Removing hydrogen sulfide (H₂S) from water using 1,3,5-triazine, commonly referred to as monoethanolamine triazine (MEA Triazine) or hexahydro-1,3,5-tris(hydroxyethyl)-s-triazine, is a chemical scavenging process widely employed in water treatment, particularly in oil and gas, wastewater, and biogas applications, to eliminate H₂S-related odor, toxicity, and corrosiveness. MEA Triazine reacts with H₂S to form water-soluble dithiazine, a non-volatile byproduct. Below is a detailed explanation of the process, including stoichiometry, reaction kinetics, typical treatment methods, treatment rates, and other relevant considerations, formatted to match the provided structure.

Chemical Reaction and Stoichiometry

MEA Triazine, a heterocyclic compound derived from monoethanolamine and formaldehyde, reacts with H₂S in water to form dithiazine, a water-soluble byproduct, and other minor products. The primary reaction is:

Reaction 1: Formation of Dithiazine

C₉H₂₁N₃O₃ (MEA Triazine) + 2H₂S → C₉H₂₁N₃O₃S₂ (dithiazine) + other products

  • Stoichiometry: 1 mole of MEA Triazine reacts with 2 moles of H₂S.

  • Molar masses:

    • H₂S: 34.08 g/mol

    • MEA Triazine (C₉H₂₁N₃O₃, ~80% active solution): ~219.28 g/mol

  • Mass ratio: ~1:3.22 (1 g of H₂S requires ~3.22 g of MEA Triazine; 68.16 g H₂S : 219.28 g C₉H₂₁N₃O₃ ÷ 2).

  • Conditions: Effective at neutral to slightly alkaline pH (pH 6–9) and temperatures of 10–70°C, with no activation required.

Key Stoichiometric Considerations:

  • The MEA Triazine dose depends on H₂S concentration and desired removal efficiency (typically 40–80% depending on the method).

  • An excess of MEA Triazine (1.2–1.5 times stoichiometric) is often used to ensure complete H₂S scavenging and account for side reactions or inefficiencies in mass transfer.

  • Example: For 1 mg/L H₂S (0.0294 mmol/L):

    • ~3.22 mg/L MEA Triazine (stoichiometric).

    • Practical dose: ~3.86–4.83 mg/L MEA Triazine. This value is then adjusted for product activity.

Reaction Kinetics

The kinetics of H₂S scavenging by MEA Triazine are influenced by:

  • pH: The reaction is fastest at pH 6–9, where H₂S and HS⁻ are present and react readily with triazine’s active sites. At pH < 6, H₂S is less dissociated, slowing the reaction slightly; at pH > 9, efficiency may decrease due to side reactions.

  • Temperature: Higher temperatures (27–49°C) increase reaction rates, following Arrhenius behavior, with minimal impact on triazine stability within typical ranges (10–70°C).

  • MEA Triazine Concentration: Higher concentrations accelerate the reaction, but over-dosing can lead to unreacted triazine, increasing costs and environmental risks.

  • Mass Transfer: In direct injection, reaction speed is limited by H₂S dissolution; contactor towers enhance mass transfer, reducing contact time.

  • Rate Law: The reaction is generally first-order with respect to H₂S and MEA Triazine:

    • Rate = k[H₂S][C₉H₂₁N₃O₃]

    • Typical k values: 100–1000 M⁻¹s⁻¹ at pH 7 and 25°C, indicating a very fast reaction.

  • Reaction Time: Scavenging is rapid, completing within 1–5 minutes via contactor vessels and 15–20 seconds via direct inejction.

Practical Considerations:

  • Dithiazine is water-soluble, avoiding turbidity, but can form solids at high H₂S loads or over-dosing, requiring system maintenance.

  • Contactor towers achieve higher efficiency (~80%) than direct injection (~40%), reducing contact time and chemical use.

  • Automated dosing systems with H₂S sensors optimize efficiency and minimize solids formation.

Typical Treatment Methods

MEA Triazine is used in wastewater treatment, oil and gas produced water, and biogas scrubbing for H₂S removal. Common methods include:

a. Direct Injection

  • Process: MEA Triazine (20–80% active solution) is injected into water via atomizers or fog nozzles, typically in pipelines or tanks.

  • Conditions: pH 6–9, dosed at 1.2–1.5 times stoichiometric requirement, achieving ~40% efficiency.

  • Advantages: Simple, low capital cost, effective for low H₂S levels (<1000 lbs/day or ~0.1–20 mg/L).

  • Challenges: Limited by mass transfer, requiring higher doses; potential for solids formation with over-dosing.

b. Contactor Towers

  • Process: Gas is bubbled through a column containing MEA Triazine solution, enhancing contact and achieving ~80% efficiency.

  • Conditions: pH 6–9, retention time of 1–5 minutes, suitable for higher H₂S loads (5–50 mg/L).

