Introduction

Hydrogen sulfide (H2S) is a highly toxic, corrosive, flammable, colorless gas with a characteristic rotten egg odor. It is generated naturally by the anaerobic decomposition of organic matter and also in industrial processes, such as petroleum refining, papermaking, wastewater treatment and natural gas production.

Due to its corrosive and hazardous properties, its elimination is a priority in various industries and its control is of great importance for operational safety in chemical processes and environmental protection. To mitigate its effects, H2S scavengers that react with this gas are used to neutralize or remove it from the system.

In this article, we outline the properties and performance of H2S scavengers, the most common types and their applications in wastewater treatment and corrosive gas control.

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What are H2S scavengers and what are they used for?

They are compounds designed to capture and neutralize hydrogen sulfide in different industrial processes. Their effectiveness depends on several factors, such as the pH of the medium, the temperature and the concentration of H2S present.

Its application is essential to:

Reduce the toxicity of the working environment.
Prevent corrosion in equipment and piping.
Comply with environmental and safety regulations.
Improve the quality of the final product in industrial processes.

How do sequestrants work in the control of H2S?

The mechanism of action of these products is based on oxidation or precipitation of sulfide into chemically inert forms. For example, some sequestrants transform H2S into insoluble sulfates or sulfides that can be easily removed from the medium in which they are found.

The following two sequestrants are used for controls

Chemical sequestrants: These compounds react with H2S to form non-volatile or insoluble products. Their main mechanisms of action include:
- Oxidation: Conversion of H2S to elemental sulfur or sulfates using oxidizing agents such as sodium hypochlorite or peroxides.
- Precipitation: Formation of insoluble compounds, such as metal sulfides (e.g., iron sulfide upon reaction with ferric salts).
- Conversion to stable organic compounds: Use of triazines and modified amines that react with H2S to generate less toxic and stable products.
Physical sequestrants: Physical processes capture H2S without altering its chemical structure. Prominent among them are:
- Absorption in liquid solutions: use of amines and alkaline solutions (sodium hydroxide or potassium carbonate) to dissolve H2S.
- Adsorption in porous solids: Use of activated carbon, zeolites or materials impregnated with metal oxides to retain the gas.
- Membrane separation: Use of selective membranes to filter and remove H2S from gaseous or liquid streams.

Types of H2S scavengers used in industry

Depending on the chemical and industrial process and the concentration of H2S, different scavengers are used such as:

Triazines: Organic compounds widely used in the oil industry to neutralize H2S in the liquid and gaseous hydrocarbon industries.
Ferric compounds: Iron (III) salts that precipitate H2S forming iron sulfide, common in water treatment plants.
Regenerable amines: Such as monoethanolamine (MEA) and diethanolamine (DEA), used in refineries and gas plants to absorb and release H2S by thermal regeneration.
Alkaline solutions: Sodium hydroxide or potassium carbonate, used in gas scrubbing processes to neutralize acids.
Solid adsorbents: Materials such as zinc oxides, activated carbon and impregnated alumina are used in the removal of H2S in gas streams.

In which industrial sectors is H2S control most critical?

This control is fundamental in various industries to mitigate odors, reduce corrosion in equipment and improve safety in industrial processes. Although its management is a priority in any sector, its toxicity and corrosive effects make certain industries require stricter control:

Oil and gas industry: In exploration, production and refining, H2S present in crude oils, derivatives and corrosive gases, must be eliminated to avoid damage to equipment and occupational hazards. For its removal, sequestrants such as triazines, iron compounds (such as ferric oxide and iron sulfate) and amine-based alkaline solutions (MEA, DEA) are used.
Wastewater treatment: The decomposition of organic matter under anaerobic conditions generates high levels of H2S, causing bad odors and corrosion in infrastructures. Nitrates, which inhibit the bacterial activity responsible for the production of H2S, are used for its neutralization, in addition to impregnated activated carbon and ferric compounds such as ferric chloride and iron sulfate.
Paper industry: In the Kraft pulp production process, sulfur compounds are released that must be eliminated to reduce environmental impacts. Sequestering agents such as sodium hypochlorite, peroxides and iron-based reagents are used, which react with H2S to form non-volatile compounds.
Chemical and petrochemical plants: In synthesis and refining processes, H2S is a common by-product, requiring its removal to avoid contamination and corrosion. Amine solutions (such as monoethanolamine, MEA), alkaline hydroxides and transition metal-based scavengers, such as zinc oxide and iron oxide, are used.
Mining and metallurgy: In the extraction and processing of sulfide ores, H2S can be released, affecting the safety and performance of equipment. For its control, oxidation reagents such as hydrogen peroxide, potassium permanganate and ferric solutions are used, which transform H2S into stable and non-toxic compounds.

Each industry applies specific strategies according to its operational needs, ensuring efficient control of H2S to reduce environmental impacts and improve safety.

What are the criteria for the selection of the sequestrant?

It depends on several technical and operational criteria, which ensure its efficiency and compatibility with industrial processes. The main factors to consider are:

Nature of the fluid to be treated

Gases: liquid sequestrants such as triazines, amines and alkaline hydroxide solutions, which react with H2S to form stable compounds, are used.
Liquids (water, crude oil, wastewater): soluble sequestrants such as ferric salts, peroxides and nitrate compounds are used.
Solids (soils, sediments): Adsorbent materials such as impregnated activated carbon or iron compounds are applied.

Concentración de H2S en el medio

Low concentrations (<100 ppm): Use of nitrate-based sequestrants or adsorbents such as activated carbon. Moderate concentrations (100-1000 ppm): Triazines, iron salts and peroxides. High
Concentrations (>1000 ppm): Amines, chemical adsorption processes (MEA, DEA) or advanced technologies such as the Claus process for sulfur recovery.

Operating conditions

pH: Some sequestrants require alkaline conditions to be effective, such as sodium hydroxide or monoethanolamine (MEA).
Temperature: The chemical reactions of the scavengers can be accelerated or inhibited depending on the process temperature.
Pressure: In high pressure systems, non-volatile sequestrants or those that do not generate unwanted by-products are preferred.

Compatibility with the industrial process

Avoidance of side reactions: Some sequestrants can generate corrosive or undesirable by-products.
Compatibility with materials: Some reagents can affect the integrity of piping, valves and equipment.
Impact on process efficiency: It should be evaluated whether the sequestrant interferes with hydrocarbon production or refining, water treatment or chemical processes.

Ease of application and handling

Market availability: Some sequestrants may be expensive or difficult to obtain.
Dosage: Products of easy handling and control are preferred, such as liquid solutions in direct injection.
Safety in handling: Toxicity and risks associated with the use of the sequestrant should be considered.

Environmental impact and regulations

Biodegradability: Some sequestrants generate less polluting residues.
Local and international regulations: Certain compounds are restricted in some industries because of their environmental effects.
Waste disposal: Safe disposal methods for the resulting products should be considered.

Conclusion

H2S scavengers play a key role in the control of hydrogen sulfides in various industries. Their application in wastewater treatment, sulfide neutralization in industrial processes, and removal of sulfide compounds helps to improve safety, reduce corrosion, and minimize environmental impacts. Proper selection and use of these compounds is essential to ensure their effectiveness and long-term sustainability.