(374e) Low-Pressure Microwave Assisted Ammonia Synthesis – Evaluation of Novel Separation Technologies, Plant-Wide Process Design, and Technoeconomic Analysis

Ammonia is one of the key chemicals for producing food and delivering energy[1]. It has been estimated that 40% of the world population rely on ammonia to produce food [2]. Due to its high energy intensity, ammonia is used as a fuel in combustion engines and fuel cells[3]. Ammonia is produced industrially via the Haber-Bosch process. This process involves an exothermic reaction between nitrogen and hydrogen at a temperature between 400^-500^oC and at a pressure of 150-300 bar[4]. As a result of the extreme conditions, the Haber-Bosch process suffers from high capital cost and high energy consumption mainly due to compression [5] accounting for 1-2% of energy consumption, about 5% of natural gas consumption, and 1.6% of CO₂ emissions worldwide[6]. An alternative to this energy-intensive process with a large carbon footprint is to produce ammonia at considerably lower pressure and temperature using microwave (MW)-assisted catalytic technology.

Recently, several lab-scale experimental studies have reported the promise of ammonia synthesis in MW-assisted reactor at ambient pressure and moderate temperature [7], [8]. However, to the best of our knowledge, no plant-wide modeling and techno-economic analysis has been presented in the open literature for the MW-assisted NH₃ synthesis yet. One of the key challenges in low pressure ammonia synthesis is the separation of unreacted N₂ and H₂ from NH₃ in a cost-effective manner. While the high-pressure Haber-Bosch process facilitates this separation readily using simple flash separators at moderate temperature, the low-pressure synthesis would require much lower temperature for flash separation thus leading to high penalty due to cryogenic conditions. Thus, one of the focus of this work is also selection of a suitable technology by evaluating a number of traditional and novel technologies.

A model of the MW reactor is developed where the reaction rate parameters are estimated by using the in-house experimental data from a laboratory-scale MW-assisted reactor. In this work, a plant-wide model is developed and several separation technologies are evaluated, namely: (i) temperature swing absorption; (ii) cryogenic distillation; (iii) cryogenic flash separation; (iv) absorption using water; (v) absorption using ionic liquid. Each of these separation technologies are optimized along with consideration of optimal heat integration. A detailed techno-economic optimization was carried out considering the tradeoff between the capital costs and the operating costs. As some of the technologies evaluated here such as the temperature swing absorption and ionic liquid absorption are novel, an uncertainly analysis is undertaken. Results shows that the novel separation technologies hold high promise for facilitating commercialization of low-pressure MW-assisted NH3 synthesis technology. Our work also identifies where more research will be desired in reducing the uncertainties in techno-economic analysis.

References

[1] E. R. Morgan, J. F. Manwell, and J. G. McGowan, “Sustainable Ammonia Production from U.S. Offshore Wind Farms: A Techno-Economic Review,” ACS Sustain. Chem. Eng., vol. 5, no. 11, pp. 9554–9567, 2017, doi: 10.1021/acssuschemeng.7b02070.

[2] J. W. Erisman, M. A. Sutton, J. Galloway, Z. Klimont, and W. Winiwarter, “How a century of ammonia synthesis changed the world,” Nat. Geosci., vol. 1, no. 10, pp. 636–639, 2008, doi: 10.1038/ngeo325.

[3] C. Zamfirescu and I. Dincer, “Ammonia as a green fuel and hydrogen source for vehicular applications,” Fuel Processing Technology, vol. 90, no. 5. pp. 729–737, 2009, doi: 10.1016/j.fuproc.2009.02.004.

[4] A. Skodra and M. Stoukides, “Electrocatalytic synthesis of ammonia from steam and nitrogen at atmospheric pressure,” Solid State Ionics, vol. 180, no. 23–25. pp. 1332–1336, 2009, doi: 10.1016/j.ssi.2009.08.001.

[5] X. Cui, C. Tang, and Q. Zhang, “A Review of Electrocatalytic Reduction of Dinitrogen to Ammonia under Ambient Conditions,” Adv. Energy Mater., vol. 8, no. 22, 2018, doi: 10.1002/aenm.201800369.

[6] K. H. R. Rouwenhorst, P. M. Krzywda, N. E. Benes, G. Mul, and L. Lefferts, “Ammonia, 4. Green Ammonia Production,” Ullmann’s Encycl. Ind. Chem., pp. 1–20, 2020, doi: 10.1002/14356007.w02_w02.

[7] C. Wildfire, V. Abdelsayed, D. Shekhawat, and M. J. Spencer, “Ambient pressure synthesis of ammonia using a microwave reactor,” Catal. Commun., vol. 115, no. May, pp. 64–67, 2018, doi: 10.1016/j.catcom.2018.07.010.

[8] C. Wildfire, V. Abdelsayed, D. Shekhawat, R. A. Dagle, S. D. Davidson, and J. Hu, “Microwave-assisted ammonia synthesis over Ru/MgO catalysts at ambient pressure,” Catal. Today, no. January 2020, 2020, doi: 10.1016/j.cattod.2020.06.013.