Investigation of thermal runaway in semibatch chemical reactors by an early warning detection device

In the last decades many efforts of chemical engineers have been focused on the development of preventive and safety systems for maintaining the stability in chemical reactors in which runaway phenomena may occur. Notwithstanding this, literature data well demonstrate that runaway events annually occur in batch and semibatch reactors with such a frequency that it cannot be ignored, even if almost the totality of these phenomena lead only to production loss with some damage to the equipment [1]. Therefore the adaptation of an efficient and robust early warning detection system, which is potentially applicable for an on-line supervision, is still a current issue in industrial chemical plants. In this work we have investigated the behaviour of an early warning detection device to control the temperature within a laboratory reactor (jacketed and stirred, 2 L) during the methyl methacrylate emulsion polymerization process carried out under semibatch conditions. The control methodology is based on techniques derived from non-linear dynamical system theory to characterize the sensitivity of chemical reactors. By evaluating the divergence of the reactor is possible to distinguish between runaway and non-runaway situations without producing false alarms [2]. The divergence is a scalar quantity defined at each point as the sum of the partial derivatives of the mass and energy balances with relation to the correspondent variables (conversion and temperature). The algorithm of the early detection device is able to calculate the divergence from only temperature measurements (in the reactor and jacket) by using phase space reconstruction techniques [3]. Experiments were carried out by feeding continuously both the monomer and the initiator solution into the reactor, allowing the reaction to self-sustain by keeping the reactor temperature within a limited range (80-85°C), which is optimal in terms of product quality and process safety. In order to test the reliability of the detection device, some failures and malfunctions were also simulated, such as the rupture of the agitator or feeding pumps. Results showed that the divergence-based detection system was able to detect dangerous situations, by distinguishing in advance runway phenomena from situations under normal operating control. In particular during the simulation of the agitator breakage, the presence of three thermo-resistance sensors located at different positions inside the reactor, allowed to identify the formation of hot-spots due to the missing of stirring. In that case the stability of the emulsion was lost and the polymerization occurred in bulk producing a higher chemical heat flow than emulsion polymerization, with potential dangerous consequences such as the explosion of the reactor. [1] K.R. Westerterp and E.J. Molga. Ind. Eng. Chem. Res., 43 (2004) 4585-4594. [2] J.M. Zaldívar and F. Strozzi. Chem. Eng. Res. Des., 88 (2010) 320-330. [3] G. Maschio, C. Ampelli, D. Di Bella and D.G. Lister. Dechema Monographs, 138 (2004), 537-541.