A methodology for automatic generation of problem specific process models to be used in simulation related to process design and operation is presented. The methodology is essentially based on information related to setting up control shells, physical description of the interior of the control shells, and, specifications of the various types of interactions between the control shells and the surroundings. From the description of the control shell, the computer aided system generates the problem specific model equations from a reference model which contains all the necessary terms for modelling a chemical process. The algorithmic aspects of the methodology are highlighted and two non-trivial applications are presented.