(401ae) Investigating High Pressure Effects on ZnCl2-Immobilized Molten Salt Membranes for NH3 Separation

Ammonia global production is expected to continue rising due to the growing population coupled with its potential use as a hydrogen carrier and clean fuel. In 2023, the global ammonia production capacity was approximately 240 MMT and it is projected to reach around 290 MMT by 2030 [1]. The industrial ammonia production uses the Haber-Bosch (H-B) process, which requires hydrogen from steam reforming and nitrogen from an air separation unit [2]. The reaction takes place over a Fe-based catalyst at high temperatures (350 – 550 °C) and pressures (150 – 300 bar) [3]. These operating conditions are considered harsh and necessary to activate the N≡N bond. The high temperatures needed for rapid kinetics restrict conversion to about 20% due to the exothermicity of the reaction, thus requiring the ammonia produced to be separated from unreacted nitrogen and hydrogen. Ammonia is conventionally separated by condensation and refrigeration, while unreacted hydrogen and nitrogen are compressed and recycled to the reactor. The conventional ammonia separation requires significant energy input for steam-powered compressors, which result to increased operational cost and environmental impact through carbon emission. Membrane technology offers a promising solution to reduce reliance on large condensation and refrigeration, consequently lowering energy consumption and CO₂ emissions [4].

Inorganic NH₃-permeable membranes offer superior thermal stability compared to polymeric membranes, making them promising for applications in high-temperature NH₃ separation. Among all NH₃-permeable membranes, the ZnCl₂-immobilized molten salt (IMS) membranes have demonstrated particularly promising performance [5,6]. At approximately 300 °C and atmospheric pressure, they achieved NH₃/N₂ and NH₃/H₂ ideal selectivities exceeding 10⁷, highlighting the strong potential of molten salt membranes for gas separation [7]. However, evaluating the NH₃ permeation performances of ZnCl₂ IMS membranes under elevated pressures is essential for their practical deployment. This study investigates the high-pressure performance of ZnCl₂ IMS membranes, revealing that increasing pressure leads to carrier saturation and a corresponding decline in NH₃ permeance.

References

[1] Global ammonia annual production capacity | Statista, (n.d.). https://www.statista.com/statistics/1065865/ammonia-production-capacity… (accessed July 23, 2024).

[2] H. Ishaq, C. Crawford, Review of ammonia production and utilization: Enabling clean energy transition and net-zero climate targets, Energy Convers Manag 300 (2024) 117869. https://doi.org/10.1016/J.ENCONMAN.2023.117869.

[3] M. Farsi, Ammonia production from syngas: Plant design and simulation, Advances in Synthesis Gas: Methods, Technologies and Applications: Volume 4: Syngas Process Modelling and Apparatus Simulation (2023) 381–399. https://doi.org/10.1016/B978-0-323-91879-4.00012-6.

[4] D. V. Laciak, G.P. Pez, P.M. Burban, Molten salt facilitated transport membranes. Part 2. Separation of ammonia from nitrogen and hydrogen at high temperatures, J Memb Sci 65 (1992) 31–38. https://doi.org/10.1016/0376-7388(92)87049-4.

[5] 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.

[6] 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.

[7] O. Ovalle-Encinia, J.Y.S. Lin, High-pressure CO2 permeation properties and stability of ceramic-carbonate dual-phase membranes, J Memb Sci 646 (2022) 120249. https://doi.org/10.1016/J.MEMSCI.2021.120249.