Monoethanolamine (MEA), CAS 141-43-5, is a primary alkanolamine with molecular formula H₂NCH₂CH₂OH and molar mass 61.08 g/mol. In the neutralisation selection matrix MEA vs Caustic Soda and Other Alkalis: Neutralisation Selection Matrix, MEA is compared with sodium hydroxide, potassium hydroxide, aqueous ammonia, sodium carbonate, magnesium hydroxide, and hydrated lime. The boiling point of neat MEA is approximately 170 °C; the freezing point is 10.5 °C; and the density is 1.012 g/cm³ at 20 °C. Industrial neutralisation formulations use aqueous MEA at 20 wt% to 30 wt% for applications where a buffered weak base or thermally regenerable acid-gas absorber is required.

What pH Plateau Can Be Expected from MEA Compared to a Strong Mineral Alkali?

The conjugate acid of MEA has a pKa of 9.50 at 25 °C. A 0.1 N aqueous MEA solution gives a pH of approximately 11.3 when measured with a glass electrode in accordance with ASTM E70; a 0.1 N sodium hydroxide solution gives a pH near 13.0. This difference is a control variable in neutralisation units. MEA holds the pH in the 8.5–10.5 band for doses that already push sodium hydroxide above 12.0. The buffer capacity of MEA is greatest within ± 1 pH unit of its pKa, so its most reliable neutralisation window is approximately pH 8.5–10.0.

On a stoichiometric basis, MEA provides 16.4 mol of proton-accepting equivalents per kilogram; anhydrous sodium hydroxide provides 25.0 mol per kilogram. The mass dose of MEA required to neutralise a fixed quantity of a strong monoprotic acid is therefore approximately 1.5× the mass of caustic soda. For sulfuric acid neutralisation, 1.00 kg of MEA neutralises approximately 0.80 kg of H₂SO₄, while 1.00 kg of sodium hydroxide neutralises approximately 1.23 kg of H₂SO₄. This mass penalty is accepted when a buffered endpoint or a regenerable solvent is required.

Acid-Gas Regeneration and Heat-Stable Salt Formation in Recirculating Neutralisers

In recirculating acid-gas neutralisation service, the most consequential difference between MEA and caustic soda is thermal regeneration. A 20–30 wt% aqueous MEA solution reacts with hydrogen sulfide and carbon dioxide in a packed absorber to form an aqueous amine salt solution; steam stripping at reboiler temperatures of 115–125 °C reverses the reaction and liberates acid gas overhead. In natural gas sweetening, structured packing or tray absorbers are designed with 10–20 theoretical stages for feed gas containing up to 1 mol% H₂S when using MEA. The rich amine from the absorber is preheated in a lean/rich exchanger and routed to the stripper. Lean loadings are typically maintained near 0.10 mol acid gas/mol amine; rich loadings of 0.35–0.50 mol acid gas/mol amine are common in published plant data for 20 wt% MEA. The reboiler duty for carbon dioxide capture with MEA is often reported as 3.5–4.0 GJ/t CO₂, though published data for high-pressure natural gas configurations show variability with acid-gas partial pressure and heat-stable salt concentration. Sodium hydroxide does not regenerate under these conditions because the sodium carbonate and sodium bicarbonate formed by CO₂ absorption require temperatures far above the practical reboiler range for decomposition.

The main process conflict in MEA neutralisation loops is the accumulation of heat-stable amine salts. Strong acids such as hydrochloric acid, sulfuric acid, and thermal degradation products such as formic acid form chloride, sulfate, and formate salts that do not dissociate at 125 °C. These salts increase viscosity, reduce free amine capacity, and raise corrosion rates. Gas plants typically control heat-stable amine salt content below 1.0 wt% of the circulating amine solution by thermal reclamation or ion exchange. Corrosion monitoring is performed with electrical resistance probes and weight-loss coupons evaluated under ASTM G31. If heat-stable salts rise above 2.0 wt%, the risk of erosion-corrosion in the reboiler and lean/rich exchanger increases sharply; published data for this specific high-salt configuration is limited and requires plant-specific corrosion testing.

Within the amine class, the primary amine MEA differs from tertiary methyldiethanolamine in CO₂ reaction kinetics. MEA forms a carbamate rapidly and absorbs CO₂ at a high rate; methyldiethanolamine does not form a stable carbamate and is used for selective H₂S removal when CO₂ slip is acceptable. The selection matrix therefore treats MEA as a high-reactivity acid-gas neutraliser for CO₂-heavy streams, while methyldiethanolamine is selected for H₂S-selective service. Caustic soda is excluded from both because it absorbs both acid gases irreversibly and generates salts requiring disposal.

