Synergistic Kinetic Hydrate Inhibition of Pectin, PVP, and PVCap with Monoethylene Glycol

Abstract

Before injecting any kinetic hydrate inhibitor (KHI) into a flow line, the standard procedure is to dissolve the chemical, which is mostly a polymer, in a solvent such as water, methanol, or glycol. It is ideal if the solvent synergizes with the KHI in hydrate inhibition. In this paper, the synergy of three KHIs, poly(vinylpyrrolidone) (PVP), poly(vinyl caprolactam) (PVCap), and a natural biodegradable polymer Pectin at 0.25 wt %, is investigated with one of the common solvents monoethylene glycol at concentrations (0.5–20 wt %). The hydrate-inhibiting performance of the KHIs and their solvent blends is evaluated using an autoclave to measure the induction time (IT) and the molar hydrate growth rate, which is approximately taken equal to the molar gas consumption rate (HGR/GCR) in constant cooling rate tests and isothermal tests. The individual hydrate inhibition testing results showed that MEG alone at high dosages (up to 20 wt %) is a very poor KHI, while Pectin is a moderate KHI with performance lower than the commercial KHIs (PVP and PVCap) at similar concentrations. The hydrate growth rate of Pectin is however, lower compared to that of both PVP and PVCap. The blends of the KHIs with MEG, showed significant hydrate inhibition synergy in all of the KHI–MEG blends with increasing MEG concentration from 0.5 to 20 wt %. The synergy was maximum at 5 wt % of MEG for all the KHIs with IT enhancement of around 50% for PVP and PVCap and 200% for Pectin. Almost 30–60% reduction in HGR was observed in the blends compared to that of their individual KHI polymer HGRs. Isothermal experiments at 1.5, 6, and 8 °C were done using KHI–MEG blends to simulate possible field/subsea conditions. Among the three KHIs at 0.25 wt % blended with 5 wt % of MEG at 6 °C, PVP–MEG and Pectin–MEG blends gave an almost equal IT of 3.5 h, while the PVCap–MEG blend gave a slightly higher IT of 4.55 h. The HGR of the Pectin–MEG blend was however, the lowest (0.055 m/h) when compared to the PVP–MEG blend HGR (0.08 m/h) and the PVCap–MEG blend HGR (0.075 m/h). Similar IT but lower HGR values were found at 1.5 °C when 20 wt % of MEG with 0.25 wt % of Pectin was used. No hydrates were formed up to 6 days with 0.25 wt % of Pectin and 5 wt % of MEG at 8 °C due to a low subcooling of 1.3 °C. In summary, adding MEG to commercial KHIs such as PVP, PVCap and natural KHI Pectin increases their IT tremendously due to synergy and at the same time lowers their HGRs. As the blends of Pectin–MEG are highly biodegradable, they can be used at locations that have relatively low subcooling requirements and where biodegradability and lower hydrate growth rate are essential as they have similar ITs as that of PVP–MEG blends while lower HGRs than both PVP–MEG and PVCap–MEG blends.