Diethanolamine (DEA; CAS 111-42-2) is an alkanolamine manufactured by reaction of ethylene oxide with ammonia; the molecule contains one secondary amino group and two terminal hydroxyl groups, with molecular formula C4H11NO2 and molar mass 105.14 g/mol. The pure compound is hygroscopic and viscous above its crystallization point of approximately 28 °C; the 99% technical grade may solidify in unheated storage, while a low-freeze 85% grade contains retained water to depress the crystallization point for cold-climate handling. Density is approximately 1.09 g/cm³ at 20 °C, and the conjugate acid pKa is approximately 8.88 at 25 °C. The secondary nitrogen retains one N–H bond, enabling carbamate formation with carbon dioxide and amide formation with fatty acids, but the two hydroxyl groups also give it diol reactivity in ester and alkyd resin synthesis. Commercial DEA is supplied in 99% technical and 85% low-freeze specifications; the latter is not a simple dilution but a formulated grade in which water content is controlled to modify low-temperature pumpability.

What Distinguishes Diethanolamine from Monoethanolamine and Triethanolamine?

The substitution pattern controls the application envelope. MEA is a primary amine and is the most reactive of the three with CO2; it is used in deep CO2 removal but has higher volatility and higher corrosive potential in carbon steel. DEA occupies an intermediate position: its lower vapor pressure reduces atmospheric losses from absorbers, but its CO2 transfer capacity is lower than that of MEA and its selective H2S removal is weaker than that of MDEA. TEA, a tertiary amine, does not form stable carbamate and is generally not used as the sole acid gas absorbent; its three hydroxyl groups and higher molar mass make it more useful as a neutralizer, emulsifier, and cement-grinding additive.

Natural gas and refinery off-gas sweetening units select DEA when acid gas streams contain carbonyl sulfide, carbon disulfide, or mercaptan species that irreversibly degrade MEA. Aqueous DEA at 25–35 wt% is circulated through a countercurrent absorber with structured packing; acid gas rich solution is regenerated in a stripping column at low pressure and approximately 115–125 °C reboiler temperature. The lower vapor pressure of DEA relative to MEA reduces amine losses in the absorber overhead, but DEA rich solutions require a lower stripping temperature to avoid thermal degradation of the amine. Published design guides report rich DEA acid gas loadings of approximately 0.35–0.45 mol acid gas/mol amine, depending on acid gas partial pressure and solution strength. Foaming is controlled by coalescing filters, hydrocarbon skimming, and particulate filtration to ≤5 µm on a side stream. Carbon steel is acceptable for many DEA sweetening circuits, provided acid gas loadings, fluid velocity, heat-stable salt concentrations, and chloride accumulation are controlled; stress-relieved welds and post-weld heat treatment are specified in wet CO2 areas. DEA is not the preferred solvent when outlet H2S below 1 ppmv must be achieved without a polishing stage or when selective H2S removal from high CO2 streams is required. Published data for a specific plant configuration should be obtained from licensor simulation packages, because acid gas partial pressure and heat-stable salt accumulation shift corrosion boundaries.

When Fatty Acid Diethanolamide Synthesis Requires Azeotropic Water Removal

In the manufacture of cocamide DEA and related alkanolamide surfactants, DEA is condensed with coconut, lauric, or oleic fatty acid at a molar ratio of approximately 1:1 or 2:1, depending on whether a 1:1 alkanolamide or a 2:1 amide is required. The reaction is conducted in a stirred stainless-steel reactor fitted with a distillation take-off, nitrogen sparge, and thermosyphon reboiler; temperature is ramped from 140 °C to 160 °C and water of reaction is removed continuously to shift equilibrium. An alkaline catalyst such as potassium hydroxide may be added at 0.05–0.2 wt%. Vacuum is applied in the final stage, typically 20–50 mbar absolute, to reduce free fatty acid to ≤2 wt%. The secondary amine content of DEA produces amides that function as foam stabilizers and viscosity builders in liquid detergent formulations, but residual DEA in the final alkanolamide is monitored by titration and is typically specified at ≤5 wt%. The use of MEA instead of DEA yields a higher-melting crystalline amide, whereas TEA condensation is slower and yields a more hydrophilic product with weaker foam stabilization.

