Runaway problems in unsteady state tubular reactors

In the specific literature of the last seventy years, the problem of identifying the operating conditions (temperatures, pressures, concentrations, residence times, etc.) in correspondence of which, for different types of reactor and operating modes (e.g., isothermal and isoperibolic), the thermal control of a reacting system can be lost, has been widely analyzed. For some industries, the conversion of chemical reactants is carried out using Plug Flow Reactors (PFRs), because continuous production is required or strongly advised. Throughout the scientific literature, the thermal behavior of these reactors have been always characterized referring to steady state operating conditions neglecting axial diffusivities (some models took into account radial diffusivities). The reason has been found in the extremely rapid dynamics that characterizes these systems and, consequently, leads them to rapidly approach steady state conditions. But in some cases, i.e., a change in operating conditions, originated by unexpected failures or just by wrong operations, can shift the process course from steady to unsteady state. Another case could be the start-up procedure: according to many industrial accidents reports that involved fast and highly exothermic reactions, this is one of most susceptible moments. For all these cases, from the safety point of view, the system dynamical behavior could be very important and cannot be neglected. The aim of this work has been to compare runaway boundaries obtained for steady state conventional operations of PFRs with that obtained under unsteady state operating conditions considering axial diffusivities. Obtained results have shown that the unsteady state aspect is very important to be considered in a safety analysis: this is because, even setting operating parameters ranges that a conventional steady state model predicts to be safe, during a start-up (or simply unsteady) operations temperatures capable of causing a runaway phenomenon can be reached.