Synergetic Performance of the Mixture of Poly(N-vinylcaprolactam) and a Pyrrolidinium-Based Ionic Liquid for Kinetic Hydrate Inhibition in the Presence of the Mineral Oil Phase

In our previous studies, it was proven that ionic liquids can be a good synergist with poly(N-vinylcaprolactam) [PVCap, a polymeric kinetic hydrate inhibitor (KHI)] against methane or natural gas hydrate formation. In particular, 1-hexyl-1-methylpyrrolidinium tetrafluoroborate (HMP-BF4) showed the best performance for natural gas hydrate inhibition among the ionic liquids tested. In this study, we report the result of performance tests on kinetic inhibition of natural gas hydrate formation with a mixture of PVCap and HMP-BF4 in the presence of an additional oil phase. At various water cuts (20, 50, and 80%) with 3.5 wt % aqueous NaCl solution and mineral oil, the KHI performance of PVCap alone and a mixture of 0.5 wt % PVCap and 0.5 wt % HMP-BF4 was evaluated. This was performed by measuring the induction time for the synthetic natural gas hydrate formation, in addition to the pressure, temperature, and torque of the system investigated. Calculations were carried out to determine the amount of water converted to hydrate and the volume fraction of hydrate in the liquid phase. The results showed that the induction time became shorter when no KHIs were tried as the water cut increased and that the trend was identical with a further diminished induction time with PVCap alone. On the other hand, the mixture of PVCap and HMP-BF4 showed an effective synergetic performance on the kinetic inhibition of natural gas hydrate formation (increased induction time). Moreover, the KHI mixture showed a longer induction time as the water cut increased. This opposite tendency in the induction time according to the water cuts and KHI applied was explained by the hydrate nucleation rate per unit volume of the aqueous phase.