This paper describes the development of a dynamic simulation model for stirred tank batch or semi-batch chemical reactor and its heating/cooling system. Heat and mass balances are established for the reactor and its jacket. As the general purpose is the thermal control of the reactor, special attention is devoted to the behaviour of the heating/cool-cooling system. The control strategy is based on the use of the thermal flux as manipulated variable. At each sampling time, the controller computes the thermal flux to be exchanged with the fluid flowing inside the jacket to get the desired reactor temperature set-point. This information is then used to select the thermal fluid on the basis of the maximal and minimal thermal flux capacities of each utility. The computer simulation program is flexible, enabling simulation of a batch or semi-batch reactor vessel, ranging from a laboratory pilot plant to full-scale production plant. To demonstrate the good performance of both simulation model and control methodology, experimental results are presented for a pilot plant and simulation studies have been performed for both pilot plant and industrial reactors.