A detailed chemical kinetic model for the supercritical water oxidation of methylamine: The importance of imine formation

Abstract

A detailed chemical kinetic model has been developed for supercritical water oxidation (SCWO) of methylamine, CH3NH2, providing insight into the intermediates and final products formed in this process as well as the dominant reaction pathways. The model was adapted from previous mechanisms, with a revision of the peroxyl radical chemistry to include imine formation, which has recently been identified as the dominant gas‐phase pathway in amine oxidation. The developed model can reproduce previous experimental data on methylamine consumption and major product formation to reasonable accuracy, although with deficiencies in describing the induction time. Our simulations indicate that oxidation of the •CH2NH2 radical to methanimine, CH2NH, is the major channel in methylamine SCWO, with subsequent hydrolysis of CH2NH providing the experimentally observed reaction products ammonia and formaldehyde. Integral‐averaged reaction rates were used to identify major reaction pathways, and a first‐order sensitivity analysis indicated that the concentration of CH3NH2 is most sensitive to OH radical kinetics. Overall, this work clarifies the importance of imine chemistry in the oxidation of nitrogen‐containing compounds and indicates that they are necessary to model these compounds in SCWO processes.

Can a single ammonia and water molecule enhance the formation of methanimine under tropospheric conditions?: kinetics of •CH2NH2 + O2 (+NH3/H2O)

“…As suggested in the previous studies (Gonza´lez et al, 2022;Onel et al, 2014;Rissanen et al, 2014;Ashraful and Silva, 2020),…”

“…The results show the formation of CH 2 N • + and • CHNH as major products and show a similarity between isoelectronic analogous systems, i.e., CH 2 O and CH 2 CH 2 . As suggested in Rissanen et al (2014 ) and Ashraful and Silva (2020 ), water may favor the formation of CH 2 N • and • CHNH; therefore, we have also analyzed the effect of water molecules on the CH 2 NH + OH reaction ( Ali et al, 2019 ). We have found that the reaction rate coefficients increase when the concentration of water molecules is not included in the calculation and decrease in the presence of water ( Ali et al, 2019 ).…”

“…Once CH 3 NH 2 is released into the Earth’s atmosphere, it reacts with the OH radical via the H-abstraction reaction, leading to the formation of a carbon-centered aminomethyl ( • CH 2 NH 2 ) radical, which is observed as a major product, and nitrogen-centered methyl amino radical (CH 3 NH • ), which is observed as a minor product ( Figure 1 ) ( Gonza´lez et al, 2022 ; Onel et al, 2014 ; Rissanen et al, 2014 ; Ashraful and Silva, 2020 ).…”

“… Photo-oxidation mechanism of methylamine ( Gonza´lez et al, 2022 ; Onel et al, 2014 ; Rissanen et al, 2014 ; Ashraful and Silva, 2020 ). …”

Review on Mechanisms and Kinetics for Supercritical Water Oxidation Processes

“…Regretfully, the conversion and C-based yield of the model simulation results are not in quantitative agreement with the experimental results, because the base model for reactions of nitrogen species is based on the gas-phase kinetics in higher temperatures around 1000 • C, instead of the lower temperature and a large amount of water used as solvent in conventional SCWO conditions. Recently, Ashraful and Silva [148] proposed an improved DCKM for methylamine oxidation in SCW, in which the peroxyl radical chemistry including imine formation as the main path for amine oxidation was added. Figure 16 demonstrates the major reaction paths in the methylamine SCWO process.…”