Kinetic Study of the Catalytic Desorption of Carbon Dioxide (CO2) from CO2-Loaded Monoethanolamine (MEA) and Blended Monoethanolamine-Methyldiethanolamine (MEA-MDEA) during Post Combustion CO2 Capture from Industrial Flue Gases
Akachuku, Ananda Udochi
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The objective of this research was to elucidate the kinetics of the catalyst-aided desorption of CO2 from CO2-loaded aqueous solutions of single monoethanolamine (MEA), and monoethanolamine blended with methyldiethanolamine (MEA-MDEA) during the post-combustion capture of CO2 from industrial flue gases. The experiments were performed over γ-Al2O3 and HZSM-5 catalysts in a complete absorber – desorber CO2 capture pilot plant unit with the absorber and desorber columns having an internal diameter of 2-inches (0.051 m) and a total height of 3.5 ft (1.067 m). The experimental kinetic data for CO2 desorption were obtained in the catalytic packed bed tubular desorber at three temperatures (348, 358, and 358 K), using MEA and MEA-MDEA concentrations respectively of 5M and 5:2M (molar ratio and total molarity of 7M) and CO2 loading ranging from ∼ 0.331-0.5 mol CO2/mol amine for different ratios of weight of catalyst/flow rate of amine (W/FAo). The kinetic performance was evaluated in terms of conversion (i.e. %CO2 desorbed), activation energy, frequency factor and rate constants. A comprehensive first order power law rate model and Langmuir-Hinshelwood-Hougen-Watson (LHHW), mechanistic models for the heterogeneous reactions, were developed. The results showed that HZSM-5 catalyst with higher Brønsted/Lewis acid site ratio exhibited dual site adsorption mechanism and provided faster kinetics and higher conversions with lower activation energy in comparison with γ-Al2O3 for both solvents. Also, a statistical analysis using four catalysts, namely, HZSM-5, γ-Al2O3, silica-alumina and HY (of widely varying characteristics in terms of BET surface area, pore size and distribution, pore volume, total acid sites, acid site strength and Brønsted/Lewis acid site ratio), as well as an inert parking scenario to determine the contributions of each characteristic to catalyst performance in CO2 desorption from CO2-loaded aqueous amines. The results showed that catalyst performance on CO2 desorption from CO2-rich MEA solution depended strongly on both the combined high acid strength with high B/L ratio as well as the high ratio of pore size to pore volume.