H₂S Scavenger (MEA-Triazine) — Engineering Fundamentals

1. Reaction chemistry

MEA-triazine — 1,3,5-tri-(2-hydroxyethyl)-hexahydro-s-triazine — is the dominant commercial chemistry (Baker Hughes Sulfa-Clear, Multi-Chem MX-200, others). The molecule has three N atoms in a six-membered ring, each carrying a 2-hydroxyethyl pendant. The ring nitrogens are the active sites:

Mechanism: the first H₂S protonates a ring nitrogen, opens the ring, and substitutes a sulfur for a nitrogen. The second H₂S repeats on the remaining ring N, leaving a stable di-thiazine (1,3,5-thiadiazinane) and one mole of free MEA. The third theoretical N is essentially inactive at neutral pH — kinetics dictate stoichiometry of 2 mol H₂S per mol triazine, not 3.

Mass basis: MW(triazine) = 219 g/mol, MW(H₂S) = 34 g/mol → 219 / (2·34) = 3.21 kg triazine per kg H₂S at the 2:1 field stoichiometry. The calc uses 3.21 kg/kg (the field-accepted 2-mol-H₂S-per-mol-triazine basis above) divided by user-specified efficiency (50–75 % typical contact-tower performance). The theoretical 3:1 maximum (2.14 kg/kg) is never achieved in the field and using it under-doses ~33 %.

2. Scavenger vs amine — when to switch

Rule of thumb: at 50 lb H₂S/day or below (≈ 100 ppmv × 5.6 MMscfd), triazine wins on lifecycle cost. Above 200 lb/day, an amine system is almost always lower TCO. The transition zone 50–200 lb/day requires a project-specific NPV — the calc shows annual product cost so you can compare against an amine quote.

3. Contact tower vs in-line dosing

Two installation modes are common:

• Contact tower (preferred): vertical bubble or packed tower with liquid hold-up, 2–10 min gas residence, achievable efficiency 60–75 %. The tower allows steady inventory of fresh triazine and a once-per-week (typical) batch turnover. This is what the calc sizes for.
• In-line direct injection: pump injects triazine into a flowing gas pipeline, mass-transfer driven by line turbulence. Efficiency drops to 30–50 % because contact time is seconds, not minutes. Useful only at very low H₂S loads < 10 ppm.

A typical contact tower has L/D ≈ 4–5, holds 50–80 % of one day's product as inventory, and is replaced (or batch swapped) when triazine active is below 30 % of original. Vendors sell on a "gallons consumed per ppm removed" basis — guarantee a contractual outlet H₂S in exchange for buying their product.

4. Spent product handling

Spent triazine is a strong-smelling, dark liquid containing the dithiazine reaction product, residual MEA, and water. It is classified as a non-hazardous industrial waste in most jurisdictions but is typically routed to a permitted disposal well (Class II in Texas, EPA UIC programme). Some plants neutralize and send to wastewater treatment, though the strong odor (mercaptan-like) often triggers complaints. Triazine is not regenerable — heat will not drive the reaction backward at any practical condition.

A key process risk is over-treating — running too much triazine causes a side reaction with H₂S in excess that forms gummy polymeric dithiane solids. These can plug downstream filters, foul cooler tubes, and accumulate at the bottom of the contact tower. Keep the operating active fraction above 30 % to avoid the polymerization regime.

5. References

• Bakke, J.M.; Buhaug, J.; Riha, J. (1996). "H₂S removal with triazine." NACE Corrosion 96, Paper 415. (triazine chemistry / 2:1 stoichiometry)
• Taylor, G.N.; Garcia-Lopez, R. (2018). "Scavenger options for natural gas operations." Hyd. Proc. 97(3).
• Baker Hughes Sulfa-Clear product literature; Multi-Chem (Halliburton) MX-200.
• GPSA Engineering Data Book, §21 — Acid Gas Treating & Trace Sulfur.