(634b) Enhancing Divalent Cation Rejection in Membrane Systems for the Treatment and Utilization of High Ionic Strength Waste Streams
AIChE Annual Meeting
2019 AIChE Annual Meeting
CO2 Industrial, Engineering and R&D Approaches
Thursday, November 14, 2019 - 8:18am to 8:36am
Currently, only about 1% of all PW in the U.S. is utilized for beneficial reuse, including irrigation and the production of drilling fluids and fracturing fluids for new wells. Additionally, PW can be utilized as a source of sodium and calcium. After organic compounds are separated from the stream, the composition of PW is largely dominated by sodium and calcium ions, containing on average about 30,000 mg/L and 5,000 mg/L respectively. The high contents of these cations in PW allow for the utilization of the water for processes such as the production of caustic soda by electrochemical techniques and the utilization of the calcium source for carbon dioxide sequestration and mineralization. For example, for caustic soda production, the removal of dissolved calcium and magnesium ions is critical because the electrochemical methods such as the chlor-alkali process or the newer bipolar membrane electrodialysis require polyvalent cation concentrations to be less than 2 mg/L to prevent the precipitation of metal hydroxides (e.g., Ca(OH)2, Mg(OH)2, etc.). Thus, to successfully utilize PW for both processes, calcium ions must be thoroughly removed from the feed while maintaining high concentrations of sodium.
The separation of cations is commonly accomplished by pressure driven membrane filtration techniques but treatment is typically limited to meeting the TDS requirements for disposal, because treatment of highly concentrated waste streams can be costly. To enhance treatment and reduce energy consumptions associated to pressure driven membrane filtration of high saline streams, this work investigates the effectiveness of zwitterion-perfluorophenylazide (PFPA) polymer grafted onto commercially available polyamide nanofiltration membranes for the enhanced separation of divalent cations from simulated PW feeds as a pretreatment step for caustic soda production. Simultaneously, the calcium-enriched stream can be used for CO2 mineralization through the formation of insoluble metal carbonates.
We have previously shown that zwitterion-PFPA polymer can potentially be used for enhanced calcium separation from dilute simulated waste streams (ionic strengths, I=0.025 M), and consequentially calcium-concentration. We show that zwitterion-PFPA grafting enhanced the Ca rejection of the commercially available NF90 nanofiltration membrane from 91% to 98%. In simulated PW streams containing either calcium or sodium chloride salts, cation rejections of unmodified NF90 membranes exhibited decreases in both calcium and sodium rejections with increasing ionic strengths from 0.05 to about 0.3 M, wherein rejections decrease to 70% and 50% respectively, from 91% and 72%. Grafted NF90 membrane increased ion rejections to 90% and 60% calcium and sodium rejection respectively, at ionic strengths of 0.5 M; at ionic strengths of 1.0M, a drop in performance for the grafted membranes were observed similar to those of unmodified membranes. Thus, the simulated PW feeds underwent multiple nanofiltration passes to reduce calcium ion concentrations to < 2 mg/L. On average, three nanofiltration passes were required to reduce calcium concentrations to the desired level using unmodified membranes, while only two passes were required for modified membranes. Energy consumption of the system only depends on the pump used for feed flow; energy values for both systems were calculated to compare efficiencies of each process. Energy consumptions for the system using modified membranes were lower than those for the system using unmodified membranes at constant pressure and flow rates, which were 7.2 and 8.0 kWh/m3 respectively, to achieve the same calcium concentrations in the permeate.
The utilization of zwitterion-PFPA polymer grafting on commercially available nanofiltration membranes has shown to enhance divalent cation rejection in simulated PW feeds containing only calcium and sodium cations. The enhanced cation separation is beneficial because it provides a calcium-rich stream that can be used for CO2 mineralization, as previously described; the sodium-rich stream can be used for caustic soda production, which can be used to increase alkalinity for CO2 mineralization. Furthermore, the grafting material used for enhanced removal of polyvalent ions can potentially be applied at the industrial scale.