It is of great importance to accurately measure or predict the Residence Time Distribution (RTD) in designing extruders and/or in setting up a proper operating condition, because chemical reactions depend significantly on the RTD and temperature when chemical reactions take place during the extrusion process. A previous method to predict the RTD can analytically determine RTD, Residence Time Distribution Function f(t) and Cumulative Residence Time Distribution Function F(t), based on a simplified two-dimensional velocity field in an extruder. However, this previous method cannot accurately take into account the three-dimensional circulatory flow inside the extruder. The present paper suggests a new method to accurately determine the RTD taking into account the three-dimensional circulatory flow and presents a new formula derived to calculate f(t). In order to demonstrate the applicability of the new method including the circulatory flow effect, RTD, f(t) and F(t) were calculated based on a three-dimensional velocity field obtained via a quasi-three-dimensional finite element analysis. It was found that the previous method has a tendency to underestimate the RTD, owing to the neglect of the three-dimensional circulatory flow in comparison with the new method.
