Guide to Ammonia Scrubbers: Design Calculations, Types, pH Control, and Maintenance

Author: Anna Frank, Equipment Selection Expert

Ammonia air scrubber — particularly packed bed columns — are recognized by the U.S. Environmental Protection Agency (EPA) as effective devices for removing alkaline compounds from gas streams. These equipment can achieve removal efficiencies of up to 99%.

These technologies are applied across multiple industries, including fertilizer production, livestock waste management, and the coke and coal chemical sectors.

In addition to ammonia wet scrubbers, the EPA also considers alternative emission control methods, such as condensation units, NH₃ capture and recirculation technologies, and regular maintenance of process equipment. It is important to note that this compound is classified as an acutely toxic substance, with an Immediately Dangerous to Life or Health (IDLH) concentration of 500 ppm. This highlights the need for stringent emission control and prevention measures to protect both human health and the environment.

An ammonia scrubber is a type of air pollution control device that removes NH₃ vapor from gas or exhaust stream using a process called wet scrubbing. Here’s a simple breakdown of how it works:
🧪 Basic Process
1. Air containing NH₃ is directed into the ammonia gas scrubber.
2. Contact with Cleaning Liquid:
The air passes through a packed bed
where it contacts a liquid — typically water or a weak acid solution.
NH3 is highly soluble in water, and even more so in acidic solutions.
3. Ammonia scrubber reaction:
If water is employed:
NH₃​(gas) + H₂O →NH₃​(aqueous)
If sulfuric acid is used:
2NH₃+H₂​SO₄​→(NH₄​)₂​SO₄
This forms ammonium sulfate, a stable salt that can be collected.
4. Clean Stream Outlet.
5. The liquid is collected and either treated, reused, or disposed of properly.
Using acid improves performance by reacting with NH3, reducing its partial pressure and allowing for greater absorption.

When developing emission cleaning equipment, it is essential to consider not only the type and concentration of pollutants but also their chemical characteristics. In cases where both alkaline and acidic components are present in the airstream, a two-stage wet system with sequentially arranged ammonia fume scrubbers is recommended.

1. First — ammonia scrubber with H2SO4:

• Efficiently removes NH3 and amines through neutralization and conversion into stable salts, such as ammonium sulfate.
• Functions as an absorber for alkaline components.

2. Second – caustic ammonia scrubber sprayed with a caustic soda (NaOH) solution:

• Targets the removal of acidic compounds, including hydrogen sulfide and organic thiols (mercaptans).
• Protects the environment from sulfur-based compounds and significantly reduces odor levels.

This approach ensures high purification performance and allows for flexible adaptation to the specific composition of the polluted airstream.

If the gas stream contains no solid particles or aerosols, packed bed towers are the most suitable choice. This equipment provides efficient mass transfer due to the large surface area for gas-liquid interaction, especially under low to moderate flow velocities.

Torch-Air offers several solutions tailored to these requirements:

Tornado Venturi Packing – a hybrid configuration engineered for high processing velocities.
It is a two-stage air purification system. First, contaminated gas passes through a Venturi tube, and then it moves through a packed bed layer.

• High removal efficiency: The Tornado Venturi Packing is capable of eliminating up to 99.9% of pollutants.
• Versatility: Suitable for various industrial applications, including chemical manufacturing, agriculture, and wastewater treatment.
• Low operating costs: Thanks to its efficient configuration and use of durable materials, the equipment requires minimal maintenance and offers long service life.

Tornado Random Packing – a vertical traditional packed-bed ammonia vapor scrubber featuring randomly distributed media.

Contaminated air enters the TORNADO RP inlet chamber, where it is sprayed with a special treatment fluid. It then passes through a bed of random packing (e.g., Raschig rings, Pall rings, or Intalox), which provides a large surface area for phase contact. In this zone, intensive interaction between the gas and liquid phases takes place. After that, the flow moves through a mist eliminator, which prevents liquid carryover, and is released into the atmosphere.

• The TORNADO RP can eliminate up to 99.9% of pollutants, including hydrogen sulfide and other harmful compounds.
• Suitable for a wide range of industrial applications, including chemical manufacturing, agriculture, and wastewater treatment.
• Requires minimal maintenance and offers long service life.
• Scalable to meet specific operational requirements.

