How Wet Scrubbers Remove Hazardous Gases in Chemical Processing Plants
The manufacturing process of many chemical production facilities generates hazardous vapours, fumes, and gas emissions that can create an adverse impact on both the environment and the health of its employees, as well as the efficiency performance of industrial operations; untreated hazardous gases hurt the environment, create risks to human health, and violate the emissions limits outlined in regulations. One of the best methods for treating and capturing hazardous chemical gases is wet scrubbing, which is a well-established air pollution control technology that utilizes both mechanical action and chemical treatment to physically and chemically collect and remove impurities from the gas stream.
This article provides an overview of wet scrubbers, the operational principles of these systems, the different designs and variations of wet scrubbers available for use in chemical processing plants, examples of how wet scrubbers are used, the benefits and limitations of using wet scrubbers for controlling emissions from chemical processing facilities, and an explanation of how wet scrubbers are critical components of effective modern-day industrial emission control strategies.
What Are Wet Scrubbers?
Air Quality Control (IAQ) Systems (wet scrubbers) are used to reduce both gas and particulate matter from the exhaust stream by bringing them into contact with a liquid, usually either water or a special scrubbing mixture, which serves to dissolve or capture a pollutant in solution before the cleaned gases are expelled into the atmosphere.
Wet scrubbers also use an absorbent medium, usually an aqueous solution, to capture and dissolve gases. Unlike dry scrubbers, which use solid sorbents, wet scrubbers work much more efficiently with solubilised and reactive gaseous pollutants. Wet scrubbers typically achieve 90% or greater removal efficiencies, but this depends entirely on the design and type of wet scrubber used.
Why Wet Scrubbers Are Important in Chemical Processing Plants
Chemical manufacturing facilities are involved in a multitude of chemical processes, which include but are not limited to: acid, base, solvent, and reactive chemical production, and as a result, the processes create hazardous gaseous products, including, but not limited to:
- (HCL) Hydrogen Chloride
- (SO2) Sulphur Dioxide
- (H2S) Hydrogen Sulfide
- (VOC) Volatile Organic Compounds
- (NH3) Ammonia
- Acidic and Basic vapours.
All of these gases can be corrosive, poisonous, and pose health hazards to both workers at the facility and the surrounding community if not properly treated before being released to the atmosphere. Wet scrubber technology is particularly beneficial in these types of scenarios because they efficiently capture not only particulates but also water-soluble gases, which often cannot easily be captured using traditional dry scrubbing technologies.
Fundamental Operating Principle of Wet Scrubbers
The Wet Scrubber is a Device that cleans contaminated gases by allowing them to make intimate contact with the Wet Scrubber Liquid.
The Pollutants will be absorbed if they have been introduced to the liquid or chemically reacted into the liquid phase as they are subjected to the action of an aerosol, bubble, or packed media.
The following are the basic steps of Wet Scrubber Operation:
1. Introduction of Contaminated Exhaust Gas into the Wet Scrubber:
The contaminated exhaust gas will enter the Wet Scrubber chamber through an inlet connection.
2. Injection of Scrubbing Liquid into the Wet Scrubber:
The scrubbing liquid will be introduced into the Wet Scrubber as a fine aerosol, through packed media, and/or into a pool of liquid.
3. Interaction of the Contaminated Gases with the Scrubbing Liquid:
The Pollutants will either be absorbed into the scrubbing liquid or neutralized through their chemical reaction with the scrubbing liquid, using the chemicals added to the liquid or used as a sintering agent.
4. Collection of Liquid Containing Captured or Dissolved Contaminants:
The liquid that has dissolved or captured the contaminants will be collected in a sump or collection basin.
5. Exit of Clean Gas from the Wet Scrubber:
The cleaned gas will flow upward from the Wet Scrubber and exit through a mist eliminator, where it will be separated from any entrained droplets, before releasing only the clean air.
By effectively transferring the contaminants from the gas stream into the Liquid Phase (which can then be managed and treated safely), a Wet Scrubber can be used to remove many types of contaminants.
Mechanisms of Pollutant Removal
If a hazardous gas is produced, a wet scrubber will remove it using one or a combination of four different means.
Absorbent Solutions: The gaseous pollutant formed is absorbed in water or sprayed by the scrubber liquid (usually an alkaline solution). This is the most common means of removing the gaseous pollutant. For example, when SO2, a type of acid gas, is dissolved in water or absorbs with a base (i.e., an alkaline solution) to make calcium sulfate or calcium sulfonate (the two main types of alkaline oxidation products). Acidic gases will dissolve similarly when they come into contact with acidic water or alkaline solutions.
Chemical Neutralizer: Some scrubber solutions are chemically active. An acid gas that is neutralized with sodium hydroxide in the scrubber solution will produce sodium chloride (NaCl) or sodium sulfate (Na2SO4). Again, there are numerous ways of creating this reaction.
Impaction and Interception: Larger particles will be captured through this process when they collide with liquid droplets in the gas stream. Smaller particles may be captured as the gas flows around packing surfaces or as part of the droplets in the liquid.
Diffusion: Fine particles and gases will move around randomly (give the particle an actual movement pattern) until they are eventually captured by the scrubber liquid using Brownian motion.
All these methods will allow a wet scrubber to collect both fine particulate materials and dissolved gases at the same time. They are adaptable to many types of processing operations.
Types of Wet Scrubbers Used in Chemical Plants
The benefits of using a particular wet scrubber design depend on both the type of pollutant and how concentrated it is:
1. Spray Tower Scrubber:
These systems utilize the action of spraying a liquid into a gas stream to create bubbles of liquid in the polluted air, which provide many surfaces for gas absorption, along with providing surfaces for capturing solid particulate matter from the air. Spray towers are very inexpensive and simple to use.
