N-Oxide Polyethers as Kinetic Hydrate Inhibitors: Side Chain Ring Size Makes the Difference

Abstract

The formation of gas hydrates in flow lines is one of the most severe problems for flow assurance in the gas and oil industry. Developing effective kinetic hydrate inhibitors (KHIs) to avoid the problem of gas hydrate formation has attracted widespread attention. In this study, a series of poly­(glycidyl amine N-oxide)­s (PGAOs) with 5–7-membered rings as side chains, poly­(pyrrolidine glycidyl amine N-oxide)­s (PPyrGAOs), poly­(piperidine glycidyl amine N-oxide)­s (PPiGAOs), and poly­(azepane glycidyl amine N-oxide)­s (PAzGAOs), with varying molecular weights, have been synthesized. The KHI performance of these glycidyl amine N-oxide polyethers has been evaluated in high-pressure rocking cells with the synthetic natural gas (SNG) mixture. The PGAOs with lower molecular weights gave better KHI performance, and at 2500 ppm, the best one gave an average T o value of 9.8 °C (ΔT = 10.4 °C), which is on a par with polyvinylcaprolactam (PVCap). Even in high concentration of brine solution, none of the PGAOs showed a cloud point up to 95 °C. Employing molecular weights of around 4 kg/mol, the KHI performance of the PGAOs follows the following trend, correlating with the ring size: PPyrGAO < PPiGAO < PAzGAO. However, at higher molecular weight, the ring size of the pendant group did not affect the KHI performance of the PGAOs. PPiGAO with the smaller piperidine ring groups gave better inhibition effect than PAzGAO when the molecular weights were at approximately 8 kg/mol. In addition, the KHI performance of one of the best PAzGAOs was tested in the concentration range from 1000 to 5000 ppm, and an increase of the KHI performance with increasing concentration of polymer was observed. The amine N-oxide functional group is critical for the KHI performance of these polymers, as poly­(pyrrolidine glycidyl amine)­s (PPyrGAs) and poly­(azepane glycidyl amine)­s (PAzGAs) with amine groups instead of the N-oxide gave a negligible inhibitory effect.

Non-Amide Polymers as Kinetic Hydrate Inhibitors─Maleic Acid/Alkyl Acrylate Copolymers and the Effect of pH on Performance

“…We also knew that amine oxide groups in polymers can give good KHI performance. One study showed that a series of polyamine oxides was significantly better as a KHI than the corresponding polyamines, as well as gave better water solubility . Therefore, we synthesized several amine oxides of the maleic polymers with dibutylamino head groups by reaction with hydrogen peroxide as previously described (Figure ).…”

Pushing the Known Performance Envelope of Kinetic Hydrate Inhibitors─Powerful Synergy of Trialkylamine Oxides with Acrylamide-based Polymers

“…They include homopolymers and copolymers of N -vinyllactams, N -isopropylmethacrylamide (NIPMAm), and hyperbranched poly­(esteramide)­s. − ,, These polymers can adsorb onto the surface of the crystal hydrate as a pseudo-guest through the polymeric pendant group and can prevent or delay nucleation of the hydrate crystal. However, polymers with other hydrophilic groups besides amides have also been shown to have good KHI performance, especially polyamine oxides. − The exact KHI mechanism is not fully understood, but it may be due to the strong hydrogen bonding afforded by both amides and amine oxides with water molecules aiding gas hydrate nucleation inhibition and crystal growth inhibition…”

Surface Science in the Research and Development of Hydrate-Based Sustainable Technologies

“…Advanced surface science is a prerequisite for the development of targeted LDHIs. ,,− As surface-active reagents, LDHIs adsorb on the surface of ensuing hydrate nuclei and retard their growth and/or aggregation. ,− This necessitates an insightful understanding of the interaction between LDHI molecules and the hydrate surface. , The attractive force between LDHI molecules and the hydrate surface (i.e., the driving force of LDHIs adsorption) acts within a subnanometer range, meaning that the adsorption process is dictated only by a few outmost water layers of the hydrate surface. Carver et al indicated that this driving force arises from the hydrogen bonding between hydrophilic sites of LDHI molecules and water molecules of the hydrate surface. , This mechanism applies to hydrate surfaces contacting a gas or hydrophobic environment.…”