The Integration of Calorimetric Techniques with an Early Warning Detection System

The reliability of a chemical reactor installed in a industrial plant is very important for the safety conditions of a process. It depends on the capability of the control and supervision system to estimate its state and to identify in time malfunctions or failure modes that can take place during the operating phases. The major problem that more frequently may occur in chemical reactors carrying out exothermic reactions is the loss of temperature control. In this situation, if the rate of heat generation by chemical reaction exceeds the rate of heat removal by the cooling system, the temperature of the reaction mass will rise , increasing in turn the heat generation rate. In this case, if no countermeasures are taken, a runaway reaction may occur. This work has been carried out within the European Research Project AWARD, (Advanced Warning And Runaway Disposal). This project intends to improve the quality, safety and reliability and reduce accidents in chemical plants by developing and testing a new device for early warning detection of runaway events (EWDS), based on the results of a previous research carried out at JRC of Ispra on the application of chaos theory techniques to discontinuous reactors. A prototype of the EWDS has been realized by Segibo and coupled to a reaction calorimeter at the University of Messina. The experiments carried out at Messina have allowed the validation of the EWDS in small scale reactors with reactions of industrial interest. The reaction studied is the free radical emulsion polymerisation of acrylic monomers, an important class of reactions of great industrial relevance. These reactions are characterized by a high exothermicity and self-accelerating reaction kinetics. Because of these characteristics polymerisation reactions have given rise to incidents due to thermal runaway. Batch and semi-batch experiments have been carried out in a jacketed, stirred reactor (capacity of 2 litres). For the batch tests, made in the temperature range 60-90°C, the emulsion is initially prepared in the reactor and brought to the reaction temperature: Before continuing the experiment the stability of the emulsion is checked; then the reaction is started by introducing an aqueous solution of the initiator into the reactor. The results obtained show that, overall for high temperatures, the reaction starts immediately after the addition of the initiator and there is such a rapid and large release of heat that the cooling system is not able to prevent a large increase in the reactor temperature. In some case rapid temperature increases of 40°C have been observed. It is not difficult to understand that, if a similar temperature increase should occur in an industrial reactor, a dangerous event could take place. The anomalous behaviours of the reactor temperature occurred during the batch experiments have been early detected by the EWDS, without any false alarms. The semi-batch experiments were made by initially charging the reactor with a partial amount of an aqueous solution of the emulsifier and bringing the system to the reaction temperature. When the system had stabilized a small quantity (~5%) of the monomer emulsion was added to the reactor. When the system had reached a stable temperature a small quantity (~5%) of the initiator solution was added and the reaction was considered to have started. After 15 minutes the semi-batch process was started by starting the constant flows of the monomer emulsion and initiator solution into the reactor. For both the batch and semi-batch experiments a constant stirrer speed in the range of 200-250 rpm has been used