Tributylphosphine Oxide as a Powerful Synergist for Kinetic Hydrate Inhibitors

Low-dosage hydrate inhibitors have been used for several decades in the oil and gas industry to prevent gas hydrates from blocking production flow lines. Trialkylamine oxides with the correct-size alkyl groups are known to be powerful clathrate hydrate crystal growth inhibitors and have been used to design both classes of LDHIs, kinetic hydrate inhibitors (KHIs), and antiagglomerants (AAs). However, there are no reports of trialkylphosphine oxides as LDHIs. Here, we report the tetrahydrofuran hydrate crystal growth inhibition properties of trialkylphosphine oxides and their synergy with classic KHI polymers including poly(N-vinyl caprolactam) (PVCap), VCap-based copolymers, and poly(N-iso-propylmethacrylamide) (PNIPMAm) in high-pressure gas hydrate rocking cell tests. Both methane and a natural gas mixture were investigated to simulate structure I and II hydrate formations. The best results were obtained for tri-n-butylphosphine oxide (TBPO) by all test methods. TBPO showed excellent synergy with PVCap and PNIPMAm for both gas types, better than n-butyl glycol ether, tributylamine oxide, or tetrabutylphosphonium bromide. For the VCap-based copolymers, the synergy with TBPO was less and decreased with increasing the polymer cloud point. This effect is proposed to be due to the relative binding strength of the polymer versus the TBPO synergist to the hydrate surface compared to their hydrophilicity.