Innovative swirling flow-type microbubble generator for multi-stage DCMD desalination system: Focus on the two-phase flow pattern, bubble size distribution, and its effect on MD performance

In this study, we proposed a novel air-assisted swirling flow-type microbubble generator (MBG) to improve the downstream transmembrane flux of a multistage direct contact membrane distillation (MDCMD) based seawater desalination system, and experimentally demonstrated its performance under various operating conditions. To simulate the downstream operating conditions of the MDCMD system, low temperature and high concentration conditions were implemented on the feed side in the experiments. It was observed that the formation of a gas-liquid two-phase flow in the feed stream demonstrated a positive influence on the performance of the direct contact membrane distillation (DCMD) process; particularly, microbubbles less than 100 mu m performed an important role in improving the heat and mass transfer. The DCMD performance using MBG was demonstrated to be more pronounced at lower feed temperatures and higher feed flow rates, where the enhancement in permeation flux was as high as 37%. The optimal air flow rate to achieve maximum DCMD performance was determined to be 50 cc/min at a given feed flow rate, while a further increase in air flow rate incurred a slug flow in the feed channel, which resulted in a decrease in the number of microbubbles and hence a decrease in the DCMD performance. In addition, the process performance was evaluated using highly concentrated brine as the feed solution, and the results demonstrated the feasibility of applying MBG to practical desalination processes.