INTEGRATED CALORIMETRIC TECHNIQUES APPLIED TO RUNAWAY REACTIONS ANALYSIS

The main objective of this thesis is the study of runaway reactions and their preventive and protective measures. The experimental approach is focused on the integration of calorimetry, a well consolidated technique in process safety assessment, with other techniques in order to overcome possible limitations and to obtain additional information about the process conditions. The first results obtained during this work are about the efficiency of the stability criteria for thermal runaway present in literature. The reaction under study was the esterification of the acetic anhydride and methanol catalysed by sulfuric acid and analysed in isoperibolic conditions as a function of jacket temperature and catalyst quantity. This mild runaway reaction represents a severe test for the reactor stability criteria. The main result was the advantage of using sensitivity based criteria because they are suitable to on-line implementation aimed at the early detection of thermal explosions. An Early Warning Detection System based on divergence criterion, which is a sensitivity based one, was applied to the data deriving from experiments on the decomposition of the hydrogen peroxide in quasi-isothermal versus runaway mode. This system was analyzed with a pseudo adiabatic calorimeter modified in our laboratory in order to study the effect of the pressure on the decomposition. The runaway reaction developed by the peroxide decomposition is very rapid and exothermic, so it was possible to test the criterion in conditions very similar to those of a full scale thermal explosion incident. The data obtained experimentally were also used to evaluate the possibility of the use of a screening instrument for vent sizing and this was found to be the greater limitation of this cost efficient technique. The rest of the thesis was dedicated at the investigation of real industrial incidents involving highly reactive substances that often undergo undesired exothermic reactions during their transport, storage or process: monomers. The first incident analysed was the self-polymerisation of commercial 63% divinylbenzene in different conditions of temperature and oxygen quantity in order to prove that an overfilling of the tank or an excessive storage temperature can make the inhibition mechanism ineffective and lead to self-initiated runaway polymerisation. The second incident was about the runaway polymerisation of methyl methacrylate added with accelerators. The experimentation allowed us to prove that accelerators affect the polymerisation rate even if with no initiator is present in the system This has been a significant finding because of the novelty of the process that has been often cause of incidents in the last years. A model of the system was formulated and the results of the simulations helped us in establishing the potential hazards associated with the use of accelerators in resin manufacture. In conclusion this work dealt with the currently relevant problem of runaway reactions and their different aspects, including possible preventive and protective measure by an experimental integrated approach, trying to find solutions that can be concretely implemented in industrial scale reactors to improve process safety of highly reactive systems.