Absorption mechanism using tertiary amine aqueous solution

https://doi.org/10.1002/ghg.2059

“The reactions involved in CO₂ absorption using tertiary amine solvent, including the dissociation and bicarbonation reaction, are summarized in Eqns (1)–(3).

Here, if all R groups are −C₂H₄OH moieties, the amine is TEA, but if one R is a −CH₃ moiety and the other two Rs are −C₂H₄OH, it is MDEA.

In tertiary amine solution before absorption, R₃N first dissociates to protonated amine (R₃NH⁺) and OH⁻ by Eqn (1). The concentrations of the generated R₃NH⁺ and OH⁻ can be determined by the dissociation constant (pK), and the pK values of MDEA and TEA are 8.54 and 7.72, respectively.³⁷ When CO₂ is injected into the solution for absorption, OH⁻, which is a strongly basic ion, firstly reacts with CO₂ to generate HCO₃⁻ (Eqn (2)), and then primary absorption is conducted by R₃N via Eqn (3). The absorption mechanism of Eqn (3) is based on a catalyst reaction in which amine does not directly react with CO₂, as shown in Fig. 1.³⁸

R₃N catalyzes the formation of OH⁻ by reacting with the water directly due to the attraction force exerted between a noncovalent pair of N atoms and the proton, and then CO₂ is combined with the OH⁻. As a result, R₃NH⁺ and HCO₃⁻ are generated when CO₂ is absorbed in a tertiary amine solution, and the theoretical molar ratio between the absorbed CO₂ and R₃N is 1.0 mol CO₂·mol⁻¹ amine. In addition to Eqn (3), water molecules can independently and additionally absorb a certain amount of CO₂ and this is regarded as the physical absorption capacity of water in amine systems. This capacity was measured as 2.34·10⁻² mol CO₂·L⁻¹ water (0.52 g CO₂·500 mL⁻¹ water) through the carbonation with 500 mL water under CO₂ at a partial pressure of 33.4 kPa at 25 °C.”