I. Introduction to wet scrubbers
Wet scrubbers are a type of scrubbing system that uses a liquid substance to remove harmful materials from industrial exhaust gases, also known as flue gas. They were the original type of scrubber developed for this purpose.
The main function of a wet scrubber is to remove acidic gases that contribute to acid rain before they are released into the environment. They work by funneling the exhaust gas through an enclosed chamber and spraying it with a wet substance such as water or a chemical solution. The liquid captures and neutralizes the acidic gases and particulate matter
II. Types of wet scrubbers
There are several different types of wet scrubbers designed to remove various pollutants from exhaust gas streams. The main categories include:
- Spray tower scrubbers
- Venturi scrubbers
- Packed-bed scrubbers
Spray tower scrubbers
Spray tower scrubbers are a simple, low energy type of wet scrubber. They consist of an open vessel or tower with spray nozzles to distribute the scrubbing liquid.
- The exhaust gas enters at the bottom of the tower and flows upward through the liquid sprays in a counter-current direction.
- The scrubbing liquid captures and absorbs pollutants as the gas passes through.
- Particulate matter is collected when particles impact and attach to the liquid droplets.
- Spray towers can also operate in a cross-current arrangement with horizontal gas flow. However, this is less efficient than counter-current operation.
Advantages: Simple design, low pressure drop, low energy usage
Disadvantages: Less efficient for small particles, potential for gas bypass around liquid
Applications: Control of dust, mists, and fumes. Removal of particulate matter.
Venturi scrubbers
Venturi scrubbers use a high-velocity throat section to accelerate the gas flow and break the scrubbing liquid into very small droplets.
- The fast moving gas stream enters a converging section and accelerates towards a narrow throat.
- Liquid is injected at the throat perpendicular to the gas flow.
- The liquid shatters into tiny droplets that collect particles through impaction and diffusion.
- A diverging section after the throat reduces the gas velocity again.
Advantages: High removal efficiency, good for sub micron particles, moderate to high energy usage
Disadvantages: Higher pressure drop, noisy operation, potential for erosion
Applications: Fine particulate control, acid gas removal, industrial exhaust gas cleaning
Packed-bed scrubbers
Packed-bed scrubbers contain layered packing material inside the scrubber chamber which provides surface area for gas-liquid contact.
- The packing material can be various shapes like rings, saddles, or structured sheets.
- The packing material creates a large surface area for the gas to contact the liquid flowing through it.
- The liquid coats the packing and establishes a thin film for the pollutants to be absorbed.
- The gas stream flows vertically up through the packing in counter-current flow.
Advantages: High removal efficiency, moderate pressure drop, smaller space requirements
Disadvantages: Limited to low dust loadings, potential for clogging and channeling
Applications: Control of inorganic gases and fumes, VOC removal, some particulate removal
III. How wet scrubbers work
Wet scrubbers operate by passing the contaminated exhaust gas through a liquid medium that removes the pollutants. There are a few different methods this gas-liquid contact occurs:
Spraying Method
- The most common method is to spray the exhaust gas with the scrubbing liquid as it passes through the scrubber chamber.
- The liquid is misted and sprinkled down onto the gas from nozzles above.
- The scrubbing liquid is typically water which is effective at removing dust and particulate matter.
- Chemicals can be added to the scrubbing liquid to help neutralize acidic gases.
- As the exhaust gas passes through the liquid mist, the pollutants collide with and attach to the droplets which causes them to be removed from the gas stream.
Bubbling Method
- Another technique is to send the exhaust gas bubbling up through a pool or bath of the scrubbing liquid.
- As the gas bubbles rise through the liquid, the pollutants are absorbed into the pool.
- This method relies on the same absorption mechanism but maximizes contact time between the gas and liquid.
Scrubbing Mechanisms
The pollutants are removed from the gas phase due to several mechanisms:
- Particle impaction - solid particles collide with and stick to liquid droplets
- Diffusion - gas molecules diffuse into liquid droplets and are absorbed
- Chemical reaction - acids and other gases react with chemicals in the liquid
The result is the exhaust gas leaving the scrubber is free of the harmful pollutants.
Scrubbing Liquids
- Water is universally used as it is highly effective at capturing particulate matter.
- Acidic gases require alkaline scrubbing solutions like sodium hydroxide or calcium hydroxide.
- Some applications use solvents like glycols to absorb specific gases.
- The selection of scrubbing liquid depends on the target pollutants.
Exhaust Gas Conditions
Wet scrubbers are capable of handling gas streams with:
- High temperatures
- High humidity
- Variable flow rates
- Corrosive compounds
IV. Wet scrubbing process
The wet scrubbing process involves intimate contact between the contaminated exhaust gas and the scrubbing liquid as they flow through the scrubber vessel. There are a few key steps:
Gas Flow
- The dirty exhaust gas containing pollutants like particulate matter, acids, VOCs, etc. enters the scrubber.
- A fan or blower system forces the gas through the scrubber chamber.
- Flow rates can vary dramatically depending on the source.
- Velocity is controlled to allow sufficient residence time for scrubbing.
