The most outstanding feature of batch distillation is its flexibility in design and operation. In particular, when chemical reactions and physical separations have some overlapping operating conditions the combination of these tasks can offer significant benefits. In this work, we propose a new operation mode for reactive batch distillation in middle-vessel columns. The special feature of this novel operation mode lies on the fact that depending on the characteristics of the reaction mixture, the reaction will also take place either along the upper column or along the lower column. The performance of the novel operation mode is demonstrated through rigorous simulation of an industrial reactive batch distillation process. Furthermore, since the performance of the start-up procedure has a large impact on the entire reactive batch distillation process, we also focus on the description of the dynamic behavior of the start-up operation starting from the cold and empty batch columns as the initial state. The proposed model includes both equation and variable discontinuity. Thus, we propose in this work a modeling approach to describe the state transitions during the start-up phase. (C) 2007 Elsevier Ltd. All rights reserved.