Critical Growth Rate of Hydrate Crystal Growth Inhibitors in the Low Growth Rate Region

We tested poly(N-vinylpyrrolidone) (PVP) K-12, K-15, K-30, K-90, poly(N-vinylcaprolactam) (PVCap), tetrapentylammonium bromide (TPeAB), tetrabutylammonium bromide (TBPB), polyacryloylpyrrolidine (PAP), poly(N-isopropylmethacrylamide) (PNIPMAM), and hyperbranched polyesteramide (PEA) as hydrate crystal growth inhibitors (HCGIs). We used a tetrahydrofuran (THF) sII hydrate system using a unidirectional growth apparatus. The HCGI concentration c = 0.5 wt % and the growth rate V ranged from 0.005 to 10 μm s^–1. The growth rates of TPeAB, PVP K-12, K-15, TBPB, and PEA were found to be critical in the region of a low growth rate with the degree of supercooling ΔT approaching 0. However, PVP K-30, K-90, PAP, PNIPMAM, and PVCap remained effective at V = 0.005 μm s^–1. These results enhance the persuasiveness for the model that the HCGI effect is explained by the residence time on the crystal surface of HCGI, τ, and time interval t* required to increase the interface curvature. In addition, we determined the threshold growth rate of THF hydrate for examining HCGI performance on sII gas hydrate. Thus, when 0.005 ≤ critical growth rate V* < 0.01 μm s^–1, the HCGIs are moderate inhibitors on sII gas hydrate. When V* < 0.005 μm s^–1, the HCGIs are strong inhibitors on sII gas hydrate.