N-Methyldiethanolamine CAS 105-59-9

Introduction to MDEA

MDEA is a colorless to pale yellow viscous tertiary alkanolamine containing one N-methyl and two hydroxyethyl substituents on a single nitrogen atom. It removes acid gases by protonating H₂S and hydrating CO₂ to bicarbonate, so natural gas treating, refinery gas scrubbing, and polyurethane catalyst intermediates are principal outlets.

MDEA has no N-H bond, so CO₂ is held mainly as bicarbonate. This supports lower regeneration steam demand and selective H₂S pickup from mixed H₂S/CO₂ streams, usually as 30–50 wt% aqueous solution. The practical handling issue is hygroscopicity: open drums absorb moisture and atmospheric CO₂, reducing active amine value and raising viscosity. Before charging, confirm amine value and water; a batch may darken slightly without failing titration. Release tests normally track amine value, water, and color.

Key Features of MDEA

• No N-H bond on tertiary nitrogen: CO₂ uptake proceeds through bicarbonate, which can reduce regeneration heat in acid-gas service.
• Two terminal hydroxyl groups: MDEA can react into polyurethane networks or be esterified and quaternized, helping bind amine fragments into the product matrix.
• High water solubility: clear aqueous solutions are practical at 30–50 wt%, so gas plants can run high amine strength without a co-solvent.
• Low vapor pressure and high flash point: evaporation losses stay low in hot absorbers and storage.
• QC checkpoints: density by ASTM D4052, viscosity by ASTM D445, flash point by ASTM D93, and water by ASTM E203 help detect dilution or CO₂ ingress before dispatch.

MDEA Chemical & Physical Properties

Applications of MDEA

Selective H₂S removal: In gas treating, the tertiary nitrogen accepts a proton from H₂S, while CO₂ enters mainly through slower bicarbonate hydration. Plants normally circulate 30–50 wt% aqueous MDEA and control absorber contact time to hold CO₂ slip.

Activated MDEA for CO₂ capture: Adding piperazine or MEA creates carbamate-forming sites inside the solvent, so CO₂ loading increases while MDEA still buffers pH and lowers regeneration heat. Typical activated blends contain 40–60 wt% total amine.

Polyurethane catalyst and reactive amine: The tertiary nitrogen accelerates isocyanate reactions, and the two hydroxyethyl groups can react with NCO groups, binding amine fragments into the polymer. Formulators often start at 0.05–0.5 phr and adjust for cure, odor, and cell opening.

Quaternary ammonium and surfactant intermediate: MDEA can be alkylated, quaternized, or esterified with fatty acids; the two hydroxyl groups improve water dispersibility and give formulators a polar head group. Intermediate synthesis is typically run near stoichiometric amine charge, and final derivative use levels often fall between 0.5–5% in finished formulations.

Storage & Safety Precautions for MDEA

Storage and moisture control: Keep drums sealed at 5–35 °C (41–95 °F), ideally under a dry nitrogen blanket. MDEA is highly hygroscopic and absorbs atmospheric CO₂, forming carbonate salts that drop the titratable amine value and spike viscosity. Use HDPE or lined carbon steel, and keep away from strong acids and isocyanates.

Safety and spill response: Treat as a skin and eye irritant (GHS H315, H319, H302). Wear chemical goggles and nitrile gloves. Absorb spills with inert sand, then wash the residue with water since MDEA is fully water-miscible. Ship as Non-DG; verify SDS section 14 for heated loads.

Field note: Purge sampling valves and cap bottles instantly. A 30-minute open sample can absorb enough CO₂ to lower the amine value by several mg KOH/g and fail a release test.

MDEA FAQ

Q: Why does MDEA (CAS 105-59-9) remove H₂S more selectively than CO₂?

A: MDEA has no N-H bond. H₂S is absorbed by fast proton transfer; CO₂ enters mainly through slower bicarbonate hydration. Typical gas treating uses 30–50 wt% MDEA. For deep CO₂ removal, add piperazine or MEA and recheck foaming and corrosion.

Q: How should I store MDEA to avoid amine loss and viscosity increase?

A: Store MDEA sealed, preferably under dry nitrogen, at 5–35 °C (41–95 °F). It absorbs moisture and CO₂, forming salts that lower amine value and raise viscosity. Use closed vents, cap samples, and test water plus amine value before use. Keep away from acids, oxidizers, and isocyanates.

Q: Can MDEA replace DEA in an existing amine unit without changes?

A: Not directly. MDEA kinetics and CO₂ chemistry differ, so circulation, lean loading, reclaimer duty, and heat duty need review. 50 wt% MDEA is not the same molar amine inventory as 30 wt% DEA. Pilot with solution analysis and gas composition; monitor corrosion and foaming.