When Effluent Total Nitrogen Limits Eliminate Ammoniacal Neutralisation Routes

MEA contributes 1 mol of total nitrogen per 61.08 g of product, equivalent to approximately 16.4 mol N/kg. Aqueous ammonia contributes 58.7 mol N/kg, while sodium hydroxide and potassium hydroxide contribute zero nitrogen. In a wastewater discharge governed by a total nitrogen limit of 10 mg/L as N, the use of MEA in once-through neutralisation of acidic effluent can exceed the limit unless the downstream biological treatment has sufficient denitrification capacity. For a discharge limit of 15 mg/L total nitrogen, the same constraint applies. Standard pH measurement for effluent compliance may follow ISO 10523 or ASTM D1293; the nitrogen fraction is measured by Kjeldahl digestion or instrumental oxidation, and the selection matrix must include the laboratory method because MEA is detected as total Kjeldahl nitrogen, not ammonia-nitrogen alone.

This nitrogen burden separates MEA from caustic soda in once-through neutralisation. Sodium hydroxide remains suitable for simple pH adjustment where the effluent permit is nitrogen-sensitive and no acid-gas regeneration is required. In closed-loop gas-treating systems, the nitrogen remains inside the process, and the selection logic shifts back toward MEA because regeneration saves chemical consumption and reduces waste salt volume.

Carbon steel in MEA service is subject to amine stress corrosion cracking and acid-gas corrosion where rich amine letdown temperatures exceed 90 °C or where heat-stable salt content is elevated. Material selection under sour service conditions is typically evaluated in accordance with NACE MR0175/ISO 15156. For lean MEA at temperatures below 90 °C and low chloride content, post-weld heat-treated carbon steel has been used; for rich amine, absorber internals, letdown valves, and reboiler tubes, Type 304 or 316 stainless steel is specified when acid-gas loading and heat-stable salts exceed the limits established by plant-specific corrosion coupons. Caustic soda presents a different risk: stress corrosion cracking of carbon steel at temperatures above approximately 60 °C and concentrations above 20 wt%, and rapid attack on aluminium, zinc, and galvanized surfaces. The neutralisation selection matrix therefore includes a metallurgical compatibility row. MEA is unsuitable for zinc and galvanized piping; it is aggressive to aluminium at high pH; and it can be handled in carbon steel only within a narrower temperature and heat-stable salt band than caustic soda in most refinery and gas-plant installations.

In batch neutralisation reactors, the use of MEA in unlined carbon steel is typically limited to temperatures below 40 °C and amine concentrations below 20 wt% unless oxygen is excluded. Above these boundaries, corrosion rates determined by electrical resistance probes can rise quickly, and published data for the specific combination of MEA, dissolved oxygen, and sulfate heat-stable salts at low pH is limited. Final material selection requires ASTM G31 immersion testing with process liquors containing the expected acid anions.

In continuous neutralisation reactors, the weak-base plateau of MEA changes pH control loop tuning. A pH controller cascaded to a positive-displacement metering pump is less likely to overshoot a setpoint near pH 8.5–9.0 when the neutraliser has a pKa of 9.50 because a given volume of base produces a smaller pH displacement near the pKa than an equivalent volume of 50 wt% sodium hydroxide. Process skids using static mixers and recirculation loops sized for 3–5 times the feed flow have been specified for citric acid-containing process water when the design target is pH 8.0–9.0. With caustic soda, the same system may require a diluted 10 wt% solution and high-flow recirculation to avoid local pH excursions above 9.5. The exothermic heat of neutralisation for MEA is lower than that of sodium hydroxide per unit mass, but in concentrated acid neutralisation, heat removal must still be calculated from the feed acid concentration and flow rate.

Storage and Transfer Viscosity at Low Ambient Temperatures

Neat MEA freezes at 10.5 °C; storage vessels in northern climates require tank heating or dilution with water to depress the freezing point. The freezing curve of MEA-water mixtures is not linear. A 85 wt% MEA solution has a lower freezing point than neat MEA, but the exact value must be confirmed for each blend. The dynamic viscosity of neat MEA at 20 °C is approximately 24.1 mPa·s, which is higher than water and higher than 50 wt% sodium hydroxide solution at the same temperature. In transfer lines, viscosity increases as temperature falls; heat tracing or larger-diameter piping is required to maintain pump suction pressure and avoid cavitation in positive-displacement pumps. Caustic soda at 50 wt% also freezes near 12 °C and becomes difficult to pump below 10 °C, so both chemicals require steam tracing in outdoor storage.

Aqueous ammonia at 25 wt% has a high vapour pressure at ambient temperature and must be stored in closed vessels with acid scrubber vents. MEA has a vapour pressure of approximately 0.4 mm Hg at 20 °C, which reduces normal inhalation exposure risk relative to ammonia during transfer. However, MEA is a skin and eye irritant; operators are required to use chemical-resistant gloves and face shields during line breaking.

Magnesium hydroxide and calcium hydroxide are selected in the matrix when alkalinity must be delivered as a slowly dissolving slurry. Magnesium hydroxide has a saturated pH of approximately 10.2 and provides 34.3 mol of hydroxide equivalents per kilogram when considered as Mg(OH)₂. It does not introduce nitrogen and is less hazardous to handle than concentrated caustic, but it can form solids and requires slurry pumps. Lime is similarly solid-based and is used when sulfate precipitation is acceptable. MEA remains a liquid option when solids are not permissible in the process.