Fluid dynamics characterization of a stirred model bio-methanation digester

This work deals with the power consumption and the fluid dynamics features of a model digester for biogas production by agricultural scraps fermentation. A combination of experimental techniques is adopted in order to obtain the full characterization of an industrial digester stirred by multiple impellers, which is scaled-down following the geometrical similarity criterion, under various impeller speeds and three liquid heights. A number of critical fluid dynamics characteristics of the reactor are identified: the velocity field is strongly dampened far from the impellers region, thus leading to wide stagnant zones; the shape of the discharge flows of the impellers and their interaction are significantly affected by the Reynolds number; under specific conditions, flow instabilities of wide amplitude take place; the power consumption is different from that of standard configurations and depends on the liquid level. Geometrical modifications are suggested for achieving better mixing and enhancing the overall process performances.