Thermodynamic and kinetic effects of ionic liquids in gas hydrate inhibition

Abstract

Gas hydrates are composed of hydrogen bonded water molecules with a guest trapped inside. Gas hydrates pose a huge risk in oil and gas processing as the resulting pipeline blockages are a major concern. Preventative measures such as thermodynamic and kinetic inhibitors are included as part of mitigation strategies. However, drilling under more extreme pressure and temperature conditions is demanding enhanced inhibitor performances. This study entails the analysis of the potential application of ionic liquids in gas hydrate inhibition. The first stage was to design and construct a hydrate rig that could accurately determine the onset of hydrate formation. The examination of procedures and evaluation techniques composed the foundations of this work. A number of experimental parameters such as agitation and cooling rates were optimised in order to ensure the reproducibility of the data obtained. Due to the stochastic nature of hydrate formation, it was important to minimise the degree of error with the formation points. Preliminary tests displayed that the hydrate rig could adequately determine the hydrate onset points. Thus, it was concluded that the methane hydrate formation point could be used to screen ionic liquids as potential thermodynamic or kinetic inhibitors. This was supported by the ability of the rig to reproduce a literature inhibitor trend, as the same order of effectiveness was obtained. The study also assessed the ability of the screening method to be used with another hydrate testing rig. Typically, data between different experimental set-ups is not transferrable. However, applied to another rig, the screening method prevailed. Once the methodology had been established, the design aspect of ionic liquids was utilised. The desirable properties of the ionic liquids included low toxicity, high biodegradability and most importantly hydrogen bonding capacity. This is an imperative property for disrupting the hydrogen bonding between the water molecules required for hydrate formation. This was accomplished through the use of the choline cation with carboxylate-based anions with different chain lengths and functional groups. The tetramethylammonium cation was selected to investigate the effect of the hydroxyl group on choline. The initial tests indicated that the small tetramethylammonium cation was more effective in combination with the longer chain carboxylate-based anions. This conclusion was different that the one obtained from the methane hydrate dissociation curves in the presence of the ionic liquids, which indicated the acetate anion was the most effective. The thermodynamic analysis was supported by the water activity values predicted by COSMO-RS. The variations in the results was suspected to be due to the kinetic inhibition ability of the long chain carboxylate-based anions. This was verified by the constant cooling tests that examined the kinetic ability of the ionic liquids. Detailed analysis of the nucléation and growth stages displayed the ability of the hexanoate and octanoate anions to produce a delayed onset of hydrate formation while also accelerating the growth. Enhanced growth is unwanted behaviour for a hydrate inhibitor. The data showed high water to hydrate conversion percentages for tétraméthylammonium hexanoate and octanoate. Reduction of the inhibitor concentration allowed the onset delay time to be decreased, while maintaining high conversion percentages. This capability demonstrates the role ionic liquids could play in gas storage applications.

Date of Award	Dec 2019
Original language	English
Awarding Institution	Queen's University Belfast
Sponsors	Northern Ireland Department for the Economy
Supervisor	Jillian Thompson (Supervisor), David Rooney (Supervisor) & Mark Muldoon (Supervisor)