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Home / Blog / Comprehensive Guide to NaOH Scrubbing Systems

Comprehensive Guide to NaOH Scrubbing Systems

NaOH scrubber refers to a purification system that uses sodium hydroxide (NaOH), often called caustic soda, to remove acidic gases from a gas stream. These systems are widely used in industry for emissions control and to minimize environmental pollution.

Principle of Operation

The NaOH scrubber process is an efficient method for removing acidic gases from industrial emissions.
The principle of operation is as follows: when an acidic gas, such as sulfur dioxide (SO₂) or hydrogen chloride (HCl), passes through a solution of NaOH, a chemical reaction occurs that converts these gases into their respective salts, which remain dissolved in the solution or precipitate out. As a result, the gas stream exiting the scrubber is less polluted.

A simple example of the reaction:
SO₂ + 2 NaOH → Na₂SO₃ + H₂O

The caustic soda solution in the scrubber usually requires regular replacement or regeneration, as the concentration of acidic gases and reaction products in the solution increases over time.


These systems are one of the methods to control and reduce atmospheric emissions from industrial facilities in order to meet environmental regulations and standards.
Wet scrubber NaOH systems are employed in various sectors and processes to remove acidic gases from gas streams. Some of them include:
  1. Power Generation: Thermal power plants, especially those burning coal, produce significant volumes of flue gases containing SO₂ and other acidic gases. Scrubbers with NaOH assist in cleaning these gases before they're released into the atmosphere.
  2. Petrochemical Industry: For treating gaseous emissions containing acidic components in oil refining processes.
  3. Glass and Cement Manufacturing: Processes involving the melting of materials can produce acidic gases that need purification.
  4. Metallurgy: Some metallurgical processes result in the formation of acidic gases, especially when smelting sulfur-rich metals.
  5. Wastewater Treatment: NaOH scrubber system can be used for odor control by removing acidic gases from air streams.
  6. Maritime Industry: With recent stringent international standards on SOx emissions for ships, scrubbers have become a popular solution for vessels to comply with these regulations.
  7. Pulp and Paper Production: Acidic gases can be produced in boiling and bleaching processes, necessitating cleaning.
These systems provide an effective solution for numerous industrial entities required to reduce their atmospheric emissions of acidic gases in line with environmental standards and regulations.

TORNADO-SP Packed Bed Scrubber
TORNADO-SP Packed Bed Scrubber

Guidelines for Scrubber Selection

The selection of a NaOH scrubber depends on several key parameters:
  • Defining the gas stream specifications: Consider the volume and concentration of acidic gases.
  • Choice of scrubber type: Consider packed or spray scrubbers.
  • Scrubber material: Suitable materials include stainless steel, plastic, and fiberglass.
  • Recirculation system: Some models use recirculation to enhance efficiency.
  • Monitoring and control system: Necessary for optimizing operation.

Factors Influencing NaOH Consumption

How does a TORNADO-ST work
The consumption of NaOH in a scrubber depends on several factors:
  • Concentration of the Acidic Gas: The higher the concentration of acidic gases like SO₂, HCl, or CO₂ in the stream, the more NaOH will be consumed.
  • Flow Rate of the Gas Stream: A higher flow rate can lead to increased consumption if the contact time between the gas and the scrubbing solution is insufficient.
  • Concentration of the NaOH Solution: A higher concentration can absorb more acidic gases, but there's a limit to its efficiency due to potential saturation.
  • Efficiency of the Scrubber Design: Some designs enhance contact between the gas and liquid, leading to more effective absorption and potentially lower NaOH consumption.
  • Desired Removal Efficiency: If a very high removal efficiency is desired, more NaOH may be consumed to achieve those results.
  • Temperature and Pressure: These can affect the solubility of the gases and the efficiency of the scrubbing process.
  • Presence of Other Contaminants: Other contaminants might react with NaOH, influencing its consumption rate.
In an industrial setting, it's crucial to monitor the NaOH consumption rate and adjust accordingly to maintain the desired removal efficiency. Periodic analysis of the scrubbing solution can give insights into the rate of NaOH consumption and the need for replenishment or regeneration.

NaOH Scrubber Design Essentials

The design of an NaOH scrubber involves specific considerations to ensure effective removal of acidic gases. Here's a brief overview of the NaOH scrubber design:
1. Tower Type: The most common designs are packed towers and spray towers.
• Packed Tower: Utilizes packing materials to increase the surface area for the gas-liquid interaction. The NaOH solution trickles down, and the gas passes upward, facilitating absorption.
• Spray Tower: The NaOH solution is sprayed into the tower where it interacts with the gas stream.
2. Packing Material: In packed towers, packing can be of various shapes like saddles, rings, or structured packing. The purpose is to enhance the gas-liquid contact area.
3. Gas & Liquid Distributors: They ensure uniform flow of gases and the NaOH solution throughout the scrubber, optimizing absorption.
4. Recirculation System: Most scrubbers have a system to recirculate the NaOH solution, maintaining a consistent concentration and improving the scrubbing efficiency.
5. Mist Eliminator: Positioned at the top of the scrubber, it prevents droplets of NaOH solution from exiting with the cleaned gas stream.
6. Material Selection: The caustic scrubber NaOH should be constructed from materials that are resistant to its caustic nature as well as the acidic gases it removes. Common materials include stainless steel, certain plastics, and fiberglass.
7. Monitoring and Control System: To monitor parameters like pH, temperature, and flow rates, ensuring the scrubber operates efficiently and safely.
8. Inlet & Outlet Design: Proper design ensures smooth flow and prevents issues like flooding (liquid accumulation).
9. Reservoir: At the scrubber's base, it holds the NaOH solution, which can be circulated back to the top or drained if it becomes too saturated.
10. Safety Measures: Venting, overflow mechanisms, and emergency shut-offs are essential components to handle any potential issues.
When designing a NaOH wet scrubber, considerations such as the specific gases to be removed, their concentration, desired removal efficiency, and operational conditions must be taken into account. Proper maintenance and periodic checks are also crucial for optimal and safe operation.

SOx Emission Control Explained

SOx scrubber refers to a purification system designed to remove sulfur oxides (SOx), mainly sulfur dioxide (SO₂) and, to a lesser extent, sulfur trioxide (SO₃), from gas streams such as flue gases. Sulfur oxides are major pollutants, especially in emissions from ships and certain industrial facilities.

When SOx scrubber uses sodium hydroxide (NaOH) or caustic soda as the neutralizing agent, it can be referred to as an "NaOH SOx scrubber". The operation of such a system involves:
  1. Flue gases passing through a NaOH solution.
  2. The SO₂ in the gas reacts with NaOH, forming sodium sulfite (Na₂SO₃) and water.
  3. The NaOH solution is regularly replaced or regenerated to maintain its efficiency in absorbing SOx.
Such a system significantly reduces SOx emissions into the atmosphere, helping industries meet environmental standards and regulations.

Elevate Your Emission Control with Torch-Air

At Torch-Air, we pride ourselves on delivering both standard and bespoke wet scrubber solutions, tailored specifically to the unique needs of your enterprise. Our expertise allows you to save on complex projects and supplementary equipment.

Interested in enhancing your production site's air purification system? Contact us today. We will promptly assess and provide you with both the manufacturing timeline and the cost estimate for a wet scrubber that best suits your needs. Trust Torch-Air to elevate your emission control standards.
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Vladimir Nikulin
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