The Limits of Modelling Based on Transport Phenomena

Since the start of Section 3.1 we have been presenting how the transport phenomena equations are used for the mathematical modelling of a process and how we transform this model into a process simulator. 

Actually, research by modelling is more and more extensively used in many applications because complex devices models, composed of different elements, can be made by assembling models the solutions of which are frequently available. This behaviour presents an impressive growth and is sustained by the extraordinary developments in numerical calculations and by the implementation of commonly used computers with a high capacity and calculus rate. Nevertheless, modelling based on the equations of transport phenomena cannot be applied to every system, because they can present some limitations, which are summarized here. 

The first limit derives from the model construction and can be called the constructive limit. It is explained by the quantity of simplifications accepted for model construction. The flow reduction by use of ideal models and the treatment of the transfer processes in equilibrium by using abstract notions - as for example, theoretical plate in distillation - represent only two of countless similar examples. 

The second limit is named the cognition limit and arises from the less controlled assumptions concerning the complicated and ill acquainted phenomena involved in the process. Considering the interface as an equilibrium Gibbs interface and introducing the turbulent flow from the turbulent diffusion coefficient are two famous examples which illustrate this class of cognition limits. 

The third limit is represented by the validity limits of the transfer phenomena equation. With respect to this last limitation. Fig. 3.6 shows the fixation of these limits with regard to the process scale evolution. 

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