Thermal decomposition behaviour and kinetics of a mixed solution: n-propyl nitrate and nitric acid solution

To provide a theoretical basis for improving the thermal safety of spent acid, its decomposition behaviour was studied. A mixed solution of n-propyl nitrate (NPN) and nitric acid was analysed using isoperibolic experiments and differential scanning calorimetry. An accurate kinetic model based on the analysis was proposed to describe the decomposition of the spent acid by applying an isoconversional method and nonlinear regression to the DSC data. We found that the thermal decomposition in the mixed solution involved two competing reactions: the decomposition of NPN in the nitric acid solution and the decomposition of nitric acid. The decomposition of the mixed solution followed a 0.9082th Prout–Tompkins equation with autocatalysis, with a differential kinetic mechanism function of f(α) = (1 − α)0.9082·α0.1291. Lastly, the thermal safety evaluation and thermodynamic parameters were studied. The self-accelerating decomposition temperature and the critical temperature of thermal runway were 338.70 K and 346.76 K, respectively, in an approximately closed system, and 347.25 K and 355.73 K, respectively, in an open system. These results show that the spent acid was more hazardous than the NPN and nitric acid solution. In addition, the thermal stability of the mixed solution in an open system was higher than that of the mixed solution in an approximately closed system.