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Microscopic insights into ammonia-water mixture phase transitions for enhanced ammonia recovery
Wei Jian Tan, Kek Seong Kim, Parthiban Siwayanan, Thomas Shean Yaw Choong, Serene Sow Mun Lock, Peng Chee Tan and Zhen Hong Ban

Molecular dynamics simulations were conducted to investigate the evaporation and condensation behavior of a 20% ammonia-water binary mixture, focusing on the role of hydrogen bonding in phase transitions. The study explored evaporation at temperatures from 298.15 K to 413.15 K and condensation from 253.15 K to 293.15 K, all at a constant pressure of 0.4 MPa. The DREIDING forcefield was employed to capture the intermolecular interactions accurately. Results showed that ammonia evaporated at a higher rate initially due to weaker hydrogen bonds within ammonia clusters, while the increasing prevalence of water-water hydrogen bonds slowed ammonia evaporation over time. In condensation, a unique bell-shaped behavior was observed, where the transition from ammonia-water to stronger water-water hydrogen bonds led to the release of ammonia from the condensed phase. This dynamic interplay between hydrogen bonding and phase transitions highlights the limitations of traditional models in predicting such behavior. By leveraging molecular simulations, this study provides a deeper understanding of ammonia behavior in evaporation and condensation, contributing to the development of more efficient separation strategies for wastewater treatment and promoting sustainable wastewater management solutions.

Keywords: molecular dynamics simulation; ammonia-water separation; ammonia recovery; hydrogen bond; evaporation-condensation rates