What is a wet scrubber and how it works?

What Is A Wet scrubber?

A wet scrubber is a mechanical system that removes contaminants from a gas stream by contacting it with a scrubbing liquid, most commonly water. Wet scrubbers are often the only air pollution control devices that can remove both particulates and gases/vapors. They are very efficient at capturing the smallest dust particles and can trap them in droplets, eliminating over 99% of airborne particles. Wet scrubbers that remove gaseous pollutants are also known as absorbers.

Wet scrubbers are widely used in industries ranging from acid, fertilizer and asphalt plants to steel plants and power plants. They provide an alternative to dust chambers and other dry dust collection methods. They are typically used to control off-gas from process equipment and can also be located downstream of dust collector/cartridge filtration systems, furnaces, dryers, mixers, bucket elevators, bagging stations and other discharge points.

How Wet scrubbers Work?

The four most commonly used types of wet scrubbers are wet scrubbers, venturi scrubbers, ejector venturi scrubbers, and packing scrubbers.

1. Wet scrubbers

Wet scrubbers collect particles and clean, cool and absorb vapors and gases. They have high collection efficiency (99% for particles 5 microns or larger), low water consumption and minimal pressure drop. The scrubber is extremely resistant to scaling and has a usable capacity of up to 200,000+ CFM. An impingement plate (or tray) scrubber uses water to flow over a porous surface. The gas stream is drawn into the scrubber and passed through the plate water. Water traps contaminants and removes them from the airflow. Impingement plate systems typically use lower energy/lower pressure drop to scrub the gas. Impact scrubbers are suitable for particle scrubbing and applications where both particulate and gaseous pollutants must be collected.

2. Venturi scrubbers

A Venturi scrubber with a cyclone is usually the best choice for efficient collection of fine particles. They used the difference between high-velocity gas and free-flowing water to create droplets that trap contaminants, keep them in suspension and deliver them as a solution of suspended solids. Compared to impingement plate scrubbers, venturi scrubber designs require more water and a higher pressure drop to remove particles. The confluent section of the venturi throat (where the water and airflow meet) produces atomized water droplets to provide 99% efficiency for particles 1 micron or larger in size. For particles 5 microns or larger, the efficiency can be increased to 99.8%.

3.Ejector Venturi scrubbers

Ejector venturi scrubbers remove soluble gases and particulates by inducing gas flow using high pressure liquid focused in the venturi throat. This eliminates the need for a separate exhaust fan or blower to deliver the contaminant-laden airflow to the filter unit. Ejector venturi scrubbers can also be used for direct contact condensation. The high liquid flow rate makes this scrubber particularly suitable for collecting viscous and/or gelatinous particles and gases that decompose on contact with water. Ejector scrubbers are often used in combination with other collection equipment such as tower scrubbers.

4.Packing scrubbers

Packed scrubbers are designed for chemical scrubbing of gaseous pollutants, typically for removal of sulfur dioxide, hydrogen chloride, ammonia, hydrogen sulfide and other gases. In a vertical scrubber, the gas flows up through the bed, while the liquid trickles down by gravity through a packing, which usually consists of small plastic or metal objects. The liquid wets the surface of the packing, which is designed to provide a large amount of surface area for gas and liquid contact. A liquid distribution header located above the packing distributes the wash solution evenly to the top of the packing. Recycle wash liquors typically contain alkaline, acidic or oxidizing detergents to achieve the desired outlet discharge. Before leaving the scrubber, the scrubbed air passes through a screen or chevron mist eliminator to remove entrained droplets.

Wet scrubber applications

Wet scrubber applications typically fall into one or more of the following four categories:

Combustible/hazardous dusts often require wet scrubbers because they are effective in reducing the risk factors for dust explosions. However, wet scrubbers are not suitable for handling all types of dust. In fact, some dusts (such as alkali metals like lithium, sodium, and potassium, and alkaline earth metals like beryllium, magnesium, and calcium) are water-reactive, which means they're more hazardous when they come in contact with water.
Moisture in the air stream can foul or clog dry dust collectors and cause filter bags to clump, but wet scrubbers can handle things like that without issue.