Monoethanolamine - abbreviated MEA, CAS 141-43-5 - is the world's most widely deployed solvent for removing CO₂ and H₂S from gas streams. From natural gas processing plants and hydrogen production facilities to post-combustion carbon capture units at power stations, 30 wt% aqueous MEA has been the benchmark absorbent for over 70 years. Its combination of high reactivity with acid gases, good absorption capacity, and relatively straightforward regeneration chemistry has kept it at the centre of amine scrubbing technology despite the emergence of newer solvent formulations.
This guide covers the absorption chemistry, process design considerations, dosing parameters, degradation management, and sourcing requirements that engineers and procurement teams need when specifying MEA for gas treatment or carbon capture applications. For full physicochemical specifications, see the Monoethanolamine product page.
🏭 Why MEA Became the Standard Absorbent
Several properties combine to make MEA uniquely suited to acid gas removal:
As a primary amine with pKa 9.50, MEA reacts rapidly with CO₂ via carbamate formation - reaction rates are significantly faster than secondary or tertiary amines. This allows compact absorber column design and shorter contact times.
MEA achieves CO₂ loadings of 0.45–0.55 mol CO₂ per mol MEA under typical absorber conditions, with a theoretical maximum of 0.5 mol/mol via carbamate chemistry. This is competitive with most alternative solvents at comparable concentrations.
MEA carbamates and bicarbonates decompose cleanly at 110–130 °C in the stripper, releasing high-purity CO₂ and regenerating the lean amine. The regeneration chemistry is well characterised, and the technology is supported by decades of operational data.
MEA is produced at large industrial scale as a co-product of the ethylene oxide / ammonia reaction. Its cost per tonne is substantially lower than engineered mixed amine solvents, proprietary formulations, or ionic liquid absorbents - a critical factor for continuous large-scale operations.
No other amine solvent has the depth of published thermodynamic, kinetic, and operational data that MEA has accumulated. This makes process simulation, scale-up, and troubleshooting significantly more reliable than for newer solvents with limited field experience.
🔬 The Absorption Chemistry
MEA reacts with CO₂ through two parallel pathways, with the dominant route depending on the CO₂ partial pressure and MEA concentration.
Pathway 1: Carbamate Formation (dominant at low CO₂ loading)
2 RNH₂ + CO₂ → RNHCOO⁻ + RNH₃⁺
where R = –CH₂CH₂OH (the hydroxyethyl group of MEA)
This zwitterionic mechanism is fast and proceeds even at low CO₂ partial pressures. It consumes two moles of MEA per mole of CO₂, which is why the theoretical maximum loading via carbamate chemistry is 0.5 mol CO₂/mol MEA. The carbamate salt (MEA carbamate) is the dominant species in the rich amine solution leaving the absorber bottom.
Pathway 2: Bicarbonate Formation (dominant at high CO₂ loading)
RNH₂ + CO₂ + H₂O → RNH₃⁺ + HCO₃⁻
This pathway consumes only 1 mol MEA per mol CO₂, but is slower than carbamate formation
At higher CO₂ partial pressures or when the lean loading is already elevated, bicarbonate formation becomes more significant. The bicarbonate pathway has a more favourable stoichiometry (1:1 rather than 2:1) but slower kinetics, which is why absorber design typically targets conditions where carbamate formation dominates in the lower absorber sections.
Regeneration: Reversing the Reaction
In the stripper (desorber), the rich amine solution is heated to 110–130 °C. Both carbamate and bicarbonate species decompose, releasing CO₂ and water vapour and regenerating the free amine:
RNHCOO⁻ + RNH₃⁺ + heat → 2 RNH₂ + CO₂↑
RNH₃⁺ + HCO₃⁻ + heat → RNH₂ + CO₂↑ + H₂O
The high heat of reaction for MEA carbamate (approximately –85 kJ/mol CO₂ absorbed) is the root cause of MEA's high regeneration energy penalty - typically 3.5–4.2 GJ per tonne CO₂ captured - which is the primary driver of research into lower-enthalpy alternative solvents for large-scale CCS applications.
Methyl diethanolamine (MDEA), a tertiary amine, reacts with CO₂ only via the slower bicarbonate pathway - it cannot form carbamates. This gives MDEA lower CO₂ absorption kinetics than MEA, but a significantly lower regeneration energy requirement (~2.0–2.5 GJ/t CO₂). In practice, many modern gas plants use activated MDEA (aMDEA) - MDEA blended with small quantities of a fast-reacting amine such as piperazine or MEA - to combine MDEA's energy efficiency with adequate absorption rates.
⚙️ Process Design Parameters
A standard MEA absorption-stripping loop consists of an absorber column, a lean-rich heat exchanger, a stripper column, a reboiler, a condenser, and associated pumps and coolers. The key operating parameters that determine system performance and MEA consumption are discussed below.
