Flows due to pressure induced dissociation-formation of gas hydrates

During the last decade, Gas Hydrates (GH) have attracted the interest of the scientific community for engineering applications. Carbon dioxide hydrate (CO₂H), for instance, may play an important role for capture and sequestration methods in order to reduce global climate change. Despite the extensive literature, the transport phenomena involved during CO₂H formation are not yet fully understood. CO₂ transfer from gas or liquid phase to the bulk of water is expected to happen not only by molecular diffusion but also driven by natural convective currents induced by CO₂ dissolution in water. Using particle tracer methods, we experimentally characterize the flow velocity of the bulk of water during CO₂H formation. For that purpose, CO₂H is grown inside an optical cell with a volume of 12 mL at various pressures and temperatures. Due to CO₂ dissolution, convection currents are noticed prior to hydrate formation. Our experimental results point to a significant correlation between this process and the subsequent hydrate formation. Two well-differentiated hydrate growth patterns were observed depending on the hydrate induction time and the corresponding CO₂ concentration distribution inside water. For long induction times, CO₂ can be provided from the water phase resulting in rapid growth. Short induction times resulted in slow growth at the interface creating a solid barrier accompanied by a significant drop in the flow velocity. In some cases, the hydrate layer appeared to be unstable and convection could restart. (paper)