Optimization of Dual-Stage Impeller Axial Positions for Mixing of Non-Newtonian Biomass Slurries: A CFD Investigation

Hydrolysis is essential for converting biomass into biofuels and value-added materials within a sustainable circular economy. However, the high viscosity, non-Newtonian rheology, and high-solids loading of biomass slurries pose significant challenges to reactor design, optimization, and scale-up, making experimental investigations costly and time-consuming. This study employs computational fluid dynamics (CFD) to investigate flow patterns and mixing characteristics in a baffled, mechanically stirred tank designed for high-solids suspensions. A single pitched-blade axial impeller is first validated against published experimental velocity data for a Newtonian fluid using ANSYS Fluent, the multiple-reference-frame (MRF) approach, and the steady Reynolds-averaged Navier–Stokes (RANS) k–ε turbulence model. Several dual-stage impeller configurations are subsequently evaluated under Newtonian conditions to optimize bulk mixing while maintaining realistic tank hydrodynamics. Two identical impellers positioned equidistantly from the tank top, bottom, and each other increase average turbulence intensity by 24% compared with a single impeller in a 100 L tank. Extending the validated model and optimized configuration to non-Newtonian slurry conditions increases turbulence intensity from 8.40% to 10.42%. The results demonstrate that shear-dependent rheology causes flow damping and high-viscosity stagnant zones that cannot be adequately captured using a Newtonian model. Axial positioning of dual-stage impellers emerges as the most influential design parameter for minimizing these zones, with a greater impact than modifying blade size or blade number. These findings provide a foundation for improved biomass slurry mixing and future integration with a thermo-electrocatalytic biomass-to-hydrogen reactor.