In this work the operation of an industrial semi-batch reactor is optimized. In the reactor a strongly exothermic polymerization reaction takes place and the objective is to minimize the duration of the batch time. Various operational as well as quality and safety related constraints have to be met during the batch and at its final time. Tn particular, a cooling system failure is taken into account explicitly since the temperature rise in this case must not exceed a corresponding limit. The optimization is based on a detailed process model derived from first principles. A reduced model is developed for optimization and trajectories for the operational variables feed flowrate and reactor temperature are calculated. The results show that significant reductions of the batch time are possible and that their extent depend on the formulated safety constraints. For a selected case the obtained optimal trajectories are verified experimentally in laboratory and production scale. (C) 2000 Elsevier Science Ltd. All rights reserved.