Methyl diethanolamine - universally abbreviated as MDEA, CAS 105-59-9 - is a tertiary alkanolamine that has become one of the most strategically important solvents in the global natural gas processing industry. Where monoethanolamine (MEA) reacts aggressively and non-selectively with all acid gases, MDEA offers a fundamentally different proposition: the ability to selectively absorb H₂S in the presence of CO₂, combined with significantly lower regeneration energy requirements than primary amine solvents.
This selectivity has made MDEA - and its activated blends with fast-reacting co-amines - the solvent of choice in a wide range of gas treating scenarios, from sour natural gas processing and refinery off-gas treatment to selective H₂S removal ahead of Claus sulphur recovery units. This guide covers MDEA's chemistry, process design parameters, its key differences from MEA and DEA, and the sourcing considerations relevant to gas processing engineers and procurement teams.
For MDEA's full physicochemical specification, refer to our Diethanolamine product page and contact our technical team for MDEA-specific enquiries.
🧪 What Is Methyl Diethanolamine?
MDEA is produced by methylating diethanolamine (DEA) - reacting DEA with formaldehyde followed by reduction, or via direct N-methylation routes - to yield a tertiary amine where the hydrogen on the nitrogen has been replaced by a methyl group:
DEA: HN(CH₂CH₂OH)₂ - secondary amine
MDEA: CH₃–N(CH₂CH₂OH)₂ - tertiary amine
This single structural change - replacing N–H with N–CH₃ - has profound consequences for absorption chemistry. As a tertiary amine, MDEA cannot form carbamates with CO₂, because carbamate formation requires a free N–H bond. CO₂ absorption by MDEA therefore proceeds exclusively via the slower bicarbonate pathway, while H₂S - which reacts as a simple proton donor regardless of amine type - is absorbed rapidly by MDEA as by any other amine base.
| CAS Number | 105-59-9 |
| Molecular Formula | C₅H₁₃NO₂ |
| Molecular Weight | 119.16 g/mol |
| Amine Type | Tertiary alkanolamine |
| Appearance | Colourless to pale yellow viscous liquid |
| Boiling Point | 247 °C at 1 atm |
| Density at 20 °C | 1.038 g/cm³ |
| pKa (conjugate acid) | 8.52 |
| Viscosity at 25 °C | ~101 mPa·s (neat); lower in aqueous solution |
| Water Miscibility | Fully miscible |
⚗️ Absorption Chemistry: Why MDEA Is Selective
The selectivity of MDEA for H₂S over CO₂ is a direct consequence of reaction mechanism and kinetics. Understanding this distinction is essential for designing an effective MDEA treating unit.
H₂S Absorption - Fast and Stoichiometric
R₃N + H₂S → R₃NH⁺ + HS⁻
Instantaneous proton transfer - rate limited only by mass transfer, not reaction kinetics
H₂S reacts with MDEA via a straightforward acid-base proton transfer - instantaneous and limited only by the rate at which H₂S molecules reach the liquid interface. This is fast regardless of whether the amine is primary, secondary, or tertiary.
CO₂ Absorption - Slow, Water-Mediated
R₃N + CO₂ + H₂O → R₃NH⁺ + HCO₃⁻
Rate-limiting step: CO₂ hydration (CO₂ + H₂O → H₂CO₃). Much slower than H₂S proton transfer.
Because MDEA cannot form carbamates, CO₂ must first hydrate to carbonic acid before reacting with the amine. The hydration step is slow - its rate constant at 25 °C is approximately 0.026 s⁻¹ - creating a significant kinetic barrier to CO₂ absorption. This is precisely what enables selectivity: in a well-designed absorber with controlled contact time, H₂S is absorbed essentially completely while a large fraction of CO₂ passes through unreacted.
MDEA's CO₂ selectivity is a double-edged sword. In applications where complete CO₂ removal is needed (e.g., LNG pre-treatment to pipeline spec, ammonia synthesis feed gas), MDEA's slow CO₂ kinetics become a liability rather than an advantage. For these applications, MDEA must be activated with a fast-reacting co-amine - typically piperazine (PZ) at 3–8 wt% - to achieve adequate CO₂ removal rates while retaining some of MDEA's energy efficiency benefit.
Regeneration Energy Advantage
The absence of carbamate formation in MDEA systems has a direct consequence for regeneration energy. MEA carbamates have a high heat of reaction (~85 kJ/mol CO₂), meaning substantial energy is required to break the carbamate bond and release CO₂ in the stripper. MDEA bicarbonates have a much lower heat of reaction (~55–60 kJ/mol CO₂ for the bicarbonate pathway):
In a large gas treating plant operating continuously, this 30–50% reduction in reboiler duty translates directly into significant fuel or steam cost savings and reduced CO₂ emissions from the regeneration process itself - an increasingly important consideration for operators with scope 1 emission reduction targets.
