About MDEA

Molecular formula: CH3N(CH2CH2OH)2; Relative molecular mass: 119.16. N-Methyldiethanolamine (MDEA) is a colorless or slightly yellowish viscous liquid that is readily soluble in water, ethanol, and ether. Boiling point: 246–249°C, relative density: 1.0425, freezing point: -21°C, latent heat of vaporization: 519.16 kJ/kg. MDEA is primarily used for the purification of acidic gases; it exhibits good selectivity, has a strong absorption capacity for acidic gases such as carbon dioxide and hydrogen sulfide, features low reaction heat and low desorption temperature, and possesses stable chemical properties—it is non-toxic and does not degrade.

I think this post should be posted in the sulfur recovery discussion area; I hope the administrator will remove it

The MDEA process for CO2 removal is a low-energy CO2 removal method developed by the German company BASF in the 1980s. This process is used in dozens of large ammonia plants around the world. Production practice has shown that this method not only features low energy consumption but also excellent absorption efficiency, enabling the CO2 level in the purified gas to be reduced to below 1%. It boasts good solution stability, no degradation, low volatility, low corrosivity, minimal corrosion to carbon steel equipment, and low solubility for hydrocarbons. 1. Process principle: The chemical name of MDEA is N-methyldiethanolamine, and it is a tertiary amine. The reaction with CO2 is as follows: CO2 + H2O → H+ + HCO3- (7) H+ + R2CH3N → R2CH3NH+ (8) R2CH3N + CO2 + H2O → R2CH3NH+ + HCO3- (9) Reaction (7) is a hydration reaction, and its rate is very slow. To increase the reaction rate, an activator is added to the N-methyldiethanolamine solution, thereby altering the reaction process. When a primary or secondary amine is added, the reaction proceeds as follows: R2NH + CO2 → RNCOOH (10) RNCOOH + R2CH3N + H2O → R2NH + R2CH3NH+•HCO3- (11) As can be seen from these reaction equations, the activator absorbs CO2 at the surface and reacts to form hydroxycarboxyl groups, which then transfer CO2 rapidly into the liquid phase to form stable bicarbonates, while the activator itself is regenerated. The N-methyldiethanolamine solution possesses the characteristics of both a chemical adsorbent and a physical solvent. 2. Process flow: The crude feed gas passes through an absorption tower where it is washed in two stages using solution. In the lower stage, absorption is carried out using a solution that has been desorbed through reduced-pressure flashing; to improve the purity of the gas, the upper stage employs a solution that has been regenerated by steam heating for washing purposes. The rich liquid coming out of the absorption tower passes through two flash tanks in sequence to have its pressure reduced; the energy used for the first stage of pressure reduction is recovered by a turbine. The recovered energy is used to drive the semi-poor liquid circulation pump. The vapor released from the high-pressure flash tank in the rich liquid contains a significant amount of hydrogen and ammonia, which can be compressed and sent back to the decarburization tower. After further pressure reduction of the solution exiting the high-pressure flash tank, the majority of the CO2 is released in the low-pressure flash tank. Most of the resulting semi-poor liquid is pumped into the lower section of the absorption tower using a circulation pump, while a small portion is sent to a steam-heated regeneration tower for regeneration; the resulting poor liquid is then fed into the upper section of the absorption tower for use. The CO2 gas containing water vapor obtained at the top of the regeneration tower is sent to a low-pressure flash tank to be used as a degassing medium. 3. Key points of process operation: (1) Ratio of lean liquid to semi-lean liquid. The ratio of lean liquid to semi-lean liquid is generally 1/3 to 1/6, and it is determined by the partial pressure of CO2 in the raw material. When the CO2 partial pressure is high and △x is large, a higher proportion can be used (such as 1/6), which reduces thermal energy consumption; the temperature of the lean liquid is generally between 55 and 70°C. (2) Temperature of the lean solution and semi-lean solution: The temperature of the semi-lean solution is generally between 70~80°C. A higher inlet temperature results in lower thermal energy consumption; however, if it is too high, it affects the temperature at the bottom of the absorption tower, reducing the solution’s absorption capacity and thereby increasing thermal energy consumption. For different feed gas conditions, there is an optimal temperature range for the solution. It ensures high purity while making full use of its physical properties, thereby minimizing thermal energy consumption. (3) CO2 removal and consumption: At an absorption pressure of 2.7 MPa, CO2 can be removed to below 0.005, with a CO2 purity of within 0.1%. The heat energy it consumes depends on the partial pressure of CO2 in the feed gas. High partial pressure, low thermal energy consumption; generally used in an adiabatic CO2 removal process. In principle, no heat energy is required, but to maintain stable absorption and desorption temperatures, thermal balance among the feed gas, purified gas, and regenerated gas is necessary. Usually, because a large amount of heat is carried away in the regenerated gas, heat must be supplied (such as using hot water or other low-energy sources) to maintain the temperature. (4) High-pressure flashing and CO2 purity: Non-polar gases such as hydrogen, nitrogen, methanol, CH, and other higher hydrocarbon compounds have low solubility in MDEA solutions; therefore, little loss of the purified gas occurs. However, when the absorption pressure is high, the CO2 content in the regenerated gas is less than 98%. If the absorption pressure is 2.7 MPa, a high-pressure flashing step is used in the process to increase the purity of CO2, with the flashing pressure being selected based on the required purity level. In this way, approximately 96% of CO2 can be recovered, with a purity of up to 99.5%. When the absorption pressure is less than 1.8 MPa, it is not necessary to use high-pressure flashing, and CO2 with a purity of over 98.5% can still be obtained. (4) Solvent loss: Since MDEA reacts with CO2 to form bicarbonates rather than urea carbamate, it does not degrade. Furthermore, the vapor pressure of MDEA itself is low (less than 0.01 mmHg at 25°C), so losses of MDEA are minimal. 4. Process characteristics: (1) The MDEA solution has good stability, is not prone to degradation, and is non-corrosive to carbon steel. (2) MDEA itself has a low vapor pressure and very low volatility. (3) The MDEA decarbonization process can remove hydrogen sulfide and organic sulfur while absorbing CO2. (4) During absorption, its solubility for non-polar gases such as H2 and N2 is relatively low; therefore, the loss of purified gas is also minimal. These properties further enhance its potential as a decarburization solvent.