BOREAS-P2 by Torch-Air is a horizontal small ammonia scrubber with random packing, designed for efficient air purification from acidic, alkaline contaminants, and their mixtures.
Contaminated gas enters the horizontal mass transfer chamber, where it passes through a layer of packing material. In this zone, intensive interaction between phases occurs, promoting the effective removal of pollutants. The cleaned stream then passes through a mist eliminator, which prevents liquid carryover, and is released into the atmosphere.

• Low operating costs.
• Modular configuration enables easy capacity expansion.
• Can be manufactured from stainless steel, carbon steel, polypropylene, polyethylene, or PVDF.
• Airflow capacity: 170 to 297,000 m³/h (100–175,000 cfm)
• Dust load: up to 50 ppm (cyclone pre-treatment recommended if needed)
• Gas-phase contaminant load: up to 2 ppm
• Environmental compliance: Incorporation of eco-friendly construction media and sustainable technologies

When necessary, two separate scrubber columns can be installed in series, each optimized for a specific class of contaminants and chemical reagents. Optimized ammonia gas scrubber design can significantly reduce NH₃ emissions and operating costs.

The ammonia tank vent scrubber is a specialized wet cleaning equipment engineered to purify air emissions from storage tanks containing volatile or hazardous substances. It effectively removes contaminants such as NH₃, hydrogen sulfide, mercaptans, and volatile organic compounds (VOCs), achieving removal efficiencies up to 99.9%, depending on the pollution level.

Contaminated vapors from the storage tank are routed into the unit through the ventilation unit. Inside, they pass through a packed bed continuously irrigated with a solution — typically water or a chemical reagent. The structured media, featuring a high surface area, promotes efficient interaction between the phases. During this process, harmful substances are either absorbed or neutralized through chemical reactions. The treated stream then moves into a separation chamber equipped with a mist eliminator, which captures residual droplets. Cleaned stream is safely released to the atmosphere, while the spent liquid is collected for recirculation or further treatment, depending on the configuration.

4. Determination of Liquid Requirements
Establish the liquid-to-gas ratio needed to ensure effective contact and absorption. This involves selecting a liquid flow rate sufficient to absorb NH₃, based on anticipated gas loading, solubility, and reaction kinetics.

5. Type and Configuration of the Ammonia Scrubber System
Packed bed towers are the preferred solution due to their superior phase contact performance and the large surface area provided by the filtration media. This configuration ensures efficient NH₃ absorption into the liquid phase, offering superior performance compared to alternatives like spray towers or Venturi scrubbers under similar conditions.

6. Column Sizing and Flow Dynamics
Determine the appropriate diameter of the column based on gas throughput and allowable velocity to prevent excessive pressure drop. The column height is chosen to provide adequate residence time and mass transfer surface, often calculated using standard mass transfer models or empirical data from similar installations.

7. Selection of Media and Internal Components
Select the appropriate packing media (structured or random) to maximize the contact area between the phases. Consider pressure loss, surface area, and material compatibility. Include demisters or mist eliminators to prevent carryover of droplets.

8. Auxiliary Equipment and Controls
Plan the supporting components: pumps for circulating the solution, chemical dosing systems, sensors for NH3 concentration, pH control, and flow measurement instruments. Automate controls to maintain stable operation and compliance with emission limits.

9. Material and Mechanical Design
Choose components engineered to withstand corrosion caused by NH₃ exposure and any scrubbing agents involved in the process. Consider temperature, pressure, and mechanical loads to ensure structural integrity and long-term performance.

10. Discharge Handling and Wastewater Management
Plan for the treatment or disposal of the process fluid, which will contain dissolved NH₃ or resulting neutralized salts. Include tanks, filtration units, or chemical treatment units as required.

Ammonia scrubber design calculation involves a systematic approach. The following steps outline the key elements required for the design process.
1. Process Data Collection

• Gas flow rate (Qₐ) m³/h or CFM
• Inlet NH₃ concentration (Cᵢ) mg/m³ or ppm
• Required removal efficiency (η)

2. Mass Balance and Load Determination

• NH₃ load (Lₙ):

Total mass of NH₃ to be removed (g/h or kg/h), essential for sizing the unit.

3. Absorption Principles

• Solubility & Reaction:

NH₃ dissolves readily in water and reacts with acids (e.g., H₂SO₄), forming ammonium salts. Incorporate Henry’s Law and equilibrium relations to estimate dissolution efficiency.

