ION EXCHANGE PROCESSES FOR CO2 MINERALIZATION USING INDUSTRIAL WASTE STREAMS (Invited Presentation)

The sequestration of CO₂ within stable mineral carbonates (e.g., CaCO₃) represents an attractive emissions reduction strategy because it offers an energy efficient, environmentally benign, and leakage-free alternative to geological storage. However, continuous addition of alkalinity is required to achieve favorable conditions for carbonate precipitation (pH>8) from aqueous streams containing dissolved CO₂ (pH<4.5) and Ca²⁺ ions (such as produced water). Herein, a pH-swing process using re-generable ion exchange media is described as a means to capture CO₂ from industrial process streams (5-50% v/v CO₂) using produced water streams rich in Ca²⁺ ions. In this process, the ion exchange media induces alkalinity in CO₂-containing aqueous streams to achieve the pH required for mineralization of CO₂ with Ca²⁺ ions (to form CaCO₃), thereby avoiding the need for expensive stoichiometric reagents such as caustic soda (e.g., NaOH). Nanofiltration and reverse osmosis steps recover Mg²⁺ and Na⁺ rich streams; the latter of which is used to regenerate the ion exchange media to complete the pH-swing cycle.

A combination of experimental ion-exchange studies with geochemical and process-modeling was used to identify the thermodynamic conditions that maximize CO₂ removal potential while minimizing energy intensity. Energy consumption was minimized under conditions where CaCO₃ yields were maximized across a variety of produced water compositions and CO₂ concentrations. The energy intensity of the process (0.22−2.10 MW·h/t of CO₂ removed) was dependent on the concentration of CO₂ in the gas phase (i.e., 5−50 vol %) and the produced water composition, with the nanofiltration and reverse osmosis steps used to recover magnesium requiring the most energy (0.07−0.80 MW·h/t of CO₂ removed). Life cycle assessments were performed to analyze the net carbon emissions of the technology which indicated a net CO₂ reduction of 0.06 to 0.39 kg CO_2e per kg precipitated CaCO₃ utilizing calcium-rich brines and gaseous streams with pCO ≥0 .12 atm. The results from this study indicate the ion exchange process can be used as a scalable method to provide alkalinity necessary for the capture and storage of CO2 in Ca-rich waste streams.