The product designated ethanolamine is supplied as a blended mixture of Monoethanolamine /Diethanolamine /Triethanolamine (Ethanolamines Blend). The three alkanolamines are produced by the reaction of ethylene oxide with ammonia; molar excess of ammonia favours monoethanolamine, while reduced ammonia-to-ethylene oxide ratios shift the product spectrum toward diethanolamine and triethanolamine. The commercial blend therefore contains site-specific proportions of the components rather than a single fixed ratio. Monoethanolamine carries CAS 141-43-5, diethanolamine CAS 111-42-2, and triethanolamine CAS 102-71-6. The mixture is hygroscopic, polar, and alkaline, with total amine value varying according to the MEA:DEA:TEA ratio. Because the three alkanolamines differ in normal boiling point—170.8 °C for MEA, 268.8 °C for DEA, and 335.4 °C for TEA—the blend can be separated by vacuum distillation, but many downstream operations use the mixed composition directly. Storage is specified in closed stainless steel or high-density polyethylene vessels under dry nitrogen, because atmospheric carbon dioxide and moisture reduce the available alkalinity and increase carbonate formation. Water content, color, and density are controlled by ASTM E203, ASTM D1209, and ASTM D4052 respectively.

Table 1 — Typical physical properties of the three ethanolamine components at commercial assay

Property	Monoethanolamine	Diethanolamine	Triethanolamine
CAS registry	141-43-5	111-42-2	102-71-6
Normal boiling point	170.8 °C	268.8 °C	335.4 °C
Freezing point	10.5 °C	28.0 °C	21.6 °C
Density at 20 °C	1.016 g/cm³	1.097 g/cm³	1.124 g/cm³

How Does the Ethanolamines Blend Function in Reversible Acid-Gas Absorption?

In refinery and gas-processing amine units, aqueous blends of monoethanolamine, diethanolamine, and triethanolamine are circulated in closed loops between the absorber and the regenerator. Monoethanolamine provides rapid carbon dioxide uptake by carbamate formation; diethanolamine contributes lower regenerative heat duty and higher selectivity for hydrogen sulfide at moderate pressures; triethanolamine, when present, buffers pH and acts as a low-kinetic tertiary amine in selective tail-gas service. The total alkanolamine concentration in lean solution is typically maintained between 20 wt% and 35 wt% in deionized water. Lean solution enters the absorber at 38–43 °C, while feed gas is cooled to 38–50 °C. Absorber pressure in natural gas units commonly ranges from 2 MPa to 7 MPa, but low-pressure refinery tail-gas absorbers may operate below 0.5 MPa.

The principal process conflict occurs at the rich-amine loading threshold. For MEA-dominated solutions, rich loading is maintained between 0.35 mol acid gas/mol amine and 0.45 mol acid gas/mol amine. Above 0.5 mol/mol, carbon steel corrosion rates in the lean/rich exchanger and regenerator overhead increase rapidly. DEA-rich blends tolerate higher rich loading, typically 0.6–0.8 mol/mol, because DEA carbamate is less stable and releases acid gas at lower reboiler temperature. The reboiler bulk temperature is limited to 121–127 °C for MEA-dominated blends and 115–125 °C for DEA-dominated blends. Higher temperatures increase carbamate reversal but accelerate amine degradation and thermal reclaimer solids formation. Heat-stable salt anions—chloride, sulfate, formate, acetate—are measured in the circulating solution by ion chromatography per ASTM D4327; total heat-stable salt loading above 500 mg/L is controlled by slipstream reclamation or ion exchange. Foaming is reduced by maintaining liquid hydrocarbon carryover below 5 mg/L and by filtering a 10–20 vol% slipstream through activated carbon cartridges rated for 3 µm particulate. Gas composition and acid gas loading are verified by gas chromatography per ASTM D1945. Published data for site-specific MEA/DEA/TEA blends in proprietary refinery units is limited because supplier ratios are adjusted to feedstock sulfur and carbon dioxide content.

Table 2 — Analytical control matrix for the ethanolamines blend

Parameter	Method	Application area	Typical acceptance window
Water content	ASTM E203	Gas treating, cement grinding	<0.5 wt%
Pt-Co color	ASTM D1209	Personal care, surfactant feedstock	<50
Density at 20 °C	ASTM D4052	Blend ratio verification	1.01–1.13 g/cm³
N-Nitrosodiethanolamine	ISO 15819:2014	DEA-containing personal care grades	Reportable
Heat-stable salt anions	ASTM D4327	Gas-treating unit control	<500 mg/L in circulating solution

When Monoethanolamine is Selected over Diethanolamine in Cement Grinding Aids

Selection of monoethanolamine over diethanolamine in cement grinding formulations is governed by mill-discharge temperature, vapor pressure, and interaction with clinker surface charge. In closed-circuit ball mills with air-swept separators, cement exit temperature often ranges from 100 °C to 120 °C. Monoethanolamine, with a normal boiling point of 170.8 °C, is at risk of partitioning into the mill air stream if injection is located too close to the hot zone. Diethanolamine and triethanolamine remain predominantly in the mill product, making them more suitable for finish milling with high separator recirculation. The liquid additive is delivered by diaphragm or peristaltic metering pump at a dosage from 0.01 wt% to 0.05 wt% of clinker feed, usually into the first mill compartment or onto the recirculating separator feed belt.

