(401v) Mixed Gas Permeation in Molten Salt Membranes: Determination of NH3 Diffusivity and Solubility

According to the U.S. Census Bureau, the global population increased from 1.6 billion in 1900 to 6.0 billion in 2000 and is projected to reach 9.0 billion by 2050 [1]. Consequently, global fertilizer production would increase to feed the world. Ammonia is one of the chemical components needed to synthesize inorganic fertilizer. Also, ammonia is progressively recognized as a potential green fuel for power generation and challenging-to-decarbonize sectors, such as aviation and shipping, offering several advantages over green hydrogen [2]. Conventionally, NH₃ is produced via the Haber-Bosch (H-B) process at 400 – 450 °C and 25 – 30 bar [3,4]. Due to the low conversion associated with the H-B process, the NH₃ must be separated from the unreacted N₂ and H₂ using large condensation and refrigeration, which further contribute to CO₂ emissions and high-energy consumption [5]. Switching the traditional NH₃ separation technology to alternative methods like membrane separation can help mitigate CO₂ emissions and reduce energy consumption [6]. Inorganic membranes offer superior thermal stability and promising separation performances at high temperatures compared to organic membranes. ZnCl₂ immobilized molten salt (IMS) membranes have shown selectivity of NH₃ over N₂ and H₂ up to 10⁷ and moderate permeance of ~200 GPU at 300 °C and atmospheric pressure [6,7]. Understanding the solubility and diffusivity of gases in IMS membranes is vital for the development of an efficient separation system. This study investigated the solubility and diffusion coefficients of NH₃ in the ZnCl₂ IMS membrane at ~300 °C and atmospheric pressure. The solubility coefficients were examined by conducting sorption experiments using a modified thermogravimetric analyzer. A time-lag method using online mass spectrometry was adopted to determine the diffusion coefficient of NH₃ in the mixture across the IMS membrane. The results revealed that the NH₃ solubility coefficient decreases with increasing NH₃ partial pressure in the feed, exhibiting the carrier saturation phenomenon. The diffusion coefficient from time-lag increases with increasing NH₃ partial pressure in the feed.

References

[1] R. Michalsky, B.J. Parman, V. Amanor-Boadu, P.H. Pfromm, Solar thermochemical production of ammonia from water, air and sunlight: Thermodynamic and economic analyses, Energy 42 (2012) 251–260. https://doi.org/10.1016/j.energy.2012.03.062.

[2] E.F. Eyisse, E. Nadimi, D. Wu, Ammonia Combustion: Internal Combustion Engines and Gas Turbines, Energies 2025, Vol. 18, Page 29 18 (2024) 29. https://doi.org/10.3390/EN18010029.

[3] A. Iulianelli, S. Liguori, J. Wilcox, A. Basile, Advances on methane steam reforming to produce hydrogen through membrane reactors technology: A review, Catal Rev Sci Eng 58 (2016) 1–35. https://doi.org/10.1080/01614940.2015.1099882.

[4] C. Smith, A.K. Hill, L. Torrente-Murciano, Current and future role of Haber–Bosch ammonia in a carbon-free energy landscape, Energy Environ Sci 13 (2020) 331–344. https://doi.org/10.1039/C9EE02873K.

[5] M.J. Bland, Optimiation of an Ammonia Synthesis Loop, Norwegian University of Science and Technology, 2015.

[6] M. Adejumo, N. Fazio, S. Liguori, NH3 separation by ZnCl2 immobilized molten salt (IMS): experimental and modeling, J Memb Sci (2025) 124053. https://doi.org/10.1016/J.MEMSCI.2025.124053.

[7] M. Adejumo, L. Oleksy, S. Liguori, Innovative NH3 separation over immobilized molten salt membrane at high temperatures, Chemical Engineering Journal 479 (2024) 147434. https://doi.org/10.1016/J.CEJ.2023.147434.