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

Gas hydrates are regarded as promising materials for various potential applications. In particular, CO2 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 CO2 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 CO2 concentration in the liquid phase was measured in situ during hydrate formation. CO2 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 CO2 transfers from the vapor phase to the liquid phase, whatever the experimental conditions tested. As soon as hydrates start forming, the CO2 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 CO2 dissolution during hydrate formation. This resulted in faster dissolution of CO2, though still transfer-limited formation kinetics. (C) 2018 Published by Elsevier Ltd.