Fluid-dynamics characteristics of a vortex-ingesting stirred tank for biohydrogen production

In this work, a novel stirred tank for the production of biohydrogen by fermentation of organic wastes is presented. The bioreactor is designed for ensuring effective mixing of the liquid phase, full contact between the substrate and the biofilm and energy efficient hydrogen recovery. A vortex-ingesting dual impeller stirred vessel equipped with a central draft tube containing the support for the attached-growth process is proposed. The local hydrodynamics features of the system have been investigated experimentally by Particle Image Velocimetry (PIV) and Digital Image Processing methods for determining the velocity fields of the two phases, the bubble size distribution and the central vortex shape at different working conditions. Additionally, power consumption was measured by a strain gauge technique for quantifying energy requirements. The results confirm that the reactor hydrodynamics characteristics are adequate for the selected application and that it can ensure the external gas recirculation towards a gas separation unit for the hydrogen recovery and purification, without additional energy input with respect to that provided for stirring.