📐 MEA Concentration in the Circulating Solvent
| Concentration | Typical Use Case | Notes |
|---|---|---|
| 15–20 wt% | High H₂S / high CO₂ streams, aggressive corrosion conditions | Lower corrosion rate; larger solvent volume and higher pumping costs |
| 30 wt% | Standard post-combustion CCS, natural gas sweetening | Industry benchmark; best-characterised corrosion / kinetics balance |
| 35–40 wt% | Compact units, high-throughput applications with corrosion inhibitors | Elevated corrosion risk; requires corrosion inhibitor addition and inhibitor management |
| >40 wt% | Rarely used in continuous systems | Severe corrosion, viscosity issues; not recommended without specific engineering assessment |
📐 Rich and Lean Loading Targets
The CO₂ loading of the circulating amine - expressed as moles of CO₂ per mole of MEA - determines both the absorption efficiency and the regeneration energy requirement.
The cyclic loading capacity - the difference between rich and lean loading - is the effective working capacity of the solvent. For 30 wt% MEA, a cyclic capacity of 0.25–0.30 mol/mol is typical under well-optimised conditions.
🌡️ Temperature Profile
| Location | Typical Temperature | Design Consideration |
|---|---|---|
| Absorber inlet (gas) | 40 – 50 °C | Gas cooling prior to absorber improves CO₂ absorption equilibrium |
| Lean amine to absorber | 40 – 45 °C | Lean amine cooler duty; lower temperature improves absorption capacity |
| Rich amine to stripper | 90 – 105 °C | After lean-rich heat exchanger; maximise heat recovery here |
| Stripper reboiler | 110 – 130 °C | Above 130 °C: accelerated thermal degradation; keep as low as feasible |
| Stripper overhead condenser | 20 – 40 °C | Condenses water from overhead CO₂ product stream |
⚠️ MEA Degradation: Causes, Products, and Management
MEA degradation is the primary operational challenge in MEA-based gas treatment. Two distinct degradation pathways operate simultaneously in most systems.
1 - Oxidative Degradation
In the presence of dissolved oxygen, MEA oxidises to form a range of nitrogen-containing and oxygen-containing degradation products including glycolate, oxalate, formate, and various amine fragments. Oxygen ingress typically occurs at the absorber inlet (flue gas applications) or through improperly sealed tanks and vents.
Key management strategies:
- ✅ Minimise dissolved oxygen in the lean amine - target <10 ppb in critical systems
- ✅ Use stainless steel or carbon steel with appropriate inhibitors; avoid copper alloys
- ✅ Add oxidative degradation inhibitors such as sodium metavanadate or EDTA-based chelants at 100–200 ppm in the circulating solvent
- ✅ Monitor formate and acetate concentrations as early indicators of oxidative degradation rate
2 - Thermal and CO₂-Induced Degradation
At stripper operating temperatures, MEA can react with CO₂ to form stable, non-regenerable compounds collectively known as heat-stable salts (HSS). The most significant is oxazolidone, formed by cyclisation of MEA carbamate at elevated temperature. N-(2-hydroxyethyl)imidazolidone (HEIA) is another major thermal degradation product.
HSS do not regenerate in the stripper. They represent a permanent loss of active amine from the circulating inventory. In a poorly managed system, HSS content can reach 5–15% of total amine, significantly reducing absorption capacity per litre of solvent circulated. Monitor total HSS by ion chromatography; initiate reclaiming when HSS exceeds 2–3% of total amine.
🔧 Reclaiming: Recovering Active MEA
A thermal reclaimer (side-stream vacuum distillation unit) is standard equipment in large MEA plants. A slip stream of 1–3% of the circulating solvent is fed to the reclaimer, where volatile MEA is distilled off and returned to the system, leaving behind a concentrated residue of HSS, corrosion products, and heavy degradation compounds that is periodically removed as waste.
Well-operated MEA plants with active reclaiming and inhibitor management achieve MEA consumption rates of 0.5–2.0 kg MEA per tonne CO₂ captured. Poorly managed systems can see losses of 5 kg/t CO₂ or higher.
🔩 Corrosion Management in MEA Systems
Corrosion is the most significant materials challenge in MEA gas treatment. The combination of CO₂, water, and amine creates an aggressive electrochemical environment, particularly in the rich amine sections of the circuit and in the stripper.
Stripper reboiler tubes, lean-rich heat exchanger, rich amine pump seals and impellers, and stripper overhead condenser. These areas see the highest temperature and CO₂ partial pressure combinations.
Carbon steel (CS) is acceptable for absorber shells and low-temperature sections. 304 or 316 stainless steel is required for reboilers, heat exchangers, and stripper internals. Avoid copper alloys, which catalyse oxidative degradation.
Sodium metavanadate (50–100 ppm as V) is the most widely used corrosion inhibitor in MEA systems. It forms a passivating iron vanadate film on carbon steel surfaces. Note that vanadium compounds require careful waste management in the reclaimer residue.
The corrosivity of MEA increases strongly with concentration above 30 wt% and with rich loading above 0.50 mol/mol. Maintaining MEA concentration at or below 30 wt% and controlling rich loading within the recommended range are the two most effective corrosion mitigation measures available to operators without hardware changes.
