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 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 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.
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.