(654c) Prediction of Reaction Rate Constant of Carbon Dioxide Absorption Into Aqueous Amine Solution

Abstract:

Carbon dioxide (CO₂) capture from a gas stream is a state-of-the-art technology and has been still developed to implement CO₂ capture and storage (CCS), which is a promising countermeasure of global warming. This study proposes a prediction scheme of a reaction rate constant for CO₂-amine-H₂O reaction system in order to provide it to process evaluation of CO₂ capture with an aqueous amine solution. Firstly, gas scrubbing tests for twelve aqueous amine solutions including primary, secondary, tertiary and hindered amines were conducted, then absorption behaviors obtained from the experiments were discussed with consideration of basicity (pKa) and reaction kinetics.

CO₂ absorption rates of aqueous amine solutions were measured through the screening test using a gas scrubbing bottle of 250 ml volume. A gas mixture of CO₂ and N₂ was supplied to the bottle with 50 ml of an aqueous amine solution at a flow rate of 700 ml/min for 60 minutes at the atmospheric pressure. The flow rate and CO₂ concentration of the feed gas were constant during the test and the CO₂ concentration of outlet gas from the bottle was analyzed with IR Analyzer. The CO₂ loading and absorption rate with time were calculated from the outlet gas CO₂ concentration. The absorption rate (N_CO2) shown in the test represented a relative CO₂-absorption rate among solvents. Test condition of CO₂ concentration was 20%, which was a model gas concentration of blast furnace gas in the steel industry. Aqueous amine solution was adjusted at amine concentrations of 2mol_-amine/L.

As a result, the absorption rate (N_CO2) had a strong correlation with reaction rate constant of CO₂-amine second order reaction rate constant, which was cited from previous published literatures. The following linear equation was given for aqueous amine solutions investigated in this study (primary, secondary and hindered amines).

log(k₂)=a N_CO2 + b

In the case of the experimental condition (solvent: 2mol_-amine/L, gas: 20%_-CO2) of this study, constants a / b were 0.0107 / -0.3726 for hindered amines. AMP (2-amino-2methyl-1-propanol), AMPD (2-amino-2metyl-1,3-propanediol), AMHD (2-amino-2-hydroxymethyl-1,3-propanediol) and 2PE (2-piperidineethanol) were considered as the hindered amines here.

Furthermore, it was shown that correlation equation between reaction rate constant and pKa for hindered amines could be expressed with the following equation,

ln(k2)=c1 pKa + c2 – c3 / T.

In brief, the scheme for prediction of reaction rate constant from the simple gas scrubbing test could make it easy to evaluate CO₂ capture with an aqueous amine solution and to accelerate development of high performance CO₂ capture solvent.

 Acknowledgments

This work was financially supported by the Ministry of Economy, Trade and Industry (METI), Japan.