Wet scrubbers are an air pollution control technology used in many industrial settings to clean exhaust gases before they are released into the atmosphere. These systems operate by forcing the contaminated gas stream into contact with a scrubbing liquid, which is most often water. The core function of this device is to transfer both particulate matter and gaseous pollutants from the gas phase into the liquid phase. This process effectively captures a wide range of contaminants, helping facilities meet environmental compliance standards.
Essential Structural Components
The main vessel, or shell, serves as the enclosed chamber where the cleansing operation takes place. Exhaust gas enters this vessel through an inlet and, after processing, the cleaned gas exits through an outlet.
A liquid delivery system is installed to introduce the scrubbing solution into the gas stream. This system typically uses a network of spray nozzles to atomize the liquid into fine droplets, or it may involve a packing material that creates a large, wetted surface area. The liquid, once contaminated, collects at the bottom of the vessel for removal or recirculation.
A mist eliminator is located near the gas outlet. Its function is to prevent scrubbing liquid droplets from being carried out with the newly cleaned air. This device works by causing the entrained liquid droplets to impact on vanes or mesh pads, where they coalesce and drain back down into the scrubber.
The Fundamental Pollutant Removal Mechanism
The process by which pollutants are removed hinges on two distinct mechanisms: one for solid particles and another for gaseous contaminants. For particulate matter, the primary removal method is inertial impaction. This occurs when the gas flow is forced to change direction around a liquid droplet, but the particle’s inertia causes it to continue on its path and physically collide with the droplet.
Interception is another physical mechanism where a particle, even if it follows the gas streamline around a droplet, is still large enough to touch the droplet’s surface and adhere to it. For very small particles, Brownian motion causes them to randomly collide with the liquid droplets. Droplets used in most systems are generally in the range of 150 to 500 micrometers to effectively capture these smaller pollutants.
Gaseous pollutant removal relies on mass transfer, which often involves chemical absorption. Soluble gas molecules, such as sulfur dioxide or hydrogen chloride, dissolve into the scrubbing liquid. Often, chemical reagents are added to the liquid to promote a reaction that neutralizes the contaminant. For example, adding an alkaline substance like lime or sodium hydroxide can chemically convert acidic gases into harmless salts dissolved in the liquid.
Variations in Wet Scrubber Design
Different structural designs are used to optimize the gas-liquid contact for specific types of pollutants, with each variation offering a trade-off between energy use and pollutant removal efficiency. The simplest design is the spray tower, which consists of a large, open chamber where the scrubbing liquid is sprayed down onto the rising gas. Spray towers are best suited for removing easily soluble gases or relatively large, coarse particles, as they operate with a very low-energy input.
Packed bed scrubbers are built to maximize the surface area available for mass transfer, making them highly effective for removing gaseous pollutants. The vessel is filled with specialized packing material, such as rings or saddles, which creates an extensive thin film of scrubbing liquid as the gas flows upward through the bed. This design maximizes the contact time and area, improving the absorption of gases like sulfur dioxide.
The Venturi scrubber represents a high-energy approach for removing fine particulate matter. The gas is forced through a narrow, converging section called the throat, which accelerates the gas flow to high velocities. The scrubbing liquid is introduced at this point, and the high-speed gas atomizes it into fine droplets, creating intense turbulence. This turbulent mixing dramatically increases the chance of fine particles colliding with and being captured by the liquid droplets.
Managing the Wastewater Byproduct
The scrubbing process inevitably results in a contaminated liquid stream known as scrubber blowdown or wastewater. This liquid contains all the captured pollutants, including heavy metals, dissolved salts, and reaction byproducts, often forming a slurry or sludge. The creation of this waste stream is a significant operational consideration, as it transfers the pollution burden from the air to the water.
Many systems employ recirculation, where a portion of the liquid is cleaned and recycled back into the scrubber to reduce water consumption. For the portion that cannot be recycled, treatment methods often include physicochemical processes like neutralizing the pH with lime, which causes dissolved solids to precipitate out. The concentrated waste solids are then removed, often using a filter press, and the remaining liquid must meet strict environmental standards before final disposal.