We begin the last part of the textbook, which refers to chemical reactors when they cannot be considered systems characterised by ideal fluid dynamics. The topic is divided into two chapters: in this one, we will focus on fluid dynamics aspects, without the need to consider systems with chemical reaction occurring. The main causes of non-ideality will be illustrated, as well as the function that can characterise a non-ideal system, i.e. the distribution of residence times (with its properties). The tracing methods for obtaining this function will then be shown and, when necessary, reference will be made to ideal systems. This is because, in some cases of interest, it is possible to apply compartment models that make the real system approachable through block diagrams consisting of ideal reactors.