Enhanced Ammonia Production with Combined Reaction-Separation: Kinetic of Reaction and Adsorption
Mohammadmahdi Malmali, Kevin Wagner, Alon V. McCormick, Edward J. Cussler
Chemical Engineering and Material Science, University of Minnesota, 421 Washington Ave. SE, Minneapolis, MN 55455-0132
According to US Department of Health ammonia is the most commonly produced industrial chemical in US. About 13 million metric tons of ammonia is consumed in US, which more than 80% of this amount was used in agriculture sector as different forms of fertilizer. Ammonia is produced via catalytic Haber-Bosch process. Despite a century of optimization and improvement in catalyst, single-pass ammonia production is reaching only up to 20% conversion. This process also requires large capital costs with large amount of sacrificial fossil fuels to obtain raw materials. We have developed a novel reaction-separation process that enables us to increase the conversion to above 95% by simultaneous and selective adsorption of ammonia with magnesium chloride. The feed for this process is obtained from totally renewable resources, in which hydrogen and nitrogen are obtained from electrolysis of water and membrane separation of air, respectively. This process has a great potential of being a local scale source of ammonia production for farmers in North America, Sub-Saharan Africa and elsewhere in the world.
Here, we present latest results for simultaneous reaction-separation process, including a detailed in-depth study of adsorption of ammonia in a fixed bed. We investigate altered MgCl2 particle sizes, bed pressure, feed composition, temperature, absorbent dilution and their effects on absorbent capacity and the sharpness of the breakthrough curve. We also investigate these effects on the desorption curves, which will be important for an industrial scale continuous operation process. Our results indicate that diffusion of the ammonia in the bulk gas to the particle surface is fast, and the process is dominated by a recrystallization of the ammonia with the magnesium chloride. We also investigated the application of knowledge gained from adsorption studies to improve the kinetics of ammonia production in the reaction separation process. The improved adsorption characteristics enables us to gain faster reaction kinetics and optimize a viable process.
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