Air Pollution Scrubbers: How Do They Work?

Air pollution scrubbers are devices that remove harmful pollutants from industrial exhaust streams, ensuring that only clean air is released into the environment. They are an essential tool for industries to comply with environmental regulations and minimise their environmental impact. There are two main types of scrubbers: wet scrubbers and dry scrubbers. Wet scrubbers use a liquid solvent, usually water, to absorb and capture pollutants, while dry scrubbers inject a dry reagent or slurry into the exhaust stream to wash out the pollutants. Wet scrubbers are generally more effective at removing pollutants and are used across various industries, including power generation, chemical manufacturing, metal processing, and wastewater treatment.

Wet scrubbers' design depends on the targeted pollutants

Wet scrubbers are designed to remove air pollutants by trapping suspended particles through direct contact with a spray of water or other liquids. The design of a wet scrubber depends on the targeted pollutants as air contaminants vary in size, shape, and electric charge.

The packed bed height size can vary based on the targeted pollutant and efficiency goals. This impacts how long the gas stream is in contact with the solvent, resulting in more effective particle absorption. The packed-bed wet scrubber's efficiency may vary between 50 and 95% depending on the application in question. These scrubber types are effective against inorganic fumes, gases, and odors.

The composition of the solvent is another variable factor. Acidic pollutants require a caustic scrubbing liquid, while caustic pollutants require an acidic scrubbing liquid. The solvent's electric charge determines which contaminants it attracts and binds to. The liquid flow rate, or the liquid-to-gas ratio, is also important. A higher liquid flow rate improves efficiency as it increases the volume of droplets, capturing more pollutants.

The target gas velocity is also significant. The speed at which the contaminated gas moves through the wet scrubber system impacts the efficiency of pollutant collection. Venturi scrubbers are highly efficient wet scrubbers, achieving efficiencies of over 98% for particles larger than 0.5 μm in diameter. They achieve high relative velocities by injecting water into the throat of a venturi channel, a constriction in the flow path.

Packed-bed wet scrubbers remove particulate contaminants and harmful gases

Packed-bed wet scrubbers are an effective solution for removing particulate contaminants and harmful gases from industrial exhaust streams. They are designed to trap and collect suspended particles and pollutants by direct contact with a spray of water or other liquid solvents. The packed bed height size can be adjusted based on the targeted pollutant and efficiency goals, optimising the duration of contact between the gas stream and the solvent liquid for more effective particle absorption.

In a packed-bed wet scrubber, the contaminated gas enters at the bottom and passes upward through the packed bed, encountering downward-flowing solvent sprays. The pollutants are then captured and collected in the solvent droplets. Before the gas exits the scrubber, it passes through a mist eliminator to capture any remaining droplets, ensuring a higher level of pollutant removal compared to dry scrubbers. This two-step process effectively removes both particulate contaminants and harmful gases, making it a valuable tool for industries aiming to minimise their environmental impact and protect human, animal, and plant life.

The versatility of packed-bed wet scrubbers lies in their ability to accommodate different solvent compositions. The solvent composition is tailored to the specific contaminants, with acidic pollutants requiring a caustic scrubbing liquid, while caustic pollutants demand an acidic scrubbing liquid. This adaptability ensures that a wide range of pollutants can be effectively addressed. Additionally, the electric charge of the solvent can be manipulated to attract and bind specific contaminants, further enhancing the scrubber's efficiency.

Packed-bed wet scrubbers are particularly effective in industrial settings, where they play a crucial role in preventing pollutants from contaminating the outside air. They are commonly employed in various industries, including wastewater treatment facilities, to control airborne fumes, vapours, and gases. By utilising packed-bed wet scrubbers, industries can adhere to legal and ethical obligations while safeguarding the environment and all living things within their vicinity.

Wet scrubbers are more effective than dry scrubbers

Wet scrubbers and dry scrubbers are both used to remove pollutants from industrial exhaust gases. Wet scrubbers use a liquid solvent to remove pollution from air or liquid, while dry scrubbers use dry sorbent materials to chemically react with and capture pollutants.

Wet scrubbers are highly effective at removing pollutants from industrial exhaust gases. They can remove organic and inorganic pollutants, including those that cause illness, deplete the ozone, or disrupt ecosystems. Wet scrubbers can remove up to 99% of particulates, including fine particles, acidic gases, and even some heavy metals. They can also handle high-humidity and high-temperature gas streams without issue and can safely manage gas streams with explosive or flammable materials. Venturi scrubbers, a type of wet scrubber, are the most efficient of the wet collectors, achieving efficiencies of more than 98% for particles larger than 0.5 μm in diameter.

Wet scrubbers tend to have lower operating costs than dry scrubbers, and sometimes they have equal or lower upfront costs. However, wet scrubbers typically have higher energy costs than dry scrubbers due to the higher differential pressure required. Wet scrubbers also consume a significant amount of water, which can be a concern in regions with water scarcity. Additionally, wet scrubber systems require careful monitoring and maintenance to prevent the growth of bacteria and fungi, which can affect performance.

Venturi scrubbers are the most efficient of the wet collectors

Venturi scrubbers are highly efficient wet collectors, achieving efficiencies of more than 98% for particles larger than 0.5 μm in diameter. They are the go-to scrubbers for demanding, high-end scrubbing applications. Their design and functionality enable them to trap and collect fine particulate and liquid mists effectively.

Venturi scrubbers inject water into the throat of a venturi channel, which is a constriction in the flow path. This injection increases the relative velocity between the droplets and the particulates, allowing the scrubber to achieve high relative velocities. The gas and water converge turbulently, and the pressure drop in the venturi and the turbulence cause atomized water droplets to form. These small droplets have a large surface-area-to-volume ratio, enabling them to collect more particles per volume of water.

The Monroe Venturi Air Scrubber, for example, utilizes a high-volume wash water injector system. It delivers an initial surface impingement shock to heavy inlet dirt loads, followed by high-velocity atomization as the inlet air passes through the fixed and dual adjustable scrubbing throats. The dual throats balance the pressure drop through the airflow system, and the increase in pressure drop in the wash zone further improves the system's overall efficiency.

Venturi scrubbers can be used independently or in combination with other scrubbers, such as Packed Tower scrubbers, to enhance overall efficiency. They are versatile and applicable in various industrial settings, making them a preferred choice for demanding air pollution control needs.

Wet scrubbers are essential equipment in various industries

The versatility of wet scrubbers allows them to be used in multiple industries, including chemical, pharmaceutical, and food production, power plants, steel mills, and fertilizer plants. Wet scrubbers are particularly effective in industries with particulate-heavy exhaust streams, such as coal combustion and metal smelting.

The design of a wet scrubber depends on the specific industrial process conditions and the nature of the air pollutants involved. They can be designed to collect particulate matter, gaseous pollutants, or both, with the packed tower and plate tower being the most common designs for removing gaseous pollutants. The packed bed height size can be adjusted based on the targeted pollutant and efficiency goals, impacting the duration of contact between the gas stream and the solvent liquid.

Wet scrubbers work by introducing the contaminated exhaust gas into the scrubbing chamber, where it comes into contact with a scrubbing liquid, usually water. Turbulence is intentionally induced to maximize contact between the gas and liquid, ensuring that pollutants are thoroughly dispersed and captured. The liquid absorbs and entraps the pollutants, preventing their escape into the environment.

Overall, wet scrubbers play a pivotal role in industrial air pollution control, providing a robust solution for mitigating harmful emissions and maintaining environmental compliance across various industries.

Frequently asked questions