Effect of promoters on CO2 hydrate formation: thermodynamic assessment and microscale Raman spectroscopy/hydrate crystal morphology characterization analysis

The use of carbon dioxide, originated from industrial processes, for gas hydrates production, is currently gaining the interest of researchers, due to the opportunity to increase the rate of methane recovery and, furthermore, to permanently store this greenhouse gas in solid form. However, the production of CO2 hydrates requires relatively high pressures and low temperatures and energy costs are often too elevated to make these applications feasible. The use of chemical promoters often allows to get around this problem and innumerable researches describe equilibrium condition for CO2 hydrates in the presence of the available and competitive promoter. This work deals with the consistent lack of information about the molecular and crystalline properties of these compounds, which play a crucial function in improving the efficiency of the process. In this research, CO2 hydrates were formed in the presence of THF and TBAB in the high-pressure optical cell. The Raman spectra and hydrates crystalline characteristics were observed in situ. The phase equilibrium results indicated that THF extended the temperature range for the stable presence of hydrates, and the optimal promoting concentration was 4.22 mol%. The same mass fraction of THF with NaCl solution still performed as a promoting effect with a narrow equilibrium temperature range. The structure of crystals was described with accuracy and the variation in crystals' morphology associated with the presence of each specific additive, was characterized. The CO2-THF sII hydrate tended to form prismatic shaped crystals, and during the decomposition, the THF molecule separated from the clathrate cage, showing an internal collapse on the crystal scale. The CO2-TBAB semi-clathrate hydrate is columnar in shape and had a similar decomposition process. This work might help comprehend CO2 hydrate storage technology and could give theoretical basis for subsequent applications in the sector. (C) 2021 Elsevier B.V. All rights reserved.

Automated summary

The utilization of CO2 in gas hydrates production has gained interest due to its potential to increase methane recovery rates and store the greenhouse gas in a solid form. However, high pressures and low temperatures are required for CO2 hydrates production, along with elevated energy costs, which can be mitigated using chemical promoters. This research focuses on the molecular and crystalline properties of CO2 hydrates formed with THF and TBAB, showing that THF extends the temperature range for stable hydrate presence and has an optimal promoting concentration of 4.22 mol%.