New insights into the effect of polyvinylpyrrolidone (PVP) concentration on methane hydrate growth. 1. Growth rate

To further understand the effects of polyvinylpyrrolidone (PVP) on methane hydrate growth, the current study evaluates initial hydrate growth rate over a wide range of PVP concentrations from 0.7 to 20 000 ppmw (2 wt%) in a stirred tank crystallizer. Tests were undertaken at temperatures ranging from 275.1 to 279.1 K, pressures from 4645 to 7183 kPa and PVP molecular weights of 10 000, 40 000 and 360 000 g/mol. Throughout all these conditions, a common sigmoidal trend between growth rate and PVP concentration was observed. Initial growth rates were the same as that of pure water at PVP concentrations under 10 ppmw but decreased rapidly to near zero growth at concentrations above 1000 ppmw. PVP reduced the initial growth rate by roughly 50% over the concentration range of 10 to 100 ppmw. Such low concentrations have not previously been investigated and provide insight into the PVP inhibition mechanism. PVP was shown to be less effective at inhibiting hydrate growth as reactor pressure increased. The various molecular weights investigated showed similar effects on growth rate at the same mass concentrations, showing no improvement in inhibition by varying polymer chain length over the range tested.