Wet Scrubbers for Pollution Control

Rising levels of air pollution have become one of the most discussed issues around the globe. Because of this, it’s critical for companies to know that one of the most effective tools against rising carbon emissions and the acidic chemicals being released into the atmosphere has been the use of wet scrubbers. These filters effectively use a combination of scrubbing fluid, absorbent material and other methods to remove chemical and particulate contaminants from emissions, lowering the amount of pollution released through manufacturing and burning of fossil fuels.

While they are most popular because of their high rate of success and versatility in filtering multiple types of pollutants, the typically low maintenance and relatively small operating costs are major industrial advantages of using wet scrubbers. This makes them a manageable option for most companies to stay in compliance with governmental environmental protection laws.

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.

How Do Wet Scrubbers Reduce Air Pollution?

In the most basic sense, scrubbers are purifiers that remove chemical or particle waste from polluted gases. The purpose of wet scrubbers is to purify vapors and gases to acceptable levels before releasing them into the environment or using it for other purposes. Wet scrubbers, the most common type of industrial purifier, use a scrubbing liquid to separate the contaminations from the mixture so that decontaminated gas can be released back into the atmosphere.

Wet scrubbers have become common because they are both versatile and effective. Moreover, depending on the system, they can provide a cost-effective method for controlling emissions. When compared to the cost of fines and penalties for violating CAA regulations, these operating costs are negligible.

Wet scrubbers are particularly popular filtration systems because they can handle both gaseous and particulate contamination. Because of this, chemical and particle treatment can occur within the same process, reducing the amount of cleansing machinery, required floor space and processing stations needed to meet regulations. Instead, the wet scrubber can treat noxious or incendiary gases regardless of how the impurities are held in the vapor, simplifying the process. It’s important to remember, however, that some wet scrubbers are better suited for gaseous substances, as particulate material can cause problems in some systems.

In addition to wet scrubbers’ efficiency and versatility, these systems also deal well with heat, corrosive materials, liquids and incendiary materials, making them ideal for especially hot exhaust gases or vapors that carry high moisture content. Dry scrubbers or other cleaning mechanisms can develop clogs in moist environments, and many purifying systems cannot handle high temperatures. Perhaps even more importantly, by varying the chemical composition of the scrubbing fluid, many wet scrubbers can handle otherwise corrosive or combustible exhausts, making them one of the safest treatment options in many fields.

Though many designs are available, packed tower scrubbers and venturi scrubbers — both regular and high energy — are among the most popular systems used. All of these designs incorporate combining the contaminated gas with a form of scrubbing fluid, though the stacked tower relies more on absorption into a packed material while the venturi uses the flow and pressure to separate the impurities from the gas.

What Industries Use Wet Scrubbers?

Wet scrubbers are used in most production and electricity generation plants since this is where the majority of air contamination comes from. However, it is not only burning fossil fuels that contribute to pollution, as there are also several other ways hazardous waste is introduced to the environment.

In the United States, electricity generation and direct emissions from industry make up half of United States greenhouse gas emissions. The most common pollutant in these fields tends to be carbon dioxide (CO2) released by burning fossil fuels. However, various chemical reactions and acidic waste products also present particular problems for companies as they try to meet government compliance standards. These fields, especially with the high levels of air toxins released, are ideal for web scrubber applications because they often have several different types of contamination, making the wet scrubbers’ versatility key in treating these vapors.

1. Electricity Production

Electricity production accounts for more than a quarter of all U.S. greenhouse gas emissions. To generate electricity, most traditional power plants use some form of heat, whether through burning fossil fuels, using nuclear energy or other means. This heat boils water and forms a high-pressure stream of steam, which in turn moves a turbine. The spinning turbine connects to a series of magnets, which create electromagnetic energy.

It is the initial step — the burning of fuels — in which scrubbing becomes essential. The majority of contaminants in this sector come from carbon dioxide emitted from these fuels, especially burning coal, which accounts for nearly two-thirds of the CO2 produced despite providing less than one-third of the nation’s electricity. In addition to this, fossil fuels and nuclear power combine for more than 85 percent of all energy production across the country, all of which result in waste materials that must be filtered before being released into the atmosphere.

2. Industry Emissions

Direct emissions from industries generating consumable goods accounted for approximately one-fifth of U.S. greenhouse gas production. These plants generally burn fossil fuels — with carbon dioxide from coal and natural gas at the forefront — for energy or heat, which is where industry contributes most to greenhouse gas generation.

Outside of general heating, particular industries tend to produce high levels of toxic pollutants. In fact, almost 90 percent of industrial material waste is generated by one of four major industries — chemical manufacturing, primary metal manufacturing, metal fabrication or petroleum processing. Though not all pollutants produced in these industries are airborne, chemicals such as cadmium used in metal-working fields and hydrogen sulfide in petroleum processing can easily find their way into the environment if proper filtration is not implemented.

