Improvement of Gas Hydrate Inhibition through the Synergistic Effect of PEO-co-VP with MEG and MeOH

The synergistic effect of two commonly used thermodynamic gas hydrate inhibitors (THIs), methanol (MeOH) and monoethylene glycol (MEG), with a kinetic gas hydrate inhibitor (KHI), poly(ethylene oxide-co-vinylpyrrolidone) (PEO-co-VP), at the underinhibited conditions was investigated using synthetic natural gas and a 200 mL high-pressure cell. The investigated concentration range was 0.0–15.0 wt % for THIs and 0.1–1.0 wt % for PEO-co-VP. Both THIs showed a concentration-dependent synergistic effect on PEO-co-VP, which became more noticeable when the THI concentration was equal to or above 10.0 wt %. Stronger synergistic effects were demonstrated by MEG than by MeOH under the same conditions. A total inhibition of gas hydrate formation over 6000 min was yielded when 15.0 wt % MEG was used with 1.0 wt % PEO-co-VP. Quantitative analysis showed that during the course of gas hydrate formation, the KHI concentration in the liquid phase decreased with time when no MEG was used, confirming that the adsorption of KHI molecules onto the formed hydrates was the predominant inhibition mechanism. However, in the presence of MEG or MeOH, the KHI concentration in the liquid phase increased after the formation of hydrates, suggesting a totally different inhibition mechanism, which we speculate is a result of the improved solubility of PEO-co-VP in the aqueous phase due to its stronger molecular interactions with MEG and MeOH. Further investigations on the effect of subcooling demonstrated that the added THIs not only can improve the inhibition performance of the KHI but also could possibly improve its workable subcooling range. The findings in this work suggest that kinetic inhibitors can be used at high subcooling if an adequate concentration of MEG is applied at the same time.