Biomethane upgrading process: thermodynamic study of solid-liquid-vapor equilibrium for methane rich mixtures

Biomethane plays an important role in the context of bio-fuels consumption and energy decarbonisation policy. For its production, raw biogas obtained from agricultural or landfill wastes has to be upgraded by removing carbon dioxide and nitrogen. Thanks to the increasing demand of Liquefied Biogas (LBG), cryogenic upgrading techniques are becoming more competitive compared to classical upgrading technologies. Optimization of energy consumption and technical challenges such as CO₂ solidification, require reliable models for the simulation of the process. The objective of the thesis is the study of an optimized cryogenic technology applied to a biogas upgrading and liquefaction process. The base of the study is the thermodynamic modeling of the phase diagrams of biomethane mixture, constituted of methane as a major component and nitrogen, oxygen and carbon dioxide. The biogas upgrading is obtained through CO₂ solidification and separation in a range of temperature of -160 to -90 deg. C at pressure from atmospheric to 30 bar. For this reason the study is focused on solid - fluid equilibria. In order to identify solid CO₂ conditions and define the phase diagrams the thermodynamic model is tested and calibrated on experimental data. In order to study the influence of nitrogen and oxygen, original solid-liquid-vapor equilibrium data are produced, providing the composition of the vapor and liquid phase. Results obtained from the model are used for understanding and optimizing the upgrading and liquefaction process. (author)