Correlations of Hydrate Dissociation

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. 

From the extensive experimental and model development work performed at CSM (during a period of over 15 years), it has been demonstrated that a heat transfer controlled model is able to most accurately predict dissociation times (comparing to laboratory experiments) without any adjustable parameters. The current model (CSMPlug see Appendix B for details and examples) is based on Fourier s Law of heat transfer in cylindrical coordinates for the water, ice, and hydrate layers, and is able to predict data for single- and two-sided depressurization, as well as for thermal stimulation using electrical heating (Davies et al 2006). A heat transfer limited process is controlled by the rate of heat supplied to the system. Therefore, a measurable intermediate (cf. activated state) is not expected for heat transfer controlled dissociation (Gupta et al., 2006). 

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). 

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