Performance is assessed by Blaine fineness per ASTM C204, mortar compressive strength per ASTM C109/C109M, and sieve residue per ASTM C430. Plant trials compare the same specific surface because grindability of clinker changes with kiln feed mineralogy. The main operational failure at addition rates above 0.05 wt% is electrostatic agglomeration and mill diaphragm coating; this appears as a rise in mill exit temperature, reduced separator efficiency, and flow interruptions from bulk storage. Dosing lines for MEA-rich grades are specified in 316L stainless steel or high-density polyethylene because carbon steel contamination above 50 mg/kg iron in the additive can promote gel formation and nozzle clogging. If the mill circuit is shut down, residual additive in the feed belt is water-flushed to avoid solidification of TEA-rich films on chutes at temperatures below 21 °C.

Semi-synthetic metalworking fluid concentrates formulated with the ethanolamines blend use triethanolamine as a fast-acting alkalinity reserve and monoethanolamine/diethanolamine as building blocks for corrosion-inhibiting amine carboxylates. Ready-to-use emulsion pH is typically held between 8.8 and 9.5; below 8.5, cast-iron corrosion protection declines and the growth of odor-producing bacteria increases. Triethanolamine is commonly present in the concentrate at 10–20 wt%, while diethanolamine is limited to 2–5 wt% where nitrosamine risk is controlled. A recognizable field failure on single-block CNC machining centers is the appearance of brown ferric soap films on chip pans when sump pH falls below 8.3; this is often caused by hard water and tramp oil consuming the amine reserve. The diluted fluid is classified under ASTM D2881, and ferrous corrosion is evaluated by the cast-iron chip test per ASTM D4627. Formulations containing nitrite must not include diethanolamine because N-nitrosodiethanolamine can form; if DEA is required, a nitrite-free biocide package is specified and total N-nitrosodiethanolamine is monitored by ISO 15819:2014.

Alkanolamide Synthesis and Surfactant Building-Block Requirements

Alkanolamide formation from the ethanolamines blend proceeds by condensation of fatty methyl ester or fatty acid with the primary or secondary amine fraction. Monoethanolamine forms solid monoethanolamides with high melting points; diethanolamine forms liquid water-soluble diethanolamides used in nonionic surfactant packages and viscosity modifiers. The reaction is carried out at 150–170 °C under vacuum from 5 kPa to 20 kPa in a jacketed stirred reactor equipped with a short-path condenser for methanol/water removal. A molar ratio of DEA to fatty acid near 1.05:1 is used to minimize free fatty acid and ester by-products. Triethanolamine functions as an acid scavenger and base source but cannot form an amide directly because it lacks a reactive hydrogen on the amino nitrogen.

Residual free amine and acid value are monitored by titration; acid value is reported per ASTM D974. Production batches with acid value above 5 mg KOH/g and Gardner color per ASTM D1544 above 8 are diverted from high-foaming personal-care formulations because of off-color and odor. Continuous vacuum stripping must keep the vapor outlet below 90 °C to prevent foam carry-over into the vacuum pump. The mixed MEA/DEA/TEA feedstock may require preheating above 30 °C before metering because DEA-rich blends can become viscous and difficult to pump at ambient conditions. Batch-to-batch variation in ethanolamine ratio changes reaction rate; primary amine reacts faster than secondary amine, so MEA-rich lots require more controlled vacuum staging to avoid entrainment of unreacted methyl ester.

Clear carbomer gels are produced by dispersing 0.4 wt% to 1.0 wt% carbomer in water and neutralizing with triethanolamine until pH 5.5–6.5 is reached. Monoethanolamine and diethanolamine are generally avoided in leave-on gels because of higher irritation potential and more pronounced yellowing on storage. In soap-stabilized emulsions, triethanolamine reacts with stearic acid at the oil-water interface; the phase-inversion viscosity peak occurs near 70–80 °C, and the resulting TEA-stearate provides emulsion stability at pH 6.0–7.5. The rheology of TEA-soap emulsions is measured by rotational viscometry per ISO 3219; the shear-thinning index between 0.5 s⁻¹ and 50 s⁻¹ is specified above 3.0 for cream structure. Regulatory status of DEA-containing alkanolamides is grade-specific under EC No 1223/2009; total N-nitrosodiethanolamine is determined by ISO 15819:2014 and should not exceed regional limits. The blend is protected from strong oxidizers and atmospheric carbon dioxide, and is stored in closed stainless steel or HDPE containers under nitrogen headspace to avoid carbonate haze and loss of neutralization capacity.

Corrosion Inhibition in Aqueous Systems and the Role of Triethanolamine

Triethanolamine additions of 0.1 wt% to 1.0 wt% are used in water-borne corrosion inhibitor packages for ferrous metals, usually with boric acid and organic acid inhibitors. The tertiary amine neutralizes acidic species and forms an adsorbed layer on carbon steel; electrochemical polarization data from rotating cylinder electrode tests show passivation only when pH remains above 8.0. Below 7.5, triethanolamine alone behaves as a weak complexing agent and may increase copper dissolution, so it is not used as the sole inhibitor in mixed-metal systems. In engine-coolant and heat-transfer fluids, TEA is evaluated by glassware corrosion testing per ASTM D1384, and reserve alkalinity is measured by ASTM D1121. Aluminum compatibility is a defined boundary: above pH 9.0 and temperature above 80 °C, TEA can attack aluminum surfaces, generating hydrogen gas and aluminate sludge. Such systems require silicate inhibitors and cast-aluminum corrosion validation per ASTM D4340. In steam condensate and boiler water treatment, the blend is dosed to maintain condensate pH between 8.5 and 9.5; suspended iron oxides above 10 mg/L indicate the need for pre-filtration because amine degradation products can adhere to heat-exchanger surfaces.