For the relief area design of safety devices in the case of runaway conditions as a precondition reaction, kinetic data are required. Generally, they are measured in dedicated (quasi adiabatic) reaction calorimeters. In the course of the design, it is then assumed that the same reaction kinetic data in the laboratory and production scale prevail. This assumption seems to only be useful in the case of homogeneous liquid phase reaction systems. In contrast to this supposition the results of experiments with a heterogeneous (immiscible) liquid-liquid reaction system show that the stirring intensity and the reactor size can considerably influence the reaction progress. A novel formal macro reaction kinetics model for thermal runaway heterogeneous reactions including the mixing intensity as scaling parameter is presented.