Mixing and liquid-to-gas mass transfer under digester operating conditions

•A dual-impeller stirred tank devoted to dark fermentation was investigated.•Hydrodynamics, mixing and mass transfer were compared using nine mixer designs.•Local flow field, vortex formation, kLa and straw suspension were also studied.•Experimental data were successfully confronted to CFD simulations.•Finally, the best compromise for mixer design in the digester was defined.