Thermal hazards evaluation of the synthesis of N-methylmorpholine-N-oxide

This study focused on the assessment of thermal safety associated with the synthesis of N-methylmorpholine-N-oxide (NMMO), a reaction known to be hazardous. First, a reaction calorimeter (SIMULAR) was used to investigate the exothermic behavior of the reaction in the semi-batch operation. The overall heat of the synthesis of 121.69 kJ mol−1 (based on the molar of N-methylmorpholine) and the maximum temperature of the synthesis reaction (MTSR) of 352.71 K under uncontrolled conditions were derived from calorimetric data and high-performance liquid chromatography results. Second, the thermal decomposition of H2O2/NMMO and NMMO under non-isothermal and adiabatic conditions and the possibility of secondary decomposition under runaway conditions were analyzed by differential scanning calorimetry and accelerating rate calorimetry to calculate the time to maximum rate (TMRad) and the temperature as TMRad is 24 h. The heat of decomposition of NMMO ranged from 314.23 to 416.23 J g−1. The activation energy (Ea), calculated using the kinetics method under non-isothermal conditions, was 40–89 kJ mol−1, which was lower than that derived under adiabatic conditions. Furthermore, the TD24 of H2O2/NMMO was lower than that of NMMO owing to the coupling effect. Finally, the risk assessment of thermal runaway was conducted on the basis of the Stoessel criticality diagram. The results indicated that the synthesis of NMMO has a class 2 risk level. Although the risk of the synthetic reaction is low, there are still hidden dangers such as thermal hazards during the NMMO synthesis in the presence of H2O2.