Heat and moisture transfer and dimension optimization of cross-flow hollow fiber membrane contactor for membrane distillation desalination

Abstract

Membrane distillation desalination (MDD) is a promising desalination technology, which can be driven by renewable energy because of its mild operating temperature and pressure, and can obtain high purity distilled fresh water by realizing non-direct contact between process liquid and cold fluid. As the core component of MDD process to achieve vapor-liquid separation, the cross-flow hollow fiber membrane contactor strengthens the heat and moisture transfer efficiency of shell side by means of continuous disturbance of fiber bundle to the air flow compared with the parallel-flow membrane contactor. To reveal the coupled heat and moisture transport between hot brine and process air in the cross-flow hollow fiber membrane contactor, a two-dimensional mathematical model was established and verified by comparison of experimental results. By analyzing the effects of operating conditions and contactor dimensions on solution and air temperature and humidity distributions, contactor dimensions were optimized to facilitate the design and performance improvement of the membrane contactor. For sensible heat transfer, the effective transfer region is mainly concentrated in a limited long narrow air inlet region, which accounts for about 17% of the total area. Whereas for moisture transfer, the effective transfer region is the square region where the air inlet segment and the solution inlet segment overlap, which accounts for about 44% of the total area. To obtain superior membrane flux, long and narrow contactors should be selected for low solution temperature (Tsi < 71 °C), low air Reynolds number (Rea < 62) and high solution Reynolds number (Res > 58); otherwise, short and wide contactors should be selected. To improve the humidification efficiency, it is advisable to choose a short and wide contactor, and reducing the solution temperature and air flow rate and increasing the solution flow rate also help to improve the humidification efficiency.