Dependence of the Kinetic Hydrate Inhibition Effect of Poly(N-vinylpyrrolidone) upon the Molecular Weight Is Influenced by Water Mobility in Millisecond Dynamics

Explosive formation of a natural gas clathrate hydrate in production lines has been a technical bottleneck in ocean field natural gas drilling and gas transport. To mitigate this problem, polymer inhibitors can be used as a low-dosage kinetic inhibitor during operation. Key factors affecting the physicochemical performance of a benchmark inhibitor, poly(N-vinylpyrrolidone) (PVP), are not yet completely known. It is found here that the ability of PVP to delay the rapid crystal growth of the clathrate hydrate is correlated to the surrounding water dynamics. PVP can produce immobilized non-freezable water molecules with fast transverse relaxation on a sub-millisecond to millisecond time scale as revealed by nuclear magnetic resonance relaxometry. The unique water dynamics are also determined as a form of non-freezable bound water measured by differential scanning calorimetry. Changes in the molecular weight of PVP lead to changes in the entire inhibition time before the rapid growth of the hydrate crystals, and an optimal size of the PVP chain is found for the best inhibition. The hydrate crystallization rate constants are lowered by 1-2 orders of magnitude when the PVP chains have near the optimal size, further providing an explanation of the chain length dependence of the inhibition time.