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Understanding the action of gas hydrate kinetic inhibitors Daraboina, Nagu

Abstract

The presence of inhibitors delayed hydrate nucleation and decreased the overall formation of methane/ethane/propane hydrate compared to pure water system. However, the two classes of inhibitors: chemical (Polyvinylpyrrolidone (PVP) and industrial inhibitor: H1W85281) and a biological (Type I and III antifreeze protein (AFP)) were distinguished by the formation of hydrates with different stabilities. A single hydrate-melting peak was seen with the AFP-III and this was consistent after re-crystallization. In contrast, multiple hydrate melting events were observed in the presence of the chemical inhibitors. In stirred reactor, onset of hydrate decomposition occurred earlier in the presence of the inhibitors compared to water controls. However, depending on the type of inhibitor present during crystallization, hydrate decomposition profiles were distinct, with a longer, two-stage decomposition profile in the presence of the chemical inhibitors. The fastest, single-stage decompositions were characteristic of hydrates in experiments with either of the AFPs. 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. 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, 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 DSC and stirred reactor experiments. These results argue that thought must be given to inhibitor-mediated decomposition kinetics when designing and screening of new kinetic inhibitors. This is a necessary practical consideration for industry in cases when due to long shut in periods; hydrate formation may be unavoidable even when inhibitors are utilized. This heterogeneity suggests that using these chemical inhibitors (PVP and H1W85281) may present a special challenge to operators depending upon the gas mixture and environmental conditions, and that AFPs may offer a more predictable, efficacious solution in these cases.

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Attribution-NonCommercial-NoDerivatives 4.0 International

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