KOH-Based Modified Solvay Process for Removing Na Ions from High Salinity Reject Brine at High Temperatures

The traditional Solvay process and other modifications that are based on different types of alkaline material and waste promise to be effective in the reduction of reject brine salinity and the capture of CO 2 . These processes, however, require low temperatures (10–20 °C) to increase the solubility of CO 2 and enhance the precipitation of metallic salts, while reject brine is usually discharged from desalination plants at relatively high temperatures (40–55 °C). A modified Solvay process based on potassium hydroxide (KOH) has emerged as a promising technique for simultaneously capturing carbon dioxide (CO 2 ) and reducing ions from reject brine in a combined reaction. In this study, the ability of the KOH-based Solvay process to reduce brine salinity at relatively high temperatures was investigated. The impact of different operating conditions, including pressure, KOH concentration, temperature, and CO 2 gas flowrate, on CO 2 uptake and ion removal was investigated and optimized. The optimization was performed using the response surface methodology based on a central composite design. A CO 2 uptake of 0.50 g CO 2 /g KOH and maximum removal rates of sodium (Na + ), chloride (Cl − ), calcium (Ca 2+ ), and magnesium (Mg 2+ ) of 45.6%, 29.8%, 100%, and 91.2%, respectively, were obtained at a gauge pressure, gas flowrate, and KOH concentration of 2 bar, 776 mL/min, and 30 g/L, respectively, and at high temperature of 50 °C. These results confirm the effectiveness of the process in salinity reduction at a relatively high temperature that is near the actual reject brine temperature without prior cooling. The structural and chemical characteristics of the produced solids were investigated, confirming the presence of valuable products such as sodium bicarbonate (NaHCO 3 ), potassium bicarbonate (KHCO 3 ) and potassium chloride (KCl).