CO₂ solubility and dissolution rate: the epic battle between ions and CO₂ for water, energy and space
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CO₂ dissolution into deep subsurface brines is regarded as a viable means of reducing CO₂ atmospheric emissions. Dissolved ions affect CO₂ solubility (CCO₂) and the rate of dissolution, but the mechanisms of the effect are not clearly understood and thus CCO₂ prediction is difficult. We measured CCO2 and solution density up to 140°C and 35.5 MPa-PCO₂ in water, NaCl, CaCl₂, Na2SO₄, and NaHCO₃ solutions up to 3.4 molal and Bravo Dome mixed brine. CCO₂ weakly correlated to ionic strength and water activity. Strong correlations (R² > 0.92) were identified between CCO₂ and each of ΔGhydr, ΔHhydr, ΔShydr, and the electrostricted water concentration, ha; calculated from ion concentration and hydration number. Traditional empirical CCO₂ prediction models require extensive experimental work to determine parameters. We use a novel prediction approach by applying a mole balance on water, then evaluating the energy required to remove water from hydrated ions to solvate CO₂. The resulting model developed using moderated multiple regression shows that CCO₂ is dependent on CO₂fugacity (f), temperature (T), ha, and the solution hydration energy (G): all of which are specified or previously catalogued variables. A model (R²=0.92) is generated from 503 data points from this study and literature and includes the squares of each variable and interactions. Interactions between f, T, ha and G evaluated using spot-light analysis indicate that: 1) competition for water molecules significantly impacts CCO₂; 2) T and f interact to exacerbate a decrease in open water structure concentration; and 3) hydrated ions may dampen thermal agitation and reduce open structure collapse caused by increased T. The interactions of this research are likely extensible to the dissolution of any non-polar gas into a salt solution. CO₂ dissolution rate measurements demonstrated that convection occurred in experimental reactors with dissolving CO₂; however, the system was diffusion limited due to a thin diffusion layer. Density measurements revealed salt solution volume decreases with increasing CO₂, which results in: 1) faster mass transfer of dissolved CO₂ and 2) increased CO₂ total storage capacity (TSC). In 1 m Na₂SO₄ at 60°C and 10 MPa volume decreases yielded a 20% TSC increase.