View A Video on Wet Scrubbers - A Quick Introduction
For removal and elimination of pollutants from an air stream, manufacturers and industrial operators have many types of air pollution control at their disposal. One such control are a type of air scrubbers, called wet scrubbers. Wet scrubbers reduce or eliminate pollutants by dissolving or absorbing them in a liquid solution. They can remove both particulates and gaseous impurities, but these require different absorption techniques or chemical reactions, so, to achieve optimal operating conditions, wet scrubbers are usually configured for the cleanup of either one or the other. The correct configuration ensures removal efficiency rates up to 95%.
Once upon a time, wet scrubbers consistently came in industrial, permanent mounted designs, but now, they are more commonly constructed as relatively small devices that range from semi-portable to mobilized. This design allows them to clean localized areas more quickly and easily. The basic configurations of all wet scrubbers are fairly similar; they consist of duct work and a fan or pump system, a collection area or chamber and another pump. The duct work and fan or pump system draws in or pushes polluted air or gas toward the collection area or chamber, where it is deposited, then pressurized by the second pump. Depending on the specific contaminant and gas combination, wet pressurize and remove pollutants either through a high pressure liquid spray or a pool of scrubbing solution. To put it another way, wet scrubbers remove pollutants by catching them in liquid droplets, collecting the liquid droplets and then absorbing or dissolving the pollutants in the droplets.
The two most common wet scrubber types are spray scrubbers and pool scrubbers. Spray scrubbers spray a pressurized cleaning solution into a moving gas/air stream, which attracts heavier particulates, which, in turn, uncouple from the gas and instead stick to the liquid. Used liquid that has been gathered in a chamber is cleaned and, frequently, put through the system again. Pool scrubbers, on the other hand, force a gas stream through a saturation chamber, or pool, of scrubber solution. Here, the solution collects and binds contaminants as a cleaned air stream passes through. If it is deemed necessary, chemical solutions may substituted or added in to help destroy or reconfigure harmful pollutants. To increase efficiency, extra filters may be added both before and after scrubbing. Pre-filters are also used to boost efficiency quite a bit; they work by gathering larger pollutants so that the scrubber can devote more attention to smaller particulates. To enhance efficiency even more, final filters may be used at a wet scrubber’s exhaust end. In the same way that a solution may be reused, scrubbed air can be returned to circulation within the machine. However, it is generally just cast into the atmosphere instead.
In addition, wet scrubbers can be further categorized by pressure drop. Wet scrubbers grouped by this measure fall into one of three categories: low-energy scrubbers, medium-energy scrubbers and high-energy scrubbers. Low-energy scrubbers have pressure drops of less than five inches of water, while medium-energy scrubbers have pressure drops between five inches and fifteen inches of water and high-energy scrubbers have pressure drops greater than fifteen inches of water. Unfortunately, however, because wet scrubbers tend to function over such a wide range of pressure drops, this form of categorization can be impractical.
Wet scrubbers can operate successfully at a variety of temperature points and with a numerous moisture contents, which makes it possible for them to decontaminate process streams in many different environments. This versatility is one of the qualities of wet scrubbers that make them popular for use in applications like: paper and pulp, steelworks, fertilizing, energy and power generation, construction, manufacturing and propane and natural-gas related processes. Processes and environments like these rampantly produce dangerous impurities like solvent fumes, hydrocarbon, volatile organic compounds (VOCs) and hazardous air pollutants (HAPs). Such air impurities can wreak havoc on both the environment and human health. For example, they can cause the rise of carbon emissions and the occurrence of acid rain, smog and overall climate change, as well as both mild and severe allergies, asthma and a host of health problems that may not emerge until later in life. Because of the seriousness of the potential problems caused industrial air pollution, government and private agencies alike have enacted air purity requirements and recommend or require the use of emission control systems like wet scrubbers. Wet scrubbers are also popular because they can handle heavy moisture/high humidity and high temperatures, they can neutralize corrosive gases, they can process both gases and particulates and they take up less space. In addition, they do not contain secondary dust sources, so there is no risk of particulates escaping during transport or via a hopper. Because they use water, wet scrubbers also pose a lower risk of fires or explosions.