Liquid Introduction
- The scrubbing liquid is dispersed into the gas stream by spray nozzles or other methods.
- Spray towers use overhead spray nozzles.
- Venturis use liquid injection at the throat.
- Packed beds rely on rain-like distribution across the packing.
- Liquid droplets are sized for optimal interfacial surface area.
Gas-Liquid Contact
- The exhaust gas is sprayed or submerged in the scrubbing liquid.
- Turbulent mixing promotes contact between the gas and liquid phases.
- Pollutants collide with and transfer into the liquid droplets.
- Contact occurs via impaction, diffusion, absorption, and chemical reaction.
Separation
- The scrubbed gas and waste liquid are separated.
- Mist eliminators like chevrons remove liquid droplets.
- Clean gas exits through the top of the scrubber.
- Waste liquid settles or drains out the bottom.
Liquid Recirculation
- The used scrubbing liquid is recycled back to the scrubber.
- A holding tank allows solids to settle out before reuse.
- Fresh scrubbing liquid is added to replenish the system.
- Bleed off of waste liquid prevents solids buildup.
This continuous wet scrubbing process allows high volumes of contaminated gas to be treated efficiently.
V. Components of a wet scrubber
Wet scrubbers consist of several main components to handle the gas flow, scrubbing liquid, and waste removal:
Gas Handling Equipment
- Ductwork - Directs the inlet dirty gas into the scrubber and routes the clean outlet gas to the stack. Should be corrosion resistant.
- Fan or blower - Provides motive force to move the gas through the scrubber. Important for contact between gas and liquid.
- Flow control dampers - Allow adjustment of gas flow rates through the scrubber.
- Gas distributors - Spread the inlet gas evenly across the scrubber cross-section.
Scrubbing Equipment
- Scrubber vessel - Enclosed chamber where gas scrubbing occurs. Often fiberglass or plastic.
- Liquid distributors - Introduce and spread the scrubbing liquid into the gas stream via spray nozzles, perforated pipes, etc.
- Packing material (for packed towers) - Provides surface area for gas-liquid contact. Made of plastic, metal, or ceramics.
- Mist eliminators - Remove liquid droplets from the scrubbed gas before discharge. Chevron blades or mesh pads.
Liquid Handling Equipment
- Pumps - Circulate the scrubbing liquid, often with corrosion resistant wetted parts.
- Holding tank - Collects the used scrubbing liquid for reuse or wastewater treatment.
- Chemical feed systems - Add fresh chemicals like caustics to the circulating liquid.
- Blowdown system - Purges a portion of waste liquid to maintain solids concentration.
Other Equipment
- Wash water booster pump - Provides periodic high pressure spray washing of interior surfaces to control solids buildup.
- Instrumentation - Monitors key parameters like pressure drop, liquid flow rate, pH, etc.
- Structural supports - Support vessel, ducts, platforms, access ladders and maintenance equipment.
Proper selection of durable, corrosion resistant materials is crucial for wet scrubbers handling harsh industrial exhaust gases.
VI. Removing particulate matter
Wet scrubbers are highly effective at removing solid particulate matter from exhaust gas streams. The key factors are:
- Amount of energy expended contacting the gas and liquid
- Size distribution of the particulate matter
- Scrubbing liquid properties
Energy Usage
- Low energy scrubbers - Pressure drop < 5 inches H2O; for particles > 5-10 μm
- Medium energy scrubbers - Pressure drop 5-25 inches H2O; for 1-5 μm particles
- High energy scrubbers - Pressure drop > 25 inches H2O; for submicron particles
Higher energy and pressure drop produces smaller liquid droplets, improving particle capture.
Particle Size Distribution
- Scrubbers target particulate matter less than 10 μm in diameter
- PM10 - Coarse particles between 2.5 and 10 μm
- PM2.5 - Fine particles between 0.1 and 2.5 μm
- Submicron particles hardest to capture
Smaller particles require higher energy scrubbing. Pre-classification can remove large particles.
Scrubbing Liquid Properties
- Liquid viscosity - Higher viscosity improves particle capture
- Liquid surface tension - Lower surface tension helps create smaller droplets
- Liquid density - Higher density boosts particle impaction
- Liquid solubility - Helps absorb soluble particulate gases
Removal Mechanisms
- Impaction - Particle collides with and sticks to a droplet
- Interception - Particle follows streamline into droplet
- Diffusion - Submicron particles diffuse to droplet surface
- Solubilization - Particle dissolves into droplet
Removal Efficiency
- Well designed wet scrubbers exceed 95% removal efficiency
- Efficiency improves with higher energy input
- Venturis offer the highest particulate removal efficiency
- Scrubbers effective down to 0.5 μm with sufficient energy
Proper scrubber sizing, liquid selection, and operation helps maximize particulate capture from exhaust gas streams.
