Attempt to Clarify the Synergistic Effect Mechanism of Clathrate Hydrate Crystal Growth Inhibitors

Certain quaternary ammonium salts (quats) can act as weak clathrate hydrate crystal growth inhibitors (HCGIs) synergistically with the kinetic hydrate inhibitor poly(N-vinylcaprolactam) (PVCap) for structure II (sII) gas hydrate formation. However, the inhibition mechanism of HCGIs on clathrate hydrates and the synergistic effect mechanism of weak HCGIs are not fully understood. To investigate this, we tested the performance of HCGIs (PVCap), tetrapropylammonium bromide (TPrAB), tetrapentylammonium bromide (TPeAB), and their mixtures (PVCap + TPrAB and PVCap + TPeAB) using the unidirectional growth method. A stoichiometric tetrahydrofuran (THF)- water solution with added HCGIs was used as the hydrate-forming agent. Our previous study proposed using the degree of supercooling of the growth interface, Delta T, as an HCGI inhibition performance index with a relatively low growth rate, V, in the region <= 10 mu m s(-1). In this study, we investigated the crystal growth rate, V, from 0.005 to 100 mu m s(-1). In the high-growth-rate region, the inhibition effect of PVCap decreased with V and finally almost disappeared. In contrast, the inhibition effect of TPeAB + PVCap remained in the high-growth-rate region. We propose a mechanistic interpretation of the synergistic inhibition mechanism of HCGI by summarizing the current and previous experimental results.