Process Simulations Reveal the Carbon Dioxide Removal Potential of a Process That Mineralizes Industrial Waste Streams via an Ion Exchange-Based Regenerable pH Swing

Abstract

The sequestration of CO 2 within stable mineral carbonates (e.g., CaCO 3 ) represents an attractive emissionsreduction strategy because it offers an energy efficient, environmentally benign, and leakage-free alternative to geological storage. However, the pH levels of aqueous streams equilibrated with CO 2containing gas streams (pH ∼ 4) are lower than the pH required for carbonate precipitation (pH > 8). Thus, the use of regenerable ion exchange materials is proposed to induce alkalinity in CO 2containing aqueous streams to achieve the pH required for mineralization without the addition of expensive stoichiometric reagents such as caustic soda (e.g., NaOH). Herein, geochemical and process-modeling software was used to identify the optimum thermodynamic conditions and to quantify the energy intensity and CO 2 reduction potential of a process that sequesters CO 2 (dissolved in wastewater) as solid calcium carbonate (CaCO 3 ). CaCO 3 yields were maximized when the initial calcium to CO 2 ratio in the aqueous phase was 1:1. The energy intensity of the process (0.22−2.10 MW•h/t of CO 2 removed) was dependent on the concentration of CO 2 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 and sodium ions requiring the most energy (0.07−0.80 MW•h/t of CO 2 removed). Energy consumption was minimized under conditions where CaCO 3 yields were maximized for all produced water compositions and CO 2 concentrations. The ratio of net CO 2 to gross CO 2 removal for the process ranged from 0.05 to 0.90, indicating a net CO 2 reduction across all conditions studied. The results from these studies indicate that ion exchange processes can be used as alternatives to the addition of stoichiometric bases to provide alkalinity for the precipitation of CaCO 3 at the CO 2 concentrations studied, thereby opening a pathway toward sustainable and economic mineralization processes.

Ion Exchange Processes for CO2 Mineralization Using Industrial Waste Streams: Pilot Plant Demonstration and Life Cycle Assessment

“…Mass of CO 2 sequestered and energy intensities from the ion exchange pilot plant were used to as inputs for the various scenarios. To extend the analysis for the entire process, previously quantified nanofiltration and reverse osmosis energy intensities [16,17] were used and make‐up deionized water is added back into the system for complete cycling of the process to produce 1 kg precipitated CaCO 3 .…”

“…[16][17][18]20] Previous bench-scale experiments and process simulations generated high purity (> 97 wt.%) CaCO 3 at yields that utilize 3.5 m 3 /h of brine from a total bed volume of 0.05 m 3 of regenerable ion exchange material (equivalent to 1 t-CO 2 captured per day). [16][17][18]20] In this work, the ion exchange process is scaled up and developed to process 300 L of brine per day to sequester 100-500 g CO 2 per day at pCO 2 = 0.03-0.20 atm. To better understand how this process operates using real-world brines of variable chemistries, two produced water sources from the United States are tested: the Niobrara mixed-shale and chalk play in the Denver-Julesburg Basin and the Utica-Point Pleasant mixed shale and limestone play in the Appalachian Basin.…”

“…In water, CO 3 2À anions are the predominant carbon species at pH > 10.3, [15] but artificially increasing the pH with caustic soda (e. g., NaOH) would make the mineralization process economically unfeasible (e. g., $336 per tonne CO 2 sequestered using caustic soda from chloralkali process compared to $25-$168 per tonne CO 2 sequestered for the ion exchange process) [16][17][18] due to increased energy intensities resulting from caustic soda production (e. g., 1.56-2.51 kWh per kg NaOH or 1.1-1.8 kg CO 2 e per kg NaOH produced depending on the production process [19] ).…”

“…Furthermore, to achieve complete regeneration of this cation exchange solid, a regeneration stream containing a pH>6 must be used so that all H + are dissociated from the cation exchange sites [24] and replaced with Na + cations. The proposed process design is shown in Figure 1 where: (1) acidity is introduced via CO 2 bubbling in deionized water; (2) H + ‐ Na + exchange using fixed‐bed reactors containing ion exchange solids to produce a carbonate‐rich effluent stream; (3) carbonate precipitation is induced by mixing with produced water or a Ca 2+ /Mg 2+ ‐rich brine; (4) precipitated solids are separated and the remaining effluent is treated via nanofiltration and reverse osmosis for the simultaneous removal of divalent cations and production of a freshwater (e. g., divalent cation concentrations less than 0.001 M) and Na‐rich (e. g., concentrations greater than 0.1 M) regeneration stream to be cycled within the process [16–18,20] …”

Enhancing Aqueous Carbonation of Calcium Silicate through Acid and Base Pretreatments with Implications for Efficient Carbon Mineralization

“…Therefore, a common method to accelerate aqueous carbonation is to acidify the solution, sometimes in combination with adjustments to other solution conditions such as temperature, salinity, and the presence of ligands. − However, acidic conditions are not conducive to carbonate precipitation, as suggested by eq where H + is a product. As such, a costly pH swing step, usually achieved through the provision of base, ion-exchange, or pressure/temperature swing involving CO 2 or ammonia, , is needed. The need to accelerate mineral dissolution through acidification has also largely prohibited the direct use of dilute CO 2 sources in accelerated aqueous carbonation without a dedicated capture process as it is difficult to achieve fast CO 2 dissolution into acidic aqueous solutions.…”