MEA CO2 Capture Solvent Support: Technical Pack and Trial Programme

Monoethanolamine (MEA, 2-aminoethanol, CAS 141-43-5) is a primary alkanolamine supplied as a clear liquid with a molecular weight of 61.08 g/mol, density near 1.016 g/cm³ at 20°C, and a boiling range centred on 170°C at 101.3 kPa. In post-combustion CO₂ capture, MEA is commonly diluted to 30 wt% in demineralised water. The technical pack and trial programme described here support evaluation of MEA as a CO₂ capture solvent by establishing feed quality limits, degradation monitoring, material compatibility boundaries, and pilot-plant data acquisition. This documentation is intended for process engineers and analytical chemists working with amine-based gas treating units; it does not replace site-specific hazard assessment or process safety review.

Which Fresh Solvent Quality Limits Apply to CO₂ Capture Service?

Fresh MEA quality for carbon capture is not determined solely by total amine content. A certificate of analysis should be accompanied by water content, colour, density, distillation range, and trace metal data. Table 1 lists acceptance indicators commonly used for commercial MEA intended for flue gas desulfurization and carbon capture. Supplier-specific limits may be tighter; the technical pack should include the exact lot certificate and analytical method. Water content influences heat duty because water is the primary stripping medium; high water content in fresh MEA concentrate shifts the final diluted solvent concentration and alters density correlations used for online concentration measurement.

The diluted working solvent should be analysed for total alkalinity and CO₂ loading. In the technical pack, the recommended procedure is titration with 0.1 N hydrochloric acid to pH 4.5, with CO₂ released by acidification and measured by gas volumetric or infrared methods. Online density measurement at 20°C or 40°C is calibrated against known MEA concentration, but the calibration must be updated when degradation products accumulate because organic acid salts and amine degradation species increase solution density and may bias the inferred MEA concentration.

Absorption of CO₂ into a 30 wt% MEA solution proceeds through predominant carbamate formation. The stoichiometry of primary amine carbamation limits the maximum theoretical loading to 0.5 mol CO₂/mol MEA under dry conditions, but aqueous bicarbonate formation allows practical loadings above 0.5 mol CO₂/mol MEA at high absorber CO₂ partial pressure. In a typical absorber at 40–50°C, a rich solvent loading of 0.45–0.50 mol CO₂/mol MEA is attainable when the flue gas contains 12–15 vol% CO₂. The reverse reaction in the stripper is thermally driven; a reboiler temperature of 110–125°C and stripper pressure of 1.5–2.0 bar absolute are often considered baseline operating coordinates. Published data for this specific configuration is limited, but the observed regeneration energy in pilot campaigns is commonly reported in the range 3.5–4.2 GJ/t CO₂ for 30 wt% MEA without advanced heat integration. The technical pack validates these boundaries with vapour-liquid equilibrium data for the specific solvent lot, not generic published curves.

Solvent Degradation Analytics and Technical Support Boundaries

Oxidative degradation in the absorber occurs when flue gas oxygen transfers into the amine phase. Typical post-combustion flue gas contains 3–8 vol% O₂; this produces carboxylic acids, ammonia, and heat-stable salts that buffer the solvent and increase viscosity and corrosion. Thermal degradation in the stripper is accelerated when the reboiler surface temperature exceeds approximately 125°C, with products including 2-oxazolidinone, N-(2-hydroxyethyl)ethylenediamine, and other condensation compounds. The technical pack includes a degradation-monitoring module that specifies: weekly anion chromatography for formate, acetate, oxalate, nitrate, and sulphate using ISO 10304-1 or equivalent; ammonia measurement in condensate by ion-selective electrode; total heat-stable salt determination by cation exchange and titration; and ultraviolet-visible colour trend at 450 nm to track degradation chromophores. The action threshold for total heat-stable salts in long-duration pilot campaigns is typically set at 1.0–1.5 wt% as formate equivalent, beyond which solvent reclamation or caustic addition should be evaluated.

Storage and transfer experience on manufacturing lines shows that MEA freezes at approximately 10.5°C. Tanks and transfer lines should be heat-traced and insulated, with storage maintained between 20°C and 35°C to avoid both freezing and excessive vapour headspace. Nitrogen blanketing is required to limit oxidative degradation and moisture uptake. A stainless steel tank in grade UNS S31603 or lined carbon steel with an inert internal coating is used for long-term storage; unprotected carbon steel is not recommended for aqueous MEA storage because of iron-catalysed degradation and corrosion. The technical pack includes a material compatibility statement prohibiting copper, copper alloys, zinc, galvanized surfaces, and aluminium in wetted parts. For gaskets and seals, EPDM or PTFE are commonly specified; nitrile rubber and natural rubber are not recommended. Vent condensers, pump seals, and level instrumentation must be rated for alkaline service and should be checked against the supplier's chemical resistance list.

