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Effect of guest-dependent reference hydrate vapor pressure in thermodynamic modeling of gas hydrate phase equilibria, with various combinations of equations of state and activity coefficient models

01-05-2022, Anil, Jugal N., Bhawangirkar, Dnyaneshwar R., Jitendra Sangwai

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.

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Thermodynamic modeling of phase equilibria of clathrate hydrates formed from CH4, CO2, C2H6, N2 and C3H8, with different equations of state

01-02-2018, Bhawangirkar, Dnyaneshwar R., Adhikari, Jhumpa, Sangwai, Jitendra S.

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.