Natural Gas Hydrate Formation and Decomposition in the Presence of Kinetic Inhibitors. 3. Structural and Compositional Changes

A synthetic natural gas mixture composed of methane, ethane, and propane in a batch reactor was used to form gas hydrates in the presence of two commercial, chemical kinetic inhibitors, polyvinylpyrrolidone (PVP) and H1W85281, and one biological inhibitor, antifreeze protein type III (AFP-III). Powder X-ray diffraction and nuclear magnetic resonance spectroscopy showed that structure II hydrates dominated, as expected, but in the presence of the chemical inhibitors, structure I was also present. Raman spectroscopy confirmed the complexity and the heterogeneity of the guest composition within these hydrates, which was also consistent with the gas analysis obtained using gas chromatography. However, in the presence of AFP-III, hydrates appeared to be relatively homogeneous structure II hydrates, with weaker evidence of structure I. When individual gas cage occupancies were calculated, both classes of inhibitors reduced large cage methane occupancy by similar to 25%. With the chemical inhibitors, these large cage methane guests appeared to be substituted by ethane, likely resulting in a decreased driving force for hydrate production. In contrast to the near full occupancy of large cages with these inhibitors, almost 10% of the large cages were not filled when hydrates were formed in the presence of AFP-III, likely contributing to the easy decomposition of such hydrates seen in other studies. Therefore, hydrates formed in the presence of these two classes of inhibitors appear to be distinct, and as a consequence, their inhibition mechanisms, as well their practical utility in the field, are likely to be marked by important differences.