UCLA

Sequestration of CO2 within stable mineral carbonates (e.g., CaCO3) represents an attractive emission reduction strategy as it offers a leakage-free alternative to geological storage of CO2 in an environmentally benign form. However, the pH of aqueous streams equilibrated with gaseous streams containing CO2 (pH < 4) are typically lower than that which is required for carbonate precipitation (pH > 8). Traditionally, alkalinity is provided by a stoichiometric reagent (e.g., NaOH) which renders these processes environmentally hazardous and economically unfeasible. This work investigates the use of regenerable ion-exchange materials to induce alkalinity in CO2-saturated aqueous solutions such that the pH shift required for mineralization occurs without the need for stoichiometric reagents. Na+-H+ exchange isotherms (at [H+] = 10-8-10-1 M) and rates were measured for 13X and 4A zeolites and TP-207 and TP-260 organic exchange resins in batch equilibrium and fixed-bed exchange experiments, respectively. At solutions equilibrated with CO2 at 1.0 atm (pH = 3.9), H+ exchange capacities for the materials were similar (1.7-2.4 mmol H+/g material) and resulted in pH increases from 3.9 to greater than 8.0. Multi-component mixtures using Ca2+ and Mg2+ cations (at 10-3-10-1 M) in CO2-saturated water were used to probe competitive ion exchange. The presence of divalent cations in solution inhibited H+ exchange, reducing capacities to as low as 0.2 mmol H+/g for both resins and zeolites. Dynamic H+ exchange capacities in fixed-bed ion exchange columns were similar-to equilibrium values for resins (~1.5 mmol/g) and zeolites (~0.8 mmol/g) using inlet solutions that were equilibrated with gaseous streams of CO2 at 1.0 atm. For the four ion exchange materials studied (e.g., ion exchange resins and synthetic zeolites), quasi-chemical linear driving-force approximations that are in first order in solid-phase capacity, effectively model contaminant breakthrough curves. Experimentally determined rate parameters reflect those determined from pore diffusion with pellets: 0.091 s-1 for R-1, 0.06 s-1 for R-2, 0.04 s-1 for Z-1, and 0.025 s-1 for Z-2, particles larger than 500 �m. Predictive H+ titration capacities for these ion exchange materials were within 5% difference of experimentally determined H+ titration capacities: 0.81 mmol H+ g-1 of R-1, 0.68 mmol H+ g-1 of R-2, 0.26 mmol H+ g-1 of Z-1, and 0.18 mmol H+ g-1 of Z-2 for pCO2 = 0.12 atm equilibrated inlet streams. These studies demonstrate that linear driving-force approximations can model experimentally determined H+ removal parameters. Additionally, experimental calcite precipitation from mixing the alkaline CO32--rich water solution obtained from the ion-exchange column with a synthetic liquid waste stream solution achieved thermodynamic maximum yields.

Geochemical and process modeling software was used to identify thermodynamically optimum conditions and to quantify the energy intensity and CO2 reduction potential of a process that sequesters CO2 (dissolved in wastewater) as solid calcium carbonate (CaCO3). CaCO3 yields are maximized when initial calcium to CO2 ratios in the aqueous phase are 1:1. The energy intensity for the ion exchange process (0.22 – 2.10 Megawatt-hour per tonne of CO2 removed (MWh/t-CO2)) is dependent upon the concentration of CO2 in the gas phase (i.e., 5-50 vol%) and the produced water composition, with nanofiltration and reverse osmosis steps used to recover magnesium and sodium ions contributing the largest energy requirements (0.07 – 0.80 MWh per t-CO2 removed). Energy consumption was minimized under conditions where CaCO3 yields were maximized for all produced water compositions and CO2 concentrations. The ratio of net CO2 to gross CO2 removal for the process ranged from 0.05 to 0.90, indicating a net CO2 reduction across all conditions studied.

Furthermore, this ion exchange process was scaled up to treat 300 L of produced water brine (oil- and gas-associated wastewater) per day for CO2 mineralization. Produced water brines are optimal for this process because these brines are (Mg2+, Ca2+)-rich, suitable for CO2 mineralization, and Na+-rich, optimal for regeneration of the spent ion exchange solids used to induce a pH swing. Proton titration capacities were quantified for aqueous streams in equilibrium with gas streams at various concentrations of CO2 (pCO2 = 0.03 – 0.20 atm; 0.10 – 0.81 mmol H+ per g ion exchange solid) and at various flow rates (0.5 – 2.0 L min-1; in equilibrium with 0.12 atm gas phase CO2; 0.65 mmol H+ per g ion exchange solid). Utilizing inlet CO2 concentration at 0.12 atm, 0.5 – 3.5 g CaCO3 per L produced water was precipitated, resulting in energy intensities between 30 – 65 kWh per tonne of CO2 sequestered from pumping and effluent mixing. The energy intensity of the process was dependent on volume ratios of the higher alkaline, ion-exchanged CO2 stream and alkaline cation-rich produced water used to precipitate CaCO3. Thermodynamic simulations for precipitated CaCO3 formation were validated through this system, with calcite as the primary precipitated CaCO3 phase (>97%) and 3% FeO solids from produced water. A life cycle assessment was performed to analyze the net carbon emissions of the technology for two produced water compositions in equilibrium with gas streams at various CO2 partial pressures (pCO2 = 0.03 – 0.20 atm) which indicated a net CO2 reduction for pCO2 ≥ 0.12 atm (-0.06 to -0.39 kg CO2e per kg precipitated CaCO3) utilizing calcium-rich brines. The results from this study indicate the ion exchange process can be used to provide alkalinity for the precipitation of carbonate solids for most of the CO2 concentrations, thereby opening a pathway toward sustainable and economic mineralization processes.