Phase boundary of gas hydrates in single and mixed electrolyte solutions: Using a novel unified equation of state

Abstract

The phase boundary of gas hydrates is key to flow assurance, energy recovery, and hydrate-based technologies. In this work, a new thermodynamic model has been proposed for calculating the phase boundary of gas hydrates based on a unified equation of state (EoS) for the non-hydrate phases and the van der Waals-Platteeuw (vdW-P) model for the hydrate phase. A modified Peng-Robinson EoS is employed to describe the interactions between water, gas, and uncharged ions, combining with a simplified explicit mean spherical approximation (MSA) term for the long-range Coulombic forces and the Born terms for the discharging-charging process of ions. A new set of reference properties have been applied and the Kihara parameters in the vdW-P model are optimized by fitting against the measured phase boundary data of hydrates formed in the pure water. The newly developed model has been validated with 734 measured phase boundary data points of pure CH4 and binary CH4-C2H6 and CH4-CO2 hydrates formed in single- and mixed-NaCl, KCl, MgCl2, and CaCl2 solutions as well as in the pure water. It is found that this model is capable of predicting the phase boundary of CH4 hydrates formed in the 1–1 electrolyte solutions (NaCl-KCl) with the average absolute relative deviation (AARD) of 3.7%. The AARD of CH4 hydrates formed in the 2–1 electrolyte solutions (MgCl2-CaCl2) is 4.2%, which is greater than that for the 1–1 electrolytes. This is acceptable given the fact that none of the ion-associated parameters have been optimized in this work. The calculated phase boundary of pure CH4 hydrate formed in mixed-electrolyte solutions deviates from the measured ones only by 2.8%. The AARD of the binary hydrates (CH4-CO2 and CH4-C2H6) formed in single- and mixed-electrolyte solutions is 3.5%. This model can also be reduced to calculate the hydrate phase boundary in pure water. The AARD of the CH4-CO2 hydrates formed in pure water is determined as 3.0%. The overall AARD of these 734 data points predicted by this newly developed model is 3.4%, which proves the capability of the newly developed model in predicting the phase boundary of hydrates in multi-component systems. It is also presented that this unified EoS is reliable to determine the gas solubility in aqueous phase with and without electrolytes.