Volume 48

Thermodynamic Modeling of CO2+THF sII hydrate phase equilibria: Implication for hydrate-based CO2 sequestration Dnyaneshwar R. Bhawangirkar, Zhenyuan Yin

Abstract

Hydrate-based CO2 sequestration has emerged as one of the possible methods for the long-term storage of CO2 as hydrates within the geological media, notably under marine sediments. In this direction, a thermodynamic model for the phase equilibria of CO2 hydrates in pure water and THF solutions (≤ 5.56 mol%), based on the fugacity approach is employed to predict the three-phase (LHV) equilibrium pressures and the fractional occupancy of CO2 and THF molecules in small and large cages. The hydrate, vapor, and liquid phases are modeled using the van der Waals-Platteeuw solid solution theory, Peng-Robinson-Stryjek-Vera equation of state, and the modified UNIFAC method, respectively. The model-predicted equilibrium pressures are in excellent agreement with experimental data with an AAD of 1.55%. The presence of THF has greatly reduced the CO2 hydrate equilibrium pressures. The cage occupancy results revealed the reduction in small cage occupancy of CO2 at (≥) 1.296 mol% THF and a continuous substantial decrease in large cage occupancy of CO2 with increased THF concentration. However, the overall hydrate stability is ensured by the large cage occupancy of the large molecule THF in larger fractions. This study is an important precursor for future hydrate-based CO2 sequestration.

Keywords gas hydrates, CO2 sequestration, mitigation technologies, thermodynamic modeling, THF, cage occupancy