Air Pollution Scrubbers
Since the Industrial Revolution, industrial processes (and some commercial and residential processes) have been creating pollution at an increasing rate. There are many types of pollution control equipment available; one type is the air pollution scrubber. These scrubbers remove particles and gases from exhaust dispelled from industrial equipment. Air pollution Scrubbers are not a single device but a diverse group of emission control systems, used for removing either Particulate Matter (PM) or gases, or both, from standard industrial emission. To scrub off different contaminants, different types of designs and solvents are available. Though, the most common is water. Moreover, scrubbers can also be utilized, as recycling equipment for recovering heat from hot gases by the flue gas-condensation process.
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Applications of Air Pollution Scrubbers
Air pollution scrubbers have applications across the board, from industrial and manufacturing processes, steelmaking, printing, pharmaceuticals, and chemical processing. Spray towers or chambers are used as a type of pollution control equipment to control benzene storage and light-oil tank emission; in this application, wash oil is used as a solvent. However, they generally have the least effective mass transfer capability among all the available options; therefore, they are used generally only for high-solubility gases and particulate matter. They are the most popular wet scrubber configuration to absorb SO2, though, having application in a wide range of industries, including in the non-ferrous metals processing of lead, aluminum, and copper, as well as in elemental sulfur plants, which releases highly concentrated SO2. They also have application in regulating exhaust from oil and coal combustion electric facilities, where they are fitted in flue-gas desulfurization (FGD) systems.
Notable Types of Air Pollution Scrubbers
Though there are many styles of scrubbers, they all operate in the same basic fashion: they force contaminated gas through a scrubbing liquid, which may be a chemical solution or just water. Air pollution scrubbers are generally divided into dry scrubbers and wet scrubbers. Dry scrubbers operate with minimal to no moisture, instead attracting and collecting pollutants through the application of a dry slurry or reagent, which is introduced into a dirty gas or exhaust stream. Chemicals in the slurry, such as limestone, draw in and capture particulates and chemicals to form, via chemical reaction, larger particles that cannot pass through a filter. Then, remaining particulates are caught when the gas stream is forced through one or more additional filters. Wet scrubbers, on the other hand, collect pollutants by forcing dirty air through a spray or pool of cleaning solution, where gases and particulates, attracted to the specialized solution, are trapped inside liquid droplets, as clean air flows free. If operators deem it necessary, additional filters may be added before and after the main scrubbing.
In addition to the basic air pollution scrubber, subtypes include:
- Chemical and Gas Scrubbers
- A type of wet scrubber, designed to remove chemical compounds and gases from contaminated gas streams. The nature of the scrubbing solution into which they are dissolved or absorbed depends upon the properties of the targeted pollutant(s).
- Chlorine Scrubbers
- A type of chemical scrubber designed specifically to target chlorine, a common toxic byproduct of chemical reactions that manufacturers are required by law to reduce or eliminate. Chlorine scrubbers generally engage a multi-stage removal system and a neutralizing agent, which is usually an alkaline solution like soda ash or caustic.
- Ammonia Scrubbers
- Another type of chemical scrubber, this time used to remove ammonia gas from an air or gas stream. Ammonia is a regulated compound, which is toxic enough that it must be treated below established limits before it can be released into the atmosphere. Ammonia scrubbers usually use a diluted sulfuric acid solution to clean the polluted air stream, which results in the formation of salt as a byproduct. This salt must be collected and removed before the air stream exits through the stack.
- Particle Scrubbers
- Also called particulate scrubbers, and, most commonly, venturi scrubbers, are smaller machines that efficiently remove particles from gas streams with both high temperatures and moisture. Incorporating fans, pumps, mist eliminators, exhaust stacks, and instrumentation and controls, particle scrubbers may remove particles separately or, potentially, they may remove gas and particles at the same time. Particle scrubbers typically collect pollutants with an efficiency above 99%.
