Analysis of Solid-Liquid Interactions in a Model Bioreactor

The aim of this work is to characterize the two-phase fluid dynamics occurring inside a model stirred bioreactor resembling at lab scale the typical design adopted industrially for the production of biogas by fermentation of agricultural scraps. As for the solid-liquid system, water and PMMA particles of average diameter equal to 180 µm, resembling the unfermented crushed scraps industrially employed are selected. The experiments are carried out by two-phase Particle Image Velocimetry, which allows to simultaneously and separately measure the velocity fields of the continuous and of the dispersed phases. The unconventional geometry of the unbaffled stirred tank makes the measurement of the flow field induced by the impellers and the evaluation of the interactions between the solids and the liquid phase particularly challenging. The single-phase mean velocity field is preliminary investigated under both transient and stationary conditions at various impeller speeds. The two phase flow measurements are carried out at average solid mass concentration of 0.5 % and at impeller speeds lower than that corresponding to the just suspended condition, therefore in all the cases a deposit of particles accumulated at the bottom of the tank, modifying its shape. The comparison of the results collected under single- and two-phase condition highlights that the un-suspended solid layer gives rise to a significant variation of the liquid mean and turbulent velocity fields, confirming the strong dependency of the flow field from the features of the tank bottom. These results are associated to the uneven distribution of particles inside the reactor and to consequent very high concentration of solids in the bottom region. Overall, the analysis of the results allows to gain insight into the fluid dynamic behaviour of the model bioreactor and to suggest possible geometrical modifications aimed at improving the performances of the industrial apparatuses.