Recent Safety and Environmental Legislation

Abstract

Several worst case scenarios were carried out for safety evaluation of an existing nitration process. Heterogeneous nitric acid decomposition is observed to be the worst credible scenario in the toluene nitration process. Fire or explosion may subsequently develop due to temperature or pressure runaway from the undesired reaction. Gassy nitric acid decomposition can easily be triggered by process derivation from the desired reaction. In this study, a heat-flux calorimeter (C80–II) and an adiabatic calorimeter (VSP2) were used to measure the thermokinetics and determine the runaway characteristics for assessing worst credible scenarios. The heat of reaction of toluene mononitration is measured to be 190.9±6 3.6/(kJ mol−1). The calculated Arrhenius parameters are Ea (kJ mol−1) = 30.9±2.1 and A (mol−1.5l1.5s−1) = (1.38 ± 0.07) × 10−2. The ‘worst case’ scenarios were chosen at the following conditions: loss of cooling, stirrer failure, batch-wise charging, and decomposition of reactants. The experimentally determined worst case was found to be a combination of a loss of cooling with batch-wise charging coupled with decomposition of nitric acid with full mixing. Adiabatic temperature rise, self-heat rate, final pressure and non-condensable gas quantity in the VSP2 experiment were used to assess thermal explosion in a nitration reactor. Unstable and critical onset temperature to runaway is determined as about 140°C for the process in this study.