Calculation of mixing times in single-phase and two-phase gas–liquid stirred tanks using positron emission particle tracking

• Mixing times obtained from PEPT measurements. • Validated and benchmarked for single-phase turbulent flow. • Existing correlations fail for aerated non-Newtonian systems. Positron Emission Particle Tracking (PEPT) enables novel insight into the mixing times of gas–liquid systems, particularly at high gassing rates, as it can interrogate opaque systems. Mixing times were obtained via analysis of Lagrangian statistics from PEPT, for single- and two-phase gas–liquid mixing experiments made in a T = 0.2 m diameter stirred tank equipped with either a radial pumping impeller (Rushton turbine, D = T /3) or axial pumping impeller (pitched blade turbine, D = 0.4 T ). Both Newtonian and non-Newtonian working fluids were employed using aqueous solutions of glycerol and carboxymethylcellulose, respectively, over a range of Reynolds numbers from 280 to 2300, in the high transitional to low turbulent flow regimes. These conditions were chosen to replicate the industrial processing of viscous media such as filamentous fungi. The approach was validated under single phase Newtonian conditions via benchmark acid-base decolourisation measurements and established empirical correlations, showing excellent agreement within ± 10%. Measurements made for gas–liquid Newtonian systems showed good agreement with established correlations in the dispersed aeration regime with both types of impellers. In non-Newtonian systems with gas void fractions of up to 9%, significant deviations in the measured mixing times from established correlations were observed, with values being up to 3–4 times greater in high viscosity, non-Newtonian media.