Residence Time Distribution in Extrusion Processes

The concept of residence time distribution (RTD) was introduced in chemical engineering in the early 1950s, then developed further and applied to plastics processing in the 1960s. When a fluid flows through a reactor, the trajectories of the particles and their velocities along these trajectories are generally different. Consequently, the time spent in the reactor is not identical for all particles, but characterized by a distribution, which describes the fluid's time history. In many extrusion applications, the quality of the final product will depend on the thermomechanical history the material has undergone during the process. This is particularly important when dealing with reactive systems, products sensitive to thermal degradation, or when you wish to correctly extrapolate a process from laboratory to industrial scale.

It is therefore crucial to have access to this thermomechanical history, characterized in part by the residence time distribution. This requires the development of appropriate measurement techniques, capable of providing accurate data rapidly and without disturbing the process. This in turn requires the development of theoretical models, with the aim of simply describing or, better, predicting the RTD as a function of operating conditions. This is the purpose of this article, in which these various aspects of RTD will be addressed, and where possible, in the different extrusion processes, namely single-screw extrusion, co- and counter-rotating twin-screw extrusion, and the co-kneader.