(515t) Prediction of the Equilibrium Conditions of Clathrate Hydrates Using Updated Pressure Dependence of the Langmuir Adsorption Constant in the Van Der Waals-Platteeuw Model

In this work, we generalized the previously developed model [Fluid Phase Equilibria 2012, 325, 80-89] for the modeling of the phase boundary of clathrate hydrates with pressure- and temperature-dependent Langmuir adsorption constant by introducing guest-guest interactions among the encapsulated guests. Briefly, in this method, the fugacity of a species in the fluid phase is determined by the Peng-Robinson-Stryjek-Vera (PRSV) EOS combined with the predictive COSMO-SAC activity coefficient model through the 1^st order modified Huron-Vidal (MHV1) mixing rule, i.e., PRSV+MHV1+COSMOSAC method. The PRSV+MHV1+COSMOSAC allows for the prediction of fluid phase behaviors (e.g., VLE) without input of any experimental data for the mixture fluids. In the solid hydrate phase, the fugacity is determined from the pressure- and temperature-dependent van der Waals-Platteeuw model, which describes the deformation of the lattice with increasing pressures.

This model successfully describes the various types of three-phase coexisting conditions of single and mixed-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 updated model introduces universal parameters of guest-guest interaction of each chemical species contribute to the potential energy for stabilizing the encapsulated guests within different cavities, resulting in a highly accurate description of the hydrate formation. We demonstrate that this approach is capable of modeling CH₄, C₂H₆, C₃H₈, iC₄H₁₀, and CO₂ of pure and their mixture gas hydrates upon 5 mixed gases. For single-gas hydrates, the average relative deviations in the equilibrium pressure are found to be 3.11 % in VIHE and VLHE regions, and the average relative deviations in the equilibrium temperature are found to be 0.33 % in LLHE region. For mixed-gas hydrates, the average relative deviations in the equilibrium pressure are found to be 5.57% in VLHE region, and the average relative deviations in the equilibrium temperature are found to be 0.29% AARD-T in LLHE region. The description of three-phase coexisting conditions of gas hydrate is over a large range of temperatures (148.8 K to 323.9 K) and pressures (5.35x10² Pa to 4.79x10⁸ Pa).