Jitendra Sangwai

A thermodynamic model to predict three phase (L-H-V and I-H-V) equilibria of gas hydrates is presented. In this model we have employed a fugacity based approach where the hydrate phase is modeled using van der Waals-Platteeuw solid solution theory and the liquid phase activity coefficients are determined from the modified UNIFAC method. For the vapour phase fugacity calculations we have investigated three equations of state (EOS): Peng-Robinson-Stryjek-Vera (PRSV), Patel-Teja (PT) and Soave-Redlich-Kwong (SRK). This model employs only parameters reported in the literature. The coexistence pressures predicted by our model for the sI hydrates of methane, carbon dioxide and ethane are in reasonable agreement with experiments, whereas our model overestimates the coexistence pressures for the sII clathrates of nitrogen and propane. The predicted cage occupancies are found to increase with increasing temperature in the L-H-V equilibria. For I-H-V equilibria the cage occupancy is observed to decrease with temperature. We have also estimated the solubility of each guest in the liquid phase (for L-H-V equilibria) using the Henry's law. The solubilities predicted using all three EOS are in good agreement for all guest molecules, with the exception of nitrogen where at relatively higher temperatures the estimates from the PRSV EOS are noticeably lower than the corresponding predictions from the PT and SRK EOS.

In this study, eight ionic liquids (ILs) from the two varieties of ILs, namely, aromatic and aliphatic ILs, have been considered to carry out experimental studies for their effect on the phase behavior of methane hydrate. We have employed five aromatic based ILs with several cations, such as 1-butyl-3-methyl imidazolium, 1-hexyl-3-methyl imidazolium, 1-octyl-3-methyl imidazolium and various anions, such as [Cl]−, [Br]−, [HSO4]−, and three aliphatic based ILs with various cations, such as di-ethyl-ammonium, tri-propyl-ammonium, tri-butyl-ammonium and [HSO4]− anion. All the experiments were performed in the hydrate equilibrium pressure and temperature ranges of 3.86–7.66 MPa and 276.68–283.18 K, respectively. It has been observed that all the investigated ILs have shown inhibition effect on methane hydrate system. Aromatic ionic liquids have shown their dominance over aliphatic ionic liquids in terms of methane hydrate inhibition. ILs with similar class of cation with varying carbon chain length have not shown significant improvement in hydrate inhibition. However, the replacement of anion by [HSO4]− in imidazolium-based ILs improves methane hydrate inhibition. 1-butyl-3-methyl imidazolium sulphate ([BMIM]+[HSO4]−) found to be the best methane hydrate inhibitor among all investigated ILs. In addition, a phase behavior model incorporating a single tuning parameter has been proposed to predict the phase behavior of methane hydrate in the presence of various ionic liquids and salt solutions. The absolute average deviation in pressure (AARD-P, %) for proposed model with an experimental data generated in this work and for various data sets from the literature has been found to be within ±2.90% of the experimental values. Both cation and anion of the ionic liquids have shown to exhibit inhibition effect on the methane hydrate phase stability. The study indicates that the selection of ionic liquids with tunable cation and anion may provide an opportunity to design the best inhibitor for methane hydrate.