Gas Hydrate Formation

Hydrates are ice-like structures that form when the right combination of water, hydrate former, temperature and pressure is present. Hydrates form at temperatures above those of pure ice formation and are not stable relative to ordinary ice.

When ice forms from liquid water, the decrease in entropy is relatively small because water itself is fairly ordered. When gas hydrates form, there is considerable reduction in entropy because the gas is condensed into a small volume. This is offset by the availability of the kinetic energy of the gas molecules, and a contribution from the van der Waals attractions between host and guest.

Methane hydrate occurs naturally both in permafrost areas and in the marine sediment in the oceans and deep lakes where pressure-temperature conditions are suitable, and where sufficient methane is delivered to the zone of hydrate stability in the uppermost sediments. Although current interest in hydrate is largely energy related, in that naturally occurring gas hydrates may be a major provider of methane for fuel, natural hydrates also appear to have an impact in global climate change and seafloor stability, as well as having a subtle effect at the base of the global food chain. Industrial hydrates, however, which are gas hydrates formed from certain mixtures of gases or otherwise formed to as to take advantage of some inherent property of the mineral species, such as extracting water molecules from seawater, are only beginning to be understood.

The hydrate crystallization process occurs in two steps: nucleation and growth. Nucleation may occur at a seed crystal or some imperfection at the surrounding vessel. On the other hand, crystals grow by the ordered deposition of material from fluid or solution state to a surface of the crystal. Hydrate nucleation and growth are dependent not only upon temperature, pressure, concentration of reactants but also on the availability of surface area. The transportation of ions to the surface, reactions at the surface and removal of reaction products from the surface need to be continually replenished with fresh solution, otherwise the solution around the crystal will become undersaturated and hydrate growth will stop.

The heat of hydrate formation and the heat of hydrate dissociation are equal in absolute magnitude but are of opposite sign. When hydrate forms heat is released from the system (exothermic) and when hydrates dissociate, heat is taken into the system (endothermic).