Thesis of Carla Rodriguez

Amphithéâtre Pierre Glorieux (CERLA) Thesis defense of Carla Rodriguez - laboratory Phlam Abstract : Nowadays, fossil fuels are constantly burnt to fulfill the increasing human and industrial demand in energy, and as a consequence, large quantities of greenhouse gases such as carbon dioxide (CO2) are released in the atmosphere and contribute to global warming. It is therefore pressing to develop efficient post-combustion CO2 mitigation techniques that are also efficient and environment-friendly, and as such, Carbon Capture and Storage (CCS) technologies involving the Hydrate-Based Separation Process (HBSP) have attracted a lot of attention. HBSP consists in encapsulating small gas molecules (e.g. CO2, nitrogen (N2), methane (CH4)) within crystalline ice-like compounds known as clathrate hydrates or hydrates. Previous works have shown that promoters like tetra-n-butyl ammonium bromide (TBAB) considerably improves the guest-gas trapping mechanism in semi-clathrate hydrate (sc). Hence, while HBSP proves to be a suitable technique for selective CO2 capture and energy recovery, advancing the fundamental understanding of processes at play is still needed before large-scale practical applications can be routinely considered. This work aims to better comprehend CO2 separation and capture processes using sc-hydrate technology, while also exploring exchange processes in hydrates to open a perspective towards industrial applications.
First, the guest distribution in the hydrate phases of CO2-based clathrate hydrates as a function of parameters (initial composition, p, T) is revisited and elucidated by ex-situ high-resolution Raman spectroscopy. Up to now, there is a gap in the literature regarding the discrimination of the contribution of the small and large cages in CO2-based hydrates, mainly due to the Fermi resonance effect. So far, only a single study has attempted to distinguish these contributions in CO2-clathrates, however with a questionable interpretation. One of the novelties of the present work is to revisit the vibrational properties of CO2-clathrates to identify distinct frequency shifts depending on the structural environment of CO2 molecules, thereby improving our knowledge of CO2 encapsulation mechanisms in hydrates. High-resolution Raman analysis and neutron diffraction analyses are additionally performed in CO2-based TBAB-semi-clathrates for characterization purposes.
Second, the influence of two different formation protocols (quick and slow crystallization protocols, commonly used in hydrate formation) on the encapsulation mechanisms, the structure, and the selectivity of CO2+N2-TBAB compounds is investigated by in-situ Raman spectroscopy. A new dissociation point (pressure and temperature) is obtained and our results highlight that slow hydrates formation rates exert a variable performance on CO2 selectivity at temperatures far from the dissociation point, while a better performance is observed when approaching dissociation. Similarly, separation factors reach their greatest values close to the dissociation, depending however on the sc crystal structure formed. Surface morphology variation is monitored by optical microscopy and exhibits a continuous transformation with temperature, starting from a rough surface coated with polygonal or stacked shaped crystals to the formation of columnar TBAB-sc crystals near dissociation. Moreover, the influence of the formation kinetics on CO2 separation and selectivity is explored.
Finally, a potential application of CO2 separation and capture by HBSP is addressed through the investigation of the exchange mechanism when exposing CO2 clathrate hydrates to N2 gas. Raman measurements revealed that the structure sI remained intact upon N2 injection into the CO2 hydrate, with a significant occupation of the small cages by N2 molecules and partial occupation of the large cages. Therefore, a solid state diffusion in the large cages may occur during the exchange reaction. Our experimental data were fitted with the Avrami model to investigate the kinetic of crystallization, which showed a linear growth during the first hours of the exchange. Keywords: CO2 capture, selectivity, Raman spectroscopy, Tetra-n-butyl ammonium bromide (TBAB), semi-clathrate hydrates