(287d) Inorganic Membranes: An Intensified Approach to Transforming Chemicals Separation

Amidst the pressing need to reduce climate change and mitigate greenhouse gas emissions, the demand for efficient separation technologies has never been more pronounced. In this context, metallic membranes present a paradigm-shifting solution. Operating at elevated temperatures and pressures, these membranes offer a transformative approach to the separation of hydrogen and ammonia, two pivotal chemicals in various industrial processes [1, 2].

The proof-of-concept of the hydrogen-selective membrane, e.g. Pd-based membrane, has been developed and studied over the years [1]. In a single unit operation, membranes deliver high purity hydrogen (by the permeation) and a separate mixture that can be sequestered at the downstream for further treatment. However, although, Pd-based membranes have shown hydrogen flux exceeding the 2015 flux targets set by US Department of Energy (~90 Nm³ H₂/m²h at 1.4 bar pressure difference) [3], their use is restricted by their relatively low microstructural stability in relevant permeation conditions owing to early onset of unselective leakage. A key to the realization and development of this promising technology is the synthesis of a reliable H₂-selective membrane material. Indeed, the long-term stability under relevant temperature and pressure conditions is still challenging due to microstructural changes occurring in the thin metallic membrane layer leading cracks and pinholes formation.

Regarding ammonia, its recovery from nitrogen and hydrogen during the production process is a critical unit operation. A series of heat exchangers and a final refrigeration stage are usually used to recover it, while the remaining gases are reheated and recycled to a catalytic converter. Research conducted by Cussler et al. suggests that substituting conventional ammonia separation methods with alternative techniques could facilitate carbon-free ammonia production at reduced operating pressures [4]. Well-known alternative approaches for ammonia separation are based on sorbents and membranes. In recent years, several studies have been reported the use of metal chlorides, such as MgCl₂, CaCl₂, SrCl₂, and CaBr₂, as the active phase of an ammonia sorbent material [2]. However, recently it has been identified a ZnCl₂ membrane stable at high temperature and highly selective towards ammonia permeation with respect hydrogen and nitrogen [2].

The present study aims to highlight the most relevant scientific results obtained on hydrogen and ammonia separation via inorganic membrane technology. Specifically, novel ternary Pd-based membranes for hydrogen permeation and ZnCl₂ immobilized molten salt membranes for ammonia separation are presented. Membrane performance in terms of permeance and ideal selectivities obtained at different temperature and pressure are reported, discussed and compared with literature.

References

[1] S Liguori, K Kian, N Buggy, BH Anzelmo, J Wilcox, Opportunities and Challenges of Low-Carbon Hydrogen via Metallic Membranes, PECS 80 (2020) 100851

[2] M. Adejumo, L. Oleksy, S. Liguori, Immobilized molten salt membrane for potential ammonia separation at high temperature, Chem. Eng J., 479, (2024), 147434

[3] Satyapal, S. 2011 Annual Progress Report: DOE Hydrogen and Fuel Cells Program; United States, 2011 https://doi.org/10.2172/1226002.

[4] C. Smith, M. Malmali, C.Y. Liu, A. V. McCormick, E.L. Cussler, Rates of Ammonia Absorption and Release in Calcium Chloride, ACS Sustain Chem Eng. 6 (2018) 11827–11835