(496b) Dynamic Modeling and Techno-Economic Optimization of a Low-Pressure Microwave Assisted Ammonia Synthesis Process

Ammonia is an essential chemical being used as a fertilizer¹, specialty chemical, hydrogen carrier, and recently being investigated more as an energy fuel² due to its high energy intensity. Industrially ammonia is produced by the Haber-Bosch (HB) process that traditionally operates at very high pressure and temperature which results in costly energy demands, high capital investments, high CO₂ footprint³, as well as safety concerns. Microwave (MW)-assisted ammonia synthesis can occur close to ambient pressure and moderate temperature ⁴⁵. MW-assisted ammonia reactors can be modular with considerably smaller footprint than the traditional HB process thus enabling the opportunity for generation of ammonia in remote and isolated locations. MW-assisted reactors also offer fast startup and shutdown, thus opening great opportunities for using intermittent renewable energy sources. However, as the HB reaction is equilibrium-limited, single-pass conversion is often limited to less than 30% and therefore unreacted reactants needs to be separated from the product. While this separation is efficiently achieved under cryogenic condition for the traditional high-pressure HB processes, this separation becomes challenging under the low-pressure condition. One potential technology for this separation under low pressure condition is to use a solid sorbent⁶. However, modeling, synthesis and optimization of low-pressure ammonia synthesis processes using solid sorbent-based separations is largely lacking in the literature. One difficulty for economic analysis and optimization of this process is due to the transient nature of the adsorption-desorption process.

A kinetic model of the MW-assisted reactor is developed by using the experimental data from an in-house reactor. A plant-wide model of the ammonia synthesis process is developed with multi-stage reactors with interstage cooling and heat recovery and recycle of unreacted reactants. Two potential sorbents are considered- an alkali metal sorbent and a zeolite-based adsorbent^7,8. Isotherm models are developed for these sorbents. A dynamic model of a fixed bed contactor is developed, and pressure temperature swing adsorption (PTSA) cycles are simulated. An economic model of the process is developed. However, as the separation process is dynamic, it is difficult to use it directly for techno-economic optimization. Surrogate models are developed for the key performance measures of interest for techno-economic optimization as a function of the decision variables (both design and operating variables) of the separation process. Using design of experiments ⁹, input-output data are generated from the dynamic contactor model. A number of candidate basis functions are considered, and optimal selection of the surrogate model and parameter is done by using an information-theoretic criterion. The surrogate models are then used to solve a mixed-integer nonlinear optimization problem for techno-economic optimization. Among several decision variables that are optimized, one of the key decision variables is the optimal operating pressure of the reaction-separation system. It was observed that operation under atmospheric pressure may not be optimal under certain circumstances.

REFERENCES

1. Humphreys J, Lan R, Tao S. Development and Recent Progress on Ammonia Synthesis Catalysts for Haber–Bosch Process. Adv Energy Sustain Res. 2021;2(1):2000043. doi:10.1002/aesr.202000043
2. Smith C, Hill AK, Torrente-Murciano L. Current and future role of Haber-Bosch ammonia in a carbon-free energy landscape. Energy Environ Sci. 2020;13(2):331-344. doi:10.1039/c9ee02873k
3. Rouwenhorst KHR, Krzywda PM, Benes NE, Mul G, Lefferts L. Ammonia, 4. Green Ammonia Production. Ullmann’s Encycl Ind Chem. Published online 2020:1-20. doi:10.1002/14356007.w02_w02
4. Wildfire C, Abdelsayed V, Shekhawat D, Spencer MJ. Ambient pressure synthesis of ammonia using a microwave reactor. Catal Commun. 2018;115(May):64-67. doi:10.1016/j.catcom.2018.07.010
5. Wildfire C, Abdelsayed V, Shekhawat D, Dagle RA, Davidson SD, Hu J. Microwave-assisted ammonia synthesis over Ru/MgO catalysts at ambient pressure. Catal Today. 2020;(January 2020). doi:10.1016/j.cattod.2020.06.013
6. Malmali M, Le G, Hendrickson J, Prince J, McCormick A V., Cussler EL. Better Absorbents for Ammonia Separation. ACS Sustain Chem Eng. 2018;6(5):6536-6546. doi:10.1021/acssuschemeng.7b04684
7. Helminen J, Helenius J, Paatero E, Turunen I. Adsorption equilibria of ammonia gas on inorganic and organic sorbents at 298.15 K. J Chem Eng Data. 2001;46(2):391-399. doi:10.1021/je000273+
8. Helminen J, Helenius J, Paatero E, Turunen I. Comparison of sorbents and isotherm models for NH3-gas separation by adsorption. AIChE J. 2000;46(8):1541-1555. doi:10.1002/aic.690460807
9. Farooq S, Ruthven DM. Heat Effects in Adsorption Column Dynamics. 2. Experimental Validation of the One-Dimensional Model. Ind Eng Chem Res. 1990;29(6):1084-1090. doi:10.1021/ie00102a020