Ball mill grinding of portland cement clinker is improved by the addition of alkanolamine-based grinding aids at 0.01–0.1 wt% of clinker mass. Diethanolamine, often blended with triethanolamine or triisopropanolamine, adsorbs on polar C3S and C3A surfaces and reduces particle agglomeration in the mill, allowing a given Blaine specific surface measured according to ASTM C204-18 to be reached with lower specific energy consumption. DEA also influences early hydration and can alter pack-set behavior of the finished cement; overdosing above approximately 0.15 wt% has been associated in mill surveys with increased storage bin coating and reduced separator efficiency. The exact response is mill-dependent: closed-circuit ball mills with high-efficiency separators show a narrower dosage window than open-circuit mills, and published data for specific DEA/triisopropanolamine blend ratios is limited.

Corrosion Inhibition Mechanisms and Amine Carboxylate Neutralization Limits

Diethanolamine is neutralized with carboxylic acids to form amine carboxylate soaps used in water-miscible metalworking fluids, ferrous corrosion inhibitor packages, and synthetic coolants. The unprotonated DEA fraction adsorbs through the nitrogen and oxygen centers on low-carbon steel, while the carboxylate portion provides a hydrophobic film; effective pH buffering is usually maintained between 8.5 and 9.5. Ferrous corrosion control is commonly evaluated by the cast iron chip test according to ASTM D4627-92(2020). Hard water containing calcium and magnesium above approximately 200 ppm as CaCO3 can form insoluble carboxylates, which deplete the inhibitor phase and increase residue on machined parts. In these systems, DEA is preferred over MEA because the secondary amine has lower vapor pressure and lower skin permeability, but DEA must not be combined with nitrite-based rust preventives unless nitrosamine formation is controlled and analytical monitoring of N-nitrosodiethanolamine is established. The same nitrogen chemistry makes DEA a neutralizing agent for anionic surfactant acids, where the target pH is typically 6.8–7.2 in personal care formulations, but residual free DEA is often limited by product specifications due to sensitization potential.

Herbicide and pesticide formulation streams use DEA as a salt former for chlorophenoxy acid herbicides and as a stabilizer in aqueous concentrates. The reaction between DEA and 2,4-dichlorophenoxyacetic acid forms a water-soluble ammonium salt that is compatible with nonionic surfactants and reduces crystallization in low-temperature storage. Liquid concentrate formulations containing DEA salts are filtered through 10 µm cartridge filters and stored in high-density polyethylene or stainless steel; mild steel is not recommended for long-term storage because of amine-promoted iron dissolution. Low-temperature storage stability is evaluated over 14 days at −10 °C, with clear-point and crystal growth observations made according to CIPAC MT 39.3. The choice between DEA, dimethylamine, and potassium salts in herbicide concentrates is determined by the desired octanol-water partition behavior, volatility, and toxicological profile; DEA salts provide lower vapor drift than methylamine salts but higher molar mass contribution per acid equivalent.

Specifications, Bulk Storage Viscosity, and Transfer Line Heat Tracing

Bulk storage of the 99% grade requires heat tracing or jacketed vessels at 35–40 °C because the crystallization point is near 28 °C; transfer lines should be limited to diameter ≥DN 25 and pump suction should be designed for a viscosity of approximately 380 mPa·s at 30 °C. Nitrogen blanketing is applied to prevent water uptake and color drift. The low-freeze 85% grade has a lower crystallization point and can be transferred at ambient temperatures above approximately −5 °C, but its water content and alkalinity must be accounted for in downstream esterification or amidation charge calculations. Because DEA is hygroscopic and can react with carbon dioxide, storage vessels should be closed-loop vented through a desiccant or nitrogen purge to limit carbonate formation in the headspace and maintain specification assay during extended storage.