Hydrate dissociation

Hydrate dissociation is of key importance in gas production from natural hydrate reservoirs and in pipeline plug remediation. Hydrate dissociation is an endothermic process in which heat must be supplied externally to break the hydrogen bonds between water molecules and the van der Waals interaction forces between the guest and water molecules of the hydrate lattice to decompose the hydrate to water and gas (e.g., the methane hydrate heat of dissociation is 500 J/gm-water). The different methods that can be used to dissociate a hydrate plug (in the pipeline) or hydrate core (in oceanic or permafrost deposits) are depressurization, thermal stimulation, thermodynamic inhibitor injection, or a combination of these methods. Thermal stimulation and depressurization have been well quantified using laboratory measurements and state-of-the-art models. Chapter 7 describes the application of hydrate dissociation to gas evolution from a hydrate reservoir, while Chapter 8 describes the industrial application of hydrate dissociation. Therefore in this section, discussion is limited to a brief review of the conceptual picture, correlations, and laboratory-scale phenomena of hydrate dissociation. 

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E. D. Sloan and F. Fleyfel. Hydrate dissociation enthalpy and guest size. Fluid Phase Equilibria, 76 123-140, 1991. 

Fig. 2. CH4 hydrate stability curves (laboratory data) showing CH4 hydrate dissociation paths (schematic) in the depressurization method.

Fig. 4. CH4 - and CO2 hydrates stability curves showing C02 enhanced CH4 hydrate dissociation zone.

Since water is normally present in large excess, the reaction can be characterized by two first-order velocity constants, for hydrate dissociation, and k for hydration. Any method which measures the rate of approach to equilibrium will give an overall rate constant k = k kJ, ... 

Gayet, P. Dichany, C. Marion, G. Graciaa, A. Lachaise, J. Nesterov, A. (2005). Experimental determination of methane hydrate dissociation curve up to 55 MPa by using a small amount of surfactant as hydrate promoter. Chem. Eng. Sci., 60 (21), 5751-5758. 

Kelkar, S.K. Selim, M.S. Sloan, E.D. (1998). Hydrate dissociation rates in pipelines. Fluid Phase Equilibria, 150, 371-382. 

Since A1(N03)3 or its salt hydrates dissociates to A13+ and NOs ions in the aqueous solution, its reactions in solutions are those of Al. It is partially hydrolyzed, producing H30" and thus accounting for the acidity of its solution in water. The products constitute a complex mixture of mono- and polynuclear hydroxo species. 

One example would be ice melting or methane hydrate dissociation when rising in seawater. Convective melting rate may be obtained by analogy to convective dissolution rate. Heat diffusivity k would play the role of mass diffusivity. The thermal Peclet number (defined as Pet = 2aw/K) would play the role of the compositional Peclet number. The Nusselt number (defined as Nu = 2u/5t, where 8t is the thermal boundary layer thickness) would play the role of Sherwood number. The thermal boundary layer (thickness 8t) would play the role of compositional boundary layer. The melting equation may be written as... 

Computer simulations provide a means of examining the early stages of hydrate formation (nucleation) on a molecular level (Baez and Clancy, 1994 Radhakrishnan and Trout, 2002 Moon et al., 2003, 2005). Computer simulation has also been applied to study hydrate dissociation (Baez and Clancy, 1994 English and MacElroy, 2004) and the effects on dissociation kinetics of external electromagnetic fields (English and MacElroy, 2004). 

Hydrate dissociation models that were later developed to simulate hydrate recovery techniques include numerical models by Masuda et al. (1997), Moridis (2002), and Hong et al. (2003) analytical models by Makogon (1997) and Tsypkin (2000). The details of these models are given in Chapter 7. 

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). 

Hydrate dissociation begins when the cell is heated from Point C in Figure 3.1b, so that the system pressure increases, at first slowly and then sharply along the steep dissociation line (between Points C and D). Finally at Point D, the hydrates are completely dissociated, as confirmed visually through the sight glass. The hydrate equilibrium condition (or hydrate dissociation temperature and pressure) is given by Point D (Section 3.3). 

Hydrate structure (which is not visible to the naked eye) remains in solution (or on an ice surface) after hydrate dissociation in the following forms ... 