2. Packed Bed Scrubber:
This type of wet scrubber uses an arrangement of packing material to provide additional surface area for contact between a gas and a liquid. When the gas comes into contact with the packing material, it will be absorbed by or captured by the scrubbing liquid that has trickled down over the packing material from above. Packed beds are particularly good for removing gaseous pollutants.
3. Venturi Scrubber Systems
Gas is forced through a small throat at a high speed, while a scrubbing liquid enters, at a very fast speed, through the constricted throat. The turbulence generated produces small droplets and allows for a very high collection efficiency for both fine particles and gaseous compounds.
4. Cyclonic Wet Scrubber Systems
The separation of pollutants in cyclonic wet scrubber design is achieved through centrifugal force. The gas enters tangentially to create a spinning motion. As the gas spins, it forces the particles outward to the scrubber’s wall, where they are captured in the scrubbing liquid that adheres to the wall’s surface.
The selection of the design will depend upon the size of the pollutant particles being removed, gas temperature, moisture content of the exhaust gas, and the collection efficiency needed.
Key Design Considerations for Optimal Performance
Efficient wet scrubber design requires an engineer to consider many factors, and the following list indicates those most influential:
1. Liquid to Gas (L/G) Ratio – The ratio of the flow of the scrubbing liquid to the flow of the gas through the scrubber impacts the performance of the scrubber as a pollutant removal system. Higher L/G ratios typically lead to better performance in pollutant capture; however, higher L/G ratios also cause higher operating costs due to increased volume of scrubbing liquid that must be handled and treated.
2. Contact Time – The longer the gas is in contact with the scrubbing liquid, the more opportunities exist for vapor absorption and reactions to occur, resulting in higher removal efficiencies.
3. pH and Chemical Composition of the Scrubbing Liquid – By selecting the appropriate scrubbing liquid with the correct pH for the pollutants being targeted in the scrubbing process, the scrubbing liquid can neutralize or assist in absorbing those pollutants (for example, the scrubbing of acidic gaseous pollutants).
4. Droplet Size and Distribution – Smaller droplets provide a greater amount of total surface area for the scrubbing liquid to interact with the gas. However, small droplets are often more difficult to remove from the gas. Designers need to achieve a balance between providing enough surface area for gas-liquid interaction while not creating difficult operational problems due to high liquid carryover.
Industrial Applications in Chemical Processing Plants
Wet scrubbers are widely used in many different industries that manufacture chemicals, including:
- Petrochemical plants; wet scrubbers are used to control acidic gas emissions.
- Chemical synthesis facilities; wet scrubbers to neutralize the corrosive vapours produced during chemical reactions;
- Pharmaceutical production; wet scrubbers are effectively used for capturing solvent vapours.
- Wastewater treatment facilities; wet scrubbers prevent unpleasant odours from polluting water.
- Combustion processes; wet scrubbers remove both particulate and gaseous pollutants from flue gases.
Wet scrubbers can effectively handle many gases that may be encountered in numerous industrial settings, including HCl, SO₂, VOCs, NH₃, and others.
Advantages of Using Wet Scrubbers
Here are some essential advantages of Wet Scrubbers in controlling emissions as a part of an Industrial Emission Control System:
The scrubber’s performance provides for excellent removal efficiencies, often above 90% for several common pollutants (including gaseous and particulate).
Most Wet Scrubber designs provide for the simultaneous capture of both gaseous and particulate pollutants; thus, they offer greater flexibility than some other types of technologies, which are designed for either particle or gas removal.
Wet Scrubbers are better suited to handle high temperature and moisture (steam) gaseous streams, which are very common in chemical manufacturing.
Some Wet Scrubber applications can produce a cooling effect on the contaminated hot exhaust stream, providing benefits downstream and reducing thermal impacts on equipment.
Wet Scrubber systems that are properly constructed using corrosion-resistant materials (i.e., Fiberglass Reinforced Plastic (FRP), special alloys) will provide for long service life in a Corrosive environment.
Wet Scrubbers are an effective means for facilitating compliance with volatility control measures for air quality and meeting the stringent regulatory requirements placed on industrial facilities with respect to Hazardous Air Pollutant (HAP) emissions.
Challenges and Limitations
Wet scrubbers have several benefits; however, these will also present several challenges.
Wastewater Generation – The scrubbing medium will become contaminated, requiring treatment prior to ultimate disposal or reuse.
Operating Expenses – The requirement to operate wet scrubbers with a larger volume of scrubbing liquid and pump the scrubbing liquid through a more extensive piping system tends to increase operating expenses when compared to other air pollution control systems.
Maintenance Requirements – Wet scrubbers will require regular maintenance/washouts to avoid clogging, corrosion, and loss of efficiency.
Design Complexity – Depending on the type and configuration of pollutant(s), a considerable amount of engineering will be needed to select the right type of wet scrubber for the job.
Conclusion
Wet scrubbers are an essential tool for removing toxic gases from chemical plants. They use gas-liquid contact, absorption, and chemical neutralisation to clean out toxic gases and odorous compounds from exhaust streams. Wet scrubbers are also very efficient at removing gases and particulate matter, making them one of the most widely used technologies in the industrial emissions control system.
Wet scrubbers do have their challenges, including wastewater disposal and operating costs; however, the benefits of improved health and environmental performance as well as compliance with regulations outweigh these challenges. All chemical plants that want to operate safely and sustainably should invest in wet scrubber systems.