(397g) Synthesis and Characterization of ZnCl2-Immobilized Molten Salt (IMS) Membrane for NH3 Separation

Ammonia (NH₃) is the world’s second most commonly produced chemical, and about 82% of its global production is used in making inorganic fertilizer [1,2]. Nearly half of the world would go hungry without it [3]. Recently, NH₃ is being considered a promising medium for hydrogen storage and transportation [4], and it has great potential to be used as a fuel to decarbonize the shipping sector [5]. Globally, more than 96% of NH₃ is produced by the energy-intensive Haber Bosch (H-B) process, contributing to 1.2% of the global anthropogenic CO₂ emission [6]. The industrial H-B process requires harsh operating conditions for its operation (150 – 250 bar and 300 – 450 °C). Due to the high temperature for rapid kinetics, the single-pass conversion is restricted to 10 – 15% [7]. As a result, the recovery of ammonia from the reactants—mixtures of nitrogen and hydrogen—is a critical unit operation in the production plants. A chain of heat exchangers and a final refrigeration stage are usually used to recover ammonia, while the remaining gases are reheated and recycled to a catalytic converter using compressors [8]. Membrane technology provides a promising alternative to intensify the H-B process through the use of an NH₃–selective membrane [9]. In this regard, there is a need to identify an inorganic membrane stable at high temperatures and highly selective towards NH₃ permeation with respect to hydrogen and nitrogen. Among all NH₃-permeable membranes, the ZnCl₂-immobilized molten salt (IMS) membrane has shown promising NH₃ separation characteristics at high temperatures [9]. In this work, ZnCl₂ IMS membrane was synthesized and evaluated for the potential separation of NH₃ from a gas mixture containing N₂ and H₂. The membrane was prepared via the direct deposition technique, and several permeation tests were performed to evaluate its permeation characteristics at temperatures between 290 °C and 350 °C and atmospheric pressure. For single gas permeation at 300 °C, NH₃ permeance is as high as 182 GPU, with NH₃/N₂ and NH₃/H₂ ideal selectivities of 11375 and >10¹⁰, respectively. For binary mixtures (10%/90% NH₃/N₂ and NH₃/H₂), NH₃ permeance as high as 825 GPU was achieved at the same operating conditions. Remarkably, higher permeance (~1100 GPU) was obtained with ternary mixtures (11%/67%/23% NH₃/H₂/N₂) at a temperature of 325 °C with NH₃ purity of 99.9%. The IMS membrane exhibits good stability over time, working for more than 180 hours with no significant performance loss. Characterization techniques such as TGA, SEM, and EDS were carried out and confirmed the feasibility of membrane use at high temperatures. This pioneering study shows that the IMS membrane can be a good candidate for downstream ammonia separation in the industrial H-B process.

References

[1] P. Tuna, C. Hulteberg, S. Ahlgren, Techno-economic assessment of nonfossil ammonia production, Environ Prog Sustain Energy. 33 (2014) 1290–1297. https://doi.org/10.1002/EP.11886.

[2] S. Ghavam, M. Vahdati, I.A.G. Wilson, P. Styring, Sustainable Ammonia Production Processes, Front Energy Res. 9 (2021) 34. https://doi.org/10.3389/FENRG.2021.580808/BIBTEX.

[3] V. Smil, Detonator of the population explosion, Nature 1999 400:6743. 400 (1999) 415–415. https://doi.org/10.1038/22672.

[4] M. Aziz, A. TriWijayanta, A.B.D. Nandiyanto, Ammonia as Effective Hydrogen Storage: A Review on Production, Storage and Utilization, Energies 2020, Vol. 13, Page 3062. 13 (2020) 3062. https://doi.org/10.3390/EN13123062.

[5] F.Y. Al-Aboosi, M.M. El-Halwagi, M. Moore, R.B. Nielsen, Renewable ammonia as an alternative fuel for the shipping industry, Curr Opin Chem Eng. 31 (2021). https://doi.org/10.1016/J.COCHE.2021.100670.

[6] 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.

[7] H. Liu, Ammonia Synthesis Catalysts, Ammonia Synthesis Catalysts. (2013). https://doi.org/10.1142/8199.

[8] K.H.R. Rouwenhorst, A.G.J. Van der Ham, L. Lefferts, Beyond Haber-Bosch: The renaissance of the Claude process, Int J Hydrogen Energy. 46 (2021) 21566–21579. https://doi.org/10.1016/J.IJHYDENE.2021.04.014.

[9] Z. Zhang, J.D. Way, C.A. Wolden, Design and operational considerations of catalytic membrane reactors for ammonia synthesis, AIChE Journal. 67 (2021) e17259. https://doi.org/10.1002/AIC.17259.