Semi-batch reactor control with NMPC avoiding thermal runaway

•The control of a semibatch reactor with highly exothermic reaction is investigated.•NMPC and runaway criteria are combined to determine the optimal feeding trajectory.•Process safety time is proposed to calculate the length of prediction horizon.•Control system is tested in case of preheating and without preheating the reagents.•Batch time and energy consumptions are calculated in the investigated cases. Several exothermic reactions are carried out in semi-batch reactors (SBR). A not suitable control system can lead to dangerous situations if thermal runaway develops. Reactor runaway can be avoided with application of a non-linear model predictive control (NMPC) with implemented runaway criterion to manipulate the feed flow rate of reagent, although the prediction horizon has to be chosen correctly. For this purpose, process safety time (PST) of the system is determined. Two different operation modes are considered. In the first case the loaded reagent is preheated to avoid accumulation, while in the second mode, only the produced heat by the reactions heat up the reactor. A simple PID controller and the proposed NMPC were tested in both cases. Operation results are compared to each other based on batch times and energy consumptions next to a safe operation. Runaway criteria can be successfully implemented in NMPC for the intensification of SBRs.