(31d) New Application of Ionic Liquids as Inhibitors for Flow Assurance

Hydrates are crystalline solids composed of hydrogen-bonded water molecules and gas molecules. Hydrates tend to form in gas pipelines and drilling fluids when there are sufficient reactants in situ, together with the necessary pressure and temperature conditions. The fluid flow blockage by hydrate in pipelines is a serious problem in natural gas transportation. To assure fluid flow, thermodynamic and kinetic inhibitors are widely used by industry to prevent hydrate formation and thus guarantee gas transportation. However, the existing inhibitors are highly uneconomical, especially at high pressures and high degrees of supercooling.

A preliminary study of imidazolium ionic liquids as a new class of inhibitors demonstrated that certain types of ionic liquids are found to shift the hydrate dissociation/stability curve to a lower temperature and slow down the hydrate nucleation rate, which is attributed to their strong electrostatic charges and hydrogen bond with water. This dual function is expected to make these novel inhibitors perform more effectively than the existing inhibitors. This current research extends the field of ionic liquids to dialkylimidazolium-based ionic liquids with halide anions. Dialkylimidazolium ionic liquids with several alkyl substitute groups in the cation heads and anions have been investigated for their potential application as novel gas hydrate inhibitors. Their effects on the equilibrium methane hydrate dissociation curve in a pressure range of 36 to 136 bar and the induction time of methane hydrate formation at 114 bar and a high degree of supercooling, i.e., about 25°C, are measured in a high-pressure, micro-differential scanning calorimeter. In order to clarify the mechanism of these ionic liquids in inhibiting the hydrate formation, the electrical conductivity and infrared spectra of ionic liquids were measured. The strength of hydrogen bonding between water molecules and anions/cations of the ionic liquids were analyzed from infrared spectra absorption.

In the pressure range of our experiments, the ionic liquids studied with concentrations of 10 wt% shift the equilibrium methane hydrate dissociation curve about 1-3.5 K to lower temperatures. Among all the ionic liquids studied, EMIM-Cl is the most effective thermodynamic inhibitor. For ionic liquids with the same cation, either EMIM or BMIM, the thermodynamic inhibition effectiveness is decreasing along the same sequence: Cl > Br > I > BF₄. For ionic liquids with the same anion, the thermodynamic inhibition effectiveness of ionic liquids with a shorter alkyl chain substituent is better than that of ionic liquids with a longer alkyl chain substituent. The effect of the anion type on the thermodynamic inhibition effectiveness is arguably more significant than that of the cation type. The electrical conductivity measurements demonstrate that the thermodynamic inhibition effectiveness of ionic liquids is closely related to the electrical conductivity of ionic liquids in aqueous solutions. Ionic liquids with higher electrical conductivity for their aqueous solutions generally show higher thermodynamic inhibition effects. The IR spectra of ionic liquids also suggest that the strength of the hydrogen bond with water may have an important role in the thermodynamic inhibition effectiveness of the ionic liquids. From the study of the induction time of hydrate formation, the kinetic inhibition effect of BMIM-I is as good as EMIM-BF₄. The mean value of the induction time of methane hydrate formation from samples containing 1 wt% BMIM-I is about 5.2 hours, compared to 5.7 hours for 1 wt% EMIM-BF₄. Thus, BMIM-I is also found to be much better than commercial kinetic inhibitors such as Luvicap and PVCap. In summary, these new ionic liquids are found to shift the equilibrium hydrate dissociation/stability curve to a lower temperature and simultaneously retard the hydrate formation by slowing down the hydrate nucleation rate.