Safe intensification of potentially runaway reactions: From semibatch to continuous processes

Fast and highly exothermic reactions are commonly carried out in semibatch reactors (SBRs) in order to better control the heat evolution by the feeding rate. In fact, for such processes, a phenomenon known as "thermal runaway", that is an uncontrolled reactor temperature increase, may be triggered whenever the rate of heat removal becomes lower than the rate of heat release. This dangerous temperature increase, occurring in practically adiabatic conditions, can trigger secondary undesired exothermic reactions or, in some cases, the decomposition of the whole reacting mixture with consequent reactor pressurization and, eventually, explosion followed by the release of high amounts of hazardous products. As a consequence, several studies on the detection of the so called "runaway boundaries" have been performed during years. However, from a practical perspective, the desired goal of whatever enterprise is to attain the maximum productivity maintaining safe conditions. Such a goal can be achieved using a series of continuous stirred tank reactors (CSTRs) operated in the isothermal temperature control mode; but a possible change of the reactor type, from discontinuous (e.g. batch or SB) to continuous (series of CSTRs), with the aim of increasing the productivity cannot be performed so easily when a potentially runaway process is involved. The main aim of this work has been to compute the number of CSTRs in a series that, guaranteeing the requested productivity and reactants conversion under safe operating conditions (runaway phenomena cannot be triggered), minimizes the volume of each reactor of the series. Such a number results to be a function of the employed kinetic scheme and the dosing policy of the co-reactants. In this work, two different dosing policies (1-co-reactant dosed into the first reactor of the series; 2) co-reactant dosed into the first NR-1 reactors of the series) will be analyzed for the relevant case study of the synthesis of N-(4-nitro, 2-phenoxyphenyl) methane sulphonamide. The obtained results have shown that it is possible to increase the overall productivity of the process, simply shifting from discontinuous to continuous operating mode, also achieving a safe intensification, that is, having lower reacting volumes at the full plant.