Wet Scrubber – Adwest Technologies, Inc.
Gas Wet Scrubber – B&W MEGTEC
Operating Principles of Wet Scrubbers
The most common pollutant in industrial exhaust air streams is particulate matter (PM), which includes, dirt, dust, soot, and smoke particles that can be seen with the naked eye. However, particles can be microscopic, too. Once inhaled, the particles can settle in the lungs and in the blood stream and cause major health implications. Particles that are larger than 10 micrometers are considered harmless, thus, are not regulated under environmental standards. However, anything smaller than 10 micrometers in size is required to be controlled at the source.
In general, the most effective emission control systems to combat the particle matter problem are scrubbers, which can either be dry or wet and have applications not limited to particulate matter. They are used primarily to arrest particulate matter because they provide many advantages over options. Wet Scrubbers can safely collect particulates from explosive dusts and can absorb gaseous pollutants, too. All types of scrubbers, basically, employ the same operating principles to remove particles from a stream. However, some scrubbers make use of diffusion, as well as electrostatics, condensation and gravitation.
Following find basic descriptions of the operating principles of wet scrubbers.
I inertial impaction, particulate matter interacts with liquid droplets which are sprayed in the scrubber chamber. When the velocity of particles in the gas and the gas itself changes, the inertial impaction occurs; the change in velocity occurs because of the interference of water droplets. In the chamber, when the polluted comes across liquid droplet, it tries to change the direction and flow around the solvent droplet. Similarly, the particles, too, try to change direction under fluid drag. However, the particles have inertia, and they follow a forward motion. This results in a fight between the inertia and the fluid drag. The inertia dominates and particles larger than 10 micrometers are collected, as they have more momentum and meet the droplet. Smaller particles (less 1 micrometer) are hard to collect by the inertial impaction, as they are dragged along the path of gases.
Within exhaust streams, fluid has its own motion, and small particles have their own random motion, which is called Brownian motion. Because of the random motion, particles diffuse into the liquid directly, which is known as direct particle-liquid contact. Once a particle contacts liquid, it cannot be reversed, and it goes down with the liquid and is collected at the bottom. Particles with a size less than 0.1 micrometer are collected by this principle.
Gravitational collection plays a minor role in scrubbing, as it usually related to impaction and direct interference. In this type of collection, the falling droplets, under the effect of gravity, crash with particles present in an air stream. When the gas velocity is high and the residence time is low, gravity plays no role.
A scrubber with electrostatic action is used when it follows an electrostatic precipitator. In electrostatic collection, liquid, particles or both are charged, and then collected on an opposite charged surface.
Types of Wet Scrubbers
Wet scrubbers come in many varieties and can be either commercial or custom made. Among emission control systems, thermal oxidizers or electronic precipitators are widely used by manufacturers. Given the wide adoption of wet scrubbers, they are not classified within a single category; they are categorized either on their capacity or on the method employed for contacting liquid and gas, as there are many different methods to scrub off the particles from the air streams and collect them.
Under the latter classification, they are categorized as, spray chambers and packed-bed scrubbers.
Spray chambers are simple and low-energy wet scrubbers, in which a loaded air stream is exposed to solvent droplets produced by nozzles in a chamber. Spray chambers are sometimes also called pre-formed spray scrubbers, as the droplets are formed before they contact an air stream. In these types of wet scrubbers, the sizes of the droplets are regulated at the spray nozzle to increase the liquid-particle contact, which is a particle collection efficiency determinant.
Spray chambers can be either in the form of towers or cyclonic chambers. Further, either spray towers can be in the form of cylinder or rectangle, which, based on the requirements, can be fitted vertically or horizontally. In vertical design, the airstream moves up through the structure and comes across a set of nozzles that produces droplets. De-misters are installed at the top, which collect wet particles as the gas stream leaves the system. Similarly, the particles are also collected at the bottom, as they drain out of the system as slurry. In horizontal design, everything works in a similar manner, except the gas moves in a horizontal direction. Cyclonic chambers also work on the same principle, but the air stream is introduced into the chamber in such a way that its motion becomes cyclonic.
