CO2 absorption in aqueous solutions of hindered amines

Sterically hindered amines constitute a new class of chemicals which have recently come into industrial use in a variety of gas-treating processes: chemical solvents in both aqueous and non-aqueous solutions, rate promotion additives for the hot carbonate process, and chemical solvents for the selective removal of hydrogen sulfide. The scant published data on the behavior of hindered amines do not allow one to estimate the actual values of the apparent kinetic constants or equilibrium constants, and even less to establish the chemical steps involved. In this paper, the results of an investigation of the behavior of one particular amine, 2-amino-2-methyl-1-propanol (AMP), as a chemical solvent for CO2 in aqueous solutions are reported. The equilibrium behavior of a hindered monoamine like AMP in aqueous solutions is dominated by the values of two equilibrium constants: the protonation constant Kp and the carbamate stability constant Kc. The value of Kp at infinite dilution has been determined experimentally, and is large enough to neglect formation of the carbonate ion. The value of Kc has been found experimentally to be significantly less than 10−1 1/gmol, as is expected for a hindered amine. A thermodynamic model has been developed and tested against experimental equilibrium data. Preliminary kinetic data seem to indicate that the reaction with CO2 is first order with respect to both CO2 and AMP. A first step in the elucidation of the chemical steps involved is presented.