Reactor Design and CFD simulation of a bio-electrical system

This Master's Thesis focuses on the first steps in the simulation and optimization of a new bio-electrical reactor used for producing biomethane from organic waste waters, which may come from: sewage, cheese factories, breweries, distilleries. The work deals with the initial development of a CFD model for a biomethane-producing electrochemical reactor, and its validation with respect to experimental data. The model will be later adopted to layout and predict the performance of cylindrical cells for different anode and cathode geometries, positioning, and morphology. Outputs of interest include current density, methane production rate, liquid-gas separation, and utilization factor of organic contaminants. This work belongs to the broader research objective of realizing a novel bioelectrical reactor to produce highly upgraded biogas, with methane content over 90%, from wastewaters and other CO2 sources, ideally ready for gas grid injection. Bio-methane production is supported by literature evidence about Bio-electrical systems, able to consume wastewaters organics by oxidation, and form methane by reduction of the carbon dioxide. Many renewable applications may be based on this technology, which exploits microbial catalyzation to lower the energy demand of the process, allowing on one side to lower the Chemical Oxygen Demand of the wastewaters, while collecting the produced bio-methane as well. The scale-up of this technology and its marketability are now becoming to be addressed, by designing and testing bigger reactors, which may benefit from theoretical models and simulations to be optimized. Therefore, the topics investigated in this work have been addressed with CFD simulations, CAD design, and material selection. Along with electrochemical theory, a short overview on Computational Fluid Dynamics has been included, with the numerical methods involved. The work by Rader and Logan, “Multi-electrode continuous flow microbial electrolysis cell for biogas production from acetate”, has been selected as Reference Study, and their experimental data have been used to perform the model validation. The development of the model for a Microbial Electrolysis Cell has been carried out by presenting the key objectives, along with the assumptions made. In order of appearance, the geometry of reactor has been presented, followed by the developed mesh grid. Governing equations, describing mathematically the physical phenomena taking place, and electrochemical assumptions are followed by a section about the multiple phases present inside the reactor. By Comparing the numerical results to the experimental data of the Reference Study, it is argued that a partial validation has been met regarding the modelling of electrochemical mechanisms inside the reactor: the relative errors were below 10%. On the other side, the simulation of multiple phases, i.e. liquid and gaseous, did not lead to a stable convergence of the numerical solution and is the main aspect to be improved. Possible future improvements to continue the research on this sensible topic include the realization of experimental tests in order to characterize the parameters influencing the model such as the microbial population and its relationship with flow rate, the permeability of the selected electrodes and reference parameters of the Tafel Equation