"Self-Preservation" of CH4 Hydrates for Gas Transport Technology: Pressure-Temperature Dependence and Ice Microstructures

Abstract

Self-preservation is a kinetic anomaly that allows for storing a substantial amount of gas locked in gas hydrate far outside its thermodynamic stability field for a period of days, weeks, or even months under very mild pressuretemperature (pT) conditions, by merely maintaining temperatures below the melting point of ice. Utilizing this phenomenon for low-cost storage and transportation of natural gas is not yet sufficiently developed to be competitive with already existing, well-established methods (e.g., liquefied natural gas (LNG), gas to liquid (GTL), compressed natural gas (CNG), or pipeline (PL)). Aside from the refinement of numerous engineering and safety aspects, a deeper understanding of the self-preservation phenomenon is needed in order to promote these technologies. We address some of these outstanding issues in a series of isothermalisobaric pressurevolumetemperature (pVT) experiments exploring the kinetics of the dissociation of pure sI methane hydrate to ice and CH4 gas in a wide pT field applicable to gas-hydrate-based technologies. By means of ex situ cryo-SEM, we correlate the kinetic data with the morphology of initially formed ice coatings recovered at various stages of the transformation. The pT dependence of the self-preservation strength is seen as a complex interplay between (1) ice microstructures (shape, arrangement, and size of ice crystals) and (2) annealing rate of the ice coating that acts as a diffusion barrier for escaping gas. Moreover, we recognize a progressive sintering of ice coatings of individual particles when close to the melting point of ice. The optimal conditions for the transport and storage at ambient pressure, where this issue is minimized and the preservation strength is still very high, have been found at similar to 250 K. Further fine-tuning of the storage capacity may involve elevating the storage pressure and active temperature control.