(414c) Continuous Powder Blending inside Twin Screw Extruder

Purpose: Our study aimed to develop fundamental understanding of powder blending at the early stages of twin screw extrusion as function of material properties (non-cohesive and cohesive powders) and process conditions using Discrete Element Method (DEM) and experimentation. Previous work in the pharmaceutical and food industry has focused on mixing when materials are melted, or homogeneity at the output of the extruder.

Methods: DEM models were developed to study powder flow/mixing inside the extruder. The simulations were performed using EDEM^TM and the Hertz–Mindlin contact model and JKR cohesion model. The geometry and dimensions of the extruder were based on the ZSK-18 Coperion Twin Screw Extruder. Experiments were subsequently conducted in the ZSK-18 to characterize powder mixing efficiency. Near-infrared (NIR) spectroscopy was used to evaluate blend uniformity. A clear extruder was built to experimentally visualize powder mixing inside the extruder. The experimental and DEM simulation factorial designs included screw speed and powder flowrate. The screw configuration consists of conveying elements and one reverse kneading element. For the DEM models, the materials properties (inputs to the simulation) and process conditions were defined based on experimental data. In addition, residence time distribution was studied experimentally and using DEM.

Results: Experimental results show that process parameters and materials flowability (non-cohesive vs cohesive) affect the mixing efficiency and mean residence time (MRT). The simulations RTD profiles are similar to an ideal continuous stirred tank reactor (CSTR) profile with a sharp peak and a tail.

Conclusion: Feasibility of powder blending inside the extruder using only one reverse kneading element was demonstrated. The developed DEM model was able to elucidate the mixing mechanisms behind the observed phenomena. DEM was also demonstrated to be a useful development and visualization tool that helped answer questions about powder blending inside the extruder.