Hydrate formers

The hypothesis was extended to nucleation of hydrates from liquid water. An alternative hypothesis was proposed by Rodger [1516]. The main difference between these two sets of theories is that Rodger s hypothesis relates the initial formation process to the surface of the water, whereas the theory of Sloan and coworkers considers clusters related to soluted hydrate formers in liquid water as the primary start for joining, agglomeration, and crystal growth. The theories of Sloan and coworkers have been discussed and related to elements of the hypothesis proposed by Rodger [1043]. 

B. Tohidi, A. Danesh, R. W. Burgass, and A. C. Todd. Effect of heavy hydrate formers on the hydrate free zone of real reservoir fluids. In Proceedings Volume, pages 257-261. SPE/Norwegian Petrol Soc Europe Prod Oper Conf (Stavanger, Norway, 4/16-4/17), 1996. 

Table 2. Structural, stability properties and heat of hydrate decomposition at 0°C of most technologically relevant hydrate formers...

Hydrate former Molecule size [A] Structure P equilibria [JVIPa.] AH H-L-V [kJ/mol]... 

Mochizuki, T. Mori, Y.H. (2006). Clathrate-hydrate film growth along water/hydrate-former phase boundaries - numerical heat-transfer study. J. Crystal Growth, 290 (2), 642-652. 

Figure 8. Comparison of molecular diameters of hydrate formers with the free diameters of the cavities (63)...

The period from 1810 to 1900 is characterized by efforts of direct composition measurements with inorganic hydrate formers, especially bromine, inorganics containing sulfur, chlorine, and phosphorus, and carbon dioxide. Other notable work listed in Table 1.2 was done by Cailletet and Bordet (1882), who first measured hydrates with mixtures of two components. Cailletet (1877) was also the first to measure a decrease in gas pressure when hydrates were formed in a closed chamber, using a precursor of an apparatus still in use at the Technical University of Delft, the Netherlands. 

In Figure 1.2, the intersection of the above three phase lines defines both a lower hydrate quadruple point Qi (I-Lw-H-V) and an upper quadruple point Q2 (Lw-H-V-Lhc)- These quadruple points are unique for each hydrate former, providing a quantitative classification for hydrate components of natural gas. 

The advantage of the method in addition to accuracy is that, in principle, it enables the user to predict properties of mixtures from parameters of single hydrate formers. Since there are only eight natural gas components (yet an infinite number of natural gas mixtures) that form hydrates, the method represents a tremendous saving in experimental effort for the natural gas industry. The modified van der Waals and Platteeuw method is detailed in Chapter 5. 

In Scotland, Danesh, Todd, and coworkers measured the inhibition of multiphase systems with methanol (Avlonitis, 1994) and mixed electrolyte solutions (Tohidi et al., 1993,1994a, 1995b,c). They also performed the most comprehensive study of systems with heavy hydrocarbons such as might be produced/transported intheNorth Sea (Avlonitis et al., 1989 Tohidi etal., 1993,1994b, 1996) including systems with structure H hydrate formers. 

Ratio of Molecular Diameters1 to Cavity Diameters0 for Natural Gas Hydrate Formers and a Few Others... 

Indicates the cavity occupied by the simple hydrate former. 

Tables 2.5a,b provide a comprehensive list of guest molecules forming simple si and sll clathrate hydrates. The type of structure formed and the measured lattice parameter, a, obtained from x-ray or neutron diffraction are listed. Unless indicated by a reference number, the cell dimension is the 0°C value given by von Stackelberg and Jahns (1954). Where no x-ray data exists, assignment of structure I or II is based on composition studies and/or the size of the guest molecule. Tables 2.5a,b also indicate the year the hydrate former was first reported, the temperature (°C) for the stable hydrate structure at 1 atm, and the temperatures (°C) and pressures (atm) of the invariant points (Qi and Q2). Both cyclopropane and trimethylene oxide can form si or sll hydrates. Much of the contents of these tables have been extracted from the excellent review article by Davidson (1973), with updated information from more recent sources (as indicated in the tables).

