Formation of gas hydrates from single-phase aqueous solutions

Experiments and theory both demonstrate that crystalline hydrates can form from a single-phase system consisting of liquid water with dissolved hydrate former under appropriate conditions of temperature, pressure, and dissolved hydrate former content. At these conditions, the pressure required for hydrate formation will be equal to or greater than that required when a separate gas or liquid phase of hydrate former is present. It is possible to form hydrates from a single phase when the mole fraction of dissolved hydrate former is greater than the mole fraction which would exist in the water-rich liquid phase at the three-phase (vapor/water-rich liquid/hydrate or VLH) hydrate equilibrium pressure. However, this would represent non-equilibrium or super-saturated conditions with respect to hydrate formation. It is also possible to form hydrates from a single water-rich phase when the mole fraction of the dissolved hydrate former is less than that which would exist in the presence of a gas phase at the three-phase VLH hydrate equilibrium pressure. The pressure requirement for the formation of hydrate increases as the mole fraction of hydrate former decreases under these conditions. The possibility of forming hydrates from dissolved hydrate formers may have application to potential commercial processes; for example, in the sequestration of CO2 in the deep ocean and in the recovery of hydrates from suboceanic and permafrost regions. (C) 2001 Elsevier Science Ltd. All rights reserved.