Different Air Pollution Scrubber For Different Needs
In today’s industrial landscape, effective pollution control equipment is essential, as the compliance standards are getting stricter to ward off the looming danger of climate change and its implications. The recent data on climate change points out that if we keep on emitting the same amount of greenhouse gases and poisonous gases into the atmosphere, we will go across the point of no return, where global devastation is inevitable. Given the changing attitude of society and enforcement towards the climate issues and pressure they induce, owners of facilities that contribute to pollution are updating their air pollution control systems. However, if you look closely, the choices can be overwhelming, as many emission control systems are available, including dust collectors, scrubbers, thermal oxidizers, blowers, air filters, mist collectors, and electrostatic precipitators.
The air pollution scrubber, however, is the most widely used equipment because of its simple design and operation. In a wet scrubber, to eliminate or neutralize toxic and harmful contaminants in an exhaust stream, before the exhaust leaves a system, a solvent, typically water, is infused into the emissions. After encountering solvent, the pollutants get condensed and remain in the water, and the filtered gases escape. Similarly, different solvents are used that suit a given application, some use powdered lime and other designs to remove contaminants from gases.
Here are some air pollution scrubbers for different applications:
- Scrubbers for Inorganic Gases Control
- When it comes to removal of inorganic pollutants, water is used as a solvent. However, for acid-gas absorption, only water as a solvent would not prove efficient, so caustic soda solution is utilized for enhancing the absorption. When air scrubbers are used in flue-gas desulfurization system, SO2 is absorbed by a reagent slurry, which is usually alkaline and loaded either with slaked lime, or limestone, or a combination of alkaline fly ash and slaked lime. Similarly, hydrogen sulfide, chromic acid, ammonia, fluorides, and chlorides are filtered out of plant exhaust.
- Scrubbers for Particulate Matter
- A standard spray chamber or tower is utilized to collect the particulate matter, which can be in the range of 10 micrometers (µm). However, different designs collect different sizes of particulate matter, though; standard scrubbers should not be used to capture very fine particulate matter because of high liquid to gas ratios. To attain higher collecting efficiency, a cyclonic spray chamber is used, which has a system, turning vanes, to create a cyclonic motion inside the tower. The movement of gases at higher velocities makes it easy to separate particles and droplets, which results in higher collection efficiency.
- Scrubbers for Volatile Organic Compound
- This type of scrubber has application in purification and separation of highly concentrated gaseous streams with organic compounds, such as in coke recovery operations and natural gas purification. Either spray tower, packed, or plate columns is employed for this application to create a counter current spray to dissolve the organics in the gas stream with the help of a liquid solvent.
Advantages and Potential Disadvantages of Spray Towers
Here are some advantages of spray towers:
- Pressure drop is a common issue with air scrubbers; however, in spray towers, the pressure drop is relatively low.
- This process equipment is considered safe, as it can safely handle explosive and flammable dusts.
- Even if the environment is highly corrosive, with small modification, like fiberglass-reinforced plastic (FRP) construction, it permits efficient operation.
- You have to invest low capital for spray towers.
- The other major problem with other type of scrubbers is plugging; the spray towers are free from plugging issue.
- Smaller space needed for installation and operation.
- Works well for collecting both PM and gases.
However, there are some disadvantages of spray towers, such as:
- Water disposal, as there is the need for proper planning for installation and operation to ensure proper functioning.
- The final waste product is wet, which at times can be hard to clean.
- In comparison to other designs, it has low mass-transfer efficiencies.
- Can be rendered useless to eliminate very fine PM.
- FRP construction makes it suitable for corrosive environment, though it remains sensitive to temperature.
- The operating cost for these air filtration systems are relatively high.
- In some applications, pre-coolers (additional spray chambers) are required for reducing the inlet air-stream temperature to circumvent the reduced absorption rates or solvent evaporation.
