Modeling of heat and mass transfer in vacuum membrane distillation for ammonia separation

Ammonia is both a contaminant and valuable commodity as chemical. The extraction of ammonia from wastewater was proven technically feasible using membrane distillation (MD). However, the fundamental separation mechanisms of ammonia in MD is not well understood, due to the complexity related to the binary transport and solution chemistry. This study aimed to develop a numerical model for the first time to describe the ammonia and vapour binary transport across hollow fibre membrane in single-stage vacuum membrane distillation (VMD). The key model constants were obtained based on measured membrane properties and unique ammonia chemistry. The modelling results indicated that the ammonia concentration factor into the permeate was strongly correlated with feed temperature at rued pH but not sensitive to feed concentration. The trade-off between ammonia selectivity and membrane flux indicated a suitable temperature range of 40-45 degrees C for efficient extraction. The predicted gas pressure of ammonia-vapour mixture showed a slow increase of process driving force and membrane flux with feed temperature. While the averaged process driving force decreased as fibre length increased, indicating an optimal length for maximum permeate production. The predicted permeate gas and membrane surface temperatures were identical along fibre length, indicating minimal conductive heat loss and hence high thermal efficiency (>90%) for ammonia extraction.