Molecular-Level Measurements of the Hydrate Phase

Equilibrium measurements of the solid hydrate phase have been previously avoided due to experimental difficulties such as water occlusion, solid phase inhomogeneity, and measurements of solid phase concentrations. Instead, researchers have traditionally measured fluid phase properties (i.e., pressure, temperature, gas phase composition, and aqueous inhibitor concentrations) and predicted hydrate formation conditions of the solid phase using a modified van der Waals and Platteeuw (1959) theory, specified in Chapter 5. 

However, over the last decade there has been a significant shift in the number of researchers recognizing the importance of implementing mesoscopic and molecular-level methods to measure the hydrate phase directly. It is clear that 

F l G U RE 6.11 (See color insert following page 390.) Schematic of the application of x-ray CT analysis to provide density profile images of different sections of a hydrate core contained in a cylindrical high pressure aluminum cell. (From Gupta, A., Methane Hydrate Dissociation Measurements and Modeling The Role of Heat Transfer and Reaction Kinetics, Ph.D. Thesis, Colorado School of Mines, Golden, CO (2007). With permission.) 

Since 1987, 39 sH hydrate formers have been reported, many of which are incorrectly listed in industrially important references such as the API Databook (Lippert et al., 1950), where methylcyclopentane and methylcyclohexane are listed as sll formers, and the Handbook of Natural Gas Engineering (Katz et al., 1959), where iso-pentane and methylcyclopentane are listed as non-hydrate formers. 

In addition to the three known natural gas hydrates, several other hydrate structures exist. Dyadin et al. (1991) found four hydrate structures and Jeffrey (1984) proposed five additional hydrate structures. These structures have yet to be confirmed in natural gas systems, although new hydrate structures have been identified using x-ray diffraction such as a tetragonal structure for bromine (Udachin et al., 1997b), a trigonal structure for dimethyl ether (Udachin et al., 2001a), 

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