Reclaiming of degraded MEA solvent is performed by thermal distillation under reduced pressure to separate high-boiling degradation products. In the technical pack, a short-path or wiped-film evaporator is specified for slipstream reclamation at 5–20 wt% of total inventory per week. The reclaimer bottom stream contains heat-stable salts and high-molecular-weight degradation products; its viscosity at 40°C can exceed 100 mPa·s, and it must be discharged through heated lines. The recovered MEA is returned to the lean solvent tank only after water content and carbonate concentration are adjusted. Published data for this specific configuration is limited, but industrial gas-treating practice indicates that reclamation reduces heat-stable salt concentration and restores absorption capacity when degradation products accumulate above 1.0 wt%.

Controlling Solvent Loss and Corrosion During Continuous Capture Trials

MEA volatility is low relative to ammonia but not negligible. At stripper overhead conditions near 95–105°C, MEA and degradation products such as ammonia report to the reflux and water wash sections. The technical pack provides a water wash model that maintains lean solvent MEA concentration in the recirculating water at <1.0 wt% to reduce total amine loss. Corrosion in carbon steel equipment is most severe in the lean/rich heat exchanger and the lower stripper where CO₂ loading and temperature intersect. Published laboratory and pilot data show that corrosion rates for carbon steel in 30 wt% MEA at 80°C with CO₂ loadings above 0.4 mol/mol can exceed 0.1 mm/year without inhibitor. Stainless steel UNS S31603 exhibits lower general corrosion but is susceptible to stress corrosion cracking under certain lean/rich cyclic conditions; therefore, weld heat treatment and control of chloride ion below 50 μg/g in makeup water are recommended. The trial programme includes corrosion coupon racks in the absorber, rich solvent line, lean solvent line, and reboiler to measure weight loss according to ASTM G1. Coupon exposure intervals of 720 h are typical.

Lean amine emissions from the absorber outlet are controlled by a two-stage water wash and demister. The technical pack sets a maximum MEA vapour concentration in treated gas of 10 mg/m³ at normal conditions, with ammonia slip limited by the same water wash. Aqueous wash water is recirculated and periodically purged to the solvent circuit. Conductivity in the wash water is maintained below 200 μS/cm to prevent excessive amine accumulation. The trial programme includes measurements of outlet gas MEA by impinger sampling and liquid chromatography, with a detection limit below 1 mg/m³. These values align with common permitting requirements for amine-based capture units, but local regulatory limits may be stricter.

Foaming in MEA absorbers is associated with hydrocarbon aerosols, iron sulphide particles, and surface-active degradation products. The technical pack specifies a foam test that injects nitrogen through a sintered frit into the solvent at 50°C and measures collapse time. Acceptable foam collapse time is below 15 s for fresh solvent; degraded solvent with heat-stable salts and suspended solids may exceed this and require filtration. Antifoam addition is restricted to a silica-free polydimethylsiloxane emulsion at a dosage below 10 μg/g active, because excess antifoam fouls the reboiler and reduces mass transfer. Published data for this specific configuration is limited, but foaming incidents in packed columns are frequently caused by inadequate flue gas pre-scrubbing rather than the amine itself.

When a Structured Trial Programme Moves from Synthetic Flue Gas to Slipstream Operation

The trial programme is divided into three stages. Stage 1 uses a laboratory absorption/desorption cell to characterise cyclic capacity, absorption rate, and solvent viscosity at 40°C and 80°C under synthetic flue gas containing 14 vol% CO₂, 5 vol% O₂, and 6 vol% H₂O. Stage 2 uses a pilot packed column with defined gas-liquid contact area and independent controls for absorber temperature, lean loading, and stripper pressure. Stage 3 is a slipstream campaign on actual flue gas, with continuous monitoring of pH, conductivity, density, and liquid level. The technical pack provides standard operating procedures for sampling, data logging, and safety shutdown interlocks. No claim of performance should be derived from short-duration runs because the MEA solvent inventory requires hundreds of hours to reach stable degradation product concentration under typical pilot conditions; published data for shorter configurations is limited.

During the trial programme, the lean solvent temperature to the absorber is maintained at 40°C by cooling water; a deviation above 55°C lowers absorption capacity and increases MEA volatility at the top of the absorber. The reboiler pressure is kept between 1.5 and 2.0 bar absolute; lower pressure strips CO₂ insufficiently, while higher pressure increases thermal degradation and reboiler surface temperature. The solvent circulation rate is set to maintain a lean loading above 0.15 mol CO₂/mol MEA to avoid excessive corrosion in the lean line. If the lean loading falls below this value, the solvent becomes more alkaline and can increase MEA slip; if rich loading exceeds 0.52 mol CO₂/mol MEA, the driving force in the absorber declines. These boundaries are included in the technical pack as soft alarms and hard interlocks for the trial unit.