Monoethanolamine (MEA; CAS 141-43-5; IUPAC 2-aminoethan-1-ol) is a primary alkanolamine with the structural formula NH₂CH₂CH₂OH and a molar mass of 61.08 g/mol. At 20 °C the compound is a clear, hygroscopic liquid with a density of 1.016 g/cm³ determined by ASTM D4052, a dynamic viscosity of 24.1 mPa·s determined by ASTM D445, and a refractive index of 1.4539 determined by ASTM D1218. The normal boiling point at 101.325 kPa is 170.2 °C, and the freezing point is 10.3 °C; bulk storage in unheated tanks therefore requires a minimum temperature of 15 °C to avoid solidification at tank walls and transfer lines. The pKa of the conjugate acid is 9.55, so MEA is substantially protonated in neutral aqueous media but retains free-amine nucleophilicity above pH 8.0. The closed-cup flash point is 85 °C under ASTM D93, and the transport classification is UN 2491 Class 8 corrosive liquid rather than Class 3 flammable. MEA is miscible with water, methanol, and acetone, but only partially miscible with aliphatic hydrocarbons. This combination of thermal and solubility boundaries drives both downstream formulation and storage design.

How Is Monoethanolamine Synthesized in Continuous Ammonolysis Plants?

Industrial MEA is produced almost exclusively by liquid-phase ammonolysis of ethylene oxide. The reaction network is sequential and composition-sensitive: ammonia plus ethylene oxide forms MEA; MEA plus additional ethylene oxide forms diethanolamine (DEA); DEA plus additional ethylene oxide forms triethanolamine (TEA). A high molar ratio of ammonia to ethylene oxide, typically 10:1 to 40:1, suppresses the secondary reactions and raises MEA selectivity above 70%. At lower ratios of 2:1 to 4:1, the product distribution shifts toward DEA and TEA. This ratio is the principal lever for grade optimization in integrated plants. The reaction is exothermic and is conducted in a cooled tubular or stirred reactor at 50 °C to 100 °C with pressure sufficient to maintain ammonia in the liquid phase. Aqueous ammonia feed contains 20 wt% to 30 wt% NH₃. Excess ammonia reduces ethylene oxide homopolymerization and color-body formation, but increases the ammonia recovery load in the stripper.

After reaction, crude MEA is concentrated in a sequence of ammonia strippers, dehydrators, and vacuum rectifiers. In the dehydration column, MEA-water nonideality and high relative volatility demand columns with 20 to 40 theoretical stages when operated at 10–50 kPa absolute. Reboiler temperature is maintained below 140 °C to limit degradation to 2-oxazolidinone and dark condensation products. MEA is recovered as a side draw or overhead product; DEA and TEA are removed as higher-boiling fractions. Published data for specific column designs is limited, but plant-scale energy consumption is concentrated in the dehydration reboiler because water is the highest-volume component in the crude mixture. No catalyst is required; excess ammonia functions as both reactant and solvent. The same nitrogen-alkylation chemistry is used to manufacture N-methylethanolamine and N,N-dimethylethanolamine by reductive methylation with formaldehyde and hydrogen over nickel-based catalysts.

In gas treating service, aqueous MEA at 15 wt% to 30 wt% is circulated through a high-pressure absorber operating between 3 bar and 70 bar, where carbon dioxide and hydrogen sulfide are chemically absorbed from natural gas, refinery gas, or ammonia synthesis gas. The CO₂ absorption follows the carbamate route, in which two MEA molecules capture one CO₂ molecule; the theoretical maximum loading is therefore 0.5 mol CO₂/mol MEA. Plant operation is normally pinned at lean loadings of 0.10–0.15 mol CO₂/mol MEA and rich loadings of 0.35–0.45 mol CO₂/mol MEA to keep regeneration energy within a practical range. The rich amine is preheated in lean/rich exchangers and stripped at reboiler temperatures of 121–132 °C. H₂S absorption is faster than CO₂ absorption and proceeds through proton transfer to form hydrosulfide, but the strong MEA–CO₂ carbamate bond raises regeneration energy above that of tertiary amines such as MDEA.

At these stripping temperatures, dissolved acid gases and oxygen ingress generate heat-stable salts, 2-oxazolidinone, N-(2-hydroxyethyl)ethylenediamine (HEEDA), and iron sulfide suspended solids. Accumulation of heat-stable salts above 5 wt% of total amine reduces capacity and increases viscosity. A thermal reclaimer is typically operated at 150–170 °C on a 1–3% slipstream to recover free MEA and discard nonvolatile degradation products. Process controls for MEA gas treating must include continuous amine strength analysis by gas chromatography and pH monitoring of the lean stream; the lean amine target is typically pH 10.0–10.5. Without reclamation, foaming, fouling, and corrosion accelerate, and the unit can lose more than 10% of its design circulation capacity within 12 months in high-oxygen service.

