chemical engineering research and design 88 (2010) 304–319 Contents lists available at ScienceDirect Chemical Engineering Research and Design journal homepage: www.elsevier.com/locate/cherd Safety aspects in modelling and operating of batch and semibatch stirred tank chemical reactors Aleksandra Milewska a , Eugeniusz Molga b,∗ a Institute of Power Engineering, Department of Thermal Processes, ul. Augustówka 36, 00-981 Warszawa, Poland b Warsaw University of Technology, Faculty of Chemical and Process Engineering, ul. Wary´ nskiego 1, 00-645 Warszawa, Poland abstract Safety aspects in modelling of batch and semibatch stirred tank reactors as well as a model based safety analysis have been considered. Applicability of two basic types of models – i.e. the perfectly mixed reactor model and the CFD model, both formulated for laboratory scale as well as pilot plant scale reactors – has been discussed. A formulation of the appropriate reactor model, which is adequate to the considered case study has been demonstrated and tested experimentally. Particular attention has been devoted to the formulation of robust CFD models employed to simulate a performance of the stirred tank reactors. It has been found that models for perfectly mixed reactors may have quite wide range of application, while the CFD models should be definitely used in case of fast reactions, high viscosity of the reacting mixture as well as of failure leading to stopping of the impeller. The CFD models are able to predict a dynamic behaviour of reactors at any circumstances, so they can play a significant role in safety analysis carried out for industrial scale reactors, for which experimental safety tests are expensive and dangerous. © 2009 The Institution of Chemical Engineers. Published by Elsevier B.V. All rights reserved. Keywords: Thermal runaway; Reactor safety; Reactor modelling; CFD modelling 1. Introduction In a chemical reactor carrying out exothermic reactions, a loss of temperature control (thermal runaway) can be provoked when, for some reasons, the rate of heat generation due to reaction progress exceeds that of heat removal by cooling. In such a case an acceleration effect of the heat generation rate over the cooling rate can be observed, which results in a local or/and global temperature increase, which in turn accelerates the reaction rate, so increasing the heat generation rate and in consequence also the reactor temperature. In industrial as well as in laboratory practice, despite of significant improvements in safety analysis observed during last two decennia, still a lot of thermal runaway events have occurred—e.g. see Westerterp and Molga (2006) and Stoessel (2008). There are numerous papers and databases specifically dealing with causes of runaway incidents. Several analyses of thermal runaway reaction incidents are presented among oth- ers by Khan and Abbasi (1999), Mclntosh and Nolan (2001) as well as by Carpenter et al. (2004). It is interesting to note, that a significant percentage of runaway events is caused by: mis- ∗ Corresponding author. E-mail address: molga@ichip.pw.edu.pl (E. Molga). Received 10 March 2009; Received in revised form 20 August 2009; Accepted 20 October 2009 charging of reactants, lack of knowledge on thermo-reaction chemistry, improper temperature control as well as loss of mixing and/or cooling. Mischarging of reactants is, most of the time a human error outside of scope of this paper, how- ever the other causes are part of the main subject of research in chemical reactor safety. The lessons learned from previ- ous incidents demonstrate that an elaboration of efficient and robust reactor model and/or safety criteria is crucial for run- away prevention. However, a prediction of safe and efficient reactor operating conditions is not sufficient to avoid thermal runaways. Also on-line runaway detection systems should be employed, which could help to detect a dangerous tempera- ture excursion caused by unexpected events—e.g. failures of stirring or/and cooling systems. In these studies an integrated approach to safety aspects of modelling and operating of stirred tank batch and semibatch chemical reactors is presented and illustrated with experi- mental and simulation results. The proposed methodology deals with a model based safety analysis, which is one of three well-known stages of safety procedure. The first step of this methodology helps to avoid thermal runaway by choice 0263-8762/$ – see front matter © 2009 The Institution of Chemical Engineers. Published by Elsevier B.V. All rights reserved. doi:10.1016/j.cherd.2009.10.014