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).

Semiclathrate hydrates of natural gas have shown potential applications in natural gas storage and transportation. Promoters, viz., tetra-n-alkyl ammonium bromide (TBAB) and tetrahydrofuran (THF) have positive impacts on the phase stability condition in lowering the required pressure for hydrate formation. As part of this work, a predictive model for the phase stability of gas hydrate, which are necessary to understand the phase behavior of methane (CH4) hydrate in promoters, has been proposed. The fugacity of hydrate former in the gaseous phase is calculated from Peng-Peng-Robinson equation of state (PR-EoS), while the fugacity of water in the liquid phase is computed from recently proposed Pitzer-Mayorga-Zavitsas-Hydration (PMZH) model for TBAB system and non-random two liquid (NRTL) model for THF system. The van der Waals Plattew model is employed for the hydrate phase. The vapor pressure of water in the empty hydrate lattice as well as Langmuir adsorption constants have been expressed in terms of concentration of the promoters. The predictions of the proposed model are found to be match well with experimental data on phase stability of CH4 hydrate formed using TBAB and THF aqueous systems. Furthermore, the developed model is employed for the prediction of phase stability conditions of the semiclathrate hydrates of CH4 in TBAB + NaCl system. The developed model is found to interpret the promotion effects of both TBAB (with or without NaCl) and THF on phase stability conditions of CH4 hydrate. AARD-P% with PMZH model are observed to be 3.21% and 8.73% for semiclathrate hydrates of CH4 in TBAB and TBAB + NaCl, respectively, and 8.56% for clathrate hydrate of CH4 in THF. The model may be extended to evaluate the phase stability conditions of hydrates of multicomponent gas systems in TBAB/THF which are necessary for real field applications.

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.

A thermodynamic model for the prediction of CO2 hydrate phase stability conditions in the presence of pure and mixed salts solutions and various ionic liquids (ILs) is developed. In the proposed model van der Waals and Platteeuw model is used to compute the hydrate phase, Peng-Robinson equation of state (PR-EoS) for the gas phase and the Pitzer-Mayorga-Zavitsas-Hydration model is employed to calculate the water activity in the liquid water phase. This model is an extension of the model developed by Tumba et al. (2011) for the prediction of methane and CO2 hydrate phase stability conditions in the presence of tributylmethylphosphonium methylsulfate IL solution. Shabani et al. (2011) mixing rule is modified by incorporating the water-inhibitor (salt/IL) interaction parameter to calculate the water activity in mixed salt solutions. The model predictions are also calculated using the Pitzer-Mayorga model separately and compared with predictions of the developed model. The model predictions are compared with experimental results on the phase stability of CO2 hydrate in the presence of ILs, pure and mixed salts as reported in literatures. The ILs are chosen from imidazolium cationic family with various anion groups such as bromide (Br), tetrafluoroborate (BF4), trifluoromethanesulfonate (TfO), and nitrate (NO3) and the common salts such as NaCl, KCl and CaCl2. Good agreement between the developed model predictions and the literature data is observed. The overall average absolute deviation (AARD%) with Pitzer-Mayorga-Zavitsas-Hydration model is observed to be within ±1.36% while Pitzer-Mayorga model accuracy were about ±1.44 %. Further, the model is extended to calculate the inhibition effect of selected inhibitors (ILs and salts) on CO2 hydrate formation.