Numerical study of particle mixing in a lab-scale screw mixer using the discrete element method

• Published in: Powder technology Vol. 308; pp. 334 - 345
• Main Authors: Qi, Fenglei, Heindel, Theodore J., Wright, Mark Mba
• Format: Journal Article
• Language: English
• Published: Lausanne Elsevier B.V 15.02.2017; Elsevier BV
• Subjects: Binary system; Biomass; Computer simulation; Demixing; Discrete element method; Double screw mixer; Experiments; Feed rate; Glass; Mathematical models; Mixing index; Multicomponent mixture; Particle mixing; Particulates; Plug flow; Plug flow chemical reactors; Simulation; Statistical analysis; Visual observation; Double screw mixer; Particle mixing; Discrete element method; Mixing index; Multicomponent mixture
• ISSN: 0032-5910; 1873-328X
• DOI: 10.1016/j.powtec.2016.12.043

This study employs the discrete element method (DEM) to simulate particulate flow and investigate mixing performance of a lab-scale double screw mixer. The simulation employs polydispersed biomass and glass bead particles based on experiments conducted in previous studies. Visual examination of particle distribution and statistical analysis of particle residence times of experimental data served as model validation. Statistical analysis indicates a maximum 9.8% difference between the experimental and simulated biomass particle mean residence time, and visual observations suggest the simulation captures the particle mixing trends observed in the experiments. Results indicate that the particle mean mixing time, non-dimensionalized by ideal flow time in the plug flow reactor, varies between 1.008 and 1.172, and it approaches 1 with increasing biomass feed rate. The mixing index profile in the axial direction shows a mixing-demixing-mixing oscillation pattern. Increasing screw pitch length is detrimental to mixing performance; decreasing the solid particle feed rate reduces the mixing degree; and increasing the biomass to glass bead size ratio decreases mixing performance. A comparison of a binary, single-sized biomass and glass particles mixture to a multicomponent mixture indicates that the binary system has similar mixing pattern as a multicomponent system. These findings demonstrate that DEM is a valuable tool for the design and simulation of double screw mixing systems. [Display omitted] •DEM is capable of predicting the granular flow in the double screw mixer.•Particle mixing time approaches to the residence time in the plug flow reactor.•Particle mixing in the mixer shows a mixing-demixing-mixing oscillation pattern.•Screw rotation speed does not affect particle mixing significantly.•Increasing pitch length and reducing solid feed rate decrease mixing performance.