MDEA refers to N-methyldiethanolamine, the molecular formula is CH3-N(CHE2CH2OH)2, and the molecular weight is 119.2. Under certain conditions, it has a strong absorption capacity for acid gases such as carbon dioxide, and has a small heat of reaction, low desorption temperature, non-toxic and non-degradable.
An amine molecule has at least one hydrocarbon group and one amino group. In general, it can be considered that the role of the hydrocarbon group is to reduce the vapor pressure and improve water solubility, and the role of the amino group is to make the aqueous solution reach the necessary pH and promote the absorption of H2S. H2S is weakly acidic, and MDEA is weakly alkaline, and the reaction generates water-soluble salts. Since the reaction is reversible, MDEA can be regenerated and recycled.
The basicity of methyldiethanolamine decreases with the increase of temperature. At low temperature, weakly basic methyldiethanolamine can combine with H2S to form amine salt, and at high temperature, amine salt can decompose into H2S and methyldiethanolamine.
At lower temperatures (20°C to 40°C), the reaction proceeds to the left (absorption), and at higher temperatures (>105°C), the reaction proceeds to the right (desorption).
The alcohol amine desulfurization method is a typical absorption-regeneration reaction process. The reaction mechanism is: H2S and CO 2 dissolved in water are slightly acidic and react with amines (weakly alkaline) to form salts that will decompose at high temperatures . Taking methyldiethanolamine (MDEA) as an example, the main reaction of its absorption of H2S and CO 2 is as follows: 2R3NH+ H2S→ (R3NH)2S
(R3NH)2S+H2S → 2R3NH2HS
R2NH+H2O+CO2→ (R3NH)2CO3
(R3NH)2CO3 +H2O+CO2→ 2R3NHHCO3
The main reaction of alcohol amine, H2S and CO2 is a reversible reaction. In the absorption tower, the balance of the above reactions moves to the right, and the acid gas components in the raw gas are removed; in the regeneration tower, the balance moves to the left, and the solvent is released. acid gas components. Like all other absorption-regeneration reaction processes, pressure and low temperature are good for absorption; decompression and high temperature are good for regeneration, but in order to prevent solvent decomposition, the regeneration temperature is usually lower than 127°C.
