CFD Prediction of Mixing Time in Anaerobic Digesters

A computational fluid dynamics (CFD) model for the calculation of mixing time in anaerobic digesters with mechanical mixing was developed. CFD simulation was carried out in two stages: (1) using a multiple reference frame (MRF) approach to predict steady-state turbulent flow, and (2) solving a transient scalar transport equation to obtain the mixing time based on the converged flow fields. Prediction of the mixing time in a cylindrical vessel with a Rushton turbine impeller was validated against experimental data from the literature. Flow simulation tests were conducted to determine economical mesh cells, appropriate turbulence model, and numerical approach. Sensitivity analysis was used to evaluate the effects of the position of an injected tracer, amount of tracer injected, and rheological properties on the mixing time. Moreover, mixing performance in a full-scale anaerobic digester with four mixing designs was examined using the mixing time.