Wet scrubbers remove particulates, vapors, and gases from the exhaust stream of a process. The goal for a wet scrubber is to remove over 99% of pollutants and particles. The smaller the particles in the waste stream, the more difficult the particles are to remove.
What is the Efficiency of a Wet Scrubber?
Efficiency begins with the selection of the wet scrubber. Different types of wet scrubbers offer different levels of efficiency. If the particulate present is relatively large, say 10 microns, then the type of scrubber needed will be different from the type of scrubber used in an application dealing with 1-micron particulate.
First, let's have a short review of the operating principles of a wet scrubber. A wet scrubber mixes a water solution with exhaust gases. The water entraps particles like dust and smoke, removing them from the waste stream. Water-soluble gases are also absorbed into the water and removed.
A wet scrubber is all about making contaminants run into a water droplet. An example of a low-efficiency wet scrubber is rain. We all know that after it rains, the air smells cleaner, and that's because falling raindrops ran into something on their way down… dust and odors.
Scrubbers create a high gas velocity and put water droplets in the way. The higher the speed, the more efficient the collection, just as driving a car at 70 miles per hour collides with more bugs than a car driving at 10 miles per hour. Wet scrubbers use different methods to create a zone where water droplets and contaminants collide.
Venturi: The Most Efficient Wet Scrubber
The most efficient wet scrubber is the venturi type. Venturi scrubbers are the go-to scrubber for high-end, demanding scrubbing applications. They atomize their scrubbing fluid to entrap fine particulate and mists better. A flow of water is introduced just above the venturi throat, causing the gas and water to converge in a turbulent manner. The pressure drop of the venturi and turbulence, causes atomized water droplets to form.
Small droplets have a large surface-area-to-volume ratio, enabling them to collect more particles per volume of water. These tiny droplets enable the scrubber to achieve collection efficiencies of 99 percent on particles as small as 1 micron in diameter. For particles 5 microns and larger, efficiency may increase to 99.8 percent.
The Venturi scrubber design requires a higher pressure drop than other types of wet scrubbers, on the order of 12-40 inches water column. A powerful exhaust fan is needed to create the pressure drop.
Impinjet Wet Scrubber Efficiency
An Impinjet wet scrubber is also a high efficiency (99%) scrubber for particles about 5 to 10 microns and larger. Its design gives ample opportunity for contaminants and water to interact. The gas passes a liquid spray section that entraps the larger particles. Then the gas passes through impingement plates, baffles that carry a flow of scrubbing liquid like a puck on an air hockey table. Combined with high-speed airflow, the baffles create tiny water droplets roughly 100 microns in diameter that entrap the smaller particles. The droplets fall and are removed as the water drains from the chamber. Gases and vapors are absorbed and can be removed to any concentration by adding more baffles. A fixed blade mist eliminator ensures that droplets are eliminated before the gas is vented or returned to the process.
Impinjet scrubbers require less water and operate with minimal pressure drop compared to Venturi scrubbers. This translates into lower energy operating costs. They are resistant to fouling, and they have capacities as high as 200,000+ CFM, more capacity than the Venturi scrubbers that Sly offers having up to 76,000+ CFM capacity. While the initial cost of a Venturi scrubber is similar to an Impinjet scrubber, the energy use will cost four to five times more compared to an Impinjet scrubber.
Occasionally we run into applications that need extremely efficient pollution control. We have a Venturi/Impinjet combination scrubber that can reach collection efficiencies of over 99.99% for such cases.
Eductor Venturi Scrubbers
A particular type of Venturi wet scrubber is called an Eductor. Eductor wet scrubbers remove particles and soluble gases using a high-pressure liquid focused into a venturi throat. The flow of the liquid induces the gas flow. This eliminates the need for an exhaust fan or blower to move the gas stream through the scrubber. The eductor design reduces complexity and lowers energy costs. Because of the force of the water, Eductor scrubbers are particularly well-suited for use with sticky particulate and to remove gases that decompose on contact with water.
Packed Tower Scrubbers
Packed Tower scrubbers provide chemical scrubbing of gaseous contaminants such as ammonia, sulfur dioxide, hydrogen sulfide, and hydrogen chloride. The exhaust gas flows upward through a bed of loose plastic or metal packing that provide a large surface area. The bed is continuously wetted from above and the liquid drips down through the packing. While the packing is not designed for particulate removal, the buildup of particulate matter can reduce efficiency. This is why Packed Tower scrubbers can be preceded by a Venturi scrubber that removes the particulate before gas enters the Packed Tower.
Monitoring Wet Scrubber Efficiency
A stack test measures scrubber efficiency. This is the test that the EPA does. It measures the amount of particulates or undesirable gases vented into the atmosphere after the scrubber has processed the gas stream. At the same time, the inspectors also record the liquid flow, differential pressure, and temperature. These factors influence the efficiency of the system. The EPA will tell the company that the airflow and the differential pressure must remain in a certain range or the company will be out of compliance. This is done to prevent companies from later lowering flow and pressure to save energy at the expense of increased pollution.
As part of normal operation, managers should monitor flow and differential pressure in their wet scrubbers. A drop in differential pressure may indicate the equipment is flooding. A drop in flow may indicate clogging or that a worker turned a wrong valve.
Scrubber efficiency relates to the type of equipment and vice versa. For complete information on the kinds of wet scrubbers that Sly designs, builds and services, get our Wet Scrubber Buyer's Guide.