Tetrahydrofuran Hydrate Crystal Growth Inhibitor Performance and Mechanism of Quaternary Ammonium and Phosphonium Salts

To gain insight into the hydrate crystal growth inhibitor (HCGIs) mechanism, we have studied various quaternary ammonium ionic liquid salts (QAILs) and phosphonium salts for their ability to prevent structure II (sII) hydrate crystal growth. We tested polyvinylpyrrolidone (PVP), polyvinylcaprolactam (PVCap), tetrapropylammonium bromide (TPrAB), tetrabutylammonium bromide (TBAB), tetrapentylammonium bromide (TPeAB), tetrahexylammonium bromide (THexAB), and tetrabutylphosphonium bromide (TBPB) in a tetrahydrofuran (THF) sII hydrate system using a unidirectional growth apparatus to determine the concentration and growth rate dependence. The growth rates of TPrAB, TBAB, and THexAB were found to be critical in the region of low growth rate, with the degree of supercooling ΔT becoming 0 for these additives in the low-growth-rate region. In contrast, PVP, PVCap, TPeAB, and TBPB remained effective at the lowest growth rate of V = 0.04 μm s–1. These trends can be explained by the residence time on the crystal surface of the HCGI, τ, and duration time of crystal growth.