  • Advantages: Higher efficiency, lower chemical use, effective for continuous systems.

  • Challenges: Higher capital cost, maintenance required for solids buildup.

c. Combined Systems

  • Process: MEA Triazine treatment is paired with filtration or separation to manage dithiazine solids or unreacted triazine.

  • Example: Direct injection followed by settling tanks or filtration to remove precipitated dithiazine in high-H₂S systems.

Typical Treatment Rates

  • H₂S Concentrations: Effective for 0.1–50 mg/L H₂S (municipal wastewater: 0.1–5 mg/L; industrial/produced water: 5–50 mg/L), reducing H₂S to <4 ppm.

  • MEA Triazine Dosage:

    • For dithiazine: 0.5–1.5 g MEA Triazine per g H₂S.

    • Practical dosing: 3–7 mg/L for low H₂S (0.1–1 mg/L); 50–150 mg/L for high H₂S (10–50 mg/L).

  • Contact Time: 15–20 seconds (contactors); 1–5 minutes (direct injection).

  • Flow Rates: Systems handle 10–20,000 m³/day, from small wastewater streams to large produced water systems.

  • pH Adjustment: Typically not required, as pH 6–9 is optimal; post-treatment acid (e.g., HCl, 10–50 mg/L) may adjust pH to 6–9 for discharge if needed.

  • Residual Triazine: Post-treatment levels should be minimized (<0.1 mg/L) to avoid aquatic toxicity, often requiring dilution or treatment.

Practical Considerations and Challenges

  • Byproducts:

    • Dithiazine is water-soluble but can precipitate as solids at high H₂S concentrations or over-dosing, causing fouling in pipelines or contactors.

    • Unreacted triazine is toxic to aquatic life, requiring careful dosing and monitoring.

  • MEA Triazine Stability: Stable under typical conditions (10–70°C, pH 6–9), but degrades at extreme pH or high temperatures. Solutions are stored in sealed containers to prevent evaporation or contamination.

  • Cost: MEA Triazine is moderately expensive (~$3–4/kg for 80% solutions), but low dosage requirements and high efficiency in contactor towers reduce overall costs.

  • Monitoring: H₂S, residual triazine, and dithiazine levels are tracked using colorimetric tests, gas chromatography, or online H₂S sensors.

  • Safety: MEA Triazine is non-hazardous but mildly irritating; handling requires standard protective equipment. Dithiazine solids require proper disposal to avoid environmental contamination.

Comparison with Hydrogen Peroxide

  • MEA Triazine Advantages: Very fast reaction, low dosage requirement (0.5–1.5 g/g H₂S), high efficiency in contactor towers (~80%), versatile for wastewater and produced water.

  • MEA Triazine Disadvantages: Non-regenerative, produces dithiazine solids requiring management, unreacted triazine is toxic, potential for pH elevation and scaling.

  • H₂O₂ Advantages: Environmentally benign, no toxic byproducts, simpler reaction control, decomposes to water and oxygen.

  • H₂O₂ Disadvantages: Slower reaction at low pH, higher cost for high H₂S levels, elemental sulfur causes turbidity requiring filtration.

Example Calculation

Scenario: Treat 1,000 m³/day of wastewater with 5 mg/L H₂S, targeting dithiazine formation.

  • H₂S mass: 5 mg/L × 1,000 m³ × 1,000 L/m³ = 5,000 g/day H₂S.

  • MEA Triazine requirement: 3.22:1 mass ratio → 5,000 g × 3.22 = 16,100 g/day MEA Triazine (stoichiometric).

  • Practical dose: 1.5× stoichiometric = 24,150 g/day MEA Triazine.

  • MEA Triazine solution: Using 80% w/w MEA Triazine (density ~1.1 g/mL):

    • Mass of solution: 24,150 g ÷ 0.80 = 30,188 g/day.

    • Volume: 30,188 g ÷ 1,100 g/L ≈ 27.4 L/day.

  • Cost estimate: At ~$3.5/kg for 80% MEA Triazine, cost ≈ $105.66/day; optimized dosing (e.g., contactor towers) reduces to ~$80–90/day (excluding equipment or solids management).

Additional Notes

  • Regulatory Limits: Treated water must meet discharge standards (e.g., H₂S < 0.1 mg/L, pH 6–9, residual triazine < 0.1 mg/L). Dithiazine and unreacted triazine may require additional treatment or dilution for sensitive ecosystems.

  • Scale-Up: Pilot testing is recommended for large systems to optimize dosing, contact time, and solids management, particularly in high-H₂S applications.

  • Environmental Impact: Unreacted triazine and dithiazine solids pose aquatic toxicity risks, requiring careful management. Unlike oxidants, MEA Triazine does not produce elemental sulfur or chlorinated byproducts, simplifying some aspects of treatment.