All simulation models in chemical engineering are based on balance equations that are derived from the basic conservation law. However in the numerical solution we forget it and do not utilize the power of the strongest law of our profession. Recently an alternative methodology has been elaborated for the direct mapping of the studied conservational processes onto a descriptive computer programme that can be executed by the help of a general purpose simulator directly. The method eliminates the manifold transformation of the individual model building, from the cognitive model via the mathematical construct and the numerical algorithm onto the computer programme. In spite of the successful practical applications the theoretical foundation of the new modelling approach failed. In this paper a possible theoretical framework is proposed for the structure based modelling of the conservational processes. The theoretical issues are illustrated by a simple example.