Development of solvent systems containing tertiaryamines for CO2 absorption

Abstract

The new tertiary amines were synthesized by varying the alkyl chain length with and without (-OH) hydroxyl groups in the structure. The effect of chemical structure of newly synthesized tertiary amines; 4-(dimethylamino)-2-butanol (DMAB), 4-(dipropylamino)-2-butanol (DPAB), 4-(dibutylamino)-2-butanol (DBAB), 4-((2-hydroxyethyl)(methyl)amino)-2-butanol (HEMAB) and 4-((2-hydroxyethyl)(ethyl)amino)-2-butanol (HEEAB) were evaluated based on CO2 equilibrium solubility and cyclic capacity, rates and heats of CO2 absorption and regeneration, the kinetics of CO2 absorption, as well as physical properties such as density, viscosity and refractive indices. The results showed that three amines (i.e. DMAB, HEMAB and HEEAB) had substantially high CO2 absorption capacity (0.88, 0.44 and 0.68 mol CO2/mol amine at 313 K temperature and 15 kPa CO2 partial pressure, respectively), and cyclic capacity (0.52, 0.26 and 0.40, respectively, at 313-353 K temperature range, 15 kPa CO2 partial pressure). These amines also had fast CO2 absorption rates (0.082, 0.111 and 0.142 mol CO2/min, respectively) and CO2 regeneration rates (0.512, 0.452 and 0.295 mol CO2/min, respectively) while maintaining a low heat of CO2 absorption (-34.17, -56.21 and -69.79 kJ/mol CO2, respectively) and heat input of CO2 regeneration (39.73, 60.48 and 72.44 kJ/mol CO2, respectively). The three blended amine solutions (MEA-DMAB, MEA-HEMAB and MEA-HEEAB) at the ratio 5:1.25 M all had high CO2 absorption capacity (0.50 and 0.48 and 0.50 mol CO2/mol amine, respectively, at 313 K temperature and 15 kPa CO2 partial pressure), and cyclic capacity (0.12, 0.13 and 0.13, respectively, at 313-353 K temperature range, 15 kPa CO2 partial pressure). These blended amines also had fast CO2 absorption rates (0.094, 0.108 and 0.129 mol CO2/min, respectively) and CO2 regeneration rates (0.070, 0.072 and 0.064 mol CO2/min, respectively) and low heat input of CO2 regeneration (130.27, 171.85 and 188.78 kJ/mol CO2, respectively). The kinetics of the reactions of CO2 absorption of DMAB, HEMAB and HEEAB in terms of the second-order reaction rate constant (k2) using the stopped-flow technique at the temperature range of 298-313 K and amine concentration range of 0.1-1.0 mol/L. The second-order reaction rate constant (k2) of CO2 in amine solutions (HEEAB and HEMAB) were the first and second highest (1096.9 and 1043.6 m3/kmol.s) comparing to DEAB, DMAB and MDEA (47.25, 19.94 and 11.15 m3/kmol.s), respectively. The Arrhenius relationship of the second-order reaction rate constant (k2) at different temperatures of all amines related to their respective activation energy (Ea) and frequency factor (A) were then evaluated. HEMAB and HEEAB showed lower Ea values (26.78 and 24.45 kJ/mol) than those of DMAB and DEAB (54.34, 52.65 kJ/mol). Physical properties such as density, viscosity and refractive indices of these newly synthesized amines were lower than that of MDEA. Based on these results, HEMAB, HEEAB and DMAB, can be considered to be promising amine components for a post-combustion CO2 capture process.