(350g) Distributed Ammonia Manufacturing

Authors: 
Malmali, M., University of Minnesota
Reese, M., University of Minnesota West Central Research and Outreach Center
McCormick, A., University of Minnesota, Twin Cities
Cussler, E. L., University of Minnesota
To eliminate the dependency on fossil fuel and the carbon dioxide emissions, carbon-free ammonia can be synthesized in small wind-powered plants, and from totally renewable resources: hydrogen from electrolysis of water and nitrogen from pressure swing adsorption of air. We are making carbon-neutral ammonia in the UMN-WCROC plant, located in the agricultural region of Morris MN.1 While this process is working, the ammonia costs twice that made conventionally, which is due to downscaling (40,000 scaled down process.) To overcome the difficulties associated with the energy demand for ammonia synthesis, and to improve the economics of the small plant, we are seeking a more efficient process to synthesize carbon-neutral ammonia.

One strategy to improve this process is to design a new class of materials for the catalysis, that allows synthesis at lower temperature and pressure. However, a century of research confirms that this is not trivial and immediately viable.2 Essentially with the conventional catalyst, high temperature is a must in order to obtain fast kinetics, while high pressure ensures the conversion. If we can efficiently remove low concentrations of ammonia formed in the reactor, then we can keep the reaction far from equilibrium, and intrinsically we obtain rates closer to forward reaction rates. Thus the constraints of equilibrium is surpassed. We have proposed the application of solid absorber for high temperature ammonia separations.3,4 In this process, ammonia can be synthesized at much lower pressure, with rates comparable to high-pressure conventional processes. The capital and operating costs of low pressure reaction-absorption process is improved since the work of compressing the fresh feed is significantly reduced, and the need for heat exchange also drops.

The low pressure reaction-absorption process suggests a strategy for making ammonia fertilizer locally, using wind-generated electricity. Alternatively, this ammonia made in rural area, can be used as a fuel, or a hydrogen carrier for fuel cell use in more urban areas.

References:

(1) 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), 3742.

(2) Schlögl, R. Catalytic Synthesis of Ammonia—A “Never-Ending Story”? Angew. Chemie Int. Ed. 2003, 42 (18), 2004.

(3) Malmali, M.; Wei, Y.; McCormick, A.; Cussler, E. L. Ammonia Synthesis at Reduced Pressure via Reactive Separation. Ind. Eng. Chem. Res. 2016, 55 (33), 8922.

(4) Wagner, K.; Malmali, M.; Smith, C.; McCormick, A.; Cussler, E. L.; Zhu, M.; Seaton, N. C. A. Column Absorption for Reproducible Cyclic Separation in Small Scale Ammonia Synthesis. AIChE J. 2017.