3. Transportation and Diesel Engines

Though not commonly thought of as an area for scrubbers to reduce greenhouse gas emissions, preliminary work also demonstrates that forms of wet scrubbing may also be a viable option to reduce carbon emissions from compression-based engines, such as those found in diesel vehicles. While most automobiles already have standard emissions filters in place, the use of scrubber technology may soon impact the carbon waste produced by millions of diesel automobiles, not to mention similar engines that are used for other applications.

What Are Packed Tower Scrubbers and How Do They Work?

There are many types of wet filters, including several varieties of packed towers and venturis. Packed tower scrubbers work similarly to common filters. A large tower is filled with various types of wet scrubber packing media, the consistency and substance of which vary based on the application, temperature conditions, purifying quality and other factors. The scrubbing solution percolates evenly down through the packing material. Gas enters the system through either the bottom or the side of the tower — depending on vertical or horizontal layout — where it interacts with the packing material as it passes through the tank. Chemical reactions or solubility with the scrubbing liquid removes chemical and particle impurities along the way. Finally, the gas passes through a mist filter near the gas release point, separating liquid from the final vapor and releasing it into the air or to another process.

While packed towers can effectively remove particle contaminants, it is often more sought after in chemical cleaning, as gases with heavy particle mixtures can eventually build up in the packed materials. However, if the chemical-contaminated gas only has moderate particle density, a packed tower may still be a good choice.

The efficiency of packed tower scrubbers can depend on many factors, such as the gas contamination, the expected level of purification and operating pressures. Testing results show that most absorber combinations have a removal of efficiency above 90 percent, with particular combinations of absorbers, liquids and gases purified to 99.9 percent efficiency.

In addition to their high chemical removal efficiency, stacked tower scrubbers handle corrosive materials exceptionally well since there are few moving or metal pieces. High-quality plastics or fiberglass make the structure itself resistant to abrasive chemicals, whereas other removal systems often incorporate more elements that are susceptible to corrosion.

Packed towers also tend to have lower pressure drops, which helps to maximize efficiency. Pressure drop is what determines how the gas moves through the system. By maintaining good pressure, systems can use relatively little energy while also maximizing purification efficiency. This is important for companies since manipulating airflow can add to the operating cost of a unit.

Another benefit of packed scrubbers is the availability of both vertical and horizontal designs, each offering a different specialization to filtration. Vertical towers, the most common of the packed tower scrubbers, use a counter-flow design. Gas enters the bottom of the scrubber and then rises vertically through the packing media. Because of their upright design, vertical scrubbers take up less floor space than horizontal scrubbers. Vertical scrubbers also generally have higher removal efficiency than horizontal scrubbers, though this depends heavily on the impurities and processing. In these scrubbers, particulate matter can easily become trapped in the vertical layers, so upright towers tend to handle chemical contamination better than particle contamination.

On the other hand, horizontal scrubbers, also known as cross-flow scrubbers, run the air stream sideways through the tank, although the liquid still flows from the top. Though they take up more square footage, horizontal tanks work well in areas with low ceilings or little headroom, or if there are other industrial advantages to keeping control equipment close to the ground. Because the air stream flows horizontally rather than vertically, particulate matter is less likely to cause problems. This makes cross-flow scrubbers a better solution for chemical gas waste with minor solid impurities.

What Are Venturi Scrubber Systems and How Do They Work?

A venturi system relies on a convergent-divergent principle. The technique uses the relative difference between pressure and velocities of fluids as they move through a constricted tunnel. The scrubber consists of two funnel-shaped structures, with the smaller portion of each meeting in the middle. This constricted point, known as the choke, is the key to the venturi scrubber. Fluid enters the top of the chamber. Then, the scrubbing fluid is mixed with the contaminated gas near the choke. The speed of the gas increases as it is forced through the choke, where it atomizes the scrubbing fluid, turning it to a mist. Then, in the divergent portion of the chamber, the gas mixes with the vaporized fluid as it slows down. At that point, the gas can be separated from the fluid through a cyclone separator and mist eliminator, leaving the contaminants behind as the purified vapor exits through the gas release.

Though a typical venturi can handle general applications, high-energy venturi systems can be used to more effectively separate particles at the micron and submicron level. The gas speed is increased — typically through a large fan which creates a high-and-low-pressure air draw — as it enters the chamber. By increasing the speed of the gas, users can also increase the atomization of the scrubbing fluid, allowing it to bond to even smaller particulate matter, down to the submicron level. However, these units consume more energy, making them more costly to operate. Consequently, these high-energy units are some of the most highly-specialized industrial wet scrubbers, reserved for industries where removing small particles is essential.

How Do I Plan My System?

If you are considering installing a new process or heating system that needs to meet environmental regulations or are looking to upgrade your current scrubbing system, it can be difficult to know exactly where to begin. While a general understanding of the types and applications of scrubbers is helpful, each industry has its own unique circumstances with the pollutants it must filter or the acceptable levels of filtration at release. Even within those fields, each shop or company has its own manufacturing needs, resource requirements, air pressure supply limitations and other considerations.

For more information about installing a wet scrubber for your company, discuss your specific needs with MACH Engineering. Our experts advise system implementation as well as design components, helping clients determine what type of system will best fit their industrial applications.

MACH Engineering

MACH Engineering