Residence Time Distribution in an Asymmetrical Twin-Screw Extruder

Residence time distribution (RTD) is an important parameter for characterizing axial conveying and mixing in a twin-screw extruder (TSE). This work studies the RTD and flow patterns in an asymmetrical TSE (ATSE) using a transparent barrel for visualization. The RTD of an aqueous solution of sodium carboxymethyl cellulose is measured by using red fine sand particles as tracers, collecting them by a moving thin film at the die exit, and counting them for further statistical calculation. The effects of the screw configuration, screw speed, and feed rate are considered. When the geometrical dimensions and operating parameters are identical, the mean residence time t̅ in the ATSE is larger than those in a classical (symmetrical) TSE. This is a signature of backflow occurring in the intermeshing regions through the radial gaps between the low flight and the inner wall of the barrel. It is confirmed by particle image velocimetry. Moreover, a response surface model is developed to quantify the effects of the screw configuration and operating parameters on t̅.