Specification Boundaries Across Commercial, Reagent, and Low-Iron MEA Grades

MEA is supplied in several grades separated by assay, water content, color, and trace iron. Commercial/technical grade is used in gas treating, metalworking fluids, and bulk chemical synthesis. Reagent grade is specified for analytical work, pharmaceutical intermediates, and applications with low trace-metal tolerance. Low-iron grade is used in high-purity cleaning formulations, electronic cleaning, and amine-terminated resin syntheses where iron-catalyzed degradation would shorten shelf life. The following table summarizes representative supplier specification ranges; these are not regulatory standards but reflect common certificate-of-analysis limits.

The grade differences are not cosmetic. DEA and TEA in MEA alter reaction stoichiometry in downstream amidation because DEA has two hydroxyl groups and can crosslink in condensation polymers. Iron above 0.5 mg/kg in low-iron applications accelerates oxidative discoloration and may promote decomposition of hydrogen peroxide or peracid-based cleaning baths. Water content above 0.3 wt% changes the mass balance in continuous processes where MEA is metered as a neat reagent and can push anhydrous reactions below 99.0% conversion. Color bodies measured as APHA are an indirect monitor of high-boiling aldehydes and condensation products that cannot be removed by simple filtration.

In aqueous detergent and personal-care formulations, MEA functions as a neutralizing amine for carbomers and acrylic acid copolymers. Neutralization of acrylic acid units raises pH to 6.5–7.5 and ionizes carboxylate groups, which swell the polymer and build yield stress. A typical starting point is 0.2–0.8 g MEA per 100 g of 0.5% carbomer dispersion; the exact demand is calculated from the polymer acid value. Over-neutralization above pH 8.0 reduces clarity and yield stress due to excessive ionic strength, while under-neutralization below pH 6.0 gives stringy rheology and incomplete polymer swelling. In semi-synthetic metalworking fluids, MEA is used at 1–3 wt% to maintain reserve alkalinity and to stabilize borate or phosphate ester additives at pH 9.0–9.5. Because commercial MEA contains DEA as a specification-limited impurity, and DEA is a secondary amine, formulations containing MEA must avoid nitrosating agents such as sodium nitrite. Finished cosmetic formulations are monitored for N-nitrosodiethanolamine by LC-MS/MS; ISO 15819:2014 specifies the determination of NDELA in cosmetics. Regulation (EC) No 1223/2009 restricts secondary amines in the presence of nitrosating agents and sets a maximum NDELA content in finished cosmetic products of 50 µg/kg where verification is required.

Bulk storage of MEA in carbon steel tanks requires a nitrogen blanket or desiccant vent because the liquid absorbs water and carbon dioxide from ambient air. The reaction with carbon dioxide forms a surface film of MEA carbamate, which can increase viscosity and plug level instruments. Tanks with internal heating coils are maintained at 20–30 °C; localized coil surface temperatures above 60 °C cause discoloration. Centrifugal pumps with 316 stainless steel wetted parts and mechanical seals are specified for continuous transfer. Polyethylene and fluoropolymer linings are suitable for storage of reagent and low-iron grades where iron pickup from carbon steel would exceed specification. Occupational exposure to MEA is regulated at an OSHA permissible exposure limit of 3 ppm (6 mg/m³) as an 8-hour time-weighted average, and the ACGIH TLV is 3 ppm with skin notation. Workplace monitoring is required because the odor threshold is reported near 2–3 ppm, making sensory detection an unreliable overexposure indicator. MEA is a corrosive liquid under Regulation (EC) No 1272/2008 with Skin Corr. 1B; H314, and is registered under REACH for manufacture and import at multiple tonnage bands. Aerobic biodegradability is classified as readily biodegradable in standard OECD 301 tests at high inoculum concentrations, but concentrated releases raise ammonia nitrogen load in receiving waters.

When Carbon Steel Is Replaced by Stainless Steel in Lean/Rich Amine Service

In rich MEA service, dissolved CO₂ and carbamate species lower pH and produce a corrosive environment that attacks carbon steel at temperatures above 90 °C. Corrosion rates on carbon steel in lean MEA service are generally acceptable below 80 °C, but rich MEA at 120–130 °C can exceed 0.25 mm/year without corrosion inhibitor or alloy upgrade, particularly in reboiler tubes, lean/rich exchanger shells, and piping downstream of the amine heater. For this reason, reboiler tubes and rich-amine piping are often specified in 304 or 316 stainless steel, while carbon steel is stress-relieved and limited to lean amine and absorber sections below 80 °C. Fluid velocity is limited to 1.5–2.0 m/s in carbon steel amine piping to avoid erosion-corrosion and to 3.0 m/s in stainless steel systems. Oxygen ingress raises corrosion rate by converting dissolved iron and sulfides into particulate iron oxide and iron sulfide, which destabilizes the amine and contributes to foaming. MEA is also incompatible with copper and copper alloys because the amine forms soluble copper-ammine complexes; brass valves and copper gaskets are excluded from MEA service. Aluminum is attacked under alkaline conditions, and zinc galvanizing is rapidly dissolved. Equipment for concentrated MEA is therefore limited to carbon steel, stainless steel, or fluoropolymer-lined carbon steel depending on temperature, acid-gas loading, velocity, and contamination level.