Guide to Hydrogen Gas Scrubbers: Types, Applications, and Sodium Hydroxide Chemical Scrubbing

Author: Michael Klepik, Chief Executive Officer

Wet scrubbers are specialized, complex devices designed for industrial applications to purify gases from pollutants.
This article explores different types of hydrogen scrubbers and their applications in removing a wide range of production by-products. From operating principles to specific applications in various industries, we will delve into the details of how these devices function and their importance in modern industrial processes.

Hollow Spray Tower Systems
There are hollow filtration systems where special nozzles spray liquid to create droplets of the required size. The airflow interacts with these droplets, which must be large enough to avoid being carried away by the flow.

Packed Bed Sodium Hydroxide Wet Scrubbers
Another effective method for air purification is the use of a packed bed sodium hydroxide wet scrubber. The packing material acts as a filler within the unit's body and can consist of large, geometrically specific structures or bulk materials composed of smaller elements. The most common designs feature vertical, wide columns that are cylindrical or partially conical, with narrowings, widenings, and attached nozzles at the ends. The use of packing significantly increases the interaction area between the phases inside the device, thereby enhancing the removal efficiency of unwanted substances.

Horizontal Builds for Limited Ceiling Height
Additionally, different models of industrial wet scrubbers with sodium hydroxide solution vary in the positioning of the mass transfer column. When ceiling height is limited, horizontal builds may be preferred. Although they occupy more floor space, they require less vertical clearance. These air purifiers are widely employed for air purification in ventilation systems, local emissions in laboratories, and small-scale production facilities. In these devices, the gas flow moves horizontally through multiple sections or chambers, where it comes into contact with the liquid.

Vertical Builds for Compact Installations
In vertical builds, air is introduced from the bottom, and liquid spraying occurs countercurrently at the top of the column, followed by the free fall of droplets against the airflow. This design prolongs the contact time between the substances to be purified and the reagent, thereby enhancing the sodium hydroxide scrubber's efficiency. Vertical units have a relatively larger height but occupy less floor space, which is advantageous for compact installations. They are widely utilized in industrial processes for the removal of gaseous pollutants.

The combustion of fossil fuels, such as coal, oil, and natural gas, has recently become the primary source of fuel for transportation, high-temperature industrial processes, and power generation. Power plants alone, which run on fossil fuels, produce approximately 30% of all CO2 emissions worldwide. CO2 is one of the main greenhouse gases that trap heat in the atmosphere. While this natural phenomenon supports life on Earth, excessive emissions of carbon dioxide from human activities lead to an increase in the Earth's average temperature. This process, known as global warming, can have devastating consequences, such as more frequent extreme weather events, a steady rise in sea levels, and shifts in climate zones. Reducing carbon dioxide emissions is one of the top priorities in modern environmental efforts. One example is the carbon dioxide scrubber with sodium hydroxide solution.

Specialized sodium hydroxide carbon dioxide scrubbers are designed for industrial applications. They operate based on the principles described above. Within the device, the vapor stream to be purified is mixed with the solution, resulting in the chemical removal of carbon dioxide from the emissions. The reaction equations can be represented as follows:
2NaOH + CO₂ → Na₂CO₃ + H₂O (pH>12)
NaOH + CO₂ → NaHCO₃ (pH<12)

In modern aluminum production facilities, a hydrogen fluoride gas scrubber is utilized. The electrolysis processes involved employ fluorine-containing electrolytes, leading to the presence of HF in the production exhaust gases. A NaOH solution can be utilized to absorb HF, similar to the previous example; however, this process produces NaF, which has low solubility in water. To streamline equipment maintenance, the acid within the hydrogen fluoride scrubber is sometimes neutralized using a potassium hydroxide solution. Potassium fluoride dissolves well in water, effectively preventing deposit formation.
HF + NaOH → NaF + H₂O
HF + KOH → KF + H₂O

These devices are also commonly employed in:

• The steel and metallurgical industries, related to metal etching and cleaning processes;
• The glass and ceramics industries;
• The production of phosphate fertilizers.

Another hazardous byproduct of human activity is the emission of HCN (hydrogen cyanide), which is particularly prevalent in the production processes of acetonitrile, acrylonitrile, and other organic compounds. Cyanides are commonly utilized in the extraction of gold and other valuable metals from ores, resulting in the production of hydrogen cyanide as a side product. Municipal and industrial waste incineration processes also generate this substance.

The operational principle of a hydrogen gas scrubber designed to remove hydrogen cyanide is analogous to the models discussed earlier. HCN is effectively neutralized by an alkaline solution inside the mass transfer column:
HCN + NaOH → NaCN + H₂O

Hydrogen cyanide is usually removed using an aqueous solution of NaOH, but occasionally, sodium hypochlorite is added to the scrubbing solution inside the hydrogen cyanide scrubber. This approach can yield more favorable reaction products:
HCN + NaOH → NaCN + H₂O
2NaCN + H₂O + 5NaClO → 2NaHCO₃ + 5NaCl + N₂

Chlorine is another gaseous byproduct of industrial processes. The challenge of air purification from chlorine is particularly significant in the following sectors:

• Production of polyvinyl chloride, pesticides, and organic solvents;
• Paper manufacturing, where chlorine compounds serve as bleaching agents;
• Waste incineration, especially of certain types of plastics;
• Municipal water disinfection and wastewater treatment.

Chlorine is typically captured in a vertical packed column using a liquid-phase reagent. Various absorbents can be employed, including caustic soda, potassium hydroxide, sodium carbonate, calcium hydroxide, sodium sulfite, ferrous chloride, and hydrogen peroxide. Among these, caustic soda is the most cost-effective and thus commonly utilized in practice.

The reaction underlying this purification method is as follows:
Cl2 + 2NaOH → NaOCl + NaCl + H₂O

To remove 2.2 lbs of chlorine, 2.48 lbs of caustic soda are required, resulting in 2.31 lbs of sodium hypochlorite as the product. However, this process involves further reactions. The resulting salt can decompose in the following ways:
3NaOCl → NaClO₃ + 2NaCl
2NaOCl → 2NaCl + O₂

Sodium hypochlorite remains relatively stable in solution if the required alkalinity level is maintained.

A critical factor in the operation and design of chlorine scrubbers with sodium hydroxide is temperature. The reaction between alkali and impurities releases a significant amount of heat. At elevated temperatures, secondary reactions and the decomposition of products as described can occur.

In certain scenarios, chemical adsorbers utilizing solid adsorbents like activated carbon may be employed to capture chlorine from gaseous streams. This equipment is commonly employed for purifying dry gas mixtures with low levels of contaminants. However, the selection of the appropriate method must consider numerous factors.

If you are uncertain about which chemical scrubber with sodium hydroxide is suitable for your requirements, feel free to reach out to our engineers. They will address all your queries and carefully consider the specific details of your air purification process. At the Torch-Air hydrogen gas scrubber factory, cutting-edge air purification solutions are developed.