A process for CO2 capture and brine salinity reduction through reaction with potassium hydroxide: A multi-stage evaluation

Solvay and modified Solvay processes are facing a major challenge in reducing brine salinity to a level suitable for agriculture and industry. This challenge arises as a result of competing reactions and mixing limits between CO₂ gas and brine. Another challenge is the high solubility of sodium bicarbonate (NaHCO₃), which results in a low overall desalination efficiency. Previous studies established the effectiveness of a modified Solvay process based on potassium hydroxide (KOH). The first objective of this study is to evaluate a multi-stage treatment for a modified Solvay process on the basis of potassium hydroxide (KOH) to achieve an additional reduction in ion removal from high-salinity brines and an increase in CO₂ capture as compared to previously obtained under optimal operating conditions. Three different methods were investigated. The first method evaluated the effectiveness of adding ammonium bicarbonate (NH₄HCO₃) in reducing the solubility of NaHCO₃. Even though the ${\text{Na}}^{+}$ and ${\text{Cl}}^{-}$ concentrations were reduced by 56.2% and 40%, respectively, the total CO₂ uptake slightly improved by 1.2% (67.8 g CO₂/1000 ml of treated brine). In the second method, the addition of extra KOH in subsequent stages was investigated to overcome the pH reduction observed in the first method. There was an $\sim$ 47.3% improvement in CO₂ uptake from the first method. Furthermore, the percentages of ${\text{Na}}^{+}$ and ${\text{Cl}}^{-}$ removal were increased to 65% and 64.5%, respectively. In the third method, the recovery of ${\text{Ca}}^{2+}$ and ${\text{Mg}}^{2+}$ was approximately 76.3% and 94.6%, respectively, following the pre-treatment step (filtration), followed by the same stages as in the second method. Reducing these ions decreased the competitive reactions and thus increased CO₂ solubility and reactivity with KOH, resulting in higher cumulative CO₂ uptake from all stages to 108.2 g CO₂/1000 ml, which was 8.3% more than the second method. Additionally, solid products were characterized using scanning electron microscopy, X-ray diffraction, FTIR and Raman analysis. Finally, the dynamic behaviour of the reactor was evaluated using step changes in the inlet gas and liquid flow rate. The results are promising in terms of the reactor system's adaptability to large-scale processes.