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

Abstract

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.

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

“…The value of V at which Δ T – Δ T 0 = 0 °C is denoted as V h *. Additionally, V l *, representing the critical V where the HCGI inhibition effect disappears in the low-growth-rate region, was previously termed V * in our previous publications. , V h * increased with an increase in the c of PVCap. However, beyond V ≥ 50 μm s –1 , some Δ T – Δ T 0 values were negative at c = 0.01 wt %, seemingly contradictory as Δ T – Δ T 0 < 0 implies a growth-promoting effect.…”

“…The black line in Figure 13c shows the synergistic inhibition effect with a mixture of weak and strong HCGIs: strong HCGIs inhibit crystal growth in the low-growth-rate region, while weak ones effectively inhibit growth in the high-growth-rate region. 22,23 In other words, the inhibition effect of the mixture ΔT mix is given by the weighted arithmetic mean of ΔT as a weighted multiplied by c…”

“…The c = 1.0, 0.5, 0.25, 0.1, and 0.01 wt %. The data plot c = 0.5 wt % within the range of 0.005 μm s –1 ≤ V ≤ 10 μm s –1 is provided in refs , .…”

“…The THF hydrate growth inhibition effect of TPeAB is stronger than that of TPrAB. Although TPeAB and TPrAB are weak, sII gas hydrate inhibitors, TPeAB in particular, are good synergists when combined with PVCap. − …”

“…Schematics of HCGI inhibition mode differences depending on τ and t * values via the Gibbs–Thomson effect. Reproduced from Muraoka et al Copyright (2021) American Chemical Society.…”