Absorption of CO2 by Single and Blended Amine Solvents in Various Gas–Liquid Contactors

Abstract

In this work the absorption of CO2 into aqueous solutions of sterically hindered and blended amines is considered. A kinetic study is carried out for the reaction of CO2 into aqueous solutions of two important single sterically hindered alkanolamines, 2-piperidineethanolamine (2-PE) and 2-amino-2-hydroxymethyl-1,3-propanediol (AHPD) at 303, 313 and 323 K for a range of amine concentrations using a fabricated wetted wall column absorber. The reaction is satisfactorily described using a zwitterionic mechanism. The hydrolysis of the carbamate ion to form bicarbonate ion considered in the reaction mechanism is confirmed by 13C NMR spectroscopy. The reaction orders are found to be around 1.0 with respect to amines for both systems. The second order rate constants, k2, are obtained from the experimental results which are correlated using Arrhenius equation. Kinetics of absorption of CO2 into a new activator such as 2-(1-piperazinyl)-ethylamine (PZEA) and into the blends of PZEA and N-methyldiethanolamine (MDEA) are also carried out. The reaction of CO2 with PZEA is described by overall second order reaction. The addition of small amounts of PZEA to (MDEA + H2O) is found to be significantly enhancing the reaction rate. In order to study the effects of different physicochemical and kinetic parameter on calculated CO2 absorption rates into aqueous solutions of (2-PE + H2O),(AHPD + H2O) and (PZEA + H2O), a parametric sensitivity analysis is investigated for which a series of simulation runs are carried out. The parameters considered for the analyses are Henry’s law constant for CO2, diffusivity of CO2 into the amine solutions and the second order reaction rate constants for the absorption of CO2. The kinetic study carried out in this work should be useful for the rational design of gas treating processes employing single or blended alkanolamine solvents. Physicochemical properties of CO2 and the aqueous alkanolamine solvents needed in the kinetic study are measured in this work, extending the data in the literature for the specific single and blended amine solvents studied in this work. The diffusion coefficients and physical solubilities of N2O in the aqueous alkanolamine solutions are measured and the diffusivities and physical solubilities of CO2 in these solvents are estimated by “N2Oanalogy”. The densities and viscosities of the aqueous amine solvents are measured over a wide range of amine concentrations and temperatures. In addition, several correlations developed in this work, will allow prediction of blend properties from single amine properties for process design and research work in gas treating. Besides this kinetic analysis, a theoretical study is carried out using hollow fiber membrane contactor (HFMC) and flat sheet membrane contactor (FSMC) to compare the absorption performance of different aqueous single and blended amine solvents for the absorption of pure and as well as 20% CO2. The performance of different single and blended amine solvents is analyzed in terms of local and average absorption flux of CO2 along the length of fiber or flat membrane. The amine solvent systems considered here are the aqueous solutions of monoethanolamine (MEA), diethanolamine (DEA), N-methyldiethanolamine (MDEA), 2-amino-2-methyl-1-propanol (AMP), 2-PE and AHPD as well as aqueous blends of (MEA + MDEA), (MEA+AMP), (DEA + MDEA), (DEA + AMP) and (PZEA + MDEA). The performance of FSMC and HFMC is also compared. The CO2 absorption flux in FSMC is found higher than that in HFMC for all high to low reacting amines.