Estudo para redução do limite mínimo de adição de nitrogênio no reator de 1,2-dicloroetano

Abstract

The main precursor to polyvinyl chloride (PVC) is 1,2-dichloroethane (DCE). In industry, one of the most widespread processes for its production is direct chlorination, which involves the reaction of ethene with chlorine in the presence of the anhydrous iron chloride catalyst (FeCl3). To ensure operational safety and prevent conditions conducive to explosions in the reactor, it is essential to determine the flammability properties of gas mixtures at the top of the reactors. To this end, interlocking systems are used that automatically control the process according to the set point defined in the presence of dangerous oxygen concentrations. To avoid such conditions, nitrogen is added to create an inert atmosphere in the reactor. Therefore, to examine the possibility of reducing the minimum nitrogen addition limit in the reactor through explosivity limit analysis, this study aimed to reassess the flammability limits of O2 and N2 mixtures used in a typical 1,2-dichloroethane production process in a direct chlorination reactor by conducting HAZOP (Hazard and Operability Study) and LOPA (Layers of Protection Analysis) audits to assess potential risks. System simulation was performed using Aspen Plus software to obtain the gas mixture composition. The flammability limits were determined based on USBM-503 literature, and the explosivity region was analyzed using the ternary diagram (fuel-oxidant-inert). Results indicate that a higher nitrogen mass flow rate leads to the carryover of DCE to the top of the reactor. Nitrogen injection into the reactor dilutes the combustible gases and reduces oxygen concentration, maintaining safe operation. The graph analysis showed that at 160% operational load with the industry-adopted minimum nitrogen limit of 120 Nm³/h, the stoichiometric point remains outside the explosion zone, potentially reducing to 104 Nm³/h at 140% operational load. However, according to the NFPA (National Fire Protection Association) safety standards, it is not possible to reduce the minimum nitrogen addition limit due to variations between oxygen concentration and the limit of oxidant (LOC) concentration and the fuel mixture that prevents explosion in industrial systems. HAZOP/LOPA analyses indicate a low risk of explosion at the top of the reactor. Thus, the results show that the current approach is the most cost-effective in terms of nitrogen use, and the system operates within safety standards.