Wet Scrubber Mist Eliminator: Types, Design and Efficiency

Author: Michael Klepik, Chief Executive Officer

A scrubber mist eliminator removes fine droplets from the gas flow, reducing moisture content and preventing the release of chemicals applied in scrubbers to neutralize pollutants. The most commonly employed filter types include plate, fiber (mesh), and cyclonic.

Plate fog catchers consist of alternating plates designed to direct the airflow along a specific path. The chevron-shaped plates are a distinctive feature of these devices. Moisture accumulates on the plates, merges into larger ones, and drain downward. This type of mist scrubber is highly effective for particles 10 microns or larger, achieving up to 99% efficacy at optimal gas velocities.

Fibrous (Mesh) aerosol separators are made up of layers of interwoven fibers or mesh that provide a large surface area for interaction with the moisture-laden airstream. They are effective for pollutants ranging from 1 to 10 microns in diameter, offering up to 98% effectiveness at low to medium flow velocities. Mesh units are often constructed from corrosion-resistant metals or polymers to withstand harsh chemical environments.

Cyclonic droplet collectors work on the principle of centrifugal force: the airflow is spun, causing suspension to be thrown toward the walls of the housing, where it coalesces and settles. These devices are most effective at high velocities and are suitable for particles ranging from 10 to 50 microns. Cyclonic devices are commonly used in systems with high moisture content, where strength and resistance to wear are essential.

Less common are Vortex and Coalescing removal systems. The former are typically found in specialized treatment systems but are rarely employed in industrial scrubbers due to their complex design. Coalescing models are highly effective for fine particles (less than 1 micron) and are typically implemented when maximum air cleaning is required, such as in compressed air systems.

When designing a wet scrubber mist eliminator, the choice of materials, design features, and placement are critical, as they directly affect both capture effectiveness and the system's durability. Materials must be resistant to corrosion and aggressive chemicals commonly found in industrial emissions. Stainless steel, polypropylene, and other corrosion-resistant polymers are common choices for equipment in highly aggressive environments, withstanding temperatures of up to 300°F and beyond.

Proper placement of the aerosol separator within the scrubber system is key to its efficiency. Typically, these units are installed after the gas-liquid contact zone, where moisture concentration is highest. If placed too far downstream, some of this moisture may evaporate, reducing capture performance. It's also essential that the gas flow remains uniform, without turbulence, which can reduce performance.

Increasing the density of fibrous or mesh fog catchers improves cleaning effectiveness but also increases flow resistance. To minimize pressure loss, engineering practice often uses multilayer mesh eliminators with varying layer densities, enabling staged droplet capture.

Using different plate geometries can further enhance capture capability. Chevron or labyrinth plate designs are the most commonly employed.

Fixed-bed scrubbers can function as fog collectors due to their packing's ability to trap fine liquid particles. Positioned within the airflow zone, the packing creates additional resistance that helps moisture settle on the packing surface. To prevent clogging or fouling, automatic or manual periodic cleaning is performed to maintain efficiency and restore flow capacity.

In large columns, a layer of packing is often placed at the top, serving as mist eliminator for wet scrubber. This layer is not continuously irrigated; cleaning occurs periodically to ensure optimal performance. This method enhances droplet capture while minimizing moisture and pollutant emissions from the airstream.