Comprehensive experimental and theoretical investigations on the effect of microbubble two-phase flow on the performance of direct-contact membrane distillation.

This study provides a comprehensive and systematic overview of the application of gas-liquid two-phase flow with microbubbles in the feed stream to improve heat and mass transfer in direct-contact membrane distillation (DCMD) processes for seawater desalination. A swirl-flow-type microbubble generator (MBG) was installed at the feed-side inlet of the DCMD module to investigate its effect on transmembrane flux. The maximum improvement in the MBG-assisted DCMD permeation flux was found to be approximately 18% at a lower feed temperature (40 °C) and optimal air flow rate (50 cc/min), and an optimal MBG geometry comprising a swirler, a nozzle tip of diameter 2 mm, and a diffuser at an angle of 30°. The results were observed to be related to the number density of microbubbles less than 100 µm in size, which plays an important role in improving heat and mass transfer in two-phase flow. In addition, the simulation results based on conventional heat transfer correlations of bubbly flow underestimated the experimental results. Therefore, this study also aims to propose and verify a new two-phase flow heat transfer correlation. The proposed correlation considers the effects of bubble size distribution to accurately predict the performance of MBG-assisted DCMD processes.

Dissolved carbon dioxide stripping while maintaining volatile components by feeding gaseous nitrogen using a microbubble generator.

Dissolved carbon dioxide (dCO2) stripping from a model solution containing sake flavor by feeding gaseous nitrogen (N2) using a microbubble (MB) generator was investigated. The effect of dCO2 stripping by N2MB increased significantly with increasing flow rate of gaseous N2 from 100 to 200 mL/min. dCO2 stripping from 3,000 mL of the model solution was achieved by feeding N2MB at a flow rate of 200 mL/min for 4 min. Volatile components from model solution containing sake flavor were hardly reduced even after feeding N2MB at a flow rate of 200 mL/min for 15 min by cooling to below 10 °C. On the other hand, non-microbubbled gaseous N2 at a flow rate of 200 mL/min was not very effective in stripping dCO2. Therefore, the use of N2MB with cooling to below 10 °C was effective in stripping dCO2 while maintaining the volatile components in model solution containing sake flavor.