Abstract
In a study designed to investigate the melting behaviour of natural gas hydrates which are usually formed in porous mineral sediments rather than in bulk, hydrate phase equilibria for binary methane and water mixtures were studied using high-pressure differential scanning calorimetry in mesoporous and macroporous silica particles having controlled pore sizes ranging from 8.5 nm to 195.7 nm. A dynamic oscillating temperature method was used to form methane hydrates reproducibly and then determine their decomposition behaviour—melting points and enthalpies of melting. Significant decreases in dissociation temperature were observed as the pore size decreased (over 6 K for 8.5 nm pores). This behaviour is consistent with the Gibbs–Thomson equation, which was used to determine hydrate–water interfacial energies. The melting data up to 50 MPa indicated a strong, essentially logarithmic, dependence on pressure, which here has been ascribed to the pressure dependence of the interfacial energy in the confined media. An empirical modification of the Gibbs–Thomson equation is proposed to include this effect.
Highlights
Methane hydrates are one example of a class of crystalline substances called ‘clathrates’ [1], where host molecules are trapped inside hydrogen-bonded solvent cages— in this case a gas hydrate where methane gas molecules are trapped inside hydrogenbonded water cages of various geometries
We report the equilibrium behaviour for confined methane hydrates over a much wider pressure range in both mesoporous and macro-porous media
Melting points Tm of confined ice in mesoporous media have been intensively investigated by various techniques, including NMR spectroscopy [28], neutron diffraction [29], differential thermal analysis [19], and calorimetry [16]
Summary
Methane hydrates are one example of a class of crystalline substances called ‘clathrates’ [1], where host molecules are trapped inside hydrogen-bonded solvent cages— in this case a gas hydrate where methane gas molecules are trapped inside hydrogenbonded water cages of various geometries. Natural gas hydrates occur in nature and contain more methane, the cleanest burning fossil fuel, than all the conventional oil and gas reservoirs put together [2,3]. Understanding their phase behaviour is fundamental to devising safe and cost-effective processes for extracting gas from them. Methane hydrates are distributed on the continental shelf and found in permafrost regions [1], usually distributed in porous soft sand-clay sediments These ‘hydrate crystal’ deposits are an increasingly favoured target for future energy production [4,5,6,7]. Most thermodynamic studies of gas hydrates have been carried out on bulk hydrates rather than under confined conditions
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