Inhibition–Promotion: Dual Effects of Polyvinylpyrrolidone (PVP) on Structure-II Hydrate Nucleation

Polyvinylpyrrolidone (PVP) is known as a kinetic hydrate inhibitor (KHI) and its inhibiting effect on hydrate nucleation and growth has been widely studied and acknowledged. In this work, the effect of PVP on methane–propane hydrate nucleation was examined in high-pressure autoclave at isothermal conditions. We examined PVP at ultralow concentrations of 50 and 100 ppm to avoid overlapping effects on nucleation and growth and maintain focus on the nucleation stage of the process. Two experimental pressures (Pexp = 6.1 or 9.0 MPa), two cooling rates (2 K/h or 6 K/h), and six subcooling levels (ΔT) ranging from 6.37–10.05 K were applied to induce nucleation. The induction time for hydrate nucleation increased with PVP present at ultralow concentrations, but intriguingly, PVP demonstrated dual effects of inhibition and promotion on hydrate nucleation rate at the experimental pressures examined. The switch from promoting to inhibiting effect on nucleation rate was seemingly not affected by pressure or cooling rate, but rather dependent on the degree of subcooling, ΔT. As compared to systems without inhibitor, PVP reduced the stationary nucleation rate at ΔT > 9 K. At subcooling levels between 6–9 K, PVP tended to promote the process through an increased nucleation rate. This is the first time a promoting effect of PVP alone on hydrate nucleation rate has been experimentally observed and reported. The cause remains unclear. We propose a hypothesis that the docking orientation of PVP polymers relative to the water cages on the surface of hydrate embryos is a function of temperature and applied subcooling level. Subsequently, a switch of the spatial configuration of PVP molecules could either inhibit or promote hydrate nucleation rate. The experimental study gives new insight into KHI working mechanisms. Further investigations are required to improve our understanding of the observed dual effects of PVP on gas hydrate nucleation.