UCLA

A new and attractive technique for reducing emissions by storing carbon dioxide geologically is to capture it within thermodynamically stable, carbonate solids, CaCO3. However, alkalinity is required to achieve favorable conditions for carbonate precipitation (pH >8) from aqueous streams containing dissolved CO2 (pH <4.5). Utilization of regenerable ion exchange solids have previously been demonstrated to induce alkalinity required for carbonate precipitation from divalent rich brines. In this study, a previously proposed ion exchange+process is scaled up to treat 300 L of produced water brine per day for CO2 mineralization. H saturation capacities were quantified for various inlet concentrations of CO2 in the gas phase (3 – 20 vol %; 0.10 – 0.81 mmol H+ per g ion exchange solid) and flow rates (0.5 – 2.0 L min-1; 12 vol %; 0.65 mmol H+ per g ion exchange solid). Utilizing inlet CO2 concentration at 0.12 vol %, 0.5 – 3.5 g CaCO3 per L produced water was precipitated, resulting in energy consumptions ranging from 30 – 65 kWh/t CO2 sequestered for the ion exchange and mineralization steps. The energy intensity of the process was dependent on volumes ratios of alkaline-rich product and produced water used to precipitate calcium carbonate. Thermodynamic simulations for precipitated calcium carbonate formation were validated through this system, with the primary phases of the precipitated solids in the form of calcite (>97%) with magnesium and iron incorporation from produced water. A life cycle assessment was performed to analyze the net carbon emissions of the technology for two produced water compositions at various inlet CO2 partial pressures (pCO2 = 0.03 – 0.20 atm) which indicated a net CO2 reduction across most conditions studied (0.533 to -0.39 kg CO2e per kg precipitated calcium carbonate). Emissions were minimized under conditions utilizing concentrated produced waters and CO2 concentrations greater than 3 vol%. Nano filtration studies showed that the XN-45 membrane can be used to generate a sodium rich permeate stream and a calcium rich retentate stream that can be recycled in the system. 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 allowing for economic mineralization processes.