Effects of nitrogen-doped graphene nanoflakes on methane hydrate formation

Abstract

Gas hydrate technologies are gaining interest in industries concerned with the transport and storage of natural gas. Graphene nanoflakes (GNFs) have been shown to improve the properties of methane hydrate-forming solutions, and nitrogen-doped GNFs (N-GNFs) may further these promotional effects. In this study, the dissolution rates of methane and molar saturation values at 2 °C and 3146 kPa, as well as methane hydrate growth rates at 2 °C and 4646 kPa, were measured in nanofluids containing N-GNFs with both low (4.28 at. %, LN-GNFs) and high (17.41 at. %, HN-GNFs) levels of doping. Loading effects for these systems were determined, and comparisons made with previous studies of as-produced GNFs (non-doped, AP-GNFs). The addition of nanoparticles had no thermodynamic effect on the system: the molar saturation value did not change at any loading. Dissolution rates were determined to be 17.54% faster for LN-GNF solutions and 21.21% faster in HN-GNF solutions compared to the pure water baseline. At lower loadings, LN-GNFs enhanced dissolution rates 2% more compared to AP-GNFs while HN-GNFs enhancement was 7% higher. However, at higher loadings, all three nanoparticles had similar levels of enhancement. Loading effects were attributed to higher N-GNF surface energy, which would improve their dispersion through greater repulsive forces but also give them a significant propensity to agglomerate at higher loadings such that the excess Gibbs free energy of the system is reduced. Hydrate growth rates were also enhanced by 77.20% for LN- and 38.32% for HN-GNFs. Compared to AP-GNFs, LN-GNFs had similar enhancement at low concentrations in clathrate-forming systems, but enhancement was about 29% lower (growth rates were 29% slower) in the higher loading regime. HN-GNFs consistently showed rate enhancements that were about 47.5% lower on average. This diminished enhancement could have resulted from the greater effective N-GNF concentration coupled with their higher propensity to agglomerate.