Process safety evaluation of the nitration synthesis process of Pendimethalin

In this research, pendimethalin was synthesized by a two-step reaction with N-(1-Ethylpropyl)− 3,4-dimethylaniline as raw material and nitric acid as nitration reagent. The research focused on the second-step reaction, which was far more dangerous than the first-step. The effects of reaction temperature (T_p), feeding time (t), stirring rate (SR) and nitric acid concentration (φ) on the yield and thermal hazard of the second step reaction were explored by experiments. The optimal process conditions were obtained. In addition, concentration variation of each component during the reaction were monitored by using HPLC for intermediate sampling. Combined with the theoretical calculation, the possible reaction mechanism was deduced. The reaction can be divided into two main paths according to the sequence of nitro substitution. In one of these paths the nitro group replaced the H first at position 2 and then at position 6, while in the other path it was completely reversed. Both paths eventually produced pendimethalin. Finally, the thermal hazard was analyzed by Stoessel criticality diagram. The risk levels for the two-step reaction were level 1 and level 3, respectively. This study provides theoretical support for the safe production of the nitration and synthesis of pendimethalin.