PhD defense of Mr. Zafar Sadain

ZAFAR Sadain, PhLAM Laboratory - UMR8523 - Team MPI

Title: CO₂ hydrates as an alternative solution to water desalination and greenhouse gas mitigation

Jury: B. CHAZALLON (PhLAM, encadrant), C. PIRIM (PhLAM, encadrant), S. PICAUD (Université de Franche-Comté, UTINAM, Rapporteur), L. FOURNAISON (UR INRAE Antony, FRISE, Rapporteure), C. FOCSA  (PhLAM, membre), L. KRIM (Sorbonne Université, MONARIS, membre), M. DUBOIS (Université de Lille, LGCgE)

Abstract:

The release of substantial amounts of CO₂ into the atmosphere, particularly through flue-gas emissions, is not environmentally sustainable and requires reduction due to its profound impact on global warming. In the meantime, the shortage of potable water is a significant challenge that may worsen in the coming years and requires immediate attention. According to the 2023 UN World Water Development Report, approximately two to three billion people worldwide are affected by water scarcity. Hydrate-based distillation (HBD) technology is one of the potential approaches that offers a promising solution for tackling both CO₂ emissions and potable water scarcity. It effectively removes salt ions and captures CO₂ simultaneously. The objective of this study is to enhance hydrate growth with the aim of improving water recovery and CO₂ capture parameters.

A unique high-pressure capillary set-up is coupled to a micro-Raman spectroscope to investigate the formation of hydrates in 0.0, 3.5, 7.0, 10.5, and 15 wt.% NaCl solutions at constant pressure. Raman spectroscopy is found to be a sensitive tool for analyzing hydrate formation in salt solutions by probing the OH-stretching band and CO₂ vibration modes as function of thermodynamic and kinetic parameters.

First, hydrates were synthesized using two different experimental protocols. The 1^st protocol investigates the influence of CO₂-saturated and CO₂-unsaturated solutions on hydrate formation in a 3.5 wt.% NaCl solution. The 3.5 wt.% NaCl solution is used to replicate the concentration of seawater.

For the first time, we evidenced that the spatial distribution of secondary hydrates in the reactor depended upon that of CO₂ in the initial NaCl salt solution. The 2^nd protocol is set to ensure equilibrium and CO₂-saturated conditions during hydrate formation in salt solutions. At 263K, secondary hydrates were observed at the interface and in the middle parts of the capillary for 0.0 and 3.5 wt.% NaCl samples. However, for 7.0, 10.5, and 15 wt.% NaCl samples, we only observed hydrates close to the interface. Our results highlight that partial and complete primary hydrate dissociations significantly impact secondary hydrate formation, along with the effect of salt inhibition.

Second, different protocols in optical microscopy are used to determine CO₂ solubility, hydrate crystal growth rate, and CO₂ capture. Our findings suggest that CO₂ solubility is the highest in pure water and decreases as salt concentration increases. Further, the growth rate of CO₂ hydrate crystals is found to decrease at high subcooling. This unexpected behavior is observed when we applied a mass transfer limitation regime with no diffusional flux from the gas phase to the crystal, i.e., in a situation that is often encountered in extraction lines due to the formation of hydrate plugs. Moreover, our new protocol allows a quantitative assessment of the total amount of CO₂ captured. A higher amount of CO₂ is captured in pure water relative to that in salt solutions. Furthermore, the captured quantity in secondary hydrates showed a strong dependence on both the distribution of primary hydrates and the concentration of salt solutions.

Finally, for the first time, in-situ Raman spectroscopy is applied to assess water recovery parameter. This involves determining the final salt concentrations (S_f) in the liquid phase in the presence of hydrates, employing calibration curves. Water recovery is determined using the OH-stretching band, which is sensitive to temperatures and salt concentrations. We observed higher water recovery in a 3.5 wt.% NaCl solution compared to other salt solutions. Water recovery depends on the distribution/formation of primary and secondary hydrates, which are also functions of partial and dissociation temperatures.