Thermodynamic Modeling Study on Phase Equilibrium of Gas Hydrate Systems for CO2 Capture

A thermodynamic model is introduced to describe equilibrium conditions of gas hydrates formed from mixtures of CO2, N-2 and H2O. The model employs the van der Waals and Platteeuw (vdW-P) solid solution theory and a modified version of cubic-plus-association equation of state that uses the Peng-Robinson equation of state for physical interactions (PR-CPA) to describe hydrate and fluid phases, respectively. When not available elsewhere, the model parameters are determined as part of this work. Pure component parameters for N-2 were calculated by fitting of the PR-CPA parameters to vapor pressures and saturated liquid densities. Moreover, solubility data for N-2 in pure water are used to fit the binary interaction parameter for the N-2-H2O system. Finally, Kihara cell potential parameters are obtained by regressing the model to the hydrate dissociation pressures of mixed hydrates. The model is validated with available experimental data in terms of equilibrium dissociation pressure and hydrate composition. Results reveal that the model is capable of describing equilibrium conditions with high accuracy. In addition to the equilibrium dissociation pressure, the model is able to predict hydrate compositions with satisfactory accuracy compared to other models, although such data were not utilized as reference data in the fitting procedure. Due to disparity amongst various data sets for the studied system, it is difficult to find unequivocally a model that perform better for all data sets. However, the introduced model shows more accurate results for most data sets and obtains satisfactory agreement in the rest. Additionally, the presented model predicts the structural transition boundary better than other similar models.