Kinetic study of CO2 hydrates crystallization: Characterization using FTIR/ATR spectroscopy and contribution modeling of equilibrium/non-equilibrium phase-behavior.

Highlights • In-situ and real-time measurement of dissolved CO 2 during CO 2 hydrate formation. • Drastic drop of dissolved CO 2 was observed during CO 2 hydrate crystallization. • CO 2 gas diffusion to the liquid was identified as the rate-limiting step. • Local equilibrium between liquid and solid phases was identified. Abstract Gas hydrates are regarded as promising materials for various potential applications. In particular, CO 2 hydrates are known for their cold storage capacities, related to their high latent heat of melting. They could be used as high efficiency Phase Change Materials in Phase Change Slurries for secondary refrigeration loops. A better understanding of the crystallization mechanism of CO 2 hydrates in slurries and of the resulting formation kinetics is still needed to evaluate and improve the efficiency of hydrate-based secondary refrigeration process. For that purpose, in the present work the real-time evolution of CO 2 concentration in the liquid phase was measured in situ during hydrate formation. CO 2 hydrates were formed within a stirred reactor equipped with an Attenuated Total Reflection probe coupled with a Fourier Transform Infra-Red spectroscopy analyzer. By comparing the measured concentration to calculations based on the assumptions of (a) a liquid-vapor equilibrium (LVE) and (b) a hydrate-liquid equilibrium (HLE), it was deduced that the crystallization kinetic is limited by CO 2 transfers from the vapor phase to the liquid phase, whatever the experimental conditions tested. As soon as hydrates start forming, the CO 2 concentration in the liquid phase almost instantaneously reaches the hydrate-vapor equilibrium (HVE) value at the experimental temperature, while the reactor pressure slowly decreases towards the LVE value defined by Henry’s law. Different stirring speeds were experimented in order to check the effect of enhancing CO 2 dissolution during hydrate formation. This resulted in faster dissolution of CO 2 , though still transfer-limited formation kinetics. [ABSTRACT FROM AUTHOR]