MEA Triazine H2S Scavenger: Reaction Chemistry Explained

MEA Triazine — formally named hexahydro-1,3,5-tris(2-hydroxyethyl)-s-triazine (CAS 4719-04-4) — is synthesised by the condensation reaction of monoethanolamine (MEA) with formaldehyde. Three moles of each reactant combine to form one mole of the triazine ring compound, releasing three moles of water: 3 HOCH2CH2NH2 + 3 CH2O yields C9H21N3O3 + 3 H2O The reaction is carried out in aqueous solution at controlled temperature. The resulting product at 78% active concentration contains the triazine dissolved in water, with small amounts of residual monoethanolamine and formaldehyde. The 78% figure refers to the weight percentage of the active hexahydrotriazine compound in the final solution. The molecular weight of the active triazine is approximately 219.3 g/mol. At 78% w/w and a solution density of approximately 1.08 g/mL, one litre of MEA Triazine 78% contains about 842 g of active triazine, corresponding to approximately 3.84 millimoles. This high molar concentration is what makes 78% the industry-standard concentration for oil and gas H2S scavenging — it delivers the maximum number of reactive moles per litre of product.

When MEA Triazine contacts hydrogen sulfide, the triazine ring undergoes nucleophilic attack by the bisulfide ion (HS-). The sulfur atom replaces nitrogen in the ring structure, and the reaction proceeds through intermediate thiadiazine species to form dithiazine as the primary product. The overall simplified reaction can be written as: one mole of hexahydro-1,3,5-tris(2-hydroxyethyl)-s-triazine reacts with up to three moles of H2S to produce dithiazine and trithiane ring compounds, while releasing monoethanolamine. The reaction is irreversible under normal operating conditions — once H2S has reacted with the triazine, it is permanently bound in the dithiazine structure and cannot be released back into the gas or liquid phase. This irreversibility is a key advantage of triazine scavengers over regenerable amine systems, where H2S can be released during upsets. The primary reaction product, dithiazine, is water-soluble at typical produced water temperatures and can be disposed of with the aqueous waste stream. Under conditions of high loading or low temperature, dithiazine can exceed its solubility limit and precipitate — this is the root cause of the solids formation problems discussed in our separate technical guide.

The theoretical stoichiometry is 1 mole of triazine per 3 moles of H2S. Working through the molecular weights: 219 g of triazine reacts with 102 g of H2S (3 times 34 g/mol). This means 1 kg of pure triazine can theoretically remove 0.465 kg of H2S. At 78% active content, 1 kg of product contains 0.78 kg of active triazine, capable of removing up to 0.363 kg of H2S. Given the product density of approximately 1.08 kg/L, one litre of MEA Triazine 78% can theoretically remove up to 0.39 kg of H2S — or equivalently, the theoretical minimum consumption is about 2.6 litres per kilogram of H2S. Why does the active concentration matter? A product at 50% active content, for example, contains only 0.50 kg of triazine per kilogram of product — 36% less active material than the 78% product. This means you need proportionally more volume to achieve the same scavenging effect, increasing transport costs, storage requirements, and the volume of liquid injected into your process. For oil and gas applications where H2S loads are significant, the 78% concentration is the clear economic choice. Lower-concentration products (such as MMA Triazine 40%) have their place in applications like water treatment where lower dosing rates are required and the smaller active content per litre makes precise low-rate dosing easier.

The triazine-H2S reaction rate is influenced by both temperature and pH: Temperature: The reaction follows Arrhenius kinetics — the rate approximately doubles for every 10 degree Celsius increase in temperature within the normal operating range of 5-80 degrees Celsius. At typical oilfield temperatures (20-60 degrees Celsius), the reaction is fast, typically reaching completion within seconds to minutes in well-mixed systems. Below 10 degrees Celsius, the reaction slows significantly, and longer contact times or higher excess factors may be required. Above 80 degrees Celsius, the triazine itself can begin to undergo thermal hydrolysis, releasing formaldehyde and reducing its scavenging capacity. pH: The active scavenging species is the bisulfide ion (HS-), which is the predominant form of dissolved H2S at pH values above approximately 7. At low pH (below 6), most dissolved sulfide exists as molecular H2S rather than HS-, and the reaction rate with triazine decreases. Triazine scavengers perform best in the pH range of 7-10. The reaction itself releases monoethanolamine, which is mildly basic, so the pH of the system tends to rise as scavenging proceeds. In acidic systems (pH below 6), the triazine ring can also undergo acid-catalysed hydrolysis, which degrades the scavenger before it can react with H2S. If your system pH is consistently below 6, pH adjustment or an alternative scavenger chemistry may be required. Understanding these kinetic factors allows operators to predict scavenger performance across different operating conditions and adjust dosing strategies accordingly. Our technical team can model expected scavenger consumption for your specific temperature, pH, and H2S profile.