(357a) Intensification of Ammonia Production

Haber-Bosch (H-B) process is cited as one of the most important inventions of the 20th century. Ammonia is sustaining the food supply for half the population. Although carefully and completely optimized through a century of continuous effort, H-B is still very energy- and capital-intensive. High pressure requirements of Haber-Bosch process imposes substantial operating (e.g., compression) and capital (compressor cost, advanced costly alloys, thick reactor casing, etc.) expenses in the ammonia production. Cost considerations force ammonia producers to take advantage of the economy of scale to drive down the manufacture cost, while small and energy-efficient processes that can be powered with off-grid renewable energy are required for ammonia-mediated hydrogen economy. Ammonia manufacturing accounts for 1-3% of world’s energy consumption, 5% of natural gas consumption, and a significant portion (~3%) of climate-changing gas emissions.3–5 These numbers incentivize us to utilize Modular Chemical Process Intensification (MCPI) methodologies to improve the energy efficiency of H-B process.

Small-scale reaction-absorption process is proposed to be a viable technology for enhanced ammonia production.^1–4 In this process, condenser is replaced by an absorber column, consisting of a packed bed of cheap, abundant metal halide salts (i.e., CaCl2) that operates at 180-250 °C. This separation unit allows lower pressure operation, which is advantageous for small-scale production. Here, we present an overview of our efforts to further intensify ammonia production via reaction-absorption process. Our target is to further improve absorbents, design catalysts that can operate at lower temperature, and optimize reaction-absorption operating conditions for lower pressure operations. Results indicate that lower pressure (P<10 bar) ammonia production is viable with optimizing operating absorptive separation. Better absorbents with the right chemistry and geometry, as well as optimized absorber conditions are key to achieve further improvements in production rates at lower pressure. This findings provide a more complete understanding of the proposed reaction-absorption process and proposes a path for further intensification of the ammonia production.

(1) Malmali, M.; Wei, Y.; McCormick, A.; Cussler, E. L. Ammonia Synthesis at Reduced Pressure via Reactive Separation. Ind. Eng. Chem. Res. 2016, 55 (33).

(2) Reese, M.; Marquart, C.; Malmali, M.; Wagner, K.; Buchanan, E.; McCormick, A.; Cussler, E. L. Performance of a Small-Scale Haber Process. Ind. Eng. Chem. Res. 2016, 55 (13).

(3) Malmali, M.; Reese, M.; McCormick, A. V.; Cussler, E. L. Converting Wind Energy to Ammonia at Lower Pressure. ACS Sustain. Chem. Eng. 2018, 6 (1).

(4) Malmali, M.; Le, G.; Hendrickson, J.; Prince, J.; McCormick, A. V.; Cussler, E. L. Better Absorbents for Ammonia Separation. ACS Sustain. Chem. Eng. 2018, 6 (5).