(3ax) Determining the Phase Boundary of Gas Hydrates Using a Novel Unified Equation of State

The phase boundary of gas hydrates divides the guest-aqueous systems into hydrate and hydrate-free two zones, which is essential to flow assurance, energy recovery, and hydrate-based technology. Determining the hydrate phase boundary of various systems accurately and efficiently is important for many applications. In this work, a new thermodynamic model has been proposed to calculate the phase boundary of gas hydrates by using a unified equation of state (EoS). The unified EoS is developed by summing up the contributions of guest, water, and electrolytes in the system to the Helmholtz free energy. The interactions between water, guest, and discharged ions are represented by a modified Peng-Robinson (PR) EoS. In the modified PR EoS, a new alpha function is defined for the water molecules and unsymmetrical binary interaction parameters (BIP) are employed for the aqueous and non-aqueous phases. The long-range interactions between cations and anions are considered using a mean spherical approximation (MSA) term. The Born term is applied to evaluate the change of Helmholtz energy induced by the discharging and charging processes. The newly developed thermodynamic model has been successfully employed in reproducing the measured phase boundary of pure and binary CH₄, C₂H₆, and CO₂ hydrates in single and mixed NaCl, KCl, MgCl₂, and CaCl₂ electrolytes, with the concentration of electrolyte solutions up to 24.12 wt%. It is revealed that the computational cost of this model is comparable to the standard model (i.e., a combination of EoS, Henry’s law, and activity model) of hydrate phase boundary. However, this model is more accurate in high salinity regions. For example, the average absolute relative deviation (AARD) of the calculated phase boundary pressure of CH₄ hydrates in 24.12 wt% NaCl solutions is 4.9% compared to 6.7% in the standard model. The overall AARD of the calculated phase boundary pressure of pure and binary CH₄, C₂H₆, and CO₂ hydrates in single and mixed NaCl, KCl, MgCl₂, and CaCl₂ solutions are within 10%.