Processes - The Residence-Time Distribution (RTD) is a chemical engineering concept introduced by Danckwerts in 1953. It has been described in a multitude of scientific papers and applied for various industrial processes. The development of Computer Fluid Dynamics will improve the comprehension and optimisation of such processes. However, this approach remains difficult in case of complex industrial processes. Therefore, the extension of the RTD concept is an alternative way to obtain hydrodynamic data and help for improvement of the processes. Actually, the models derived from tracer experiments are often restricted to the use of simple elementary reactors such as perfect mixing cells in series or plug flow with axial dispersion. The resulting information is often not sufficient for the understanding of complex processes. Better understanding may be obtained by the creation of complex networks of interconnected elementary reactors. However, that can lead to two problems to be solved: the first one is the possibility to realise an easy simulation of any complex network, the second one is to create realistic models on a sound physical basis. Indeed, complex models contain so many parameters that two different models may give the same result or the same model may give an identical result with different sets of parameters, A software package has been developed to simulate the response to an input of any complex network of elementary reactors properly interconnected. Processes with multiple inlets or outlets can be modelled convolution and optimisation procedures. The software may, equally be used to determine the parameters of different models giving the same response, and the subsequent examination of the physical soundness of these parameters leads to the choice of a realistic model In addition, local measurements may be validated through the possibility to simulate the local response within the model and to optimise the corresponding parameters. Also, a general procedure has been developed to optimise the different flow rates of models in complex industrial processes with many undetermined recirculation flow rates. Future developments including RTD under transient state and automatic generation of flow models are also presented. They are illustrated through experiments and literature analysis. Finally, different perspectives of recent concepts are suggested.
