This paper describes an expert system to assist process development. This system, PRINCE, aims to specify the parameters of a refining unit while respecting the constraints on the operating conditions and products. The originality of the procedure lies in the juxtaposition and cooperation of different types of knowledge, i.e. production rules containing expertise as well as equations; the use of numerical procedural codes for simulation. An overall task architecture manages the concatination and communication among all these types of knowledge.