Monoethanolamine (MEA) for CO₂ Capture: Solvent Comparison, Degradation and Makeup Rate

Post-combustion CO₂ capture with aqueous monoethanolamine operates as a closed solvent loop in which the absorber removes CO₂ from flue gas and the stripper regenerates the rich amine. The benchmark solvent is 30 wt% MEA in demineralized water, with a lean loading of 0.25–0.35 mol CO₂/mol MEA and a rich loading of 0.45–0.50 mol CO₂/mol MEA. This window maintains acceptable mass-transfer driving force while limiting carbamate polymerization and carbon steel corrosion. Absorber internals are typically structured packing such as Mellapak 250X or IMTP random packing with bed heights of 15–25 m; absorber pressure drop is controlled at 5–15 kPa to limit blower parasitic load. Solvent density and water content are monitored by ASTM D4052-22 and ASTM E203-24, and the CO₂ capture rate is quantified according to ISO 14064-1:2018. For conventional single-loop operation without advanced heat integration, reboiler duty is 3.8–4.2 GJ/t CO₂ for coal-fired flue gas containing 12 vol% CO₂ at 90% capture. MEA is classified under REACH as acute toxicity category 4, skin corrosion category 1B, and eye damage category 1; access-zone exposure is typically controlled below 1 ppmv as an 8-hour time-weighted average.

Production-scale amine loops show that batch-to-batch solvent quality variability is driven mainly by heat-stable salt accumulation and reclaimer operation. A forced-circulation reclaimer operating on a 1–3% slipstream removes nonvolatile degradation products but also returns trace degradation intermediates that can alter lean loading by 0.02–0.05 mol CO₂/mol MEA when reclaimer temperature is not tightly controlled. Reclaimer temperature is therefore held at 140–150 °C and vacuum at 0.2–0.5 bar; deviation above 155 °C produces excessive 2-oxazolidinone and darkens the solvent.

Particulate matter from fly ash and reclaimer sludge must be removed to avoid erosion and foaming. Lean and rich solvent filters are typically cartridge filters rated at 5–10 µm with a differential-pressure alarm at 50–100 kPa. Continuous filtration of 10–20% of the circulating flow maintains suspended solids below 20 mg/L. Solvent management including filtration, thermal reclaiming and antifoam control accounts for 20–30% of variable operating cost at utility-scale coal-fired capture plants.

What Limits Rich-Loading Attainment in MEA Absorbers at Differential Pressure Below 15 kPa?

Rich-loading attainment is governed by the liquid-film mass-transfer coefficient, absorber temperature and intercooling duty. The absorption reaction is second order overall, first order in CO₂ and first order in free MEA. Above 35 wt% MEA, solvent viscosity increases from 1.8 mPa·s to 2.6 mPa·s at 40 °C, reducing liquid-film renewal and raising packed-bed pressure drop. Absorber intercooling to 40–50 °C improves rich loading by 10–15% compared with adiabatic operation at bulk temperatures above 60 °C. Rich loading above 0.50 mol CO₂/mol MEA increases equilibrium CO₂ back-pressure at the absorber outlet, yielding treated-gas CO₂ slip of 0.8–1.2 vol% at 90% capture. It also accelerates corrosion in the rich-solvent circuit and increases downstream carbamate polymerization. The control window is therefore narrow: lean loading 0.25–0.35 mol/mol, rich loading 0.45–0.50 mol/mol, absorber sump temperature 45–55 °C, and stripper bottom temperature 118–122 °C.

When Solvent Comparison Is Normalized to 90% Carbon Dioxide Removal

Comparison of amine solvents is valid only when absorber packing depth, liquid-to-gas ratio, flue gas composition and capture duty are held constant. The following data are normalized to coal-fired flue gas containing 12 vol% CO₂, 4 vol% O₂, 200 ppmv SO₂ after wet flue-gas desulfurization, and 50 ppmv NO₂. Reported values are drawn from pilot-plant campaigns and vendor heat-integrated configurations; published full-scale piperazine-only data is limited. MEA remains the kinetic benchmark, but its reboiler duty and degradation rate are the highest among the compared solvents. MDEA/PZ blends use piperazine as a rate promoter while relying on tertiary MDEA for reduced carbamate stability. AMP has high stoichiometric CO₂ capacity but slower absorption kinetics. The operational boundaries in the table are the demonstrated limits for stable long-term operation.

MEA’s higher makeup rate arises from oxidative degradation and reclaimer losses rather than volatility alone. The 0.15/0.60 AMP lean/rich window is wider because AMP reacts stoichiometrically with CO₂ to form bicarbonate rather than a stable carbamate, but the absorption rate at 40 °C is lower than MEA by a factor of 2–3. In a retrofit scenario, replacing MEA with a 40/5 wt% MDEA/PZ blend can reduce reboiler duty by 10–15% and lower makeup by 50–70%, but the circulating solvent viscosity increases and reclaiming becomes more complex. Piperazine-based solvents require preheating above 40 °C to avoid solid precipitation in low-temperature piping.

In conventional stripper operation at 1.8 bar gauge, sensible heat accounts for 10–12% of reboiler energy, heat of CO₂ desorption accounts for 50–55%, and stripping steam accounts for 30–35%. Advanced solvents reduce the stripping steam share by operating at higher rich loading and lower water vaporization per unit CO₂.

