Experimental measurements and thermodynamic modeling of dissociation pressure of CO2 hydrate in sulfate solutions

Carbon capture and storage (CCS) has been a popular strategy to mitigate climate change and has attracted significant attention from both industry and academia. CO₂ can be stored in the form of CO₂ hydrate in deeper locations in an ocean, which makes it a potential option for CO₂ sequestration. Dissolved salts in the ocean brine can significantly affect the dissociation pressure of CO₂ hydrate. Sulfate salts are one of the most common salt species in the oceanic brine. Although various experimental studies have been conducted to investigate the phase behavior of CO₂ hydrate in brine, few studies focus on the effect of sulfate salts on the dissociation pressure of CO₂ hydrate. In this study, we build an in-house experimental setup to investigate the effect of monovalent and divalent sulfate salts on the dissociation pressure of CO₂ hydrate. The dissociation pressure of CO₂ hydrate in Na₂SO₄, K₂SO₄, MgSO₄, and CuSO₄ aqueous solutions is measured using the isochoric pressure search method at different concentrations over the temperature range of 274.36 – 282.15 K and the pressure range of 1.50 – 4.03 MPa. A hybrid methodology incorporating the Soave-Redlich-Kwong Equation of State (SRK EOS), the van der Waals-Platteeuw (vdW-P) model, and the Pitzer model is applied to predict the dissociation pressure of CO₂ in sulfate solutions. In addition, the dissociation enthalpies of CO₂ hydrate in these sulfate solutions are calculated using the Clausius-Clapeyron equation based on the measured dissociation points. The experimental results show that the dissociation pressure of CO₂ hydrate in Na₂SO₄, K₂SO₄, and MgSO₄ solutions is higher than that in pure water, and the dissociation pressure of CO₂ hydrate in sulfate solutions increases with an increasing salt concentration. Conversely, CuSO₄ barely affects the dissociation pressure of CO₂ hydrate, which is mainly attributed to the lower molar concentration of the ions compared with the other salt solutions and the ion specificity of Cu²⁺. The prediction results are in alignment with the experimental data measured in this study, which proves the feasibility of the thermodynamic model in predicting the dissociation pressure of CO₂ hydrate in sulfate solutions. Additionally, the calculated dissociation enthalpies of CO₂ hydrate show a dependence on both temperature and salt concentration. It is also revealed that Cu²⁺ exhibits ion specificity in affecting the dissociation enthalpy of CO₂ hydrate, likely due to its more distinct ability to impact the cage occupancy of CO₂ hydrate than the other ions. These findings enhance our understanding of the impact of sulfate salts on the dissociation behavior of CO₂ hydrate and offer valuable insights for CO₂ sequestration.