Safe and optimal operation of complex production processes is one of the most important research and development problems in process engineering. This problem is most relevant at the design of the optimal feeding profile of fed-batch reactors due to the nonlinear and unstable dynamical behavior of these processes. This paper shows that the possible (thermal) runaway of these reactors should be forecasted till the runaway can be avoided by the control of the process. This requires predictive stability analysis. For this purpose a model based technique has been worked out, where Ljapunov indirect stability analysis of state variables along the simulated trajectories are used to detect the boundary of the controllable region of the process. The developed methodology has been applied to determine the safe operating regimes of a pilot plant fed-batch reactor.