472251 Viability of Low Pressure Ammonia Synthesis Via Reactive Separation
Mahdi Malmali, Alon McCormick, and Edward L. CusslerDepartment of Chemical Engineering and Materials Science, University of Minnesota, Minneapolis, MN 55455
Ammonia is a crucial chemical primarily used as an agricultural fertilizer; it is the second most manufactured chemical in the world with more than 160 million tons produced per year1. Ammonia is produced via the Haber-Bosch process, where hydrogen (from thermochemical steam-reforming of methane) and nitrogen are combined and react over a promoted iron-based catalyst at temperatures ranging from 380° to 520° C and pressures from 150 to 300 bar2. Reaction at such an elevated pressures only achieves partial conversions (typically less than 30%), which necessitates the recycle of unreacted gas using costly phase separation processes. After more than a century of research and optimization, this process is still highly capital- and energy-intensive3, hence only economically viable in large-scale centralized plants.
We seek to develop technologies for small-scale synthesis of ammonia with zero-carbon footprint4,5. This process is powered by electricity produced from stranded intermittent wind or solar energy and can make hydrogen from electrolysis of water and nitrogen from pressure swing adsorption of air. A major drawback for widespread application of such a process is the high cost of the synthesized ammonia. The pressure in this process can be reduced to improve the economics of the process. This is achieved by simultaneous removal of the ammonia from the reaction system and circumventing the reaction equilibrium limitation.
We will initially present the ammonia reaction rate kinetics at low pressures. Then we discuss our findings of the reaction-separation process, which consists of a catalytic reactor and absorber unit; we report the absorption of ammonia on CaCl2, which is utilized to selectively remove the ammonia from the system at high temperatures. Our experimental results demonstrate the high capacity of the absorption material at close-to-reaction temperature, which suppresses the equilibrium and increases production rates in our reaction-separation system; conversions as high as 80% at significantly reduced pressure (< 20 bar) are achieved. The effect of different operating conditions, including reaction temperature, reaction pressure, absorption temperature, gas transport, etc. on the kinetics of ammonia synthesis will be discussed in detail. For proof-of-concept, we compare the results obtained from high pressure reaction and low pressure reaction-separation process.
(1) http://minerals.usgs.gov/minerals/pubs/commodity/nitrogen/mcs-2011-nitro.pdf. (2) Catalytic Ammonia Synthesis: Fundamentals and Practice; Springer Science & Business Media, 2013. (3) Vojvodic, A.; Medford, A. J.; Studt, F.; Abild-Pedersen, F.; Khan, T. S.; Bligaard, T.; Nørskov, J. K. Exploring the Limits: A Low-Pressure, Low-Temperature Haber–Bosch Process. Chem. Phys. Lett. 2014, 598, 108.
(4) 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.
(5) Himstedt, H. H.; Huberty, M. S.; McCormick, A. V.; Schmidt, L. D.; Cussler, E. L. Ammonia Synthesis Enhanced by Magnesium Chloride Absorption. AIChE J. 2015, 61 (4), 1364.