(394a) Advances in Absorbent-Enhanced Ammonia Production

Decarbonized ammonia (NH₃) production has been identified as a key avenue to agricultural and more broadly energy sustainability. One renewable production concept is to use wind- or solar-powered electrolysis to obtain feedstock hydrogen (H₂). This electrolysis-based concept is well suited to small production scales due to the geographically distributed nature of renewable resources and the increased operational flexibility in the face of renewable intermittency afforded by smaller process units. However, the conventional ammonia production process which uses condensation to separate NH₃ from unreacted H₂ and nitrogen (N₂) does not scale down well due to its high pressure (above 85 bar) and cryogenic NH₃ separation (below 0ºC) and thus associated high capital cost [1].

The University of Minnesota proposed to replace the condenser used in the conventional process with a bed of ammonia-selective absorbent [2-4]. This absorbent allows for more complete removal of ammonia at lower pressure (between 20 and 30 bar) and higher temperature, eliminating the refrigeration need. We have previously developed a mathematical model and design optimization framework for this process [5]. Net present cost minimization of a first design iteration indeed revealed a lower required capital investment than the conventional process, but with approximately 4 times higher energy intensity [6]. Further experimental efforts have been undertaken to reduce this energy intensity and/or further reduce capital cost. Specifically, experimental work at UMN has focused on quantifying NH₃ absorption-desorption cycling parameter effects (i.e. absorption and desorption temperatures, cycle time) and improving this cycling [7,8] while other institutions have developed ammonia synthesis catalysts with improved activity at low temperatures and pressures [9,10].

In this work we describe key learnings from these experimental advances and incorporate their quantitative results into our model of the absorbent-enhanced process. We also investigate high temperature heat pump infrastructure to heat integrate exothermic NH_3­ absorption and endothermic NH₃ desorption, where the latter is thermodynamically constrained to operate at higher temperature than the former. We use our updated model to elucidate the potential benefits of these advances with respect to levelized cost of ammonia production, total capital investment, and energy intensity by optimizing absorbent-enhanced flowsheets and benchmarking these results against the condenser-based process. We consider distributed production scales from 5,000 up to 500,000 ton/year. For each production scale, we consider hydrogen sourced from both autothermal reforming of natural gas and electrolysis and perform sensitivity analysis to natural gas and electricity prices for these respective means of hydrogen production. This comprehensive technoeconomic analysis allows us to identify the production scales and market conditions for which the absorbent-enhanced process is best suited and guides the most important next steps in process development.

References

[1] Demirhan, Tso, Powell, & Pistikopoulos. (2018). Sustainable ammonia production through process synthesis and global optimization. AIChE J. 65(7), e16498.

[2] Himstedt, Huberty, McCormick, Schmidt, & Cussler. (2015). Ammonia synthesis enhanced by magnesium chloride absorption. AIChE J. 61(4), 1364-1371.

[3] Malmali, Wei, McCormick, & Cussler. (2016). Ammonia synthesis at reduced pressure via reactive separation. Ind. Eng. Chem. Res. 55, 8922-8932.

[4] Smith, Malmali, Liu, McCormick, & Cussler. (2018). Rates of ammonia absorption and release in calcium chloride. ACS Sustain. Chem. Eng. 6(9), 11827-11835.

[5] Palys, McCormick, Cussler, & Daoutidis. (2018). Modeling and optimal design of absorbent enhanced ammonia synthesis. Processes 6, 91.

[6] Palys, McCormick, Cussler & Daoutidis. (2019). Comparative Technoeconomic Analysis of Conventional and Absorbent-Enhanced Ammonia Synthesis. In 2019 AIChE Annual Meeting. Orlando, FL.

[7] Kale, Ojha, Biswas, Militti, McCormick, Schott, Dauenhauer, & Cussler. (2020). Optimizing ammonia separation via reactive absorption for sustainable ammonia synthesis. ACS Appl. Energy Mat. 3(3), 2576-2584.

[8] Ojha, Kale, Dauenhauer, McCormick, & Cussler. (2020). Desorption in ammonia manufacture from stranded wind energy. ACS Sust. Chem. Eng. 8(41), 15475-15483.

[9] Luz, Parvathikar, Carpenter, Bellamy, Amato, Carpenter, & Lail. (2020). MOF-derived nanostructured catalysts for low-temperature ammonia synthesis. Catal. Sci. Technol. 10(1), 105-112.

[10] Smith, C., & Torrente‐Murciano, L. (2021). Exceeding single‐pass equilibrium with integrated absorption separation for ammonia synthesis using renewable energy – Redefining the Haber‐Bosch loop. Adv. Energy Mater. 2003845.