4. Liquid-to-Gas Ratio (L/G)

• Empirical or theoretical basis:

Select a suitable L/G ratio (typically in the range of 2–10 L/m³) based on process requirements, using:

Where Q_L is liquid flow rate (L/h), and Q_a is air flow rate.
5. Tower Sizing

• Packed bed dimensions:
• Diameter (D):

Where V_g is superficial gas velocity (m/s).

• Height (H):

Based on mass transfer requirements and specific packing characteristics. Use:

N_t: number of transfer units
H_t: height of one transfer unit, determined experimentally or from vendor data.

6. Mass Transfer Calculations
Mass transfer is a critical step in ammonia scrubber calculation for accurate column sizing.

• NTU/HTU Method: Apply the Number of Transfer Units (NTU) and Height of Transfer Unit (HTU) method for precise column height estimation.
• KGa (overall mass transfer coefficient): Estimate using correlations that consider media type, fluid properties, and movement conditions.

7. Neutralization Chemistry

• For acid scrubbing:

2NH3₃+H₂SO₄→(NH₄)2SO₄
Calculate neutralizing agent requirement based on stoichiometry and molar flow rates.

8. Auxiliary Infrastructure Design
Pump selection: Based on required head and flow.
Mist eliminators: To capture entrained droplets.
Instrumentation: Include flow meters, pH control, and NH3 detectors.

9. Material Selection
Choose corrosion-resistant fabrication elements (e.g., FRP, PVC, stainless steel) based on chemical compatibility and temperature.

The efficiency of an ammonia scrubber is closely tied to the pH level of the scrubbing liquid, as NH3 is a weak base that reacts readily with acids to form ammonium salts (e.g., ammonium sulfate, NH₄HSO₄).

• At low pH (acidic solution): The fluid contains a high concentration of H⁺ ions, which react with NH₃ to form NH₄⁺ (ammonium ion).

• At neutral to high pH: The solution becomes less effective because free NH₃ is less likely to convert to NH₄⁺, reducing the driving force for absorption. Ammonia stays in the gas phase, and removal efficiency drops.

Ammonia scrubber with phosphoric acid involves using H₃PO₄ to neutralize NH₃. The process converts NH₃ into ammonium phosphate (NH₄H₂PO₄) or other phosphate salts. This method is particularly useful when a more moderate pH range is needed for NH₃ absorption compared to stronger acids like sulfuric acid. The resulting byproducts can be employed in fertilizer production or other chemical applications.

Maintaining the appropriate ammonia scrubber pH is critical to ensure maximum absorption performance.

Ammonia scrubber efficiency with pH is highest when the system is maintained in an acidic range, typically between pH 1 and 3. H2SO4 is commonly employed as the scrubbing medium due to its strong acidity and ability to form stable ammonium salts.

When selecting ammonia scrubber material, key factors to consider include:
✅Corrosion Resistance:
This alkaline compound is highly corrosive, so materials must withstand its effects. Stainless steel (e.g., 316) and fiberglass-reinforced plastic (FRP) are commonly employed due to their resistance to both this compound and other chemicals involved in the scrubbing process, such as H₂SO₄.
✅Chemical Compatibility:
Equipment components must be compatible with NH₃ as well as any neutralizing agents employed, like H₂SO₄ or sodium hydroxide. FRP is ideal for acidic environments, while stainless steel handles both acids and alkalis well.
✅Temperature and Pressure Resistance:
The selected construction components must withstand the specified operating temperature (typically 40°F to 140°F) and pressure levels. Stainless steel is suitable for higher temperatures and pressures, while FRP is typically employed for lower-pressure installations.
✅Structural Integrity:
Materials need to support the weight and mechanical stresses of the unit without deformation or failure. Stainless steel provides robust strength, whereas FRP is lighter but may require additional support for heavy setups.
In summary, selecting the right material is critical to ensuring the equipment's longevity, performance, and reliability.

In an acid scrubber ammonia, regular maintenance is essential to maintain high performance and ensure long-term operation. Over time, media materials can accumulate contaminants, which reduces phase contact efficiency. Periodic inspections and cleaning of the packing prevent clogging and preserve optimal performance for purification. Additionally, monitoring the pH levels and replacing worn-out components helps prevent corrosion and ensures the unit's reliability.