(446c) Experimental and Computational Evaluation of Ammonia Separation Using Eutectic Molten Salt Membranes

Developing an efficient ammonia separation process is crucial for improving the overall energy efficiency of ammonia production. The Haber-Bosch (HB) process, the primary method for ammonia production, operates under harsh conditions with low single-pass conversion (only about 20%) [1]. Effective ammonia separation and the recycling of unreacted gases are crucial to maximize process efficiency. Adopting membrane separation as an alternative to the energy-intensive condensation method can potentially reduce the overall energy consumption of the HB process and simultaneously achieve high-purity ammonia [2]. Herein, we propose using an immobilized molten salt membrane (IMS). These membranes have demonstrated an exceptional ammonia separation performance, achieving high ammonia selectivity and permeance at high temperatures. Zinc chloride (ZnCl₂) IMS has been reported to achieve incredible ammonia selectivity (>10⁷) and permeance (up to 182 GPU) at a temperature of 300 °C, but showed low long-term stability [3]. In this study, we employ eutectic mixing of zinc chloride (ZnCl₂) with lithium chloride (LiCl), copper chloride (CuCl₂), nickel chloride (NiCl₂), and manganese chloride (MnCl₂) to develop a highly stable IMS membrane for efficient ammonia separation at high temperatures. Binary eutectic salts will be prepared using the static melting method and characterized using a thermo-gravimetric analyzer (TGA) and differential scanning calorimeter (DSC). In addition, ammonia absorption capacities and solubility of the eutectic salts will be studied by sorption experiments using a customized TGA. The preliminary experimental results showed improved ammonia absorption capacities and thermal stability for the eutectic salts as compared to pure ZnCl₂. The eutectic salts with the best stability will be used to synthesize IMS membranes, and permeation experiments will be performed under different operating conditions to examine their ammonia separation performance. Furthermore, to understand the mechanism of ammonia absorption of these molten salts, a computation analysis using density functional theory (DFT) will be employed to study the interaction of ammonia with the molten salts ions.

Reference

[1] C. Smith, L. Torrente-Murciano, Exceeding Single-Pass Equilibrium with Integrated Absorption Separation for Ammonia Synthesis Using Renewable Energy—Redefining the Haber-Bosch Loop, Adv Energy Mater 11 (2021) 2003845. https://doi.org/https://doi.org/10.1002/aenm.202003845.

[2] 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. https://doi.org/10.1039/c9ee02873k.

[3] 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/https://doi.org/10.1016/j.cej.2023.147434.