NMR and Viscosity Investigation of Clathrate Hydrate Formation and Dissociation

Abstract

To better understand clathrate hydrate mechanisms, nuclear magnetic resonance (NMR) and viscosity measurements were employed to investigate tetrahydrofuran (THF) hydrate formation and dissociation processes. In NMR experiments, the proton spin lattice relaxation time (T1) of THF in deuterium oxide (D2O) was measured as the sample was cooled from room temperature down to the hydrate formation region. The D2O structural change around THF during this process was examined by monitoring the rotational activation energy of THF, which can be obtained from the slope of ln(1/T1) vs 1/T. No evidence of hydrate precursor formation in the hydrate region was found. T1 measurements of THF under constant subcooling temperature indicate that THF hydration shells do not undergo much structural rearrangement during induction. The T1 of THF was also measured as the sample was warmed back to room temperature after hydrate dissociation. T1 values of THF after hydrate dissociation were consistently smaller than those before hydrate formation and never returned to original values. It was proposed that this difference in T1 after hydrate dissociation indicates that the THF−D2O solution is more microscopically homogeneous than before hydrate formation. In viscosity experiments, a Champion Technologies hydrate rocking cell (CTHRC) was used to probe the residual viscosity phenomenon after Green Canyon (GC) gas hydrate as well as THF hydrate dissociation. The residual viscosity reported in the literature was observed after GC hydrate dissociation but not after THF hydrate dissociation. Because GC hydrate behavior involves significant amounts of gas mass transfer while THF hydrate does not, one might conclude that the residual viscosity observed after GC hydrate dissociation was likely caused by the supersaturated gas concentration and its general effect on solvent viscosity, not necessarily by a clathrate water structure lingering from the solid.