing facilities that require additional exhaust processing, dry scrubbers require less space. In some cases, you can even retrofit them into existing areas. For removing particulate matter, fabric filters work very efficiently with dry scrubbers. Plus, these models have very low maintenance costs.

Disadvantages of Dry Scrubbers

Dry scrubbers are not ideal for all facilities. The greatest disadvantage to these scrubbers is their inability to remove all pollutants. Plus, they do not remove as many acids from gases as wet scrubbers. While sufficient, dry scrubbers do not perform as well as wet scrubbers for air pollution control.

Other Types of Industrial Scrubbers

While wet and dry scrubbers are the two most commonly used types of industrial scrubbers, electrostatic precipitators are another option. Often, this device combines with a dry scrubber to act as a particulate filter. However, it can also work as a standalone scrubber in some applications, with a removal rate of up to 99% of particulate matter from gas.

An electrostatic precipitator (ESP) requires construction and a static charge to match the type of dust or contaminants it removes. The exhaust gas stream passes through perforated discharge plates to control the flow. Discharge electrodes charge the gas with either a positive or negative charge, depending on what particles the system removes.
As the charged gas passes by the collection pipe or plate, the charged particles stick to the pipes or plates because the collection surfaces carry an opposite charge. The final step requires cleaning off the collection plates or pipes to prepare them for more processing.

ESPs can remove the collected pollutant with a dry or wet method. The most common method is the dry process that uses a rapper to vibrate the collection plates or pipes to knock off the pollutant. In wet models, water rinses off the particles. Facilities that have a sticky particulate matter that may not come off effectively using a rapper benefit more from using a wet ESP.

Disposing of the pollutants removed with ESP depends on the type of particle and what method you use to remove it. For example, dry ESPs that remove particulate matter from coal-fired power plants gather soot and ash, also known as fly ash. In the United States, facilities recycle up to 43% of this fly ash to cut down on greenhouse gases and eliminate disposal problems.

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Contact MACH Engineering for Your Wet Scrubber Design, Components and More

MACH Engineering can design the right wet scrubber for your facility. With so many factors at play, the specifics of the wet scrubbers require expert design to achieve the desired results. Trust us at MACH Engineering to assist with design, installation supervision, tower internals, packing and service. Contact us today for more information.