An improved model for the phase equilibrium of methane hydrate inhibition in the presence of ionic liquids

Abstract

In this work, a thermodynamic model is developed and used to predict the phase stability conditions for methane hydrate-ionic liquid (IL)-water system. The hydrate phase is computed from modified van der Waals-Platteeuw model. The Peng-Robinson equation of state (PR-EoS) and developed activity model as a combination of Pitzer-Mayorga-Zavitsas-hydration model is used to evaluate the fugacities of gas and liquid phases, respectively. The hydrate phase stability prediction is also computed using the liquid phase activity predicted by NRTL and Pitzer-Mayogra models, separately, and is compared with the results predicted from the developed model. The model predictions are compared with experimental results on the phase stability of methane hydrate reported in open literatures for 21 ILs. The 21 ILs chosen from various ionic groups such as tetraalkylammonium, pyrrolidinium, imidazolium cationic family with various anion group such as halides (Cl, Br, I), sulphate (HSO4, ethylsulphate), tetrafluoroborate (BF4) and dicyanamide (DCA). The absolute average relative deviation in predicted pressure (AARD-P) with developed Pitzer-Mayorga-Zavitsas-hydration-model is improved to 1.60% and non-random two liquid (NRTL), Pitzer-Mayorga model showed 2.02% and 1.77% with 120 data points in the temperature range of 272.1-291.59K and pressure range of 2.48-20.67MPa. For 120 data points of phase stability conditions of 21 ILs, 39.2% of the predicted equilibrium pressures (47 data points) were within relative absolute deviation of 0.0-1.0%, 29.2% of the equilibrium pressures (35 data points) were within absolute deviation of 1.01-2.5%, 25.8% of data (31 data points) were within 2.51-7.5% which are mainly for data with low concentrations of ILs and only 5.8% of data (7 data points) showed relative absolute deviations above 7.5% which are observed mainly for data with high concentrations of ILs. Further, the model is used to calculate the inhibition effect of selected 21 ILs on methane hydrate formation.