Effects of Hydrophobic and Hydrophilic Graphene Nanoflakes on Methane Dissolution Rates in Water under Vapor–Liquid–Hydrate Equilibrium Conditions

Abstract

Several industries have steadily gained interest in gas hydrate technologies for their potential use in natural gas transport and storage applications. Additives which optimize the efficiencies of these technologies, particularly nanoparticles, have lately been subject to an increasing investigative focus. Graphene nanoflakes (GNFs) have previously been proven to enhance hydrate systems, particularly methane hydrate systems. In this study, the dissolution rates of methane and molar saturation values were measured in nanofluids containing both hydrophobic (as-produced) and hydrophilic (plasma-functionalized) GNFs at 2 °C and 3146 kPa. For both types of GNFs, the effect of loading in the aqueous phase was equally determined. Dissolution rate enhancement was limited at low concentrations of around 0.5 ppm for hydrophobic GNFs due to small-scale agglomeration while significantly increasing dissolution kinetics by about 18.84% at concentrations of 5 ppm. The performance eventually decreased at higher concentrations (10 ppm) due to large-scale agglomeration. Hydrophilic GNFs, which exhibited no agglomeration, enhanced dissolution rates further with each successive loading until a 44.45% plateau at 10 ppm. This plateau may have been a limit of the system or a result of mean free path limitations. Either type of GNFs nearly triples the dissolution rates of methane investigated in previous studies on multi-walled carbon nanotubes due to their higher specific surface area.

Non-Einsteinian Viscosity Behavior in Plasma-Functionalized Graphene Nanoflake Nanofluids and Their Effect on the Dynamic Viscosity of Methane Hydrate Systems

“…There have been previous studies where the addition of O-GNFs did not significantly change how much methane dissolved into the liquid phase of the system. 22 However, non-Einsteinian behavior may only depend on an accumulation of surface effects, so small amounts of methane could still impart a significant effect on the viscosity. Therefore, there may be a weak, negative effect of nanoparticle addition on the pressure dependence of viscosity in this system.…”

“…24 However, O-GNFs have previously been measured to enhance the rates of methane gas dissolution in water by about 45%. 22 Therefore, this may not be a significant factor considering that only a single condition at a single concentration was measured to be affected. Lastly, hydrate formation is exothermic, and sufficient heat evolution can result in the system becoming self-limiting, notably creating less favorable nucleation conditions.…”

“…Usually, chemical treatment or surfactant addition is required to avoid nanoparticle agglomeration and settling. , Now, oxygen or nitrogen functional groups can be added onto GNF surfaces via thermal plasma decomposition and chemical functionalization processes imparting hydrophilicity onto the particles. Recent studies have shown that plasma-functionalized GNFs significantly improve hydrate formation, with oxygen functionalized GNFs (O-GNFs) nearly quadrupling methane hydrate growth rates. − However, while promotion is essential for improving hydrate technologies, it is insufficient to demonstrate their viability alone. Several new hydrate technologies propose using either semi-batch or, more often, continuous processes during full operation.…”

All-Atom Molecular Dynamics of Pure Water–Methane Gas Hydrate Systems under Pre-Nucleation Conditions: A Direct Comparison between Experiments and Simulations of Transport Properties for the Tip4p/Ice Water Model

“…These include gas separations [ 19 , 20 ], pre- and post-combustion carbon capture [ 21 , 22 , 23 ], the transport and storage of natural gas [ 24 , 25 ], and the desalination of water [ 26 ]. These emerging applications have led to research on promoter additives such as graphene nanoflake and multi-walled carbon nanotube nanofluids [ 27 , 28 , 29 ]. Many of the technologies above involve the continuous flow of pre-nucleation and nucleating gas hydrate systems that require to be maintained in the flow state for operation and involve aqueous systems (no oil).…”

Dynamic viscosity of methane hydrate systems from non-Einsteinian, plasma-functionalized carbon nanotube nanofluids

“…10 Nanoparticle additives often include metal oxides or carbon-based nanoparticles like graphene nanoflakes or carbon nanotubes. [11][12][13] Notably, hydrate yields in aqueous multi-walled carbon nanotube (MWCNT) systems have been measured to be 4.5 times higher than in pure water. 14,15 However, these nanoparticles are made of carbon and naturally hydrophobic, so they agglomerate in and settle out of aqueous solutions.…”