Small Scale Low-Pressure Ammonia Synthesis


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Ammonia is one of the most important chemical commodities in the US and will be a key component in helping the world meet the rising demand for food and energy. Ammonia is needed in distributed locations for agriculture (as fertilizer for small grain and corn production), for indirect hydrogen storage1 (transported as a liquid at moderate pressure to hydrogen stations), or as a liquid fuel2 (for internal combustion engines or solid oxide fuel cells).

Recently, there has been significant effort to develop scalable technologies for conversion of intermittent energies (e.g., solar, wind) into energy dense carbon-neutral liquid fuels, and ammonia is considered to be a promising option. For the synthesis of carbon-neutral ammonia, hydrogen and nitrogen are delivered from electrolysis of water and pressure swing adsorption of air. Once produced, nitrogen and hydrogen are fed to a catalytic reactor to produce anhydrous ammonia. All these processes are carbon-free and powered by wind turbines or solar arrays.

In the first part of this talk, we are going to present a benchmark for the performance of a small-scale ammonia synthesis pilot plant powered by wind energy3. The Renewable Hydrogen and Ammonia Pilot Plant located in Morris, MN, is the first local farm-to-coop scale system for ammonia production. This plant targets production of small amounts of ammonia for local demand, thus representing distributed production. We will present the analysis of small plant runs, along with some laboratory-scale kinetic studies, to assess the performance of different units, including reaction, phase separation and recycle. A simple but insightful model is developed, in order to understand the performance of the small-scale Haber-Bosch plant and to determine the optimal conditions for operating. This model is successfully employed to predict the performance of each unit, and gives us insight into selecting appropriate unit operating conditions. Such a straightforward model may help us to understand the design of other small-scale processes in the future.

In the second part of this talk, we are going to specifically talk about the possibility of ammonia production at reduced pressure4; ammonia can be made at high temperature but lower pressure if the product ammonia is rapidly separated. We will present our systematic studies on the performance of the reaction-separation process, where we have qualitatively investigated the absorptive separation of ammonia using calcium chloride in a reaction-separation process. Absorption displays a rapid separation, which reduces the constraint of reversible reaction and enables us to obtain appropriate reaction rates at relatively lower pressure. The effect of different operating conditions – reaction temperature, pressure, absorption temperature and gas transport – on production rates will be discussed.

(1) Sørensen, R. Z.; Hummelshøj, J. S.; Klerke, A.; Reves, J. B.; Vegge, T.; Nørskov, J. K.; Christensen, C. H. Indirect, Reversible High-Density Hydrogen Storage in Compact Metal Ammine Salts. J. Am. Chem. Soc. 2008, 130 (27), 8660.
(2) Wojcik, A.; Middleton, H.; Damopoulos, I.; Van herle, J. Ammonia as a Fuel in Solid Oxide Fuel Cells. J. Power Sources2003, 118 (1-2), 342.
(3) 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
(4) Malmali, M.; Wei, Y.; McCormick, A.; Cussler, E. L. Ammonia Synthesis at Reduced Pressure via Reactive Separation. Ind. Eng. Chem. Res. 2016, acs.iecr.6b01880.