FIGURE 3.20 Successive cooling curves for hydrate formation with successive runs listed as Sj < S2 < S3. Gas and liquid water were isochorically cooled into the metastable region until hydrates formed in the portion of the curve labeled Sj. The container was then heated and hydrates dissociated along the vapor-liquid water-hydrate (V-Lyy-H) line until point H was reached, where dissociation of the last hydrate crystal was visually observed. (Reproduced from Schroeter, J.R, Kobayashi, R., Hildebrand, M.A., Ind. Eng. Chem. Fundam. 22, 361 (1983). With permission from the American Chemical Society.)... 

Persistent hydrate crystallites (long-range ordered structure), which were shown from neutron scattering to remain in solution for several hours after increasing the temperature above the hydrate dissociation temperature (Buchanan et al 2005). 

The memory effect has important implications for the gas industry. For example, after hydrates initially form in a pipeline, hydrate dissociation should be accompanied by the removal of the water phase. If the water phase is not removed, the residual entity (i.e., residual structure, persistent crystallites, or dissolved gas) will enable rapid reformation of the hydrate plug. Conversely, if hydrate formation is desired, the memory effect suggests that hydrate formation can be promoted by multiple dissociation and reformation experiments (provided the melting temperature is not too high, or melting time is not too long). 

The modern conceptual picture of dissociation of a hydrate core/plug typically involves radial hydrate dissociation rather than the previously suggested axial... 

The different hydrate dissociation models that have been developed by various research groups are summarized in Table 7.10. The majority of these models are based on heat transfer limited dissociation. Some of the models have been developed to incorporate both heat transfer and kinetics. 

Figure 3.35 (See color insert following page 390.) X-ray CT imaging shows radial dissociation of a hydrate core. Image number 1 -8 (top number on each image) recorded over 0-245 min (bottom number on each image). The cell pressure was decreased from 4.65 to 3.0 MPa over 248 min. The hydrate core temperature decreased from 277 to 274 K with time, following the three-phase methane hydrate equilibrium line. (From Gupta, A., Methane Hydrate Dissociation Measurements andModeling The Role of Heat Transfer and Reaction Kinetics, Ph.D. Thesis Colorado School of Mines, Golden, CO (2007). With permission.)...

Rehder et al. (2004) measured the dissociation rates of methane and carbon dioxide hydrates in seawater during a seafloor experiment. The seafloor conditions provided constant temperature and pressure conditions, and enabled heat transfer limitations to be largely eliminated. Hydrate dissociation was caused by differences in concentration of the guest molecule in the hydrate surface and in the bulk solution. In this case, a solubility-controlled boundary layer model (mass transfer limited) was able to predict the dissociation data. The results showed that carbon dioxide hydrate dissociated much more rapidly than methane hydrate due to the higher solubility in water of carbon dioxide compared to methane. 

Nuclear magnetic resonance studies of methane hydrate dissociation suggest that intrinsic kinetics is not likely to play a dominant role in the dissociation process (Gupta et al., 2006). Methane hydrate dissociation was shown to progress in the absence of an intermediate state (or activated state), with no preferential decay of large to small cavities. Similar measurements have been performed for Xe hydrate dissociation (Moudrakovski et al., 2001b). 

The following summary statements are made about hydrate dissociation, based upon the previous subsections ... 

Hydrate dissociation is an endothermic process, typically controlled by heat transfer. 

Using a heat transfer limited model based on Fourier s Law in cylindrical coordinates (developed at the Colorado School of Mines), hydrate dissociation under field conditions provides an order of magnitude (higher) prediction, which is acceptable in the industrial setting (see Chapter 8 and Appendix B for more details, including a description of CSMPlug to determine dissociation dimensions and times). 

The engineer may finally turn to the computer simulation packages mentioned above to obtain a mixture hydrate dissociation pressure of 1.26 MPa. 

As a first approximation, the temperature depression for hydrate inhibition might be considered to be similar to the depression of the freezing point of ice by an equivalent mass fraction of the inhibitor. However, Nielsen and Bucklin (1983) derived an equation indicating that the hydrate depression temperature will always be less than the ice depression temperature by a factor equal to [(heat of fusion of ice)/(heat of hydrate dissociation)], which has a numerical value between 0.6 and 0.7 as a function of the hydrate structure. This is illustrated in Figure 4.2d, by the fact that at constant pressure, the ice depression temperature (i.e., distance between... 

Calculate the three-phase hydrate dissociation temperature,... 

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