Hydrates crystal structure

Figure 2. Packing of the molecules in the diphenylguanidinium nicotinate hydrate crystal structure. The dashed lines represent hydrogen bonds and some atoms were omitted for clarity.

The initial predictive method by Wilcox et al. (1941) was based on distribution coefficients (sometimes called Kvsi values) for hydrates on a water-free basis. With a substantial degree of intuition, Katz determined that hydrates were solid solutions that might be treated similar to an ideal liquid solution. Establishment of the Kvsj value (defined as the component mole fraction ratio in the gas to the hydrate phase) for each of a number of components enabled the user to determine the pressure and temperature of hydrate formation from mixtures. These Kysi value charts were generated in advance of the determination of hydrate crystal structure. The method is discussed in detail in Section 4.2.2. 

Katz s two predictive techniques provided industry with acceptable predictions of mixture hydrate formation conditions, without the need for costly measurements. Subsequently, hydrate research centered on the determination of the hydrate crystal structure(s). Further refinements of the Kvsi values were determined by Katz and coworkers (especially Kobayashi) in Chapter 5 of the Handbook of Natural Gas Engineering (1959), by Robinson and coworkers (Jhaveri and Robinson, 1965 Robinson andNg, 1976), and by Poettmann (1984). 

Hydrate Crystal Structures and Hydrate Type Definitions... 

Tabushi et al. (1981) suggested that the 15-hedron (51263) is absent from Figure 2.5 and in all clathrates except bromine due to an unfavorable strain relative to the other cavities in si and sll. In their review of simple and combined cavities, Dyadin et al. (1991) suggested that in addition to the cavities found in si, sll, and sH, there are 4258 and 51263 cavities. In Jeffrey s (1984) list of a series of seven hydrate crystal structures (Table 2.3), additional cavities to those found in si, sll, and sH are 51263, 4454,43596273, 4668. 

Jeffrey s (1984) List of a Series of Seven Hydrate Crystal Structures... 

One might anticipate similar results for structures I and II in the absence of measurements. In many of the properties that are derived from structure, the differences between the hydrate crystal structures are not appreciable. One might intuitively expect properties on the basis of the water crystal structure to exhibit less variation between hydrate structures than between hydrate and ice properties, in view of the fact that the 512 cavity is common to each hydrate structure. 

The temperatures and pressures for the sH system are very similar to those found in si and sll diagrams. If ahydrate forms in a pipeline with both gas and condensate/oil phases present, examining the pressure-temperature conditions may be insufficient to determine the hydrate crystal structure. 

Even with such limitations, the A vsi-value method represented a significant advance in hydrate prediction ability. It was conceived prior to the determination of the hydrate crystal structures and it is a fine representation of the intuitive insight that characterizes much of Katz s work. The Kysi-value method was the first predictive method, and it was used as the basis for the calculations in the gravity method, so it is logical that the A si-value method should be more accurate. 

Structure identification and relative cage occupancies. The hydration number and relative cage occupation for pure components and guests were measured by Sum et al. (1997), Uchida et al. (1999), and Wilson et al. (2002). Raman guest spectra of clathrate hydrates have been measured for the three known hydrate crystal structures si, sll, and sH. Long (1994) previously measured the kinetic phenomena for THF hydrate. Thermodynamic sl/sll structural transitions have been studied for binary hydrate systems (Subramanian et al., 2000 Schicks et al., 2006). 

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