(618g) Lower Pressure Ammonia Synthesis

Ammonia is a very important chemical, mainly produced through the Haber-Bosch process. This process requires high temperature (>400 °C) and pressure (>150 bar) in order to ensure fast kinetics and high conversions, respectively.¹ As a result, ammonia synthesis is known to be very complex and energy-intensive.² To alleviate the complexity and energy requirements of ammonia synthesis, and to reduce the CO2 emissions, we are proposing an innovative reaction-absorption process to synthesize carbon-free ammonia in small plants.³ This green ammonia can be synthesized in wind-powered plants, with hydrogen from electrolysis of water and nitrogen from pressure swing adsorption of air.⁴

In our reaction-absorption process, we use the conventional catalyst and a solid absorber: hydrogen and nitrogen catalytically convert to ammonia at much lower pressure and absorber removes ammonia from the reactor at high temperature. The absorber not only enables us to reducing the constraints of reverse reaction, but also it removes the ammonia more completely. Thus the concentration of ammonia decreases in the recycle flow, leading to an increase in the driving force for the synthesis. In our low pressure reaction-absorption process, the work of compressing the fresh feed is significantly reduced, and the need for heat exchange also drops.

In this talk, I am going to discuss our most recent findings of the reaction-absorption process. Our results indicate that the ammonia production rates can be sustained at pressures as low as 10-20 bar. Our studies show that the ammonia synthesis via reaction-absorption process is no longer controlled by the chemical kinetics or restricted by the reverse reaction, but governed by the absorption of ammonia and the recycle of unreacted hydrogen and nitrogen. I will also discuss some of the latest improvements in the supported absorbents design for better ammonia uptake.⁵

References:

(1) Catalytic Ammonia Synthesis: Fundamentals and Practice; Springer Science & Business Media, 2013.

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

(3) Malmali, M.; Wickramasinghe, S. R. Continuous Hydrolysis of Lignocellulosic Biomass via Integrated Membrane Processes. In Integrated Membrane Systems and Processes; Basile, A., Charcosset, C., Eds.; John Wiley & Sons, 2016; pp 61–78.

(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) 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.