Mixing assessment of an industrial anaerobic digestion reactor using CFD

A computational fluid dynamic model was developed for an industrial anaerobic digestion reactor to assess its mixing efficiency. The mixing was evaluated by inspecting different parameters, including velocity patterns, dead zones and residence time distribution (RTD). Single-phase simulations were conducted considering and neglecting the presence of solids content and its effect on the viscosity of the slurry. The fluid was considered Newtonian in the absence of solids and non-Newtonian when solids were present. Dead zones were observed in both mixing and expanded sludge bed sections. These zones comprised, respectively, 49 vol % and 10 vol % of the mixing section and the expanded sludge bed section. The formation of the dead zones can be attributed to inefficiency of the mixing section in pumping flow upward into the middle of the reactor, especially, near the internal circulation pipe. Hence, a modification considering the discussed reasons would be beneficial to avoid the formation of dead zones. In addition, a compartment model representing the RTD is proposed, in which different patterns of plug flow, continuous mixed zones, dead volumes and recycle flow are considered.