Elementary reaction rate model for MPA oxidation in supercritical water

Abstract

An organophosphorus elementary reaction rate model has been developed to characterize rates and mechanistic pathways for methylphosphonic acid (MPA or PO(OH)₂CH₃) oxidation in supercritical water (SCW). The reaction network contains 242 reactions and 41 species, including 92 new phosphorus-containing reaction rates and 13 new phosphorus-containing species. Critical hydrogen abstraction and β-scission pathways omitted from existing organophosphorus combustion models have been added to the present model. With these additions, the MPA SCWO model accurately predicts the experimental concentration profiles of MPA and its measured intermediates and products, H₃PO₄, CO, CO₂, and CH₄ at a pressure of 246 bar, [MPA]₀ = 1 mM, and stoichiometric conditions at a residence time range of 3 to 10 s and a temperature range of 751 to 845 K in supercritical water (P. A. Sullivan and J. W. Tester, AIChE J., 2004, 50, 673). Modeling results qualitatively agree with the experimental observation that the MPA oxidation rate increases with increasing oxygen concentration. The model does not correctly reproduce the observed experimental trend of increasing MPA oxidation rates with increasing water density; possible reasons for this discrepancy are discussed. The model correctly predicts product selectivity for the carbon-containing products, CO, CO₂, and CH₄ at varying τ, T, Φ, and P, indicating that the newly proposed reactions are important to properly predict the branching between the MPA reaction pathways.