Preventing of unwanted gas hydrate in pipelines and other hydrocarbon transmission apparatus, mitigating its extent, and removing it once hydrate masses affect flow assurance and safety has focused on inhibiting hydrate nucleation and growth. Formation of hydrate masses reduces pipeline flow and occasionally results in complete blockages. Removal of unwanted hydrate that affect flow and safety are very costly as the processes may involve close-down of transmission systems and application of costly engineering, operations, and chemicals.
Newer approaches to mitigation of unwanted hydrate focus on causing hydrate to form in such a manner that blockages may be prevented. Formation of hydrate slurries that are chemically inhibited from adhering to surfaces and agglomerating may result in ?cold flow' of hydrate slurries. A new method that also follows the approach of causing hydrate to form in a controlled manner is described here. Hydrate asymmetrical restraint technology (HART?) extracts water from gaseous and liquid hydrocarbons by causing hydrate to form on a porous restraint by artificially chilling the restraint while conditions for the spontaneous formation of hydrate nearby are not suitable for hydrate nucleation and growth. This method is based on a growth model in which diffusion of dissolved water vapor toward a site of hydrate growth has the effect of extracting water from the hydrate-forming gas-rich growth media. Water, along with wet gas that has formed the hydrate, is extracted from solid mats or plugs of hydrate that have formed on the porous restraint by lowering pressure on that portion of the hydrate that is no longer in contact with the mixture that provided reactants. Dynamic recyrstallization under shear strain caused by differential pressure allows the maintenance of the solid hydrate mats or plugs that form a seal between the two regions of different pressure
Introduction Natural gas hydrates are non-stoichiometric crystalline compounds of natural gas (dominantly methane but also ethane, butane, and propane, along with other hydrocarbons) and water, which naturally occur both at very low temperatures and moderate pressures in permafrost regions, and in the low temperature - high-pressure regimes present in the deep oceans (Fig. 1). Water molecules form an open latticework that surrounds voids or guest sites, which are largely occupied by gas molecules. The hydrate crystal structure is thermodynamically stabilized by van der Waals interactions between the water lattice molecules and the guest gas molecules (Kvenvolden, 1993). Not all guest sites need be occupied for hydrate to be stable. Hydrate is very different from water ice, with which it is often compared, as hydrate is stable at temperatures considerably higher than the freezing point of water ice and it is not as isobaric as water ice (Fig. 1). In appearance, hydrates usually take the form of inter-grown, translucent-to-opaque, white-to-grey polycrystalline aggregates that have poorly defined crystal morphology.