Thermal hazard assessment of nitrobenzene/dinitrobenzene mixtures

During the production of nitrobenzene by an adiabatic nitration process, the main byproducts are mono and dinitrophenols as well as 2,4,6-trinitrophenol (picric acid) and 1,3-dinitrobenzene. The byproducts can become concentrated if a distillation step to remove high boiling point impurities is used. In the present work, representative samples of nitrobenzene containing 20-30% dinitrobenzene and less than 1% dinitrophenol, 1% picric acid, and 1% sodium hydroxide were tested by Differential Scanning Calorimetry (DSC) and Accelerating Rate Calorimetry (ARC) to investigate their thermal stability relative to the pure substances. The DSC thermal curves for pure nitrobenzene and the various nitrobenzene-dinitrobenzene mixtures exhibited exothermic activity from about 300 °C to 500 °C and enthalpy changes of about −2.5 × 103 J g−1, which is very energetic. The impurities (dinitrophenol, picric acid, and sodium hydroxide) had no significant effect on the DSC results. During the ARC experiments, the various nitrobenzene-dinitrobenzene mixtures were found to be less thermally stable than pure nitrobenzene and pure dinitrobenzene, with exotherms beginning in the 263-280 °C temperature range. Analysis of ARC data indicates that short-term exposure of nitrobenzene mixtures containing up to 20 mass% dinitrobenzene to temperatures up to 208 °C should not pose a serious runaway reaction hazard.