Carbon Dioxide Capture by Alkaline Water with a Semi-Batch Column and Ultra-Fine Microbubble Generator

Increased emissions of carbon dioxide (CO2) from industrial activities are the main cause of the growing problem of global warming and climate change, highlighting the needs for efficient CO2 capture and storage (CCS) techniques. The present work aims to investigate the possibility of CO2 sequestration using sodium hydroxide (NaOH) in a semi-batch column with an integrated gas lift tower and an ultra-micro bubbles generator, a novel setup designed to enhance mass transfer rates and capture efficiency. Unlike the previously reported setups, our system achieves a 50% faster capture rate with improved mass transfer, enhanced gas-liquid interaction and higher removal efficiency due to finer bubble dispersion, as confirmed by experimental findings. Preliminary tests to ascertain the effectiveness of CO2 removal were carried out across various CO2 gas flow rates (3, 5, 7 L/min), NaOH volumes (2, 3, 4 L) and concentrations (0.1, 0.2, 0.3 M). The results indicated that both gas flow rate and NaOH concentration have profound impacts on the CO2 capture rate. Increasing either of these parameters, or using low concentrations of NaOH, leads to a rapid drop in pH due to a faster rate of neutralization and the formation of carbonic acid (H2CO3), a weak acidic solution. For instance, with 0.1 M NaOH and 2 L volume, the pH decreased from 13.07 to 7.02 within 1.5 min at gas flow rate of 7 L/min, while with 0.3 M NaOH, pH reduced to 7.3 after 6 min. Higher volumes and concentrations of NaOH caused a decrease in the capture rate of CO2 due to reversed reaction with formed sodium carbonate. For instance, with 0.3 M NaOH and 4 L volume, the pH reduced from 13.58 to 8 after 5 min at 7 L/min gas flow rate. Scaling up to a 100 L semi-batch column with an ultra-fine micro bubble generator, as a new approach, reduced the time taken by half in the capture of CO2. Additionally, the study also investigated the comparison of tap versus deionized water in CO2 capture reaction. The results demonstrated that dissolved minerals in tap water, particularly Ca2+ and Mg2+ ions, affected precipitate formation and capture efficiency differently than deionized water, offering practical insights for CCS in varied water sources.