Clathrate-hydrate film growth along water/methane phase boundaries-an observational study

Methane hydrate films, each growing along a horizontal interface between the quiescent liquid-water and methane-gas phases, were observed through a low magnification microscope to reveal the morphology and advancing motion of the film front, and the linear film growth rate (advancing speed of the film front) at various degrees of system subcooling from the phase-equilibrium temperature corresponding to the system pressure was quantitatively measured. At a lower subcooling (approximate to 1.2 K), the film front was often covered by laterally aligned facets showing a regular roughness with sharp peaks and valleys and a non-isotropic growth. As the subcooling increased, the film front tended to have an irregular, non-faceted, smaller-scale roughness and exhibited a smoother, isotropic growth. For such an isotropic, quasi-steady film growth at subcoolings from approximate to 2.5 to approximate to 10 K, we collected the film-growth rate data. The obtained data were compared to other relevant data or empirical correlations previously reported by different research groups. The consistency or inconsistency between the different data sources were examined in detail. (C) 2013 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim