SIMULATION OF FLOW, MASS TRANSFER AND BIO-CHEMICAL REACTIONS IN ANAEROBIC DIGESTION
The aims of the dissertation were to understand and simulate flow, mass transfer and bio-chemical reactions in anaerobic digesters. Computational fluid dynamics (CFD) and anaerobic digestion model No. 1 (ADM1) were used to assist the development of a new high solid anaerobic digestion system. The kinetic theory of granular flow (KTGF) was introduced and a multi-fluid model was developed to describe the phenomena of settling and suspension in the anaerobic digestor which is critical to increase biomass retention and improve digestion performance. To assess flow fields in a HSAD, a mechanical mixing model was constructed to predict flow characteristics. The model results show impeller A-310 was not suitable for agitating a high solid digester due to its impaired mixing performance and large shear rate. The helical ribbon had a better potential to be more suitable for the mixing of high solids digesters than the Impeller A-30. Its low shear environment is suitable for microbial flocs in anaerobic digestion. Through optimization, the power of the helical ribbon used in high solids digesters can be reduced significantly. A comprehensive model based on ADM1 was developed to describe a new two-stage HSAD system. The predictions indicate that a high rate HSAD system could be achieved by adjusting major parameters such as pH, recycled methanogenic bacteria, and UASB section area. Recycled methanogenic bacteria increased methane concentration and decreased hydrogen concentrations in the HSAD reactor, while pH increased in the batch mode, but decreased in the continuous mode with an increase of recycling rate. The UASB height had little impact on the acetic acid concentration and methane production. The mechnisim of a high rate digestor was explored by a multi-fluid model with KTGF. The simulation results suggest that the dairy manure particles tend to have soft and deformable fluid properties due to the lower contribution of collisional and kinetic components. The evaluation for biomass retention allowed the determination of the optimum SRT in anaerobic digestion. Aided with CFD simulation, the scale-up effect of the hydrodynamic nature from the bottle reactor to the column reactor was reduced.