Jitendra Sangwai

In offshore gas transmission pipeline systems, typically gas and water are produced under high pressure and low temperature conditions causing the formation of gas hydrates blocking pipelines. Thermodynamic modeling is necessary to understand the phase stability of hydrate in the presence of green solvents namely, ionic liquids (ILs). In this work, the thermodynamic models are based on the computation of fugacity of hydrate phase using Van der Waals and Platteeuw solid solution theory combined with Peng - Robinson equation of state (PR-EoS) for fugacity of hydrate former in the gas phase and the computation of fugacity of aqueous water phase using activity coefficient models such as the non - random two - liquid (NRTL) model and Pitzer - Mayorga model. The model results are compared with available experimental data from open literature and observed to be in good agreement with the reported literature. Finally, the hydrate suppression temperature due to ILs on methane hydrate is calculated to know the inhibition effectiveness of IL on methane hydrate formation in offshore pipeline system. The overall accuracy of Pitzer-Mayorga model is found to be 5.8 % while NRTL model's accuracy was 6.3 % for various ILs and methane hydrate system. Model results further indicated that ILs with shorter alkyl chain length exhibit better inhibition effect. The model developed in this work shows potential application in the determination of hydrate phase stability using green solvent for offshore oil field applications. Copyright © 2014 by the International Society of Offshore and Polar Engineers (ISOPE).

Using the van der Waals solid solution theory and the fugacity-based approach of Klauda and Sandler, a thermodynamic model to predict the phase equilibria of gas hydrates using an Equation of State (EOS) and an Activity Coefficient Model (ACM) is presented. Here, we have studied five EOS models, that are, Peng-Robinson-Stryjek-Vera (PRSV), Patel-Teja (PT), Soave-Redlich-Kwong (SRK), Peng-Robinson (PR), and Redlich-Kwong (RK) to calculate the gas phase fugacity and four ACMs, that are, modified UNIFAC (mUNIFAC), UNIQUAC, Wilson and NRTL to model the liquid phase non-ideality. The Wilson and NRTL activity coefficient models have previously been used for systems containing additional chemicals in the liquid phase but not for pure water in coexistence with gas hydrates, to the best of our knowledge, and this has been explored in this work. By making the empty hydrate vapor pressure parameters guest dependent, the model improves upon the predictions from our previous model for sI and sII hydrates significantly and has drastically reduced the errors especially in modeling sII hydrates. The best combinations of EOS and ACM in L-H-V equilibrium and best EOS for I-H-V equilibrium, respectively, for each compound, to be used in the guest dependent model from the 100 combinations of EOS and ACM (for L-H-V equilibrium) and 25 possible EOS options (for I-H-V equilibrium) examined are: RK–mUNIFAC and SRK for CH4, RK–mUNIFAC and PRSV for CO2, RK–mUNIFAC and PR for C2H6, SRK–mUNIFAC and SRK for N2, and PR–NRTL and RK for C3H8. While different EOS work well for CH4, CO2, C2H6 and C3H8 hydrates in L-H-V and I-H-V equilibria, the SRK EOS works well for N2 hydrates in both L-H-V and I-H-V equilibria. We have also reported the solubilities of guest species in liquid water in equilibrium with hydrates and the hydrate cage occupancies from the models producing the least error as data on these facets is limited.

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.