Additionally, spray towers cannot be applied for air pollution control of very fine PM, as for this purpose, the liquid to gas ratios—greater than 22.4 gallons per thousand cubic feet—would have to be very high to be profitable and eco friendly. Depending upon the application, the efficiency of collecting contaminants varies from 70 to more than 99 percent. When the requirement is higher, cyclonic spray towers, which produce cyclonic motion in the chamber, are generally utilized, instead of standard spray chambers.
Pollution Control Equipment that Uses Absorption
Just like adsorption, the concept of absorption is employed extensively in the industry for purification of gas streams, usually in pollution control equipment when water-soluble inorganic contaminants are involved in exhaust air streams. The concept is widely utilized in air pollution scrubbers.
In the absorption process, the soluble components of a gas mixture are dissolved in a solvent, which can either be a chemical or physical process. In physical absorption, the absorbed compound dissolves in the solvent, physically, without any reaction, whereas in the chemical absorption, the absorbed compound and the solvent react, chemically.
The solvents that are usually used in this equipment are:
- Mineral Oils
- Nonvolatile Hydrocarbon Oils
- Aqueous Solutions
These factors determine if air pollution scrubbers are suitable for air pollution control:
- Whether a suitable solvent is available in the region.
- The required removal efficiency.
- The concentration of pollutant in the inlet vapor.
- The capacity requirements when handling waste gas.
- The recovery cost of the contaminant.
- The cost for disposing the used solvent.
Air pollution control devices that work on the basis of absorption principles include:
- Packed Towers
- Designed in such a way that they provide a large surface area to assist contact between the gas and solvent. They have columns filled with packed material and can handle higher liquid rates, while achieving higher removal efficiencies. They are advantageous because they have lower consumption requirements in comparison to other types of gas scrubbers. Their biggest disadvantages are that they encounter high system pressure drops and can get clogged quite often. This can escalate the maintenance costs. Moreover, wastewater disposal and operation costs can also be higher.
- Plate or Tray Columns
- Have vertical cylinders, where the liquid and gas come in contact, following a stepwise configuration on trays. The liquid or solvent enters from the top of the column. As the gas moves upwards, the solvent moves through a downspout. The gas moves through openings, and as the gas and solvent meet, the gas bubbles into the solvent and gets cleaned in the process. These scrubbers are advantageous, as they can be cleaned easily, and can handle large temperature fluctuations efficiently, unlike packed towers. However, they too have disadvantages, as when high gas flow rates are involved, the towers encounter larger pressure drops.
- Venturi Scrubbers
- When pollutants, such as Particulate Matter (PM) and sulfur dioxide, are controlled at the source, Venturi scrubbers are used. To remove submicron particles, ranging from 0.5 and 5.0 micrometers, they employ Venturi tubes, which have sections that gradually converge and diverge, for cleaning exhaust airstreams. The solvent is atomized to collect particles. As the gas-liquid contact time is less, these scrubbers are used only for highly soluble gases.
- Spray Chambers
- In spray towers, the liquid droplets are introduced in countercurrent gas stream, where it comes in contact with the contaminants. They are easy to maintain and operate and also need relatively low energy to operate, which makes them economical. Their effective mass-transfer capability, however, is the worst in comparison to others scrubbers. Therefore, their use is limited to particulate removal and highly soluble gases, including ammonia and sulfur dioxide.
Factors to Consider When Purchasing Air Pollution Scrubbers
Before deciding what type of scrubber is best for an application, manufacturers must consider the conditions under which it will operate as well as any and all compliance requirements. A scrubber must be able to efficiently attract, collect, and scrub the targeted pollutant. Some pollutants, for example, will require a specific chemical reaction or filters to capture them, while others may only need a water stream. The efficiency at which a manufacturer requires or wishes to work is also an important consideration. Process conditions to be considered include flow rate, pollutant concentration, and temperature. If, after these considerations, the choice is still unclear, a potential customer need not worry; generally, air pollution scrubber manufacturers are more than willing to help customers understand their exact needs and navigate their choices.