🏗️ Natural Gas Sweetening vs Post-Combustion CCS: Key Differences
MEA is used in both natural gas sweetening and post-combustion carbon capture, but the operating environment and design priorities differ significantly between the two applications.
| Parameter | Natural Gas Sweetening | Post-Combustion CCS |
|---|---|---|
| Feed gas pressure | 20–80 bar | Near atmospheric (0.1–0.15 bar CO₂ partial pressure) |
| CO₂ content in feed | 1–50 mol% | 3–15 vol% (flue gas) |
| H₂S co-removal | Often required (pipeline spec <4 ppm) | Not present in most flue gas streams |
| O₂ in feed gas | Typically absent | 3–8 vol% - major oxidative degradation driver |
| SOₓ / NOₓ in feed | Usually absent | Present; form heat-stable salts; require upstream removal |
| MEA consumption | 0.3–1.0 kg/t CO₂ equivalent | 0.5–2.0 kg/t CO₂ (higher due to O₂ degradation) |
| Primary design focus | Product gas specification (H₂S, CO₂ content) | Capture rate (>90%), energy penalty minimisation |
📋 Practical Dosing and Make-Up Guide
This section summarises the practical parameters needed to specify MEA for a new system or manage make-up requirements in an existing plant.
Initial Solvent Charge
Ongoing Make-Up Rate
The following make-up rates are indicative for a 30 wt% MEA system treating flue gas in a post-combustion CCS application. Actual values will vary with feed gas composition, inhibitor programme, and reclaimer efficiency.
| Loss Mechanism | Typical Loss Rate | Primary Mitigation |
|---|---|---|
| Vapour carry-over (absorber overhead) | 0.1–0.3 kg/t CO₂ | Water wash section on absorber overhead; mist eliminator |
| Oxidative degradation | 0.2–1.0 kg/t CO₂ | O₂ scavenger, inhibitor addition, minimise air ingress |
| Thermal / CO₂-induced degradation | 0.1–0.5 kg/t CO₂ | Reboiler temperature control (<130 °C); reclaimer operation |
| Total - well-managed plant | 0.5–1.5 kg MEA / t CO₂ | Full inhibitor + reclaimer programme |
For gas treatment and CCS applications, specify MEA 99% with the following parameters: purity ≥99.0%, DEA content ≤0.5%, colour APHA ≤20, water content ≤0.3%, iron content ≤1 ppm. Request a Certificate of Analysis and batch traceability documentation with each delivery. For large continuous operations, IBC (1,000 kg) or ISO tank (20–25 t) supply is most cost-effective.
🔄 MEA Alternatives: When to Consider a Different Solvent
MEA is not always the optimal choice. The following scenarios favour considering an alternative amine solvent:
Consider MDEA or DEA. Their lower CO₂ reactivity allows H₂S to be preferentially absorbed when CO₂ slip is acceptable. MEA removes both gases non-selectively.
Consider piperazine-promoted MDEA (aMDEA) or proprietary low-enthalpy solvents such as Cansolv DC-103 or KS-1. These can reduce regeneration energy by 20–40% versus 30 wt% MEA.
MEA corrosion becomes severe at high rich loadings encountered with high-CO₂ feeds. K₂CO₃ (hot potassium carbonate) or MDEA blends may be preferable for bulk CO₂ removal in these conditions.
MEA requires the lean amine to be cooled to 40–45 °C before the absorber. Processes with limited cooling water or high ambient temperatures may achieve better economics with a higher-boiling tertiary amine solvent.
For most standard natural gas sweetening applications and first-generation post-combustion CCS projects, the combination of low MEA cost, well-understood process design, and available engineering expertise continues to favour MEA as the default solvent choice. The transition to lower-enthalpy solvents is underway in the CCS sector, but MEA remains the reference case against which all alternatives are benchmarked.
❓ Frequently Asked Questions
📝 Summary
Monoethanolamine at 30 wt% remains the reference solvent for CO₂ absorption from gas streams - its combination of fast reaction kinetics, adequate loading capacity, predictable regeneration chemistry, and low material cost has sustained its dominance across both gas treatment and carbon capture applications for seven decades. The principal operational challenges are degradation management (oxidative and thermal) and corrosion control, both of which are well understood and manageable with appropriate inhibitor programmes, reclaimer operation, and materials selection.
For engineers specifying MEA for a new project, the key parameters to fix early are solvent concentration (30 wt% recommended), rich and lean loading targets, reboiler temperature ceiling (<130 °C), and make-up supply logistics. For procurement teams placing orders, specifying MEA 99% with low DEA content, colour, and iron documentation ensures the solvent is fit for purpose from the first charge.
Sinolook Chemical supplies monoethanolamine (MEA 99%) in 200 kg drums and 1,000 kg IBC totes, with full documentation including CoA, SDS, and REACH registration support. ISO tank quantities available for large continuous operations.