VII. Monitoring wet scrubber performance
Continuous monitoring of key operating parameters is crucial to ensure wet scrubbers maintain optimal pollutant removal efficiency. Critical process measurements include:
Pressure Differential
- Measures pressure drop through the scrubber
- Indicates energy expended contacting gas and liquid
- Higher pressure drop improves scrubbing
- Set point varies by scrubber design
- Monitors plugging of packing or mist eliminators
Liquid Flow Rate
- Flow rate of recirculating scrubbing liquid
- Adequate flow needed for proper liquid distribution
- Flow proportional to gas flow rate
- Monitored for pump failure or clogged nozzles
- Typically use magnetic flow meters
Outlet Liquid Concentration
- Measures scrubbing liquid solids content
- Indicates loading of dissolved or suspended contaminants
- Critical for particulate matter removal
- Samples taken manually or online analyzers
Gas Flow Rate
- Flow rate of inlet dirty gas
- Proportional to liquid flow rate
- Significant fluctuations can reduce efficiency
- Measure using pitot tube or anemometer
Scrubber Pressure Drop
| Parameter | Instrument | Alarm Limits |
|---|---|---|
| Pressure Differential | Differential Pressure Transmitter | High/Low |
| Liquid Flow Rate | Magnetic Flow Meter | High/Low |
| Liquid Concentration | Solids Content Analyzer | High |
| Gas Flow Rate | Pitot Tube | High/Low |
The above measurements are key indicators of wet scrubber health and should be closely monitored.
VIII. Advantages of wet scrubbers
Wet scrubbers offer numerous advantages that make them a widely used air pollution control technology:
Effective Removal of Pollutants
- Achieve >95% removal efficiency for particulate matter
- Effectively collect dusts, mists, and fumes
- Remove soluble gases like SO2, VOCs, etc.
- Handle saturated or supersaturated gas streams
- Can collect particles down to 0.5 microns
Tolerate Wide Temperature Range
- Can handle gas temperatures over 500°F
- Scrubbing liquid cools the hot gases
- No temperature limitation or condensation concerns
- Useful for hot, saturated exhaust gases
Compact Design
- Scrubbing condenses gases and reduces volume
- Smaller vessel sizes, ducts, and stacks
- Lower capital costs than other options
- Easily retrofit into existing systems
Simple Operation
- No moving parts aside from pumps
- Less maintenance than electrostatic precipitators
- Automatic monitoring and control systems
- Require minimal operator attention
Corrosion Resistant
- FRP or plastic construction available
- Withstand corrosive gases and scrubbing liquids
- Long service life with minimal deterioration
Wet scrubbers are an effective, proven, simple air pollution control technology suitable for many applications. The advantages make them a popular choice across industry sectors.
IX. Disadvantages of wet scrubbers
While wet scrubbers have numerous advantages, they also come with some drawbacks:
Maintenance Requirements
- Pump and nozzle maintenance for liquid delivery
- Cleaning of mist eliminators and packing
- Solids buildup in tanks and pipes
- Periodic wash down of interior surfaces
- Replacement of corroded parts
Requires regular inspections and maintenance.
Corrosion Issues
- Both gas and liquid can be highly corrosive
- Acidic exhaust gases accelerate corrosion
- Scrubber materials must withstand corrosion
- FRP/plastic construction recommended
- Stainless steels required for metallic components
- Erosion around spray nozzles
Must take steps to minimize corrosion.
Wastewater Management
- Considerable volumes of contaminated water
- Must be treated before discharge
- Solids captured must be disposed properly
- Potential groundwater contamination
Produces liquid waste requiring specialized treatment.
Energy Requirements
- Significant power for pumps and blowers
- High pressure drop designs use more energy
- Operating costs can be substantial
Notable energy consumption, especially for high pressure drop units.
Plugging Issues
- Buildup of solids in packing and piping
- Mist eliminators prone to fouling
- Can lead to channeling and short circuiting
- Causes operating problems
Solids accumulation requires mitigation.
While these downsides exist, proper design and operation can minimize the disadvantages.
X. Applications of wet scrubbers
Wet scrubbers are utilized in a variety of industries and processes due to their ability to handle tough gas cleaning challenges:
Manufacturing Facilities
- Chemical plants - Acid gas removal from vents and stacks
- Refineries - Hydrocarbon recovery, SO2 and NOx control
- Steel mills - Control of rolling mill emissions
- Pulp & paper mills - Collection of lime dust and boiler emissions
- Food processing - Odor control and mist elimination
Power Generation
- Coal-fired power - Flue gas desulfurization (FGD scrubbers)
- Waste-to-energy - Acid gas and particulate control
- Biomass boilers - Removal of ash, alkali metals, and HCl
Mining
- Coal mining - Methane drainage gas cleaning
- Metal mining - Process off-gas scrubbing
Engine Exhaust
- Marine vessels - Scrubber systems to remove SOx and PM
- Diesel gensets - Particulate removal from exhaust
- Jet engine testing - Cooling and silencing
Industrial Processes
- Asphalt plants - Containment of bitumen fumes
- Smelters - Removal of acid fumes and dust
- Kilns and dryers - Heat recovery and PM reduction
Wet scrubbers serve an important emission control role across many industries and processes with challenging gas streams.