Without oxygen scavenging, MEA oxidative degradation is the dominant loss mechanism. Absorber sump dissolved oxygen from flue gas containing 4–8 vol% O₂ initiates a radical chain reaction producing ammonia, formate, acetate, glycolate and oxalate. The reaction accelerates above 55 °C and is catalyzed by iron, copper and chromium ions leached from carbon steel and stainless steel surfaces. Copper alloys are incompatible with MEA loops because dissolved copper ions accelerate both oxidative degradation and stress corrosion cracking. Sulfur oxides from flue gas at 5–200 ppmv SO₂ form sulfate heat-stable salts, while NOₓ forms nitrate and nitrite. When total heat-stable salts reach 2.0–3.0 wt% of solvent mass, the loop typically exhibits foaming in the absorber, increased filter loading, and reboiler tube scaling. Caustic injection with 30–50 wt% sodium hydroxide recovers free MEA from protonated amine salts, but the resulting sodium sulfate accumulates and must be removed. A forced-circulation thermal reclaimer operating at 140–150 °C and 0.2–0.5 bar vacuum recovers MEA and water from a 1–3% slipstream; reclaimer sludge generation is typically 1–3 kg/m³ of circulated solvent per day. Amine aerosol carryover from the absorber outlet is 10–50 mg/Nm³ without wet electrostatic precipitation and 1–5 mg/Nm³ with it; this loss is not recovered in the water wash and therefore contributes to makeup.

Thermal Degradation and Carbamate Polymerisation in the Stripper Sump

Stripper sump thermal degradation is governed by temperature, residence time and carbamate concentration. At 118–122 °C and 1.5–2.2 bar gauge, MEA carbamate dehydrates to 2-oxazolidinone and further reacts to 1-(2-hydroxyethyl)-2-imidazolidinone. These cyclic degradation products are not regenerable and accumulate in the circulating solvent. The degradation reaction has an apparent activation energy of 70–80 kJ/mol and first-order dependence on carbamate concentration. Reboiler residence time should be kept below 10–15 min; forced-circulation reboilers with tube-side velocity 1.5–2.5 m/s avoid local hot spots. Thermal degradation is monitored by colour according to ASTM D1209-05; Pt-Co values above 300 indicate excessive thermal stress. Secondary-amine promoters such as piperazine can form nitrosamines in the presence of nitrite; therefore nitrite is controlled below 10 mg/L. Chloride is kept below 50 mg/L to reduce pitting risk in 316L stainless steel reboiler tubes.

The lean/rich cross-exchanger is a plate-and-frame unit with an approach temperature of 5–10 °C. Fouling by oxalate precipitates increases the approach temperature and reduces regeneration efficiency. Cleaning cycles with citric acid at 5 wt% are required when pressure drop across the exchanger exceeds 150 kPa. Continuous monitoring of the lean/rich exchanger pressure drop and approach temperature provides an early indication of oxalate scaling before reboiler fouling becomes severe.

Makeup rate is established by mass balance around the circulating inventory. The measured loss streams include reclaimer sludge, mechanical entrainment from the absorber outlet, filter backwash, pump seal leakage, and nonregenerable degradation products. A typical MEA balance for a pulverized-coal unit at 90% capture and 12 vol% CO₂ shows a total makeup of 1.5–3.0 kg MEA/t CO₂. Oxidative degradation contributes 60–70% of this total, thermal reclaimer losses 20–30%, and aerosol carryover 0.2–0.5 kg/t CO₂ when no wet electrostatic precipitator is installed. Analytical monitoring uses total alkalinity titration with 0.1 N HCl and ion chromatography according to EPA Method 300.1 for organic acid anions. Heat-stable salts are quantified by cation-exchange separation followed by titration and expressed as wt% of total amine equivalent. Table 2 lists degradation markers and operating thresholds for a 30 wt% MEA loop.

Alternatives to thermal reclaiming are electrodialysis and ion exchange. Electrodialysis removes sulfate and nitrate while recovering free amine, but stack membranes are fouled by high-molecular-weight degradation products. Ion exchange with strong-base anion resin removes heat-stable salts but requires sodium hydroxide regeneration and generates brine waste. Thermal reclaiming is preferred when degradation product concentration exceeds 4 wt% because it also removes nonionic cyclic degradation products, which electrodialysis cannot. The trade-off is that thermal reclaimers lose 2–5% of circulating MEA with the sludge.

Operational boundaries for MEA-based capture are defined by solvent chemistry rather than column capacity. The solvent must be kept below 50 °C in the absorber sump to limit oxidative degradation and below 122 °C in the stripper to limit carbamate polymerization. Chloride concentration is maintained below 50 mg/L to avoid pitting corrosion in 316L stainless steel reboilers. The solvent should not be contacted with copper alloys, zinc, galvanized piping, or strong oxidizing acids. Demineralized water with conductivity below 5 µS/cm is used for dilution to avoid calcium scale and chloride ingress. Makeup MEA should meet ≥99.0 wt% purity with low iron and chloride; nitrogen blanketing of storage tanks reduces oxidative degradation.