Work

Thickening, temperature and guest concentration of the meltwater layer growing over a slowly melting gas hydrate

Year: 2025

Type: article

Abstract: In this study, the melting of a porous methane hydrate driven to self-preservation conditions is investigated experimentally and modelled by adapting/extending the classical Stefan problem of phase change with a moving free boundary to the two-component (water+guest molecules) system of interest here. Experimentally, the very slow buildup of a layer of a supercooled meltwater is observed by optical microscopy at the boundary of the porous hydrate initially facing the gas phase, whose thickness increases as the square root of time, a typical signature of a Stefan-like melting process. As shown by Stefan’s model, this very slow buildup is the expression of temperatures within the melting hydrate and meltwater that remain extremely close to the imposed outside temperature: the melting process is quasi-isothermal. The (supercooled) liquid water is supersaturated with the guest (methane) molecule, which pops up as a bubble when the meltwater layer thickness exceeds a few hundreds of microns; this gas bubble expels the liquid water and the melting process starts again at the hydrate/gas interface, etc. The guest (methane) dissolved in this layer, which is monitored by microRaman spectroscopy, exhibits a concentration gradient from a high value at the melting hydrate front to the (low) equilibrium concentration at the meltwater/guest interface. The Stefan problem is solved analytically for a semi-infinite hydrate, and numerically by a finite difference method. The melting process is driven by mass transfer effects rather than being controlled by heat transfer, as has also been observed with other hydrate phase change processes.

Source: Chemical Engineering Science

DOI: 10.1016/j.ces.2025.122375

Authors: Belkacem Samar, Marc Dambrine, Helmut Harbrecht, Arnaud Desmedt, Daniel Broseta