(525a) Thermodynamic Model for Phase Boundary of Gas Hydrates with and without Additives

In this work, we propose a new Langmuir adsorption constant for the modeling of the phase boundary of clathrate hydrates over a wide range of conditions. The proposed pressure- and temperature- dependent Langmuir adsorption constant is designed to produce a reduced free volume available to the encapsulated gas molecules as the pressure increases. The Peng-Robinson-Stryjek-Vera (PRSV) equation of state, combined with the COSMO-SAC activity coefficient liquid model through the modified Huron–Vidal (MHV1) mixing rule and with this new Langmuir model  can be used to describe various types of three phase coexisting conditions of gas hydrates, from vapor-ice-hydrate equilibrium (VIHE) at low temperatures, to vapor-liquid-hydrate equilibrium (VLHE) at higher temperatures, and to liquid-liquid-hydrate equilibrium (LLHE) at high pressures, using a single set of parameters. The average relative deviations in the equilibrium pressure are found to be 4.57 % for 12 pure gas hydrates over a large range of temperatures (148.8 K to 323.9 K) and pressures (5.35x10² Pa to 8.27x10⁸ Pa). Furthermore, the retrograde behavior observed in CH₄, CO₂, C₃H₈, and i-C₄H₁₀pure gas hydrate systems can all be successfully modeled by the change of free volume at high pressures.

By using this method, the prediction of phase boundary change in gas hydrates with the addition of organic inhibitors and electrolytes is achieved. We examined the accuracy of this method using 5 organic inhibitors and 9 electrolytes, and over a range of temperature (259.0 K to 303.6 K) and pressure (1.37x10⁵ Pa to 2.08x10⁸ Pa). The average relative deviations in the predicted equilibrium temperatures are found to be 0.72 % with organic inhibitors and 0.18 % with electrolytes, respectively. We believe that this method is useful for many gas hydrate related engineering problems such as the screening of inhibitors for gas hydrates in flow assurance.