Process hazard and thermal risk evaluation of m-xylene nitration with mixed acid

The exothermic pattern of m-xylene nitration with mixed acid in semi-batch mode was studied by reaction calorimetry. Adiabatic temperature rise (ΔT_ad) and reaction enthalpy (∆H) were obtained to identify the thermal hazard. Process conditions including stirring rate, reaction temperature and strength of mixed acid were studied to optimize the reaction. Optimal yield (95.5%) was reached under the conditions of sufficient stirring rate, 70% strength of mixed acid and reaction temperature of 40 °C. Meanwhile, the thermal stability of nitration products was investigated by differential scanning calorimetry (DSC) and accelerating rate calorimetry (ARC). Pyrolysis kinetic parameters were calculated under non-isothermal and adiabatic conditions. Furthermore, the maximum temperature of the runaway system for synthesis reaction (MTSR) was determined to analyze the reaction hazard. The thermal risk of nitration of m-xylene was also assessed by the risk matrix and Stoessel criticality diagram. In addition, the nitration mechanism of m-xylene was presented. The active site for the electrophilic reaction of m-xylene was predicted via theoretical approach which was in consistent with the selectivity obtained by the experiment. These findings provide guidance for the safe operation of m-xylene nitration and can be used for further scale-up.