In this project, the hydrodynamics of two laboratory-stirred tank reactors (CSTR) for producing biogas from bean straw hydrolysate were characterized numerically. Simulations of the equations of the quantity of motion were performed over a range of stirring speeds between 20 and 400 revolutions per minute. The results based on the velocity profiles, streamlines, axial and radial mixing time scales, and the average swirl diffusivity, suggest reactor operation at stirring speeds no greater than 200 rpm.