Methane hydrate dissociation

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. 

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... 

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). 

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.)...

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). 

N.J. English et al., Molecular-dynamics simulations of methane hydrate dissociation. J. Chem. Phys. 123, 244503 (2005)... 

V.P. Melnikov, A.N. Nesterov, A.M. Reshemikov, A.G. Zavodovsky, Evidence of Liquid Water Formation during Methane Hydrates Dissociation Below the Ice Point, Chem. Eng. Sci. 64 (5) (2009) 1160 1166. 

Jahren A. H., Arens N. C., Sarmiento G., Guerrero J., and Amundson R. (2001) Terrestrial record of methane hydrate dissociation in the early Cretaceous. Geology 29, 159-162. 

Figure 2 Experimental and predicted methane hydrate dissociation (si) conditions in the presence MEG (Model predictions are independent from experimental data) (Error bars 1 °C, only for visual purpose)...

Methane hydrate and propane hydrate crystallize in Structures I and II respectively, their dissociation pressures at — 3°C have been determined and were found to be 23.1 and 1.48 atm. Above a... 

The combination of C02 injection and methane production over specific PT regimes allows the heat effects of C02 hydrate formation and methane hydrate decomposition to nullify each other resulting in a sustainable delivery process which both reduces C02 emissions to combat global warming and recovers methane to supplement the declining reserves of conventional natural gas (Fig. 4). This gas hydrate phase-behaviour in response to the dissociation and formation processes clearly demonstrates the potential of C02 enhanced CH4 recovery from the Mallik gas hydrate deposit. 

Link, D. D. Ladner, E. P. Elsen, H. A. Taylor, C. E. (2003). Formation and dissociation studies for optimizing the uptake of methane by methane hydrates. Fluid Phase Equilibria, 211, 1-10. 

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. 

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. 

FIGURE 3.36 Average rates for methane hydrate samples reaching 50% dissociation at 0.1 MPa, following destabilization by rapid release of P. The anomalous preservation regime is between 242 and 271 K. Square symbols experiments in which P is maintained at 2 MPa, Diamonds 0.1 MPa rapid depressurization tests on sll methane-ethane hydrate, showing no comparable preservation behavior at 268 K. (Reproduced from Stern, L.A., Circone, S., Kirby, S.H., Durhan, W., Can. J. Phys., 81, 271 (2003). With permission from the National Research Council.)... 

Figure 4.24 Hydrate dissociation lines for mixtures of methane and ethane. (Reproduced from Sloan, E.D., Fleyfel, F., Fluid Phase Equilib., 76, 123 (1992). With permission from Elsevier Science Publishers.)...

Half a century later, the work of Carson and Katz (1942) provided a second reason for considering the dissociation condition of the hydrate equilibrium point (see Chapter 3, Figure 3.1b for more details). Their work clearly showed the solid solution behavior of hydrates formed by gas mixtures. This result meant that hydrate preferentially encapsulated propane from a methane + propane gas mixture, so that a closed gas volume was denuded of propane (or enriched in methane) as more hydrates formed. On the other hand, upon hydrate dissociation, when the last crystal melted the initial gas composition was regained, minus a very small amount to account for solubility in the liquid phase. 

In the CSM laboratory, Rueff et al. (1988) used a Perkin-Elmer differential scanning calorimeter (DSC-2), with sample containers modified for high pressure, to obtain methane hydrate heat capacity (245-259 K) and heat of dissociation (285 K), which were accurate to within 20%. Rueff (1985) was able to analyze his data to account for the portion of the sample that was ice, in an extension of work done earlier (Rueff and Sloan, 1985) to measure the thermal properties of hydrates in sediments. At Rice University, Lievois (1987) developed a twin-cell heat flux calorimeter and made AH measurements at 278.15 and 283.15 K to within 2.6%. More recently, at CSM a method was developed using the Setaram high pressure (heat-flux) micro-DSC VII (Gupta, 2007) to determine the heat capacity and heats of dissociation of methane hydrate at 277-283 K and at pressures of 5-20 MPa to within 2%. See Section 6.3.2 for gas hydrate heat capacity and heats of dissociation data. Figure 6.6 shows a schematic of the heat flux DSC system. In heat flux DSC, the heat flow necessary to achieve a zero temperature difference between the reference and sample cells is measured through the thermocouples linked to each of the cells. For more details on the principles of calorimetry the reader is referred to Hohne et al. (2003) and Brown (1998). 

Lievois, J.S., Development of an Automated, High Pressure Heat Flux Calorimeter and Its Application to Measure the Heat of Dissociation of Methane Hydrate, Ph.D. Thesis, Rice University, TX (1987). 

Experiments, both in the field and laboratory, are very expensive. Models of methane production from hydrate can save substantial expense of time, effort, and capital. This section of Chapter 7 gives guidelines from the models for hydrate dissociation. 

The above discussion demonstrates two principles for hydrate dissociation (1) hydrates will not occur outside the thermodynamic restrictions of the phase equilibria, that is, hydrates require the appropriate temperature, pressure, as shown in the area to the left of the lines in Figure 7.18, as well as methane and water and (2) when hydrates are dissociated, even at constant temperature as shown in Figure 7.18, heat must flow from the surrounding media to the hydrates, causing a cooling. This last point is also intuitive, because gas and water molecules from dissociated hydrates have more energy than they do in nondissociated hydrates. Thus energy must flow to the hydrate surface in order to dissociate it. 

Low chlorinity zones were coincident with zones of anomalously low recovered core temperatures on the ship catwalk. For example, while some of the background core temperatures were at 10-12°C, cores in suspected hydrate regions had temperatures as low as 1°C, perhaps caused by endothermic dissociation of hydrate. The extrapolated geothermal gradient of 33.5°C/km yielded a temperature of 18.3°C at the BSR (440 mbsf), well within the temperature stability field of methane hydrate. 

Valentine D. L., Blanton D. C., Reeburgh W. S., and Kastner M. (2001) Water column methane oxidation adjacent to an area of active hydrate dissociation. Eel River Basin. Geochim. Cosmochim. Acta 65, 2633—2640. 

Dickens G. R., Castillo M. M., and Walker J. C. G. (1997) A blast of gas in the latest Paleocene simulating first-order effects of massive dissociation of oceanic methane hydrate. Geology 25(3), 259-262. 

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