Enhancing ammonia selectivity in membrane distillation: The role of membrane structural characteristics.

Ammonia is emerging as a promising carbon-free fuel in the pursuit of carbon neutrality due to its combustion process, which produces no carbon dioxide emissions. Membrane distillation (MD) for recovering ammonia from wastewater is an alternative to conventional energy-intensive production methods. However, the efficiency of MD processes is hindered by the low purity of the recovered ammonia stream, primarily due to excessive water permeation. In this study, we investigated the influence of membrane properties on water and ammonia permeation using poly(vinylidene fluoride- co -hexafluoropropene) (PVDF-HFP) nanofibrous membranes fabricated in-house via electrospinning. Additionally, we compared the impact of manipulating membrane characteristics with adjusting operating conditions, employing a commercial PVDF membrane as the conventional approach for controlling ammonia purity. Our findings revealed that increasing membrane thickness (from 82 to 283 μm) under isothermal conditions substantially enhanced the ammonia separation factor (from 4.07 to 7.28) by approximately 80 %, whereas reducing the feed temperature from 50 °C to 43 °C yielded only a 20 % improvement. Mechanistic modeling of MD for the water-ammonia system illustrated the critical role of membrane thickness in improving ammonia selectivity by markedly reducing water flux while maintaining relatively consistent ammonia flux. The combination of experimental and simulation outcomes suggests that modifying membrane properties or selecting membranes with specific characteristics can enhance ammonia selectivity without significant alterations to operating conditions or configuration. • NH 3 recovery from wastewater was applied using membrane distillation (MD). • Influence of membrane properties on MD permeation was studied to enhance NH 3 selectivity. • Increased membrane thickness under isothermal conditions enhanced NH 3 separation factor. • Mechanistic modeling highlighted the critical role of membrane thickness in NH 3 selectivity. • Modifying membrane properties offers a viable approach to enhance NH 3 selectivity in MD. [ABSTRACT FROM AUTHOR]