🏭 Industrial Applications of MDEA
MDEA's primary application. In sour gas processing where the feed contains both H₂S and CO₂, MDEA allows H₂S to be selectively removed to pipeline spec (<4 ppm H₂S, <2% CO₂) while retaining a portion of the CO₂ - avoiding the over-treatment cost of removing CO₂ that would simply need to be replaced by inert gas downstream.
Claus sulphur recovery units require a feed gas with an H₂S/CO₂ ratio high enough for stable combustion. MDEA-based selective treating concentrates H₂S in the acid gas stream by limiting CO₂ co-absorption, improving Claus unit efficiency and reducing the risk of sub-stoichiometric combustion.
When complete CO₂ removal is required - LNG pre-treatment, ammonia synthesis, hydrogen production - MDEA is blended with a fast-reacting activator such as piperazine (PZ, 3–8 wt%) or MEA (5–10 wt%). The activator provides rapid CO₂ kinetics while MDEA provides the energy efficiency and capacity. This aMDEA approach is increasingly displacing straight MEA in large CO₂ removal applications.
Refinery fuel gas and hydrogen streams often contain H₂S from catalytic cracking and hydrotreater operations. MDEA selectively removes H₂S from these streams while retaining CO₂ and light hydrocarbons, making it preferable to MEA in fuel gas treating where CO₂ removal is neither required nor desired.
In biogas upgrading to biomethane, activated MDEA is used for CO₂ removal in chemical absorption units. The lower regeneration energy of MDEA versus MEA improves the economics of biomethane production, particularly in smaller-scale units where energy cost is a significant fraction of operating expenditure.
In steam methane reforming with carbon capture (blue hydrogen), aMDEA is increasingly preferred over MEA for the CO₂ absorption step. The lower reboiler duty reduces the energy penalty of capture and improves the carbon intensity of the hydrogen produced - a key metric for low-carbon hydrogen certification schemes.
📊 MDEA vs MEA vs DEA: Technical Comparison
The table below compares the three principal alkanolamine solvents across the parameters most relevant to gas treating design and operations.
| Parameter | MEA | DEA | MDEA |
|---|---|---|---|
| Amine type | Primary | Secondary | Tertiary |
| Typical gas treating conc. | 25–30 wt% | 25–35 wt% | 40–55 wt% |
| CO₂ absorption mechanism | Carbamate (fast) | Carbamate (moderate) | Bicarbonate only (slow) |
| H₂S / CO₂ selectivity | None | Moderate | High ✅ |
| Max theoretical CO₂ loading (mol/mol) | 0.5 (carbamate) | 0.5 (carbamate) | 1.0 (bicarbonate) |
| Reboiler duty (GJ/t CO₂) | 3.5 – 4.2 | 3.0 – 3.8 | 2.0 – 2.5 ✅ |
| Thermal stability | Moderate | Moderate | Excellent ✅ |
| Corrosivity at typical conc. | High | Moderate–High | Low–Moderate ✅ |
| Solvent losses (degradation) | High (0.5–2.0 kg/t CO₂) | Moderate | Low ✅ |
| Suitable for complete CO₂ removal | ✅ Yes | ⚠️ Partial | ⚠️ Only with activator (aMDEA) |
| Relative material cost | Low | Low–Moderate | Moderate–High |
⚙️ MDEA Process Design Parameters
Solvent Concentration
MDEA is typically used at significantly higher concentrations than MEA - commonly 40–55 wt% in aqueous solution. Its lower corrosivity compared to MEA at equivalent concentrations allows this higher loading, which in turn increases the capacity per unit volume of circulating solvent and reduces pumping costs. For selective H₂S service, 45–50 wt% MDEA is standard. For activated MDEA (aMDEA) used in bulk CO₂ removal, 40–45 wt% MDEA with 3–8 wt% piperazine is typical.