List of Simple si and si I Hydrate Formers, and the Hydrate Structure and Properties... 

Similar statements could be made about the AH values for other simple hydrate formers that occupy similar size cavities, such as C2H6 (AZZj = 72 kJ/mol Handa, 1986) and CO2 (AH = 73 kJ/mol Long, 1994) in the 51262 cavity, or CH4 and H2S (AZZj within 3% of each other Long, 1994) that occupy both 512 and 51262 as simple hydrates. 

Hydrate structures containing hydrate former + Xe help gas have been confirmed by 129Xe NMR spectroscopy by Ripmeester and Ratcliffe (1990a). 

Tohidi et al. (2001) also suggested that the stability of simple methane and nitrogen hydrates could be increased by using sH large guest formers. They suggested that the C6 Cio fraction of real petroleum fluids are potential sH hydrate formers, though no evidence exists so far that real reservoir fluids are more likely to form structure H. 

Structural Changes in Binary (Double) Hydrates. Although CH4 and C2H6 are both si hydrate formers, Subramanian et al. (2000) showed that a binary CH4/C2H6 mixture can exhibit sl/sll transitions with varying pressure and/or composition. In contrast, a binary CH4/CO2 mixture, where again both pure components are si hydrate formers, forms only si hydrate. 

Hester and Sloan (2005) extended the size-structure correlations for double hydrates. A simple scheme of guest size-structure boundaries was proposed to predict the sl/sll structural transitions for double hydrates consisting of si hydrate formers (Figure 2.16). Raman spectroscopy and neutron diffraction measurements were performed to test the limits of these structural transitions. 

The induction time is marked as 1 and includes the time taken for crystal nuclei to form which are not visible to macroscopic probes. The induction time is defined in practice as the time elapsed until the appearance of a detectable volume of hydrate phase or, equivalently, until the consumption of a detectable number of moles of hydrate former gas. The induction time is often also termed the hydrate nucleation or lag time (Section 3.1). (The induction or lag time is the time taken for hydrates to be detected macroscopically, after nucleation and onset of growth have occurred, whereas nucleation occurs on too small a size scale to be detected. Therefore, the term nucleation time will not be used in this context. Instead, the term induction time or induction period will be used. The induction time is most likely to be dominated by the nucleation period, but also includes growth up to the point at which hydrates are first detected.)... 

In a review of the thermodynamics of water, Franks and Reid (1973) showed that the optimum molecular size range for maximum solubility was similar to hydrate stability. Franks and Reid noted, this is not intended to imply that long-lived clathrate structures exist in solution—only that the stabilization of the water structure by the apolar solutes resembles the stabilization of water in a clathrate lattice. Glew (1962) noted that, within experimental error, the heat of solution for ten hydrate formers (including methane, ethane, propane, and hydrogen sulfide) was the same as the heat of hydrate formation from gas and ice, thereby suggesting the coordination of the aqueous solute with surrounding water molecules. 

Film growth at liquid water-hydrate former interface... 

Film growth at liquid water-hydrate former interface Film growth at liquid water-hydrate former interface... 

Shell growth on gas (hydrate former) bubble surface Shell growth on gas (hydrate former) bubble surface Shell growth on gas (hydrate former) bubble surface Shell growth on liquid hydrate former droplet surface Shell growth on liquid hydrate former droplet surface Shell growth on liquid hydrate former droplet surface Shell growth on droplet surface of aqueous solution of hydrate former... 

In summary, the microimaging technique provides a powerful tool to study directly the mechanism of converting water droplets to hydrate particles. The results reported indicate that provided the gas hydrate former can diffuse into the interior droplet, hydrate growth can proceed in the bulk interior droplet away from the hydrate shell-water interface, as well by growing out from the hydrate shell resulting in shell thickening. 

The data were modeled with one fitted parameter (K ) for hydrate growth of simple hydrate formers of methane, ethane, carbon dioxide. Since all these model components form si hydrate, the model should be used with caution for sll and sH. 

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