Aqueous monoethanolamine (MEA) for natural gas & refinery sweetening is managed through circulation-rate-based dosage and corrosion control rather than fixed ppm addition. The active concentration in carbon steel absorbers and regenerators is maintained at 15–20 wt% MEA. Monoethanolamine has a molecular weight of 61.08 g/mol and reacts with H₂S to form bisulfide and sulfide species, while CO₂ reacts through a carbamate-forming pathway with a theoretical uptake of 0.5 mol CO₂/mol MEA. Practical rich acid gas loading is limited to 0.30–0.35 mol total acid gas/mol MEA because higher loadings lower the solvent pH and increase carbon steel corrosion rates. The required circulation rate is calculated from:

L = q_AG / (C_amine × Δα)

where q_AG is acid gas removal rate in kmol/h, C_amine is active MEA concentration in kmol/m³, and Δα is the net rich-lean loading differential in mol acid gas/mol MEA. For a 100 MMscfd gas stream containing 6.0 mol% total acid gas and a net loading differential of 0.25 mol/mol, the removal duty is approximately 15,810 lbmol/day, requiring roughly 3.86 million lb/day of pure MEA. This corresponds to 19.3 million lb/day of 20 wt% solution, or about 1,600 gal/min at 25°C. The actual pump size is then corrected for absorber pressure, lean amine temperature, and foaming tendency. Centrifugal pumps with stainless steel impellers and carbon steel casings are commonly selected, with mechanical seals specified for lean/rich differential service.

What Limits Rich Amine Loading in Carbon Steel Circuits?

Rich amine loading is the dominant process variable controlling general and localized corrosion in MEA systems. At loadings above 0.35 mol acid gas/mol MEA, the solution pH drops and the concentration of acidic bicarbonate and bisulfide species increases. Carbon steel may form a semi-protective iron sulfide or iron carbonate film, but film stability is compromised by high wall shear, particulates, heat-stable salt contamination, and oxygen ingress. Published corrosion coupon evaluations in MEA service show general corrosion rates below 0.1 mm/y when loading, temperature, and heat-stable salt concentration are held within design limits. Upset conditions with oxygen ingress or reboiler skin-temperature excursions produce localized rates exceeding 0.25 mm/y at flow-accelerated corrosion zones.

Materials selection in sour gas and refinery MEA service follows ISO 15156-2 for sulfide stress cracking and API RP 945 for amine stress corrosion cracking. Carbon steel weldments in lean and rich amine circuits require post-weld heat treatment and hardness control at ≤200 HB or ≤22 HRC to reduce amine stress corrosion cracking susceptibility. Rich loading limits are used with continuous acid gas analysis and daily solvent titration to prevent operating drift outside the 0.30–0.35 mol/mol envelope for carbon steel.

Refinery fuel gas systems with variable H₂S/CO₂ ratios and intermittent hydrotreater purge gas require MEA circulation to be trimmed against acid gas composition rather than total gas flow alone. The MEA dosage response uses continuous online gas analyzers and a ratio-controlled lean amine flow loop. In such service, lean loading is maintained between 0.05 and 0.15 mol/mol to avoid excessive regeneration energy and to keep reboiler duty within the design curve. Refinery MEA units that are not selective for H₂S may produce acid gas below 50 mol% H₂S when inlet CO₂/H₂S ratios exceed 3:1. This can require oxygen enrichment in the Claus furnace or a selective MDEA swing unit. Published data for this specific configuration is limited.

MEA has a higher vapor pressure than secondary or tertiary alkanolamines, so absorber overhead water wash stages are required to reduce amine carryover. The lean solvent temperature is maintained 5–10°C above the absorber gas temperature to avoid hydrocarbon condensation and foaming. Filtration of the circulating solvent through mechanical filters and activated carbon beds controls particulate iron sulfides and hydrocarbon impurities that otherwise accumulate at the rich/lean exchanger and reboiler tubes.

Reboiler Tube Wall Loss and Heat Flux Thresholds

Corrosion rates in MEA reboilers are controlled by limiting steam-side temperature, tube wall temperature, and heat flux. Bulk solvent temperature in MEA regeneration towers is commonly held at 120–127°C. Tube skin temperature can exceed bulk temperature by 20–30°C when steam pressure is not controlled. Reboiler tube wall loss is highest at the hot tube outlet where acid gas breakout occurs; vapor blanketing and local dry-out accelerate carbon steel attack. MEA system design practice sets a maximum tube wall heat flux of 30–40 kW/m² for carbon steel bundles. When steam cannot be maintained below 150°C skin temperature, 316L stainless steel reboiler tubes are specified.

Rich amine velocity in carbon steel piping is maintained above 1 m/s to prevent solids deposition and below 2.1 m/s to limit flow-accelerated corrosion. Elbow and pump suction velocities are reviewed by ultrasonic thickness measurement at intervals established under API 510 pressure vessel inspection requirements. Reboiler return lines are a high-priority monitoring location because they combine high temperature, acid gas breakout, and turbulent two-phase flow.

When a Thermal Reclaimer Is Required Instead of Solely HSS Neutralization

MEA oxidation and thermal degradation generate heat-stable salts including formate, acetate, oxalate, glycolate, sulfate, and chloride anions. These salts cannot be regenerated by steam and are measured by ion chromatography. Total heat-stable salt concentration above 1.0 wt% of circulating solution is associated with increased corrosion in hot lean and rich/lean exchanger circuits. Caustic addition can neutralize some heat-stable salts but produces non-regenerable sodium salts and may precipitate solids that foul reboiler tubes. In MEA systems with high heat-stable salt accumulation, a side-stream thermal reclaimer is operated at 1–3% of circulation. The reclaimer is a kettle or forced-circulation evaporator, vacuum-operated to keep bottoms temperature below 150°C. 316L stainless steel internals are specified for the reclaimer, recovered amine cooler, and reclaimer overhead piping. Reclaimer sludge contains amine degradation polymers, iron sulfides, and inorganic salts; it is drained to a closed disposal system on a batch schedule.

Corrosion control programs for MEA units combine process analytical instruments with weight-loss coupons and electrical resistance probes. Coupon holders are installed in rich amine, lean amine, reboiler return, and reclaimer overhead lines. Coupon evaluation follows ASTM G1-03, while probe data are recorded at 1 h intervals and correlated with pH, dissolved iron, heat-stable salt concentration, and acid gas loading. Dissolved iron in lean MEA is maintained below 10 mg/L as an early indicator of system corrosion.

Compliance documentation for MEA sweetening units requires alignment with sour service materials selection, corrosion monitoring, and inspection standards. The following matrix summarizes the applicable standard designations and record types used during mechanical integrity reviews.