Packed bed scrubbers are designed to provide higher liquid-particle contact, with the help of specially designed layers of packing material, which can be in the shape of spiral rings, berl saddles, and raschig rings. The packing remains fastened with the help of wire mesh retainers and plates that support them.
Packed-bed scrubbers come in two forms—vertical and horizontal. In vertical design, the air stream moves up from the bottom, and the scrubbing liquid is poured from the top evenly to create a layer of liquid on the packing material. The air stream follows a predetermined path and encounters liquid, which collects particles. At the top, an air filtration system, commonly, mist collector, is also fitted to collect excessive particulates and scrubbing liquid. The movement of the gas stream is countercurrent in vertical designs, whereas in horizontal designs, it is crosscurrent.
In either packed-bed scrubber design, plugging is a major issue, however, these machines more efficiently remove particulates. They are majorly used for gas scrubbers, as maintenance cost remains low. Moreover, as an alternative to conventional packing material, which is highly susceptible to plugging, mobile-bed scrubbers are commercially available that provide more mobile packing material, making scrubbers less susceptible to plugging.
Wet Scrubbers-Application Limitations and Efficiency Determinants
According to modern environmental regulations, many types of emissions are controlled to meet the requirements set by the EPA. Among all pollutants, particulate matter and volatile organic compounds are considered the most dangerous and are filtered out at their sources. To meet EPA requirements, various types of air-pollution control systems are available. However, other than thermal oxidizers, wet scrubbers are arguably the most extensively used emission control systems across various industries, as they provide many advantages.
Specialized Applications of Wet Scrubbers
Wet scrubbers provide specialized uses when particulate matter is involved in an exhaust air streams since they can collect explosive and inflammable dust, as well as moist dust. However, wet scrubbers are also used to capture mist, essentially suspended liquids, or gases independently, or while capturing particulate matter.
Limitations of Standard Scrubbers
However, standard wet scrubbers have some limitations too, as they cannot be used efficiently when particulate matter of lower than 1 micrometer in size is involved. Nevertheless, specialized scrubbers, such as condensation and charged and venture scrubbers, achieve good efficiency even with sub-micrometer particles. Moreover, scrubbers are very prone to corrosion. Therefore, the composition of the air stream should be considered before using a wet scrubbing device.
Additionally, wet scrubbers are not used to collect dust that can be sold, as to reclaim dust from the slurry is expensive and difficult.
Factors Affecting the Efficiency of Wet Scrubbers
- The collection efficiencies of wet scrubbers varies based on numbers of factors, however, particulate size determines it to great measure, as anything lower than one micrometer is hard to collect with standard scrubber. Other than the particulate size, efficiency of conventional scrubbers is determined by inlet dust loading and energy input.
- Scrubbers that rely on inertial impaction have limited efficiency but it can be leveraged if the relative velocity of the particulate matter present in the airstream and the liquid droplets is increased. To achieve this, the velocity of the gas stream is increased, which increases the momentum and capture of particles in the stream. However, if the velocity of the air stream is increased, the pressure drop will also drop, hence increasing the overall operation cost.
- Another aspect that can influence wet scrubbing efficiency is particle resident time. Particles more than one micrometer in size are collected efficiently with standard resident time. However, small particle capture is determined by Brownian movement and requires extra resident time for capture. Therefore, to increase the efficiency of a scrubber, manufacturers can increase the resident time. However, it the resident time should be monitored for an adverse effect on other system functions.
- Recently, it has been found that the collection efficiency of scrubbers is linked proportionally to the concentration of dust at the inlet. Thus, as the loading increases, the efficiency of a scrubber also increases.
The application limitations and ways to regulate the efficacy of a scrubber will help any user to operate a scrubber optimally.
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