Mechanism of CO2 Dissolution

The dissolution of CO2 in aqueous solution of amine is combination of chemical reactions and physical dissolution. The chemical reactions involved depend on the type of the amine. 

In case of primary or secondary amine, the presence of hydrogen(s) branched on nitrogen atom allows formation of carbamate. The carbon dioxide reacts first with one molecule of amine to form a zwitterion (Eq. 14.1). Then the zwitterion reacts with a second amine molecule to form a carbamate (Eq. 14.2). The stoechiometry for chemical dissolution in primary or secondary amines is then limited to 2 molecules of amine per molecule of carbon dioxide (Eq. 14.3). 

In ease of sterically hindered amine the carbamate is instable and primary or secondary amines will react as the tertiary amines. The instability of carbamate also 

The increase of partial pressure of carbon dioxide forces the physical dissolution. This mechanism is purely mechanical and results in the apparition of molecular CO2 in the solution. This physical dissolution is particularly considerate in the treatment of natural gas in which partial pressure of carbon dioxide can reach hundreds of bars. In the case of CO2 capture in industrial effluent, partial pressure of CO2 remains below atmospheric pressure and carbon dioxide is mainly chemically absorbed with formation of carbamate and hydrogen carbonate. 

The solution concentration and the choice of the amine will be adapted to the conditions of temperature, pressure and composition of the gas stream to be treated. It has been shown that amines can also be combined to improve kinetic of absorption and quantity of dissolved gas [12]. In some cases, physical solvents can also be added to the solution in order to increase physical dissolution. 

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