Absorber Design for Selectivity
Achieving good H₂S/CO₂ selectivity with MDEA requires careful absorber design. Selectivity is maximised by:
- 🎯 Minimising gas-liquid contact time - shorter packed bed height or fewer trays limits CO₂ absorption while allowing the faster H₂S absorption to proceed to completion
- 🎯 Low liquid-to-gas (L/G) ratio - reducing solvent circulation relative to gas rate limits CO₂ co-absorption without affecting H₂S removal
- 🎯 Low absorber temperature - operating the absorber at 35–45 °C rather than the higher temperatures sometimes used for MEA systems improves selectivity by reducing CO₂ absorption kinetics further
- 🎯 Using high lean loading - unlike MEA where lean loading must be minimised, MDEA systems can tolerate higher lean CO₂ loading (0.005–0.01 mol/mol) without significantly affecting H₂S removal, reducing reboiler duty further
Temperature Profile
| Location | MDEA System | vs MEA |
|---|---|---|
| Absorber operating temperature | 35 – 45 °C | Lower than MEA absorber (40–50 °C) to improve selectivity |
| Lean amine to absorber | 35 – 40 °C | Slightly cooler than MEA to support selectivity |
| Stripper reboiler | 105 – 120 °C | Lower than MEA (110–130 °C) - less degradation, less energy |
| Flash drum (optional) | 60 – 80 °C | Often used in MDEA systems to recover co-absorbed hydrocarbons before stripping |
🛡️ MDEA Stability: Why It Outlasts MEA in Service
MDEA's tertiary amine structure makes it significantly more resistant to both oxidative and thermal degradation than MEA or DEA:
Carbamate-derived heat-stable salts (the primary thermal degradation products in MEA systems) cannot form from MDEA. The principal degradation pathway - bicarbonate cycling - is fully reversible in the stripper. MDEA consumption rates in well-managed systems are typically 0.05–0.3 kg/t CO₂ equivalent treated - a factor of 5–10 lower than MEA.
In the presence of dissolved oxygen (relevant for flue gas treating), MDEA oxidises more slowly than MEA due to the absence of the reactive N–H bond that is the primary site for oxidative attack. In natural gas treating where O₂ is absent, oxidative degradation is essentially a non-issue for MDEA.
Many MDEA plants operate for years without requiring thermal reclaiming of the solvent inventory. Where reclaiming is performed, it is typically triggered by accumulation of heat-stable sulphur salts (thiosulphate, sulphate) from H₂S oxidation products rather than amine degradation products. This significantly simplifies plant operations and reduces waste generation compared to MEA systems.
While MDEA is resistant to CO₂-induced degradation, it reacts with carbonyl sulphide (COS) and carbon disulphide (CS₂) - minor components in some gas streams - to form thiazolidine degradation products. If the feed gas contains significant COS or CS₂ concentrations (>50 ppm), include a COS hydrolyser upstream of the MDEA absorber, or specify an aMDEA blend with a COS-hydrolysis promoter. This is a niche concern but relevant for certain refinery off-gas and partial oxidation syngas treating applications.
📦 Sourcing MDEA: Specification and Supply
MDEA for gas treating is available in a narrow range of commercial grades. Unlike MEA - which has a long history of cosmetic and pharmaceutical applications driving high-purity grades - MDEA is primarily an industrial product and most commercial supply is positioned for gas treating service.
| Parameter | Typical Specification | Significance |
|---|---|---|
| MDEA Assay | ≥ 98.5 wt% | Higher purity reduces DEA co-product concentration in circulating solvent |
| DEA Content | ≤ 1.0 wt% | DEA impurity reduces selectivity; can form N-nitrosamines in certain contexts |
| Water Content | ≤ 0.5 wt% | Affects dilution calculation when blending to target concentration |
| Colour (APHA) | ≤ 30 | Excessive colour indicates degraded or contaminated material |
| Iron Content | ≤ 2 ppm | Iron catalyses corrosion and can form sludge deposits in heat exchangers |
Packaging and Supply Logistics
MDEA is a stable liquid at ambient temperature with a low vapour pressure and no solidification concern (melting point –21 °C). Standard carbon steel storage tanks are suitable; nitrogen blanketing is recommended for long-term storage to prevent surface oxidation and colour development. Shelf life is 24 months in sealed containers under recommended storage conditions.
❓ Frequently Asked Questions
📝 Summary
Methyl diethanolamine occupies a distinct and important niche in amine gas treating. Its tertiary amine structure - no N–H bond, no carbamate formation - gives it a unique combination of H₂S/CO₂ selectivity, low regeneration energy, excellent thermal stability, and low corrosivity that no primary or secondary amine can match. In selective H₂S service, it is unrivalled. In bulk CO₂ removal, activated MDEA blends bridge the kinetics gap while retaining most of the energy efficiency advantage over MEA.
For procurement teams specifying MDEA, the key parameters are assay (≥98.5%), DEA impurity level (≤1%), and colour - with ISO tank supply being the most cost-effective option for continuous large-scale operations. For engineers evaluating a conversion from MEA to MDEA, the absorber sizing and reboiler heat recovery are the critical design parameters to assess before committing to the retrofit.
Sinolook Chemical supplies methyl diethanolamine (MDEA ≥98.5%) and diethanolamine (DEA 99%) for gas treating and industrial applications, with full CoA, SDS, and REACH documentation. ISO tank, IBC, and drum supply available. Technical support for aMDEA blend formulation and gas treating applications.
